Synthesis of hexagonal and cubic ZnIn₂S₄ nanosheets for the photocatalytic reduction of CO₂ with methanol

Abstract

ZnIn₂S₄ nanosheets with hexagonal and cubic structures have been prepared through a liquid ultrasonic exfoliation method and another strategy involving a lamellar hybrid, respectively. The structure, morphology, and composition properties of the as-prepared samples were characterized using X-ray powder diffraction, UV-vis spectrophotometry, transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. The photocatalytic activities of the as-prepared ZnIn₂S₄ samples were evaluated by methyl formate formation from CO₂ photoreduction in methanol as a solution of absorbing CO₂ and reducing agent, and the activity over hexagonal ZnIn₂S₄ was better than over cubic ZnIn₂S₄. In addition, both hexagonal and cubic ZnIn₂S₄ nanosheets displayed much higher reactivity than ZnIn₂S₄ microspheres prepared by the hydrothermal method. The electronic structures of the two phases of ZnIn₂S₄ were investigated in the light of density functional theory.

---

Introduction

Shortage of energy and carbon resources and increase of CO₂ concentrations in the atmosphere, due to excessive dependence on fossil fuels, have aroused great concern. Photocatalytic reduction of CO₂ to fuels or value-added chemicals employing inexpensive semiconductors is an attractive solution for both the rising demand for clean energy and the need for greenhouse gas reduction.¹ The development of environmentally friendly semiconductors has taken a great perspective for photocatalytic applications. To date, various semiconductors such as TiO₂,^2–4 Bi₂WO₆,^5,6 ZnGe₂O₄,^7,8 Na₂V₆O₁₆·xH₂O⁹ have been reported for the photocatalytic conversion of CO₂. Although some progress has been made in this area, the photocatalytic activity of CO₂ reduction is very low yet. In general, the photocatalytic performances of semiconductors are strongly affected by various parameters, including morphology, crystalline phase, surface area, surface state and crystallinity.^10–13

In recent years, metal sulfides, such as ZnS, CdS and ZnIn₂S₄, have been intensively studied as active photocatalysts in virtue of their unique catalytic functions.^14,15 Their valence band, made of 3p orbitals of the sulfur atoms, is shifted upwards compared with those of the oxide analogues, while the conduction band electrons are also more reductive.¹⁶ ZnIn₂S₄ is a ternary chalcogenide with two distinct polymorphs based on cubic and hexagonal lattices.¹⁷ The structure of hexagonal ZnIn₂S₄ is essentially based on a stacking of packets of S–Zn–S–In–S–In–S layers. Zn and half of the In atoms are tetrahedrally coordinated by S atoms, whereas the other In atoms are octahedrally coordinated. In the cubic ZnIn₂S₄, the Zn atoms are tetrahedrally coordinated by S atoms, but the In atoms are octahedrally coordinated.¹⁸ A variety of ZnIn₂S₄ nanomaterials with specific morphologies like nanotubes, nanoribbons, nanowires, and flower-like microspheres were reported for degradation of contaminants as well as photocatalytic water splitting.^19–26 The band gap and conduction band of ZnIn₂S₄ opens up the possibility for photocatalytically reducing CO₂ to formic acid. In addition, ZnIn₂S₄ has a layered structure with high chemical stability. Very recently, sheet-like materials have attracted intensive interests because of their improved catalytic performance and potential applications in optoelectronics, energy storage, thermal conductors, and so on, which benefit from their high specific surface areas and large fraction of uncoordinated surface atoms with respect to the corresponding bulks. Synthesis of nanosheets materials is, therefore, a popular strategy to effectively increase the active sites and subsequently the photocatalytic activity. From the thermodynamic point of view, however, the surface energy of an individual nanosheet is quite high with two main exposed planes, and thus they tend to aggregate to the surface planes to decrease the surface energy by reducing exposed areas. Accordingly, the efficient synthesis of nanosheet photocatalysts with high photocatalytic activity still remains a challenge.

