Electronic and optical bandgaps of WO3 and WO3−x: Adsorbed water-induced Burstein–Moss shift

Abstract

The reported bandgap of stoichiometric WO3 spans a range of 2.6–3.4 eV. The present study evaluated the underlying reason for this wide variation using multiple spectroscopic techniques that preferentially probe the surface, near-surface, or bulk region of WO3 and WO3−x. The results show that the bulk electronic bandgap of WO3 is ∼2.2 eV, which is much lower than the value of 2.7–2.8 eV measured through optical, photoelectrochemical, and photoemission probes that are more surface sensitive. The difference between the electronic and optical bandgap is likely due to the presence of a surface e− accumulation layer on the stoichiometric WO3 due to doping by adsorbed water. In contrast, the presence of oxygen vacancy defects in the bulk lattice of WO3−x causes degenerate bulk doping, a Burstein–Moss shift of Fermi energy to higher electron energies within the CB. However, exposure of WO3−x to ambient humid O2 causes surface oxidation and the formation of an e− depletion layer on the surface. This study highlights the important role of ambient O2 and water in modulating the carrier concentration and electron affinity of semiconductors.