Showing papers by "Tolek Tyliszczak published in 2008"

Nanoscale chemical imaging of a working catalyst by scanning transmission X-ray microscopy

Abstract: The modern chemical industry uses heterogeneous catalysts in almost every production process. They commonly consist of nanometre-size active components (typically metals or metal oxides) dispersed on a high-surface-area solid support, with performance depending on the catalysts' nanometre-size features and on interactions involving the active components, the support and the reactant and product molecules. To gain insight into the mechanisms of heterogeneous catalysts, which could guide the design of improved or novel catalysts, it is thus necessary to have a detailed characterization of the physicochemical composition of heterogeneous catalysts in their working state at the nanometre scale. Scanning probe microscopy methods have been used to study inorganic catalyst phases at subnanometre resolution, but detailed chemical information of the materials in their working state is often difficult to obtain. By contrast, optical microspectroscopic approaches offer much flexibility for in situ chemical characterization; however, this comes at the expense of limited spatial resolution. A recent development promising high spatial resolution and chemical characterization capabilities is scanning transmission X-ray microscopy, which has been used in a proof-of-principle study to characterize a solid catalyst. Here we show that when adapting a nanoreactor specially designed for high-resolution electron microscopy, scanning transmission X-ray microscopy can be used at atmospheric pressure and up to 350 degrees C to monitor in situ phase changes in a complex iron-based Fisher-Tropsch catalyst and the nature and location of carbon species produced. We expect that our system, which is capable of operating up to 500 degrees C, will open new opportunities for nanometre-resolution imaging of a range of important chemical processes taking place on solids in gaseous or liquid environments.

PolLux: a new facility for soft x-ray spectromicroscopy at the Swiss Light Source.

Abstract: We report on the successful installation and operation of a scanning transmission x-ray microspectroscope (STXM) at the PolLux facility at the Swiss Light Source. This integration of an advanced STXM with improved sample handling capabilities and a novel beamline provides unique capabilities. PolLux uses linearly or circularly polarized x-rays from a bending magnet with an extended photon energy range (200–1400 eV). It is therefore well suited to determine a sample’s quantitative chemical composition, molecular orientation, or thickness of organic as well as condensed matter materials. The local magnetic state of magnetic thin films is accessible through fast helicity switching by steering the electron beam off axis through the bending magnet. Ex vacuo girder movers allow fast and highly reproducible (<1 μm) alignment of the instrument with respect to the photon beam. The present spatial resolution is ∼20 nm, limited by the zone plates utilized. The instrument has the stability and positional resolution t...

Quantitative organic and light-element analysis of comet 81P/Wild 2 particles using C-, N-, and O-μ-XANES

Abstract: Synchrotron-based soft X-ray micro-analysis was performed on particles extracted from the Stardust aerogel collector in order to obtain detailed organic functional group information on any organic solids captured as part of the Principal Examination suite of analyses for samples from comet 81P/Wild 2. It is observed that cometary organic carbon captured in aerogel is present in a number of different manifestations and often intimately associated with silicates. Carbon X-ray absorption near edge structure (XANES) spectra reveal considerable chemical complexity in all of the organic particles studied so far. Universally, the comet 81P/Wild 2 organic particles contain low concentrations of aromatic and/or olefinic carbon relative to aliphatic and heteroatom-containing functional groups, e.g., amide, carboxyl, and alcohol/ethers. N-XANES confirms the presence and assignments of these functional groups. In general, the XANES data record considerable chemical complexity across the range of organic samples currently analyzed. The atomic ratios, N/C and O/C, derived from XANES data reveal a wide range in heteroatom content; in all cases these elemental ratios are higher than that of primitive meteoritic organic matter. The wide range in chemistry, both in elemental abundances and specific organic functional groups, suggests that the comet 81P/Wild 2 organic solids may have multiple origins.

