Recombination Pathways in the Degussa P25 Formulation of TiO2: Surface versus Lattice Mechanisms

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Abstract
Charge migration between electron trapping sites within the mixed-phase titania photocatalyst Degussa P25 has been studied. In addition to previously described lattice electron trapping sites on both anatase and rutile phases, surface electron trapping sites and an anatase−rutile interface trapping site specific to Degussa P25 are identified. The relationship between these sites and recombination with surface hole trapping sites is also determined. It is experimentally shown that upon band-gap illumination holes appear at the surface and preferentially recombine with electrons in surface trapping sites. These findings indicate that in mixed-phase TiO2, such as Degussa P25, photogenerated holes are trapped exclusively on the particle surface, while photogenerated electrons are trapped within the nanoparticle lattice. Recombination reactions are dominated by surface reactions that follow charge migration. These findings indicate that, in mixed-phase TiO2, such as Degussa P25, a random flight mechanism of recombination predominates. Such knowledge simplifies the mechanistic mathematical models used for process design and points the way for improving future oxidative titania catalysts.