Carbonized MoS2: Super-Active Co-catalyst for High-Efficient Water

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Carbonized MoS2: Super-Active Co-catalyst for High-Efficient Water Splitting on CdS Mengmeng Shao, Yangfan Shao, Shengjie Ding, Rui Tong, Xiongwei Zhong, Lingmin Yao, Weng Fai Ip, Baomin Xu, Xing-Qiang Shi, Yi-Yang Sun, Xuesen Wang, and Hui Pan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05917 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

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ACS Sustainable Chemistry & Engineering

Carbonized MoS2: Super-Active Co-catalyst for HighEfficient Water Splitting on CdS Mengmeng Shao1†, Yangfan Shao1,2†, Shengjie Ding1, Rui Tong1, Xiongwei Zhong1,3, Lingmin Yao4, Weng Fai Ip5, Baomin Xu3, Xing-Qiang Shi2, Yi-Yang Sun6, Xuesen Wang7, and Hui Pan1,5* 1

Joint Key Laboratory of the Ministry of Education, Institute of Applied Physics and Materials Engineering, University of Macau, Taipa, Macao SAR, China 2 Department of Physics, Southern University of Science and Technology, No. 1088 Xueyuan Road, Nanshan District, Shenzhen 518055, China 3 Department of Materials Science and Engineering, Southern University of Science and Technology, No. 1088 Xueyuan Road, Nanshan District, Shenzhen 518055, China 4 School of Physics and Electronic Engineering, Guangzhou University, 230 Wai Huan Xi Road, Guangzhou Higher Education Mega Center, Guangzhou 510006, China 5 Department of Physics and Chemistry, Faculty of Science and Technology, University of Macau, Taipa, Macao SAR, China 6 State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 585 Heshuo Road, Jiading, Shanghai 201899, China 7 Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542 * Corresponding Author: [email protected]; Tel: (853)88224427; Fax: (853)88222426 † These authors contributed equally to this work

ABSTRACT: Searching photocatalysts for efficient hydrogen production has been a challenging issue for solar-energy harvesting. Using co-catalyst is proved to be an effective approach to improve the efficiency of photocatalyst in water-splitting. Here, we report that carbonized MoS2 (MoS2/Mo2C) can be a super-active co-catalyst in solar-driven hydrogen production. We show that MoS2/Mo2C decorated CdS achieves a high photocatalytic hydrogen evolution rate (34 mmol/h/g, ~112 times higher than pure CdS) and excellent apparent quantum efficiency (41.4 % at 420 nm). The outstanding photocatalytic performance of MoS2/Mo2C/CdS is attributed to the metallic characteristic of MoS2/Mo2C and suitable Gibbs free energy of hydrogen adsorption, 1

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leading to enhanced light absorption, fast separation and transportation of photoinduced carriers, and optimal activity in hydrogen evolution reaction (HER). We further show that MoS2/Mo2C as co-catalyst can also dramatically improve the photocatalytic activity of g-C3N4. Our findings demonstrate that the carbonized transition metal disulfide can be active as co-catalyst in photocatalysis, providing guidance on exploring novel photocatalysts for energy harvesting. KEYWORDS: MoS2/Mo2C, CdS, Photocatalytic hydrogen evolution, Co-catalyst

INTRODUCTION The excessive consumption of fossil fuels and aggravation of environmental crisis due to the rapid development of society have prompted the searching of renewable green energies to meet ever-increasing energy demand.1,2 Solar energy has been considered as one of the cleanest and renewable energy sources.3 Converting solar power into hydrogen by photocatalytic process is one of ideal and attractive ways for solar energy harvesting and storage because hydrogen has the highest energy density, and is clean, abundant, and renewable.4-6 To obtain high efficiency of solar-to-hydrogen conversion, photocatalyst has to maximally utilize visible light, has suitable band edges with water redox potentials, and enable fast separation of electron-hole pairs.7-9 It is difficult to find a single-component photocatalyst to satisfy all the requirements. One effective strategy is to design co-catalysts integrated with photocatalysts for enhanced conversion efficiency, which can facilitate the separation of photogenerated electronhole pairs especially.10-14 For this purpose, the co-catalysts need to trap and transfer 2


