Photoelectrochemical properties of n-type NiTiO3

Abstract

The photoelectrochemical behavior of a sintered Nb doped n‐NiTiO₃ electrode in relation to the photoassisted oxidation of water has been studied. Three bands mediate the different photoprocesses observed: A conduction band (Ti⁴⁺:3d) several tenths of one electron volt above the H⁺/H₂ redox level; the O²⁻:2 p⁶ band about 3 eV below the conduction band; and the Ni²⁺:3d⁸ ‘‘band’’ above the oxygen band that seems just to overlap the O₂/H₂O level. Electronic transitions induced by UV illumination are mainly O²⁻→Ti⁴⁺ transitions; holes photogenerated in the oxygen band move through this band, by the effect of the internal electric field, to the electrode surface where they react with water molecules with an efficiency similar to that observed in n‐TiO₂ or SrTiO₃. Visible light produces Ni²⁺→Ti⁴⁺ electronic transitions. Hole mobilities in the Ni²⁺:3d⁸ ‘‘band’’ appear to be adequate for the photogenerated holes to reach the s.c. surface, where they become trapped at Ni³⁺ surface centers at an energy level too high to oxidize water efficiently. However, the geometry of the NiTiO₃ ilmenite structure makes the oscillator strength for the Ni²⁺→Ti⁴⁺ charge transfer too weak to have an acceptable photoresponse to visible light. Therefore NiTiO₃ is not a good candidate for water photoelectrolysis in a photoelectrochemical cell. This conclusion is probably valid for other mixed‐metal oxides with ilmenite structure.