Understanding the advantage of hexagonal WO₃ as an efficient photoanode for solar water splitting: A first-principles perspective

Polycrystalline WO₃ has been suggested as an alternative photoanode material for the water splitting reaction. However, the band gap and band edge positions of the most commonly used γ-monoclinic WO₃ phase are found to be not optimal for effective water oxidation. In this work, by using first-principles density-functional theory calculations with an ab initio thermodynamic model, we demonstrate the potential advantage of using h-WO₃ (and its surfaces) over the larger band gap γ-WO₃ phase for the anode in water splitting. Notably, after addressing the relative thermodynamic stability of the various h-WO₃ surfaces, we carefully quantify and compare the electronic band structure of these two bulk phases of WO₃ (using their valence and conduction band edges as descriptors). We then provide a simple perspective as to illustrate how the surface band edges of h-WO₃ match up with the redox potential of water and other possible cathode materials.