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.

作为一种典型的光催化剂，TiO ₂是具有科学技术意义的材料。在光催化和其他应用中，TiO ₂经常被还原，表现为具有独特物理化学性质的n型半导体。在这篇综述中，我们总结了对还原的，未掺杂的TiO ₂中过量电子的基本性质和应用的理解的最新进展。我们讨论了在金红石和锐钛矿型TiO ₂的本体中以及表面的过量电子的特征，着重于它们的定位，空间分布，能级和动力学性质。我们研究了光激发TiO ₂的电子态的特定特征，用于固有的氧空位和Ti间隙缺陷，以及用于表面羟基。我们讨论了金红石相和锐钛矿相中过量电子行为的异同。最后，我们考虑过量电子对反应性的影响，重点是过量电子与被吸附物之间的相互作用。