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Growing N-doped multiphase TiO2 nanocomposites on reduced graphene oxide: characterization and activity under low energy visible radiationLütfiye Y. Ozer,a Yuyoung Shin,b Alexandre Felten,c Habeebllah Oladipo,a Oluwadamilola Pikuda,a Christopher Muryn,b Cinzia Casiraghi,b and Giovanni Palmisano*aa Department of Chemical Engineering, Khalifa University of Science and Technology, Masdar Institute, Masdar City, PO Box 54224, Abu Dhabi, United Arab Emirates. E-mail: gpalmisano@masdar.ac.ae b School of Chemistry, University of Manchester, Oxford road, Manchester M139PL, United Kingdom.c Synthesis, Irradiation and Analysis of Materials (SIAM), University of Namur, rue de Bruxelles 61, 5000 Namur, Belgium.Abstract: Reduced graphene oxide (G) was used as a platform to grow a mixed catalyst made of brookite and rutile nanoparticles doped with nitrogen, resulting in excellent performance for the oxidation of 4-nitrophenol (4-NP) in water under low energy (>425 nm) radiation. The samples were fully characterized by X-Ray Diffractometry (XRD), Raman Spectroscopy, Electron Microscopy, X-Ray photoelectron spectroscopy (XPS), photoluminescence (PL), Z-potential analysis, UV-Visible Diffuse Reflectance Spectrophotometry (UV-Vis DRS), and porosimetry. The improved hole-electron separation, demonstrated by PL, is boosted by the exceptional properties of reduced graphene oxide, which attracts and conveys electrons to dissolved oxygen, in turn initiating the oxidation process. The optimal amount of reduced graphene oxide was found to be 1% w/w based on 4-nitrophenol (4-NP) conversion rates. No leaching of carbon into water was revealed, even under irradiation, pointing to the suitability of the composite catalyst in water.Keywords: Photocatalysis, Reduced graphene oxide-TiO2, multiphase TiO2, N-doping, Visible radiation1. IntroductionPhotocatalysis is an increasingly popular technology to mitigate pollution and environmental issues ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1016/j.cattod.2007.01.026", "ISSN" : "09205861", "abstract" : "The photocatalytic oxidation of different benzene derivatives has been investigated in order to understand how the substituent group affects the selectivity to hydroxylated compounds. Experimental runs were performed by using TiO2 (Merck) aqueous suspensions at natural pH irradiated by near-UV light. The organic molecules used as substrate contained an electron withdrawing group (EWG) (nitrobenzene, cyanobenzene, benzoic acid, 1-phenylethanone), an electron donor one (EDG) (phenol, phenylamine, N-phenylacetamide) or both an EWG and an EDG (4-chlorophenol). The results clearly indicated that the primary photocatalytic oxidation of aromatic compounds containing an EDG gives rise mainly to ortho and para mono-hydroxy derivatives while in the presence of an EWG all the mono-hydroxy derivatives are obtained. This finding can open a new green route for the synthesis of hydroxylated aromatic compounds. Moreover, in the presence of both an ED and an EW group, as in the case of 4-chlorophenol and hydroxy-cyanobenzenes, the attack of the hydroxyl radical takes place only in the positions activated by \u2013OH. A competing reaction pathway to total oxidation was also observed from the starting of the irradiation; this pathway was more important for compounds containing an EWG. This evidence can be explained by considering the strong interaction of these molecules with the TiO2 surface. In fact, adsorption in the dark, measured for all the compounds, resulted to be significant only for molecules having strongly EWG's.", "author" : [ { "dropping-particle" : "", "family" : "Palmisano", "given" : "Giovanni", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Addamo", "given" : "Maurizio", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Augugliaro", "given" : "Vincenzo", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Caronna", "given" : "Tullio", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Paola", "given" : "Agatino", "non-dropping-particle" : "Di", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "L\u00f3pez", "given" : "Elisa Garc\u00eda", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Loddo", "given" : "Vittorio", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Marc\u00ec", "given" : "Giuseppe", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Palmisano", "given" : "Leonardo", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Schiavello", "given" : "Mario", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Catalysis Today", "id" : "ITEM-1", "issue" : "1-2", "issued" : { "date-parts" : [ [ "2007", "4" ] ] }, "page" : "118-127", "title" : "Selectivity of hydroxyl radical in the partial oxidation of aromatic compounds in heterogeneous photocatalysis", "type" : "article-journal", "volume" : "122" }, "uris" : [ "" ] }, { "id" : "ITEM-2", "itemData" : { "DOI" : "10.1016/j.apcata.2015.09.020", "ISSN" : "0926860X", "abstract" : "Hydrogen is the ideal candidate to fulfill the growing energy demand in a sustainable manner because of its high energy content and no emission of greenhouse gases from its combustion. Currently most of hydrogen generation techniques involve the employment of fossil fuels, with consequent production of toxic greenhouse gases. The possibility to produce hydrogen by means of photocatalytic processes using the solar radiation as energy source fits in perfectly with the switch to a more sustainable energy production. The solar photocatalytic hydrogen generation can be achieved by reforming organic substances contained in civil or industrial wastewaters. This could allow to combine water decontamination with production of an energy carrier starting from a renewable source, the solar radiation. Within this perspective, a novel nano-TiO2 photocatalytic system based on the solar reforming of formic acid in presence of cupric ions and chlorides has been investigated. The effect on hydrogen generation rate of the initial concentrations of formic acid, chloride and cupric ion, and pH values has been evaluated. For both formic acid and chloride ions, at least up to a starting concentration of 103mM, the higher the initial concentration, the higher the rate of hydrogen generation. Hydrogen production has turned out to be noticeably dependent on cupric ion concentration. An almost opposite behavior has been found varying the starting cupric ion concentration in the range 2.5\u201320mM, with the highest value of hydrogen production rate recorded for Cu(II) initial concentration equal to 5.0mM. The pH value has been identified to be a crucial parameter of the system. A decrease in hydrogen production has been also observed rising pH of the solution from 1.0 to 4.0. A characterization of the solid samples recovered at the end of the runs has been performed by X-ray Diffractometry. These experimental outcomes have been rationalized within a consistent reaction mechanism able to predict the system behavior under different operating conditions. This work opens the way to the development of new competitive processes able to use waste organic streams for hydrogen generation through photacatalytic system based on solar energy.", "author" : [ { "dropping-particle" : "", "family" : "Clarizia", "given" : "Laura", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Somma", "given" : "Ilaria", "non-dropping-particle" : "Di", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Marotta", "given" : "Raffaele", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Minutolo", "given" : "Patrizia", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Villamaina", "given" : "Roberta", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Andreozzi", "given" : "Roberto", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Applied Catalysis A: General", "id" : "ITEM-2", "issued" : { "date-parts" : [ [ "2016" ] ] }, "page" : "181-188", "title" : "Photocatalytic reforming of formic acid for hydrogen production in aqueous solutions containing cupric ions and TiO2 suspended nanoparticles under UV-simulated solar radiation", "type" : "article-journal", "volume" : "518" }, "uris" : [ "" ] }, { "id" : "ITEM-3", "itemData" : { "DOI" : "10.1016/j.cattod.2014.05.008", "ISSN" : "09205861", "abstract" : "A series of titania samples consisting of pure brookite, brookite/anatase and brookite/layered titanate hybrids were synthesized by a hydrothermal method. The influence of experimental variables, including electrolyte concentration, temperature and duration of hydrothermal treatment, on structural and physical properties of the resulting materials was investigated. The crystalline phase composition of the materials was analyzed by X-ray diffraction, the morphology was examined by scanning electron microscopy (SEM) and the specific surface areas were measured according to the Brunauer\u2013Emmett\u2013Teller method. Pure brookite could be obtained by hydrothermal treatment at 180\u00b0C with addition of 0.25M NaCl, 0.5M NaCl or 0.1M NaOH, and adjustment of reaction time. Brookite samples prepared by addition of NaCl as source of Na+ ions showed a flower-type morphology, whereas those synthesized with NaOH consisted in micrometric-size aggregations of nanoparticles. The photocatalytic activity of the materials was evaluated by means of methyl orange bleaching and by As(III) to As(V) oxidation.", "author" : [ { "dropping-particle" : "", "family" : "L\u00f3pez-Mu\u00f1oz", "given" : "M.J.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Revilla", "given" : "A.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Alcalde", "given" : "G.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Catalysis Today", "id" : "ITEM-3", "issued" : { "date-parts" : [ [ "2015" ] ] }, "page" : "138-145", "title" : "Brookite TiO2-based materials: Synthesis and photocatalytic performance in oxidation of methyl orange and As(III) in aqueous suspensions", "type" : "article-journal", "volume" : "240" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "[1\u20133]", "plainTextFormattedCitation" : "[1\u20133]", "previouslyFormattedCitation" : "[1\u20133]" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }[1–3]. As such, it enables the degradation of organic species in water without the use of aggressive oxidizing chemicals. TiO2 has been the most widely reported photocatalyst owing to its low cost, high efficiency and good stability. However, the ability of TiO2 to be activated for oxidation processes is restricted to the need of UV light. Consequently, its modification with nitrogen has been proved as one of the best way to boost reactivity under visible light radiation, which is the majority of solar spectrum, and the only light available indoors ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1016/j.jes.2016.02.018", "ISSN" : "10010742", "PMID" : "28391938", "abstract" : "A novel visible light-active photocatalyst formulation (NdT/OP) was obtained by supporting N-doped TiO2 (NdT) particles on up-conversion luminescent organic phosphors (OP). The photocatalytic activity of such catalysts was evaluated for the mineralization process of spiramycin in aqueous solution. The effect of NdT loading in the range 15-60wt.