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electrons to protons fast and show excellent hydrogen evolution reaction (HER) with low activation energy.15,16 Generally, the noble metals, such as Pt, are effective cocatalysts to boost solar-driven water splitting.17 However, the scarcity and high cost limit their practical application. Therefore, the earth-abundant non-noble metal cocatalysts with super efficiency and low cost are highly demanded. Various non-noble-metal co-catalysts have been investigated to enhance the photocatalytic hydrogen generation, such as earth-abundant transition metals (for example Co, Ni, Mo and W), and their alloys, sulfides, nitrides, carbides and phosphides.18-23 Among them, the transition metal sulfides have been widely explored and demonstrated as one of the most promising alternatives to Pt-based co-catalysts.2426

For example, MoS2 was reported to be used as co-catalyst for improved solar-driven

H2 production.27,28 However, MoS2 only showed remarkable HER activity at metallic edges and defects, which limited its application as co-catalyst.29,30 To improve its HER activity, a lot of approaches were adopted, such as increasing active edge sites, introducing defects on basal plane, and tuning phase, morphology and electronic structure.31-34 For example, Kumar et al. and He et al. prepared ultrathin MoS2 with many edge sites exposed to greatly improve its HER activity and CdS loaded with the ultrathin MoS2 displayed high photocatalytic H2 evolution rate.35,36 Although HERactive MoS2 as co-catalyst was reported to improve photocatalytic performance, further studies are still needed. Recently, the MoS2/Mo2C hybrids were fabricated and their HER performance and electrical characteristics were investigated in detail. The results show that MoS2/Mo2C hybrid display excellent HER activity and electrical 3



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properties.37-39 Therefore, the MoS2/Mo2C composite could be a potential co-catalyst to promote the photocatalytic hydrogen generation, which is worthwhile to explore and has not been reported so far. Herein, we report that MoS2/Mo2C can be a super-active co-catalyst for photocatalytic H2 production. We first investigate the possibility of MoS2/Mo2C as cocatalyst for photocatalytic H2 production by first-principles calculations. Then, the composite is fabricated by a facile chemical vapor carbonization process. Integrating MoS2/Mo2C with CdS, the high stability and splendid photocatalytic hydrogen evolution rate (34 mmol/h/g) are achieved, which is around 112 times higher than pure CdS. The super-effective photocatalytic activity of MoS2/Mo2C/CdS is attributed to the high conductivity and active HER ability of MoS2/Mo2C, leading to fast electron transfer and separation of the photo-induced carriers.

EXPERIMENTAL SECTION Synthesis of MoS2/Mo2C. All the chemicals were used in this work without further purification, and the deionized water (DI water) was used in all experiments. Firstly, the precursor-MoS2 was prepared by one-pot hydrothermal process.40 Typically, 1.24 g ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O, Aladdin, 98%) and 2.66 g thiourea (CH4N2S) were added into 30 mL DI water and stirred for 60 min. Then, the above solution was transferred to 50 mL Teflon-lined stainless-steel autoclave and kept at 200 ℃ for 24 h. The resulting black solids were filtrated, washed with DI water and ethanol several times, and dried in vacuum to obtain the MoS2. 4


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Secondly, the carbonized MoS2 was prepared by a simple chemical vapor carbonization process. The as-prepared MoS2 (125 mg) was mixed with 1 g dicyandiamide powders (C2H4N4, Aladdin, 98%), which were covered with alumina crucible and placed in the center of the tube furnace. The mixture solids were calcined at 450 °C for 2 h and then partially carbonized at 950 °C for 2 h under Ar-H2 gases (5% H2) with 10 °C/min heating rate. After reaction, the tube furnace was naturally cooled to room temperature under Ar-H2 gases to form MoS2/Mo2C complex, which was labeled as MoSC-950. Additionally, different carbonized temperatures, including 800, 900, 1000, and 1050 °C, were carried out by the same method to investigate the effect on the structure, and labeled as MoSC-800, MoSC-900, MoSC-1000 and MoSC-1050, respectively. Synthesis of MoS2/Mo2C/CdS. The MoS2/Mo2C/CdS was also synthesized by hydrothermal process. Firstly, 0.73 g cadmium chloride hemipentahydrate (CdCl2·2.5H2O, Aladdin, 98%) was dissolved in 30 mL DI water, and a certain amount of MoSC-950 was added into the above solution. After stirred for 60 min, 0.84 g thiourea was added into the suspension and stirred for another 60 min (nCd:nS=1:3). Then, the above mixture was transferred to 50 mL Teflon-lined stainless-steel autoclave and kept at 180 ℃ for 12 h. The resulting dark yellow solids were filtrated, washed with DI water and ethanol several times, and dried in vacuum to obtain the MoS2/Mo2C/CdS, where the weight of CdS was based on the Cd source. The different weight ratios of MoSC-950 to CdS were prepared (0.5, 1, 2, 3 and 4 wt%) by adding different amounts of MoSC-950, and the resulting samples were marked as MoSC/CdS-0.5, MoSC/CdS5



