Insights into the gas phase oxidation of Ru(0001) on the mesoscopic scale using molecular oxygen.

We present an extensive mesoscale study of the initial gas phase oxidation of Ru(0001), employing in situ low-energy electron microscopy (LEEM), micro low-energy electron diffraction (µ-LEED) and scanning tunneling microscopy (STM). The initial oxidation was investigated in a temperature range of 500-800 K at a constant oxygen pressure of p(O2) = 4 × 10(-5) mbar. Depending to the preparation temperature a dramatic change of the growth morphology of the RuO2 film was observed. At lower temperature (580 K) the RuO2(110) film grows anisotropically oriented along the high symmetry directions of the Ru(0001) substrate. At higher temperature (680 K), new rotational domains of RuO2(110) begin to appear, which are slightly rotated by up to 20° with respect to the high symmetry direction. These rotated RuO2(110) domains grow along slightly rotated step edges and reveal an isotropic growth morphology. Both the growth speed and the nucleation rate differ from that of the oxide growth at lower temperature (580 K).

Oxidation-state analysis of ceria by X-ray photoelectron spectroscopy.

Three different methods to determine the oxide-phase concentration in mixed cerium oxide by hard X-ray photoelectron spectroscopy are applied and quantitatively compared. Synchrotron-based characterization of the O 1s region was used as a benchmark to introduce a method based on the weighted superposition of the Ce 3d spectra of the pure Ce(3+) and Ce(4+) phases, which was shown to lead to reliable and highly accurate determination of the mean oxidation state in mixed cerium oxides. The results obtained reveal a linear relation between the third distinct final state (u''') satellite peak intensity of the Ce(4+) phase and the Ce(4+) concentration by proper inclusion of Ce(3+)-related plasmon satellite peaks, which contradicts previous claims of nonlinear behavior. In contrast, quantitative conventional peak-fitting procedures were shown to be well suited for the Ce 2p region due to its relatively simple structure. Additional satellite features observed in the Ce 3d spectrum of CeO2 were proposed to originate from plasmon contributions.