Crystal nucleation and growth at a liquid-liquid interface is studied on the atomic scale by in situ Å-resolution X-ray scattering methods for the case of liquid Hg and an electrochemical dilute electrolyte containing Pb 2+ , F − , and Br − ions. In the regime negative of the Pb amalgamation potential Φ rp = − 0:70 V, no change is observed from the surface-layered structure of pure Hg. Upon potential-induced release of Pb 2+ from the Hg bulk at Φ > Φ rp , the formation of an intriguing interface structure is observed, comprising a well-defined 7.6-Å-thick adlayer, decorated with structurally related 3D crystallites. Both are identified by their diffraction peaks as PbFBr, preferentially aligned with theirc axis along the interface normal. X-ray reflectivity shows the adlayer to consist of a stack of five ionic layers, forming a single-unit-cellthick crystalline PbFBr precursor film, which acts as a template for the subsequent quasiepitaxial 3D crystal growth. This growth behavior is assigned to the combined action of electrostatic and shortrange chemical interactions.electrochemistry | liquid metal L iquid-liquid and liquid-gas interfaces provide exciting new possibilities for material synthesis (1, 2). Contrary to solid interfaces, which exhibit strain and stress, heterogeneities, and defects such as steps, which all strongly affect growth processes, fluid systems provide soft, defect-and stress-free interfaces. The high mobility of reagents, products, and deposited particles in liquid phases facilitates the growth process as well as the selfassembly of ordered particle arrays at the interface.A large variety of materials has been prepared via deposition at liquid-liquid interfaces, such as metals (1), oxides (3, 4), chalcogenides (5, 6), polymers (7), plasmonic materials (2), and nanoparticle catalysts of ceria (3), Pd (8, 9), and Pt (10). As demonstrated by Carim et al., deposition at liquid-liquid interfaces even allows the synthesis of group IV semiconductors such as Ge from oxide materials via a simple one-step, room-temperature electrochemical process (11). Different methods for nanoparticle manufacturing, such as deposition by reduction of metal ions (12) or electrochemical deposition (11), are available at the liquidliquid interface, allowing for particle modification and growth control via adjustment of concentration or interfacial potential.Despite the absence of long-range order, liquid interfaces provide the possibility to control the crystallinity, shape, and orientation of deposits. Examples are the growth of single-crystalline CuO and CuS films (4), the surfactant-induced oriented growth of calcite crystals (13), and the formation of pyramidal PbS crystallites with defined, high surface area facets (5). These phenomena were rationalized by energetic effects, such as the interface energies, surface charges, and specific chemical interactions, as well as by the growth kinetics. However, detailed insight into the phase formation mechanisms is generally precluded by lack of atomicscale data on the in...

The study of liquid-liquid interfaces with X-ray scattering methods requires special instrumental considerations. A dedicated liquid surface diffractometer employing a tilting double-crystal monochromator in Bragg geometry has been designed. This diffractometer allows reflectivity and grazing-incidence scattering measurements of an immobile mechanically completely decoupled liquid sample, providing high mechanical stability. The available energy range is from 6.4 to 29.4 keV, covering many important absorption edges. The instrument provides access in momentum space out to 2.54 Å(-1) in the surface normal and out to 14.8 Å(-1) in the in-plane direction at 29.4 keV. Owing to its modular design the diffractometer is also suitable for heavy apparatus such as vacuum chambers. The instrument performance is described and examples of X-ray reflectivity studies performed under in situ electrochemical control and on biochemical model systems are given.

The atomic-scale structure of the mercury-electrolyte (0.01 M NaF) interface was studied as a function of temperature and potential by x-ray reflectivity and x-ray diffuse scattering measurements. The capillary wave contribution is determined and removed from the data, giving access to the intrinsic surface-normal electron density profile at the interface, especially to the surface layering in the Hg phase. A temperature dependent roughness anomaly known from the Hg-air interface is found to persist also at the Hg-electrolyte interface. Additionally, a temperature dependence of the layering period was discovered. The increase in the layer spacing with increasing temperature is approximately four times lager than the increase expected from thermal expansion. Finally, the interface is found to broaden towards the electrolyte side as the potential becomes more negative, in agreement with the Schmickler-Henderson theory. Our results favor a model for the interface structure, which is different to the model formerly used in comparable studies.

Detailed in operando studies of electrochemically induced PbBrF deposition at the liquid mercury/liquid electrolyte interface are presented. The nucleation and growth were monitored using time-resolved X-ray diffraction and reflectivity combined with electrochemical measurements, revealing a complex potential-dependent behavior. PbBrF deposition commences at potentials above −0.7 V with the rapid formation of an ultrathin adlayer of one unit cell thickness, on top of which (001)-oriented three-dimensional crystallites are formed. Two potential regimes are identified. At low overpotentials, slow growth of a low surface density film of large crystals is observed. At high overpotentials, crossover to a potential-independent morphology occurs, consisting of a compact PbBrF deposit with a saturation thickness of 25 nm, which forms within a few minutes. This potential behavior can be rationalized by the increasing supersaturation near the interface, caused by the potential-dependent Pb2+ deamalgamation, which changes from a slow reaction-controlled process to a fast transport-controlled process in this range of overpotentials. In addition, growth on the liquid substrate is found to involve complex micromechanical effects, such as crystal reorientation and film breakup during dissolution.