Insight into the Effect of Highly Dispersed MoS2 versus Layer

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Article 2

Insight into the Effect of Highly Dispersed MoS versus Layer Structured MoS on the Photocorrosion and Photoactivity of CdS in Graphene-CdS-MoS Composites 2

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Min-Quan Yang, Chuang Han, and Yi-Jun Xu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08016 • Publication Date (Web): 22 Sep 2015 Downloaded from http://pubs.acs.org on September 23, 2015

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The Journal of Physical Chemistry

Insight into the Effect of Highly Dispersed MoS2 versus Layer Structured MoS2 on the Photocorrosion and Photoactivity of CdS in Graphene-CdS-MoS2 Composites Min-Quan Yang, Chuang Han, and Yi-Jun Xu*

State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, 350002, P. R. China & College of Chemistry, New Campus, Fuzhou University, Fuzhou, 350108, P. R. China *

Corresponding Author. Tel./Fax: +86 591 83779326; E-mail: [email protected]

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Abstract Designing semiconductor CdS-based photocatalyst for H2 evolution from water with high activity and stability is extremely desirable for practical application. Here, we report the important morphology and structure influence of MoS2 co-catalyst on the photocorrosion and photoactivity of CdS that is carpeted on the graphene (GR) surface. Homogeneous dispersion of MoS2 nanoparticles by controlled photodeposition (PD) method produces the GR-CdS-MoS2 (PD) composite, which does not have the characteristic stacked layer structure of MoS2. However, this GR-CdS-MoS2 (PD) composites exhibit much higher activity and particularly anti-photocorrosion than the GR-CdS-MoS2 (HT) counterparts, which feature the characteristic MoS2 layer structure, toward photocatalytic water splitting under visible light irradiation. The characterization results indicate that homogeneous dispersion of tiny MoS2 for GR-CdS-MoS2 (PD) markedly improves the separation and transfer of charge carriers and provides the increased number of catalytic active sites afforded by the absence of stacked layer structure of MoS2 co-catalyst. This work provides the direct evidence of negative effect of staked layer structure of MoS2 on boosting the activity and photostability of CdS on the GR surface, which would guide the more rational use of MoS2 and GR as co-catalyst toward achieving a highly active and stable semiconductor-based composite photocatalyst for H2 evolution.

Keywords: CdS, MoS2, graphene, photoactivity, stability, structure and morphology

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Photocatalytic and photoelectrocatalytic water splitting to produce H2 have received considerable attention owing to their potential to resolve the energy and environmental issues.1-18 Cadmium sulfide (CdS), one of the most well-known semiconductor materials with a narrow band gap (ca. 2.4 eV) and more negative conduction band edge than the redox potential of H+/H2,3, 19-20 has been extensively studied as a promising visible-light-responsive material for photocatalytic H2 evolution form water. However, CdS suffers from an inherent drawback of photocorrosion problem; that is, the sulfide ion (S2-) in CdS is highly prone to oxidation by photogenerated holes accompanied with leaching of toxic cadmium ion (Cd2+),3, 19-21 which makes CdS very unstable and is fatal for its cyclic operation and large-scale application. In addition, the facile recombination of photogenerated charge carriers before migrating to the surface for reactions and the low surface reaction efficiency to consume the electron-hole pairs are other important drawbacks that obviously deteriorate the solar energy conversion efficiency of CdS.5-6 Thus, CdS alone generally cannot give high H2 evolution activity and stability even in the presence of sacrificial agent.5-6 These issues seriously hinder the wide practical application of CdS photocatalytic material.

To improve the activity and anti-photocorrosion of CdS photocatalyst, integrating proper co-catalyst with CdS has proven to be an effective strategy.6, 22 In this respect, molybdenum disulfide (MoS2) has been commonly studied as a noble metal free low-cost and abundant co-catalyst,5, 15, 23-26 which is comparative and even superior to noble metal co-catalyst for enhancing the photocatalytic H2 evolution efficiency of semiconductor. On the other hand, graphene (GR), featuring the unique physico-chemical properties (e.g., the 2D sheet structure, high electron conductivity/mobility, flexible support platform .etc),27-34 has also been shown to be an ideal co-catalyst to assemble with semiconductor for enhancing the photoactivity.14, 27-28, 33-40 The integration of CdS with MoS2 and/or GR in an appropriate manner can provide efficient catalytic active sites for proton reduction and improve the separation and transfer of photogenerated charge carriers, thereby increasing the activity of CdS for photocatalytic H2 evolution from water.5, 10, 37-38, 41

However, it is worth noting that, for the GR-CdS-MoS2 composites available in literatures,5, 10, 41-42 MoS2 often exists in the form of stacking layers. Whereas, the co-catalytic activity of MoS2 is suggested to be derived from the uncoordinated sulfur edge sites while its basal planes remain 3



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catalytically inert.7, 43-44 The efficacy of MoS2 co-catalyst on boosting photocatalytic H2 evolution is strongly correlated with the number of active sulfur atoms on its exposed edges, which is predominantly affected by the structure and morphology of MoS2.43, 45-47 Thus, the stacking of MoS2 layers must block a substantial amount of catalytic edge sites on MoS27, 47 and lower the contact surface between MoS2 and CdS, which would be detrimental to fully exert the co-catalyst role of MoS2. Furthermore, it still remains unclear for the key issue on how to rationally use MoS2 co-catalyst to efficiently improve the anti-photocorrosion of CdS for the ternary GR-CdS-MoS2 composites, which together with the activity concern is equally important for practical application of CdS-based composite photocatalysts.

To circumvent the aforementioned disadvantages, a potentially ideal strategy is the fabrication of homogeneously dispersed MoS2 co-catalyst onto the surface of GR-CdS, possessing unstacked MoS2 structure and intimate interfacial contact between MoS2 and CdS. Such an ensemble of composite structure would in principle provide more active sites associated with MoS2 and timely facilitate the separation of charge carriers via decreasing the charge transfer distance, which thereby improves the long-term photocatalytic activity and particularly anti-photocorrosion of CdS in a more efficient way.

Herein, we report a simple, viable strategy for homogeneously loading the tiny MoS2 nanoparticles onto the surface of GR-CdS composites by a controlled photodeposition (PD) method at ambient conditions. In particular, the significant effect of structure and morphology of MoS2 co-catalyst on boosting the activity and anti-photocorrosion of CdS on the GR surface has been revealed by comparison with GR-CdS-MoS2 (HT) that is prepared by the widely-adopted hydrothermal (HT) method. The GR-CdS-MoS2 (PD) composite with optimal activity for water splitting to produce H2 does not have the stacked layer structure of MoS2, which is distinctly different from the optimal GR-CdS-MoS2 (HT) composite featuring the characteristic stacked layer structure of MoS2. Importantly, the GR-CdS-MoS2 (PD) composite shows much higher photoactivity and particularly anti-photocorrosion than the GR-CdS-MoS2 (HT) counterpart. The characterization results indicate that homogeneous dispersion of tiny MoS2 for GR-CdS-MoS2 (PD) markedly improves the separation and transfer of photogenerated charge carriers and provides the increased number of catalytic active sites afforded by the absence of stacked layer structure of MoS2 co-catalyst. Our work for the first time 4


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offers the direct evidence of negative effect of staked layer structure of MoS2 on improving the activity and photostability of CdS dispersed on the GR surface, highlighting the importance of the more rational use of MoS2 co-catalyst toward achieving a highly both active and stable semiconductor-based composite photocatalyst toward H2 evolution.

