Enhanced photoelectrochemical performance of plate-like WO3 induced by surface oxygen vacancies
Introduction
Since the pioneering work by Fujishima and Honda in 1972 [1], extensive efforts have been made to investigate for the photoelectrolysis of water in to H2 and O2 [2], [3], [4]. Among semiconductors, tungsten oxide (WO3) has attracted great interest because of its high stability in acidic conditions and relatively small band gap (2.7 eV) [5], [6], [7], [8]. However, various charge recombination pathways have been identified as a key factor limiting the performance of practical devices.
Enormous efforts have been devoted to improve the WO3 performance in the visible region, including nanotechnology [9], [10], [11], surface modification [12], [13], [14], [15], doping [16], [17], [18], and so on. For example, one-dimensional (1D) and two-dimensional (2D) nanostructures of WO3 (e.g. nanorods [6], [19], [20], nanowire [11] and nanoplate [10], [21], [22], [23], [24]) have the superior ability of promoting the transport and separation of photogenerated charge carriers. Photoelectrodes of WO3 nanoplate, nanowires, nanobelts and nanorod have been fabricated and exhibited enhanced photoelectrochemical (PEC) activity, demonstrating the great potential of such WO3 nanostructures [10], [19], [25].
Furthermore, it is commonly believed that oxygen vacancies play a key role in the properties of WO3 [26]. Substoichiometric WO3 − x is formed by creating oxygen vacancies in WO3, which is thermodynamically stable at room temperature [27]. Desai et al. found that the WO3 − x could be used as a passive layer to protect tungsten metal from further dissolution in chemical mechanical polishing [28]. Li et al. demonstrated that the donor density of WO3 − x was increased three orders of magnitude by introduction of oxygen vacancy, resulting in an order of magnitude enhancement in photocurrent density [7]. However, they got substoichiometric WO3 − x by annealing pristine WO3 in hydrogen atmosphere at 350–450 °C.
In this communication, we reported a simple method of introduction of oxygen vacancies into the surface of the plate-like WO3 films. Substoichiometric WO3 − x was obtained by hydrogen peroxide (H2O2) treatment, which offers an advantage of low temperature processing. The oxygen vacancies formed in the presence of surface of WO3 demonstrated enhanced PEC performance over pristine WO3 platelet.
Section snippets
Experiment
The plate-like WO3 films were prepared on a fluorine-doped tin oxide (FTO) glass slide using our reported hydrothermal method [10]. Then the films were calcined in air at 450 °C for 1 h with a heating rate of 2 °C/min. The as-prepared WO3 samples were immersed in a 20% H2O2 solution. The solution was kept at 20 °C for a certain time, and then, the films were cleaned with de-ionized water to remove the residual H2O2 solution and dried under vacuum at 80 °C. Scanning electron microscopy (SEM) images
Results and discussion
Hydrothermal method is a novel, low-cost and low-temperature approach to realize the controllable synthesis of vertically oriented WO3 plate-like arrays grown directly on a transparent FTO conductive glass [10], [11]. A uniform film consisting of plate-like nanostructures was obtained after annealing (Fig. 1a). The cross-sectional view image (inset in Fig. 1a) shows that the flakes are tetragonal in shape with height of ca. 1.4 μm and grown vertically on the FTO conductive glass. Fig. 1b–e show
Conclusion
Plate-like WO3 films with surface oxygen vacancies were prepared via a facile economic H2O2 treatment method. The concentration of oxygen vacancies could be controlled by tuning the treatment time in H2O2 solution. The H2O2 treated WO3 photoanodes exhibit an enhanced photoelectrochemical water splitting performance, which is attributed to high separation efficiency of photoinduced electron–hole pairs. The above results show that introducing appropriate surface oxygen vacancies could improve
Conflict of interest
The author's declare that there are no conflicts of interest.
Acknowledgments
We acknowledge the financial support from the NSFC (No. 51304253), and the Fundamental Research Funds for the Central Universities of Central South University (2015zzts021).
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2023, Chemical Engineering JournalCitation Excerpt :The O 1s profiles of WO3, Ov-WO3 and Ov-WO3@CB[5] can be primarily fitted into two peaks (Fig. 2B). The dominant band at 530.4 eV is assigned to the lattice oxygen of WO3 [58], while the extra shoulder band at 531.4 eV is probably attributed to surface adsorbed oxygen species or –OH, particularly adjacent to oxygen defects [64,65]. The adsorption rate of O was increased to 26.7 % possibly on account of rich oxygen vacancies on the Ov-WO3 surface, while that of WO3 was 16.6 %.