Anomalous Small-angle X-ray Scattering Measurements Research Articles

Anomalous Small-Angle X-Ray Scattering of Fuel-Cell Catalyst Samples

Anomalous small-angle X-ray scattering (ASAXS) is a technique capable of extracting element-specific structural information from multi-element materials, and is considered useful in the structure analysis of metal alloy catalyst particles for fuel cells. ASAXS utilizes the resonant term of the atomic scattering factor of the element of interest. The resonant term has a real part (f1) and an imaginary part (f2), and f1 mainly contributes to the anomalous scattering phenomenon. The amplitude of f1 varies with X-ray energy, and decreases sharply around the absorption edges of the element. The decrease is as large as -18 electrons in the case of platinum (Pt) LIII-edge. ASAXS experiments are possible only in synchrotron radiation facilities where X-ray energies are freely tunable.ASAXS experiments require X-ray recordings at 3 or more different X-ray energies to mathematically isolate the element-specific component. Although the energy dependence of f1 seems substantial at around the absorption edge, its contribution to the total scattering intensity from a multi-element system is subtle, and high-precision measurements are required to obtain correct structural information.In actual measurements, experimental errors can arise from many sources, even in present-day third-generation synchrotron radiation facilities. They include fluctuations of the X-ray beam intensities and positions, the calibration error of X-ray energies, and time-dependent changes of sample properties. Here we evaluate the effect of such errors by using model calculations. The model consists of a mixture of 3-nm Pt particles and 5-nm gold (Au) nanoparticles.The scattering profiles of this mixture were calculated for X-ray energies of 11.504, 11.555 and 11.560 keV (Curve A in Figure 1. The LIII edge is at 11.564 keV). The scattering amplitude of the Pt particles was calculated by using the f1 and f2 values from the Cromer-Liberman tables (Brennan & Cowan, Rev. Sci. Instr., 63, 1992), using equations described by Sanada et al. (J. Am. Chem. Soc., 135, 2013). The scattering from Au particles (Curve B) was assumed to be independent of energy. The extracted scattering profile for Pt was identical to that calculated for a sample with Pt particles alone (Curve D, compare with C). Next, the scattering intensities at a specific energy were deliberately changed by a few percent before extracting the Pt contribution. For intensities at 11.56 keV, even a reduction of as slight as 0.2% resulted in catastrophic consequences (Curve F). With a 5% increase of intensity, the generated Pt profile superficially looked normal, but in reality the scattering curve retained the features of the Pt-Au mixture, meaning that the Pt-specific component was not successfully extracted (Curve D).These results indicates that the accuracy requirements in ASAXS measurements are very stringent, and every effort should be made to eliminate factors that can cause errors and uncertainties.In the meeting, data recorded from actual catalyst samples will also be presented, including those from MIRAI (fuel-cell vehicle manufactured by Toyota).This work was performed under the NEDO FC-Platform project. Figure 1

ASAXS measurements on ferritin and apoferritin at the bioSAXS beamline P12 (PETRA III, DESY).

Small-angle X-ray scattering is widely utilized to study biological macromol-ecules in solution. For samples containing specific (e.g. metal) atoms, additional information can be obtained using anomalous scattering. Here, measuring samples at different energies close to the absorption edges of relevant elements provides specific structural details. However, anomalous small-angle X-ray scattering (ASAXS) applications to dilute macromolecular solutions are challenging owing to the overall low anomalous scattering effect. Here, pilot ASAXS experiments from dilute solutions of ferritin and cobalt-loaded apoferritin are reported. These samples were investigated near the resonance X-ray K edges of Fe and Co, respectively, at the EMBL P12 bioSAXS beamline at PETRA III, DESY. Thanks to the high brilliance of the P12 beamline, ASAXS experiments are feasible on dilute protein solutions, allowing one to extract the Fe- or Co-specific anomalous dispersion terms from the ASAXS data. The data were subsequently used to determine the spatial distribution of either iron or cobalt atoms incorporated into the ferritin/apoferritin protein cages.

