Acknowledgements The authors gratefully acknowledge the support of the Natural Sci- ence Foundation of Hebei Province (No. E2020203123).

In recently year, constructing heterojunction has been proved to be an important method to improve the efficiency of medical pollution degradation and photocatalytic hydrogen production. However, as we known that the single heterojunction cannot get the satisfactory photocatalytic efficiency. Herein, a new double heterojunction photocatalyst was prepared by phosphating Co3O4 on g-C3N4 nanosheets (CNNS/Co3O4@CoP). The asprepared CNNS/4%Co3O4@7%CoP exhibit excellent catalytic activity, which degradation rate of TC under visible-light irradiation could reach to 92% within 3 h, and the H2 production could reach to 1677.38 mu mol g-1 h-1. Photoelectrochemical tests and density functional theory (DFT) analysis show that the synergistic effect between p-n junction (CNNS/Co3O4) and Ohmic junction (Co3O4@CoP) can significantly facilitate charge transfer, prolong the lifetime of photogenerated carriers, and improve the photocatalytic degradation and hydrogen production activity. This work provides an evidential proof of rational designing double heterojunction, and confirms the synergistic effect of double heterojunction can drive charge transfer and separation effectively, which promoting photocatalytic performances.