Self-assembled MoS2-GO framework as efficient cocatalyst of

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Self-assembled MoS2-GO framework as efficient cocatalyst of CuInZnS for visible-light driven hydrogen evolution Ting Huang, Yuting Luo, Wei Chen, Jiacheng Yao, and Xiaoheng Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03693 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 21, 2018

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ACS Sustainable Chemistry & Engineering

Self-assembled MoS2-GO framework as efficient cocatalyst of CuInZnS for visiblelight driven hydrogen evolution Ting Huang, Yuting Luo, Wei Chen, Jiacheng Yao, Xiaoheng Liu*

Key Laboratory for Soft Chemistry and Functional Materials of Ministry Education, Nanjing University of Science & Technology, Nanjing 210094, P.R.China *corresponding author

Tel. 86-25-84315943

Fax 86-25-84315054

E-mail: [email protected]; [email protected]

Address: Xiaolingwei 200, Xuanwu district Nanjing 210094, P.R China KEYWORDS: Hydrogen evolution; Photocatalysis; Visible light response; MoS2

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ABSTRACT

A ternary heterostructured CuInZnS/MoS2-GO photocatalyst was constructed by a simple two-step hydrothermal method. Three-dimensional hierarchical architecture of MoS2-GO hydrogel was first synthesized through a facile hydrothermal method. The obtained MoS2-GO hydrogel with ultralow density and high surface area was redispersed into water and composite with CuInZnS. The resulting catalysts were analyzed by systematic characterizations including X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), Raman, and UV-vis diffuse reflectance spectra (DRS) et al. The noble metal-free composite exhibited dramatically enhanced photocatalytic performance toward hydrogen evolution. The enhanced solar water splitting performance could be ascribed to the synergetic effect of GO and MoS2. GO served as an electron acceptor and transporter while MoS2 provide abundant active sites for hydrogen evolution. We hope this work may give some perspectives on the construction of noble-metal free catalyst for visible light driven hydrogen production.


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Introduction The rapid development of human society increased the reliance on traditional fossil fuels and the combustion of fossil fuels also give rise to a series of environmental issues.1, 2 In the wake of the global energy and environmental issues, great efforts have been devoted to the search of clean and renewable fuels to replace traditional fossil fuels. The high specific energy of combustion and clean combustion product of hydrogen make it a promising candidate in future energy system.3,

4

In 1972,

Fujishima and Honda successfully generate hydrogen on a titanium photoelectrode.5 Their pioneering work sparked worldwide interest for converting solar energy to chemical energy in the form of hydrogen.6-8 Photocatalytic water splitting is an attractive strategy to produce H2 in the presence of semiconductor photocatalysts. During the photocatalytic process, noble metals were usually introduced as an efficient cocatalyst to enhance its catalytic performance. The loading of noble metals on semiconductors can facilitate a more efficient separation of the photogenerated electron–hole pair and introduce active sites for H2 evolution.9, 10 Limited by their scarcity and high price, the search of inexpensive and efficient cocatalyst to replace noble metals is still undergoing. The past few years witnessed a significant progress in the synthesis of 2D layered structures.11,

12

Among them, MoS2, a typical 2D transition metal dichalcogenide

composed of earth abundant elements, is viewed as good candidate as cocatalysts for photocatalytic water splitting.13-15 Theoretical studies indicate that the edges of MoS2 are easily bound to hydrogen atoms, which contribute to the reduction of H+ by electrons.16, 17 However, MoS2 suffers the problem of poor electrical conductivity,18 which restricts its cocatalytic activity. Loading MoS2 on conductive substrates is a good way to overcome this drawback. The hybrid of MoS2 layers with graphene has



