Synthesis of MoS₂/SrZrO₃ heterostructures and their photocatalytic H₂ evolution under UV irradiation

Abstract

A novel heterojunction of a MoS₂/SrZrO₃ photocatalyst was successfully synthesized via a simple hydrothermal process and applied to photocatalytic H₂ evolution under UV light irradiation. The samples were characterized by X-ray diffraction, UV-vis absorption spectroscopy, scanning and transmission electron microscopy, X-ray photoemission spectroscopy, energy dispersive X-ray spectroscopy and EDX mapping. The heterostructure with an optimal content of 0.05 wt% MoS₂ exhibits the highest H₂ evolution rate of 5.31 mmol h⁻¹. This is due to the junction between SrZrO₃ and MoS₂, which suppresses the recombination of photogenerated electrons and holes. Our work indicated that the prepared MoS₂/SrZrO₃ heterostructured photocatalyst can be used as an effective material for water splitting.

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1. Introduction

Photocatalytic water splitting has attracted considerable attention due to the depletion of fossils and energy crisis challenging science,^1–3 and hydrogen has been considered as a clean, economical and environmentally friendly energy in modern society.^4–6 Since the first report by Fujishima and Honda on electrochemical photolysis of water at a TiO₂ electrode,⁷ great efforts have been made to explore hydrogen evolution to solve energy and environmental issues.^4,8 Selecting novel photocatalysts possessing a wide range of light response and higher quantum efficiency, such as, heterogeneous photocatalysts, metal coordination photocatalysts,⁹ hierarchically structured nanocrystals^10,11 etc., have been intensively and extensively investigated because of their special band structure and carrier transportation.^12,13

Photocatalysts containing d⁰ and d¹⁰ metal irons, such as, Ti⁴⁺, Zr⁴⁺, Nb⁵⁺, Ga³⁺ and Sb⁵⁺ have been demonstrated as higher photocatalytic activities for H₂ evolution under UV irradiation.^4,14 Among them, perovskite type oxide ABO₃, where A is an alkaline metal or an alkaline earth metal and B is a transition metal,¹⁵ such as, SrZrO₃, BaZrO₃, SrTiO₃, BiVO₄ and etc., have been considered as one of the most promising photocatalysts due to its unique properties as high stability and nontoxicity.¹⁶ Furthermore, perovskite-type SrZrO₃, with a wide band gap of 5.6 eV, has attracted much attention owing to its applications in high-temperature materials,¹⁷ luminescence properties,¹⁸ hydrogen gas sensors¹⁹ and catalysts.²⁰ And excellent stabilities and more photocatalytic active sites make SrZrO₃ a suitable photocatalyst applied to the water splitting and dye degradation reactions under UV light irradiation.

Heterogeneous photocatalysis is considered as one of the most promising methods solving energy and environmental crisis, and perovskite type oxide compounds have been selected as one of the suitable materials.^4,14,21 On the other hands, layer-structured transition metal sulfide especially for MoS₂, which is consist of metal Mo attached with two S and stacking together to furnish sandwich-like structure, is also exploited for photocatalytic applications owing to its narrow band gap and high thermal stability,^22,23 and a great number of heterogeneous photocatalysts containing MoS₂ have been synthesized during the past decades.^22,24–28 Li et al. synthesized MoS₂/CdS catalyst and investigated the photocatalytic activities before and after loading MoS₂ under visible light irradiation, founding a greatly enhancement of H₂ evolution after loading MoS₂.²⁹ Liu et al. prepared a novel photocatalyst with 3D hierarchical heterostructure, MoS₂/TiO₂, exhibiting an excellent photocatalytic H₂ evolution and dye degradation, impling the matched energy band of MoS₂/TiO₂ heterostructure favoring the charge transfer and suppression the recombination of photogenerated electron and hole between MoS₂ and TiO₂.²⁷

