The defect disorder models are derived using defect-related properties determined in the gas/solid equilibrium. These models are therefore valid at elevated temperatures. On the other hand, there is a need to use defect chemistry models for predicting the properties of TiO2 at the temperatures corresponding to the performance of Ti02-based photoelectrodes and photocatalysts, which is room temperature. It is therefore essential to assess the effects accompanied by cooling from the temperature of processing (or equilibration) to room temperature.
The measurements of the electrical conductivity may be used for in situ monitoring of the effects associated with cooling . These effects may be considered in terms of the following factors influencing defect disorder:
• Gas/Solid Kinetics. The decreasing temperature results in decreasing the thickness of the surface layer that is penetrated by oxygen from the gas phase (equilibrated).
Ionization Degree of Defects. The TiO2 defect disorder models derived in the present chapter assume that defects are fully ionized. The decrease of temperature, however, may result in a change of ionization degree of ionic defects. For example, lowering the temperature may lead to the formation of singly ionized and neutral oxygen vacancies, which are formed according to the following reactions :
In analogy, cooling may lead to a change in the ionization degree of titanium vacancies:
A schematic representation of the TiO2 electronic structure, showing the energy levels of fully ionized defects in TiO2, is in Figure 4.6 [18-20]. All these defects may exhibit different ionization degrees during cooling. The related ionization energy levels for titanium vacancies, oxygen vacancies, and titanium interstitials were determined by He et al.  (Table 4.4). These energy levels are represented in Figure 4.7.