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1, MoSC/CdS-2, MoSC/CdS-3 and MoSC/CdS-4, respectively. Moreover, MoSC-950 was also replaced by MoSC-800, MoSC-900, MoSC-1000, and MoSC-1050, respectively, to prepare the composites with respect to the effect of temperature. The above materials were marked as MoSC/CdS-800, MoSC/CdS-900, MoSC/CdS-1000, and MoSC/CdS-1050, respectively. For comparison, pure CdS (by the same process without adding MoS2/Mo2C) and MoS2/CdS (MoS2 substituted MoS2/Mo2C) were also prepared. The weight ratios of above MoSC (different temperature) and MoS2 to CdS were 2 wt%. Materials Characterization. The crystalline phase of samples was recorded by Xray diffraction (XRD) on the Rigaku Smartlab system with Cu Kα radiation (λ = 0.154 nm). The morphology and microstructure information of samples were captured using the scanning electron microscopy (SEM, Zeiss Sigma), transmission electron microscopy (TEM, Talos F200S) and energy dispersive spectrometer (EDS) element mapping. The X-ray photoelectron spectroscopy (XPS) with monochromatic Al Kα Xray (Thermo Fisher Scientific, ESCALAB 250Xi) was applied to analyze chemical composition of samples. The molecular structure was investigated by Micro Raman System (Horiba LABHRev-UV) with the excitation wavelength of 532 nm. The light absorption of samples was measured by UV-vis diffuse reflectance spectrum (UV-vis DRS) obtained from spectrometer with integrating sphere (Shimadzu UV-2600, BaSO4 as reference). The photoluminescence (PL) spectrum were acquired through fluorescence spectrometer (Shimadzu RF-5301) with excitation of 325 nm. The timeresolved PL curves were measured on spectrofluorometer (Edinburgh FS5) with 6


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excitation of 405 nm and the signal collected at 460 nm. The N2 adsorption-desorption isotherms were measured by Micromeritics 3Flex instrument to analyze the BrunauerEmmett-Teller (BET) surface area of samples. Electrochemical and Photoelectrochemical Measurements. The electrochemical and photoelectrochemical measurements were carried out using a standard threeelectrode electrochemical workstation (AMETEK solartron analytical), where Pt foil and Hg/HgO were used as counter and reference electrodes, respectively, and the samples deposited on carbon paper were used as working electrodes. The electrochemical impedance spectroscopy (EIS) and photocurrent were implemented in mixed aqueous solution (0.1M NaSO4 + 0.1M Na2SO3 + 0.01M Na2S). The Abet Technologies solar simulator (AM 1.5, 100 mW/cm2) was used as the light source. Moreover, the EIS was recorded at 10 mV amplitude in the frequency ranging from 0.1 to 100000 Hz under open-circle potential and light irradiation. The photocurrent was measured under chopped light illumination with the potential of 0 V vs Hg/HgO. Photocatalytic Hydrogen Generation. The photocatalytic performance of MoSC/CdS composites in hydrogen generation from water was evaluated in a sealed side-irradiation reaction vessel with sodium sulfite as hole scavenger. The solar simulator (AM 1.5, ABET TECHNOLOGIES) with light intensity of 100 mW/cm2 was adopted as the light source. The reaction vessel was around 21 cm far away from the light source and the illuminated area was 12.56 cm2. Typically, 50 mg of photocatalyst and 85 mL Na2SO3 (0.25 M)/Na2S (0.35 M) solution were added into the reaction vessel. Before light irradiation, the system was sonicated to disperse photocatalyst, evacuated 7



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with nitrogen to remove the air, and sealed tightly. During the photocatalysis, 0.4 mL of gas product was sampled at regular intervals, and the H2 content was quantified by gas chromatography (Agilent GC7890B, TCD detector, N2 as carrier gas). The apparent quantum efficiency (AQE) was acquired under the same photocatalytic process with a 420 nm monochromatic light adopted as the light source (obtained through 420 nm bandpass filter). The power intensity of incident light at five different points were determined to gain the average power intensity, which was about 14.3 mW/cm2. The AQE at 420 nm monochromatic light was calculated as follows:41 AQE (%) = (Number of produced H2 × 2) / (Number of incident photons) × 100%. DFT Calculations. All the calculations based on the density functional theory (DFT) were performed by the Vienna ab initio simulation package (VASP) with the projector augmented wave (PAW) scheme to study the electronic and catalytic properties of MoS2 and MoS2/Mo2C.42 The Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) exchange and correlation functional were used. K-point sampling for integration over the first Brillouin zone was based on the Monkhorst and Pack scheme.43 The detailed calculation methods can be found in the computational section of Supporting Information.