Herein, a novel heterostructured MoS₂/SrZrO₃ photocatalyst was synthesized by facile hydrothermal method. Then the as-prepared sample was employed to investigated photocatalytic H₂ evolution from aqueous solutions containing Na₂S and Na₂SO₃ under UV irradiation, and 0.05 wt% MoS₂ loading on the SrZrO₃ crystal was found to show best performance with H₂ evolution rate of 5.31 mmol h⁻¹. MoS₂ was proved to be an effective separation of photogenerated carriers in the MoS₂/SrZrO₃ heterojunction and the possible mechanism of H₂ evolution was discussed. To the best of our knowledge, this is the first report on the preparation of MoS₂/SrZrO₃ heterojunction and their application in the photocatalytic activity for water splitting under UV light irradiation.

2. Experimental section

2.1 Chemicals and materials

Zirconium(IV) oxychloride octahydrate (ZrOCl₂·8H₂O), strontium nitrate (Sr(NO₃)₂), potassium hydroxide (KOH), sodium molybdate dehydrate (Na₂MoO₄·2H₂O), thioacetamide (C₂H₅NS), acetic acid and ethanol. All of the chemicals were purchased from Shanghai Chemical Reagent Company in analytical grade, and were used without further purification.

2.2 Synthesis of SrZrO₃ photocatalyst

SrZrO₃ photocatalyst were synthesized by hydrothermal method according to the previous literature.³⁰ Typically, 1.164 g Sr(NO₃)₂ and 1.608 g ZrOCl₂·8H₂O were dissolved in 60 mL 12 mol L⁻¹ KOH solution and stirred 1 h at ambient temperature by using a magnetic stirrer. Then the mixture was transferred to a 100 mL Teflon-lined stainless steel autoclave, heated up to 200 °C and kept for 24 h. After cooling naturally, the product was centrifuged and washed with distilled water, diluted acetic acid to remove some SrCO₃ by-products, and ethanol several times. After that, the sample was vacuum dried at 60 °C for 6 h. It is worth noting that the KOH solution was pre-prepared or the temperature would rise up to over 100 °C resulting in the inhomogeneous formation of SrZrO₃ crystal.

2.3 Synthesis of MoS₂ crystal

The MoS₂ crystal was synthesized by hydrothermal method. Typically, 0.807 g Na₂MoO₄·2H₂O and 1.2522 g C₂H₅NS was dissolved in a 100 mL beaker containing 20 mL distilled water and 20 mL ethanol, and the mixture was stirred 30 min under a magnetic stirrer. Then the solution was transferred in a 100 mL Teflon-lined stainless-steel autoclave, sealed and heated to 200 °C for 48 h. After cooling naturally, the product was centrifuged, washed with distilled water and ethanol several times. The sample was vaccum dried at 60 °C for 6 h.

2.4 Synthesis of MoS₂/SrZrO₃ heterocatalyst

The formation process of MoS₂/SrZrO₃ heterocatalyst was described as follows. 0.5 g as-prepared SrZrO₃ was dissolved in the mixture containing 20 mL distilled water and 20 mL ethanol, a certain quality Na₂MoO₄·2H₂O and C₂H₅NS was then added into the solution and the molar ration of Na₂MoO₄·2H₂O and a certain quality Na₂MoO₄·2H₂O and C₂H₅NS was then added into the solution and the molar ration of Na₂MoO₄·2H₂O and C₂H₅NS was 1 : 5. The obtained suspension was sonicated for 10 min and stirred 30 min in order to disperse uniformly. The solution was sealed in a 100 mL Teflon-lined stainless-steel autoclave and heated in an electric oven at 200 °C for 48 h. After cooling to the room temperature, the product was separated by centrifugation, washed with distilled water and ethanol several times. Finally, the catalysts were dried in vacuum at 60 °C for 6 h. The prepared samples with 0.01, 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 7, 10 wt% MoS₂, defined as the weight ration of MoS₂ to SrZrO₃, were named as SM1, SM2, SM3, SM4, SM5, SM6, SM7, SM8, SM9, SM10.