Herein, hexagonal ZnIn₂S₄ and cubic ZnIn₂S₄ nanosheets were selectively synthesized. Both hexagonal and cubic ZnIn₂S₄ have been reported to show photocatalytic activity for water splitting under visible light.^27–29 In this work, novel investigation of ZnIn₂S₄ for the photocatalytic reduction of CO₂ has been performed in pure methanol, which acts as absorbent for concentrating CO₂, sacrificial reagent for balancing the hole on the valence band and reactant for esterifying the formic acid producing on the conduction band. It was found that the morphology and crystal phase had a great influence on the photocatalytic activity and stability of ZnIn₂S₄ for reducing CO₂. Furthermore, first-principles density functional calculations were performed to better understand the influence of crystal structures on the electronic structures and photocatalytic properties.

Experimental section

Synthesis

All chemicals are analytical grade and all the reagents were used as received without further purification.

In the synthesis of hexagonal ZnIn₂S₄ nanosheets, Zn(NO₃)₂·6H₂O (0.150 g), In(NO₃)₃·4.5H₂O (0.391 g) and thiourea (TU, 0.308 g) were dissolved in 40 mL of deionized water and transferred to a 70 mL Teflon liner. The Teflon liner was sealed in the stainless steel autoclave and heated at 180 °C for 18 h. After being cooled to room temperature, the yellow precipitate was collected by centrifugation and washed with ethanol and the distilled water for three times, respectively. And then it was dried at 60 °C to obtain the bulk product. Hexagonal ZnIn₂S₄ nanosheets were fabricated by one-pot exfoliation of the obtained product in IPA via a sonication. 30 mg of as-prepared ZnIn₂S₄ and 15 mL IPA were added into 25 mL flask. The flask was sealed and sonicated for 5 h, and then the dispersions were centrifuged at 2000 rpm for 6 min to remove aggregates. Thus, hexagonal ZnIn₂S₄ nanosheets were obtained.

For the synthesis of cubic ZnIn₂S₄ nanosheets, Zn(NO₃)₂·6H₂O (0.150 g), In(NO₃)₃·4.5H₂O (0.391 g) and thiourea (TU, 0.308 g) were added into a mixed solvent of diethylenetriamine (DETA) and distilled water (18 mL, V_DETA : V_H2O = 4 : 1) to form a homogenous solution under constant strong stirring, and transferred to a 70 mL Teflon liner. The Teflon liner was sealed in the stainless steel autoclave and heated at 180 °C for 18 h. The samples were collected and washed three times with ethanol and water, respectively. Cubic ZnIn₂S₄ nanosheets were synthesized by a hydrothermal method using the above prepared hybrids as precursors.

ZnIn₂S₄ microspheres were prepared by a CA-assisted hydrothermal method.

Characterization

The scanning electron microscopy (SEM) images were taken with a Hitachi S-4800 scanning electron microscope (SEM, 5 kV). Transmission electron microscopy (TEM) and higher-resolution transmission electron microscopy (HRTEM) images were obtained with JEM-2100F system. The X-ray diffraction patterns (XRD) of the products were recorded with Bruker D8 Focus Diffraction System using a Cu Kα source (λ = 0.154178 nm). Surface composition and chemical states were analyzed by XPS (PHA-5400, SPECS, America) with Mg Kα ADES (hν = 1253.6 eV) source at a residual gas pressure of below 10–8 Pa. UV-vis DRS was recorded by Shimadzu UV-2550 spectrophotometer by using BaSO4 as a reference in wavelength of 200–800 nm. Photoluminescence (PL) spectra were measured at room temperature using HORIBA JY Fluorolog-3 Spectrofluorometer. The FTIR spectra were obtained with Nicolet Nexus spectrometer.

