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Metallic 1T-LixMoS2 Cocatalyst Significantly Enhanced the Photocatalytic H2 Evolution over Cd0.5Zn0.5S Nanocrystals under Visible Light Irradiation Hong Du,†,‡ Hong-Li Guo,† Ya-Nan Liu,† Xiao Xie,† Kuang Liang,† Xiao Zhou,† Xin Wang,† and An-Wu Xu*,† †

Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, P. R. China ‡ College of Chemistry and Chemical Engineering, Xinjiang Normal University, Urumqi 830054, P. R. China S Supporting Information *

ABSTRACT: In the present work, metallic 1T-LixMoS2 is utilized as a novel cocatalyst for Cd0.5Zn0.5S photocatalyst. The obtained LixMoS2/Cd0.5Zn0.5S hybrids show excellent photocatalytic performance for H2 generation from aqueous solution containing Na2S and Na2SO3 under splitting visible light illumination (λ ≥ 420 nm) without precious metal cocatalysts. It turns out that a certain amount of intercalating Li+ ions ultimately drives the transition of MoS2 crystal from semiconductor triagonal phase (2H phase) to metallic phase (1T phase). The distinct properties of 1T-LixMoS2 promote the efficient separation of photoexcited electrons and holes when used as cocatalyst for Cd0.5Zn0.5S photocatalyst. As compared to 2H-MoS2 nanosheets only having edge active sites, photoinduced electrons not only transfer to the edge sites of 1T-LixMoS2, but also to the plane active sites of 1T-LixMoS2 nanosheets. The content of LixMoS2 in hybrid photocatalysts influences the photocatalytic activity. The optimal 1T-LixMoS2 (1.0 wt %)/Cd0.5Zn0.5S nanojunctions display the best activity for hydrogen production, achieving a hydrogen evolution rate of 769.9 μmol h−1, with no use of noble metal loading, which is about 3.5 times higher than that of sole Cd0.5Zn0.5S, and 2 times higher than that of 2H-MoS2 (1.0 wt %)/Cd0.5Zn0.5S samples. Our results demonstrate that Li+intercalated MoS2 nanosheets with high conductivity, high densities of active sites, low cost, and environmental friendliness are a prominent H2 evolution cocatalyst that might substitute for noble metal for potential hydrogen energy applications. KEYWORDS: 1T-LixMoS2/Cd0.5Zn0.5S nanojunctions, photocatalytic H2 production, water splitting, visible light, renewable energy, clean energy, metallic phase, noble-free cocatalyst

1. INTRODUCTION Due to the increasing worldwide energy crisis and environmental pollution, photocatalytic hydrogen production from water splitting has aroused huge attention and is regarded as a promising strategy to produce H2 by utilizing solar energy.1−3 Some semiconductor photocatalysts without any noble metal cocatalyst were developed for hydrogen generation.4,5 Metal sulfides have been confirmed to be potential materials for photocatalytic H2 evolution from water with visible light, due to their proper bandgaps and excellent photocatalytic activity.6−8 Among chalcogenides, CdS with a narrow but direct bandgap (2.3 eV), which is greater than the minimum energy (1.23 eV) © XXXX American Chemical Society

required for photocatalytic water splitting, is available for absorbing the visible light and has sufficient conductive band potential for the reduction of proton to H2.9,10 Nevertheless, CdS alone has very low photocatalytic activity because of the fast recombination of photoinduced charge carriers. Nice performances are often achieved with the use of noble metal cocatalysts. However, there are still some key issues (such as photocorrosion, the requirement of noble metals as cocatalysts, Received: November 24, 2015 Accepted: January 25, 2016

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DOI: 10.1021/acsami.5b11377 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Scheme 1. Synthetic Procedures of Intercalation of Lithium Ions into MoS2 Nanosheets and Preparation of 1T-LixMoS2/ Cd0.5Zn0.5S Photocatalysts

