Interaction of CO and deuterium with bimetallic, monolayer Pt-island/film covered Ru(0001) surfaces.

The adsorption properties of structurally well defined bimetallic Pt/Ru(0001) surfaces, consisting of a Ru(0001) substrate partly or fully covered by monolayer Pt islands or a monolayer Pt film, were studied by temperature programmed desorption (TPD) using CO and deuterium as probe molecules. Additionally, the adsorption of CO was investigated by infrared reflection absorption spectroscopy (IRAS). The presence of the pseudomorphic platinum islands or monolayer film leads to considerable modifications of the adsorption properties for both adsorbates, both on the Pt covered and, to a smaller extent, on the bare Ru part of the surfaces. In addition to distinct weakly bound adspecies, which are adsorbed on the monolayer Pt islands, we find unique contributions from island edge desorption, from spill-over processes during the desorption run, and a general down-shift of the peak related to desorption from Pt-free Ru(0001) areas with increasing Pt coverage. These effects, which we consider as characteristic for adsorption on bimetallic surfaces with large contiguous areas of the respective types, are discussed in detail.

The interaction of CO with PdAg/Pd(111) surface alloys--a case study of ensemble effects on a bimetallic surface.

The interaction of CO with structurally well-defined PdAg/Pd(111) surface alloys was investigated by temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) to unravel and understand contributions from electronic strain, electronic ligand and geometric ensemble effects. TPD measurements indicate that CO adsorption is not possible on the Ag sites of the surface alloys (at 120 K) and that the CO binding strength on Pd sites decreases significantly with increasing Ag concentration. Comparison with previous scanning tunneling microscopy (STM) data on the distribution of Pd and Ag atoms in the surface alloy shows that this modification is mainly due to geometric ensemble effects, since Pd(3) ensembles, which are the preferred ensembles for CO adsorption on non-modified Pd(111), are no longer available on Ag-rich surfaces. Consequently, the preferred CO adsorption site changes with increasing Ag content from a Pd(3) trimer via a Pd(2) dimer to a Pd monomer, going along with a successive weakening of CO adsorption. Additionally, the CO adsorption properties of the surface alloys are also influenced by electronic ligand and strain effects, but on a lower scale. The results are discussed in comparison with previous findings on PdAg bulk alloys, supported PdAg catalysts and PdAu/Pd(111) model systems.

Product gas evolution above planar microstructured model catalysts--a combined scanning mass spectrometry, Monte Carlo, and Computational Fluid Dynamics study.

The transport and distribution of reaction products above catalytically active Pt microstructures was studied by spatially resolved scanning mass spectrometry (SMS) in combination with Monte Carlo simulation and fluid dynamics calculations, using the oxidation of CO as test reaction. The spatial gas distribution above the Pt fields was measured via a thin quartz capillary connected to a mass spectrometer. Measurements were performed in two different pressure regimes, being characteristic for ballistic mass transfer and diffusion involving multiple collisions for the motion of CO(2) product molecules between the sample and the capillary tip, and using differently sized and shaped Pt microstructures. The tip height dependent lateral resolution of the SMS measurements as well as contributions from shadowing effects, due to the mass transport limitations between capillary tip and sample surface at close separations, were evaluated and analyzed. The data allow to define measurement and reaction conditions where effects induced by the capillary tip can be neglected ("minimal invasive measurements") and provide a basis for the evaluation of catalyst activities on microstructured model systems, e.g., for catalyst screening or studies of transport effects.

Quadratic X-ray magneto-optical effect upon reflection in a near-normal-incidence configuration at the M edges of 3d-transition metals.

We have observed a quadratic x-ray magneto-optical effect in near-normal-incidence reflection at the M edges of iron. The effect appears as the magnetically induced rotation of approximately 0.1 degrees of the polarization plane of linearly polarized x-ray radiation upon reflection. A comparison of the measured rotation spectrum with results from x-ray magnetic linear dichroism data demonstrates that this is the first observation of the Schäfer-Hubert effect in the x-ray regime. Ab initio density-functional theory calculations reveal that hybridization effects of the 3p core states necessarily need to be considered when interpreting experimental data. The discovered magneto-x-ray effect holds promise for future ultrafast and element-selective studies of ferromagnetic as well as antiferromagnetic materials.

Size-dependent spin structures in iron nanoparticles.