Photocatalytic reduction of CO₂

The photocatalytic activities of various ZnIn₂S₄ catalysts were evaluated through photocatalytic reduction of CO₂ in a batch slurry reactor. Initially, 10 mg of catalysts was suspended in 10 mL of methanol. Prior to the irradiation, pure CO₂ was subsequently bubbled through the suspended solution for at least 30 min to purge air and approach saturation in methanol. Thereafter, the reactor was tightly sealed and the reaction solution was stirred continuously by a magnetic stirrer to prevent catalyst sedimentation. The reactor was illuminated by a 250 W high-pressure mercury lamp with an emission wavelength of mainly 365 nm. The temperature of the reaction solution was kept at 25 ± 2 °C by using a thermostatic water bath during experiments. After reaction for 4 h, the suspension was centrifuged and the product in liquid sample was determined by GC-MS of Agilent 5975C and quantified by GC-FID of Agilent 7890A GC equipped with 60 m of HP Wax capillary column. IR spectra of CO₂ in liquid were recorded on line using a ReactIR 15 from Mettler-Toledo AutoChem fitted with a AgX optic fiber and Dicomp (Diamond Composite) insertion probe.

Theoretical calculations

Plane-wave density functional theory (DFT) calculations with the CASTEP program package³⁰ were performed to obtain the band structures, densities of state (DOS), and partial densities of state (PDOS) of hexagonal and cubic ZnIn₂S₄. The core electrons were replaced by ultrasoft pseudopotentials with a plane-wave basis cutoff energy of 400 eV, and the interactions of exchange and correlation were treated within the framework of the local density approximation (LDA). The respective k-point sets of 6 × 6 × 2 and 4 × 4 × 4 were used for hexagonal and cubic ZnIn₂S₄.

Results and discussion

For the preparation of hexagonal ZnIn₂S₄ nanosheets, bulk ZnIn₂S₄ was first prepared by a simple hydrothermal reaction. Fig. S1 (ESI†) displayed the representative scanning electron microscope (SEM) images of the as-synthesized ZnIn₂S₄, showing a typical layered structure. It was found that the bulk products composed of many nonuniform flakes. The ZnIn₂S₄ nanosheet dispersion was obtained through ultrasonication-induced exfoliation of as-prepared bulk ZnIn₂S₄ in isopropanol (IPA), which was regarded as the exclusive way to prepare single-layered compounds.³¹ The morphology of as-obtained ZnIn₂S₄ nanosheets was investigated via SEM and transmission electron microscopy (TEM). As shown in Fig. 1a–c, numerous nanosheets with laminar morphology could be observed. The TEM image (Fig. 1d and e) also confirmed their sheet-like morphological feature. The representative high-resolution TEM image (Fig. 1f) showed clear lattice fringes of 0.32 and 0.41 nm, which matched those of the (102) and (006) plane of hexagonal ZnIn₂S₄. X-ray diffraction (XRD) pattern in Fig. 1g clearly revealed that all the diffraction peaks from the XRD pattern could be indexed to the hexagonal ZnIn₂S₄ (JCPDS, no. 03-065-2023). No peaks attributable to other phases were observed and indicated the formation of pure hexagonal ZnIn₂S₄. The surface valence state and the chemical composition of the hexagonal ZnIn₂S₄ nanosheets were characterized by XPS analysis. Fig. 1h showed the survey XPS spectrum of the hexagonal ZnIn₂S₄, and the Zn2p, In3d, and S2p were examined (see Fig. S3, ESI†). The results indicated that only Zn²⁺, In³⁺, S²⁻ were present in the nanosheets.

Fig. 1 SEM images (a, b and c), TEM (d and e) and HRTEM (f) images, XRD pattern (g) and XPS spectrum (h) of the hexagonal ZnIn₂S₄ nanosheets.