RESULTS AND DISCUSSION Theoretical Calculations of MoS2/Mo2C. A highly active co-catalyst in photocatalytic reaction should not only possess metallic characteristics, which will facilitate the transfer of photo-induced electrons from photocatalyst to reactive sites, 8


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but also display favorable HER performance.44-46 We performed the theoretical calculations to investigate the possibility of MoS2/Mo2C as a co-catalyst (the structural models are displayed in Figure S1) based on density functional theory (DFT). The calculated total density of states (DOS) shows that MoS2/Mo2C exhibits strong metallic characteristic, as indicated by the continuous states crossing and increased DOS around the Fermi level (Figure 1a and b). Interestingly, the charge density differences on MoS2 with H and MoS2/Mo2C with H (Figure S2) show that the electrons accumulation between H and MoS2/Mo2C is much wider than that between H and MoS2, which will benefit the following catalytic reaction of H2 evolution. The Gibbs free energy of Hadsorption, ΔGH, is a general descriptor to evaluate the HER activity, and the zero ΔGH is desired for outstanding HER performance.47 The∣ΔGH∣value of MoS2/Mo2C is much smaller than those of MoS2, Mo2C48 and Ti3C211 in wide hydrogen coverage (Figure 1c and d). Importantly, MoS2/Mo2C displays the near-zero value of ΔGH (about 0.12 eV) at hydrogen coverage of 8/9 (Figure 1c), which is comparable to Pt (ΔGH = -0.09 eV, Figure 1d).49 Our theoretical calculations clearly demonstrate the superior HER performance of MoS2/Mo2C and its potential as a co-catalyst in boosting photocatalytic hydrogen evolution. To confirm our design, MoS2/Mo2C was prepared and used as cocatalyst for solar-driven hydrogen production. The successful formation of MoS2/Mo2C composites are confirmed and characterized by using SEM-EDS (Figure S3), XRD patterns, Raman spectra, TEM and HRTEM images (Figure S4).

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Figure 1. The total density of states (DOS) for MoS2 (a) and MoS2/Mo2C (b), the Fermi level is at 0 eV. (c) The calculated free-energy (ΔGH) of MoS2/Mo2C as a function of hydrogen coverages. (d) The comparison on ΔGH of MoS2/Mo2C, MoS2, Pt, WS2 (ref 50) and Ti3C2.

Structures and Characterizations of MoS2/Mo2C/CdS. The promising co-catalyst, MoS2/Mo2C, was loaded on CdS to investigate its photocatalytic activity. The crystal structures of the as-prepared CdS, MoS2/CdS and MoS2/Mo2C/CdS were recorded on XRD patterns. The XRD results reveal that all as-prepared CdS, MoS2/CdS and MoS2/Mo2C/CdS distinctly display characteristic diffraction peaks of CdS without obvious peaks of MoS2 and Mo2C (Figure 2a and Figure S5). The fine views from 10 to 22° in XRD patterns of MoS2/Mo2C/CdS (insets of Figure 2a and Figure S5) show small diffraction peak at around 14°, which is indexed to (002) crystal plane of MoS2.51 The Raman spectra (Figure 2b) further confirm the existence of CdS and MoS2 in the samples. The longitudinal optical (LO) phonon vibration modes of 1LO (302 cm-1) and 2LO (601 cm-1) belong to CdS.52 The other two peaks at 384 cm-1 and 409 cm-1 are 10