A high-resolution chemical and structural study of framboidal pyrite formed within a low-temperature bacterial biofilm

Abstract: A novel, anaerobically grown microbial biofilm, scraped from the inner surface of a borehole, 1474 m below land surface within a South African, Witwatersrand gold mine, contains framboidal pyrite. Water flowing from the borehole had a temperature of 30.9 degrees C, a pH of 7.4, and an Eh of -50 mV. Examination of the biofilm using X-ray diffraction, field emission gun scanning electron microscope equipped for energy dispersive X-ray microanalysis demonstrated that the framboids formed within a matrix of bacteria and biopolymers. Focused ion beam sectioning of framboids followed by NEXAFS measurements using both scanning transmission X-ray microscopy and X-ray photoelectron emission microscopy revealed that the pyrite crystals grew within an organic carbon matrix consisting of exopolysaccharides and possibly extracellular DNA, which is intuitively important in sulfide mineral diagenesis. Growth of individual pyrite crystals within the framboid occurred inside organic templates confirms the association between framboidal pyrite and organic materials in low-temperature diagenetic environments and the important role of microenvironments in biofilms in regulating geochemical cycles.

Polarization selective magnetic vortex dynamics and core reversal in rotating magnetic fields.

Abstract: We report on the observation of magnetic vortex dynamics in response to rotating magnetic fields in submicron platelets. Unlike linear fields or spin polarized currents, which excite both vortex core polarization states, an in-plane rotating field can selectively excite one of the polarization states. We demonstrate by direct imaging with time-resolved scanning x-ray microscopy that the rotating field only excites the gyrotropic mode if the rotation sense of the field coincides with the vortex gyration sense and that such a field can selectively reverse the vortex polarization.

Time-resolved x-ray microscopy of spin-torque-induced magnetic vortex gyration.

Abstract: Time-resolved x-ray microscopy is used to image the influence of alternating high-density currents on the magnetization dynamics of ferromagnetic vortices. Spin-torque-induced vortex gyration is observed in micrometer-sized permalloy squares. The phases of the gyration in structures with different chirality are compared to an analytical model and micromagnetic simulations, considering both alternating spin-polarized currents and the current's Oersted field. In our case the driving force due to spin-transfer torque is about 70% of the total excitation while the remainder originates from the current's Oersted field. This finding has implications to magnetic storage devices using spin-torque driven magnetization switching and domain-wall motion.

Chemical speciation of sulfur in marine cloud droplets and particles: Analysis of individual particles from the marine boundary layer over the California current

Abstract: [1] Detailed chemical speciation of the dry residue particles from individual cloud droplets and interstitial aerosol collected during the Marine Stratus Experiment (MASE) was performed using a combination of complementary microanalysis techniques. Techniques include computer controlled scanning electron microscopy with energy dispersed analysis of X rays (CCSEM/EDX), time-of-flight secondary ionization mass spectrometry (TOF-SIMS), and scanning transmission X-ray microscopy with near edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS). Samples were collected at the ground site located in Point Reyes National Seashore, approximately 1 km from the coast. This manuscript focuses on the analysis of individual particles sampled from air masses that originated over the open ocean and then passed through the area of the California current located along the northern California coast. On the basis of composition, morphology, and chemical bonding information, two externally mixed, distinct classes of sulfur containing particles were identified: chemically modified (aged) sea salt particles and secondary formed sulfate particles. The results indicate substantial heterogeneous replacement of chloride by methanesulfonate (CH3SO3−) and non-sea-salt sulfate (nss-SO42−) in sea-salt particles with characteristic ratios of nss-S/Na > 0.10 and CH3SO3−/nss-SO42− > 0.6.

Direct Observation of Spin-Torque Driven Magnetization Reversal through Nonuniform Modes

Abstract: We present time-resolved x-ray images with 30 nm spatial and 70 ps temporal resolution, which reveal details of the spatially resolved magnetization evolution in nanoscale samples of various dimensions during reversible spin-torque switching processes. Our data in conjunction with micromagnetic simulations suggest a simple unified picture of magnetic switching based on the motion of a magnetic vortex. With decreasing size of the magnetic element the path of the vortex core moves from inside to outside of the nanoelement, and the switching process evolves from a curled nonuniform to an increasingly uniform mode.

