The possibility of incident sunlight passively splitting water to constituent hydrogen and oxygen, through the designed placement of the semiconductor conduction band (CB) and the valence band (VB) with respect to the H2O/H2 reduction potential and the H2O/O2 oxidation potential, respectively, is of immense scientific and technological interest.
The role of non-stoichiometry in a hierarchically structured WO3-x electrode, constituted from nanoscale fuzziness as well as microscale wire morphology, on the photoelectrochemical response is investigated. Through x-ray photoelectron spectroscopy (XPS) studies, the relative amounts of the various oxidation states of the constituent W are probed with respect to the observed response. It is concluded that an intermediate/optimal number of vacancies, yielding a W6+/ (W5+ + W4+) ratio of around 2, would be beneficial for increasing the photocurrent. It is posited that defect engineering combined with optimized band structure modulation could be used for enhanced photocurrent density as well as electrode stability. The work would help considerably elucidate the role of defects as well as charge carriers for oxygen evolution reaction (OER) efficiency increase.
Given that OER proceeds through the interaction of hole charge carriers (h+) with water and that the presence of h+ implies surface instability, we indicate that a charge compensating mechanism through electrons from oxygen vacancies may be beneficial. It is hoped that the consideration of such defect engineering aspects, perhaps at the atomic level, would be helpful in designing higher efficiency electrodes for water oxidation.