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

Tungsten trioxide (WO 3 ) 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 WO 3 —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 WO 3 found either at high temperature or under pressure exhibit a valence band maximum higher in energy than that of room temperature monoclinic WO 3 by ∼0.35 eV and smaller electronic gaps, better suited to efficiently absorb sunlight. In addition, we found that orthorhombic WO 3, 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 WO 3 at ambient conditions and hence to improve the properties of tungsten trioxide as a photoanode material.