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A Z-scheme BiVO 4 /NiCo 2 O 4 (BVO/NCO) heterojunction structure was constructed by modifying optimal ultrathin nickel–cobalt (NiCo 2 O 4) spinel nanosheets on BiVO 4 as an efficient photocatalyst toward water oxidation, which deliver an excellent photocatalytic O 2 evolution performance of 1640.9 μmol∙g−1∙h−1 by reducing the energy barriers and enhancing the separation and transfer of photogenerated carriers. [Display omitted] The sluggish water oxidation process is a severe obstacle for solar-driven water splitting. Therefore, it is imperative to develop a suitable photocatalyst with reduced energy barrier for strong oxidation. In this study, a Z-scheme BiVO 4 /NiCo 2 O 4 (BVO/NCO) heterojunction system was designed by decorating ultrathin nickel–cobalt (NiCo 2 O 4) spinel nanosheets on BiVO 4 as an efficient photocatalyst for water oxidation. The unique structure of the system significantly reduced the energy barrier and improved the oxidation ability of BiVO 4 to efficiently enhance the separation and transfer of the photogenerated carriers. Thus, the photocatalyst delivered an excellent O 2 evolution performance of 1640.9 μmol∙g−1∙h−1 and showed 124% improved efficiency as compared to pristine BiVO 4 and a quantum efficiency of 5.39% at 400 nm for O 2 evolution. Additionally, the theoretical calculations revealed that the formation of *OOH was the rate-determining step for water oxidation. The decoration with NiCo 2 O 4 significantly reduced the energy barrier between *O and *OOH, which eventually improved the photocatalytic performance of BVO/NCO. The results hold great promise for the potential application of spinel-based materials in efficient photocatalytic O 2 evolution and offer fundamental insights into the design of efficient water oxidation heterojunctions. [ABSTRACT FROM AUTHOR]