RESUMO
Since catalytic performance of platinum-metal (Pt-M) nanoparticles is primarily determined by the chemical and structural configurations of the outermost atomic layers, detailed knowledge of the distribution of Pt and M surface atoms is crucial for the design of Pt-M electrocatalysts with optimum activity. Further, an understanding of how the surface composition and structure of electrocatalysts may be controlled by external means is useful for their efficient production. Here, we report our study of surface composition and the dynamics involved in facet-dependent oxidation of equilibrium-shaped Pt3Co nanoparticles in an initially disordered state via in situ transmission electron microscopy and density functional calculations. In brief, using our advanced in situ gas cell technique, evolution of the surface of the Pt3Co nanoparticles was monitored at the atomic scale during their exposure to an oxygen atmosphere at elevated temperature, and it was found that Co segregation and oxidation take place on {111} surfaces but not on {100} surfaces.
RESUMO
Mn3Ta2O8, a stable targeted material with an unusual and complex cation topology in the complicated Mn-Ta-O phase space, has been grown as a ≈3-cm-long single crystal via the optical floating-zone technique. Single-crystal absorbance studies determine the band gap as 1.89 eV, which agrees with the value obtained from density functional theory electronic-band-structure calculations. The valence band consists of the hybridized Mn d-O p states, whereas the bottom of the conduction band is formed by the Ta d states. Furthermore, out of the three crystallographically distinct Mn atoms that are four-, seven-, or eight-coordinate, only the former two contribute their states near the top of the valence band and hence govern the electronic transitions across the band gap.