Polarization-dependent Total Reflection Fluorescence X-ray Research Articles

Can amine ligands atomically disperse Cu atoms on TiO2(110)? Cu deposition on TiO2(110) premodified with o-anthranilic acid

Abstract The three-dimensional (3D) structure of evaporated Cu atoms on a TiO2(110) surface premodified with o-anthranilic acid (o-AA) was examined using polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS) to elucidate the effect of the premodification on the Cu dispersion over the TiO2(110) surface. Cu was found to be atomically dispersed with the formation of N–Cu–O bonds (where the N is provided by the o-AA and the O is present in the TiO2 lattice) on the surface, and the factors for controlling the atomic dispersion are discussed.

Bent-crystal Laue analyser combined with polarization-dependent total reflection fluorescence X-ray absorption fine structure (BCLA-TRFXAFS) and its application to electrode surface

Bent-crystal Laue analyser combined with polarization-dependent total reflection fluorescence X-ray absorption fine structure (BCLA-TRFXAFS) and its application to electrode surface

Development of Operando Polarization-Dependent Total Reflection Fluorescence X-ray Absorption Fine Structure Technique for Three-Dimensional Structure Determination of Active Metal Species on a Model Catalyst Surface under Working Conditions

Development of <i>Operando</i> Polarization-Dependent Total Reflection Fluorescence X-ray Absorption Fine Structure Technique for Three-Dimensional Structure Determination of Active Metal Species on a Model Catalyst Surface under Working Conditions

Extracting the local electronic states of Pt polycrystalline films surface under electrochemical conditions using polarization-dependent total reflection fluorescence x-ray absorption near edge structure spectroscopy

The local atomic information about the interface between the 30 nm-thick Pt polycrystalline films and the solution with and without perfluorosulfonic acid polymers (Nafion®) for the model cathode catalyst of fuel cell has been captured under electrochemical conditions using polarization-dependent total reflection fluorescence x-ray absorption near edge structure spectroscopy (PTRF–XANES). The results show that the formation of sub-monolayer-equivalent PtO or adsorbed hydrogen/oxygen species in the surface region can be successfully observed in the PTRF–XANES spectra when the thickness of the solution layer and the incidence angle are properly controlled. This capability enables us to examine the metal /(Nafion®/) solution interface structure through XANES together with other surface analysis methods, which will enhance comprehensive understanding of the nature of the interface of the fuel cell system.

Premodified Surface Method to Obtain Ultra-Highly Dispersed Metals and their 3D Structure Control on an Oxide Single-Crystal Surface.

Precise control of the three-dimensional (3D) structure of highly dispersed metal species such as metal complexes and clusters attached to an oxide surface has been important for the development of next-generation high-performance heterogeneous catalysts. However, this is not easily achieved for the following reasons. (1) Metal species are easily aggregated on an oxide surface, which makes it difficult to control their size and orientation definitely. (2) Determination of the 3D structure of the metal species on an oxide powder surface is hardly possible. To overcome these difficulties, we have developed the premodified surface method, where prior to metal deposition, the oxide surface is premodified with a functional organic molecule that can strongly coordinate to a metal atom. This method has successfully provided a single metal dispersion on an oxide single-crystal surface with the 3D structure precisely determined by polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS). Here we describe our recent results on ultra-high dispersions of various metal atoms on TiO2 (110) surfaces premodified with mercapto compounds, and show the possibility of fine tuning and orientation control of the surface metal 3D structures.

Polarization-dependent Total Reflection Fluorescence X-ray Absorption Fine Structure (PTRF-XAFS) Studies on the Structure of a Pt Monolayer on Au(111) Prepared by the Surface-limited Redox Replacement Reaction

We studied the initial stage of a Pt monolayer produced by surface-limited redox replacement (SLRR) using polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS). Different from the widely accepted understanding that a metallic monolayer island is formed, our XAFS showed that the Pt monolayer, initially present on the Au(111) substrate, was mainly in the form of a planar [PtCl4]2- complex with its molecular plane parallel to the Au(111). This result provides a new insight into the mechanism of SLRR.

