Number Of Surface Atoms Research Articles

Review of: "The important feature of all nano structures is that in which the number of surface atoms is more than the number of volume atoms."

Review of: "The important feature of all nano structures is that in which the number of surface atoms is more than the number of volume atoms."

Study of size effect on thermophysical properties of metallic nanosolids

In the present study, a phenomenological model based on thermodynamic variables is developed to study the thermophysical properties of nanomaterials with respect to size in nanoscale. The model input parameters are lattice packing fraction depending on crystal structure and atomic diameter of nanosolid. The shape parameter is incorporated in the model to study the variation in physical properties of metallic nanosolids with shape. The size and shape effect on melting temperature , Debye temperature θ, Specific heat capacity , thermal conductivity and electrical conductivity σ is studied in metallic nanosolids. It is observed from the results obtained that both melting temperature and Debye temperature get reduced with reduction in size of nanosolid. Also Thermal conductivity and electrical conductivity in nanosolids decrease as size reduces. This is due to the increase in the number of surface atoms with size reduction and pronounced quantum confinement in nanomaterials. Also, the drastic change in number of surface atoms with the change in shape of the nanomaterial of same size brings about change in its thermophysical properties. The present model results are found consistent with the available experimental and simulated results of previous workers and may be useful for experimental researchers exploring the physical properties of nanomaterials.

Boron Nitride- and Graphene-Supported Trimetallic Yolk–Shell and Hollow Nanoparticles

In this study, the stability of yolk–shell and hollow Ni@PdAu and PdAu@Ni nanoparticles in free state and supported on boron nitride and graphene substrates was investigated at 300 K using molecular dynamics simulations. To this end, analyses of several parameters such as relative stability, surface energy, coordination number, number of surface atoms, bond length, displacement vector, and atomic strain were employed. Results show that the yolk–shell and hollow Ni@PdAu nanoparticles are more stable than yolk–shell and hollow PdAu@Ni nanoparticles in free and supported states due to the less surface energy. Furthermore, results show that the hollow nanoparticles are more stable than yolk–shell nanoparticles in all states. In addition, results show that the void and hollow core in yolk–shell and hollow nanoparticles are unstable and collapse due to the contraction and expansion of the core and shell, which lead to the formation of the core–shell and quasi–Janus structures in hollow and yolk–shell nanoparticles, respectively. Moreover, nanoparticles create a quasi–ellipsoid structure with low density and high coordination number on the boron nitride. The yolk–shell and hollow nanoparticles are the most stable on the boron nitride due to the smaller displacement and variation of the atomic bond length and lower atomic strain, which are caused by the strong nonbonded interaction, good atomic match, and placement of the Ni, Au, and Pd atoms on the middle position of the boron nitride hexagonal structure. Generally, this study demonstrated that the stability of yolk–shell and hollow nanoparticles can be tuned by the substrate–nanoparticle interaction.

Crystal Growth, Optical Properties, and Photocatalytic Performances of ZnO‐CuAl2O4 Hybrid Compounds: Theoretical and Experimental Studies

AbstractIn this study, a two‐step synthesis of the ZnO‐CuAl2O4 nanocomposite with houseleek‐like grains structures is developed for the first time. The proposed preparation procedure is led to an increase in bandgap energy (Eg) of the synthesized nanocomposite in comparison with the synthesized ZnO nanoparticles. Theoretically, based on a relation between Eg and the number of surface atoms, the increase in Eg value has been ascribed to the morphology of the synthesized samples. Experimentally, the photocatalytic performance of the synthesized composites, and pristine ZnO and CuAl2O4 are evaluated by comparing the degradation of methyl orange (MO) solution as a model of azo dye pollutants under UV irradiation. ZnO content and solution pH are selected as variables. Experimental results are showed that acidic pH (≈4) leads to the highest photodegradation performance due to the anionic nature of the MO solution. On the other hand, CuAl2O4‐25%ZnO attained the highest degradation efficiency and high adsorption behavior of the composite is the main reason for this achievement.

