Structure and Electrochemical Behavior of



STRUCTURE AND ELECTROCHEMICAL BEHAVIOR OF NITROGEN DOPED DIAMOND-LIKE CARBON THIN FILMS WITH OR WITHOUT PLATINUM AND RUTHENIUM DOPING with or without Platinum and Ruthenium DopingSTRUCTURE AND ELECTROCHEMICAL BEHAVIOR OF NITROGEN DOPED DIAMOND-LIKE CARBON THIN FILMS WITH OR WITHOUT PLATINUM AND RUTHENIUM DOPING KHUN NAY WINKHUN NAY WINSCHOOL OF MECHANICAL AND AEROSPACE ENGINEERING20112011STRUCTURE AND ELECTROCHEMICAL BEHAVIOR OF NITROGEN DOPED DIAMOND-LIKE CARBON THIN FILMS WITH OR WITHOUT PLATINUM AND RUTHENIUM DOPING KHUN NAY WINSchool of Mechanical and Aerospace EngineeringA thesis submitted to the Nanyang Technological Universityin partial fulfillment of the requirement for the degree ofDoctor of Philosophy2011AbstractHuman activities have released toxic metals such as Zn, Pb, Cd, Cu and Hg, etc. into the environment. Nowadays, the presence of toxic metals in the aquatic ecosystem implicates directly to biota and indirectly to human beings. Therefore, fast detection and determination of toxic metals in aqueous solutions are a tough challenge for analysts.Diamond-like carbon (DLC) is a type of carbon, which consists of both sp2 (graphite like) and sp3 (diamond like) bonds, and an environmentally friendly material. In addition, DLC films can be produced at room temperature and achieve similar properties to those of diamond films, so they have been explored as electrode materials for heavy metal tracing. However, high electrical resistivity of DLC films has confined their electrochemical applications. DLC films used as electrodes for electrochemical applications must be conductive. Nitrogen is an effective donor in DLC films because of its five valance electrons. Therefore, nitrogen is used as a dopant for making conductive nitrogen doped diamond-like carbon (N-DLC) films. However, an incorporation of nitrogen in DLC films lowers the corrosion resistance of DLC films by degrading sp3-bonded cross-linking structure through increased sp2 bonds though it can increase the electrical conductivity of the films. Poor corrosion resistance of N-DLC films can affect the electrochemical performance of the films such as sensitivity, long-time response stability, durability and repeatability.It has been known that noble metals such as Pt and Ru are inherently corrosion resistant and have outstanding catalytic properties. Pure Pt is much more expensive compared to pure Ru, so alloying of Pt with Ru is an economical way to reduce the cost of the electrode. Besides, Ru is also alloyed in Pt to keep good mechanical properties of the films due to its ability to harden platinum. It has been reported that PtRu phases incorporated in an amorphous carbon structure exist as nano aggregates. When these PtRu aggregates appear on the surface of DLC films, they play a role of catalytic property which facilitates charge transfer at electrode/solution interface. It is expected that incorporation of Pt/Ru/N in DLC films may promote the electrical conductivity and electrochemical performance of the films such as corrosion resistance and sensitivity for heavy metal tracing. Therefore, the electrochemical behavior of nitrogen doped DLC (N-DLC) films with or without Pt and Ru and its dependence on the film structure together with some other physical characteristics were investigated in this research for practical application.The corrosion behavior of the N-DLC films deposited on p-Si substrates using a filtered cathodic vaccum arc (FCVA) deposition system by varying nitrogen flow rate was investigated in 0.6 M NaCl solutions with both potentiodynamic polarization and immersion tests at room temperature. The results revealed that the increased nitrogen content in the N-DLC films degraded the corrosion resistance of the films. Potential windows of the N-DLC films measured in 0.5 M HCl, 0.1 M KCl, 0.1 M NaCl, 0.1 M KOH, and 0.1 M NaOH were 2.4, 2.32, 3.2, 3.1 and 3.25 V, respectively. Though the N-DLC film electrodes offered (1) wide potential windows in different types of solutions, (2) a very low and stable background to improve the signal-to-background and signal-to-noise ratios, (3) the repeatability of voltammograms, and (4) long-time response stability, their voltammograms were apparently affected by their electrical conductivity, types of alkaline species and unbalanced H+ and OH- ions. These film electrodes also provided a significant stripping response for determination of single-element (Zn2+, Pb2+, Cu2+, and Hg2+) and multi-elements (Pb2+ + Cu2+ + Hg2+) simultaneously in KCl solutions. It was observed that the sensitivity of the film electrodes to the elements detected was apparently influenced by nitrogen concentration in the films, deposition time and potential, and concentration and type of elements. The simultaneous analysis of heavy metals using linear sweep anodic stripping voltammetry (LSASV) produced sharp and well-defined peaks with good peak separations. The novel N-DLC film electrodes under development showed a great promise for the detection of heavy metals. Pt/Ru/N doped DLC (PtRuN-DLC) films were deposited on p-Si substrates by a DC magnetron sputtering deposition system by varying DC power applied to a Pt50Ru50 target. The increased Pt and Ru incorporation in the PtRuN-DLC films improved the corrosion resistance of the films in the NaCl solution at lower potential though more sp2 bonds were formed in the films via metal-induced graphitization. However, the PtRuN-DLC films with higher Pt/Ru content degraded early than the ones with lower Pt/Ru content at higher potential. Noble metal incorporation appears to be a promising way to improve the corrosion resistance of the N-DLC films. Furthermore, N-DLC and PtRuN-DLC films were deposited on p-Si substrates using a DC magnetron sputtering system under the same deposition conditions except Pt and Ru doping for the PtRuN-DLC film in order to get more understanding of the effect of Pt and Ru incorporation on the structural and electrochemical properties of the N-DLC films. Though the N-DLC film electrodes showed wide potential windows in acidic solutions such as 0.1 M H2SO4 and 0.1 M HCl and a neutral solution of 0.1 M KCl, it was found that the Pt and Ru doping significantly narrowed down the potential windows of the N-DLC film electrodes in these solutions due to their catalytic activities. The N-DLC film electrodes showed a good electrocatalytic activity in Fe(CN)64-/Fe(CN)63- redox reactions. However, an increased kinetic limitation upon the PtRuN-DLC film electrodes with Pt and Ru doping shifted the oxidization peak to a more positive value and the reduction peak to a more negative value compared to those obtained from the N-DLC film electrodes. It could be deduced that the introduction of Pt and Ru into the N-DLC films improved the corrosion resistance of the films but significantly degraded the electrochemical behavior of the films. AcknowledgementsI would like to express my sincere appreciation and gratitude to my supervisor, Assoc. Prof. Liu Erjia, for his invaluable advice and encouragement throughout the duration of this project. He has not only provided enthusiasm and support but also imparted his personal wisdom that will last forever. Without his help, the project would be impossible to accomplish and the attainment would be much compromised. Co-Supervisor, Dr. Zeng Xianting, is also gratefully acknowledged.My sincere thanks would be extended to Research student, Wang Huili, for his valuable discussion in the research. I would like to show my special thanks to the technicians in Materials Lab 1 and Fuel cell Lab, School of Mechanical and Aerospace Engineering, Nanyang Technological University, for their technical assistance and support.I sincerely thank Prof. Sam Zhang and his students for their assistance for my sample preparation and analysis. I would like to greatly thank Assoc. Prof. Jiang San Ping for his PtRu alloy target and analytical instruments used for my research work.I would like to thank my parents: Mr. Nay Win and Dr. Khin Myint Myint, for their love and support to my research. My deep gratitude also goes to my parents-in-law: Mr. Zaw Win and Madam Mya Mya Thin for their care and support to my family during my PhD study. This thesis is dedicated to my wife Madam Zar Chi Win and my daughter Shin Thant Mon. Thank also goes to Nanyang Technological University, Singapore for providing a PhD scholarship to this research. The financial support from the Environment & Water Industry Development Council (EWI), Singapore is gratefully acknowledged. Last but not least, I would like to thank my friends and many others who have in one way or another contributed to the completion of the work.List of PublicationsJournal papersN. W. Khun, E. Liu, “Enhancement of adhesion strength and corrosion resistance of nitrogen or platinum/ruthenium/nitrogen doped diamond-like carbon thin films by platinum/ruthenium layer”, Diamond and Related Materials, 19 (2010) 1065.N. W. Khun, E. Liu, “Effect of platinum and ruthenium incorporation on voltammetric behavior of nitrogen doped diamond-like carbon thin films”, Electroanalysis, 21 (2009) 2590.N. W. Khun, E. Liu, G. C. Yang, W. G. Ma, S. P. Jiang, “Structure and corrosion behavior of platinum/ruthenium/nitrogen doped diamond-like carbon thin films”, Journal of Applied Physics, 106 (2009) 013506.N. W. Khun, E. Liu, X. T. Zeng, “Corrosion behavior of nitrogen doped diamond-like carbon thin films in NaCl solutions”, Corrosion Science, 51 (2009) 2158.N. W. Khun, E. Liu, “Linear sweep anodic stripping voltammetry of heavy metals from nitrogenated tetrahedral amorphous carbon thin films”, Electrochimica Acta, 54 (2009) 2890.N. W. Khun, E. Liu, “Cyclic voltammetric behavior of nitrogen doped tetrahedral amorphous carbon films deposited by filtered cathodic vacuum arc”, Electroanalysis, 20 (2008) 1851.PatentE. Liu, W. G. Ma, A. P. Liu, G. C. Yang, N. W. Khun, Z. M. Wang, “Diamond-like carbon based microelectrode sensor and associated electrochemical cell for detection of trace heavy and toxic metals in aqueous solutions”, US Patent (12/842,857).Table of ContentsPageAbstract iAcknowledgements vList of Publications viiTable of Contents ixList of Nomenclatures xvList of Figures xviList of Tables xxiiiChapter 1 Introduction 1 1.1. Synopsis 1 1.2. Objective 7 1.3. Scope 7 1.4. Organization 8Chapter 2 Literature review 10 2.1. Carbon 10 2.1.1. Diamond 11 2.1.2. Graphite 12 2.1.3. Diamond films 13 2.1.4. Diamond-like carbon (DLC) 13 2.2. Doping of DLC films 16 2.3. Deposition methods of DLC films 21 2.3.1. Magnetron sputtering deposition 22 2.3.2. Cathodic arc 24 2.3.3. Pulsed laser deposition (PLD) 25 2.4. Surface morphological characteristics of DLC films 26 2.5. Adhesion strength of DLC films 28 2.6. Electrochemistry 31 2.6.1. Corrosion mechanisms 31 2.6.1.1. Linear polarization 34 2.6.1.2. Potentiodynamic polarization 35 2.6.1.3. Tafel plot 36 2.6.1.4. Electrochemical impedance spectroscopy 37 2.6.2. Corrosion properties of DLC films 41 2.7. Electrode materials for electroanalysis 44 2.8. Electrochemistry of DLC films 49 2.8.1. Cyclic voltammetry 52 2.8.2. Anodic stripping voltammetry 55 2.8.3. Stripping analysis of DLC films 56 2.8.4. Potential applications of DLC films as electrodes for electroanalysis 59 2.9. Summary 61Chapter 3 Experimental details 63 3.1. Sample preparation 63 3.2. Characterization 65 3.2.1. Film structure 65 3.2.2. Film surface activity, morphology and topography 67 3.2.3. Adhesion strength of the film to p-Si substrate 67 3.3. Electrochemical measurement 68 3.3.1. Sample preparation 68 3.3.2. Electrochemical workstation 68 3.3.3. Setup of electrochemical cell 69 3.3.4. Potentiodynamic polarization test 70 3.3.5. Immersion test 70 3.3.6. Electrochemical impedance spectroscopy 71 3.3.7. Cyclic voltammetry 71 3.3.8. Anodic stripping voltammetry 72 3.4. Summary 72Chapter 4 Structural and Electrochemical Properties of Nitrogen Doped Diamond-like Carbon Thin Films 74 4.1. Introduction 74 4.2. Structural properties of N-DLC thin films 76 4.2.1. Chemical composition of N-DLC thin films 76 4.2.2. Raman results of N-DLC thin films 78 4.2.3. Surface morphology of N-DLC thin films 79 4.2.4. Adhesion strength of N-DLC thin films 80 4.3. Electrochemical performance of N-DLC thin films 83 4.3.1. Corrosion behavior of N-DLC thin films 83 4.3.1.1. Potentiodynamic polarization results of N-DLC thin films 83 4.3.1.2. Immersion results of N-DLC thin films 88 4.3.2. Linear sweep cyclic voltammetric behavior of N-DLC thin films 92 4.3.2.1. Cyclic voltammetry of N-DLC thin films in acidic solution 92 4.3.2.2. Cyclic voltammetry of N-DLC thin films in neutral solutions 94 4.3.2.3. Cyclic voltammetry of N-DLC thin films in hydroxide solutions 96 4.3.2.4. Cyclic voltammetry of reversible couple (Ferricyanide) 98 4.3.3. Linear sweep anodic stripping voltammetric behavior of N-DLC thin films 99 4.3.3.1. Linear sweep anodic stripping voltammograms of Lead 99 4.3.3.2. Linear Sweep Anodic Stripping Voltammograms of Zinc and Lead 106 4.3.3.3. Linear Sweep Anodic Stripping Voltammograms of Copper 107 4.3.3.4. Linear Sweep Anodic Stripping Voltammograms of Mercury 109 4.3.3.5. Linear Sweep Anodic Stripping Voltammograms of Simultaneous Lead, Copper and Mercury 111 4.3.3.6. Concentration effect for the ions traced by LSASV 113 4.4. Conclusions 115Chapter 5 Structural and Electrochemical Properties of Platinum/ruthenium/nitrogen Doped Diamond-like Carbon Thin Films 118 5.1. Introduction 118 5.2. Structural properties of PtRuN-DLC thin films 119 5.2.1. Chemical composition of PtRuN-DLC thin films 119 5.2.2. Microstructure of PtRuN-DLC thin films 121 5.2.3. XPS results of PtRuN-DLC thin films 122 5.2.4. Raman results of PtRuN-DLC thin films 126 5.2.5. Surface activity of PtRuN-DLC thin films 129 5.2.6. Surface morphology of PtRuN-DLC thin films 131 5.2.7. Adhesion strength of PtRuN-DLC thin films 131 5.3. Electrochemical performance of PtRuN-DLC thin films 133 5.3.1. Corrosion behavior of PtRuN-DLC thin films 133 5.4. Conclusions 138Chapter 6 Comparative Study of Structural and Electrochemical Properties of Nitrogen-doped and Platinum/ruthenium/nitrogen-doped Diamond-like Carbon Thin Films 140 6.1. Introduction 140 6.2. Structureal properties of N-DLC and PtRuN-DLC thin films 141 6.2.1. Chemical composition of N-DLC and PtRuN-DLC thin films 141 6.2.2. Microstructure of PtRuN-DLC thin films 142 6.2.3. Chemical bonding structure of N-DLC and PtRuN-DLC thin films measured by XPS 142 6.2.4. Chemical bonding structure of N-DLC and PtRuN-DLC thin films measured by Raman spectroscopy 144 6.2.5. Surface activity of N-DLC and PtRuN-DLC thin films 145 6.2.6. Surface morphology of N-DLC and PtRuN-DLC thin films 146 6.2.7. Adhesion strength of N-DLC and PtRuN-DLC thin films 147 6.3. Electrochemical performance of N-DLC and PtRuN-DLC thin films 149 6.3.1. Corrosion behavior of N-DLC and PtRuN-DLC thin films 149 6.3.2. Linear sweep cyclic voltammetric behavior of N-DLC and PtRuN-DLC thin films 151 6.3.2.1. Cyclic voltammetry of N-DLC and PtRuN-DLC thin films 151 6.3.2.2. Cyclic voltammetry of reversible couple (Ferricyanide) 156 6.4. Conclusions 158Chapter 7 Conclusions 160 7.1. Conclusions on N-DLC films prepared by a FCVA technique 160 7.2. Conclusions on PtRuN-DLC films prepared by a DC magnetron sputtering technique 162 7.3. Conclusions on N-DLC and PtRuN-DLC films prepared by a DC magnetron sputtering Technique 163 7.4. Recommendations for future work 165References 166List of Nomenclaturesa-CAmorphous carbonAFMAtomic force microscopyASVAnodic stripping voltammetryBDDBoron doped diamondCVCyclic voltammetryCVDChemical vapor deposition DCDirect currentDLCDiamond-like carbonN-DLCNitrogen doped diamond-like carbonDPASVDifferential pulse anodic stripping voltammetryEASElectrochemically active speciesFCVAFiltered cathodic vacuum arcFWHMFull width at half maximumLSASVLinear sweep anodic stripping voltammetryPtRuN-DLCPlatinum/ruthenium/nitrogen doped diamond-like carbonOCPOpen circuit potentialPLD Pulsed laser deposition PVDPhysical vapor depositionSCEStandard calomel electrodeSEMScanning electron microscopyta-CTetrahedral amorphous carbonXPSX-ray photoelectron spectroscopyList of FiguresPageFig. 2.1: The sp3, sp2, sp1 hybridized bonding. 10Fig. 2.2: Ideal diamond structure. 11Fig. 2.3: Crystal structure of graphite. 12Fig. 2.4: Structure of amorphous carbon. 14Fig. 2.5: Ternary phase diagram of bonding in amorphous carbon-hydrogen alloys. 15Fig. 2.6: Schematic DOS of a carbon showing σ and π states. 17Fig. 2.7: Schematic of the levels of substitutional nitrogen in diamond and ta-C. 18Fig. 2.8: Various nitrogen configurations in DLC, showing the doping configuration. One dot means an unpaired electron. Two dots mean a lone pair (non-bonding). 19Fig. 2.9: Diagram showing how nitrogen modifies bonding in graphitic layer, by introducing five-membered rings and warping, or inter-layer bonding. 20Fig. 2.10: General feature of sputter coater. 23Fig. 2.11: FCVA source design. 24Fig. 2.12: Configuration of a PLD chamber. 26Fig. 2.13: SEM images of ta-C films deposited at various substrate biases: (a) 0 V, (b) 250 V, (c) 300 V and (d) 400 V. 27Fig. 2.14: Electrical double layer for electrode submerged in an electrolyte. 32Fig. 2.15: Cyclic polarization curve. 33 Fig. 2.16: Linear polarization. 34Fig. 2.17: Potentiodynamic polarization curve. 35Fig. 2.18: Tafel plot. 37Fig.2.19: (a) An equivalent circuit proposed for a porous free coating. (b) Nyquist plot and (c) Bode magnitude and (d) phase plots of the coating. 39Fig.2.20: (a) An equivalent circuit proposed for a coating with porosities. (b) Nyquist plot and (c) Bode magnitude and (d) phase plots of the coating. 41Fig. 2.21: Cyclic voltammetric i-E curves for boron doped diamond electrodes in (a) 1 M KCl and (b) Fe(CN)6-3/-4 + 1 M KCl. 46Fig. 2.22: Cyclic voltemmetric i-E curves obtained (a) N-DLC electrode (dash line) and glassy carbon electrode (solid line) in 1 mM Fe(CN)6/0.1 M H2SO4 and (b) N-DLC electrode in 0.5 M H2SO4. 48Fig. 2.23: Bright-field transmission electron micrograph of a Pt-DLC composite film. Platinum nanoparticles self-assemble into arrays (dark regions) within the DLC matrix (light regions). 50Fig. 2.24: Typical cyclic voltammogram. 53Fig. 2.25:?Typical DPASVs obtained at N-DLC film electrode in a 0.1 M KCl (pH 1.0) solution. 57Fig. 3.1: Schematic configuration of a FCVA deposition system. 63 Fig. 3.2: Schematic configuration of a magnetron sputtering system. 64Fig. 3.3: Basic feature of a micro-scratch tester. 68Fig. 3.4: (a) Size of a film-coated sample and (b) film-coated samples. 68Fig. 3.5: (a) Schematic configuration and (b) outlook of a three-electrode electrochemical cell. 69Fig. 4.1: Fitted XPS C 1s spectra of N-DLC films deposited with nitrogen flow rates of (a) 0.5 and (b) 20 sccm. 77Fig. 4.2: Raman spectra of N-DLC films deposited with different nitrogen flow rates. The inset shows ID/IG and AD/AG as a function of nitrogen flow rate. 78Fig. 4.3: (a) Ra values of N-DLC films versus nitrogen flow rate. AFM images of N-DLC films deposited with nitrogen flow rates of (b) 0.5 and (c) 20 sccm. 80Fig. 4.4: (a) Critical loads of N-DLC films with respect to nitrogen flow rate and (b) SEM micrograph of a N-DLC film (0.5 sccm N2) scratch tested till a critical load of 456 mN. HP and LP indicate high pressure and low pressure areas, respectively. The inset is the progressive loading curve measured, from which the critical load is determined. 81Fig. 4.5 Potentiodynamic polarization curves of N-DLC films measured in a 0.6 M NaCl solution at room temperature. 84 Fig. 4.6: SEM micrographs showing surface morphologies of N-DLC films after potentiodynamic polarization tests: (a) 0.5 sccm, Ecorr = -85.72 mV vs. SCE and (b) 3 sccm, Ecorr = -57.41 mV vs. SCE, where the insets in the bottom right corners show their enlarged views of locations A and B, respectively. 85Fig. 4.7: Corrosion potentials (Ecorr) and polarization resistances (Rp) of N-DLC films as a function of nitrogen flow rate. 86Fig. 4.8: SEM micrographs of the corroded areas of N-DLC film coated samples after immersion tests in 0.6 M NaCl solutions with different pH values: (a) pH 2, (b) pH 4.5 and (c) pH 12 for the films deposited with 3 sccm N2 and (d) pH 4.5 for the film deposited with 20 sccm N2 for comparison. All the tests are conducted for 336 hr at room temperature and ambient atmosphere. 89Fig. 4.9: Cyclic voltammograms of N-DLC film electrodes with different nitrogen flow rates measured in 0.5 M HCl solution at a scan rate of 100 mV/s. 93Fig. 4.10: Cyclic voltammograms of N-DLC film electrodes with different nitrogen flow rates measured in 0.1 M KCl solution at a scan rate of 100 mV/s. 94Fig. 4.11: Cyclic voltammogram of N-DLC (20 sccm N2) film electrode measured in 0.1 M NaCl solution at a scan rate of 100 mV/s. 95Fig. 4.12: Cyclic voltammogram of N-DLC (20 sccm N2) film electrode measured in 0.1 M KOH solution at a scan rate of 100 mV/s. 96Fig. 4.13: Cyclic voltammogram of N-DLC (20 sccm N2) film electrode measured in 0.1 M NaOH solution at a scan rate of 100 mV/s. 97Fig. 4.14: Cyclic voltammograms of N-DLC (20 sccm N2) film electrode measured in 5 mM K3Fe(CN)6 /0.1 M NaCl solution at different scan rates: (a) 30, (b) 50, (c) 70, (d) 90, (e) 110, and (f) 130 mV/s. 98Fig. 4.15: Stripping voltammograms obtained from N-DLC film electrodes deposited with (a) 3 and (b) 20 sccm N2 in a 1 × 10-3 M Pb2+ + 0.1 M KCl solution as a function of deposition potential. Dependence of (c) stripping peak current and (d) stripping potential of Pb on deposition potential. The deposition time and scan rate are 120 s and 36.36 mV/s, respectively. 100Fig. 4.16: A linear sweep cyclic voltammogram obtained from a N-DLC film electrode deposited with 3 sccm N2 in the same solution as the one used for Fig. 4.13 with a scan rate of 36.36 mV/s. 101 Fig. 4.17: Stripping voltammograms obtained from a N-DLC film electrode (20 sccm N2) in a 1 × 10-6 M Pb2+ + 0.1 M KCl solution as a function of deposition potential. The deposition time and scan rate are 120 s and 36.36 mV/s, respectively. 102 Fig. 4.18: Stripping voltammograms obtained from N-DLC film electrodes deposited with (a) 3 and (b) 20 sccm N2 in the same solution as the one used for Fig. 4.13 as a function of deposition time, dependence of (c) stripping peak current and (d) stripping potential of Pb2+ on deposition time, and (e) relationship between stripping peak current and potential. The deposition potential and scan rate are -1.2 V and 36.36 mV/s, respectively. 104 Fig. 4.19: Stripping voltammograms obtained from a N-DLC electrode (20 sccm N2) in two different 0.1 M KCl solutions containing 1 × 10-2 M Zn2+ and 1 × 10-3 M Pb2+, respectively. The scan rate, deposition time and deposition potential are 36.36 mV/s, 120 s and -1.2 V, respectively. 106Fig. 4.20: Stripping voltammograms obtained from a N-DLC film electrode (20 sccm N2) in a 2 × 10-5 M Cu2+ + 0.1 M KCl solution as functions of (a) deposition potential and (c) deposition time. Dependence of stripping peak current and stripping potential on (b) deposition potential and (d) deposition time, respectively. The deposition time is 120 s (a and b), the deposition potential is -1.2 V (c and d), and the scan rate used for all the tests is 36.36 mV/s. 108Fig. 4.21: Stripping voltammograms obtained from a N-DLC film electrode (20 sccm N2) in a 1.1 × 10-6 M Hg2+ + 0.1 M KCl solution as functions of (a) deposition potential and (b) deposition time. Dependence of stripping peak current on (c) deposition potential and (d) deposition time. The deposition time is 120 s (a and c), the deposition potential is -1.2 V (b and d), and the scan rate for all the tests is 36.36 mV/s. 110Fig. 4.22: (a) Stripping voltammograms obtained from a N-DLC film electrode (20 sccm N2) in a 0.1 M KCl solution containing 8.9 × 10-6 M Pb2+ + 2.5 × 10-5 M Cu2+ + 9.2 × 10-6 M Hg2+ as a function of deposition time. Dependence of (b) stripping peak current and (c) stripping potential on deposition time. The deposition potential and scan rate are -1.2 V and 36.36 mV/s, respectively. 112 Fig. 5.1: N/(C+Ru+Pt+N), Pt/(C+Ru+Pt+N) and Ru/(C+Ru+Pt+N) atomic ratios with respect to DC power applied to Pt50Ru50 target during film depositions. 120 Fig. 5.2: TEM micrograph of a PtRuN-DLC film deposited with a DC power of 30 W applied to Pt50Ru50 target. 122Fig. 5.3: XPS spectrum of a PtRuN-DLC film deposited with DC power of 30 W applied to Pt50Ru50 target. 122Fig. 5.4: Fitted XPS spectra of a PtRuN-DLC film deposited with DC power of 15 W applied to Pt50Ru50 target: (a) C 1s + Ru 3d, (b) N 1s, (c) Pt 4f and (d) Ru 3p. The insets show the relevant XPS spectra of PtRuN-DLC films as a function of DC power applied to Pt50Ru50 target. 126 Fig. 5.5: Raman spectrum together with fitted G and D peaks of a PtRuN-DLC film deposited with DC power of 15 W applied to Pt50Ru50 target. The inset shows the Raman spectra of PtRuN-DLC films deposited with different DC powers applied to Pt50Ru50 target. 127Fig. 5.6: Results from the fitted Raman spectra of PtRuN-DLC films as shown in the inset of Fig. 5.5: (a) peak positions, (b) FWHMs and (c) ID/IG ratios of D and G peaks. 129Fig. 5.7: Water contact angles of PtRuN-DLC films as a function of DC power applied to Pt50Ru50 target. The insets show the water droplets on the surfaces of the films deposited with DC powers of (a) 15 and (b) 30 W applied to Pt50Ru50 target. 130 Fig. 5.8: AFM images showing surface topographies of PtRuN-DLC films deposited with DC powers of (a) 15 and (b) 30 W applied to Pt50Ru50 target. 131Fig. 5.9: Critical loads of PtRuN-DLC films as a function of DC power applied to Pt50Ru50 target. 132Fig. 5.10: (a) Potentiodynamic polarization curves, (b) corrosion current (Icorr) and polarization resistance (Rp), and (c) corrosion potential (Ecorr) of PtRuN-DLC films as a function of DC power applied to Pt50Ru50 target. The insets in (c) show SEM micrographs of corroded areas of PtRuN-DLC films deposited with DC powers of 30 (top left) and 25 W (bottom right) applied to Pt50Ru50 target. 135Fig. 6.1: TEM image of PtRuN-DLC film. 142Fig. 6.2: Fitted XPS C 1s and C 1s + Ru 3d spectra of (a) N-DLC and (b) PtRuN-DLC films, respectively, and fitted XPS N 1s spectra of (c) N-DLC and (d) PtRuN-DLC films. 143 Fig. 6.3: Raman spectra of N-DLC and PtRuN-DLC films, where G and D represent fitted G and D peaks, respectively. 145Fig. 6.4: Water droplets on (a) N-DLC and (b) PtRuN-DLC film surfaces. 146 Fig. 6.5: AFM images showing surface morphologies of (a) N-DLC and (b) PtRuN-DLC films. 147 Fig. 6.6: SEM micrographs showing surface morphologies of scratched (a) N-DLC and (b) PtRuN-DLC films. The inset in (a) shows the scratch track of the N-DLC film. 147 Fig. 6.7: (a) Nyquist and (b) Bode plots of N-DLC and PtRuN-DLC films measured in 0.1 M HCl solution. The frequency range is 105 – 10-2 Hz and the amplitude is 10 mV. The inset in (a) shows an equivalent circuit for electrochemical reactions on N-DLC and PtRuN-DLC coated samples. 150Fig. 6.8: Cyclic voltammograms measured from N-DLC (solid line) and PtRuN-DLC (dash line) film electrodes in (a) 0.1 M H2SO4 solution, (b) 0.1 M HCl solution and (c) 0.1 M KCl solution, where scan rate is 100 mV/s. 153Fig. 6.9: Cyclic voltammograms measured from (a) N-DLC film electrodes with different scan rates and (b) N-DLC (solid line) and PtRuN-DLC (dash line) film electrodes in 1 mM K3Fe(CN)6/0.1 M HCl solution, where scan rate is 10 mV/s. 157List of TablesPageTable 1.1: Approximate potential ranges for platinum, mercury and carbon in aqueous and non-aqueous electrolytes. 3Table 3.1: Process parameters of N-DLC films. 63Table 3.2: Process parameter of PtRuN-DLC films. 64Table 3.3: Process parameter of N-DLC and PtRuN-DLC films. 65Table 6.1: Chemical compositions and sp2/sp3 ratios of N-DLC and PtRuN-DLC films. 141Table 6.2: Results determined from fitted Raman spectra as shown in Fig. 6.3. 145Table 6.3: Water contact angles, surface roughnesses and critical loads of N-DLC and PtRuN-DLC films. 146Table 6.4: Results determined from EIS spectra based on the proposed equivalent circuit as shown in the inset of Fig. 6.7a. 151Table 7.1: Major findings from the three major work chapters (4 to 6). 164Chapter 1Introduction1.1. SynopsisCarbon is a unique and intriguing material with a diversity of technological applications. With a 1s2 2s2 2p2 electronic ground state configuration, carbon naturally exists in many allotropic forms such as graphite, diamond, bucky ball (C60) and so on. In the last few decades an important scientific and technological breakthrough occurred with the discovery that diamond thin films can be successfully grown by many chemical and physical vapor deposition techniques, artificially.Diamond-like carbon (DLC) is a type of carbon material that contains both sp2 (graphite-like) and sp3 (diamond-like) bonds. DLC has attracted researchers’ attention three decades ago due to its outstanding properties similar to those of diamond. The unique properties of DLC films and their modifications, together with the possibility to adjust the properties by choosing the right deposition parameters, make them suitable for a variety of applications. Some of the exploited properties of DLC materials are high hardness, high wear resistance, low friction coefficient, high chemical inertness, good infrared transparency, high electrical resistivity, and low dielectric constant. DLC has found novel applications in many fields. For example, DLC films deposited at low temperatures can be a suitable wear-protective layer on products made of plastics (e.g. on sunglass lenses made of polycarbonate) [1]. DLC films have been widely used in hard disk drives in which ultra thin DLC films are used for both magnetic disks and heads. Some other tribological applications of DLC films are for cutting tools, bearings, gears and seals. DLC is biocompatible so that DLC films can protect biological implants from corrosion and serve as diffusion barriers due to their chemical inertness and dense structures impermeable to biofluids. Depositions of DLC films on stainless steel and titanium alloys used for the components of artificial heart valves also satisfy both mechanical and biological requirements, thus resulting in the improved performance of these components [2]. Nowadays, human activity has released toxic metals into the environment. Pollutants in water include a wide spectrum of chemicals, trace metals, pathogens, and chemical or sensory changes due to the contamination, over-use and mismanagement of water resources [3]. Many of the chemical substances are toxic, for example, pathogens can produce waterborne diseases in human or animal hosts. Preventing of pollution is treated by determination of wastewater discharge to the aquatic environment and water emission limitations. The presence of toxic metals such as mercury, lead, copper, etc. in the environment has been a source of worry to environmentalists, government agencies and health practitioners because these metals in the aquatic ecosystem have far-reaching implications directly to the biota and indirectly to human beings. A concentration of Pb > 4.8 × 10-7 M is detrimental to foetuses and children with possible development of neurological problems [3]. For Cu, a concentration of 7.87 × 10-5 M can give adverse chronic effects [3]. Mercury occurs in deposits throughout the world. It is harmless in insoluble forms, such as mercuric sulfide, but it is poisonous in soluble forms such as mercuric chloride or methylmercury. Mercury can enter the environment from natural sources such as volcanoes, stationary combustion such as power plants, productions such as gold, non-ferrous metals, mercury itself, etc., waste disposal including municipal and hazardous water, crematoria, and the deposal of certain products such as auto parts, batteries, etc. [4]. Case control studies have shown the effects of trace mercury, such as tremors, impaired cognitive skills, and sleep disturbance in human beings, with chronic exposure to mercury even at low concentrations in the range of 3.4 ×10-9 – 2 × 10-7 M [5, 6]. A study has shown that acute exposure (4-8 hr) to calculated elemental mercury level of 5.48 × 10-6 to 2.14 × 10-4 M can cause chest pain, dyspnea, cough, hemoptysis, impairment of pulmonary function, and evidence of interstitial pneumonitis [7]. Although copper and zinc have been found to have low toxicity to human beings, prolonged consumption of a large dose can result in some health complications. Therefore, fast detection and determination of heavy metals are a tough challenge for electrochemical analysts. Electroanalytical techniques are relatively simple, quick, cheap and easy to use in situ for measurements in rivers or lakes [8, 9]. Anodic stripping voltammetry (ASV), adsorptive stripping voltammetry (ASV) and cathodic stripping voltammetry (CSV) have allowed the determination of a wide range of inorganic and organic species down to concentrations of an order of 10-10 M under favorable conditions.Table 1.1: Approximate potential ranges for platinum, mercury and carbon in aqueous and non-aqueous electrolytes [9]. TBAP tetrabutylammonium perchlorate DMF dimethylformamide TBABF4 tetrabutylammonium tetrafluoroborate CAN acetonitrileIn the past, mercury was the first metal to be extensively used for electroanalytical purposes in the form of the dropping mercury electrode because of its cyclic operation – continual drop growth, release and renewal – avoiding many of the problems of electrode poisoning in complex matrix. The drawback of the mercury electrodes is that the positive parts of their electrochemical potential windows in aqueous solutions are very small as shown in Table 1.1 [9]. Therefore, the detection limit can be too high and there are some experimental manipulation difficulties because of its toxicity. There are some metallic ions that cannot be determined at mercury (for example, Au and Ag). Therefore, solid electrode materials have been developed in order to permit oxidation reactions to be studied but there are no electrode materials being as good as mercury for studying reduction owing to its extended negative limits. However, carbon has become common in the form of glassy carbon, graphite, carbon paste and carbon fiber electrodes. Although glassy carbon is one of the most widely used electrode materials in electroanalytical applications due to its robustness, smooth surface and wide electrochemical window, irreproducible background contribution and gradual loss of surface activity frequently affect its electroanalytical performance. Noble metals like Pt and Au have been commonly used for microelectrodes. However, in an aqueous solution, detection using such metal electrodes is often impossible at negative potentials due to high currents produced by hydrogen evolution reactions on Pt [9]. Metal and graphite electrodes are limited by surface oxidation and reduction in terms of sensitivity [10].Boron doped diamond (BDD) film electrodes prepared by chemical vapor deposition (CVD) can have remarkable properties such