Microwave photodielectric and photoconductivity studies on titanium dioxide exposed to continuous, polychromatic irradiation Part I: A novel analytical tool to assess the photoactivity of titanium dioxide

A novel analytical method is described which allows the charge-carrier dynamics taking place when titanium dioxide is subjected to ultrabandgap irradiation to be measured. Samples are exposed continuously to polychromatic light and photophysical processes monitored in real-time. The method relies on the perturbations that take place in the stored energy characteristics of a microwave cavity. In such a device, the electric and magnetic fields of the microwave energy reach a maximum when they are resonant with the cavity. Titanium dioxide powders held in the cavity, and simultaneously irradiated with visible light, produce free-carriers which reduce the stored energy density of the cavity. This results in a shift in the value of the resonant frequency and an attenuation of the microwave power, which are in proportion to the population of free-carriers produced in the sample. The frequency measurements are tempered by the presence of localised (trapped) carriers. Uncoupling the changes in power and frequency allows the dynamics of free and trapped carriers to be quantified. The results derived from this real-time method are consistent with those reported from time-resolved microwave conductivity (TRMC) measurements, which follow the decay of conductivity following a pulse of high intensity light. In addition, the real-time method follows the build-up of charge-carriers during irradiation as well as their decay immediately after irradiation. Data is presented for anatase, rutile and a sample of mixed morphology (75% anatase:25% rutile). The influences of sample size, humidity, temperature, light-intensity and wavelength on the microwave response, are considered. (C) 1998 Elsevier Science S.A.