Photoelectrochemical properties of the crednerite CuMnO_2 and its application to hydrogen production and M~(n+) reduction (M~(n+) = Cd~(2+), Pd~(2+), Zn~(2+), Ni~(2+),and Ag~+)

摘要

The crednerite CuMnO_2 was synthesized by sol-gel method under N_2 flow. The oxide crystallizes in a monoclinic symmetry ascribed to the Jahn-Teller Mn~(3+) ion and the formation is completed above 600 °C. The XPS analysis confirms the mono and trivalent states of copper and manganese. CuMnO_2 exhibits a direct optical gap of 1.70 eV and the p-type conductivity is due to oxygen insertion in the Cu layers. An exchange current density (0.2 mA cm~(-2)) and a corrosion potential (~ - 0.816 V_(SCE)) indicate a stable compound in neutral solution (pH ~ 7). The flat band potential (E_(fb), = 0.17 V_(SCE)) and the holes density (N_(A-) = 7.25 × 10~(15) cm~(-3)) were determined from the capacitance measurement (C~(-2) -E). The conduction band made up of Cu-3d orbital is located at (3.43 ± 0.02) eV below vacuum and the electrons can reduce heavy metals to their elemental states. Indeed, CuMnO_2 was successfully used for the photodeposition of Cd~(2+), Pd~(2+), Zn~(2+), Ni~(2+), and Ag~+ (30 mg L~(-1)) under visible-light illumination. The best reduction was obtained with Zn~(2+) (94%) because of the optimal band bending, due to the redox potential of Zn~(2+) and its adsorption. By contrast, Ag~+ (76%) was weakly deposited owing to its positive potential. The metal photodeposition proceeds competitively with the water reduction and decelerates over time. The species showing the high H_2 evolution exhibits the lower M~(n+) reduction and the medium pH makes the M~(n+) deposition easier. The best photocatalytic H_2 evolution (1.02 mL g~(-1) min~(-1)) occurred with Cd~(2+).