Characterization of BaZrO3 nanocolumns in Zr-added (Gd, Y)Ba2Cu3Ox superconductor tape by anomalous small-angle x-ray scattering

BaZrO3 nanocolumns in (Gd, Y)Ba2Cu3Ox superconductor tapes have been characterized by anomalous small-angle x-ray scattering (ASAXS). As Zr is distributed only in the nanocolumns, the scattering from the nanocolumns can be separated using ASAXS measurements at the Zr K absorption edge. The ASAXS results showed that the distance between the nanocolumns decreases from 37 to 28 nm with increasing the content of Zr from 15 to 25 mol%. The average diameter of the nanocolumns increases up to Zr content of 20 mol%, while the average diameter of the 25 mol% Zr sample is similar to that of the 20 mol% Zr sample. These results confirm that ASAXS measurements are useful for the characterization of BaZrO3 nanocolumns.

Crystallization of ZrTiO4 Nanocrystals in Lithium-Alumino-Silicate Glass Ceramics: Anomalous Small-Angle X-ray Scattering Investigation

We report on anomalous small-angle X-ray scattering (ASAXS) investigations of the formation and structure of nanosized ZrTiO4 crystals in lithium-alumino-silicate (LAS) glass during heat treatment at 750 °C for different periods of time. For a sample annealed for 30 min, ASAXS measurements near the X-ray absorption edge of Ti and Zr reveal the formation of particles (ZrTiO4) surrounded by a shell-like region. The sample annealed for 240 min shows the formation of two different types of particles (spherical core–shell: ZrTiO4 and large spherical particles: LiAlSi2O6). Additionally, ASAXS results allow quantitative determination of the average composition and volume fraction of the nanocrystals, the surrounding region (shell), and the remaining glass matrix. Data analysis reveals the formation of an alumina enriched region around ZrTiO4. This alumina enriched layer makes the glass network rigid and hinders further growth of ZrTiO4 nanoparticles. For a prolonged annealing time (240 min), a new phase (LiAlSi2O6) is formed additionally, and the (Al/Si) ratio in the shell surrounding the ZrTiO4 crystals decreases. Moreover, the unannealed sample also shows the formation of a Ti enriched phase during cooling of the samples. The obtained quantitative information helps to understand the crystallization and growth mechanisms of ZrTiO4 nanocrystals in LAS glass ceramics.

The Impact of the Turing Number on Quantitative ASAXS Measurements of Ternary Alloys

During the last ten years, anomalous small-angle x-ray scattering (ASAXS) became a precise quantitative method resolving scattering contributions two or three orders of magnitude smaller compared with the overall small-angle scattering, which are related to the so-called pure-resonant scattering contribution. The technique gives access to the analysis of weak concentration fluctuations in systems undergoing spinodal decomposition, which cannot be resolved by transmission electron microscopy (TEM) images because these systems are to a large degree homogeneous. In addition to the structural information, precise quantitative information about the different chemical concentrations localized in the nanosized structures of ternary alloys are obtained from these scattering contributions. The application of the Gauss elimination algorithm to the vector equation established by ASAXS measurements at three x-ray energies is demonstrated for two ternary alloys, one in the state of spinodal decomposition. The example deals with the quantitative analysis of the resonant invariant (RI-analysis). From the integrals of the pure-resonant scattering contribution, the chemical concentrations in the nanoscaled phases are determined. Moreover the Turing numbers of the vector equation stated by the ASAXS measurement are calculated giving a decisive quantitative measure thereby indicating whether the quantitative parameters obtained from the matrix inversion are significant.

