ESR study of photooxidation of quinoline on rutile and effects of surface states

Abstract

Election spin resonance (ESR) was used to study photolysis of quinoline on TiO 2 (rutile) pigments in the presence of gaseous oxygen and to study the effects of inorganic redox couples added as surface states ro recombine photoproduced electron-hole pairs in the TiO 2. The ESR resonance lines of most interest are at a g value of 2.004, arising from photoproduced radicals of quinoline and oxygen. Other lines observed were at g values of 1.974 due to a solid-state defect (oxygen ion vacancy) in TiO 2 and 2.009 probably due to chemisorbed oxygen. The conditions that produced the resonance line at g = 2.004 were examined in detail. Photoirradiation of quinoline-treated TiO 2 in 3 Torr O 2 with light of bandgap energy (< 400 nrn) resulted in a slow increase in the intensity of the 2.004 line. Subsequent exposure to light and dark conditions resulted in a much more rapid rise and fall in the intensity of the 2.004 line. In this case the photolysis is most rapid with 420 nm radiation. We conclude that during the intial photolysis the electron-hole pairs react chemically to produce the quinoline radical cation, Q +, and the radical oxygen anion, O − 2. These radicals combine slowly to form a relatively stable diamagnetic complex, probably QO 2. When formed this complex can be rapidly photolyzed by wavelengths of about 420 nm to form the radical complex Q +…O 2 −, which in the dark reverts to QO 2. Increasing the surface concentration of inorganic redox additives from 10 −6 to 10 −2 monomolecular layers decreased the photolysis of quinoline-treated TiO 2 as determined from a decrease in the 2.004 line. This is interpreted as a competition for the photoproduced holes between the redox additive and the quinoline. The Ce 3+ Ce 4+ redox couple additive was more effective as a recombination center than the Fe(CN) 4 4− Fe(CN) 6 3− and IrCl 6 3− IrCl 3 2− additives. Evidence of electron transfer between a bulk defect center (oxygen ion vacancy) in TiO 2 and a surface state due to Ce 4+ was deduced from the change in intensity of the 1.974 line. Water vapor competes with quinoline for photoproduced holes and decreases the 2.004 line.