2.5 Characterization of photocatalyst

X-ray diffraction (XRD) patterns were recorded on Bruker D8-Discover with Cu Kα radiation (λ = 1.54178 Å). The transmission electron microscopy (TEM, Hitachi H-600) and the scanning electron microscopy (SEM, FEI Inspect F50) were used to characterize the morphologies and size of the synthesized product. UV-vis absorption spectra were measured on a Shimadzu UV-3100 spectrometer. The chemical composition analysis of the sample surfaces was characterized by X-ray photoelectron spectra (XPS) (VG Multiab-2000) using a PHI Quantum 2000 XPS system with a monochromatic Al Kα source and charge neutralizer. All of the spectra were calibrated to C 1s peak at 284.6 eV.

2.6 Photocatalytic activity test

Photocatalytic reactions of hydrogen evolution were carried out in a 250 mL Pyrex flask at room temperature and atmospheric pressure, and 100 W mercury lamp (365 nm, Shanghai Special Lighting Factory) was used as the light source. Typically, photocatalyst (0.2 g) was dissolved in 250 mL aqueous solution containing Na₂S (0.35 M) and Na₂SO₃ (0.25 M) as electron donors. Prior to irradiation, the suspension was sonicated for 20 min and bubbled with N₂ for 30 min to remove the dissolved oxygen. Then the photocatalyst was irradiated with ultraviolet light from the Hg lamp. The amount of H₂ was determined by gas chromatograph (GC-14C, Shimadzu, Japan, TCD, nitrogen as a carrier gas and 5 Å molecular sieve column).

3. Results and discussion

In order to investigate the changes of phase structures and crystal form of the as-prepared samples, X-ray diffraction measurement was performed. Fig. 1 shows the XRD patterns of SrZrO₃, MoS₂ and MoS₂/SrZrO₃ synthesized by hydrothermal method. All the peaks were assigned as orthorhombic perovskite SrZrO₃ structure (JCPDS card 44-0161) with cell parameter a = 5.819 Å, b = 8.204 Å and c = 5.797 Å in Pnma (62) space group, and several typical peaks at 2θ = 21.6°, 24.2°, 30.7°, 36.2°, 37.9°, 44.1°, 45.5°, 54.7°, 64.0°, 72.7° were observed and corresponding to (020), (111), (200), (211), (220), (202), (212), (240), (400) and (402) planes, respectively. For pure MoS₂ crystal, peaks are ascribed to (002), (100) and (110) planes with hexagonal phase MoS₂ (JCPDS card 37-1492).^27,31 However, no obvious peaks of heterojunctions attributed to MoS₂ phase were observed in the MoS₂/SrZrO₃ samples mainly due to its low loading content, high distribution of the MoS₂ component and weak crystallization in these catalysts.^32–34

Fig. 1 XRD patterns of SrZrO₃, SM1, SM3, SM5, SM8, SM10, MoS₂ and standard pattern of SrZrO₃.

Fig. 2 demonstrated the UV-vis absorption spectra of pure SrZrO₃ and MoS₂/SrZrO₃ heterostructures. For pure SrZrO₃ sample, the absorption at 230 nm was supposed to the instinct band gap absorption, and the band gap of SrZrO₃ is 5.4 eV which is close to 5.6 eV that literature reported.^18,35,36 And the absorption of MoS₂ was blue-shifted relative to the bulk MoS₂ having a band edge at 1040 nm.^27,37 Noticeably, a similar UV-vis absorption curves were observed, and the absorption edges of heterojunction is red-shifted gradually when the increasing amount of MoS₂ crystal was loaded on the surface of SrZrO₃. Further, the absorption band of MoS₂/SrZrO₃ was located at the UV region, indicating an excellent UV absorption at this area.

Fig. 2 UV-vis absorption spectra of SrZrO₃, SM1, SM3, SM5, SM8, SM10 and MoS₂ samples.