In view of the previous reports,^32,33 the inorganic–organic hybrid nanomaterials could be used as a template for the preparation of sheet-like materials. Here we adopted the inorganic–organic hybrid ZnIn₂S₄–diethylenetriamine (DETA) as the starting materials. Another kind of ZnIn₂S₄ nanosheets were successfully synthesized when DETA molecules were removed by hydrothermal treatment.³³ The SEM and TEM images of as-obtained ZnIn₂S₄ presented in Fig. 2a and b clearly showed their sheet-like morphology. It could be seen that the FTIR spectrum (Fig. 2c) of the as-prepared cubic nanosheets was highly identical to that of hexagonal nanosheets, indicating that the hybrid precursors had been transformed into inorganic materials and the surfactant of cubic nanosheets were removed. The XPS spectrum shown in Fig. S5 (ESI†) further confirmed the composition of the products as ZnIn₂S₄. The XRD pattern showed peaks of 2θ values at 14.4°, 23.7°, 27.8°, 33.7°, 44.2° and 48.4°, which corresponded to the (111), (220), (311), (400), (511) and (440) crystallographic planes of cubic ZnIn₂S₄ (JCPDS, 00-048-1778) (Fig. 2d). As could be seen from the HRTEM image in Fig. S6 (ESI†), the interplanar spacing was measured to be 0.32 nm, which agreed with the (311) plane of cubic ZnIn₂S₄.

Fig. 2 SEM (a) and TEM (b) images of cubic ZnIn₂S₄ nanosheets, FTIR spectra (c) of ZnIn₂S₄–DETA precursor (black), cubic ZnIn₂S₄ (red) and hexagonal ZnIn₂S₄ (blue), and XRD pattern (d) of cubic ZnIn₂S₄.

A comparison of the UV-vis diffuse reflectance spectra of the hexagonal and cubic ZnIn₂S₄ nanosheets were depicted in Fig. 3a. A strong light absorption rise at wavelengths shorter than 550 nm could be observed for both samples, which could be assigned to intrinsic band gap absorption. The steep band edge stemmed from the band gap transition other than impurity level transition.³⁴ Obviously, there was an enhanced absorbance for hexagonal ZnIn₂S₄ nanosheets. The band gaps (E_g) of the hexagonal and cubic ZnIn₂S₄ were determined by plotting (αhν)² as a function of the photon energy (Fig. 3b), with α being the absorption coefficient, h being Planck's constant, and v being the frequency. The E_g values were estimated as 2.34 eV for hexagonal ZnIn₂S₄ and 2.41 eV for cubic ZnIn₂S₄. Compared with cubic ZnIn₂S₄, the band gap for the hexagonal ZnIn₂S₄ had an obvious blue shift. Furthermore, the position of the conduction and valence bands in ZnIn₂S₄ nanosheets was determined by the following equation:

E_VB = X − E_c − 0.5E_g

where E_c is the energy of free electrons on the hydrogen scale (4.5 eV), X is the electronegativity of the semiconductor, and E_g is the band-gap energy of the semiconductor. The edge of the valence band E_VB of hexagonal and cubic ZnIn₂S₄ were determined to be 1.56 V and 1.53 V versus NHE, and the conduction band E_CB were estimated to be −0.85 V and −0.81 V, respectively.

Fig. 3 UV-vis spectra (a), (αhv)² vs. hv curves (b) of the hexagonal and cubic ZnIn₂S₄ nanosheets.

The photoluminescence (PL) spectroscopy is often employed to study the efficiency of charge carrier trapping, immigration, transfer and separation, and the recombination rate of the photogenerated electron–hole pairs in the semiconductors. Fig. 4 showed the PL spectra of the hexagonal and cubic ZnIn₂S₄ nanosheets in the wavelength range of 500–630 nm with excitation wavelength of 430 nm. The two samples presented similar PL spectrum and the main emission peaks at about 560 nm was attributed to the emission of band-to-band transition with the energy of light approximately equal to the band gap energy of ZnIn₂S₄. The lower fluorescence emission intensity of hexagonal ZnIn₂S₄ nanosheets indicated that the electron–hole recombination rate of hexagonal ZnIn₂S₄ was lower than that of cubic ZnIn₂S₄.

Fig. 4 PL spectra of the cubic (a) and hexagonal (b) ZnIn₂S₄ nanosheets.

