Transition between icosahedral and cuboctahedral nanoclusters of lead

We have used ab initio methods to study the possible transition between icosahedral (ico) and cuboctahedral (fcc) structures in lead nanoclusters of sizes up to 309 atoms. Spontaneous fcc-to-ico transition in Pb(13) was observed in the ab initio molecular dynamics (MD) simulations at various temperatures. The transition path can be described predominantly by an angular variable s, which can, generally be applied to the similar transitions in clusters of larger sizes and was observed to follow the Mackay model. We have calculated the two-dimensional energy surface that describes the transition in Pb(13) and found a barrierless fcc-to-ico transition path, which is consistent with the observed spontaneous transition in the ab initio MD simulations. The atomic displacements in the transition were identified as one of the vibrational eigenmodes of these two Pb(13) clusters. For clusters of larger sizes (Pb(n), where n = 55, 147, and 309), the possible transitions following similar paths were determined not to be barrierless and the sizes of the barriers were determined by the ab initio elastic band method.     

Binding energy of large icosahedral and cuboctahedral Lennard-Jones clusters

It is widely believed that the lowest energy configurations for small rare gas clusters have icosahedral symmetry. This contrasts with the bulk crystal structures which have cuboctahedral fcc symmetry. It is of interest to understand the transition between this finite and bulk behavior. To model this transition in rare gas clusters we have undertaken optimization studies within the Lennard-Jones pair potential model. Using a combination of Monte Carlo and Partan Search optimization methods, the lowest energy relaxed structures of Lennard-Jones clusters having icosahedral and cuboctahedral symmetry were found. Studies were performed for complete shell clusters ranging in size from one shell having 13 atoms to 14 shells having 10,179 atoms. It was found that the icosahedral structures are lower in energy than the cuboctahedral structures for cluster sizes having 13 shells or fewer. Additional studies were performed using the more accurate Aziz-Chen [HFD-C] pair potential parameterized for argon. The conclusions appear to be relatively insensitive to the form of the potential.     

Effect of catalysis on the stability of metallic nanoparticles: Suzuki reaction catalyzed by PVP-palladium nanoparticles

The small size of nanoparticles makes them attractive in catalysis due to their large surface-to-volume ratio. However, being small raises questions about their stability in the harsh chemical environment in which these nanoparticles find themselves during their catalytic function. In the present work, we studied the Suzuki reaction between phenylboronic acid and iodobenzene catalyzed by PVP-Pd nanoparticles to investigate the effect of catalysis, recycling, and the different individual chemicals on the stability and catalytic activity of the nanoparticles during this harsh reaction. The stability of the nanoparticles to the different perturbations is assessed using TEM, and the changes in the catalytic activity are assessed using HPLC analysis of the product yield. It was found that the process of refluxing the nanoparticles for 12 h during the Suzuki catalytic reaction increases the average size and the width of the distribution of the nanoparticles. This was attributed to Ostwald ripening in which the small nanoparticles dissolve to form larger nanoparticles. The kinetics of the change in the nanoparticle size during the 12 h period show that the nanoparticles increase in size during the beginning of the reaction and level off toward the end of the first cycle. When the nanoparticles are recycled for the second cycle, the average size decreases. This could be due to the larger nanoparticles aggregating and precipitating out of solution. This process could also explain the observed loss of the catalytic efficiency of the nanoparticles during the second cycle. It is also found that the addition of biphenyl to the reaction mixture results in it poisoning the active sites and giving rise to a low product yield. The addition of excess PVP stabilizer to the reaction mixture seems to lead to the stability of the nanoparticle surface and size, perhaps due to the inhibition of the Ostwald ripening process. This also decreases the catalytic efficiency of the nanoparticles due to capping of the nanoparticle surface. The addition of phenylboronic acid is found to lead to the stability of the size distribution as it binds to the particle surface through the O(-) of the OH group and acts as a stabilizer. Iodobenzene is found to have no effect and thus probably does not bind strongly to the surface during the catalytic process. These two results might have an implication on the catalytic mechanism of this reaction.     

