Semiconducting properties of CuBi2O4 prepared at low temperature: application to oxygen evolution under visible light

The spinel CuBi2O4 was synthesized by a facile step using the co-precipitation at low temperature (140 degrees C) for the first time. The physical properties of the synthesized oxide such as functionality, morphology and optic were characterized by the FT-IR, XRD and UV-Visible analysis. The diffuse reflectance gives a direct optical gap of 1.77 eV, a further indirect transition was also determined. The conduction mechanism is assigned to the low polaron lattice and curiously, CuBi2O4 acts as n-type semiconductor unlike other spinels. The potentials of the electronic bands are determined by photo-electrochemistry, which predicts the interfacial reactions on the spinel. A flat band potential of 0.38 V-SCE was determined from the capacitance measurement in Na2SO4 (0.1 M) medium. According to the capacitance measurement, the valence band (1.95 V-SCE) is more anodic than the O-2/H2O level (similar to 0.65 V-SCE) at pH similar to 7. Therefore, oxygen is spontaneously evolved on the junction spinel/Na2SO4 electrolyte under visible light, due to the low O-2 overvoltage. A quantum conversion efficiency of 5% is obtained within 14 min under visible light (6.28 x 10(16) photons s(-1)), with an average rate O-2 evolution of 75 mu mol g(-1) min(-1).