Direct photoinduced synthesis of amorphous CoMoSx cocatalyst and

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Direct photoinduced synthesis of amorphous CoMoSx cocatalyst and its improved photocatalytic H2-evolution activity of CdS Wenjing Liu, Xuefei Wang, Huogen Yu, and Jiaguo Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02971 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on August 4, 2018

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ACS Sustainable Chemistry & Engineering

Direct photoinduced synthesis of amorphous CoMoSx cocatalyst and its improved photocatalytic H2-evolution activity of CdS Wenjing Liu‡, Xuefei Wang‡, Huogen Yu†,‡*, Jiaguo Yu§

†

State Key Laboratory of Silicate Materials for Architectures, Wuhan University of

Technology, 122 Luoshi Road, Wuhan, Hubei 430070, People’s Republic of China ‡

Department of Chemistry, School of Chemistry, Chemical Engineering and Life

Sciences, Wuhan University of Technology, 122 Luoshi Road, Wuhan, Hubei 430070, People’s Republic of China §

State Key Laboratory of Advanced Technology for Material Synthesis and

Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan, Hubei 430070, People’s Republic of China

Tel: 0086-27-87756662, Fax: 0086-27-87879468 [email protected]

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ABSTRACT Compared with the well-known Pt cocatalyst, molybdenum sulfide is one of the most prospective alternatives for developing highly efficient photocatalytic H2-evoultion materials. To further improve its H2-evoultion activity, it is highly required to further optimize the electronic and surface structures of molybdenum sulfide cocatalyst. Herein, a novel amorphous molybdenum-based bimetallic sulfide (a-CoMoSx) was prepared and loaded on the CdS surface by a photoinduced electron-reduction method to promote the interfacial H2-production rate of CdS. It is found that loading a-CoMoSx cocatalyst can markedly enhance the H2-generation activity of CdS photocatalyst, which is significantly higher than that of pure CdS and a-MoS2/CdS by a factor of 26.3 and 3.1 times. More importantly, in addition to CdS, the a-CoMoSx can also serve as the general electron cocatalyst to obviously promote the H2-production activity of well-known TiO2 (typical UV-responsive titanium dioxide) and g-C3N4 (novel visible light-responsive organic semiconductors). According to the present results, an electron-cocatalyst mechanism of a-CoMoSx is proposed to explain the enhanced photocatalytic H2-production performance, namely, amorphous CoMoSx can rapidly capture electrons and then quickly transfer the electrons to the active sites (its unsaturated S atoms and defect sites) to effectively enhance the interfacial H2-production reaction. In consideration of its facile synthesis, low cost, and superior performance, the amorphous CoMoSx appears to be one of the most prospective cocatalysts for photocatalytic water splitting. Keywords: Photocatalysis; H2 production; Amorphous CoMoSx; CdS; Cocatalyst


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Introduction Using semiconductor photocatalytic materials to convert solar energy to hydrogen has attracted extensive attention in solving the current energy problems.1-7 However, for most single photocatalysts, photogenerated electron-hole pairs will easily recombine on its surface or inside the photocatalyst after light irradiation, resulting in low quantum efficiency.8-11 To significantly increase the rate of photocatalytic H2 evolution reaction (HER), various strategies have been widely utilized

such

as electron-cocatalyst

modification,12-16

doping

with

foreign

elements,17-20 and semiconductor coupling.21-25 Among of them, electron-cocatalyst modification is regarded as one of the most efficient methods owing to its simple preparation method and remarkable effect. It is widely reported that the electron cocatalyst such as noble metals (such as Pt,26-27 Pd28 and Au29) show excellent promotion action for hydrogen production. However, they are expensive and rarity, making them economically inefficient. Thence, developing new, low-cost and efficient electron cocatalyst is highly anticipated and challenging. Molybdenum sulfide, one of the most prospective alternatives to the noble metal Pt cocatalyst, has widely been used in the field of photocatalytic H2 production.30-33 Its basic principle of enhancing H2 production is that the unsaturated S atoms of molybdenum sulfide can quickly capture protons (H+) in the solution, and then accelerate the interfacial H2-production reaction.34-35 However, it is difficult to obtain high H2-production performance of molybdenum sulfide owing to its weak conductivity and limited active sites.36-37 Recently, many studies have reported that