% on bulk and surface characteristics of NdT/OP catalysts was investigated by several chemico-physical characterization techniques. The photocatalytic performance of NdT/OP catalysts in the removal of spyramicin from aqueous solution was assessed through photocatalytic tests under visible light irradiation. Total organic carbon (TOC) of aqueous solution, and CO and CO2 gas concentrations evolved during the photodegradation were analyzed. A dramatic enhancement of photocatalytic activity of the photostructured visible active NdT/OP catalysts, compared to NdT catalyst, was observed. Only CO2 was detected in gas-phase during visible light irradiation, proving that the photocatalytic process is effective in the mineralization of spiramycin, reaching very high values of TOC removal. The photocatalyst NdT/OP at 30wt.% of NdT loading showed the highest photocatalytic activity (58% of TOC removed after 180min irradiation against only 31% removal after 300min of irradiation of NdT). We attribute this enhanced activity to the high effectiveness in the utilization of visible light through improved light harvesting and exploiting. OP particles act as \"photoactive support\", able to be excited by the external visible light irradiation, and reissue luminescence of wavelength suitable to promote NdT photomineralization activity.", "author" : [ { "dropping-particle" : "", "family" : "Sacco", "given" : "Olga", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Vaiano", "given" : "Vincenzo", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Sannino", "given" : "Diana", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Ciambelli", "given" : "Paolo", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Journal of Environmental Sciences", "id" : "ITEM-1", "issued" : { "date-parts" : [ [ "2017", "4" ] ] }, "page" : "268-276", "title" : "Visible light driven mineralization of spiramycin over photostructured N-doped TiO 2 on up conversion phosphors", "type" : "article-journal", "volume" : "54" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "[4]", "plainTextFormattedCitation" : "[4]", "previouslyFormattedCitation" : "[4]" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }[4]. The most used crystal phase of TiO2 is anatase (with a band gap of 3.2 eV), very active in the UV region; however, rutile and brookite-rutile nanocomposites have been recently reported to be active in the visible, especially upon nitrogen doping ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1021/jp031267y", "ISSN" : "1520-6106", "author" : [ { "dropping-particle" : "", "family" : "Diwald", "given" : "Oliver", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Thompson", "given" : "Tracy L.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Zubkov", "given" : "Tykhon", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Walck", "given" : "Scott D.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Yates", "given" : "John T.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "The Journal of Physical Chemistry B", "id" : "ITEM-1", "issue" : "19", "issued" : { "date-parts" : [ [ "2004", "5" ] ] }, "page" : "6004-6008", "title" : "Photochemical Activity of Nitrogen-Doped Rutile TiO ₂ (110) in Visible Light", "type" : "article-journal", "volume" : "108" }, "uris" : [ "" ] }, { "id" : "ITEM-2", "itemData" : { "DOI" : "10.1016/j.jcat.2016.12.010", "ISSN" : "00219517", "abstract" : "N-doped brookite-rutile catalysts were prepared using the sol-gel method with different nitrogen precursors, namely urea (CH4N2O), propionitrile (C3H5N), ammonium hydroxide (NH4OH), ammonium nitrate (NH4NO3) and ethylene diamine (C2H8N2). Testing the photoactivity of the prepared catalysts with 4-nitrophenol revealed that ammonium nitrate was the best doping agent with a nominal N-content of 0.8% (w/w), yielding a 5-fold increase in the pseudo-first order constant of 4-nitrophenol disappearance and a 3-fold increase in the pseudo-first order constant of TOC disappearance, with respect to undoped TiO2, when irradiated with LED visible light (>425nm). In the same experimental conditions, commercial catalysts such as Evonik P25 and mixtures of commercial rutile and brookite failed to work. XRD allowed to identify two crystal phases, i.e. brookite and rutile, and to show that the most active catalyst had the highest brookite and lowest amorphous content, along with the largest rutile crystallites. Through HRTEM, the morphology and crystallinity were further investigated: brookite particles were much smaller and roundish with respect to rutile and the intimate contact between the two phases was also well highlighted. N-doping did not produce oxygen vacancies as shown by Raman spectroscopy; thus, the doping can be considered interstitial rather than substitutional. Surface hydroxylation did not promote oxidation ability, as revealed by TGA-DTA: the most reacting catalyst is the least hydroxylated one. BET revealed that the samples are partially mesoporous (type IV hysteresis), although no template/surfactant was used, and the pore size and volume seemed to affect their activity. UV-vis DRS allowed to extrapolate the band gaps, only slightly narrower for N-doped samples, which, however, showed a pronounced absorption of visible radiation compared to undoped TiO2. Photoluminescence showed that the emission due to electron-hole recombination decreases with the N-loading, eventually reaching a minimum plateau for doping amounts just above the optimal one.", "author" : [ { "dropping-particle" : "", "family" : "Pikuda", "given" : "Oluwadamilola", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Garlisi", "given" : "Corrado", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Scandura", "given" : "Gabriele", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Palmisano", "given" : "Giovanni", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Journal of Catalysis", "id" : "ITEM-2", "issued" : { "date-parts" : [ [ "2017" ] ] }, "page" : "109-116", "title" : "Micro-mesoporous N-doped brookite-rutile TiO2 as efficient catalysts for water remediation under UV-free visible LED radiation", "type" : "article-journal", "volume" : "346" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "[5,6]", "plainTextFormattedCitation" : "[5,6]", "previouslyFormattedCitation" : "[5,6]" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }[5,6]. Rutile has a narrower band gap (3.0 eV) than anatase and, coupled with brookite, it can enhance charge separation. Nitrogen atoms can be incorporated either interstitially or substitutionally, acting as attraction centers for holes ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1016/j.jssc.2007.11.012", "ISSN" : "00224596", "author" : [ { "dropping-particle" : "", "family" : "Peng", "given" : "Feng", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Cai", "given" : "Lingfeng", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Yu", "given" : "Hao", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Wang", "given" : "Hongjuan", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Yang", "given" : "Jian", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Journal of Solid State Chemistry", "id" : "ITEM-1", "issue" : "1", "issued" : { "date-parts" : [ [ "2008", "1" ] ] }, "page" : "130-136", "title" : "Synthesis and characterization of substitutional and interstitial nitrogen-doped titanium dioxides with visible light photocatalytic activity", "type" : "article-journal", "volume" : "181" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "[7]", "plainTextFormattedCitation" : "[7]", "previouslyFormattedCitation" : "[7]" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }[7], and then foster the formation of oxygen vacancies, thus promoting absorption of visible light and, eventually, reactivity. Beside nitrogen doping, addition of reduced graphene oxide (G) can significantly enhance reactivity under visible radiation [8] due to its remarkable properties, such as high electron mobility, high surface area to volume ratio due to its 2-dimensional nature, and transparency [ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1039/C7NR00704C", "ISSN" : "2040-3364", "abstract" : "The as-synthesized TiO2 nanorods a-TNR (amorphous TiO2 layer covering the crystalline anatase TiO2 core) and TNR (fully crystalline anatase TiO2) were decorated with reduced graphene oxide (rGO) to synthesize two series of TiO2 + rGO composites with different nominal loadings of GO (from 4 to 20 wt%). The structural, surface and electronic properties of the obtained TiO2 + rGO composites were analyzed and correlated to their performance in the photocatalytic oxidation of aqueous bisphenol A solution. X-ray photoelectron spectroscopy (XPS) analyses revealed that charge separation in TiO2 + rGO composites is improved due to the perfect matching of TiO2 and rGO valence band maxima (VBM). Cyclic voltammetry (CV) experiments revealed that the peak-to-peak separations (\u0394Ep) are the lowest and the oxidation current densities are the highest for composites with a nominal 10 wt% GO content, meaning that it is much easier for the charge carriers to percolate through the solid, resulting in improved charge migration. Due to the high charge carrier mobility in rGO and perfect VBM matching between TiO2 and rGO, the electron\u2013hole recombination in composites was suppressed, resulting in more electrons and holes being able to participate in the photocatalytic reaction. rGO amounts above 10 wt% decreased the photocatalytic activity; thus, it is critical to optimize its amount in the TiO2 + rGO composites for achieving the highest photocatalytic activity. BPA degradation rates correlated completely with the results of the CV measurements, which directly evidenced improved charge separation and migration as the crucial parameters governing photocatalysis.", "author" : [ { "dropping-particle" : "", "family" : "\u017derjav", "given" : "Gregor", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Arshad", "given" : "Muhammad