Reduced Graphene Oxide Nanocomposite with

Sep 12, 2014 - Zhiping Yan, Xingxing Yu, Ali Han, Peng Xu, and Pingwu Du*. CAS Key Laboratory of Materials for Energy Conversion, Department of ...
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Noble-Metal-Free Ni(OH)2‑Modified CdS/Reduced Graphene Oxide Nanocomposite with Enhanced Photocatalytic Activity for Hydrogen Production under Visible Light Irradiation Zhiping Yan, Xingxing Yu, Ali Han, Peng Xu, and Pingwu Du* CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, and the Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), University of Science and Technology of China, Hefei, China 230026 S Supporting Information *

ABSTRACT: A series of nickel hydroxide-modified cadmium sulfide/reduced graphene oxide (Ni(OH)2-CdS/rGO) nanocomposites were synthesized and characterized. The photocatalytic activity of the as-prepared Ni(OH)2-CdS/rGO materials for hydrogen production from water under visible light irradiation (λ > 420 nm) was investigated. The results demonstrated that Ni(OH)2 is an efficient cocatalyst for photocatalytic hydrogen production and rGO can significantly enhance the rate of photocatalysis. The optimal Ni(OH)2 loading was found to be 1.0 wt %, giving a rate for hydrogen production of 4731 μmol·h−1·g−1, which is nearly 10 times higher than that of CdS/rGO photocatalyst under the same condition. This work demonstrated the synergetic effect of Ni(OH)2 and rGO to enhance catalytic activity for visible light-driven hydrogen production.

1. INTRODUCTION The increasing global demand for clean energy has stimulated a new wave of research activity on efficient utilization of solar energy. Photocatalytic hydrogen production over semiconductors has attracted extensive attention during the past few decades. A variety of active photocatalysts for hydrogen production, including metal oxides,1−4 metal hydroxides,5,6 oxynitrides,7,8 sulfides,9,10 and metal-free semiconductors have been widely studied in the literature.11,12 However, most oxide semiconductors, such as titanium dioxide (TiO2), mainly absorb the ultraviolet photons of solar light due to their large bandgaps (3.0 eV for rutile and 3.2 eV for anatase). Cadmium sulfide (CdS), one of the most studied metal sulfide materials,13−15 has a high visible light-driven photoactivity for hydrogen production because of its low band gap energy (2.4 eV). However, prolonged irradiation of CdS suspensions leads to decomposition of CdS into Cd2+ and S (or sulfate in the presence of oxygen) because of the high oxidation ability of the holes.16 To efficiently transport the holes, other semiconductors with suitable bandgaps have been combined with CdS to form heterojunction composites, which facilitates interfacial charge transfer and further improves the stability of CdS during photocatalysis. The typical semiconductors used for this purpose are TiO2,17,18 Cu2O,19 and ZnO.20,21 In addition, cocatalysts made of noble metals are widely used to promote hydrogen evolution reaction, such as Pt,22 Au,23,24 Rh,25 and Pd.26 Unfortunately, the high cost of noble metals hamper their practical application. Despite these attempts, the photocorrosion and low-separation efficiency of electron−hole pairs © 2014 American Chemical Society

are still the principal problem of CdS-based photocatalytic composites. Therefore, it is essential to develop photocatalysts with high photocatalytic activity and good stability for efficient hydrogen production. Graphene, possesses large specific surface area (>2600 m2 −1 g ), high thermal conductivity (>5000 W m−1 K−1), excellent mobility of charge carriers (>200 000 cm2 V−1 s−1), good stability, and tunable surface properties.27−29 Thus, since its discovery in 2004,30−32 graphene has attracted immense attention in many applications such as nanoelectronics, sensors, catalysts, and energy conversion. A variety of semiconductor/ reduced graphene oxide composites have been reported in the literature for photocatalysis. These semiconductors mainly include metal oxides (e.g., Cu2O,33 Fe2O3,34 NiO,35 and WO336), metal sulfides (e.g., ZnS37 and MoS238), metallates (e.g., Bi2WO639 and BiVO440), and other nanomaterials (e.g., C3N441). In this present study, we report the use of Ni(OH)2 as an efficient cocatalyst on CdS/rGO composite photocatalyst for visible light-driven hydrogen production. Recently, our group42 and other groups43−45 reported on the use of Ni(OH)2 as a cocatalyst to promote the efficiency of photocatalytic hydrogen production. In the presence of rGO, the rate of photocatalytic hydrogen production can be highly enhanced. The semiconductor composites were synthesized via a simple precipReceived: July 1, 2014 Revised: August 25, 2014 Published: September 12, 2014 22896

