Molecular level energy and electron transfer processes at nanocrystalline titanium dioxide interfaces.

摘要

This thesis describes photo-induced molecular electron and energy transfer processes occurring at nanocrystalline semiconductor interfaces. The Introductory Chapter provides background and describes how these materials may be useful for solar energy conversion.; In Chapter 2, results describing 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, immersed in an acetonitrile bath are presented. The data indicates that light excitation forms predominately long-lived metal-to-ligand charge-transfer, MLCT, excited states under these conditions. 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 x 108 s-1.; Chapter 3 extends the work described in Chapter 2 to LiClO4 acetonitrile solutions. Li+ addition results in a red shift in the MLCT absorption and photoluminescence, PL, 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. 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 photophysical and photoelectrochemical properties of porous nanocrystalline anatase TiO2 electrodes modified with Ru(deeb)(bpy)2 2+, Os(deeb)(bpy)22+, and mixtures of both are described in Chapters 4 and 5. In regenerative solar cells with 0.5 M LiI/0.05 M I2 acetonitrile electrolyte, both compounds efficiently inject electrons into TiO2 producing monochromatic incident photon-to-current efficiencies (IPCE), IPCE (460 nm) = 0.70 + 0.05 for Ru(dcb)(bpy)2 2+/TiO2 and 0. 10 + 0.05 for Os(dcb)(bpy)2 2+/TiO2. Os(dcb)(bpy)22+ extends the spectral sensitivity of the TiO2 material beyond 700 rim. Application of a negative bias to the derivatized TiO2 surfaces results in inefficient interfacial electron transfer and no significant photocurrent. Instead, lateral energy transfer cross the nanocrystalline TiO2 surface from Ru(dcb)(bpy)22+* to Os(dcb)(bpy) 22+ is observed. The energy transfer process can be switched off with a positive applied bias ten times with no significant deterioration. The results demonstrate control of molecular excited states at nanostructured interfaces.