Visible-light driven generation of reactive radicals over BiFeO₃/TiO₂ nanotube array: experimental evidence and energetic mechanism

BiFeO₃-based materials have attracted broad interests due to their multiferric and photoactive properties, but there remains some controversial uncertainty about the electronic structures of various nanoparticles of BiFeO₃. In this work, BiFeO₃ nanoparticles smaller than 100 nm have grown into TiO₂ nanotube array by an ultrasonic-immersion method. Some evidences on the energy band position of BiFeO₃ nanoparticles in BiFeO₃/TiO₂ nanotube array were from the visible-light driven photo responses. Positive surface photovoltage polarity of the composite is the same as that of the pure TiO₂ nanotube array, which reveals the photo-induced electron transfer from BiFeO₃ nanoparticles to TiO₂ nanotubes. Furthermore, both ·OH and ·O₂⁻ radicals could be detected by the electron paramagnetic resonance (EPR) spin-trapping method. According to the EPR result, the valence band edge of BiFeO₃ nanoparticles could be deduced as more positive than the potential at which ·OH radical could be generated. A scheme on the energy band alignment and photo-induced charges transfer processes is then proposed based on the observed visible-light photo activity of BiFeO₃/TiO₂ nanotube array