Tunable Photodeposition of MoS2 onto a Composite of Reduced

Publication Date (Web): July 28, 2014 ... The photocatalytic HER rate under the synergic condition is 4.3 times as high as that under antisynergic con...
0 downloads 0 Views 4MB Size
Download PDF
Article pubs.acs.org/JPCC

Tunable Photodeposition of MoS2 onto a Composite of Reduced Graphene Oxide and CdS for Synergic Photocatalytic Hydrogen Generation Yuexiang Li,* Hao Wang, and Shaoqin Peng Department of Chemistry, Nanchang University, Nanchang 330031, China S Supporting Information *

ABSTRACT: Recently, MoS2 as an excellent cocatalyst for hydrogen evolution reaction (HER) has attracted extensive attention. In this work, MoS2 was controllably loaded on the composite of reduced graphene oxide (rGO) and CdS (rGO/CdS) by a facile photoreduction method at different pHs. At low pH 7, MoS2 deposits on the surface of the CdS particles of the composite. However, at high pH 11, it loads on the exposed rGO. When MoS2 is on the rGO, the transfer of the photoexcited electron from CdS to rGO is compatible with the HER at MoS2 (synergic effect), whereas the transfer is incompatible with the HER when it is on the CdS (antisynergic effect). Moreover, the MoS2 deposited on the CdS decreases the photoabsorption and photoactivity of CdS, and the effect is avoided when MoS2 is on the rGO. The photocatalytic HER rate under the synergic condition is 4.3 times as high as that under antisynergic condition. This work would open a promising way to design and fabricate the efficient composite photocatalysts.

hydrogen production.16 However, in this work, the precious metal Pt was used as a cocatalyst. For the G/CdS composite photocatalyst, loading the MoS2 cocatalyst onto graphene should be more effective for HER than onto CdS. The edge sites of MoS2 are catalytically active for HER, whereas their basal planes were inert.21 Graphene as a supporting matrix can improve the directing growth of few layer MoS2 sheets with the abundant exposed edges.22,23 When MoS2 grows directly on the functionalized rGO, there is strong chemical and electrical coupling between MoS2 and rGO, which enhances the charge transport through their interface. As a result, the HER performance of the rGO/MoS2 hybrid is much better than that of MoS2 alone.22,23 On the other hand, MoS2 (band gap energy of 1.17 eV) can shield the light absorption of CdS when it is deposited on CdS. If MoS2 can be selectively loaded on the rGO of rGO/CdS, the parasitic light absorption can be avoided. As a result, the total HER activity enhances.3 In this work, we synthesized first a rGO/CdS nanocomposite by a solvothermal method using dimethyl sulfoxide (DMSO) as a sulfide source and reducing agent.16 MoS2 was then loaded onto the rGO/CdS by a facile photodeposition method using (NH4)2MoS4 as a precursor, and its selective deposition onto rGO can be achieved by only adjusting the solution pH. To the best of our knowledge, no studies on controllable deposition of MoS2 on rGO/CdS for the photocatalytic H2 production have been reported. The HER activity of the prepared composite photocatalyst with MoS2 was evaluated in a lactic acid

1. INTRODUCTION Conversion of solar energy into hydrogen via semiconductor photocatalysts has attracted extensive attention during the past decades for the utilization of hydrogen energy.1−7 CdS is an attractive photocatalyst due to the narrow band gap (2.4 eV) for effective absorption of sunlight and the suitable conduction band edge for HER.8−10 To accelerate the HER, the cocatalyst loaded on the semiconductors is necessary because it can offer low activation energy for HER. Precious metal Pt, a highly efficient HER cocatalyst, is too expensive and rare to be suitable for the scalable production. Molybdenum disulfide (MoS2) is a promising nonprecious catalyst for HER since it is earthabundant, inexpensive, and efficient.11 It has been reported that MoS2 loaded on the photocatalyst CdS is an excellent HER cocatalyst.12,13 However, in these works, the MoS2 was deposited on the CdS surface via a hightemperature process using (NH4)2MoS4 as a precursor in the presence of toxic H2S gas. Recently, Mai et al. loaded MoS2 cocatalyst onto a semiconductor ZnxCd1−xS via a simple in situ photodeposition method under mild conditions.14 The hybrid ZnxCd1−xS/MoS2 exhibits efficient photoactivity for hydrogen evolution. Graphene (G) as a two-dimensional (2D) platform has been considered as an excellent supporting matrix for semiconductor particles, because it possesses excellent electron conductivity and high specific surface area.15 It can receive and transport photogenerated electrons of semiconductors upon light irradiation. In order to enhance the activity of the semiconductors, various graphene-based composite photocatalysts have been reported.8,16−20 For example, Li et al. reported that a CdS− graphene composite showed highly efficient photocatalytic © 2014 American Chemical Society