Shahid", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Djinovi\u0107", "given" : "Petar", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Junker", "given" : "Ita", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Kovac", "given" : "Janez", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Zava\u0161nik", "given" : "Janez", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Pintar", "given" : "Albin", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Nanoscale", "id" : "ITEM-1", "issue" : "13", "issued" : { "date-parts" : [ [ "2017" ] ] }, "page" : "4578-4592", "publisher" : "The Royal Society of Chemistry", "title" : "Improved electron-hole separation and migration in anatase TiO2 nanorod/reduced graphene oxide composites and their significance on photocatalytic performance", "type" : "article-journal", "volume" : "9" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "[8]", "plainTextFormattedCitation" : "[8]", "previouslyFormattedCitation" : "[8]" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }8]. To the best of our knowledge, there was only one attempt to obtain a N-doped TiO2 synthesized on graphene through a sol-gel route, giving rise to anatase phase; moreover, the catalyst was not used for water remediation purposes [ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1039/c4nr06435f", "ISSN" : "2040-3372", "PMID" : "25697910", "abstract" : "Highly monodispersed nitrogen doped TiO2 nanoparticles were successfully deposited on graphene (N-TiO2/Gr) by a facile in-situ wet chemical method for the first time. N-TiO2/Gr has been further used for photocatalytic hydrogen production using a naturally occurring abundant source of energy i.e. solar light. The N-TiO2/Gr nanocomposite composition was optimized by varying the concentrations of dopant nitrogen and graphene (using various concentrations of graphene) for utmost hydrogen production. The structural, optical and morphological aspects of nanocomposites were studied using XRD, UV-DRS, Raman, XPS, FESEM, and TEM. The structural study of the nanocomposite shows existence of anatase N-TiO2. Further, the details of the components present in the composition were confirmed with Raman and XPS. The morphological study shows that very tiny, 7-10 nm sized, N-TiO2 nanoparticles are deposited on the graphene sheet. The optical study reveals a drastic change in absorption edge and consequent total absorption due to nitrogen doping and presence of graphene. Considering the extended absorption edge to the visible region, these nanocomposites were further used as a photocatalyst to transform hazardous H2S waste into eco-friendly hydrogen using solar light. The N-TiO2/Gr nanocomposite with 2% graphene exhibits enhanced photocatalytic stable hydrogen production i.e. \u223c5941 \u03bcmol h(-1) under solar light irradiation using just 0.2 gm nanocomposite, which is much higher as compared to P25, undoped TiO2 and TiO2/Gr nanocomposite. The enhancement in the photocatalytic activity is attributed to 'N' doping as well as high specific surface area and charge carrier ability of graphene. The recycling of the photocatalyst shows a good stability of the nanocomposites. This work may provide new insights to design other semiconductor deposited graphene novel nanocomposites as a visible light active photocatalyst.", "author" : [ { "dropping-particle" : "", "family" : "Bhirud", "given" : "Ashwini P", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Sathaye", "given" : "Shivaram D", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Waichal", "given" : "Rupali P", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Ambekar", "given" : "Jalindar D", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Park", "given" : "Chan-J", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Kale", "given" : "Bharat B", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Nanoscale", "id" : "ITEM-1", "issue" : "11", "issued" : { "date-parts" : [ [ "2015", "3", "21" ] ] }, "page" : "5023-5034", "title" : "In-situ preparation of N-TiO2/graphene nanocomposite and its enhanced photocatalytic hydrogen production by H2S splitting under solar light", "type" : "article-journal", "volume" : "7" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "[9]", "plainTextFormattedCitation" : "[9]", "previouslyFormattedCitation" : "[9]" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }9]. While the modification of anatase or anatase-rutile TiO2 with reduced graphene oxide has been reported in the last years by a number of scholars [ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1016/j.apsusc.2015.06.150", "author" : [ { "dropping-particle" : "", "family" : "Luo", "given" : "Lijun", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Yang", "given" : "Ye", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Zhang", "given" : "Ali", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Wang", "given" : "Min", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Liu", "given" : "Yongjun", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Bian", "given" : "Longchun", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Jiang", "given" : "Fengzhi", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Pan", "given" : "Xuejun", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Applied Surface Science", "id" : "ITEM-1", "issued" : { "date-parts" : [ [ "2015", "10" ] ] }, "page" : "469-479", "title" : "Hydrothermal synthesis of fluorinated anatase TiO2/reduced graphene oxide nanocomposites and their photocatalytic degradation of bisphenol A", "type" : "article-journal", "volume" : "353" }, "uris" : [ "" ] }, { "id" : "ITEM-2", "itemData" : { "DOI" : "10.1021/am301287m", "abstract" : "A series of TiO2\u2013reduced graphene oxide (RGO) nanocomposites were prepared by simple one-step hydrothermal reactions using the titania precursor, TiCl4 and graphene oxide (GO) without reducing agents. Hydrolysis of TiCl4 and mild reduction of GO were simultaneously carried out under hydrothermal conditions. While conventional approaches mostly utilize multistep chemical methods wherein strong reducing agents, such as hydrazine, hydroquinone, and sodium borohydride are employed, our method provides the notable advantages of a single step reaction without employing toxic solvents or reducing agents, thereby providing a novel green synthetic route to produce the nanocomposites of RGO and TiO2. The as-synthesized nanocomposites were characterized by several crystallographic, microscopic, and spectroscopic characterization methods, which enabled confrimation of the robustness of the suggested reaction scheme. Notably, X-ray diffraction and transmission electron micrograph proved that TiO2 contained both anatas...", "author" : [ { "dropping-particle" : "", "family" : "Sher Shah", "given" : "Md. Selim Arif", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Park", "given" : "A Reum", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Zhang", "given" : "Kan", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Park", "given" : "Jong Hyeok", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Yoo", "given" : "Pil J.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "ACS Applied Materials & Interfaces", "id" : "ITEM-2", "issue" : "8", "issued" : { "date-parts" : [ [ "2012", "8", "22" ] ] }, "page" : "3893-3901", "publisher" : "American Chemical Society", "title" : "Green Synthesis of Biphasic TiO2 \u2013Reduced Graphene Oxide Nanocomposites with Highly Enhanced Photocatalytic Activity", "type" : "article-journal", "volume" : "4" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "[10,11]", "plainTextFormattedCitation" : "[10,11]", "previouslyFormattedCitation" : "[10,11]" }, "properties" : { "noteIndex" : 2 }, "schema" : "" }10,11] the present study deals with a new composite material (N-TiO2-G) obtained by synthesizing N-doped brookite-rutile nanoparticles in the presence of reduced graphene oxide (G). The particles form and grow on the reduced graphene oxide sheets, resulting in a number of advantages in terms of 4-NP photodegradation in water, irradiated by visible LED, as detailed in the following. To the best of our knowledge, TiO2 modified with reduced graphene oxide has never been used before to oxidize 4-NP.2. ExperimentalSynthesis of reduced graphene oxideA modified Hummer method was used to prepare graphite oxide by using graphite flakes as the precursors. 3 g graphite flakes, 2.5 g potassium persulfate and 2.5 g phosphorous pentoxide were poured into 12 mL concentrated sulfuric acid (H2SO4) and stirred vigorously for 4.5 h at 80 oC. The mixture was cooled down to room temperature naturally. Afterwards, 0.5 L deionized (DI) water was added and aging was prolonged for 12 h under stirring. The solution was filtered, washed and dried to obtain a black solid. The pretreated graphite flakes underwent oxidation by 15 g potassium permanganate (KMnO4) in 120 mL concentrated sulfuric acid (H2SO4) in an ice bath for 1 h. The mixture was stirred at 35oC for 24 h to obtain a very viscous dark-brown paste, diluted with a slow addition of 250 mL of DI water and further stirred for 4 h. 30 mL (30 w%) of H2O2 were slowly added to quench the solution thus producing a golden-brown solution. The mixture was filtered and washed with DI water until a pH of 6 was obtained in the washing solution. The graphite oxide was dried at 40 oC. Following the oxidation process, graphite oxide (0.5 g L-1) was exfoliated in water using a ultrasound bath (100 W) for 10 h, ultimately producing a multilayered graphene oxide (GO). The reduction of GO was performed by using reducing agent, that is sodium borohydride (NaBH4), by stirring at room temperature for 5 h. Reduced graphene oxide (G) black powder was finally dried by a rotary evaporator at 50 oC. Washing with DI water 10 times with the aid of a centrifuge removed the residual ions producing a pure G powder.Fabrication of N-TiO2-GN-TiO2-G was prepared by using a sol-gel method. Titanium (IV) butoxide (TBOT) was used as TiO2 precursor. TBOT, 2-propanol and HCl (4 M) in a volumetric ratio of 1:5.461:1.283, respectively, were mixed until a clear solution (total volume: 116.24 mL) was obtained under stirring at 70 oC for 20 h. The samples were dried by means of rotary evaporator at 70oC. Following that, the powder was calcined at 450oC for 4 h under nitrogen flow to minimize oxidative/thermal degradation of G. G and ammonium nitrate, used as N-doping agent with 0.8% (w/w), were dispersed/dissolved in the water prior to mixing with TBOT, HCl and 2-propanol