Modeling the thermodynamic properties of bimetallic nanosolids

A model has been developed to account for size, shape, surface segregation, composition and dimension dependent cohesive energy of bimetallic nanosolids, and further been extended to predict the size dependent thermodynamic properties, such as melting temperature, Curie temperatures, ordering temperature and phase diagram. The cohesive energy, melting temperature, Curie temperatures and ordering temperature of bimetallic nanosolids decrease with decreasing the particle size. The depression is dramatic in the lower range of size, while it becomes smoothly in large size. For nano phase diagram, the solidus and liquidus curves drop and the two-phase zones become small, as the size of the nanosolids decreases. The two-phase zones of the nano phase are always lower than the regions indicated in the bulk Ag-Pd alloy phase diagram, and they may deteriorate into a curve at a critical size. It is also found that the thermodynamic properties of nanosolids not only depend on the compositions, the atomic diameter and the cohesive energy of each component, but also depend on the size and the shape. The model predictions are consistent with the corresponding simulation, semi-empirical model and experimental data.     

First-principles study of small aluminum clusters: Oxygen adsorptions, oxidation and phase stability

It is important to understand the properties of individual nanometals before we can exploit full potential of their applications, for example, as energetic materials, enhancing additives, or catalysts. Here, we present a density functional theory study of the structure and properties of clean Al₁₃ clusters, oxygen adsorptions on the cluster surface, and the completely oxidized clusters. The relative stability of various phases at various oxygen pressures and temperatures is investigated based on the so-called “atomistic thermodynamics”, which was previously employed for studying metals. The effect of temperature and oxygen pressure on the phase stability is taken into account via the oxygen chemical potential and reflected in the (P, T) phase diagram. Our results show that only intact and completely oxidized clusters are thermodynamically stable, and that the O adsorption phases are never thermodynamically stable. Also, our results show that the Al₁₃ clusters are extremely easy to get oxidized. The present study provides valuable insight into the basic behaviors of small Al clusters in the presence of oxygen and a theoretical basis for exploring practical applications of these clusters.     

Oxidation of nanocrystalline aluminum by variable charge molecular dynamics

We investigate the oxidation of nanocrystalline aluminum surfaces using molecular dynamics (MD) simulations with the variable charge model that allows charge dynamically transfer among atoms. The interaction potential between atoms is described by the electrostatic plus (Es+) potential model, which is composed of an embedded atom method potential and an electrostatic term. The simulations were performed from 300 to 750 K on polycrystalline samples with a mean grain size of 5 nanometers. We mainly focused on the effect of the temperature parameter on the oxidation kinetic. The results show that, beyond a first linear regime, the kinetics follows a direct logarithmic law (governed by diffusion process) and tends to a limiting value corresponding to a thickness of ∼3 nm. We also characterized at 600 K the effects of an external applied strain on the microstructure and the chemical composition of oxide films formed at the surface. In particular, we obtained a partially crystalline oxide films for all temperatures and we noticed a strong correlation between the degree of crystallinity of the oxide film and the oxidation temperature.     

Modeling cohesive energy and melting temperature of nanocrystals

A model has been developed to account for the size dependent cohesive energy and melting temperature of nanocrystals. This model can deal with the thermodynamic properties of nanoparticles (spherical and non-spherical), nanowires and nanofilms with free surface or non-free surface (embedded in a matrix). The cohesive energy depression of nanocrystals has been predicted, and the conditions of superheating are obtained. It is found that the present theoretical results are consistent with the available experimental values.     

