Diffusion-Controlled Charge Transfer from Excited Ru(bpy)₃²⁺ into Nanosized TiO₂ Colloids Stabilized with EDTA

Abstract

Transparent colloidal TiO 2 was prepared in aqueous EDTA solution. The average particle size was 7 nm, measured by TEM. In the presence of EDTA, the colloids were stable within a pH range from 2 to 10. In contrast to previous observations involving Ru(bpy) 3 2+ and TiO 2 where no emission quenching occurred up to pH 10, an efficient quenching of the excited Ru(bpy) 3 2+ ( 3 MLCT) by the colloidal TiO 2 was obtained in the presence of EDTA. A detailed kinetics investigation by laser flash photolysis showed the quenching process to be diffusion-controlled. The fact that not all the excited-state population returned to the ground state in a simple exponential process led to the conclusion that Ru(bpy) 3 3+ was being formed and the quenching was by injection of electrons into the colloid. The overall quantum yield of formation of Ru(bpy) 3 3+ was increased from 5.8% to 12.9% as the ionic strength increased from 0.1 to 1.5 M at pH 2.8. It was concluded that the increase in ionic strength weakens the electrostatic interaction between the geminate redox photoproducts Ru(bpy) 3 3+ ...e -(TiO 2 ) and hence promotes the cage release of Ru(bpy) 3 3+ . With increases in temperature, both the charge-transfer (triplet quenching) rate and the release of the geminate species Ru(bpy) 3 3+ ...e -(TiO 2 ) were enhanced. The activation energy for the former process was measured as 22.0 ( 0.3 kJ mol -1 , consistent with the typical values (8-25 kJ mol -1 ) reported for the diffusion-controlled reactions in homogeneous solutions. For the separation of the geminate species, a lower activation energy of 7.1 ( 0.5 kJ mol -1 was evaluated, implying a typical diffusion-controlled process. A detailed pH dependence study indicated that the charge-transfer rate increased as the pH decreased below 6, while at pH > 6 no charge transfer could be observed. This is surmised to be due to driving force dependence.

Photoinduced interaction between xanthene dyes and colloidal CdS nanoparticles

“…From the scheme we observed that the electron transfer process is feasible. Similar type of electron transfer between dyes and semiconductors has been previously reported [42][43][44][45][46].…”

The Measure of TiO2 Photocatalytic Efficiency and the Comparison of Different Photocatalytic Titania

“…The lack of a pronounced effect of particle size under our conditions means that the competition between the oxidation of water by reaction 2 and reduction of the holes depends little, if at all, on particle size. The rate of reaction 2 is not expected to depend on particle size, because the rate of electron transfer through the TiO 2 /water interface is only little affected by the size. − This conclusion applies to reduction of adsorbed oxygen and its intermediates as well as interfacial electron transfer to the holes from adsorbed molecules. Note that in an earlier publication slower reduction rates have been reported for TiO 2 particles of 1 nm diameter .…”

“…The rate of reaction 2 is not expected to depend on particle size, because the rate of electron transfer through the TiO 2 /water interface is only little affected by the size. [74][75][76] This conclusion applies to reduction of adsorbed oxygen and its intermediates as well as interfacial electron transfer to the holes from adsorbed molecules. Note that in an earlier publication slower reduction rates have been reported for TiO 2 particles of 1 nm diameter.…”

Luminescence quenching of tris(2,2′-bipyridine)ruthenium(II) by 2,6-dimethylphenol and 4-bromo-2,6-dimethylphenol in sol–gel-processed silicate thin films

“…The luminescent excited state of Ru(bpy) 2+ 3 is assigned to the metal-to-ligand chargetransfer (MLCT) state, whose properties are very sensitive to polarity and viscosity of the environment due to the MLCT character. The dynamic properties of the photoexcited state of Ru(bpy) 2+ 3 and related complexes adsorbed on semiconductors are of much interest, because the complexes in photoexcited state transfer an electron to certain semiconductors such as TiO 2 , resulting in dye sensitization and charge separation [9][10][11]. These processes are important for solar energy conversion and artificial photosynthetic applications.…”