Interaction Patterns of Water and Oxide Crystals Revealed Using 3D Analysis of Hydration Structures

Oxide crystals such as sapphire and quartz are widely used in various industrial applications, but their atomic-scale structures are poorly understood. Sapphire and quartz primarily comprise aluminum oxide and silicon dioxide, respectively. These compounds have a high affinity for water, which affects their chemical reactivity.

By understanding the water-binding properties of these oxides, scientists can utilize them for further innovative applications. However, conventional microscopic methods only offer insights into the two-dimensional topography of their surfaces.

Understanding the Hydrated Form of Oxide Crystals

At Kanazawa University in Japan, a research group led by Keisuke Miyazawa from the NanoLSI developed a 3D microscopy technique to investigate the interaction of the surfaces of oxide crystals with water.

The experiment started with analyzing the surface structures and hydration structures of sapphire and α-quartz in water. This was made possible using an advanced form of microscopy called 3D atomic force microscopy (3D-AFM). Oxide crystals are known for having hydroxyl groups, the main "water-binding" molecules, closely linked with the oxides. For this reason, the researchers studied the hydroxyl group and its hydration structures on both crystals when submerged in water.

The researchers discovered that the hydration layer on sapphire was not uniform due to the nonuniform local distributions of the surface hydroxyl groups. Meanwhile, there is a uniform hydration layer on α-quartz because of the atomically flat distributions of the surface hydroxyl groups.

Upon measuring the interaction force of the oxides for water, it was found that a greater force was needed to break the water-crystal bonds on sapphire than in α-quartz. Additionally, this affinity was much higher in areas where the oxide was in close proximity to the hydroxyl groups.

The result of the study revealed that the hydration structures of oxides depend on the location and density of hydroxyl groups and the hydrogen bonding strength of the hydroxyl groups. It also shows that 3D-AFM can potentially unravel the interaction of water with several surfaces, allowing experts to understand the solid-liquid interactions better.

What is 3D Atomic Force Microscopy (3D-AFM)?

Three-dimensional atomic force microscopy refers to the cutting-edge technology used to observe solvent molecule distributions at solid-liquid interfaces by mapping forces in 3D space. It is used in lipid membranes, mica, DNA surfaces, and proteins to reveal their interfacial hydration structures.

Atomic force microscopy is an advanced form of microscopy where a sharp tip is placed on a cantilever and follows the surface of a molecule. While this procedure is done, signals are emitted by the tip based on its movement, helping to identify the topography of the molecule. This technique has been applied to various flat or nearly flat molecules, but it becomes limited when used on molecules with three-dimensional topography.

As a high-resolution non-optical imaging technique, 3D-AFM enables the experts to scan larger sample areas without the risk of crashing the tip into an unexpected obstacle like an atomic step or a different type of molecule that can exist on the surface. Moreover, this approach allows the molecule and the substrate to be imaged simultaneously, simplifying the determination of the orientation and location of the molecule on the surface.