Received: June 2, 2014 Revised: July 17, 2014 Published: July 28, 2014 19842

dx.doi.org/10.1021/jp5054474 | J. Phys. Chem. C 2014, 118, 19842−19848

The Journal of Physical Chemistry C

Article

(an electron donor) solution under visible light irradiation (λ ≥ 420 nm), and the synergic and antisynergic effects of the MoS2 have been found.

the suspension was measured with a Malvern NANO ZS90 Zetasizer. Photocatalytic Reaction. Photocatalytic reaction was conducted in a 200 mL Pyrex flask with a flat window for irradiation whose top was sealed with a silicone rubber septum. Typically, 50 mg of the prepared rGO/CdS/MoS2 and 100 mL of 10 vol % lactic acid solution were added to the cell, and the resultant mixture was dispersed in an ultrasonic bath for 5 min. The suspension was bubbled with N2 for 30 min to remove oxygen completely and then irradiated with a 350 W Xe lamp equipped with a cutoff filter (λ ≥ 420 nm). The incident intensity was 34 mW·cm−1. The produced H2 was analyzed on a gas chromatograph (thermal conductivity detector, 13X molecular sieve packed column, N2 gas carrier). In the stability test for rGO/CdS/MoS2-11, the reaction system was flushed with N2 for 30 min each 5 h of irradiation. The apparent quantum efficiency at 420 nm was estimated by the following equation.

2. EXPERIMENTAL SECTION Materials Preparation. The reagents were analytical grade, except for graphite powder (chemical pure), and were used without any further purification. Graphene oxide (GO) was synthesized by a modified Hummers’ method.8 A composite of reduced graphene oxide and CdS (labeled as rGOx/CdS, where x represents wt % of rGO) was prepared by a similar manner as described in ref 16. Typically, a given amount of the synthesized GO and 2.66 g of Cd(Ac)2·2H2O were added in 80 mL of DMSO, and the mixture was dispersed in an ultrasonic bath. The mixture was transferred into a 100 mL Teflon-lined autoclave, remained at 180 °C for 12 h, and cooled to room temperature. The precipitates from the mixture were collected by filtration, then washed with ethanol and distilled water several times to remove the residue of DMSO, and last dried in an oven at 60 °C for 12 h. The obtained samples with weight ratios of GO to Cd(Ac)2·2H2O being 0, 0.5, 1.0, 1.5, and 2.5 wt % were denoted as rGO0/CdS (pure CdS), rGO0.5/ CdS, rGO1.0/CdS, rGO1.5/CdS, and rGO2.5/CdS, respectively. The pure reduced graphene oxide (rGO) was prepared by the same process without adding Cd(Ac)2·2H2O. Photodeposition of MoS2 on rGOx/CdS. A 0.080 g portion of rGOx/CdS and 13 mL of 1.0 mg·mL−1 (3.8 × 10−3 mol·L−1) (NH4)2MoS4 were added into 87 mL of 15 vol % triethanolamine (TEOA) solution whose pH was adjusted by HCl and NaOH to 7, 8, 9, 10, 11, or 12, respectively. The suspension was bubbled with N2 for 30 min to remove oxygen completely and then irradiated with a 350 W Xe lamp equipped with a cutoff filter (λ ≥ 420 nm) for 60 min. The incident intensity was 34 mW·cm−1. The MoS42− was reduced into the loaded MoS2 by the photoexcited electrons of rGOx/CdS. The product was obtained by filtration, washing with distilled water several times, and drying in an oven at 60 °C for 12 h. The loaded amount of MoS2 was determined by measuring the MoS42− concentrations in both the reacted solution and the washed solution on a spectrophotometer at 468 nm.24 The obtained sample was labeled as rGOx/CdS/MoS2-y, where y indicates the deposition pH. Characterization. X-ray diffraction patterns of the prepared samples were obtained on a Bede D1 System multifunction X-ray diffractometer. The optical properties were analyzed on a UV−vis diffuse reflectance spectrometer (HITACHIU-3310). The X-ray photoelectron spectra were measured on a UGESCALAB210. The C 1s peak set at 284.80 eV was used as an internal reference for absolute binding energy. The transmission electron microscopy (TEM) and high-resolution electron microscopy (HRTEM) images were observed with a JEM-2100F electron microscope equipped with an energy-dispersive X-ray spectroscopy (EDS). The specific surface areas were obtained on an ST-08 surface analyzer by the BET method using N2 adsorbent. Infrared (IR) spectra were recorded on a Nicolet 380 spectrometer by a transmission method using the KBr pellet technique. A 0.010 g portion of the as-prepared rGO1.5/CdS or rGO was added into 100 mL of distilled water, and the mixture was dispersed in an ultrasonic bath. A 2.5 mL aliquot of the dispersion was diluted with 22.5 mL of 9.5 × 10−3 mol·L−1 TEOA solution whose pH was adjusted by HCl and NaOH. After 30 min of ultrasonic dispersion, the zeta potential of

