An Atomic-Scale Study of TiO2(110) Surfaces Exposed to Humid Environments

Rutile titanium dioxide (TiO2) (110)-(1 × 1) surfaces prepared in an ultrahigh vacuum (UHV) were transferred to humid environments and thereafter returned to UHV to be examined by the scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) techniques. STM images showed that hydroxyl groups terminating 5-fold-coordinated Ti atoms (OHt groups) and hydroperoxyl (OOH) groups, with densities of 0.9 and 0.2 nm–2, respectively, formed after exposure to water vapor at a pressure of several tens of pascals. The density of the OHt groups was found to exceed that predicted when they were assumed to have originated from another type of OH groups involved in bridging O atoms (OHb groups) intrinsically present on the (1 × 1) surface. On the basis of this observation, we hypothesize that dissociation of H2O molecules by evacuation of the water layer produces OHt groups. The OHb groups, produced with the OHt groups by this dissociation, react with O2 molecules dissolved in the water layer, forming OOH groups. Surfaces exposed to laboratory air or immersed in liquid water were also found to be rich in OHt groups; consequently, evacuation-induced processes occurred on those surfaces. Clarification of the effects of evacuation of the water layer implies the possibility of characterizing (1 × 1) surfaces in humid environments by ex situ analysis under UHV.