as wide potential window in aqueous and non-aqueous media, low and stable background current, weak adsorption of polar molecules, high resistance to electrode deactivation and fouling, long-time response stability and superbmicrostructural and morphological stability at high temperatures [11, 12]. BDD electrodes have been successful in stripping voltammetric analysis of Pb, Mn, Cd, Cu and Ag [13-15]. However, fabrication of diamond films demands a high substrate temperature which is impossible for microfabrication of semiconductors and microelectrode arrays [16]. In addition, difficulties in controlling the deposition parameters limit the applications of diamond films. Recently, there has been much interest in the use of conductive DLC films for electrochemical measurements [17]. DLC films can be deposited over a large surface area with high uniformity of near-atomic level. A large variety of microstructures can be obtained, ranging from graphite-like to diamond-like depending on the deposition methods and process parameters employed, which has been reflected by the considerable variations of physical and mechanical properties measured. The acceptance of dopants makes them electrically conductive. Smoother surface topography, better adhesion to the underlying substrate, and relative ease in fabrication compared to diamond films make DLC films a desirable choice in certain applications. The unique combination of their chemical and mechanical properties opens a possibility for their applications as electrode materials in electrochemistry. DLC films also exhibit a low double-layer capacitance, large electrochemical window, and low background density. In addition, compared to the traditionally used mercury coated graphite electrode, DLC is environmentally friendly. When combined with the advantage of ready deposition at ambient temperature and convenient masking for fabricating microelectrode arrays and sensors, the properties of DLC films have suggested a valuable potential in electroanalysis. DLC films produced by physical vapor deposition (PVD) and chemical vapor deposition (CVD) methods at room temperature have been explored as electrode materials for heavy metal tracing [18-24]. However, high electrical resistivity and residual stress of DLC films have confined their electrochemical applications. The electrical conductivity of DLC films is very important for electrochemical analysis because it can abruptly affect the sensitivity of the film electrodes. High residual stress in DLC films apparently reduces the adhesion strength of the films. In a corrosion environment, poor adhesion strength of DLC films allows undermining and delaminating of the films by attacking the interfacial bonds between the films and their underlying substrates with electrochemically active species permeated through the porosities in the films. The characteristics of DLC films can be altered by incorporating different elements such as H, N, F, Si, Pt, Au, Ni, Ti, V, etc. in the films [25-27]. It is known that an introduction of nitrogen into DLC films can lower both the electrical resistivity and corrosion resistance of the films because of increased sp2 sites in the films [18]. However, the introduction of nitrogen into DLC films lowers the corrosion resistance of the films due to the increased sp2 sites in the films. The poor corrosion resistance of N doped DLC films can abruptly affect the electrochemical properties of the films such as sensitivity, repeatability, long-time response stability and durability when the DLC films are used as film electrodes for electrochemical purposes. Therefore, an improvement of the corrosion resistance of N doped DLC (N-DLC) films becomes important for electrochemical applications.Pt and Ru possess high resistances to chemical attack, excellent high-temperature characteristics, and stable electrical properties. Pt does not oxidize in air and is insoluble in hydrochloric and nitric acids though it does dissolve in aqua regia [28]. Therefore, Pt/Ru/N doped DLC (PtRuN-DLC) thin films to be developed as novel materials in this project are expected to have the following potential applications in the areas of water quality technology, reclaimed water technology and water technologies in industrial applications: (a) qualitative and quantitative electrochemical analyses (e.g. detection of heavy metals such as Zn, Pb, Cu, Hg, etc) and (b) water purification and disinfection. The novelty of this project is:N-DLC thin films with or without Pt and Ru doping to be developed are novel materials for electrochemical applications.Investigation of the structural and electrochemical properties of the novel thin films.1.2. ObjectiveThe main objective of this project is to develop N-DLC thin films with or without Pt and Ru doping and to study their structural and electrochemical characteristics, which forms two parts:Optimization of the deposition conditions of the N-DLC and PtRuN-DLC thin films by controlling the doping levels of N, Pt and Ru in the films in order to improve the electrical conductivity of the films and, at the same time, maintain the good electrochemical performance of the films.Investigation of the structural and electrochemical properties of the N-DLC and PtRuN-DLC films.1.3. ScopeThe N-DLC and PtRuN-DLC thin films will be deposited on p-type Si substrates using filtered cathodic vacuum arc (FCVA) and DC magnetron sputtering deposition methods. The film deposition conditions, such as substrate bias, sputtering power density, gas flow rate and deposition duration, will be optimized.The film chemical composition will be measured using X-ray photoelectron spectroscopy (XPS).The film microstructure will be observed using transmission electron microscopy (TEM).The film chemical structure will be diagnosed using XPS and micro-Raman spectroscopy. The film surface activity, morphology and topography will be studied using contact angle measurement, scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively.The adhesion of the films to the substrates will be evaluated using micro scratch test.The corrosion resistance of the films in NaCl and HCl solutions will be studied using potentiodynamic polarization test, immersion test and electrochemical impedance spectroscopy (EIS).The cyclic voltammetric behavior of the films in different aqueous solutions, such as HCl, H2SO4, KCl, NaCl, KOH, and NaOH, will be studied using linear sweep cyclic voltammetry.Anodic stripping voltammetric behavior of the films is to be evaluated in terms of trace heavy metals, such as Zn, Pb, Cu, Hg, etc., by using linear sweep anodic stripping voltammetry.The relationship between the microstructure and electrochemical properties of the films will be systematically investigated.1.4. OrganizationThis thesis has seven chapters. Chapter 1 provides background, objectives and scope of the dissertation. Chapter 2 summarizes the state-of-the-art on the development of DLC films. Chapter 3 details the experimental methodologies used for the current research. Results are systematically analyzed and discussed in Chapters 4 to 6. Based on the analysis of the results, conclusions are drawn in the last chapter. Chapter 2Literature review2.1. CarbonCarbon is one of the commonest elements throughout the Universe. Carbon is a chemical element that has the symbol C and atomic number 6. Carbon is the lightest element of 4A group. The electronic configuration for carbon is written as 1s22s22p2. An abundant nonmetallic, tetravalent element, carbon has several allotropic forms. As the free element it forms allotropes from differing kinds of carbon-carbon bonds. Different forms include the hardest naturally occurring substance (diamond) and one of the softest substances (graphite) known. Depending on its allotropic form and the impurities, carbon can be an insulator, conductor, or semiconductor. Fig. 2.1: The sp3, sp2, sp1 hybridized bonding [29].Carbon forms a great variety of crystalline and disordered structures because it is able to exist in three hybridizations, sp3, sp2 and sp1 (Fig. 2.1). In the sp3 configuration, as in diamond, a carbon atom’s four valance electrons are each assigned to a tetrahedrally directed sp3 orbital, which makes a strong σ bond to an adjacent atom. In the three-fold coordinated sp2 configuration as in graphite, three of the four valance electrons enter trigonally directed sp2 orbitals, which form σ bonds in a plane. The fourth electron of the sp2 atom lies in a pπ orbital, which lies normal to the σ bonding plane. This π orbital forms a weaker π bond with a π orbital on one or more neighboring atoms. In the sp1 configuration, two of the four valence electrons enter σ orbitals, each forming a σ bond directed along the ± x-axis, and the other two electrons pπ orbitals in the y and z directions. Although it forms an incredible variety of compounds, most forms of carbon are comparatively unreactive under normal conditions, e.g., it does not react with sulfuric acid, hydrochloric acid, chlorine or any alkalis. 2.1.1. DiamondDiamond is composed of the single element carbon and the structure is shown in Fig. 2.2. The diamond structure is cubic and can be viewed as two interpenetrating FCC structure displaced by (?, ?, ?) ao along the body diagonal. The cube edge length a0 is 3.567 ? and the nearest-neighbor carbon distance is 1.544 ?. Each of the carbon atoms in the structures is tetrahedrally bonded to the four nearest carbon atoms at the corners of a regular tetrahedron by strong covalent sp3 bonds. Fig. 2.2: Ideal diamond structure [2]. Diamond is one of the best known allotropes of carbon, whose hardness and high dispersion of light make it useful for industrial applications and jewelry. Most notable are its extreme hardness, its high dispersion index, and high thermal conductivity, with a melting point of 3820 K (3547 °C / 6420 °F) and a boiling point of 5100 K (4827 °C / 8720 °F). Naturally occurring diamond has a density ranging from 3.15 to 3.53 g/cm?. Diamond is the hardest natural material known to man - its hardness set to 10 (hardest) on Mohs scale of mineral hardness and having an absolute hardness value of between 90, 167, and 231 GPa in various tests, which makes it an excellent abrasive. Other specialized applications also exist or are being developed, including use as semiconductors: Diamond with a band gap of 5.45 eV is considered among the best electrical insulators. Through doping, diamond can be made into an excellent p-type semiconductor. 2.1.2. GraphiteGraphite is one of the allotropes of carbon. Graphite holds the distinction of being the most stable form of solid carbon ever discovered. Graphite is composed of the single element carbon and the structure is shown in Fig. 2.3. Fig. 2.3: Crystal structure of graphite [2].Each carbon atom is covalently bonded to three other surrounding carbon atoms. The flat sheets of carbon atoms are bonded into hexagonal structures. These exist in layers, which are not covalently connected to the surrounding layers. Instead, different layers are connected together by weak forces called van der Waals forces. The unit cell dimensions are a = b = 2.456 ?, c = 6.694 ?. The carbon-carbon bond length in the bulk form is 1.418 ?, and the interlayer spacing is c/2 = 3.347 ?. Each carbon atom possesses a sp? orbital hybridization. The π orbital electrons delocalized across the hexagonal atomic sheets of carbon contribute to graphite's conductivity. In an oriented piece of graphite, conductivity parallel to these sheets is greater than that perpendicular to these sheets. Graphite can conduct electricity due to the vast electron delocalization within the carbon layers. These electrons are free to move, so are able to conduct electricity. However, the electricity is only conducted within the plane of the layers. The bond between the atoms within a layer is stronger than the bond of diamond, but the force between two layers of graphite is weak. Therefore, layers of it can slip over each other making it soft.2.1.3. Diamond filmsIn the past two decades an important scientific and technological breakthrough occurred with the discovery that diamond thin films can be successfully grown by a large variety of chemical deposition technique. The feature common to all these methods is that decomposed hydrocarbon radicals impinge upon a hot (> 900 ?C) surface in the presence of atomic hydrogen. Since the procedure enhances the formation of sp3 over sp2 bonds, diamond can be grown. The films grown by these techniques are usually polycrystalline, consisting of agglomerations of randomly oriented, small diamond crystallites (several micron in size), and the films thus tend to have rough surface morphologies. Raman spectroscopic studies of diamond films usually show a sharp peak at 1332 cm-1, typical for sp3 bonded carbon, superimposed on a broad peak at about 1500 cm-1 which is due to graphitic sp2 bonding.2.1.4. Diamond-like carbon (DLC) Diamond-like carbon (DLC) is a metastable form of amorphous carbon (Fig. 2.4) containing a significant fraction of sp3 bonds. DLC has some extreme properties similar to diamond, such as hardness, elastic modulus and chemical inertness, but these are achieved in an isotropic disordered thin film with no grain boundaries. Diamond-like carbon (DLC) films lack any long-range order and contain a mixture of sp3, sp2 and sometimes even sp1 coordinated carbon atoms in a disordered network. The bond types have a considerable influence on the material properties of amorphous carbon films. If the sp2 type is predominant, the film will be softer, and if the sp3 type is predominant, the film will be harder. Fig. 2.4: Structure of amorphous carbon [2].The structure of DLC modeled by Robertson [30] is a random network of covalently bonded carbon atoms in the different hybridizations, with a substantial degree of medium range order on the 1 nm scale. DLC is a name attributed to a variety of amorphous carbon materials with carbon atoms bonded in mainly sp3 and sp2 hybridizations, some containing up to about 50 at% hydrogen (a-C:H), other containing less than 1 % hydrogen (a-C). The DLC films contain a significant fraction of sp3 bonds, giving them attractive physical and mechanical properties that are similar to those of diamond films. The properties of DLC films are determined by the relative ratio of the two hybridizations (sp3 and sp2). The a-C:H films typically contain sp3 fractions smaller than 50%, while the a-C films can contain up to 85% sp3 bonds. A-C:H is commonly used to designate the hydrogenated form of diamond-like carbon, while a-C is used to designate the non-hydrogenated carbon [31].The compositions of the various forms of a-C:H alloys on a ternary phase diagram are displayed in Fig. 2.5, as first used by Jacob et al. [32]. There are many a-Cs with disordered graphitic ordering, such as glassy carbon and evaporated a-C, which lie in the lower left hand corner. High degree of sp3 bonding in a-C can be extended by sputtering method. If the fraction of sp3 bonding reaches a high degree, the a-C is denoted as tetrahedral amorphous carbon (ta-C), to distinguish it from sp2 a-C [33]. Non-hydrogenated ta-C, which has small fractions of sp2 bonds, has a very rigid network. Fig. 2.5: Ternary phase diagram of bonding in amorphous carbon-hydrogen alloys [32].Various materials derived from DLC films have been developed to change and improve their properties. Such materials are similar in structure to DLC but containing Ti, Zr, W, Nb, Cr, Ni, Fe, Mo, La2O3 or WC and non-metals such as Si, N, B, F and P in addition to carbon and/or hydrogen [34-38]. Most modifications made to DLC are to reduce its internal stresses, surface energy and friction coefficient, or to modify its electrical properties [39, 40].Both the hydrogenated and non-hydrogenated DLC are metastable materials and their structures will change towards graphite–like carbon by either thermal activation or irradiation with energetic photons or particles [41, 42]. It has been reported that thermal activation can induce changes in DLC films and cause the conversion of some sp3 carbon bonds to sp2 bonds [42].The properties of DLC films, such as hardness, elastic modulus and internal stress, are directly correlated to the fraction of sp3 bonds in the films. The hardness of DLC films is in the range of 10-30 GPa [43], with a corresponding Young’s modulus 6-10 times larger. The hardness of ta-C films can reach higher values (in the range of 40-80 GPa) [44]. The high internal stress limits the thickness of films that can be used for any application, often to less than 1 ?m thick. The stress can be reduced by incorporating N, Si, or metals in the films or by building multilayered structures comprising soft and hard films, although the reduction in the stress is often associated with a reduction in hardness and elastic modulus of the films [45, 46]. 2.2. Doping of DLC filmsDLC films with a high sp3 content appear more suitable for device fabrication, since they are smooth, have no grain boundaries and a high electrical resistivity and band gap up to about 3 eV[47]. The availability of a doping method is a necessary condition for their use as active elements in semiconductor devices. Given the structural and chemical similarities of diamond and silicon, it can be expected that the doping of DLC films can be achieved in the same way as in the case of amorphous silicon (a-Si) films. However, no evidence of the formation of even a simple p-n junction based on DLC on a Si substrate has been reported till now. Doping process is a way to both decrease the stress in DLC films and make them electrically conductive. There are two processes for doping in DLC films: ion implantation and in-situ doping in film deposition.By implanting acceptor or donor ions, such as B+, BF+, P+, C+ and metal ions, the conductivity of DLC films is found to increase once a certain threshold dose has been reached. But the measurement has not revealed any significant difference between C and B implantation despite the valence difference between C and B. It is therefore concluded from the work that the electrical effect was due to implantation-induced damage and not to chemical doping caused by the particular implanted ions. Inhomogeneous dopant concentration, damage and graphitization make ion implantation for DLC films difficult in practice.In-situ doping, there are three ways to choose. First, a gas, containing the dopant atoms, is introduced into a gas mixture during film growth (such as boron doping or nitrogen doping). Second, dopant elements are impregnated into a solid carbon source. Third, dopant ion beams, which are developed independently, are introduced into deposition processes. Any semiconductor is only really useful when it can be doped both n- and p-types. This is a particular problem in all wide-gap semiconductors. Carbon can exist in three hybridizations, sp3, sp2 and sp1. The σ bonds of all carbon sites form occupied σ states in the valance band and empty σ* states in the conduction band, separated by a wide σ-σ* gap (Fig. 2.6). The π bonds of sp2 and sp1 sites form filled π states and empty π* states, with a much narrower π-π* gap [48, 49]. Fig. 2.6: Schematic DOS of a carbon showing σ and π states [48].Semiconductor can only be doped to one polarity, either because of the dopant levels that are too deep, or the low solubility, or because of auto-compensation. Solubility is not an issue, as the flexibility of a random network allows atoms of any size to be incorporated. The main problem in amorphous semiconductor is that network flexibility allows atoms to also exert their chemically preferred valance [50] and also form a trivalent non-doping site [51]. Undoped ta-C is slightly p-type with EF lying just below mid gap [52]. Nitrogen has a similar size to carbon, so nitrogen is soluble. Nitrogen is an obvious candidate as a donor in ta-C. It is noted that ta-C has a narrower gap than diamond (Fig. 2.7), so that level like N which is deep in diamond can be shallow in ta-C [53]. Generally, the doping efficiency of nitrogen is low in amorphous semiconductors because N can adopt so many bonding configurations in a carbon network. Apart from the sp3 substitutional sites which are doping, these are the sp2 substitutional site (doping) and many non-doping sites such as pyridine, pyrrole and nitrile. It is because it encourages carbon to form sp2 bonding. Fig. 2.7: Schematic of the levels of substitutional nitrogen in diamond and ta-C [53].Bonding in carbon nitride systems is complex since both atoms; carbon and nitrogen can exist in three hybridizations, sp3, sp2 and sp1. Therefore, at least nine different bonding configurations are possible, as shown in Fig. 2.8. Fig. 2.8: Various nitrogen configurations in DLC, showing the doping configuration. One dot means an unpaired electron. Two dots mean a lone pair (non-bonding) [54].In its simple trivalent configuration (a) N30, nitrogen forms three σ bonds with the remaining two electrons in a lone pair. Nitrogen in a four-fold coordinated substitutional site (b) N4+, uses four electrons in σ bonds with the remaining unpaired electron available for doping. A variant of this site is the (c) N4+ – C3- pair; which forms a trivalent carbon site and the unpaired electron transfers to the carbon to give a positive N+ site/C- defect pair. The remaining configurations correspond to π bonding. Nitrogen can substitute for carbon in a benzene ring (d) pyridine and (e) doped pyridine-like. Another alternative, (f) pyrrole, is of nitrogen bonded to three neighbors in a five-fold ring, here three electrons are in bonds and the other two are used to complete the sextet of the aromatic ring. The other variants of π bonding are; with nitrogen two-fold coordinated, where a double bond unit (g) has two electrons in σ bonds and one in a π bond, leaving a non-bonding pair, and a double bond unit (h) which uses three electrons in σ bonds, one in a π bond and the fifth in an antibonding π* state, available for doping [54]. The last configuration (i) is the triple bond with an isolated lone pair; as in cyanide. Substitutional N can dope graphite C by donating π electrons. The N uses three of its valance electrons to form three σ bonds, and one electron to fill the π states and its fifth electron enters the π* state, giving a ‘π-doping’. Fig. 2.9: Diagram showing how nitrogen modifies bonding in graphitic layer, by introducing five-membered rings and warping, or inter-layer bonding [29].New types of local bonding are found in sputtered N-DLC films. Nitrogen has one more electron than carbon, so that nitrogen in a five-fold pyrrole ring now gives the six π electrons needed for aromatic stability [29]. These rings can introduce warping in a graphite layer (Fig. 2.9). Another effect of nitrogen is to favor inter-layer bonding. Substituting nitrogen for C breaks a π bond and leaves an unpaired electron of the remaining C which is available to make σ bond to a similar atom on an adjacent layer.Bai et al. reported [55] that N incorporation increased the sp2 bonds which contributed to lower resistivity of the N-DLC films. This is due to the three factors: (1) N could act as a weak donor; the higher the donor density in the films is, the higher the electrical conductivity of the films is, (2) N raises the Fermi level towards the conduction band and (3) N narrows the band gap by graphitization of the bonding [55]. The last factor is dominant for the increase of conductivity with annealing temperature and N concentration.The wide-gap diamond-like carbon is not in itself electrochemically active; however, it acquires electrochemical activity upon introducing platinum into the DLC bulk during the film deposition. In Pt incorporated DLC films, Pt usually cannot be evenly doped [56]. The effect of platinum is shown to be of a threshold nature: the electrochemical current appears at approximately 3 at% and saturates at approximately 10 at% of Pt [56]. By contrast, the differential capacitance increases continuously with increasing Pt content. The observed effects are explained in terms of a model assuming non-uniform characters of both the electrical conductance in the DLC bulk and the catalytic effect of Pt at the electrode/solution interface. A DLC film is a sp3-hybridized carbon matrix, which contains areas of sp2-hybridized states. The DLC film has a hopping type conductance, which is associated with charge carrier hopping between the sp2-hybridized states. The metal may also be non-uniformly distributed in the DLC matrix: atoms of metals form clusters. Isolated metal clusters are electrically inactive in the film bulk. However, when appearing on the film surface, the Pt clusters play the role of catalytic-active sites, which facilitate the charge transfer at the electrode/solution interface. With increasing Pt concentration in the DLC film, the distance between the Pt clusters decreases both in the film bulk and on its surface. When a threshold value of the Pt concentration (approx. 3 at%) is reached, the distances between the Pt clusters and the nearest sp2-hybridized states on the surface drop down to some critical values which ensure their effective exchange with charge carriers. Therefore, effective current flows occur both in the electrode bulk and through the electrode/solution interface. 2.3. Deposition methods of DLC filmsIn order to grow DLC film instead of graphitic films, one has to circumvent the natural tendency of carbon to form stable sp2 graphite-like bonds as opposed to sp3 metastable diamond-like bonds. In order to obtain the metastable structure of DLC, such films are deposited by plasma assisted chemical vapor deposition or physical vapor deposition techniques (sputtering or ion beams) using a variety of precursors. The deposition is performed in hydrogen-containing environment to obtain DLC films containing 10-50 at% hydrogen. The hydrogen is required for obtaining “diamond-like carbon” properties for these materials. The deposition of hydrogen free DLC requires a carbon source and energy for the carbon species. The superhard properties of DLC films are achieved by the high energies of the impinging particles that form the films due to the films grown by subplantation [44]. The required high energies of the depositing species are achieved by different variations of cathodic arc discharges, such as filtered arc, pulsed arc, laser controlled arc, pulsed laser depositions or mass selected ion beams. 2.3.1. Magnetron sputtering depositionThe most common industrial process for the deposition of DLC is sputtering [57]. The most common form uses the DC or RF sputtering of a graphite target by Ar plasma. Because of the low sputter yield of graphite, magnetron sputtering is often used to increase the deposition rate.In DC sputtering as shown in Fig. 2.10, the electrons that are ejected from the cathode are accelerated away from the cathode and are not efficiently used for sustaining the discharge. By the suitable application of a magnetic field, the electrons can be deflected to stay near the target surface and by an appropriate arrangement of the magnets, the electrons can be made to circulate in a closed path on the target surface. This high flux of electrons creates a dense plasma near the cathode at low pressures so that ions can be accelerated from the plasma to the cathode without loss of energy due to physical and charge-exchange collisions. This allows a high sputtering rate with a lower potential on the target than with the DC diode configuration. The most common magnetron source is the planar magnetron where the sputter-erosion path is a closed circle on a flat surface. The magnetic field in magnetron sputtering can be formed using permanent magnet or electromagnetic or a combination of the two. Fig. 2.10: General feature of sputter coater [57].The disadvantage of the magnetron sputtering configurations is that the plasma is confined near the cathode and is not available to activate reactive gases in plasma near the substrate for reactive sputter deposition. This disadvantage can be overcome by applying an unbalanced magnetron configuration where the magnetic field can be configured to pass across to the substrate such that some electrons can escape from the cathode region. This system helps the formation of sp3 bonding in carbon film.Reactive sputter deposition from an elemental target relies on: (a) the reaction of the depositing species with a gaseous species, such as nitrogen and (b) reaction with a co-depositing species to form a compound. The reactive gas is in the molecular state (e.g., N2) and “activated” by dissociation of molecular species to more chemically reactive radicals (e.g., N2 + e-→ 2N0). Typically, the reactive gas has a low atomic mass (N=14) and is thus not effective in sputtering. It is therefore desirable to have a heavier inert gas, such as argon, to aid in sputtering. Mixing argon with the reactive gas also aids in activating the reactive gas by the penning ionization processes. The appropriate gas composition and flow for reactive sputter deposition can be established by monitoring the partial pressure of the reactive gas as a function of reactive gas flow [57]. 2.3.2. Cathodic arcA usual method for laboratory and industrial use is the cathodic arc [33]. An arc is initiated in a high vacuum by touching the graphite cathode with a small carbon striker electrode and withdrawing the striker. This produces an energetic plasma with a high ion density. The power supply is a low voltage and high current supply. The cathode spot is formed by an explosive emission process. This creates particulates as well as the desired plasma.AnodeFocusing coil Scanning coilDouble coilGrinderStrikerCathode Platform Fig. 2.11: FCVA source design [57]. The particulates can be filtered by passing the plasma along a toroidal magnetic filter duct. This is known as filtered cathodic vacuum arc (FCVA). The toroidal currents produce a magnetic field of about 0.1 T along the axis of the filter. The electrons of the plasma spiral around the magnetic field lines and so they follow them along the filter axis. This motion produces an electrostatic field, which causes the positive ions to follow the electrons around the filter axis. This motion produces a transport of the plasma around the filter. The plasma beam is condensed onto a substrate to produce the ta-C. A DC or RF self-bias voltage applied to the substrate is used to increase the incident ion energy. The particulates are generally submicron-size particles. These can still pass through the filter section by bouncing off the walls. The filtering can be improved by a factor of 100, by adding baffles along the filter section and by including a second bend to give a double bend or ‘S-bend’ filter [58].The advantages of the FCVA are that it produces a highly ionised plasma with an energetic species, a fairly narrow ion energy distribution, and high growth rates of 1 nm s-1 for a low capital cost. 2.3.3. Pulsed laser deposition (PLD)Pulse laser deposition, known well for over 2 decades, has gained prominence in the deposition of a wide variety of thin film materials such as superconductors, semiconductors, dielectrics, metals and biomaterials, among others. Pulsed laser deposition (PLD) (Fig. 2.12) is a thin film deposition technique where a high power pulsed laser beam is focused inside a vacuum chamber to strike a target of the desired composition. Pulsed excimer lasers such as Ar give very short, intense energy pulses, which can be used to vaporize materials as an intense plasma [59]. When the laser pulse is absorbed by the target, energy is first converted to electronic excitation and then into thermal, chemical and mechanical energy resulting in evaporation, ablation, plasma formation and even exfoliation. The ejected species expand into the surrounding vacuum in the form of a plume containing many energetic species including atoms, molecules, electrons, ions, clusters, particulates and molten globules, before depositing on the typically hot substrate. The kinetic energy of this expansion gives an ion energy analogous to the ion energy of the cathodic arc. In this way, pulsed laser deposition produces ta-C films similar to those from the FCVA methods [59-61]. In the pulsed laser depositions, the quality of the films is controlled by several parameters such as laser parameters (laser fluence, laser energy, ionization degree of ablated materials), surface temperature, substrate surface, background pressure, etc [59-61]. Fig. 2.12: Configuration of a PLD chamber [59].2.4. Surface morphological characteristics of DLC filmsRoughness evalution can be used for which a DLC film can be grown continuous and pin-hole free. The knowledge of the surface evalution mechanism of DLC films allows one to know if the loss of continuity in films is an intrinsic and unavoidable problem related to the nature of the deposited film or if it is a technical problem, which could be improved with better process condition. Roughness is a measurement of the small-scale variation with the height of a physical surface. Roughness is sometimes an undesirable property, as it may cause friction, wear, drag and fatigue, but it is sometimes beneficial, as it allows surfaces to trap lubricants and prevents them from welding together. Roughness evaluation studies can be conducted by atomic force microscopy (AFM). Amplitude parameters, such as root mean square roughness value (Rq) and arithmetic mean roughness value (Ra), are used in AFM studying of topography of DLC films. Ra provides only information on the departure of the surface from the mean line and is insensitive to extreme profile peaks and valleys. Although both Rq and Ra having the same trend, the value of Rq is higher than that of Ra because Rq is more sensitive to peaks and valleys due to the squaring function. In the studies of Zhang et al. [62] and Lifshitx et al. [63], surface roughness changed with substrate bias, deposition time and C+ energy due to bombarding effect. During the film deposition, increasing the target power or the pulse bias leads to a higher electrical field, so that the carbon ions reaching the substrate surface possess higher kinetic energy. When the kinetic energy reaches a certain value, the sp3 bonds favorably formed result in the densification of DLC films and then a smooth surface can be achieved. Therefore, the surface roughness (about 0.1 to 1.5 nm) of DLC films fabricated by a FCVA system is lower than that (about 1 to 3 nm) of DLC films deposited by a sputtering system because the FCVA system can produce the higher kinetic energy of the impinging species. When the ion energy exceeds the optimum value, the surplus energy will cause graphitization, so more sp2 bonds are formed in the form of graphic clusters spreading in the sp3 matrix. The increased sp2 bonds in DLC films decrease the density of the films, leading to the higher surface roughness. Zhang el al. [64] reported that average roughness increased with annealing temperature due to heat treatment. Surface roughness of Si