The morphologies of as-prepared catalysts were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM), which was shown in Fig. 3. The morphology of SrZrO₃ (Fig. 3a) shows a cubic structure with an average side length of 2 μm, and the sizes and shapes of pure SrZrO₃ and SM8 remained almost the same after loading MoS₂, which is demonstrated by the comparison of Fig. 3c with Fig. 3a. For pure MoS₂ (Fig. 3b), the successfully synthesized MoS₂ was layer-structured, which is flower-like nanostructure with average diameter of 200 nm. Furthermore, the MoS₂ small nanoflakes loading on the surface of SrZrO₃ were observed from Fig. 3d. The elemental composition of the photocatalyst was examined by energy dispersive X-ray spectroscopy (EDX) analysis, Fig. 3f. The presence of Sr, Zr, O, Mo and S elements were confirmed by EDX. The HRTEM image of MoS₂/SrZrO₃ in Fig. 3e indicates the lattice spacing with an interplanar distance of 0.291 nm and 0.61 nm, which are corresponding to the (200) and (002) d-spacing of the orthorhombic phase of SrZrO₃ and hexagonal MoS₂,^5,27,38 respectively. Herein, the HRTEM image is just an indirect proof demonstrating the existence of MoS₂ in MoS₂/SrZrO₃ heterojunctions.

Fig. 3 (a) SEM images of SrZrO₃, (b) SEM images of MoS₂, (c and d) SEM images of SM8, (e and g) HRTEM images of SM8, (f) EDX pattern of SM8 photocatalyst in (c).

To confirm the formation of MoS₂/SrZrO₃ heterojunction, the SEM and TEM analysis of SrZrO₃ have been characterized before and after loading MoS₂. In addition to this, SEM-EDX mapping has also been detected to show the elemental distribution of Sr, Zr, O, Mo and S, which was presented in Fig. 4 and the selected area of EDX-mapping analysis was shown in Fig. 4a. Obviously, homogeneous distributions of Sr, Zr and O were observed in the particle (Fig. 4b–d) which is consistent to the SEM image. The distribution of elements of Mo and S were also displayed uniformly with low density (Fig. 4e and f).

Fig. 4 (a) SEM image of SM8, and (b–f) the corresponding EDX mapping of MoS₂/SrZrO₃ at the region shown in (a), indicating spatial distribution of Sr, Zr, O, Mo, S, respectively.

To further investigated the chemical composition and elemental status of SrZrO₃ and MoS₂/SrZrO₃, XPS analysis was carried out and presented in Fig. 5. The content of MoS₂ lower than 0.1% cannot be detected by XPS, and the different of all heterojunctions is the smoothness with different MoS₂ content, thus, we chose SM8 to investigate the XPS of the elements. The XPS peak for C 1s at 284.6 eV is due to adventitious carbon from the XPS instrument. Fig. 5a gives the comparison of XPS spectra of SrZrO₃ and SM8 samples. The compositions of high resolution XPS of Sr, Zr and O elements for SrZrO₃ and SM8 were shown in Fig. 5b–d. No apparent differences except O were observed when MoS₂ loading on the surface of SrZrO₃, indicating the introduced MoS₂ has little influence on the chemical valence.³³ Two peaks of Sr 3d in Fig. 4b at 135.3 and 133.7 eV are assigned to Sr 3d_3/2 and Sr 3d_5/2, and the binding energies corresponding to Zr 3d_3/2 and Zr 3d_5/2 are 182.2 and 179.9 eV, respectively.³² The binding energy difference of O 1s was 533.7 eV due to the absorbed oxygen in SrZrO₃ sample,³⁴ and the binding energy located at 531.8 eV is assigned to O 1s. The peaks of Mo 3d (Fig. 5e) at 232.6 and 229.4 eV are attributed to Mo(+4) 3d_3/2 and Mo(+4) 3d_5/2, respectively.²² It can be clearly seen from Fig. 5f that the measured binding energies of S 2p_1/2 and S 2p_3/2 are corresponding to 163 and 161.9 eV, respectively.^22,27,39 The atomic percentage of Mo and S analyzed by XPS was 3.876 : 7.481, which is close to 1 : 2. Taken all, the existence of MoS₂ and SrZrO₃ in SM8 sample was further confirmed by high resolution of XPS spectra, which is consistent of XRD and TEM analysis.