Thermal stability and reaction properties of passivated Al/CuO nano-thermite

Thermal stability and reaction properties of Al-CuO system, a mixture of 50-200 nm aluminum nanoparticles passivated by nitrocellulose and 12 nm copper (II) oxide, were investigated with microstructure characterization, differential thermal analysis (DTA), and thermogravimetric analysis (TGA). Transmission electron microscopy observation confirmed that the passivation coating successfully hinders the oxidization. TGA revealed that the passivation shell does not influence the ignition temperature of the thermite reaction. Reaction chemistry of the nano-thermite was elucidated by heating the composite both in inert ambient and vacuum. It was found that the thermite reaction composes of three continuing steps: At 570 °C, Al is oxidized into Al₂O₃ by reacting with CuO, which forms Cu₂O and produces a significant amount of heat. Subsequently two endothermic reactions occur. Starting at 800 °C, alumina reacts with Cu₂O and forms CuAlO₂. Above this temperature CuAlO₂ will decompose and eventually produce alumina, Cu, and O₂ at 1000 °C. Since the nano-thermite reaction pathway differs greatly from bulk thermite reactions, these results are important to develop a nano-thermite platform that can be used for a novel low cost, low temperature, and copper based microjoining and advance IC packaging.     

Modeling size dependence of melting temperature of metallic nanoparticles

A model has been developed to account for the dependence of melting temperature of nanoparticles on their size, shape and lattice type. This model is consistent with reported experimental data and shows better consistency than the liquid drop and bond energy model. A general equation is proposed which correlates with the bond energy model formula and has high potential for application in research and development. The model also leads to an equation showing the limiting size for nanoparticles.     

Size-dependent linear and non-linear optical response of single carrier two-dimensional quantum dots

We explore the pattern of size dependence of linear and non-linear optical (NLO) responses of one-electron quantum dots in two dimensions with or without anharmonicity in the confinement potential. For some fixed values of transverse magnetic field strength (ω_c) and harmonic confinement potential (ω₀), the influence of the size of the dot on the linear (), the first (β) and the second (γ) NLO responses of the system computed through a finite field linear variational route is analysed. Size-dependent maximization is predicted to be feasible for the quadratic hyperpolarizability.     

Fabrication of novel Cu microspheres assembled with nanoparticles by a solvothermal reduction route

Copper microspheres assembled with nanoparticles have been synthesized by a simple solvothermal route at 160 °C for 24 h. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometer (EDX), transmission electron microscopy (TEM) and electron diffraction (ED) techniques. The results show that the diameters of the microspheres range from 2 to 4 μm. The formation mechanism of the morphology control over the copper microspheres assembled by nanoparticles was investigated; the use of polyvinylpyrrolidone (PVP) as the surfactant and the choice of N,N-dimethylformamide (DMF) as the reducing agent were found to be important for the final generation of copper microspheres.     

Fabrication and application of highly ordered mesoporous Co3O4, NiO, and their metals

Highly ordered mesoporous Co₃O₄, NiO, and their metals were synthesized by nanocasting method using there corresponding mesoporous SBA-15 silica as a template. The obtained porous metal oxides have high surface areas, large pore volume, and a narrow pore size distribution. The N₂-adsorption data for mesoporous metal oxides have provided the BET area of 257.7 m² g⁻¹ and the total pore volume of 0.46 cm³ g⁻¹. The mesoporous metals were employed as a catalyst in the synthesis of (S)-3-pyrrolidinol from chiral (S)-4-chloro-3-hydroxybutyronitrile, and a high yield to (S)-3-pyrrolidinol-salt was obtained on the mesoporous Co metal catalyst.     

Temperature-dependence of phonons,solid state properties and liquid structure of noble metals: A comparison of pair-potentials

Two groups of effective pair-potentials are studied from the viewpoint of their suitability in being able to describe solid state properties and liquid state structure of noble metals Cu, Ag and Au over a wide temperature range. Since the effective pair-potentials are usually empirical in nature, with parameters obtained by fitting to some reference state properties, the objective of the present study is to determine whether a particular parametrization scheme has any definite advantage over another. We consider Morse potentials with parameters determined by equilibrium lattice parameter, cohesive/sublimation energies as well as bulk modulus values of the solid at low/room temperatures. The other group of potentials considered is Erkoç potentials, where the parameters were determined first by studying dimers and further modified using bulk stability condition and bulk cohesive energy values. The potentials were then used to study the energetics of microclusters containing 3–7 atoms. Quasiharmonic results for the solid obtained at different temperatures and Monte Carlo simulation for the liquid state show that phonon spectra, thermal expansion, temperature-dependence of specific heats and liquid structure are much better described by the latter group. The first group of potentials may have an advantage in reproducing the temperature-dependence of elastic constants and bulk moduli, since they are based on room temperature values of these properties, which show only weak temperature-dependence in general for all metals. It is argued that potentials based on parameters fitted to the properties at a single volume are less versatile in capturing the temperature-dependence of various thermodynamic properties over a wide range. Potentials capable of reproducing the energetics of clusters of different co-ordination numbers and volumes per atom may fare better in this regard.     

