Visible-light driven generation of reactive radicals over BiFeO<inf>3</inf>/TiO<inf>2</inf> nanotube array: experimental evidence and energetic mechanism

© 2015, Springer Science+Business Media Dordrecht. BiFeO&lt;inf&gt;3&lt;/inf&gt;-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 BiFeO&lt;inf&gt;3&lt;/inf&gt;. In this work, BiFeO&lt;inf&gt;3&lt;/inf&gt; nanoparticles smaller than 100 nm have grown into TiO&lt;inf&gt;2&lt;/inf&gt; nanotube array by an ultrasonic-immersion method. Some evidences on the energy band position of BiFeO&lt;inf&gt;3&lt;/inf&gt; nanoparticles in BiFeO&lt;inf&gt;3&lt;/inf&gt;/TiO&lt;inf&gt;2&lt;/inf&gt; 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 TiO&lt;inf&gt;2&lt;/inf&gt; nanotube array, which reveals the photo-induced electron transfer from BiFeO&lt;inf&gt;3&lt;/inf&gt; nanoparticles to TiO&lt;inf&gt;2&lt;/inf&gt; nanotubes. Furthermore, both ·OH and ·O&lt;inf&gt;2&lt;/inf&gt; &lt;sup&gt;-&lt;/sup&gt; radicals could be detected by the electron paramagnetic resonance (EPR) spin-trapping method. According to the EPR result, the valence band edge of BiFeO&lt;inf&gt;3&lt;/inf&gt; 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 BiFeO&lt;inf&gt;3&lt;/inf&gt;/TiO&lt;inf&gt;2&lt;/inf&gt; nanotube array