Element doping is a general and effective approach to modify the electrocatalytic performances, but the low intrinsic activity in each electroactive site still limits the further improvements. Herein, we provide an effective strategy by simultaneously introducing P doping and Se vacancies to enhance the intrinsic activities in NiSe2 nanosheet arrays (A-NiSe2 vertical bar P) through Ar plasma treatment. Owing to the increased active sites and enhanced electrical conductivity, the resulted A-NiSe2 vertical bar P shows the enhanced hydrogen evolution performances. Theoretical calculations reveal that introduction of Se vacancies plays a significant role in lowering the adsorption free energy of H* in Ni, Se and P sites, leading to promoted intrinsic activities in A-NiSe2 vertical bar P. Further, A-NiSe2 vertical bar P as bifunctional electrocatalysts only needs 1.62 V to reach 10 mA cm(-2) for overall water splitting. Our study and understanding of A-NiSe2 vertical bar P may highlight the importance of element doping and vacancies in enhancing the catalytic activities in overall water splitting. (C) 2020 Elsevier Inc. All rights reserved.