In situ synthesis of CdS/CdWO4/WO3 heterojunction films with enhanced photoelectrochemical properties
Introduction
To solve the problems of the global energy crisis and environmental pollution, development and utilization of clean, renewable energy resources has become very urgent. Hydrogen production by solar water splitting is a promising technology to achieve a resource-conserving and environmentally-friendly society [1], [2]. Since the first report on photoelectrochemical (PEC) water splitting using TiO2 photocatalysts in 1972 [3], many studies have been conducted, investigating materials such as ZnO [4], Fe2O3 [5], Cu2O and CuO [6] to find suitable photoelectrodes for converting solar energy into hydrogen. Among the various transition metal oxides, tungsten trioxide (WO3) has been widely used as a promising photocatalyst due to its suitable band gap of 2.6 eV [7], non-toxicity, good stability in acidic solution [8] and a sufficiently positive valence band edge [9]. Despite this, its solar-to-hydrogen (STH) efficiency for photoelectrochemical water splitting is limited by its poor utilization of visible light [10] and fast photo-generated charge recombination [11]. Therefore, many significant efforts have been made to enhance the PEC performance of WO3, including elemental-doping [11], [12], [13], [14], surface modification [15] and construction of hybrid heterojunction structures [16], [17], [18]. Additionally, coupling with narrow-gap semiconductors in materials such as Fe2O3/WO3 [19], Cu2O/WO3 [20], BiVO4/WO3 [18] and CdS/WO3 [21] has been shown to be a promising approach to enhancing light absorption.
Among the various narrow bandgap semiconductor materials, metal sulfides have been widely used as photosensitizers for various wide bandgap semiconductor photoanodes, such as CdS [22], CuS [23], Bi2S3 [24] and Ag2S [25]. Special attention has been paid to CdS due to its appropriate bandgap (Eg ≈ 2.4 eV) and suitable conduction band position [26]. CdS sensitized wide bandgap semiconductor electrodes exhibited enhanced visible light absorption and improved PEC performance, with CdS/TiO2 [27] and CdS/ZnO [28] as the main examples. However, little work has been done on the WO3 films. This is mainly because of the intrinsic instability of WO3 films in an alkaline solution. Nevertheless, the alkaline Na2S solution is usually used as the source for element S in the preparation of CdS. Furthermore, an alkaline electrolyte containing 0.25 M Na2S and 0.35 M Na2SO3 aqueous solution is required to maintain the stability of CdS for PEC water splitting [29]. Therefore, the films stability in an alkaline environment should be improved. Liu et al. prepared a CdS quantum dots sensitized WO3 photoelectrode with a TiO2 buffer-layer, which showed high chemical stability in a strongly alkaline electrolyte solution [30]. The CdS modification of the TiO2/WO3 electrode resulted in an increase in the visible light absorption and a 10 times higher photocurrent density. Moreover, Aslam et al. synthesized WO3/CdWO4 hybrid photocatalyst, which exhibited much higher photocatalytic efficiency than that of either WO3 or CdWO4 under visible light irradiation [31]. Wang et al. synthesized CdS modified CdWO4 in situ, which exhibited ca. 3.4 and 34 fold higher PEC H2 evolution rates than that of CdS or CdWO4 alone under the same conditions [32].
Following the approach of the studies mentioned above, we first synthesized the CdS/CdWO4/WO3 heterojunction photoelectrode in situ by replacing the TiO2 buffer-layer with CdWO4. Similar to TiO2, CdWO4 plays important roles in the PEC performance. On one hand, it improves WO3 film stability in an alkaline environment; on the other hand, it inhibits the combination between the electrons in the conduction band (CB) of WO3 and the holes in the valence band (VB) of CdS at the interface of the composite. The as-prepared CdS/CdWO4/WO3 film shows enhanced PEC performance. To the best of our knowledge, this work is the first report of ternary composite photocatalyst synthesis by the in situ conversion process.
Section snippets
Synthesis of WO3 plate-like films
The WO3 plate-like films were prepared using a typical hydrothermal synthesis process according to our previous report [33]. The solution for hydrothermal growth was prepared by mixing 60 ml of Na2WO4 solution (0.7 mM), 6 ml of HCl (3 M) and 0.2 g of ammonium oxalate ((NH4)2C2O4) as the structure-directing agent. The hydrothermal synthesis process was carried out at 140 °C for 6 h. Then, the films were taken out and dried in the oven.
Synthesis of CdS/CdWO4/WO3 heterojunction films
First, a dipping-annealing process was used to synthesize the
Characterization of the synthesized photoanodes
Fig. 1 shows the XRD patterns of the as-prepared films. It was observed that WO3 was indexed as monoclinic tungsten oxide (JCPDS data card no. 43-1035). The diffraction peaks at 15.1°, 17.6°, 28.9°, 29.6° and 35.4° were assigned to the (010), (100), (−111), (111) and (002) planes of monoclinic CdWO4 (JCPDS data card no. 14-0676). After hydrothermal sulfidation, the diffraction peaks of CdWO4 became weaker, but new diffraction peaks at 26.5° and 43.9° appeared; theses peaks were assigned to the
Conclusion
In this study, for the first time, CdS/CdWO4/WO3 heterojunction films were reported as an efficient photoanode for solar H2 generation. The CdS hollow spheres and CdWO4 buffer-layer are formed on WO3 plates through the in situ conversion method. CdS acted as the photosensitizer and the CdWO4 buffer-layer increases the efficiency of photo-generated carrier separation. The as-prepared CdS/CdWO4/WO3 films show enhanced PEC performance. Based on EIS and Mott-Schottky experiments, it was shown that
Acknowledgements
This study was supported by the National High Technology Research and Development Program of China (2011AA050528), the National Nature Science Foundation of China (51304253).
References (54)
- et al.
J. Power Sources
(2015) - et al.
J. Solid State Chem.
(2008) - et al.
Electrochim. Acta
(2015) - et al.
Appl. Surf. Sci.
(2012) - et al.
Int. J. Hydrogen Energ
(2015) - et al.
Electrochim. Acta
(2013) - et al.
J. Hazard Mater
(2011) - et al.
Mater Lett.
(2014) - et al.
Int. J. Hydrogen Energ
(2012) - et al.
Sol. Energ Mat. Sol. C
(2007)
Int. J. Hydrogen Energ
Appl. Surf. Sci.
Sol. Energ Mat. Sol. C
Mater Res. Bull.
Appl. Surf. Sci.
Chem. Soc. Rev.
J. Phys. Chem. Lett.
Nature
J. Mater. Chem. A
Phys. Chem. Chem. Phys.
ACS Appl. Mater Interf.
Chem. Commun.
J. Phys. Chem. C
J. Phys. Chem. Lett.
Rsc Adv.
Appl. Surf. Sci.
Appl. Phys. Lett.
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