Abstract: In this paper the Photoelectrochemical behaviour and the energy level construction of the reduced n-NiNb₂O₆ semiconductor single crystal electrodes were studied by measuringthe current-potential characteristics and the spectral responses.Two absorption bands were observed,the most intense at 3.24eV being due to oxygen-niobium photoexcitation,the other at 1.75eV in the visible region and being due to nickel-niobium charge transfer.The mechanism of anode dark corrosion,photrvcorrosion,and photoelectrolysis were discussedand the NiNb₂O₆ electron energy diagram and the major band cleavage energy levels were also given out.

Key words: semiconductor photoelectrochemistry, dark corrosion, photo-corrosion