Two-dimensional (2D) polar semiconductors have garnered significant attention due to their inherent polarization, which effectively suppress the rapid recombination of photogenerated electrons and holes. However, intrinsic 2D polar semiconductors suffer from surface inertness, leading to low reaction efficiency. Doping modification, a widely used strategy for enhancing catalyst activity, holds great promise for further improving the performance of 2D polar photocatalysts. In this study, 2D $g\text{−}{\mathrm{C}}_3{\mathrm{N}}_5$ with out-of-plane polarization and thirteen unequal doping sites is employed to systemically explore the doping effect on photocatalytic water splitting. The doped $g\text{−}{\mathrm{C}}_3{\mathrm{N}}_5$ can induce a surface dipole that is more than twice as large as that of the pristine material, while simultaneously exhibiting enhanced spatial separation of photogenerated electrons and holes. More importantly, doping effectively enhances local activity, facilitating smooth adsorption of the *OOH group in the oxygen evolution reaction and promoting an ideal hydrogen evolution reaction with near-zero Gibbs free energy change. Additionally, the introduced impurity states in the band gap broaden the optical absorption into the visible-light range, highlighting the potential of doped $g\text{−}{\mathrm{C}}_3{\mathrm{N}}_5$ for practical application. This work presents a feasible strategy for designing high-performance photocatalysts based on 2D polar semiconductors.

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• Photocatalysis
• 2-dimensional systems
• Density functional theory
• First-principles calculations