Greatly increased visible-light photocatalytic activity of SnS2/carbon nanotube composite for Cr(VI) reduction: Insights into effects of solid acid structure

•p-π bond was successfully fabricated in SnS2/CNT composite by ball-milling method.•p-π bond and persistent radical sites make the composite has a solid acid structure.•Solid acid structure is conducive to the proton-coupled electron transfer reaction.•SnS2/CNT composite shows 800% higher photocatalytic activities compared with SnS2.•Structure-activity relationship in Cr(VI) reduction by the composite is proposed.