Adsorption and reactions of O2 on anatase TiO2.

Abstract

CONSPECTUS: The interaction of molecular oxygen with titanium dioxide (TiO2) surfaces plays a key role in many technologically important processes such as catalytic oxidation reactions, chemical sensing, and photocatalysis. While O2 interacts weakly with fully oxidized TiO2, excess electrons are often present in TiO2 samples. These excess electrons originate from intrinsic reducing defects (oxygen vacancies and titanium interstitials), doping, or photoexcitation and form polaronic Ti(3+) states in the band gap near the bottom of the conduction band. Oxygen adsorption involves the transfer of one or more of these excess electrons to an O2 molecule at the TiO2 surface. This results in an adsorbed superoxo (O2(-)) or peroxo (O2(2-)) species or in molecular dissociation and formation of two oxygen adatoms (2 × O(2-)). Oxygen adsorption is also the first step toward oxygen incorporation, a fundamental reaction that strongly affects the chemical properties and charge-carrier densities; for instance, it can transform the material from an n-type semiconductor to a poor electronic conductor. In this Account, we present an overview of recent theoretical work on O2 adsorption and reactions on the reduced anatase (101) surface. Anatase is the TiO2 polymorph that is generally considered most active in photocatalysis. Experiments on anatase powders have shown that the properties of photoexcited electrons are similar to those of excess electrons from reducing defects, and therefore, oxygen on reduced anatase is also a model system for studying the role of O2 in photocatalysis. Experimentally, the characteristic Ti(3+) defect states disappear after adsorption of molecular oxygen, which indicates that the excess electrons are indeed trapped by O2. Moreover, superoxide surface species associated with two different cation surface sites, possibly a regular cation site and a cation close to an anion vacancy, were identified by electron paramagnetic resonance spectroscopy. On the theoretical side, however, density functional theory studies have consistently found that it is energetically more favorable for O2 to adsorb in the peroxo form rather than the superoxo form. As a result, obtaining a detailed understanding of the nature of the observed superoxide species has proven difficult for many years. On reduced anatase (101), both oxygen vacancies and Ti interstitials have been shown to reside exclusively in the susbsurface. We discuss how reaction of O2 with a subsurface O vacancy heals the vacancy while leading to the formation of a surface bridging dimer defect. Similarly, the interaction of O2 with a Ti interstitial causes migration of this defect to the surface and the formation of a surface TiO2 cluster. Finally, we analyze the peroxo and superoxo states of the adsorbed molecule. On the basis of periodic hybrid functional calculations of interfacial electron transfer between reduced anatase and O2, we show that the peroxide form, while energetically more stable, is kinetically less favorable than the superoxide form. The existence of a kinetic barrier between the superoxo and peroxo states is essential for explaining a variety of experimental observations.