Doping of WO3 for Photocatalytic Water Splitting: Hints from Density Functional Theory

The electronic properties of doped tungsten oxide (WO3) have been studied using DFT calculations with hybrid functionals. While the position of the top of the valence band (VB) in WO3 is good for O-2 evolution in water splitting, the conduction band (CB) is too low for H-2 production. Furthermore, the band gap should be reduced to improve activity with visible light. Doping can be used to alter the position of the energy levels, thus resulting in a more efficient photocatalyst. Replacing W in the lattice by isovalent Mo or Cr ions narrows the band gap but shifts the CB edge further down. Replacing O by S has the effect to narrow the energy gap by introducing localized occupied states above the VB and shifts the CB minimum upward-two effects that go in the right direction. Substitution of W with low-valent Ti, Zr, or Hf ions widens the band gap and shifts the CB edge to higher energies. However, a low-valent ion replacing W induces the formation of compensating defects: in Hf-doped WO3 oxygen vacancies (V-O) have negative formation energies. The simultaneous presence of substitutional Hf and of an O vacancy results in a shift of both VB and CB to higher energies and a reduction of the band gap, with potential benefit for photocatalytic hydrogen production. Codoping with Hf + 2F or Hf + V-O + S has also been investigated.