ΦH2 = 2 ×

mole of hydrogen evolved mole of incident photon

The reaction conditions were identical to those for the above photocatalytic reaction except for the light source. An LED lamp (420 nm, UVEC-4, Shenzhen LAMPLIC Science Co Ltd., China) instead of the Xe lamp was used as the light source, and the light intensity was 2.6 mW·cm−2.

3. RESULTS AND DISCUSSSION Performance of rGOx/CdS. Effects of rGO content on properties of rGOx/CdS were investigated. The X-ray diffraction (XRD) patterns (Figure 1) of pure CdS and

Figure 1. XRD patterns of CdS and rGOx/CdS.

rGOx/CdS show three peaks at 26.5, 44.0, and 52.0°, which are assigned to the diffractions of the (111), (220), and (311) planes of cubic CdS (JCPDS 80-0019), respectively. Compared with that of pure CdS, the crystallinity of CdS particles of the composite enhances with the increase of rGO content, consistent with the previous report.16 The average crystallite sizes of the different samples were calculated via the Scherrer formula using the (111) facet diffraction peak (Table 1). 19843

dx.doi.org/10.1021/jp5054474 | J. Phys. Chem. C 2014, 118, 19842−19848

The Journal of Physical Chemistry C

Article

displays three diffraction rings that correspond to the (111), (220) and (311) planes of the cubic CdS, which is in good agreement with the XRD results from Figure 1. The size of CdS nanoparticles (Figure 2b) is 11−15 nm, which is consistent with the datum (13 nm) obtained by the Scherrer formula (Table 1). The CdS nanoparticles display three lattice fringes with lattice spacings of 0.336, 0.206, and 0.177 nm, corresponding to the (111), (220), and (311) lattice planes of cubic CdS, respectively. Site and Content of MoS2 Deposited on rGOx/CdS. When rGOx/CdS was irradiated in the presence of (NH4)2MoS4, the photoexcited electron of CdS would reduce the Mo6+. The chemical state of Mo photodeposited on rGO1.5/ CdS at pH 11 (rGO1.5/CdS/MoS2-11) was determined by X-ray photoelectron spectroscopy (Figure 3). The Mo 3d doublet peaks at 232.4 and 229.4 eV indicate the presence of a +4 oxidation state of Mo, while the S 2p doublet peaks at 162.1 and 163.3 eV suggest the existence of S2−.12,13 The results confirm that the deposited Mo exists in MoS2, which is in agreement with Nguyen et al.’s report.14 IR spectra (Figure S2, Supporting Information) show that, for rGO1.5/CdS/MoS2-11, there is a characteristic peak at 480 cm−1, corresponding to γas(Mo−S) of MoS2,25 whereas, for rGO1.5/CdS, there is not. This confirms also the existence of MoS2 in rGO1.5/CdS/ MoS2-11. The depositing sites of the MoS2 on rGO1.5/CdS prepared at different pHs were characterized with TEM, HRTEM, and energy-dispersive X-ray spectroscopy (EDS). As shown in Figure 4b, the layered MoS2 loads on CdS particles of rGO1.5/ CdS prepared at pH 7, whereas, at pH 11, MoS2 cannot be observed on the CdS particles (Figure 4d), indicating indirectly that MoS2 is loaded on the rGO of rGO1.5/CdS. In addition, Figure 4b shows that the deposited MoS2 is amorphous. We investigated also the deposited sites of MoS2 by EDS (Figure S3, Supporting Information). Mo contents of two characteristic areas, the area of agglomerated CdS particles (area 1, without exposed rGO) and the area of dispersed CdS particles (area 2, with exposed rGO), were detected. For the deposition at low pH 7, MoS2 loads only on the agglomerated CdS particles (area 1), whereas, at medium pH 9, it deposits on both areas. However, at high pH 11, MoS2 deposits almost on the exposed rGO (area 2). These confirm further the above HRTEM observation. The deposition of MoS2 is controllable: at low pH,

