Atomic-Resolution Imaging of the Ice Nucleating Silver Iodide Surface: Does this Polar Surface Reconstruct at the Atomic Scale?

Abstract

Silver iodide (AgI) particles are known for their outstanding ice nucleation ability. The effective ice nucleation has been explained by the structural similarity of the AgI surfaces and the basal plane of ice Ih. However, the relevant AgI surfaces are polar, i.e., thermodynamically instable. This fact implies the existence of a stabilization mechanism. The nature of this stabilization mechanism remains, however, unknown. Additionally, calculations suggest that exclusively the silver-terminated and not the iodine-terminated surfaces nucleate ice. So far, no atomically resolved images at any AgI–water interface exist. This is most likely due to the delicate synthesis and the photolytic degradation of AgI. Here, atomic-resolution images of the AgI–water interface of freshly cleaved crystals are provided. Most importantly, the atomic structure of these surfaces closely resembles the bulk-truncated geometry, regardless of the termination. In an attempt to reduce ion adsorption, crystals cleaved in ethanol are investigated. Also in this case, a bulk-truncated geometry is obtained. The measurements thus reveal no indication for a reconstruction of the AgI cleavage planes neither in water nor in ethanol. This study provides atomic-scale insights into the interfacial structure, which is essential for understanding the excellent ice nucleation ability of AgI.