A New Indicator for Single Metal Dispersion on a TiO2(110) Surface Premodified with a Mercapto Compound

Ni and Pt structures evaporated onto a TiO2(110) surface premodified with ortho-mercaptobenzoic acid (o-MBA) were studied using polarization-dependent total reflection fluorescence X-ray absorption fine structure analysis to determine the effects of the premodification on the dispersion of the metal atoms over the TiO2(110) surface. Ni was found to be atomically dispersed with the formation of S–Ni–O bonds (where the S is provided by the o-MBA and the O is present in the TiO2 lattice) on the surface. In contrast, Pt underwent aggregation to form small clusters. The varying behavior of these metals on the o-MBA-modified TiO2(110) surface is discussed based on the energy difference between sulfur–metal–oxygen and metal–metal bond formations, and we propose a new indicator for single metal dispersion on the TiO2(110) surface.

Portable ultrahigh-vacuum sample storage system for polarization-dependent total-reflection fluorescence x-ray absorption fine structure spectroscopy

A portable ultrahigh-vacuum sample storage system was designed and built to investigate the detailed geometric structures of mass-selected metal clusters on oxide substrates by polarization-dependent total-reflection fluorescence x-ray absorption fine structure spectroscopy (PTRF-XAFS). This ultrahigh-vacuum (UHV) sample storage system provides the handover of samples between two different sample manipulating systems. The sample storage system is adaptable for public transportation, facilitating experiments using air-sensitive samples in synchrotron radiation or other quantum beam facilities. The samples were transferred by the developed portable UHV transfer system via a public transportation at a distance over 400 km. The performance of the transfer system was demonstrated by a successful PTRF-XAFS study of Pt4 clusters deposited on a TiO2(110) surface.

Thin Film Structures of Metal-Organic Framework [Cu3(BTC)2(H2O)3]n on TiO2(110)

Thin film structures of [Cu3(BTC)2(H2O)3]n (HKUST-1), which is one of Metal-organic frameworks (MOFs), prepared via step-by-step liquid phase epitaxy (LPE) method, on a TiO2(110) surface were investigated by polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS), X-ray diffraction (XRD), and scanning electron microscopy (SEM). For the ultra thin film, PTRF-XAFS enlightened that HKUST-1 eventually formed even at 1 cycle LPE. For further growth, XRD and SEM verified well-oriented < 111 > HKUST-1 thin films on the surface. These results proposed that the adsorption structure of linkers on the surface plays an important role to control the orientation of MOF films.

Anisotropic growth of a nickel trimer formed on a highly-stepped TiO2(1 1 0) surface

The structures of Ni clusters formed on a highly-stepped TiO2(110) surface were studied by polarization-dependent total reflection fluorescence X-ray absorption fine structure analysis. When 0.8 monolayers of Ni were deposited, three-dimensional Ni clusters with 1–2nm diameters and heights less than 1nm were formed. Conversely, when 0.07 monolayers of Ni were deposited, an anisotropic Ni trimer with a Ni–Ni distance of 0.260nm was created at the [001] step. We revealed that the surface modification to enhance the metal–anion interaction can control the deposited metal structure.

Fine tuning and orientation control of surface Cu complexes on TiO2(110) premodified with mercapto compounds: the effect of different mercapto group positions

Three-dimensional structures of vacuum-deposited Cu species formed on TiO2(110) surfaces premodified with three mercaptobenzoic acid (MBA) isomers were studied using polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS). We explored the possibility of fine tuning and orientation control of the surface Cu structures, including their coordination and configuration against the surface, according to the different mercapto group positions of the three MBA isomers (o-, m-, and p-MBA). Almost linear S-Cu-O (lattice O of TiO2) surface compounds were formed on the three MBA-modified TiO2(110) surfaces; however, the orientation of the Cu species on the o- and m-MBA-modified TiO2(110) surfaces (40-45° inclined from the surface normal) was different from that on the p-MBA-modified TiO2(110) surface (60° from the surface normal). This work suggests that the selection of a different MBA isomer for premodification of a single crystal TiO2(110) surface enables fine tuning and orientation control of surface Cu complexes.

