Ohmic-functionalized type I heterojunction: Improved alkaline water splitting and photocatalytic conversion from CO2 to C2H2

•A hollow dodecahedron-shaped N-doped C (NC)/CoSe2 is fabricated via self-sacrificing strategy.•The higher HER rate of CdS/N-doped C/CoSe2 (CCE50) achieved in alkaline media may be attributed to the accumulation of O2.•CCE50 converts CO2 to C2H2 at a rate of 2.8 μmol h−1 g cat−1.•The introduction of an Ohmic contact has resulted in a 15.7-fold increase in the HER for type I CdS/NC.•Extra valence electrons of NC raise the Fermi level to regulate electronic structures.