Anisotropic Electron Transport Limits Performance of Bi₂WO₆ Photoanodes

Abstract

Bi 2 WO 6 is one of the simplest members of the versatile Aurivillius oxide family of materials. An intriguing model system for Aurivillius oxides, BiVO 4 exhibits low water oxidation onset potentials (~0.5-0.6 V RHE ) for driven solar water oxidation. Despite this, Bi 2 WO 6 also produces low photocurrents in comparison to other metal oxides. Due to a lack of in-situ studies, the reasons for such poor performance are not understood. In this study, Bi 2 WO 6 photoanodes are synthesised by aerosol-assisted chemical vapour deposition. The charge carrier dynamics of Bi 2 WO 6 are studied in-situ under water oxidation conditions and the rate of both bulk recombination and water oxidation is found to be comparable to other metal oxide photoanodes. However, the rate of electron extraction is at least 10 times slower than slowest kinetics previously reported in an oxide photoanode. First-principles analysis indicates that the slow electron extraction kinetics are linked to a strong anisotropy in the conduction band. Preferred or epitaxial growth along the conductive axes is a strategy to overcome slow electron transport and low photocurrent densities in layered materials such as Bi 2 WO 6.

The kinetics of metal oxide photoanodes from charge generation to catalysis

“…Furthermore, for some metal oxides, charge-carrier mobility has been reported to be anisotropic due to anisotropic lattice spacing. 84,90 Anisotropic electron and hole mobilities have been reported to, for example, aid spatial charge separation and enhance the performance of BiVO4 photocatalysts. 91 A unifying model to account for these variations would likely lead to convergence of the reported carrier lifetimes.…”

Analysis of charge trapping and long lived hole generation in SrTiO3 photoanodes

“…25 In SrTiO 3 , the electron trapping may limit the size of the photocurrent but it does not result in a slow extraction rate, possibly due to the immobile nature of the deeply trapped electrons limiting their influence on the charge extraction kinetics. 55 The immobile nature of these trapped charges also aids explanation of the small degree of bias dependence of the decay kinetics.…”

Sonochemical regulation of oxygen vacancies for Bi2WO6 nanosheet-based photoanodes to promote photoelectrochemical performance

“…1,2 However, there are still problems in photogenerated carrier separation and transmission steps, which seriously limit the performance of semiconductor-based photocatalysts. 3,4 To address these challenges, numerous strat-egies have been established, including interfacial modification, 5 crystal facet engineering, 6 and defect engineering. 7 Therein, by introducing oxygen vacancies (Vo), it is possible to enhance the separation and transmission of charge carriers by constructing efficient charge transfer channels and promote catalytic activity by creating active surface sites.…”