Scientists on the TU Wien and the University of Vienna have uncovered the detailed structure of the aluminum oxide surface, a challenge that has baffled researchers for a long time.
Aluminum oxide (Al2O3), also often known as alumina, corundum, sapphire, or ruby, is among the finest insulators utilized in a big selection of applications: in electronic components, as a support material for catalysts, or as a chemically resistant ceramic, to call a couple of. Knowledge of the precise arrangement of the surface atoms is vital to understanding how chemical reactions occur on this material, akin to those in catalytic processes. Atoms contained in the material follow a hard and fast arrangement, giving rise to the characteristic shapes of crystals. On the surface, nevertheless, the structure deviates from that contained in the crystal. The strongly insulating nature of alumina hindered experimental studies, and the surface structure evaded precise determination for greater than half a century. Researchers at TU Wien and the University of Vienna have now solved the complex structure of the Al2O3 surface, a puzzle listed in 1997 as considered one of the “Three mysteries of surface science.” The research group led by Jan Balajka and Ulrike Diebold recently published their findings within the journal Science.
High-resolution microscopy identifies surface atoms
The research team used noncontact atomic force microscopy (ncAFM) to research the surface structure. This method generates images of the surface structure by scanning a pointy tip mounted on a quartz tuning fork at a detailed distance from the surface. The frequency of the tuning fork changes because the tip interacts with the atoms on the surface without touching the fabric. Johanna Hütner, who performed the experiments, explains: “In an ncAFM image, one can see the placement of atoms, but not their chemical identity. We overcame the dearth of chemical sensitivity by precisely controlling the tip. Attaching a single oxygen atom to the tip apex allowed us to differentiate between oxygen and aluminum atoms on the surface. The oxygen atom on the tip is repelled from other oxygen atoms on the surface and drawn to aluminum atoms of the Al2O3 surface. Mapping the local repulsion or attraction enabled us to visualise the chemical identity of every surface atom directly.”
Restructuring stabilizes the surface without changing its composition
The researchers found that the surface rearranges to permit the aluminum atoms on the surface to penetrate into the fabric and form chemical bonds with the oxygen atoms within the deeper layers. This rearrangement of the primary two atomic layers significantly reduces the energy, effectively stabilizing the structure. In contrast to previous beliefs, the numerical ratio of aluminum to oxygen atoms stays unchanged.
The three-dimensional model of the aluminum oxide surface was optimized with machine learning methods. The important challenge was to match the restructured surface with the underlying crystal. “The structure may be very complex, leading to an enormous variety of possibilities for the way the experimentally inaccessible atoms below the surface might be arranged. The state-of-the-art machine learning algorithms combined with conventional computational methods allowed us to look at quite a few possibilities and create the stable three-dimensional model of the aluminum oxide surface,” states Andrea Conti, who carried out the computational modeling.
“Through the collaborative effort of experimental and computational research, we not only tackled a long-standing mystery by determining the detailed structure of this enigmatic insulator, but in addition discovered structure design principles applicable to a whole class of materials. Our results pave the best way for advancements in catalysis, material science, and other fields,” says Jan Balajka, who led the research.
Parts of the experimental setup housing the noncontact atomic force microscope have been patented: Passive vibration isolation for high-resolution microscopy.