doped DLC films decreases with increased Si content because the increased Si content decreases craters and surface flaws distributed in the outer surface of the coating [65]. Maharizi et al. studied [66] the dependence of the roughness of DLC films on the type of the substrates. The roughness of the N doped DLC (N-DLC) films increases with increased N content. These features are caused possibly by the nitrogen doping in the films or nitrogen normal bonding [67, 68]. Fig. 2.13: SEM images of ta-C films deposited at various substrate biases: (a) 0V, (b) 250 V, (c) 300 V and (d) 400 V [69].Figure 2.13 shows the SEM pictures of ta-C films deposited at various substrate biases. Carbon clusters are observed on the surfaces of ta-C films, and the sizes of carbon clusters are varied with the substrate bias [69]. It is found that the lowest cluster size gives the smoothest surface of the film.The surface roughness of DLC films can affect the electrochemical properties of the films because the low surface roughness (about 0.1 to 3 nm) of the films induces the low double-layer capacitance at the film/electrolyte interfaces, resulting in an improvement of signal-to-background ratio. The high surface roughness of the films can negatively affect the corrosion resistance of the films because the high surface roughness may have a high probability of defects or porosities that can lead to the dissolution of the underlying substrates by allowing permeation of an electrolyte through the defects or porosities [29].2.5. Adhesion strength of DLC filmsAdhesion is an important property of the film/substrate system. Chemical bonding between DLC films and their substrates is essential if the adhesion of the films is to be adequate. Mechanical locking of the film to the surface of a rough substrate can play a part but there is still a necessity for chemical bond formation if high degree of adhesion is to be achieved. Generally, chemical bonding between the films and substrates is promoted by deposition at high temperature whereas residual stress in the films is best controlled by deposition at low temperature. The residual stress in DLC thin films consists of two main components: thermal stress and intrinsic growth stress. Thermal stress is attributed to the thermal expansion mismatch between films and substrates. The expansion of DLC is low compared with many ceramics, even with Si and very low compared with most metals and thus compressive thermal stress will be induced as the substrate temperature increases during deposition. For many metallic alloys, where the expansion is too high, this leads to spallation of the films due to the large compressive thermal stress unless the deposition temperature is reduced. High temperature deposition also causes the DLC films to graphitization which in turn reduces the thermal expansion because of reduced mechanical properties. There is thus a compromise deposition temperature at which the adhesion of the films will be maximized; this is clearly influenced by the intended operating temperature. The intrinsic growth stress in DLC films is induced by enhanced cross-linkage and bond distortions in the films caused by bombardment of energetic impinging species, which abruptly affects the adhesion strength of the films. Therefore, the adhesion strength of DLC films should be correlated to sputtering power density, chamber pressure and substrate bias. The effects of the sputtering power, chamber pressure and substrate bias used during DLC film depositions on the adhesion strength of the films can be explained in terms of kinetic energy of the sputtered species which is proportional to the sputtering power density and bias voltage and inversely proportional to the root square of the chamber pressure [70]. For example, increasing the chamber pressure decreases mean free path of the ions; therefore decreasing of the chamber pressure promotes the collision of ions onto the substrates which leads to higher residual stress. The sputtering power and bias also impart more kinetic energy to the ions [71, 72]. Not enough cleaning of substrate and landing of impurities from the chamber walls and substrate holder on the substrate surface due to too high substrate bias are also reasons degrading the adhesion strength of DLC films. As decreasing the kinetic energy of the ions promotes the amount of sp2, an incorporation of nitrogen can also form new sp2 sites and encourages the sp2 sites to cluster [24]. As the sp2 bonds are shorter than the sp3 bonds, the increased sp2 sites would reduce the strain in the films. C=N bonds resulted from nitrogen incorporation in the carbon matrix have the shortest bond length compared to C-C and C=C [24]. It is reported [73] that depositing DLC films at above room temperature or annealing them lower the residual stress in the films due to a conversion of sp3 bonded configuration to sp2 bonded one. It is clear that the increased sp2 sites in DLC films improve the adhesion of the films by reducing the residual stress in the films.The scratch test is being increasingly used to qualitatively evaluate film adhesion. A normal force at the moment of film detachment is called a critical load and gives a comparative value of the film adhesion. In the study of Hedenqvist et al. [74], the maximum normal force that the film-substrate system could sustain increased with increased film thickness and substrate hardness. Hintermann et al. [75] proposed that the life of DLC coated tools and their performance were considerably improved by high adhesive and cohesive strengths of the DLC films because the bad adhesion leads to flaking while the poor cohesion causes chipping. The effects of the interface topography, coating thickness and elastic mismatch on the interfacial stress were investigated in the study of Wiklund et al. [76]. The effect of the film thickness on the adhesion strength of DLC films was also investigated by Perry et al. [77] and Sheeja et al. [78]. It was found that the adhesion strength of DLC films was elevated by increasing the film thickness because the thicker films would need more loads for the indenter to break through the films into the substrates. However, beyond a critical film thickness, the critical load is reduced. This was likely due to weakening of the film structures by the developed compressive stress in the films with increased film thickness. The influence of deposition process parameters such as substrate temperature and bias, target power and chamber pressure on the film adhesion was also found in the studies of Matinez et al. [79], and Zhang et al. [71, 72]. In the study of Gupta et al. [80], the thinner film exhibited instant damage when the normal load exceeded the critical load, whereas the thicker films exhibited gradual damage through the formation of tensile cracks. The introduction of H, N, F, Si and metals (e.g. Ti, V) into DLC films apparently reduces the residual stresses in the films via degraded cross-linking structures [25-27].It is difficult to quantitatively express the adherence because the critical load depends on several parameters related to the testing and to the coating system. Both intrinsic parameters, such as scratching speed, loading rate, diamond tip radius and shape, diamond wear, and extrinsic parameters, such as substrate hardness, coating thickness, substrate and coating roughness and friction coefficient, are considered in order to improve the interpretation of the critical load results [81]. 2.6. ElectrochemistryIntroductionElectrochemistry is the study of phenomena caused by charge separation. It deals with the study of charge transfer processes at the electrode/solution interface, either in equilibrium at the interface, or under partially or totally kinetic control. Most of the charge transfer processes are transfer of electrons, which can be represented in the simplest case of oxidized species, O, and reduced species, R, by On+ + ne- ? R (2.1)where O receives n electrons in order to be transformed into R. 2.6.1. Corrosion mechanismsGenerally, corrosion is a process that takes place when essential properties within a given material begin to deteriorate, after exposure to elements that recur within the environment. Corrosion can be separated into two types: (a) uniform or general corrosion, and (b) localized corrosion. General corrosion uniformly occurs over the entire film surface. Localized corrosion occurs at a small discrete location on a film surface and is usually characterized by rapid, deep penetration through the film. Localized corrosion can be several orders of magnitude faster than general corrosion. Corrosion reactions are often complex heterogeneous reactions whose rates are typically determined by several factors such as (a) usual kinetic considerations, (b) electrolyte chemical composition, (c) mass transfer between electrolyte and material surface, and (d) various surface effects such as adsorption/deposition and surface roughness. Fig. 2.14: Electrical double layer for electrode submerged in an electrolyte [82].The corrosion phenomenon can be explained by electrical double layer (EDL) model as shown in Fig. 2.14. Electrode (film) ions leave their structure when an electrode is submerged in an electrolyte. Water molecules surround the electrode ions as they leave and the hydrated ions are free to diffuse away from the electrode. The negative charges (caused by excess electrons) on the electrode surface attracts positively charged ions and a percentage of them remains near the surface, instead of diffusing into the bulk electrolyte. The water layer around the ions prevents most of them from making direct contact with the excess surface electrons. The positive ions in the electrolyte are also attracted to the negatively charged electrode surface. Consequently, the electrolyte layer adjacent to the electrode surface contains water molecules and ions, and has a distinctly different chemical composition than the bulk electrolyte. The negatively charged surface and the adjacent electrolyte layer are collectively referred to as the electrical double layer (EDL) [82]. Fig. 2.15: Cyclic polarization curve [82]. A corrosion reaction can proceed in both the forward and reverse directions and equilibrium is achieved when forward and reverse reaction rates are equal. There is no net loss of electrode atoms when an EDL is at equilibrium, because electrode atoms are oxidized at the same rate that electrode ions are reduced. When corrosion occurs, electrochemically active species, which can be reduced by the excess electrons such as hydrogen ion reduction, diffuse from the bulk electrolyte to the electrode surface and discharge the EDL at the point on the electrode surface where electrons are removed, causing more electrode atoms to leave in an effort to re-establish original EDL conditions.The Nernst equation mathematically relates EDL composition to electrical potential [82]: E=Eo- (RT/nF) ln(ao/ar) (2.2)E: measured potential (V); Eo: open circuit potential when all species have unit activity; a: concentration of chemical species; R: ideal gas constant (8.314 J/K.mol); T: temperature (K); n: number of electrons in anodic half reaction; F: Faraday constant, (96485 C/mol) [82].The cyclic polarization curve shown in Fig. 2.15 is used for a brief overview of DC polarization corrosion measurement methods. The applied potentials are plotted as a function of electrical current density.2.6.1.1. Linear polarizationElectrode polarization causes certain processes to occur in an electrolyte that can limit electrical current during electrochemical corrosion. The working electrode polarization is controlled by a potentiostat supplying electrons to either the counter or working electrodes. There are many factors other than potentiostat current that can control the magnitude of electrode polarization. These are: (a) working electrode chemical composition, (b) working and counter electrode surface condition, (c) working and counter electrode geometrical shapes, (d) counter electrode size, (e) electrical double layer chemical composition, and (f) electrolyte chemical composition. Fig. 2.16: Linear polarization [82].Working electrode electrical current is zero at open circuit potential (OCP) and electrode potential polarity switches from cathodic to anodic as the scan proceeds past the OCP. The slope for a linear polarization curve as shown in Fig. 2.16 is the change in potential divided by the corresponding change in current density. This relation is written mathematically as [82]: Rp=?E/?i (?cm2) (2.3)Rp is referred as the corrosion or polarization resistance.2.6.1.2. Potentiodynamic polarization A potentiodynamic polarization curve has a cathodic branch that is similar to that for a Tafel plot. It also has an anodic branch, but the anodic branch extends over a wider potential range and is often much more complex than the Tafel plot anodic branch. Fig. 2.17: Potentiodynamic polarization curve [82].Several quantities appear in the potentiodynamic polarization anodic branch as shown in Fig.2.17. Epp is the primary passivation potential after which current either decreases, or becomes essentially constant over a finite potential range. Eb is the breakdown potential where current increases with increasing potential. The passive region is the portion of the curve between Epp and Eb. The active region of the curve is the portion of the PDS curve where potentials are less than Epp. The transpassive region of the curve is the portion of the curve where potentials are greater than Eb.Quantities like Epp, Eb and passive region width can be used to characterize the corrosion behavior, and evaluate how a passive film effectively protects a film from corrosion. General corrosion, and sometimes pitting, occurs in the active region; little or no corrosion occurs in the passive region, and pitting corrosion can occur in the transpassive region. 2.6.1.3. Tafel plotA Tafel plot shown in Fig. 2.18 has anodic and cathodic branches, corresponding to the anodic and cathodic half reactions for corrosion. Tafel plots are classified as activation controlled when the corrosion rate is determined by how fast an electrode is capable of transferring its electrons to electrochemically active species (EAS). A characteristic of activation control is an increasing current density magnitude with increasing potential for both branches. Tafel plots are classified as diffusion controlled when an EAS diffusion rate determines the corrosion rate. Diffusion control theoretically causes cathodic current density to become constant. The constant, or slowly changing, cathodic current is referred to as the diffusion limited current. Diffusion can restrict access of the EAS to an electrode surface when: (1) an electrolyte has a limited supply of EAS (eg. At pH 14, the hydrogen ion concentration is 10-14 M), (2) EAS diffuses very slowly through the electrolyte to an electrode surface, and (3) corrosion reaction products restrict EAS access to an electrode surface [82]. Fig. 2.18: Tafel plot [82].Tafel equation [82]: ?V = β × ln (Icorr / Io) (2.4)where ?V: the overpotential, (V); β: Tafel slope, (V/decade); Icorr: the current density, (A/cm2); Io: the exchange current density, (A/cm2).The corrosion current (Icorr) can be calculated using the following equation [82, 83]: Icorr = [βa × βc] / [2.3 × Rp × (βa + βc)] (2.5)where Rp: the polarization resistance (?cm2), Rp = [βa × βc] / [2.3 Icorr (βa + βc)]; βa: the anodic Tafel slope, (V/decade); βc: the cathodic Tafel slope, (V/decade); (βa × βc) / (βa + βc) = Tafel constant.2.6.1.4. Electrochemical impedance spectroscopyDiamond-like carbon (DLC) thin films can be deposited by physical vapor deposition (PVD) and chemical vapor deposition (CVD) to get attractive properties for electronic, optical, mechanical, biomedical and electrochemical applications. However, there are almost always pores, even in good quality PVD or CVD DLC films [84]. Pores with dimensions of approximately 0.5 nm are randomly distributed in the films, and water molecules with dimensions of approximately 0.32 nm can easily permeate into the film [84]. DLC films are usually electrochemically nobler than the substrates, so the presence of nanopores in the films can rapidly lead to the electrochemical dissolution of the substrate. Although atomic force microscopy (AFM) can measure such small sizes, it is not easy to locate the sites of nanopores in a large area of measurement. Therefore, detecting the nanopores and assessing their effect on the corrosion protection behavior of DLC films holds the key to fully understand the electrochemical behavior of the films. It is difficult to measure the pores on nanometer scales, even with a scanning electron microscope (SEM). In the polarization method, a partially coated surface exhibits a mixed potential when exposed to a corrosive electrolyte, depending on the number of pores and the degree of polarization of each electrode component. In the case where a film has pores, the polarization result is a combined output of the film and substrate. The polarization behavior of DLC film itself is not clear so the polarization method is not suitable to measure the porosity in DLC films. It is reported [84] that electrochemical impedance spectroscopy (EIS) is a sensitive technique in detecting nanopores. Though the polarization study can provide data about the corrosion processes occurring at the electrochemical interface, AC methods, such as electrochemical impedance spectroscopy (EIS), offer potentially more information especially regarding the performance of coatings with small defects on active substrates. EIS can be used to quantify coating integrity and long term corrosion performance. Impedance spectroscopy is based on applying a small sinusoidal potential modulation to the electrode system and monitoring the amplitude and phase shift of current response over a range of frequencies. This technique has been successfully applied to the evaluation of the protective character of polymer on metal as well as dielectrics such as Al2O3 on steel [85-87]. Fig.2.19: (a) An equivalent circuit proposed for a solid coating. (b) Nyquist plot and (c) Bode magnitude and (d) phase plots of the coating [82].Figure 2.19a shows an equivalent circuit of a perfect coating without defects or porosities. The circuit includes a solution resistance (Rs), a charge transfer resistance (Rct) and a double layer capacitance (Cdl). The total impedance magnitude for the circuit can be mathematically expressed [88]: Z = Rs + [Rct / (1 + ω2Rct2Cdl2)] + [j(ωRct2Cdl) / ( 1 + ω2Rct2Cdl2)] (2.6)where Z is the total impedance in ?, Rs and Rct are the solution and charge transfer resistances, respectively, C is the capacitor capacitance in F, ω=2π (AC voltage frequency) and j is the square root of -1 and referred to as an imaginary number. In equation 2.6, the second term, [Rct / (1 + ω2Rct2Cdl2)], is referred to as the real impedance and the third term, [j(ωRct2Cdl)/( 1 + ω2Rct2Cdl2)], is referred to as imaginary impedance. The imaginary impedance magnitude is zero when the phase angle is zero. The imaginary impedance magnitude is infinity when the phase angle is 90 degree. In the Nyquist plot shown in Fig. 2.19b, the Rs can be found by reading the real axis value at the high frequency intercept. This is the intercept near the origin of the plot. The real axis value at the other (low frequency) intercept is the sum of the Rct and Rs. The diameter of the semicircle is therefore equal to the Rct. The Rs and the sum of the Rs + Rct can be read from the Bode magnitude plot shown in Fig. 2.19c. Plot slope is zero when polarization is through resistances and the slope is less than zero (negative) when capacitive reactance becomes part of the circuit response to a polarization. The Rs, Rct and capacitive reactance are also noted in the Bode phase plot shown in Fig. 2.19d. The inflection point frequency shown in Fig. 2.19d is equal to [88]: Inflection point frequency = 1 / (1.77RctCdl) (2.7)Comparing Bode magnitude and phase plots shows that phase plot inflection corresponds to the area where the Bode magnitude slope is negative. Comparing phase and magnitude plots also illustrates that magnitude plot resistances, Rs and Rct, correspond to polarizing voltage frequencies where phase angle is zero. Organic coating capacitive reactance and resistance properties can produce a second circuit, which contains bulk resistivity (Rp) and capacitance (Cc) of the coating, in an equivalent circuit shown in Fig. 2.20a and appear a high frequency circle in the Nyquist plot shown in Fig. 2.20b. The bulk conductivity of the coating can be attributed to its original conductivity and the electrolytic conduction when the coating is composed of porosities that allow migration of ions to the substrate [82]. Water and ions typically diffuse into an organic coating, after a coated sample is submerged in an electrolyte, and change coating dielectric properties. Water and ions also move inside a coating in response to AC polarizations. Therefore, water and ion movement through a coating causes the electrolytic conduction in the bulk of the coating and is also restricted by coating morphology, producing a coating, or pore resistance. Fig. 2.20: (a) An equivalent circuit proposed for a coating with porosities. (b) Nyquist plot and (c) Bode magnitude and (d) phase plots of the coating [82].Notice that there are two semicircles in the Nyquist plot shown in Fig. 2.20b; one comes from the electrochemical reactions in interface region between the coating and electrolyte at low frequencies and one is attributed to bulk electrical properties of the coating at high frequencies. The associated magnitude plot has two slopes in Fig. 2.20c and the associated phase plot in Fig. 2.20d has two inflection points. 2.6.2. Corrosion properties of DLC filmsDiamond-like carbon (DLC) is an attractive candidate material for devices which need to withstand exposure to a range of harsh environment. The unique combination of high hardness, high wear resistance, low friction, electrical insulation, high corrosion resistance and chemical inertness make DLC films ideal for protective coatings in corrosive environment. Diamond is resistant to all acids, even at elevated temperatures although it can be etched by fluxes of caustic alkalis, oxysalts, etc. The polycrystalline nature of diamond films and small pores or pinholes within them are the major drawbacks of the diamond films. Chemical attack at the grain boundaries or attack of the underlying substrate through pinholes lead to unacceptable performance in many coating systems and diamond is not exceptional. However, DLC is amorphous and has no grain boundaries compared to diamond, which is a great benefit for the high corrosion resistance of DLC.DLC films can potentially be used in applications requiring good corrosion resistance, such as protection of metals in magnetic recording and micro electronics industries because of their intrinsic stability under most aqueous conditions. Corrosion of metals with DLC overcoats is usually initiated at microscopic pinholes that lead to the exposure of the metals to the environment. Requirements for effective protection include chemical inertness together with microstructural demands, such as high density, smooth surface without microporositites and diffusion pathways, homogeneous stoichiometry, stress-free layers and good adhesion between the film and the substrate material. The electrical conductivity and porosity level of the carbon overcoat promote a galvanically-induced corrosion mechanism between the overcoat and the substrate. To minimize the galvanic action between the substrate and the overcoat, it is necessary to develop overcoats with high electrical resistance, in an ideal case with dielectric properties. However, the electrodes used for electrochemical purposes, such as tracing heavy metals, must need to have high electrical conductivity, clearly pointing out that the best balance should be carefully optimized.Morrison et al. [89] reported that DLC-metal composite films (Ag-DLC, Pt-DLC, AgPt-DLC) exhibited low corrosion rate in phosphate buffered saline (PBS) electrolyte. The ultra-thin ta-C films showed superior corrosion protective properties with respect to the sputtered film when deposited on a Co-alloy disk (Co78Cr12Pt10) [90]. DLC films reduced the corrosion rate of steel substrates in corrosive solution (3.5 wt% NaCl) by functioning as a physical barrier and restraining from anodizing [91]. Liu et al. [92] showed that DLC films could significantly improve anti-corrosion properties of AlTiC substrates when the film thickness was more than a few tens of nanometers [80]. After potentiodynamic corrosion tests of H-DLC and DLC films in 0.5 M NaCl aqueous solution were carried out, comparison between the corrosion parameters of the DLC:H and DLC coated Ti-6Al-4V alloys showed that DLC film presented higher corrosion potential and polarization resistance than those of the H-DLC film [93]. The corrosion studies indicated that coating Ti (Ti-13Nb-13Zr) alloy with DLC film could improve the corrosion resistance in simulated body fluid environment [94]. Introduction of Si into DLC films led to a significant improvement in the corrosion resistance of the films in 2 M HCl solution, as revealed by an increase in the charge transfer resistance and a decrease in the anodic current [95]. An increase of bias voltage could improve corrosion resistance of the films in a simulated body fluid (0.89 % NaCl solution). This could be attributed to the formation of dense and low-porosity films which impeded permeation of the solution [96]. Zeng et al. [84] proved that DLC films could serve as protective layers over their underlying Si substrates in sulfuric acid solution because the films with few nanopores could effectively prevent their underlying silicon substrates from corrosion. Sharma et al. [97] found that increasing deposition time increased the corrosion resistance of the films, showing that the increased thickness of the films improved the corrosion resistance of the films by lessening the possible number of the porosities in the films [97]. Liu et al. [98] reported that adhesion strength had a great influence on the effectiveness of corrosion protection since the porosities in the films could allow the electrolyte to permeate to the interfaces between the films and substrates and attacked the interfacial bonds between them; the better the adhesion strength of the films was, the better the corrosion performance of the films was. Papakonstantinou et al. [99] found that the corrosion resistance of ultrathin DLC films in corrosive solution substantially increased with immersion time due to filling of the pores with passivating materials and subsequently stopping access of the solution to their substrates. It can be seen that the porosity density is one of the most important parameters assessing the effectiveness of corrosion protection of DLC films [25-27, 100, 101].2.7. Electrode materials for electroanalysis A number of experimental design factors have to be considered if it is decided to perform an electrochemical analysis. These design factors depend on the technique employed, electrode material, and electrode and cell configurations. The useful potential ranges of electrode materials are determined by oxidation and reduction of a solvent, decomposition of a supporting electrolyte, electrode dissolution or formation of a layer of insulating/semiconducting substance on its surface. Electrode materials for voltammetry must conduct electrons. Thus, their choice is limited to metals, other solids with metallic conductivities and good semiconductors. Usually, it is also desired that the electrode material is inert in the region of potential in which the electroanalytical determination is carried out. The reliability and repeatability of experiments can be aided by assuring a constant flux of electroactive species to the electrode. This is done by using controlled convective flow over the electrode or by creating a sufficient high concentration gradient. The additional advantage of this approach is that, because of the greater mass transport, sensitivity is increased and detection limits are lowered.Detection limits can also be affected by other electrode reactions which can occur in the same potential range. The most prevalent of these is the reduction of oxygen since its solubility in solutions open to the atmosphere is up to 10-4 M. Oxygen must be removed from the solution by passage of an inert gas, prepurified nitrogen or argon, through the solution to diminish the oxygen partial pressure to a very low value. The poisoning of electrode surfaces has been one of the main limitations to the widespread use of electroanalysis by non-experts. Mercury was the first metal to be extensively used, in the form of a dropping mercury electrode; however, mercury’s useful potential range is limited by its oxidation which means that, essentially, only reductions can be investigated. Therefore, solid electrode materials are developed which permit oxidation reactions to be studied.Sp2-carbon is a versatile material that has a wide range of applications in electrochemistry. Because of its reasonable electrical conductivity and good corrosion resistance, carbon has found widespread acceptance in electrodes. Platinum possesses high resistance to chemical attack, excellent high-temperature characteristics, and stable electrical properties. All these properties have been exploited for industrial applications. Platinum does not oxidize in air at any temperature, but can be corroded by cyanides, halogens, sulfur, and caustic alkalis. This metal is insoluble in hydrochloric and nitric acid, but does dissolve in the mixture known as aqua regia. Pt dissolution is more severe in phosphoric acid than in perchloric acid. Because of its chemical inertness, it is chosen as electrodes for electroanalysis. However, in aqueous solutions, detection of analyses is often not possible at negative potentials using such electrodes due to the high Faradic currents produced by hydrogen evolution reaction. Mercury electrodes can eliminate this problem of Faradic currents. However, the use of the mercury results in the generation of hazardous waste.Sp2-carbon, e.g. glassy carbon, is widely used for electroanalysis in aqueous media, as it exhibits a relatively wide potential window. Glassy carbon has a number of properties, including high conductivity, impermeability, and unreactivity, which make it an excellent electrode material. However, this material has serious limitations, including high background currents and deactivation via fouling. This is due to the irreversible absorption of product formed in the electrochemical oxidation reaction. It is an inherent property of glassy carbon to undergo deactivation upon exposure to the laboratory environment or working solution, which is due to factors such as oxidation and adsorption of contaminants and reaction products. The porous nature of the electrode may also complicate the voltammetric response since redox sites which lie in deep pores will be subject to greater uncompensated solution resistance than sites close to the outer surface of the electrode [102]. Fig. 2.21: Cyclic voltammetric i-E curves for boron doped diamond electrodes in (a) 1 M KCl and (b) Fe(CN)6-3/-4 + 1 M KCl [11].The attractive features of diamond films include a wide potential window in aqueous media, very low capacitance, and high electrochemical stability. In addition, diamond electrodes have recently been found to exhibit additional characteristics of interest for electro-analysis. The diamond electrode material is stable with respect to platinum, gold, palladium, and silver and shows a wide potential range for the water decomposition in various electrolytes such as NaCl, H2SO4, HNO3, HCl, KOH, KNO3, and Na2SO4 [103]. Hupert et al. [11] studied cyclic voltammograms obtained at diamond electrode in 1 M KCl and Fe(CN)6-3/-4 + 1 M KCl solutions as found in Fig. 2.21.Boron-doped diamond thin-films possess a number of important and practical electrochemical properties, unequivocally distinguishing them from other commonly used sp2-bonded carbon electrodes, such as glassy carbon, pyrolytic graphite, and carbon paste. These properties are (i) a low and stable background current, leading to improved signal-to-background (SBR) and signal-to-noise (SNR) ratios; (ii) a wide working potential window in aqueous and non-aqueous media; (iii) superb micro-structural and morphological stability at high temperatures (e.g. 180 ?C); (iv) good responsiveness for several aqueous and non aqueous redox analytes without any conventional pretreatment; (v) weak adsorption of polar molecules, leading to improved resistance to electrode deactivation and fouling; (vi) long-term response stability [11, 104]. Electrically conductive diamond films yield film electrodes of remarkable properties. The diamond conductivity basically comes from either the damage generated sp2 (graphite) contents in case of ion-implanted diamond or the doped-boron in case of CVD-diamond films [105]. The latter films have shown an exceptionally wide electrochemical window, a low capacitance charging background current (3.7 to 7.1 ?F/cm2) for aqueous systems and remarkable stability comparable with the chemical resistance of pure carbon diamond [106, 107]. Thus, diamond is becoming an interesting material to consider for electrolysis. However, some restraints upon its applications arise, such as high substrate temperature required (typically 1175 K), relatively small film area presently available, and relative difficulty in deposition parameter control limit. Compared to diamond, DLC materials are also chemically stable and can achieve a wide range of excellent mechanical, electrical and chemical properties. Wide cyclic voltammograms shown in Fig. 2.22 indicate that N-DLC films are promising electrodes for electrical applications [97]. In the study of Zeng et al. [20], the repeatability of the voltammogram showed the durability of N-DLC film to high anodic potential, which was identified by scanning the voltammogram more than 20 times with essentially identical profile. The wide electrochemical window (approximately -1.20 to +2.21 V vs. SCE) (Fig. 2.20b) of the N-DLC film electrode was comparable with that of diamond film electrode, which is about -1.25 to +2.30 V (vs. SHE) [108]. For the graphite electrodes, the potential window is significantly smaller. The potential window of about -0.30 to +1.80 V was reported for glassy carbon electrode [109]. For a highly oriented pyrolytic graphite (HOPG) electrode, it was about -0.40 to +1.60 V [109].Fig. 2.22: Cyclic voltammetric i-E curves obtained (a) N-DLC electrode (dash line) and glassy carbon electrode (solid line) in 1 mM K3Fe(CN)6/0.1 M H2SO4 and (b) N-DLC electrode in 0.5 M H2SO4 [20].Surface cleanliness is an important parameter influencing the responsiveness of all electrodes. In a very general way, adsorbed contaminants can either