A review of catalyst-enhanced magnesium hydride as a hydrogen storage material

Magnesium hydride remains an attractive hydrogen storage material due to the high hydrogen capacity and low cost of production. A high activation energy and poor kinetics at practical temperatures for the pure material have driven research into different additives to improve the sorption properties. This review details the development of catalytic additives and their effect on the activation energy, kinetics and thermodynamic properties of magnesium hydride.     

Stability of anti-reflection coatings via the self-assembly encapsulation of silica nanoparticles by diazo-resins

A modified silica nanoparticle (MSNP) solution was formed by the encapsulation of negatively charged silica nanoparticles by the UV-crosslinkable polycation oligomer diazo-resin (DAR). Appropriate DAR encapsulation concentrations were determined by use of zeta-potential and dynamic light scattering measurements. The MSNPs were used in conjunction with poly(styrene sulfonate) (PSS) to grow homogenous ionic self-assembled multilayer anti-reflection coatings. Stability was induced within the films by the exposure of UV-irradiation that allowed for crosslinking of the DAR and PSS. The films were characterized by UV/vis/IR spectroscopy and field emission scanning electron microscopy. The transmission and reflection levels were >98.5% and <0.05%, respectively. The refractive indices resided in the 1.25–1.26 range. The solvent stability was tested by sonication of the films in a ternary solvent (H₂O/DMF/ZnCl₂ 3:5:2 w/w/w).     

Electrocatalytic oxidation of methanol on Pt catalyst supported on nitrogen-doped graphene induced by hydrazine reduction

Hydrazine is often used to reduce graphene oxide (GO) to produce graphene. Recent observations suggested that when hydrazine is used to reduce GO, the resulting reduced graphene actually contains certain amounts of nitrogen dopants, which may influence the properties of the obtained material, and in some cases may be deployed for beneficial advantage. In this work, we prepared graphene oxide by the chemical oxidation method, then used either hydrazine or sodium borohydride (as a control) to reduce the graphene oxide to graphene and to explore the nature of the nitrogen functionalities introduced by hydrazine reduction. Pt nanoparticles were then deposited on the nitrogen doped (hydrazine-reduced) and undoped (control) graphene substrates, and the morphology, structure, and electrocatalytic methanol oxidation activity were characterized and evaluated. The results show that the nitrogen functional groups introduced into the graphene by hydrazine reduction greatly improve the electrocatalytic activity of the underlying Pt nanoparticles towards the methanol oxidation reaction.     

Effects of temperature and vacancy defects on tensile deformation of single-walled carbon nanotubes

This study adopts the Tersoff-Brenner interaction potential function in a series of molecular dynamic (MD) simulations which investigate the mechanical properties under tensile loading of (10,0) zigzag, (8,3) chiral and (6,6) armchair single-walled carbon nanotubes (SWCNTs) of similar radii. The Young's modulus values of the (10,0), (8,3) and (6,6) nanotubes are determined to be approximately 0.92, 0.95, and 1.03 TPa, respectively. Of these nanotubes, the results reveal that the (6,6) nanotube possesses the best tensile strength and toughness properties under tension. Although it is noted that under small tensions, the mechanical properties such as Young's modulus are essentially insensitive to helicity, under larger plastic deformations, they may be influenced by helicity effects. Finally, the simulations demonstrate that the values of the majority of the considered mechanical properties decrease with increasing temperature and increasing vacancy percentage.     