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Figure 1. (A) TEM image of rGO. (B) TEM image of Ni(OH)2-CdS/rGO sample (1.0 wt % Ni(OH)2). (C, D) HR-TEM images of Ni(OH)2-CdS/ rGO sample (1.0 wt % Ni(OH)2).

and held at 180 °C for 12 h. After the autoclave was cooled to room temperature, the product was washed with water three times and then dried in an oven at 60 °C for 12 h. (The weight ratio of GO:CdS = 1:2.) The bare CdS was prepared under the same conditions without adding GO. Synthesis of Ni(OH)2-CdS/rGO composites: 0.5 g of CdS/ rGO was first dispersed in 20 mL of 1.0 M NaOH solution, and then a calculated amount of Ni(NO3)2·6H2O was added. A clear solution was formed after stirring for 12 h at room temperature. After that, the mixture was centrifuged, washed by distilled water and ethanol 3 times, and dried in an oven at 353 K overnight to obtain the final Ni(OH)2-CdS/rGO nanocomposites. (The mass ratios of Ni(OH)2 in Ni(OH)2-CdS/ rGO composite are 0%, 0.5%, 1.0%, 2.0%, and 3.0%, respectively.) Ni(OH)2-CdS was prepared by the same method for the control experiment. 2.3. Characterization. Transmission electron microscopy (TEM) as well as high resolution transmission electron microscopy (HR-TEM) images were collected using a JEM2010 electron microscope, operated at an acceleration voltage of 200 kV. The crystal structure and phase identification of the composite photocatalysts were performed by powder X-ray diffraction (XRD, D/max-TTR III) using graphite monochromatized Cu Kα radiation of 1.54178 Å, operating at 40 kV and 200 mA. The scanning rate was 5° min−1 from 10° to 70° (2θ). Raman spectra were obtained using Renishaw inVia Raman microscope with 514 nm argon ion laser at room temperature. X-ray photoelectron spectroscopy (XPS) measurement was performed using an ESCALAB 250 X-ray photoelectron spectrometer. Ultraviolet−visible diffuse reflectance spectra were recorded using a UV−visible spectrophotometer (SOLID 3700 UV−vis spectrometer). The transient photoluminescence (PL) decay spectra were carried out on a FluoroHub-B spectrometer at room temperature.

itation method and characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), highresolution transmission electron microscopy (HR-TEM), ultraviolet−visible diffuse reflectance spectroscopy (UV−vis DRS), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) to identify their physical properties. The effect of Ni(OH)2 on the photocatalytic hydrogen evolution of CdS/ rGO composite is investigated and discussed in detail.