block specific surface sites, thus inhibiting surface sensitive redox reactions, or increase the electron-tunneling distance for redox analytes, thereby lowering the probability of tunneling (i.e. the rate of electron transfer). DLC films are not as susceptible to contamination (air or solution borne contaminants), as other electrodes are, because of their hydrophobic surfaces. In addition, DLC films can be deposited on various substrates like metals, ceramics, polymers and semiconductors at low temperature and achieve the following characteristics:Conformal and well-adherent,Pinhole free,High corrosion resistance,Low coefficient of friction,High thermal conductivity,High hardness,Excellent biocompatibility,Wide electrochemical potential window andControllable resistivity.All processes required for production of DLC film electrodes are compatible with semiconductor technologies. Thus, DLC film electrodes can directly be amenable to mass production. These characteristics demonstrate a great promise of using conductive DLC as an environmental-friendly disposable electrode material.2.8. Electrochemistry of DLC filmsNowadays, special interest has been devoted to the preparation of optically transparent carbon electrodes and the production of microelectrodes by PVD techniques such as electron beam evaporation and sputtering [110, 111]. Sputtered carbon films at room temperature are generally amorphous. Their surface smoothness can be obtained near atomic scale, which would induce a low capacitance charging background current if the film is used for electroanalysis. However, depending on the sputtering methods and the deposition parameters, a large variety of microstructures can be obtained, ranging from graphite-like to diamond-like carbon. This is reflected in the considerable variability of physical, mechanical and electrochemical parameters. Diamond-like carbon has been used for several applications such as mechanical, optical, electrical, tribological, biological and electrochemical applications. Furthermore, DLC can be thought of as a more universal application especially when large surface area electrodes are required, because uniform and smooth DLC films can be deposited over large surface areas of substrates without grain boundaries. Therefore, the unique combination of variable electrical conductivity and diamond-like chemical and mechanical properties opens the possibility for applications of these materials in several areas such as microelectronics and electrochemical application, such as electrodepositing anode and analytic electrode, etc. Although there are many reports on the mechanical and electrical properties of DLC films, limited reports have been published on their electrochemistry. It is said that DLC films are chemically inert under biochemical conditions, and a good passivation coating for engineering materials (Ti, Al/C and stainless steel) [112, 113]. Some successful applications of DLC films as protective coatings for biomaterials have been reported [114]. Fig. 2.23: Bright-field transmission electron micrograph of a Pt-DLC composite film. Platinum nanoparticles self-assemble into arrays (dark regions) within the DLC matrix (light regions) [89].In Pt incorporated DLC films (Fig. 2.23), Pt usually cannot be evenly doped, and does not act as carriers [89, 115]. A wide-band gap diamond-like carbon is not in itself electrochemically active; however, it acquires electrochemical activity upon introducing platinum into the DLC bulk during the film deposition [116]. Schlesinger et al. [117] studied the potential limits of Ti-doped amorphous carbon thin film electrodes with diamond-like character, deposited by sputtering, in several kinds of aqueous solutions. They found that the films exhibited a low double-layer capacitance, a large electrochemical window, and a relatively high activity toward ferricyanide reduction, comparing with conventional carbon electrodes such as glassy carbon. Nickel doped DLC (Ni-DLC) thin film coated samples were used as working electrodes to electrocatalyze glucose oxidation in 0.1 M NaOH aqueous solutions [118]. It was found that direct electrochemical response of glucose at the Ni-DLC thin film electrodes gradually developed with increased glucose concentration in the electrolytic solutions. It was deduced that the Ni nanoparticles distributed on the Ni-DLC film electrode surfaces played a major role in promoting the glucose oxidation [118].Yoo et al. [119] investigated electrodes made of N incorporated DLC (N-DLC) thin films. They found that the electrodes exhibited excellent electrochemical characteristics. They had (1) wide potential windows in aqueous system than those of boron doped diamond, (2) reversible and excellent analytical behavior, (3) electron-transfer kinetics intermediate between polished pyrolytic graphite and Boron doped diamond for the quinine/hydroquinone couple and excellent analytical behavior, and (4) considerably higher catalytic activity for Cl2/Cl- than that of boron doped diamond as well as durability to high anodic potentials. N-DLC films showed wide potential windows in different aqueous solutions and had high signals to heavy metals such as lead, copper and mercury at ?M level [19, 20, 23, 24]. Lagrini et al. [120] found that nitrogen partial pressure influenced potential windows of N-DLC films in LiClO4 solution. When these good characteristics of DLC films are combined with the advantage of ready deposition at ambient temperatures, the properties of DLC suggest valuable additions to the repertoire of electrochemistry. A more detailed examination of the preparative parameters in relation to better understanding of electrode behavior is required before broad and practical applications of DLC coated electrodes can be realized. 2.8.1. Cyclic voltammetryCyclic voltammetry is a type of potentiodynamic electrochemical measurement. In a cyclic voltammetry experiment, a potential is applied to the system and the Faradic current response is measured and plotted versus the applied potential to give the cyclic voltammogram as shown in Fig. 2.24. The current response over a range of potentials (a potential window) is measured, starting at an initial value and varying the potential in a linear manner up to a pre-defined limiting value. At this potential (often referred to as a switching potential), the direction of the potential scan is reversed, and the same potential window is scanned in the opposite direction. This means that species formed by oxidation on the first (forward) scan can be reduced on the second (reverse) scan. This technique is commonly used, since it provides a fast and simple method for initial characterization of a redox-active system. In addition to providing an estimate of the redox potential, it can also provide information about the rate of electron transfer between the electrode and the analyte, and the stability of the analyte in the electrolyzed oxidation states.Voltammogram can be obtained by measurement of the current response as a function of applied potential (or the potential response as a function of applied current); in other words, they depend on the registering of current-potential profiles. Potential sweep methods consist of scanning a chosen region of potential and measuring the current response arising from the electron transfer and associated reactions that occur. They are widely used for the investigation of electrode processes, which is a first step towards developing an electroanalytical procedure. Naturally, they can also give quantitative information, since the currents obtained are directly proportional to concentration. Cyclic voltammetry (CV) is one of the most frequently used electrochemical methods because of its relative simplicity and high information content. CV has a great advantage for elucidating the mechanism of electrode reactions that are complicated by chemical (C) reactions that either precede or follow the electron (E) transfer step. There are many possible reaction mechanisms: EE, EC, CE, and ECE, etc. which are important to consider when multiple peaks or strange-looking CV waves are experimentally encountered. Fig. 2.24: Typical cyclic voltammogram [9].In cyclic voltammetry, the basic response to potential is a peak-shaped curve. The current begins to rise as potentials are reached where electrode reaction can occur. This creates a concentration gradient which sucks in more electroactive species until depletion effects set in and the current begins to fall again. The total current obtained is a sum of a contribution from a Faradic current and a residual current. The Faradic current is associated with the redox activity of the analyte. The residual current is primarily composed of four components: (1) expected faradic current from electrochemical reactions, (2) charging current, (3) variable current from electrochemical reactions of the electrode surface, and (4) trace currents from electroactive impurities in solution. The first effect can be explained by electrochemical rate theory which provides that finite current flows exist at potentials considerably removed from the point where the sharp rise in oxidation or reduction current occurs. The second effect comes from the charging or capacitive current resulting from charging the double layer associated with the electrode-solution interface. The third residual current results from extraneous processes primarily associated with the electrode surface. Thus, according to the pass history of the electrode, the surface may have become oxidized or covered with a layer of absorbed gas. Potentials applied to these electrodes may then give rise to currents from dissolution of the oxides or gas films. In addition, no background electrolyte is normally free of traces of oxidizable and reducible impurities. These substances contribute to the overall residual current. In cyclic voltammetry of reversible reactions, the product of the initial oxidation or reduction is then reduced or oxidized, respectively, on reversing the scan direction. If a redox system remains in equilibrium throughout the potential scan, the electrochemical reaction is said to be reversible.? In other words, equilibrium requires that the surface concentrations of O and R are maintained at the values required by the Nernst Equation.? For completely irreversible reactions only the oxidation or reduction corresponding to the initial sweep direction appears, since re-reduction or re-oxidation, respectively, cannot occur, there is no reverse peak. The majority of redox couples fall between the two extremes and exhibit quasi-reversible behavior. This means that the reverse peak appears but is smaller than the forward peak. In aqueous solution, usually, the ions except H+ and OH- are to be detected. It requires that over potentials for hydrogen and oxygen evolutions are larger than that of expected ion redox. The difference between the potentials for hydrogen and oxygen evolution is called electrochemical potential window. The cyclic voltammetry is used to measure the potential windows of electrodes: the wider the potential window is, more elements in solution can be detected. 2.8.2. Anodic stripping voltammetryPollutants in water include a wide spectrum of chemicals, pathogens, and physical chemistry or sensory changes due to the contamination, over-use and mismanagement of water resources [25]. Many of the chemical substances are toxic. The presence of toxic metals such as mercury, lead and copper in the environment has been a source of worry to environmentalists, government agencies and health practitioners because these metals in the aquatic ecosystem far-reaching implications directly to the biota and indirectly to human beings. Tracing and determination of these poisonous metals in solutions are of major importance in electrochemical analysis. Although electrochemical stripping voltammetric measurement is a simple, quick and cheap way of tracing metals, their sensitivity changes with electrode properties (electrical conductivity, surface roughness, surface cleanliness), operation parameters (scan rate, deposition potential, deposition time, methods) and environmental parameters (current flow, pH value of solution, metal concentration, metal solubility). Anodic stripping voltammetry (ASV) is a voltammetric method for quantitative determination of specific ionic species. The ASV has been widely used for detection of heavy metals in various solutions because of its remarkly low detection limits [121, 122]. Linear sweep anodic stripping voltammetry (LSASV) is one of the anodic stripping voltammetric methods to trace heavy metals. In linear sweep voltammetric (LSV) measurement, the current response is plotted as a function of voltage rather than time, unlike potential step measurement. The LSV technique shows better defined and selective peaks. This is due to less interference of species that can be absorbed by the working electrode, contrasting with differential pulse and square wave voltammetric techniques [123].Anodic stripping voltammetry usually incorporates 3 or 4 steps. In the first step, the potential is held at a lower potential that is enough to reduce the analyte and deposit it on the electrode. Its efficiency depends on the rate of transport of the species to be accumulated onto the electrode surface; a constant rate of transport will lead to better reproducibility and repeatability and a linear dependence on accumulation. Thus, constant stirring is used in order to increase sensitivity and decrease detection limit. After the first step, the electrode is kept at the lower potential. The purpose of this second step is to allow the deposited material to distribute more evenly in the mercury. If a solid inert electrode is used, this step is unnecessary. The third step involves raising the working electrode to a higher potential (anodic), and stripping (oxidizing) the analyte. As the analyte is oxidized, it gives off electrons which are measured as a current. In this step, potential control used in stripping voltammetry leads to a current peak whose height (and area) is proportional to the concentration of the accumulated species. The last step is a cleaning step; in the cleaning step, the potential is held at a more oxidizing potential than the analyte of interest for a period of time in order to fully remove it from the electrode.2.8.3. Stripping analysis of DLC filmsAnodic stripping voltammetry (ASV) has been widely used for detection of heavy metals in various solutions because of its remarkably low detection limits. Other advantages of stripping voltammetry include the capability of simultaneous multi-element determination, and relatively inexpensive instrumentation as compared with spectroscopic techniques used for trace metal analysis. In addition, its low operating power makes them attractive as portable and compact instruments for on-site monitoring of trace metals. In the past, mercury-coated carbon-based electrodes such as graphite and glassy carbon (GC) have been used for ASV extensively [124, 125]. The advantage of the mercury electrode is to increase the negative potential range available, particularly important in the case of the commonly analyzed zinc; the elements are dissolved in or form an amalgam with the mercury. However, the major drawbacks of the mercury electrode are disposal problems and cost. Moreover, mercury toxicity has been a great concern of environmentalists and has motivated the sensor technology to develop mercury-free electrodes. In recent years, boron-doped diamond film electrodes have been proposed as suitable electrodes for the stripping analysis of heavy metals [126-128]. A diamond film electrode has been developed. It consists of a thin film of carbon (with a preponderance of sp3 carbon geometry) deposited on an inert substrate such as silicon by chemical vapor deposition (CVD). This carbon is doped with boron, which has a high conductivity value. The boron doped diamond (BDD) electrode has several qualities, such as a large electroactivity range in water and a low background current (one magnitude lower than the one generated by glassy carbon electrode). Furthermore, chemical and mechanical robustness of this electrode made it suitable for working in corrosive media. Fig. 2.25:?Typical DPASVs obtained at N-DLC film electrode in an 0.1 M KCl (pH 1.0) solution [18].DLC films have similar chemical characteristics to that of diamond, such as excellent chemical inertness, high fouling resistance and large potential window. DLC can also be doped into electrically conductive films and be deposited at room temperature. DLC’s amorphous structure makes the DLC film surface much smoother than that of polycrystalline diamond film, resulting in considerable decrease in residual current due to the lower capacitance. The N-DLC film electrodes compare very favorably with conventional carbon based electrodes such as glassy carbon, graphite and highly oriented pyrolytic graphite (HOPG). The N-DLC film electrode exhibits a low double-layer capacitance, a wide electrochemical potential window, and a relatively high electrochemical activity toward ferricyanide reduction. In addition, the electrode exhibits a catalytic activity for Cl2/Cl-, durability to high anodic potential and a high signal for the trace analysis of Pb2+. These characteristics demonstrate great promise of the N-DLC film as a novel electrode material for electrochemical analysis. Some researchers have started stripping analysis of heavy metals using N-DLC electrodes as shown in Fig. 2.25 [18].Conductive N-DLC film electrodes have been used to investigate the possibility of detecting heavy metals such as lead, copper and cadmium by differential pulse anodic stripping voltammetry (DPASV) in the absence of mercury film [18, 19]. Liu et al. [18] found that N-DLC film could trace single metals such as Hg2+, Pb2+ and Cu2+ and multi-metals (Pb2++ Cu2+). Zeng et al. [19] also reported that N-DLC film deposited with DC magnetron sputtering could detect multiple heavy-metals (Cd2+, Pb2+, Cu2+) at the same time. Recently, Khun et al. [23, 129] proved that N-DLC films fabricated by FCVA had high potential to detect single metals such as Zn2+, Pb2+, Cu2+ and Hg2+ and multi-metals such as Pb2+ + Cu2+, Cu2+ + Hg2+ and Pb2+ + Cu2+ + Hg2+.Rehacek et al. [130, 131] developed N-DLC electrodes as supports for bismuth electroplating on highly conductive Si substrates. It was found that the electrodes had a potential to replace toxic mercury used most frequently for determination of heavy metals (Zn2+, Cd2+, Pb2+) by anodic stripping voltammetry. 2.8.4. Potential applications of DLC films as electrodes for electroanalysisMicroelectrodes for electrochemical analysis have been evaluated in recent years in attempts to increase their detection sensitivity by minimizing the effect of analytes depletion and double-layer charging current during the measurement of reversible redox species [132].Microelectrodes are electrodes which have at least one dimension in the micrometer range and thus have particular characteristics which are a direct function of their small size. Microelectrodes can have many different forms. For example, the hemispherical diffusion profile induces an improved mass transfer compared to that of macro-electrodes. This brings much higher current density which translates into a high signal-to-noise ratio. Reduction of size leads to lower capacitative contribution to the total current and the possibility of attainment of steady-state currents within a short time. The very small surface areas of the microelectrodes also mean that the double-layer capacitance is extremely small. This leads to allowing a dramatic improvement to the ratio of faradic to capacitive current in analytic measurement (high sensitivity and low responding time). The small size enables them to be inserted in places where other electrodes are too large. Microelectrodes allow working in very high resistive media without loss of sensitivity. This is due to the small currents and to the facts that the ohmic drop is limited to a small area close to the electrode. The high current density leads to good signal resolution and low detection limits. The sensing is done within the diffusion layer and this induces a very low dependence on hydrodynamic conditions. With such microelectrodes, the smaller the microelectrode diameters are, the lower the flow dependence is. It is reported that the double layer capacitance of the DLC films is much smaller than that of platinum electrode, which is an advantage of the DLC film electrodes over the Pt electrodes [133].Fouling can arise from interfering species or from the analyte itself or its electrode reaction products. Surface modification can be done to prevent fouling. There is no doubt that flow system to the electrode is an extremely important area of electroanalysis. Particular advantages of continuous flow (stirring) to the electrode arise from the fact that fresh solution is constantly being transported to the electrode surface so that there is no reagent depletion and no build-up of products since they are continuously washed away. However, in flow streams for on-line monitoring the fouling of the electrode can be a big problem since there is continuous contact between the electrode and fresh contaminant. At present, microelectrodes with submicro-meter dimensions are starting to be fabricated. If they are to be used singly, the instrumentation becomes primarily important since the currents registered are at the pico-ampere level. The use of arrays of microelectrodes all with the same function can increase the total measured current whilst retaining the particular properties of microelectrodes with respect to high concentration gradient. DLC films would be a possible and novel material for fabrication of microelectrodes. In order to study the electrochemical characteristics of the DLC films in detail and eventually apply them to single microelectrode and array microelectrode sensors, further work will need to be conducted.Fabricating DLC films into micro-patterns is an essential step for producing single microelectrodes and array microelectrodes in mass for electroanalysis. In theory, it may be easier to deposit DLC films than diamond films on micro-fibers. However, the technology of coating the conductive DLC films on metallic or non-metallic fibers is still in its infancy. Although there are some publications about diamond film patterning and some other carbon (mainly with sp2), there are very few reports on DLC films in this field [130, 131]. It clearly points out that there is still a necessity to develop DLC films as array micro-film-electrodes for electrochemical purposes. 2.9. SummaryDiamond-like carbon (DLC) films generally have a mixture of sp3 and sp2 bonding. DLC films with a high sp3 content can possess properties close to those of diamond, including high hardness, high wear resistance, low friction coefficient, high electrical resistivity, excellent chemical inertness, and high optical transparency. As amorphous materials, they can be deposited with near atomic smoothness, adding the relative ease of synthesis compared to that of diamond films. DLC films are promising materials to replace diamond films in many fields. There are many ways to synthesize DLC films with FCVA and magnetron sputtering being among the most widely used methods. DLC films are usually dielectric. In order to use them in electronics and electrochemistry, it is necessary to dope them into a good electrical conductor, for example, with nitrogen doping. Till now, only a few methods have been successful in doping DLC films. A wide-gap diamond-like carbon is not in itself electrochemically active; however, it acquires electrochemical activity upon introducing platinum into the DLC bulk during the film deposition. The effect of platinum is shown to be of a threshold nature: the electrochemical current appears at approximately 3 at% and saturates at approximately 10 at% Pt.DLC films can have a low double layer capacitance and a large potential window. The electrochemical behavior of DLC films would be different according to their microstructure and dopants employed. However, there have been rarely systematic studies on the electrochemistry of DLC materials. The literature review reveals that conductive DLC films can be a favorable novel electrode material in the future. Fabrication of DLC films into fine patterns is an essential step to promote the films for the applications in electronics and electrochemical analysis. In principle, it is easier to pattern DLC films compared to diamond films. A detailed examination of preparative parameters in relation to a better understanding of pure and doped DLC film electrode behavior in electrochemistry is needed before the DLC film electrodes are used for practical applications. Micro-fabrication of DLC films should be taken into consideration in order to use DLC films as microelectrodes for future electrochemical applications.Chapter 3Experimental details3.1. Sample preparationDoped diamond-like carbon (DLC) thin films were fabricated using two different methods: filtered cathodic vacuum arc (FCVA) and DC magnetron sputtering deposition methods. Fig. 3.1: Schematic configuration of a FCVA deposition system. The nitrogen doped DLC (N-DLC) thin films (used in Chapter 4) were deposited on highly conductive p-Si (111) (1-6 10-3 cm) (Ra ~ 0.12 nm) substrates using a FCVA system (nano films) shown in Fig. 3.1. A pure graphite (99.995%C) target was used as the carbon source and nitrogen gas was introduced into the deposition chamber for doping. The thickness of the films was about 100 nm. All the film depositions were carried out at room temperature and the other deposition parameters are listed in Table 3.1.Table 3.1: Process parameters of N-DLC films.N2 flow rate (sccm)0.5351020Chamber pressure (Torr)2.8×10-63.5×10-55.0×10-59.7×10-51.9×10-4Bias (V) (pulse)1500Substrate temperature Room temperature39116002286001. Gas feed line 2. Magnetrons 3. Cryogenic pump 4. Rotary pump5. Butterfly valve 6. Substrate holder 7. Load lock 8. Gate valve Fig. 3.2: Schematic configuration of a magnetron sputtering system [62].Table 3.2: Process parameters of PtRuN-DLC films.DC power on Pt50Ru50 target (W)15202530DC power on C target (W)650Substrate bias (V)-30Substrate rotation (rpm)33Substrate temperature Room temperatureWorking pressure (mTorr)3Ar flow rate (sccm)50Deposition time (min)120N2 flow rate (sccm)10The platinum/ruthenium/nitrogen doped DLC (PtRuN-DLC) thin films (used in Chapter 5) were deposited on highly conductive p-Si (100) (0.02–0.005 Ωcm) (Ra ~ 0.12 nm) substrates with a size of 2 cm × 2 cm using a DC magnetron sputtering deposition system (Penta Vacuum) (Fig. 3.2). Prior to the film depositions, the Si substrates were ultrasonically cleaned with ethanol for 20 min followed by deionized water cleaning and air drying. Thereafter, the Si substrates were pre-sputtered with Ar+ plasma at a substrate bias of -250 V for 20 min in order to remove oxide layers and contaminations on the substrate surfaces. High-purity graphite (99.999 %) and Pt50Ru50 (99.99 %) targets of 4 inch in diameter were co-sputtered. The thickness of the PtRuN-DLC films increased from 220 to 300 nm with increased DC power applied to the Pt50Ru50 target. The detailed deposition parameters are summarized in Table 3.2.The N-DLC and PtRuN-DLC thin films (used in Chapter 6) were deposited on p-Si (100) (0.0035-0.001 Ωcm) (Ra ~ 0.12 nm) substrates with a size of 2 cm × 2 cm using a DC magnetron sputtering deposition system (Penta vacuum) shown in Fig. 3.2. Prior to the film depositions, the Si substrates were cleaned according to the procedure used in the preparation of the PtRuN-DLC films as described above. The thicknesses of the N-DLC and PtRuN-DLC films were 160 and 250 nm, respectively. The detailed deposition parameters are summarized in Table 3.3.Table 3.3: Process parameters of N-DLC and PtRuN-DLC films.SamplesN-DLCPtRuN-DLCDC power on Pt50Ru50 target (W)?-40N2 flow rate (sccm)15DC power on C target (W)850Substrate bias (V)-90Substrate rotation (rpm)20Substrate temperature Room temperatureWorking pressure (mTorr)3Ar flow rate (sccm)50Deposition time (min)303.2. Characterization3.2.1. Film StructureThe microstructure of the films was investigated using transmission electron microscopy (JEOL-JEM-2010) which was operated at an accelerating voltage of 200 kV. The samples were prepared by dispersing a small amount of the scratched powder in ethanol with ultrasonic treatment. A drop of the dispersion was taken by using a pipet and put on a holly-carbon copper grid, followed by being dried at room temperature.Since nitrogen is a weak dopant in carbon, techniques like core-level analysis are required to establish the chemical bond of nitrogen with carbon. The chemical composition and binding energy of the films were measured with X-ray photoelectron spectroscopy (XPS) (Kratos Axis Ultra) with a hemispherical analyzer using a pass energy of 40 eV for C 1s, C 1s + Ru 3d, N 1s, Pt 4f, Ru 3p and O 1s core level spectra and 160 eV for the survey scans. The energy resolutions were 1 eV for the survey scans and 0.1 eV for the narrow scans, respectively. A monochromatic Al Kα X-ray radiation (hυ = 1486.71 eV) was employed for the measurements. Fitting of the XPS spectra was performed by decomposing the spectra into different components with a Gaussian line shape and by approximating the contribution of the background with Shirley method. A calibration was done by C 1s (approximately 285 eV) spectrum of a single crystal diamond. The structure of the films was analyzed using micro Raman spectroscopy (Renishaw S2000) with a He-Ne laser (632 nm) over the range of 800-2000 cm-1. An objective lens (×50) was used for a better signal-to-noise ratio. Each Raman instrument used had a spectral resolution of 1 cm-1 and a spatial resolution of 1 ?m. The Raman spectrum acquired was deconvoluted using Gaussian functions into two peaks: graphitic (G) and disordered (D) peaks. Average values of the Raman parameters such as positions, full-widths-at-half-maximum (FHWM) and intensity ratios (ID/IG) of D and G peaks were taken from five random measurements on each sample.3.2.2. Film surface activity, morphology and topography The surface activity of the films was studied with water contact angle measurement (a sessile liquid drop method, FTA 200). An average value of water contact angles was taken from five random measurements per sample.The surface roughness of the films was measured using atomic force microscopy (AFM) (Digital Instruments, S-3000) with a tapping mode Si3N4 cantilever in the scan size of 1 ?m × 1 ?m. An average value of surface roughness (Ra) was determined from five measurements per sample.The film surface morphology was observed using scanning electron microscopy (SEM) (JEOL-JSM-5600LV). Before SEM studies, each sample was coated with a gold layer to avoid charging effect. 3.2.3. Adhesion strength of the film to the p-Si substrateA microscratch tester (Shimadzu SST-101) (Fig. 3.3) was used to qualitatively evaluate the film adhesive strength. During a scratch test, the spherical-shaped diamond stylus, which had 15 ?m in radius, was drawn over the sample surface under a normal force that was progressively increased until the film was detached. The normal force at the moment of the film detachment was measured as a critical load that was a comparative value of the film adhesive strength. The scan amplitude and frequency, scratching rate, and down speed for all the tests were set as 50 ?m, 30 Hz, 10 ?m/s, and 2 ?m/s, respectively. Five measurements were conducted on each sample and an average value of critical loads was taken. Fig. 3.3: Basic feature of a micro-scratch tester [62].3.3. Electrochemical measurements3.3.1. Sample preparationFor all the electrochemical tests, the DLC film coated silicon samples were cut into 2 cm × 2 cm square pieces as shown in Fig.3.4 and a gold layer was deposited on the backsides of the Si substrates to get a good electrical contact during the tests. (a) (b) Fig. 3.4: (a) Size of a film-coated sample and (b) film-coated samples.3.3.2. Electrochemical workstationThe potentiodynamic polarization tests were carried out using a potentiostat/galvanostat (EG&G 263A) having a potential resolution of 250 ?V and a current resolution of less than 2 pA. A bionano electrochemical workstation (LK6200) having a potential resolution of 0.1 mV and a current resolution of less than 0.1 pA was applied for the linear sweep cyclic and anodic stripping voltammetric measurements. The electrochemical impedance spectroscopic (EIS) measurements were conducted using an Autolab Type II potentiostat/galvanostat with GPES 4.9 software (Eco Chemie, Netherlands) having a potential resolution of 150 ?V and a current resolution of less than 30 fA.3.3.3. Setup of Electrochemical cellA flat cell kit (K0235, Princeton Applied Research) (Fig. 3.5) was used for all the electrochemical measurements. The cylindrical shape of the flat cell and the placement of the counter electrode directly opposite to the working electrode provide an optimum current distribution over the surface of the working electrode. Offsetting the reference electrode and using a capillary of 1.5 mm in outside diameter as a Luggin probe result in the minimum shielding of the working electrode surface. Consequently, this device minimizes any iR drop in the electrolyte associated with the passage of current in the electrochemical cell. (a) (b) Fig. 3.5: (a) Schematic configuration and (b) outlook of a three-electrode electrochemical cell.First, a small piece of the film coated silicon sample was placed at the hole on the side of the cell. It should be noted that the surface coated with film must face towards the compartment of the electrochemical cell which will be filled with electrolytes as it is the area for electrochemical testing. The film coated silicon sample was then tightened using the clamp to seal off the hole completely. Secondly, the wires were connected to the three electrodes, namely, the reference, counter and working electrodes of the electrochemical cell. Thirdly, the cell compartment was filled up with different solutions depending on the requirements of experiments under going. The electrochemical cell was then ready for all the electrochemical tests. All the experiments were performed at room temperature. The tested area on the films was a circle of 1 cm in diameter. The potentials were measured with respect to a standard saturated calomel reference electrode (SCE) (244 mV vs.SHE at 25 ?C), and a platinum mesh counter electrode was used. 3.3.4. Potentiodynamic polarization testPotentiodynamic polarization tests were conducted using a potentiostat/galvanostat (EG&G 263A) to evaluate the corrosion performance of the DLC films in NaCl solutions with different concentrations when the corrosion potential was nearly stable after immersion in the solution for about 15 to 30 min. In this experiment, the potential scanning range was varied according to experiment’s requirements and a scan rate of 0.8 mV/s was used. All the corrosion parameters, such as corrosion potentials (Ecorr) and currents (Icorr), were taken using Tafel’s technique.3.3.5. Immersion testImmersion tests were performed in NaCl solutions with different pH values which were compensated with HCl (HCl → H+ + Cl-) and NaOH (NaOH → Na+ + OH-) for 366 hr at room temperature. For the immersion tests, the samples were cut into 1 cm × 1 cm square pieces and directly immersed and suspended in the corrosive media without any packaging of the samples. 