constructing molybdenum-based bimetallic sulfide is one of the most significant and effective methods to optimize its electronic and surface structures for enhanced electrocatalytic H2-production activity. For example, Wu et al.38 found that compared with the CoS and MoS, the CoMoS nanosheets possessed higher electrocatalytic H2-evolution performance due to the formation of porous, defect-rich, and vertically aligned nanostructure. Dai et al.39 also indicted the excellent electrocatalytic HER performance of the CoMoS (nCo/nMo=0.2) due to the synergistic effect of both structural and electronic benefits, the balance of active sites, the existence of abundant defects, and electronic conductivity. In addition, Lv et al.40 successfully prepared various Ni-doping MoS2 microspheres with high active surface area and density of electrochemically active sites for improved H2-evolution performance. Many other reports also strongly demonstrated that molybdenum-based bimetallic sulfide could serve as the excellent H2-production active centers in electrocatalytic reactions.41-43 Based on their similar mechanism for electroncatalytic and photocatalytic H2 evolution, it is quite expected that the molybdenum-based bimetallic sulfide can function as a high-efficiency electron cocatalyst to promote the photocatalytic H2 production of various photocatalysts. Moreover, compared with crystalline cocatalyst, amorphous bimetallic sulfide modified photocatalysts may exhibit a better photocatalytic H2-production performance because the amorphous phase is in a highly irregular arrangement with more unsaturated or defect atoms.44-47 In this study, the CdS surface has been successfully modified with amorphous molybdenum-based bimetallic sulfide (a-CoMoSx) to form the a-CoMoSx/CdS


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photocatalyst by a simple photoinduced electron-reduction method. It can be found that the photocatalytic H2-production activity of a-CoMoSx/CdS is significantly higher than that of blank CdS, MoSx/CdS and Co(Ⅱ)/CdS photocatalysts. Moreover, the a-CoMoSx can also serve as the universal and high-efficiency electron cocatalyst to significantly promote the H2-production performance of common photocatalytic materials (such as g-C3N4 and TiO2). On the basis of above experimental results, an electron-cocatalyst mechanism of a-CoMoSx is put forward to explain the enhanced photocatalytic H2-production activity of various photocatalysts. As far as we know, this is the first report about the photoinduced synthesis of a-CoMoSx on the surface of CdS to significantly boost its photocatalytic H2-production performance. This study may offer new ideas about developing high-performance electron cocatalyst to enhance the photocatalytic H2 generation.

Experimental methods Preparation of CdS photocatalyst CdS sample was prepared via facile precipitation method. In brief, 0.0831 g of Na2S·9H2O was dissolved in 5 mL of deionized water to form Na2S solution, and 0.105 g of Cd(NO3)2·4H2O was dissolved in 15 mL of deionized water to form Cd(NO3)2 solution. Then, the NaⅡS solution was directly added dropwise into the Cd(NO3)2 solution under strong stirring at room temperature. After stirring for 2 h and aging for overnight, the resulting orange sample was washed with distill water to obtain the CdS photocatalyst. Preparation of a-CoMoSx/CdS photocatalyst



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The amorphous CoMoSx-modified CdS photocatalyst (a-CoMoSx/CdS) was prepared by a photoinduced electron-reduction method.48-50 First, a certain amount of Co(NO3)2 (1 mg/mL) and (NH4)2MoS4 (1 mg/mL) solution was mixed to obtain the homogeneous [Con(MoS4)3-n](NH4)2 solution. Next, the above prepared 50 mg CdS sample was dispersed into 80 mL of aqueous solution containing 10 vol% of lactic acid, and then a certain amount of [Con(MoS4)3-n](NH4)2 solution was quickly added into the above suspension. After purged with N2 for 15 min to eliminate oxygen, the suspension was irradiated by a xenon lamp (λ≥420 nm) for 2 h to load the amorphous CoMoSx on CdS surface. Finally, the fabricated samples were washed and dried at 60Ⅱ for 12 h to obtain a-CoMoSx/CdS sample. In this case, the total content of CoMo was controlled to be 1 wt% of CdS (To clearly observe the presence of Co and Mo element by TEM, XPS and ICP technologies, the 3 wt% of CoMo amount was also loaded on the CdS surface to prepare a-CoMoSx/CdS(3 wt%)). To investigate the effect of Co/Mo ratio on the photocatalytic performance, the molar ratio of Co to Mo

