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Highly Durable N-Doped Graphene/CdS Nanocomposites with Enhanced Photocatalytic Hydrogen Evolution from Water under Visible Light Irradiation Li Jia,† Dong-Hong Wang,† Yu-Xi Huang,‡ An-Wu Xu,*,† and Han-Qin Yu‡ † ‡
Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale and Department of Chemistry, University of Science and Technology of China, Hefei 230026, People's Republic of China
bS Supporting Information ABSTRACT: A series of N-doped graphene (N-graphene)/CdS nanocomposites were synthesized by calcination and characterized by X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, N2 adsorption analysis, ultravioletvisible diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy. The photocatalytic activity of as-prepared N-graphene/CdS for hydrogen production from water under visible light irradiation at λ g 420 nm was investigated. The results show that N-graphene/CdS nanocomposites have a higher photocatalytic activity than pure CdS. Transient photocurrents measured with a photoelectrochemical test device show that the photocurrent of the N-graphene/CdS sample is much increased as compared to the sole CdS. This enhanced photoresponse indicates that the photoinduced electrons in the CdS prefer separately transferring to the N-doped graphene. As a consequence, the radiative recombination of the electronhole pairs is hampered and the photocatalytic activity is significantly enhanced for the N-graphene/CdS photocatalysts. The amount of N-graphene is an important factor affecting photocatalytic activity of N-graphene/CdS nanocomposites; the optimum amount of N-graphene is ca. 2 wt %, at which the N-graphene/CdS sample displays the highest reactivity. Photocatalytic activity of graphene/CdS and GO/CdS composites for H2 production from water under visible light irradiation was also measured. The relative order of reactivity for the synthesized catalysts was found to be N-graphene/CdS > graphene/CdS > GO/CdS > CdS. Furthermore, the N-graphene/CdS photocatalyst does not show deactivation for H2 evolution for longer than 30 h, indicating that the cocatalyst of N-graphene as a protective layer can prevent CdS from photocorrosion under light irradiation. Our findings demonstrate that N-graphene as a cocatalyst is a more promising candidate for development of high-performance photocatalysts in the photocatalytic H2 production.
1. INTRODUCTION Hydrogen, one of the primary candidates as a future solar energy carrier, has recently attracted increased attention. One of the best ways to produce H2 from renewable sources is water splitting under solar irradiation by photocatalysts, which has been a challenging yet significant research topic due to growing environmental concerns and increasing energy demand. As a means to achieve this goal, semiconductor photocatalysts have been extensively studied. Particularly, a variety of visible lightcatalyzed materials19 including CdS1016 were investigated. CdS is probably one of the most studied metal sulfide materials as a photochemical water splitting catalyst because of its relatively narrow band gap (2.4 eV).1720 However, prolonged irradiation of CdS suspensions leads to decomposition of CdS into S (or sulfate in the presence of oxygen) and Cd2þ.14,21,22 CdS for photochemical water splitting using EDTA as a sacrificial agent were first investigated by Mills et al.23 Darwent and Porter found that photocorrosion of the CdS catalyst occurred when the irradiation time was over than 4 h.24,25 Taking into account the photochemical instability and improving photocatalytic activity of CdS, more and more research has r 2011 American Chemical Society