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been widely accepted as an efficient cocatalyst for further optimization of photocatalytic reactions because of the high exposure of active sites and good electron transfer abilities.19, 20 Table 1 summarizes the recent development of MoS2-GO based composites, including the employed approach for synthesis and their photocatalytic performance for hydrogen evolution. By combining MoS2 with GO as cocatalysts, the new hybrid cocatalyst can obviously enhance the hydrogen evolution rate of the ternary nanocomposite. TiO2 and ZnO are the most frequently used semiconductor metal oxide for photocatalytic hydrogen evolution. For example, Kumar et al.21 synthesized ternary heterojunction nanocomposites consisting of ZnO and MoS2-GO cocatalyst. The heterogeneous catalysts showed the highest photocatalytic H2 production of 28.616 mmol･h-1･g-1 under solar light irradiation, which is 56 times as high as the bare ZnO. Xiang et al.22 first report the synergetic effect of Graphene and MoS2 on TiO2 nanoparticles for use in solar light driven hydrogen generation. The photocatalytic activity is significantly enhanced in presence of the MoS2-Graphene cocatalyst. However, the main limitation of ZnO and TiO2 is the UV excitation arising from their large band gaps.23-25 Thus tremendous effort has been devoted to the nanocomposite of MoS2-GO with other visible light responsive photacatalyst, such as CdS26-28, ZnIn2S429, 30, C3N431-34 . Among them, CdS is the currently most studied visible light driven photocatalyst coupling with MoS2-GO. Liu et al.35 reported the preparation

of

MoS2–graphene/CdS

nanocomposite

with

a

mixed

nanoparticles/nanorods morphology. The quantum yield (QY) was as high as 65.8% and the performance of photocatalytic H2 evolution was obviously enhanced. Nevertheless, Ben Ali et al.36 successfully synthesized CdS nanorods/GO/MoS2 nanocomposites. The incorporation of GO can obviously enhance the hydrogen


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evolution rate compared to the CdS/MoS2 solely. It is worth noting that Cd-based metal sulfide is composed of toxic heavy metal. So the exploration of low toxic, visible light responsive photocatalyst is still undergoing. The construction of multinary particles hold a unique advantage for the development of efficient visible-light responsive catalysts for hydrogen generation. The rational design of chemical composition in multinary semiconductor can produce low toxic, visible light responsive semiconductors constituted by earth abundant elements. Such novel semiconductor hold great promise to replace the currently most studied Cd-based toxic heavy metal sulfide. Besides, the bandgap and bandgap alignment of the multinary nanoparticles can be adjusted by varying the composition and stoichiometry of the multinary nanoparticles, which is of great importance for solar light driven H2 evolution because the conduction band position of photocatalysts must be more negative than the redox potential of water (0 V vs NHE).37 In this work, we have successfully prepared a noble-metal free photocatalyst constructed by CuInZnS, graphene oxide and MoS2 via a two-step hydrothermal mehod. The hydrogel of MoS2–GO was first fabricated and then the as-obtained nanohybrid was further coupled with multinary metal sulfide CuInZnS. The hierarchical architecture exhibited significantly enhanced photocatalytic performance for hydrogen evolution. To clarify the mechanism underlying the observed phenomenon, a tentative electron transfer scheme was also proposed. The introduction of MoS2-GO can effectively suppress the recombination of photo-generated carriers thus lengthen the life span of holes and electrons. Besides, the active site of MoS2 can boost the catalytic activity for hydrogen evolution. We hope this work may provide some insight into the development of noble-metal free photacatalyst for hydrogen evolution.


