In situ synthesis of CdS/CdWO4/WO3 heterojunction films with enhanced photoelectrochemical properties

doi:10.1016/j.jpowsour.2016.06.079

Journal of Power Sources

Volume 325, 1 September 2016, Pages 591-597

https://doi.org/10.1016/j.jpowsour.2016.06.079 Get rights and content

Highlights

•
CdS/CdWO₄/WO₃ heterojunction photoanode was fabricated through a facile method.
•
The heterojunction films exhibit increased visible light absorption.
•
The heterojunction films exhibit high photoelectrochemical activity.
•
The in situ formation of heterojunction optimizes the charge transfer.

Abstract

CdS/CdWO₄/WO₃ heterojunction films on fluorine-doped tin oxide (FTO) substrates are for the first time prepared as an efficient photoanode for photoelectrochemical (PEC) hydrogen generation by an in situ conversion process. The samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet visible spectrometry (UV–vis) and X-ray photoelectron spectroscopy (XPS). The CdS hollow spheres (∼80 nm) sensitized WO₃ plate film with a CdWO₄ buffer-layer exhibits increased visible light absorption and a significantly improved photoelectrochemical performance. The photocurrent density at 0 V (vs. Ag/AgCl) of the CdS/CdWO₄/WO₃ anode is ∼3 times higher than that of the CdWO₄/WO₃ anode, and ∼9 times higher than that of pure WO₃ under illumination. The highest incident-photon-to-current-efficiency (IPCE) value increased from 16% to 63% when the ternary heterojunction was formed. This study demonstrates that the synthesis of ternary composite photocatalysts by the in situ conversion process may be a promising approach to achieve high photoelectric conversion efficiency.

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).

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In situ synthesis of CdS/CdWO4/WO3 heterojunction films with enhanced photoelectrochemical properties

Highlights

Abstract

Introduction

Section snippets

Synthesis of WO3 plate-like films

Synthesis of CdS/CdWO4/WO3 heterojunction films

Characterization of the synthesized photoanodes

Conclusion

Acknowledgements

J. Power Sources

J. Solid State Chem.

Electrochim. Acta

Appl. Surf. Sci.

Int. J. Hydrogen Energ

Electrochim. Acta

J. Hazard Mater

Mater Lett.

Int. J. Hydrogen Energ

Sol. Energ Mat. Sol. C

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.

In situ synthesis of CdS/CdWO₄/WO₃ heterojunction films with enhanced photoelectrochemical properties

Synthesis of WO₃ plate-like films

Synthesis of CdS/CdWO₄/WO₃ heterojunction films