Effects of Nanoparticles’ Size and Shape on the Thermodynamic Properties of PI/SiO2 Nanocomposites

In this paper, the effects of SiO2 size (ranging from 5.7[Formula: see text]Å to 7.50[Formula: see text]Å) and shape (spherical, cylindrical and rod) on the interface structure and the thermodynamic performance of polyimide (PI)/SiO2 composites were investigated by molecular dynamics simulation. Also, whether the nanofiller with isocyanate propyl triethoxysilane (ICTOS) can highly enhance interaction in PI/SiO2 (bonded PI/SiO[Formula: see text] was investigated by experiment. To enhance the interfacial intensity efficiency, the coupling agent ICTOS was used to modify the surface of the silica nanoparticle. The results showed that the size and shape of SiO2 not only affect the interfacial number of hydrogen bonds and the interfacial area, but also impact on the glass transition temperature of the composites. As the radius of the embedded nanosilica increased, the bonded energy and [Formula: see text] gradually decreased, showing a prominent filler size effect. At the same time, the [Formula: see text] of spherical-type system was significantly greater than the nanosystems of other shapes. This is because as the number of surface atoms and superficial area of the spherical filler are higher than the nanofillers of other shapes, the combination between spherical-type silica and the PI chains is more compact than those between cylindrical and rod-like silica and the PI chains. The contributions of different nanofillers to the filler size dependency of [Formula: see text] of the nanocomposites can be elucidated by comparing the interfacial interaction and net structure between the PI matrix and silica which exhibit prominent filler shape and size dependency. Finally, the results were confirmed by the nonbonded interaction energy and reduced radial distribution function (RRDF), and the highly interaction interface is found to bring about a greater reinforcing effect.

Size effects on melting point based on surface energy and coordination number

Size effect on melting point is closely related with the surface energy of nanoparticles, since the increased surface/volume ratio. As the nanoparticle’s size decreases, there have some different surface shape existing, which makes the coordination number are different from each other. Both the surface energy and coordination number of surface atoms are the necessary consideration in understanding the thermal stability of nanoparticles. So, a new model free of any adjustable parameters, based on size-dependent surface energy and calculation of the surface atomic coordination number of nanoparticles, is developed to predict the size-dependent melting point of nanoparticles. The model can be utilized to predict the thermal stability of low-dimensional materials within a large size range. The theoretical predictions in terms of the model are consistent with experimental evidences.

Coexistence of nanowire-like hex and (1 × 1) phases in the topmost layer of Au(100) surface

Scanning tunneling microscopy and density functional theory calculations were used to study the structure of the Au(100) surface after ion bombardment. The results indicate a development of two phases: the quasi-hexagonal (hex) and the (1 × 1) on the surface. A decrease in the number of surface atoms caused by ion bombardment leads to the development of a coexistence of phases inside the surface layer. Our experimental data and theoretical results rule out a scenario that the unreconstructed (1 × 1) domains under study represent a subsurface atomic layer revealed by ion sputtering. The hex phase is anisotropic, with the quantized width in accord with (6n + 1) formula. In certain conditions, nanowire-like structures consisting of seven atomic rows of alternating height are developed on the surface.

Determination of surface properties and elastic constants of FCC metals: a comparison among different EAM potentials in thin film and bulk scale

Three independent elastic constants C11, C12, and C44 were calculated and compared using available potentials of eight different metals with FCC crystal structure; Gold, Silver, Copper, Nickel, Platinum, Palladium, Aluminum and Lead. In order to calculate the elastic constants, the second derivative of the energy density of each system was calculated with respect to different directions of strains. Each set of the elastic constants of the metals in bulk scale was compared with experimental results, and the average relative error was for each was calculated and compared with other available potentials. Then, using the Voigt-Reuss-Hill method, approximated values for Young and shear moduli and Poisson’s ratio of the FCC metals in the bulk scale were found for each potential. Furthermore, to observe the surface effects on the metals in nanoscale, surface elastic constants of the thin films of the metals have been calculated. In the study of the thin films of materials in nanoscale, the number of surface atoms is considerable compared to all atoms of the object. This leads to an increase in the surface effects, which influence the elastic properties. By considering this fact and employing related definitions and equations, the properties of the thin films of the metals were calculated, and the surface effects for different crystallographic directions were compared. Subsequently, in some cases, comparisons among characteristics of the metals in the thin film and bulk material were made.

