Total x-ray scattering setup for crystalline particles at SPring-8 BL15XU NIMS beamline.

We report a total x-ray scattering (TXS) system for structural analysis of crystalline particle materials at the BL15XU NIMS beamline of SPring-8 in Japan. To achieve a high angular resolution over a high Q region up to 25 Å-1, the TXS system was capable of measuring to 120° at an x-ray energy of 29.02 keV with five CdTe pin detectors. The sample alignment and measuring system were controlled by LabView software. The x-ray pair distribution function (PDF) results for Ni bulk powder and Pt and AgRh nanoparticles were successfully simulated by the PDFgui program. In addition, Rietveld refinement results were also obtained from x-ray diffraction patterns, reflecting long-range order in the Pt nanoparticles. We expect that this TXS system may be useful for understanding structural information of crystalline nanoparticles, including amorphous features at their surface region.

The relationship between crystalline disorder and electronic structure of Pd nanoparticles and their hydrogen storage properties.

We investigated the relationship between crystalline disorder and electronic structure deviations of Pd nanoparticles (NPs) and their hydrogen storage properties as a function of their particle diameter (2.0, 4.6 and 7.6 nm) using various synchrotron techniques. The lattice constant of the 2.0 nm-diameter Pd NPs was observed to be larger than that of the 4.6 or 7.6 nm-diameter Pd NPs. With increasing particle diameter the structural ordering was improved, the lattice constant and atomic displacement were reduced and the coordination numbers increased, as determined using high-energy X-ray diffraction, reverse Monte Carlo modelling and X-ray absorption fine structure spectroscopy. The structural order of the core part of the larger NPs was also better than that of the smaller NPs. In addition, the bond strength of the Pd-H formation increased with increasing particle diameter. Finally, the surface order of the Pd NPs was related to enhancement of the hydrogen storage capacity and Pd-H bond strength.

Correlation between the electronic/local structure and CO-oxidation activity of Pd x Ru1-x alloy nanoparticles.

Pd x Ru1-x nanoparticles (NPs) were observed to display enhanced CO oxidation activity with the maximum performance obtained at the composition x = 0.5. To unveil the origin of this superior CO oxidation activity, we investigated the local structure, valence state, and electronic properties of Pd x Ru1-x NPs using synchrotron-based X-ray techniques. Site specific information obtained from X-ray absorption fine structure (XAFS) spectroscopy revealed that the local disorder around Pd and Ru atoms and their valence state can be systematically tuned by varying the Pd composition. Furthermore, the XAFS results indicated a strong correlation among the structural and valence state and the observed CO oxidation catalytic properties of Pd x Ru1-x NPs. Hard X-ray photoelectron spectroscopy (HAXPES) analysis suggested that the capability of CO oxidation requires an optimum balance between the adsorption and desorption energy for CO adsorption and eventually conversion to CO2. A comparison between the experimental valence band (VB) HAXPES spectra of Pd x Ru1-x NPs and the linear combination of VB HAXPES spectra of Pd and Ru NPs revealed that the charge transfer from Pd to Ru occurs in the Pd x Ru1-x alloy at intermediate compositions, causing electron enrichment of the Ru surface. In addition, the maximum red-shift in the edge-position relative to that of bulk Pd/Ru and high structural disorder were observed for the PdRu alloy at the intermediate composition. This coupled behavior of structure and electronic properties followed the experimental trend of CO oxidation activity in this system.

Local Geometry and Electronic Properties of Nickel Nanoparticles Prepared via Thermal Decomposition of Ni-MOF-74.

Metal-organic frameworks (MOFs) provide highly selective catalytic activity because of their porous crystalline structure. There is particular interest in metal nanoparticle-MOF composites (MNP@MOF) that could take advantage of synergistic effects for enhanced catalytic properties. We present an investigation into the local geometry and electronic properties of thermally decomposed Ni-MOF-74 calcined at different temperatures and time durations. Pair distribution function analysis using high-energy X-ray diffraction reveals the formation of fcc-Ni nanoparticles with a mixture of MOF phase in samples heated at 623 K for 12 h. Elevating the calcination temperature and lengthening the time duration assisted complete precipitation of Ni nanoparticles in the MOF matrix. Local structures and valence states were investigated using X-ray absorption fine structure spectroscopy. Evidence of ligand-to-metal charge transfer and gradual reduction of Ni2+ is apparent for those samples heated above 623 K for 12 h. In addition, the Ni lattice was found to be slightly compressed as a result of surface stresses in the nanosized particles or surface ligand environment. Electronic structure investigation using hard X-ray photoelectron spectroscopy shows a significant narrowing of the valence band and a decrease in the d-band center (toward the Fermi level) when the heating temperature is increased, thus suggesting promising catalytic properties for NiNP@MOF composite.

Size effects on rhodium nanoparticles related to hydrogen-storage capability.

To unveil the origin of the hydrogen-storage properties of rhodium nanoparticles (Rh NPs), we investigated the electronic and crystal structures of the Rh NPs using various synchrotron based X-ray techniques. Electronic structure studies revealed that the hydrogen-storage capability of Rh NPs could be attributed to their more unoccupied d-DOSs than that of the bulk near the Fermi level. Crystal structure studies indicated that lattice distortion and mean-square displacement increase while coordination number decreases with decreasing particle size and the hydrogen-absorption capability of Rh NPs improves to a greater extent with increased structural disorder in the local structure than with that in the mean structure. The smallest Rh NPs, having the largest structural disorder/increased vacancy spaces and the smallest coordination number, exhibited excellent hydrogen-storage capacity. Finally, from the bond-orientational order analysis, we confirmed that the localized disordering is distributed more over the surface part than the core part and hydrogen can be trapped on the surface part of Rh NPs which increases with a decrease in NP diameter.