focused on CdS composites combined with other semiconductors, such as TiO2, CdSe, ZnO, ZnS, and so on. Despite these efforts, the low separation efficiency of electronhole pairs and photocorrosion remain the principal problems of CdS-based water-splitting catalysts. Generally, to enhance photocatalytic activity by these semiconductormatrix systems, it is essential to retard the recombination of electronhole species in the semiconductors by molecular electron relay semiconductor structures or efficient electron transport matrices, such as conductive polymer films or carbon nanotubes (CNTs).2629 The delocalized conjugated materials are well matched with the photocatalysts in energy level and an intensive interface hybrid effect emerges between these materials, causing rapid charge separation and slow charge recombination in the electron-transfer process. The superior electrical conductivity and the springy atom-thin two-dimensional (2D) feature of graphene would make itself an excellent electron-transport matrix.3033 Graphene, a new Received: March 12, 2011 Revised: May 7, 2011 Published: May 24, 2011 11466
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The Journal of Physical Chemistry C class of 2D carbonaceous material with atom-thick layer features, different from ball-like C60 and 1D carbon nanotubes, has attracted much attention recently because it has many potential applications, such as in electronic, optical, and catalytic fields.3437 Because of its high specific surface area with a large interface, tunable band gap, and high electron mobility up to 10 000 cm2 V1 s1, graphene will be a very prospective electron acceptor in photovoltaic devices.38 Like single-walled carbon nanotubes, conducting/semiconducting fullerenes, graphene is also expected to be used as an efficient acceptor to enhance photoinduced charge transfer for improved catalytic activity.3941 Zhang et al. have recently reported the use of graphene sheets as an electron transfer channel for reducing the recombination of the photogenerated electron holes, and leading to improving photoconversion efficiency of the graphene/TiO2 composites.42 Generally speaking, doping was found to be helpful for tailoring the electronic properties. More recently, heteroatoms (e.g., nitrogen, boron) doped into graphene sheets have been investigated to modulate their electronic and catalytic properties.4345 In this work, we attempt to use N-doped graphene (N-graphene) as an electron transfer channel for reducing the recombination of the photogenerated electronholes, resulting in enhancing photoconversion efficiency of the photocatalytic materials. Bridging CdS nanocrystals with N-graphene nanosheets yields a series of nanocomposites, N-graphene/CdS, which induces direct splitting of water into hydrogen under visible light irradiation (λ g 420 nm). N-graphene/CdS photocatalysts were found to display higher photocatalytic activity than the sole CdS. More importantly, no noticeable decrease in the activity of as-prepared N-graphene/CdS was observed during the photocatalytic water splitting for at least 30 h.
2. EXPERIMENTAL SECTION 2.1. Preparation. All regents were analytical grade and used without any further purification. For the synthesis of N-graphene/CdS hybrid catalysts, graphene oxide was prepared from natural graphite powder (99%, Shanghai Sinopharm Chemical Reagent Co., Ltd.) using a modified Hummers’ method,46 and then was annealed in NH3 gas to obtain N-graphene materials. Briefly, GO samples were put in a porcelain boat, after flowing N2 first and then NH3 for about 5 min, the furnace was heated up to 500 °C at a heating rate of 5 °C/min for 2 h. N-graphene/CdS photocatalysts were synthesized in two steps using N-graphene and cadmium sulfide as the staring materials. Take the preparation of N-graphene (2 wt %)/CdS nanocomposites for example, 0.0609 g of N-graphene was dispersed into 15 mL water with sonicating for 3 days, resulting in a metastable purplish-gray dispersion solution. The dispersion was then allowed to settle overnight. The precipitates at bottom were removed (about 0.0490 g), and 50 mL ethanol was added. The solution was then sonicated for an additional 30 min (resulting in a stable purplishgray dispersion that does not see any significant precipitation when lay on a laboratory for days or weeks). A 0.9134-g portion of CdCl2 3 2.5H2O was dissolved in above solution and magnetically stirring for 15 min, and then an appropriate amount of Na2S solution was added drop-by-drop into the suspension. After being magnetically stirred for 2 days, CdS nanoparticles were obtained. And then N-graphene/CdS composites were annealed in nitrogen atmospheres at 400 °C for 2 h, washed several times with distilled water, and dried at 60 °C. In the similar procedure, N-graphene (0.5 wt %)/CdS, N-graphene
