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 parti cles is known to control the energetics of the conduction band and therefor e the ability for molecular sensitizers to inject electrons into the semico nductor upon irradiation. In photoelectrochemical energy conversion devices employing dye-sensitized titanium dioxide mesoporous electrodes, back elec tron transfer is generally intercepted by the use of the iodide/triiodide c ouple 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 meas ured by flash photolysis in propylene carbonate. The rate of this reaction was found to depend on the nature and concentration of added cations such a s Mg2+, Li+, Na+, and K+. A brusque acceleration of the process was in part icular observed at a critical concentration. Electrophoretic measurements s howed that this step in the dye regeneration reaction kinetics corresponds to the reversal of particle surface charge upon adsorption of potential-det ermining species, which causes I- to efficiently adsorb onto the oxide. The se observations strongly suggest that the specific adsorption of cations on TiO2 nanoparticles governs the formation of (I-, I-) ion pairs on the surf ace, and allows the more energetically favorable and faster mechanism invol ving oxidation of I- to I-2(. -) radical to take place.