Electronic Structure Evolution with Composition Alteration of RhxCuy Alloy Nanoparticles.

The change in electronic structure of extremely small RhxCuy alloy nanoparticles (NPs) with composition variation was investigated by core-level (CL) and valence-band (VB) hard X-ray photoelectron spectroscopy. A combination of CL and VB spectra analyses confirmed that intermetallic charge transfer occurs between Rh and Cu. This is an important compensation mechanism that helps to explain the relationship between the catalytic activity and composition of RhxCuy alloy NPs. For monometallic Rh and Rh-rich alloy (Rh0.77Cu0.23) NPs, the formation of Rh surface oxide with a non-integer oxidation state (Rh(3-δ)+) resulted in high catalytic activity. Conversely, for alloy NPs with comparable Rh:Cu ratio (Rh0.53Cu0.47 and Rh0.50Cu0.50), the decreased fraction of catalytically active Rh(3-δ)+ oxide is compensated by charge transfer from Cu to Rh. As a result, ensuring negligible change in the catalytic activities of the NPs with comparable Rh:Cu ratio to those of Rh-rich and monometallic Rh NPs.

Origin of the catalytic activity of face-centered-cubic ruthenium nanoparticles determined from an atomic-scale structure.

The 3-dimensional (3D) atomic-scale structure of newly discovered face-centered cubic (fcc) and conventional hexagonal close packed (hcp) type ruthenium (Ru) nanoparticles (NPs) of 2.2 to 5.4 nm diameter were studied using X-ray pair distribution function (PDF) analysis and reverse Monte Carlo (RMC) modeling. Atomic PDF based high-energy X-ray diffraction measurements show highly diffuse X-ray diffraction patterns for fcc- and hcp-type Ru NPs. We here report the atomic-scale structure of Ru NPs in terms of the total structure factor and Fourier-transformed PDF. It is found that the respective NPs have substantial structural disorder over short- to medium-range order atomic distances from the PDF analysis. The first-nearest-neighbor peak analyses show a significant size dependence for the fcc-type Ru NPs demonstrating the increase in the peak height due to an increase in the number density as a function of particle size. The bond angle and coordination number (CN) distribution for the RMC-simulated fcc- and hcp-type Ru NP models indicated inherited structural features from their bulk counterparts. The CN analysis of the whole NP and surface of each RMC model of Ru NPs show the low activation energy packing sites on the fcc-type Ru NP surface atoms. Finally, our newly defined order parameters for RMC simulated Ru NP models suggested that the enhancement of the CO oxidation activity of fcc-type NPs was due to a decrease in the close packing ordering that resulted from the increased NP size. These structural findings could be positively supported for synthesized low-cost and high performance nano-sized catalysts and have potential application in fuel-cell systems and organic synthesis.

Atomic disorder of Li0.5Ni0.5O thin films caused by Li doping: estimation from X-ray Debye-Waller factors.

Cubic type room-temperature (RT) epitaxial Li0.5Ni0.5O and NiO thin films with [111] orientation grown on ultra-smooth sapphire (0001) substrates were examined using synchrotron-based thin-film X-ray diffraction. The 1[Formula: see text]1 and 2[Formula: see text]2 rocking curves including six respective equivalent reflections of the Li0.5Ni0.5O and NiO thin films were recorded. The RT B1 factor, which appears in the Debye-Waller factor, of a cubic Li0.5Ni0.5O thin film was estimated to be 1.8 (4) Å2 from its 1[Formula: see text]1 and 2[Formula: see text]2 reflections, even though the Debye model was originally derived on the basis of one cubic element. The corresponding Debye temperature is 281 (39) K. Furthermore, the B2 factor in the pseudo-Debye-Waller factor is proposed. This parameter, which is evaluated using one reflection, was also determined for the Li0.5Ni0.5O thin film by treating Li0.5Ni0.5O and NiO as ideal NaCl crystal structures. A structural parameter for the atomic disorder is introduced and evaluated. This parameter includes the combined effects of thermal vibration, interstitial atoms and defects caused by Li doping using the two Debye-Waller factors.

Hard X-ray photoelectron spectroscopy of LixNi1-xO epitaxial thin films with a high lithium content.

The core-level and valence-band electronic structures of LixNi1-xO epitaxial thin films with x = 0, 0.27, and 0.48 were studied by hard X-ray photoelectron spectroscopy. A double peak structure, consisting of a main peak and a shoulder peak, and a satellite structure were observed in the Ni 2p3/2 core-level spectra. The intensity ratio of the shoulder to main peak in this double peak structure increased with increasing lithium content in LixNi1-xO. This lithium doping dependence of the Ni 2p3/2 core-level spectra was investigated using an extended cluster model, which included the Zhang-Rice (ZR) doublet bound states arising from a competition between O 2p - Ni 3d hybridization and the Ni on-site Coulomb interaction. The results indicated that the change in the intensity ratio in the main peak is because of a reduction in the ZR doublet bound states from lithium substitutions. This strongly suggests that holes compensating Li doping in LixNi1-xO are of primarily ZR character.