Reactivity of titanium dioxide with oxygen at room temperature and the related charge transfer

The measurements of electron work function were applied for in situ monitoring of the charge transfer during oxidation and reduction for well-defined titanium dioxide, TiO2, at room temperature. The TiO2 specimen was initially standardized at 1173 K in the gas phase of controlled oxygen activity, at P(O-2) = 10 Pa, and then cooled down in the same gas phase. The work function changes were monitored during oxidation at room temperature at P(O-2) = 75 kPa and subsequent reduction at P(O-2) = 10 Pa. It is shown that oxidation of TiO2 at room temperature results in fast oxygen chemisorption, involving initially the formation of singly ionized molecular oxygen species, followed by the formation of singly ionized atomic oxygen species, and subsequent slow oxygen incorporation. Although all these processes lead to work function increase, the components of the work function changes related to the individual processes may be distinguished based on different kinetics. The obtained work function data indicate that oxidation results in rapid surface coverage with singly ionized molecular oxygen species, which are subsequently dissociated leading to the formation of singly ionized atomic species. The related chemisorption equilibria are established within 2 and 5 h, respectively. Oxygen incorporation leads to slow work function changes, which achieve a maximum within 100 h. The determined work function data were assessed by using a theoretical model that describes the electrical effects related to different mechanisms of TiO2 oxidation. The work function data indicate that oxygen incorporation leads to structural changes of the outermost surface layer resulting, in consequence, in a change of the external work function component. Reimposition of the initial gas phase, P(02) = 10 Pa, leads to partial desorption of weakly adsorbed molecular species formed during oxidation.