The surface science of titanium dioxide - Niser

82 U. Diebold / Surface Science Reports 48 (2003) 53±229
Fig. 14. STM image …200 Ð 200 І of a TiO 2 (1 1 0) surface, sputtered and annealed in UHV to 1100 K for 10 min,
showing point defects. Features labeled with `A' have been assigned as oxygen vacancies. The position of the line scan shown
is indicated in the image. From [116].
vacancydensityestimates from spectroscopic measurements. The size of the bright spots is of the order
of one single atom (FWHM 5 AÊ in line scans along the [0 0 1] direction) when taking into account
convolution with the tip. Theyalways appear as isolated spots with no apparent short-range ordering
but a slight tendencyto be staggered perpendicular to the rows. It is well-known that oxygen vacancies
are healed bydosing a reduced TiO 2 sample with oxygen [127,130,131]. When the sample in Fig. 14
was stepwise exposed to oxygen, the density of the bright spots (A) decreased. This, and the contrast
expected from electronic structure calculations (missing bridging oxygens give rise to a protrusion in
empty-states charge-density contours, see Fig. 9b) led to the assumption of the bright spots being
missing oxygen vacancies [110,116]. As was reported in [116], their appearance in STM is stronglytipdependent.
The dark features (B in Fig. 14) are less common (surface concentration of 1±2%) than the oxygen
vacancies. Theycan extend over several unit cells. Upon adsorption of oxygen, the number of the dark
spots stays constant within the statistical error. They were tentatively assigned as subsurface oxygen
vacancies [109], however, ®rst-principles total-energycalculations show that such a con®guration is
highlyunlikely[132]. Their nature is unclear at this point.
The `A' features, assigned to oxygen vacancies in Fig. 14, are mobile in STM images, albeit in a
somewhat erratic fashion [116]. Theycan be removed byscanning with a high bias voltage [116,133],
hence one needs to consider that theyrepresent adsorbates, speci®callyH atoms adsorbed on a bridging
O atom. In [116] this possibilitywas discarded because the densityof these features decreases upon
exposure to molecular oxygen, as expected for a `®lling' of vacancies, and because they could not be
¯ashed off. Suzuki et al. [133], however, argued that these features represent H atoms on bridging
oxygen atoms because their number density decreases upon irradiation with 20 eV-electrons and
increases upon exposure of the surface to atomic hydrogen. A few `A' features were resistant against
irradiation with electrons and were attributed to H atoms trapped on an oxygen vacancy. These authors
concluded that H adatoms, either on bridging oxygen atoms or on O vacancies, still exist on TiO 2 (1 1 0)
when surfaces were prepared bysputtering and annealing. The H was supposed to stem from the bulk.
Recent STM of clean and water-covered surfaces showed that oxygen vacancies and hydroxylated
bridging oxygen atoms are imaged slightly different in STM images [134], see Fig. 15. The main