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

Graphical abstract Highlights • Bimetal Cu Ni OS oxysulfide nanoflower was prepared by a feasible method. • Cu Ni OS performed high redox activity in the dark without additional reagents. • The Cu+/Cu2+ ratio in Cu Ni OS affected its catalytic activity for redox reactions. • Redox reactions involve the electron or hole transport between Cu+ and Cu2+. • Kinetic mechanism is schematically proposed to explain for the fast redox reactions. Abstract Cu Ni OS 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 (N 2 H 4) amount added during the Cu Ni OS synthesis on the redox reactions was evaluated. The N 2 H 4 -free Cu Ni OS showed the fast methylene blue (MB) degradation capability and the [N 2 H 4 ]-high Cu Ni OS had the high Cr(VI) reduction capability all in the dark condition. The adjustable redox reactions by the Cu Ni OS catalyst system are demonstrated and explained. The catalytic reaction mechanism is elucidated, depending upon the Cu+/Cu2+ ratio for different charge transports. Cu Ni OS 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. [ABSTRACT FROM AUTHOR]