Yu Gong

Crystallization mechanism analysis of noncrystalline Ni-P nanoparticles through XRD, HRTEM and XAFS

The crystallization behavior of noncrystalline Ni–P nanoparticles prepared by a liquid-phase pulsed-discharge method was studied through XRD, HRTEM, and X-ray absorption fine structure (XAFS) spectra from both Ni K-edge and P K-edge. A competitive growth between Ni3P and Ni crystalline phases was found. The main phases within the particles are crystalline Ni3P and Ni, while the metastable phase Ni5(P, Ni)2 is presented as a coated shell outside the particles. The appearance of feature D in the P K-edge XANES spectrum could be used as a characteristic for the formation of long-range ordered Ni3P. The standard deviation (ΔR/R) of the fitting P–Ni bond length from the theoretical value of a Ni3P crystal could be used to define the crystallization process. Although the nanoparticles were observed as XRD amorphous at 250 °C, their magnetic properties can be attributed to the formation of FCC-Ni clusters. A crystallization mechanism has been proposed to describe the crystallization process of the as-prepared noncrystalline Ni–P nanoparticles.

Mythen detector for X-ray diffraction at the Beijing synchrotron radiation facility

ABSTRACT Time-resolved X-ray powder diffraction may be used to elucidate the structure of polycrystalline matter, but a fast-response full-profile detector is often necessary. This article describes a Mythen detector incorporated in beamline 4B9A of the Beijing Synchrotron Radiation Facility. Basic parameters of this detector are reported, and flat-field correction, angular calibration, eccentric error, and other data treatment for this detector are discussed. X-ray diffraction measurement of lanthanum boride demonstrates that this detector provides high-quality, time-resolved X-ray powder diffraction measurements.

Microstructural change of degummed Bombyx mori silk: an in situ stretching wide-angle X-ray-scattering study.

The microstructural change of degummed Bombyx mori silk was examined by in situ wide-angle X-ray-scattering (WAXS) with applied stretching force. WAXS patterns confirmed that the crystalline and amorphous regions coexist in the silk fibers. The crystallites with β-sheet structure have an orthorhombic unit cell with lattice parameters: a=9.10 Å, b=9.71 Å and c=6.80 Å. The crystallite size, crystallite orientation and crystallinity were also estimated based on the WAXS patterns. The results demonstrate that the crystallite size is almost unchanged with the stretching strain. The crystallinity is approximately linearly increasing with the applied stretching force. However, the change of the unit-cell orientation degree with c-axis along the fiber axis behaves as a fast stage and an approximately unchanged stage during the in situ stretching process. All these experimental phenomena confirm that the microstructure of the degummed silk fibers can be well explained by the model of oriented β-sheet structure with a banded feature.

Structural Change of Human Hair Induced by Mercury Exposure

Mercury is one of the most hazardous pollutants in the environment. In this paper, the structural change of human hair induced by mercury exposure was studied. Human hair samples were, respectively, collected from the normal Beijing area and the Hg-contaminated Wanshan area of the Guizhou Province, China. Inductively coupled plasma mass spectroscopy was used to detect the element contents. A small angle X-ray scattering technique was used to probe the structural change. Three reflections with 8.8, 6.7, and 4.5 nm spacing were compared between the normal and the Hg-contaminated hair samples. The results confirm that the 4.5 nm reflection is from the ordered fibrillar structure of glycosaminoglycan (GAG) in proteoglycan (PG) that composes the matrix around the intermediate filaments. The increase of Ca content makes the regular oriented fibrillar structure of GAG transform to a random oriented one, broadening the angular extent of the reflection with 4.5 nm spacing. However, overdose Hg makes the core proteins where the ordered fibrils of GAG are attached become coiled, which destroys the ordered arrangements of fibrillar GAG in PG, resulting in the disappearance of the reflections with 4.5 nm spacing. The disappearance of the 4.5 nm reflection can be used as a bioindicator of overdose Hg contamination to the human body. A supercoiled-coil model of hair nanoscale structure and a possible mechanism of mercury effect in human hair are proposed in this paper.

Noncrystalline structure of Ni-P nanoparticles prepared by liquid pulse discharge.

