Vibrational Properties of Pd Nanocubes

The atomic disorder and the vibrational properties of Pd nanocubes have been studied through a combined use of X-ray diffraction and molecular dynamics simulations. The latter show that the trend of the mean square relative displacement as a function of the radius of the coordination shells is characteristic of the nanoparticle shape and can be described by a combined model: A correlated Debye model for the thermal displacement and a parametric expression for the static disorder. This combined model, supplemented by results of line profile analysis of the diffraction patterns collected at different temperatures (100, 200, and 300 K) can explain the observed increase in the Debye–Waller coefficient, and shed light on the effect of the finite domain size and of the atomic disorder on the vibrational properties of metal nanocrystals.This research was partly supported by the Autonomous Province of Trento, within the framework of the programmatic Energy Action 2015–2017

Anisotropic atom displacement in Pd nanocubes resolved by molecular dynamics simulations supported by x-ray diffraction imaging

Nearly identical Pd nanocubes yield an x-ray powder diffraction pattern with interference fringes affording access to unprecedented structural details of nanocrystal size, shape, and complex atomic displacement for a billion-sized population. The excellent agreement between diffraction data and molecular dynamics (MD) provides strong experimental validation of MD simulations and the proposed data-interpretation paradigm. These results show that individual atomic displacements within the nanocubes are not only a function of disrupted bonds and the crystallographic plane of the adjacent surface, but are complex strain gradients extending across all surfaces of the particle strongly influenced by atomic displacements. This observation of nonuniform surface strain and the manner in which it is affected by different sizes, shapes, and locations within each facet could be the key to understanding many surface related properties of shaped nanocrystals including those associated with important catalysis applications

Reshaping of Truncated Pd Nanocubes: Energetic and Kinetic Analysis Integrating Transmission Electron Microscopy with Atomistic-Level and Coarse-Grained Modeling

Stability against reshaping of metallic fcc nanocrystals synthesized with tailored far-from-equilibrium shapes is key to maintaining optimal properties for applications such as catalysis. Yet Arrhenius analysis of experimental reshaping kinetics, and appropriate theory and simulation, is lacking. Thus, we use TEM to monitor the reshaping of Pd nanocubes of ∼25 nm side length between 410 °C (over ∼4.5 h) and 440 °C (over ∼0.25 h), extracting a high effective energy barrier of Eeff ≈ 4.6 eV. We also provide an analytic determination of the energy variation along the optimal pathway for reshaping that involves transfer of atoms across the nanocube surface from edges or corners to form new layers on side {100} facets. The effective barrier from this analysis is shown to increase strongly with the degree of truncation of edges and corners in the synthesized nanocube. Theory matches experiment for the appropriate degree of truncation. In addition, we perform simulations of a stochastic atomistic-level model incorporating a realistic description of diffusive hopping for undercoordinated surface atoms, thereby providing a visualization of the initial reshaping process

Ice-templating of core/shell microgel fibers through 'Bricks-and-Mortar' assembly

通讯作者地址: Stucky, GD (通讯作者), Univ Calif Santa Barbara, Mat Res Lab, Dept Chem & Biochem, Santa Barbara, CA 93106 USA 地址: 1. Univ Calif Santa Barbara, Mat Res Lab, Dept Chem & Biochem, Santa Barbara, CA 93106 USA 电子邮件地址: [email protected] 'bricks-and-mortar' assembly approach can be used to create well-defined fibers on the microscale with alternating organic-inorganic arrangement through a simple ice-templating strategy. ne fibers are constructed from closely packed monodisperse inorganic nanoparticles@PNIPAm core/shelf microgels. A series of functional 'bricks', such as core/shell and hollow nanoparticles are prepared

Kinetics, Energetics, and Size Dependence of the Transformation from Pt to Ordered PtSn Intermetallic Nanoparticles

The outstanding catalytic activity and chemical selectivity of intermetallic compounds make them excellent candidates for heterogeneous catalysis. However, the kinetics of their formation at the nanoscale is poorly understood or characterized, and precise control of their size, shape as well as composition during synthesis remains challenging. Here, using well-defined Pt nanoparticles (5 nm and 14 nm) encapsulated in mesoporous silica, we study the transformation kinetics from monometallic Pt to intermetallic PtSn at different temperatures by a series of time-evolution X-ray diffraction studies. Observations indicate an initial transformation stage mediated by Pt surface-controlled intermixing kinetics, followed by a second stage with distinct transformation kinetics corresponding to a Ginstling-Brounstein (G-B) type bulk diffusion mode. Moreover, the activation barrier for both surface intermixing and diffusion stages are obtained through the development of appropriate kinetic models for analysis of experimental data. Our density-functional-theory (DFT) calculations provide further insights into the atomistic-level processes and associated energetics underlying surface-controlled intermixing

A General Route to Diverse Mesoporous Metal Oxide Submicrospheres with Highly Crystalline Frameworks

通讯作者地址: Wang, JF (通讯作者), Chinese Univ Hong Kong, Dept Phys, Shatin, Hong Kong Peoples R China 地址: 1. Chinese Univ Hong Kong, Dept Phys, Shatin, Hong Kong Peoples R China 2. Univ Calif Santa Barbara, Dept Chem & Biochem, Santa Barbara, CA 93106 USA 电子邮件地址: [email protected], [email protected] NSF 0233728 NASA URETI NCC-1-02037 HK RGC Research 2060332 NSFC/RGC joint Research Scheme N_CUHK448/06 290031