Chemical Physics
Imaging surface structure and premelting of ice Ih with atomic resolution
This study provides a molecular perspective on the origin and mechanism of ice premelting, resolving longstanding debates on ice surface structures.
Imagine trying to see the detailed pattern on a delicate snowflake before it melts. It's incredibly difficult. For decades, scientists faced a similar problem trying to see the surface of ice at the smallest possible scale—the level of individual atoms. They knew the surface was important, but couldn't get a clear picture. In this study, researchers used a revolutionary microscope with a tip so fine it's like a record player needle for atoms. By working in an extremely cold, stable environment, they gently 'felt' the surface of the ice without breaking it. They discovered the surface isn't a single, perfect crystal pattern like a tiled floor. Instead, it's a patchwork quilt of two slightly different patterns stitched together. They also witnessed the very first moment of melting, which started right at the 'seams' of this quilt, not everywhere at once.
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Atomic-resolution imaging of the basal (0001) surface structure of hexagonal water ice (ice Ih) achieved using qPlus-based cryogenic atomic force microscopy.
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The crystalline ice-Ih surface consists of mixed Ih- and cubic (Ic)-stacking nanodomains, forming periodic superstructures.
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Density functional theory reveals the reconstructed surface is stabilized by minimizing electrostatic repulsion between dangling OH bonds.
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The ice surface becomes disordered with increasing temperature, indicating the onset of the premelting process.
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Surface premelting occurs from defective boundaries between Ih and Ic domains and is promoted by planar local structure formation.
Ice surfaces are closely relevant to many physical and chemical properties, such as melting, freezing, friction, gas uptake and atmospheric reaction. Despite extensive experimental and theoretical investigations, the exact atomic structures of ice interfaces remain elusive owing to the vulnerable hydrogen-bonding network and the complicated premelting process. Here we realize atomic-resolution imaging of the basal (0001) surface structure of hexagonal water ice (ice Ih) by using qPlus-based cryogenic atomic force microscopy with a carbon monoxide-functionalized tip. We find that the crystalline ice-Ih surface consists of mixed Ih- and cubic (Ic)-stacking nanodomains, forming periodic superstructures. Density functional theory reveals that this reconstructed surface is stabilized over the ideal ice surface mainly by minimizing the electrostatic repulsion between dangling OH bonds. Moreover, we observe that the ice surface gradually becomes disordered with increasing temperature (above 120 Kelvin), indicating the onset of the premelting process. The surface premelting occurs from the defective boundaries between the Ih and Ic domains and can be promoted by the formation of a planar local structure. These results put an end to the longstanding debate on ice surface structures and shed light on the molecular origin of ice premelting, which may lead to a paradigm shift in the understanding of ice physics and chemistry.