Tungsten Oxide Clathrates for Water Oxidation: A First Principles Study

Tungsten oxide (WO₃) is a good photoanode material for oxidizing water, but it is not an efficient absorber of sunlight because of its large band gap (2.6 eV). Recently, stable clathrates of WO₃ with interstitial N₂ molecules (<i>x</i>N₂·WO₃, <i>x</i> = 0.034–0.039) were synthesized, with a band gap of 1.8 eV. We studied the structural and electronic properties of these clathrates using ab initio calculations, and we analyzed the physical origin of the gap reduction reported experimentally. We found that both structural changes caused by the insertion of N₂, and a small charge overlap between N₂ and WO₃, are responsible for the gap decrease. We compared the effect of N₂ intercalation to that of other closed shell species, in particular CO and rare gas atoms. Our calculations predicted that CO insertion lowers the band gap by about the same amount as N₂ but it leads to a change of both the oxide valence and conduction band positions, while the presence of N₂ only affects the conduction band minimum. We also predicted that, in the case of Xe, a strong hybridization between Xe 5p and O 2p states modifies the valence band edge of WO₃, leading to a reduction of the band gap by approximately 1 eV