solution. When needed, ultrasound treatment and ultrafast (3000 rpm) magnetic stirring were applied to properly break down and disperse G. Bare N-doped TiO2 was prepared as well. The amount of G was fixed at 0.1%,0.5% 1%, 1.5%, 2% (w/w) with respect to TiO2 and the corresponding samples were denoted as N-TiO2/G_0.1%, N-TiO2-G_0.5%, N-TiO2-G_1%, N-TiO2-G_1.5% and TiO2-G_2%, respectively.Characterization and reactivity – Experimental detailsA Nova NanoSEM was used to observe the morphology and dimensions of samples and to perform EDX mapping. Samples were prepared by dropping a water suspension on a stainless steel stub previously cleansed with water, acetone and ethanol under ultrasounds. Mapping of the constitutive elements of the composite materials was carried out by using a distribution mapping technique by energy dispersive X-ray spectroscopy (EDX) combined with SEM. EDX mapping was recorded using a spot size of 5 nm, X-ray energy of 20 eV with a 256x200 resolution and 200 ?s dwells (64 frames). Carbon, titanium and oxygen mapping, using red, green and blue colors respectively, provides the visualization of elemental contents. Powder X-Ray diffraction (XRD) was performed in a 2θ range of 10-90o by using PANalytical Empyrean diffractometer with Cu Kα radiation of 1.54 ? at 45kV and 40 mA to determine the crystal structure of the prepared samples. The powders were ground in a mortar before analysis, and a holder with fixed thickness of samples was used to get comparable results.Raman spectroscopy was performed by a Renishaw Raman spectroscope equipped with a 514.5 nm laser line and laser power below 0.5 mW. The Raman spectra were collected in back scattering configuration in the range 3500 cm1-50 cm1. The powder was pressed in a holder to get a homogeneous surface before the analysis.Fourier transform infrared (FT-IR) analysis was carried out by using a Bruker Vertex 80v FT-IR (128 scans) in the 4000 cm1-400 cm1 region in diffuse reflectance mode (DRIFT). The powders were ground in a mortar before analysis and pressed in a holder to get a flat surface.X-ray Photoelectron Spectroscopy (XPS) was performed on an Escalab 250Xi from Thermo with a monochromatic Al Kα source (1486.6 eV). Photoelectrons are collected at an angle of 0° relative to the sample surface normal. Samples were fixed using double-sided carbon tape. Survey and high-resolution spectra were acquired using a spot size of 250 ?m and pass energy of 150 eV and 20 eV respectively. A flood gun with combined electron and low energy ions was also used during analysis to prevent surface charging. Surveys were measured in steps of 1.0 eV with 50 ms dwell time per data point. Nitrogen, carbon, and oxygen 1s high-resolution spectra were measured within the spectral range of interest with 0.1 eV steps and 50 ms dwell time per data point. Analysis of the data was carried out with CasaXPS software. A Shirley background was used in curve-fitting along with a GL(30) line-shape (70% Gaussian, 30% Lorentzian using the Gaussian/Lorentzian product form) for the C 1s, N 1s and O 1s spectra. Samples were referenced to the C 1s emission by adventitious hydrocarbon contamination at 284.8 eV.TEM analysis was carried out by using a Tecnai G2 transmission electron microscope to observe the morphology and dimensions of samples. Samples for TEM were deposited on a Forvar/Carbon 300 or 400-mesh Cu grid purchased from Tedpella; the powder was previously suspended in water and 2L were dropped on the TEM grid twice (waiting for the complete drying before the second deposition).The adsorption/desorption isotherms of the catalysts were recorded by using a Quantachrome NOVA 2000e surface area and pore size analyzer by using N2 as adsorbent. The sample was degassed in static conditions under vacuum at 400 oC for 4 hours prior to analysis. The specific surface area of catalysts were calculated using the multipoint Brunauer-Emmett-Teller (BET) method, as an average desorption and adsorption values in the P/Po range of 0-0.35.The pore size distribution was calculated from the desorption curve in the whole range of pressures by using two different models, BJH and DFT.UV-Vis diffuse reflectance spectra (DRS) were recorded by using a Shimadzu UV-2600 Spectrophotometer in a range 200-800 nm. The powder was pressed in a holder to get a flat surface before the analysis. The optical band gaps were calculated by considering indirect transitions, as typically applied to TiO2 based samples.Photoluminescence was recorded by using a Perkin Elmer LS-55, preparing the samples by pressing in a holder to get a flat surface before the analysis. The analyses were run by using dry powders with the following parameters: excitation wavelength: 300 nm, scanning speed of 500 nm min1, excitation slit width 2.5 nm, emission slit: 7.5 nm.The point of zero charge was assessed by Brookhaven ZetaPALS (Zeta potential Brookhaven Instruments Corporation, USA). The Zeta potential of the samples was recorded at different pH’s in water suspensions of 1 mg/L at 25oC and the equipment run 10 cycles per measurement. The pH of samples was adjusted by using 0.3M HCl and 1M NaOH and the same analysis was triplicated.The irradiation system employed for the reactivity study consisted of a flat visible LED source (electric power absorbed: 33.1 W) emitting at wavelengths greater than 425 nm (see figure below). A 400mL beaker was placed containing 250mL suspension mixed at a 1800 rpm by using a magnetic stirrer. The radiation intensity reaching the surface of the suspension was 203 W m2 (measured in the 450-950nm range by using a DeltaOhm 9721 radiometer and the matching probe). Before the runs, each sample was added progressively into 250 mL DI water (every time treating with ultrasounds for 2 min) until reaching a final transmitted radiation intensity from the bottom of the beaker of ca. 10 % with respect to the transmitted light in the presence of only DI water: in this way it was assured that all particles were irradiated. The powder weight required to have this transmittance was the one used for the reactivity tests (Table S1), carried out in water suspensions of 5 mg L1 of 4-nitrophenol (4-NP), at the natural pH of 5.5. At this pH, only molecular 4-NP exists in solution and the amount of 4-nitrophenate can be neglected. This is relevant since the latter one has a strong absorption in the visible radiation and would drastically affect the reactivity under radiation greater 425 nm, such that used in the present study.Oxygen was bubbled into the suspension for 2 hour in the dark before turning the lamp on, after which the adsorbed 4-NP was checked. Then light was then switched on and oxygen was flowed during all the run. Samples were withdrawn periodically from the reactor by using a syringe and filtered using a 0.2 μm PTFE filter. The total irradiation time was 20.5 h. The extent of degradation of 4-NP was measured as a function of the visible absorbance of each sample at a wavelength of 315 nm by using a Thermo Scientific HPLC (Dionex UltiMate 3000 Photodiode Array Detector) with an Acclaim-120 C18 Reversed-phase LC column working at 25°C (eluent: 33% water, 33% acetonitrile, 34% methanol; flow rate: 0.2 mL min1). The reproducibility of the experimental runs was always over 95%. The stability of N-TiO2-G_1% was assessed by recycling the catalyst for 3 runs. After the each photocatalytic reaction, the suspension was centrifuged and filtered to recover the photocatalyst, which was washed with DI water, dried and used in the next cycle of 4-NP oxidation, in the same experimental conditions described above. 3. Results and DiscussionGO was prepared by using a modified Hummer’s method ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1155/2014/276143", "ISSN" : "1687-4110", "abstract" : "Graphene oxide (GO) films with two-dimensional structure were successfully prepared via the modified Hummer method. It is proven that redox method is a promising way to synthesize GO films on a large scale. Comprehensive characterizations of the properties of GO films were conducted. TEM and DFM analyses showed that GO sheets prepared in this study had single and double lamellar layer structure and a thickness of 2~3 nm. X-ray diffraction (XRD) was selected to measure the crystal structure of GO sheet. Fourier-transform infrared spectra analyzer (FT-IR) was used to certify the presence of oxygen-containing functional groups in GO films. The tests of UV-VIS spectrometer and TGA analyzer indicated that GO sheet possessed excellent optical response and outstanding thermal stability. Elemental analyzer (EA) and X-ray photoelectron spectroscope (XPS) analyzed the components synthetic material. Simultaneously, chemical structure of GO sheet was described in this study. Discussion and references for further research on graphene are provided.", "author" : [ { "dropping-particle" : "", "family" : "Song", "given" : "Jianguo", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Wang", "given" : "Xinzhi", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Chang", "given" : "Chang-Tang", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Journal of Nanomaterials", "id" : "ITEM-1", "issued" : { "date-parts" : [ [ "2014" ] ] }, "page" : "1-6", "publisher" : "Hindawi Publishing Corporation", "title" : "Preparation and Characterization of Graphene Oxide", "type" : "article-journal", "volume" : "2014" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "[12]", "plainTextFormattedCitation" : "[12]", "previouslyFormattedCitation" : "[12]" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }[12] and reduced using sodium borohydride. After that, Titanium tetrabutoxide (TBOT) was hydrolyzed in a controlled way, in the presence of evenly dispersed G and a nitrogen source. The obtained powder was annealed at 450 °C under nitrogen flow to limit graphene oxidative thermal degradation. The thermal treatment resulted in the degradation of excess G layers built up over the nanoparticles (Figs. 1, 2 and S2) during the annealing of the composite catalyst, thus ensuring the photocatalytic activity of the materials. EDX mapping on titanium, oxygen and carbon on a representative composite shows the very different Ti signal before and after treatment (Figs. 1 and 2), due to the degradation of G over TiO2 particles during the annealing. Incidentally, it should be pointed out that EDX is not very sensitive for oxygen and, especially, for carbon at the low used loading.XRD allowed to verify the successful oxidation of graphite to graphite oxide first and to graphene oxide (GO) after exfoliation, and