Photodegradation of Graphene Oxide Sheets by TiO2 Nanoparticles after a Photocatalytic Reduction

Photodegradation of Graphene Oxide Sheets by TiO2Nanoparticles after a Photocatalytic ReductionO.Akhavan,*,†,‡M.Abdolahad,§A.Esfandiar,‡and M.Mohatashamifar|Department of Physics,P.O.Box11155-9161,Sharif Uni V ersity of Technology,Tehran,Iran,Institute forNanoscience and Nanotechnology,P.O.Box14588-89694,Sharif Uni V ersity of Technology,Tehran,Iran,Nano-Electronics and Thin Film Laboratory,Department of Electrical and Computer Engineering,North KargarA V enue,P.O.Box14395/515,Uni V ersity of Tehran,Tehran,Iran,and Electronic Research Center,Tehran,IranRecei V ed:April18,2010;Re V ised Manuscript Recei V ed:June6,2010TiO2nanoparticles were physically attached to chemically synthesized single-layer graphene oxide nanosheetsdeposited between Au electrodes in order to investigate the electrical,chemical,and structural properties ofthe TiO2/graphene oxide composition exposed to UV irradiation.X-ray photoelectron spectroscopy showedthat after effective photocatalytic reduction of the graphene oxide sheets by the TiO2nanoparticles in ethanol,the carbon content of the reduced graphene oxides gradually decreased by increasing the irradiation time,while no considerable variation was detected in the reduction level of the reduced sheets.Raman spectroscopyindicated that,atfirst,the photocatalytic reduction resulted in a significant increase in the graphitized sp2structure over the disorders in the graphene oxides.After that,as the carbon content decreased by UV irradiation,further disorders appeared in the reduced graphene oxide sheets,confirming degradation of the reduced sheetsafter the photocatalytic reduction.Based on the current-voltage characteristic,the optimum time for thephotocatalytic reduction resulted in a sharp decrease in the electrical resistivity of the reduced graphene oxide.However,longer photocatalytic processes caused a high increase in the resistivity,due to dominating thephotodegradation process over the nearly completed photocatalytic reduction.1.IntroductionIn the universe of carbon nanostructures,graphene as a one-atom-thick sheet made up of sp2-bonded carbons in a hexagonal lattice with unique physical and chemical characteristics has attracted special attention.In fact,after empirical discovery of graphene in2004by Novoselov et al.,1it has rapidly come into view as an extraordinary nanomaterial with promising capability in some research and technologicalfields in material science and condensed-matter physics.2-14Among the various fabrication procedures of graphene,the chemical exfoliation method has been extensively used as an effective,reliable,and low-cost method.15,16In this method,the chemically oxidized graphite is cleaved by ultrasonic dispersion or rapid thermal expansion,and the synthesized graphene oxide nanosheets should be reduced to obtain graphene nanosheets. After reduction of graphene oxides,high electrical conduc-tance and transparency can be achieved,so that reduced graphene oxides are known as promising candidates for large-area transparent conductors.17However,due to the toxicity of chemical reductants such as hydrazine and high temperatures (>500°C)required in the thermal reductions,the usual chemical and thermal reductions are not completely compatible with the current electronic and chemistry technologies,and so,their extensive applications have been limited.Furthermore,it was shown that in the Au-doped graphene sheets,18the Au nano-structures act as antireduction resist during a chemical reduc-tion.19Hence,in addition to improving the current reducing processes,other effective reducing methods,including micro-wave and(photo)catalytic