Mystery of the surface structure of aluminum oxide revealed

The secret of the complex surface structure of aluminum oxide (Al2O3) - also known as corundum, sapphire or ruby - and its surface atoms has been solved by a team of researchers after several decades.

Aluminum oxide (Al2O3), also known as corundum, sapphire or ruby, is used in a variety of applications: as an insulator in electronic components, as a carrier material for catalysts or as a chemically inert ceramic, to name just a few examples. In order to understand how chemical reactions take place on this material - for example in catalytic processes - the arrangement of the surface atoms must be known. Inside the material, the atoms follow a fixed order. This results in the characteristic shapes of crystals. On the surface, however, the structure differs from that inside the crystal. Researchers at the Vienna University of Technology and the University of Vienna have now solved the puzzle of the complex structure of the Al2O3 surface, a task that was already listed as one of the "three mysteries of surface science" in 1997. The research group led by Jan Balajka and Ulrike Diebold recently published their results in the renowned journal Science.

The research team used atomic force microscopy to analyze the surface structure. In this method, the surface is scanned at close range with a sharp tip mounted on a quartz tuning fork. The frequency of the tuning fork changes when the tip interacts with the atoms on the surface without touching the material. This produces an image of the surface. Johanna Hütner, who carried out the experiments, explains: "In a topography image, you can see the position of the atoms, but not their chemical identity. By modifying the tip, we were able to achieve chemical sensitivity. A single oxygen atom was placed at the very end of the tip, which allowed us to distinguish between oxygen and aluminum atoms on the surface. The oxygen atom at the tip is repelled by other oxygen atoms on the surface and attracted to the aluminum atoms on the Al2O3 surface. This local repulsion or attraction made it possible to directly visualize the chemical identity of the individual surface atoms together with their position."

The researchers discovered that the surface restructures in such a way that the aluminum atoms can penetrate the surface and form chemical bonds with the oxygen atoms in the deeper layers. This rearrangement of the first two atomic layers significantly reduces the energy and stabilizes the structure, while the numerical ratio of aluminium to oxygen atoms remains unchanged.

The 3D model of the aluminium oxide surface was optimized using machine learning methods. The biggest challenge was to find out how the surface is connected to the underlying crystal. "The structure is very complex, which leads to a multitude of possibilities as to how the experimentally inaccessible atoms could be arranged under the surface. State-of-the-art machine learning algorithms combined with conventional computational methods allowed us to explore numerous possibilities and determine the three-dimensional structure of the alumina surface," explains Andrea Conti, who carried out the modeling.

"By combining experimental and computational research, we have not only solved the long-standing puzzle of the insulator's atomic structure, but also discovered principles for structure formation that apply to a whole class of materials. Our results pave the way for advances in catalysis, materials science and other fields," says Jan Balajka, who led the research. Parts of the setup in which the contactless atomic force microscope is embedded have also been patented. (OM-10/24)