Sylvio Haas

ASAXS study on the formation of core–shell Ag/Au nanoparticles in glass

Nanosized metal particles of various configurations embedded in surface regions of glass have great potential as nonlinear optical materials for photonic devices. We have prepared Ag/Au nanoparticles in core-shell configuration in soda-lime silicate glass by double-ion implantation and investigated their structural characteristics by anomalous small-angle x-ray scattering (ASAXS) and transmission electron microscopy. Measurements at x-ray energies slightly below the Au L(3) edge indicate the formation of bimetallic Ag/Au shells in some of the nanoparticles for high-dose ion implantation. An element-specific analysis of the ASAXS results allowed us not only to validate and quantify the core-shell structure, but simultaneously also the composition of the shells. Hollow nanoparticles were found for an Au-Ag implantation sequence, whereas an Ag-Au sequence generates a diluted core composition. The shift of the maximum position of optical absorption of the samples due to surface plasmon resonance of bimetallic nanoparticles, as monitored by optical spectroscopy, revealed the considerable influence of the respective particle configuration.

Distribution of counterions around lignosulfonate macromolecules in different polar solvent mixtures.

Lignosulfonate is a colloidal polyelectrolyte that is obtained as a side product in sulfite pulping. In this work we wanted to study the noncovalent association of the colloids in different solvents, as well as to find out how the charged sulfonate groups are organized on the colloid surface. We studied sodium and rubidium lignosulfonate in water-methanol mixtures and in dimethyl formamide. The number average molecular weights of the Na- and Rb-lignosulfonate fractions were 7600 g/mol and 9100 g/mol, respectively, and the polydispersity index for both was 2. Anomalous small-angle X-ray scattering (ASAXS) was used for determining the distribution of counterions around the Rb-lignosulfonate macromolecules. The scattering curves were fitted with a model constructed from ellipsoids of revolution of different sizes. Counterions were taken into account by deriving an approximative formula for the scattering intensity of the Poisson-Boltzmann diffuse double layer model. The interaction term between the spheroidal particles was estimated using the local monodisperse approximation and the improved Hayter-Penfold structure factor given by the rescaled mean spherical approximation. Effective charge of the polyelectrolyte and the local dielectric constant of the solvent close to the globular polyelectrolyte were followed as a function of the methanol content in the solvent and lignosulfonate concentration. The lignosulfonate macromolecules were found to aggregate noncovalently in water-methanol mixtures with increasing methanol or lignosulfonate content in a specific directional manner. The flat macromolecule aggregates had a nearly constant thickness of 1-1.4 nm, while their diameter grew when counterion association onto the polyelectrolyte increased. These results indicate that the charged groups in lignosulfonate are mostly at the flat surfaces of the colloid, allowing the associated lignosulfonate complexes to grow further at the edges of the complex.

On the determination of partial structure functions in small-angle scattering exemplified by Al89Ni6La5 alloy

A comparison between the resonant scattering curve obtained by anomalous small-angle X-ray scattering at the X-ray absorption edge of Ni and the complementary small-angle neutron scattering curve from an Al89Ni6La5 alloy sample is reported. The sample does not comply with the two-phase approximation. The two resulting scattering curves are approximately proportional to each other in this particular case. The anomalous small-angle X-ray scattering resonant curve at the Ni absorption edge equals the Ni–Ni partial structure factor and, owing to the favourable neutron scattering lengths of Ni, La and Al, the neutron scattering curve is also proportional to that partial structure factor.

