Synthesis and characterization of the semiconductor Li0.93Cu0.07Nb3O8: Application to oxygen evolution under visible light

The semiconductor Li0.93Cu0.07Nb3O8 is prepared by soft chemistry in aqueous electrolyte via Cu2+ → Li+ exchange between copper nitrate and LiNb3O8. The substituted niobate crystallizes in an orthorhombic symmetry and the semiconducting and photo-electrochemical properties are investigated for the first time. The oxide exhibits a dark brown color and the UV-Visible spectroscopy gives an optical gap of 1.42 eV, due to the crystal field splitting of Cu2+ in octahedral site. The thermal variation of the conductivity shows that Nb: 4d-electrons are localized and the data are fitted by a small-polaron hopping model σ = σo exp {−0.053 eV/kT} with a carrier density thermally activated. The capacitance measurement done in ionic electrolyte (Na2SO4, 10−2 M) indicates n type semiconductor with mixed valences Nb5+/4+, due to the hetero-valent substitution Li+/Cu2+, with a flat band potential of 0.28 VSCE and electrons density of 2.17×1017 cm−3. The Nyquist diagram shows mainly the bulk contribution with a diffusion process. The valence band (6.39 eV below vacuum) derives from O2-: 2p orbital with a small admixture of Cu2+: 3d character while the conduction band is made up of Nb5+: 4d orbital. The material is successfully tested for the oxygen generation with an evolution rate of 87 µmol mn−1 g−1 under visible light (29 mW cm−2) and a quantum yield of 0.35%.