The effect of the Cu+/Cu2+ ratio on the redox reactions by nanoflower CuNiOS catalysts

CuNiOS bimetal oxysulfide catalysts with catalytic activity in the dark were synthesized by a feasible solution method below 100 °C. It has a hexagonal CuS covellite structure with the Cu+/(Cu++Cu2+) molar ratio above 0.6. S6+ was formed to balance the deficiency in positive charge due to a high Cu+ content. The effect of hydrazine (N2H4) amount added during the CuNiOS synthesis on the redox reactions was evaluated. The N2H4-free CuNiOS showed the fast methylene blue (MB) degradation capability and the [N2H4]-high CuNiOS had the high Cr(VI) reduction capability all in the dark condition. The adjustable redox reactions by the CuNiOS catalyst system are demonstrated and explained. The catalytic reaction mechanism is elucidated, depending upon the Cu+/Cu2+ ratio for different charge transports. CuNiOS also exhibited relatively good stability and durability during its reusability tests. The catalytic reactions under the mild conditions without heat and electrical and thermal energies are encouraging for green synthesis.