5 research outputs found

    Direct Atomic-Scale Observation of Intermediate Pathways of Melting and Crystallization in Supported Bi Nanoparticles

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    Uncovering the evolutional pathways of melting and crystallization atomically is critical to understanding complex microscopic mechanism of first-order phase transformation. We conduct in situ atomic-scale observations of melting and crystallization in supported Bi nanoparticles under heating and cooling within an aberration-corrected TEM. We provide direct evidence of the multiple intermediate state events in melting and crystallization. The melting of the supported nanocrystal involves the formation and migration of domain boundaries and dislocations due to the atomic rearrangement under heating, which occurs through a size-dependent multiple intermediate state. A critical size, which is key to inducing the transition pathway in melting from two to four barriers, is identified for the nanocrystal. In contrast, crystallization of a Bi droplet involves three stages. These findings demonstrate that the phase transformations cannot be viewed as a simple single barrier-crossing event but as a complex multiple intermediate state phenomenon, highlighting the importance of nonlocal behaviors

    Direct Atomic-Scale Observation of Intermediate Pathways of Melting and Crystallization in Supported Bi Nanoparticles

    No full text
    Uncovering the evolutional pathways of melting and crystallization atomically is critical to understanding complex microscopic mechanism of first-order phase transformation. We conduct in situ atomic-scale observations of melting and crystallization in supported Bi nanoparticles under heating and cooling within an aberration-corrected TEM. We provide direct evidence of the multiple intermediate state events in melting and crystallization. The melting of the supported nanocrystal involves the formation and migration of domain boundaries and dislocations due to the atomic rearrangement under heating, which occurs through a size-dependent multiple intermediate state. A critical size, which is key to inducing the transition pathway in melting from two to four barriers, is identified for the nanocrystal. In contrast, crystallization of a Bi droplet involves three stages. These findings demonstrate that the phase transformations cannot be viewed as a simple single barrier-crossing event but as a complex multiple intermediate state phenomenon, highlighting the importance of nonlocal behaviors

    Real-Time Dynamical Observation of Lattice Induced Nucleation and Growth in Interfacial Solidā€“Solid Phase Transitions

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    Uncovering dynamical processes of lattice induced epitaxial growth of nanocrystal on the support is critical to understanding crystallization, solid-phase epitaxial growth, Oswald ripening process, and advanced nanofabrication, all of which are linked to different important applications in the materials field. Here, we conduct direct <i>in situ</i> atomic-scale dynamical observation of segregated Bi layers on SrBi<sub>2</sub>Ta<sub>2</sub>O<sub>9</sub> support under low dose electron irradiation to explore the nucleation and growth from an initial disordered solid state to a stable faceted crystal by using aberration-corrected transmission electron microscopy. We provide, for the first time, atomic-scale insights into the initial prenucleation stage of lattice induced interfacial nucleation, size-dependent crystalline fluctuation, and stepped-growth stage of the formed nanocrystal on the oxide support at the atomic scale. We identify a critical diameter in forming a stable faceted configuration and find interestingly that the stable nanocrystal presents a size-dependent coalescence mechanism. These results offer an atomic-scale view into the dynamic process at solid/solid interfaces, which has implications for thin film growth and advanced nanofabrication

    A Convenient Route for Au@Tiā€“SiO<sub>2</sub> Nanocatalyst Synthesis and Its Application for Room Temperature CO Oxidation

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    Small gold nanoparticles of size less than 5 nm encapsulated inside titanium modified silica shell have been reported. Here, a modified solā€“gel method, which is a one-step process, produces Au@Tiā€“SiO<sub>2</sub> nanocatalyst with a good control of titanium loading. With a titanium loading of 0.9 and 2.2 wt % in silica, unprecedented low temperature activity (full conversion) is observed for this catalyst for CO oxidation reaction compared to Au@SiO<sub>2</sub> catalyst. A combination of optimum sized gold nanoparticles with a large amount of oxygen vacancies created due to Ti incorporation in silica matrix is considered to be the reason for this enhanced catalytic activity. The size of gold nanoparticles is maintained even after high temperature pretreatments, which show the benefit of encapsulation. The effect of the various pretreatments on the catalytic activity has also been reported

    High-Temperature Magnetism as a Probe for Structural and Compositional Uniformity in Ligand-Capped Magnetite Nanoparticles

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    To investigate magnetostructural relationships in colloidal magnetite (Fe<sub>3</sub>O<sub>4</sub>) nanoparticles (NPs) at high temperature (300ā€“900 K), we measured the temperature dependence of magnetization (<i>M</i>) of oleate-capped magnetite NPs ca. 20 nm in size. Magnetometry revealed an unusual irreversible high-temperature dependence of <i>M</i> for these NPs, with dip and loop features observed during heatingā€“cooling cycles. Detailed characterizations of as-synthesized and annealed Fe<sub>3</sub>O<sub>4</sub> NPs as well as reference ligand-free Fe<sub>3</sub>O<sub>4</sub> NPs indicate that both types of features in <i>M</i>(<i>T</i>) are related to thermal decomposition of the capping ligands. The ligand decomposition upon the initial heating induces a reduction of Fe<sup>3+</sup> to Fe<sup>2+</sup> and the associated dip in <i>M</i>, leading to more structurally and compositionally uniform magnetite NPs. Having lost the protective ligands, the NPs continually sinter during subsequent heating cycles, resulting in divergent <i>M</i> curves featuring loops. The increase in <i>M</i> with sintering proceeds not only through elimination of a magnetically dead layer on the particle surface, as a result of a decrease in specific surface area with increasing size, but also through an uncommonly invoked effect resulting from a significant change in Fe<sup>3+</sup>/Fe<sup>2+</sup> ratio with heat treatment. The interpretation of irreversible features in <i>M</i>(<i>T</i>) indicates that reversible <i>M</i>(<i>T</i>) behavior, conversely, can be expected only for ligand-free, structurally and compositionally uniform magnetite NPs, suggesting a general applicability of high-temperature <i>M</i>(<i>T</i>) measurements as an analytical method for probing the structure and composition of magnetic nanomaterials
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