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• Co@N-doped carbon was prepared by pyrolysis of ZIF-8@ZIF-67 to modify CdSe QDs. • The k of Cr(VI) reduction over 2%Co@C/CdSe is 33.9 times higher than that of CdSe. • The best H 2 evolution rate of 2%Co@C/CdSe is 139 times larger than that of CdSe. • Enhanced charge separation and light absorption is responsible for high activity. Solar-driven water splitting is regarded as one promising green way to produce H 2. Photo-reduction of hexavalent chromium (Cr(Ⅵ)) to hypotoxic Cr(III) can control the Cr(VI)-induced pollution. A key in these processes is to develop a photocatalyst with high specific surface area, good light absorption and efficient charge carrier separation. A Co-encapsulated N-doped carbon (Co@NC) was prepared by pyrolysis ZIF-8@ZIF-67 precursor to modify CdSe quantum dots (QDs). The obtained catalyst was used to reduce Cr(Ⅵ) and split water under visible light irradiation. Modification CdSe with Co@NC made obtained Co@NC/CdSe possessing enhanced visible-light absorption and photoexcited charge separation. The optimal 2%Co@NC/CdSe exhibited the excellent Cr(Ⅵ) reduction performance with a rate constant of 0.28 min–1 in the absence of sacrificial agent, as well as high H 2 evolution rate of 6.48 mmol g–1 h–1 in 10% lactic acid solution. The H 2 O 2 and electrons were the active species to reduce Cr(VI). This work gives a beneficial insight for improving the photocatalytic activity of QDs. [Display omitted] [ABSTRACT FROM AUTHOR]