catalysts

The science of surface

surface_1

Structure of an oxide surface (view in 3-D). “For a long time we have not understood oxide surfaces,” says Laurence Marks. “We only have had relatively simple models constructed from crystal planes of the bulk structure, and these have not enabled us to predict where the atoms should be on a surface.” (Courtesy: Northwestern)

NORTHWESTERN (US)—A research team has shown how, using methods commonly taught to undergraduate chemistry students, one can understand how atoms are arranged on a material’s surface.

A collaboration between researchers at Northwestern University’s Center for Catalysis and Surface Science and scientists at the University of Oxford produced the new approach for understanding surfaces, particularly metal oxide surfaces, widely used in industry as supports for catalysts.

This knowledge of the surface layer of atoms is critical to understanding a material’s overall properties. The findings were published online in the journal Nature Materials.

The team used a combination of advanced experimental tools coupled with theoretical calculations.

“For a long time we have not understood oxide surfaces,” says Laurence Marks, professor of materials science and engineering in the McCormick School of Engineering and Applied Science at Northwestern. “We only have had relatively simple models constructed from crystal planes of the bulk structure, and these have not enabled us to predict where the atoms should be on a surface.

“Now we have something that seems to work,” Marks says. “It’s the bond-valence-sum method, which has been used for many years to understand bulk materials. The way to understand oxide surfaces turns out to be to look at the bonding patterns and how the atoms are arranged and then to follow this method.”

In the study, Northwestern graduate student James Enterkin analyzed electron diffraction patterns from a strontium titanate surface to work out the atomic structure. He combined the patterns with scanning-tunneling microscopy images obtained by Bruce Russell at Oxford.

Enterkin then combined them with density functional calculations and bond-valence sums, showing that those that had bonding similar to that found in bulk oxides were those with the lowest energy.

“This simple and intuitive, yet powerful concept [the bond-valence-sum method] is widely used to analyze and predict structures in inorganic chemistry. Its successful description of the surface reconstruction of SrTiO3 (110) shows that this approach could be relevant for similar phenomena in other materials,” writes Ulrike Diebold from the Institute of Applied Physics in Vienna, Austria in a “News and Views” article from the same issue of Nature Materials.

The National Science Foundation and the Northwestern University Institute for Catalysis in Energy Processing, funded through the U.S. Department of Energy, Office of Basic Energy Science, supported the research.

Northwestern University news: www.northwestern.edu/newscenter/

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