Two-dimensional (2D) van der Waals (vdW) bilayers, with their intrinsic interlayer electric fields for enhanced carrier separation, provide a promising platform for efficient photocatalysis. However, the microscopic mechanisms linking these internal fields to higher photocatalytic activities remain elusive. Here, we construct ${\text{HfN}}_2$ bilayers with different stacking orders to explore how interlayer polarization governs carrier dynamics for higher photocatalytic water-splitting characteristics. First-principles calculations show that the AB stacking, which breaks interlayer symmetry, possesses a spontaneous polarization, leading to an intrinsic type-II band alignment and strongly suppressed electron-hole recombination. Thereby, AB stacking exhibits prolonged carrier lifetimes and enhanced solar-to-hydrogen conversion compared with the ${\text{AA}}^{\prime }$ stacking. Moreover, such spontaneous built-in fields can also generate stronger electron overpotentials, lowering hydrogen evolution reaction barriers and reducing external bias requirements during hydrogen evolution. Via interlayer sliding, the completely reversed interlayer polarization can exchange the oxidation and reduction planes, suggesting a viable route toward their switchable photocatalytic applications. Since the interlayer polarization originates purely from stacking asymmetry, these insights are broadly applicable to other 2D vdW bilayers. Therefore, our findings offer a feasible pathway for the next design and optimal regulation of 2D high-efficient photocatalysts.

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• Hydrogen production
• Photocatalysis
• Sliding ferroelectricity
• Solar energy harvesting devices
• Bilayer films
• Semiconductor compounds
• van der Waals systems
• First-principles calculations