Modifying the Electron-Trapping Process at the BiVO₄ Surface States via the TiO₂ Overlayer for Enhanced Water Oxidation

Abstract

BiVO4 is one of the most promising photoanode candidates to achieve high-efficiency water splitting. However, overwhelming charge recombination at the interface limits its water oxidation activity. In this study, we show that the water oxidation activity of the BiVO4 photoanode is significantly boosted by the TiO2 overlayer prepared by atomic layer deposition. With a TiO2 overlayer of an optimized thickness, the photocurrent at 1.23 V RHE increased from 0.64 to 1.1 mA·cm–2 under front illumination corresponding to 72% enhancement. We attribute this substantial improvement to enhanced charge separation and suppression of surface recombination due to surface-state passivation. We provide direct evidence via transient photocurrent measurements that the TiO2 overlayer significantly decreases the photogenerated electron-trapping process at the BiVO4 surface. Electron-trapping passivation leads to enhanced electron photoconductivity, which results in higher photocurrent enhancement under front illumination rather than back illumination. This feature can be particularly useful for wireless tandem devices for water splitting as the higher band gap photoanodes are typically utilized with front illumination in such configurations. Even though the electron-trapping process is eliminated completely at higher TiO2 overlayer thicknesses, the charge-transfer resistance at the surface also increases significantly, resulting in a diminished photocurrent. We demonstrate that the ultrathin TiO2 overlayer can be used to fine tune the surface properties of BiVO4 and may be used for similar purposes for other photoelectrode systems and other photoelectrocatalytic reactions.

WO3 Photoanode with Predominant Exposure of {202} Facets for Enhanced Selective Oxidation of Glycerol to Glyceraldehyde

“…In recent years, metal oxides such as BiVO 4 , − TiO 2 , − and WO 3 − have been extensively studied as photoanodes in PEC water oxidation reaction . They display good oxidation abilities because of the low valence band position determined by the O 2p orbital .…”

Reinforcement of Visible-Light Harvesting and Charge-Transfer Dynamics of BiVO4 Photoanode via Formation of p–n Heterojunction with CuO for Efficient Photoelectrocatalytic Water Splitting

“…Photoelectrode-assisted water splitting is a promising way to produce hydrogen without any harmful carbon emission. , In PEC water splitting, the semiconductor photoelectrodes absorb incident photons and generate electron–hole pairs, which undergo redox reactions with water and produce molecular oxygen (anode) and hydrogen (cathode). − In recent decades, several efforts have been developed to obtain highly efficient and stable photoanodes, because the kinetics of water oxidation is much slower than water reduction at the cathode, and therefore, the water oxidation at photoanode is considered as a key step in overall PEC water splitting. , In this context, various n-type semiconductor metal oxides, such as TiO 2 , ZnO, WO 3 , Fe 2 O 3 , BiVO 4 , Bi 2 WO 6 , Bi 2 MoO 6 , and so on, have been employed as photoanodes in PEC water splitting. − Among them, the BiVO 4 has been demonstrated as one of the most promising photoanodes because of its remarkable features, namely, low cost, ability to absorb ∼11% light energy in the visible region by means of its narrow band gap (∼2.4 eV), and suitable valence band edge for water oxidation. Moreover, the BiVO 4 exhibits a maximum theoretical photocurrent density of 7.5 mA cm –2 under AM 1.5G illumination (100 mW cm –2 ), resulting in a high solar-to-hydrogen (STH) conversion efficiency of ∼9.2%. − However, in practice, the poor charge separation, high charge recombination, and poor water oxidation kinetics have reduced the performance of the BiVO 4 photoanode.…”

Plasmon-Mediated Electrochemical Activation of Au/TiO2 Nanostructure-Based Photoanodes for Enhancing Water Oxidation and Antibiotic Degradation

“…Electrochemical impedance spectroscopy (EIS) measurements were performed to further uncover the role of the electron-rich surface states in PEC water oxidation . Nyquist plots showed two semicircles when the applied frequency was scanned (Figure a), demonstrating that water oxidation is indeed mediated by the surface hole-trapping states , rather than a direct hole transfer from the sp band of Au to surface-adsorbed H 2 O molecules.…”