1 research outputs found
Subsurface Polaron Concentration As a Factor in the Chemistry of Reduced TiO<sub>2</sub> (110) Surfaces
Surface
reactivity of rutile TiO<sub>2</sub> (110) surfaces has
long been ascribed to bridging oxygen vacancies (V<sub>O</sub>), but
recently, excess electrons introduced by donor defects are being considered
as the main players. However, the spatial distribution of them is
not yet clear due to difficulties in interpreting filled state images
of scanning tunneling microscopy (STM). In this study, several different
images available in the literature are consistently interpreted using
density functional theory (DFT). The key factors are polarons in the
second layer below Ti<sub>5c</sub> row (Ti<sub>5c‑2nd</sub> polarons) and a temperature dependence of their concentration. Bright
blobs in the experimental images are interpreted as Ti<sub>5c‑2nd</sub> polarons. At 78 K, their concentration reaches 33.3% ML, where 1
ML is defined as the density of (1 × 1) unit cells, regardless
of V<sub>O</sub> coverage. In contrast, at 5 K, it is twice the V<sub>O</sub> coverage. This discrepancy is understood by the ionization
of donor defects other than V<sub>O</sub>, most probably subsurface
Ti interstitials, and subsequent diffusion of polarons to Ti<sub>5c‑2nd</sub> sites at high temperature. This mechanism explains seemingly contradicting
reports on oxygen chemisorption on this surface, which suggests that
the so-called oxygen-vacancy model needs to be modified at temperature
above at least 78 K