Resin-derived carbon to in-situ support Cu-Cu2-xS heteroparticles for efficient photocatalytic reduction of Cr(VI)

•Sulfonic resin is utilized for fabricating the ternary Cu-Cu2-xS/C.•Supported Cu-Cu2-xS monoparticles synchronously include both metallic Cu and Cu2-xS phases.•Cu-Cu2-xS heterostructure endows the Cu-Cu2-xS/C product with a good photoactivity.•Photocatalytic mechanism of the Cu-Cu2-xS/C for the reduction of Cr(VI) is explored.