Au Clusters on TiO2(110) (1 × 1) and (1 × 2) Surfaces Examined by Polarization-Dependent Total Reflection Fluorescence XAFS

The structure of Au clusters on the TiO2(110) surface and the cluster-substrate interaction were investigated by polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS). The TiO2(110) (1 × 1) and (1 × 2) surfaces were compared in terms of substrate structural effects on Au cluster structure. We could not find evidence for the direct Au-substrate bonding and observed that the substrate morphology did not significantly affect the size and structure of Au clusters. Finally, we precisely determined that the Au clusters favored a geometry with icosahedral Au55 symmetry. We discuss the possible interaction mechanism between Au and TiO2(110) that leads to these results.

Atomically dispersed Cu species on a TiO 2(1 1 0) surface precovered with acetic anhydride

Preparing highly dispersed monoatomic Cu species on a TiO 2(1 1 0) surface was successfully achieved on a TiO 2(1 1 0) surface that had been precovered with an acetic anhydride layer. Polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS) spectra indicated that the three-dimensional structures of the monoatomic Cu species were stabilized on the surface by two 0.196-nm-long C–O bonds, namely, Cu–O (acetate) and Cu–O (bridging surface oxygen) bonds. The Cu species diffusion was effectively blocked by the acetate species with its molecular plane perpendicular to the [0 0 1] diffusion direction created by the dissociative adsorption of acetic anhydride.

Origin of Self-Regulated Cluster Growth on the TiO2(110) Surface Studied Using Polarization-Dependent Total Reflection Fluorescence XAFS

Three-dimensional structures of Ni clusters deposited on a TiO2(110) surface were studied using polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS) analysis. Three-dimensional Ni clusters were formed with a slightly anisotropic structure at 1 × 1014 atoms/cm2. The PTRF-XAFS results for a 1 × 1014 atoms/cm2 sample revealed the formation of one-atomic-layer Ni clusters with a distorted (110)fcc plane structure. We found a Ni−Ni distance of 0.240 nm with Ni−O interaction at 0.23 nm. The structure was stabilized by interaction of surface oxygen atoms. The origin of cluster size self-regulation was the strong Ni−O interaction and matching of the crystal lattice between TiO2(110) and distorted Ni(110) structures.

Structure Analysis of Exfoliated Unilamellar Crystallites of Manganese Oxide Nanosheets

Structure analysis of unilamellar manganese oxide nanosheets obtained via exfoliation of layered manganese oxides was carried out utilizing synchrotron radiation (SR) X-ray in-plane diffraction and polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS) analyses. A combination of SR excitation and the total reflection of incoming X-rays provides signals strong enough for both analyses even from a monolayer of the MnO(2) nanosheets having a concentration of 0.7 microg cm(-2). In addition, the mean oxidation state of constituent manganese ions in the MnO(2) sheets was estimated on the basis of XANES spectra, and bond valence sum calculations with the bond length obtained from the present EXAFS analyses. The obtained structural data revealed that the two-dimensional lattice of the MnO(2) sheets underwent a slight elongation upon delamination. These changes correspond to approximately 1% expansion of sheet area and 1-2% expansion of thickness, which can be understood by reduction of the mean oxidation number of manganese ions in the sheet through the exfoliation process.

Three-Dimensional Structure Analyses of Cu Species Dispersed on TiO2(110) Surfaces Studied by Polarization-Dependent Total-Reflection Fluorescence X-ray Absorption Fine Structure (PTRF-XAFS)