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(1 wt %)/CdS, and N-graphene (5 wt %)/CdS composites were synthesized when 0.0127 g, 0.0300, and 0.1229 g of N-graphene were used, respectively. For comparison, CdS blank N-graphene was prepared by the same method. The graphene were obtained through reduction of GO using sodium borohydride as a reducing agent. Briefly, 0.0119 g of GO was dissolved into 20 mL ethanol with sonicating for 30 min, and then 0.0240 g of NaBH4 was added, followed by magnetically stirring for 48 h at room temperature. 2.2. Characterization. The X-ray powder diffraction (XRD) patterns of the samples were performed on a Rigaku/Max-3A X-ray diffractometer with Cu KR radiation (λ = 1.54178 Å), the operation voltage and current maintained at 40 kV and 40 mA, respectively. Transmission electron microscopic (TEM) images, high-resolution transmission electron microscopic (HRTEM) images were performed on a JEOL-2010 microscope with an accelerating voltage of 200 kV. Raman spectra were recorded with an InVia microscopic confocal Raman spectrometer (Renishaw, England) using a 514.5 nm laser beam. A Shimadzu spectrophotometer (Model 2501 PC) was used to record the UVvis diffuse reflectance spectra of the samples with the region of 200 to 800 nm. The X-ray photoelectron spectroscopy (XPS) was performed at the Photoemission Endstation in the National Synchrotron Radiation Laboratory (NSRL, Hefei, P. R. China). Nitrogen adsorption measurements were performed at 77 K using a Micromeritics ASAP 2010 system utilizing Barrett EmmettTeller (BET) calculations for surface area. Photoelectrochemical test systems were composed of a CHI 660D electrochemistry potentiostat (Shanghai Chenhua Limited, China), a 300 W xenon lamp with cutoff filters (λ g 420 nm), and a homemade three-electrode cell with using Pt wire as the counter electrode and Ag/AgCl as reference electrode, and 0.5 M Na2SO4 as electrolyte. CdS and N-graphene/CdS electrodes were prepared by depositing suspensions made of CdS or N-graphene/CdS and absolute ethanol (the concentration of CdS or N-graphene/CdS is 50 g/L) onto IndiumTin oxide (ITO) glass using the doctorblade coating method with a glass rod and scotch tape as a frame and spacer, respectively. The electrodes were dried and then calcined at 100 °C for 1 h. During measurements, the electrodes were pressed against a 0-shape of an electrochemical cell with a working area of 4.0 cm2. Photocatalytic reactions were conducted at 25 °C in a gasclosed circulation system with outer irradiation. The light source was a 300 W Xe lamp (PLS-SXE300/300UV, Trusttech Co., Ltd. Beijing). Briefly, 0.2 g of N-graphene/CdS sample was dispersed in 300 mL of an aqueous solution containing 0.1 mol L1 Na2S/ 0.1 mol L1 Na2SO3 in a 500 mL Pyrex glass reactor. Before illumination, the solution was vigorously stirred for 5 min, then the flask was illuminated by the Xe lamp combined with a UV cutoff filter (λ g 420 nm) with magnetic stirring. The amount of hydrogen production was measured with online gas chromatography (GC7890, TCD, molecular sieve 5 Å, N2 carrier, Shanghai Techcomp Limited).
3. RESULTS AND DISCUSSION The phases of the as-obtained products were determined by X-ray diffraction (XRD) measurements. XRD patterns of pristine graphite, as-prepared graphite oxide (GO) and reduced GO (N-graphene) are shown in Figure S1 (see Supporting Information), respectively. A sharp (002) diffraction peak appearing at 10.80° is attributed to GO. This indicates that the pristine 11467
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Figure 1. HRTEM images of N-graphene (2 wt %)/CdS photocatalyst prepared at 400 °C.
Figure 2. (a) XPS spectra of the pristine graphite and the N-doped graphene. (b) XPS N 1s spectrum of the N-graphene. The N 1s peak is fitted into two Lorentzian peaks at 398.6 and 401.4 eV, which could be indexed to pridinic N and quaternary N, respectively.
