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 ultraband gap irradiation to be measured. Samples are exposed continuously to po lychromatic light and photophysical processes monitored in real-time. The method relies on the perturbations that take place in the stored e nergy characteristics of a microwave cavity. In such a device, the ele ctric and magnetic fields of the microwave energy reach a maximum when they are resonant with the cavity. Titanium dioxide powders held in t he cavity, and simultaneously irradiated with visible light, produce f ree-carriers which reduce the stored energy density of the cavity. Thi s results in a shift in the value of the resonant frequency and an att enuation of the microwave power, which are in proportion to the popula tion of free-carriers produced in the sample. The frequency measuremen ts are tempered by the presence of localised (trapped) carriers. Uncou pling the changes in power and frequency allows the dynamics of free a nd trapped carriers to be quantified. The results derived from this re al-time method are consistent with those reported from time-resolved m icrowave conductivity (TRMC) measurements, which follow the decay of c onductivity following a pulse of high intensity light. In addition, th e real-time method follows the build-up of charge-carriers during irra diation as well as their decay immediately after irradiation. Data is presented for anatase, rutile and a sample of mixed morphology (75% an atase:25% rutile). The influences of sample size, humidity, temperatur e, light-intensity and wavelength on the microwave response, are consi dered. (C) 1998 Elsevier Science S.A.