Three-dimensional analysis of structures for dispersed Cu species on a TiO2(110) surface has been performed by a polarization-dependent total-reflection fluorescence X-ray absorption fine structure (PTRF-XAFS) technique. The dispersed Cu species were prepared by a chemical reaction between Cu(DPM)2 and a TiO2(110) surface and reduced with H2. The Cu(DPM)2 dissociatively adsorbed on the 2-fold site of bridging oxygen atoms of the TiO2(110) surface to form a Cu(DPM)(O-)2 species. Subsequent reduction of the Cu species with H2 at 363 K produced Cu3 trimers that were bound to the bridging oxygen atoms with the trimer plane inclined at about 30° from the TiO2(110) normal. There were direct Cu−O bonds at the Cu3 trimer/TiO2(110) interface in addition to Cu−Cu bonds. Further reduction at 473 K produced three-dimensional prismatic small Cu6 clusters by vertical growth in the [110] direction of the Cu fcc lattice. Orientations of the Cu3 triangle and Cu6 prism clusters may be regulated by the interaction between t...

In situ Characterization of Supported Metal Catalysts and Model Surfaces by Time‐Resolved and Three‐Dimensional XAFS Techniques

AbstractFor Abstract see ChemInform Abstract in Full Text.

In situ characterization of supported metal catalysts and model surfaces by time-resolved and three-dimensional XAFS techniques

This paper emphasizes novel issues in in situ characterization techniques for catalysts and model surfaces, namely, energy dispersive X-ray absorption fine structure (DXAFS) for time-resolved structure analysis and polarization-dependent total-reflection fluorescence X-ray absorption fine structure (PTRF-XAFS) for 3D structure analysis, which have been developed in the past 5–10 years. DXAFS is a powerful technique for elucidating dynamic structural changes at active metal sites on catalyst surfaces that involve a sequence of formation and breaking of metal–metal, metal–adsorbate, and metal–support bonds; it opens a new area of structure kinetics, revealing kinetic parameters during catalytic reactions, catalyst preparations, and catalyst deactivation and regeneration. PTRF-XAFS is another powerful technique for elucidating 3D structures of active metal sites at single crystal model surfaces, determining structural parameters in three different directions independently with high surface sensitivity, which opens up a new area of catalytic structural chemistry, presenting information on asymmetric and anisotropic bond arrangements and transformations. The issues addressed in this paper are relevant to important key issues in understanding new catalytic structures and reaction mechanisms and to developing new concepts and strategies for novel catalytic materials, which can hardly be addressed by other techniques.

Development of an in situ polarization-dependent total-reflection fluorescence XAFS measurement system.

An in situ polarization-dependent total-reflection fluorescence X-ray absorption fine structure (PTRF-XAFS) spectroscopy system has been developed, which enables PTRF-XAFS experiments to be performed in three different orientations at various temperatures (273-600 K) and pressures (10(-10) approximately 760 torr). The system consists of a measurement chamber and a preparation chamber. The measurement chamber has a high-precision six-axis goniometer and a multielement solid-state detector. Using a transfer chamber, also operated under ultra-high-vacuum conditions, the sample can be transferred to the measurement chamber from the preparation chamber, which possesses low-energy electron diffraction, Auger electron spectroscopy and X-ray photoelectron spectroscopy facilities, as well as a sputtering gun and an annealing system. The in situ PTRF-EXAFS for Cu species on TiO2 (110) has been measured in three different orientations, revealing anisotropic growth of Cu under the influence of the TiO2 (110) surface.

Polarization-Dependent Total-Reflection Fluorescence XAFS Study of Mo Oxides on a Rutile TiO2(110) Single Crystal Surface

The structure of well-dispersed Mo oxides on TiO2(110) was investigated by means of polarization-dependent total-reflection fluorescence X-ray absorption fine structure (PTRF-EXAFS) in three different directions of the single-crystal surface. A model sample for supported Mo oxide catalysts was prepared by impregnation of (NH4)6Mo7O24·4H2O dissolved in ultrapure water, followed by calcination at 773 K. Direct comparison of the observed EXAFS and the calculated one was performed based on three-dimensional model structures. We found that anisotropic Mo dimers were preferentially formed on the TiO2(110) surface, with Mo−Mo distance (0.335 nm) parallel to the [110] direction of the surface. It was also found that the Mo dimers were attached to the surface, showing Mo−Ti (interfacial) distance at 0.296 nm.