reductions,are considered as some of the important research subjects for production of high quality reduced graphene oxide nanosheets at lower and lower tem-peratures.For example,an improved thermal reduction method by which an effective reduction of graphene oxide sheets occurred at temperatures lower than500°C was studied previously.20Microwave reduction of graphene oxide sheets was also investigated by Chen et al.21Concerning the(photo)catalytic reduction processes,Xu et al.22showed that Au,Pt,and Pd metallic nanoparticles adsorbed on graphene oxide sheets can reduce the graphene oxide sheets with ethylene glycol in a catalytic process.In other works,photocatalytic reduction of graphene oxide sheets by TiO2nanoparticles and thinfilms in ethanol solution was investigated.23,24However,it is well-known that metal oxide semiconductors,such as TiO2,can easily decompose carbonaceous bonds in a photocatalytic process.25,26 Therefore,photocatalytic reduction of graphene oxides by using metal oxide semiconductor photocatalysts and its effects on the quality of the reduced graphene sheets should be examined in more detail.In this research,based on the procedure reported by Williams et al.23(only applied in the photocatalytic reduction of graphene oxide),gradual photodegradation of chemically synthesized graphene oxide sheets physically combined with TiO2nano-particles was systematically investigated,particularly after completion of the photocatalytic reduction of the oxide sheets. In this regard,the effect of UV irradiation and its duration on the oxygen-containing carbonaceous bonds,carbon contents including disorder and the graphitized carbons,and the current-voltage characteristic of the TiO2/graphene oxide sheets were examined for different periods of irradiation time.*Corresponding author.Tel.:+98-21-66164566.Fax:+98-21-66022711.E-mail address:oakhavan@.†Department of Physics,Sharif University of Technology.‡Institute for Nanoscience and Nanotechnology,Sharif University ofTechnology.§University of Tehran.|Electronic Research Center.J.Phys.Chem.C2010,114,12955–129591295510.1021/jp103472c 2010American Chemical SocietyPublished on Web07/14/20102.Experimental SectionThe improved Hummers method15,27was applied to oxidizenatural graphite powder(particle diameter of45µm,Sigma-Aldrich)in order to synthesize the graphite oxide.In a typicalprocedure,50mL of H2SO4was added into a500mLflaskincluding2g of graphite at room temperature.Theflask wascooled to0°C in an ice bath.Then6g of potassiumpermanganate(KMnO4)was added slowly to the above mixtureand allowed to warm to room temperature.The suspension wasstirred continuously for2h at35°C.Then,it was cooled in anice bath and subsequently diluted by350mL of deionized(DI)water.Then H2O2(30%)was added to reduce the residualpermanganate to soluble manganese ions,that is,until stoppingthe gas evolution.Finally,the resulting suspension wasfiltered,washed with1M HCl and twice with DI water,and dried at60°C for24h to obtain brownish graphite oxide powders.The obtained graphite oxide powder was dispersed in water(1mg/mL)to obtain a suspension.Then the suspension was sonicatedfor30min to obtain a graphene oxide suspension.The graphene oxide samples were prepared by drop-castingthe graphene oxide suspension onto two chemically patternedAu electrodes(with∼1.5µm space between the electrodes) deposited on SiO2/Si substrates by using e-beam evaporation. After drying in air,the samples were annealed in air at200°C for30min.Then,the annealed samples were immersed in a prepared TiO2suspension(40mg/mL commercial TiO2nano-particles with particle diameter of15nm and surface area of 240(50m2g-1(Hurricane Co.,Iran)in DI water),and the suspension was sonicated for30min