Table 1. Properties of CdS and rGOx/CdS

a

rGO content (x) (wt %)

specific surface area (m2·g−1)

crystal size of CdSa (nm)

0 (CdS) 1.0 1.5 2.5

28.0 49.0 57.0 62.5

10.6 12.5 13.0 13.7

The size is calculated by the Scherrer formula using the (111) facet.

The crystallite size of CdS particles of rGOx/CdS increases from 10.6 nm (pure CdS) to 13.7 nm (rGO2.5/CdS) with increasing rGO content, indicating that rGO improves the growth of CdS nanoparticles.16 The BET surface area enhances also from 28.0 to 62.5 m2 g−1, which can be attributed to that the atomic layered rGO as the supporting matrix of CdS nanoparticles prevents their agglomeration. As a result, a greater specific surface area of the composites can provide more surface active sites for photocatalytic reaction. UV−vis diffuse reflectance absorption spectra of samples rGOx/CdS (Figure S1, Supporting Information) show that their absorbance over about 550 nm increases with increasing rGO content due to the light absorption of rGO.16 Figure 2 shows transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of rGO1.5/CdS. As shown in Figure 2a, CdS particles grow on the rGO sheet and there are the exposed areas of rGO (graphene). The selected area electron diffraction (SAED) pattern (inset in Figure 2a)

Figure 2. TEM (a) and HRTEM (b) images of rGO1.5/CdS. Inset of (a): the SAED pattern.

Figure 3. XPS spectra of the Mo 3d and S 2p regions for rGO1.5/CdS-MoS2-11. 19844

dx.doi.org/10.1021/jp5054474 | J. Phys. Chem. C 2014, 118, 19842−19848

The Journal of Physical Chemistry C

Article

on CdS of rGO1.5/CdS decreases quickly with increasing the pH. As shown in Table 2, with the increase of the pH, the deposition amount of MoS2 on rGO1.5/CdS decreases from 7 to 11. However, at pH 12, although the adsorption amount is the lowest, the loading content is the highest. Figure 5 shows

Figure 4. TEM and HRTEM images of rGO1.5/CdS/MoS2-7 (a, b) and rGO1.5/CdS/MoS2-11 (c, d).

it deposits selectively on the CdS particles, whereas, at high pH, it loads on the exposed rGO sheet. The controllable deposition of MoS2 on rGO/CdS can be shown in Scheme 1.

Figure 5. UV−vis absorption spectra of rGO1.5/CdS and rGO1.5/ CdS/MoS2 prepared at different pHs (7, 11, and 12).

UV−vis diffuse reflectance absorption spectra of samples rGOx/ CdS/MoS2 deposited at different pHs. The absorbance of the samples over about 550 nm increases with the increase of MoS2 loading content owing to the absorption of the narrow-band semiconductor MoS2. Both rGO1.5/CdS/MoS2-7 and rGO1.5/ CdS/MoS2-12 possess high MoS2 loading, but for the absorbance edge of the former, a remarkable red shift happens compared with that of rGO1.5/CdS, whereas, for that of the latter, the red shift is smaller, suggesting also MoS2 loading on CdS at pH 7 and on rGO at pH 12. Zeta potentials of rGO1.5/CdS and rGO (Figure 6) were measured to understand the different deposition contents and

Scheme 1. Schematic Illustration of Tunable Photodeposition of MoS2 on rGO1.5/CdS

Table 2. Comparison of Adsorption of MoS42− on CdS, rGO, and rGO1.5/CdS with Load of MoS2 on rGO1.5/CdS by the Photodeposition Method at Different pH Values from Aqueous (NH4)2MoS4 Solution amount of MoS42− adsorbed on CdS, rGO, and rGO1.5/CdS (%)

amount of MoS2 loaded on rGO1.5/CdS

pH

pure CdS

pure rGO

rGO1.5/CdS

(%)

(wt %)

