Optimizing the Band Edges of Tungsten Trioxide for Water Oxidation: A First-Principles Study

Abstract

Tungsten trioxide (WO3) is a promising photoanode material for water oxidation, due to its stability in aqueous environments under different pH conditions. However, the optical gap of monoclinic WO3—the stable form of the solid at ambient conditions—is larger than desired for optimal sunlight absorption, and the position of its valence band maximum with respect to the water oxidation potential requires a large overpotential to drive the oxidation reaction. Using first-principles calculations we showed that several phases of WO3 found either at high temperature or under pressure exhibit a valence band maximum higher in energy than that of room temperature monoclinic WO3 by ∼0.35 eV and smaller electronic gaps, better suited to efficiently absorb sunlight. In addition, we found that orthorhombic WO3, stable at high temperature, has also improved carrier mobilities for both holes and electrons. We propose that epitaxial growth on appropriate substrates may be a promising way to stabilize metastable phases of...