The experiments of photocatalytic reduction of CO₂ were carried out in methanol under ambient conditions using a 250 W high-pressure mercury lamp with the main radiation peak at about 365 nm, and the predominant reaction product was methyl formate (MF). For comparison, the photocatalytic performance of ZnIn₂S₄ microspheres prepared by a CA-assisted hydrothermal method (see Fig. S7, ESI†) was also evaluated. As shown in Fig. 5, ZnIn₂S₄ microspheres presented lower photocatalytic activity for MF production although they had the highest BET surface area (Table S1†). In contrast, the activities of ZnIn₂S₄ nanosheets were remarkably enhanced, and hexagonal ZnIn₂S₄ exhibited higher activity (762.36 μmol g⁻¹) for MF production than cubic ZnIn₂S₄ (629.62 μmol g⁻¹). The improved photocatalytic activities of ZnIn₂S₄ nanosheets might be the result of their unique morphological and structural features which facilitate the electron transport to the catalyst surface.³⁵ The sheet-like structures could also supply large exposed surface areas for harvesting light and converting the reactants, while most the internal surfaces of ZnIn₂S₄ microspheres could hardly be illuminated and excited. Additionally, the large exposed surface could usually offer high density of photocatalytic active sites. The photocatalytic activity of hexagonal ZnIn₂S₄ was better than that of cubic ZnIn₂S₄ under light irradiation, which was strongly related to the crystallographic structure and band structure. Moreover, the durability of the hexagonal and cubic ZnIn₂S₄ as the photocatalysts was measured. As shown in Fig. S8,† the hexagonal ZnIn₂S₄ still maintained most of its intrinsic photocatalytic activity after three runs. However, the photocatalytic activity of cubic ZnIn₂S₄ decreased obviously during the 3rd run.

Fig. 5 Comparison of the photocatalytic MF production activity of different ZnIn₂S₄ photocatalysts: microspheres, cubic and hexagonal nanosheets.

Control experiments demonstrated that there was no detectable MF generation without methanol, photocatalysts or light irradiation. In order to confirm the MF formation through CO₂ reduction, instead of photo-oxidation of methanol, online ATR-FTIR spectroscopy was applied to monitor the CO₂ content of the reaction solution. Fig. S9† showed the relative concentration of CO₂ (the peak height at 2343 cm⁻¹) as a function of reaction time under light irradiation. When the hexagonal ZnIn₂S₄ was kept in the dark, no reduction was seen in the relative concentration of CO₂, while CO₂ consumption was markedly observed during light irradiation. In addition, the hexagonal ZnIn₂S₄ consumed more CO₂ than cubic ZnIn₂S₄, consistent with the photocatalytic activity shown in Fig. 5. The above experiments provided clear evidence that CO₂ was being reduced in the reaction. The photoreduction process of CO₂ mainly comprised two simultaneous steps of reduction and oxidation. In this study, methanol was used as reductant and solvent because of its strong reducibility and the high solubility of CO₂. The main product was methyl formate and the formyl group of methyl formate was produced via reduction of CO₂ to formic acid, which then reacted with methanol to produce the ester. The band structure indicated that the photogenerated electrons and holes in the illuminated ZnIn₂S₄ nanosheets could react with adsorbed CO₂ and methanol to produce HCOOH and CH₂OH, as presented in Fig. 6. According to published report,³⁶ CO₂ was almost the entire carbon source for the photochemically produced formic acid, and methanol acting as a reducing agent was oxidized into formaldehyde during the photocatalytic reaction. Thus, methyl formate could be produced through the esterification of formic acid and methanol under UV light irradiation.

Fig. 6 Calculated band positions of the hexagonal and cubic ZnIn₂S₄ nanosheets.