7 9 11 12

26 14 2.8 2.0

8.4 8.7 7.8 7.6

35 26 8.6 5.6

52 28 17 77

4.8 2.6 1.5 7.0

Table 2 indicates that both pure CdS and rGO can adsorb MoS42−. With increasing the pH, the adsorption amount of MoS42− on pure CdS decreases quickly, whereas that on pure rGO remains almost constant. The adsorption amount of MoS42− on the composite rGO1.5/CdS decreases also with the increase of the pH. When CdS particles are loaded on rGO, their agglomeration decreases (Table 1). As a result, the true adsorption amount of MoS42− on CdS of rGO1.5/CdS should be higher than that on pure CdS. Note that most of the rGO surface of rGO1.5/CdS is covered with CdS nanoparticles (Figure 1). Thus, the actual adsorption amount of MoS42− on rGO of rGO1.5/CdS should be much lower than that on pure rGO. On the basis of the above analyses, we conclude that the adsorption of MoS42− on CdS of rGO1.5/CdS is major, whereas that on the rGO is minor, and the adsorption amount of MoS42−

Figure 6. Zeta potentials of rGO1.5/CdS and rGO at different pHs.

sites of MoS2 at different pH values. The potentials of the rGO1.5/CdS and rGO at pH 7 are negative. With increasing pH, the zeta potential of rGO1.5/CdS becomes more negative, 19845

dx.doi.org/10.1021/jp5054474 | J. Phys. Chem. C 2014, 118, 19842−19848

The Journal of Physical Chemistry C

Article

(pH 11), the activity reaches the highest value of 99 μmol h−1, enhancing by a factor of 4.3 compared to that at pH 7. However, when the depositing pH is 12, the activity decreases to some extent (80 μmol h−1). For the photodeposition of MoS2 on rGO1.5/CdS at pH 11, the optimal deposition time is 60 min (Figure S5, Supporting Information). When rGOx/CdS/MoS2-y is irradiated by visible light, the photoexcited electrons of CdS particles can either diffuse to the surface of CdS and or inject into rGO. In general, rGO as an acceptor of the electrons generated in the CdS can effectively reduce recombination probability of the electron−hole pairs and thus enhance photocatalytic reactivity.16 However, for the sample rGO1.5/CdS/MoS2-7, due to MoS2 loading on CdS, the photoexcited electrons that injected into rGO could not effectively reduce water into hydrogen due to the absence of MoS2 cocatalyst. In this case, rGO would by contraries act as a recombination center of the electron−hole pairs, leading to the low HER activity. This indicates that the electron transferring from CdS to rGO is antisynergic with the HER at CdS. For rGO1.5/CdS/MoS2-11, because of MoS2 depositing on rGO, the effective separation of the photoinduced electron−hole pair by rGO is compatible with HER at the MoS2 on rGO (synergic effect). Scheme 2 describes the incompatibility and compati-

which is consistent with property of CdS (the negatively charged surface).26 The zeta potential of rGO decreases also with increasing pH, but enhances at pH 12. On the whole, the zeta potential of rGO is more positive than that of rGO1.5/ CdS. Three factors influence the deposition content and site of MoS2. First, a higher adsorption amount of MoS42− (Table 2) at lower pH enhances the deposition content. Second, compared to that of CdS, a lower negatively charged surface of rGO is beneficial to MoS42− diffusing and depositing onto rGO. Third and most importantly, because the CdS surface becomes more negative with increasing pH, the electrostatic repulsion between the CdS surface charges and the electrons generated in CdS increases, which decreases greatly the electron diffusion from the bulk to the surface of CdS. As a result, at higher pH, the electron can more effectively inject into rGO (more efficient separation of the electron−hole pairs), leading to selective deposition of MoS2 on rGO. At pH 12, due to the smaller negatively charged surface of rGO, MoS42− can efficiently diffuse to rGO and be reduced on rGO. Thus, the MoS2 deposition content is the largest under this condition. Because the surface charge of metal oxides or sulfides can be adjusted by pH,27 this method would be suitable for many semiconductor/rGO composites. Photocatalytic Performance. Both rGO content and MoS2 loading influence markedly the photocatalytic HER activity of CdS (Figure S4, Supporting Information). CdS alone exhibits low the activity (1.3 μmol h−1). With loading MoS2 on CdS at pH 11 (CdS/MoS2-11), the H2-production rate reaches 28 μmol h−1, 21 times higher than that for pure CdS, indicating that MoS2 is an excellent HER catalyst. With increase of rGO content (wt %), the activity of rGOx/CdS/MoS2-11 enhances, reaches a maximal value at 1.5 wt % of rGO (rGO1.5/CdS/ MoS2-11) and then decreases, similar to the result of ref 16. The apparent quantum efficiency of rGO1.5/CdS/MoS2-11 for HER at 420 nm is 9.8%. Figure 7 shows the deposition pH effect of MoS2 on the photocatalytic hydrogen evolution in lactic acid solution. For the sample rGO1.5/CdS/MoS2-7 (pH 7), a relative low activity (23 μmol h−1) can be observed. For rGO1.5/CdS/MoS2-11

