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Photoinduced Charge Carrier Dynamics in Colloidal Quantum Dots with Application in Photocatalysis

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This dissertation describes the fundamental studies of photoinduced charge-carrier transfer from colloidal metal chalcogenide quantum dots (QDs) to surface-adsorbed molecular redox partners. In addition, we also present the use of visible-light absorbing QDs in photocatalytic applications. CdS QDs are used as photocatalysts in a C-C coupling reaction, in which no sacrificial reagent is used. The QDs efficiently carry out both redox half reactions. We show that the rate-limiting step of this reaction is hole transfer to one of the starting materials, pyrrolidine, and through a simple ligand exchange procedure, we increase the disorder in the ligand shell of the QDs, thereby making it more permeable to pyrrolidine. As such, we are able to increase the efficiency of the reaction by a factor of 2.3. We also present a study in which we use CdS QDs to photosensitize a Cu(II)-hydroxyl coordination complex for the photocatalytic hydrogen atom abstraction from 9,10-dihydroanthracene (DHA) to produce anthracene. Both the CdS QDs and the QD/Cu(II)-hydroxyl system are able to carry out this photochemical reaction. However, in one case, the QD/Cu(II)-hydroxyl system performs up to 11 times better than the CdS QDs alone. This lower performance is probably due to the fact that when the QDs act as the solo photocatalysts, the reaction proceeds through the stepwise oxidation/deprotonation of DHA. In contrast, when the reaction proceeds through the photosensitization of the Cu(II)-hydroxyl species, the hydrogen atom transfer mechanism consists of a concerted Cu(III) centered reduction, and hydroxyl-centered protonation. We suggest methods for circumventing some of the limits presented by this system. Spontaneous electron transfer from PbS QDs to a surface adsorbed tetracyanoquinonedimethane molecule is investigated. Additionally, we investigate the charge carrier dynamics of PbS QDs when excited in the preformed PbSx+/TCNQx- ion pair. We observe the ultrafast excited state decay of the PbS QDs in the preformed ion pair. We show that this decay is because the localized charges present in the preformed ion pair act as recombination centers for the photogenerated electron and hole. We present a study in which we are able to accumulate two electrons onto a cyclophane redox acceptor from photoexcited CdS QDs using a single photon pulse. This process is achieved through a multi-photon absorption mechanism, which generates the biexcitonic state of the CdS QDs. The QDs are then able to sequentially transfer two electrons to the electrostatically bound cyclophane from their biexcitonic state. We show that the rates for the first and second electron transfer steps are 1 ps and 5 ps respectively. We further show that the charge-separated state in the singly reduced cyclophane is on the order of hundreds of microseconds, whereas it is on the order of tens of nanoseconds in the doubly reduced state. We propose, through electron paramagnetic resonance (EPR) spectroscopy experiments, that the long charge-separated state in the singly reduce case is because the electron hops between the two arms of the cyclophane molecule with a lifetime on the order of hundreds of nanoseconds. We further design a system in which we tether a hole accepting dimer to the surface of CdS QDs, through a carboxylate linker. The goal for this design is to achieve the accumulation of two oxidative equivalents onto the hole acceptor from the biexcitonic state of the QDs. We show that in the current implementation of the system, single-hole transfer occurs in approximately 5 ps, but multi-oxidation of the acceptor cannot be achieved. We propose a strategy to circumvent this limit.

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  • 01/09/2019
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