also the reduction of graphene oxide (GO) to reduced graphene oxide (Fig. S3). In fact, the characteristic diffraction peaks of graphite, corresponding to (002) and (004) plans, disappear after oxidation, and GO peak at 10.25° disappears after reduction. The diffraction peak at 8.2°, obtained for G, can be ascribed to the establishment of a 1.1 nm interplanar spacing in the stacking of the carbon layers, similarly to what previously observed ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1039/C4RA15705B", "ISSN" : "2046-2069", "abstract" : "Octadecylamine functionalized ultra-thin reduced graphene oxide nanoparticles were synthesized and dispersed in the supramolecular order of discotic liquid crystals for the first time. The insertion and properties of the graphene nanoparticles in the columnar mesophase were studied using field emission scanning electron microscopy, atomic force microscopy, Raman spectroscopy, UV-vis spectroscopy, photoluminescence spectroscopy, polarized optical microscopy, differential scanning calorimetry, X-ray diffraction and DC conductivity. Experimental studies indicate the stacking of two-dimensional graphene nanoparticles in the supramolecular order of the columnar mesophase. The dispersion of graphene nanoparticles improves the order in the columnar phase and thus enhances the conductivity of the system.", "author" : [ { "dropping-particle" : "", "family" : "Kumar", "given" : "Manish", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Kumar", "given" : "Sandeep", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "RSC Adv.", "id" : "ITEM-1", "issue" : "19", "issued" : { "date-parts" : [ [ "2015" ] ] }, "page" : "14871-14878", "publisher" : "The Royal Society of Chemistry", "title" : "Stacking of ultra-thin reduced graphene oxide nanoparticles in supramolecular structures for optoelectronic applications", "type" : "article-journal", "volume" : "5" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "[13]", "plainTextFormattedCitation" : "[13]", "previouslyFormattedCitation" : "[13]" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }[13]. A broad (002) peak can be observed at ca. 29.0° and it can be attributed to the layer-to-layer distance (0.31 nm) ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1016/j.apsusc.2011.05.131", "ISSN" : "01694332", "abstract" : "Graphene and nitrogen doped graphene have been prepared by modified Hummers\u2019 method and the following ammonia heat-treatment process, respectively. The effects of N-doping on the structure of graphene have been systematically investigated by various characterization techniques. SEM, TEM, BET, Raman and XRD analysis were used to distinguish the difference of the microstructures; and FT-IR, XPS, especially XANES were performed to elucidate the bonding information such as C\u2013N. The effect of nitrogen doping on the structure of graphene has been obtained. More defects are present on nitrogen doped graphene as elucidated by BET, XRD, Raman, and XANES characterizations. XANES analysis also indicates that the N-doping decreases the surface oxygen-containing groups.", "author" : [ { "dropping-particle" : "", "family" : "Geng", "given" : "Dongsheng", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Yang", "given" : "Songlan", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Zhang", "given" : "Yong", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Yang", "given" : "Jinli", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Liu", "given" : "Jian", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Li", "given" : "Ruying", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Sham", "given" : "Tsun-Kong", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Sun", "given" : "Xueliang", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Ye", "given" : "Siyu", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Knights", "given" : "Shanna", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Applied Surface Science", "id" : "ITEM-1", "issue" : "21", "issued" : { "date-parts" : [ [ "2011" ] ] }, "page" : "9193-9198", "title" : "Nitrogen doping effects on the structure of graphene", "type" : "article-journal", "volume" : "257" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "[14]", "plainTextFormattedCitation" : "[14]", "previouslyFormattedCitation" : "[14]" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }[14]. The broad diffraction peak at 43° is associated to a turbostratic signal characteristic of disordered carbon nanomaterials ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1039/c5ra21112c", "ISSN" : "2046-2069", "abstract" : "This study demonstrates the electronic applications of graphene synthesized from the graphite electrode of waste dry cell zinc\u2013carbon batteries. Graphite powder [G (R)] is successfully recovered from the graphite electrode of waste batteries by acid treatment and used as starting material for synthesis of graphene oxide (GO) following Hummers method. Finally, reduced graphene oxide (RGO) was obtained from the chemical reduction of GO by hydrazine hydrate. RGO thus obtained was characterized by X-ray diffraction, Raman spectroscopy, Fourier-transform infrared spectroscopy, UV-vis absorption spectroscopy, dynamic light scattering, energy dispersive X-ray spectra and transmission electron microscopy to get detailed information about the structure and morphology of the RGO. All the above characterization results confirmed the restoration of sp2 conjugation and removal of functional groups after the reduction of GO and also the sheet like morphology of RGO. The surface charge and stability of RGO in an aqueous medium are examined by measuring zeta potential. An electrochemical study demonstrated that, at different sweep rates, the current is the highest for RGO and lowest for GO and the current increases with an increasing sweep rate for all materials. The loop area of all the samples at the 100 mV s\u22121 sweep rate is the highest. The galvanostatic charging/discharging measurements have also been performed for both the GO and RGO samples at a current density of 1 mA g\u22121. Electro-conductivity measurement shows that RGO has higher conductivity than GO due to the restoration of the sp2 structure. The current voltage (I\u2013V) characteristics show a non-linear behavior of GO and the ohmic nature of RGO.", "author" : [ { "dropping-particle" : "", "family" : "Roy", "given" : "Indranil", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Sarkar", "given" : "Gunjan", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Mondal", "given" : "Soumya", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Rana", "given" : "Dipak", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Bhattacharyya", "given" : "Amartya", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Saha", "given" : "Nayan Ranjan", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Adhikari", "given" : "Arpita", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Khastgir", "given" : "Dipak", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Chattopadhyay", "given" : "Sanatan", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Chattopadhyay", "given" : "Dipankar", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "RSC Adv.", "id" : "ITEM-1", "issue" : "13", "issued" : { "date-parts" : [ [ "2016" ] ] }, "note" : "XRD result of RGO turbostratic band", "page" : "10557-10564", "publisher" : "The Royal Society of Chemistry", "title" : "Synthesis and characterization of graphene from waste dry cell battery for electronic applications", "type" : "article-journal", "volume" : "6" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "[15]", "plainTextFormattedCitation" : "[15]", "previouslyFormattedCitation" : "[15]" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }[15]. Looking at the XRD diffractograms of N-TiO2 and N-TiO2-G (Fig. S4), we can observe all the characteristic peaks of brookite and rutile (25.4?, 30.8?, 48.1? for brookite; 27.4?, 41.2?, 54.3?, 56.6?, 69.0? for rutile), and exclude the presence of anatase (75.1? is an intense diffraction peak of the crystal plane (215) of anatase and, in this case, it is absent). Graphene peaks are not visible in the XRD of the composite materials, since its percentage is far below XRD detection limit. Using the Scherrer’s equation ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1016/j.jcat.2016.12.010", "ISSN" : "00219517", "abstract" : "N-doped brookite-rutile catalysts were prepared using the sol-gel method with different nitrogen precursors, namely urea (CH4N2O), propionitrile (C3H5N), ammonium hydroxide (NH4OH), ammonium nitrate (NH4NO3) and ethylene diamine (C2H8N2). Testing the photoactivity of the prepared catalysts with 4-nitrophenol revealed that ammonium nitrate was the best doping agent with a nominal N-content of 0.8% (w/w), yielding a 5-fold increase in the pseudo-first order constant of 4-nitrophenol disappearance and a 3-fold increase in the pseudo-first order constant of TOC disappearance, with respect to undoped TiO2, when irradiated with LED visible light (>425nm). In the same experimental conditions, commercial catalysts such as Evonik P25 and mixtures of commercial rutile and brookite failed to work. XRD allowed to identify two crystal phases, i.e. brookite and rutile, and to show that the most active catalyst had the highest brookite and lowest amorphous content, along with the largest rutile crystallites. Through HRTEM, the morphology and crystallinity were further investigated: brookite particles were much smaller and roundish with respect to rutile and the intimate contact between the two phases was also well highlighted. N-doping did not produce oxygen vacancies as shown by Raman spectroscopy; thus, the doping can be considered interstitial rather than substitutional. Surface hydroxylation did not promote oxidation ability, as revealed by TGA-DTA: the most reacting catalyst is the least hydroxylated one. BET revealed that the samples are partially mesoporous (type IV hysteresis), although no template/surfactant was used, and the pore size and volume seemed to affect their activity. UV-vis DRS allowed to extrapolate the band gaps, only slightly narrower for N-doped samples, which, however, showed a pronounced absorption of visible radiation compared to undoped TiO2. Photoluminescence showed that the emission due to electron-hole recombination decreases with the N-loading, eventually reaching a minimum plateau for doping amounts just above the optimal one.", "author" : [ { "dropping-particle" : "", "family" : "Pikuda", "given" : "Oluwadamilola", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Garlisi", "given" : "Corrado", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Scandura", "given" : "Gabriele", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Palmisano", "given" : "Giovanni", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Journal of Catalysis", "id" : "ITEM-1", "issued" : { "date-parts" : [ [ "2017" ] ] }, "page" : "109-116", "title" : "Micro-mesoporous N-doped brookite-rutile TiO2 as efficient catalysts for water remediation under UV-free visible LED radiation", "type" : "article-journal", "volume" : "346" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "[6]", "plainTextFormattedCitation" : "[6]", "previouslyFormattedCitation" : "[6]" }, "properties" : { "noteIndex" : 8 }, "schema" : "" }[6], the rutile crystallites was 22-29 nm in size, well larger than those of brookite crystallites (11-13 nm).Figure 1. EDX mapping of N-TiO2-G_0.5% before annealing: (a) image, (b) carbon mapping, (c) titanium mapping, (d) oxygen mapping.Figure 2. EDX mapping of N-TiO2-G_0.5% after annealing: (a) image, (b) carbon mapping, (c) titanium mapping, (d) oxygen mapping.The Raman spectra of graphite, graphite oxide, GO and G (Fig. S5) highlight that, upon graphite oxidization to graphite oxide, the characteristic sharp bands of the former disappear. The predominant Raman bands in GO and G are the D and G bands, and the ratio between the intensity of D and G bands looks greater for G, compared to GO, thus confirming the successful reduction of the latter one. Moreover, the 2D band is absent because both GO and G are highly defective ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1103/PhysRevB.88.035426", "ISSN" : "1098-0121", "author" : [ { "dropping-particle" : "", "family" : "Eckmann", "given" : "Axel", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Felten", "given" : "Alexandre", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Verzhbitskiy", "given" : "Ivan", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Davey", "given" : "Rebecca", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Casiraghi", "given" : "Cinzia", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Physical Review B", "id" : "ITEM-1", "issue" : "3", "issued" : { "date-parts" : [ [ "2013", "7", "15" ] ] }, "page" : "035426", "publisher" : "American Physical Society", "title" : "Raman study on defective graphene: Effect of the excitation energy, type, and amount of defects", "type" : "article-journal", "volume" : "88" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "[16]", "plainTextFormattedCitation" : "[16]", "previouslyFormattedCitation" : "[16]" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }[16].Raman spectra of the composite materials are shown in Fig. 3, where brookite signals at ca. 150 (A1g), 246 (A1g), 322 (B1g), 365 (B2g), 402 (A1g), 449 (B2g), and 640 (A1g) cm1 can be clearly observed ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1016/j.vibspec.2012.08.003", "ISSN" : "09242031", "abstract" : "The zone-center phonons and the frequency-dependent dielectric function of the brookite phase of TiO2 are studied experimentally by means of polarized Raman and infrared spectroscopy. The Raman- and infrared-active modes are unambiguously identified by symmetry. The mode frequencies are in good agreement with those predicted by the density function calculations of lattice dynamics.", "author" : [ { "dropping-particle" : "", "family" : "Iliev", "given" : "M.N.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Hadjiev", "given" : "V.G.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Litvinchuk", "given" : "A.P.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Vibrational Spectroscopy", "id" : "ITEM-1", "issued" : { "date-parts" : [ [ "2013" ] ] }, "number-of-pages" : "148-152", "title" : "Raman and infrared spectra of brookite (TiO2): Experiment and theory", "type" : "report", "volume" : "64" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "[17]", "plainTextFormattedCitation" : "[17]", "previouslyFormattedCitation" : "[17]" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }[17]. Rutile signals are more difficult to be identified and they correspond to frequencies of 126 (A1g), 440 (Eg) and 611 (B1g) cm1, as also suggested by literature ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1007/s11051-004-1714-3", "ISSN" : "1388-0764", "author" : [ { "dropping-particle" : "", "family" : "Jensen", "given" : "Henrik", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Joensen", "given" : "Karsten D.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "J\ufffdrgensen", "given" : "Jens-Erik", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Pedersen", "given" : "Jan Skov", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "S\ufffdgaard", "given" : "G.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Journal of Nanoparticle Research", "id" : "ITEM-1", "issue" : "5", "issued" : { "date-parts" : [ [ "2004", "10" ] ] }, "page" : "519-526", "publisher" : "Kluwer Academic Publishers", "title" : "Characterization of nanosized partly crystalline photocatalysts", "type" : "article-journal", "volume" : "6" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "[18]", "plainTextFormattedCitation" : "[18]", "previouslyFormattedCitation" : "[18]" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }[18]. However, the exact determination of rutile frequencies is hampered by the high number of brookite bands, which partially overlap with rutile bands. The Raman signals of graphene cannot be seen, due to the small amount used, and to the TiO2 Raman background in the spectra region of the graphene Raman peaks.Figure 3. Raman spectra of TiO2, N-TiO2, N-TiO2-G_2%.After reduction of GO, the FTIR highlighted the disappearance of the band centered at 1700 cm?1, assigned to C=O stretching, pointing to the effective reduction of GO to G (Fig. S6). The difference between N-TiO2 and N-TiO2-G_1% is not obvious. The C=C band centered at 1600 cm–1 partially overlaps with –OH signals making it very difficult to appreciate any difference.XPS surveys of GO and G also support the effective reduction of the former to the latter. As shown in high resolution carbon spectra for GO and G (Fig. 4), the oxidized carbon peaks (C–O at 286.1 eV and C=O at 288.1 eV) decreased from 36 to 21.8% and from 5.4 to 4.5%, respectively, while sp2 carbon peak (C–C at 284.8 eV) increased from 17.9 to 28.7% after the reduction. In Fig. 4c-d, high resolution (HR) carbon spectra for N-TiO2 with 0.1% and 1% G loadings are shown. Due to the very small G content in N-TiO2 and the possible contamination of adventitious carbon from the atmosphere, it is difficult to conclusively attribute the sp2 carbon signal to G. However, small increase of carbon content is observed for the higher G loading sample, possibly originated from increased G in N-TiO2.TEM characterization of graphite oxide, GO and G are shown in Supporting information (Fig. S7), where the 2-dimensional morphology typical of these materials with thinner and smaller particle sizes moving from graphite oxide down to G is evident. TEM of N-TiO2 samples modified with G (Fig. 5 and Fig. S8) highlighted the presence of bigger particles of rutile (20-100 nm) with respect to brookite (5-20 nm), as suggested by XRD as well.Figure 4. XPS high resolution carbon spectra for (a) GO, (b) G, (c) N-TiO2-G_0.1% and (d) N-TiO2-G_1%.Figure 5. TEM images of N-TiO2-G_1%. (a) Nanoparticles of rutile and brookite (contact highlighted) with the corresponding EDP in inset; (b) crystal fringes of rutile (110) plane; (c,d) G edge highlighted in an agglomerate of particles grown on it (c,d were shown as preview in the review article ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1016/j.jphotochemrev.2017.06.003", "ISSN" : "13895567", "abstract" : "\u00a9 2017 Elsevier B.V. The remarkable physicochemical properties of graphene (GR) and derivatives can be leveraged in the photocatalytic activity of GR-semiconductor photocatalysts. The hitherto state of knowledge on the role of GR in these composite materials is insufficient and leaves many questions unanswered, thus it is imperative to fully understand the interaction mechanisms between GR and inorganic semiconductors. Detailed study and optimization of the features related to the interface are still very much sought to efficiently design photocatalysts targeting their eventual commercialization. This review shows that photocatalytic activity of such composites depends not only on high GR electron mobility and charge transfer, but also on the properties of the interface (such as interface morphology, size, crystal phases and facet, dimensionality of composites, etc.). Focusing on the last advancements in this field, this review analyses the challenges involved in the synthetic strategies of GR-semiconductor photo(electro)catalysts in various applications including pollutant degradation, organic synthesis, hydrogen evolution and photoreduction of carbon dioxide (CO 2 ). Mechanism of interaction between GR and semiconductors are thoroughly discussed by examining the proposed mechanism in the diverse areas where the composite materials are employed in photo(electro)catalytic processes. In addition, various synthetic and characterization technique available hitherto are presented, since they are pivotal to the understanding of the composites properties (such as morphology, crystal phases and exposed facets, degree of crystallinity, dimensionality etc.), and even to shed more light on interaction mechanisms of the photocatalyst constituents. As a future outlook, it is envisaged that research will not only focus on optimizing GR electrical and chemical properties, yet in the synthesis of GR-semiconductor photocatalysts attention needs also be placed on the properties of the resulting composites, using suitable synthetic methods and proper characterizations to assess their performance.", "author" : [ { "dropping-particle" : "", "family" : "Ozer", "given" : "L.Y.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Garlisi", "given" : "C.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Oladipo", "given" : "H.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Pagliaro", "given" : "M.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Sharief", "given" : "S.A.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Yusuf", "given" : "A.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Almheiri", "given" : "S.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Palmisano", "given" : "G.