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

Rutile titanium dioxide (TiO 2 ) (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 (OH t 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 OH t 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 (OH b groups) intrinsically present on the (1 × 1) surface. On the basis of this observation, we hypothesize that dissociation of H 2 O molecules by evacuation of the water layer produces OH t groups. The OH b groups, produced with the OH t groups by this dissociation, react with O 2 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 OH t 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.