Selective control of localised vs. delocalised carriers in anatase TiO2 through reaction with O2

TL;DR

This study demonstrates selective control over localized and delocalized electronic states in anatase TiO2 by oxygen exposure, revealing that surface and subsurface vacancies respond differently, which is crucial for oxide surface applications.

Contribution

The paper combines experimental and theoretical methods to distinguish and control surface versus subsurface vacancy-induced states in anatase TiO2.

Findings

In-gap states are quenched by O2 exposure, while metallic states are less affected.

Surface vacancies are surface-localized, subsurface vacancies are more stable.

Subsurface vacancies are responsible for metallic states, surface vacancies for in-gap states.

Abstract

Two-dimensional (2D) metallic states induced by oxygen vacancies at oxide surfaces and interfaces provide new opportunities for the development of advanced applications, but the ability to control the behavior of these states is still limited. We used Angle Resolved Photoelectron Spectroscopy combined with density functional theory to study the reactivity of states induced by the oxygen vacancies at the (001)-(1x4) surface of anatase TiO2, where both 2D metallic and deeper lying in-gap states (IGs) are observed. Remarkably, the two states exhibit very different evolution when the surface is exposed to molecular O2: while IGs are almost completely quenched, the metallic states are only weakly affected. The energy scale analysis for the vacancy migration and recombination resulting from the DFT calculations confirms indeed that only the IGs originate from and remain localized at the surface, whereas the metallic states originate from subsurface vacancies, whose migration and recombination at the surface is energetically less favorable rendering them therefore insensitive to oxygen dosing.