graphite was oxidized into GO by expanding the d-spacing from 3.5 Å to 6.78 Å.47,48 Disappearance of the characteristic (002) diffraction of graphite at 26.3° illustrates that oxidation took place, resulting in the formation of a well ordered, lamellar structure. The XRD peak at 26.3°, corresponding to 0.34 nm, as observed in the graphite, says the reduction of GO into N-doped graphene by calcination in NH3 gas.44 Figure S1B of the Supporting Information, SI, displays the XRD patterns of the N-graphene/CdS composites with different N-graphene contents. It is obvious that the composites with different weight addition ratios of N-graphene exhibit similar XRD patterns. It has to be noted that all samples are principally composed of a hexagonal CdS phase and show no diffraction peaks of the N-graphene. The one reason for this phenomenon may be due to the N-graphene content being much lower, meaning that the main characteristic peak of N-graphene at 25.3°might be shielded by the main peak of CdS at 25°, and the other one may be because of the layer-stacking regularity almost disappeared after redox of graphite.4749 The microstructures of the obtained samples were examined with transmission electron microscopy (TEM). TEM images of N-graphene and N-graphene/CdS composites with different N-graphene contents are shown in Figure S2 of the SI. The bare N-graphene (reduced GO) has a sheet-like morphology with a clear, smooth surface. The crumpled membrane indicates that the N-graphene sheets are flexible. From the nonuniform contrast throughout the N-graphene sheets, we consider that these Ngraphene sheets consist of a few layers rather than a single-layer
N-graphene (Figure S2a of the SI). It is clearly seen that for Ngraphene (2 wt %)/CdS composites (Figure S2c of the SI), the CdS nanoparticles are almost uniformly deposited on the Ngraphene sheets. As estimated from the TEM images, the size of CdS nanoparticles in these samples is in the range 50 to 300 nm. To better study the interface structure between the two phases, the obtained samples were further examined with high-resolution transmission electron microscopy (HRTEM). Figure 1 shows the HRTEM images of N-graphene/CdS. The HRTEM image in Figure 1a shows a perfectly crystallized nanoparticle of with a lattice spacing of ca. 0.35 nm, which corresponds to the (100) plane of hexagonal CdS. It can be clearly seen from Figure 1a that the Ngraphene nanosheet with the basal plane is perfectly bridged with CdS nanoparticles. CdS particle is bridged on the edge of the Ngraphene nanosheet, and the interface between CdS and graphene is clearly seen (pointed to by red arrows). From Figure 1b, it is seen that the graphitic layers are curved, interrupted and have many defects. This crystalline morphology was also found in case of N-doped graphene by chemical vapor deposition43 and N-doped multiwalled carbon nanotubes,50 and would be attributed to the substitution of N atoms. The interlayer separation of the Ngraphene, estimated from HRTEM image (Figure 1b) is about 0.34 nm. The N-graphene has a near basal spacing with that of the (100) plane of CdS, which is propitious to interlock tightly to form thermodynamically stable interfaces. Some CdS particles are intimately deposited on the N-graphene nanosheets. The intimate contact between N-graphene and CdS favors the formation of 11468
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Figure 3. UVvis diffuse reflectance spectra of pure CdS and the prepared composite samples, CdS (a), N-graphene (0.5 wt %)/CdS (b), N-graphene (1 wt %)/CdS (c), N-graphene (2 wt %)/CdS (d), and N-graphene (5 wt %)/CdS (e).
Figure 4. H2 evolution of CdS, N-graphene/CdS composites with different contents of N-graphene. Measurement conditions: 0.2 g sample, 300 mL aqueous solution containing 0.1 M Na2S and 0.1 M Na2SO3, and light source: 300 W Xe lamp (λ g 420 nm).
