Elsevier

Applied Surface Science

Volume 361, 15 January 2016, Pages 133-140
Applied Surface Science

Enhancement of the photoelectrochemical performance of CuWO4 films for water splitting by hydrogen treatment

https://doi.org/10.1016/j.apsusc.2015.11.129Get rights and content

Highlights

  • Hydrogen-treated CuWO4 film was synthesized on FTO substrates.

  • Hydroxyl groups and oxygen vacancies were confirmed for the H-treated CuWO4.

  • The donor density of H-treated CuWO4 was improved one order of magnitude.

  • The photocurrent density for H-treated CuWO4 tripled compared with pristine CuWO4.

Abstract

CuWO4 films with feature particle sizes of 100–200 nm and thickness up to 700–900 nm on fluorine-doped tin oxide (FTO) substrates were prepared by hydrothermal synthesis. The prepared CuWO4 films were treated in hydrogen atmosphere at constant temperature 300 °C for different annealing time and used for photoelectrochemical (PEC) water oxidation. Compared with pristine CuWO4 film, the optimized hydrogen-treated CuWO4 film presented three times enhanced photocurrent density of 0.75 mA/cm2 at 1.2 V vs. Ag/AgCl in 0.1 M Na2SO4 solution under the illumination. The donor density of hydrogenated CuWO4 film determined by Mott–Schottky analysis was improved one order of magnitude as well. The enhanced photoelectrochemical activity could be attributed to the formation of oxygen vacancies after hydrogen treatment, which facilitated the charge transport and collection.

Introduction

Using semiconductor photoelectrodes for Photoelectrochemical (PEC) water splitting has attracted extensive attention in the last few years [1], [2]. As far as we know, the most challenge of PEC water splitting technology is stimulated around the development of new photoanode materials with high efficiency and good stability. For the past few decades, some metal oxides have been the primary choice of photoelectrodes for PEC water oxidation because of their low cost and high chemical stability. However, these metal oxides have not been satisfactory because they have narrow spectral response range and high charge carrier recombination [3]. For instance, TiO2 suffers from low photoelectric efficiency that only absorbs strongly in UV light due to its wide bandgap (3.0–3.2 eV) [4]. Fe2O3 has a suitable bandgap of 2.2 eV, however, its low charge-carrier mobility and a short hole-diffusion length limit the photoelectrochemical activity [5], [6]. Similarly, for WO3, its band gap (2.7 eV) only allows absorbing a small part of visible spectrum and its chemical property is unstable under neutral pH electrolytes [7], [8], [9], [10]. The essential reason which are constrained these metal oxides is their valence bands are typically composed of O 2p character and lies below the water oxidation potential [11]. Hence, more and more attention focus on the ternary metal oxides, such as CuWO4 [12], ZnWO4 [13], BiVO4 [14], [15]. For ternary metal oxides, metal based d orbitals and O 2p orbitals co-contribute to the valence band maxima. As a result, ternary metal oxides could fine-tune the position of valence and conduction bands as well as the band gap energy.

At present, an attractive and promising ternary metal oxide for photoanode is n-type CuWO4 with a favorable band gap of 2.2–2.4 eV. Because of the hybridized Cu-3d and O-2p orbitals contributing to the valence band maxima, CuWO4 exhibits good bandgap property for water splitting in neutral pH electrolyte [15]. Nevertheless, no significant improvement in the efficiency of CuWO4 as a photocatalyst for water oxidation is observed. The possible reason is that the midgap states introduced by the empty Cu (3d x2−y2) would be detrimental to the carrier mobility [16], [17]. Therefore, it is necessary to improve the transport properties and electron transfer kinetics of CuWO4, either by modifying structure [15], [18], [19], [20], doping [21], [22], or fabrication of a heterojunction [23], [24], [25]. Recently, hydrogen treatment is demonstrated that can significantly enhance the PEC performance of TiO2 [26], WO3 [27] and BiVO4 [11] photoanodes as a result of increased donor density and electrical conductivity by introducing a moderate amount of oxygen vacancies. Therefore, hydrogen treatment is expected to have a similar effect on CuWO4.

In this work, we successfully synthesized densely packed CuWO4 thin film on fluorine-doped tin oxide (FTO) substrates through a simple hydrothermal synthesis route. And then, the prepared CuWO4 film is treated in hydrogen atmosphere at a constant temperature for different time. The photoelectrocatalytic activity of hydrogen-treated CuWO4 (denoted as H-treated CuWO4) thin film for PEC water splitting is explored. Indeed, we find that hydrogen treatment is a simple and general strategy that can considerably enhance the PEC performance of CuWO4 by improving their donor density.

