Surface oxygen vacancies on WO₃ nanoplate arrays induced by Ar plasma treatment for efficient photoelectrochemical water oxidation

Highlights

• Ar plasma treatment introduces surface oxygen vacancies (O_V) and W⁵⁺/W⁴⁺ defects on WO₃ nanoplate arrays.

• Surface defects extend light absorption and promote e⁻/h⁺ generation and transfer.

• Surface O_V and W⁵⁺/W⁴⁺ defects endow WO₃ nanoplate arrays with improved photoelectrochemical performance.

Abstract

The construction of surface defects on semiconductor oxide photoanodes has been identified as a promising route for attaining improved photoelectrochemical (PEC) performance. Here, Ar plasma treatment has been used to etch the surface of hydrothermally synthesized WO₃ nanoplate array films, which generated surface oxygen vacancies (O_vs) with the concomitant reduction of surface W⁶⁺ to W⁵⁺/W⁴⁺. As a result, the visible-light absorption and photogenerated charge separation of WO₃ was enhanced, as evidenced by X-ray photoelectron spectroscopy (XPS), UV/Vis diffuse-reflectance spectroscopy (DRS), and photoluminescence (PL) spectroscopy. A WO₃ photoanode subjected to Ar plasma treatment for 120 s exhibited a photocurrent density of 1.32 mA cm⁻² and an O₂ evolution rate of 1.0 μmol cm⁻²·min⁻¹ under simulated solar light irradiation at a bias potential of 1.23 V vs. reversible hydrogen electrode, approximately 1.7- and 5-times higher, respectively, than those of a pristine sample. This improvement is mainly attributed to the facilitative effect of surface oxygen vacancy defects, which promote electron–hole generation and separation rates and decrease the interfacial charge-transfer resistance between the WO₃ photoanode and electrolyte. Our Ar plasma surface modification also has potential for developing other photoanodes for efficient PEC performance.

Introduction

The production of H₂ and O₂ by PEC water splitting for solar energy harvesting and storage has been extensively investigated by scientific and technological researchers [[1], [2], [3]]. The development of economical and practical PEC devices requires efficient semiconductor photoanodes with adequate sunlight absorption, high charge separation efficiencies, and good surface reactivity [[4], [5], [6], [7], [8]]. Among the widely investigated photoanode materials, WO₃ is a promising n-type semiconductor due to its appropriate band gap (2.8 eV) for the absorption of 12% of solar light, excellent stability, and favorable valence band (VB) edge position [9,10]. However, a WO₃ photoanode is hampered by low hole mobility (ca. 10 cm²/(V·S)) and a short hole diffusion length (L_P ≈ 150 nm) [11]. Moreover, the absorption coefficient α of WO₃ is around 10⁴ cm⁻¹, i.e., the optical penetration depth 1/α is several micrometers [12]. Hence, PEC water oxidation at a WO₃ photoanode is curtailed by limited sunlight absorption, unsatisfactory photogenerated charge separation and transference, and annoying back-reaction. Extensive efforts have been made to improve PEC water oxidation activity at a WO₃ photoanode, such as by morphological control [13], doping [14], the introduction of heterojunctions [15], and modification with co-catalysts [16,17]. Among these approaches, the introduction of defects in a WO₃ photoanode is a promising strategy for increasing the absorption of sunlight and promoting charge separation and transfer across the electrode/electrolyte interface, thereby achieving efficient PEC water oxidation activity.

Oxygen vacancy defects in oxide semiconductors are pervasive, and have a significant effect on the optoelectronic properties of materials and devices [[18], [19], [20], [21], [22], [23], [24]]. The introduction of oxygen vacancy defects has emerged as an important strategy for modulating the electronic structures, conductivity, and catalytic performance of oxide semiconductors [25]. Oxygen vacancies improve the electron delocalization of semiconductor materials, and hence increase their conductivity [26]. Moreover, oxygen vacancies have been introduced into WO₃ photoanodes as shallow donors below the conduction band minimum (CBM), which served to raise the Fermi level to the CB, improving the optical absorption and electronic conductivity and facilitating the formation of more active sites, leading to significantly enhanced PEC water oxidation activity [27, 28]. However, bulk oxygen vacancy defects also act as charge recombination centers and suppress delivery of the photogenerated VB holes to the surface of WO₃ photoanodes. Park et al. reported that conventional vacancies in the bulk region also diminish electron mobility to some extent because of their state localization. Therefore, modulating the functions of vacancies using a finely controlled synthetic method is important for achieving charge polarization in a PEC photoanode [29]. In this context, the introduction of surface oxygen vacancies at an appropriate density may be more suitable for extending the photoresponse and adjusting the surface electronic structure and surface properties of WO₃ photoanodes, while avoiding undesired bulk recombination. Several studies have focused on the introduction of oxygen vacancy defects into oxide semiconductors for the improvement of PEC and photocatalytic performance. For instance, Ma et al. reported that Ar-plasma-etched Co3O4 nanosheets with surface oxygen vacancies and Co2+ exhibited tenfold higher PEC photocurrent density than that of pristine Co3O4 [30]. Rahimnejad et al. incorporated surface oxygen vacancy defects into WO3 nanoparticles by a radiofrequency hydrogen plasma technique, which narrowed the band gap of WO3 and enhanced the photocatalytic activity [31]. Liu et al. introduced both surface and bulk oxygen vacancy defects into a WO3-x photocatalyst by vacuum heat treatment, and concluded that the former were beneficial for photocatalytic O2 evolution by promoting light absorption and photogenerated charge separation, while the latter served as recombination centers of the carriers [32]. Presently, WO₃ photoanodes are often modified with both bulk and surface oxygen vacancy defects for improvement of their PEC water oxidation performance [33]. Research on introducing only surface oxygen vacancies on WO₃ photoanodes for efficient PEC performance with comprehensive analysis has hitherto been lacking.

Herein, we report the introduction of surface oxygen vacancy defects, and concomitantly W⁵⁺/W⁴⁺, into hydrothermally prepared WO₃ nanoplate array films by Ar plasma treatment. The surface oxygen vacancy defects and W⁵⁺/W⁴⁺ were increasingly generated by prolonging the Ar plasma treatment time, leading to enhanced sunlight absorption and photogenerated charge separation in WO₃, and elevation of the Fermi level. The PEC activity for water oxidation of the samples has been assessed. The results showed that the treated WO₃ photoanode with an optimal density of surface oxygen vacancy defects and W⁵⁺/W⁴⁺ gave 1.7- and 5-fold higher photocurrent density and O₂ evolution rate in a PEC process than those of a pristine WO₃ photoanode. This may be rationalized in terms of the surface defects increasing the photogenerated carrier density and reducing the interfacial charge-transfer resistance between the WO₃ photoanode and the electrolyte.