junctions between the two materials, as a result, being helpful for improving the charge separation and thus the photocatalytic activity. Raman spectroscopy is a powerful tool for investigating the electronic and phonon structure in pristine and doped carbon materials. Figure S3 of the SI shows the Raman spectrum of the graphite oxide and N-graphene sample. A D band at ca. 1354 cm1 and a G band at ca. 1594 cm1, which are typical Raman features of GO, could be observed from the Raman spectra of the original GO before reduction. In contrast, a D band at ∼1352 cm1 and a G band at ∼1592 cm1 with an increased ID/IG ratio are observed for the N-graphene sample. There are many factors affecting the position of the G band, such as defects, strains, and doping, etc. Yang et al. have pointed out that the D-band upshifts while G-band downshifts with the introduction of N into carbon layers and shifts are enhanced with the increase in the N/C atomic ratios.50 Our Raman data shows that N-graphene sample has a high intensity of D band than that of GO, demonstrating the N-doping of the graphitic sheets,51 as the D band only takes place in the sp2 C with defects, and N doping introduces a large amount of topological defects.52 The N doping can also be confirmed by the X-ray photoelectron spectroscopy (XPS) measurements. An obvious N peak can be clearly detected in the N-graphene sample, while it is absent in case of the pristine graphite (Figure 2a). The N 1s peak in the XPS spectrum of the N-graphene (Figure 2b) is fitted into two peaks, a lower energy peak and a higher energy peak. The lower energy peak is near 398.6 eV, corresponding to pyridinic nitrogen (sp2 hybridization), which refers to the N atoms locating in a π conjugated system and contributing to the π system with p-electrons. The higher energy peak is located at 401.4 eV, which could be indexed to quaternary N (also called “graphitic nitrogen”), in which N atoms are incorporated into the graphene sheet and bonded to three carbon atoms.51a,53 The N content in the Ngraphene sample is ∼4.3%, as estimated from the XPS data. The BET surface area of the N-graphene modified CdS composites and sole CdS is given in Table S1 of the SI. All samples show relatively low BET surface area (2.32.8 m2 g1) because of severely sintering during the synthesis process. For the N-graphene/CdS composites with different N-graphene contents, there is no obvious difference in the BET surface area for these samples, pure CdS sample has a little higher BET surface area than that of N-graphene/CdS composites. Obviously, BET data
indicates that the surface area is not the determining factor for enhanced photocatalytic activity due to their little difference. The UVvis diffuse reflectance spectra of as-prepared CdS and N-graphene/CdS photocatalysts are shown in Figure 3. The powder color changed from orange to gray with increasing Ngraphene content. Compared with pure CdS, for the N-graphene/ CdS photocatalysts, a wide background absorption in the visible light region is observed. This can be attributed to the presence of carbon in the N-graphene/CdS composites, reducing reflection of light.42,54 For the N-graphene/CdS sample, the carbon species could modify the surface of CdS and absorb the visible light, which can lead to the absorption edge of N-graphene/CdS kindly shifts to the visible light range, as compared to pure CdS. In addition, the presence of N-graphene leads to a continuous absorption band in the range of 500800 nm, which is in agreement with the observed gray color of the samples. Hydrogen evolution measurements from photocatalytic water splitting using the N-graphene/CdS composites were conducted, together with those on N-graphene and CdS sample for a comparison. Figure 4 shows the varied amount of H2 evolution from an aqueous solution under visible light irradiation (λ g 420 nm) for various photocatalysts. Almost no H2 was detected when N-graphene alone was used as the catalyst. The photocatalysts of N-graphene/CdS using N-graphene as a cocatalyst exhibit the much higher ability for H2 evolution than the sole CdS. The amount of Ngraphene is an important factor affecting photocatalytic activity; with the N-graphene content in the nanocomposites increasing, the amount of H2 evolution is at first increased and then decreased. The optimum amount of N-graphene is ca. 2 wt %, at which each Ngraphene/CdS sample shows the most reactivity in H2 production. N-graphene (2 wt %)/CdS shows the highest H2 evolution rate of 210 μmol h1 among different N-graphene/CdS composites, the amount of H2 evolution enhanced to ca. 1200 μmol within 5 h, which is 5 times higher than that of pure CdS (40 μmol h1). However, the 5 wt % sample shows the lowest activity for H2 evolution, the reason could be that the superfluous N-graphene in composites should increase in the opacity and light scattering, leading to a decrease of light absorption passing through the reaction suspension solution, as found in graphene/TiO2 photocatalysts.42 The results show that a suitable loading content of N-graphene is crucial for optimizing the 11469
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Figure 5. The energy level diagram for N-graphene/CdS nanocomposites in relation to the redox potentials for water spitting process in Na2S/Na2SO3 aqueous solution.
