Cooperative effect of adsorbed cations and iodide on the interception of back electron transfer in the dye sensitization of nanocrystalline TiO2

Specific adsorption of cations (H+, Li+,....) on TiO2 nanocrystalline particles is known to control the energetics of the conduction band and therefore the ability for molecular sensitizers to inject electrons into the semiconductor upon irradiation. In photoelectrochemical energy conversion devices employing dye-sensitized titanium dioxide mesoporous electrodes, back electron transfer is generally intercepted by the use of the iodide/triiodide couple as a charge mediator. Kinetics of the oxidation of I- by the oxidized state of cis-Ru-II-(dcbpy)(2)(NCS)(2) sensitizer adsorbed on TiO2 was measured by flash photolysis in propylene carbonate. The rate 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 to the reversal of particle surface charge upon adsorption of potential-determining species, which causes I- to efficiently adsorb onto the oxide. These observations strongly suggest that the specific adsorption of cations on TiO2 nanoparticles governs the formation of (I-, I-) ion pairs on the surface, and allows the more energetically favorable and faster mechanism involving oxidation of I- to I-2(. -) radical to take place.