CQDs/ZnIn2S4 (CQDs represents carbon quantum dots) nanocomposites were prepared via a microwave-assisted hydrothermal method. The photocatalytic activity for the reduction of Cr(VI) to Cr(III) over the as-prepared CQDs/ZnIn2S4 nanocomposites under visible light irradiations was studied. The study found that the reduction of Cr(VI) to Cr(III) over hexagonal ZnIn2S4 can be significantly increased by loading CQDs as a co-catalyst. After 60 min of visible light illumination (λ > 420 nm), almost 100% Cr(VI) can convert to Cr(III) with (NH4)2C2O4 as a scavenger. Furthermore, the CQDs/ZnIn2S4 nanocomposites are demonstrated to execute as bifunctional photocatalysts with appreciable activity in the hybrid systems (Cr(VI)/RhB) under visible light, which made it a potential application material for industrial wastewater treatment. Combined with photoelectrochemical analyses, it can be found that the introduction of CQDs can reduce the recombination of photogenerated electron-hole pairs. Finally, a possible photocatalytic reaction mechanism is also proposed. [ABSTRACT FROM AUTHOR]