| Abstract
| - Specific adsorption of cations (H+, Li+, ...) on TiO2 nanocrystalline particles is known to control the energeticsof the conduction band and therefore the ability for molecular sensitizers to inject electrons into thesemiconductor upon irradiation. In photoelectrochemical energy conversion devices employing dye-sensitizedtitanium dioxide mesoporous electrodes, back electron transfer is generally intercepted by the use of theiodide/triiodide couple as a charge mediator. Kinetics of the oxidation of I- by the oxidized state of cis-RuII(dcbpy)2(NCS)2 sensitizer adsorbed on TiO2 was measured by flash photolysis in propylene carbonate. Therate of this reaction was found to depend on the nature and concentration of added cations such as Mg2+, Li+,Na+, and K+. A brusque acceleration of the process was in particular observed at a critical concentration.Electrophoretic measurements showed that this step in the dye regeneration reaction kinetics corresponds tothe reversal of particle surface charge upon adsorption of potential-determining species, which causes I- toefficiently adsorb onto the oxide. These observations strongly suggest that the specific adsorption of cationson TiO2 nanoparticles governs the formation of (I-, I-) ion pairs on the surface, and allows the moreenergetically favorable and faster mechanism involving oxidation of I- to I2•- radical to take place.
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