XAFS (X-ray absorption Fine Structure) was available in the advents of synchrotron radiation (SR) and short range order theory. In 1982, Photon Factory starts in Japan and since then XAFS has been our daily tool for surface analysis. The Frontier of surface science is nanoscience and nanotechnology. XAFS has been developed to be an appropriate technique for the investigation of nanomaterials which require the applicability of XAFS to light, thin, short and small targets. We review the recent development in XAFS in these key words “light, thin, short and small”. We also discuss the future of XAFS in stand point of new light sources, XFEL and ERL. However both of them are a bit different from SR but complementary ones. These new light sources will make it possible to open a new horizon in XAFS in surface science.
Dynamic structural change of palladium loaded on USY zeolites was monitored by means of an in situ Quick-XAFS method inthe H2, O2 atmosphere and solvents. We found that metal Pd clusters were readily generated on exposure to a H2 flow at room temperature. The size of the Pd clusters could be regulated simply by changing the number of times that H2 and O2 flows were alternately introduced into the in situ cell. Then structural change of Pd was monitored during temperature programmed reduction and oxidation. We could observe the coalescence and dispersion behavior of Pd depending on the temperature and oxidation-reduction cycles. The Pd/USY catalysts exhibited prominent activity in the Suzuki-Miyaura coupling when the reaction was performed in o-xylene. Pd K-edge and Pd L3-edge XAFS analyses revealed the formation of atomic Pd with a cationic character in the o-xylene as a solvent.
Dispersive XAFS is applicable to the millisecond time-resolved sequential measurement. DXAFS apparatus consists of a bent crystal (polychromator) and a position sensitive detector. There is no mechanical movement during the measurement. The time resolution of DXAFS is suitable to analyze chemical reactions started by introducing a reactant. XAFS spectrum of short-lived reaction intermediate can be obtained from DXAFS experiment. The formation of intermediate CoO species has been confirmed during the oxidation process of supported Co catalyst to Co3O4. Reaction kinetics is also an important information obtained from the time change of X-ray absorbance. The reaction mechanism of H2 reduction process of supported metal oxide catalysts was analyzed by the DXAFS technique. It is clarified that the oxygen migration process is a key step in the reduction of supported metal oxides.
XAFS measurements with the spatial resolution are attractive in the analytical applications of synchrotron X-ray microprobe. The present status of the X-ray focusing elements such as zone plate and aspherical total reflection mirror is explained and the analytical systems for micro XAFS measurements are reviewed. Micro XAFS measurements were applied to the Cr concentrated rust layer of weathered steel, and the local structural changes of iron in the Cr enriched region were observed.
X-ray absorption fine structure (XAFS) at the liquid-liquid (oil-water) interface has been developed to elucidate the molecular structure of the interfacial species. It reveals the concentration, valence state, molecular orientation, and solvation structure of the chemical spices at the interface. Here we describe this technique.
Magnetic materials are generally synthesized and used as alloys and compounds. They are also stacked as a multilayer film for spintronics device such as a reading-head sensor of a hard disk drive. The evaluation of magnetization is the most fundamental characterization in studies of magnetic materials. Especially, in alloys and compounds involving more than two magnetic elements, a partial magnetization with respect to each element, we call as an element specific magnetization, promises to provide the deeper understanding of their magnetic property. X-ray magnetic circular dichroism (XMCD) in absorption spectroscopy provides an element specific magnetization. As XMCD became increasingly popular, high-magnetic-field environment for XMCD measurements also became very important in order to investigate paramagnetic, antiferromagnetic, and meta-magnetic materials. Under the circumstance, a high-magnetic-field XMCD measurement technique of the soft-X-ray regime has been developed using a non-destructive pulse magnet having capability of generating 40 T at the twin helical undulators beamline, BL25SU, of SPring-8. In this review, we first introduce the concept and the technical features of high magnetic field XMCD and then show recent examples of the experiments.