Assignment of Polarization-Dependent Peaks in Carbon K-Edge Spectra from Biogenic and Geologic Aragonite

Abstract: Many biominerals, including mollusk and echinoderm shells, avian eggshells, modern and fossil bacterial sediments, planktonic coccolithophores, and foraminifera, contain carbonates in the form of biogenic aragonite or calcite. Here we analyze biogenic and geologic aragonite using different kinds of surface- and bulksensitive X-ray absorption near-edge structure (XANES) spectroscopy at the carbon K-edge, as well as highresolution scanning transmission X-ray microscopy (STXM). Besides the well-known main π* and σ* carbonate peaks, we observed and fully characterized four minor peaks, at energies between the main π* and σ* peaks. As expected, the main peaks are similar in geologic and biogenic aragonite, while the minor peaks differ in relative intensity. In this and previous work, the minor peaks appear to be the ones most affected in biomineralization processes, hence the interest in characterizing them. Peak assignment was achieved by correlation of polarization-dependent behavior of the minor peaks with that of the main π* and σ* peaks. The present characterization provides the background for future studies of aragonitic biominerals.

New data on the structure of uranium monocarbide

Abstract: Uranium monocarbide (UC) or ternary alloys are considered to be possible candidates for future nuclear fuels. Although the crystallographic and electronic structure of UC has been addressed in past investigations, discrepancies in the literature data have fostered a new investigation of the UC phase. We report here a reinvestigation of the UC phase by complementary X-ray spectroscopy and quantum chemical calculations. A combination of X-ray powder diffraction and extended X-ray absorption fine structure analysis at the uranium LIII edge led to the crystallographic determination of the UC phase of the NaCl type. For electronic structure investigation, a combination of uranium X-ray absorption near-edge spectroscopy at the LIII edge and at the NIV,V edges with quantum chemical calculations allowed us to define the evolution of the metal charge in comparison with metallic uranium on the one hand and uranium dioxide on the other hand.

Photofragmentation of closo-carboranes part 1: Energetics of decomposition.

Abstract: The ionic fragmentation following B 1s and C 1s excitation of three isomeric carborane cage compounds [closo-dicarbadodecaboranes: orthocarborane (1,2-C2B10H12), metacarborane (1,7-C2B10H12), and paracarborane (1,12-C2B10H12)] is compared with the energetics of decomposition. The fragmentation yields for all three molecules are quite similar. Thermodynamic cycles are constructed for neutral and ionic species in an attempt to systemically characterize single-ion closo-carborane creation and fragmentation processes. Lower energy decomposition processes are favored. Among the ionic species, the photon-induced decomposition is dominated by BH+ and BH2+ fragment loss. Changes in ion yield associated with core to bound excitations are observed.

Scanning Transmission X-ray Spectromicroscopy of Actinide Complexes

Abstract: The fundamental characterization and understanding of 5f electron behavior in actinide complexes is imperative to provide an enhanced basis for the rational and accelerated development of improved processes relevant to nuclear energy. Soft x-ray absorption spectroscopy utilizing the scanning transmission x-ray microscope (STXM) at the Advanced Light Source-Molecular Environmental Science (ALS-MES) Beamline 11.0.2 has been used to probe the electronic characteristics of a nitrogen donor ligand 2,6-Bis(2-benzimidazyl)pyridine (BBP) and its resulting U(IV) complex. The nitrogen K- and carbon K-edges have been collected from both ligand and uranium complex, as well as the uranium 4d-edge from the complex. Upon complexation, the light element absorption spectra change markedly and the uranium spectra from the complex is compared to the reference spectrum obtained from U(IV)Cl4. The evolution of the spectral features are described and interpreted within a simple conceptual framework. Based on spectral evidence alone, the uranium is bound through the pyridine-like nitrogens and the oxidation state of the uranium is consistent with a U(IV) species.

Structural, Magnetic and Electronic Properties of Surface Oxidised Fe Nanoparticles

Abstract: A combined XRD, Mossbauer, SEM, STXM and NMR study of naturally oxidised, ball milled iron powders is presented. The XRD patterns show the peaks of the bcc-Fe phase with the line widths increasing with the milling time. This corresponds to a flattening of the crystallites, as confirmed by SEM, and increased strain due to the accumulation of defects. The effect is consistent with the variation of the Mossbauer line-widths with the milling time. Scanning Transmission Xray Microscopy (STXM) measurements provided oxygen maps of the particles and revealed that the dominant oxide in the nanometric oxide layer is magnetite. The 57Fe spin echo NMR study reveals a dominant signal corresponding to a bcc-Fe core and a much weaker resonance corresponding to a magnetite amount of less than 1%.