2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals, including graphite powder, sulfuric acid (H2SO4, 98.0%), sodium nitrate (NaNO3, 99.0%), hydrogen peroxide (H2O2, 30%), potassium permanganate (KMnO4, 98.0%), sodium sulfide (Na2S, 98.0%), sodium sulfite (Na2SO3, 98.0%), cadmium acetate dihydrate (Cd(OAc)2· 2H2O, 98.5%), thiourea (NH2CSNH2, 99.0%), sodium hydroxide (NaOH, 96.0%), nickel nitrate hexahydrate (Ni(NO3)2·6H2O, 98.0%), sodium sulfate (Na2SO4, 99.0%), and ethanol (99.5%), were all commercially acquired (Aldrich or Acros) and used without further purification. 2.2. Synthesis of the Photocatalysts. Synthesis of graphene oxide (GO): GO was synthesized from graphite powder by a modified Hummers method.46 GO prepared by this way is widely used to synthesize reduced graphene oxide in the literature.3,47 Fabrication of CdS/rGO: 0.4 g of GO was added into 40 mL of distilled water and ultrasonicated for 2 h to form a stable dispersion. Then, a calculated amount of Cd(OAc)2·2H2O and NH2CSNH2 (mole ratio of Cd(OAc)2·2H2O:NH2CSNH2 = 1:3) was added into the above GO aqueous solution, and the mixed solution was stirred for 3 h at room temperature. Thereafter, the pH was tuned to 10 with 0.5 M NaOH solution, and the reaction was run for an additional 2 h at room temperature under vigorous stirring. Next, the resulting solution was transferred into a 100 mL Teflon-lined autoclave 22897

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can be observed that all the as-prepared Ni(OH)2-CdS/rGO composites exhibit XRD patterns similar to that of CdS/rGO. The three broad peaks located at 2θ = 26.9°, 44.2°, and 52.2° correspond respectively to the (111), (220), and (311) planes of the cubic CdS.51,52 It should be noted that all samples are mainly composed of CdS crystalline phases and show no diffraction peaks of the Ni(OH)2 and rGO, probably due to the low amount of Ni(OH)2 and the relatively low diffraction intensity of rGO. Figure S1 shows the diffraction patterns of pure Ni(OH)2. The results also indicate that the presence of Ni(OH)2 and rGO does not influence the crystalline phases of CdS. The Ni(OH)2-CdS/rGO sample was further studied by Xray photoelectron spectroscopy (XPS) (Figure 3). Figure 3A shows that the main elements on the surface of the composite are Cd (405.65 eV), C (284.85 eV), S (162.05 eV), O (531.95 eV), and Ni (856.45 eV). Figure 3B shows the high resolution C 1s XPS spectra of the Ni(OH)2-CdS/rGO sample containing 1.0 wt % Ni(OH)2. Four different peaks maximized at 284.69, 286.60, 287.85, and 288.97 eV are observed. The main peak of C 1s located at 284.69 eV is attributed to the sp2 carbon in the graphitic structure, whereas the other three peaks at 286.60, 287.85, and 288.97 eV are assigned respectively to C−O, C O, and COO- functional groups in the narrow areas of the C 1s region. The presence and locations of these peaks are in good agreement with previous studies.53,54 In Figure 3C, the sample exhibits two large Cd 3d peaks: a higher peak at 405.65 eV corresponding to Cd 3d5/2 and a lower peak at 412.40 eV corresponding to Cd 3d3/2.55 The Ni 2p XPS spectrum is shown in Figure 3D. The binding energy of Ni 2p3/2 centered at 855.78 eV is the typical Ni2+ phase of Ni(OH)2 and the fitting peak at 874.70 eV is assigned to the binding energy of Ni2+ 2p1/2.56 Due to the low content of nickel, the intensities of these two peaks are not high. The XPS results further confirm the existence of Ni(OH)2 and CdS in the Ni(OH)2-CdS/rGO sample. After visible light irradiation for 12 h, the XPS spectrum of Ni 2p of the sample exhibits a slight change (see Figure S2). A new peak of Ni 2p3/2 appears at 854.6 eV, indicating the formation of NiO. One possible explanation is that the photoinduced electrons are transferred to reduce partial Ni2+ to Ni0 atoms in Ni(OH)2 nanoparticles, further forming Ni particles. Because of the unstability of Ni particles in air, they are easily oxidated into NiO. As shown in Figure S3, the peaks of Ni 2p3/2 at 855.78 eV and Ni2+ 2p1/2 at 874.70 eV remain unchanged, suggesting that the sample still contains Ni(OH)2 after the photocatalytic H2 production for 12 h. Figure S3 shows the Raman spectra of GO and Ni(OH)2CdS/rGO samples. Both samples have two prominent peaks at 1350 and 1570 cm−1, corresponding to the well documented D band and G band, respectively.9,40 The significant change from GO to Ni(OH)2-CdS/rGO was also reflected in the Raman spectra. The calculated I(D)/I(G) intensity ratios for GO and Ni(OH)2-CdS/rGO are 0.94 and 1.13, respectively. The higher ratio of I(D)/I(G) in the as-prepared Ni(OH)2-CdS/rGO nanocomposite suggests a successful reduction of GO to rGO. Figure 4 shows the UV−vis diffuse reflectance absorption spectra of the CdS/rGO and Ni(OH)2-CdS/rGO composites with different ratios of Ni(OH)2. It is observed that the absorption intensity of the composites in the visible region increases with the Ni(OH)2 content. Furthermore, the optical absorption band edge of the Ni(OH)2-CdS/rGO nanocomposites shows obvious redshift compared with that of

