Photoelectrochemical characterization of the delafossite CuFeO2: Application to removal of divalent metals ions

The catalytic electrodeposition of a variety of metal ions M2+ (Zn2+, Cd2+, Hg2+, Ni2+, Pb2+, Cu2+, Ag+), all of environmental issue, was performed under visible light in aqueous air-equilibrated CuFeO2 suspensions. The delafossite CuFeO2 was synthesized by nitrate way and the surface/bulk ratio is greatly increased. In the dark, it exhibits a long term chemical stability with a corrosion rate of 1.3 × 10−8 mol m−2 year−1 in KCl (1 M). The black oxide, with a gap of 1.32 eV, determined from the reflectance diffuse spectrum, was characterized by photoelectrochemistry. The flat band potential was found to be 0.16 VSCE and a correlation exists between the redox potential of M2+/0 couple and the conduction band of CuFeO2 located at −1 VSCE. Ag+ is slightly photoelectrodeposited because of its low adsorption and its positive potential. By contrast, Cu2+ and Cd2+ are efficiently reduced owing to the strong dark adsorption and the large driving force at the interface. The quantum yield of M2+ deposition works out to be 0.1%; the metal deposition proceeds competitively with the water reduction and decelerates over time. The specie showing the high H2-evolution exhibits the lower M2+ reduction and the medium pH should make the M2+ deposition easier. The best photocatalytic H2-evolution (0.18 ml g−1 min−1) was obtained in solution containing Cd2+ (30 ppm). This is a surprising result and merits a more detailed investigation.