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related to the E12g and A1g vibration modes of MoS2.52 However, the characteristic Raman peaks of Mo2C in the composites cannot be detected even in the fine views from 400 to 1000 cm-1 (inset of Figure 2b) due to its low content and the strong PL signal of CdS that covers the Raman signals of Mo2C in the range from 800 to 1000 cm-1. Although no distinct Mo2C is found in the XRD patterns and Raman spectra, its existence is evidenced in TEM images and EDS elemental mappings (Figure 3). The SEM images show that CdS presents a leaf-like morphology (Figure 3a and b), and its EDS elemental mappings indicate the components of Cd and S elements (Figure 3c). Compared to pure CdS, MoS2/Mo2C/CdS contains leaf-like CdS and nanosheet-like MoS2/Mo2C (the circled area in Figure 3d). The EDS result (Figure 3f) confirms the existences of Mo and C elements as well as Cd, S and O elements. The HRTEM images (Figure 3g and h) shows three different lattice spacings, namely 0.35, 0.27 and 0.24 nm, which are corresponding to CdS (100), MoS2 (100) and Mo2C (002), respectively. Clearly, the EDS results and HRTEM images confirm the successful loading of MoS2/Mo2C onto CdS.

Figure 2. (a) XRD patterns of CdS, MoS2/CdS, MoS2/Mo2C/CdS and standard XRD cards of CdS. (b) Raman spectra of CdS, MoS2/CdS and MoS2/Mo2C/CdS. Insets in (a) and (b): the fine views of XRD (10-22°) and Raman spectra (400-1000 cm-1), respectively. 11



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Figure 3. SEM images and EDS results of CdS (a-c) and MoS2/Mo2C/CdS (d-f); (g) TEM and (h) HRTEM images of MoS2/Mo2C/CdS. The Cu film is used as the substrate in the SEM measurement.

The XPS analysis was performed to further acquire the specific composition of MoS2/Mo2C/CdS (Figure 4). The survey XPS spectrum clearly shows the existences of Cd, Mo, S, C and O elements in MoS2/Mo2C/CdS (Figure 4a), where oxygen is resulted from the partial oxidation of the sample surface. The high-resolution XPS spectrum of Cd 3d (Figure 4b) obviously displays two characteristic peaks of Cd2+ at 411.7 and 404.9 eV, which are related to Cd 3d3/2 and Cd 3d5/2, respectively. The XPS signals (Figure 4c) at 162.5 and 161.4 eV are assigned to S 2p1/2 and S 2p3/2, respectively, implying the -2 oxidation state of S in the composite.53,54 The peak at 225.7 eV (S 2s) is also related to S2- (Figure 4d). Moreover, the doublet peaks (Figure 4d) at 231.9 and 12


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228.5 eV are attributed to Mo 3d3/2 and Mo 3d5/2 of Mo4+, which belong to MoS2.55 The other doublet peaks in Mo 3d are ascribed to Mo 3d3/2 (231.1 eV) and Mo 3d5/2 (227.5 eV) of Mo2+, which are corresponding to Mo2C.56 The XPS spectra of CdS, MoS2/CdS and MoS2/Mo2C/CdS were compared to investigate the effects of MoS2 and MoS2/Mo2C on electronic structures of CdS. Positive shifts of Cd 3d and S 2p are observed in MoS2/CdS and MoS2/Mo2C/CdS compared with pure CdS (Figure S6), which results from the electronic coupling and electron transfer between CdS and MoS2/Mo2C (or MoS2). Therefore, our systematic characterizations, including XRD, Raman, SEM, TEM and XPS, fully demonstrate that the MoS2/Mo2C/CdS composite is successfully obtained via the facile carbonization and hydrothermal process. The electronic coupling and electron transfer in MoS2/Mo2C/CdS and MoS2/CdS are favorable to improve photocatalytic activity.57,58

Figure 4. XPS spectrum of survey (a), Cd 3d (b), S 2p (c) and Mo 3d (d) of the MoS2/Mo2C/CdS.

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Figure 5. Photocatalytic activity and spectroscopy/(photo)electrochemical analysis. (a) Photocatalytic H2-evolution rates of MoSC/CdS composites with MoS2 carbonized at different temperatures and (b) MoSC/CdS with different contents of MoS2/Mo2C carbonized at 950 ℃. (c) Stability test of photocatalysis over MoSC/CdS-2. (d) Comparison of photocatalytic activities of MoS2 and MoSC-950 loaded on CdS and g-C3N4, respectively. (e) UV-vis DRS spectra, (f) PL spectra, (g) Time-resolved PL decay curves, (h) EIS Nyquist plots and (i) Transient photocurrent responses of CdS, MoS2/CdS and MoSC/CdS-2.