Structural analysis of magnetic nanocrystals embedded in silicate glasses by anomalous small‐angle X‐ray scattering

The present work reports the structural analysis of magnetic nanocrystals of Fe- and Mn-containing phases formed during annealing of the glass 13.6Na2O–62.9SiO2–8.5MnO–15.0Fe2O3−x (mol%) at 823 K for different periods of time. X-ray diffraction investigation shows the formation of a mixture of MnFe2O4 and Fe3O4 phases. Furthermore, small-angle X-ray scattering and anomalous small-angle X-ray scattering (ASAXS) experiments were used to extract quantitative information about the structure of the formed nano-sized crystals, as well as their phase compositions, volume fractions and size distributions. ASAXS analysis reveals the formation of spherical core–shell particles with the size of the particles between 10 and 50 nm. The concentrations of iron atoms in the core are higher than those in the shell and in the remaining matrix. With increasing annealing time, the ratio of Mn to Fe increases in the nanoparticles and the crystalline phase moves toward the pure jacobsite phase. Evaluated compositions reveal the presence of SiO2 in the shell. This shell acts as a diffusion barrier for the nanocrystal-forming ions, and the growth of the particles is kinetically constrained.

Structural Investigation of Carbon Supported Ru-Se Based Catalysts using Anomalous Small Angle X-Ray Scattering

Currently the commercial applicability of proton exchange membrane fuel cells (PEMFC) is almost exclusively bound to the employment of expensive and rare platinum. However as, recently successfully demonstrated, selenium modified ruthenium based catalysts also exhibit high catalytic activity for the oxygen reduction reaction in fuel cells [1]. One advantage of ruthenium is that it has only one-tenth the costs of platinum. Furthermore, in direct methanol fuel cell (DMFC) the undesired crossover of methanol through the membrane from the anode space into the cathode compartment is still an unsolved issue. Therefore, state of the art platinum cathode catalysts suffer from an activity loss under these conditions due to methanol oxidation. As alternative, carbon supported ruthenium nano-particles modified with selenium were suggested featuring absolute methanol tolerance. Although intense studies of these catalysts were performed during the last decade, no definite conclusion with respect to the nature of the catalytically active sites and the constitution of the RuSe nano-particles could be drawn. We performed an Anomalous Small Angle X-ray Scattering (ASAXS) experiment to clarify the structural and chemical features of these catalytically active Semodified ruthenium nano-particles, such materials were prepared by thermolysis of Ru3(CO)12 in an organic solvent with and without the presence of dissolved selenium. The results are due to be discussed considering data from XRD, XPS, and EXAFS measurements, as well as TEM images [2,3]. Particularly, data about the selenium distribution over the catalysts surface are aspired, because catalytic efficiency depends strongly on the selenium content of the catalysts. Investigating RuSe electrocatalysts we will show that the ASAXS method represents a powerful tool to determine size distributions and volume fraction of structures on a nano scale domain. Furthermore, it becomes a sensitive method on chemical composition fluctuations by taking into account the so-called anomalous or resonant behaviour of the atomic scattering amplitude of an element, containing in the sample, near its x-ray absorption edge. We investigated with ASAXS a complete set of samples, including the final working RuSe catalyst supported on a commercial carbon black, and some intermediate preparation states like non-modified (selenium free) ruthenium nano-particles or the bare carbon support only. We will discuss scattering curves taken in the vicinity of the selenium and the ruthenium K absorption edge, respectively. Figure 1. shows the large Q range of two scattering curves taken at 5eV below the selenium K absorption edge. The sample which contains selenium (red circles) shows a small hump at Q ~ 7 nm while the sample without selenium (blue triangle) does not. Thus, selenium within the sample generates structural features clearly detectable and analysable by SAXS. Taken into account the energy dependences (the anomalous dispersion effect) of the scattering intensity of all measured samples, a structure model of the catalytically active metallic nano-particles has been deduced, suggesting a nearly spherical Ru particle, with a mean diameter of 2.3 nm decorated with Se aggregates. The selenium structure onto the ruthenium nanocrystallites features a diameter less than 0.5 nm. The real shape of these ruthenium supported selenium clusters is not yet finally clarified. However, it is suggested to represent a symmetric ring like observed for free selenium clusters. [4] We investigated the same systems with small angle neutron scattering (SANS) confirming the structure model obtained from ASAXS. The comparison of the volume size distribution of the particles of both methods will be discussed. We also confirmed that the selenium modified ruthenium nano-particles are extremely resistant against particle growth. Even after annealing at 800°C the average particle diameter of the ruthenium particles was found to remain below 2.5 nm.