Published: March 30, 2011

r 2011 American Chemical Society 7355 dx.doi.org/10.1021/jp200953k | J. Phys. Chem. C 2011, 115, 7355–7363

ARTICLE

pubs.acs.org/JPCC

Preparation and Enhanced Visible-Light PhotocatalyticH2-Production Activity of Graphene/C3N4 CompositesQuanjun Xiang,† Jiaguo Yu,*,† and Mietek Jaroniec*,‡

†State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology,Luoshi Road 122#, Wuhan 430070, People's Republic of China‡Department of Chemistry, Kent State University, Kent, Ohio 44242, United States

1. INTRODUCTION

Hydrogen production via photocatalytic water splitting undervisible-light irradiation offers a viable strategy for solving simul-taneously the incoming energy and environmental problems.1

To date, various oxide, sulfide, and oxynitride semiconductorphotocatalysts have been developed for the aforementionedphotocatalytic reaction.1�10 However, the development of effi-cient, sustainable, visible-light-responsive photocatalytic materi-als remains a significant challenge.1 Recently, Wang et al.11

reported that a metal-free polymeric photocatalyst, graphiticcarbon nitride (g-C3N4), showed a good photocatalytic perfor-mance for hydrogen or oxygen production via water splittingunder visible-light irradiation. The metal-free g-C3N4 photoca-talysts possess very high thermal and chemical stability as well asinteresting electronic properties , which make them valuablematerials for photocatalysis-driven applications. However, thephotocatalytic efficiency of bare g-C3N4 is limited due to the highrecombination rate of photogenerated electron�hole pairs. Toresolve this problem, many methods have been proposed toimprove the photocatalytic performance of g-C3N4, for example,by designing an appropriate textural porosity,12�16 doping,17�21

and coupling g-C3N4 with metals,22,23 etc. In particular, there is a

great interest in combining g-C3N4 with graphene to improve itsconductivity and catalytic performance.24

Graphene, a two-dimensional macromolecular sheet of carbonatoms with a honeycomb structure, has attracted much attentiondue to its outstanding mechanical, thermal, optical, and electricalproperties and wide applications in nanoelectronics, biosensing,polymer composites, capacitors, and catalysis.25,26 Especially, ithas high thermal conductivity (∼5000 W m�1 K �1), assures anexcellent mobility of charge carriers (200 000 cm2 V�1 s�1), andpossesses an extremely high specific surface area (∼2600m2/g).27

Furthermore, graphene can be easily produced at a low cost fromnatural graphite through chemical oxidation�dispersion�reduc-tion procedures.28 Natural graphite can be effectively oxidized andexfoliated into graphene oxide (GO), which possesses a richassortment of oxygen-containing groups.28 The reduction of GOgives graphene, an excellent support for various graphene-basedcomposites, the applicability of which is rapidly growing.29�37 Forexample, Zhang et al.32 reported graphene�P25 TiO2 composites

Received: January 28, 2011Revised: March 18, 2011

ABSTRACT: Graphene and graphitic carbon nitride (g-C3N4)composite photocatalysts were prepared by a combined im-pregnation�chemical reduction strategy involving polymeriza-tion of melamine in the presence of graphene oxide (precursors)and hydrazine hydrate (reducing agent), followed by thermaltreatment at 550 �C under flowing nitrogen. The resultinggraphene/g-C3N4 composite photocatalysts were characterizedby X-ray diffraction, transmission electron microscopy, UV�vi-sible spectrophotometry, nitrogen adsorption, X-ray photoelec-tron spectroscopy, Fourier transform infrared spectroscopy,Raman spectroscopy, and photoluminescence spectroscopy.The transient photocurrent response was measured for several on�off cycles of intermittent irradiation. The effect of graphenecontent on the rate of visible-light photocatalytic hydrogen production was studied for a series of graphene�graphitic carbon nitridecomposite samples containing Pt as a cocatalyst in methanol aqueous solutions. This study shows that graphene sheets act aselectronic conductive channels to efficiently separate the photogenerated charge carriers and, consequently, to enhance the visible-light photocatalytic H2-production activity of g-C3N4. The optimal graphene content was determined to be ∼1.0 wt %, and thecorresponding H2-production rate was 451 μmol h�1 g�1, which exceeded that of pure g-C3N4 by more than 3.07 times. Theproposed mechanism for the enhanced visible-light photocatalytic activity of g-C3N4 modified by a small amount of graphene wasfurther confirmed by photoluminescence spectroscopy and transient photocurrent response. The metal-free graphene/g-C3N4

composites showed high visible-light photocatalytic activity, which makes them promising nanomaterials for further applications inwater treatment and dye-sensitized solar cells.