3.3.6. Electrochemical impedance spectroscopyElectrochemical impedance spectroscopy (EIS) was used to measure the electrochemical properties of the films in a HCl solution. The EIS measurements were carried out using an Autolab Type II potentiostat/galvanostat with GPES 4.9 software (Eco Chemie, Netherlands). Bode and Nyquist plots of the film coated samples were acquired at the open circuit potentials in the frequency range of 105 to 10-2 Hz with an AC excitation signal of 10 mV.3.3.7. Cyclic voltammetryLinear sweep voltammetry (LSV) was used for all the voltammetric measurements. In the linear sweep voltammetric measurement, the current response was plotted as a function of voltage. This method gave an exact form of voltammogram which could be rationalized by voltage and mass transport effects. In addition, the characteristics of the linear sweep voltammogram recorded depended on several factors including: (1) the rate of electron transfer reaction, (2) the chemical and electrochemical reactivity of active species and (3) the voltage scan rate. Cyclic voltammetric experiments were carried out using a bionano electrochemical workstation (LK6200). All the cyclic voltammograms were obtained with three electrodes immersed in a flat cell kit containing an electrolytic solution at a scan rate of 100 mV/s. The cyclic voltammetry was performed to measure the potential windows of the films. The difference between the potentials for hydrogen and oxygen evolution was called a potential window. The wider the potential window, the more elements in solution could be detected. After every CV testing, a potential of 0.5 V vs. SCE was applied on the film electrode for 2 min for discharge cleaning, and then the electrode was washed in distilled water to further clean the electrode surface.3.3.8. Anodic stripping voltammetryLinear sweep anodic stripping voltammetry (LSASV) was appled for tracing heavy metals, and all the LSASV measurements were carried out using a bionano electrochemical workstation (LK6200). Before the first electrochemical testing, the N-DLC film electrodes were cleaned with acetone and distilled water. After each stripping analysis experiment, the electrode was held at 0.5 V vs. SCE for 2 min to clean away the metals deposited on the electrode surface. After that, the electrode was washed with distilled water. The pH value of the electrolyte was adjusted with HCl and NaOH. With linear sweep voltammetry, the sweep rate used was about 36.36 mV/s. All the chemicals employed were analytical reagent grade. All the stripping voltammograms were compensated with the ohmic drop measured using EIS.3.4. SummaryThe N-DLC films were deposited on the p-Si substrates using a filtered cathodic vacuum arc (FCVA) system by varying nitrogen flow rate from 0.5 to 20 sccm. The PtRuN-DLC films were fabricated on the p-Si substrates using a DC magnetron sputtering system by varying the DC power applied to the Pt50Ru50 target from 15 to 30 W. The N-DLC and PtRuN-DLC films were deposited on the p-Si substrates using the DC magnetron sputtering system under the same deposition conditions, except co-sputtering the Pt50Ru50 target.The film chemical composition was measured using X-ray photoelectron spectroscopy (XPS). The film microstructure was observed using transmission electron microscopy (TEM) and the film chemical structure was diagnosed using XPS and micro-Raman spectroscopy. The film surface activity, morphology and topography were studied using contact angle measurement, scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively. The adhesion strength of the films to the substrates was evaluated using micro scratch test. The corrosion resistance of the films in aqueous solutions, such as NaCl and HCl, was studied using potentiodynamic polarization test, immersion test and electrochemical impedance spectroscopy (EIS). The cyclic voltammetric behavior of the films in different aqueous solutions, such as HCl, H2SO4, KCl, NaCl, KOH, and NaOH, was studied using linear sweep cyclic voltammetry. Anodic stripping voltammetric behavior of the films was evaluated in terms of tracing heavy metals, such as Zn, Pb, Cu, Hg, etc., by using linear sweep anodic stripping voltammetry.Chapter 4Structure and Electrochemical Properties of Nitrogen Doped Diamond-like Carbon Thin Films4.1. IntroductionDiamond-like carbon (DLC) is well-known as a suitable candidate material for use in harsh environments. A unique combination of high hardness, low friction coefficient, high wear resistance, and excellent chemical inertness of DLC materials makes them ideal protective coatings in corrosive environments [29, 134]. Human activities have released toxic metals such as mercury (Hg), lead (Pb), copper (Cu) and zinc (Zn), etc. into the environment. Nowadays, the presence of toxic metals in the aquatic ecosystem implicates directly to biota and indirectly to human beings [3, 135]. Pb is of great concern because of the high toxicity of its compounds and accumulation in various organisms [3]. It was reported that a Pb concentration ≥ 0.4 ?M in drinking water is detrimental to fetuses and children with possible development of neurological problems [3]. Hg and its compounds are also highly toxic, which can accumulate in vital organs and tissues, such as heart muscle, liver and brain, and cause kidney injury, central nerve system disorder, intellectual deterioration, and even death [3]. Some other effects caused by Hg are tremors, impaired cognitive skills, and sleep disturbance in human being with chronic exposure to mercury even at low concentrations of ?M [3, 135]. Although Cu and Zn have been found to have relatively low toxicity to human being, prolonged consumption of a large dose can result in some health complications [3, 135]. For Cu, a concentration ≥ 8×10?5M can adverse chronic effects [3, 135]. Therefore, fast detection and determination of trace heavy metals are a tough challenge for analysts. Hanging mercury drop electrodes (HMDE), noble metal electrodes, and glassy carbon and graphite electrodes have been widely used for electroanalysis, but the poisoning of Hg used in HMDE electrodes to the environment, and surface oxidation and reduction of the metal and carbon electrodes limit their use for electrochemical applications. Boron doped diamond (BDD) film electrodes have been successfully introduced for stripping voltammetric analysis of Pb, Mn, Cd, Cu and Ag, but fabrication of these films demands a high substrate temperature [18-22].DLC films can be produced by physical vapor deposition (PVD) and chemical vapor deposition (CVD) methods at room temperature and achieve similar properties to those of diamond films, so they have been explored as electrode materials for heavy metal tracing [18-24]. However, high electrical resistivity and residual stress of DLC films have confined their electrochemical applications. The electrical conductivity of DLC films is very important for electrochemical analysis because it can abruptly affect the sensitivity of the film electrodes. A high residual stress in DLC films apparently reduces the adhesion strength of the films. In a corrosion environment, poor adhesion strength of DLC films allows undermining of the films by attacking the interfacial bonds between films and substrates with electrochemically active species permeated through the porosities in the films. The characteristics of DLC films can be altered by incorporating different elements such as H, N, F, Si, Pt, Au, Ni, Ti, V, etc. in the films [25-27]. It was reported that the introduction of nitrogen into DLC films could lower the electrical resistivity because of the increased sp2 sites in the films [18]. However, the increased sp2 sites in DLC film with nitrogen incorporation lower the corrosion resistance of the films due to increased prompt dissolution of the films. The poor corrosion resistance of DLC films can abruptly affect the electrochemical properties of the films such as sensitivity, repeatability, long-time response stability and durability when the DLC films are used as film electrodes for electrochemical purposes. Therefore, an improvement of the corrosion resistance of nitrogen doped diamond-like carbon (N-DLC) films becomes important for electrochemical applications.Many electroanalytical techniques are simple, quick, cheap, and easy to use for in situ measurements in rivers or lakes [8]. Cyclic voltammetry (CV) is one of the most frequently used electrochemical methods because of its relative simplicity and high information content. CV has a great advantage for elucidating the mechanisms of electrode reactions. Linear sweep voltammetry is a simple and useful technique in stripping analysis, which can produce well defined and selective stripping peaks due to a lower interference of species that can be adsorbed by working electrode, compared to differential pulse and square wave techniques [136]. A voltammogram measured by this method and rationalized by the potential and transport of metal ions allows a study on the kinetic behavior of the ions in solutions having a wide range of metal ion concentrations. This chapter investigates the influence of nitrogen concentration on the bonding structure, surface morphology, adhesion strength and electrochemical performance of nitrogen doped diamond-like carbon (N-DLC) thin films deposited on p-Si substrates using a filtered cathodic vacuum arc (FCVA) deposition system in terms of nitrogen flow rate.4.2. Structural properties of N-DLC thin films4.2.1. Chemical composition of N-DLC thin filmsThe chemical composition of the N-DLC films deposited with different nitrogen flow rates was measured using X-ray photoelectron microscopy (XPS). From the XPS results, it is found that the N content on the N-DLC film surfaces significantly increases from about 0.98 to 6.48 at% when the nitrogen flow rate is increased from 0.5 to 20 sccm. After plasma cleaning for 300 s at a chamber pressure of about 3.5 × 10-8 Torr, the N contents in the bulks of the N-DLC films deposited with the nitrogen flow rates of 0.5 and 20 sccm are about 0.52 and 4.34 at%, respectively. Fig. 4.1 shows that the fitted XPS spectra of the N-DLC films deposited with different nitrogen flow rates. Fitting of the C 1s spectra is performed by decomposing each of them into four components with a Gaussian line shape and by approximating the contribution of the background by Shirley method. The peak found about 284.1 eV is attributed to the C–C sp2 bonding, and the C–C sp3 bonding contributes to the peak observed at about 285 eV [18]. The peak at about 286.1 eV is attributed to the C-O bonding formed at the surface of the films due to their exposure to the air and the peak locating at about 288.5 eV comes from the C-N bonding [18]. Fig. 4.1: Fitted XPS C 1s spectra of N-DLC films deposited with nitrogen flow rates of (a) 0.5 and (b) 20 sccm.The sp3 carbon content is determined as a ratio of its corresponding peak over the total C 1s peak area [18]. It is found that the sp3 content of the N-DLC films significantly decreases from about 54.64 to 46.42 % with increased nitrogen concentration in the films probably due the increased amount of the sp2 bonds in the films [54]. 4.2.2. Raman results of N-DLC thin filmsThe chemical structure of the N-DLC films was characterized using micro-Raman spectroscopy. Figure 4.2 shows the Raman spectra of the N-DLC films as a function of nitrogen flow rate. The Raman spectra of the N-DLC films are fitted using a Gaussian function for the G peaks and a Lorentzian function for the D peaks as shown in Fig. 4.2. The FCVA process can generate high kinetic energy of impinging carbon species that in turn form N-DLC films with a high fraction of sp3 bonds. 1442720113030 Fig. 4.2: Raman spectra of N-DLC films deposited with different nitrogen flow rates. The inset shows ID/IG and AD/AG as a function of nitrogen flow rate.As shown in Fig. 4.2, the Raman spectra of the films correspond to the sp2 bonded carbon embedded in the sp3 networks. It is well known that a typical Raman spectrum of DLC is composed of G and D peaks in which the G peak is due to the stretching vibrations of any pairs of sp2 sites in chains or aromatic rings and the D peak comes from the breathing mode of those sp2 sites only in aromatic rings [137]. Therefore, all the Raman spectra were fitted using Gaussian functions for both G and D peaks. Ferrari et al. [137] proposed that sp2 bonds could exist not only as rings but also as chains in a dense matrix of DLC depending on its sp3 content. Usually, the introduction of nitrogen into a carbon network induces the formation of new sp2 sites and encourages the sp2 sites to form clusters because of preferential π bonding of nitrogen [54]. Therefore, the increased nitrogen content in the N-DLC films enhances the amount and clustering of the sp2 sites. The increased intensity ratios (ID/IG) and integrated area ratios (AD/AG) between the D and G peaks of the N-DLC films with increased nitrogen flow rate (inset in Fig. 4.2) reveal that the increased nitrogen content in the N-DLC films promotes the clustering of the aromatic rings, indicating an increase in graphitic phases [29, 138].4.2.3. Surface morphology of N-DLC thin filmsFigure 4.3a shows the Ra values of the N-DLC films as a function of nitrogen flow rate. When the nitrogen flow rate changes from 0.5 to 20 sccm, the surface roughness (Ra) of the N-DLC films slightly increases from about 0.12 to about 0.23 nm (91.7% increment) as shown in Fig. 4.3a. It indicates that increasing the nitrogen concentration in the N-DLC films increases the surface roughness of the films due to a reduced density of the films caused by increased sp2 sites [24]. In addition, the aggregation of the nitrogen inclusions in the N-DLC films also contributes to the surface roughness of the films due to the difference in electronegativity values between carbon (~2.55, pauling scale) and nitrogen (~3.04) [24].The N-DLC film deposited with 0.5 sccm N2 has fine asperities as shown in Fig. 4.3b, which is due to a higher fraction of sp3 carbon bonding formed in the film. Larger surface asperities can be seen on the N-DLC film deposited with 20 sccm N2 (Fig. 4.3c), which are possibly caused by the increased number of sp2 sites and aggregation of nitrogen atoms [67, 68]. (a)508444559055205549530480 (b) (c)Fig. 4.3: (a) Ra values of N-DLC films versus nitrogen flow rate. AFM images of N-DLC films deposited with nitrogen flow rates of (b) 0.5 and (c) 20 sccm.4.2.4. Adhesion strength of N-DLC thin filmsIn a scratch test, a critical load is determined when an abrupt change in tangential force is observed, which comes from an instant failure between DLC film and Si substrate. The critical loads for the N-DLC film coated samples increase from 445 to 477 mN (7.2% increment) with increased concentration as shown in Fig. 4.4a. (a)1426845148590LPLPHPpP (b)Fig. 4.4: (a) Critical loads of N-DLC films with respect to nitrogen flow rate and (b) SEM micrograph of a N-DLC film (0.5 sccm N2) scratch tested till a critical load of 456 mN. HP and LP indicate high pressure and low pressure areas, respectively. The inset is the progressive loading curve measured, from which the critical load is determined.It is well known that the adhesive strength of a film is strongly influenced by its residual stress. Sullivan and co-workers [73] proposed that the reduction in residual stress was due to the conversion of a small fraction of sp3 sites to sp2 sites during the film deposition. The sp2 bonds having shorter bond lengths than those of the sp3 bonds would reduce the strain in the film plane. In addition, since the C=N bonds have shorter bond lengths compared to the C-C and C=C bonds, the increased C=N bonds with increased nitrogen concentration give rise to a higher critical load by reducing the strain in the film [24].The inset in Fig. 4.4b shows the load-cartridge output signal from the scratch test of the specimen with the N-DLC film deposited with 0.5 sccm N2 from which a critical load of 456 mN is determined. A cartridge output signal is a voltage ratio obtained from the motions of the stylus and the cartridge that is also oscillating horizontally above the sample during the scratch testing [139]. The cartridge output signals are an indication of tangential forces experienced by the stylus during the scratch testing. No fluctuation can be seen on the cartridge output signals with respect to normal load, except for an abrupt increase in the critical load. A stable linear relationship between tangential force and normal load indicates a uniform film adhesion throughout the film-substrate interface. From the SEM micrograph (Fig. 4.4b), no cohesive failure of the film can be found within the scratched path of the film, while only the fractured area with flaking at the critical load can be viewed as a brittle fracture. It implies that a high fraction of sp3 bonds can enhance the cohesive strength of the N-DLC films so that the fracture occurs only at the critical load. This is in agreement with the work by Gupta et al. in which a thinner film exhibited an instant damage when the normal load exceeded the critical load [80]. When the cartridge vibrates, the stylus is pulled in the direction of the vibration. The traction induced results in a nonuniform pressure distribution that causes the shearing between the film and substrate, leading to the peeling-off and breaking of the film.4.3. Electrochemical performance of N-DLC thin films4.3.1. Corrosion behavior of N-DLC thin films4.3.1.1. Potentiodynamic polarization results of N-DLC thin filmsFigure 4.5 presents the potentiodynamic polarization curves of the N-DLC films as a function of nitrogen flow rate. From the curves, it is found that the cathodic branches do not change with nitrogen concentration except for the sample deposited with 3 sccm N2. However, the bonding structure of the N-DLC films, which varies with nitrogen concentration, affects the anodic branches of their polarization curves. It shows that the N-DLC film deposited with 3 sccm N2 has the highest corrosion resistance.In the polarization curves of the N-DLC films deposited with 0.5 and 3 sccm N2, a reduction in corrosion current is found at a potential above 500 mV vs. SCE. However, no such a reduction in corrosion current is found from the rest curves, which can be explained based on the observation of the surface morphologies of the corroded films. Figures 4.6a and b shows the SEM micrographs of the corroded areas of the N-DLC films deposited with 0.5 and 3 sccm N2, respectively, after the polarization testing in the 0.6 M NaCl solution at room temperature. Some localized solid products can be seen on the film surfaces, which usually agglomerate around the defects or pores. These solid products would reduce the corrosion currents by blocking the diffusion paths of the electrochemically active species from the electrolytic solution to the film surfaces or to the underlying substrates. That the reduction of the corrosion currents after the formation of these solid products takes place is consistent with the decreased anodic currents in the anodic branches of the polarization curves of the N-DLC films deposited with 0.5 and 3 sccm N2 (insets of Fig. 4.6). Fig. 4.5: Potentiodynamic polarization curves of N-DLC films measured in a 0.6 M NaCl solution at room temperature. In Fig. 4.7, the corrosion potentials (Ecorr) of the N-DLC films are determined by fitting the potentiodynamic polarization curves measured at the fixed scan rate of 0.8 mV/s (Fig. 4.5). The corrosion current (Icorr) and anodic (βa) and cathodic (βc) Tafel slopes are determined at the same time. The polarization resistance (Rp) values in Fig. 4.7 are then calculated using the following formula [83]: Rp = βa × βc / 2.3 Icorr (βa + βc) (4.1)where Rp is in k? cm2; βa and βc are in terms of V/I-decade; and Icorr is in ?A/cm2.A33318451581150A (a)BB32937451554480 (b)Fig. 4.6: SEM micrographs showing surface morphologies of corroded N-DLC films after potentiodynamic polarization tests in 0.6 M NaCl solution: (a) 0.5 sccm, Ecorr = -85.72 mV vs. SCE and (b) 3 sccm, Ecorr = -57.41 mV vs. SCE, where the insets in the bottom right corners show their enlarged views of locations A and B, respectively.It is found that the trend of Rp is similar to that of Ecorr as shown in Fig. 4.7. When the nitrogen flow rate is increased from 0.5 to 3 sccm, the Ecorr shifts from -85.7 to -57.4 mV vs. SCE and the Rp increases from 16.8 to 150.1 k? cm2. However, the Ecorr shifts to more negative values from -57.4 to -97.6 mV vs. SCE and the Rp turns to decrease from 150.1 to 15.8 k? cm2 (89.5% decrement) with further increased nitrogen flow rate to 20 sccm. The corrosion results point out that the N-DLC film deposited with 3 sccm N2 shows the highest corrosion resistance among the films used in this study. Fig. 4.7: Corrosion potentials (Ecorr) and polarization resistances (Rp) of N-DLC films as a function of nitrogen flow rate.The decreased corrosion resistance of the N-DLC films with higher nitrogen flow rates than 3 sccm is attributed to several factors such as the bonding structure, electrical resistivity, surface roughness and porosity density of the films. An increase of nitrogen content in the films promotes the graphitic phases as depicted by the increased ID/IG and AD/AG ratios (inset of Fig. 4.2). The increased graphitic phases in the N-DLC films lead to an earlier dissolution of the films because the increased sp2 bonds via the increased graphitization degraded the sp3-bonded cross-linking structure which was responsible for preventing the anodic dissolution of the films. Therefore, the decreased corrosion resistance of the N-DLC films with increased nitrogen concentration is the reason decreasing the corrosion potentials (Ecorr) of the films.In addition, the corrosion properties of the N-DLC films are related to the kinetics of electrochemical reactions taking place at the film-solution interface. It is known that the introduction of nitrogen into the films reduces the electrical resistivity of the films [18, 20, 23]. Another parameter which can determine the electrical conductivity of the DLC films is sp2/sp3 ratio, i.e. the higher the sp2/sp3 ratio, the lower the electrical resistivity of the film is [140]. The increased electrical conductivity of the N-DLC films with increased nitrogen concentration promotes the electron transfer to or from the films during the corrosion testing, thus accelerating the electrochemical reactions in the electrical double layers (EDL).Moreover, due to the presence of many tiny anodic and cathodic sites on the surface of the films caused by the aggregated nitrogen or the electrochemical potential difference between the films and substrates [67, 98, 141], a galvanically-induced corrosion could occur between them after the electrochemically active species access the film surface and permeate into the substrate through the pores. Such corrosion becomes pronounced with increased nitrogen flow rate. Furthermore, the increased surface roughness of the N-DLC films may also contribute to the decreased corrosion resistance of the films because of a larger exposed surface area to the electrolyte during the corrosion testing.These combined effects have resulted in the corrosion behavior of the N-DLC films as shown in Fig. 4.7. A lower corrosion resistance of the N-DLC film deposited with 0.5 sccm N2 than that of the film deposited with 3 sccm N2 may be attributed to more porosities formed in the film deposited with 0.5 sccm N2, which probably results from an unbalance between the kinetic energy of the sputtered C species arriving at the growing film surface and the surface mobility of the adatoms on the growing surface [142, 143]. An increase of nitrogen gas in the deposition chamber under the fixed argon flow rate reduces the mean free path of the sputtered species, thus decelerating the kinetics of the sputtered species and lessening the formation of porosities. Above combined factors can explain the higher corrosion resistance of the N-DLC film deposited with 3 sccm N2 than that of the one deposited with 0.5 sccm N2.4.3.1.2. Immersion results of N-DLC thin filmsImmersion test offers a simple and cheap way of studying the corrosion behavior of the N-DLC films immersed in the corrosive media for a longer time. In this investigation, the N-DLC film coated samples (3 sccm N2) were immersed in the 0.6 M NaCl solutions with different pH values compensated with HCl (HCl → H+ + Cl-) and NaOH (NaOH → Na+ + OH-) for 336 hr. Fig. 4.8 shows the surface morphologies of the samples after the immersion tests where no detachment of the films is found. The sample immersed in a solution of pH 2 is more severely corroded than other two samples tested in the solutions of pH 4.5 and pH 12, because of unbalanced hydrogen (H+) and hydroxide (OH-) ions in the solutions [141]. The corrosion on the surface comes from the reaction of the film with the electrochemically active species in the electrolyte. When the N-DLC film atoms are dissolved as ions into the aqueous solution, the electrons released will flow to the electrochemically active species in the solution where the hydrogen ions resulting from the water dissociation (H2O → OH- + H+) gain these released electrons in the cathodic reaction (H+ + e- → H) [90]. At pH < 7, the H+ ions mainly influence the corrosion while at pH ≥ 7, the OH- ions deplete the H+ ions in the solution through hydrolysis resulting in a reduced corrosion. Therefore, an apparent change in corrosion characteristics on the surface of the N-DLC film with respect to pH value is observed in the SEM micrographs (Fig. 4.8a–c).At the same time, the effect of Cl- ions, which are increased by compensating the solution with HCl to make the solution more acidic, on the corrosion resistance of the N-DLC films should also be taken into account besides the effect of H+ ions. Since the catalytic activity of the Cl- ions accelerates the corrosion process of the N-DLC films, it should be noted that the increase in the concentration of the Cl- ions with the compensation of the solution can give an additional effect on the decrease in the corrosion resistance of the films with decreased pH as discussed above. (b) (c) (d)Fig. 4.8: SEM micrographs showing corroded areas of N-DLC film coated samples after immersion tests in 0.6 M NaCl solutions with different pH values: (a) pH 2, (b) pH 4.5 and (c) pH 12 for the films deposited with 3 sccm N2 and (d) pH 4.5 for the film deposited with 20 sccm N2 for comparison. All the tests are conducted for 336 hr at room temperature and ambient atmosphere.The initiation of a pit on the film surface occurs when a small local site on the film is exposed to the damaging species such as chloride and hydrogen ions. It is known that the pits initiate at defects, surface compositional heterogeneities and porosities because these imperfections degrade the cross-linking structure of the film. The DLC films may always contain certain open pores that allow a direct access of the electrolyte to the underlying substrate and some closed pores which are not absolutely open or which diameters are only big enough to allow some specific ions or molecules in the electrolyte to gradually migrate to the substrate surface [84]. When the electrolyte accesses the film surface, the pores become the initiation sites for the corrosion. An increase in immersion time causes the closed pores to become open and the open pores to grow, resulting in an increase of the porosity density in the film. The pores serving as crevices allow the entrapment of the permeated electrochemically active species inside them and a buildup of the positive hydrogen ions through hydrolytic reactions. The buildup of the positive hydrogen ions increases the acidity of the entrapped electrolyte inside the pores when the openings of the pores are covered with the corrosion products resulted from the corrosion of the film. In addition, such a buildup of the positive charges in the pores becomes a strong attractor to negative ions, e.g. chlorine, which can be corrosive in their own right, resulting in a more severe corrosion inside the pores than the surrounding. Eventually, the growth of the pores by connecting adjacent pores forms the pits as shown in Fig. 4.8a. However, a collection of the pits (Fig. 4.8a), which serves as an active site with respect to the surrounding (a cathodic site) and shows a more severe corrosion than the surrounding, indicates that the localized occurrence of the pits probably results from the aggregation of nitrogen in the amorphous carbon structure. The aggregated nitrogen locally degrades the sp3 bonded cross-link structure through the increased sp2 sites and the degraded cross-link structure can be easily attacked by the electrochemically active species. As explained above, the insufficient repassivation of the film also accelerates the localized dissolution of the film. When the pH value of the electrolyte is decreased, the increased concentration of hydrogen ions increases the acidity of the electrolyte accessible to the N-DLC film as well as the entrapped electrolyte inside the pores, thus accelerating the growth of the pits. Therefore, after the immersion tests, a more apparent pitting of the N-DLC sample (3 sccm N2) is found with the solution of pH 2 (Fig. 4.8a) than those tested with the solutions of pH 4.5 (Fig. 4.8b) and pH 12 (Fig. 4.8c).A DLC film is usually electrochemically nobler than a Si substrate, so a galvanically-induced corrosion can occur between the film and the substrate when the electrolyte has permeated into the substrate through some pores or defects of the film [84]. The increased electrical conductivity of the film with nitrogen doping may facilitate the galvanically-induced corrosion between the film and the substrate. With a prolonged immersion, more oxygen existing in the electrolyte confined within the pores and undermining areas of the film is consumed leading to an increase in concentration of H+ ions in the entrapped electrolyte. The accumulated H+ ions can attract the negatively charged ions like chlorine ions in the surrounding electrolyte. A highly acidic local environment with increased immersion time can cause a substantial increase in corrosion rate, resulting in large undermining areas. The increased electrolyte trapped inside the enlarged undermining areas leads to a higher surface tension and eventually cracking and damaging happen to the film as revealed in Fig. 4.8b for the N-DLC film (3 sccm N2). From the comparison between Fig. 4.8b and d, the N-DLC film deposited with a higher N2 flow rate (20 sccm) has a lower undermining effect due to its higher adhesion strength and at the same time, however, a higher pitting rate attributed to a higher nitrogen doping level. Therefore, it can be deduced that the adhesive strength of the films is an important parameter to lessen the undermining effect of the imperfections in the films exposed to the corrosive media.4.3.2. Linear sweep cyclic voltammetric behavior of N-DLC thin films4.3.2.1. Cyclic voltammetry of N-DLC thin films in acidic solutionThe cyclic voltammograms of the N-DLC film electrodes measured in a 0.5 M HCl solution at a scan rate of 100 mV/s are illustrated in Fig. 4.9. A difference between the potentials for hydrogen and oxygen evolutions in a cyclic voltammogram gives an electrochemical potential window. The wider the potential window, the more the elements in the solutions can be detected for metal tracing analysis. The potentials for hydrogen and oxygen evolutions on the surface of the N-DLC film electrode (3 sccm N2) are about -1.25 V and +1.15 V, respectively. The N-DLC film electrode (20 sccm N2) has a lower negative potential value of about -0.8 V for hydrogen evolution. It is clearly seen that the potential window of the films decreases with increasing nitrogen concentration, which can be explained in terms of electrical resistivity of the films as nitrogen doping reduces the electrical resistivity of the N-DLC film electrodes [55, 120, 144, 145]. The promoted electrical conductivity reduces an electron transfer potential through the N-DLC film electrode and results in early hydrogen evolution (2H+ + 2e- → H2↑) at a lower negative potential value in the reduction. From the cyclic voltammogram shown in Fig.4.9, no change in potential for the oxygen evolution (4OH- → 2H2O + O2↑ + 4e-) is found in the oxidization half cycle at the scan rate of 100 mV/s. It is clearly found that the potential window is mainly affected by the potential for the hydrogen evolution in the acidic aqueous solution where the main influent ions are H+ ions. Fig. 4.9: Cyclic voltammograms of N-DLC film electrodes with different nitrogen flow rates measured in 0.5 M HCl solution at a scan rate of 100 mV/s.A contribution of background current to the cyclic voltammogram of the N-DLC film electrode (3 sccm N2) can be found. The background current at solid electrode results from extraneous processes primarily associated with the electrode surface. Thus, according to the past history of the electrode, the surface may have become oxidized. The potential applied to the electrode may then give rise to a current from the dissolution of the oxides. An XPS quantitative analysis of the stoichiometry of the N-DLC film electrode surfaces indicates that the N-DLC film electrodes (3 sccm N2) have a higher surface oxygen fraction than the film electrodes deposited with 20 sccm N2. This results from a higher affinity of oxygen with carbon since there is a greater difference in electronegativities between oxygen (~3.44, Pauling scale) and carbon (~2.55) with respect to nitrogen (~3.04). Thus, it can be deduced that the background current is mainly attributed to the dissolution of the oxidized layers. The occurrence of the peak observed in the reduction cycle may be due to the catalytic activity for Cl2/Cl- [20, 146].4.3.2.2. Cyclic voltammetry of N-DLC thin films in neutral solutionsFigure 4.10 shows the cyclic voltammograms of the N-DLCfilm electrodes tested in a 0.1 M KCl (pH 1) solution at a scan rate of 100 mV/s. The pH value of the solution was adjusted by HCl (HCl → H+ + Cl-). The effect of nitrogen incorporation on the potential windows of the N-DLC film electrodes in the solution can also be seen. The film electrodes deposited with 3 and 20 sccm N2 have potential windows of approximately 2.32 and 1.9 V, respectively. Fig. 4.10: Cyclic