Experimental section Catalyst preparation Materials. Hydrochloric acid (HCl), potassium persulfate (K2S2O8), phosphorus pentoxide (P2O5), concentrated sulfuric acid (H2SO4, 98%), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 30%), nitric acid (HNO3, 65%), N,N-dimethylformamide (DMF, C3H7NO), dimethyl sulfoxide (C2H6OS2, DMSO), acetone (C3H6O), lactic acid (C3H6O3), absolute ethanol (C2H5OH), cadmium acetate (Cd(CH3COO)2·2H2O), sodium molybdate (Na2MoO4·2H2O) and thioacetamide (C2H5NS) were all obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Graphite powder was supplied from Qingdao Zhongtian Company, China. All of the reagents were used as received without further purification. The deionized (DI) water used in the experiment was from local sources.

Synthesis. (a) Synthesis of graphene oxide (GO) and the GR-CdS composites. GO was synthesized from natural graphite powder by a modified Hummers method 48-51 that involves a strong oxidation process in solution. The GR-CdS composites were fabricated based on a one-step solvothermal method as presented in previous works.52-55 The details were presented in the Supporting Information.

(b) Preparation of GR-CdS-MoS2 (PD) composites. The preparation of GR-CdS-MoS2 (PD) composites was based on a very facile one-step photodeposition method at ambient conditions. (NH4)2MoS4 was used as precursor for MoS2,56 which was synthesized according to the literature.5754 In brief, 5 g of (NH4)6Mo7O24•4H2O was added into 80 ml of ca. 20 wt% aqueous solution of (NH4)2S at ambient temperature and stirred for 12 h. After that, the red precipitation was thoroughly washed with ethanol for four times, then dried and stored under nitrogen.57 For the photodeposition of MoS2 onto GR-CdS, 40 mg of the prepared GR-CdS composite was firstly dispersed in 40 mL mixture of

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ethanol (8 mL) and DI water (32 mL) that contained certain amount of (NH4)2MoS4 and deaerated with N2 for 30 min. Then, the suspension was irradiated with visible light (λ > 420 nm) for 1 h under continuous N2 bubbling at the flow rate of 50 mL min−1. After that, the products were separated by filtration and washed with DI water for three times. Followed by drying at room temperature with a gentle stream of N2, the GR-CdS-MoS2 (PD) composites with different weight ratios of MoS2 (0.5%, 1%, 2% and 5%) can be obtained.

(c) Preparation of GR-CdS-MoS2 (HT) and GR-CdS (HT) composites. The GR-CdS-MoS2 (HT) composites were synthesized by a hydrothermal method.5,

15, 58

Typically, 0.2 g of the prepared

GR-CdS hybrids was ultrasonicated thoroughly in 60 mL DI water. Then, a certain amount of sodium molybdate (Na2MoO4·2H2O) and thioacetamide (C2H5NS) with the molar ratio of 1:2 was added into the above mixture. After stirring at room-temperature for 1 h, the mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave, sealed and heated at 200 °C for 24 h. The resulting products were cooled to room temperature and collected, washed thoroughly with distilled water for three times, and then dried in an oven at 60 °C. The GR-CdS-MoS2 (HT) composites with different weight ratios of MoS2 (0.5%, 1%, 2% and 5%) can be obtained. For the preparation of GR-CdS (HT), the procedure was the same as that for the synthesis of GR-CdS-MoS2 (HT) without the addition of Na2MoO4·2H2O and C2H5NS. Catalyst characterization The crystal-phase properties of the samples were analyzed with a powder X-ray diffractometer (Philip X' Pert Pro MPP) using Ni-filtered Cu Kα radiation (λ = 1.5418 Å) in the 2θ range from 5° to 80° with a scan rate of 0.02° per second. UV-vis diffuse reflectance spectra (DRS) were recorded on a Cary-500 UV-vis-NIR spectrometer in which BaSO4 powder was used as the internal standard to obtain the optical properties of the samples. Transmission electron microscopy (TEM) images were collected by using a JEOL model JEM 2010 EX microscope at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurement was performed on a Thermo Scientific ESCA Lab 250 spectrometer which consists of a mono chromatic Al Kα as the X-ray source, a hemispherical analyzer, and a sample stage with multiaxial adjustability, to obtain the surface composition of the samples. All of the binding energies were calibrated by the C 1s peak at 284.6 eV. The 6


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photoluminescence (PL) spectra were obtained using an Edinburgh Analytical Instrument PLS920 system. Nitrogen adsorption-desorption isotherms and the Brunauer-Emmett-Teller (BET) surface areas were collected at 77 K using Micromeritics ASAP2010 equipment. The concentration of the leached Cd2+ and Mo4+ in solution was quantified by an inductively coupled plasma emission spectroscopy (ICP, Perkin Elmer Optima 2000DV).

Photoelectrochemical measurements were performed in a homemade three electrode quartz cell with a PAR VMP3 Multi Potentiostat apparatus. A Pt plate was used as the counter electrode, and Ag/AgCl electrode was used as the reference electrode. The working electrode was prepared on fluorine-doped tin oxide (FTO) glass that was cleaned by ultrasonication in ethanol for 30 min and dried at 80 °C. Typically, 3 mg of the sample powder was ultrasonicated in 0.5 mL of DMF to disperse it evenly to get a slurry. The slurry was spread onto FTO glass, whose side part was previously protected using Scotch tape. After air drying, the working electrode was further dried at 100 °C for 2 h to improve adhesion. Then, the Scotch tape was unstuck, and the uncoated part of the electrode was isolated with epoxy resin. The exposed area of the working electrode was 0.25 cm2. The photocurrent measurement was carried out on a BAS Epsilon workstation without bias and the electrolyte was 0.2 M aqueous Na2SO4 solution (pH = 6.8) without an additive. The visible light irradiation source was a 300 W Xe arc lamp system equipped with a UV-CUT filter (λ > 420 nm). The cathodic polarization curves were obtained using the linear sweep voltammetry (LSV) technique with a scan rate of 0.2 mV s-1. The electrochemical impedance spectroscopy (EIS) measurement was carried out using a CHI-660D workstation, (CH Instrument, USA) in the three electrode cell in the presence of 0.5 M KCl solution containing 0.01 mM K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) under open circuit potential conditions.

Photocatalytic H2 evolution The photocatalytic H2 evolution was performed in a Pyrex reaction cell connected to a closed gas circulation and evacuation system. In a typical photocatalytic experiment, 40 mg of the prepared photocatalysts was dispersed with constant stirring in an 80 mL mixed solution of lactic acid (8 mL) and water (72 mL). Prior to irradiation, the solution was degassed for 20 min, followed by irradiation with a 300 W Xe arc lamp (PLS-SXE 300C, Beijing Perfectlight Co., Ltd.) equipped with a filter to cut off light of wavelength below 420 nm (λ > 420 nm). The reactant solution was stirred and 7



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maintained at low temperature by a flow of cooling water during the photocatalytic reaction. The amount of produced H2 was determined with online gas chromatography (GC 7890, MS-5 A column, Ar carrier, Fuli Co., China) equipped with a thermal conductivity detector.