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7356 dx.doi.org/10.1021/jp200953k |J. Phys. Chem. C 2011, 115, 7355–7363

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synthesized under hydrothermal conditions and showed theirenhanced photocatalytic activity for degradation of methyleneblue in aqueous solutions. Paek et al.36 reported graphene�SnO2

compositematerials with a three-dimensional delaminated flexiblestructure, exhibiting an improved electrochemical performancedue to the presence of graphene sheets facilitating electrontransfer. Cao et al.37 developed a one-step method to producegraphene�CdS nanocomposite materials with good structuraland optoelectronic properties. The aforementioned studies showthat the graphene-based composite materials can improve theperformance of various optoelectronic and energy conversiondevices. However, to date, there are only few reports on thesynthesis and application of graphene/g-C3N4 composites.

24 Veryrecently, Sun et al.24 synthesized the layered graphene/g-C3N4

composites with improved conductivity and electrocatalytic per-formance. However, to the best of our knowledge, there is noreport on the use of these composites for hydrogen production viaphotocatalytic water splitting.

In this study, we report the fabrication of graphene/g-C3N4

composite photocatalysts by a combined impregnation�chemicalreduction strategy involving polymerization of melamine in thepresence of graphene oxide (precursors) and hydrazine hydrate(reducing agent), followed by thermal treatment at 550 �C underflowing nitrogen. Specifically, the effect of the graphene contenton the rate of photocatalytic hydrogen production in methanolaqueous solutions under visible-light irradiation is discussed. Toour knowledge, this is the first report on the visible-light photo-catalytic activity of graphene/g-C3N4 composites toward hydro-gen production, which was achieved by introduction of graphenesheets into graphitic C3N4 to facilitate electron transfer. Also, thisstudy provides new insights into the enhancement of photocata-lytic activity of composite materials by incorporation of graphenesheets as electron-conducting channels.

2. EXPERIMENTAL SECTION

2.1. Sample Preparation. All chemicals were reagent-gradeand, distilled water was used in the whole experiment. Grapheneoxide sheets were provided by Xiamen Knano Graphene Tech-nology Corporation Limited, Xiamen, People's Republic ofChina. Graphene/g-C3N4 composite photocatalysts were pre-pared by an impregnation�chemical reduction strategy and, atthe end, calcined at 550 �C. In a typical synthesis,melamine (0.5 g)was dispersed in distilled water (150 mL), and then the specifiedvolume of graphite oxide dispersed in aqueous solution (0.5 g/L)was added. The weight percentages of graphene in the photo-catalyst samples were 0, 0.25, 0.5, 1.0, 2.0, and 5.0 wt % (Table 1);the resulting samples were labeled as GCx, where x = 0, 0.25, 0.5,1.0, 2.0, and 5.0, respectively. Graphite oxide in the synthesismixture was reduced by adding hydrazine hydrate (1 mL) understirring at 90 �C for 24 h.38a After that, the product was collected

by centrifugation, washed five times with methanol and distilledwater, and then dried in an oven at 80 �C for 6 h. Finally, theobtained solid samples were heated to 550 �C for 4 h and kept atthis temperature for another 4 h under flowing nitrogen.2.2. Characterization. Powder X-ray diffraction (XRD) pat-

