Subsurface Polaron Concentration As a Factor in the Chemistry of Reduced TiO2 (110) Surfaces

• Surface reactivity of rutile TiO2 (110) surfaces has long been ascribed to bridging oxygen vacancies (V-o), 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-sc row (Tisc-2nd polarons) and a temperature dependence of their concentration. Bright blobs in the experimental images are interpreted as Tisc-2nd polarons. At 78 K, their concentration reaches 33.3% ML, where 1 ML is defined as the density of (1 x 1) unit cells, regardless of v(o) coverage. In contrast, at 5 K, it is twice the Vo coverage. This discrepancy is understood by the ionization of donor defects other than V-o, most probably subsurface Ti interstitials, and subsequent diffusion of polarons to Tisc-2nd 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.

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NATURVETENSKAP  -- Kemi -- Fysikalisk kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Physical Chemistry (hsv//eng)

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