The DFT calculations were carried out to get an insight into the electronic structures of hexagonal and cubic ZnIn₂S₄. As could be seen from Fig. 7a and b, for the two phases, the valence band was dominated by the contributions of Zn3d and In5s5p orbitals, as well as a small fraction of contribution from S3p. The conduction band was predominantly In5s5p and S3p orbitals, with a fringe of contribution from Zn4s4p. The band structures shown in Fig. 7c and d indicated that both of them were typical direct-band-gap semiconductors, which was consistent with the analysis of UV-visible absorption spectra. The band gap of hexagonal and cubic ZnIn₂S₄ was 0.28 and 1.36 eV, while the experimental value was 2.34 and 2.41 eV, respectively. This deviation is due to the limitation of the LDA functional that underestimates the band gaps in semiconductor simulation. In addition to the narrower band gap, the conduction bands of hexagonal ZnIn₂S₄ were slightly more dispersive than those of cubic ZnIn₂S₄, indicating that the photogenerated electrons in hexagonal ZnIn₂S₄ possessed a smaller effective mass and therefore higher migration ability, leading to promoting the photoreduction activity.³⁷

Fig. 7 The DOS of (a) hexagonal ZnIn₂S₄ and (b) cubic ZnIn₂S₄. The calculated band structures of (c) hexagonal ZnIn₂S₄ and (d) cubic ZnIn₂S₄.

Conclusions

In summary, hexagonal ZnIn₂S₄ nanosheets have been synthesized via a simple liquid ultrasonic exfoliation approach, and cubic ZnIn₂S₄ nanosheets through another strategy involving lamellar inorganic–organic hybrid ZnIn₂S₄–DETA precursors. It was revealed that ZnIn₂S₄ could be used as an effective photocatalyst for CO₂ reduction, and both hexagonal and cubic ZnIn₂S₄ nanosheets exhibited high photocatalytic activity due to their large exposed surface area and thin thickness. Besides, hexagonal ZnIn₂S₄ showed higher activity and better stability for MF production. The results from this work illustrate the crucial role of particular crystalline phase of ZnIn₂S₄ with well defined shapes on photocatalytic reduction of CO₂. In addition, the as-prepared ZnIn₂S₄ nanosheets may find their broad applications in the fields of water splitting, photoelectrochemistry, water treatment and dye-sensitized solar cells.

Acknowledgements

The authors appreciate the support from the National Natural Science Foundation of China (NSFC, no. 21176192), the Tianjin natural science foundation (no. 12JCZDJC29400) and Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, no. IRT0936).