Fig. 5 (a) XPS survey spectra of sample SrZrO₃ and SM8, (b–f) high resolution XPS spectra of Sr, Zr, O, Mo and S.

In order to investigated the photocatalytic performance of SrZrO₃ photocatalyst before and after loading MoS₂, a serious of experiments have been carried out and the results have been shown in Fig. 6a. It is clear that the rate of H₂ evolution was obviously enhanced when loading a relative small amount of MoS₂ with 0.05 wt%. The rate of H₂ evolution for pure SrZrO₃ photocatalyst was 1.95 mmol h⁻¹, while the rate of H₂ evolution of MoS₂/SrZrO₃ was 5.31 mmol h⁻¹, nearly 2.7 times of pure samples, which is due to the effect of heterojunction between MoS₂ and SrZrO₃. As can be seen, H₂ production was gradually decreased or even lower than pure SrZrO₃ as the increasing amount of MoS₂. It is mainly due to several reasons: the decrease of SrZrO₃ surface active sites resulting from the excessive loading MoS₂ cluster; deterioration of catalytic properties of SrZrO₃ cluster; decrease of irradiation passing through the sacrifice solution.⁴⁰ The stability and recycling performance of photocatalysts are important factors in the commercial applications, thus, time course of H₂ production over MoS₂/SrZrO₃ was performed (Fig. 6b). Clearly, the average rate of H₂ evolution was nearly unchanged after 30 h reaction, indicating an excellent stability of photocatalyst.

Fig. 6 (a) Photocatalytic activity of MoS₂/SrZrO₃ catalysts with loading different amount of MoS₂, and (b) time course of H₂ evolution over 0.05 MoS₂ (0.05 wt%)/SrZrO₃ photocatalyst.

A possible mechanism of photocatalytic H₂ evolution over MoS₂/SrZrO₃ heterojunction was proposed and depicted in Fig. 7. The SrZrO₃ photocatalyst absorbed enough energy greater than band gap under UV light irradiation, and generated electron was easily transferred to conduction band, leaving photo-generated holes in the valence band. The catalytic activities without MoS₂ loading were lower because of the recombination between photo-generated electrons and holes. While, the H₂ evolution was significantly enhanced after loading MoS₂ because of the rapid transfer of the photo-generated electrons from the conduction band of SrZrO₃ samples to MoS₂ which is due to the higher conduction band of SrZrO₃ than that of MoS₂. At the same time, the H⁺ was reduced to H₂ with the MoS₂ trapping of photo-generated electrons, and whereas holes of SrZrO₃ crystals in the valence band oxidize S²⁻ and SO₃²⁻on the surface of SrZrO₃. The heterojunctions between SrZrO₃ and MoS₂ was considered as an important factor for the remarkably improved H₂ evolution.

Fig. 7 Mechanism of photocatalytic water splitting over MoS₂/SrZrO₃ photocatalysts under UV light.

4. Conclusions

In summary, the MoS₂/SrZrO₃ heterojunction was synthesized for the first time through two-step hydrothermal reaction. The highest of H₂ evolution rate of 5.31 mmol h⁻¹ was obtained with 0.05 wt% MoS₂ loading on the surface of SrZrO₃ under UV irradiation, and the mechanism of process of H₂ evolution was also proposed. Furthermore, the amount of MoS₂ plays an important in photocatalytic activities, which were might due to the changes of active sites, band gaps and irradiation opacity. Obviously, the influences of MoS₂ in MoS₂/SrZrO₃ heterogeneous photocatalyst were matching the band energy and suppressing the recombination between photogenerated electron and holes, resulting in the enhancement of H₂ evolution.

Acknowledgements

This work was financially supported by the National Nature Science Foundation of China (no. 31070517) and the National Basic Research Program of China (973 Program, no. 2013CB932902).

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Footnote

† These authors contributed equally to this work.

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