Photocatalytic Hydrogen Evolution. Our calculations predicted that MoS2/Mo2C would be a promising co-catalyst in photocatalytic reaction. To verify our prediction, MoS2/Mo2C has been successfully fabricated and loaded onto CdS, and the photocatalytic H2 evolution activities of MoS2/Mo2C/CdS composites were evaluated experimentally. The pure CdS shows a photocatalytic H2 evolution rate about 0.3 mmol/h/g. Its photocatalytic activity is dramatically enhanced after loading MoS2/Mo2C (Figure 5a). We see that MoSC/CdS-950 shows the highest photocatalytic H2-evolution rate, indicating the suitable carbonization temperature is 950 °C because 14


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of optimal MoS2/Mo2C ratio and fine nanosheet morphology. As discussed in materials characterizations (Figure S3 and Figure S4), the low temperature (800 and 900 ℃) leads to insufficient carbonization of MoS2, while the high temperature (1000 and 1050 ℃) results in excessive carbonization and serious agglomerate, and both are not beneficial to improve the photocatalytic activity. Moreover, the effect of loading content on the photocatalytic activity is also evaluated (Figure 5b). We find the hydrogen evolution rate of CdS is enhanced by more than 17 times at a loading of 0.5 wt% MoSC-950, demonstrating the highly active MoSC-950. CdS loaded with 2.0 wt% MoSC-950 shows the highest photocatalytic hydrogen evolution rate of 34 mmol/h/g, which is 112 times higher than that of pure CdS, and an apparent quantum efficiency (AQE) at 420 nm up to 41.4%. Further increasing the loading results in the reduction of photocatalytic activity (MoSC/CdS-3 and MoSC/CdS-4), possibly due to the blocking of sun-light and covering of active sites. Additionally, pure MoS2 calcined without carbon source at 950 ℃ was also prepared and loaded on CdS to confirm the catalytic activity of MoSC-950. Clearly, the hydrogen production rate of MoSC/CdS-2 is much higher than MoS2/CdS-950 (5.6 mmol/h/g) (Figure S7). After four-cycle tests, MoSC/CdS-2 still shows rather high photocatalytic activity without significant reduction (Figure 5c). A slight decrease occurred after 20 hours reaction is ascribed to the redox products that cover the surface of MoSC/CdS and the partial photocorrosion of CdS. Meanwhile, we see that the crystal structure, chemical status, composition and light harvesting capability of MoSC/CdS-2 can be 15



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maintained except slightly weakened intensities of XRD, XPS and UV-vis DRS (Figure S8) after prolonged photocatalysis, indicating the high stability of MoSC/CdS-2. Our experimental results confirm the theoretical prediction that MoS2/Mo2C is an excellent co-catalyst for photocatalytic reaction. To provide further evidence, g-C3N4 was used as the host-photocatalyst and loaded with the MoSC-950. Our experiments show that MoSC/g-C3N4 has greatly enhanced photocatalytic performance (1.1 mmol/h/g), which is about 16 and 5.8 times as high as pure g-C3N4 (0.07 mmol/h/g) and MoS2/g-C3N4 (0.19 mmol/h/g) (Figure 5d), respectively. Furthermore, comparing with other CdS-based photocatalysts with different co-catalysts in literatures, our MoSC/CdS-2 still shows better or competitive photocatalytic activity (Table S1). Therefore, MoS2/Mo2C is expected to be a highly effective co-catalyst to replace the noble metal in photocatalytic hydrogen evolution process. In addition, the effects of photocatalyst dose, scavenger concentration, types of scavenger and reaction pH on photocatalytic performance of MoSC/CdS-2 were also investigated (Figure S9). As shown in Figure S9a, the photocatalytic activity of MoSC/CdS-2 is obviously improved as its dosage increases from 10 to 50 mg. While further increasing the amount of MoSC/CdS-2 results in the reduction of hydrogen evolution rate because the excessive MoSC/CdS-2 would block or scatter incident light.59,60 Figure S9b shows the influence of scavenger concentration on photocatalytic performance. We find that the hydrogen production rate of MoSC/CdS-2 increases with the increment of scavenger (Na2SO3-Na2S) concentration up to 0.25 M-0.35 M. However, the hydrogen evolution rate displays downward trend with further increasing 16