was

controlled to be 0.5:2.5, 1:2, 1.5:1.5 and 2:1, and their corresponding samples were denoted as a-CoMoSx/CdS(0.5:2.5), a-CoMoSx/CdS(1:2), a-CoMoSx/CdS(1.5:1.5) and a-CoMoSx/CdS(2:1), respectively. When the molar ratio of Co/Mo was 1:2, the resultant a-CoMoSx/CdS(1:2) exhibited the highest H2 evolution rate. To simply the sample name, the above a-CoMoSx/CdS(1:2) was denoted as a-CoMoSx/CdS in the following experiments. For comparison, single-component cocatalyst-modified CdS samples such as MoSx-modified CdS (a-MoSx/CdS), Co(Ⅱ)-modified CdS (Co(Ⅱ)/CdS) and


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Pt-modified CdS (Pt/CdS) photocatalysts were synthesized via a similar process as the a-CoMoSx/CdS sample. In this case, the WCo/WCdS, WMo/WCdS and WPt/WCdS are controlled to be 1 wt%. In addition, the dual cocatalyst-modified CdS sample (Co(II)-MoSx/CdS) was also synthesized by a two-step photodeposition process under the same conditions as that of a-CoMoSx/CdS sample via first loading of a-MoSx and subsequent loading of Co(II) cocatalysts. In this study, the total content of CoMo was controlled to be 1 wt% of CdS, and the molar ratio of Co to Mo in the Co-MoSx/CdS sample was 1:2. Characterization X-ray diffraction (XRD) results were obtained by a Rigaku III X-ray diffractometer (Japan). The morphology and structure were analyzed via a field emission scanning electron microscope (JSM-7500F, FESEM, JEOL, Japan) with an X-Max 50 energy-dispersive X-ray spectrometer (EDS, Oxford Instruments, Britain), a transmission electron microscope (JEM-2100F, TEM, JEOL, Japan) and high angle annular dark field (HAADF). X-ray photoelectron spectroscopy (XPS) analysis was performed on a KRATOA XSAM800 XPS system. UV-vis spectrum was tested on a UV-2450 spectrophotometer (Shimadzu, Japan). Photocatalytic H2 generation performance Photocatalytic H2-generation activity was measured in a three-necked Pyrex flask (100 mL) with stirring at ambient temperature, similar to our previous reports.51-53 Typically, 50 mg of photocatalyst was re-dispersed in 80 mL of lactic acid solution (10 vol%). After purged with N2 for 15 min to eliminate oxygen, the system



was sealed with sealing films and then irradiated by a xenon lamp (λ≥420 nm). To ensure the samples in suspension state during photocatalytic H2-production experiments, continuous stirring was applied. 0.4 mL of gas was intermittently sampled, and H2 was tested by a gas chromatograph (a thermal conductivity detector and a 5 Å molecular sieve column, Shimadzu GC-2014C, Japan). Photoelectrochemical measurement The photoelectrochemical performance was tested by an electrochemical workstation (CHI660E, China) in a standard three-electrode system. The photocatalyst-loaded FTO glass, Pt slice, and Ag/AgCl standard electrode works as the working, counter, and reference electrode, respectively. In addition, the three-electrode system employs the aqueous solution consisting of Na2SO4 (0.5 M) and lactic acid (10 vol %) as the electrolyte, which was continuously purged with N2. A 3-W LED was utilized as the visible light source (420 nm with a 90 mW cm-2 light intensity). The working electrode was synthesized on a fluorine-doped tin oxide glass (FTO) via spinning coating. For instance, the mixture containing 10 mg of photocatalyst, 1mL of anhydrous ethanol and 1 mL of Nafion-ethanol solution (1 wt%) was ultrasonicated to form a homogeneous suspension. Eventually, the suspension was coated on the FTO surface (where one side was protected with Scotch tape) and then was dried at 40 oC for 24 h. Linear sweep voltammetry (LSV) was conducted with a bias range of 0.2-1.2 V, and the transient photocurrent responses (i–t curve) was analyzed under repeating light irradiation (light on/off) at a bias potential of +0.5 V,


