The Hidden Roughness of Sapphire Surface

The Hidden Roughness of Sapphire Surface

Why do certain surfaces behave very differently from what theoretical calculations suggest? Scientists long assumed that the aluminum oxide surface should be highly reactive and capable of splitting water molecules. In experiments, however, this behavior is barely observed.

At TU Wien, researchers have found an answer that may also help explain the behavior of many other materials: At the atomic scale, the surface looks completely different from what had been assumed. Instead of a smooth and regularly ordered surface, the outermost atoms are arranged in an irregular way, which dramatically changes chemical properties of the surface.

A Surprisingly Unreactive Surface

"For decades, researchers assumed that cutting aluminum oxide along its basal plane would create a surface terminated by a regular layer of aluminum atoms," says Jan Balajka, corresponding author of the study. Such a surface should be highly reactive and catalyze chemical reactions, for example the dissociation of water molecules into hydrogen atoms and OH groups. But experiments proved disappointing: The observed reactivity fell far short of theoretical predictions.

Imaging the Surface with Atomic Resolution

Researchers in the surface physics group of Prof. Ulrike Diebold at the Institute of Applied Physics at TU Wien investigated the surface using a combination of density functional theory calculations and noncontact atomic force microscopy. This precise imaging technique can resolve the surface atom by atom.

The results were surprising. "The surface is not smooth and regularly ordered," says Ulrike Diebold. "Instead, we found that it is remarkably irregular and rough at the atomic scale."

Only tiny regions of the surface consist of the ordered aluminum atoms previously expected to cover the entire surface. After just a few nanometers, this regular structure breaks down and the surface becomes rough, with local height variations spanning several atomic layers.

Geometry Determines Chemistry

"This atomic-scale disorder has a decisive effect on the chemical behavior of the surface," explains Jan Balajka. "The previously accepted theory may be correct for the small regular regions, but most of the surface is rough and inhomogeneous, and therefore behaves very differently."

The results show that atomic-scale structure must be taken into account when considering chemical reactions on surfaces - not only for aluminum oxide, but for many other materials used in catalysis, thin-film growth and other technological applications.

The study shows that the chemical behavior of a material cannot be understood solely from its chemical composition. The atomic-scale structure of the surface is equally important. Even surfaces that appear perfectly smooth under an ordinary microscope may, on the scale of individual atoms, consist of a highly irregular landscape with very different local chemical properties.

Original publication

J.I. Hütner-Reisch et al., AFM imaging reveals the unreconstructed α-Al2O3(0001) surface to be inhomogeneous and rough, Nature Communications 17, 4692 (2026). , opens an external URL in a new window

Contact

Dr. Jan Balajka
Institute of Applied Physics
TU Wien
+43 1 58801 13476, +43 1 58801 13467
jan.balajka@tuwien.ac.at

Prof. Ulrike Diebold
Institute of Applied Physics
TU Wien
+43 1 58801 13425
ulrike.diebold@tuwien.ac.at