Scientists have revealed the intricate structure of the aluminum oxide surface, a problem that has perplexed researchers for many years.
Researchers at TU Wien and the University of Vienna have successfully deciphered the complex structure of the aluminum oxide surface, a mystery that has puzzled scientists for decades.
Aluminum oxide (Al2O3), commonly referred to as alumina, corundum, sapphire, or ruby, is recognized as one of the finest insulators with a variety of applications, including use in electronic devices, as a support medium for catalysts, and in ceramics that resist chemical reactions. A clear understanding of the precise arrangement of surface atoms is essential for grasping how chemical reactions occur on this material, especially in catalytic processes. While atoms within the material are organized in a specific pattern, which leads to the distinctive shapes of crystals, the structure at the surface differs from that of the inner crystal. Due to the strong insulating properties of alumina, experimental investigations have faced challenges, causing the exact surface structure to remain unspecified for over fifty years. Researchers from TU Wien and the University of Vienna have now resolved the intricate structure of the Al2O3 surface, a puzzle designated as one of the “Three Mysteries of Surface Science” in 1997. The research team, under the direction of Jan Balajka and Ulrike Diebold, recently published their findings in the journal Science.
Utilizing High-Resolution Microscopy to Identify Surface Atoms
The research team implemented noncontact atomic force microscopy (ncAFM) to scrutinize the surface structure. This technique captures images of the surface by scanning a fine tip attached to a quartz tuning fork at a minimal distance from the surface. As the tip interacts with surface atoms, the frequency of the tuning fork changes without making contact with the material. Johanna Hütner, who conducted the experiments, elaborates: “In an ncAFM image, we can observe the positions of the atoms, but not their chemical identities. We addressed the limitation in chemical sensitivity by meticulously controlling the tip. By attaching a single oxygen atom to the tip, we could differentiate between oxygen and aluminum atoms on the surface. The oxygen atom at the tip is repelled by other oxygen atoms on the surface but attracted to aluminum atoms of the Al2O3. By mapping this local repulsion or attraction, we could directly visualize the chemical identity of each atom on the surface.”
Surface Restructuring Stabilizes Composition Without Alteration
The researchers discovered that the surface undergoes a rearrangement, allowing aluminum atoms at the surface to dive into the material and form bonds with oxygen atoms in the deeper layers. This rearrangement of the top two atomic layers drastically lowers the energy, effectively enhancing the stability of the structure. Contrary to previous assumptions, the numerical ratio of aluminum to oxygen atoms does not change.
To generate a three-dimensional model of the aluminum oxide surface, machine learning techniques were utilized. The main challenge involved correlating the restructured surface with the underlying crystal. “The complexity of the structure leads to a multitude of arrangements for the atoms that are not accessible in experiments. The advanced machine learning algorithms, in conjunction with traditional computational methods, enabled us to explore numerous configurations and create a stable three-dimensional model of the aluminum oxide surface,” states Andrea Conti, who executed the computational modeling.
“Through the joint efforts of experimental and computational research, we not only tackled a longstanding enigma by revealing the intricate structure of this fascinating insulator, but we also uncovered design principles that can be applied to various materials. Our findings open up new avenues for progress in catalysis, material science, and beyond,” remarks Jan Balajka, the lead researcher.
Some components of the experimental setup used for the noncontact atomic force microscope have been patented: Passive vibration isolation for high-resolution microscopy.
