PHOTOSENSITIZATION OF QUANTUM-SIZE TIO2 PARTICLES IN WATER-IN-OIL MICROEMULSIONS

Ultrasmall TiO2 particles (2R = 9.0 +/- 0.5 Angstrom) were generated i n situ in a water-in-oil (w/o) microemulsion composed of water, cetyld imethylbenzylammonium chloride (CDBA), and benzene, by controlled hydr olysis of TiCl4. Effective photosensitization of the TiO2 particles by Ru(II) tris(bipyridine), Ru(bpy)(3)(2+), proceeds in the w/o microemu lsion. The photosensitization of TiO2 was studied by following the emi ssion decay of excited RU(bpy)(3)(2+) at different TiO2 concentrations . The emission decay curves of excited Ru(bpy)(3)(2+) were analyzed by assuming a Poisson distribution of the TiO2 particles over the revers e micelles and realizing that excited Ru(bpy)(3)(2+) photosensitizes T iO2 by two photosensitization pathways: (a) intramicellar electron inj ection quenching (k(q)) and (b) particle incorporation by intermicella r exchange (k(c)) followed by fast intramicellar quenching. The derive d values of intramicellar electron injection and intermicellar exchang e rate constants were kg = 7 X 10(6) s(-1) and 1.4 x 10(9) M(-1) s(-1) , respectively. The kinetic analysis allowed to estimate the mean aggl omeration number of the TiO2 particles to be 11 +/- 2, which correspon ds to a particle diameter of 9.0 +/- 0.5 Angstrom, Effects of the wate r-pool size on the TiO2 particle structure and photosensitization proc ess were also investigated. As the water-pool size increases, the inte rmicellar exchange rate constant is enhanced but the intramicellar ele ctron injection rate is unaffected. The agglomeration number is higher in larger water pools.