terns were obtained on a D/Max-RB X-ray diffractometer(Rigaku, Japan) with Cu KR radiation (λ = 0.15418 nm) at ascan rate (2θ) of 0.05� s�1. The accelerating voltage and theapplied current were 40 kV and 80 mA, respectively. Transmis-sion electron microscopy (TEM) analysis was conducted on aTecnai G2 F20 S-TWIN microscope at an accelerating voltageof 200 kV. UV�visible absorbance spectra were obtained forthe dry-pressed disk samples with a UV�visible spectrophot-ometer (UV-2550, Shimadzu, Japan). BaSO4 was used as areflectance standard in a UV�visible diffuse reflectance ex-periment. The Brunauer�Emmett�Teller specific surfaceareas (SBET) and porosity of the samples were evaluated onthe basis of nitrogen adsorption isotherms measured at�196 �C using a Micromeritics ASAP 2020 gas adsorptionapparatus (USA). All the samples were degassed at 180 �Cbefore nitrogen adsorption measurements. The BET surfacearea was determined using adsorption data in the relativepressure (P/P0) range of 0.05�0.3. The desorption data wereused to determine the pore size distribution via the Bar-ret�Joyner�Halender (BJH) method.39 The nitrogen adsorp-tion volume at the relative pressure (P/P0) of 0.99 was used todetermine the single-point pore volume and the average poresize.40�42 X-ray photoelectron spectroscopy (XPS) measure-ments were performed by using an ultra-high-vacuum VGESCALAB 210 electron spectrometer equipped with a multi-channel detector. The spectra were excited using Mg KR(1253.6 eV) radiation (operated at 200 W) of a twin anodein the constant analyzer energy mode with a pass energy of 30eV. The Fourier transform infrared spectra (FTIR) of thesamples were recorded using an IRAffinity-1 FTIR spectro-meter. Raman spectra were recorded at room temperature using amicro-Raman spectrometer (Renishaw InVia) in the backscatter-ing geometry with a 514.5 nm Arþ laser as an excitation source.Photoluminescence (PL) spectra were measured at room tempera-ture on a Fluorescence Spectrophotometer (F-7000, Hitachi,Japan). The excitation wavelength was 315 nm, the scanningspeed was 1200 nm/min, and the PMT voltage was 700 V. Thewidths of the excitation slit and emission slit were both 5.0 nm.2.3. Photocatalytic H2-Production Activity. The photocata-

lytic H2-production experiments were performed in a 100 mLPyrex flask at ambient temperature and atmospheric pressure, andthree openings of the flask were sealed with a silicone rubberseptum. A 350 W Xe arc lamp through a UV-cutoff filter(>400 nm) was used as the light source (20 cm away from thephotocatalytic reactor). The focused intensity on the flask was ca.180 mW/cm�2; it was measured on an FZ-A visible-light

Table 1. Effects of Graphene Content on the Physical Properties of the Graphene/g-C3N4 Samples Studied

samples graphene (wt %) SBET (m2/g) average pore size (nm) pore volume (cm3/g) porosity (%)

GC0 0 11.6 13.6 0.05 15.6

GC0.25 0.25 13.4 13.6 0.06 18.2

GC0.5 0.5 16.0 14.0 0.07 20.6

GC1.0 1.0 20.4 14.4 0.07 20.6

GC2.0 2.0 21.8 15.6 0.07 20.6

GC5.0 5.0 26.6 17.6 0.09 23.7

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7357 dx.doi.org/10.1021/jp200953k |J. Phys. Chem. C 2011, 115, 7355–7363

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radiometer (made in the photoelectric instrument factory ofBeijing Normal University, China) with the wavelength rangeof 400�1000 nm. In a typical photocatalytic experiment, 80mg ofphotocatalyst powder was suspended in 80 mL of aqueoussolution containing 25% methanol scavenger in volume. Theloading of 1.5 wt % Pt cocatalyst was conducted by directlydissolving H2PtCl6 into the above 80 mL mixed solution. Next,the suspensions were stirred and followed by illumination (350WXe arc lamp) for 20 min at room temperature. Before irradiation,suspensions of the cocatalyst were dispersed in an ultrasonic bathand nitrogenwas bubbled through the reactionmixture for 40minto remove the dissolved oxygen and to ensure the anaerobicconditions in the reaction system. A 0.4 mL portion of gas wassampled intermittently through the septum, and hydrogen wasanalyzed by a gas chromatograph (GC-14C, Shimadzu, Japan,TCD, nitrogen as a carrier gas and 5 Å molecular sieve column).All glassware was carefully rinsed with distilled water prior to use.The apparent quantum efficiency (QE) was measured andcalculated according to eq 1:

QE ½%� ¼ number of reacted electronsnumber of incident photons

� 100

¼ number of evolved H2 molecules� 2number of incident photons

� 100

ð1Þ2.4. Photoelectrochemical Measurements. Photocurrent

was measured on an electrochemical analyzer (CHI660C In-struments) in a standard three-electrode system using theprepared samples as the working electrodes with an active areaof ca. 0.5 cm2, a Pt wire as the counter electrode, and Ag/AgCl(saturated KCl) as a reference electrode. A 300 W Xe arc lampthrough a UV-cutoff filter (>400 nm) served as a light source.The integrated light intensity measured with an FZ-A visible-light radiometer was 120 mW/cm2. Na2SO4 (1 M) aqueoussolution was used as the electrolyte. Working electrodes wereprepared as follows: 0.2 g of photocatalyst (samples GC0 andGC1.0) were ground with 0.06 g of polyethylene glycol (PEG,molecular weight: 20 000) and 0.5 mL of water to make a slurry.The slurry was then coated onto a 2 cm� 1.2 cm F-doped SnO2-coated glass (FTO glass) electrode by the doctor blade techni-que. Next, these electrodes were dried in an oven and calcined at450 �C for 30 min. All investigated electrodes have a similar filmthickness (10�11 μm).