Using photoemission electron microscopy, we study the magnetization orientation in single 5-25 nm iron particles coupled to a ferromagnetic cobalt support. We find a noncollinear alignment between the particle and substrate magnetization above a particle size of approximately 6 nm and a parallel alignment for smaller sizes. Numerical calculations reveal a transition from an exchange-dominated to an anisotropy-dominated regime on increasing the particle height: the smaller particles are in a single-domain collinear state while larger particles exhibit a spin-spiral magnetic structure determined by the magnetic anisotropy energy.

X-ray photoelectron spectrum in surface interfacing of gold nanoparticles with polymer molecules in a hybrid nanocomposite structure.

In situ Au3+--> reaction and dispersion of the resulting gold in the form of nanoparticles (NPs) in a reactive polymer medium, such as poly(vinyl alcohol) (PVA) molecules in hot water, are explored to devise a composite Au-PVA structure of self-standing films. The Au-NPs (0-2 wt%) tailor a violet to purple to a yellowish color of films from a colorless base polymer. A distinct surface interface (of thickness 2-5 nm) from the core (20-30 nm diameter) is resolved in transmission electron microscope images. The x-ray photoelectron spectrum (XPS) in the 81-91 eV range accounts for the effect which causes asymmetric shapes of the 4f(7/2) and 4f(5/2) Au bands. In a model structure, the asymmetry arises in the contribution of the surface Au atoms, which extend a chemical bridging with surface O atoms from the PVA molecules in a surface layer. A similar asymmetry also arises in C 1s and O 1s bands in the counterpart PVA molecules at 287.79 and 532.35 eV, respectively. At Au content as large as 2 wt%, no separate XPS band arises on a negligibly small fraction of surface Au atoms when Au-NPs have been grown in thin laminates (or cuboids and prisms).

Scanning mass spectrometer for quantitative reaction studies on catalytically active microstructures.

We describe an apparatus for spatially resolving scanning mass spectrometry which is able to measure the gas composition above catalytically active microstructures or arrays of these microstructures with a lateral resolution of better than 100 mum under reaction conditions and which allows us to quantitatively determine reaction rates on individual microstructures. Measurements of the three-dimensional gas composition at different vertical distances and separations between active structures allow the evaluation of gas phase mass transport effects. The system is based on a piezoelectrically driven positioning substage for controlled lateral and vertical positioning of the sample under a rigidly mounted capillary probe connecting to a mass spectrometer. Measurements can be performed at pressures in the range of <10(-2)-10 mbars and temperatures between room temperature and 450 degrees C. The performance of the setup is demonstrated using the CO oxidation reaction on Pt microstructures on Si with sizes between 100 and 300 mum and distances in the same order of magnitude, evaluating CO(2) formation and CO consumption above the microstructures. The rapidly decaying lateral resolution with increasing distance between sample and probe underlines the effects of (lateral) gas transport in the room between sample and probe. The reaction rates and apparent activation energy obtained from such measurements agree with previous data on extended surfaces, demonstrating the feasibility of determining absolute reaction rates on individual microstructures.

Controlling the interparticle spacing of Au-salt loaded micelles and Au nanoparticles on flat surfaces.

The self-organization of diblock copolymers into micellar structures in an appropriate solvent allows the deposition of well ordered arrays of pure metal and alloy nanoparticles on flat surfaces with narrow distributions in particle size and interparticle spacing. Here we investigated the influence of the materials (substrate and polymer) and deposition parameters (temperature and emersion velocity) on the deposition of metal salt loaded micelles by dip-coating from solution and on the order and inter-particle spacing of the micellar deposits and thus of the metal nanoparticle arrays resulting after plasma removal of the polymer shell. For identical substrate and polymer, variation of the process parameters temperature and emersion velocity enables the controlled modification of the interparticle distance within a certain length regime. Moreover, also the degree of hexagonal order of the final array depends sensitively on these parameters.

Perpendicular spin orientation in ultrasmall Fe islands on W(110).

We have studied the magnetism of Ag-coated Fe islands on W(110) by nuclear resonant scattering of synchrotron radiation at the 14.4 keV resonance of (57)Fe. Separated islands with an average diameter of 2.0 nm and monolayer thickness are formed at a Fe coverage of theta = 0.57 bulk monolayers. Time spectra of the nuclear decay were measured in the temperature range from 4.5 to 300 K. We find strong evidence for perpendicular spin orientation, which most likely results from the interplay of shape anisotropy and elastic strain in the islands.