Synergistic Role of Facet-Engineered Surface and Ferroelectric Polarization in Photoelectrochemical Water Reduction over Pure BiFeO₃ Thin Film

Ferroelectric semiconductors have drawn significant attention in photoelectrocatalysis due to their spontaneous ferroelectric polarization, facilitating charge separation and transfer by shielding charge recombination. Nevertheless, the impact of the facet-engineered surface and ferroelectric polarization direction on the polarization strength of a ferroelectric material, surface band bending at the ferroelectrics/electrolyte interface, and charge transport property remains vague. Here, we synthesized p-type BiFeO3 (BFO) epitaxial thin films by controlling the (001)-facet and (111)-facet, as well as the upward polarization (Pup) and downward polarization (Pdown) to systematically investigate ferroelectric polarization-induced internal electric field (Eint) inside BFO and interfacial charge transport mechanism, which in turn affect the overall performance of photoelectrochemical (PEC) water splitting. Our observations demonstrated that the Eint strength and the charge transport behavior of a BFO can be modulated by facet orientations and polarization directions, leading to huge surface band bending at its BFO/electrolyte interface during PEC reactions. Notably, the BFO film with (111)-facet and Pup state showed a ∼5.1-fold enhancement in Eint compared to the BFO film with (001)-facet and Pdown state. This was attributed to a ∼1-order reduction in leakage current in (111)-facet-Pup. These experimental results were further supported by density functional theory (DFT) calculations. Meanwhile, the (111)-facet-Pup exhibited a ∼2.8-fold increase in incident photon-to-current efficiency (IPCE) and a ∼4.8-fold improvement in charge transport density, indicating an effective charge separation and reduction in electron-hole recombination with such a synergistic effect. This work emphasizes the synergistic effect of facet-engineered surface and ferroelectric polarization on manipulating charge transfer between ferroelectrics/electrolyte interface and polarization magnitude, offering a generic strategy for optimizing the functionality of ferroelectric-based photoelectrodes for solar-driven water splitting.