Photocurrent measurements were performed on a CHI 602E electrochemistry potentiostat in a standard three-electrode configuration with the photocatalyst-coated ITO as the working electrode, an Ag/AgCl as a reference electrode, and Pt wire as the counter electrode. Irradiation was provided by a 300 W xenon lamp with a cutoff filter (λ > 420 nm). Sodium sulfate (Na2SO4, 0.5 M) solution was used as the electrolyte. The working electrodes were prepared by dropping a suspension (15 μL) made of Ni(OH)2-CdS/rGO, Ni(OH)2-CdS, or CdS/ rGO (the concentration of Ni(OH)2-CdS/rGO, Ni(OH)2CdS, or CdS/rGO being 50 mg/mL) onto the surface of a precleaned ITO plate using a pipet. The working electrodes were dried at room temperature. The scan rate was 50 mV/s, and all the potentials in this paper correspond to Ag/AgCl. 2.4. Photocatalytic Hydrogen Production. Photocatalytic reactions were carried out in an outer irradiation-type photoreactor (pyrex glass). A 300 W Xe-lamp equipped with a 420 nm cutoff filter was used to provide the visible light irradiations. Briefly, 0.01 g of Ni(OH)2-CdS/rGO sample was dispersed in a 20 mL aqueous solution containing 0.35 M Na2S and 0.25 M Na2SO3 as sacrificial reagents. Before irradiation, the solution was stirred for 10 min in the dark and purged with nitrogen for 15 min to remove air. After that, 5 mL of methane was added into the reactor as the internal standard. The amount of H2 evolved was determined with a gas chromatograph (GC) equipped with a TCD detector.

3. RESULTS AND DISCUSSION Figure 1A shows a typical TEM image of rGO produced in our experiments. The bare rGO has a sheet-like morphology with a clear, smooth surface. After reaction, it is clearly seen that rGO sheets are uniformly covered with nanoparticles, and there is no apparent aggregation of particles on the rGO scaffold (Figure 1B). To better study the interfacial structure, the obtained sample was further examined with HR-TEM (Figure 1C,D). As estimated from the HR-TEM images, the size of nanoparticles in these samples is in the 6−8 nm range. Figure 1D shows a distinct lattice fringe at a distance of 0.335 nm, which corresponds to the (111) plane of cubic CdS.48,49 The lattice spacing of 0.790 nm (Figure 1C,D) corresponds to the (003) crystal phase of α-Ni(OH)2.50 The crystalline phase properties of the Ni(OH)2-loaded CdS/rGO semiconductor composites with different percentage of Ni(OH)2 are reflected by the XRD patterns in Figure 2. It

Figure 2. XRD patterns of Ni(OH)2-CdS/rGO composites with different weight ratios of Ni(OH)2. 22898

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Figure 3. (A) XPS survey spectrum of Ni(OH)2-CdS/rGO composite containing 1.0 wt % Ni(OH)2; (B) High-resolution XPS spectra of C 1s peaks mainly located at 284.69, 286.60, 287.85, and 288.97 eV; (C) High-resolution XPS spectra of Cd 3d peaks at 405.65 and 412.40 eV; (D) Highresolution XPS spectra of Ni 2p peaks mainly at 855.78 and 874.70 eV.