Cd0.5Zn0.5S solid solutions to dramatically enhance photocatalytic H2 evolution.41 In this work, we report that metallic 1T-LixMoS2 as a cocatalyst is loaded on Cd0.5Zn0.5S solid solution for enhancing photocatalytic hydrogen production from water with visible light illumination. A simple method is developed to accomplish the phase transtion from semiconducting 2H-MoS2 to metallic 1T-LixMoS2 by calcination of simply mixed 2H-MoS2 and lithium carbonate (Li2CO3, which is mild and safe in use, was utilized to substitute for n-butyl lithium) at 700 °C. 1TLixMoS2/Cd0.5Zn0.5S heterojunctions are prepared under hydrothermal conditions and show higher activity than 2HMoS2/Cd0.5Zn0.5S and sole Cd0.5Zn0.5S. Metallic 1T-LixMoS2 provides a potential candidate for cost-effective, highly efficient, and stable cocatalysts utilized in other photocatalytic systems with improved performance.

and the toxicity of cadmium) hindering its practical application.11,12 The good method to improve the performance of CdS has been confirmed by introduction of other materials to form solid solutions, displaying tunable controllable band structure and remarkable catalytic properties.13−16 The bandgap of Cd1−xZnxS material generated between ZnS and CdS could be readily tuned by adjusting the value of x to meet the premise of photocatalysis.17−20 Even though the conduction band position and the bandgap of Cd1−xZnxS solid solutions can be modified to improve the photocatalytic property, the rapid recombination of photoexcited electrons and holes in semiconductors still remains a key problem restricting the further improvement of photocatalytic efficiency. For the purpose of preventing the fast recombination of electron/ hole pairs to improve the hydrogen generation from water, one of the common methods is the introduction of cocatalyst.2,21,22 A suitable cocatalyst can provide more effective proton reduction active sites, lower the activation energies to facilitate the surface reactions, and prevent the recombination of photogenerated e/h pairs.23 Pt is used as an effective cocatalyst to promote water splitting into H2 due to active metal surface and its electronic structure.24,25 However, the wild use of Pt is restricted by its high cost. Therefore, it is of significance to develop cost-effective cocatalysts for enhancing photocatalytic activity and stability. As nonprecious metal cocatalyst, MoS2 is much cheaper than noble metals such as Pt. Furthermore, similar to graphene, MoS2 is not only a promising electrocatalyst, but also an efficient cocatalyst when coupled with a semiconductor, showing significantly enhancement in H2 generation and the removal of contaminants.1,11,26−28 Two-dimensional (2D) layered MoS2 material is composed of stacked atom layers bound by van der Waals interactions between each sandwiched S−Mo−S monolayers. Polymorphism is one of the unique properties in MoS2 determining its distinct electronic characteristics. MoS2 has two polytypes: the 2H (trigonal prismatic D3h) phase and 1T (octahedral Oh) phase, due to the different arrangement of S atoms. Different electronic structures exist between the two phases: the 2H phase is semiconductor, but the 1T phase is metallic with high electrical conductivity (10− 100 S cm−1).29−31 An interesting aspect of alkali metal ion intercalation as a result of the weak interlayer interaction is the ability to alter completely the electronic structures of the host materials. It was reported that 2H-MoS2 semiconductors becomes metallic 1T-MoS2 with Li+ intercalation.32−36 Latest studies have elucidated that 1T-LixMoS2 are much more active in electrocatalytic H2 evolution reaction than MoS2.37−39 However, the report on 1T-LixMoS2 as cocatalyst for enhanced photocatalytic H2 production over semiconductor is still limited.40 Our previous work also proved that metallic MoO2, as an efficient H2 production cocatalyst, was loaded on