Scheme 2. Schematic Illustration of Incompatibility between the Photoexcited Electron Transfer from CdS to rGO and the HER at MoS2 for rGO1.5/CdS/MoS2-7 (a), and the Compatibility for rGO1.5/CdS/MoS2-11 (b)

bility for the charge separation and the HER over rGO1.5/ CdS/MoS2 deposited at pH 7 and 11, respectively. Moreover, MoS2 deposited on CdS decreases the light absorbance of CdS. For rGO1.5/CdS/MoS2-7, there is the parasitic light absorption of MoS2 for the CdS, whereas, for rGO1.5/CdS/MoS2-11, there is not. This is the second reason why rGO1.5/CdS/ MoS2-11 exhibits higher activity than rGO1.5/CdS/MoS2-7. Yu et al. reported that the HER catalytic activity of MoS2 decreases with increasing the layer number, which is related to the hopping of electrons in the vertical direction of MoS2 layers.28 The layer number of the loaded MoS2 should increase compared to that of rGO1.5/CdS/MoS2-11 due to the highest loading content of MoS2 on rGO (Table 2). Thus, the activity of rGO1.5/CdS/MoS2-12 is lower than that of rGO1.5/CdS/ MoS2-11. A control experiment was conducted. A composite of MoS2 and GO with a weight ratio of 1.0 (denoted as MoS2/ GO) was synthesized first by pyrolysis of a mixture of GO and (NH4)2MoS4 (see the Supporting Information),29 and then CdS was deposited on the composite by the above-mentioned solvothermal method to obtain MoS2/rGO1.5/CdS in which the weight percentages of GO and MoS2 are identical to those for rGO1.5/CdS/MoS2-11 (1.5 wt %). During the process, GO was reduced to rGO. The composite (MoS2/rGO1.5/CdS)

Figure 7. Photocatalytic hydrogen evolution over rGO1.5/CdS/ MoS2-y prepared at different pHs. Reaction conditions: catalyst, 50 mg; 10 vol % lactic acid solution, 100 mL; light source, 350 W xenon lamp with a cutoff filter (λ ≥ 420 nm; light intensity, 34 mW cm−1); 1 h irradiation. 19846

dx.doi.org/10.1021/jp5054474 | J. Phys. Chem. C 2014, 118, 19842−19848

The Journal of Physical Chemistry C

■

exhibits much higher activity (72 mol h−1) than rGO1.5/CdS/ MoS2-7 (23 mol h−1) and the activity is close to that of rGO1.5/CdS/MoS2-11 (99 mol h−1) under the same photocatalytic reaction condition, confirming further the presence of the synergic effect when MoS2 is on the rGO. The time course for photocatalytic H2 production over rGO1.5/CdS/MoS2-11 is shown in Figure 8. The steady

ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China (21163012, 21366022) and 973 project (2009CB220003).

■

hydrogen-generation rate maintains in four reaction cycles for 20 h of irradiation, indicating that the composite photocatalyst is very stable.

4. CONCLUSIONS In conclusion, MoS2 was controllably loaded on rGO1.5/CdS by a simple photodeposition method adjusting the precursor solution pH. At low pH 7, MoS2 deposits on the surface of CdS particles, whereas, at high pH 11, it loads on the exposed rGO. When MoS2 is on rGO, the separation of photoexcited charges by rGO is synergic with HER at MoS2 on rGO, and the parasitic light absorption of MoS2 for CdS can be removed. The two factors improve the photocatalytic hydrogen production. Therefore, the hydrogen-production rate of rGO1.5/CdS/ MoS2-11 is 4.3 times as high as that of rGO1.5/CdS/MoS2-7. The findings would open a promising way to design and fabricate the efficient composite photocatalysts. ASSOCIATED CONTENT

S Supporting Information *

Experimental details for syntheses of graphene oxide (GO), MoS2/GO, and MoS2/rGO1.5/CdS; measured method for adsorption amount of MoS42− on CdS, rGO, and rGO1.5/CdS; and figures. This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

