  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.
