The photophysical and photoelectrochemical properties of Ru(deeb)(bpy)2(PF6)2, where bpy is 2,2′bipyridine and deeb is 4,4′-(COOEt)2-2,2′-bipyridine, anchored to nanocrystalline TiO2 (anatase) or ZrO2 films are reported. In neat acetonitrile (or 0.1 M tetrabutylammonium perchlorate) long-lived metal-toligand charge transfer (MLCT) excited states are observed on both TiO2 and ZrO2. Addition of LiClO4 results in a red shift in the MLCT absorption and photoluminescence, PL, spectra on both TiO2 and ZrO2, and a concentration-dependent quenching of the PL intensity on TiO2. The Li + -induced spectroscopic changes were found to be reversible by varying the electrolyte composition. Time-resolved absorption measurements demonstrate that the presence of lithium cations increases the quantum yield for interfacial charge separation with no discernible influence on the rate of charge recombination. A second-order kinetic model quantified charge recombination transients. A model is proposed wherein Li + ion adsorption stabilizes TiO2 acceptor states resulting in energetically more favorable interfacial electron transfer. The generality of this model was explored with different electrolytes and sensitizers. In regenerative solar cells, the addition of Li + increases both the efficiency and long wavelength sensitivity of the cell.
Coordination compounds of the general type Ru(dmb)2(LL)(PF6)2, where dmb is 4,4‘-(CH3)2-2,2‘-bipyridine and LL is 4-(CH3)-4‘-(COOH)-2,2‘-bipyridine, or 4-(CH3)-4‘-((CH2)3COOH)-2,2‘-bipyridine, or 4-(CH3)-4‘-((CH2)3COCH2COOC2H5)-2,2‘-bipyridine were prepared for the attachment to semiconductor metal oxide surfaces. The optical and redox properties of these compounds in dichloromethane solution are reported. Binding to porous nanostructured TiO2 films was analyzed with the Langmuir adsorption isotherm model. Photoelectrochemical measurements of the modified TiO2 electrodes in regenerative solar cells are reported. The results indicate that intimate electronic coupling between the surface link and the chromophoric ligand is not a strict requirement in the design of sensitizers for photovoltaic applications. Interfacial kinetics for recombination of the electron in the solid with the oxidized form of the sensitizer were quantitated by excited state absorption spectroscopy.
The optical and electrochemical properties of nanocrystalline Ti02 electrodes in acidic aqueous solution are reported. Cyclic voltammetry, UVlvisMIR attenuance, EPR, photocurrent spectroscopy, and electrochemical impedance measurements are presented. At negative applied potentials the Ti02 films turn black, and an EPR spectrum appears which we attribute to Ti(II1) species. The results are described in terms of a model in which the reduction of Ti02 leads to band edge unpinning.
Excitation of Ru(deeb)(bpy)2 2+, bis(2,2‘-bipyridine)(2,2‘-bipyridine-4,4‘-diethylester)ruthenium(II) hexafluorophosphate, bound to nanocrystalline TiO2 thin films and immersed in an acetonitrile bath at 25 °C under an argon atmosphere, results in the formation of a species, the transient spectral characteristics for which are, consistent with a metal-to-ligand charge transfer, MLCT, excited state. The spectrum decays by kinetics that are inconsistent with a simple first-order process. Modeling of the data as a function of irradiance has been accomplished assuming parallel unimolecular and bimolecular excited-state deactivation processes. The quantum yield for excited-state formation depends on the excitation irradiance, consistent with triplet−triplet annihilation processes that occur with k ≥ 1 × 108 s-1.
An alternative mechanism for dye sensitization of wide bandgap semiconductors has been realized: reductive quenching of the dye excited state(s) followed by thermal interfacial electron transfer. The processes were identified using nanosecond-time-resolved absorption and photoluminescence spectroscopies after selective excitation of Ru(deeb)(bpy)2(PF6)2, where bpy is 2,2‘-bipyridine and deeb is 4,4‘-(COOC2H5)2-2,2‘-bipyridine, sensitizers anchored to a nanocrystalline (anatase) TiO2 film immersed in a 0.1 M tetrabutylammonium perchlorate acetonitrile solution with phenothiazine electron donors. With the electrolyte changed to 0.1 M lithium perchlorate, this same assembly undergoes the generally accepted mechanism for dye sensitization: the sensitizer excited state(s) transfer an electron to the semiconductor and is subsequently reduced by the phenothiazine donor.
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