Noncrystalline nickel phosphide (Ni-P) nanoparticles have drawn great attention due to their high potential as catalysts. However, the structure of noncrystalline Ni-P nanoparticles is still unknown, which may shed light on explaining the catalysis mechanism of the Ni-P nanoparticles. In this paper, noncrystalline Ni-P nanoparticles were synthesized. Their morphology, particle size, element contents, local atomic structures, as well as the catalysis in the thermal decomposition of ammonium perchlorate were studied. The results demonstrate that the as-prepared Ni-P nanoparticles are spherical with an average diameter of about 13.5 nm. The Ni and P contents are, respectively, 78.15% and 21.85%. The noncrystalline nature of the as-prepared Ni-P nanoparticles can be attributed to cross-linkage between P-doping f.c.c.-like Ni centers and Ni3P-like P centers. The locally ordered Ni centers and P centers are the nuclei sites, which can explain well the origin of initial nuclei to form the crystalline phases after high-temperature annealing. The starting temperature of high-temperature decomposition of ammonium perchlorate was found having a significant decrease in the presence of the noncrystalline Ni-P nanoparticles. Therefore, the as-prepared noncrystalline Ni-P nanoparticles can be used as a potential catalyst in the thermal decomposition of ammonium perchlorate.

Shape evolution with temperature of a thermotolerant protein (PeaT1) in solution detected by small angle X-ray scattering

The protein elicitor from Alternaria tenuissima (PeaT1) presented excellent thermotolerance and potential application in agriculture as a pesticide. Previous synchrotron radiation circular dichroism study demonstrated that the secondary structures in PeaT1 protein are reversible with temperature change. To further clarify the mechanism of its thermotolerance, synchrotron radiation small angle x‐ray scattering (SAXS) technique was used to study the shape change of PeaT1 protein with temperature in this article. Ab initio structure restorations based on the SAXS data revealed that PeaT1 protein has a prolate shape with a P2 symmetry axis along the prolate anisometric direction. With temperature increase, a gooseneck vase‐like (25°C), to jug‐like (55°C), then to oval (85°C) shape change can be found, and these shape changes are also approximately reversible with temperature decrease. PeaT1 protein contains two homogenous molecules, and each of them consists of F, NAC, T, and UBA domains. The structures of the four domains were predicted. Simulated annealing algorithm was used to superimpose the domain structures onto the SAXS shapes. It was found that all the structural domains have position rotation and translation with temperature change, but the NAC domains are relatively stable, playing a role of frame. This shape change information provides clues for further exploring its biological function and application. Proteins 2013.

Revisiting local structural changes in GeO2 glass at high pressure

Despite the great importance in fundamental and industrial fields, understanding structural changes for pressure-induced polyamorphism in network-forming glasses remains a formidable challenge. Here, we revisited the local structural transformations in GeO2 glass up to 54 GPa using x-ray absorption fine structure (XAFS) spectroscopy via a combination diamond anvil cell and polycapillary half-lens. Three polyamorphic transitions can be clearly identified by XAFS structure refinement. First, a progressive increase of the nearest Ge-O distance and bond disorder to a maximum at ~5-16 GPa, in the same pressure region of previously observed tetrahedral-octahedral transformation. Second, a marked decrease of the nearest Ge-O distance at ~16-22.6 GPa but a slight increase at ~22.6-32.7 GPa, with a concomitant decrease of bond disorder. This stage can be related to a second-order-like transition from less dense to dense octahedral glass. Third, another decrease in the nearest Ge-O distance at ~32.7-41.4 GPa but a slight increase up to 54 GPa, synchronized with a gradual increase of bond disorder. This stage provides strong evidence for ultrahigh-pressure polyamorphism with coordination number  >6. Furthermore, cooperative modification is observed in more distant shells. Those results provide a unified local structural picture for elucidating the polyamorphic transitions and densification process in GeO2 glass.