Study of Fe, Co, and Mn-based perovskite-type catalysts for the simultaneous control of soot and NOX from diesel engine exhaust

This paper discusses, the nanometric size effects, including intrinsic factors such as structure, the nature of A-site ion substitution, B-site ions, on the redox properties of nanometric La1−xKxFeO3, La1−xKxCoO3, La1−xNaxMnO3, La1−xKxMnO3, perovskite-type oxide catalysts. The external factors of catalyst and soot particulate, the content of NO and O2 and the ratio of catalyst and gas flow rate on the catalytic activity for the simultaneous removal of NOX and soot particulates were systematically investigated. The catalysts were characterized by means of SEM, EDX, XRD and FT-IR and the catalytic potential for the simultaneous removal of nitrogen oxides (NOx) and diesel particulate matter (soot) were evaluated by temperature-programmed oxidation reactions. The entire catalyst samples obtained the perovskite structure, with high activities for the simultaneous elimination of soot and NOX. Alternatively, the number of surface atoms is much higher than the normal bulk catalysts and the size of the nanoparticle is also smaller. These characteristics are beneficial for the adsorption and activation of reactant molecules including NO or O2, or even soot particle.

Facet-Dependent Activity of Pt Nanoparticles as Cocatalyst on TiO2Photocatalyst for Dye-Sensitized Visible-Light Hydrogen Generation

The photocatalytic activities of polyoriented and preferential Pt(111) nanoparticles supported on TiO2(Pt(poly)/TiO2and Pt(111)/TiO2) were investigated by the photocatalytic hydrogen generation from water under visible-light irradiation. The photocatalytic hydrogen production rate of Pt(111)/TiO2was 1.6 times higher than that of Pt(poly)/TiO2. The corresponding apparent activation energy on Pt(111)/TiO2was about 2.39 KJ/mol, while on Pt(poly)/TiO2, it was about 4.83 KJ/mol. The difference in the apparent activation energies was probably due to the diversity in the number of surface atoms at corners and edges between the Pt(poly) and Pt(111) nanoparticles. The photocurrent of Pt(111)/TiO2was also bigger than that of Pt(poly)/TiO2, implying that the surface structure of Pt(111) nanoparticles can improve the transfer efficiency of photo-induced electrons from the conduction band of TiO2to Pt nanoparticles. As a result, the surface structure of Pt nanoparticles played an important role in the reactivity and kinetics performance of hydrogen evolution. Therefore, the photocatalytic properties of Pt/TiO2strongly depended on the surface structure of Pt nanoparticles.

Simulation of solid-friction dependence on number of surface atoms and theoretical approach for infinite number of atoms

We investigate the dependence of solid friction on the number of surface atoms in a unit cell under periodic boundary conditions. A two-dimensional friction model between a single asperity and an elastic solid surface is examined. The solid surface is modeled using a coupled-oscillator lattice that consists of an infinite number of inner solid atoms; the dynamics are simulated by the recently proposed semi-infinite dynamic lattice Green's function method. A significant dependence of the friction on the number of surface atoms is observed. The dependence is attributed to the requirement for a large number of surface atoms for the excitation of the energy-dissipative surface phonons with nonzero wave numbers. In order to eliminate the problematic dependence, a correction method combined with the continuum contact theory is developed to evaluate friction in the limit of the infinite surface atoms. In addition, we found a relationship between the friction and the power spectrum of the temporal fluctuation in the force; the latter quantity does not significantly depend on the number of surface atoms.

Molecular dynamics study of Laplace pressure in solid-state nanostructures

The paper provides a comprehensive molecular dynamics study of nanostructures compressed by a system of surface atoms to analyze their surface tension. Surface tension is here understood as phenomena resulting from the presence of surface atoms. All main properties of nanostructures are conditioned by a highly developed surface. The number of surface atoms and their energy are comparable to those of bulk atoms.