to physically hybridize the TiO2nanoparticles on the surface of the deposited graphene oxide sheets.Then,the TiO2/graphene oxide samples were annealed at200°C for30min.To study the photocatalytic effect of the TiO2nanoparticles on the graphene oxide sheets,atfirst,the TiO2/graphene oxide samples were dispersed in ethanol(C2H5OH,Merck,>99.9%)). Then,the samples were irradiated by a110mW/cm2mercury lamp(peak wavelengths at275,350,and660nm)for different periods of time at room temperature.Surface morphology of the TiO2/graphene oxide sheets deposited on the Au electrodes(especially those that were deposited on the hollow space between the two electrodes)was studied by using a Philips XL30scanning electron microscopy (SEM).Atomic force microscopy(AFM)images were obtained by using a Park Scientific model CP-Research(VEECO).The substrates used for AFM imaging were freshly cleaved mica substrates.X-ray photoelectron spectroscopy(XPS)was em-ployed to study the relative concentration of carbon and the chemical states of the TiO2/graphene oxide sheets irradiated at different irradiation times.The data were obtained using a hemispherical analyzer with an Al K R X-ray source(hν) 1486.6eV)operating at a vacuum better than10-7Pa.In the XPS data analysis,peak deconvolution was performed using Gaussian components after a Shirley background subtraction. Raman spectroscopy was performed at room temperature using a Raman Microprobe(HR-800Jobin-Yvon)with532nm Nd: YAG excitation source.Current-voltage curves of the TiO2/ graphene oxide sheets exposed to UV irradiation for the different times were obtained by using a Keithley485Autoranging Picoammeter.3.Results and DiscussionFigure1shows surface morphology of the TiO2/graphene oxide sheets deposited on the Au electrodes,especially those located on the hollow space between the two electrodes,as connecting bridges of the electrodes.The width of the electrodes was250µm,and the average distance between them was about 1.5µm.After deposition of the TiO2/graphene oxide sheets, we observed41sheets connecting the two electrodes.Here,the SEM image shows only two graphene oxide sheets connecting the electrodes.The dimension of the majority of the graphene oxide sheets was found to be a few micrometers,as can be seen for some of them in Figure1.The light spots on the graphene oxide sheets can be assigned to the TiO2nanoparticles attached on the surface.To better observe and characterize the topography of the TiO2/ graphene oxide sheets,AFM was used as an appropriate technique,as shown in Figure2.The AFM image(Figure2a) shows two partially overlapped platelets nearly covered by nanoparticles with average size of18nm.The diameter histogram of the surface particles was presented in Figure2b. Although,based on the average size,the surface particles can be attributed to the TiO2nanoparticles,a fraction of them can be also assigned to the residual carbons and/or solvents attached to defect sites of the graphene oxide sheets.The height profile diagram of the AFM image(Figure2c)containing the sharp peaks(with the height of∼10-20nm)confirmed attachment of the TiO2nanoparticles(with the average size of15nm)on the surface of the graphene sheets.The height profile also showed that the thickness of the graphene oxide sheets was∼0.9 nm,which is in good consistency with the typical thickness of the single-layer graphene oxides(∼0.8nm).16In fact,the typical thickness of graphene oxide shows a∼0.44nm increase in graphene thickness(∼0.36nm)because of the presence of epoxy and hydroxyl groups on both sides of the oxide surface.16,28 To study the effect of UV irradiation and its duration on the chemical state of the TiO2/graphene oxide sheets,XPS was utilized.The deconvoluted