The recycling test of catalytic H2 evolution over the as-prepared photocatalyst was done as follows. Typically, after the reaction of the first run under visible light irradiation, the photocatalyst was separated by filtration and washed with deionized water for 3 times. Then, the fresh reaction solution of 80 mL of lactic acid aqueous solution (10 vol.%) was mixed with this used catalyst to subject to the second run photocatalytic activity test. By analogy, the following three runs of photocatalytic recycling tests were performed.

The apparent quantum efficiency (AQE) of H2 evolution was conducted under the same photocatalytic condition. The 300 W Xe lamp equipped with a 420 nm band-pass filter was used as the light source and the number of incident photons was measured using a Si photodiode (ILT 950). The apparent quantum efficiency (AQE) is calculated according to eq (1): AQE =

=

Number of reacted electrons Number of incident photons

× 100%

Number of evolved H 2 molecules × 2 × 100% Number of incident photons

(1)

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Results and discussion Scheme 1A has shown the typical synthesis of the ternary GR-CdS-MoS2 (PD) composite photocatalysts. At first, the series of GR-CdS composites with different weight addition ratios of GR are prepared by a one-step solvothermal process, which has been well established to produce GR-CdS with intimate interfacial contact.50, 52, 55 Then, MoS2 is loaded onto GR-CdS through a facile and effective room-temperature photodeposition (PD) technique by using (NH4)2MoS4 as the precursor of MoS2,56 for which the tiny MoS2 without stacked layer structure can be homogeneously photodeposited onto the surface of GR-CdS. For comparison, Scheme 1B has displayed the synthesis process of GR-CdS-MoS2 (HT) composites, which is performed via a widely reported hydrothermal method5, 25, 41-42, 58 by using Na2MoO4 and thioacetamide as the precursor of MoS2. By this method, the as-prepared MoS2 generally appears as characteristic stacking layers.5, 25, 41-42, 58 In comparison with the high-temperature and long-time hydrothermal method for preparing MoS2,5,

25, 41-42, 58

or the

method of reduction by toxic H2S stream and annealing at high temperature reported in previous works,26, 59-60 it is clear that the photodeposition method is more convenient and simple.

The photodeposition of MoS2 onto the surface of GR-CdS can be directly evidenced by the X-ray photoelectron spectroscopy (XPS) analysis. The full spectrum of (GR-CdS)-MoS2 (PD) in Figure S1A (Supporting Information) reveals the peaks of Cd, S, Mo and C elements, which is well in accordance with the nominal composition of the composite photocatalysts. Figure 1A displays the high-resolution XPS spectrum of Mo 3d, which gives two peaks at 228.9 and 232.1 eV and can be assigned to the Mo 3/d5/2 and Mo 3/d3/2, respectively, demonstrating that Mo is in the +4 valence state.7, 26, 47

In the S 2p XPS spectrum (Figure 1B), the peaks at 161.5 and 162.7 eV are corresponding to

S2−.10, 26 Figure 1C displays the typical high-resolution XPS spectrum of Cd 3d. The peaks at 405.3 and 412.0 eV are assigned to Cd2+ in CdS.10, 26,

61

In addition, Figure 1D shows the C 1s XPS

spectrum of (GR-CdS)-MoS2 (PD), which displays the significant decrease of oxygen-containing functional groups as compared to that of original GO (Figure S1B, Supporting Information), indicating the effective reduction of GO to GR by the solvothermal treatment process.50, 52, 55 The XPS analysis results clearly indicate the successful photodeposition of MoS2 onto GR-CdS.

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Figure 2A-B displays the X-ray diffraction (XRD) patterns of the (GR-CdS)-MoS2 (PD) composites with different weight addition ratios of GR and MoS2. It can be seen that all of the (GR-CdS)-MoS2 (PD) composites possess analogous XRD patterns. The well-resolved broad diffraction peaks located at ca. 26.5°, 43.9°, and 52.1° are indexed to the (111), (220), and (311) crystal planes of cubic phase CdS, respectively.50, 53-54 No typical diffraction peaks of GR and MoS2 have been detected, which could be ascribed to the relatively low content and weak diffraction intensity of GR and MoS2.41 The optical properties of the (GR-CdS)-MoS2 (PD) composites have been determined by UV-vis diffuse reflectance (DRS) spectra, as shown in Figure 2C-D. The sample of blank CdS displays an absorption edge around 520 nm, corresponding to the band gap of ~2.4 eV. The addition of different amounts of both GR and MoS2 has obvious influence on the optical property of the as-prepared (GR-CdS)-MoS2 (PD) composites. With the increased ratios of MoS2 and GR, the light absorption of the (GR-CdS)-MoS2 (PD) composites in the visible light range (500-800 nm) is gradually enhanced, which can be attributed to the intrinsic background absorption of black or dark colored GR and MoS2.15

The photoactivity of the (GR-CdS)-MoS2 (PD) composites has been evaluated for photocatalytic H2 evolution from water under visible light irradiation with the addition of lactic acid, which has been well proven to be a good hole scavenger without obvious influence on the origin of the produced H2.62 As shown in Figure 3A-B, it can be observed that although CdS is a well known visible light driven photocatalyst, the rate of H2 evolution is still very low (19 µmol h-1) because of the rapid recombination of photogenerated electron-hole pairs and the lack of active sites for proton reduction.6 The photodeposition of MoS2 co-catalyst onto CdS results in a significant improvement in the H2 evolution activity, as shown in Figure S2 (Supporting Information). With the photodeposition of 1%MoS2, the composite of CdS-1%MoS2 displays the optimal H2 evolution activity (305 µmol h-1). In the next, the modification of CdS with two co-catalysts of GR and MoS2 displays further improved photoactivity toward H2 evolution, as reflected by Figure 3. This can be attributed to that GR as an electron conductive platform with low Fermi energy level is able to facilitate the electrons transfer from photoexcited CdS to the MoS2 co-catalyst with abundant catalytic active sites and then react with adsorbed proton to form H2.5 The change of the contents of GR (Figure 3A) and MoS2 (Figure 3B) plays an important influence on the photoactivity of the GR-CdS-MoS2 (PD) composites. When the 10


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contents of GR and MoS2 both reach 1.0%, the sample of (1%GR-CdS)-1%MoS2 (PD) displays the highest H2 production rate of 513 µmol h-1, corresponding to an apparent quantum efficiency (AQE) of 26.8% at 420 nm. Further increase of the GR and MoS2 contents leads to a gradual reduction of the H2 evolution activity of the (GR-CdS)-MoS2 (PD) composites. These results clearly demonstrate the importance of controlling the addition ratios of GR and MoS2 for achieving an optimal synergistic interaction between the co-catalysts (GR and MoS2) and CdS to obtain the best photoactivity of (GR-CdS)-MoS2.