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Table 1. Summary of MoS2/GO based photocatalysts and their performance for hydrogen evolution. Catalyst

Synthesis Method

Sacrificial agents

Light source

Activity

Morphology

Ref

CdS- MoS2-GO

ultrasonic method

Lactic acid

Simulated solar light

234 mmol･h-1･g-1

Nanorods/ sheet

26

ZnO-MoS2-GO

hydrothermal

Na2S and Na2SO3

Sunlight

28.616 mmol･h-1･g-1

Particles/sheet

21

g‑C3N4-MoS2-GO

ion exchange method

Na2SO3

450 W Xe lamp with AM

1.65 mmol･h-1･g-1

quantum dots/ sheet

31

CdS- MoS2-GO

hydrothermal process

Lactic acid

7.1 mmol･h-1･g-1

Nanorods/ sheet

36

ZnO-MoS2-GO

solvothermal method

A 300 W Xe lamp

288.4 µmol･h-1･g-1

Particles/sheet

23

ZnIn2S4-MoS2-GO

two-step process

0.25 M Na2S and

300 W xenon lamp

0.25 M Na2SO3

λ>420 nm

4169 µmol･h-1･g-1

sheet/sheet

29

CdS-MoS2-GO

hydrothermal

1913 µmol･h-1･g-1

Particles/sheet

27

ZnS- MoS2-GO

hydrothermal

300 W Xenon lamp

2258µmol･h-1･g-1

Particles/sheet

24

CdS-MoS2–GO

hydrothermal

λ>420 nm

2.32 mmol･h-1･g-1

mixed particles and

TiO2-MoS2-GO

hydrothermal

xenon arc lamp

165.3µmol･h-1･

Particles/sheet

1.5G filter 300 W Xe Lamp with a UV filter

0.25 M Na2S and 0.25 M Na2SO3.

10% vol lactic acid 0.005 M Na2S and 0.005 M Na2SO3 lactic acid 25% of ethanol

300 W xenon lamp λ>400 nm


rods/sheet

35

22

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Experimental Preparation of MoS2-GO hybrid GO was prepared by modified Hummers method.38 In a typical synthesis of MoS2-GO hybrid, 4 mmol Na2MoO4 and 16 mmol thiourea were dispersed in 30ml DI water. The homogenous solution was then dropped into an aqueous suspension of GO. The resulting whole mixture was then transferred to a 100 mL PPL-lined autoclave and heated at 220‑ for 24h. After cooling to room temperature, the hydrogel of MoS2-GO was obtained. The collected hydrogel was washed by water several times and freeze dried. By varying the initial amount of their precursors, we can get the hybrid of MoS2 and GO with different mass fractions. Weight ratio of MoS2 to graphene oxide at 95:5, 90:10, 80:20, 70:30 were labeled as M95G5, M90G10, M80G20 and M70G30, respectively. Preparation of CuInZnS/MoS2-GO To prepare the CuInZnS/MoS2-GO nanocomposite, we first prepare the MoS2-GO suspension. In brief, 0.1 g of MoS2-GO hybrid was added to 100 mL of water, followed by strong ultrasound at 800 W output power. A dark brown suspension was finally obtained after 3 h continuous ultrasonic effect. The CuInZnS nanoparticles were synthesized by a similar method of Tang.39 Zn(Ac)2･2H2O(219 mg), CuCl (2.4 mg), InCl3･4H2O (65.89 mg), and excessive thioacetamide (200 mg) were added to appropriate amount suspension of MoS2-GO. The loading amount of cocatalyst was chosen as 0, 0.5%, 1%, 1.5%, 2%. The whole mixture was sealed in a 100 ml autoclave and maintained at 180 ℃ for 18 h. The obtained sample was washed by deionized water and ethanol five times, finally dried in an oven for 12 h.



Characterization The structures and morphologies of the prepared products were examined using transmission electron microscopy (TEM, JEOL-2100) and field-emission scanning electron microscopy (FE-SEM, S-4800). The crystal structure and phase identification were analyzed via X-ray diffraction (Bruker D8 Advance, Cu Kα λ=1.5417°) ranging