2. EXPERIMENTAL SECTION Synthesis of MoS2 Nanosheets. Hexa-ammonium heptamolybdate tetrahydrate (0.5 mmol, (NH4)6Mo7O24·4H2O) and thiourea (15 mmol) were added to 20 mL of double distilled water under vigorous stirring for 20 min at room temperature. Then, the mixture solution was transferred into a Teflon-lined stainless steel autoclave (25 mL) and heated to 220 °C. The reaction was kept to proceed for 18 h. After cooling, washing by water and ethanol for several times, and drying at 60 °C under vacuum, the final product was acquired. Li+-Intercalated MoS2 (LixMoS2). Typically, 0.5 mmol of Li2CO3 and 0.5 mmol of prepared MoS2 nanosheets were ground in the agate mortar for 15 min to ensure homogeneity and then heated at 700 °C for 4 h in nitrogen atmosphere at a heating rate of 5 °C/min; 1TLixMoS2 nanosheets were obtained. LixMoS2/Cd0.5Zn0.5S Photocatalyst Synthesis. LixMoS2 (1.0 wt %)/Cd0.5Zn0.5S composites with LixMoS2 nominal content of 1.0 wt % were fabricated by sequentially adding 3 mmol of cadmium acetate (Cd(CH3COO)2·2H2O), 3 mmol of zinc acetate (Zn(CH3COO)2· 2H2O), and 7.5 mmol of thioacetamide (TAA) to a 15 mL portion of a mixed solvent composed of distilled water (12 mL) and ethylenediamine (EN, 3 mL) and 8 mg of LixMoS2. After stirring for 30 min, the mixture solution was subsequently transferred into Teflon-lined stainless steel autoclave (25 mL), and heated to 230 °C for 30 min. After naturally cooling to room temperature, the product was acquired by centrifugation, washed with water and ethanol, and then dried at 60 °C for 12 h in the vacuum oven. LixMoS2 (0.5 wt %)/Cd0.5Zn0.5S and LixMoS2 (1.5 wt %)/Cd0.5Zn0.5S samples were fabricated by the same method. For comparison, 2H-MoS2 (1.0 wt %)/Cd0.5Zn0.5S with MoS2 nominal content of 1.0 wt % was also synthesized following the same route. Characterization. The morphologies and structures of specimens were characterized by transmission electron microscopy (TEM, JEOL2010) images and high-resolution TEM images (HRTEM, JEOL2010) with an accelerating voltage of 200 kV. X-ray diffraction (XRD) patterns of prepared samples were measured by a Rigaku diffactometer (MXPAHF, Japan) with Cu Kα irradiation (λ = 0.1540 nm), with the operating voltage 40 kV and current 200 mA. The UV−vis absorption spectral data was obtained on a Shimadzu UV-2510 spectrophotometer recorded in the region from 300 to 800 nm. The Raman B


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Figure 1. XRD patterns of synthesized MoS2 and LixMoS2 samples (A), Cd0.5Zn0.5S, MoS2 (1.0 wt %)/Cd0.5Zn0.5S, and LixMoS2 (1.0 wt %)/Cd0.5Zn0.5S (B). spectra were recorded under ambient conditions using JY LABRAMHR confocal micro-Raman spectroscopy (514.5 nm excitation laser). Brunauer−Emmett−Teller (BET) surface areas were taken at 77 K by nitrogen sorption (a Micromeritics ASAP 2020 system). X-ray photoelectron spectroscopy (XPS) spectra were carried out on the Photoemission Endstation in the National Synchrotron Radiation Laboratory (NSRL, Hefei, P. R. China). Photocatalytic H2 Production. The photocatalytic hydrogen evolution reactions from water splitting were performed in a gas-closed circulation and high-vacuum system using a 300 W Xe lamp with a cutoff filter (λ ≥ 420 nm). A 0.1 g portion of photocatalyst was added in an aqueous solution (25 mL) containing Na2S (0.25 M) and Na2SO3 (0.25 M) in a 500 mL Pyrex glass reactor, and then the suspension was ultrasonically dispersed for 30 min. The amount of H2 production was determined by using online gas chromatography (Agilent 6820, TCD detector, N2 carrier) with a thermal conductivity detector and nitrogen as carrier gas. The activity of photocatalysts was compared on the basis of the average H2 generation rate in the first 5 h (μmol h−1).

