The solid solutions of BiFeO3–BaTiO3 (BFO − BTO) with the composition in the morphotropic phase boundary (MPB) were synthesized for piezo-photocatalytic application. The MPB samples were exploited for such an application because they have two special properties, i.e., a large piezoelectric effect and the coexistence of both BFO and BTO phases. BFO has a small bandgap for efficient absorption of sunlight, but its conduction band potential is too high for photoexcited electrons to produce superoxide radicals. In contrast, despite a large bandgap, BTO has its conduction and valence band potentials both located properly for photoexcited electrons and holes to produce superoxide and hydroxyl radicals. Piezoelectric modulation of band structures allows both BFO and BTO phases to take their advantages. Methylene blue (MB) was used as a model pollutant for various catalytic processes. In sole piezocatalytic action, a degradation efficiency of 86% was observed after 120 min under the irradiation of 200 W ultrasonic wave. In comparison, a photodegradation efficiency of 68% was measured after 120 min illumination under a simulated sunlight generated from a 100 W Xenon lamp. A greatly improved degradation efficiency of 97% was achieved under both the sunlight illumination and ultrasonic vibration. As confirmed by the scavenger tests, the synergic action of piezo-photocatalysis was indeed productive in the creation of superoxide and hydroxyl radicals. This can be well explained by the piezopotential-induced band tilting, which not only helps to reduce the recombination rate, but also allows both photoexcited electrons and holes to be functional.
Hu et al. (Thu,) studied this question.