3. RESULTS AND DISCUSSION

3.1. Phase Structures and Morphology. A series of gra-phene/g-C3N4 composite samples denoted as GCx, where xstands for wt % of graphene in the sample, were studied bypowder XRD to identify the phase structures. Figure 1 shows theXRD patterns of the GCx samples with x varying from 0 to 5.0 incomparison to the pattern of graphene oxide. The XRD patternsrecorded for pure g-C3N4 (sample GC0) and graphene/g-C3N4

composites (samples GC0.25, GC0.5, GC1.0, GC2.0, andGC5.0) feature two distinct diffraction peaks. The high-intensitypeak at 27.41�, corresponding to an interlayer distance of0.326 nm, reflects the interlayer stacking of aromatic segmentsand can be indexed as the (002) peak observed for graphiticmaterials. The peak at 13.08�, which corresponds to a distance d =0.676 nm, is indexed as (100) and can be associated with an in-plane structural packing motif. These two diffraction peaks are ingood agreement with the recent reports on g-C3N4 prepared bypolymerization of cyanamide, dicyandiamide, and melamine.43,44

For graphene oxide, the XRD peak at 9.8� (Figure 1, pattern g)corresponds to the (001) interlayer spacing of 0.90 nm, whichmatches well the values reported in the literature.45 Notably, noapparent peaks of graphene oxide and graphene were observed inall the graphene/g-C3N4 composites. This can be ascribed to thefact that the regular stacking of graphene oxide was destroyedduring the reduction process with hydrazine hydrate and couldnot be resolved by XRD. However, the presence of graphenesheets in the graphene/g-C3N4 composites can be easily evi-denced by Fourier transform infrared andRaman spectroscopy, asdiscussed later.TEM was further used to study the morphology and micro-

structures of the graphene/g-C3N4 composites along with gra-phene oxide for the purpose of comparison. Figure 2a shows atypical TEM image of graphene oxide. As can be seen in thisfigure, graphene oxide has a two-dimensional structure consistingof sheets with micrometer-long wrinkles. The TEM image of theGC1.0 graphene/g-C3N4 composite sample shows that its sur-face morphology is layered as in the case of graphite. Incomparison to graphene oxide, the graphene/g-C3N4 compositestudied possessed a more compact structure because g-C3N4 wassandwiched between graphene sheets through polymerization ofmelamine molecules preadsorbed on the GO sheets (having a lotof hydroxyl, carboxyl, and carbonyl groups).3.2. UV�vis Analysis. The absorbance of the GCx (x = 0,

0.25, 0.5, 1.0, 2.0, 5.0) samples was measured by UV�vis diffusereflectance spectroscopy (UV�vis spectra are shown inFigure 3). For all the samples studied, a significant increase in

Figure 1. XRD patterns of the GC0 (a), GC0.25 (b), GC0.5 (c), GC1.0(d), GC2.0 (e), and GC5.0 (f) samples and graphene oxide (g).

Figure 2. TEM images of graphene oxide (a) and the GC1.0sample (b).

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the absorption at wavelengths shorter than 460 nmcan be assignedto the intrinsic band gap of g-C3N4 (≈2.7 eV). A comparison ofthe absorption spectrum of the GC0 sample (pure g-C3N4) withthose obtained for the graphene/g-C3N4 samples studied showsthat the latter exhibit a broad background absorption in the visible-light region. This can be attributed to the presence of graphene inthe graphene/g-C3N4 composites. Also, the composite samplesshow stronger broad background absorption with increasinggraphene content, which is in agreement with the color changingfrom yellow to gray (inset in Figure 3). A further observationindicates that all the graphene/g-C3N4 composite samples showalmost the same absorption edge as that of pure g-C3N4, implyingthat carbon was not incorporated to the lattice of g-C3N4 and thelayerd graphenewas only a substrate for immobilization of g-C3N4.3.3. BET Surface Area and Pore Size Distributions. The