from 10° to 80°. X-ray photoelectron spectroscopy (XPS, Phi Quantera II SXM) was used to analyze the elemental compositions and oxidation state. The UV–vis diffuse reflectance (DRS) experiment were carried out on a UV–vis spectrophotometer (Shimadzu UV-2600, Japan) using BaSO4 as a reference in the wavelength range of 200–800 nm. Raman spectra was acquired by a Raman microscope (Renishaw Invia) using argon ion laser and collected from 100 to 2000 cm-1. Electrochemical measurement The electrochemical measurements were measured by CHI660B electrochemical station (Chenhua, China). A conventional three-electrode cell was constructed using Ag/AgCl electrode and Pt wire as reference electrode and counter electrode, respectively. To prepare the working electrode, the samples were dispersed in absolute ethanol with a concentration of 2.0 mg/mL. After ultrasonication for few minutes, 10 µL of the suspension was dropped onto a cleaned FTO conducting glass with a square of 1 cm × 2 cm and maintained at 60℃ for twelve hours. The photocurrent curve of the working electrode was measured in 0.5 M Na2SO4 and irradiated by a 300W Xe lamp equipped with a 420 nm cutoff filter at an interval of 20 seconds. The electrochemical impedance spectroscopy were obtained from 100000 Hz to 0.01 Hz with an AC voltage of 5 mV under visible light irradiation. 0.1 M KCl


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solution containing 2.5 mM K3[Fe(CN)6]/K4[Fe(CN)6] was employed as the electrolyte.

Photocatalytic activity To conduct photocatalytic reactions, 50 mg of the samples were dispersed in a 300 mL aqueous solution of Na2S 0.35M/Na2SO3 0.25M. The whole mixture was then transferred to a reaction vessel and evacuated several times prior to irradiation. A 300 W Xe lamp coupled with a 420 nm cut-off filter was employed as visible light source. The amount of hydrogen was analyzed by a gas chromatograph (JieDao, GC1609) using Ar as carrier gas with a flow rate of 15 mL･min-1.

Discussion We first prepared the nanohybrid of MoS2 and GO and denote them as MxGy, in which x and y represent the mass percentage of MoS2 and GO, respectively. For example, M90G10 represents the MoS2-GO hybrid was composed of MoS2 (90 wt%) and graphene oxide (10 wt%). Figure 1(a) shows the precursor solution before and after hydrothermal reaction. A three-dimensional assembly was formed after hydrothermal reaction at 220℃ for 24h. As can be seen from that, different molar ratios of MoS2 and GO give rise to similar assembly of the product, which is observed as 3D assemblies of 2D materials. Further structural insights were obtained by TEM, as depicted in Figure 1(b-i). Sheet-like MoS2 was in situ grown on graphene oxide thus GO serve as a template for the growth of MoS2. The thickness of the MoS2 grown on the graphene oxide become thinner with increasing the adding amount of graphene oxide. It is considerable since more graphene oxide can provide more growing template for MoS2.



Figure 1.(a) Photograph of precursor solution before hydrothermal reaction and the obtained products with different mass ratio of MoS2 and GO (b,f) TEM images of M95G5, (c,g) M90G10, (d,h), M80G20, (e,i)M70G30

The crystallinity and phase structure of CuInZnS and CuInZnS/M90G10 samples were characterized by XRD, as exhibited in Figure 2. Different XRD patterns of pure CuInZnS and its composites with different contents of MoS2-GO cocatalyst were compared. The nanocomposite with different mass fraction of M90G10 cocatalyst were labeled as CIZS/MG-y (y=0.5, 1, 1.5 and 2). Previous studies reported that pure CIZS possesses a cubic zinc blende structure(JCPDS No. 65-0309).40 The major reflections at 28.6, 47.5, 56.4° could be indexed to the (111), (220) and (311) planes. For the CIZS/M90G10 composites, no characteristic peaks for MoS2 or GO could be


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observed, mainly due to the relatively low diffraction intensity and loading amount.41 So the presence of MoS2 and graphene oxide was further demonstrated by XPS and Raman analysis.

Figure 2. XRD pattern of the CIZS nanoparticles and the composite of CIZS and M90G10.

