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• CdS/KPW/meso-g-C 3 N 4 tandem heterojunction is fabricated for the first time. • It has good light response and narrow band gap. • It exhibits excellent visible light photocatalytic activity. • It can be due to the synergy between the Z-scheme and type-II heterojunction. • The tandem heterojunction favors the spatial charge carrier separation greatly. It is known that fabrication of tandem heterojunctions between different types of heterojunctions can promote the charge separation. Herein, novel cadmium sulfide quantum dots (CdS QDs)/dodecahedral phosphotungstic acid potassium K 3 PW 12 O 40 (KPW)/oxygen-doped mesoporous graphite carbon nitride (meso-g-C 3 N 4) nanosheets tandem heterojunctions are prepared by the hydrothermal method combined with direct template calcination and in-situ chemical sedimentation strategy. The results show that tandem heterojunctions formed by the Z-Scheme heterojunction between CdS QDs and KPW and the type-II heterojunction between CdS QDs and meso-g-C 3 N 4 can extend the optical response into visible light region. Importantly, under visible light irradiation, photocatalytic hydrogen production rate and photocatalytic Cr6+ removal rate over CdS/KPW/meso-g-C 3 N 4 is higher than that of KPW and CdS/KPW. This remarkable photocatalytic performance is due to the effective charge separation and transfer of the special tandem heterojunction structure. This novel tandem heterojunction will offer new insights for fabricating other high-performance photocatalytic systems. [ABSTRACT FROM AUTHOR]