Spatial Control of Oxygen Vacancy Concentration in Monoclinic WO₃ Photoanodes for Enhanced Solar Water Splitting

Abstract

Oxygen vacancies (OVs) are a mixed blessing for the photoelectrochemical (PEC) water oxidation performance of monoclinic tungsten trioxide (m-WO 3 ) photoanodes. Although it is widely accepted that a moderate concentration of OVs is beneficial for the PEC performance of the m-WO 3 photoanodes, this argument assumes a uniform distribution of OVs throughout the m-WO 3 crystal. In this case, only the overall concentration of OVs needs to be considered. However, the spatial non-uniformity of OV defects in m-WO 3 photoanodes has not been thoroughly examined. In this study, by employing a m-WO 3 nanorod array as a model photoanode, the aim is to show that a higher OV concentration near the surface of m-WO 3 compared to that in the bulk is advantageous for the PEC performances of this material. In addition, a laser-assisted defect control (LADC) process is employed to manipulate the spatial distribution of OVs in the m-WO 3 photoanodes to achieve enhanced PEC performances. Moreover, a one-step laser deposition process is introduced to obtain an ultrathin FeNi oxygen evolution catalyst overlayer on the defect-controlled m-WO 3 photoanodes, further improving PEC performance, photostability, and Faradaic efficiency.

Laser-induced deposition of Ni, Co-doped FeOOH cocatalysts on WO3 photoanodes and elucidating their roles in water oxidation in terms of carrier dynamics

“…Notably, as previously reported by Kong et al , the PEC performance of monoclinic WO 3 may considerably fluctuate depending on its chemical properties, such as the spatial distribution of oxygen vacancies. 21 Therefore, for consistency in comparison, we employed only the WO 3 /bare samples with photocurrent values in the range of 1.7–2.0 mA cm −2 at 1.23 V RHE . Fig.…”

“…An OEC overlayer passivates the WO 3 surface and selectively promotes water oxidation in the Na 2 SO 4 solution, increasing the FE to ∼100% and enhancing photostability. 20,21 Iron oxyhydroxide (FeOOH) is one of the most frequently used OECs for WO 3 photoanodes. 12,20,22,23 FeOOH often cooperates with Ni-based OEC, forming FeOOH/NiOOH heterojunction OEC 24 or FeNi layered double hydroxide (LDH).…”

Hydrogen Spillover-Accelerated Selective Hydrogenation on WO3 with ppm-Level Pd

“…However, the hexagonal phase (PDF #85-2459) was confirmed for the PdHD/WO 3 -350 and PdHD/WO 3 -450 catalysts (Figure c). Considering that the generation of oxygen vacancies on WO 3 would inevitably lead to the reduction of W 6+ to W 5+ ions, − the fraction of W 5+ was selected as the descriptor to display the amount of surface oxygen. For the as-synthesized WO 3 supports, the surface W 5+ fraction was only 2.1% from the X-ray photoelectron spectroscopy (XPS) analysis of the W 4f peak (Figures d and S3), revealing their low concentration of oxygen vacancies.…”

Tandem cells for unbiased photoelectrochemical water splitting

“…To further improve the photocurrent for the photoanode, a semiconductor with an intrinsic narrower band gap than TiO 2 was examined. 97,98 The WO 3 is a n-type semiconductor with a band gap of 2.5–2.8 eV, which can utilise ∼12% of sunlight in theory. 99 Since the first report in 1976 by Hodes et al , WO 3 has attracted extensive attention because of its relatively narrow band gap, high chemical stability in aqueous solutions especially in acid solution, simple fabrication methods, and a long minority carrier diffusion length (150 nm).…”