Magnetically separable BiFeO3 nanoparticles with a γ-Fe2O3 parasitic phase: controlled fabrication and enhanced visible-light photocatalytic activity

A facile sol–gel approach with a fixed calcination temperature is developed to prepare BiFeO3 (BFO) nanoparticles, and the gel-drying temperature is adjusted to control the appearance of a γ-Fe2O3 parasitic phase. The room temperature ferromagnetism of the samples is strongly dependent on the gel-drying temperature. When the gel-drying temperature increases from 80 to 140 °C, the saturated magnetization of the corresponding samples jumps from 0.22 emu g−1 to 1.2 emu g−1, allowing the nanoparticles to be magnetically separated in solution. From examination by transmission electron microscopy and X-ray photoelectron spectroscopy, it is confirmed that the γ-Fe2O3 parasitic phase is nucleated during the gel-drying process under high temperatures above 120 °C, and remains in the subsequent annealing process, resulting in the anomalous enhanced magnetization. Comparing with pure BFO nanoparticles prepared under low gel-drying temperature, the BFO/γ-Fe2O3 samples exhibit significantly increased visible-light photocatalytic ability towards rhodamine B. The formation of a heterojunction structure between the BFO and γ-Fe2O3 phases is proposed to be responsible for the enhanced photocatalytic activity. Further enhanced photocatalytic activity is obtained in this study when adding a small amount of H2O2 during photocatalysis, indicating the samples have photo-Fenton-like catalytic activity.