Abstract: Mo₂C MXene (Mo₂CT_x) has been recognized as one of the most effective cocatalysts for photocatalytic H₂production, but its intrinsic cocatalytic performance are usually impeded by the strong H intermediates (H*) adsorption on catalytic Mo atoms. In this case, an ingenious strategy to modulate H* adsorption affinity of active Mo atoms in Mo₂CT_x is developed to effectively weaken Mo-H_ads bond by fabricating Mo₂CT_x-MoSe_2-yS_y heterojunction cocatalyst, achieving efficient photocatalytic H₂ production. Herein, the Mo₂CT_x-MoSe_2-yS_y heterojunction is skillfully synthesized via a facile NaBH₄-assisted solvothermal approach, followed by coupling with TiO₂ through a ultrasonication-assisted method to prepare TiO₂/Mo₂CT_x-MoSe_2-yS_y composite photocatalyst. As expected, the resultant TiO₂/Mo₂CT_x-MoSe_1.5S_0.5 sample demonstrates a superior photocatalytic H₂-production rate of 1142.76 μmol g^-1 h^-1, surpassing TiO₂/Mo₂CT_x and TiO₂/Mo₂CT_xδ-MoSe₂ by 4.76 and 1.95 times. Characterization results and density functional theory (DFT) calculation reveal a directional free-electron transfer from MoSe_1.5S_0.5to Mo₂CT_x, thus essentially charging electrons to d orbitals of Mo atoms on Mo₂CT_x to generate electron-rich Mo^δ- sites. The electron-rich Mo^δ- sites can increase the antibonding-orbital occupancy of Mo-H_ads, ultimately weakening the Mo-H_ads bond for acquiring the near-balanced H* adsorption/desorption dynamics on Mo₂CT_xto boost H₂ production performance. This study opens new avenues for the rational design of Mo₂CT_x-based cocatalysts for optimizing the efficiency of active sites.

Key words: Photocatalytic H₂ production, Cocatalysts, Mo₂CT_x-MoSe_2-yS_y heterojunction, Mo₂C MXene, Antibonding-orbital occupancy