3. RESULTS AND DISCUSSION The synthetic process of 1T-LixMoS2/Cd0.5Zn0.5S hybrid photocatalysts is illustrated in Scheme 1. MoS2 nanosheets were first prepared by hydrothermal treatment, then LixMoS2 was obtained by calcining the mixture of MoS2 and Li2CO3, and finally LixMoS2/Cd0.5Zn0.5S nanojunctions were fabricated by solvothermal method (see Experimental Section). The crystal structure of obtained samples was measured by X-ray diffraction (XRD) patterns. As shown in Figure 1, all diffraction peaks for synthesized MoS2 nanosheets (Figure 1A) are in accordance with the standard data of trigonal phase of MoS2 (JCPDS 74-0932). Strong (00l) reflections in the Li+intercalated MoS2 sample demonstrate good crystallinity and ordered stacking of the two-dimensional (2D) layers in the asprepared Li+-intercalated MoS2 nanosheets.42,43 The diffraction peaks of Cd0.5Zn0.5S, 2H-MoS2 (1.0 wt %)/Cd0.5Zn0.5S, and 1TLixMoS2 (1.0 wt %)/Cd0.5Zn0.5S samples match well with the standard peaks of the wurtzite hexagonal phase (Figure 1B). In addition, the diffraction peaks of LixMoS2 or MoS2 were not detected by XRD patterns most probably due to the very low content (1.0 wt %) of cocatalyst in nanocomposites. The transmission electron microscopy (TEM) image (see Figure 2A) exhibits the morphology of obtained 2H-MoS2 as the ultrathin 2D nanosheets, and the dense flakes are dominated by edges. Interplanar spacing of 0.30 nm is observed from a high-resolution TEM (HRTEM) image (Figure 2B), which matches well the (100) crystallographic planes of trigonal 2H-MoS2. After lithiation, the morphology of obtained LixMoS2 (Figure 2C) is featured with a flexible 2D structure

Figure 2. TEM and HRTEM images of 2H-MoS2 (A, B), 1T-LixMoS2 (C, D), and 1T-LixMoS2 (1.0 wt %)/Cd0.5Zn0.5S (E, F). Inset in part E shows the corresponding particle distribution.

inherited from pristine 2H-MoS2. To reveal the structures in more detail, the LixMoS2 sample was measured by HRTEM. From Figure 2D, it can be observed that the lattice space values of LixMoS2 were found to be 0.27 nm, matching well the crystallographic planes of octahedral 1T-MoS2.44 Figure 2E presents a TEM image of the LixMoS2/Cd0.5Zn0.5S hybrid structure. It reveals that Cd0.5Zn0.5S NPs with a diameter of about 15 nm (inset in Figure 2E) are closely supported on LixMoS2 nanosheets. The HRTEM image shows that the fringes with a lattice spacing of d = 0.32 nm are ascribed to the wurtzite Cd0.5Zn0.5S (002) plane. The lattice fringes of d = 0.27 nm on the nanosheets correspond to the 1T-LixMoS2 phase (Figure 2F). Triple layers are also visible with an interlayer distance of 0.65 nm, as indicated in Figure 2F. The 2H and 1T phase change was verified by Raman spectroscopy analysis because the symmetry elements in their C