The morphology and nanostructure of the pure CIZS and the CIZS/M90G10 nanocomposites were characterized by TEM. As depicted in Figure 3 a and b, pure CIZS possess a typical spherical-like structures with the diameter around 100 nm. The TEM image of CIZS/M90G10 nanohybrid are shown in Figure 3 c and d. CIZS are insitu anchored on the surface of sheet-like MoS2-GO cocatalyst with good interfacial contact. This interfacial facilitate the charge transfer and contribute to the enhanced photocatalytic performance.



Figure 3. TEM images of the CIZS nanoparticles (a, b) and the CIZS/M90G10 nanocomposite (c, d).

The FESEM images were obtained to further observe the microstructure of the samples. As can be seen in Figure 4a, sphere-like CIZS particles was observed and average particles size was about 100 nm. An enlarged FESEM image in Figure 4b shows that the CIZS particles were composed of small particles thus it has a rough surface. For the CIZS/MoS2-GO nanocomposite, we can observe that CIZS particles were deposited on the surface of MoS2-GO. Such intimate interfaces can facilitate the charge transfer from CIZS to GO and MoS2. The EDS was conducted to analyze the


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element composition of CIZS, which is provided in supporting information (Figure S2). The EDS analysis of the selected field reveals the existence of Cu, In, Zn, and S elements. The corresponding element composition ratio was given in Table S1 in supporting information.

Figure 4. FESEM images of CIZS(a, b) and CIZS/M90G10 samples (c, d).

To demonstrate the coexistence of MoS2 and graphene oxide in the hydrogel, Raman spectroscopy analysis was conducted on the cocatalyst M95G5, M90G10, M80G20 and M70G30. As can be seen in Figure 5, two dominant peaks located at 1343 and 1582 cm-1 can be assigned to the characteristic D band and G band peaks of graphene oxide. The G band is originated from the vibration of ordered sp2 C atoms, while the D band is associated with the edges, defects and disordered carbon.42, 43 Two reflections centered at 376.8 cm-1 and 403.0 cm-1 could be attributed to MoS2, which correspond to E12g and A1g modes of the hexagonal MoS2 crystal.44-46 The co-



existence of MoS2 and GO peaks in the raman spectra demonstrate that layeredstructured MoS2 was in situ deposited on the surface of graphene oxide.

Figure 5. Raman spectra of M95G5, M90G10, M80G20 and M70G30.

The constituent elements and chemical states of the composite were confirmed by X-ray photoemission spectroscopy (XPS). Obvious peaks in the survey spectrum (Figure S1) indicate the presence of Cu, In, Zn, Mo, S in the composite. Highresolution spectra of individual elements were further acquired to expolre the valence states of the elements. As shown in Figure 6a, two peaks at 931.8 and 951.8 eV can be ascribed to Cu 2p3/2 and Cu 2p1/2. The high resolution in In 3d spectra (Figure 6b) displays two peaks at 444.5 and 452.1 eV. The XPS result of Zn 2p region was shown in figure 6c, which contains Zn 2p3/2 at 1021.6 eV and Zn 2p1/2 at 1044.8 eV. In the case of Mo 3d region (Figure 6d), two peaks at 228.8 and 232.3 eV can be assigned to the doublet Mo3d5/2 and Mo3d3/2, respectively. The value is in accordance with the


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reported values for the MoS2 and verified the existence of MoS2 in the composite sample.47 As can be seen from Figure 6e, the XPS spectra of S 2p can be deconvoluted into two peaks centered at 161.4 and 162.6 eV, corresponding to the S 2p3/2 and S 2p1/2.48, 49 The C1s spectra of the composite in Figure 6(f) could be fitted into three peaks, 284.6 eV for sp2 bonded carbon (C-C), 286.1 eV for carbonyls (C=O) and 288.3 eV for carboxyl (O=C-O) functional groups, respectively

Figure 6. XPS spectra of the involved element in the CIZS/M90G10 nanocomposite (a) Cu; (b) In; (c) Zn; (d) Mo; (e) S; (f) C.