photocatalytic activity of N-graphene/CdS nanocomposites. Briefly, more graphene should block light absorption and weaken light intensity arriving at catalyst’s surface, and lower than 2% sample indicates a small amount of CdS/graphene junctions formed. And thus, the activity of samples with higher or lower than 2% N-doped graphene decreased, our study found that the optimal amount of N-doped graphene is about 2% in the nanocomposites. These results suggest a synergistic effect between the N-graphene sheets and CdS nanoparticles. Here, the enhancement of the catalytic activity for H2 evolution could be attributed to the predominant electrical conductivity of N-graphene, indicating that the photoinduced electrons transport to the surface of the composites much easily to prevent the recombination between photogenerated electrons and holes. The photocatalytic activity of a given catalyst is usually affected by various factors including crystallinity, particle size, morphology, specific surface area and density of defects. In the present work, the similarity of crystallinity, morphology, surface area (particle size), and band gap energy between N-graphene/CdS and CdS should not be responsible for the remarkable enhancement of H2 production. Heterogeneous junction between N-graphene and CdS can be formed when the N-graphene/CdS nanocomposites were treated at high temperatures, and the junction will lead to a more efficient interelectron transfer between the two components, as found in MoS2/CdS photocatalyst.10 The work function of graphene has been computed to be 4.42 eV,55 CdS semiconductor has the conducting band ca. 3.8 eV and a band gap of about 2.25 eV. The conducting band of CdS is smaller than work functions of graphene, such that the photogenerated electrons transfer from CdS to graphene is energetically favorable.56 In addition, the 2D planar-conjugation structure endows graphene with the very excellent conductivity of electrons. Meanwhile, the photogenerated holes transfer to anions and the oxidation of the sulfide ions and sulfite ions occurred (see Figure 5). It is a fact that the presence of graphene in the N-graphene/CdS photocatalyst can effectively inhibit the electronhole pair recombination. These lead to the fact that the presence of graphene in the N-graphene/CdS nanocomposites can effectively inhibit the electron hole pair recombination. Remarkably, the photocatalytic water splitting on as-prepared catalysts proceeds without any noticeable decrease in the activity for longer than 30 h reaction (Figure 6). XRD patterns of the N-graphene/CdS sample had no notable differences before and after the photocatalytic recycles, implying that N-graphene/CdS is photostable and not photocorroded. The rapid
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Figure 6. A typical time course of hydrogen yield over 0.2 g of Ngraphene/CdS (λ g 420 nm) from an aqueous solution containing 0.1 M Na2S and 0.1 M Na2SO3 under visble-light irradiation, six runs in one continuous reaction.
transferring of electron and high separation efficiency of electron hole pairs lead to the dramatically enhanced photoactivity and completely inhibited photocorrosion. The photostability of the N-graphene/CdS is better than that of the traditional Pt/CdS systems, it is found that the rate of H2 production over Pt/CdS sample gradually decreases after an irradiation for 10 h. Figure S4 of the SI shows the rate of H2 evolution using CdS, a simple physical mixture of N-graphene and CdS, and various N-graphene (2 wt %)/CdS samples that were prepared at different calcination temperatures. The mechanical mixed sample of N-graphene (0.004 g) and CdS (0.2 g), in weak contact, shows a slightly higher rate of H2 evolution than pure CdS, but not as much as N-graphene (2 wt %)/CdS did. This strongly suggests that the intimate contact between N-graphene and CdS is very crucial for the transfer of photogenerated carriers. The importance of the interfacial structure in prompt charge migration was also suggested in several previous studies.10,57,58 We carried out N-graphene (2 wt %)/CdS annealing in N2 flow at different temperatures from 200 to 800 °C. The rate of H2 evolution on different N-graphene (2 wt %)/CdS photocatalysts annealing in N2 between 200 and 800 °C were in a range of ∼10210 μmol h1, with 400 °C annealing affording the highest H2 evolution rate of 210 μmol h1. The junctions