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Journal of Photochemistry and Photobiology C: Photochemistry Reviews", "id" : "ITEM-1", "issued" : { "date-parts" : [ [ "2017" ] ] }, "title" : "Inorganic semiconductors-graphene composites in photo(electro)catalysis: Synthetic strategies, interaction mechanisms and applications", "type" : "article-journal" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "[19]", "plainTextFormattedCitation" : "[19]", "previouslyFormattedCitation" : "[19]" }, "properties" : { "noteIndex" : 11 }, "schema" : "" }[19]).The significantly different size and shape of brookite and rutile was recently validated by the analysis of electron diffraction patterns (EDP) and d-spacing (through HRTEM) of analogous brookite-rutile samples prepared without G ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1016/j.jcat.2016.12.010", "ISSN" : "00219517", "abstract" : "N-doped brookite-rutile catalysts were prepared using the sol-gel method with different nitrogen precursors, namely urea (CH4N2O), propionitrile (C3H5N), ammonium hydroxide (NH4OH), ammonium nitrate (NH4NO3) and ethylene diamine (C2H8N2). Testing the photoactivity of the prepared catalysts with 4-nitrophenol revealed that ammonium nitrate was the best doping agent with a nominal N-content of 0.8% (w/w), yielding a 5-fold increase in the pseudo-first order constant of 4-nitrophenol disappearance and a 3-fold increase in the pseudo-first order constant of TOC disappearance, with respect to undoped TiO2, when irradiated with LED visible light (>425nm). In the same experimental conditions, commercial catalysts such as Evonik P25 and mixtures of commercial rutile and brookite failed to work. XRD allowed to identify two crystal phases, i.e. brookite and rutile, and to show that the most active catalyst had the highest brookite and lowest amorphous content, along with the largest rutile crystallites. Through HRTEM, the morphology and crystallinity were further investigated: brookite particles were much smaller and roundish with respect to rutile and the intimate contact between the two phases was also well highlighted. N-doping did not produce oxygen vacancies as shown by Raman spectroscopy; thus, the doping can be considered interstitial rather than substitutional. Surface hydroxylation did not promote oxidation ability, as revealed by TGA-DTA: the most reacting catalyst is the least hydroxylated one. BET revealed that the samples are partially mesoporous (type IV hysteresis), although no template/surfactant was used, and the pore size and volume seemed to affect their activity. UV-vis DRS allowed to extrapolate the band gaps, only slightly narrower for N-doped samples, which, however, showed a pronounced absorption of visible radiation compared to undoped TiO2. Photoluminescence showed that the emission due to electron-hole recombination decreases with the N-loading, eventually reaching a minimum plateau for doping amounts just above the optimal one.", "author" : [ { "dropping-particle" : "", "family" : "Pikuda", "given" : "Oluwadamilola", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Garlisi", "given" : "Corrado", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Scandura", "given" : "Gabriele", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Palmisano", "given" : "Giovanni", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Journal of Catalysis", "id" : "ITEM-1", "issued" : { "date-parts" : [ [ "2017" ] ] }, "page" : "109-116", "title" : "Micro-mesoporous N-doped brookite-rutile TiO2 as efficient catalysts for water remediation under UV-free visible LED radiation", "type" : "article-journal", "volume" : "346" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "[6]", "plainTextFormattedCitation" : "[6]", "previouslyFormattedCitation" : "[6]" }, "properties" : { "noteIndex" : 11 }, "schema" : "" }[6].Fig. 5 shows that brookite and rutile nanoparticles grow on G sheets with dimensions ranging from ca. 100 to 1000 nm, and the good contact among rutile, brookite and G is remarkable. This is obviously propitious for an efficient charge transfer and stable separation of electrons and holes, as further demonstrated by photoluminescence spectra shown in the following. In samples containing 1% G, it was possible to detect G in few points, although its high transparency to the electron beam, along with the masking effects of nanoparticles on it hampered a perfect visualization (Fig. 5c-d). The EDP of an area where brookite, rutile and G were present (inset of Fig. 5a) resulted in bright circles typical of G, along with distinct spots determined by crystalline particles.A hysteresis of type IV can be seen in the BET isotherms of all the samples (Figs. S9-14 in Supporting information), pointing to a capillary condensation in mesopores. The pore sizes are in the mesoporosity region, roughly ranging from 2.9 to 30 nm and pore size distributions are available as Supporting information. The specific surface area is smaller for the sample with 1% G, with wider pores compared to the other samples, as shown in Table 1.Table 1. Textural properties of the catalysts under study.BET SSA(m2 g1)PHW (DF model) (nm)PR (BJH model) (nm)TPV (cc g1)TiO241.94.668.290.144N-TiO256.83.737.530.178N-TiO2-G_0.1%56.0 3.737.600.161N-TiO2-G_0.5%56.8 3.907.620.159N-TiO2-G_1%50.1 4.679.440.179N-TiO2-G_2%59.13.908.360.173SSA,specific surface area; PHW, pore half width; PR, pore radius; TPV, total pore volume.In UV-vis DRS (Fig. S15) we can notice a slightly better ability to absorb visible radiation with respect to N-TiO2, although the calculated band gap of N-TiO2 (2.95 eV) for indirect semiconductors is very close to that of N-TiO2-G_2% (2.93 eV), similarly to what previously reported for modified TiO2 ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1002/adfm.200701478", "ISSN" : "1616301X", "author" : [ { "dropping-particle" : "", "family" : "Zhang", "given" : "Li-Wu", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Fu", "given" : "Hong-Bo", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Zhu", "given" : "Yong-Fa", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Advanced Functional Materials", "id" : "ITEM-1", "issue" : "15", "issued" : { "date-parts" : [ [ "2008", "8", "11" ] ] }, "page" : "2180-2189", "publisher" : "WILEY\u2010VCH Verlag", "title" : "Efficient TiO 2 Photocatalysts from Surface Hybridization of TiO 2 Particles with Graphite-like Carbon", "type" : "article-journal", "volume" : "18" }, "uris" : [ "" ] }, { "id" : "ITEM-2", "itemData" : { "DOI" : "10.1016/j.cattod.2016.03.040", "ISSN" : "09205861", "abstract" : "Different amounts of graphene oxide were chemically reduced with hydrazine in the presence of nanometric TiO2 and SiO2. The photocatalytic performance of the resulting hybrid materials was compared with pristine supports using phenol and methylene blue (MB) under two different irradiation conditions (UV\u2013vis and Vis only light). MB is strongly adsorbed on the hybrid materials. Significant MB degradation rates were observed on pristine TiO2 and hybrid TiO2-reduced graphene oxide (rGO) material under both irradiation conditions. In the presence of the hybrid catalyst, the degradation of MB under Vis is due to the dye-sensitized mechanism, while under UV\u2013vis there is an additional semiconductor-based photocatalytic mechanism. Conversely, the presence of rGO reduces the rate of photocatalytic transformation for the poorly adsorbed phenol under UV irradiation, and a negligible degradation rate was observed under Vis. The UV\u2013vis absorption spectra of aqueous suspensions of hybrid materials with different rGO loading indicate a strong interaction of the two materials and a reduction of the light absorption due to the presence of rGO. Among many mechanisms reported on the role of rGO, it is inferred that the working mechanism involves electron transfer from photoexcited states of rGO onto the titania, and holes migration from titania to rGO, where adsorbed substrates are oxidized. This oxidation is possible only if the substrate HOMO has higher energy (less positive standard redox potential) than the empty states of excited rGO, supposedly for MB and not for phenol. Then, reduced graphene is advantageous when substrates are adsorbed and when the charge separation is possible (coupled with a proper semiconductor like TiO2). Alone, or coupled with low work function oxides like SiO2, rGO could be ineffective.", "author" : [ { "dropping-particle" : "", "family" : "Minella", "given" : "M.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Sordello", "given" : "F.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Minero", "given" : "C.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Catalysis Today", "id" : "ITEM-2", "issued" : { "date-parts" : [ [ "2017", "4" ] ] }, "page" : "29-37", "title" : "Photocatalytic process in TiO2/graphene hybrid materials. Evidence of charge separation by electron transfer from reduced graphene oxide to TiO2", "type" : "article-journal", "volume" : "281" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "[20,21]", "plainTextFormattedCitation" : "[20,21]", "previouslyFormattedCitation" : "[20,21]" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }[20,21].The photoluminescence (PL) emission upon excitation at 300 nm (Fig. 6) shades light on the strong improvement of charge separation on the composite materials compared to TiO2 and N-TiO2. G surface has a positive Mulliken charge, resulting in an opposite interface dipole at the border with TiO2 nanoparticles: consequently a strong tendency exists for electron transfer to G, resulting in an attenuation of charge recombination ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "ISBN" : "9781466591271", "abstract" : "\"Graphene is the strongest material ever studied and can be an efficient substitute for silicon. There is no other major reference work of this scope on the topic of graphene, which is one of the most researched materials of the twenty-first century. The set includes contributions from top researchers in the field and a foreword written by two Nobel laureates in physics. This volume in the set focuses on fabrication methods\"-- Key points for transferring graphene grown by chemical vapor deposition / Elisabet Prats-Alfonso, Philippe Godignon, Rosa Villa, and Gemma Gabriel -- Fabrication considerations for graphene devices / G. Burwell -- Synthesis methods for graphene / Kal Renganathan Sharma -- Synthesis and application of graphene nanoribbons (GNRS) / Emma Aryee -- Preparation of electrically conductive graphene-based aerogels to modify the supercapacitor electrode surface / Pietro Russo -- Synthesis strategies for the graphene / Rajesh Kumar -- Atomic-scale exfoliation and adhesion of nano-carbon / Kouji Miura -- Fabrication and applications of biocompatible graphene oxide and graphene / Xuejun Pan -- Fabrication methods of graphene nanoribbons / Shazed Aziz -- Functionalized graphene: synthesis and its applications in electrochemistry / Mohmmad Reza Ganjali -- Electrophoretic deposition of graphene-based materials and their energy-related applications / Aldo R. Boccaccini -- Preparation of graphene by solvent dispersion methods and its functionalization through non-covalent and covalent approaches / Wesley R. 