Abstract
Silver iodide (AgI) is an efficient ice-nucleating material. This has been related to the close lattice match to hexagonal ice, which helps to nucleate ice crystals on its surface under supersaturated conditions. In turn, the structure of water molecules adsorbed on its surface, embodied in the coordination of hydrogen bonds, has not been addressed so far. We suspected that AgI may induce ice-like coordination among adsorbed water molecules already under subsaturated conditions. X-ray photoelectron spectroscopy was used to probe the structure and composition at the AgI surface. We determined the chemical properties of the surface, the thickness of adsorbed water, and the amount of contaminating carbon species. Auger electron yield near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to assess the hydrogen bonding (HB) structure. The O K-edge NEXAFS spectra indicated that the HB structure of the adsorbed water on AgI under subsaturated conditions showed similarity to that of ice, which helps facilitate the stabilization of ice embryos at saturation. The approach used here opens up important perspectives for characterizing adsorbed water molecules on a wide variety of solids, which provides an important basis for understanding ice nucleation and other interfacial processes at the molecular level.
Highlights
Ice nucleation and ice particles are exerting a substantial impact on various physical and chemical processes of the atmosphere
The number of water molecules reversibly adsorbed on the surface of AgI particles at equilibrium was monitored via the O 1s photoemission signal intensity excited by 870 eV photons, as a function of relative humidity (Figure 1)
As the relative humidity increases (Figure 1, b and a), a new broad feature appears at 532.5 eV, whose relative intensity is higher at 90% RH than at 60% RH, which we attribute to adsorbed water molecules.[53]
Summary
Ice nucleation and ice particles are exerting a substantial impact on various physical and chemical processes of the atmosphere. The atmosphere contains a multitude of condensed materials, which can stabilize an ice embryo and initiate ice nucleation at much higher temperatures and lower supersaturation. This is referred to as heterogeneous ice nucleation. It is the dominant mechanism by which ice forms in mixed phase clouds or by which cirrus ice particles form.[5]
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