On the low-temperature properties of

Magnetic, electrical transport and heat capacity measurements have been performed on polycrystalline sample of the compound TmRu₂Si₂, which crystallizes with the tetragonal ThCr₂Si₂-type crystal structure. The results show paramagnetic behaviour down to 0.3 K, at variance with the literature data. On the basis of the collected data, possible origins of non-magnetic behaviour are discussed. Mixed valence of Tm ions or hybridisation between Tm 4f and Ru 4d states are the most probable reasons for paramagnetic behaviour of the TmRu₂Si₂.     

Estimation of critical grain size at different temperatures and thermodynamic stability of γ→α in NC Fe by QDA and EOS methods

Thermodynamic properties of Nanocrystalline (NC) materials are essentially different from the conventional coarse-grained materials (with the same chemical composition). The role of grain boundary is very important in the characterization of thermodynamic functions and thermal properties of NC materials when the grain size is less than 100 nm. Therefore, the traditional thermodynamics being applied for coarse-grained materials is not applicable for NC materials. In this study, Quasiharmonic Debye Approximation (QDA) and Equation of State (EOS) methods are used to calculate the Gibbs free energy in NC Fe. Since the Gibbs free energy for Fe, predicted by EOS and QDA methods, is inaccurate (especially at temperatures higher than the ambient temperature), a term called as ΔG^Excess is proposed to modify the results. Thus, the Modified QDA (MQDA) and Modified EOS (MEOS) methods are introduced for this purpose. Thereafter, the change in the Gibbs free energy for γ-Fe to α-Fe phase transformation (ΔG^γ→α) via the grain size is calculated by MQDA and MEOS methods. The results obtained by the two methods are also compared and discussed. Finally, the critical grain size, at which ΔG^γ→α=0, can be estimated at different temperatures, is found to increase with increasing temperature.     

Effect of annealing temperature on the microstructures and photocatalytic property of colloidal ZnO nanoparticles

Colloidal ZnO nanoparticles were prepared in ethanol solutions and annealed at different temperatures (150-500 °C) subsequently. The size, morphology and surface characteristics of ZnO nanoparticles were examined by TEM, XRD, UV-vis absorption spectrum and FTIR technique. With the increase of annealing temperature, the mean size of ZnO nanoparticles was increased from 10 to 90 nm, while the bonding structure of acetate groups coordinating with zinc ions evolved from unidentate to bidentate type. The UV-induced degradation results of methyl orange verified that the photocatalytic process of colloidal ZnO nanoparticles without annealing and the sample annealed at 150 °C was unstable for the weakly bonding unidentate type of acetate groups. However, the sample annealed above 150 °C demonstrated their photocatalytic stability in the whole catalytic process for the stable bidentate bonding type of acetate groups. In addition, the change of particle size in the annealing process significantly affected the catalytic activity of photocatalysts. ZnO nanoparticles annealed at 300 °C would be a prospective photocatalysts with a high catalytic activity and stability compared with the other samples.     

Hydrothermal synthesis of well-dispersed ultrafine N-doped TiO2 nanoparticles with enhanced photocatalytic activity under visible light

Ultrafine nitrogen-doped TiO₂ nanoparticles with narrow particle size distribution, good dispersion, and high surface area were synthesized in the presence of urea and PEG-4000 via a hydrothermal procedure. TEM observation, N₂ adsorption, XRD, UV-vis spectroscopy, the Raman spectroscopy and XPS analysis were conducted to characterize the synthesized TiO₂ particles. The synthesized TiO₂ particles were a mixture of 49.5% anatase and 50.5% rutile with a size of around 5 nm. The photocatalytic activities were tested in the degradation of an aqueous solution of a reactive Brilliant Blue KN-R under both UV and visible light. The synthesized TiO₂ particles showed much higher photocatalytic activity than a commercial P25 TiO₂ powder under both UV and visible light irradiations. The high performance is associated to N doping, the reduced particle size, good dispersion, high surface area, and a quantum size effect.