Ferroelectric Bi_1+xFeO₃ Thin Film for Enhanced Photoelectrochemical Water Splitting Performance

BiFeO₃ thin films have been widely studied for photoelectrochemical water splitting application because of its narrow bandgap and good ferroelectricity which can promote the separation of photo-generated charges. Bismuth is well known as volatile and excess bismuth is usually added into the precursor to compensate the loss of bismuth during heat treatment. However, the exact quantity of excess bismuth and how excess bismuth affects its PEC performance have not been studied. Herein, self-doped Bi_1+xFeO₃ (x from 0 to 0.3) thin films are prepared via simple chemical solution deposition method. The grain size of films increases firstly and then decrease with increasing x. The loss of bismuth after annealing is confirmed by EDX and it estimates that a stoichiometric BiFeO₃ thin film can be achieved between x=0.05 and 0.1. Enhanced photocathodic photocurrent density is observed in slightly bismuth-rich films (x=0.15 and 0.2) which can be ascribed to the co-existence of rhombohedral and orthorhombic crystal structures. Moreover, the switching of ferroelectric domains in BFO thin films is observed using piezoresponse force microscopy, confirming the ferroelectric polarization. In addition, the PEC performance can be controlled via the ferroelectric polarization using electrochemical (EC) poling. Our work offers a simple and low-cost approach to enhance the photocathodic current density, which could help the further development of BiFeO₃-based thin films for PEC water splitting application.