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soft with reducing the number of layers, causing a peak shift from 407 to 403 cm−1 for MoS2 transfer from bulk to a single monolayer. On the other hand, E12g phonons become hard with reduction of the number of layers, corresponding to a peak shift from 384 cm−1 for bulk to 382 cm−1 for monolayer MoS2.42,46 Our LixMoS2 sample shows E12g at ca. 382 cm−1 and A1g peak at 406 cm−1, suggesting that the number of layers becomes large, in line with the results of HRTEM observations (Figure 2A,C). The Raman shifts of Li+-intercalated MoS2 nanosheets confirm the presence of the 1T phase; however, quantitative determination of the phase composition is difficult owing to its weak signal. Instead, the phase compositions were determined by X-ray photoelectron spectroscopy (XPS) measurements. As displayed in Figure 4, the binding energies of Mo 3d (Figure 4A) and S 2p (Figure 4B) for the LixMoS2 sample shift toward lower positions as compared to 2H-MoS2, indicating the formation of 1T-LixMoS2.42 It is found that Mo 3d and S 2p peaks of LixMoS2/Cd0.5Zn0.5S are still located at lower positions, suggesting the good stability of prepared 1T-MoS2. The Mo 3d spectra of pristine MoS2 consist of peaks at 229.6 and 232.7 eV, which is assigned to Mo4+ 3d5/2 and Mo4+ 3d3/ 2 components of 2H-MoS2, respectively.48 After lithiation, the deconvolution of the peaks reveals additional peaks for the 1T phase at 228.6 eV for Mo 3d5/2 and 231.9 eV for Mo 3d3/2, and peaks for 2H phase at 229.0 and 232.5 eV are also observed, confirming coexistence of the two phases.42,49 At the same time, in the S 2p region of the spectra, additional S 2p3/2 and S 2p1/2 peaks for the 1T phase at 162.5 and 161.5 eV are found besides the known doublet peaks for the 2H phase at 163.1 and 161.9 eV (Figure 4C).42,50 These results are in line with previous reports.42,51 The new peaks of Mo 3d and S 2p appear from 1T-LixMoS2, in that the sulfur atoms glide from the original trigonal prismatic coordination (2H-MoS2) to octahedral coordination (1T-MoS2) induced by lithium ion

structures are different. The Raman shifts (Figure 3, upper trace) of as-grown MoS2 at 387, 412, and 456 cm−1 can be

Figure 3. Raman spectra of as-grown MoS2 and LixMoS2 samples.

clearly observed, which correspond to the in-plane E12g and out-of-plane A1g modes and longitudinal acoustic phonon modes of 2H-MoS2.37 After Li+ intercalation, three additional new peaks appeared at 150, 219, and 327 cm−1 shifts that correspond to modes of the 1T MoS2 phase (see Figure 3, bottom trace).45 Raman shifts of LixMoS2 (Figure 3, bottom trace) illustrate the E12g, A1g, and longitudinal acoustic phonon modes as signatures of the 2H phase located at 382, 406, and 454 cm−1, respectively, suggesting that the obtained sample still contains a certain content of the 2H-MoS2 phase. Note that the significantly suppressed intensity of these peaks implies that the concentration of the 2H polymorph significantly reduced, and the transition from 2H phase to 1T phase occurs. Previous studies demonstrated that the position of E12g and A1g peaks is subject to the number of monolayers.46,47 A1g phonons become

Figure 4. High-resolution XPS spectra of Mo 3d (A) and S 2p (B) regions for MoS2 (black), LixMoS2 (blue), and LixMoS2 (1.0 wt %)/Cd0.5Zn0.5S (red). XPS spectra of Mo 3d and S 2p peaks for MoS2, LixMoS2, and LixMoS2 (1.0 wt %)/Cd0.5Zn0.5S (C). Mo 3d and S 2p peaks were deconvoluted into peaks highlighted in red and blue assigned to 2H-MoS2 and 1T-LixMoS2, respectively. Crystal structure of the 2H and 1T phase (D). D


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Figure 5. N2 sorption isotherms of 1T-LixMoS2 (1.0 wt %)/Cd0.5Zn0.5S (A) and corresponding pore size distribution (B) derived from adsorption isotherm.