C(1s)XPS spectra of the TiO2/ graphene(oxide)samples have been shown in Figure3.The deconvoluted peak centered at the binding energy ranging from 284.8to285.0eV was assigned to the C s C,C d C,and C s H bonds.The deconvoluted peaks centered at the binding energy ranges of286.0-286.5,287.4-287.7,and289.0-289.5eV were attributed to the C s OH,C d O,and O d C s OH oxygen-containing carbonaceous bands,respectively.29-31Before the light irradiation(Figure3a),high amounts of the oxygen-containing carbonaceous bands were detected in the carbon peak, consistent with the presence of the graphene oxides on the Figure1.SEM image of the TiO2/graphene oxide sheets on Au electrodes deposited on SiO2/Si substrate.12956J.Phys.Chem.C,Vol.114,No.30,2010Akhavan etal.surface of the as-prepared samples.No peaks relating to formation of Ti-C and or Ti-O-C bonds were found in the XPS spectra,indicating physical(not chemical)attachment of the TiO2nanoparticles to the graphene oxide sheets.To quantitatively investigate and compare the change in concentra-tion of the oxygen-containing carbonaceous bands,the peak area ratios of the C s OH,C d O,and O d C s OH bonds to the C s C,C d C,and C s H bonds were calculated and presented in Table1.By exposing the TiO2/graphene oxides immersed in ethanol to the UV light irradiation for1h(Figure3b),the concentration of the oxygen-containing bonds substantially decreased,indicat-ing photocatalytic reduction of the graphene oxide sheets in the TiO2/graphene oxide composition.After2h irradiation(Figure 3c),the relative concentration of the C s OH,C d O,and O d C s OH bonds showed about76,85,and81%reduction relative to the corresponding concentrations of the sample before irradia-tion,respectively.The remarkable decrease in the concentration of the oxygen-containing bonds of the graphene oxides after 2h UV irradiation indicated their effective photocatalytic reduction in the TiO2/graphene(oxide)composition.By increas-ing the time of the UV exposure to4,10,and24h,no considerable change in the concentration of the oxygen-containing bonds was observed.But,based on the decrease in the peak area ratio of the C(1s)to Au(4f)core levels(A C/A Au), it was found that the amount of carbon on surface of the samples also decreased during the photocatalytic reduction,particularly after10and24h irradiation.In fact,after24h irradiation,85% of the surface carbons of the TiO2/graphene(oxide)composition disappeared,while no considerable change in their chemical states was found.This means that the TiO2nanoparticles in the TiO2/graphene(oxide)composition gradually degraded the graphene oxide sheets after reducing them in a photocatalytic process.To further examine the degradation of the reduced graphene oxide sheets of the TiO2/graphene(oxide)composition in the photocatalytic process,Raman spectroscopy was utilized,as shown in Figure4.In fact,Raman spectroscopy is known as a suitable technique to study the ordered/disordered crystal structures of carbonaceous materials.The usual characteristics of carbon materials in Raman spectra are the G band(∼1580 cm-1),which is usually attributed to the E2g phonon of C sp2 atoms,and the D band(∼1350cm-1)as a breathing mode of κ-point phonons of A1g symmetry,32,33which is attributed to local defects and disorders,particularly located at the edges of graphene and graphite platelets.34Concerning this,a smaller I D/I G peak intensity ratio in a Raman spectrum can be assigned to lower defects and disorders of the graphitized structures, smaller fraction of sp3/sp2-bonded carbon,and/or larger size of the in-plane graphitic crystallite sp2domains.The Raman spectra shown in Figure4display the G line at about1585cm-1and the D line at1350cm-1.The obtained values of the I D/I G ratio were also presented in Table1.It was found that the I D/I G ratio decreased from1.26to0.81after2h UV irradiation.This canFigure2.