Reaction of O2 with Subsurface Oxygen Vacancies on TiO2 Anatase (101)

TL;DR

This study combines experimental microscopy and theoretical analysis to investigate how oxygen molecules interact with and transform on the surface of niobium-doped TiO2 anatase, revealing mechanisms of adsorption and vacancy reactions.

Contribution

It provides new insights into the behavior of oxygen molecules on TiO2 surfaces, especially regarding vacancy-mediated transformations and charge transfer processes.

Findings

O2 adsorbs as superoxo on Ti sites

O2 transforms into peroxo and bridging dimer species

Oxygen vacancies are pulled to the surface and react with adsorbed O2

Abstract

Oxygen (O2) adsorbed on metal oxides is important in catalytic oxidation reactions, chemical sensing, and photocatalysis. Strong adsorption requires transfer of negative charge from oxygen vacancies (VOs) or dopants, for example. With scanning tunneling microscopy, we observed, transformed, and, in conjunction with theory, identified the nature of O2 molecules on the (101) surface of anatase (titanium oxide, TiO2) doped with niobium. VOs reside exclusively in the bulk, but we pull them to the surface with a strongly negatively charged scanning tunneling microscope tip. O2 adsorbed as superoxo (O2)- at fivefold-coordinated Ti sites was transformed to peroxo (O2)2- and, via reaction with a VO, placed into an anion surface lattice site as an (O2)O species. This so-called bridging dimer also formed when O2 directly reacted with VOs at or below the surface.