Yanna Chen

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.

Synchrotron X-ray diffraction characterization of the inheritance of GaN homoepitaxial thin films grown on selective growth substrates

The crystallinity of one n-GaN (Si-doped) and two p-GaN (Mg-doped) homoepitaxial thin films selectively grown on GaN substrates was evaluated by using synchrotron X-ray diffraction. A reflection-mode monochromatic X-ray topography image from the n-GaN homoepitaxial thin film shows a mesh-shape structure that is similar to that of the selective-growth GaN substrate. Moreover, transmission-mode white-beam X-ray topography images from the GaN substrate and the n-GaN homoepitaxial thin film show similar regular dot-shape diffraction patterns. This suggests that, following hydride vapor phase epitaxy, the structural characteristics of the selectively grown GaN substrates inherited from their corresponding foreign substrates (dot-patterned sapphire) were inherited by the subsequent n-GaN homoepitaxial thin film, although the crystal quality of the homoepitaxial thin film had been deteriorated. White-beam topography images from two p-GaN homoepitaxial thin films grown on the same GaN substrate wafer and cut from adjacent areas indicate that the p-GaN thin films were non-uniform.

Author Correction: Electronic origin of hydrogen storage in MOF-covered palladium nanocubes investigated by synchrotron X-rays

The previously published version of this Article contained an error in Fig. 1. In panel c, the thickness of the HKUST-1 layer (3 nm) was incorrectly labelled. This error has been corrected in both the PDF and HTML versions of the Article.

Lattice constant, bond-orientational order, and solid solubility of PdPt bimetallic nanoparticles

Crystal structure deviations of bimetallic Pd-Pt nanoparticles (NPs) were investigated using synchrotron high-energy X-ray diffraction. The samples comprised Pd NPs and bimetallic Pd-Pt NPs featuring those with a Pd-core/Pt-shell structure as well as PdPt solid-solution NPs with varying diameters of 6.1, 6.7, 8.1, and 11.2 nm. The Rietveld method was used to find the lattice constant of the Pd NPs, which was larger than the bulk value, while reverse Monte Carlo modeling revealed that they possessed a highly disordered structure. The lattice constants of the Pd-Pt bimetallic NPs were found to vary with the Pt content. For Pd-core/Pt-shell structure NPs, the lattice constants decreased with the Pt content owing to lattice relaxation at the core/shell interface. After a reaction process of hydrogen absorption/desorption, the lattice constant of the resulting PdPt solid-solution NPs increased with the NP Pt content, which followed Vegards law. The degree of disorder of the solid-solution NPs was evaluated by the PBOO values (i.e., structural parameters describing the deviation of the local structure from an ideal crystal structure) and was found to decrease with the Pt content. A short-range-order parameter was introduced using a three-dimensional reverse Monte Carlo configuration structure to evaluate the atomic-scale solid solubility of the Pd0.92Pt0.08 and Pd0.79Pt0.21 NPs. The parameter values obtained were close to 0, indicating that the Pd and Pt atoms were essentially randomly dispersed in the solid-solution NPs.

Evaluation of lattice curvature and crystalline homogeneity for 2-inch GaN homo-epitaxial layer

We evaluated the lattice curvature, crystallinity, and crystalline homogeneity of a GaN layer on a free standing GaN substrate using lattice orientation measurements, θ rocking curves, and reciprocal space mapping from synchrotron X-ray diffraction topography, and X-ray diffraction. The lattice curvature of the 2-inch GaN homo-epitaxial layer was a concave bend, which had a curvature radius of approximately 20.7 m. The GaN layer was epitaxially grown on the GaN substrate and had good crystallinity with a high homogeneity.