Figure 4. UV−vis diffuse reflectance spectra of the Ni(OH)2-CdS/ rGO nanocomposites with different weight ratios of Ni(OH)2. Figure 5. Photoluminescence decay measured for Ni(OH)2-CdS and Ni(OH)2-CdS/rGO photocatalysts in aqueous solutions. Test conditions: 1 × 10−5 M Ni(OH)2-CdS, 1 × 10−5 M Ni(OH)2-CdS/ rGO solution; excitation wavelength = 450 nm.

CdS/rGO, probably attributing to the interfacial interaction between Ni(OH)2 and CdS/rGO. However, when the ratio of Ni(OH)2 is higher than 1.0 wt %, the absorption intensity decreased, probably because the excess Ni(OH)2 nanoparticles block some absorbance of CdS/rGO. This inference is consistent with the photocatalytic activity for H2 evolution over Ni(OH)2-CdS/rGO, as discussed in the following part. In the previous studies,42,57,58 photoluminescence (PL) decay analysis was used to study the lifetimes of charge transfer processes. Herein, we performed the PL decay measurement for Ni(OH)2-CdS and Ni(OH)2-CdS/rGO, as shown in Figure 5. It can be clearly observed that slower PL decay occurs for Ni(OH)2-CdS than the decay for Ni(OH)2-CdS/rGO nanocomposite. The Ni(OH)2-CdS sample exhibits an average PL lifetime of 0.61 ns. However, with the addition of rGO, the lifetime decreases to 0.21 ns. The faster decay of PL in Ni(OH)2-CdS/rGO demonstrates the efficient electron transfer

process from CdS to Ni(OH)2 via rGO, indicating the vital role of rGO. Photocatalytic hydrogen evolution experiments were carried out over the as-prepared photocatalysts in the presence of Na2S and Na2SO3 as sacrificial reagents under visible light irradiation. Figure 6A shows the rates of H2 evolution on Ni(OH)2-CdS/ rGO composites with different amounts of Ni(OH)2. The corresponding H2 evolution rates are 1011, 4731, 1900, and 844 μmol·h−1·g−1 for the samples containing 0.5, 1.0, 2.0, and 3.0 wt % Ni(OH)2 as the cocatalyst, respectively. It is found that the rate of H2 evolution initially rises with the increasing ratio of Ni(OH)2. An optimum rate of H2 evolution is observed for the Ni(OH)2-CdS/rGO sample containing 1.0 wt % Ni(OH)2. A further increase in the amount of Ni(OH)2 results 22899

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repeated cycles. As shown in Figure 7, the Ni(OH)2-CdS/rGO photocatalyst shows excellent stability and maintains a similar

Figure 7. Cyclic H2-evolution curves for 0.01 g of Ni(OH)2-CdS/rGO sample (1.0 wt % Ni(OH)2) in a 20 mL of aqueous solution containing 0.175 M Na2S and 0.125 M Na2SO3. A 300 W xenon arc lamp was used as the light source with a long-pass cut filter (λ > 420 nm). The experiment was run at room temperature.

photocatalytic activity for more than 12 h, indicating good stability of this material for H2 production. In order to further investigate the photoelectrochemical properties, the electrodes coated by CdS/rGO and Ni(OH)2CdS/rGO were measured using an electrochemical potentiostat both under visible light irradiation (λ > 420 nm) and without irradiation. Figure 8A shows the current−voltage curve of the

Figure 6. (A) Hydrogen evolution rate in a 20 mL aqueous solution containing 0.35 M Na2S and 0.25 M Na2SO3 suspended with 0.01 g of Ni(OH)2-CdS/rGO samples containing 0.5, 1.0, 2.0, and 3.0 wt % Ni(OH)2, respectively. (B) Hydrogen evolution rate over CdS/rGO, Ni(OH)2-CdS (1.0 wt % Ni(OH)2), and Ni(OH)2-CdS/rGO (1.0 wt % Ni(OH)2) in a 20 mL aqueous solution containing 0.35 M Na2S and 0.25 M Na2SO3. A 300 W xenon arc lamp was used as the light source with a long-pass cut filter (λ > 420 nm). The experiments were run at room temperature.