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while the electrochemical impedance spectroscopy (EIS) was carried out in the 10-3-106 Hz at the open circuit voltage with an AC amplitude of 10 mV.

Results and discussion Synthesis strategy of a-CoMoSx/CdS photocatalyst The formation route of a-CoMoSx/CdS photocatalyst is schematically explained via a simple photoinduced deposition process, as shown in Figure 1. First, CdS nanoparticles are prepared from a direct precipitation method and then are re-dispersed into lactic acid solution to form orange suspension solution (Figure 1a). After the addition of a certain amount of [Con(MoS4)3-n](NH4)2 solution, the [Con(MoS4)3-n]2- can be easily adsorbed on the CdS particle surface. Under visible-light irradiation, the photogenerated electron can migrate from the conduction band of CdS to its surface, causing the rapid reduction of adsorbed [Con(MoS4)3-n]2and the in situ formation of CoMoSx, which can be clearly observed by the color change from orange to darkslategray (Figure 1b). The above formation mechanism can be clearly shown as follows:43, 54 CdS + hν → h+ + e-

(1)

CoⅡ(MoⅡS4)22- + 2e- → Co0(MoⅡS4)24-

(2)

Co0(MoⅡS4)24- → (MoⅡS4)CoⅡ(MoⅡS4)4- → CoMoSx

(3)

For comparison, (NH4)2MoS4 and Co(NO3)2 were also added into the CdS suspension to prepare a-MoSx/CdS (Figure 1c) and Co(II)/CdS (Figure 1d) photocatalysts under an identical condition, respectively. Since all the cocatalyst-modified samples were



prepared under room temperature and mild condition, it can reasonably be deduced that the present CoMoSx, MoSx and Co(II) cocatalysts are amorphous and can only be loaded on the CdS surface.

Figure 1. Schematic illustration for the synthesis of various samples: (a) CdS, (b) a-CoMoSx/CdS, (c) a-MoSx/CdS, and (d) Co(II)/CdS.

Morphology and microstructures of a-CoMoSx/CdS photocatalysts To demonstrate the successful synthesis of a-CoMoSx/CdS photocatalysts, the samples are firstly characterized by the XRD, FESEM and TEM. Figure 2 shows that the prepared CdS, a-CoMoSx/CdS, a-MoSx/CdS and Co(II)/CdS samples mainly consist of cubic phase CdS (JCPDS 90-0440). Compared with pure CdS sample, the diffraction peaks of a-CoMoSx/CdS, a-MoSx/CdS and Co(II)/CdS photocatalysts have no obvious change, demonstrating that the crystal structure and crystallite size of CdS


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have not been affected after modification with cocatalysts. In addition, no any peaks of molybdenum sulfide or cobalt-molybdenum sulfide can be observed, suggesting

20

(222)

(311)

(220)

(200)

(111)

the amorphous state of cocatalysts.

Relative 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


a b c d

30 40 50 2 Theta (degree)

60

Figure 2. XRD patterns of various samples: (a) CdS, (b) a-CoMoSx/CdS, (c) a-MoSx/CdS, and (d) Co(II)/CdS.

The FESEM images of various CdS photocatalysts and their corresponding EDX spectra are shown in Figure 3. The CdS sample (Figure 3A) exhibits many aggregated nanoparticles with a size of 10-40 nm. After the a-CoMoSx cocatalyst is modified on the surface of CdS (Figure 3B), the a-CoMoSx/CdS exhibits a similar structure to pure CdS owing to the low content of a-CoMoSx cocatalyst. However, the signals of Co and Mo elements are clearly shown in its corresponding EDX data besides the main Cd and S elements. For the a-MoSx/CdS (Figure 3C) and Co(II)/CdS (Figure 3D) samples, they also show similar morphology characteristics as those of the a-CoMoSx/CdS sample. In the light of their corresponding EDX results, a-MoSx and



Co(II) cocatalysts have also been successfully deposited on the surface of CdS photocatalyst.