insertion (Figure 4D).25 From the deconvolution of the XPS spectra, the content of 1T phase LixMoS2 was estimated to be about 56%. It should be noted that LixMoS2 cocatalyst in LixMoS2/Cd0.5Zn0.5S hybrid photocatalysts still contains about 52% 1T phase, and a little decrease of 1T phase concentration is found. The S 2p deconvoluted dark green peak for LixMoS2/ Cd0.5Zn0.5S at 161.7 eV could be ascribed to Cd(Zn)−S bonds.33 In addition, the Zn 2p and Cd 3d peaks at binding energies 1021.3 eV (Zn 2p3/2), 405.16 eV (Cd 3d5), and 411.7 eV (Cd 3d3) are ascribed to the Cd0.5Zn0.5S molecular environment (Figure S1). The porous structure and surface area of LixMoS2 (1.0 wt %)/Cd0.5Zn0.5S were studied by N2 adsorption measurements. The N2 adsorption−desorption isotherms (Figure 5A) could be categorized as type III with an H3 hysteresis loop in the P/P0 range of 0.8−1.0, demonstrating the presence of mesopores and macropores in the sample. The pore size distribution (Figure 5B) of the sample shows a broad distribution (from 50 to 500 nm), suggesting the presence of macropores (>50 nm). The BET surface area of LixMoS2 (1.0 wt %)/Cd0.5Zn0.5S is about 81 m2 g−1, which is larger than that of Cd0.5Zn0.5S (65 m2 g−1) and 2H-MoS2 (1.0 wt %)/Cd0.5Zn0.5S (76 m2 g−1) (Figure S2). The large surface area will give rise to high photocatalytic activity because of more surface active sites for the adsorption of reactants. Figure 6 shows the rate of the photocatalytic H2 production on Cd0.5Zn0.5S, 2H-MoS2/Cd0.5Zn0.5S, and 1T-LixMoS2/ Cd0.5Zn0.5S samples under visible light illumination. It is

found that no obvious H2 was detected when MoS2 as well as LixMoS2 alone were employed as the catalyst under the same conditions, indicating that 2H-MoS2 and 1T-LixMoS2 are not active for photocatalytic H2 generation. The bare Cd0.5Zn0.5S sample without cocatalyst exhibits a rate of 222.4 μmol h−1; when 2H-MoS2 was used as a cocatalyst with a content of 1.0 wt %, the H 2 production rate of 2H-MoS 2 (1.0 wt %)/Cd0.5Zn0.5S was efficiently enhanced to achieve 403.9 μmol h−1, because 2H-MoS2 nanosheets with active edges are beneficial for the carrier separation and work as H2 production cocatalyst. Consequently, the photocatalytic H2 evolution activity is improved. As compared to 2H-MoS2 only displaying active edge sites, 1T-LixMoS2 nanosheets not only exhibit edge active sites, but also have basal plane active sites, which will further enhance photocatalytic activity.51,52 When 1T-LixMoS2 was employed as the cocatalyst, the hydrogen evolution efficiency is significantly enhanced. The rate of H2 evolution over 1T-LixMoS2 (1.0 wt %)/Cd0.5Zn0.5S with a content of 1.0 wt % cocatylyst increases to 769.9 μmol h−1; this is ca. 3.5 times and 2 times higher than that of bare Cd0.5Zn0.5S catalyst and 2H-MoS2 (1.0 wt %)/Cd0.5Zn0.5S photocatalysts. These results demonstrate that the 1T phase is superior to the 2H phase as a cocatalyst. It is noted that 1T-LixMoS2 (1.0 wt %)/Cd0.5Zn0.5S hybrid photocatalyst exhibits a higher H2 generation rate than Pt (0.5 wt %)/Cd0.5Zn0.5S (426.7 μmol h−1) by visible light illumination (see Figure S3). It is reasonable that 1T-LixMoS2 can be an alternative for rare metals as a cocatalyst to enhance photocatalytic performances. In addition, Figure S4 shows that the rate of H2 evolution on LixMoS2 (1.0 wt %)/Cd0.5Zn0.5S increased from 217.1 to 769.9 μmol h−1 as the Li+ contents are increased from 0.5 to 1.0, corresponding to the 1T phase content from 39% to 56% (Figure S5 and Table S1), and the highest H2 evolution rate was achieved at x = 1.0. However, the H2 evolution rate decreased with further increasing Li+ content to x = 1.5; the rate of H2 evolution is 693.7 μmol h−1, corresponding to the 1T phase content of 49% (Figure S5 and Table S1). Therefore, the results suggest that a higher content of the 1T phase would give rise to better activity. The influence of the loading amount of 1T-LixMoS2 cocatalyst on the photocatalytic performance was explored. From Figure 7 it can be seen that, with the LixMoS2 content in the composites increasing, the H2 evolution rate first increases and then decreases. The optimum content of 1T-LixMoS2 is about 1.0 wt %, at which the LixMoS2/Cd0.5Zn0.5S sample exhibits the highest activity in H2 evolution with 769.9 μmol h−1. The amount of H2 production increases to 3829.5 μmol

