Assemblies of gold nanoparticles at liquid-liquid interfaces: from liquid optics to electrocatalysis

The interface between two immiscible liquids, i.e. oil and water, is an extremely attractive scaffold to self-assemble nanoparticles (NP) in arranged films. The liquid-liquid interfaces (LLIs) are defect-free and pristine by the nature, they are transparent and self-healing as well as mechanically flexible. Therefore, the present thesis is devoted to self-assembly of gold NPs into nanofilms at various LLIs and further investigation of their optical, mechanical and electrochemical properties. The thesis consists of two large parts: (i) self-assembly of gold nanoparticles into Metal Liquid-Like Droplets (MeLLDs) and (ii) electrocatalytic capability of nanoparticles placed at the interface. In the first part we showed that the irreversible adsorption of AuNPs at LLIs was achieved by charging nanoparticles with a lipophilic electron donor, tetrathiafulvalene (TTF). The established role of TTF was to charge directly the gold core of citrate stabilized AuNPs, reducing the Coulombic repulsion between separate NPs, as well as to "glue"€ nanoparticles together, providing the mechanical stability of the film. The present process was facile and required only vigorous shaking of organic phase with TTF and aqueous phase with AuNPs. Detailed study of optical properties demonstrated that MeLLDs could be used as liquid mirrors and filters with tuned optical response by varying NPs size, concentration and the solvent’s nature. This study opens a new way to understand requirements for voltage-induced self-assembly of NPs at the interface of two immiscible electrolyte solutions (ITIES). Further, we investigated ion and electron transfer properties of nanoparticle assemblies. A new method was developed to prepare nanofilm in four-electrode electrochemical cell. It consisted in precise microinjection of pre-concentrated solution of AuNPs in methanol directly at the ITIES. Primary achievement of this technique was prevention pollution of aqueous and organic phases. The nanofilm occupied roughly 30% of available surface area and did not interfere with the ion transfer across the ITIES. However, these films showed a capability to be charge by electron donors in organic phase with subsequent formation of the corresponding ions. In the second part we focused on electron transfer (ET) reactions at ITIES between redox couple in adjunct phases and interfacial redox electrocatalysis phenomenon at AuNPs film modified ITIES. Firstly, the difference between pure heterogeneous ET (HET) reactions and ET-IT mechanism was distinguished by numerical simulations. In the latter case, homogeneous ET reaction is followed by ion-transfer (IT). The model system consisted of ferrocene (Fc) in organic phase and ferry/ferro-cyanide (Fe(CN)6) in aqueous phase illustrated co-existence of HET and ET-IT mechanisms, but the main contribution to the overall observed current belonged to the IT across ITIES. Secondly, we developed a concept of Fermi level equilibration to describe observed interfacial ET for the model system with and without AuNPs at ITIES. Functionalization of the interface with AuNPs significantly improved kinetics of the interfacial reaction by changing reaction mechanism to a bipolar pathway, where NP provided electrical conductance between two phases. Therefore, we highlighted the interfacial redox electrocatalitic property of nanofilms towards HET from the viewpoint the developed theory.

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