Micro- and Nanostructure of the Surface of Polycrystalline Platinum Foil in the Oxygen Atmosphere at 1000-1400 K

Abstract

The micro- and nanostructure of the surface of polycrystalline platinum foil (Pt(poly)) after annealing in the O₂ atmosphere at P_O2 = 0.2 bar and 1 bar and T = 1000-1400 K is analyzed by scanning electron microscopy (SEM), powder X-ray diffraction, and electron backscatter diffraction (EBSD). With an increase in the temperature from 1000 K to 1400 K the main part of grains maintains the surface structure close to the (110) face, with the grain surface rearranging from the (311) stair structure to the (100) surface structure, and partially to (111). At these temperatures the values of unit cell parameter a almost do not change in coherence scattering region D and appear to be close to those obtained for initial Pt(poly) (3.925 Å, 25 nm). In the electron secondary and backscatter mode, SEM images in the magnification range from ×100 to ×300k at E₀ of 5 keV, 10 keV, and 20 keV demonstrate the microgranular structure with 10-150 µm (~30 µm) grains and different brightness levels, as well as the 10-100 nm high facet on the grain surface. The neighboring grains with different brightness contain facets of different shapes, sizes, and orientations. After annealing at 1100 K layered structures with a layer thickness up to 100 nm are found on the surface of bright grains while square and rectangular facets with step heights up to 100 nm and 25 nm respectively are observed on the surface of grains with gray and dark levels of brightness. The fixed level of brightness and the structure of facets can be related to the grain surface structure close to (111), (100), and (110) crystallographic planes respectively. At high temperatures (1000-1400 K) and O₂ pressure (~1 bar) the surface can be densely covered with adsorbed O_ads atoms stimulating the release of adsorbed Pt_ads atoms from the bulk to the surface. The high concentration of mobile Pt_ads atoms causes the surface rearrangement involving the formation of facets on the grain surface, which decrease the surface energy and form the stable surface structure.