Figure 7a exhibits the UV-vis diffuse reflectance spectra of CIZS and CIZS/M90G10 composite. Pure CIZS shows broad absorption in the visible-light region. All the composite samples with various MoS2-GO ratios exhibit the enhanced absorption in the visible light region. With increasing MoS2-GO content, the light absorption of the composite samples rises rapidly and the corresponding color changes from yellow to grey. Such phenomenon was induced by the background absorption of graphene oxide and MoS2. The bandgap of the CIZS was estimated according to the Tauc plot, αhν = A(hν−Eg)n/2, in which α, h, ν, and Eg represent the



absorption coefficient, Planck constant, light frequency and band gap energy, respectively. The coefficient n is determined at 1 for direct transition semiconductors. Based on the above discussion, the corresponding tauc plot of pure CIZS is displayed in Figure 7b. By extrapolating the linear part of the plot, we can get the band gap value at 2.6 eV. We also conduct Mott–Schottky experiment to determine the CB position of the sample (Figure S3, SI) and the conduction band (CB) position of CIZS was estimated to be -0.53 V (vs. NHE).

Figure 7 (a) UV-vis diffuse reflectance spectra of CIZS and the CIZS/M90G10 nanocomposite; (b) Tauc plot of pure CIZS sample.

The photocatalytic activity of pure and different cocatalyst modified CIZS were all studied for comparison. The experiment were conducted under the irradiation of a 300 W Xenon lamp equipped with 420 nm filter. To clarify the synergetic effect of GO and MoS2, we first compared the photocatalytic performance of pure CIZS and CIZS/MoS2. As can be seen in Figure 8a, bare CIZS shows negligible photocatalytic activity, which is around 32 µmol･g-1･h-1. The poor activity might result from the fast recombination of electron–hole pairs. With increasing MoS2 content, the H2 evolution rate increased initially and then decreased. When the loading amount of MoS2 is


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1wt%, the composite exhibited a H2 production rate of 426 µmol･g-1･h-1, which is 13 times of the pure one. The result demonstrate that MoS2 alone can be an efficient cocatalyst for photocatalytic hydrogen evolution. The photocatalytic hydrogen evolution activity of CIZS in presence of MoS2-GO with varied contents are presented in Figure 8b. The loading amount of different MoS2-GO was fixed at 1wt%. As can be seen in Figure 8b, a much higher hydrogen production rate was obtained when CIZS was coupled with MoS2-GO hybrid. When M90G10 was coupled with CIZS, the photocatalytic H2 production rate is increased to 827 µmol･g-1･h-1, which is 29 times as high as the pure one. Further expansion of graphene oxide content in MoS2-GO cocatalyst just lead to a decline in H2 evolution activity. The result demonstrated that MoS2 and GO can have a synergetic effect on CIZS for photocatatic hydrogen production. The photocatalytic activity of MoS2-GO loaded samples is obviously superior to MoS2 and GO loaded alone. Introducing graphene oxide to this system can accelerate the electron transfer from the CIZS to MoS2. Based on the above observation, we also evaluate the photocatalytic activity of CIZS loaded with different M90G10 contents, which is shown in Figure 8c. The addition of M90G10 cocatalyst can obviously enhance the hydrogen evolution rate. Specifically, the optimum loading amount was observed at 1 wt%. More addition of M90G10 cocatalyst just lead to the decrease of activity, mainly because cocatalyst covered on the surface of composites and block the absorption of CIZS.



Figure 8(a) Comparison of H2-production rates on CIZS and CIZS/MoS2 with varied ratios; (b) Comparison of H2-production rates on CIZS and CIZS coupled with different MoS2-GO at the loading amount 1wt%; (c) Comparison of H2-production rates on CIZS and CIZS/M90G10 with varied contents.