Figure 3. FESEM images and EDX spectra of various samples: (A) CdS, (B) a-CoMoSx/CdS, (C) a-MoSx/CdS, and (D) Co(II)/CdS.

To further explore the microstructures of a-CoMoSx/CdS, its corresponding TEM results are displayed in Figures 4A-C. It is found that the a-CoMoSx/CdS(3 wt%) sample (Figure 4B) has a particle size of 10-20 nm, and the typical lattice spacing is measured to be 0.334 nm (Figure 4C), corresponding to the (002) facet of cubic CdS. In addition, the cubic CdS nanoparticle surface is obviously wrapped by a layer of amorphous CoMoSx. Figures 4D-D5 shows the elemental-mapping results of a-CoMoSx/CdS nanocrystal photocatalyst. It is found that that the Mo and Co


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elements are uniformly dispersed on the CdS surface, suggesting the formation of amorphous CoMoSx cocatalyst by the present photoinduced method.

Figure 4. (A, B) TEM, (C) HRTEM and (D-D5) HAADF-STEM and EDS mapping images of the a-CoMoSx/CdS(3 wt%) photocatalyst.

The chemical state of amorphous CoMoSx on the CdS surface can be analyzed by XPS. Figure 5A displays the XPS survey spectra of pure CdS and a-CoMoSx/CdS(3 wt%) samples. It can be seen that the samples mainly show the characteristic peaks of Cd 3d and S 2p from bulk CdS nanocrystals, while the O



element may be from adsorbed H2O or -OH on the CdS photocatalyst surface. In addition, there is no characteristic XPS peaks about amorphous CoMoSx in the XPS survey spectra, probably due to its very limited content. To further carefully investigate the chemical state for the above elements, the corresponding high-resolution XPS spectra are shown in Figures 5B-D. It is found that compared with blank CdS with a binding energy of 405.08 eV for Cd 3d5/2 and 161.32 eV for S 2p (Figure 5B and 5C), the binding energy of Cd and S elements in the a-CoMoSx/CdS shifts to a lower position (E(Cd 3d5/2)=404.95 eV; E(S 2p)=161.24 eV), probably due to the strong coupling interface between CdS and a-CoMoSx.29, 55 For the XPS spectra of Mo element (Fig. 5D), the binding energies at 229.29 and 232.47 eV can be assigned to Mo4+ 3d5/2 and Mo4+ 3d3/2, respectively.39, 56 To further ascertain the exact content of Co and Mo elements in the a-CoMoSx/CdS sample, the inductively coupled plasma-optical emission spectrometry (ICP-OES) was also measured. It can be found that the Co and Mo amounts are 0.11 wt% and 1.19 wt% in the a-CoMoSx/CdS(3 wt%), respectively. Therefore, the above results further indicate that the CdS surface has been successfully modified with the a-CoMoSx to form a-CoMoSx/CdS photocatalysts.


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B

402

S

S 2p3/2 S 2p1/2

b

a

159

160

161 162 163 164 Binding energy (eV)

165

Cd Cd 3d5/2

Cd 3d3/2

b

a

405 408 411 Binding energy (eV)

D Relative intensity (a.u.)

C


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


228

414

Mo Mo 3d5/2

Mo 3d3/2

b a

230 232 Binding energy (eV)

234

Figure 5. (A) XPS survey spectra and (B-D) the high-resolution XPS spectra of (B) Cd 3d, (C) S 2p, and (D) Mo 3d for various samples: (a) CdS, (b) a-CoMoSx/CdS(3 wt%).