(a)AFM image of the TiO2/graphene oxide sheets on a mica substrate,(b)diameter histogram of the surface particles,and(c)height profile diagram of the line shown in the AFM image.The sharp peaks in the depth profile correspond to the TiO2nanoparticles,and the vertical distance of each couple of the markers is given above them.Figure3.Peak deconvolution of C(1s)XPS core level of the TiO2/ graphene(oxide)sheets after(a)0,(b)1,(c)2,(d)4,(e)10,and(f) 24h UV irradiation time.TABLE1:Peak Area(A)Ratios of the Oxygen-Containing Bonds to the CC Bonds and the Total Carbon Bands to the Au Band(Obtained by XPS)and the Peak Intensity Ratios of I D/I G(Obtained by Raman Analysis)of the TiO2/ Graphene(Oxide)Samples at the Different Irradiation TimesXPS Raman irradiationtime(h)A COH/A CCA CO/A CCA OCOH/A CCnormalized(A C/A Au)a I D/I G00.68 1.260.161 1.2610.230.370.060.820.9220.160.180.030.730.8140.150.180.030.630.95100.130.210.030.32 1.10 240.140.200.030.15 1.55a A C)A CC+A COH+A CO+A OCOH.Photodegradation of Graphene Oxide Sheets J.Phys.Chem.C,Vol.114,No.30,201012957be assigned to formation of further sp 2bonds after the UV-assisted photocatalytic reduction of the graphene oxide sheets,in good consistency with the XPS results.But,by further increasing the time of UV irradiation,the I D /I G ratio increased so that after 24h irradiation the I D /I G ratio increased to a high value of 1.55.Such considerable increase in the I D /I G ratio was assigned to the appearance of further carbonaceous defects in the reduced graphene oxide sheets due to the photocatalytic degradation of them by the TiO 2nanoparticles.In fact,since the ratio of I G /I D is proportional to the in-plane graphitic crystallite size,35,36it can be concluded that the crystallite size of sp 2domains of the graphene sheets photodegraded by the TiO 2nanoparticles after 24h decreased about 19and 48%of the crystallite size of the as-prepared graphene oxide sheets and the graphene oxide sheets photocatalytically reduced in 2h,respectively.Raman spectroscopy is also utilized to examine the single-,bi-,and multilayer characteristics of graphene and/or graphene oxide layers.For example,it was previously reported that the peak position of the G band of the single-layer graphenes (1585cm -1)shifts about 6cm -1into lower frequencies after stacking further graphene layers (for 2-6layers G band shifts to 1579cm -1).28,34-39In addition,shape and position of the 2D band are the key parameters indicating formation and the layer numbers of graphene sheets.28,37-40The 2D peak position of the single-layer graphene sheets is usually observed at 2679cm -1,while the 2D band of multilayer (2-4layers)shifts to higher wavenumbers by 19cm -1.34Here,the observed 2D bands of the graphene (oxide)sheets with a nearly symmetrical shape were centered at around 2680cm -1,indicating formation of single-layer graphene (oxide)sheets.Therefore,the Raman analysis confirmed presence of single-layer graphene (oxide)sheets,as also found by using the height profile analysis of the AFM image.The current -voltage (IV )characteristic of the TiO 2/graphene (oxide)sheets irradiated for the different periods of time was also studied,as shown in Figure 5.The linear behavior of the current -voltage curve of the TiO 2/graphene (oxide)sheets indicated the metallic nature of the sheets and formation of ohmic contact between the sheets and the ing the obtained IV curves,sheet resistance (Rs)of the TiO 2/graphene (oxide)sheets was calculated for different irradiation times,as shown in the inset of Figure 5.The sheet resistance