Inducing n-type photoanodic behavior in p-type bismuth ferrite via ferroelectric polarization

Bismuth ferrite (BFO) has shown great promise as a photoelectrode for photoelectrochemical (PEC) reactions arising from its multiferroic properties at room temperature and suitable band gap for solar harvesting. Further, despite being a p-type semiconductor, the internal electrical field in BFO can induce significant anodic photocurrent, indicating a regulation of the p/n-type behavior of the material by exploiting its switchable polarization capacity. However, the mechanism behind how the polarization field controls the p/n-type behavior of a BFO photoelectrode remains ambiguous. Here, we report on the effects of polarization on surface chemistry, charge dynamics and ultimately PEC performance, and how these effects enable BFO to switch from p-type to n-type behavior. Conventionally, prompting a p-to-n type behavioral switch in BFO requires the introduction of oxygen vacancies by annealing/vacuum treatment at an elevated temperature, where the majority charge carrier in BFO is changed. In contrast, the origin of induced n-type behavior in down-polarized BFO is different and is attributed to a negatively-charged surface, gradient energy modulation, and negatively shifted band energies. The strategy of exploiting the polarization states of BFO photoelectrodes to switch n/p-type behavior offers a facile approach for developing a tunable photoelectrode in PEC systems.