Excess electrons in reduced rutile and anatase TiO₂

As a prototypical photocatalyst, TiO₂ is a material of scientific and technological interest. In photocatalysis and other applications, TiO₂ is often reduced, behaving as an n-type semiconductor with unique physico-chemical properties. In this review, we summarize recent advances in the understanding of the fundamental properties and applications of excess electrons in reduced, undoped TiO₂. We discuss the characteristics of excess electrons in the bulk and at the surface of rutile and anatase TiO₂ focusing on their localization, spatial distribution, energy levels, and dynamical properties. We examine specific features of the electronic states for photoexcited TiO₂, for intrinsic oxygen vacancy and Ti interstitial defects, and for surface hydroxyls. We discuss similarities and differences in the behaviors of excess electrons in the rutile and anatase phases. Finally, we consider the effect of excess electrons on the reactivity, focusing on the interaction between excess electrons and adsorbates.