The Effect of Surface Chemistry on the Photocatalytic Applications of Semiconductor Quantum Dots

McClelland, Kevin Patrick

doi:https://doi.org/10.21985/n2-zkct-mm24

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The Effect of Surface Chemistry on the Photocatalytic Applications of Semiconductor Quantum Dots

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This dissertation describes modifications to the surface of colloidal semiconductor quantum dots (QDs) to improve their use as photocatalysts for charge transfer reactions. The surface atoms and ligands of these nanoparticles can interfere with direct charge transfer to a reagent by inhibiting binding, forming trap states, or acting as competitive electron or hole acceptors. Many of these effects can be mitigated through passivation of the surface either by shelling the QDs or by adding strong-binding ligands. However, by understanding how the surface chemistry affects charge transfer, it can simultaneously be used to enhance both the selectivity and activity of photocatalytic reactions using QDs. Using ligands with phosphonate binding groups, rather than strong-binding thiolates, to solubilize CdS QDs in polar media, leaves many surface cadmium atoms undercoordinated. When performing photocatalytic oxidation reactions, such as the oxidation of benzyl alcohol to benzaldehyde or hydrobenzoin, this exposed Cd2+ is reduced to form Cd0. Metallic cadmium can then act as a co-catalyst to convert the formed benzaldehyde to hydrobenzoin. Promoting the photodeposition of Cd0 by adding excess Cd2+ can, therefore, result in high selectivity for coupled product. Inversely, inhibiting cadmium reduction by adding a competitive electron scavenger gives almost entirely aldehyde. The photocatalysis with thiolate ligands showed no surface reduction and poor alcohol conversion due to strong passivation of the surface and competitive oxidation of the ligand. With less oxidizing CdSe QDs, the same thiolate ligands create surface hole traps which decrease the rate of hole transfer to a reagent. While a slower rate of hole transfer would inhibit many photocatalytic reactions, it can be useful for photo-induced electron transfer reversible addition-fragmentation chain transfer (PET-RAFT) polymerization as it proceeds through an activation/deactivation mechanism by electron transfer/charge recombination. The decreased rate of charge recombination therefore increases the time spent in the active state, and, in conjunction with the high stability of the QDs, makes the QD more efficient than a leading molecular photocatalysts for the aqueous polymerization of acrylamide and acrylate monomers. The large size of these nanoparticles furthermore allows for a unique method of purification that promotes photocatalyst recycling and block co-polymer synthesis.

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