Steady-state photodielectric microwave spectroscopy: characterisation and behaviour of titanium dioxide pigments

Abstract

This paper reports on some recent findings in our current work carried out on an evaluation of the photoactivity and photoconductivity of titanium dioxide pigments using steady-state photodielectric microwave Spectroscopy. The two crystalline forms of titanium dioxide, anatase and rutile with and without a surface coating are investigated. Each pigment exhibited a different microwave resonant frequency absorption profile, depending upon the nature of the crystalline modification and the presence of a surface coating. In this work. the effect of time, intensity and wavelength of irradiation and the temperature and humidity of the microwave cavity on the resonant frequency absorption changes of the pigments have been examined. Thus, on steady-state illumination of the pigments in the microwave cavity with polychromatic light the-resonant frequency losses exhibited shifts, the nature of which were again dependent upon the pigment type and coating. The anatase pigments exhibited, greater responses compared to the rutile pigments, particularly with regard to the shift in the resonant frequency values and the change in the Q factor of the resonant peak. The resonant frequency values of the anatase samples were also at higher values than those obtained for the rutile samples. Furthermore, the frequency shifts were in opposite directions, with the anatase modification exhibiting a more rapid shift to lower frequencies and rutile a slower shift to higher frequencies. It would appear that when the anatase samples were irradiated over a prolonged period of time the resonant peak reverts to a higher frequency value. Such processes are related to the differences in the thermodynamics of the crystal lattice. In the case of anatase, it is proposed that free charge carriers are produced on irradiation, whereas with the rutile form the energy input is only sufficient to cause excitation of the charge carriers to a higher energy level within the band gap, and this effectively introduces a space charge separation of the charge carriers, similar to that of an induced polarisation process. Coated pigments exhibited slower photo responses. In this case. the coating on the pigment particle surface is acting as a trap recombination centre. This process would reduce the number of charge carriers generated. An increase in the cavity temperature increases the photodielectric responses of both pigment modifications due to the promotion of charge carriers to higher energy levels, thus increasing the conductivity of the pigments. At higher temperatures rutile behaves like anatase. An increase in moisture had the reverse effect, due to the moisture on the surface of the pigment reducing its photoactivity by reacting 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 other semi-conductor materials such as selenium and zinc oxide, are also presented in the light of the above behaviour. © 1993.