Electron-transfer reactions at semiconducting anodic niobium oxide films

Anodic niobium oxide in a n-type semiconductor due to an oxygen deficit. Electron donor densities and the dependence of excitation energies upon donor density were determined from measurements of space-charge capacitances as a function of electrode potential. Rates of Fe(III) reduction and of Fe(II) oxidation were measured at oxide films of different donor densities as functions of electrode potential and pH value. The results indicate electronic states in the space-charge layer below the oxide surface to participate in the electron-transfer reactions. Experimental polarisation curves are compared to curves calculated theoretically. From the shape of the polarisation curve flat-band potentials of the oxide are estimated. The observed polarisation curves are compatible with λ = 1·2 eV for the reorganisation energy of the Fe(CN)^3?₆/Fe(CN)^4?₆ redox couple.