in a decrease of the rate for photocatalytic H2 evolution. The visible light absorption of CdS/rGO is partly shielded by the loaded Ni(OH)2 particles, leading to a decrease of irradiation passing through the photocatalysts. Similar observations have been previously reported.42,59 To further confirm the critical roles of Ni(OH)2 and rGO in the photocatalytic experiments, CdS/rGO, Ni(OH)2-CdS, and Ni(OH)2-CdS/rGO samples were used for comparison under the same condition, as shown in Figure 6B. It was found that CdS/rGO and Ni(OH)2-CdS have relatively low activities, with the rates of hydrogen production at 511 and 1839 μmol·h−1·g−1, respectively. However, a significant improvement of the photocatalytic activity is observed when Ni(OH)2 particles were attached to CdS/rGO composites. The corresponding H2 evolution rate is 4731 μmol·h−1·g−1, nearly 10 times more than that of CdS/ rGO. Control experiment using pure Ni(OH)2 was also performed for photocatalytic hydrogen production under the same condition as that for Ni(OH)2-CdS/rGO. However, no hydrogen was detected even after 10 h of visible light irradiation, indicating the pure Ni(OH)2 itself did not show any photocatalytic activity. All of the results indicate that Ni(OH)2 can be used as an efficient cocatalyst for H2 production and rGO plays an important role in photoinduced electron transfer. Further experiment was performed to confirm the stability of the Ni(OH)2-CdS/rGO composites on photocatalytic activity under visible light irradiation. The reaction system was evacuated every 3 h and the process was carried out for

Figure 8. (A) Photocurrents of CdS/rGO and Ni(OH)2-CdS/rGO (1.0 wt % Ni(OH)2) in a 0.5 M Na2SO4 solution with and without visible light irradiation: (a) CdS/rGO (light off), (b) CdS/rGO (light on), (c) Ni(OH)2-CdS/rGO (light off), (d) Ni(OH)2-CdS/rGO (light on). (B) Photocurrent responses of CdS/rGO, Ni(OH)2-CdS, and Ni(OH)2-CdS/rGO samples under chopped irradiation with an electrode potential of 0 V versus Ag/AgCl. A 300 W xenon arc lamp was used as the light source with a long-pass cut filter (λ > 420 nm). 22900

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method. The Ni(OH)2-CdS/rGO composites show high photocatalytic H2 production activity with an optimal rate as high as 4731 μmol·h−1·g−1 for the sample containing 1.0 wt % Ni(OH)2, which is nearly 10 times higher than that of CdS/ rGO composite. The result indicates Ni(OH)2 is a good cocatalyst to enhance the photocatalytic activity for hydrogen production. The positive synergetic effect of CdS, rGO, and Ni(OH)2 can efficiently suppress charge recombination, improve interfacial charge transfer, and provide active sites for photocatalytic H2 evolution. This study provides a new noble-metal-free system in the development of rGO-based semiconductor nanocomposites for visible light-driven photocatalytic H2 evolution.

CdS/rGO and Ni(OH)2-CdS/rGO photoelectrodes. As can be seen, the Ni(OH)2-CdS/rGO electrode generates much higher currents than the CdS/rGO electrode under the same applied potential without light irradiation (Figure 8A, plot a,c). Both samples show increasing currents when the irradiation is on, and the Ni(OH)2-CdS/rGO sample still shows higher currents compared to CdS/rGO (Figure 8A, plot b,d). Therefore, Ni(OH)2 is clearly demonstrated to be a good cocatalyst for H2 evolution. Figure 8B shows the photocurrent responses of CdS/rGO, Ni(OH)2-CdS, and Ni(OH)2-CdS/rGO under intermittent visible light illumination (λ > 420 nm). All the photoelectrodes show very low dark current (