For comparison, Figure 3C-D has displayed the photocatalytic H2 evolution activity over (GR-CdS)-MoS2 (HT) composites prepared via the widely reported hydrothermal method.5, 25, 41-42, 58 It is seen that the trend of photoactivity change observed in the (CdS-GR)-MoS2 (HT) composites is similar with that of (GR-CdS)-MoS2 (PD) samples. The introduction of MoS2 and GR as dual co-catalysts for semiconductor CdS results in an obvious improvement in the photocatalytic H2 evolution activity. Through the optimization of each component proportion, the sample of (1%GR-CdS)-1%MoS2 (HT) shows the best photocatalytic H2 production rate of 313 µmol h-1 with apparent quantum efficiency of 16.7% at 420 nm. The optimum weight addition ratios of GR and MoS2 in the (GR-CdS)-MoS2 (HT) are the same with that in (GR-CdS)-MoS2 (PD) composites. However, the photocatalytic performances of the samples are significantly different. The photocatalytic H2 evolution rate of (1%GR-CdS)-1%MoS2 (PD) composite is much higher than that of (1%GR-CdS)-1%MoS2 (HT). Moreover, it is remarkable to find that all of the (GR-CdS)-MoS2 (PD) composites display higher photoactivity than the (GR-CdS)-MoS2 (HT) samples with the same addition ratios of GR and MoS2. The result implies that the photodeposition of MoS2 is more beneficial than the hydrothermal loading of MoS2 onto GR-CdS for improving the photoactivity of the resulting GR-CdS-MoS2 composites.

To further understand the different co-catalytic effect between the photodeposited MoS2 and the hydrothermal synthesized MoS2 co-catalyst on enhancing the photoactivity of the resulting GR-CdS-MoS2 toward H2 evolution, we have prepared the sample of CdS-1%Pt as reference catalyst for photoactivity comparison by the typical photodeposition method using H2PtCl6 as the precursor. Pt is a very high efficiency noble metal co-catalyst for photocatalytic H2 evolution reaction and has been 11



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widely used in the reported research works.5-7 The time-online photoactivity test in Figure 4 indicates that the loading of Pt onto CdS significantly enhances the H2 evolution activity. The H2 production amount over CdS-1%Pt reaches 1505 µmol after 4 h visible light irradiation, which is higher than that of the optimal (1%GR-CdS)-1%MoS2 (HT) (1236 µmol) composite. However, the value is still much lower than the photoactivity of (1%GR-CdS)-1%MoS2 (PD) (2231 µmol) and even slightly lower than that of CdS-1%MoS2 (PD) (1578 µmol). The results further indicate that the photodeposition of MoS2 is a simpler yet more efficient approach than the hydrothermal synthesis of MoS2 for enhancing the activity of GR-CdS composite toward H2 evolution.

The stability is as important as activity for the practical application of semiconductor-based photocatalytic materials. To evaluate the photostability of the composites, the recycling test over blank CdS, CdS-1%MoS2 (PD), CdS-1%Pt, (1%GR-CdS)-1%MoS2 (PD) and (1%GR-CdS)-1%MoS2 (HT) has been measured and compared in Figure 5A. The result shows that after five cycles, the H2 evolution activity of blank CdS has deteriorated by ca. 57%, displaying a significant photoactivity declination, as shown in Figure 5B. On the contrary, the H2 evolution amounts over the CdS-1%Pt, CdS-1%MoS2 (PD) and (1%GR-CdS)-1%MoS2 (PD) composites remain relatively stable in the five reaction cycles for 20 h of visible light irradiation; the photoactivity deterioration is lower than 10%. However, it is notable that the H2 evolution amount of (1%GR-CdS)-1%MoS2 (HT) composite has reduced by ca. 32% after five times recycling test, which is lower than that of CdS, but is much higher than

the

photoactivity

declination

of

(1%GR-CdS)-1%MoS2 (PD),

implying

that

the

anti-photocorrosion of (1%GR-CdS)-1%MoS2 (HT) is weaker than (1%GR-CdS)-1%MoS2 (PD). ICP analysis of the reaction solution of (1%GR-CdS)-1%MoS2 (PD) after continuous visible light irradiation of 20 h shows an Cd2+ leaching of about 4.2% (Table S1, Supporting Information), which is close to the value of (1%GR-CdS)-1%MoS2 (PD) for stirring in dark of 20 h (3.9%). The similar leaching of Cd2+ could be ascribed to the partially dissolution of CdS ingredient at the acidic condition due to the presence of lactic acid. No obvious Mo4+ leaching has been detected in both of the cases. The

result further confirms the

good photostability and strong anti-photocorrosion of

(1%GR-CdS)-1%MoS2 (PD). Therefore, the slight photoactivity decrease of (1%GR-CdS)-1%MoS2 (PD) after five cycles might be caused by loss of the photocatalyst during each round of collection and rinsing, which has been commonly reported as an important reason accounting for the photoactivity 12


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decrease of photocatalysts during the cycle test process.24,

63

In addition, the ICP analysis of the

reaction solution of (1%GR-CdS)-1%MoS2 (HT) after visible light irradiation of 20 h has also been performed. The Cd2+ leaching has been determined to be 15.7%, which is significantly higher than that of (1%GR-CdS)-1%MoS2 (PD). The results have further demonstrated that the photostability and anti-photocorrosion of (1%GR-CdS)-1%MoS2 (PD) is higher than that of (1%GR-CdS)-1%MoS2 (HT). This photostability differences among these samples can also be directly observed from the color change of the reaction solution. As exemplified in Figure S3 (Supporting Information), the color of the reaction solution of (1%GR-CdS)-1%MoS2 (HT) and blank CdS has got darker after 4 h visible light irradiation, while no obvious color change of (1%GR-CdS)-1%MoS2 (PD) has been observed. The above results clearly indicate that (i) the incorporation of CdS with GR and MoS2 dual co-catalysts not only increases the photocatalytic activity but also enhances the anti-photocorrosion of the CdS photocatalyst; (ii) the MoS2 synthesized from the photodeposition method is more beneficial than the hydrothermal synthesized MoS2 for improving the photoactivity and anti-photocorrosion of CdS on the GR surface.

To explore the underlying reasons for the different acceleration role between the photodeposited MoS2 and the hydrothermal synthesized MoS2 and disclose the origin of GR and MoS2 on enhancing the photoactivity and anti-photocorrosion of CdS, a series of joint characterization techniques along with controlled experiments have been carried out in the next. Figure 6 displays the transmission electron microscopy (TEM) analysis of GR-CdS-MoS2 (PD), which reflects the microscopic morphology and structure information of the composite. It can be seen form Figure 6A that the GR-CdS-MoS2 (PD) sample displays the sheet-like structure of GR and the CdS nanoparticles overspread on the GR surface densely. The morphology and structure of GR-CdS-MoS2 (PD) is similar with that of GR-CdS composite, as shown in Figure S4 (Supporting Information), indicating that the photodeposition of MoS2 has no obvious influence on the morphology and structure of the GR-CdS

samples.

Figure

6B

shows

the

high-resolution

TEM

(HRTEM)

image

of

(1%GR-CdS)-1%MoS2 (PD), which displays distinct lattice fringes with a lattice spacing of 0.33 nm, corresponding to the (111) crystallographic plane of cubic CdS.50,

53

No typically stacked layer

structure of MoS2 has been observed from the HRTEM analysis on the sample of (1%GR-CdS)-1%MoS2 (PD). However, the energy-dispersive X-ray (EDX) analysis on selected areas 13



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of (1%GR-CdS)-1%MoS2 (PD) has shown that the sample contains C, Cd, S and Mo elements, as shown in Figure 6D and E. In addition, the elemental mapping analysis of (1%GR-CdS)-1%MoS2 (PD) has also proven that the elements of C, Cd, S and Mo are present in the randomly selected testing area with even distribution, as displayed in the bottom panels of Figure 6 and Figure S5 (Supporting Information). The results imply that MoS2 have carpeted the GR-CdS sheet structure uniformly after the photodeposition process. Therefore, it can be deduced that the disappearance of lattice fringes of MoS2 in the (1%GR-CdS)-1%MoS2 (PD) is attributed to the tiny size and homogeneous dispersion of the photodeposited MoS2, which makes it hard to observe the structure of MoS2 from the HRTEM analysis.