voltammograms of N-DLC film electrodes with different nitrogen flow rates measured in 0.1 M KCl solution at a scan rate of 100 mV/s.The potential window decreases with increased level of the incorporated nitrogen, resulting from the increased electrical conductivity. This is in agreement with the result mentioned in the preceding section where the contribution of the background current was attributed to the dissolution of the oxidized layer on the N-DLC film electrode (3 sccm N2). It is noticed from this experiment that a higher nitrogen content in the N-DLC film electrodes produces a lower background current leading to an improved signal-to-background ratio in the electroanalytical measurement. Fig. 4.11: Cyclic voltammogram of N-DLC (20 sccm N2) film electrode measured in 0.1 M NaCl solution at a scan rate of 100 mV/s.It has been shown that the nitrogen concentration apparently affects the potential windows of the N-DLC film electrodes in the KCl and HCl solutions. The N-DLC film electrodes (20 sccm N2) are used for further analysis of the electrochemical potential windows in different solutions in order to investigate their outstanding electrochemical properties. The cyclic voltammogram of the N-DLC film electrode (20 sccm N2) recorded in a 0.1 M NaCl solution at the scan rate of 100 mV/s is shown in Fig. 4.11. The pH value of the NaCl solution is not compensated by HCl. The potential window of the N-DLC film electrode (20 sccm N2) in the NaCl solution is approximately 3.2 V, about 1.3 V higher than the potential window of the same electrode measured in the KCl aqueous solution. The difference in potential window can be explained by different pH values. The KCl solution compensated by HCl has a higher concentration of active H+ ions than the NaCl aqueous solution uncompensated by HCl. Both unbalanced hydrogen and hydroxide ions and different electroactive species existing in the KCl and NaCl aqueous solutions can abruptly affect the cyclic voltammetric behavior of the N-DLC films in the solutions. No peak occurring in the reduction half cycle measured in the NaCl solution indicates a lower catalytic activity for Cl2/Cl- in the solution, which may be one of the reasons why the cyclic voltammogram in the NaCl solution has a wider potential window.4.3.2.3. Cyclic voltammetry of N-DLC thin films in hydroxide solutionsFigures 4.12 and 4.13 show the cyclic voltammetric I–E curves of the N-DLC film electrodes (20 sccm N2) recorded in 0.1 M KOH and 0.1 M NaOH solutions at a scan rate of 100 mV/s. The N-DLC film electrode tested in the NaOH solution has a wider potential window of approximately 3.25 V compared to approximately 3.1 V measured in the KOH solution. This may be attributed to different electroactive alkaline species of the solutions. Fig. 4.12: Cyclic voltammogram of N-DLC (20 sccm N2) film electrode measured in 0.1 M KOH solution at a scan rate of 100 mV/s.The potential windows of the N-DLC film electrodes in the solutions containing a higher concentration of OH- ions are wider, compared to those of the film electrodes in the solutions containing a higher concentration of H+ ions. This can be explained by acid-base reactions in which H+ ions and OH- ions are mainly involved. An alkaline hydroxide (KOH, NaOH) dissociates into a cation and one or more hydroxide ions in water, making the solution basic. These hydroxide ions react with hydrogen ions to form water (OH- + H+ → H2O), resulting in a decrease in acidity of the solution. Fig. 4.13: Cyclic voltammogram of N-DLC (20 sccm N2) film electrode measured in 0.1 M NaOH solution at a scan rate of 100 mV/s.Hydrogen evolution lately occurs in a hydroxide solution containing a low concentration of active H+ ions, resulting in a wider potential window. A background current attributed to the charging effect of electrical double layers can be seen on both the cyclic voltammograms of the N-DLC films measured in the KOH and NaOH aqueous solutions. The OH- ions decomposed from KOH or NaOH near the electrode combine with hydronium ions (H3O+) to form water molecules (OH- + H3O+ → 2H2O) which lead to an adsorbed water layer on the electrode surface and a water hydration sheath surrounding the ions, resulting in the separation of the charges. Because the charging effect is becoming greater with increasing the concentration of the OH- ions, the cyclic voltammograms of the N-DLC film electrodes measured in the hydroxide solutions have higher backgrounds attributed to the electrical double layers.4.3.2.4. Cyclic voltammetry of reversible couple (Ferricyanide)Figure 4.14 shows the scan rate dependence of the cyclic voltammograms measured at the N-DLC film electrodes (20 sccm N2) using the reversible ferri-ferrocyanide couple as a redox system [120, 128]: [Fe(CN)6]3- + e- ? [Fe(CN)6]4- (4.2) Fig. 4.14: Cyclic voltammograms of N-DLC (20 sccm N2) film electrode measured in 5 mM K3Fe(CN)6 /0.1 M NaCl solution at different scan rates: (a) 30, (b) 50, (c) 70, (d) 90, (e) 110, and (f) 130 mV/s.The electrolyte used was 0.1 M NaCl with pH 1 compensated by H2SO4. It is found that a peak-potential separation (?Ep) value increases from 92 to 124 mV (34.8% increment) when the scan rate is increased from 30 to 130 mV/s. This is due to the effect of the increased kinetic limitation to shifting an oxidation to more positive potentials and a reduction to more negative potentials [147]. As the scan rate is increased, the timescale of the experiment becomes smaller so that, eventually, an equilibrium is not reached at the film electrode surface and a kinetic effect begins to appear. It is found that the ratio between anodic and cathodic current peaks (Ip,a/Ip,c) fluctuates between 1.02 and 1.06. An Ip,a/Ip,c ratio slightly greater than unity indicates that the Fe(CN)64-/Fe(CN)63- redox reaction at the N-DLC film electrode exhibits a quasi reversible behavior, which means that though the reverse peak appears, it is slightly smaller than the forward one [20].4.3.3. Linear sweep anodic stripping voltammetric behavior of N-DLC thin films4.3.3.1. Linear sweep anodic stripping voltammograms of LeadThe stripping voltammograms of Pb obtained from the N-DLC film electrodes deposited with 3 and 20 sccm N2 in a 1 × 10?3 M Pb2+ + 0.1 M KCl solution (pH 1) as a function of deposition potential are shown in Fig. 4.15a and b. The deposition time and scan rate used are 120 s and 36.36 mV/s, respectively. It can be clearly seen that the stripping peaks of Pb increase with more negative deposition potentials for both nitrogen flow rates. The increased negative deposition potentials cause the mobility of the Pb2+ ions to rise, leading to an increase in the amount of reduced Pb atoms on the surfaces of the film electrodes, which is confirmed by the increased stripping peak currents of Pb with the increase of the negative deposition potentials as shown in Fig. 4.15c. However, the increase of stripping peak current of Pb slows down beyond the deposition potential of ?1.4 V, which may be partially attributed to hydrogen evolution disrupting the reduction of the Pb2+ ions on the surfaces of the film electrodes during accumulation [147].The stripping peaks of Pb obtained from the N-DLC film electrode deposited with 20 sccm N2 are more pronounced than those from the film deposited with 3 sccm N2, which is due to a decrease in the electrical resistivity of the film electrode with a higher nitrogen content in it, leading to an increase in kinetics of electron transfer through the film electrode [18, 52, 147]. The increased kinetics of the electron transfer results in more consumption of the Pb2+ ions in the interface region between the film electrode and the solution. Therefore, a greater concentration gradient created by the increased consumption of the ions promotes the transport of the Pb2+ ions from the solution to the interface and leads to an increase of the reduced Pb atoms on the surface of the film electrode [18]. Consequently, a less restriction in the transport of the ions with a higher nitrogen content in the film electrode gives higher stripping peak currents of Pb.3152775-28575 29432251905Fig. 4.15: Stripping voltammograms obtained from N-DLC film electrodes deposited with (a) 3 and (b) 20 sccm N2 in a 1 × 10-3 M Pb2+ + 0.1 M KCl solution as a function of deposition potential. Dependence of (c) stripping peak current and (d) stripping potential of Pb on deposition potential. The deposition time and scan rate are 120 s and 36.36 mV/s, respectively.That the stripping potentials of Pb shifting to more positive values with more negative deposition potentials for both nitrogen flow rates, as observed in Fig. 4.15d, indicate a quasi-reversible reaction of Pb on the surface of the film electrode in the solution with 1 × 10?3 M Pb2+. It is well known that the linear sweep voltammetric technique gives a voltammogram rationalized by the potential and transport of metal ions. The concentration of redox species at the interface depends on the transport of these species from the bulk solution. When a flux of metal ions to the electrode surface is slower than an electrode reaction, an equilibrium between oxidized and reduced species involved in the electrode reaction is established at the film electrode surface, which implies a reversible reaction corresponding to a case where the electrode reaction is much faster than the transport of the metal ions. Therefore, it is expected that there is a kinetic limit upon the electrode reaction compared to the transport of the Pb2+ ions because the ion concentration is high enough to cause a high concentration gradient between the solution and the interface region, so that the transport of the ions becomes faster than the electrode reaction. Furthermore, the increased kinetic limitation with more negative deposition potentials shifts the oxidation peak to more positive potentials and the reduction peak to more negative potentials [147], which explains an opposite shift between the stripping and deposition potentials of Pb as shown in Fig. 4.15d. Fig. 4.16: A linear sweep cyclic voltammogram obtained from a N-DLC film electrode deposited with 3 sccm N2 in the same solution as the one used for Fig. 4.14 with a scan rate of 36.36 mV/s. A linear sweep cyclic voltammogram obtained from the N-DLC film electrode (3 sccm N2) in the same solution confirms the quasi-reversible reaction of Pb as shown in Fig. 4.16, which illustrates a weaker reverse peak compared to the forward one and has a peak-potential separation of 0.112 V and a ratio of 3.28 between anodic and cathodic current peaks. Fig. 4.17: Stripping voltammograms obtained from a N-DLC film electrode (20 sccm N2) in a 1 × 10-6 M Pb2+ + 0.1 M KCl solution as a function of deposition potential. The deposition time and scan rate are 120 s and 36.36 mV/s, respectively. Figure 4.17 shows the effect of the deposition potential on the stripping response of the N-DLC film electrode (20 sccm N2) in a 1 × 10?6 M Pb2+ + 0.1 M KCl solution (pH 1), where the deposition time is 120 s and the scan rate is 36.36 mV/s. The stripping peaks of Pb become stronger with more negative deposition potentials. It is clearly observed that the increased negative deposition potentials result in a linear shift of the stripping potentials of Pb to more negative values in Fig. 4.17, which indicates the reversible reaction of Pb on the surface of the film electrode because a low concentration gradient between the bulk solution and the interface region causes a slow transport of the Pb2+ ions so that the kinetics of the electrode reaction becomes faster than the transport of the Pb2+ ions. It is clearly seen that the metal ion concentrations at mM and ?M levels induce different stripping voltammetric behavior of Pb on the surfaces of the N-DLC film electrodes.An influence of deposition time on the stripping voltammograms obtained from the N-DLC film electrodes deposited at different nitrogen flow rates in the same solution as the one used for Fig. 4.15 is shown in Fig. 4.18a and b. The deposition time is responsible for the amount of analytes available on the surface of the film electrode at the stripping stage and, therefore, for the sensitivity. Since an increase in deposition time leads to a proportional increase in the amount of reduced Pb atoms on the film electrode surface, the stripping peaks of Pb become stronger with a longer deposition time as revealed in Fig. 4.18a and b. The replotted Pb anodic stripping peak currents with respect to deposition times (Fig. 4.18c) show a near linear upward shift for both nitrogen flow rates [128]. It can be seen from Fig. 4.18c that a higher nitrogen content in the N-DLC film electrode (20 sccm N2) apparently promotes the sensitivity of the film electrode to the Pb2+ ions, resulting in a higher trend of the stripping peak current versus deposition time. However, a slowdown in the increase of stripping peak current of Pb is found beyond the deposition time of 120 s for the film deposited with 20 sccm N2 (Fig. 4.18c). Since the concentration of the Pb2+ ions is fixed and the quantity of the Pb2+ ions is limited in the solution, the accumulation of the Pb2+ ions on the surface of the film electrode eventually reaches a saturation when the time is prolonged. A more apparent slowdown in the response of the N-DLC film electrode deposited with 20 sccm N2 observed beyond the deposition time of 120 s compared to the one with 3 sccm N2 is due to an earlier saturation of the stripping peak current with the increased sensitivity of the film electrode. 3057525-15240 29241750 Fig. 4.18: Stripping voltammograms obtained from N-DLC film electrodes deposited with (a) 3 and (b) 20 sccm N2 in the same solution as the one used for Fig. 4.14 as a function of deposition time, dependence of (c) stripping peak current and (d) stripping potential of Pb2+ on deposition time, and (e) relationship between stripping peak current and potential. The deposition potential and scan rate are -1.2 V and 36.36 mV/s, respectively. A shift of the stripping potentials of Pb to more positive values with the increase of the amount of reduced Pb atoms on the surface of the film electrode during the accumulation, leading to a proportional increase in the concentration of the stripped Pb2+ ions during the stripping, is found in Fig. 4.18d for both nitrogen flow rates when the deposition time is increased from 20 to 140 s. The increased concentration of the stripped Pb2+ ions found in the half oxidization reaction of Pb (Pb → Pb2+ + 2e-, E0) can be correlated to the increased stripping potential according to the Nernst equation [147]: E = E0 – (RT/nF) ln [Pb2+] (4.3)where E0, R, T, n, and F are the standard half cell potential of Pb, the universal gas constant, the absolute temperature, the number of electrons transferred in the half-reaction, and the Faraday constant, respectively. It is found that the increased sensitivity of the film electrode also causes a shift of the stripping potential of Pb to more positive values, resulting from the increased concentration of the stripped Pb2+ ions. The increased nitrogen content in the N-DLC film electrodes promotes the sensitivity of the electrodes to the metal ions, which is related to the electrical conductivity of the electrodes. It is found that the stripping peak currents of Pb measured from the N-DLC film electrode deposited with 3 sccm N2 are consistently lower than those from the film electrode deposited with 20 sccm N2 as shown in Fig. 4.18c. An ohmic drop caused by a lower N doping level can be estimated by a stripping potential shift at a stripping peak current of Pb according to Fig. 4.18e. It is clear that the N-DLC film electrode deposited with a lower N2 flow rate (3 sccm) indicates a higher stripping resistance compared to the one deposited with a higher N2 flow rate (20 sccm).The measured standard deviation of stripping peak currents for five stripping cycles of Pb2+ (N2 flow rate = 20 sccm, deposition potential = -1.2 V, deposition time = 120 s, scan rate ≈ 36.36 mV/s) is about 1.9 %, demonstrating the good measurement repeatability and stability of the N-DLC film electrodes. 4.3.3.2. Linear Sweep Anodic Stripping Voltammograms of Zinc and LeadFigure 4.19 shows the stripping voltammograms of Zn and Pb separately measured by using a N-DLC film electrode (20 sccm N2) in two different 0.1 M KCl solutions (pH 1) containing 1 × 10?2 M Zn2+ and 1 × 10?3 M Pb2+, respectively. The deposition potential, deposition time and scan rate used for both metals are ?1.2 V, 120 s and 36.36 mV/s, respectively. Fig. 4.19: Stripping voltammograms obtained from a N-DLC electrode (20 sccm N2) in two different 0.1 M KCl solutions containing 1 × 10-2 M Zn2+ and 1 × 10-3 M Pb2+, respectively. The scan rate, deposition time and deposition potential are 36.36 mV/s, 120 s and -1.2 V, respectively.The stripping peaks of Zn and Pb are found at ?0.861 and ?0.354 V vs. SCE, respectively. Though the concentration of the Zn2+ ions is higher than that of the Pb2+ ions, the stripping peak current of Zn is much lower than that of Pb because of a smaller deposition overpotential of Zn compared to that of Pb. It is well known that the larger the negative deposition potential is, the closer the hydrogen evolution is. It is likely that a larger negative deposition potential of Zn may also contribute to a lower stripping peak current because of the presence of a closer hydrogen evolution [128]. The difference between the start and end points of the Zn stripping band is attributed to the edge of the potential window though the N-DLC film electrode is able to detect the Zn2+ ions due to its larger negative part of the potential window. However, it is believed that the N-DLC film electrode used in this experiment can have a higher efficiency for the detection of the Zn2+ ions with low concentrations by lowering the disturbance of the potential window measured. The results for Zn prove a low detection limit of the N-DLC film electrodes for tracing heavy metals. 4.3.3.3. Linear Sweep Anodic Stripping Voltammograms of CopperFigure 4.20a shows that the Cu stripping peaks are significantly enhanced upon applying more negative deposition potentials in the range of ?0.8 to ?1.4 V in a 2 × 10?5 M Cu2+ + 0.1 M KCl solution (pH 1) at a scan rate of 36.36 mV/s for 120 s. The stripping peaks of Cu at lower negative deposition potentials may be affected by the Cu+ ions stabilized with the Cl? ions through the following EC mechanism [148-150]: Cu0 ? Cu+ + e- (4.4) Cu+ + 2Cl- ? CuCl-2 (4.5)For stabilization of the intermediate Cu(I) species, the deposition potentials required to obtain a good efficiency during the accumulation are more negative. In addition, the formation of copper oxides at the film electrode surface may affect the stripping process of Cu by chemical kinetics [151]. It can be deduced that the effective negative deposition potentials should be larger than ?1.0 V with the concentration of Cu2+ ions used in this study. It is noted that the stripping peak current of Cu first linearly increases with increased negative deposition potential up to ?1.2 V and then decreases with the further increase of the negative deposition potential as shown in Fig. 4.20b, probably, because of a concomitant hydrogen evolution during the accumulation [151]. -68770528575276034566675 2769870131445-706755160020Fig. 4.20: Stripping voltammograms obtained from a N-DLC film electrode (20 sccm N2) in a 2 × 10-5 M Cu2+ + 0.1 M KCl solution as functions of (a) deposition potential and (c) deposition time. Dependence of stripping peak current and stripping potential on (b) deposition potential and (d) deposition time, respectively. The deposition time is 120 s (a and b), the deposition potential is -1.2 V (c and d), and the scan rate used for all the tests is 36.36 mV/s. As shown in Fig. 4.20b, a reduction of the Cu2+ ions at more negative deposition potentials seems to have caused the stripping of Cu0 at more negative stripping potentials, noting that a reversible reaction of Cu at the surface of the film electrode is attributed to a faster electrode reaction compared to transport of the Cu species due to a low concentration of Cu species. However, a non-linear relationship of the stripping potentials of Cu with the deposition potentials reveals that the reaction of Cu at the surface of the N-DLC film electrode is not perfectly reversible [151, 152].Figure 4.20c shows the effect of the deposition time on the stripping voltammograms of Cu in the same solution. All the stripping voltammograms of Cu obtained at the deposition potential of ?1.2 V produce sharp and well-defined peaks. As shown in Fig. 4.20d, the stripping peak current of Cu first linearly increases with the increase of the deposition time up to 200 s and then the increase of the current slows down that is due to the fixed concentration of Cu2+ ions in the solution. A shift of the stripping potential of Cu to more positive values with prolonged deposition time is found in Fig. 4.20d, resulting from an increased amount of reduced Cu atoms on the surface of the film electrode.4.3.3.4. Linear Sweep Anodic Stripping Voltammograms of MercuryThe stripping voltammograms recorded in a 0.1 M KCl solution (pH 1) containing Hg2+ ions of 1.1 × 10?6 M as functions of deposition potential and time are shown in Fig. 4.21a and b. The deposition potential and scan rate used are ?1.2 V and 36.36 mV/s, respectively. The sharp stripping peaks of Hg indicate that there is almost no restriction to the flow of the Hg2+ ions in the solution, which is supported by a monotonic increase of the stripping peak current of Hg with not only more negative deposition potentials but also longer deposition durations as shown in Fig. 4.21c and d. However, a deposition time in the range of 40 – 120 s cannot give an apparent increase of the stripping peak current with increased deposition time, indicating that the deposition time should be greater than 120 s to effectively detect the Hg2+ ions at ?M levels. 3019425-6352990850-3810Fig. 4.21: Stripping voltammograms obtained from a N-DLC film electrode (20 sccm N2) in a 1.1 × 10-6 M Hg2+ + 0.1 M KCl solution as functions of (a) deposition potential and (b) deposition time. Dependence of stripping peak current on (c) deposition potential and (d) deposition time. The deposition time is 120 s (a and c), the deposition potential is -1.2 V (b and d), and the scan rate for all the tests is 36.36 mV/s.The stripping potential of Hg observed at around 0.106 V without a correlation with the deposition potential and time reveals that the scan rate of 36.36 mV/s used in this study is slow enough to correspond to the reaction kinetics of Hg0 in the solution containing the Cl? ions during the stripping due to the oxidization of Hg itself, resulting in an immediate current response without a delay to the applied voltage.4.3.3.5. Linear Sweep Anodic Stripping Voltammograms of Simultaneous Lead, Copper and MercuryThe linear sweep anodic stripping voltammograms (LSASV) obtained from the N-DLC film electrode (20 sccm N2) in a solution (pH 1) containing 8.9 × 10?6 M Pb2+ + 2.5 × 10?5 M Cu2+ + 9.2 × 10?6 M Hg2+ + 0.1 M KCl were measured over the deposition time of 40 – 280 s as shown in Fig. 4.22a. The deposition potential and scan rate used for the simultaneous detection and determination of Pb2+, Cu2+ and Hg2+ are ?1.2 V and 36.36 mV/s, respectively. Both the unrestricted transport of the Hg2+ ions and the oxidization of the Hg atoms give rise to more pronounced stripping peaks of Hg while a complicated oxidation process of Cu during the stripping results in the broadest stripping peaks of Cu out of the three bands [153]. The current response of the N-DLC film electrode is evidently significant to differentiate all the tested elements, which demonstrates that the three ions can easily be determined simultaneously with good peak separations. It is found that the stripping peak currents for Pb, Cu and Hg are linearly related to the deposition time as shown in Fig. 14.22b. Pb, the last metal deposited, has the lowest stripping peak currents out of the three because of not only its lowest concentration but also its smallest deposition overpotential compared to the others. The correlations between the stripping potentials of multispecies such as Pb2+, Cu2+ and Hg2+ and the deposition time are shown in Fig. 4.22c. The stripping potential of Pb, the first metal stripped, is observed in the range of ?0.514 to ?0.488 V vs. SCE depending on the deposition time. Cu comes next in the range of ?0.121 to ?0.077 V vs. SCE, followed by Hg in the range of 0.13 to 0.148 V vs. SCE. The differences in the stripping potentials of Pb, Cu and Hg are attributed to their different redox potentials. It should be noted that all the stripping potentials of the three species shift to more positive values with increased deposition time. Hg has the smallest variation in stripping potential (0.018 V) compared to Cu (0.044 V) and Pb (0.026 V) within the range of deposition time, owing to its sharpest stripping peaks resulting from the most unrestricted transport of the Hg2+ ions and ready oxidation of the Hg atoms [147].301942515240 Fig. 4.22: (a) Stripping voltammograms obtained from a N-DLC film electrode (20 sccm N2) in a 0.1 M KCl solution containing 8.9 × 10-6 M Pb2+ + 2.5 × 10-5 M Cu2+ + 9.2 × 10-6 M Hg2+ as a function of deposition time. Dependence of (b) stripping peak current and (c) stripping potential on deposition time. The deposition potential and scan rate are -1.2 V and 36.36 mV/s, respectively. 4.3.3.6. Concentration effect for the ions traced by LSASVIn the plot of the stripping peak current of Pb against the deposition time, the stripping peak current of Pb increases linearly up to 120 s and such an increase slows down beyond 120 s for the N-DLC film electrode (20 sccm N2), while such slowdown is not obvious for the N-DLC film electrode (3 sccm N2) (Fig. 4.18c). The observed slowdown in the increase of the stripping peak current of Pb at longer deposition time for the N-DLC film electrode (20 sccm N2) implies the depletion of the Pb2+ ions in the solution, which has caused a small concentration gradient between the interface region and the bulk solution, leading to a slower transport of the Pb2+ ions. A similar stripping behavior is observed for Cu as depicted in Fig. 4.20d. The change in the stripping peak current of Hg with deposition time is small in the range of 40 – 120 s and large from 120 to 280 s (Fig. 4.21d). The detection of small concentrations of metal ions needs certain time to give rise to a linear relationship between stripping peak current and deposition time. However, the stripping peak currents of the multi-elements (Pb2+, Cu2+, and Hg2+) increase almost linearly as shown in Fig. 4.22b, which can be explained by the effect of metal concentration. The concentrations of the Pb2+ ions used for Fig. 4.18c and 4.22b are 1 × 10?3 M and 8.9 × 10?6 M, respectively. The mM level of the Pb2+ ions can produce a larger concentration gradient resulting in a faster depletion of the ions in the solution even at a shorter deposition time compared to the ?M level of the Pb2+ ions.The stripping voltammetric measurements for Cu and Hg were conducted with very small concentrations of 10?5 and 10?6 M, respectively. In the case of Cu, the stripping peak current in Fig. 4.22b shows almost a linear relationship with the deposition time because the concentration (2.5 × 10?5 M) of the Cu2+ ions used for Fig. 4.22b is a little higher than that (2 × 10?5 M) used for Fig. 4.20d. Thus, it may be a great favor for stripping analysis at very small concentrations of Cu2+ ions. Besides, the presence of Hg during the simultaneous multiple-element tracing can improve the sensitivity of the film electrode to the Cu2+ ions [15]. It is clear that the stripping peak currents of the Hg2+ ions found in Fig. 4.22b show a linear relationship with the deposition time due to a larger concentration (9.2 × 10?6 M Hg2+) compared to that shown in Fig. 4.21d. A larger concentration of Hg2+ not only depresses the depletion of the Hg2+ ions but also produces a larger concentration gradient which can provide a faster transport of the Hg2+ ions during stripping analysis. In the plot of the stripping potential of Pb versus deposition time, the increase of the stripping potential of Pb slows down at the deposition time beyond 120 s (Fig. 4.18d). A similar phenomenon is also found for Cu when the deposition time is beyond 200 s (Fig. 4.20d). However, the stripping potentials obtained from the simultaneous tracing of the multi-elements have maintained a linear relationship with the deposition time as shown in Fig. 4.22c. As the increased concentration of the stripped ions can be correlated to the increased stripping potential according to the Nernst equation, the trends observed for the stripping potentials in Figs. 4.18d, 4.20d and 4.22c are consistent with the trends for the stripping peak currents as shown in Figs. 4.18c, 4.20d and 4.22b in terms of deposition time. The detection ability of the N-DLC film electrodes to single elements (Pb2+, Cu2+ and Hg2+) and three simultaneous elements (Pb2+ + Cu2+ + Hg2+) with respect to deposition potential and time in the 0.1 M KCl solutions with the metal ion concentrations of around ?M levels indicates much lower detection limits of these elements compared to that reported in the literature [18]. Such low detection limits are relatively reasonable to detect the levels of Pb, Cu and Hg in water, which could be directly detrimental to the health of aquatic ecosystems and indirectly to human beings.4.4. ConclusionsThe effect of nitrogen concentration on the structure and electrochemical performance of the N-DLC films deposited on conductive p-Si substrates using a filtered cathodic vacuum arc deposition system by varying nitrogen flow rate from 0.5 to 20 sccm was systematically investigated. The Raman results showed that the sp2 bonds in the films increased with nitrogen flow rate. The increased sp2 sites promoted the surface roughness and adhesion strength of the films. As found from the potentiodynamic polarization experiments, the corrosion resistance of the N-DLC films decreased with increased nitrogen flow rate because the increased sp2 sites in the N-DLC films with increased nitrogen incorporation degraded the sp3-bonded cross-linking structure that was main responsible for preventing prompt dissolution of the films. The increased N content in the N-DLC films increased the electrical conductivity of the films, which in turn accelerated the electron transfer kinetics affecting the corrosion rate of the films. The corrosion results indicated that the nitrogen flow rate of 3 sccm (1.67 %N) used during the film depositions gave rise to the highest corrosion resistance of the N-DLC films. The immersion tests were employed to study the corrosion behavior of the N-DLC films in similar media for long runs. It was observed that the pH value of the solutions affected the anti-corrosion performance of the N-DLC films, i.e., the lower the pH value, the more severe the corrosion of the film was.Potential windows of the N-DLC films measured in solutions, such as 0.5 M HCl, 0.1 M KCl, 0.1 M NaCl, 0.1 M KOH, and 0.1 M NaOH, were about 2.4, 2.32, 3.2, 3.1, and 3.25 V, respectively. Although the N-DLC film electrodes offered i) wide potential windows with different types of solutions, ii) very low and stable background currents to improve the signal-to-background and signal-to-noise ratios, iii) repeatability of voltammograms, iv) durability of the N-DLC film electrode to high anodic potential and v) long-time response stability, their voltammograms were apparently affected by their electrical conductivity, type of alkaline species and unbalanced H+ and OH- ions. Therefore, it was found that the lower nitrogen content (3 sccm N2, 1.67 %N) in the N-DLC films resulted in the wider potential windows of the films in the aqueous solutions due to the higher electrical resistivity of the films. In addition, the background current was attributed to not only the concentration of OH- ions but also the surface cleanliness (surface oxidized layer). The N-DLC films used in this study had the desired voltammetric characteristics suitable for electrochemical analysis.The N-DLC film coated samples were used as working electrodes to identify single elements (Zn2+, Pb2+, Cu2+ and Hg2+) and simultaneous multi-elements (Pb2+ + Cu2+ + Hg2+) in deaerated and unstirred 0.1 M KCl solutions (pH 1) using linear sweep anodic stripping voltammetry (LSASV). The results showed that the current response of the N-DLC film electrodes was significant to detect all the tested trace metal ions (Zn2+, Pb2+, Cu2+, and Hg2+) and the three ions (Pb2+ + Cu2+ + Hg2+) could be simultaneously identified with good stripping peak potential separations. It was found that the increase of nitrogen content in the N-DLC film electrodes increased the sensitivity of the film electrodes to the trace metal ions so the higher nitrogen content (20 sccm N2, 6.48 % N) in the N-DLC films gave rise to the higher sensitivity of the films to the trace metals. It was also found that electrochemical deposition parameters such as metal ion concentration, deposition potential and deposition time systematically affected the stripping voltammetric behavior of the N-DLC film electrodes. It was noted that the increased nitrogen content in the N-DLC film electrodes promoted the sensitivity of the film electrodes to trace metals but apparently narrowed down the potential windows of the film electrodes, pointing out that the nitrogen content in the N-DLC film electrodes needed to be optimized between 1.67 and 6.48 % (between 3 and 20 sccm N2) to get the best balance between the high sensitivity and the wide potential windows of the film electrodes along with the high corrosion resistance. The degraded corrosion resistance of the N-DLC film electrodes with N incorporation clearly pointed out that it was still a nessesity to improve the corrosion resistance of the N-DLC film electrodes for electrochemical purposes. Chapter 5Structure and Electrochemical Properties of Platinum/ruthenium/nitrogen Doped Diamond-like Carbon Thin Films5.1. Introduction A unique combination of high hardness, high wear resistance, low friction, high corrosion resistance, excellent chemical inertness and electrical insulation makes diamond-like carbon (DLC) films a favorable candidate for protective coatings [154, 155]. Nowadays, DLC films are interested in electrochemical applications because the conductive films can be successfully made by nitrogen doping [18-20, 23, 24]. In addition, an introduction of nitrogen into DLC films enhances the adhesion strength of the films via the reduced residual stress in the films [100]. However, the incorporation of nitrogen in DLC films degrades the corrosion resistance of the films by increasing sp2 sites in the films [100]. It is well known that Pt and Ru are widely used materials in electrochemical applications. Pt possesses high resistance to chemical and thermal