The UV-Vis absorption spectra of pure CdS, a-CoMoSx/CdS, a-MoSx/CdS and Co(II)/CdS samples are displayed in Figure 6. It is clearly found that the pure CdS sample (Figure 6a) exhibits obvious visible-light absorption peak in the range of 400-550 nm owing to its narrow band structure (ca. 2.40 eV).57-58 After surface modification by a-CoMoSx (Figure 6b) and a-MoSx (Figure 6c), the samples have stronger and broader absorption in the visible region of 550-800 nm due to the loading of a-CoMoSx and a-MoSx. For the Co(II)/CdS sample, there is a slightly improved



absorption in the visible-light region compared with the CdS, which can be ascribed to the d-d electron transition of Co(II) element. In fact, the above UV-Vis spectra results are in good accordance with their corresponding photograph images (inset in Figure 6).

Figure 6. UV-Vis spectra and (inset) the corresponding photographs of various samples: (a) CdS, (b) a-CoMoSx/CdS, (c) a-MoSx/CdS, and (d) Co(II)/CdS

Photocatalytic activity and mechanism The hydrogen-generation performance of various photocatalysts is evaluated by their H2-generation rate under visible light, as shown in Figure 7. It is obvious that cubic phase CdS shows a weak photocatalytic H2-generation performance (6.8 µmol h-1) owing to the fact that the photogenerated carriers in CdS are easily recombined. After surface modification by a small amount of a-MoSx or Co(II), the H2-production rates of a-MoSx/CdS and Co(II)/CdS photocatalysts reach 57.7 and 23.7 µmol h-1,


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respectively, which are clearly higher than that of blank CdS. With further combination of MoSx and Co(II) to form a-CoMoSx, all the a-CoMoSx/CdS samples exhibit significantly improved H2-generation performance. In particular, the a-CoMoSx/CdS(1:2) exhibits the highest H2-generation activity (178.5 µmol h-1), which is distinctly higher than that of blank CdS, a-MoSx/CdS and Co(II)/CdS by a factor of 26.3, 3.1 and 7.5 times, respectively. In addition, the a-CoMoSx/CdS photocatalyst clearly shows a higher photocatalytic H2-evolution activity compared with Pt/CdS. Hence, it is believed that the amorphous CoMoSx has a great potential to replace the expensive and scarce Pt for the photocatalytic HER. To further determine the promoting action of the a-CoMoSx on the H2-generation activity of CdS, the MoSx and Co(II) cocatalysts are successively loaded on the CdS surface to form the Co(II)-MoSx/CdS photocatalyst. It is found that the Co(Ⅱ)-MoSx/CdS sample (Figure 7A-h) clearly exhibits a lower H2-generation activity (100.3 µmol h-1) than the a-CoMoSx/CdS photocatalyst (178.5 µmol h-1), indicating that the improved H2-generation performance of a-CoMoSx/CdS is mainly attributed to the formation of amorphous bimetallic sulfides (a-CoMoSx) instead of the dual cocatalysts (Co(Ⅱ)-MoSx). In addition to the above lactic acid solution, the a-CoMoSx/CdS still exhibits a higher H2-evolution performance than the CdS photocatalyst in the well-known Na2S-Na2SO3 system, as shown in Figure 7B. To further demonstrate the stability of a-CoMoSx/CdS photocatalyst for photocatalytic hydrogen generation, the cycle experiment was carried out under identical conditions (Figure 7C). It can be seen that the a-CoMoSx/CdS(1:2) sample maintains a relatively stable H2-production



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rate. Moreover, the turnover number of the CoMoSx cocatalyst can be calculated to be ca. 1838, clearing suggesting the excellent catalytic activity of CoMoSx cocatalyst.

Figure 7. (A) Photocatalytic H2-evolution activities of various samples: (a) CdS, (b) a-MoSx/CdS,

(c)

a-CoMoSx/CdS(0.5:2.5),

a-CoMoSx/CdS(1.5:1.5),

(f)

(d)

a-CoMoSx/CdS(2:1),

a-CoMoSx/CdS(1:2), (g)

Co(II)/CdS,

(e) (h)

Co(Ⅱ)-MoSx/CdS, and (i) Pt/CdS; (B) photocatalytic H2-evolution rates of CdS and a-CoMoSx/CdS in different sacrifice reagents and (C) the cycling runs of the a-CoMoSx/CdS(1:2) photocatalyst.