Figure 6C has displayed the typical TEM image of (1%GR-CdS)-2%MoS2 (PD), from which it can be seen that with the increased weight ratio of MoS2 (2%), tiny MoS2 particles with characteristic slabs have appeared, suggesting the morphology and structure transformation of the photodeposited MoS2. Moreover, when the weight ratio of MoS2 increases to 5%, as shown in Figure 6F, the deposited MoS2 is prone to aggregate on the surface of GR-CdS. The enlarged inset in Figure 6F displays the typical layer structure with an interlayer spacing of 0.62 nm, which is corresponding to the (002) plane of hexagonal MoS2.5, 15 The results have further demonstrated the successful loading of MoS2 onto the GR-CdS surface via the simple photodeposition method. More importantly, it has shown that the morphology and structure of the as-deposited MoS2 can be controlled by simply tuning the weight content of MoS2 in the (GR-CdS)-MoS2 (PD) composites. With the low photodeposition amount, the tiny MoS2 without stacked layer structure can be homogeneously dispersed on the surface of GR-CdS. The morphology and structure of MoS2 in the optimal (1%GR-CdS)-1%MoS2 (PD) is distinctly different with that of MoS2 obtained from the widely reported hydrothermal method,5, 25, 41-42, 58 which generally features the characteristic stacked layer structure, as shown by the case of (1%GR-CdS)-1%MoS2 (HT) in Figure 7.

Figure 7A-B has displayed the typical TEM images of (GR-CdS)-MoS2 (HT) (taking (1%GR-CdS)-1%MoS2 (HT) with the optimal photoactivity as an example) composite obtained from the hydrothermal method by using Na2MoO4 and thioacetamide as the precursor of MoS2, from which it can be seen that the as-prepared MoS2 displays typical layer structure with the thickness of 4-8 nm. 14


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The HRTEM image in Figure 7C displays the interlayer distance of 0.62 nm, corresponding to the (002) plane of hexagonal MoS2. Figure 7D shows the clear lattice fringes with a lattice spacing of 0.33 nm, which can be assigned to the (111) facet of cubic CdS. The results indicate that MoS2 has also been successfully synthesized and integrated with GR-CdS via the hydrothermal method. However, the morphology and structure of the hydrothermal synthesized MoS2 in GR-CdS-MoS2 (HT) samples is significantly different with that of the photodeposited MoS2 in the GR-CdS-MoS2 (PD) composites. Since that the co-catalytic activity of MoS2 is stemmed from the active sulfur atoms on its exposed edges, while the basal planes are catalytically inert,7, 43-44 the homogeneously photodeposited MoS2 with tiny size would provide more catalytic active sites than the hydrothermal synthesized MoS2 with stacked layer structure,43-44 which is able to facilitate the separation and transfer of photoexcited electron-hole pairs, prolong the lifetime of charge carriers generated from band gap excitation of CdS more efficiently, thus resulting in the higher photocatalytic H2 evolution activity of GR-CdS-MoS2 (PD) than that of GR-CdS-MoS2 (HT). This inference has been faithfully confirmed by the following comparative analysis of photoelectrochemical and photoluminescence (PL) spectra.

Figure

8A

displays

the

polarization

curves

of

blank

CdS,

CdS-1%MoS2

(PD),

(1%GR-CdS)-1%MoS2 (PD) and (1%GR-CdS)-1%MoS2 (HT) samples, from which it can be seen that the loading of MoS2 onto CdS obviously enhances the current density, indicating that MoS2 can efficiently promote the photoactivity of reduction of water to H2.10, 60 With the introduction of a proper small amount of GR, the (1%GR-CdS)-1%MoS2 (PD) composite displays a further improved current density as compared to CdS-1%MoS2, which can be attributed to the fact that the introduction of GR greatly increases the electrical conductivity of this system, allowing a more rapid electron transport through the catalytic system.7, 10 Moreover, the higher current density of (1%GR-CdS)-1%MoS2 (PD) than that of (1%GR-CdS)-1%MoS2 (HT) indicates that the (1%GR-CdS)-1%MoS2 (PD) has more available catalytic edge sites.7, 10 The result has consolidated our standpoint that the photodeposited MoS2 with tiny size and homogeneous deposition would possess more catalytic edge sites than the hydrothermal synthesized MoS2 with stacked layer structure.

Figure 8B shows the transient photocurrent response of the four samples. Under visible light irradiation, the photocurrent transient response for (1%GR-CdS)-1%MoS2 (HT) is higher than blank 15



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CdS, while (1%GR-CdS)-1%MoS2 (PD) displays the highest photocurrent density, indicating that (1%GR-CdS)-1%MoS2 (PD) possesses the longest lifetime of photogenerated charge carriers.35, 64-65 The higher photocurrent of (1%GR-CdS)-1%MoS2 (PD) than that of CdS-1%MoS2 (PD) has demonstrated the important synergistic acceleration effect of GR and MoS2 on promoting the separation of electron-hole pairs generated from the band gap photoexcitation of CdS under visible light. Figure 8C displays the electrochemical impedance spectroscopy (EIS) Nyquist plots of the four samples under visible light irradiation. It is clearly seen that, the four electrode materials all show semi-cycles at high frequencies. The smallest arc radius of the EIS Nyquist plot of (1%GR-CdS)-1%MoS2 (PD) implies that it has the fastest interfacial electron transfer than that of (1%GR-CdS)-1%MoS2 (HT) and CdS-1%MoS2.50, 65 In addition, the most efficient transfer of charge carriers and longest lifetime of photogenerated electron-hole pairs over (1%GR-CdS)-1%MoS2 (PD) have been further verified by the photoluminescence (PL) analysis, which is often employed to study the surface processes involving the photoexcited energy/electron transfer and recombination.35, 66 As shown in Figure 8D, the PL intensity of the (1%GR-CdS)-1%MoS2 (PD) is much lower than those of both blank CdS and CdS-1% MoS2. In particular, the PL intensity of (1%GR-CdS)-1%MoS2 (PD) is also lower than that of (1%GR-CdS)-1%MoS2 (HT) with the same content of GR and MoS2. The PL spectra show that the photodeposition of tiny size MoS2 onto GR-CdS results in a highest quenching degree of PL intensity, which suggests that the recombination of photogenerated electron-hole pairs over (1%GR-CdS)-1%MoS2 (PD) is most efficiently inhibited among these four samples.35, 50, 66

The photoelectrochemical and photoluminescence (PL) spectra analysis has clearly demonstrated that the incorporation of GR and MoS2 dual co-catalysts with CdS provides increased catalytic active sites for proton reduction and significantly promotes the separation and transfer of photogenerated electron-hole pairs. In addition, the result has also corroborated the important morphology and structure influence of MoS2 on the separation and transfer efficiency of electron-hole pairs, i.e. the tiny MoS2 without stacked layer structure would provide more active sites and facilitate the separation of charge carriers more efficiently than the layered MoS2, which thus results in the higher photoactivity of (GR-CdS)-MoS2 (PD) composites than the (GR-CdS)-MoS2 (HT) counterparts.