BET surface areas and porous structures of the graphene/g-C3N4

composite samples and pure g-C3N4 were investigated bynitrogen adsorption. Figure 4 shows nitrogen sorption isothermsand the corresponding pore size distribution curves (inset inFigure 4) for the GC0 and GC1.0 samples. The nitrogenadsorption�desorption isotherms for the aforementioned twosamples are type IV (Brunauer�Deming�Deming�Teller(BDDT) classification), indicating the presence of mesopores(mesopores have widths between 2 and 50 nm).46�48 These

isotherms exhibit H3 hysteresis loops associated with mesoporespresent in aggregates composed of primary particles, giving riseto slitlike pores,49 which is consistent with TEM analysis(Figure 2). Moreover, the adsorption branch of nitrogen iso-therms shows a steady increase at P/P0 approaching unity,suggesting the formation of large mesopores and small macro-pores. The pore-size distributions (inset in Figure 4) of the GC0and GC1.0 samples are very broad, indicating the existence ofmesopores and macropores. Table 1 shows that all graphene/g-C3N4 composite samples have larger specific surface areas thanthat of pure g-C3N4. This is due to the presence of graphene inthe composites, which has an extremely high surface area(theoretical value of ∼2600 m2/g). It is reasonable to concludethat the BET specific surface area increases with increasingamount of graphene, which is beneficial for enhancing thephotocatalytic activity of the composites studied.3.4. XPS Analysis. The chemical composition of the gra-

phene/g-C3N4 composites studied and the chemical status of Cand N elements in the samples were characterized by XPS.Figure 5 shows the XPS survey spectrum for the graphene/g-C3N4 composite loaded with 1.0 wt % graphene (sampleGC1.0). As expected, the aforementioned sample contains onlyC and N elements, with sharp photoelectron peaks appearing atbinding energies of 285 eV (C 1s) and 399 eV (N 1s). No peaksfor other elements were found. This indicates that the compositephotocatalysts studied are primarily composed of carbon andnitrogen.Figure 6A shows the high-resolution C 1s XPS spectra of the

GC1.0 sample in comparison to that of graphene oxide. As can beseen from this figure, both samples have the C 1s peak at 285.1eV, which can be ascribed to carbon and defect-containing sp2-hybridized carbon atoms present in graphitic domains.21 Forgraphene oxide, the C 1s peak at 286.9 eV is usually assigned tothe oxygen-containing carbonaceous bands (C�OH),24 indicat-ing the presence of oxygen-containing functional graphene oxide.The main peak at 288.2 eV for GC1.0, corresponding toNdC�N2 coordination, is ascribed to carbon atoms that haveone double and two single bonds with three N neighbors.43,50 Nosignal of the oxygen-containing carbonaceous bands was found inthe graphene/g-C3N4 composite (sample GC1.0). This indicatesthat the introduced graphene oxide sheets have been completelyreduced into graphene during the synthesis process. Figure 6Bshows the high-resolutionN 1s XPS spectra of the GC1.0 sample.The asymmetrical and broad features of the observed N 1s XPSpeaks suggest the coexistence of distinguishable models. A signal

Figure 3. UV�vis diffuse reflection spectra of the GC0, GC0.25,GC0.5, GC1.0, GC2.0, and GC5.0 samples. The inset shows theircorresponding colors.

Figure 4. Nitrogen adsorption�desorption isotherms and the corre-sponding pore size distribution curves (inset) of the GC0 (a) andGC1.0(b) samples.

Figure 5. XPS survey spectrum of the GC1.0 sample.

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deconvolution after Gaussian curve fitting is shown in Figure 6B,pointing out chemically different N species in the g-C3N4, withtheir N 1s binding energies at about 398.6, 399.8, and 401.5 eV.The main N 1s peak at a binding energy (BE) of 398.6 eV can beassigned to sp2-hybridized nitrogen (CdN�C).21 The two weakpeaks at about 399.8 and 401.5 eV can be attributed to tertiarynitrogen (N�(C)3) and amino functional groups having ahydrogen atom (C�N�H), respectively.24 The presence ofthe N�(C)3 groups confirms the polymerization of melamine.24

The existence of amino functional groups suggests that theg-C3N4 product prepared by pyrolysis of melamine was incom-pletely condensed, which is consistent with the previousreport.21,51 Moreover, the area ratio of N�(C)3 and C�N�H

peaks also suggests that the degree of condensation reaction andthe molar percentage are about 90% and 10%, respectively.3.5. IR and Raman Spectra. Figure 7 shows a comparison of

the FTIR spectra of graphene oxide, pure g-C3N4 (sample GC0),and the graphene/g-C3N4 composite (sample GC1.0). In the caseof pure g-C3N4 (Figure 7, spectrum a), several strong bands in the1200�1650 cm�1 region dominate the spectrum, with the peaksat about 1241, 1322, 1406, 1571, and 1631 cm�1, which corre-spond to the typical stretching modes of CN heterocycles.44