between the Ngraphene layers and CdS particles and less defects in graphene when calcined at 400 °C, could be responsible for the highest photocatalytic activity of the sample calcined at 400 °C. For comparison, graphene (pure graphene)/CdS and GO/ CdS photocatalysts were also prepared by the same way and the photocatalytic activity was measured. The graphene was obtained through reduction GO by using sodium borohydride as a reducing agent.47 Figure 7 shows the photocatalytic activity in the presence of pure CdS, GO/CdS, N-graphene/CdS and graphene/CdS at different reaction times, from which it can be seen that the photocatalytic activity of N-graphene/CdS are much higher than graphene/CdS and GO/CdS. It is found that the relative order of reactivity for the synthesized various catalysts is N-graphene/CdS > graphene/CdS > GO/CdS > CdS. The results show that the rate H2 evolution were about 210 μmol h1 for N-graphene/CdS composites while 99 μmol h1 for graphene/CdS sample and 95 μmol h1 for GO/CdS (Figure 7), 11470
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Figure 7. Photocatalytic H2 production activity of CdS, GO (2 wt %)/ CdS, graphene (2 wt %)/CdS and N-graphene (2 wt %)/CdS photocatalysts with different reaction time. Catalyst (0.2 g); 0.1 M Na2S and 0.1 M Na2SO3 solution (300 mL); light source, Xe lamp (300 W).
which is consistent with previous studies showing that impurities significantly enhance the conductivity of graphene,44,59 although the normalized resistance of the NH3 annealed GO was still more than 100 times higher than that of pristine graphene due to the irreversible defects such as large vacancies and disrupted conjugation in the plane resulted from harsh oxidization.30,60 In addition, in conductive materials, core-hole screening imparts an inherent line asymmetry in the XPS spectra of the C 1s peak, and the asymmetry increased due to localized lattice disorder arising from change in bonding configuration, especially a change from an sp2 to an sp3 bonding structure upon N doping. The increased asymmetry would denote a change in the electronic density of state, which enhanced the graphene metallic/conductive character,61 as found for CNTs.62 Hence, the photocatalytic activity of Ngraphene/CdS is better than graphene/CdS. Lahaye et al. have studied graphite oxide with a first-principles density functional theory calculation proved that a low degree of oxidation, graphite oxide (GO) is a semiconductor.63 A recent study found that graphene oxide as a semiconductor showed photocatalytic activity for hydrogen evolution, and the hydrogen yield is very low (less than 5 μmol/h) under visible light irradiation,64 thus contributing little to H2 evolution of our GO/CdS sample. For the graphene sample, the Fermi level is 0 eV relative to the normal hydrogen electrode,54 so it would be difficult for graphene itself to generate H2, as demonstrated by our experiments. N-doped graphene/CdS photocatalyst exhibits the highest activity among N-graphene/CdS, graphene/CdS and GO/ CdS sample, the possibility is that N-doped graphene has the highest electrical conductivity, and the hybrid effect26 also results in greatly enhanced photocatalytic hydrogen production for N-graphene/CdS heterojunctions. It was suggested that the carbon atoms on the graphene sheets are accessible to protons that can readily transform to H2 by accepting photogenerated electrons.64 With photogenerated electrons accumulating on graphene, the Fermi level of graphene would shift upward and closer to the conduction band of CdS due to the metallic behavior of graphene, and it would be possible for N-graphene/CdS heterojunctions to generate hydrogen on the surface of graphene.54 Recently, Teng et al. reported that graphene oxide works as a metal-free photocatalyst to produce hydrogen from a watermethanol mixture, indicating hydrogen evolution occurs on the surface of graphene derivatives.64
Figure 8. (A) Transient photocurrents of the N-graphene/CdS nanocomposites and CdS with (λ g 420 nm) and without irradiation. (a) CdS (light off), (b) N-graphene/CdS (light off), (c) CdS (light on), (d) N-graphene/CdS (light on). (B) Current response vs time of the Ngraphene/CdS under chopped irradiation with an electrode potential of 0 V vs Ag/AgCl (λ g 420 nm), and (C) current response vs time of CdS under chopped irradiation with an electrode potential of 0 V vs Ag/AgCl (λ g 420 nm).