In addition, it is notable that the crystal structure and size of CdS in the GR-CdS-MoS2 (HT) is 16


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different with that in GR-CdS-MoS2 (PD), as shown in Figure S3 and S6 (Supporting Information). This might be caused by the high-temperature and long-time hydrothermal treatment process for preparing GR-CdS-MoS2 (HT). In order to evaluate the possible contribution of the crystal structure and size change on the photoactivity difference between GR-CdS-MoS2 (PD) and GR-CdS-MoS2 (HT). We have comparatively tested the photocatalytic H2 evolution activity of GR-CdS and GR-CdS (HT), for which the GR-CdS (HT) composites are prepared via the same procedure as that for the synthesis of GR-CdS-MoS2 (HT) without the addition of Na2MoO4·2H2O and C2H5NS. As displayed in Figure 9A-B, the photoactivity comparison shows that the GR-CdS (HT) samples display higher photoactivity than the GR-CdS composites with the same content of GR, indicating that the hydrothermal treatment of GR-CdS results in the enhanced photoactivity. The optimal ratios of GR in the GR-CdS and GR-CdS (HT) composites are also the same (1%). Figure 9C has shown the result of cycle test over blank CdS, 1%GR-CdS and 1%GR-CdS (HT). After five cycles, the photoactivity declinations of blank CdS, 1%GR-CdS and 1%GR-CdS (HT) are 57%, 41% and 34%, respectively, demonstrating that 1%GR-CdS (HT) has higher photostability than 1%GR-CdS. The results prove that the hydrothermal treatment not only improves the photoactivity but also enhances the anti-photocorrosion of the GR-CdS composite. Therefore, the crystal structure and size change of CdS caused by the hydrothermal treatment process could not be the reason accounting for the lower photoactivity and stability of GR-CdS-MoS2 (HT) than that of GR-CdS-MoS2 (PD).

Furthermore, Table S2 (Supporting Information) has shown the nitrogen adsorption-desorption isotherm results of blank CdS, CdS-1%MoS2 (PD), (1%GR-CdS)-1%MoS2 (PD) and (1%GR-CdS)-1% MoS2 (HT) samples. The Brunauer-Emmett-Teller (BET) surface area of these samples is determined to be ca. 51 m2 g-1, 72 m2 g-1, 35 m2 g-1 and 10 m2 g-1, respectively, following the order of CdS-1%MoS2 (PD) > blank CdS > (1%GR-CdS)-1%MoS2 (PD) > (1%GR-CdS)-1%MoS2 (HT). However, the photoactivity order of these samples is (1%GR-CdS)-1%MoS2 (PD) > CdS-1%MoS2 (PD) > (1%GR-CdS)-1%MoS2 (HT) > > blank CdS. Therefore, it is reasonable to infer that the surface area is not the primary reason accounting for the significant photoactivity difference between (1%GR-CdS)-1%MoS2 (PD) and (1%GR-CdS)-1%MoS2 (HT). On the contrary, the different content of catalytic active sites, separation and transfer of photogenerated electron-hole pairs caused by the morphology and structure difference between the photodeposited MoS2 and the hydrothermal 17



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Page 18 of 36

synthesized MoS2 are the key and predominant factors that determine the photoactivity difference between (GR-CdS)-MoS2 (PD) and (GR-CdS)-MoS2 (HT).

To further confirm the above analysis, we have prepared the (1%GR-CdS (HT))-1%MoS2 (PD) composite via the photodeposition of MoS2 onto the surface of 1%GR-CdS (HT). The BET surface area of (1%GR-CdS (HT))-1%MoS2 (PD) is determined to be ca. 11 m2 g-1, which is close to 10 m2 g-1 of (1%GR-CdS)-1%MoS2 (HT). However, the photocatalytic performances of the two samples are remarkably different, as shown in Figure S7 (Supporting Information). The comparison results show that both the photoactivity and stability of the (1%GR-CdS (HT))-1%MoS2 (PD) composite are much higher than those of the (1%GR-CdS)-1%MoS2 (HT) sample. These data further prove the significant influence of morphology and structure of MoS2 on the photoactivity of the resulting (GR-CdS)-MoS2 composites, which is the key factor determining the different photocatalytic performance between (GR-CdS)-MoS2 (PD) and (GR-CdS)-MoS2 (HT).

Thus far, the tentative reaction mechanisms for photocatalytic H2 evolution from water over the (GR-CdS)-MoS2 (PD) and (GR-CdS)-MoS2 (HT) composites has been proposed, as schematically illustrated in Figure 10. Upon the visible light irradiation, the electrons in the valence band (VB) of CdS are photoexcited to the conduction band (CB), leaving holes in the valence band. The matched energy level and the close neighbourhood of these components favor the vectorial transfer of photogenerated electrons in the CB of CdS to GR and MoS2,15 while the holes are consumed by the scavenger of lactic acid. GR and MoS2 as dual co-catalysts obviously improve the lifetime and transfer of photogenerated charge carriers and simultaneously provide a source of reactive sites, thereby increasing the overall photocatalytic H2 evolution efficiency of CdS. As for the different photocatalytic performance between (GR-CdS)-MoS2 (PD) and (GR-CdS)-MoS2 (HT) composites, it is mainly caused by the morphology and structure difference of the synthesized MoS2. For the (GR-CdS)-MoS2 (PD) composites, the photodeposited tiny MoS2 particles are homogeneously dispersed on the surface of GR-CdS without stacked layer structure. They are able to provide more catalytic active sites and shorter charge transfer distance than the hydrothermal synthesized MoS2 in the (GR-CdS)-MoS2 (HT) counterparts that feature the characteristic layer structure. The separation and transfer of photogenerated charge carriers in the (GR-CdS)-MoS2 (PD) composite is more timely and efficient 18


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than that in (GR-CdS)-MoS2 (HT), which thus dramatically facilitates the proton reduction to generate H2 and consumption of the holes by the scavenger of lactic acid. As a result, the GR-CdS-MoS2 (PD) composites display the higher visible light photoactivity and anti-photocorrosion than the GR-CdS-MoS2 (HT) counterparts.

Conclusions In summary, we have reported a very facile photodeposition method for homogeneously loading the tiny MoS2 particles without characteristic layer structure onto the surface of GR-CdS to form the ternary GR-CdS-MoS2 (PD) composites with the optimum photoactivity. The integration of MoS2 and GR dual co-catalysts with CdS significantly enhances the visible light activity and stability of CdS toward photocatalytic H2 evolution form water. In particular, the GR-CdS-MoS2 (PD) composites exhibit much higher photocatalytic H2 evolution activity and anti-photocorrosion than the hydrothermal synthesized GR-CdS-MoS2 (HT) counterparts that feature the characteristic MoS2 layer structure with the thickness of several nanometers. The superior photocatalytic performance of GR-CdS-MoS2 (PD) is attributed to the homogeneous dispersion of tiny MoS2 in the composites, which provides the increased catalytic active sites and more effective separation and transfer of charge carriers due to the absence of stacked layer structure. This study demonstrates that the stacking of MoS2 is detrimental for efficiently exerting its co-catalyst role, disclosing the important morphology and structure influence of the synthesized MoS2 co-catalyst on the photocatalytic performance of the semiconductor photocatalysts. It is hoped that this work could enrich the facile fabrication of other high-performance and stable semiconductor-based composites via the rational use and preparation of MoS2 and GR co-catalysts for photocatalytic H2 evolution from water.