Additionally, the characteristic breathing mode of triazine unitsat 806 cm�1 was observed.13,44 For graphene oxide, there are fourrepresentative absorption peaks in the 1000�1800 cm�1 region(Figure 7, spectrum c). The peaks at 1068 and 1406 cm�1 areattributed to the C�O stretching vibrations and tertiary C�OHgroups stretching, respectively.52 The other two absorptions at1631 and 1723 cm�1 can be attributed to the O�H bendingvibration of epoxide groups and skeletal ring and the CdOstretching of COOH groups, respectively.52 In the case of thegraphene/g-C3N4 composite (sample GC1.0), the characteristicbands for g-C3N4 still remain (Figure 7, spectrum b), but thetypical absorption peaks of graphene oxide decrease dramaticallyin intensity or even disappear as compared with those of the puregraphene oxide, indicating the reduction of graphene oxide.Significantly, a new peak emerges at 1557 cm�1, which isattributed to the skeletal vibration of the graphene sheets,52

indicating the presence of these sheets in the graphene/g-C3N4

composite. In addition, the broad bands in the 3400�3800 cm�1

region and at 2377 cm�1 are clearly observed for all the samples,which correspond to the physically adsorbed H2O and CO2 fromthe atmosphere, respectively.53

TheRaman spectra further confirm the presence of graphene inthe graphene/g-C3N4 composites revealed by FTIR. Figure 8shows the Raman spectra of graphene oxide, pure g-C3N4

(sample GC0), and the graphene/g-C3N4 composite (sampleGC1.0). Several characteristic peaks of g-C3N4 at 458, 693, 734,964, 1217, and 1296 cm�1 are observed (Figure 8, spectrum a). Inthe Raman spectrum of graphene oxide (Figure 8, spectrum c),two typical bands of graphene oxide can be found at 1354 and1600 cm�1, corresponding to the well-documented D and Gbands, respectively.38 For the graphene/g-C3N4 composite, allthe Raman bands for g-C3N4 can be found. Significantly, the twocharacteristic peaks at about 1343 cm�1 (D band) and 1586 cm�1

(G band) for the graphitized structures are observed in the Ramanspectrum of the graphene/g-C3N4 composite. Also, in comparison

Figure 7. IR spectra of the GC0 (a) and GC1.0 (b) samples andgraphene oxide (c).

Figure 8. Raman spectra of the GC0 (a) and GC1.0 (b) samples andgraphene oxide (c).

Figure 6. High-resolution XPS spectra of C 1s (A) for the GC1.0sample (a) and graphene oxide (b) and N 1s (B) for the GC1.0 sample.

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to graphene oxide, the D band moves to 1343 cm�1 and the Gband shifts to 1586 cm�1, which matches the value of pristinegraphite (1586 cm�1), indicating the reduction of grapheneoxide. A further observation indicates that the graphene/g-C3N4 composites show an increased D/G intensity ratio incomparison to that of pure graphene oxide. This change suggestsa decrease in the average size of the in-plane sp2 domains uponreduction of the exfoliated graphene oxide,38,54 confirming theexistence of graphene sheets in the graphene/g-C3N4

composites.3.6. Photocatalytic H2-Production Activity. Photocatalytic

hydrogen production on various samples loaded with 1.5 wt % Ptwas evaluated under visible-light irradiation (>400 nm) usingmethanol as a scavenger. Control experiments indicated that noappreciable hydrogen production was detected in the absence ofeither irradiation or photocatalyst, suggesting that hydrogen wasproduced via photocatalytic reactions. Figure 9A presents acomparison of the visible-light photocatalytic H2-productionactivity of the GC0, GC0.25, GC0.5, GC1.0, GC2.0, and GC5.0samples. As can be seen from this figure, the graphene content hasa significant influence on the photocatalytic activity of g-C3N4. Forthe graphene content = 0, the GC0 sample shows an obviousvisible-light photocatalytic activity and the H2 production ratereaches 147 μmol h�1 g�1, owing to the moderate band gap andunique electronic structure of g-C3N4. In the presence of a smallamount of graphene sheets, the activity of the GC0.25 sample isremarkably enhanced. The photocatalytic activity of the samplesfurther increased with increasing graphene content from 0.25 to1.0%. The highest H2-production rate, obtained for the GC1.0