Comparative study of nanometric Co-, Mn- and Fe-based perovskite-type complex oxide catalysts for the simultaneous elimination of soot and NOx from diesel engine exhaust

In this feature article, the effects of the intrinsic factors including structure, nanometric size effect, nature of B-site ions, A-site ion substitution, redox properties of three systems of nanometric La1−xMxCoO3, La1−xMxMnO3, La1−xKxFeO3 perovskite-type oxide catalysts, and the external factors containing contacting model of catalyst and soot, the content of O2 or NO, the ratio of catalyst and the gas flow rate on their catalytic performances for the simultaneous elimination of soot particles and NO were systematically investigated. The catalysts were characterized by means of XRD, FT-IR, SEM, XPS and H2-TPR, and the catalytic performances for the simultaneous removal of nitrogen oxides and diesel soot were evaluated by temperature-programmed oxidation reactions. All the prepared samples had the perovskite structure, possessed nano-sizes and gave high activities for the simultaneous elimination of soot particles and NO. On the one hand, the number of surface atoms is much greater than the normal bulk catalysts and the size of nanoparticle is small. These characters are favorable for the adsorption and activation of reactant molecules including the O2 or NO, even soot particle. On the other hand, the surface atoms of small nano-particles formed on the surface of the nanometric perovskite oxide samples possess high free energy, which improves the mobility of catalyst, and thus it favors the contact between the catalysts and soot. Besides the intrinsic factors, the external factors of reaction conditions containing contact model of soot and catalyst, the content of O2 or NO, the ratio of catalyst to soot and gas flow rate also play important roles in affecting the catalytic performances for the simultaneous removal of NO and soot. The reaction mechanisms for soot oxidation and the simultaneous removal of soot particles and NO over La1−xKxMnO3 were also briefly discussed.

Electronic properties and charge transfer phenomena in Pt nanoparticles on γ-Al2O3: size, shape, support, and adsorbate effects

This study presents a systematic detailed experimental and theoretical investigation of the electronic properties of size-controlled free and γ-Al(2)O(3)-supported Pt nanoparticles (NPs) and their evolution with decreasing NP size and adsorbate (H(2)) coverage. A combination of in situ X-ray absorption near-edge structure (XANES) and density functional theory (DFT) calculations revealed changes in the electronic characteristics of the NPs due to size, shape, NP-adsorbate (H(2)) and NP-support interactions. A correlation between the NP size, number of surface atoms and coordination of such atoms, and the maximum hydrogen coverage stabilized at a given temperature is established, with H/Pt ratios exceeding the 1 : 1 ratio previously reported for bulk Pt surfaces.

Peculiar features of heat capacity for Cu and Ni nanoclusters

The heat capacity of copper and nickel clusters (from 2 to 6 nm in diameter) was investigated in the temperature range 200–800 K using molecular dynamics method and a modified tight-binding potential. The simulation results demonstrate a very good agreement with the available experimental data at T = 200 K and a fairy good agreement at higher temperatures. A number of regular trends are revealed in computer experiments which agree with the corresponding theoretical predictions. A conclusion is made that in the case of single free clusters the heat capacity may exceed the capacity of the corresponding bulk material. It is found that at 200 K, the copper nanocluster (D = 6 nm) heat capacity is higher by 10% and for nickel cluster by 13%. The difference diminishes with increasing the nanoparticles size proportionally to the relative number of surface atoms. A conclusion is made that very high values of the nanostructure heat capacity observed in laboratory experiments should not be attributed to free clusters, i.e., the effect in question is caused by other reasons.