Stability and reusability are important for photocatalysts in practical applications. To evaluate the stability of the samples, a cycling test was conducted. The hydrogen evolution on CIZS/M90G10 was carried out for four times under the same experimental condition. Form Figure 9a we can see that the amount of evolved H2 was almost proportional to the irradiation time. During the consecutive 4 runs, no noticeable decrease can be detected in hydrogen production rate. Figure 9b shows the average H2-production rate during the four runs. A relatively stable hydrogen production rate was observed, suggesting the good stability and reusability for photocatalytic H2 evolution.

Figure 9 (a) Cyclic H2 production on 1.0 wt% CuInZnS/M90G10 photocatalyst; (b) the average H2 evolution rate in different runs.

To estimate the charge carriers transfer of the samples, the transient photocurrent responses were recorded for several on–off cycles. As shown in Figure 10a, CIZS modified electrode show a relatively low photocurrent response, which is about 0.35µA･cm-2. An enhanced photocurrent was observed when CIZS was composite


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with M95G5. CIZS/M90G10 obtianed the highest photocurrent resoponse (1.12µA･ cm-2) among all tested samples, which is 3.2 times of pure CIZS. The enhanced photocurrent response may be attributed to the heterojunction formed between CIZS and MoS2-GO cocatalyst, which increases the transfer and separation efficiency of photogenerated charges. To further investigate the charge transfer and recombination processes, we compared electrochemical impedance spectra (EIS) of pure CIZS and CIZS/MoS2-GO heterojunction electrodes. The as obtained EIS spectra was shown in Figure 10b. The arc radius of the Nyquist curves of the samples is related to the charge transfer efficiency of the samples.50 Normally, smaller radius means lower charge transfer impedance at the electrode−electrolyte interface. Compared with pure CIZS, an obvious smaller radius was observed for CIZS/MoS2-GO composite, which indicates the decrease of solid state interface layer resistance and charge transfer resistance. When M90G10 was coupled with CIZS, the nanocomposite displays the smallest arc radius among all the samples, suggesting the fastest interfacial electron transfer.

Figure 10 (a) Transient photocurrent responses of pure CuInZnS and CuInZnS composite with different MoS2 and GO ratios; (b) EIS spectra of of the corresponding samples.

Mechanism



To clarify the main procedures for H2 evolution, an illustration of possible mechanism for interface electron transfer was proposed. As shown in Figure 11, the photogenerated electrons of CuInZnS would transfer from VB to CB upon the irradiation of visible light, leaving photogenerated holes in the valence band. Without cocatalyst loading, the photogenerated electron-hole pairs are easy to recombine, resulting in a poor photocatalytic activity for H2 evolution. As for the CIZS/ MoS2GO composite, the photo-generated electrons in the CB of CIZS can effective ly transfer to the layered MoS2-GO through the strong interaction and heterojunction among the three components. The conduction band position of CIZS was estimated to be -0.53 V vs. NHE by Mott–Schottky plots (Figure S3, SI), which is more negative than that of graphene/graphene• − and MoS2 nanosheets51, providing the driving force for electron transfer from CIZS to GO or MoS2. In addition, the good electrical conductivity of GO could facilitate the electron transfer and separation. Previous studies also verified that the abundant unsaturated active S atoms at the edge of MoS2 can easily bond to the H+ in the solvent so the edges of MoS2 crystallites can act as the active sites for H2 evolution reaction after they accepted electrons from GO. In conclusion, graphene oxide can serve as an electron acceptor and charge transporter while MoS2 act as active sites for H2 evolution. So in the as constructed system, photogenerated electrons could be transfered from the CB of CIZS to MoS2 nanosheets facilitated by graphene oxide. Then the electrons can reduce the absorbed H+ at the edge of MoS2 and evolve H2. Thus GO and MoS2 can have the synergistic effect on the photocatalytic H2 evolution. Besides, the excited electrons from the conduction band of CIZS could directly transfer to that of MoS2 nanosheets owing to the lower conduction band of MoS2 nanosheet. Na2S/Na2SO3 were used as sacrificial reagent to consume the holes on the VB of CIZS.