Figure 6. Visible light photocatalytic performances of samples under irradiation by the same lighting source: MoS2 (a), LixMoS2 (b), Cd0.5Zn0.5S (c), 2H-MoS2 (1.0 wt %)/Cd0.5Zn0.5S (d), and 1TLixMoS2 (1.0 wt %)/Cd0.5Zn0.5S (e). Reaction conditions: catalyst, 0.1 g; 100 mL aqueous solution with 0.25 M Na2S and 0.25 M Na2SO3; light source, 300 W xenon lamp with a λ ≥ 420 nm filter. E


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stability of Cd0.5Zn0.5S loaded with LixMoS2, we compare Mo 3d and S 2p XPS spectra of LixMoS2/Cd0.5Zn0.5S after and before photocatalytic reactions (Figure S8 and Table S2). As estimated from the deconvolution of the XPS spectra, only a small fraction (2.0%) of 1T phase transforms back to the 2H phase, suggesting good stability of Li x MoS 2 (1.0 wt %)/Cd0.5Zn0.5S for long-time photocatalytic H2 evolution. XPS results are in line with the cycled measure for photocatalytic water splitting. The above results confirm that 1T-MoS2 as a cocatalyst for improving electron/hole separation of Cd0.5Zn0.5S is better than 2H-MoS2. Recent reports39,40 have found that the 1T-MoS2 phase displays better activity in electrocatalytic H2 generation with good stability in aqueous solution compared to 2H MoS2, and that the 1T phase not only leads to enhancing the chargetransfer kinetics of MoS2 but also increases the additional active sites on the basal plane,51 as clearly shown in Scheme 2. Taken

Figure 7. Photocatalytic H2 evolution rates of LixMoS2/Cd0.5Zn0.5S hybrid photocatalysts with different contents of LixMoS2 under visible light irradiation: Cd0.5Zn0.5S (a), LixMoS2 (0.5 wt %)/Cd0.5Zn0.5S-0.5 (b), LixMoS2 (1.0 wt %)/Cd0.5Zn0.5S (c), and LixMoS2 (1.5 wt %)/Cd0.5Zn0.5S (d).

within 5 h; this is 3.5 times higher than that of Cd0.5Zn0.5S alone. Further increasing the LixMoS2 content to 1.5 wt % in the hybrid photocatalyst results in a gradual decline in photocatalytic activity, as clearly presented in Figure 7. The possible reason could be that superfluous LixMoS2 cocatalyst in composites can lead to an increase in the opacity and light scattering, while resulting in light absorption decrease of photocatalyst, as already found in our previous reports.41,53,54 This is in line with the results of UV−vis diffuse reflectance analysis (see Figure S6, the bandgap of Cd0.5Zn0.5S is 2.58 eV). These results indicate that an optimized loading amount of LixMoS2 cocatalyst is critical to maximize the photocatalytic H2 evolution rate on LixMoS2/Cd0.5Zn0.5S. To investigate the durability and recycling performance of the LixMoS2/Cd0.5Zn0.5S hybrid photocatalysts, the cycling tests of the photocatalytic H2 generation from water using Na2SO3 and Na2S as sacrificial agents were performed by using the same photocatalyst repeatedly for four times with a 300 W xenon lamp (λ ≥ 420 nm). As shown in Figure 8, there is no