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Zhang -- Wafer-scale chemical vapor deposition of high quality graphene on evaporated cu film / Li Tao and Deji Akinwande -- Novel graphene sensors for chemical and biological applications / Joonwon Bae -- New methods in aqueous graphene (graphene oxide) synthesis for biosensing / Nowak Christoph -- Graphene chemiresistors as pH sensors: fabrication and characterization / Jie Xu -- Wet chemical fabrication of graphene and graphene oxide and spectroscopic characterization / Amanda Ellis -- Mechanical cleavage of graphite to graphene via graphite intercalation compounds / Shioyama Hiroshi -- Synthesis of graphene by pyrolisis of organic matter / Boris Kharisov -- Graphene nanoribbons synthesis by gamma irradia\u2026", "author" : [ { "dropping-particle" : "", "family" : "Aliofkhazraei", "given" : "Mahmood", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Ali", "given" : "Nasar", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Milne", "given" : "W. I. (William I.)", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Ozkan", "given" : "Cengiz S.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Mitura", "given" : "Stanislaw", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Gervasoni", "given" : "Juana L.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "id" : "ITEM-1", "issued" : { "date-parts" : [ [ "0" ] ] }, "title" : "Graphene science handbook. Fabrication methods", "type" : "book" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "[22]", "plainTextFormattedCitation" : "[22]", "previouslyFormattedCitation" : "[22]" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }[22]. Accordingly, a lower photoluminescence emission can be observed in the whole emission spectrum (380-585 nm) for N-TiO2-G_1% compared to N-TiO2 and TiO2. From the spectra in Fig. 6 one can notice that the shape of the emissions are similar and the main emission is centered at ca. 420 nm. The band-to-band transition occurs around this wavelength corresponding to the band gap energy. Emission at greater wavelengths is produced by electron trapping in TiO2 oxygen vacancies and transitions from intra-band gap states to valence band. However, the intensity of the peak in the presence of G is lower compared to N-TiO2, highlighting the ability of G to attract electrons on an energy level close to the conduction band of brookite and rutile and to convey them away from TiO2, thus limiting their recombination with holes. This is possible given the work function of reduced graphene oxide (4.5±0.2 eV) ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1021/acs.jpcc.5b09234", "abstract" : "Graphene oxide (GO) has shown great potential as a component in various devices due to its excellent solution processability and two-dimensional structure. However, the oxygenated form of graphene has a moderate charge-transport capability. The latter parameter may be enhanced through controlled deoxygenation of GO with subsequent tuning of its work function (WF). Various reduction approaches were employed to investigate the effect of the oxygen content on the work function of GO derivatives as thin films on an indium tin oxide substrate. Such films were reduced by stepwise thermal annealing in ultrahigh vacuum up to 650 \u00b0C, by chemical reduction with hydrazine, or by a combination of chemical and thermal reduction processes. The effect of the GO film thickness and the flake size on the WF was also investigated. UV photoelectron spectroscopy and X-ray photoelectron spectroscopy were used to correlate the WF of GO derivatives with their oxygen content. 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Photoluminescence emission spectra of N-TiO2-G at different G loadings. Excitation wavelength: 300 nm. The point of zero charge of G in water is ranging from 3 to 5 due to the presence of carboxylic acid groups and phenolic hydroxyl groups in G ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1021/cr4007347", "ISSN" : "0009-2665", "author" : [ { "dropping-particle" : "", "family" : "Navalon", "given" : "Sergio", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Dhakshinamoorthy", "given" : "Amarajothi", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Alvaro", "given" : "Mercedes", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Garcia", "given" : "Hermenegildo", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Chemical Reviews", "id" : "ITEM-1", "issue" : "12", "issued" : { "date-parts" : [ [ "2014", "6", "25" ] ] }, "page" : "6179-6212", "publisher" : "American Chemical Society", "title" : "Carbocatalysis by Graphene-Based Materials", "type" : "article-journal", "volume" : "114" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "[24]", "plainTextFormattedCitation" : "[24]", "previouslyFormattedCitation" : "[24]" }, "properties" : { "noteIndex" : 13 }, "schema" : "" }[24]. On the other hand, the point of zero charge of TiO2 can be drastically different depending on the surface characteristics of the semiconductor. In the present case, the Zeta-potential analysis performed on the composite materials indicated that Nitrogen doping and G loading had a minor effect on the point of zero charges, which ranged between 4.7 and 5.3 without any trend (Supporting information, Fig. S15 and Table S2). The minor change by doping with nitrogen can be ascribed to the electronegativities of nitrogen and oxygen in TiO2, which are not very different. On the other hand, G is not affecting the point of zero charge of photocatalysts most probably because it is located below the semiconductor nanoparticles, with a negligible interface with water molecules.In terms of activity in the removal of 4-NP from water, all the composite catalysts showed a negligible ability to adsorb the organic species without any applied radiation. After switching the light on, the photocatalytic activity of brookite-rutile sample enhanced drastically via N-doping, whereas the contemporary N-doping and loading of 1% G, gave a further increase in the pseudo-first order kinetic constant (k) of 4-NP disappearance via photooxidation under UV-free visible radiation (Fig. 7) evaluated through an exponential best fitting procedure of the following equations:-r4-NP=-VXdC4-NPdt=kC4-NP(Eq. 1)where –r4-NP is the disappearance rate of 4-NP, V is the reactor volume, t is the irradiation time and X is either the mass (Fig. 7a) or the surface area (Fig. 7b) of the catalyst in the reactor. The improvement of G over N-TiO2, in terms of kinetic constant, was 40% and 60% on mass and surface basis, as showed in Fig. 7a and 7b, respectively. Notably, the optimum catalyst has been filtered, washed and dried after reaction and reusing it for three times did not result in any noticeable deactivation (Fig. 7b). 4-NP conversion achieved with the optimum catalyst was 45 % after 22.5 h irradiation (Fig. 7a). Table 1 reveals that the best performing catalyst, N-TiO2-G_1%, is also the one with the widest pores and the highest total pore volume. Then, porosity is relevant in affecting reactivity thanks to a favorable adsorption on catalyst surface, which is more accessible in porous particles. Specific surface area is instead the lowest for the same sample.Although the investigation of reaction mechanism was not the aim of the present study, extensive past research has shown that, in the course of 4-NP photocatalytic oxidation, NO2 group can be replaced by a hydroxyl group forming hydroquinone or alternatively an hydroxyl radical can enter the ortho-position activated by the phenolic group in 4-NP, producing 3,4-dihydroxynitrobenzene. Ring opening and conversion to CO2 are further oxidation steps, although the parallel conversion to CO2 of the starting substrate, without any formation of organic intermediates is also viable, as shown in a number of studies [ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1016/0043-1354(95)00240-5", "author" : [ { "dropping-particle" : "", "family" : "Dieckmann", "given" : "M. 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ConclusionThe reported study showed that the contemporary i) N-doping and ii) growth of brookite-rutile samples on reduced graphene oxide yield a high efficiency in the oxidation of a model pollutant (4-NP) from water through photodegradation under UV-free visible radiation. The effect of reduced graphene oxide in the composite material is to: (i) improve the charge separation and (ii) extend the light absorption threshold. Likely, ?-conjugation of reduced graphene oxide can play a major positive role.Figure 7. Pseudo-first order constants of 4-NP conversion by using N-TiO2-G at different G loadings (a) on weight basis; (b) on surface area basis. The conversion percentage of catalyst after 2 h is reported above the error bar of each catalysts (a).AcknowledgementsFlorent Ravaux and Thomas Delclos are gratefully acknowledged for their assistance at the TEM, Raman, XRD and FTIR. Abu Dhabi Education Council (ADEC Award for Research Excellence 2015 A2RE 2015, project code EX2016-000005) is gratefully acknowledged for funding. CC and YS acknowledge the Engineering and Physical Sciences Research Council (EPSRC) in the framework of the project Graphene-Based Membranes (EP/K016946/1).ReferencesADDIN Mendeley Bibliography CSL_BIBLIOGRAPHY [1]G. Palmisano, M. Addamo, V. Augugliaro, T. Caronna, A. Di Paola, E. Garcia López, V. Loddo, G. Marcì, L. Palmisano, M. Schiavello, Selectivity of hydroxyl radical in the partial oxidation of aromatic compounds in heterogeneous photocatalysis, Catal. Today. 122 (2007) 118–127. doi:10.1016/j.cattod.2007.01.026.[2]L. Clarizia, I. Di Somma, R. Marotta, P. Minutolo, R. Villamaina, R. Andreozzi, Photocatalytic reforming of formic acid for hydrogen production in aqueous solutions containing cupric ions and TiO2 suspended nanoparticles under UV-simulated solar radiation, Appl. 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