attacks as well as stable electrical properties. It is insoluble in hydrochloric and nitric acids, but can be corroded by cyanides, halogens, sulfur, caustic alkalis and aqua regia [28]. Ruthenium is often used in platinum alloys to enhance their wear resistance due to its ability to harden these materials and improve their electrochemical performance because of their outstanding catalytic properties coming from bimetallic functions. Ru can be attacked by halogens at high temperatures. Besides, it can be dissolved in alkaline solutions, but it is stable in acidic solutions [28, 156]. Magnetron sputtering deposition is one of the most interesting techniques employed to deposit DLC films. This method can produce DLC films with lower residual stresses compared to those produced by cathodic arc discharge or laser deposition methods. A reduced residual stress in DLC films allows thicker films to be deposited, which promotes the corrosion resistance of the films by lessening possible porosities in the films since the presence of nanopores in the DLC films can rapidly lead to the electrochemical dissolution of their underlying Si substrates due to permeation of water, environmental oxygen, and ions. It is expected that noble metal and nitrogen doped DLC films fabricated by DC magnetron co-sputtering can be a new type of film material interesting for electrochemical applications.In this chapter, the effect of Pt/Ru/N incorporation in DLC films prepared by a DC magnetron sputtering deposition system on the chemical composition, micro-structure, bonding configuration, surface activity and morphology, adhesion strength, and corrosion behavior of the films was systematically investigated.5.2. Structural properties of PtRuN-DLC thin films5.2.1. Chemical composition of PtRuN-DLC thin filmsFigure 5.1 shows the N/(C+Ru+Pt+N), Pt/(C+Ru+Pt+N), and Ru/(C+Ru+Pt+N) atomic ratios as a function of DC power applied to the Pt50Ru50 target, which are determined from the integrated areas of the XPS N 1s, C 1s + Ru 3d, and Pt 4f peaks. It is found that the DC power increased from 15 to 30 W gives rise to increases in the Pt/(C+Ru+Pt+N) from 0.022 to 0.042 and the Ru/(C+Ru+Pt+N) from 0.024 to 0.051 and a decrease in the N/(C+Ru+Pt+N) from 0.127 to 0.085.It is found from Fig. 5.1 that the Ru content in the PtRuN-DLC films is consistently higher than the Pt one for all the DC powers applied to the Pt50Ru50 target. Higher Ru content in the PtRuN-DLC films is probably attributed to more Ru atoms located in the outermost surface layers of the bimetallic Pt/Ru aggregates that appear on the film surfaces as shown in Fig. 5.2. This may be due to a core-shell structure of Pt/Ru aggregates, as proposed by Liu et al. [157], in which one element that is enriched in the core always exhibits a stronger preference for the selection of homometallic bonding than the other element that existed in the shell. In addition, Pt atoms can exhibit a preference of forming homometallic bonding, and thus there would be more neighboring atoms around the cores of the Pt aggregate [157]. Therefore, a Pt–Ru core-shell structure would be thought to possess an inner core enriched with Pt and an outer shell enriched with Ru [22], which is in agreement with the finding by Babu et al. [158]. Fig. 5.1: N/(C+Ru+Pt+N), Pt/(C+Ru+Pt+N) and Ru/(C+Ru+Pt+N) atomic ratios with respect to DC power applied to Pt50Ru50 target during film depositions. All the PtRuN-DLC films used in this study were deposited under the same process parameters except the sputtering power applied to the Pt50Ru50 target. Therefore, it can be deduced that the increased incorporation of Pt and Ru with higher DC power applied to the Pt50Ru50 target mainly contributes to the decreased nitrogen content in the films, indicating the difficult reactive nature of noble metals with nitrogen. Moreover, in the DC magnetron co-sputtering processes, the increased DC power applied to the target increases the flux of the sputtered species to the substrate surfaces, and their kinetic energy can be estimated from the following relationship [159, 160]: Uk ∞ (Dw × Vs) / Pg0.5 (5.1)where Uk is the kinetic energy, Dw is the target power density, Vs is the substrate bias, and Pg is the gas pressure. Since the substrate bias voltage and process pressure are fixed in all the deposition processes, while Ar has a larger ionization rate than that of N2, varying DC power applied to the Pt50Ru50 target eventually results in the differences in the N contents in the PtRuN-DLC films [161].In addition, an increase in surface oxygen percentage adsorbed on the PtRuN-DLC film surfaces with increased DC power applied to the Pt50Ru50 target was observed during the XPS compositional analysis. For example, the quantity of the surface oxygen with respect to 15 W is about 1.5 times larger than that corresponding to 30 W. The increase in surface oxygen percentage results from a greater difference in electronegativities between O (~3.44, pauling scale) and C (~2.55), Pt (~2.28), or Ru (~2.2) as compared to the one between C and N (~3.04) [28]. Bewilogua et al. [162] proposed that weakly cross-linked metal doped DLC films could react with oxygen much more promptly. It may be proposed that the increased Pt/Ru aggregates on the surfaces of the films with increased DC power on the Pt50Ru50 target have resulted in more adsorbed oxygen because a nitrogen doped amorphous carbon network surrounded by undissolved Pt/Ru aggregates is not strongly cross-linked.5.2.2. Microstructure of PtRuN-DLC thin filmsIn Fig. 5.2, the dark regions correspond to the Pt/Ru aggregates and the light areas correspond to the N-DLC matrices, where the sizes of the aggregates range between 2 and 5 nm. The TEM image in Fig. 5.2 demonstrates that the Pt/Ru aggregates are segregated within the N-DLC matrices.Fig. 5.2: TEM micrograph of a PtRuN-DLC film deposited with a DC power of 30 W applied to Pt50Ru50 target.5.2.3. XPS results of PtRuN-DLC thin filmsFigure 5.3 shows the XPS spectrum of the PtRuN-DLC film deposited with 30 W applied to the Pt50Ru50 target, where several peaks such as C 1s, O 1s, N 1s, Pt 4s, Pt 4p, Pt 4d, Pt 4f, Ru 3s, and Ru 3d can be clearly identified. The O 1s peak probably comes from surface oxidation during the exposure of the sample to the atmosphere. Fig. 5.3: XPS spectrum of a PtRuN-DLC film deposited with a DC power of 30 W applied to Pt50Ru50 target. The C 1s + Ru 3d spectrum of the PtRuN-DLC film deposited with 15 W applied to the Pt50Ru50 target is shown in Fig. 5.4a from which it is observed that the C 1s peak entirely covers Ru 3d3/2 and partially overlaps with Ru 3d5/2. By fitting the spectrum with a Gaussian line shape and by approximating the contribution of the background with the Shirley method [18], the peak at about 284.1 eV is attributed to the C–C sp2 bonding as the C–C sp3 bonding contributes to the peak observed at about 285 eV [18, 163]. C–O bonds resulting from the oxygen adsorbed after the sample’s exposure to the air contribute to the peak located at around 286.2 eV [164, 165]. The peak at around 287.5 eV is attributed to C–N bonding [164,165]. The two peaks at about 280.4 and 281.6 eV in the Ru 3d5/2 region are qualitatively attributed to Ru0 and Ru–O bonds, respectively, resulting from their spin-orbit coupling effect [166, 167]. The inset in Fig. 5.4a shows the C 1s + Ru 3d spectra of the PtRuN-DLC films deposited with different DC powers applied to the Pt50Ru50 target. A slight shift in the C 1s peaks to lower binding energies (285 to 284.7 eV) with an increase in the DC power indicates that there are more sp2 bonds in the films, resulting from the increased Pt and Ru contents in the films [140]. It is consistently found that the sp3 content of the PtRuN-DLC films increases from about 34.11 to 21.36 % with an increase in the DC power. It is known that a higher nitrogen incorporation in an amorphous carbon structure gives rise to a higher fraction of sp2 bonds. However, in this study, the increase in the sp2 bonds in the PtRuN-DLC films revealed by the shift in the C 1s peak positions to lower binding energies cannot be correlated with the decrease in the nitrogen content in the films with increased DC power applied to the Pt50Ru50 target. Therefore, it is proposed that the introduction of Pt and Ru into the films probably increases the sp2 bonds in the nitrogen doped amorphous carbon structures, which is in agreement with the literature [89, 168, 169]. The Ru 3d5/2 peaks also apparently shift to lower binding energies with the increase in DC power applied to the Pt50Ru50 target, resulting from an increase in neutral Ru as confirmed by the stronger Ru0 peaks than the Ru–O peaks. In addition, such an increased Ru content in the films results in a decrease in Coulombic interactions between photon-emitted electrons and ion cores, leading to an apparent shift in the peaks to lower binding energies. The XPS analyses of Ru 3d, Pt 4f, and Ru 3p spectra (Figs. 5.4a, c and d) show that their deconvolued components shift to higher binding energies with their increased oxidization states due to the extra Coulombic interactions between the photon-emitted electrons and the ion cores.In Fig. 5.4b, the N 1s spectrum is deconvoluted into three components peaked at approximately 398.8, 400.5, and 401.8 eV with Gaussian line shapes and a Shirley background. It is found that the surface oxygen contributes to the broad peak centered at about 401.8 eV, resulting in N–O bonds. As reported in the literature [170], the peaks at around 398.8 and 400.5 eV can be attributed to nitrogen atoms each of which is bonded by either two carbon atoms with one having π bond (N-sp2) or three C atoms all σ bonded (N-sp3). From the inset of Fig. 5.4b, a slight shift in the N 1s peaks to lower binding energies (398.8 to 398.5 eV) with increased DC power applied to the Pt50Ru50 indicates that the increased Pt and Ru contents with increased DC power have caused the configuration of nitrogen incorporated amorphous carbon networks to change from N-sp3 to N-sp2, as confirmed by an apparent increase in N-sp2/N-sp3 area ratio from about 2.39 to 3.04. It can be further pondered that the occupancy of N in aromatic rings is increased by the increased Pt and Ru contents in the films. Fig. 5.4: Fitted XPS spectra of a PtRuN-DLC film deposited with DC power of 15 W applied to Pt50Ru50 target: (a) C 1s + Ru 3d, (b) N 1s, (c) Pt 4f and (d) Ru 3p. The insets show the relevant XPS spectra of PtRuN-DLC films as a function of DC power applied to Pt50Ru50 target. The Pt 4f region composed of three spin-orbit doublets is shown in Fig. 5.4c, where the Pt 4f components located at about 71.1, 71.9, and 73.2 eV are attributed to Pt0, Pt2+, and Pt4+, respectively, due to neutral Pt and its oxides [166, 167, 171]. The Pt 4f spectrum illustrates that the PtRuN-DLC films used in this study consist of a significant amount of oxidized Pt species. An increase in neutral Pt having lower binding energy and weaker extra Coulombic interaction with increased DC power applied to the Pt50Ru50 target shifts the spin-orbit coupling peaks of Pt 4f(7/2,5/2) to lower binding energies, as shown in the inset of Fig. 5.4c. Two spin-orbit couplings of Ru 3p(3/2,1/2) recorded at about 461.9 and 463.4 eV are shown in Fig. 5.4d, where the main peak at around 461.9 eV is attributed to Ru0 while 463.4 eV is related to Ru–O [166, 167, 171]. An apparent shift in the Ru 3p(3/2,1/2) peaks to lower binding energies with increased DC power applied to the Pt50Ru50 target (inset in Fig. 5.4d) is not found.5.2.4. Raman results of PtRuN-DLC thin filmsFigure 5.5 shows the deconvoluted Raman spectrum of the PtRuN-DLC film deposited with DC power of 15 W applied to the Pt50Ru50 target. The spectrum is well fitted with two Gaussian peaks for G (graphite) and D (disordered) bands lying on a linear background. The inset in Fig. 5.5 shows the Raman spectra of the PtRuN-DLC films deposited with different DC powers applied to the Pt50Ru50 target. The asymmetrical shapes of the Raman spectra are changed to a symmetrical shape by developing the D peak (inset in Fig. 5.5) with the increased Pt and Ru contents in the films. Both G and D peaks shift from about 1548 to 1535 cm?1 and from about 1380 to 1374 cm?1, respectively, with increased DC power, as shown in Fig. 5.6a. A redshift of both peak positions is mainly attributed to the introduction of Pt and Ru into the nitrogen doped amorphous carbon structures, resulting in an increase in the specific mass of the entire networks [64, 172, 173]. In addition, their inherent longer bond vibrating at a lower frequency is also a fact of the redshift of both peaks [174, 175]. Fig. 5.5: Raman spectrum together with fitted G and D peaks of a PtRuN-DLC film deposited with DC power of 15 W applied to Pt50Ru50 target. The inset shows the Raman spectra of PtRuN-DLC films deposited with different DC powers applied to Pt50Ru50 target.The Raman peak intensities of the PtRuN-DLC films decrease with increased Pt and Ru contents in the films (inset in Fig. 5.5), which are similar to those of the metal doped DLC films, as reported in the literature [176, 177], due to increased inactive phases. The inactive phases result from the undissolved PtRu aggregates within the nitrogen doped amorphous carbon matrices because of their larger atomic radii compared to that of C. In addition, similar electronegativity values among Pt, Ru, and C are one of the reasons for the undissolved PtRu aggregates in the carbon matrices [173, 178]. Furthermore, the increased PtRu aggregates with increased Pt and Ru contents in the films result in a contraction effect on the surrounding matrices, causing a reduction in the vibrational frequencies of the neighboring bonds around them by absorbing their vibrational energies. This may also contribute to the downward shifts of both peak positions.The full-widths-at-half maximum (FWHMs) of both G and D Raman peaks decrease with increased DC power from 15 to 30 W (Fig. 5.6b). Alternatively, such a decreased FWHM for the G peak with increased Pt and Ru contents in the films can be correlated with a decrease in residual stress in the films. Ting and Lee [179] and Morrison et al. [89] reported that the reduction in G-band width with respect to metal incorporation could be attributed to a decrease in residual stress within DLC films.The information about the carbon bonding structure such as graphite clustering and structure disordering can be obtained from an intensity ratio (ID/IG) between D and G peaks [137, 93]. Tan and Cheng [180] reported that the tendency of nitrogen was to promote clustered sp2 bonding, which in turn reduced the sp3 content with increased nitrogen in DLC films. According to the XPS results, the ID/IG ratio would have downshifted with decreasing nitrogen content in the films. However, such a correlation between the ID/ IG and nitrogen content is not found in this study. Therefore, the observed upshift of the ID/IG ratio from about 1.81 to 2.43 with increased DC power (Fig. 5.6c) can be related to an increase in graphite-like phases induced by the increased Pt and Ru contents in the films [169, 181].It was reported in the literature [169, 182] that metal phases could enhance graphitization of amorphous carbon around them because the metal phases could act as catalysts due to a high sputtered carbon energy. A sputtering process can provide sufficient energy to locally heat amorphous carbon on a metal surface according to a thermal spike mode [183]. Moreover, the amorphous carbon contacting with the metal phases transforms into graphite at a relatively low temperature [182-184]. Therefore, it can be depicted that a local increase in sp2 bonds in the amorphous carbon matrices is induced by metal-induced graphitization [182-185], which is also consistent with the work reported in the literature [116, 186, 187], in which metal atoms in the carbon matrices catalyze the formation of sp2 sites. Fig. 5.6: Results from the fitted Raman spectra of PtRuN-DLC films as shown in the inset of Fig. 5.5: (a) peak positions, (b) FWHMs and (c) ID/IG ratios of D and G peaks.5.2.5. Surface activity of PtRuN-DLC thin filmsFigure 5.7 shows the distilled water contact angles of the PtRuN-DLC films deposited with different DC powers applied to the Pt50Ru50 target. It is well known that the wettability of a solid surface is strongly affected by its surface characteristics. The increased water contact angle from about 79° to 83° (5.1% increment) for the PtRuN-DLC films with increased DC power applied to the Pt50Ru50 target indicates the decreased surface energy or wettability of the films. A similar result was reported by Choi et al. [188] for their study on the effect of Ag concentration on the surface energy of Ag doped DLC films. The formation of a surface oxide on a DLC film surface due to the adsorption of atmospheric oxygen could reduce the water contact angle by increasing the film surface energy via strong polarities induced by C–O, Pt–O, and Ru–O bonds [189, 190]. However, the increased water contact angles of the PtRuN-DLC films with increased Pt and Ru contents in the films (Fig. 5.7) illustrate that the surface energies of these films are not apparently influenced by the adsorbed surface oxygen. (a)2074545196215(b)3627120996315 Fig. 5.7: Water contact angles of PtRuN-DLC films as a function of DC power applied to Pt50Ru50 target. The insets show the water droplets on the surfaces of the films deposited with DC powers of (a) 15 and (b) 30 W applied to Pt50Ru50 target. Chen and Hong [190] and Grischke et al. [191] reported that the surface energy of DLC films could be affected by the incorporated nitrogen on the films due to carbon-nitrogen bonds that greatly enhanced the surface polarity and imposed attractive forces to react with polar H2O molecules. The decreased nitrogen content in the PtRuNDLC films with increased DC power applied to the Pt50Ru50 target (Fig. 5.1) probably increases the water contact angle as the polarity of the films is weakened [190]. Besides, the PtRu aggregates on the PtRuN-DLC film surfaces would also cause a decrease in surface polarity by decreasing the fraction of carbon-nitrogen bonds. Therefore, the increased Pt and Ru contents in the PtRuN-DLC films with increased DC power are one of the reasons for the increased water contact angle.5.2.6. Surface morphology of PtRuN-DLC thin filmsFrom the AFM images shown in Fig. 5.8, the film with higher Pt and Ru contents (Fig. 5.8b) has larger asperities than the film with lower Pt and Ru contents (Fig. 5.8a). The surface roughness (Ra) of the PtRuN-DLC films increases from about 1.3 to 1.9 nm (46.2% increment) with increased DC power applied to the Pt50Ru50 target from 15 to 30 W due to increased sp2 sites and PtRu aggregates in the films [169, 192].(a) (b)Fig. 5.8: AFM images showing surface topographies of PtRuN-DLC films deposited with DC powers of (a) 15 and (b) 30 W applied to Pt50Ru50 target.5.2.7. Adhesion strength of PtRuN-DLC thin filmsAn applied normal load corresponding to an abrupt change in tangential force, which comes from an instant adhesive failure between the PtRuN-DLC film and the Si substrate, is taken as a critical load [193]. It is found in Fig. 5.9 that the critical loads of the PtRuN-DLC films increase from about 338 to 407 mN (20.4% increment), while the DC power applied to the Pt50Ru50 target is increased from 15 to 30 W, indicating that the adhesion strength of the films to the substrates increases with increased Pt and Ru incorporation [169]. Fig. 5.9: Critical loads of PtRuN-DLC films as a function of DC power applied to Pt50Ru50 target.It is well known that the adhesion strength of the films is strongly influenced by the residual stress in the films, which results from the enhanced cross-linkage and bond distortion in the films caused by the bombardment of the high energetic impinging ions during the film deposition [194-196]. However, compressive residual stress can be decreased by increased sp2/sp3 ratio in the films since the sp2 bonds are shorter than the sp3 bonds [73]. Therefore, the increased sp2 sites in the PtRuN-DLC films with increased Pt and Ru contents have enhanced the adhesion strength of the films to the Si substrates. In addition, the undissolved PtRu aggregates could relax the rigidity of the amorphous carbon structures. The increased Pt and Ru contents in the films with increased DC power lead to a proportional increase in the critical load. Moreover, a nitrogen doped amorphous carbon structure can have an inherently lower residual stress than a pure amorphous carbon structure because of shorter C=N bonds compared to C–C or C=C bonds. Thus, several factors attributed to the noble metal incorporation, such as low solubilities of noble metals in amorphous carbon structures, formation of noble metal carbon nanocomposites, and graphitization of DLC structures, apparently influence the adhesion of the DLC films to the substrates by causing the relaxation of the stress in the films [197, 198].It is found that the thickness of the PtRuN-DLC films increases from about 220 to 300 nm with increased DC power applied to the Pt50Ru50 target. Since the residual stress in DLC films came along increased film thickness [29], the critical loads of the PtRuN-DLC films should decrease with increased film thickness. However, the increased critical loads of the PtRuN-DLC films with increased film thickness indicate that the influence of the film thickness on the adhesion strength of the films in terms of residual stress is not significant. On the other hand, a thicker film may need a higher load for the indenter to break through the film [78]. Therefore, it may be supposed that the increased thickness of the PtRuN-DLC films is one of the reasons increasing the critical loads of the films. 5.3. Electrochemical performance of PtRuN-DLC thin films5.3.1. Corrosion behavior of PtRuN-DLC thin filmsFigure 5.10a shows the potentiodynamic polarization curves of the PtRuN-DLC films measured in 0.1 M NaCl solutions in atmospheric environment as a function of DC power applied to the Pt50Ru50 target, which are analyzed using a Tafel technique to obtain corrosion parameters such as corrosion potential (Ecorr) and current (Icorr). Thereafter, polarization resistance (Rp) is calculated from the anodic (βa) and cathodic (βc) Tafel slopes and corrosion current (Icorr) according to the equation 4.1.From Fig. 5.10a, it is found that there are mainly active and transpassive regions in the anodic branches of the polarization curves. The observed passivated region in the anodic branch of the polarization curve of the PtRuN-DLC film deposited with 20 W applied to the Pt50Ru50 target may come from the formation of some passivated PtRu aggregates (Pt–O and/or Ru–O) by reacting with water through the following equation [73, 192, 197, 198]: Me + H2O → Me(OH)3 → MeO(OH)2 → MeO2 (Me= Pt, Ru) (5.2)since the carbon matrixes do not repassivate. A decrease in Icorr from 3.05 to 0.42 ?A and an increase in Rp from 44.29 to 162.1 k? (266% increment) (Fig. 5.10b) with increased DC power applied to the Pt50Ru50 target indicate that the introduction of Pt and Ru into the N-DLC films can induce an improved corrosion resistance of the films. A shift in Ecorr to more positive values (17 – 149.5 mV vs. SCE) (see Fig. 5.10c) reveals that the PtRuN-DLC films require a higher applied potential for the dissolution of the films with increased Pt and Ru contents in the films. In addition, Pt and Ru may be electrochemically more stable than C, which also contributes to the shift in Ecorr to more positive values by promoting their cathodic protective behavior so that the polarization occurs at more positive potentials. The increased water contact angles of the films with increased DC power applied to the Pt50Ru50 target reveal that a decreased film surface energy improves the corrosion resistance of the films by lowering the surface activity of the films.The above mentioned effects on the corrosion resistance of the films are so strong that the increased sp2 sites in the films with the increase in the DC power applied to the Pt50Ru50 target cannot be simply correlated with the increased corrosion resistance of the films at lower potentials. However, a significant increase in the anodic current observed beyond the applied potential of 1 V can be related to the increased sp2 sites in the films because these sites have reduced the rigidity of the sp3-bonded structures, resulting in an easy, prompt dissolution of the films with increased sp2-bonded fraction at higher applied potentials [92]. 34937709144001884045133350 Fig. 5.10: (a) Potentiodynamic polarization curves, (b) corrosion current (Icorr) and polarization resistance (Rp), and (c) corrosion potential (Ecorr) of PtRuN-DLC films as a function of DC power applied to Pt50Ru50 target. The insets in (c) show SEM micrographs of corroded areas of PtRuN-DLC films deposited with DC powers of 30 (top left) and 25 W (bottom right) applied to Pt50Ru50 target. In addition, the increased anodic current of the films is also attributed to the increased PtRu aggregates in the nitrogen doped amorphous carbon structures, resulting from the weak interatomic interactions between the PtRu aggregates and the matrices because these weak interfacial bonds can be easily attacked by electrochemically active species. In reality, the film dissolution comes from the reaction of the film with the electrochemically active species in the electrolyte. When the atoms of a film are ionized in an electrolytic solution, the electrons released will flow through the path of the lowest resistance of the film to the places where cathodic reactions occur. Therefore, the most important reactions come from the reduction in dissolved oxygen, water molecules, and hydrogen ions in the electrolyte with the released electrons via the following possible reactions [11, 28, 90, 99, 199]: 2H+ + 2e- → H2 (5.3) 2H2O + 2e- → H2 (g) + 2 OH- (5.4)2H2O + O2 + 4e- → 4OH- (5.5) C + 6OH- → CO32- + 3H2O + 4e- (5.6) C + (O2-) → CO + 2e- (5.7) CO + (O2-) → CO2 + 2e- (5.8)where (O2-) represents a reduced state of oxygen in the form of H2O, OH-, and/or Me-O, etc. The catalytic activity of Cl- ions causing the anodic dissolution of Pt and Ru from the PtRuN-DLC films in the NaCl solution by formation of soluble chloride complexes accelerates the corrosion process, probably through the following possible reactions [28, 200, 201]: 4OH- + 4NaCl → 4NaOH + 4Cl- (5.9) NaCl → Na+ + Cl- (5.10) Ru2+ + 5Cl- → RuCl52- + 1e- (5.11) Pt + 4Cl- → PtCl42- + 2e- (5.12) Pt + 6Cl- → PtCl62- + 4e- (5.13)Furthermore, these electrochemically active species permeated into the interfaces between the films and the underlying substrates through some pores or defects in the films will attack the weak interfacial bonds between them. The attack becomes more severe with higher Pt and Ru contents in the films because these aggregates create easier paths, allowing the permeation of the electrochemically active species into the interfaces by developing interfaces between the aggregates and the C matrix. Galvanically-induced corrosion, which occurs between the films and the substrates due to different electrochemical potentials between them, accelerates the attack to the interfaces. Consequently, the corrosion products formed at the film–substrate interfaces may also force the films to delaminate from the substrates. A higher applied potential during the anodic scanning can also promote the above phenomena. Therefore, the PtRuN-DLC films having higher Pt and Ru contents (e.g., deposited with 25 and 30 W applied to the Pt50Ru50 target) show a more severe delamination as revealed by the SEM micrographs (insets in Fig. 5.10c). When the delamination of the films occurs, the corrosion current starts to increase. However, such delamination is not found in the PtRuN-DLC films deposited with 15 and 20 W applied to the Pt50Ru50 target, confirming that lower amounts of sp2 sites and PtRu aggregates contained in the films are responsible for hindering a high anodic current flow from the films at higher applied potentials. The increased corrosion resistance of the PtRuN-DLC films may also be attributed to the decreased surface energy of the films because the decreased attraction between the water molecules in the aqueous solution and the films reduces the concentration of the water molecules near the film surfaces and subsequently decelerates the cathodic reactions (Equations from 5.3 to 5.5) which are responsible for the anodic dissolution of the films.The increased surface roughness of the PtRuN-DLC films with increased DC power applied to the Pt50Ru50 target may also contribute to the corrosion of the films because of a larger exposed surface area to the corrosive medium, thus causing a more prompt dissolution. It can be deduced that the corrosion resistance of the PtRuN-DLC films with increased Pt and Ru contents in the NaCl solution increases at lower applied potentials but degrades at higher applied potentials.5.4. ConclusionsThe chemical composition, microstructure, bonding structure, surface activity and morphology, adhesion strength and corrosion resistance of the Pt/Ru/N incorporated DLC films prepared with DC magnetron co-sputtering were investigated in terms of the DC sputtering power applied to the Pt50Ru50 target. The nitrogen content was reduced with increased Pt and Ru contents in the films. The XPS C 1s peaks of the films entirely overlapped with Ru 3d3/2 and partially overlapped with Ru 3d5/2. According to the fitted N 1s peaks, the area ratio between N-sp2 and N-sp3 bands increased from about 2.39 to 3.04 with higher Pt and Ru contents in the films. The Pt 4f region was composed of three sets of spin-orbit doublets, namely, Pt0, Pt2+, and Pt4+, peaked at about 71.1, 72.1, and 75.1 eV, respectively, attributed to the pure Pt and its oxides. Two sets of spin-orbit couplings of Ru 3p(3/1,1/2) were recorded at about 461.9 and 463.4 eV that were attributed to Ru0 and Ru–O, respectively. Both G and D Raman peak positions decreased while the ID/IG peak intensity ratio increased with higher Pt and Ru contents in the films, whose trends were in agreement with the XPS results. The surface roughness and adhesion strength of the PtRuN-DLC films increased with increased Pt and Ru contents. It was found that the increased Pt and Ru contents in the films also enhanced the water contact angles on the film surfaces. The corrosion resistance of the PtRuN-DLC films increased with the increase in the Pt and Ru contents in the films when the applied potential was less than 1 V. However, with higher polarization potentials beyond 1 V, the films with higher Pt and Ru contents showed higher anodic currents in the 0.1 M NaCl solution due to the delamination of the films. Therefore, the Pt/(C+Ru+Pt+N) and Ru/(C+Ru+Pt+N) should be optimized between 0.033 (20 W) and 0.035 (25 W) and between 0.037 (20 W) and 0.041 (25 W), respectively, to lessen the anodic dissolution of the PtRuN-DLC films.Chapter 6Comparative Study of Structural and Electrochemical Properties of Nitrogen-doped and Platinum/ruthenium/nitrogen-doped Diamond-like Carbon Thin Films6.1. IntroductionNitrogen doped diamond-like carbon (N-DLC) thin films have been developed for electrochemical applications because of their wide electrochemical potential windows and high signals to heavy metals at ?M concentration [23, 24]. In addition, nitrogen doping significantly improves the electrical conductivity and adhesion strength of DLC films [23, 24, 100]. However, the introduction of nitrogen into DLC films degrades the corrosion resistance of the films [100]. It is well known that noble metals such as platinum (Pt) and ruthenium (Ru) possess high corrosion resistance and excellent catalytic activity. It was reported in the previous chapter (see Chapter 5) that increasing Pt and Ru contents in the N-DLC films improved the corrosion resistance of the films and at the same time, their adhesion strength. Though the influence of Pt/Ru concentration on the corrosion performance of the N-DLC films has been investigated in the previous chapter, the effect of Pt/Ru doping on the structure and electrochemical performance of the N-DLC films has not be reported yet.In this chapter, nitrogen doped DLC films without (N-DLC) or with (PtRuN-DLC) Pt and Ru doping were deposited on highly conductive p-Si substrates using a DC magnetron sputtering deposition system to investigate the influence of Pt and Ru doping on the chemical composition, bonding structure, micro-structure, surface activity and morphology, adhesion strength, corrosion resistance and cyclic voltammetric behavior of these films. 6.2. Structural properties of N-DLC and PtRuN-DLC thin films6.2.1. Chemical composition of N-DLC and PtRuN-DLC thin filmsThe atomic ratios of the N/(C+N), N/(C+Ru+Pt+N), Pt/(C+Ru+Pt+N) and Ru/(C+Ru+Pt+N) are determined from the integrated areas of the XPS C 1s, N 1s, Pt 4f and Ru 3d spectra and presented in Table 6.1. It is found that the N/(C+N) ratio of the N-DLC film is about 0.207. The N/(C+Ru+Pt+N), Pt/(C+Ru+Pt+N) and Ru/(C+Ru+Pt+N) ratios for the PtRuN-DLC film are about 0.181, 0.032 and 0.049, respectively. It is found that the Ru content is higher than the Pt content for the PtRuN-DLC film, which may be attributed to a core-shell structure of PtRu aggregates [101, 157, 158].Table 6.1: Chemical compositions and sp2/sp3 ratios of N-DLC and PtRuN-DLC films.SampleN/(C+N)N/(C+Ru+Pt+N)Pt/(C+Ru+Pt+N)Ru/(C+Ru+Pt+N)Csp2/Csp3Nsp2/Nsp3N-DLC0.207-??-?-0.751.3PtRuN-DLC?-0.1810.0320.0491.422.21Furthermore, the chemical compositions of the N-DLC and PtRuN-DLC films are analyzed after Ar+ plasma cleaning for 5 min at a chamber pressure of 3.5 × 10-8 Torr. It is found that the N/(C+N) ratio in the N-DLC and N/(C+Ru+Pt+N) ratio in the PtRuN-DLC films decrease to about 10% and 5%, respectively. However, it is found that the Pt/(C+Ru+Pt+N) and Ru/(C+Ru+Pt+N) ratios in the PtRuN-DLC film increase to about 21% and 29%, respectively, also implying that the Ru content is still higher than the Pt content in the bulk of the film. The decreased N contents in the bulks of the films than those on the film surfaces after 5 min long plasma cleaning are due to the existence of the adsorbed nitrogen atoms on the film surfaces. It can be seen that the decreased adsorbance of nitrogen probably increases the Pt and Ru contents in the bulk of the PtRuN-DLC film. 6.2.2. Microstructure of PtRuN-DLC thin filmsFigure 6.1 shows a TEM micrograph of the PtRuN-DLC film, which clearly shows that Pt and Ru exist as nano-aggregates embedded in the N-DLC matrix. The sizes of the nano-aggregates are in a range of 2-5 nm. Fig. 6.1: TEM image of PtRuN-DLC film.6.2.3. Chemical bonding structure of N-DLC and PtRuN-DLC thin films measured by XPSEach C 1s spectrum of the N-DLC (Fig. 6.2a) and PtRuN-DLC (Fig. 6.2b) films is mainly composed of four components located at around 285, 284.1, 286, and 287.3 eV corresponding to C-C sp3, C-C sp2, C-O (adsorbed surface oxygen), and C-N bonds, respectively. However, it is observed that the C 1s spectrum of the PtRuN-DLC film overlaps with Ru 3d3/2 and partially overlaps with Ru 3d5/2, so additional spin-orbit doublets of Ru0 at about 280.5 eV and Ru-O at about 281.7 eV are found in Fig. 6.2b [166, 167]. The existence of the Ru-O component at the higher binding energy than the Ru0 is due to extra coulombic interactions between photon-emitted electrons and ion cores. It is found that the C-C sp2/C-C sp3 ratio, which is estimated from the relevant peak areas, of the PtRuN-DLC film (about 1.42) is higher than that of the N-DLC film (about 0.75) as shown in Table 6.1. The sp3 content (about 28.19%) of the PtRuN-DLC film is lower than that that (about 42.5%) of the N-DLC film. All the DLC films used in this study were deposited under the same process parameters except the sputtering power applied to the Pt50Ru50 target during the deposition of the PtRuN-DLC film. It can be deduced that the introduction of Pt and Ru mainly contributes to the increased C-C sp2/C-C sp3 ratio of the PtRuN-DLC film due to metal-induced graphitization [101, 116, 182-186]. At the same time, the effect of the DC sputtering power applied to the Pt50Ru50 target used during the deposition of the PtRuN-DLC film should also be taken into account because it may be one of the reasons promoting the metal-induced graphitization via producing the energetic metal species. 