sample (graphene content = 1.0 wt%), is 451 μmol h�1 g�1 with a2.6% apparent quantum efficiency. This value exceeds that of pureg-C3N4 (sample GC0) and N-doped TiO2 nanoparticles (used asa reference photocatalyst) by a factor of 3.07 and 1.21, respectively.When the graphene content is higher than 1.0 wt %, a furtherincrease in graphene content (2.0 and 5.0 wt %) causes a rapidreduction in the photocatalytic activity. This decrease can berelated to the increase in the opacity and light scattering, leadingto a decrease of irradiation passing through the reaction suspen-sion solution (see Figure 3).55�57 The present results are similar tothe previous studies showing that a suitable loading content ofgraphene is crucial for optimizing the photocatalytic activity ofgraphene/TiO2 nanocomposites.54

The stability of graphene/g-C3N4 nanocomposites (sampleGC1.0) was evaluated by performing the recycle experiments ofthe photocatalyst under similar conditions (see Figure 9B). Afterfour recycles, H2 evolution did not decrease, which indicatessufficient stability of this material for hydrogen generation.The high H2-production activity of the GC0.5 and GC1.0

samples under visible-light irradiation can be understood asillustrated in Figure 10. Under visible-light irradiation, electrons(e�) are excited from the valence band (VB) populated by N 2porbitals to the conduction band (CB) formed by C 2p orbitals ofg-C3N4, creating holes (h

þ) in the VB. Normally, these chargecarriers quickly recombine and only a fraction of electrons areinjected into the Pt nanoparticles due to a Schottky barrier.48,58

The injected electrons accumulate on the Pt nanoparticles andcan effectively reduce H2O (or Hþ) to produce H2, while holesaccumulate at the valence band of g-C3N4 and can react withmethanol as a sacrificial reagent. However, when g-C3N4 isimmobilized on the surface of graphene sheets to form thelayered composites, these photogenerated electrons on the CBof g-C3N4 tend to transfer to graphene sheets due to theirexcellent electronic conductivity, leading to hole�electronseparation. The transferred electrons will accumulate on the Ptnanoparticles loaded on the graphene sheets via a percolationmechanism31 and then participate in H2 generation. The majorreaction steps in this photocatalytic water-splitting mechanismunder visible-light irradiation are summarized by the followingeqs 2�5.

graphene=g-C3N4 hνsf graphene ðe�Þ=g-C3N4 ðhþÞ ð2Þ

graphene ðe�Þ þ Pt f grapheneþ Pt ðe�Þ ð3Þ

Pt ðe�Þ þ 2Hþ f PtþH2 ð4Þ

Figure 9. (A) Comparison of the photocatalytic activity of the samplesGC0, GC0.25, GC0.5, GC1.0, GC2.0, and GC5.0, and N-doped TiO2

for the photocatalytic H2 production from methanol aqueous solutionunder visible-light irradiation. (B) Cyclic H2-evolution curve for theGC1.0 sample.

Figure 10. Proposed mechanism for the enhanced electron transfer inthe graphene/g-C3N4 composites.

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g-C3N4 ðhþÞ þ CH3OHþ 6OH� f g-C3N4 þ CO2 þ 5H2O

ð5Þ

In the previous studies, photoluminescence (PL) analysis wasused to reveal the efficiency of charge carrier trapping, transfer,and separation and to investigate the fate of photogeneratedelectrons and holes in semiconductors, because the PL emissionresults from the recombination of free charge carriers. Herein, wepresent the suitable PL measurement for pure g-C3N4 (sampleGC0) and the graphene/g-C3N4 composite (sample GC1.0), asshown in Figure 11. As can be seen from this figure, the PLspectrum of the graphene/g-C3N4 composite is similar to that ofpure g-C3N4. For these two samples, one main emission peakappears at about 460 nm, which is equivalent to 2.7 eV. Thisstrong peak is attributed to the band�band PL phenomenonwith the energy of light approximately equal to the band-gapenergy of g-C3N4. In fact, the band�band PL signal is attributedto excitonic PL, which mainly results from the n�π* electronictransitions involving lone pairs of nitrogen atoms in g-C3N4.

