Assembly and reactivity of nanoparticles at liquid / liquid interfaces

The interface between two immiscible electrolyte solutions (ITIES), or liquid|liquid interface, provides a defect-free junction for fundamental studies of adsorption phenomena, heterogeneous charge transfer reactions, and phase-formation processes. Recent research interests include the organization and reactivity of metallic and semiconducting nanoparticles (NPs) at ITIES. In a general sense, the present thesis is devoted to addressing this issue, including the voltage-induced assembly of charged NPs, the photoreactivity of semiconducting NPs and the redox properties of alkanethiolate monolayer-protected gold nanoclusters. The adsorption of ionic species at the liquid|liquid interface was first modeled theoretically. The aim of the simulations was to envisage the effect of the adsorption of charged NPs on the interfacial charge distribution across the liquid|liquid interface. For simplicity, in the calculations the charged NPs were considered as large ions, which were further taken as point charges. Various adsorption isotherms were considered, including potential independent, Langmuir and Frumkin models. The simulation indicates that the ionic adsorption has a strong influence on the potential distribution across the interface. Under certain conditions, it results in a non-monotonous potential profile with a trap at the interface. The voltage induced assembly of mercaptosuccinic acid (MSA) stabilized gold and cadmium selenide (CdSe) NPs were investigated at the polarizable water|1,2-dichloroethane (DCE) interface. The studies reveal that the surface concentration of NPs at the liquid|liquid boundary is reversibly controlled by the applied bias potential. There was no evidence of irreversible aggregation or deposition of the particles at the interface. Furthermore, the adsorbed CdSe NPs at the water|DCE interface demonstrated the photoreactivity characteristic of a self-assembled ultrathin p-type semiconductor photoelectrode. The surface structure also has significant effect on the optical properties of CdSe NPs. The redox properties of hexanethiolate monolayer-protected gold nanoclusters (MPCs) were studied in detail at electrochemical interfaces. First, the absolute standard redox potential of MPCs in solutions was theoretically derived from electrostatic considerations. The significant effect of the solvent polarity was verified experimentally by studies in various organic solvents. Second, the redox properties of self-assembled MPCs on a gold electrode were studied in organic electrolytes and room temperature ionic liquids (RTILS). The effect of the electrostatic interaction between MPCs and electrode on the redox properties of MPCs was theoretically considered in terms of the method of images in classical electrostatics and justified experimentally. In RTILS, the rectification of the successive oxidation of self-assembled MPCs by the anionic component of RTILS was observed. Third, MPCs were further used as redox quenchers at the polarizable water|DCE interface. Photocurrent responses originating from the heterogeneous quenching of photoexcited porphyrins by MPCs dissolved in the DCE phase were observed. As MPCs can function as both electron acceptors and electron donors, the photocurrent results from the superposition of two simultaneous processes, which correspond to the oxidation and reduction of MPCs. The magnitude of the net photocurrent is essentially determined by the balance of the kinetics of these two processes, which can be controlled by tuning the Galvani potential difference between the two phases. We show that, within the available potential window, the apparent electron transfer rate constants follow the classical Butler-Volmer dependence on the applied potential difference.

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