Dynamics of water adsorption on TiO2 monitored by work function spectroscopy

Water adsorption dynamics on two TiO2 (110) rutile surfaces at room temperature has been investigated using the work function (WF) change as a function of time. The first surface was prepared in a standard way using sputtering/annealing cycles, whereas the second one was long term annealed at 620K in moderate vacuum conditions (the residual gas pressure of about 1×10-7mbar) and cleaned afterwards. The WF change show striking difference as compared to those obtained for highly reduced TiO2 (110) rutile or the (2×1) reconstructed surfaces. For the first kind of surface we show that the observed adsorption dynamics can be qualitatively explained by the present understanding of the water adsorption on non-reconstructed TiO2 (110) rutile surface according to which the bridging oxygen vacancies and Ti rows are the main adsorption sites. Although generally similar to the former results, water adsorption dynamics on the second kind of the surface has an additional feature that can be only explained by a new adsorption site, which we suggest to be due to (2×1) reconstructed regions coexisting with the non-reconstructed TiO2 (110) surface.