Nuclear spin–lattice relaxation in nanocarbon compounds caused by adsorbed oxygen

Nuclear spin–lattice relaxation measurement is an effective tool for studying electronic structure and magnetic properties of nanosized compounds. The present work deals with the effect of oxygen molecules, adsorbed onto the surface of carbon nanoparticles – in which the number of surface atoms is comparable with that in the sample's volume – on nuclear spin–lattice relaxation rate of the carbon nuclei. We measured 13C spin–lattice relaxation in as-prepared (air rich) and out-gassed samples of detonation nanodiamond (DND), activated carbon fibers (ACF) and glassy carbon (GC) samples having multishell onion-like structure. Our measurements showed that the paramagnetic oxygen molecules (the only magnetic agent in ambient air), being physisorbed onto the surface and in structural voids of ACF and GC, create an additional relaxation channel and definitely affect the 13C spin–lattice relaxation as do the unpaired electrons of the internal dangling bonds. Air removal results in 1.5–2 times elongation in T 1. In contrast, the relaxation time is nearly the same for as-prepared and out-gassed DND samples. The reason is that in DND oxygen molecules have access only to the surface carbon nuclei whereas the rest of carbons remain unaffected by oxygen. Thus the main relaxation agents in DND particles are dangling bonds with unpaired electron spins, which mask the relaxation effect of paramagnetic oxygen. These findings are in a good agreement with our EPR data, which show that oxygen affects the inherent paramagnetic defects in the aforementioned nanocarbons.

Localised Vibrational Mode in CuO:Sn (5 at%) Nanoparticles

An experimental and theoretical investigation of defect modes in tin-doped cupric oxide (Sn-doped CuO) nanoparticles synthesized via a one-step solid-state reaction was reported. The defect mode at 455 cm-1due to Sn doping in CuO nanoparticles, calculated using a molecular model, was compared with the experimental value of 458 cm-1obtained from the FTIR vibrational spectrum. The Debye-Waller factor (DWF) of CuO nanoparticles was determined using Rietveld refinement of the XRD pattern and the Wilson’s plot, and the results were discussed on the basis of the number of surface atoms and thermal vibrations. The effect of defect modes upon the DWF of Sn-doped CuO nanoparticles was also discussed.

Mechanisms underlying two kinds of surface effects on elastic constants

Recently, people are confused with two opposite variations of elastic modulus with decreasing size of nano scale sample: elastic modulus either decreases or increases with decreasing sample size. In this paper, based on intermolecular potentials and a one dimensional model, we provide a unified understanding of the two opposite size effects. Firstly, we analyzed the microstructural variation near the surface of an fcc nanofilm based on the Lennard-Jones potential. It is found that the atomic lattice near the surface becomes looser in comparison with the bulk, indicating that atoms in the bulk are located at the balance of repulsive forces, and the elastic moduli decrease with the decreasing thickness of the film accordingly. In addition, the decrease in moduli should be attributed to both the looser surface layer and smaller coordination number of surface atoms. Furthermore, it is found that both looser and tighter lattice near the surface can appear for a general pair potential and the governing mechanism should be attributed to the surplus of the nearest force to all other long range interactions in the pair potential. Surprisingly, the surplus can be simply expressed by a sum of the long range interactions and the sum being positive or negative determines the looser or tighter lattice near surface respectively. To justify this concept, we examined ZnO in terms of Buckingham potential with long range Coulomb interactions. It is found that compared to its bulk lattice, the ZnO lattice near the surface becomes tighter, indicating the atoms in the bulk are located at the balance of attractive forces, owing to the long range Coulomb interaction. Correspondingly, the elastic modulus of one-dimensional ZnO chain increases with decreasing size. Finally, a kind of many-body potential for Cu was examined. In this case, the surface layer becomes tighter than the bulk and the modulus increases with deceasing size, owing to the long range repulsive pair interaction, as well as the cohesive many-body interaction caused by the electron redistribution.

Comment on ‘Modelling of surface energies of elemental crystals’

Jiang et al (2004 J. Phys.: Condens. Matter 16 521) present a model based on thetraditional broken-bond model for predicting surface energies of elemental crystals. It isfound that bias errors can be produced in calculating the coordination numbers ofsurface atoms, especially in the prediction of high-Miller-index surface energies.

Dose and Response Metrics in Nanotoxicology: Wittmaack Responds to Oberdoerster et al. and Stoeger et al.

Dose and Response Metrics in Nanotoxicology: Wittmaack Responds to Oberdoerster et al. and Stoeger et al.