Abstract: A composite of SnS₂ and carbon nanotube (CNT) was successfully synthesized as a visible-light-driven photocatalyst with a mechanochemical method. In comparing with SnS₂, the SnS₂/CNT composite improved much the photocatalytic removal of Cr(VI) in acidic condition, which was confirmed to its special solid acid structure. During the synthesis of the SnS₂/CNT composite by ball milling, S-C p-π bonding was formed between sulfur atoms in SnS₂ and carbon atoms in CNT. The generated S-C p-π bonding enhanced the transfer of photo-induced electrons in the bulk phase of the composite photocatalyst under visible light irradiation. Moreover, the persistent radical sites on CNT were able to trap photo-induced electrons and served as conjugated acid sites in the photocatalysis, which increased surface concentration of protons through their association with H⁺. These made the catalyst have a solid acid structure with plentiful surface protons, promoting the interfacial electron transfer between the catalysts and Cr(VI) and increasing the source of H⁺ for the reduction of Cr(VI). As a result, the photocatalytic reduction rate of Cr(VI) on the SnS₂/CNT composite were dramatically enhanced, being about 800% that on SnS₂. On the basis of various characterizations and probe experiments, we confirmed the significance of the solid acid structure of the photocatalyst and clarified the catalytic mechanism of the new photocatalyst. Our finding may provide a new strategy to prepare highly active photocatalysts for proton-involved reactions.

Key words: SnS₂/CNT composite, S-C p-π bonding, Solid acid, Ball milling, Cr(VI) reduction