(1) Chen, X. B.; Shen, S. H.; Guo, L. J.; Mao, S. S. SemiconductorBased Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110, 6503−6570. (2) Li, Y. X.; Lin, S. Y.; Peng, S. Q.; Lu, G. X.; Li, S. B. Modification of ZnS1‑x‑0.5yOx(OH)y-ZnO Photocatalyst with NiS for Enhanced Visible-Light-Driven Hydrogen Generation from Seawater. Int. J. Hydrogen Energy 2013, 38, 15976−15984. (3) McKone, J. R.; Lewis, N. S.; Gray, H. B. Will Solar-Driven WaterSplitting Devices See the Light of Day? Chem. Mater. 2014, 26, 407− 414. (4) Li, Y. X.; Lu, G. X.; Li, S. B. Photocatalytic Hydrogen Generation and Decomposition of Oxalic Acid over Platinized TiO2. Appl. Catal., A 2001, 214, 179−185. (5) Li, Q.; Meng, H.; Zhou, P.; Zheng, Y. Q.; Wang, J.; Yu, J. G.; Gong, J. R. Zn1−xCdxS Solid Solutions with Controlled Bandgap and Enhanced Visible-Light Photocatalytic H2-Production Activity. ACS Catal. 2013, 3, 882−889. (6) Zhang, X. H.; Yu, L. J.; Zhuang, C. S.; Peng, T. Y.; Li, R. J.; Li, X. G. Highly Asymmetric Phthalocyanine as a Sensitizer of Graphitic Carbon Nitride for Extremely Efficient Photocatalytic H2 Production under Near-Infrared Light. ACS Catal. 2014, 4, 162−170. (7) Li, Y. X.; Xie, C. F.; Peng, S. Q.; Lu, G. X.; Li, S. B. Eosin YSensitized Nitrogen-Doped TiO2 for Efficient Visible Light Photocatalytic Hydrogen Evolution. J. Mol. Catal. A 2008, 282, 117−123. (8) Peng, T. Y.; Li, K.; Zeng, P.; Zhang, Q. G.; Zhang, X. G. Enhanced Photocatalytic Hydrogen Production over Graphene Oxide−Cadmium Sulfide Nanocomposite under Visible Light Irradiation. J. Phys. Chem. C 2012, 116, 22720−22726. (9) Li, Y. X.; Tang, L. F.; Peng, S. Q.; Li, Z. C.; Lu, G. X.; Li, S. B. Phosphate-Assisted Hydrothermal Synthesis of Hexagonal CdS for Efficient Photocatalytic Hydrogen Evolution. CrystEngComm 2012, 14, 6974−6982. (10) Li, Y. X.; Hu, Y..F.; Peng, S. Q.; Lu, G. X.; Li, S. B. Synthesis of CdS Nanorods by an Ethylenediamine Assisted Hydrothermal Method for Photocatalytic Hydrogen Evolution. J. Phys. Chem. C 2009, 113, 9352−9358. (11) Yan, Y.; Xia, B. Y.; Ge, X. M.; Liu, Z. L.; Wang, J. Y.; Wang, X. Ultrathin MoS2 Nanoplates with Rich Active Sites as Highly Efficient Catalyst for Hydrogen Evolution. ACS Appl. Mater. Interfaces 2013, 5, 12794−12798. (12) Zong, X.; Yan, H.; Wu, G.; Ma, G.; Wen, F.; Wang, L.; Li, C. Enhancement of Photocatalytic H2 Evolution on CdS by Loading MoS2 as Cocatalyst under Visible Light Irradiation. J. Am. Chem. Soc. 2008, 130, 7176−7177. (13) Zong, X.; Wu, G. P.; Yan, H. J.; Ma, G. J.; Shi, J. Y.; Wen, F. Y.; Wang, L.; Li, C. Photocatalytic H2 Evolution on MoS2/CdS Catalysts under Visible Light Irradiation. J. Phys. Chem. C 2010, 114, 1963− 1968. (14) Nguyen, M.; Tran, P. D.; Pramana, S. S.; Lee, R. L.; Batabyal, S. K.; Mathews, N.; Wong, L. H.; Graetzel, M. In Situ Photo-Assisted Deposition of MoS2 Electrocatalyst onto Zinc Cadmium Sulphide Nanoparticle Surfaces to Construct an Efficient Photocatalyst for Hydrogen Generation. Nanoscale 2013, 5, 1479−1482. (15) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. (16) Li, Q.; Guo, B. D.; Yu, J. G.; Ran, J. R.; Zhang, B. H.; Yan, H. J.; Gong, J. R. Highly Efficient Visible-Light-Driven Photocatalytic Hydrogen Production of CdS-Cluster-Decorated Graphene Nanosheets. J. Am. Chem. Soc. 2011, 133, 10878−10884. (17) Huang, Q.; Tian, S.; Zeng, D.; Wang, X.; Song, W.; Li, Y.; Xiao, W.; Xie, C. Enhanced Photocatalytic Activity of Chemically Bonded