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scavenger concentration because lots of scavenger molecules around the surface of MoSC/CdS-2 would hinder the hydrogen production and reduce the light harvesting of MoSC/CdS-2.61,62 Figure S9c shows the effect of scavenger type on photocatalytic activity. We see that the photocatalytic reaction with lactic acid and Na2SO3-Na2S scavengers show higher hydrogen evolution rate than methanol scavenger. Although the reaction system with lactic acid displays better photocatalytic performance than that with Na2SO3-Na2S, Na2SO3-Na2S is also an efficient and common scavenger. Figure S9d shows the influence of reaction pH on photocatalytic activity, where the pH value is regulated in alkaline range as CdS and Na2SO3-Na2S scavenger are unstable under acidic condition. It is observed that the photocatalytic activity decreases with the increase of reaction pH, which results from the poor hydrogen evolution activity under strong alkali condition. Furthermore, in order to fully assess the practical application of MoSC/CdS-2, the photocatalytic process under aerobic condition was performed (Figure S10). A reduced photocatalytic hydrogen evolution rate is observed under aerobic condition because MoSC/CdS-2 is more easily oxidized and backward reaction will occur under aerobic condition.63 Photocatalytic Mechanism. The photocatalytic reaction is mainly affected by three factors: optical adsorption, carrier mobility and separation, and surface catalytic activity.10,64 Firstly, we can see that both MoS2/CdS and MoSC/CdS-2 show enhanced light absorption in the entire region of 220-800 nm, and slight red shift of absorption edge, demonstrating the enhanced utilization of light (Figure 5e). Other MoSC/CdS composites also exhibit similar phenomenon (Figure S11). Moreover, the conduction 17



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band (CB) levels of CdS, MoS2/CdS and MoSC/CdS were measured by Mott-Schottky plots, and their energy band diagram (Figure S12) was constituted based on the MottSchottky plots and DRS spectra. We can find that the CB flat-band values of CdS, MoS2/CdS and MoSC/CdS are all more negative than the reduction potential of H+/H2, implying the efficient electrons transfer for H2 evolution. MoS2/CdS and MoSC/CdS show positive shift in CB flat-band value compared with pure CdS due to the intimate contact between CdS and MoSC (or MoS2), which is favorable to electron transfer in MoSC/CdS and MoS2/CdS and improves photocatalytic activity accordingly.59,65 Secondly, the PL spectra and EIS plots reveal the separation and transfer of photoinduced electrons and holes. Two conspicuous emission peaks around 506 and 713 nm are observed under the excitation light of 325 nm, which are attributed to the intrinsic band gap and surface defects excitation, respectively (Figure 5f). The strong PL spectrum of pure CdS indicates the high recombination rate of carriers. Obviously, the PL intensity of CdS decreases after loaded with MoS2 or MoSC. The MoSC/CdS shows the lowest PL intensity, implying its remarkable separation of photo-induced charges. Additionally, the PL decay curves (Figure 5g and Table S2) reveal that the average PL lifetime (τave) is enhanced in the composites, as compared to pure CdS. Especially, MoSC/CdS shows even longer τave than MoS2/CdS, further demonstrating the separation of photo-induced carriers in MoSC/CdS are much more efficient. In terms of the transfer of photo-induced carriers, the EIS curves (Figure 5h and Figure S13) reveal that MoSC/CdS possesses lowest carrier transfer resistance, as indicated by the smallest semicircle diameter. Consequently, our PL and EIS results confirm that the 18


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charge separation and transfer in MoSC/CdS are much better than MoS2/CdS and CdS. Our DFT calculations also show that the Fermi level of MoS2/Mo2C (0.29 eV vs. SHE, Figure S14b) is still more positive than the CB level of CdS (-0.802 eV vs. SHE, Figure S12d), although the Fermi level of MoS2/Mo2C is lower than MoS2 (1.02 eV vs. SHE, Figure S14a). The suitable band alignment results in the easy transfer of photo-induced electrons from CdS to MoS2/Mo2C, which promptly shuttles the electrons to its surface active sites for hydrogen production due to its strong metallic property (Figure 1b). The observed high photocurrent of MoSC/CdS further demonstrates the efficient separation and transfer of carriers, which is nearly 3 times higher than that of CdS (Figure 5i). Finally, the surface catalytic activity on the H2 evolution is experimentally confirmed by the electrocatalytic performance and BET analysis. The polarization curves (Figure S15) show that all the MoS2/Mo2C samples display lower overpotential than pure MoS2. Especially, MoSC-950 has the lowest overpotential among those samples. After 1000 stability testing cycles, MoSC-950 maintains electrocatalytic activity with negligible reduction, illustrating its high HER activity and stability. Moreover, the N2 adsorptiondesorption isotherms were measured to investigate the surface area of CdS, MoS2/CdS and MoSC/CdS. The results (Figure S16) show that the surface area of MoSC/CdS is higher than those of MoS2/CdS and CdS, indicating more active sites on MoSC/CdS and the photo-induced electrons accumulated on the above active sites lead to fast H2 evolution. Therefore, the combined outstanding optical adsorption, carrier separation and mobility, and surface catalytic activity of MoSC/CdS lead to highly impressing photocatalytic H2 evolution rate, which are mainly attributed to the ultra-high active co19



