Electric impedance spectroscopy of titania: Influence of gas treatment and of surface area

The electric charge transport in oxygen-treated titania is accomplished via two different routes. One of these pathways (A) occurs by electronic transport through the bulk phase with additional contributions from transfer through grain boundaries and between the electrode and the grains. As a parallel conduction pathway (B) ions can move along the grain surface parallel to the electric field and can surmount the grain boundaries between the grains. Because of the low voltage applied, the ion discharge at the electrode by electrochemical reaction is impeded and contributes a capacitive impedance only. According to this mechanism an equivalent circuit model was proposed that fits excellently the experimental data. When samples with a low surface area are treated in oxygen, the charge is transported mostly by electrons via pathway A. The electrons are formed by removal of oxygen leaving a surface oxygen vacancy and two electrons in the titania. Since the formation of these charge carriers is highly endothermic, the apparent activation energy of this mechanism is higher than that of the ionic conduction and thus the ionic transport along the surface becomes more important at lower temperatures. The contribution of the ionic conduction becomes important only in samples with a high surface area. When titania is treated in hydrogen the conductance is drastically increased, since surface oxygen ions are removed more easily due to reaction with hydrogen. The apparent activation energy for electronic conduction becomes smaller because the enthalpy of vacancy formation is decreased by the formation energy of water. Thus, in hydrogen treated titania the conduction is accomplished exclusively by electronic transport.