Size-dependent catalytic and melting properties of platinum-palladium nanoparticles

While nanocatalysis is a very active field, there have been very few studies in the size/shape-dependent catalytic properties of transition metals from a thermodynamical approach. Transition metal nanoparticles are very attractive due their high surface to volume ratio and their high surface energy. In particular, in this paper we focus on the Pt-Pd catalyst which is an important system in catalysis. The melting temperature, melting enthalpy, and catalytic activation energy were found to decrease with size. The face centered cubic crystal structure of platinum and palladium has been considered in the model. The shape stability has been discussed. The phase diagram of different polyhedral shapes has been plotted and the surface segregation has been considered. The model predicts a nanoparticle core rich in Pt surrounded by a layer enriched in Pd. The Pd segregation at the surface strongly modifies the catalytic activation energy compared to the non-segregated nanoparticle. The predictions were compared with the available experimental data in the literature

Nanomaterial properties: size and shape dependencies

Nanoscience and nanotechnology are among the most widely used terms in the modern scientific and technological literature. The idea of nanotechnology appeared for the first time in the famous talk “There is plenty of room at the bottom” given by the physicist Richard Feynman at the American Physical Society meeting at Caltech on December 29, 1959. Feynman described a process by which the ability to manipulate individual atoms and molecules might be developed, using one set of precise tools to build and operate another proportionally smaller set and so on down to the needed scale. In the course of this, he noted, scaling issues would arise from the changing magnitude of various physical phenomena: gravity would become less important whereas surface effects would become increasingly more significan

Gold−Copper Nano-Alloy, “Tumbaga”, in the era of nano: phase diagram and segregation

Gold–copper (Au–Cu) phases were employed already by pre-Columbian civilizations, essentially in decorative arts, whereas nowadays, they emerge in nanotechnology as an important catalyst. The knowledge of the phase diagram is critical to understanding the performance of a material. However, experimental determination of nanophase diagrams is rare because calorimetry remains quite challenging at the nanoscale; theoretical investigations, therefore, are welcomed. Using nanothermodynamics, this paper presents the phase diagrams of various polyhedral nanoparticles (tetrahedron, cube, octahedron, decahedron, dodecahedron, rhombic dodecahedron, truncated octahedron, cuboctahedron, and icosahedron) at sizes 4 and 10 nm. One finds, for all the shapes investigated, that the congruent melting point of these nanoparticles is shifted with respect to both size and composition (copper enrichment). Segregation reveals a gold enrichment at the surface, leading to a kind of core–shell structure, reminiscent of the historical artifacts. Finally, the most stable structures were determined to be the dodecahedron, truncated octahedron, and icosahedron with a Cu-rich core/Au-rich surface. The results of the thermodynamic approach are compared and supported by molecular-dynamics simulations and by electron-microscopy (EDX) observations

Size and Shape Dependencies of Nanomaterial Properties: Thermodynamic Considerations

ABSTRACT A top-down approach using classical thermodynamics is presented in this paper to deduce size and shape dependencies of different material properties. Particular attention is focused on the thermal expansion coefficient. The theory developed here can also be used to deduce information on surface energies

Schottky defects in nanoparticles

Vacancies play a major role in the electrical and thermal transport as well as the mechanical behavior of materials. To understand the processes occurring in nanomaterials during heat treatment and mechanical deformation, the size effects on the vacancy formation energy and entropy have to be considered. As these material properties are hardly measurable experimentally, particularly in nanoparticles, a theoretical model calculating the size-dependent vacancy formation energy and entropy has been developed in this study. It has been found that size reduction makes the vacancy much easier to form; then the vacancy concentration increases when size reduces and temperature increases. © 2011 American Chemical Society

αshape, birth of one universal parameter?

