Spontaneous reduction of Cr(VI) using InSnS₂ under dark condition

The flower-like In³⁺-doped SnS₂ (InSnS₂) was synthesized by microwave-assisted method for the photoreduction of toxic Cr(VI) to less-toxic Cr(III). InSnS₂ showed a narrow bandgap (2.0 eV) and a broad spectrum response (400–800 nm), suitable to apply the photocatalytic reduction of Cr(VI) under visible-light irradiation. In our latest report, we found that the enhanced photocatalytic activity was caused by the indium doping and the formic acid, which acted as the inhibitor to hole–electron recombination and hole scavenger, respectively. Herein, as a further study, the contact time of Cr(VI) with InSnS₂ in the dark condition was adjusted to ensure the adsorption–desorption equilibrium, and its effect on the reduction activity was evaluated. The results show that spontaneous reduction of Cr(VI) was observed in the initial adsorption process in the dark condition via the synergistic effect of In³⁺ release and HCOOH, and at later the reduction of Cr(VI) only occurred by the photocatalytic reaction under visible-light irradiation. Finally, the adsorption of Cr(VI) on photocatalyst before light irradiation was found as an important factor to enhance the photoreduction of Cr(VI).