Scheme 2. Schematic Illustration of Charge-Transfer Processes for Photocatalytic Reactions on 1T-LixMoS2/ Cd0.5Zn0.5S Hybrids

together, fast electron migration from Cd0.5Zn0.5S to metallic 1T-LixMoS2 with high conductivity and activated basal plane results in effective spatial electron/hole pair separation, thus enhancing the photocatalytic H2 efficiency (Scheme 2). It is noteworthy that the metallicity of 1T-MoS2 promotes the proton reduction reaction to H2 due to its high work function (4.2 eV).55 Upon visible light irradiation, the electrons generated on Cd0.5Zn0.5S nanorods transfer not only to the edge sites, just like in the case of the 2H-MoS2, but also to the plane active sites on 1T-MoS2 for catalytic reactions. Hence, the travel distance of electrons is remarkably shortened, and the possibility of charge recombination is also decreased. Our results demonstrate that 1T-LixMoS2 nanosheets can be employed as an effective cocatalyst to enhance photocatalytic H2 generation.

4. CONCLUSIONS In summary, the photocatalytic hydrogen production of 1TLixMoS2/Cd0.5Zn0.5S composites with visible light illumination is significantly increased using metallic 1T-LixMoS2 as a novel cocatalyst. It turns out that 1T phase MoS2 nanosheets is an excellent cocatalyst on account of high conductivity and high density of both edge active sites and the basal plane active sites. The optimal 1T-1LixMoS2/Cd0.5Zn0.5S sample reveals much higher photocatalytic activity as compared to sole Cd0.5Zn0.5S and the hybrid counterpart 2H-MoS 2/Cd0.5Zn0.5S. 1TLixMoS2/Cd0.5Zn0.5S nanojunctions also exhibit good durability and recycling performance, indicating 1T-LixMoS2 cocatalyst can protect Cd0.5Zn0.5S against photocorrosion. Our results demonstrate that metallic 1T-LixMoS2 can be considered to be an efficient cocatalyst as an alternative for precious metals for H2 generation from water splitting. Intercalation-induced

Figure 8. Time-cycle photocatalytic hydrogen production over 1TLixMoS2 (1.0 wt %)/Cd0.5Zn0.5S with visible light illumination (λ ≥ 420 nm).

noticeable loss of photocatalytic activity after each run, revealing remarkable stability and recycling performance of LixMoS2/Cd0.5Zn0.5S, in which Cd0.5Zn0.5S was loaded with LixMoS2 as cocatalyst in hydrogen evolution after 20 h illumination. XRD patterns (Figure S7) of the LixMoS2/ Cd0.5Zn0.5S sample have no obvious change after the photocatalytic cycles, indicating that LixMoS2/Cd0.5Zn0.5S photocatalyst is stable and not photocorroded. To prove the structure F


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transition of MoS2 crystal from the semiconducting 2H phase to the metallic 1T phase could also be expected in other semiconductor hosts. The lithiated MoS2 nanosheets, obtained by the calcination of Li2CO3 and MoS2 mixture, provide an environmentally friendly and economical approach for easy preparation of various high-efficiency photocatalytic systems with enhanced performance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11377. Details for XPS spectra, nitrogen adsorption/desorption isotherms, photocatalytic activity, UV−vis diffused reflection spectra, and elemental compositions from XPS, and XRD patterns (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86 0551 63602346. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support from the National Basic Research Program of China (2011CB933700), the National Natural Science Foundation of China (51561135011, 51572253, 21271165), and Scientific Research Grant for Hefei Science Center of CAS (2015SRGHSC048) is gratefully acknowledged.

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REFERENCES

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