Domain-engineered BiFeO₃ thin-film photoanodes for highly enhanced ferroelectric solar water splitting

In photoelectrochemical (PEC) water splitting, charge separation and collection by the electric field in the photoactive material are the most important factors for improved conversion efficiency. Hence, ferroelectric oxides, in which electrons are the majority carriers, are considered promising photoanode materials because their high built-in potential, provided by their spontaneous polarization, can significantly enhance the separation and drift of photogenerated carriers. In this regard, the PEC properties of BiFeO₃ thin-film photoanodes with different crystallographic orientations and consequent ferroelectric domain structures are investigated. As the crystallographic orientation changes from (001)_pc via (110)_pc to (111)_pc, the ferroelastic domains in epitaxial BiFeO₃ thin films become mono-variant and the spontaneous polarization levels increase to 110 μC/cm². Consequently, the photocurrent density at 0 V vs. Ag/AgCl increases approximately 5.3-fold and the onset potential decreases by 0.180 V in the downward polarization state. It is further demonstrated that ferroelectric switching in the (111)_pc BiFeO₃ thin-film photoanode leads to an approximate change of 8,000% in the photocurrent density and a 0.330 V shift in the onset potential. This study strongly suggests that domain-engineered ferroelectric materials can be used as effective charge separation and collection layers for efficient solar water-splitting photoanodes.