of the as-prepared TiO 2/graphene oxides was evaluated to be as high as ∼1011Ω/sq.The IV diagram of the as-prepared graphene oxides was very similar to that of the as-prepared TiO 2/graphene oxides (shown in Figure 5).This similarity indicated that the TiO 2nanoparticles connected only physically (not chemically)to the surface of the graphene oxide sheets,consistent with the XPSanalysis.After the photocatalytic reduction of the graphene oxide sheets,the sheet resistance sharply decreased,so that,for example,the Rs value of the TiO 2/graphene oxide sheets decreased to 4.6×106Ω/sq,after 2h photocatalytic reduction.The increase in the conduction of the graphene (oxide)sheets can be assigned to decrease in the oxygen contents of the sheets,as previously studied for the graphene sheets exposed to oxygen plasma.41Moreover,comparing our optimum Rs value of the photocatalytic reduced graphene oxides with the ones reported for graphene oxides reduced by hydrazine (∼108Ω/sq)and graphene oxides reduced by both hydrazine and thermal reduction at 400°C (∼105Ω/sq)42,43shows that application of the photocatalytic process in a suitable time (here,2h)can be substantially effective in chemical reduction of the graphene oxide sheets.However,longer times of UV irradiation resulted in increase in the sheet resistance of the reduced graphene oxide sheets (for example,after 24h irradiation,the Rs value increased to the large value of 5.1×109Ω/sq).Since,the XPS analysis showed that the level of reduction of the reduced graphene oxide sheets was unchanged for the irradiation times longer than 2h,the high increase in the Rs value of the graphene oxide sheets irradiated for the longer times was assigned to photodegradation of the reduced graphene oxide sheets by the TiO 2nanoparticles,consistent with the analysis of Raman spectra.4.ConclusionsThe chemically synthesized graphene oxide sheets were physically combined by TiO 2nanoparticles after deposition between Au thin film electrodes.Both Raman and AFM analyses confirmed formation of single-layer graphene oxide sheets.Based on the significant reduction of the oxygen-containing carbonaceous bands and the I D /I G ratio of the XPS and Raman spectra,it was found that the photocatalytic reduction of the graphene oxides by the TiO 2nanoparticles was nearly completed after 2h UV irradiation,respectively.In addition,using XPS no considerable change was observed in the reduction level of the reduced graphene oxides for the longer irradiation times.However,the longer irradiations resulted in decrease of the carbon content of the reduced graphene oxides (based on the XPS)and increase of the carbon defects (based on the Raman),indicating degradation of the reduced sheets by the TiO 2nanoparticles in the photocatalytic process.Consistently,the IV measurements showed that the Rs value of the graphene oxide sheets substantially decreased from ∼1011to 4.6×106Ω/sq after the photocatalytic reduction for 2h.But,by increasing the irradiation time from 4to 24h,the Rs value highly increasedFigure 4.Raman spectra of the TiO 2/graphene (oxide)sheets after (a)0,(b)1,(c)2,(d)4,(e)10,and (f)24h UV irradiation time.Figure 5.Current -voltage diagram of the TiO 2/graphene (oxide)samples at different irradiation times.The inset shows sheet resistance of the TiO 2/graphene (oxide)nanosheets at the different irradiation times.12958J.Phys.Chem.C,Vol.114,No.30,2010Akhavan etal.from1.3×107to5.1×109Ω/sq.These results showed that the photodegradation of graphene oxide sheets by TiO2occurred along with their photocatalytic reduction.Therefore,to achieve photocatalytically reduced graphene oxide sheets with minimum carbon defects,obtaining an optimum irradiation time in which the photodegradation does not act as a dominant mechanism is necessary.Acknowledgment.O.A.thanks the Research Council of Sharif University of Technology and also the Iran Nanotech-nology Initiative Council for thefinancial support of the work. References and Notes(1)Novoselov,K.S.;Geim,A.K.;Morozov,S.V.;Jiang,D.;Zhang, Y.;Dubonos,S.V.;Grigorieva,I.V.;Firsov,A.A.Electric Field Effect in Atomically Thin Carbon Films.Science2004,306,666–669.