Notes and references

1. S. C. Roy, O. K. Varghese, M. Paulose and C. A. Grimes, ACS Nano, 2010, 4, 1259.
2. H. Zhao, L. Liu, J. M. Andino and Y. Li, J. Mater. Chem. A, 2013, 1, 8209.
3. K. Mori, H. Yamashita and M. Anpo, RSC Adv., 2012, 2, 3165.
4. W. N. Wang, W. J. An, B. Ramalingam, S. Mukherjee, D. M. Niedzwiedzki, S. Gangopadhyay and P. Biswas, J. Am. Chem. Soc., 2012, 134, 11276.
5. H. Cheng, B. Huang, Y. Liu, Z. Wang, X. Qin, X. Zhang and Y. Dai, Chem. Commun., 2012, 48, 9729.
6. Y. Zhou, Z. Tian, Z. Zhao, Q. Liu, J. Kou, X. Chen, J. Gao, S. Yan and Z. Zou, ACS Appl. Mater. Interfaces, 2011, 3, 3594.
7. Q. Liu, Y. Zhou, J. Kou, X. Chen, Z. Tian, J. Gao, S. Yan and Z. Zou, J. Am. Chem. Soc., 2010, 132, 14385.
8. Q. Liu, Y. Zhou, Z. Tian, X. Chen, J. Gao and Z. Zou, J. Mater. Chem., 2012, 22, 2033.
9. S. Feng, X. Chen, Y. Zhou, W. Tu, P. Li, H. Li and Z. Zou, Nanoscale, 2014, 6, 1896.
10. U. Diebold, Surf. Sci. Rep., 2003, 48, 53.
11. D. Ravelli, D. Dondi, M. Fagnoni and A. Albini, Chem. Soc. Rev., 2009, 38, 1999.
12. M. D. Hernández-Alonso, F. Fresno, S. Suárez and J. M. Coronado, Energy Environ. Sci., 2009, 2, 1231.
13. H. Tong, S. Ouyang, Y. Bi, N. Umezawa, M. Oshikiri and J. Ye, Adv. Mater., 2012, 24, 229.
14. S. N. Habisreutinge, L. S. Mende and J. K. Stolarczyk, Angew. Chem., Int. Ed., 2013, 52, 2.
15. J. Zhou, G. Tian, Y. Chen, X. Meng, Y. Shi, X. Cao, K. Pan and H. Fu, Chem. Commun., 2013, 49, 2237.
16. K. Maeda and K. Domen, J. Phys. Chem. C, 2007, 111, 7851.
17. Y. Chen, R. Huang, D. Chen, Y. Wang, W. Liu, X. Li and Z. Li, ACS Appl. Mater. Interfaces, 2012, 4, 2273.
18. X. Hu, J. C. Yu, J. Gong and Q. Li, Cryst. Growth Des., 2007, 7, 2444.
19. X. Gou, F. Cheng, Y. Shi, L. Zhang, S. Peng, J. Chen and P. Shen, J. Am. Chem. Soc., 2006, 128, 7222.
20. L. Shi, P. Yin and Y. Dai, Langmuir, 2013, 29, 12818.
21. Z. Chen, D. Li, W. Zhang, C. Chen, W. Li, M. Sun, Y. He and X. Fu, Inorg. Chem., 2008, 47, 9766.
22. F. Fang, L. Chen, Y. B. Chen and L. M. Wu, J. Phys. Chem. C, 2010, 114, 2393.
23. Y. Chen, S. Hu, W. Liu, X. Chen, L. Wu, X. Wang, P. Liu and Z. Li, Dalton Trans., 2011, 40, 2607.
24. N. S. Chaudhari, A. P. Bhirud, R. S. Sonawane, L. K. Nikam, S. S. Warule, V. H. Rane and B. B. Kale, Green Chem., 2011, 13, 2500.
25. B. Chai, T. Peng, P. Zeng, X. Zhang and X. Liu, J. Phys. Chem. C, 2011, 115, 6149.
26. Z. Xu, Y. Li, S. Peng, G. Lu and S. Li, RSC Adv., 2012, 2, 3458.
27. Z. Lei, W. You, M. Liu, G. Zhou, T. Takata, M. Hara, K. Domen and C. Li, Chem. Commun., 2003, 2142.
28. J. Shen, J. Zai, Y. Yuan and X. Qian, Int. J. Hydrogen Energy, 2012, 37, 16986.
29. L. Shang, C. Zhou, T. Bian, H. Yu, L.-Z. Wu, C.-H. Tung and T. Zhang, J. Mater. Chem. A, 2013, 1, 4552.
30. M. D. Segall, P. J. D. Lindan, M. J. Probert, C. J. Pickard, P. J. Hasnip, S. J. Clark and M. C. Payne, J. Phys.: Condens. Matter, 2002, 14, 2717.
31. Y. Sun, Z. Sun, S. Gao, H. Cheng, Q. Liu, J. Piao, T. Yao, C. Wu, S. Hu, S. Wei and Y. Xie, Nat. Commun., 2012, 3, 1057.
32. J. Zhang, J. Yu, Y. Zhang, Q. Li and J. R. Gong, Nano Lett., 2011, 11, 4774.
33. Y. Yu, J. Zhang, X. Wu, W. Zhao and B. Zhang, Angew. Chem., Int. Ed., 2012, 51, 897.
34. A. Kudo, I. Tsuji and H. Kato, Chem. Commun., 2002, 1958.
35. K. Li, A. D. Handoko, M. Khraisheh and J. Tang, Nanoscale, 2014, 6, 9767.
36. K. Sekizawa, K. Maeda, K. Domen, K. Koike and O. Ishitani, J. Am. Chem. Soc., 2013, 135, 4596.
37. P. Li, S. Ouyang, G. Xi, T. Kako and J. Ye, J. Phys. Chem. C, 2012, 116, 7621.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra13191f

---