Hole diffusion across leaky amorphous TiO2 coating layers for catalytic water splitting at photoanodes

We investigate the mechanism of hole diffusion across leaky amorphous TiO2 (am-TiO2) layers. Through ab initio molecular dynamics simulations, we construct an atomistic model of am-TiO2 consistent with the experimental characterization. We first demonstrate that the oxygen vacancies do not occur in am-TiO2, as they can be assimilated by the amorphous structure upon structural rearrangement. Hence, their role in hole diffusion is ruled out. In contrast, O-O peroxy linkages are formed in pristine am-TiO2 upon injection of excess holes, with an associated defect level lying at 1.25 +/- 0.15 eV above the valence band of the material. We show that such linkages can provide a viable mechanism for hole diffusion in am-TiO2, as illustrated by a diffusion path of 1.2 nm with energy barriers lower than 0.5 eV in our atomistic model of am-TiO2.