Acknowledgment The support from the Key Project of National Natural Science Foundation of China (U1463204), the National Natural Science Foundation of China (20903023 and 21173045), the Award Program for Minjiang Scholar Professorship, the Natural Science Foundation (NSF) of Fujian Province for Distinguished Young Investigator Grant (2012J06003), the Independent Research Project of State Key Laboratory of Photocatalysis on Energy and Environment (NO. 2014A05), the 1st Program of Fujian 19



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Province for Top Creative Young Talents, and the Program for Returned High-Level Overseas Chinese Scholars of Fujian province is gratefully acknowledged.

Supporting Information Available: Additional experimental details, characterization and photoactivity results. This material is available free of charge via the Internet at http://pubs.acs.org.

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Photocatalytic

H2

Production

Activity

of

Multiarmed

CdS Nanorods.

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Phys. Chem. C 2013, 117, 21724-21734. (52) Cao, A.; Liu, Z.; Chu, S.; Wu, M.; Ye, Z.; Cai, Z.; Chang, Y.; Wang, S.; Gong, Q.; Liu, Y. A Facile One-step Method to Produce Graphene–CdS Quantum Dot Nanocomposites as Promising Optoelectronic Materials. Adv. Mater. 2010, 22, 103-106. (53) Zhang, N.; Zhang, Y.; Pan, X.; Fu, X.; Liu, S.; Xu, Y.-J. Assembly of CdS Nanoparticles on the Two-Dimensional Graphene Scaffold as Visible-Light-Driven Photocatalyst for Selective Organic Transformation under Ambient Conditions. J. Phys. Chem. C 2011, 115, 23501-23511. (54) Zhang, N.; Zhang, Y.; Yang, M.-Q.; Tang, Z.-R.; Xu, Y.-J. A Critical and Benchmark Comparison on Graphene-, Carbon Nanotube-, and Fullerene-Semiconductor Nanocomposites as Visible Light Photocatalysts for Selective Oxidation. J. Catal. 2013, 299, 210-221. (55) Zhang, N.; Yang, M.-Q.; Tang, Z.-R.; Xu, Y.-J. CdS–Graphene Nanocomposites as Visible Light Photocatalyst for Redox Reactions in Water: A Green Route for Selective Transformation and Environmental Remediation. J. Catal. 2013, 303, 60-69. (56) Kanda, S.; Akita, T.; Fujishima, M.; Tada, H. Facile Synthesis and Catalytic Activity of MoS2/TiO2 by a Photodeposition-Based Technique and its Oxidized Derivative MoO3/TiO2 with a Unique Photochromism. J. Colloid Interface Sci. 2011, 354, 607-610. (57) Genuit, D.; Afanasiev, P.; Vrinat, M. Solution Syntheses of Unsupported Co(Ni)–Mo–S Hydrotreating Catalysts. J. Catal. 2005, 235, 302-317. (58) Wang, Z.; Hou, J.; Yang, C.; Jiao, S.; Zhu, H. Three-dimensional MoS2-CdS-γ-TaON Hollow Composites for Enhanced Visible-Light-Driven Hydrogen Evolution. Chem. Commun. 2014, 50, 1731-1734. (59) Wei, L.; Chen, Y.; Lin, Y.; Wu, H.; Yuan, R.; Li, Z. MoS2 as Non-noble-metal Co-catalyst for Photocatalytic Hydrogen Evolution over Hexagonal ZnIn2S4 under Visible Light Irradiations. Appl. Catal., B 2014, 144, 521-527. (60) Hou, Y.; Laursen, A. B.; Zhang, J.; Zhang, G.; Zhu, Y.; Wang, X.; Dahl, S.; Chorkendorff, I. Layered Nanojunctions for Hydrogen-Evolution Catalysis. Angew. Chem. Int. Ed. 2013, 52, 3621-3625. (61) Xiao, F.-X.; Miao, J.; Wang, H.-Y.; Liu, B. Self-Assembly of Hierarchically Ordered CdS Quantum Dots-TiO2 Nanotube Array Heterostructures as Efficient Visible Light Photocatalysts for Photoredox Applications. J. Mater. Chem, A 2013, 1, 12229-12238. 25



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Page 27 of 36

Scheme 1. The schematic illustration for the preparation of GR-CdS-MoS2 (PD) (A) and GR-CdS-MoS2 (HT) (B) composites. Note: RT refers to room-temperature, TAA refers to thioacetamide.

Mo 3d5/2 228.9 eV

Mo 3d3/2 232.1 eV

228

B

Mo 3d Intensity (a.u.)

Intensity (a.u.)

A

230

232

234

236

Cd 3d3/2

405.3 eV

412.0 eV

404

406

408

410

412

414

Binding Energy (eV)

S 2p1/2 162.7 eV

161

D

Cd 3d

Cd 3d5/2

161.5 eV

162

416

163

164

165

166

Binding Energy (eV)

167

C 1s

C-C, C=C & C-H

Intensity (a.u.)

C

S 2p

S 2p3/2

160

Binding Energy (eV)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C-OH C-O-C & C=O HO-C=O

284

286

288

290

292

Binding Energy (ev)

Figure 1. X-ray photoelectron spectra (XPS) of Mo 3d (A); S 2p (B); Cd 3d (C) and C 1s (D) of GR-CdS-MoS2 (PD) composite. 27



B

(1% GR-CdS)-1% MoS2 (0.5% GR-CdS)-1% MoS2 CdS-1% MoS2

10

20

30

40

(1% GR-CdS)-1% MoS2 (1% GR-CdS)-0.5% MoS2 CdS-1% MoS2 (111)

Blank CdS

(311)

(220)

(111)

Blank CdS

(1% GR-CdS)-2% MoS2

50

60

70

80

10

20

C

Blank CdS CdS-1% MoS2

1.6

D

(5% GR-CdS)-1% MoS2

0.8 0.4

300

400 500 600 700 Wavelength(nm)

60

70

80

800

(1% GR-CdS)-0.5% MoS2

Absorbance(a.u.)

(1% GR-CdS)-1% MoS2 (10% GR-CdS)-1% MoS2

0.0

50


(0.5% GR-CdS)-1% MoS2

1.2

40

2 Theta (degree)

2 Theta (degree) 1.6

30

(311)

Intensity (a.u.)

(5% GR-CdS)-1% MoS2

(1% GR-CdS)-5% MoS2

Intensity (a.u.)