Figure 8. Time course of photocatalytic hydrogen evolution over rGO1.5/CdS/MoS2-11. Reaction conditions as those in Figure 7, except irradiation time.

■

Article

AUTHOR INFORMATION

Corresponding Author

*Tel: 86-791-83969983. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 19847

dx.doi.org/10.1021/jp5054474 | J. Phys. Chem. C 2014, 118, 19842−19848

The Journal of Physical Chemistry C

Article

TiO2/Graphene Composites Based on the Effective Interfacial Charge Transfer through the C−Ti Bond. ACS Catal. 2013, 3, 1477−1485. (18) Xiang, Q. J.; Yu, J. G. Graphene-Based Photocatalysts for Hydrogen Generation. J. Phys. Chem. Lett. 2013, 4, 753−759. (19) Xiang, Q. J.; Yu, J. G.; Jaroniec, M. Synergetic Effect of MoS2 and Graphene as Cocatalysts for Enhanced Photocatalytic H2 Production Activity of TiO2 Nanoparticles. J. Am. Chem. Soc. 2012, 134, 6575−6578. (20) Xiang, Q. J.; Yu, J. G.; Jaroniec, M. Graphene-Based Semiconductor Photocatalysts. Chem. Soc. Rev. 2012, 41, 782−796. (21) Jaramillo, T. F.; Jorgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendor, I. Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts. Science 2007, 317, 100−102. (22) Li, Y. G.; Wang, H. L.; Xie, L. M.; Liang, Y. Y.; Hong, G. S.; Dai, H. J. MoS2 Nanoparticles Grown on Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2011, 133, 7296−7299. (23) Liang, Y. Y.; Li, Y. G.; Wang, H. L.; Dai, H. J. Strongly Coupled Inorganic/Nanocarbon Hybrid Materials for Advanced Electrocatalysis. J. Am. Chem. Soc. 2013, 135, 2013−2036. (24) Zong, X.; Xing, Z.; Yu, H.; Bai, Y.; Lu, G. Q.; Wang, L. Z. Photocatalytic Hydrogen Production in a Noble-Metal-Free System Catalyzed by in Situ Grown Molybdenum Sulfide Catalyst. J. Catal. 2014, 310, 51−56. (25) Nagaraju, G.; Tharamani, C. N.; Chandrappa, G. T.; Livage, J. Hydrothermal Synthesis of Amorphous MoS2 Nanofiber Bundles via Acidification of Ammonium Heptamolybdate Tetrahydrate. Nanoscale Res. Lett. 2007, 2, 461−468. (26) Guindo, M. C.; Zurita, L.; Durkn, J. D. G.; Delgado, A. V. Electrokinetic Behavior of Spherical Colloidal Particles of Cadmium Sulfide. Mater. Chem. Phys. 1996, 44, 51−58. (27) Bebie, J.; Schoonen, M. A. A.; Fuhrmann, M.; Strongin, D. R. Surface Charge Development on Transition Metal Sulfides: An Electrokinetic Study. Geochim. Cosmochim. Acta 1998, 62, 633−642. (28) Yu, Y. F.; Huang, S. Y.; Li, Y. P.; Steinmann, S. N.; Yang, W. T.; Cao, L. Y. Layer-Dependent Electrocatalysis of MoS2 for Hydrogen Evolution. Nano Lett. 2014, 14, 553−558. (29) Wang, H. W.; Skeldon, P.; Thompson, G. E. Thermogravimetric−Differential Thermal Analysis of the Solid-State Decomposition of Ammonium Tetrathiomolybdate during Heating in Argon. J. Mater. Sci. 1998, 33, 3079−3083.

19848

dx.doi.org/10.1021/jp5054474 | J. Phys. Chem. C 2014, 118, 19842−19848