Therefore, for the N-graphene/CdS photocatalyst, we propose that there are the hydrogen evolution sites on both the surface of the CdS and N-graphene, but a majority of hydrogen evolution occurs on the surface of graphene derivatives. Meanwhile, the photogenerated holes also transport/transfer to the surface of the CdS easily and the oxidation of the sulfide ions and sulfite ions 11471
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The Journal of Physical Chemistry C occurred. It is a fact that the presence of graphene in the N-graphene/CdS photocatalyst can effectively inhibit the electronhole pair recombination and enhance photocatalytic activity in H2 evolution. Transient photocurrents were measured on a photoelectrochemical test device fabricated by drop casting the N-graphene/CdS nanocomposite dispersed alcohol onto an ITO substrate (see Experimental Section). Figure 8A shows typical currentvoltage curves of the device with (λ g 420 nm) and without irradiation of the N-graphene/CdS and CdS electrode, respectively. The current is drastically increased when the irradiation is on, and the N-graphene/ CdS sample (Figure 8A(d)) shows increased current as compared to CdS (Figure 8A(c)), while without irradiation, no currents are generated for CdS (Figure 8A(a)). This enhanced photoresponse indicates that the photoinduced electrons and holes in the CdS prefer separately transferring to the N-doped graphene. As a consequence, the radiative recombination of the electronhole pairs is hampered and the photocatalytic activity is enhanced significantly for the N-graphene/CdS photocatalyst. These photocurrent results support the photocatalytic activity results very well. Figure 8B,C shows the photocurrent response versus time of as-prepared N-graphene/CdS and CdS respectively. It is found that the photocurrent response of the N-graphene/CdS nanocomposites is fairly reversible and stable, as shown in Figure 8B, from which we can see that the current can reproducibly increase violently under each irradiation and recover rapidly in the dark. While the photocurrent response of the CdS is decreased under several on/off irradiation cycles, as clearly seen from Figure 8C. The stable photocurrent property of our N-graphene/ CdS photocatalyst is also in agreement with previous observations for reduced graphene oxide/CdSe nanocomposites, which showed stable photocurrent.65 N-doped graphene acts as protective layers to prevent CdS from photocorrosion, and can make the N-graphene/ CdS very stable under the photocatalytic reaction conditions. A more recent study has also demonstrated that graphene can protect Si against oxidation into SiO2 using graphene/Si heterojunction for photoelectrochemical systems.54 Further work is underway to establish the roles of the cocatalyst and photocatalyst in photocatalytic H2 production.
4. CONCLUSIONS In summary, by loading N-graphene as a cocatalyst, we have successfully synthesized N-graphene/CdS heterostructures as photocatalyst for splitting water under visible light irradiation. The as-prepared N-graphene/CdS nanocomposites have been demonstrated to be highly active photocatalysts for hydrogen evolution under visible light irradiation and the highest photocatalytic activity was found for the sample with a content of 2 wt % N-graphene doping. This finding further implies that the performance of catalysts can be increased by the formation of N-graphene/CdS heterojunctions. Our study not only presents a possibility for the use of N-graphene as a substitute for noble metals in the photocatalytic H2 production but also confirms an important concept that the proper junction structure between cocatalyst and semiconductor is crucial for high photocatalytic activity. N-graphene as a cocatalyst can prevent CdS from photocorrosion under light irradiation; N-graphene can be used as a charge collector to promote separation and transfer of photogenerated carriers, and finally N-graphene can make hydrogen evolution easier and show considerable photocurrent under moderate conditions. By profiting from these functions of N-graphene, the N-graphene/CdS heterojunction shows
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remarkable photocatalytic ability for the continuous and stable production of hydrogen from water. It can be expected that our findings will be the starting point for N-doped graphene as a protective material not only for CdS but also for other semiconductors and thus promote their potential application for hydrogen production by using solar energy.
’ ASSOCIATED CONTENT
bS
Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: 86-551-3602346; Fax: 86-551-3600724; E-mail: anwuxu@ ustc.edu.cn.
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