Interfaces between liquid metals and liquid electrolytes, in particular the liquid Hg - electrolyte interface, have played a key role in the development of the theory of the electrical double layer and electrochemical adsorption. More recently, liquid-liquid interfaces have regained interest in the field of material synthesis. Unlike solid interfaces, where strain and stress, heterogeneities, and defects strongly influence growth processes, fluid systems provide soft, defect- and stress-free interfaces. In addition, the growth process profits from the high mobility of atoms, molecules, and particles in both liquids, which allows growth from both sides of the phase boundary. A large variety of metallic and non-metallic nanomaterials has been prepared via electro­chemical and electroless deposition at such liquid-liquid interfaces. As demonstrated by Maldonado and coworkers, electrodeposition at liquid metal electrodes even allows the growth of nanostructured crystalline semiconductors via a simple one-step, room-temperature electrochemical process [1].

Understanding of the fundamental processes in nucleation and growth at liquid-liquid interfaces is hampered by difficulties in studying these interfaces experimentally on the atomic scale. Most surface-sensitive techniques, especially also scanning probe microscopy methods, cannot access these fluidic phase boundaries. For this reason, the majority of studies relies on electrochemical measurements, optical microscopy, and ex situ investigation of the deposit and thus provide little insight on the initial steps of the growth process. We have shown in the past that hard X-ray scattering methods, such as X-ray reflectivity (XRR) and grazing incidence X-ray scattering (GIXS), are unique tools for determining the atomic liquid-liquid interface structure. In this talk, we present case studies of electrochemically induced growth at liquid interfaces from the first monolayer up to several ten nanometer thick films.

The first part discusses the growth of ionic compounds, using lead halides on Hg electrodes as an example. In PbBr₂ containing NaF we observed previously growth of a PbBrF layer by operando X-ray scattering. This growth exhibited a complex nucleation and growth behavior, involving a crystalline precursor layer prior to 3D crystal growth [2]. The well-defined subnanometer thick precursor layer provided a template for the subsequent quasi-epitaxial growth of oriented 3D crystallites. Detailed studies on the potential-dependent nucleation and growth kinetics revealed with increasing overpotential a crossover from a low surface density film of large crystals to a compact PbBrF deposit with a saturation thickness of 25 nm [3,4]. In addition, growth on the liquid substrate was found to involve micromechanical effects, such as crystal reorientation and film breakup during dissolution. More recently, we extended these studies to growth in solutions containing only one type of halide anion (Br, Cl, or F). Also here, the formation of precursor layers was observed, indicating that this growth behavior is a general phenomenon.

In the second part, joint X-ray scattering studies with Maldonado and coworkers on the electrochemical liquid-liquid-solid deposition of semiconductors from aqueous electrolyte are presented, focusing on Ge electrodeposition on Hg and Hg_xIn_1-x alloy electrodes [5]. We provide evidence for the adsorption of GeO₃^- anions on the liquid metal surface and the formation of a crystalline GeO₂ adlayer at the positive end of the double layer region. Ge electrodeposition results in nanocrystals, which are separated from the Hg electrode by a water cushion. Furthermore, pronounced Hg surface segregation is found in Hg_xIn_1-x, which protects the electrode surface from oxidation in the potential regime of Ge deposition.

[1] Carim, A. I., Collins, S. M., Foley, J. M. & Maldonado, J. Am. Chem. Soc. 133, 13292 (2011)

[2] A. Elsen, S. Festersen, B. Runge, C.T. Koops, B. M. Ocko, M. Deutsch, O. Seeck, B. M. Murphy, O. M. Magnussen, Proc. Nat. Acad. Sci., 110, 6663 (2013)

[3] B.M. Murphy, S. Festersen, O.M. Magnussen, Nanoscale, 8, 13859 (2016)

[4] S. Festersen, B. Runge, C. Koops, F. Bertram, B.M. Ocko, M. Deutsch, B.M. Murphy, O.M. Magnussen, Langmuir, 36, 10905 (2020)

[5] D. Pattadar, Q. Cheek, A. Satori, Y. Zhao, P.R. Giri, B. Murphy, O.M. Magnussen, S. Maldonado, Cryst. Growth Des., 21, 1645 (2021)