Electron and Hole Polaron Formation and Transport in Monoclinic WO₃ Studied by Hybrid Functional Approach

Abstract

WO3 is a well-known semiconductor used in photocatalytic water splitting. However, its charge carrier transport properties are still not well understood. In this work, using the hybrid density functional theory (DFT), we investigated the electron and hole transport in WO3. We found that the electron can form a 2D-like polaron localized on several W sites, which is consistent with the experimental and other theoretical works. The electron polaron is mainly in W 5d state but with contributions from the W 6s orbital. The activation energy of electron polaron transfer within the ac plane is several orders of magnitude lower than that perpendicular to the planes. This indicates that the electron polaron transfer can only occur within the ac plane but not perpendicular to the planes. With regard to the charge carrier hole, we found that it will form a small hole polaron on a single oxygen site which is in the O 2p state. The calculated mobility of the hole polaron is much lower than that of the mobility of electron, which indicates that the hole is highly localized. This work provides the fundamental understanding of charge carrier transport in WO3 and may be helpful to design high-efficiency WO3-based photocatalysts. It is also beneficial to study the charge carrier transfer properties in other W-based materials.