Temperature-driven directional coalescence of silver nanoparticles

Silver nanoparticles were synthesized with a chemical reduction method in the presence of polyvinylpyrrolidone as stabilizing agent. The thermal stability behavior of the silver nanoparticles was studied in the temperature range from 25 to 700°C. Thermal gravimetric analysis was used to measure the weight loss of the silver nanoparticles. Scanning electron microscopy and high-resolution transmission electron microscopy were used to observe the morphology and the change in shape of the silver nanoparticles. In situ temperature-dependent small-angle X-ray scattering was used to detect the increase in particle size with temperature. In situ temperature-dependent X-ray diffraction was used to characterize the increase in nanocrystal size and the thermal expansion coefficient. The results demonstrate that sequential slow and fast Ostward ripening are the main methods of nanoparticle growth at lower temperatures (<500°C), whereas successive random and directional coalescences are the main methods of nanoparticle growth at higher temperatures (>500°C). A four-stage model can be used to describe the whole sintering process. The thermal expansion coefficient (2.8 × 10(-5) K(-1)) of silver nanoparticles is about 30% larger than that of bulk silver. To our knowledge, the temperature-driven directional coalescence of silver nanocrystals is reported for the first time. Two possible mechanisms of directional coalescence have been proposed. This study is of importance not only in terms of its fundamental academic interest but also in terms of the thermal stability of silver nanoparticles.

Local structural changes during the disordered substitutional alloy transition in Bi2Te3 by high-pressure XAFS

A2B3-type 3D topological insulators, Bi2Te3 and Sb2Te3, have been reported to transform into disordered substitutional alloys under high pressure. However, γ → δ phase transition and the local structure changes around Bi during the formation of the disordered Bi-Te binary alloy in Bi2Te3 still remain unclear. Here, high-pressure X-ray absorption fine structure (XAFS) combined with high-pressure X-ray diffraction has been used to explore the local structural transformations in the three structural phase transitions of Bi2Te3. The Bi L3-X-ray absorbing near edge structure (XANES) spectra of δ-Bi2Te3 clearly showed that a new absorption feature at energy of about 13 465 eV would emerge during the γ → δ phase transition. Through simulation of the XANES spectra by varying the cluster size, we confirmed that the new absorption peak arises from the medium-range order in bcc structure. The Bi L3-EXAFS results in δ phase reveal that the Bi atoms in the third shell exhibit abnormal elongations with pressure until merging with the shrinking Te shell. Our findings indicate a two-step structural transition of the disordered substitutional alloys via an ordered bcc intermediate phase.A2B3-type 3D topological insulators, Bi2Te3 and Sb2Te3, have been reported to transform into disordered substitutional alloys under high pressure. However, γ → δ phase transition and the local structure changes around Bi during the formation of the disordered Bi-Te binary alloy in Bi2Te3 still remain unclear. Here, high-pressure X-ray absorption fine structure (XAFS) combined with high-pressure X-ray diffraction has been used to explore the local structural transformations in the three structural phase transitions of Bi2Te3. The Bi L3-X-ray absorbing near edge structure (XANES) spectra of δ-Bi2Te3 clearly showed that a new absorption feature at energy of about 13 465 eV would emerge during the γ → δ phase transition. Through simulation of the XANES spectra by varying the cluster size, we confirmed that the new absorption peak arises from the medium-range order in bcc structure. The Bi L3-EXAFS results in δ phase reveal that the Bi atoms in the third shell exhibit abnormal elongations with pressure until m...

In-situ crystallization study of amorphous Ni-P nanoparticles with high P content

The crystallization behavior of amorphous Ni-P nanoparticles produced by liquid pulsed-discharge was studied by using in situ high temperature XRD at beamline 4B9A of Beijing Synchrotron Radiation Facility. Transmission electron microscope (TEM) was used to observe the morphology and Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) was used to analyze the chemical composition of the as-prepared Ni-P nanoparticles. TEM results show that the average size of the as-prepared nanoparticles is about 13.5 nm. ICP-AES identifies the Ni-P nanoparticles contain 13.16 wt. % (21.85 at. %) of P and 86.84 wt. % (78.15% at. %) of Ni. Eight XRD patterns were, respectively, collected at 300, 373, 473, 573, 673, 773, 873 and 973K under low-vacuum condition (0.1 Pa). XRD results show that the as-prepared Ni-P nanoparticles are amorphous, no peaks of crystalline phases can be observed until 573K. Afterwards, the crystallization of the amorphous phase undergoes the formation and decomposition of some metastable phases. Finally, the obtained stable phases are the bct Ni3P and fcc Ni cryatalline phases. Both are randomly distributed in the sample. The crystallization mechanisms of the as-prepared amorphous Ni-P nanoparticles has also been discussed at the end of this paper.