(2)Novoselov,K.S.;Geim, A.K.;Morozov,S.V.;Jiang, D.; Katsnelson,M.I.;Grigorieva,I.V.;Dubonos,S.V.;Firsov,A.A.Two-Dimensional Gas of Massless Dirac Fermions in Graphene.Nature2005, 438,197–200.(3)Zhang,Y.B.;Tan,Y.;Stormer,H.L.;Kim,P.Experimental Observation of Quantum Hall Effect and Berry’s Phase in Graphene.Nature 2005,438,201–204.(4)Meyer,J.C.;Geim,A.K.;Katsnelson,M.I.;Novoselov,K.S.; Booth,T.J.;Roth,S.The Structure of Suspended Graphene Sheets.Nature 2007,446,60–63.(5)Katsnelson,M.I.;Novoselov,K.S.Graphene:New Bridge Between Condensed Matter Physics and Quantum Electrodynamics.Solid State Commun.2007,143,3–13.(6)Schedin,F.;Geim,A.K.;Morozov,S.V.;Hill,E.W.;Blake,P.; Katsnelson,M.I.;Novoselov,K.S.Detection of Individual Gas Molecules Adsorbed on Graphene.Nat.Mater.2007,6,652–655.(7)Robinson,J.T.;Perkins,F.K.;Snow,E.S.;Wei,Z.Q.;Sheehan, P.E.Reduced Graphene Oxide Molecular Sensors.Nano Lett.2008,8, 3137–3140.(8)Arsat,R.;Breedon,M.;Shafiei,M.;Spizziri,P.G.;Gilje,S.;Kaner, R.B.;Kalantar-zadeh,K.;Wlodarski,W.Graphene-Like Nano-Sheets for Surface Acoustic Wave Gas Sensor Applications.Chem.Phys.Lett.2009, 467,344–347.(9)Wang,X.;Zhi,L.J.;Tsao,N.;Tomovic,Z.;Li,J.L.;Mullen,K. Transparent Carbon Films as Electrodes in Organic Solar Cells.Angew. Chem.,Int.Ed.2008,47,2990–2992.(10)Gomez-Navarro,C.;Weitz,R.T.;Bittner,A.M.;Scolari,M.; Mews,A.;Burghard,M.;Kern,K.Electronic Transport Properties of Individual Chemically Reduced Graphene Oxide Sheets.Nano Lett.2007, 7,3499–3503.(11)Gilje,S.;Han,S.;Wang,M.;Wang,K.L.;Kaner,R.B.A Chemical Route to Graphene for Device Applications.Nano Lett.2007,7,3394–3398.(12)Liang,X.;Fu,Z.;Chou,S.Y.Graphene Transistors Fabricated via Transfer-Printing in Device Active-Areas on Large Wafer.Nano Lett. 2007,7,3840–3844.(13)Stampfer,C.;Schurtenberger,E.;Molitor,F.;Gu¨ttinger,J.;Ihn, T.;Ensslin,K.Tunable Graphene Single Electron Transistor.Nano Lett. 2008,8,2378–2383.(14)Bao,W.;Zhang,H.;Bruck,J.;Lau,C.N.;Bockrath,M.;Standley,B.Graphene-Based Atomic-Scale Switches.Nano Lett.2008,8,3345–3349.(15)Stankovich,S.;Dikin,D.A.;Piner,R.D.;Kohlhaas,K.A.; Kleinhammes,A.;Jia,Y.;Wu,Y.;Nguyen,S.T.;Ruoff,R.S.Synthesis of Graphene-Based Nanosheets via Chemical Reduction of Exfoliated Graphite Oxide.Carbon2007,45,1558–1565.(16)Schniepp,H.C.;Li,J.L.;McAllister,M.J.;Sai,H.;Herrera-Alonso,M.;Adamson,D.H.;Prud’homme,R.K.;Car,R.;Saville,D.A.; Aksay,I.A.Functionalized Single Graphene Sheets Derived from Splitting Graphite Oxide.J.Phys.Chem.B2006,110,8535–8539.(17)Kim,K.S.;Zhao,Y.;Jang,H.;Lee,S.Y.;Kim,J.M.;Kim,K.S.; Ahn,J.-H.;Kim,P.;Choi,J.-Y.;Hong,rge-Scale Pattern Growth of Graphene Films for Stretchable Transparent Electrodes.Nature2009, 457,706–710.(18)Muszynski,R.;Seger,B.;Kamat,P.V.Decorating Graphene Sheets with Gold Nanoparticles.J.Phys.Chem.C2008,112,5263–5266.(19)Jasuja,K.;Berry,V.Implantation and Growth of Dendritic Gold Nanostructures on Graphene Derivatives:Electrical Property Tailoring and Raman Enhancement.ACS Nano2009,3,2358–2366.(20)Akhavan,O.The Effect of Heat Treatment on Formation of Graphene Thin Films from Graphene Oxide Nanosheets.Carbon2009,48, 509–519.(21)Chen,W.;Yan,L.;Bangal,P.R.Preparation of Graphene by the Rapid and Mild Thermal Reduction of Graphene Oxide Induced by Microwaves.Carbon2010,48,1146–1152.