(10% GR-CdS)-1% MoS2

(220)

A

Absorbance(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 36

(1% GR-CdS)-1% MoS2

1.2

(1% GR-CdS)-2% MoS2 (1% GR-CdS)-5% MoS2

0.8 0.4 0.0

300

400 500 600 700 Wavelength(nm)

800

Figure 2. X-ray diffraction (XRD) patterns and UV-vis diffuse reflectance spectra (DRS) of blank CdS, CdS-1%MoS2 (PD), (GR-CdS)-1%MoS2 (PD) composites with different weight ratios of GR (A, C) and (1%GR-CdS)-MoS2 (PD) composites with different weight ratios of MoS2 (B, D).

28


Page 29 of 36

A

500

B

H2 evolution (µ mol)

H2 evolution (µmol)

500

400

400

300

300 200

200

100

100

0

) ) ) ) dS PD) S 2 (PD (PD (PD (PD S ( o nk C MoS 2 % MoS 2 % MoS 2 Bla 1% Mo 2-1% M % 1 1 1 ) S S) S) S) CdS R-Cd - Cd - Cd -Cd GR GR GR %G (1% (5% (0.5 (10%

C

500

400 300 200 100 0

0

) ) ) ) ) dS (P D (P D (P D (PD (PD oS 2 nk C MoS 2 % MoS 2 % MoS 2 % MoS 2 Bla -1% M % 5 1 2 5 . 0 ) S) S) CdS -CdS)CdS -Cd -Cd GR % GRGR GR ( 1% (1% (1 ( 1%


500


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


) ) ) ) dS (HT (HT (HT (HT nk C oS 2 oS 2 oS 2 MoS 2 M Bla M M % % % -1 -1 )-1% S)-1 dS) dS) CdS -Cd R-C R-C GRGR %G %G % % 1 0 5 ( 5 . 1 ( ( (0

D

400 300 200 100 0

) ) ) ) dS (HT (HT (HT (HT nk C oS 2 MoS 2 MoS 2 MoS 2 M Bla % % % -2 -5 -1 .5% dS) dS) dS) S)-0 R-C R-C R-C -Cd %G GR %G %G 1 1 1 ( ( ( % (1

Figure 3. Photocatalytic H2 evolution over blank CdS, CdS-1%MoS2 (PD), (GR-CdS)-1%MoS2 (PD) with different weight ratios of GR (A), (1%GR-CdS)-MoS2 (PD) with different weight ratios of MoS2 (B), (GR-CdS)-1%MoS2 (HT) with different weight ratios of GR (C) and (1%GR-CdS)- MoS2 (HT) with different weight ratios of MoS2 (D). Light source: 300 W Xe lamp, λ > 420 nm. Reaction time: 1 h. Reaction solution: 80 mL of lactic acid aqueous solution (10 vol.%). Catalyst: 40 mg.

29



2400



2000

CdS-1% Pt (1% GR-CdS)-1% MoS2 (PD)

1600

(1% GR-CdS)-1% MoS2 (HT)

1200 800 400 0 1

2

3

4

Irradiation time (h) Figure 4. Comparison of photocatalytic H2 evolution activity over blank CdS, CdS-1%MoS2 (PD), CdS-1%Pt, (1%GR-CdS)-1%MoS2 (PD) and (1%GR-CdS)-1%MoS2 (HT). Light source: 300 W Xe lamp, λ > 420 nm. Reaction solution: 80 mL of lactic acid aqueous solution (10 vol.%). Catalyst: 40 mg.

A

2400

Blank CdS CdS-1% MoS2 (PD)

CdS-1% Pt (1% GR-CdS)-1% MoS2 (PD) (1% GR-CdS)-1% MoS2 (HT)

2000

80

B

Blank CdS


2800


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 36

60

1600

40

1200 800

20

400 0

0 0

4

8

12

16

20

0

4

Irradiation time (h)

8

12

16

20


Figure 5. Recycling test of photocatalytic H2 evolution over blank CdS, CdS-1%MoS2 (PD), CdS-1%Pt, (1%GR-CdS)-1%MoS2 (PD) and (1%GR-CdS)-1%MoS2 (HT) (A); magnified recycling test result of blank CdS (B). Light source: 300 W Xe lamp, λ > 420 nm. Reaction solution: 80 mL of lactic acid aqueous solution (10 vol.%). Catalyst: 40 mg.

30


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Typical TEM image of (GR-CdS)-MoS2 (PD) (A); HRTEM images of (1%GR-CdS)-1%MoS2 (PD) (B), (1%GR-CdS)-2%MoS2 (PD) (C) and (1%GR-CdS)-5%MoS2 (PD) (F); EDX of the (1%GR-CdS)-1%MoS2 (PD) composite (D, E); the high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) image (G), and mapping results of the elements of C (H), Cd (I), and Mo (J) and S (K).

31



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Page 32 of 36

Figure 7. Typical TEM (A-B) and HRTEM (C-D) images of (1%GR-CdS)-1%MoS2 (HT) composite.

32


A

-5

-15


-20

(1% GR-CdS)-1% MoS2 (PD)

Photocurrent (µ A)

2

0

-10

5

B

Blank CdS CdS-1% MoS2 (PD) (1% GR-CdS)-1% MoS2 (PD)

4


3 2 1


-25

0 -0.50

-0.45

-0.40

-0.35

-0.30

40

-0.25

80

Potential/V vs. Ag/AgCl

300 250

(1% GR-CdS)-1% MoS2 (PD) (1% GR-CdS)-1% MoS2 (HT)

′

160

200

240

Blank CdS CdS-1% GR (PD) (1% GR-CdS)-1% MoS2 (PD)

D


C

120

Irradiation time (s)

200 150 100

Intensity (a.u.)

350

-Z′′ (ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Current density (µA/cm )

Page 33 of 36


50 0 200

400

600

Z′ (ohm)

800

450

500

550

600

650

700

750

800

Wavelength (nm)

Figure 8. Polarization curves (A), transient photocurrent responses (B), electrochemical impedance spectroscopy (EIS) Nyquist plots (C) and photoluminescence (PL) spectra (D) of blank CdS, CdS-1% MoS2 (PD), (1%GR-CdS)-1%MoS2 (PD) and (1%GR-CdS)-1%MoS2 (HT) electrodes composites.

33



80

A

(GR-CdS) (GR-CdS)-HT

60

B

250


70


Blank CdS 1% GR-CdS 1% GR-CdS (HT)

200

50

150

40 30

100

20 10

50 0

0 0%

0.5%

1%

5%

1

10%

2

240

3

4


Weight ratio of GR


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 36

C

Blank CdS 1% GR-CdS 1% GR-CdS (HT)

180

120

60

0 0

4

8

12

16


20

Figure 9. Photocatalytic H2 evolution over blank CdS, GR-CdS and GR-CdS (HT) composites with different weight ratios of GR under visible light irradiation for 1 h (A); time-online photoactivity comparison (B) and recycling test of photocatalytic H2 evolution (C) over blank CdS, 1%GR-CdS and 1%GR-CdS (HT). Light source: 300 W Xe lamp, λ > 420 nm. Reaction solution: 80 mL of lactic acid aqueous solution (10 vol.%). Catalyst: 40 mg.

34


Page 35 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10. Schematic illustration for the photocatalytic H2 evolution over the (GR-CdS)-MoS2 (PD) (A) and (GR-CdS)-MoS2 (HT) (B) composites under visible light irradiation (λ > 420 nm).

35



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TOC Graphic

36


Insight into the Effect of Highly Dispersed MoS2 versus Layer

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