Electrodeposition of Au and AgAu Nanoparticles from Reverse Micelles – Tuning Particle Properties and Understanding Solvent and Confinement Effects by Ensemble and Single Entity Approaches
Authors: Schroeder Rossi Thais , Spallek Marius , Jaugstetter Maximilian , Gerwin Maximilian , Yang Fengli , Ortega Eduardo , Roldan Cuenya Beatriz , Tschulik Kristina 
Published: February 2024

The synthesis and reactivity of nanoparticles (NPs) has vastly been studied for years,[1-3] yet still there are several open questions about how a synthesis method and its parameters influence the final properties of the obtained NPs. In the classical wet-chemical synthesis, capping agents are widely used to adjust the morphology and size, preventing further growth of NPs. However, their presence can be disadvantageous for some applications, e.g. electrocatalysis, where they decrease reaction kinetics. To overcome this issue, we use electrodeposition as synthetic route, and the size of the obtained NP is tuned by the amount of precursor that is loaded inside a reverse micelle. The NP formation is induced by applying a suitable potential to reduce the entrapped precursor at the electrode. This deposition can be done by ensemble or single entity approaches.
Experimentally speaking, both approaches consist of an electrode immersed in a media with suspended precursor-loaded reverse micelles. However, for ensemble electrodeposition a macroelectrode is used, a standard chronoamperometry is recorded and average information about the system is provided. On the other hand, single entity approach relies on random collision of reverse micelles at a micro- or nanoelectrode, which appears as spikes on the chronoamperometric response, and data can be determined based on statistical analysis of single particle events.[4,5] It has been used to characterize different types of NPs, their size, composition and kinetic reaction parameters.[6,7]

Focusing on the electrosynthesis, this work investigates the formation of Au and AgAu NPs by both ensemble and single entity approaches. Besides the synthesis method, the effect of the solvent and the 0D confinement is also under evaluation to better understand how one can tune NPs’ shape and structure, i.e. core-shell vs alloy arrangements. The solvent effect is investigated by using room temperature ionic liquid or organic solvents during electrodeposition. The reverse micelles act as a nanoreactor cage, confining the precursors, and therefore changing the type of this cage helps us to understand the influence of the 0D confinement. For this, reverse micelles of two different materials were used: SDS (sodium dodecyl sulphate) and PS-P2VP (polystyrene-b-poly(2-vinylpyridine)), with the former forming nanodroplets of an aqueous phase protected by a thin wall, and the latter assembling to a sphere with a polymeric core, completely binding the precursor.
The resulting NPs were imaged by AFM, SEM and TEM and their composition resolved by EDX. Initial catalytic CV and LSV measurements towards hydrogen evolution reaction were done to provide further insights about the NPs’ properties. From these characterizations of the NPs, it has been demonstrated that the synthesis approach, the applied potential, the solvent, and the type of RM influence the final NPs’ properties. Hence, the size, composition, structure and catalytic activity of the resulting NPs can by tuned by electrochemical parameters, avoiding the necessity to select different capping and reducing agents as done in wet-chemical synthesis.
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