313372515621031667452267585 Fig. 6.2: Fitted XPS C 1s and C 1s + Ru 3d spectra of (a) N-DLC and (b) PtRuN-DLC films, respectively, and fitted XPS N 1s spectra of (c) N-DLC and (d) PtRuN-DLC films. The N 1s spectra of the N-DLC and PtRuN-DLC films (Fig. 6.2c and d) are composed of three components, i.e. N-sp2 at about 398.5 eV, N-sp3 at about 400.2 eV and broad N-O component at about 401.4 eV [78]. The N-sp2/N-sp3 ratio of the PtRuN-DLC (about 2.21) is higher than that (about 1.3) of the N-DLC film, indicating the preferential existence of N in the sp2-bonded configuration.6.2.4. Chemical bonding structure of N-DLC and PtRuN-DLC thin films measured by Raman spectroscopyFigure 6.3 shows the Raman spectra of the N-DLC and PtRuN-DLC films from which it is clearly seen that the introduction of Pt and Ru into the N-DLC film apparently depresses its Raman spectrum due to the inactive phases that result from the undissolved PtRu aggregates. It is found that the PtRuN-DLC film causes a downshift of the G peak position from about 1535 to 1525 cm-1 due to an increased specific mass of the entire network compared to the N-DLC film [64, 172, 173]. However, the change in the D peak position is not so significant. The introduced Pt and Ru can serve as catalysts for the formation of sp2 sites via graphitization of amorphous carbon around them [29, 116, 182, 184, 186, 187]. Therefore, clustering of the aromatic rings resulted from the metal-induced graphitization promotes the density of the breathing vibration of the rings, which seems to cause an upward shift of the D peak position from about 1360 to 1363 cm-1 instead of a downshift with increased specific mass of the network.The decreased full-widths-at-half-maximum (FWHMs) of both the G peak from about 204 to 184 cm-1 and the D peak from about 384 to 361 cm-1 with the introduction of Pt and Ru are associated with decreases in bond and ring disorders [29]. An increase of intensity ratio (ID/IG) between D and G peaks from about 1.3 to 1.59 with Pt and Ru doping indicates an increase in cluster size [202]. The decreased FWHMs and increased ID/IG ratio clearly point out that the introduction of Pt and Ru during the film deposition results in metal-induced graphitization of the amorphous carbon structure. Fig. 6.3: Raman spectra of N-DLC and PtRuN-DLC films, where G and D represent fitted G and D peaks, respectively.Table 6.2: Results determined from fitted Raman spectra as shown in Fig. 6.3.SampleG peak (cm-1)D peak (cm-1)FWHMG (cm-1)FWHMD (cm-1)ID/IGN-DLC1535 (± 1)1360 (± 1)204 (± 1)384 (± 1)1.3PtRuN-DLC1525 (± 1)1363 (± 1)184 (± 1)361 (± 1)1.596.2.5. Surface activity of N-DLC and PtRuN-DLC thin filmsThe water contact angles of the N-DLC and PtRuN-DLC films were determined with a sessile water drop method. From Fig. 6.4 and Table 6.3, it is found that the water contact angles of the N-DLC and PtRuN-DLC films are about 59 and 68.1°, respectively, pointing out that the PtRuN-DLC film surface is more hydrophobic (15.4% increment) than the N-DLC film surface. The incorporation of Pt and Ru in the N-DLC film weakens the polarity of the film because the PtRu aggregates reduce polar carbon-nitrogen bonds that can promote the surface energy of the film [190]. Fig. 6.4: Water droplets on (a) N-DLC and (b) PtRuN-DLC film surfaces. Table 6.3: Water contact angles, surface roughnesses and critical loads of N-DLC and PtRuN-DLC films.SampleContact angle (?)Ra (nm)Critical load (mN)N-DLC59 (± 0.9) 0.81 (± 0.05 )359 (± 7) PtRuN-DLC68.1 (± 0.9)1.18 (± 0.1 )393 (± 9)6.2.6. Surface morphology of N-DLC and PtRuN-DLC thin filmsFigure 6.5 shows the surface morphologies of the N-DLC and PtRuN-DLC films measured with AFM. The small asperities found on the N-DLC film surface (Ra ≈ 0.8 nm, Table 6.3) are attributed to the effect of nitrogen inclusions [24]. When Pt and Ru are introduced, together with nitrogen, into the amorphous carbon structure, it is found that the surface roughness of the PtRuN-DLC film is increased to Ra ≈ 1.2 nm (50% increment) (Table 6.3) and the surface asperities become larger, resulting from the increased sp2 sites and the undissolved PtRu aggregates in the film [101]. (a) (b)Fig. 6.5: AFM images showing surface morphologies of (a) N-DLC and (b) PtRuN-DLC films.6.2.7. Adhesion strength of N-DLC and PtRuN-DLC thin filmsThe influence of Pt and Ru doping on the adhesion strengths of the N-DLC and PtRuN-DLC films is investigated by measuring a critical load at which a sudden change in tangential force is observed [139]. 1409701152525Spallation (a) (b)Fig. 6.6: SEM micrographs showing surface morphologies of scratched (a) N-DLC and (b) PtRuN-DLC films. The inset in (a) shows the scratch track of the N-DLC film. It is observed that the critical load of the N-DLC film is about 359 mN as presented in Table 6.3. The SEM micrograph in Fig. 6.6a shows the morphology of the scratched N-DLC film, which is observed at the location where the indenter was stopped at the relevant critical load. The inset in Fig. 6.6a shows full path of the scratch test conducted on the N-DLC film. The observed brittle fracture of the N-DLC film deposited on the Si substrate, which occurs by removing the film material as platelets from the region bounded by free surface and lateral cracks developed during scratching, only at the critical load reveals a high cohesive strength of the film and implies that the kinetic energy of the sputtered C species is sufficient to form the rigid amorphous carbon network with a high enough sp3 fraction even with the incorporation of N in the film.It is found that the incorporation of Pt and Ru into the N-DLC film apparently promotes the adhesion strength of the film so the critical load of the PtRuN-DLC film is even higher (about 393 mN) (9.5% increment) than that of the N-DLC film as shown in Table 6.3 due to a higher degree of metal-induced-graphitization. The PtRu aggregates existing in the N-DLC film also degrade the sp3-bonded cross-linking structure, leading to a lower residual stress in the film. In addition, the PtRu aggregates could lower the crack density by blocking the crack propagation inside the film, which allows the removal of the scratched materials as platelets as shown in Fig. 6.6b. The observed spallations in some scratched areas reveal that the incorporation of Pt and Ru into the N-DLC film improves the adhesion strength of the film so that the detachment of the film at the critical load comes from the bulk region of the Si substrate instead of the interface between the film and the substrate. It is found that the introduction of Pt and Ru into the N-DLC film increases the thickness of the film, which is confirmed by the higher thickness (about 250 nm) of the PtRuN-DLC film than that (about 160 nm) of the N-DLC film. Robertson [29] reported that the residual stress in DLC films came along increased film thickness. However, the increased critical load of the PtRuN-DLC film with increased film thickness compared to that of the N-DLC film indicates that the influence of the film thickness on the adhesion strength of the films in terms of residual stress is not significant. On the other hand, a thicker film may need a higher load for the indenter to break through the film [78]. Therefore, it may be supposed that the higher thickness of the PtRuN-DLC film is one of the reasons giving rise to the higher critical load than that of the N-DLC film. 6.3. Electrochemical performance of N-DLC and PtRuN-DLC thin films6.3.1. Corrosion behavior of N-DLC and PtRuN-DLC thin filmsFigure 6.7 shows the Nyquist and Bode plots of the N-DLC and PtRuN-DLC films measured in 0.1 M HCl solution over the frequency range of 105 - 10-2 Hz with an amplitude of 10 mV. The inset in Fig. 6.7a shows an equivalent circuit to simulate the frequency responses of the N-DLC and PtRuN-DLC films in the HCl solution.In the circuit, R1 is the bulk resistance of the solution, R2 represents the charge transfer resistance, R3 is the bulk resistance of the film, and Q2 and Q3 are the constant-phase-elements (CPE) to replace the double-layer capacitance at the film/solution interface and the capacitance of the film, respectively. The results deduced from the Nyquist and Bode plots shown in Fig. 6.7a are summarized in Table 6.4. Q2 and Q3 in the circuit describe the deviation of the actual electrochemical process from an ideal one with n = 1, where a CPE resembles as a capacitor. Q2 and Q3 are attributed to several factors such as aggregations of N and PtRu, non-uniform composition, and resultant non-uniform electron transport and electrochemical reaction rate.A higher R2 value of the PtRuN-DLC film than that of the N-DLC film indicates that the introduction of Pt and Ru increases the charge transfer resistance of the PtRuN-DLC film, meaning an enhanced anti-corrosion behavior of the film, which is in agreement with Ref. [101]. Fig. 6.7: (a) Nyquist and (b) Bode plots of N-DLC and PtRuN-DLC films measured in 0.1 M HCl solution. The frequency range is 105 – 10-2 Hz and the amplitude is 10 mV. The inset in (a) shows an equivalent circuit for electrochemical reactions on N-DLC and PtRuN-DLC coated samples.A higher R3 value of the PtRuN-DLC film than that of the N-DLC film reveals an increase in the bulk resistance of the film with Pt and Ru doping. It was reported that the electrical conductance of a DLC film electrode is associated with charge carriers hopping between sp2-hybridized states [116, 203]. The introduction of N into the films promotes the electrical conductivity of the films [20]. However, in the noble metal doped DLC film, the isolated metal aggregates are electrically inactive in the film bulk.An effective current can occur only when the gap distances between the metal aggregates themselves, between sp2-hybridized states themselves, or between the metal aggregates and the nearest sp2-hybridized states are reduced to certain critical values by increasing metal concentration to ensure an effective exchange of the charge carriers [116, 203]. Therefore, it is supposed that the Pt and Ru contents in the PtRuN-DLC film used in this study would be below critical value to give rise to sufficient conducting paths for the charge carriers to pass through. Besides, the PtRu aggregates would non-uniformly distribute within the DLC matrix when the film was deposited by co-sputtering the C and Pt50Ru50 targets. Pleskov et al. [116] found that a noble metal was not an effective dopant in a DLC film as the incorporated noble metal did not increase the concentration of free charge carriers. Table 6.4: Results determined from EIS spectra based on the proposed equivalent circuit as shown in the inset of Fig. 6.7a.SampleR1 (Ω.cm2)R2 (Ω.cm2)Q2 (sn/Ω.cm2)n2R3 (Ω.cm2)Q3 (sn/Ω.cm2)n3N-DLC9079 × 10466.3 × 10-70.96649 × 10412.4 × 10-80.834PtRuN-DLC84.632 × 10635.4 × 10-70.93211 ×10526 × 10-70.8726.3.2. Linear sweep cyclic voltammetric behavior of N-DLC and PtRuN-DLC thin films6.3.2.1. Cyclic voltammetry of N-DLC and PtRuN-DLC thin filmsFigure 6.8a shows the cyclic voltammograms obtained from the N-DLC and PtRuN-DLC film electrodes in a 0.1 M H2SO4 solution at a scan rate of 100 mV/s, where the N-DLC film shows a broader potential window from approximately -0.65 to +2 V vs. SCE over which water decomposition occurs. The overpotentials for hydrogen (2H+ + 2e- → H2↑) and oxygen (4OH- → 2H2O + O2↑ + 4e-) evolutions for the N-DLC film are larger than those from glassy carbon (-0.3 to +1.8 V) and highly oriented pyrolytic graphite (HOPG) (-0.4 to +1.6 V) electrodes [204]. It is found that the introduction of Pt and Ru into the N-DLC film electrode causes the hydrogen evolution to start at a higher potential of about 0.05 V vs. SCE, which is probably due to the electrocatalytic effect of the incorporated Pt and Ru (M representing Pt and Ru) that facilitates the hydrogen evolution on the film electrode in the acidic media based on the following mechanism [205-209]:Step 1. Primary discharge (Volmer Reaction): M + H+ + e- ? MHad (6.1) Step 2. Combined with either electrochemical desorption (Heyrovsky Reaction): MHad + H+ + e- ? M + H2↑ (6.2)or recombination desorption (Tafel Reaction): MHad + MHad → 2M + H2↑ (6.3)Furthermore, an earlier oxygen evolution is found at the PtRuN-DLC film electrode, which is also attributed to the catalytic behavior of the Pt and Ru possibly through the following process [164, 165]: M + H2O → MOH + H+ + e- (6.4) MOH + e- → M + OH- → 4OH- → 2H2O + O2↑ + 4e- (6.5)It is observed that the oxygen evolution from the PtRuN-DLC film electrode begins at about 0.25 V lower than that from the N-DLC film electrode. A similar catalytic effect of Pt and Ru doping is found in Fig. 6.8b, in which the cyclic voltammograms obtained from the N-DLC and PtRuN-DLC film electrodes in a 0.1 M HCl solution at a scan rate of 100 mV/s are presented. The negative part of the potential window of the N-DLC film electrode in the HCl solution is about 0.25 V larger than that measured in the H2SO4 solution (Fig. 6.8a), indicating a lower negative potential limit of the N-DLC film electrode in the HCl solution due to a higher overpotential for the hydrogen evolution. It is noted that H2SO4 (H2SO4 → 2H+ + SO42-) has two possible H+ ions to donate, making it twice as acidic as HCl (HCl → H+ + Cl-) having the same concentration of 0.1 M, for an earlier hydrogen evolution. Fig. 6.8: Cyclic voltammograms measured from N-DLC (solid line) and PtRuN-DLC (dash line) film electrodes in (a) 0.1 M H2SO4 solution, (b) 0.1 M HCl solution and (c) 0.1 M KCl solution, where scan rate is 100 mV/s.Though the H2SO4 solution can provide two H+ ions than the HCl solution, it does not significantly influence the overpotential for the hydrogen evolution from the PtRuN-DLC film electrode, which is confirmed by similar hydrogen evolution potentials in the both H2SO4 and HCl solutions as shown in Fig. 6.8a and b. This may be due to a limited electrical conductivity of the PtRuN-DLC film electrode which cannot provide sufficient electrons (e-) to match the number of H+ ions for the hydrogen evolution according to Equation 6.1-6.3.The experimental results from this study point out that the different acidic electrolytes influence the negative parts of the cyclic voltammograms measured from the N-DLC film electrode rather than the PtRuN-DLC film electrode. The earlier hydrogen evolution on the PtRuN-DLC film electrode in the both H2SO4 and HCl solutions is assumed to be mainly attributed to the purely catalytic effect of the Pt and Ru aggregates existing on the film surface because these aggregates facilitate the charge transfer at the film/solution interface, which is in agreement with the finding by Pleskov and coworkers [116] that the effect of a noble metal is purely catalytic in acidic media. In Fig. 6.8b, the oxygen evolution on the PtRuN-DLC film electrode in the HCl solution is found nearly at the same potential as that in the H2SO4 solution (Fig. 6.8a), indicating that there is no obvious influence from acidic solution type on the positive portions of the cyclic voltammograms. It is clear that the catalytic effect of Pt/Ru on the oxygen evolution according to Equations 6.4 and 6.5 is not influenced by the concentration of H+ ions. The catalytic activity for Cl2/Cl- in the HCl solution causes the oxygen evolution on the N-DLC film electrode starting at about 0.15 V lower than that observed in the H2SO4 solution, which is in agreement with the report by Zeng et al. [20].In Fig. 6.8c, the cyclic voltammogram measured from the N-DLC film electrode in a KCl solution at a scan rate of 100 mV/s shows a rather larger negative part that is about 0.55 V wider than that measured in the H2SO4 solution (Fig. 6.8a) and about 0.3 V wider than that measured in the HCl solution (Fig. 6.8b). The KCl solution cannot support as many H+ ions as those from the H2SO4 and HCl acidic solutions for the hydrogen evolution, which explains why the cyclic voltammogram from the N-DLC film electrode in the KCl solution has a larger negative portion. The introduction of Pt and Ru into the N-DLC film electrode also results in an earlier hydrogen evolution in the KCl solution possibly according to the following steps [207]:Step 1. Primary discharge: M + H2O + e- ? MHad + OH- (6.6)Step 2. Coupled with either electrochemical desorption: MHad + H2O + e- ? H2↑+ OH- + M (6.7)or H recombination desorption: MHad + MHad ? H2↑ + 2M (6.8) A larger negative part (about 0.8 V wider) of the cyclic voltammogram obtained from the PtRuN-DLC film electrode than those measured in the both H2SO4 and HCl solutions also reveals that a much lower concentration of H+ ions in the KCl solution than those in the H2SO4 and HCl solutions is a main reason for the wider negative part of the potential window. The potential for the oxygen evolution from the PtRuN-DLC film electrode in the KCl solution appears to be slightly lower than those measured in the H2SO4 and HCl solutions, which is probably due to a higher concentration of OH- ions in the KCl solution according to Equations 6.4 and 6.5. The observed potential for the oxygen evolution from the N-DLC film electrode in the KCl solution is similar to the one found in the HCl solution (Fig. 6.8b) and slightly lower than that observed in the H2SO4 solution (Fig. 6.8a), indicating the effect of the catalytic activity of Cl- ions on the oxygen evolution potential in the KCl solution. A contribution of a background current to the cyclic voltammogram measured from the N-DLC film electrode in the KCl solution is found. This is due to a charging effect of the double layer since a high concentration of OH- ions near the electrode surface supports the formation of water molecules that lead to an adsorbed water layer on the electrode surface, resulting in the separations of the charges.6.3.2.2. Cyclic voltammetry of reversible couple (Ferricyanide)Figure 6.9a shows the cyclic voltammograms measured from the N-DLC film electrode with two different scan rates using a reversible ferri-ferrocyanide couple as a redox system [24, 120]: [Fe (CN)6]3- + e- ? [Fe (CN)6]4- (6.9)It is found that peak-potential separation ΔE and ratio of anodic to cathodic peak currents (Ip,a/Ip,c) obtained from the N-DLC film electrode at the scan rate of 10 mV/s are about 73 mV and 1.06, respectively. The N-DLC film electrode having a low ΔE indicates that the film electrode can have a good electrocatalytic activity for the Fe(CN)64-/Fe(CN)63- redox reaction. The Ip,a/Ip,c ratio greater than unity implies that the Fe(CN)64-/Fe(CN)63- redox reaction at the N-DLC film electrode is quasi-reversible, meaning that though a reverse peak current appears, it is slightly smaller than the forward one [20, 24]. When the scan rate is decreased to 5 mV/s, the ΔE and Ip,a/Ip,c obtained from the N-DLC film are 65 mV (11% decrement) and 0.99, respectively. As the scan rate is lowered, the time scale of the experiment becomes larger so that an equilibrium condition is achieved at the electrode surface and the kinetic effect begins to diminish. Therefore, the decreased kinetic limitation shifts oxidation to less positive potential and a reduction to less negative potential [24].The measurements of the cyclic voltammogram as shown in Fig. 6.9a were repeated for more than 3 times with essentially identical profiles, revealing that a good repeatability of the voltammograms. Fig. 6.9: Cyclic voltammograms measured from (a) N-DLC film electrodes with different scan rates and (b) N-DLC (solid line) and PtRuN-DLC (dash line) film electrodes in 1 mM K3Fe(CN)6/0.1 M HCl solution, where scan rate is 10 mV/s.It is clearly seen in Fig. 6.9b that the incorporation of Pt and Ru into the N-DLC film electrode increases the ΔE and Ip,a/Ip,c to 200 mV (174% increment) and 1.15, respectively. It is well known that linear sweep voltammetry measures a voltammogram rationalized by the potential and transport of species. When a flux of species to the electrode surface is slower than an electrode reaction, an equilibrium between oxidized and reduced species involved in the electrode reaction is established at the film electrode surface, which implies a reversible reaction corresponding to a case where the electrode reaction is much faster than the transport of the species. When there is a kinetic limit upon the electrode reaction compared to the transport of the species, the kinetic limitation shifts an oxidation peak to a more positive potential and a reduction peak to a more negative potential. Therefore, more apparent shifts of the redox peaks at the PtRuN-DLC film electrode to opposite potentials than those at the N-DLC film electrode indicate an increased kinetic limitation upon the PtRuN-DLC film electrode, which is attributed to the increased electrical resistivity of the film electrode by doping Pt and Ru. The increased electrode kinetic limitation associated with the increased electrical resistivity of the film electrode causes the Fe(CN)64-/Fe(CN)63- redox reaction at the PtRuN-DLC film electrode to be more quasi-reversible, which is confirmed by the increased Ip,a/Ip,c ratio. 6.4. ConclusionsNitrogen doped DLC films without (N-DLC) or with (PtRuN-DLC) Pt and Ru doping were deposited on highly conductive p-Si substrates using a DC magnetron sputtering deposition system to investigate the influence of Pt and Ru doping on the chemical composition, bonding structure, micro-structure, surface activity and morphology, adhesion strength, corrosion resistance and cyclic voltammetric behavior of these films. The introduction of Pt and Ru into the N-DLC films promoted the sp2 sites via metal-induced graphitization, increased the surface roughness, and enhanced the adhesion strength and corrosion resistance of the films. The enhancement of the corrosion resistance was confirmed by the increased charge transfer resistance of the PtRuN-DLC film. The observed constant phase element behavior of all the samples indicated that aggregations of N and PtRu in the DLC films induced non-uniform currents and consequently non-uniform electrochemical reaction rates. Though the N-DLC film electrodes showed wider potential windows in acidic solutions such as H2SO4 and HCl and a neutral solution of KCl, the potential for hydrogen evolution was significantly affected by the concentration of H+ ions in the solutions. It was found that the Pt and Ru doping apparently narrowed down the potential windows of the N-DLC film electrodes in these solutions due to their catalytic activities. The N-DLC film electrodes showed a good electrocatalytic activity in Fe(CN)64-/Fe(CN)63- redox reactions. However, an increased kinetic limitation upon the PtRuN-DLC film electrode shifted the oxidization peak to a more positive value and the reduction peak to a more negative value compared to those obtained from the N-DLC film electrode. Chapter 7Conclusions 7.1. Conclusions on N-DLC films prepared by a FCVA techniqueIn this study, high quality N-DLC thin films were deposited on conductive p-Si substrates using a filtered cathodic vacuum arc (FCVA) deposition system by varying nitrogen flow rate from 0.5 to 20 sccm. The chemical composition, bond structure, surface morphology and adhesion strength of the N-DLC films were characterized using X-ray photoelectron spectroscopy (XPS), micro-Raman spectroscopy, atomic force microscopy (AFM) and scanning electron microscopy (SEM), and micro-scratch test, respectively. The corrosion resistance of the N-DLC films was evaluated using potentiodyanmic polarization and immersion tests. The cyclic voltammetric behavior and anodic stripping voltammetric performance of the N-DLC films were investigated with linear sweep voltammetric method. The increased nitrogen flow rate significantly increased the N content in the N-DLC films. The Raman results indicated that the sp2 bonds in the N-DLC films increased with increased nitrogen incorporation because nitrogen preferred π bonding. The increased sp2 bonds and nitrogen aggregation in the N-DLC films increased the surface roughness of the N-DLC films. The increased critical loads of the N-DLC films with increased nitrogen flow rate revealed that the increased nitrogen incorporation in the films promoted the adhesion strength of the films due to the increased sp2 sites in the films. It was found from the potentiodynamic polarization tests that the corrosion resistance of the N-DLC films in the 0.6 M NaCl solution decreased with increased nitrogen incorporation because the increased sp2 sites increased the prompt dissolution of the films by degrading the sp3-bonded cross-linking structure. The corrosion results indicated that the nitrogen flow rate of 3 sccm (1.67% N) used during the film depositions resulted in the highest corrosion resistance of the N-DLC films. The immersion tests pointed out that the pH value of the solution significantly affected the corrosion performance of the N-DLC films, i.e. the lower the pH value, the more severe the corrosion was.The N-DLC thin films showed excellent electrochemical behavior in different aqueous solutions. The electrochemical potential windows of the N-DLC films measured in 0.5 M HCl, 0.1 M KCl, 0.1 M NaCl, 0.1 M KOH and 0.1 M NaOH were about 2.4, 2.32, 3.2, 3.1 and 3.25 V, respectively. Although the N-DLC film electrodes offered (1) wide potential windows in different types of solutions, (2) a very low and stable background to improve the signal-to-background and signal-to-noise ratios, (3) repeatability of voltammograms, (4) durability of the film electrodes to high anodic potential, and (5) long-time response stability, their voltammograms were apparently affected by their electrical conductivity, type of alkaline species and unbalanced H+ and OH- ions. It was found that the lower nitrogen content (3 sccm N2, 1.67 %N) in the N-DLC films resulted in the wider potential windows of the films in the aqueous solutions due to the higher electrical resistivity of the films..The N-DLC films provided a significant stripping response for determination of single-elements (Zn2+, Pb2+, Cu2+ and Hg2+) and multi-elements (Pb2+ + Cu2+ + Hg2+) simultaneously in the KCl solution. However, the sensitivity of the N-DLC films was significantly influenced by the nitrogen content in the films so the higher nitrogen content (20 sccm N2, 6.48%N) in the N-DLC films gave rise to the higher sensitivity of the films to the trace metals. It was observed that the sensitivity of the film electrodes to the metal elements was apparently influenced by deposition time and potential, concentration of elements in the solution, pH value, and scan rate. The simultaneous detection of the heavy metals using linear sweep anodic stripping voltammetry produced sharp and well-defined peaks with good peak separations. It was noted that the increased nitrogen content in the N-DLC film electrodes promoted the sensitivity of the film electrodes to the trace metals but apparently narrowed down the potential windows of the film electrodes, pointing out that the nitrogen content in the N-DLC film electrodes needed to be optimized between 1.67 and 6.48 % (between 3 and 20 sccm N2) to get the best balance between the high sensitivity and the wide potential windows of the film electrodes along with the high corrosion resistance.The novel N-DLC film electrodes under development showed a great promise for the detection of trace metals at ?M concentration. However, the degraded corrosion resistance of the N-DLC films with nitrogen incorporation pointed out that it was important to improve the corrosion resistance of the films because the poor corrosion resistance of the films could affect the electrochemical performance of the films such as sensitivity, repeatability, long-time response stability, durability, etc. Since noble metals, such as Pt and Ru, had high corrosion resistance and bi-catalytic activities, it was expected that the incorporation of Pt and Ru into the N-DLC films would improve the corrosion resistance of the films and at the same time, their sensitivity. 7.2. Conclusions on PtRuN-DLC films prepared by a DC magnetron sputtering techniquePlatinum/ruthenium/nitrogen-doped DLC (PtRuN-DLC) thin films were deposited on conductive p-Si substrates using a DC magnetron sputtering deposition system by varying DC power applied to Pt50Ru50 target from 15 to 30 W. The influence of Pt and Ru on the chemical composition, micro-structure, bonding structure, surface activity and morphology, adhesion strength and corrosion resistance of the N-DLC films were investigated using X-ray photoelectron microscopy (XPS), transmission electron microscopy (TEM), micro-Raman spectroscopy, water contact angle measurement, atomic force microscopy (AFM), scanning electron microscopy (SEM), micro-scratch test and potentiodynamic polarization test.The Raman results indicated that the increased Pt and Ru contents in the PtRuN-DLC films with increased DC power applied to the Pt50Ru50 target significantly increased graphitization of the films. TEM micrograph showed that the incorporated Pt and Ru existed as aggregates in the nitrogen doped amorphous carbon matrix. The increased metal-induced graphitization and PtRu aggregation with increased Pt and Ru contents in the PtRuN-DLC films increased the surface roughness and adhesion strength of the films. The increased water contact angles on the PtRuN-DLC film surfaces revealed that the PtRuN-DLC film surfaces became more hydrophobic with increased Pt and Ru contents. The corrosion resistance of the PtRuN-DLC films increased with increased Pt and Ru contents in the films when the applied potential was below 1 V. However, with higher polarization potentials beyond 1 V, the films with higher Pt and Ru contents showed higher anodic currents in the 0.1 M NaCl solution due to the delamination of the films. Therefore, the Pt/(C+Ru+Pt+N) and Ru/(C+Ru+Pt+N) should be optimized between 0.033 (20 W) and 0.035 (25 W) and between 0.037 (20 W) and 0.041 (25 W), respectively, to prevent the anodic dissolution of the PtRuN-DLC films.7.3. Conclusions on N-DLC and PtRuN-DLC films prepared by a DC magnetron sputtering techniqueFurthermore, the N-DLC and PtRuN-DLC thin films deposited on p-Si substrates using a DC magnetron sputtering deposition system were used to investigate the effect of Pt and Ru doping on the chemical composition, micro-structure, bonding structure, surface activity and morphology, adhesion strength, corrosion resistance and cyclic voltammetric behavior of the films.It was found that the PtRuN-DLC film had the higher degree of graphitization than the N-DLC film, which was responsible for the higher surface roughness and adhesion strength of the PtRuN-DLC film than those of the N-DLC film. The larger water contact angle on the PtRuN-DLC film than that on the N-DLC film indicated that the PtRuN-DLC film surface was more hydrophobic than the N-DLC film surface. In EIS results, the higher R2 and R3 values of the PtRuN-DLC film than those of the N-DLC film implied that the introduction of Pt and Ru into the N-DLC film significantly increased the charge transfer resistance and bulk electrical resistivity of the film.Table 7.1: Major findings from three major work chapters (4 to 6)N-DLC films (FCVA technique) Chapter 4PtRuN-DLC films (Magnetron sputtering technique) Chapter 5N-DLC and PtRuN-DLC films (Magnetron sputtering technique) Chapter 6Increasing N incorporation in N-DLC films1. Improved the adhesion strength of the films, but decreased the corrosion resistance of the films.2. Wide electrochemical potential windows in acidic, neutral and basic solutions.3. Decreased the electrochemical potential windows of the films (disadvantage)4. Increased the film electrodes’ sensitivity to toxic metals such as single elements (Pb2+, Cu2+ and Hg2+) and multi elements (Pb2+, Cu2+ and Hg2+).Increasing Pt and Ru incorporation in PtRuN-DLC films1. Increased the adhesion strength and the corrosion resistance of the films at lower applied potentials (less than 1 V).2. Increased the anodic dissolution of the films at higher applied potentials.Pt and Ru incorporation in N-DLC films1. Increased the adhesion strength and corrosion resistance of the films.2. Decreased the electrochemical potential windows and degraded the electrochemical performance (EC) of the films.The linear sweep cyclic voltammetric measurements of the N-DLC and PtRuN-DLC films clearly indicated that the introduction of Pt and Ru into the N-DLC film significantly narrowed down the potential windows of the film because of their catalytic activities though the N-DLC film showed wide potential windows in 0.1 M H2SO4, 0.1 M HCl and 0.1 M KCl solutions of about 2.65, 2.9 and 3.2 V, respectively. The N-DLC film showed a good electrocatalytic activity in Fe(CN)64-/Fe(CN)63- redox reactions. However, an increased kinetic limitation upon the PtRuN-DLC film shifted the oxidization peak to a more positive value and the reduction peak to a more negative value compared to those obtained from the N-DLC film. Such the increased kinetic limitation upon the PtRuN-DLC film was attributed to the increased electrical resistivity of the film with Pt and Ru incorporation, which was consistent with the EIS results. It could be deduced that the introduction of Pt and Ru into the N-DLC film improved the corrosion resistance of the film but apparently degraded the electrochemical performance of the film. The major findings from the three major work chapters (4 to 6) were summarized in Table 7.1.7.4. Recommendations for future workThe experimental results obtained from this project have clearly revealed that N-DLC films have a great promise for detection of heavy metals such as Pb2+, Cu2+ and Hg2+. 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