Defective BiFeO3 with surface oxygen vacancies: Facile synthesis and mechanism insight into photocatalytic performance

Oxygen vacancies particularly for those located on the photocatalyst surface are believed to play an important role in its photocatalytic process. In this work, surface oxygen vacancies were introduced into hydrothermally-synthesized BiFeO3 (BFO) nanocrystals through high pressure hydrogenation treatment, and with increasing hydrogenation temperature the oxygen vacancy concentration would increase. The X-ray diffraction (XRD) results indicated no pronounced structure change when the hydrogenation temperature was below 200 degrees C. The formation of surface oxygen vacancies on BFO nanocrystals was verified by a-variety of techniques such as high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) and Kelvin probe force microscopy (KPFM). The hydrogenated BFO nanocrystals exhibited nearly four times methyl orange (MO) photodegradation efficiency higher than pristine BFO under visible light irradiation, and the inherent correlation between surface oxygen vacancies and enhanced photo catalytic activity was established. Our findings demonstrate that the formation of surface oxygen vacancies on the hydrogenated BFO nanocrystals could narrow the band gap of BFO resulting in the improved light absorption capability, act as trapping centers for photoinduced electrons thus to facilitate the separation of the photo generated electron-hole pairs as well as the production of predominant active species (hydroxyl radicals) for MO photodegradation, suppress the recombination of photogenerated electrons and holes, and also favor the adsorption of MO molecules. All these factors could contribute to the observed enhanced photocatalytic activity of the hydrogenated BFO nanocrystals for MO degradation. In addition, the as-formed surface oxygen vacancies were also stable at room temperature.