STEADY-STATE PHOTODIELECTRIC MICROWAVE SPECTROSCOPY - CHARACTERIZATION AND BEHAVIOR OF TITANIUM-DIOXIDE PIGMENTS

This paper reports on some recent findings in our current work carried out on an evaluation of the photoactivity and photoconductivity of ti tanium dioxide pigments using steady-state photodielectric microwave s pectroscopy. The two crystalline forms of titanium dioxide, anatase an d rutile with and without a surface coating are investigated Each pigm ent exhibited a different microwave resonant frequency absorption prof ile, depending upon the nature of the crystalline modification and the presence of a surface coating. In this work, the effect of time, inte nsity and wavelength of irradiation and the temperature and humidity o f the microwave cavity on the resonant frequency absorption changes of the pigments have been examined. Thus, on steady-state illumination o f the pigments in the microwave cavity with polychromatic light the re sonant frequency losses exhibited shifts, the nature of which were aga in dependent upon the pigment type and coating. The anatase pigments e xhibited greater responses compared to the rutile pigments, particular ly with regard to the shift in the resonant frequency values and the c hange in the Q factor of the resonant peak. The resonant frequency val ues of the anatase samples were also at higher values than those obtai ned for the rutile samples. Furthermore, the frequency shifts were in opposite directions, with the anatase modification exhibiting a more r apid shift to lower frequencies and rutile a slower shift to higher fr equencies. It would appear that when the anatase samples were irradiat ed over a prolonged period of time the resonant peak reverts to a high er frequency value. Such processes are related to the differences in t he thermodynamics of the crystal lattice. In the case of anatase, it i s proposed that free charge carries are produced an irradiation, where as with the rutile form the energy input is only sufficient to cause e xcitation of the charge carriers to a higher energy level within the b and gap, and this effectively introduces a space charge separation of the charge carriers, similar to that of an induced polarisation proces s. Coated pigments exhibited slower photo responses. In this case, the coating on the pigment particle surface is acting as a trap recombina tion centre. This process would reduce the number of charge carriers g enerated. An increase in the cavity temperature increases the photodie lectric responses of both pigment modifications due to the promotion o f charge carriers to higher energy levels, thus increasing the conduct ivity of the pigments. At higher temperatures rutile behaves like anat ase, An increase in moisture had the reverse effect, due to the moistu re on the surface of the pigment reducing its photoactivity by reactin g with charge carriers, thus reducing the amount of photocurrent. The data are discussed in relation to the known photoactivity behaviour of titanium dioxide pigments in polymeric media. Preliminary data on oth er semi-conductor materials such as selenium and zinc oxide, are also presented in the light of the above behaviour.