Surface chemistry of nanostructures: 1) interactions of mixed monolayers of carboxylic acids on titania, 2) synthesis and immobilization of aqueous cadmium selenide quantum dots
Nevins, Jeremy S.
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This thesis will focus on (1) characterization of mixed monolayers of thiol-terminated (T) and methyl-terminated (Me) carboxylic acids on nanocrystalline TiO 2 thin films, (2) the synthesis of aqueous CdSe quantum dots (QDs), with particular emphasis on the influence of capping-group functionality and reaction conditions on the kinetics and mechanism of particle growth, and (3) attachment of CdSe QDs to TiO 2 thin films and their photoelectrochemical performance as a function of surfactant in QD-sensitized solar cells (QDSSCs). Mixed monolayers have been used in many applications, such as chemical sensing, biomolecular recognition, molecular electronics, catalysis, and as building blocks for materials assembly. Mixed monolayers of T and Me on TiO 2 underwent dimerization-induced compositional changes. Me was displaced on the surface by T because of the formation of intermolecular disulfide bonds between thiol groups of T adsorbed to the TiO 2 surface. The compositional changes were found to vary as a function of solvent, alkyl chain length of T, steric bulk of adsorbates, and surface-binding and terminal functional groups. The findings illustrate that dimerization and other intermolecular interactions between adsorbates may dramatically influence the composition and terminal functionalization of mixed monolayers. Semiconductor QDs are attractive alternatives to molecular chromophores and bulk semiconductors for light-harvesting applications in photovoltaics and photocatalysis. Aqueous QDs are of particular interest due to their straightforward, cost-effective, and environmentally-benign syntheses. CdSe QDs were synthesized in basic aqueous suspensions at room temperature under ambient conditions by mixing a cadmium precursor, selenide precursor, and one of several carboxylate-functionalized capping groups (cysteinate, mercaptopropionate, and mercaptosuccinate). The photophysical properties of the QDs varied with capping-group functionality, concentration of precursors, and pH of the aqueous reaction mixture. Varying these parameters allowed for systematic control of the kinetics and mechanism of particle growth, as well as the size and size distribution of QDs at equilibrium. Under certain conditions, "magic-sized" clusters (MSCs) of CdSe, rather than regular QDs, were preferentially synthesized. The carboxylated capping groups of aqueous QDs were used as bifunctional linkers, allowing for facile attachment to nanocrystalline TiO 2 thin films. Equilibrium binding experiments were performed to quantify the adsorption of regular QDs and MSCs to nanocrystalline TiO 2 thin films. Finally, photoelectrochemistry was used to quantify the influence of capping-group functionality on the efficiency of electron injection from adsorbed QDs into TiO 2 and the power-conversion efficiency of QDSSCs.