Tailoring of the solid state properties of Al-Nb mixed oxides: A photoelectrochemical study

Al-Nb containing mixed oxides were grown by anodizing sputter-deposited Al-Nb alloys of different compositions. A photoelectrochemical investigation was carried out in order to estimate the band gap, flat band potential, and conductivity type of these oxides as a function of their composition. The dependence of the band gap on the composition of mixed sp-d metal oxides has been rationalized by using a semiempirical correlation between the difference of electronegativity and band gap of oxides proposed in the literature some years ago and recently tested for regular d-d metal mixed oxides. The band gap increase observed as a function of Al content into the oxides seems mainly depending on the monotonic shift of the conduction band mobility edge, E CBM, derived from 5d orbitals of Nb5+ in the presence of an almost constant energy location of the valence band mobility edge, E VBM.