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catalyst, MoS2/Mo2C. Based on the joint theoretical and experimental results, we conclude that the enhanced photocatalytic performance of MoSC/CdS is attributed to: optimal optical adsorption, efficient separation and fast transfer of carriers, and high surface catalytic activity (Figure 6). Under the light irradiation, the improved light-harvesting capability of MoSC/CdS facilitates the generation of electron-hole pairs. Then, MoS2/Mo2C with suitable Fermi level and strong metallic characteristic can effectively extract the electrons and shuttle them to surface for catalytic reaction. Finally, MoS2/Mo2C with favorable HER activity promotes the rapid reduction of proton to H2 on the surface through the above photo-induced electrons.

Figure 6. The mechanism of photocatalytic hydrogen evolution on MoS2/Mo2C/CdS. The main photocatalytic hydrogen evolution process is: (1) the charge carriers are produced on CdS under light irradiation; (2) the photo-induced electrons mainly transfer to MoS2/Mo2C, and partially to surface active sites of CdS; (3) the transferred electrons are shuttled to surface active sites of MoS2/Mo2C; and (4) hydrogen molecules are generated mostly on MoS2/Mo2C and partially on CdS.

CONCLUSION In this work, a highly effective co-catalyst, MoS2/Mo2C, for photocatalytic hydrogen 20


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generation is developed by partially carbonizing MoS2 through a simple chemical vapor reaction process. Our combined computational and experimental studies show that MoS2/Mo2C can dramatically enhance the solar-driven hydrogen production. We find that the MoS2/Mo2C/CdS composite display ultra-high photocatalytic performance with the hydrogen evolution rate of 34 mmol/h/g, which is 112 and 5 times as high as those of CdS and MoS2/CdS, respectively, and the hydrogen production rate of MoS2/Mo2C/g-C3N4 is 16 and 5.8 times higher than those of pure g-C3N4 and MoS2/gC3N4, respectively. The superior activity is attributed to the co-catalyst (MoS2/Mo2C) that results in optimal optical adsorption, efficient separation and fast transfer of carriers and high surface catalytic activity on HER. We expect MoS2/Mo2C would be utilized as co-catalyst in practical application for solar-energy harvesting. Our findings also provide guidance for developing effective co-catalysts in photocatalysis.

ASSOCIATED CONTENT The Supporting Information accompanies this paper available: Figures S1-S11 and Tables S1-S2 on additional characterization, computational model, calculation and discussion.

AUTHOR INFORMATION Corresponding Author *(H.P.) E-mail: [email protected]. ORCID 21



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Hui Pan: 0000-0002-6515-4970 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Science and Technology Development Fund from Macau SAR (FDCT-132/2014/A3) and Multi-Year Research Grants (MYRG201700027-FST and MYRG2018-00003-IAPME) from Research & Development Office at University of Macau as well as a Singapore MOE AcRF grant (R-144-000-365-112). The DFT calculations were performed at High Performance Computing Cluster (HPCC) of Information and Communication Technology Office (ICTO) at University of Macau.

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Hisatomi, T.; Kubota, J.; Domen, K. Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting. Chem. Soc. Rev. 2014, 43, 7520-7535, DOI 10.1039/C3CS60378D.

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Reddy, D. A.; Choi, J.; Lee, S.; Kim, Y.; Hong, S.; Kumar, D. P.; Kim, T. K. Hierarchical dandelion-flower-like

cobalt-phosphide

modified

CdS/reduced

graphene

oxide-MoS2

nanocomposites as a noble-metal-free catalyst for efficient hydrogen evolution from water. Catal. Sci. Technol. 2016, 6, 6197-6206, DOI 10.1039/C6CY00768F.

For Table of Contents Use Only

The high photocatalytic hydrogen evolution performance of MoS2/Mo2C/CdS is mainly contributed to the enhanced utilization of light, efficient separation and transportation of electron-hole pairs, and fast surface redox reaction. 27



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Carbonized MoS2: Super-Active Co-catalyst for High-Efficient Water

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