50

Obviously, in comparison with pure g-C3N4 (sample GC0), theintensity of the PL signal for the GC1.0 composite sample ismuch lower. This indicates that the composite has a lowerrecombination rate of electrons and holes under visible-lightirradiation, which is mainly due to the fact that the electrons areexcited from the valence band to the conduction band and thentransfer to graphene sheets, preventing a direct recombination ofelectrons and holes. This may be ascribed to graphene sheets ingraphene/g-C3N4 composites becoming the separation center ofthe photogenerated electrons and holes because these sheets areconsidered to be a good electron-acceptor material to effectivelyhinder the electron�hole pair recombination due to its two-dimensional π-conjugation structure.26

To give further evidence to support the above suggestedphotocatalytic water-splitting mechanism, the transient photo-current responses of pure g-C3N4 (sample GC0) and graphene/g-C3N4 composite (sample GC1.0) electrodes were recorded forseveral on�off cycles of irradiation. Figure 12 shows the I�tcurves for the aforementioned two samples with several on�offcycles of intermittent irradiation. As can be seen from this figure,the photocurrent value rapidly decreases to zero as soon as theirradiation of light turns off, and the photocurrent comes back toa constant value when the light is on again, which is reproducible.This indicates that most of photogenerated electrons are

transported to the back contact across the samples to producephotocurrent under visible-light irradiation. Notably, for theGC0 sample, the photocurrent curves have an anodic photo-current spike at the initial time of irradiation. After the spikecurrent has been attained, a continuous decrease in the photo-current with time can be observed until a constant current isreached. The photocurrent decay indicates that the recombina-tion processes are occurring. During decay, the holes accumu-lated at the g-C3N4 surface competitively recombine withelectrons from the conduction band of g-C3N4, instead of beingtrapped or captured by reduced species in the electrolyte. That is,the decay is largely determined by the efficiency of the transfer ofconduction band electrons toward the back contact.59 On thecontrary, for the GC1.0 sample, the anodic photocurrent spike isnot as sharp as that observed in the previous sample and it isalmost equal to the stable photocurrent, which indicates that thecomposite has a lower recombination rate of electrons and holes.After the equilibration of competitive separation and recombina-tion of electron�hole pairs, the photocurrent reached a constantvalue.59,60 In the present case, the graphene/g-C3N4 compositesample shows a higher photocurrent intensity than g-C3N4. Thisobvious enhancement of photocurrent also indicates a smallerrecombination and amore efficient separation of photogeneratedelectron�hole pairs for the graphene/g-C3N4 composites at theinterface between graphene and g-C3N4: holes diffuse toward theg-C3N4 surface, where they are trapped or captured by reducedspecies in the electrolyte, while the electrons are effectivelytransported to the back contact via graphene sheets.

4. CONCLUSIONS

In summary, a series of graphene/g-C3N4 composite photo-catalysts with high visible-light photocatalytic H2-productionactivity were prepared by the impregnation�chemical reductionstrategy using polymerization of melamine in the presence ofgraphene oxide as precursors and hydrazine hydrate as a reducingagent, followed by calcination at 550 �C under a flowing nitrogenatmosphere. It was shown that the graphene loading slightlyinfluences the textural properties (specific surface area) andoptical characteristics (UV�vis absorption) of the g-C3N4

powders. After introduction of graphene sheets, g-C3N4 isimmobilized on the surface of graphene sheets to form a layeredcomposite. Graphene sheets act as conductive channels toefficiently separate the photogenerated charge carriers and toenhance the visible-light photocatalytic H2-production activity of

Figure 11. Comparison of photoluminescence spectra of the GC0 (a)and GC1.0 (b) samples.

Figure 12. Transient photocurrent responses of the GC0 (a) andGC1.0 (b) samples in 1 M Na2SO4 aqueous solution under visible-lightirradiation at 0.5 V vs Ag/AgCl.

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The Journal of Physical Chemistry C ARTICLE

g-C3N4. The optimal graphene content was found to be 1.0 wt %,and the corresponding H2-production rate was 451 μmol h�1

g�1, which exceeded that of pure g-C3N4 by more than 3.07times. The PL and photocurrent response data further confirmthat graphene can act as a good acceptor of the photogeneratedelectrons, and consequently, the electron�hole pair recombina-tion is hindered. This work demonstrates that graphene is a verypromising candidate for the development of high-performancephotocatalysts and inexpensive metal-free graphene/g-C3N4

composites with high visible-light photocatalytic activity thatcan find potential applications in water treatment and dye-sensitized solar cells.

’AUTHOR INFORMATION

Corresponding Author*E-mail: jiaguoyu@yahoo.com (J.Y.), jaroniec@kent.edu (M.J.).

’ACKNOWLEDGMENT

This work was partially supported by the National NaturalScience Foundation of China (20877061 and 51072154) and theNatural Science Foundation of Hubei Province (2010CDA078).Also, this work was financially supported by the National BasicResearch Program of China (2007CB613302).

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