Anomalous Small-angle X-ray Scattering Study on Aggregation of a Block Copolymer in a Selective Solvent

Anomalous small-angle X-ray Scattering (ASAXS) analysis for poly(4-vinylphenol)-b-poly(4-bromostyrene) (PVPh-b-PBrS) in N,N,N',N'-tetramethyl ethylene diamine (TMEDA) solution was examined. The ASAXS measurements, executed at three different energies, provided the separation of the scattering of the Br atoms from whole system. The energy-dependent term in the SAXS profile, which is corresponding to the form factor of PBrS domain, sufficiently agree with the SAXS profile calculated for hard sphere with 41 nm radius. Therefore, the PBrS chains assemble into spherical core with 41 nm radius in TMEDA solution.

ASAXS, SAXS and SANS investigations of vulcanized elastomers filled with carbon black

Small-angle X-ray scattering (SAXS) performed down to small q values (q <or= 10(-3) A(-1)) is a powerful method for investigating the arrangement of filler aggregates in filled elastomers under uniaxial strain. Meanwhile, for vulcanized samples, zinc oxide is used as an additive. Owing to their high contrast, the ZnO particles remaining in the manufactured composite are strong X-ray scatterers. In the low-q domain, their scattering hides that of filler aggregates (carbon black, pyrogenic silica) and must be quantified in order to be suppressed. To this end, anomalous SAXS (ASAXS) and small-angle neutron scattering (SANS) have been performed. It is shown that ASAXS measurements can be performed down to small q values (q <or= 10(-3) A(-1)). Therefore ASAXS is well adapted to separate the contributions of ZnO and filler scattering. For neutron scattering the contrast of the ZnO particles is similar to that of carbon. Because the amount of ZnO is much smaller than that of filler, ZnO scattering can be neglected. Owing to multiple scattering effects, however, SANS can only be used for very thin samples (less than about 0.25 mm). It is shown that, providing multiple scattering is avoided, ASAXS and SANS yield similar scattering curves for the filler aggregates.

Localization of Dibromophenol in DPPC/Water Liposomes Studied by Anomalous Small-Angle X-ray Scattering

The localization of 2,4-dibromophenol molecules along the bilayer normal was investigated by anomalous small-angle X-ray scattering (ASAXS) with synchrotron radiation. The ASAXS measurements, executed at three different energies, provide the separation of the scattering of the bromine atoms of dibromophenol molecules from that of the whole system. Using a full q-range model, the localization of the dibromophenol molecules was characterized at a lower (0.1) and a higher (1) dibromophenol/lipid molar ratio corresponding to the gel and to the interdigitated phases of the vesicle matrix, respectively.

Recent developments and ASAXS measurements at the ultra small angle X-ray scattering instrument of HASYLAB

The wiggler beamline BW4 at the synchrotron radiation facility HASYLAB (DESY) is mainly designed for Ultra Small Angle X-ray Scattering (USAXS) and usually operated with detector–sample distances up to 13 m and at photon energies between 4 and 16 keV. With a new optical design the largest observable correlation distances have now been increased up to 9×10 3 Å. A grazing incidence set-up [P. Müller-Buschbaum et al., Europhys. Lett. 42 (5) (1998) 517], vapor chamber, furnace, tensile testing machine and other instruments make the USAXS beamline attractive for a variety of scattering experiments [A. Endres et al., Rev. Sci. Instrum. 11 (1997) 68; A. Karl et al., J. Macromolecular Sci.-Phys. B 38 (5&6) (1999) 901; S. Minko et al., J. Macromolecular Sci., Phys. B 38 (5&6) (1999) 913]. A fully evacuated beampath allows high quality measurements with very low background signal. A photodiode mounted in the primary beam stop registers the primary beam flux simultaneously to the data acquisition and thus provides a precise method for measuring absolute scattering intensities. Anomalous small angle X-ray scattering (ASAXS) experiments at the K absorption edge of titanium were recently performed to investigate phase separated fluorine titania silicate glass. From these measurements we expect some new results concerning the distribution of titanium atoms within the diffusion zone outside the phase separated particles.