According to the above results, a possible mechanism illuminating the increased H2-generation activity of a-CoMoSx/CdS is proposed and shown in Figure 8.


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According to previous reports, the unsaturated S atoms of molybdenum sulfide can serve as effective active sites to quickly capture H+ from reaction system, and subsequently boost the direct reduction of H+ by photogenerated electrons. In fact, in view of its weak conductivity and limited active sites, it can be deduced that the H2-generation activity of the MoSx/CdS is still quite limited. For the CoMoSx cocatalyst, it has been widely reported that the addition of Co can greatly improve the conductivity of MoSx via regulating the electronic structure of MoS2 material (where the intermediate sulfur atoms were simultaneously bonded to Mo and Co atoms).39 In this study, it is reasonable to conclude that the photogenerated electron on the CdS CB can be promptly transported to adsorbed H+ by the CoMoSx cocatalyst (as electron-transfer mediator), which is beneficial for HER activity. On the other hand, compared with the MoSx, the addition of Co into the CoMoSx usually exhibits more defect sites, which is suitable for effective adsorption of H+ and interfacial H2 evolution.38, 59 Therefore, the enhanced photocatalytic performance of a-CoMoSx/CdS can be well explained by the fact that after light excitation, the a-CoMoSx first works as an electron capturer (step (1)) to quickly capture electrons from the CB of CdS, then functions as an electron-transfer mediator (step (2)) to efficiently transport the above captured electrons, and finally serves as the H2-evolution active site (step (3)) to boost the interfacial H2-production reaction.



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Figure 8. Schematic diagram illustrating the photocatalytic H2-evolution mechanism of a-CoMoSx/CdS photocatalysts: (1) the rapid capture of photogenerated electrons by a-CoMoSx; (2) the effective transfer of photogenerated electrons from the capture sites to active sites; (3) the efficient interfacial H2-evolution reactions on the a-CoMoSx active sites.

To confirm the above assumption that amorphous CoMoSx can function as the high-efficiency cocatalyst for CdS in the photocatalytic process, the PEC results were evaluated in a standard three-electrode PEC reactor. Figure 9A displays the polarization curves of various samples without light irradiation. It can be found that the current density of each sample gradually increases in the potential range of -0.7 to -1.2 V, indicating that the effective reduction reaction of H+ to H2 can occur on the surface of CdS semiconductor electrode within this voltage range. Moreover, compared with the blank sample, all the cocatalyst-modified CdS samples (a-CoMoSx/CdS,

a-MoSx/CdS

and

Co(II)/CdS)

can


obviously

reduce

the

Page 21 of 36

H2-generation overpotential owing to the rapid reduction of H+ to H2. Among of them, the a-CoMoSx/CdS sample has the most positive potential (namely the minimum overpotential of H2 evolution) and the largest current density, suggesting that the a-CoMoSx can serve as the high-efficiency active site to markedly boost the H2-production rate of CdS photocatalyst.60 Figure 9B displays the i–t curve of various photocatalysts. It is obvious that compared with the pure CdS, a-MoSx/CdS and Co(II)/CdS photocatalysts, the a-CoMoSx/CdS photocatalyst displays an increased transient photocurrent density, demonstrating that the photogenerated electrons in the a-CoMoSx/CdS system have more efficient separation from photogenerated holes, which will cause a rapider interfacial H2-production reaction. Beyond that, the EIS plots of a-CoMoSx/CdS sample shows a smaller arc radius than that of the CdS, a-MoSx/CdS and Co(II)/CdS, indicating a faster charge transfer in the a-CoMoSx/CdS photocatalyst and confirming that the addition Co atoms can improve the electrical conductivity of molybdenum sulfide.

A -2

J (mAcm )

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


0

-2 a d c b

-4 -1.2

-1.1

-1.0

-0.9

-0.8

E (V vs. Ag/AgCl)



C

8 a

-Z''/1000 (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

Page 22 of 36

6 d

4

c

2 0

b

0

1

2 3 Z'/1000 (ohm)

4

5

Figure 9. The curves of (A) linear sweep voltammograms, (B) transient photocurrent responses and (C) electrochemical impedance spectra for various samples: (a) CdS, (b) a-CoMoSx/CdS, (c) a-MoSx/CdS and (d) Co(II)/CdS.