The description of different effects observed in nature by only one general equation is the "Holy Grail" for all physicists. This goal has been achieved for characteristic temperatures through a top-down approach (studying size effects from macroscopic laws) and is presented in this chapter. Here, we show the general equation based on the surface area to volume ratio of nanostructures and statistics (Fermi-Dirac or Bose-Einstein) followed by the particles involved in the investigated phenomena. From the distinction between fermions and bosons, so-called particles which follow a Fermi-Dirac or a Bose-Einstein statistics respectively, this equation indicates the universal behaviour of size and shape effects on different material properties like melting, ferromagnetism, vibration and superconduction. The same shape parameter used in this universal equation can be used to determine the melting enthalpy, the phase diagrams of alloys, the energy bandgap and also the creep behavior of nanomaterials. Theoretical predictions show satisfactory agreement with experimental data taken from literature. © (2010) Trans Tech Publications

Synthesis of Nickel-Based Nanoparticles by Pulsed Laser Ablation in Liquids: Correlations between Laser Beam Power, Size Distribution and Cavitation Bubble Lifetime

Pulsed laser ablation in liquids (PLAL) is a colloidal synthesis technique attracting significant interest from the scientific community due to the quality of the nanoparticles being produced. In this type of synthesis protocol, the cavitation bubble plays a vital role during the synthesis of nanoparticles. This work studied the effect of the laser beam power on cavitation bubble lifetime. Three different laser beam power values (5.8 W, 7.5 W and 10.5 W) were used to irradiate a pure nickel target in de-ionized (DI) water to synthesize nickel-based nanoparticles. The optimal repetition rate maximizing the production of nanoparticles was determined by atomic emission spectroscopy for each laser beam power. It was determined that the optimal repetition rate increased exponentially with laser beam power, while the cavitation bubble lifetime decreased logarithmically with the laser beam power. Moreover, the effect of the laser beam power on the cavitation bubble lifetime also had an effect on the size distribution of the nanoparticles being produced; the smallest size distribution was obtained at the highest laser beam power

Synthesis of fluorine doped zinc oxide by reactive magnetron sputtering

ZnO and fluorine doped ZnO (FZO) thin films were prepared by d.c. reactive magnetron sputtering using a zinc target in an Ar/O2(/F2) mixture. In a first attempt ZnO films were synthesized in order to optimize the matrix properties in terms of crystalline properties and transparency. The parameters studied were the d.c. power (Pdc), the total pressure (PTot) and the O2 content in the discharge (%O 2). The highest grain size of ∼25 nm is obtained for P dc = 70 W, PTot = 30 mtorr and %O2 = 7.5%. F2 was then introduced in the discharge. The influence of the presence of fluorine on the crystallographic, chemical, electrical and optical properties of the deposited films were evaluated. Our X-ray photoelectron spectroscopy and X-ray diffraction (XRD) data suggest that only a certain part of the measured fluorine atoms substitute for oxygen atoms in the ZnO structure. The rest of fluorine could be adsorbed as F2 on the grain boundaries or located in interstices of the ZnO structure. XRD data reveal a decrease in the crystallite size with an increase in the fluorine content. Above a fluorine concentration of ∼2% the FZO films become amorphous. The electrical properties have been investigated by Hall effect measurements. The optimal synthesis conditions (∼2% of fluorine in the film) were a charge carrier density of ∼1020 cm-3, an electrical resistivity of 10-2 Ω cm, and a charge mobility of 4 cm2 V s -1. Finally, all deposited FZO films had >80% transmission in the visible range. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Theoretical predictions of wurtzite III-nitride nano-materials properties

In this paper, top-down and bottom-up approaches are used to predict material properties of group III-nitride nanostructures. The. first approach calculates the melting temperature, melting enthalpy, Debye temperature and energy bandgap of InN, GaN and AlN through classical thermodynamics. The second approach calculates the surface energies in the liquid and solid states of the considered nitrides materials through molecular dynamics. Moreover, the liquid and solid surface energies of the zinc-blende and wurtzite III-V materials are compared. Finally, the phase diagram of a ternary III-nitride nanomaterial, AlGaN, is presented and the variation of its energy bandgap with composition is predicted