Dongbai Sun

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.

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.

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.

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.

Microwave-absorbing properties of CoNi nanoparticles

The CoNi alloy nanoparticles with different molar ratios of Co:Ni were prepared by the liquid-phase reduction, and the morphology, chemical composition and structure of the particles were investigated by field-emission scanning electron microscopy and X-ray powder diffraction. The complex permittivity and permeability of the nanoparticles were investigated by network analyzer in the frequency range of 1–18 GHz. The reflection loss was calculated based on the determined complex permittivity and permeability. The field-emission scanning electron microscopy results showed that all samples with different Co:Ni molar ratios were spherical and evenly distributed with the diameter ranging from 60 to 200 nm. Compared with other samples with different Co:Ni molar ratios, the sample with Co:Ni molar ratio of 1:2 displayed higher microwave-absorbing properties due to the higher imaginary parts of the complex permittivity and permeability.

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.