To explore whether the a-CoMoSx cocatalyst can act as a universal cocatalyst to boost the H2-generation activity of other photocatalysts, typical UV-responsive titanium dioxide (TiO2) and novel visible light-responsive organic semiconductors (g-C3N4) are also loaded by the a-CoMoSx under the same experimental condition as the a-CoMoSx/CdS and their corresponding H2-production activity is displayed in Figure 10. It can be seen that the hydrogen production performance of the a-CoMoSx/TiO2 (or a-CoMoSx/g-C3N4) photocatalyst is significantly increased by 21.7 (or 24.8) times compared to pure TiO2 (or g-C3N4), manifesting that amorphous CoMoSx can serve as a general and high-efficiency electron cocatalyst for H2 evolution.


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Figure 10. The photocatalytic performance of typical photocatalytic materials before and after a-CoMoSx modification.

Conclusions In summary, the CdS surface has been successfully modified with amorphous CoMoSx to prepare the a-CoMoSx/CdS photocatalyst by a facile photoinduced deposition process. Photocatalytic experiments show that the resultant a-CoMoSx/CdS sample displays a markedly higher photocatalytic H2-production performance (178.5 µmol h-1) than the blank CdS (6.8 µmol h-1), a-MoSx/CdS (57.7 µmol h-1) and Co(Ⅱ)/CdS (23.7 µmol h-1) samples by a factor of 26.3, 3.1 and 7.5 times, respectively. Based on the present results, an electron-cocatalyst mechanism of amorphous CoMoSx is put forward to explain the increased photocatalytic H2-production performance, namely, the a-CoMoSx first works as an electron capturer to quickly capture electrons from the CB of CdS, then functions as electron-transfer mediator to steadily transport the photogenerated electrons, and finally serves as the



H2-evolution active site to boost the interfacial H2-production reaction. In addition, the resultant a-CoMoSx can also be used as a universal H2-production cocatalyst to significantly increase the H2-production activity of conventional semiconductor materials (TiO2 and g-C3N4). In consideration of its high efficiency and low cost, the amorphous CoMoSx cocatalyst should have great potential in photocatalytic H2 evolution.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Tel: 0086-27-87756662, Fax: 0086-27-87879468

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (21771142, 51472192). This work was also financially supported the Fundamental Research Funds for the Central Universities (WUT: 2015IB002, 185220004).


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(58) Wei, R.-B.; Kuang, P.-Y.; Cheng, H.; Chen, Y.-B.; Long, J.-Y.; Zhang, M.-Y.; Liu, Z.-Q. Plasmon-Enhanced Photoelectrochemical Water Splitting on Gold Nanoparticle Decorated ZnO/CdS Nanotube Arrays. ACS Sus. Chem. Eng. 2017, 5, 4249-4257, DOI 10.1021/acssuschemeng.7b00242. (59) Li, P.; Liu, X.; Zhang, C.; Chen, Y.; Huang, B.; Liu, T.; Jiang, Z.; Li, C. Selective Hydrodesulfurization of Gasoline on Co/MoS2±x Catalyst: Effect of Sulfur Defects in MoS2±x. Appl. Catal., A 2016, 524, 66-76, DOI 10.1016/j.apcata.2016.06.003. (60) Choi, Y.; Kim, H.-i.; Moon, G.-h.; Jo, S.; Choi, W. Boosting up the Low Catalytic Activity of Silver for H2 Production on Ag/TiO2 Photocatalyst: Thiocyanate as a Selective Modifier. ACS Catal. 2016, 6, 821-828, DOI 10.1021/acscatal.5b02376.



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A novel amorphous molybdenum-based bimetallic sulfide (CoMoSx) was loaded on the CdS surface by a photoinduced electron-reduction method to promote the interfacial H2-production rate of CdS.


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Direct photoinduced synthesis of amorphous CoMoSx cocatalyst and

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