Synergistic effect of sliding ferroelectricity and piezoelectricity optimizes the separation of photogenerated electron and hole in two-dimensional van der Waals homostructures

Efficient carrier separation is a crucial factor in photocatalysis, typically achieved in type-II or $Z$-scheme heterostructures. However, in homostructures, carrier separation remains challenging due to degenerate band alignment. Here, we demonstrate enhanced photogenerated electron-hole separation at a homointerface, enabling superior water-splitting catalysis by leveraging the synergistic effects of sliding ferroelectricity and piezoelectricity. Using two-dimensional ${\ensuremath{\beta}}_{2}\text{\ensuremath{-}}M{A}_{2}{\mathrm{Z}}_{4}$ ($M$ = Zr, Hf; $A$ = Si, Ge; $Z$ = N) homostructures as a model system, we reveal that this synergy allows for precise control over the orientation and intensity of polarization. The interlayer vertical polarization increases from \ensuremath{\sim}2.5 to \ensuremath{\sim}8.8 pC/m, with a moderate polarization level facilitating Gibbs free-energy reduction toward zero, thereby enhancing the hydrogen evolution reaction (HER). Additionally, the reversible polarization characteristic enables dynamic modulation of carrier distribution across individual sublayers, enabling an ON-OFF switching mechanism for catalysis. Specifically, one polarization direction promotes HER, while the opposite favors the oxygen evolution reaction. Our findings provide a viable strategy for designing high-efficiency ferroelectric and piezoelectric catalysts for water splitting.