(22)Xu,C.;Wang,X.;Zhu,J.Graphene-Metal Particle Nanocom-posites.J.Phys.Chem.C2008,112,19841–19845.(23)Williams,G.;Seger,B.;Kamat,P.V.TiO2-Graphene Nanocom-posites.UV-Assisted Photocatalytic Reduction of Graphene Oxide.ACS Nano2008,2,1487–1491.(24)Akhavan,O.;Ghaderi,E.Photocatalytic Reduction of Graphene Oxide Nanosheets on TiO2Thin Film for Photoinactivation of Bacteria in Solar Light Irradiation.J.Phys.Chem.C2009,113,20214–20220.(25)Gaya,U.I.;Abdullah, A.H.Heterogeneous Photocatalytic Degradation of Organic Contaminants over Titanium Dioxide:A Review of Fundamentals,Progress and Problems.J.Photochem.Photobiol.,C2008, 9,1–12.(26)Kuo,T.-J.;Lin,C.-N.;Kuo,C.-L.;Huang,M.H.Growth of Ultralong ZnO Nanowires on Silicon Substrates by Vapor Transport and Their Use as Recyclable Photocatalysts.Chem.Mater.2007,19,5143–5147.(27)Hummers,W.S.;Offeman,R.E.Preparation of Graphitic Oxide. J.Am.Chem.Soc.1958,80,1339–1339.(28)McAllister,M.J.;Li,J.L.;Adamson,D.H.;Schniepp,H.C.; Abdala,A.A.;Liu,J.;Alonso,M.H.;Milius,D.L.;Car,R.;Prud’homme, R.K.;Aksay,I.A.Single Sheet Functionalized Graphene by Oxidation and Thermal Expansion of Graphite.Chem.Mater.2007,19,4396–4404.(29)Yang,D.;Velamakanni,A.;Bozoklu,G.;Park,S.;Stoller,M.; Piner,R.D.;Stankovich,S.;Jung,I.;Field,D.A.;Ventrice,C.A.,Jr.; Ruoff,R.S.Chemical Analysis of Graphene Oxide Films after Heat and Chemical Treatments by X-Ray Photoelectron and Micro-Raman Spectros-copy.Carbon2009,47,145–152.(30)Chiang,T.C.;Seitz,F.Photoemission Spectroscopy in Solids.Ann. Phys.2001,10,61–74.(31)Yumitori,S.Correlation of C1s Chemical State Intensities with the O1s Intensity in the XPS Analysis of Anodically Oxidized Glass-Like Carbon Samples.J.Mater.Sci.2000,35,139–146.(32)Tuinstra,F.;Koenig,J.L.Raman Spectrum of Graphite.J.Chem. Phys.1970,53,1126–1130.(33)Ferrari,A.C.;Robertson,J.Interpretation of Raman Spectra of Disordered and Amorphous Carbon.Phys.Re V.B2000,61,14095–14107.(34)Graf,D.;Molitor,F.;Ensslin,K.;Stampfer,C.;Jungen,A.;Hierold,C.;Wirtz,L.Spatially Resolved Raman Spectroscopy of Single-and Few-Layer Graphene.Nano Lett.2007,7,238–242.(35)Ferrari,A.C.Raman Spectroscopy of Graphene and Graphite: Disorder,Electron-Phonon Coupling,Doping and Nonadiabatic Effects. Solid State Commun.2007,143,45–57.(36)Cancado,L.G.;Takai,K.;Enoki,T.;Endo,M.;Kim,Y.A.; Mizusaki,H.;Jorio,A.;Coelho,L.N.;Magalhaes-Paniago,R.;Pimenta, M.General Equation for the Determination of the Crystallite Size L a of Nanographite by Raman Spectroscopy.Appl.Phys.Lett.2006,88,163106.(37)Kudin,K.N.;Ozbas,B.;Schniepp,H.C.;Pruı¨homme,R.K.; Aksay,I.A.;Car,R.Raman Spectra of Graphite Oxide and Functionalized Graphene Sheets.Nano Lett.2008,8,36–41.(38)Dato,A.;Radmilovic,V.;Lee,Z.;Phillips,J.;Frenkach,M. Substrate Free Gas-Phase Synthesis of Graphene Sheets.Nano Lett.2008, 8,2012–2016.(39)Ferrari,A.C.;Meyer,J.C.;Scardaci,V.;Casiraghi,C.;Lazzeri, M.;Mauri,F.;Piscanec,S.;Jiang,D.;Novoselov,K.S.;Roth,S.;Geim, A.K.The Raman Fingerprint of Graphene and Graphene Layers.Phys. Re V.Lett.2006,97,187401.(40)Calizo,I.;Balandin, A. A.;Bao,W.;Miao, F.;Lau, C.N. Temperature Dependence of the Raman Spectra of Graphene and Graphene Multilayers.Nano Lett.2007,7,2645–2649.(41)Kim,K.;Park,H.J.;Woo,B.-C.;Kim,K.J.;Kim,G.T.;Yun, W.S.Electric Property Evolution of Structurally Defected Multilayer Graphene.Nano Lett.2008,8,3092–3096.(42)Becerril,H.A.;Mao,J.;Liu,Z.;Stoltenberg,R.M.;Bao,Z.;Chen, Y.Evaluation of Solution-Processed Reduced Graphene Oxide Films as Transparent Conductors.ACS Nano2008,2,463–470.(43)Eda,G.;Fanchini,G.;Chhowalla,rge-Area Ultrathin Films of Reduced Graphene Oxide as a Transparent and Flexible Electronic Material.Nat.Nanotechnol.2008,3,270–274.JP103472CPhotodegradation of Graphene Oxide Sheets J.Phys.Chem.C,Vol.114,No.30,201012959。