Section snippets

Synthesis of CuWO4 films

Fluorine doped tin oxide (FTO, TEC 8)-coated glass substrates were first cleaned by sonication in toluene, acetone and ethanol, respectively, subsequently rinsed with deionized (DI) water, and finally dried in an air stream. FTO substrates were seeded with a thin layer of CuWO4 before growing the films. The CuWO4 seed solution was prepared as follows, 0.2 mmol H2N10O41W12·xH2O, 0.2 mmol CuCl2·2H2O, 0.5 ml H2O2 (30%) solution and 10 ml deionized water were mixed together and stirred until it became

Structural and compositional characterization

When CuWO4 nanostructure was fabricated on FTO through a hydrothermal synthesis process, a uniform and densely film with thickness of about 700–900 nm was obtained, as shown in the cross-section SEM image of Fig. 1e. The top-view image shows a highly densely material consisting of aggregated nanoparticles ranging from 100–200 nm grew on the FTO substrate (Fig. 1a and b). The SEM image of H-treated CuWO4 (30 min) film was chosen as an example shown in Fig. 1c and 1d. The hydrogenation did not

Conclusions

In summary, we successfully prepared hydrogenated CuWO4 film on FTO substrates by annealing the film in hydrogen atmosphere. The hydrogen treatment represents a novel facile method to substantially enhance the photoelectrocatalytic activity of CuWO4 for PEC water oxidation. In addition, we studied the optimum conditions for the H-treated CuWO4 film and found that hydrogen treatment time of 30 min was the optimal time. Based on this treatment time, H-treated CuWO4 electrode achieved a maximum

Acknowledgements

This research was supported by the sharing fund of Chongqing University's large-scale equipment and Chongqing University Postgraduates’ Innovation Project (2015).

References (51)

  • S. Penner et al.

    The structure and composition of oxidized and reduced tungsten oxide thin films

    Thin Solid Films

    (2008)
  • B.A. Deangelis et al.

    X-ray photoelectron spectroscopy study of nonstoichiometric tungsten oxides

    J. Solid State Chem.

    (1977)
  • S.C. Moulzolf et al.

    Stoichiometry and microstructure effects on tungsten oxide chemiresistive films

    Sens. Actuators B

    (2001)
  • T. Robert et al.

    Characterization of oxygen species adsorbed on copper and nickel oxides by X-ray photoelectron spectroscopy

    Surf. Sci.

    (1972)
  • A. Fujishima et al.

    Electrochemical photolysis of water at a semiconductor electrode

    Nature

    (1972)
  • A. Kudo et al.

    Heterogeneous photocatalyst materials for water splitting

    Chem. Soc. Rev.

    (2009)
  • H.C. He et al.

    Nanostructured Bi2S3/WO3 heterojunction films exhibiting enhanced photoelectrochemical performance

    J. Mater. Chem. A

    (2013)
  • J.H. Kennedy et al.

    Photooxidation of water at α-Fe2O3 electrodes

    J. Electrochem. Soc.

    (1978)
  • S. Kerisit et al.

    Kinetic Monte Carlo model of charge transport in hematite (α-Fe2O3)

    J. Chem. Phys

    (2007)
  • R. Liu et al.

    Water splitting by tungsten oxide prepared by atomic layer deposition and decorated with an oxygen-evolving catalyst

    Angew. Chem.

    (2011)
  • G.M. Wang et al.

    Computational and photoelectrochemical study of hydrogenated bismuth vanadate

    J. Phys. Chem. C

    (2013)
  • X.J. Bai et al.

    Visible photocatalytic activity enhancement of ZnWO4 by graphene hybridization

    ACS Catal.

    (2012)
  • W.J. Luo et al.

    Effects of surface electrochemical pretreatment on the photoelectrochemical performance of Mo-Doped BiVO4

    J. Phys. Chem. C

    (2012)
  • J.E. Yourey et al.

    Electrochemical deposition and photoelectron- chemistry of CuWO4, a promising photoanode for water oxidation

    J. Mater. Chem.

    (2011)
  • J.E. Yourey et al.

    Water oxidation on a CuWO4−WO3 composite electrode in the presence of [Fe(CN)6]3−: toward solar Z-scheme water splitting at zero bias

    J. Phys. Chem. C

    (2012)
  • Cited by (102)

    View all citing articles on Scopus
    View full text