In situ X-ray studies of adlayer-induced crystal nucleation at the liquid-liquid interface

Crystal nucleation and growth at a liquid-liquid interface is studied on the atomic scale by in situ angstrom-resolution X-ray scattering methods for the case of liquid Hg and an electrochemical dilute electrolyte containing Pb2+, F-, and Br- ions. In the regime negative of the Pb amalgamation potential Phi(rp) = -0.70 V, no change is observed from the surface-layered structure of pure Hg. Upon potential-induced release of Pb2+ from the Hg bulk at Phi > Phi(rp), the formation of an intriguing interface structure is observed, comprising a well-defined 7.6-angstrom-thick adlayer, decorated with structurally related 3D crystallites. Both are identified by their diffraction peaks as PbFBr, preferentially aligned with their (c) over right arrow axis along the interface normal. X-ray reflectivity shows the adlayer to consist of a stack of five ionic layers, forming a single-unit-cell-thick 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 short-range chemical interactions.