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Figure 11 Schematic illustrations of the possible charge transfer for photocatalytic H2 production.

Conclusion A ternary complex CIZS/MoS2-GO was successfully prepared and applied for photocatalytic H2 production. Sphere-like CIZS nanoparticles were in situ anchored on the layered MoS2-GO cocatalyst. The CIZS/M90G10 exhibited best photocatalytic performance with a hydrogen production rate as high as 827 µmol･g-1･h-1, which is 29 times as high as the pure one. The synergistic effect of the MoS2 and GO cocatalyst results in better charge separation through the effective suppression of their recombination and rapid transfer of the charge carriers to the active sites for hydrogen evolution because of the intimate contact and favorable band potentials. In the constructed system, GO can facilitate the electron transfer while MoS2 can provide more active sites for hydrogen evolution, both of them play a positive role in the



photocatalytic H2 production activity. Such noble-metal-free photocatalyst hold a great potential for the production and conversion of sustainable energy.

ASSOCIATED CONTENT Supporting Information XPS survey spectrum of CIZS/M90G10 nanocomposite, EDX spectrum of CIZS sample, table about the element composition of CIZS sample, Mott-Schottky plot for CIZS particles based photoelectrode.

Acknowledge This project is supported financially by the National Natural Science Foundation of China (Grant no.51572126).

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Layer-structured MoS2-GO was synthesized and used as cocatalyst of CIZS for visible light driven photocatalytic H2 evolution.



Figure 1.(a) Photograph of precursor solution before hydrothermal reaction and the obtained products with different molar ratio of MoS2 and GO (b,f) TEM images of M95G5, (c,g) M90G10, (d,h), M80G20, (e,i)M70G30GO 160x133mm (300 x 300 DPI)


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Figure 2. XRD pattern of the CIZS nanoparticles and the composite of CIZS and M90G10. 80x56mm (300 x 300 DPI)



Figure 3. TEM images of the CIZS nanoparticles (a, b) and the composite (c, d). 160x160mm (300 x 300 DPI)


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Figure 4. FESEM images of CIZS(a, b) and CIZS/MoS2-GO samples (c, d). 160x120mm (300 x 300 DPI)



Figure 5. Raman spectra of M95G5, M90G10, M80G20 and M70g30. 160x113mm (300 x 300 DPI)


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Figure 6. XPS spectra of the involved element in the composite (a) Cu; (b) In; (c) Zn; (d) Mo; (e) S; (f) C. 160x75mm (300 x 300 DPI)



Figure 7 (a) UV-vis diffuse reflectance spectra of CIZS and the CIZS/M90G10 nanocomposite; (b) Tauc plot of pure CIZS sample. 160x56mm (300 x 300 DPI)


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Figure 8(a) Comparison of H2-production rates on CIZS and CIZS loaded with different amount of MoS2; (b) Comparison of H2-production rates on CIZS and CIZS coupled with different MoS2-GO at the loading amount 1wt%; (c) Comparison of H2-production rates on CIZS and CIZS loaded with different amount of M90G10. 160x37mm (300 x 300 DPI)



Figure 9(a) Cyclic H2 production on 1.0 wt% CuInZnS/M90G10 photocatalyst; (b)corresponding bar graph of the average H2-production rate. 160x56mm (300 x 300 DPI)


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Figure 10 (a) Transient photocurrent responses of pure CuInZnS and CuInZnS composite with different kind of MoS2-Graphene; (b) the Electrochemical impedance spectra (EIS) of the samples. 160x56mm (300 x 300 DPI)



Figure 11 Schematic illustrations of the possible charge transfer for photocatalytic H2 production. 80x60mm (300 x 300 DPI)


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Self-assembled MoS2-GO framework as efficient cocatalyst of

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