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SURFACE CHEMSITRY
Metal oxidation is generally considered a bad thing, especially when it's rust on a car or a dull sheen on silver. However, thin oxide films on metal surfaces can be useful when it comes to technical applications such as catalysts, semiconductors, or corrosion-inhibiting layers. Kinetics studies have provided a good understanding of such oxide-film growth, but little is known at the atomic level about how these films nucleate and start to grow. University of Maryland scientists now provide an up-close look at metal-oxide film formation by using time-lapse scanning tunneling microscopy to view lead oxide growth on the surface of lead crystallites [Science, 297, 2033 (2002)]. Chemistry postdoctoral researcher Konrad Thürmer, associate chemistry professor Janice E. Reutt-Robey, and physics professor Ellen D. Williams prepared ultrapure micrometer-sized lead crystals on a ruthenium substrate, then exposed the crystals to O2, H2O, or N2O at 370 K. They observed temperature-induced microscopic reshaping of the surface features of the crystals but no apparent oxide formation, even after exposure to higher concentrations of the gases. "After watching monatomic steps move about on the crystallites for hundreds of hours, we observed no oxide formation but came away with a much better understanding of microscopic crystal-reshaping processes," Reutt-Robey notes. When the researchers allowed nanometer-sized impurities to remain in the crystals during their preparation, however, PbO grains were observed to rapidly nucleate and grow on a crystal surface when exposed to O2. Although the impurities are not yet identified, the researchers believe they could be clusters of RuO, which are known to catalyze O2 dissociation. The Maryland scientists observed that the impurities gradually breach the crystal's surface during reshaping, serving as nucleation sites for PbO grains. The metal oxide formation then becomes autocatalytic, and the nanometer-thick PbO grains quickly grow anisotropically. "Impurities thus play the dual roles of triggering oxide formation and setting the location of individual oxide grains on the surface," Reutt-Robey says. The work provides a clearer understanding of the initial oxidation of metal clusters, note Herbert Over of Justus-Liebig University in Giessen, Germany, and Ari P. Seitsonen of the University of Zurich in a related article. "The deliberate introduction of impurities may thus pave the way for the controlled structuring of metal surfaces by nanometer-sized oxide particles," the researchers write.
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