Photosensitization of quantum-size TiO₂ particles in water-in-oil microemulsions

Ultrasmall TiO₂ particles (2R = 9.0 ± 0.5 Å) were generated in situ in a water-in-oil (w/o) microemulsion composed of water, cetyldimethylbenzylammonium chloride (CDBA), and benzene, by controlled hydrolysis of TiCl₄. Effective photosensitization of the TiO₂ particles by Ru(II) tris(bipyridine), Ru(bpy)₃²⁺, proceeds in the w/o microemulsion. The photosensitization of TiO₂ was studied by following the emission decay of excited Ru(bpy)₃²⁺ at different TiO₂ concentrations. The emission decay curves of excited Ru(bpy)₃²⁺ were analyzed by assuming a Poisson distribution of the TiO₂ particles over the reverse micelles and realizing that excited Ru(bpy)₃²⁺ photosensitizes TiO₂ by two photosensitization pathways: (a) intramicellar electron injection quenching (k_q) and (b) particle incorporation by intermicellar exchange (k_e) followed by fast intramicellar quenching. The derived values of intramicellar electron injection and intermicellar exchange rate constants were k_q = 7 × 10⁶ s^-1 and 1.4 × 10⁹ M^-1 s^-1, respectively. The kinetic analysis allowed to estimate the mean agglomeration number of the TiO₂ particles to be 11 ± 2, which corresponds to a particle diameter of 9.0 ± 0.5 Å. Effects of the water-pool size on the TiO₂ particle structure and photosensitization process were also investigated. As the water-pool size increases, the intermicellar exchange rate constant is enhanced but the intramicellar electron injection rate is unaffected. The agglomeration number is higher in larger water pools.