The fuel cell is a promising alternative to environmentally unfriendly devices that are currently powered by fossil fuels. In the polymer electrolyte membrane fuel cell (PEMFC), the main fuel is hydrogen, which through its reaction with oxygen produces electricity with water as the only by-product. To make PEMFCs economically viable, there are several problems that should be solved; the main one is to find more effective catalysts than Pt for the oxygen reduction reaction (ORR), 1/2 O 2 + 2 H + + 2 e À = H 2 O. The design of inexpensive, stable, and catalytically active materials for the ORR will require fundamental breakthroughs, and to this end it is important to develop a fundamental understanding of the catalytic process on different materials. Herein, we describe how variations in the electronic structure determine trends in the catalytic activity of the ORR across the periodic table. We show that Pt alloys involving 3d metals are better catalysts than Pt because the electronic structure of the Pt atoms in the surface of these alloys has been modified slightly. With this understanding, we hope that electrocatalysts can begin to be designed on the basis of fundamental insight.

On-stream deactivation during a water gas shift (WGS) reaction over gold supported on a ceria-zirconia catalyst was examined. Although the fresh catalyst has very high low temperature (<200 °C) for WGS activity, a significant loss of CO conversion is found under steady-state operations over hours. This has been shown to be directly related to the concentration of water in the gas phase. The same catalyst also undergoes thermal deactivation above 250 °C, and using a combined experimental and theoretical approach, a common deactivation mechanism is proposed. In both cases, the gold nanoparticles, which are found under reaction conditions, are thought to detach from the oxide support either through hydrolysis, <250 °C, or thermally, >250 °C. This process reduces the metal-support interaction, which is considered to be critical in determining the high activity of the catalyst.

We present a general theoretical methodology and related open-access computer program for carrying out the calculation of photoelectron, Auger electron, and x-ray emission intensities in the presence of several x-ray optical effects, including total reflection at grazing incidence, excitation with standing-waves produced by reflection from synthetic multilayers and at core-level resonance conditions, and the use of variable polarization to produce magnetic circular dichroism. Calculations illustrating all of these effects are presented, including in some cases comparisons to experimental results. Sample types include both semi-infinite flat surfaces and arbitrary multilayer configurations, with interdiffusion/roughness at their interfaces. These x-ray optical effects can significantly alter observed photoelectron, Auger, and x-ray intensities, and in fact lead to several generally useful techniques for enhancing surface and buried-layer sensitivity, including layer-resolved densities of states and depth profiles of element-specific magnetization. The computer program used in this study should thus be useful for a broad range of studies in which x-ray optical effects are involved or are to be exploited in next-generation surface and interface studies of nanoscale systems.

We present experimental and theoretical results related to multiatom resonant photoemission, in which the photoelectron intensity from a core level on one atom is influenced by a core-level absorption resonance on another. We point out that some prior experimental data has been strongly influenced by detector nonlinearity and that the effects seen in new corrected data are smaller and of different form. Corrected data are found to be well described by an extension of resonant photoemission theory to the interatomic case, provided that interactions beyond the usual second-order Kramers-Heisenberg treatment are included. This microscopic theory is also found to simplify under certain conditions so as to yield results equivalent to a classical x-ray optical approach, with the latter providing an alternative, although less detailed and general, physical picture of these effects. The potential utility of these effects as near-neighbor probes, as well as their implications for x-ray emission and x-ray scattering experiments, are also discussed.

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The time evolution of the growth process of vertically aligned single-walled carbon nanotubes (or V-SWNTs) on a flat substrate was examined by scanning electron microscopy (SEM), resonant Raman spectroscopy, and angle-resolved X-ray absorption near-edge structure (XANES). This detailed characterization gives evidence for the growth of a thin layer (crust) of randomly oriented single-walled carbon nanotubes during the first stages of the growth process. This crust is responsible for the unique forest-like morphology exhibited by this type of SWNT structure.

Ferromagnetic Co-doped rutile TiO2 single crystals were synthesized by high-temperature ion implantation and characterized by a variety of techniques. Co is uniformly distributed to a depth of ∼300 nm with an average concentration of ∼2 at. %, except in the near-surface region, where the concentration is ∼3 at. %. Ferromagnetic behavior is exhibited at room temperature with an effective saturation magnetization of ∼0.6 μB/Co atom. The Co is in a formal oxidation state of +2 throughout the implanted region, and no Co(O) is detected.

IntroductionBuried solid-solid interfaces are ubiquitous in nanoscience and nanotechnology, and their properties are thus becoming more important in many areas of application, for example in magnetic read heads, magnetic data storage devices, and semiconductor logic devices. Due to the continuous shrinkage of the various layers and elements in these devices into the nanometer range, the thickness of the interfaces and their degree of roughness, as well as the variation across them of composition, of the chemical and magnetic states of their various constituents, and of the detailed valence electronic structure, all represent important microscopic properties that can strongly influence macroscopic properties such as electrical conductivity and magnetism.