Study of the formation and stability of the Pd and Pt metallic nanoparticles on carbon support

Catalysts containing Pd and Pt on a Sibunit carbon support were studied by the temperature-programmed reduction, in situ X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy (XAFS). The reduction of Pd and Pt species in samples 2%Pd/C and 2%Pt/C calcined in an air flow at 370°C was studied. Reduction of the 2%Pd/C sample begins at 50—60 °C and is completed at 250—300°C. Particles of various dispersion are formed during reduction. Long-distance peaks observed in the EXAFS spectra point to the presence of a fraction of relatively large crystallites. The average Pd—Pd coordination number (5) at 200 °C gives evidence that a number of very small Pd nanoparticles, oligomeric clusters, is present. Reduction at T > 200°C results in sintering of a small fraction of the Pd particles. Reduction of Pt in 2%Pt/C sample begins at 120—150 °C and is completed at 300—350°C. The sintering-resistant monodispersed Pt particles are formed under these conditions.     

Description of metallic and covalent clusters with icosahedral symmetry: the polytope model

Compact and tetravalent clusters with icosahedral local or global symmetries are generated by mapping from ideal structures in curved space onto a tangent euclidean 3D space. The observed elastic energy of the clusters can thus be interpreted as an intrinsic curvature associated to a frustrated local order. It is then proposed a kind of classification of the very rich family of possible clusters using a limited set of parameters.     

Force fields for metallic clusters and nanoparticles

Atomic force fields for simulating copper, silver, and gold clusters and nanoparticles are developed. Potential energy functions are obtained for both monatomic and binary metallic systems using an embedded atom method. Many cluster configurations of varying size and shape are used to constrain the parametrization for each system. Binding energies for these training clusters were computed using density functional theory (DFT) with the Perdew-Wang exchange-correlation functional in the generalized gradients approximation. Extensive testing shows that the many-body potentials are able to reproduce the DFT energies for most of the structures that were included in the training set. The force fields were used to calculate surface energies, bulk structures, and thermodynamic properties. The results are in good agreement with the DFT values and consistent with the available experimental data.     

Nanoscale cF96 cubic versus icosahedral phase in devitrified Hf-based metallic glasses

The devitrification behaviour of the rapidly solidified quaternary hafnium-based alloys of Hf₆₅NM_17.5TM₁₀Al_7.5 composition (NM- noble metal, TM- 3d transition metal) was studied using differential scanning calorimetry. The purpose of the study was to evaluate the influence of composition on the devitrification products at the primary stage. The structure was analysed by using X-ray diffraction and transmission electron microscopy. The formation of the nanoparticles of the Hf-based quasicrystalline icosahedral or cubic cF96 Fd₃⁻m Ti₂Ni-type phase about 10 nm in size was observed at the primary devitrification stage in different metallic glasses with close compositions. The icosahedral phase consists of 137-atom Bergmann rhombic triacontahedra. The alloys in systems in which the stable Hf-based cF96 phase exists do not show the formation of the icosahedral phase from the amorphous matrix. The cF96 phase and the icosahedral phase formed by primary devitrification from the amorphous phase may inherit the local order of the icosahedral clusters in the amorphous matrix.     

Fabrication of metallic nanoparticles by electrochemical discharges

A method for the fabrication of metallic nanoparticles in large quantities by electrochemical discharges is presented. In an aqueous electrolyte, large current density (∼1 A/mm² at ∼20 V) leads to the formation of a ‘gas film’ around the electrode through which discharges occur. When metal ions are additionally present in the electrolyte and when the applied potential is cathodic, metal nanoparticles (typically 10–150 nm) are produced. The nanoparticles are formed in the solution and the gas film prevents them from depositing on the electrode. To control the size of the particles a method based on ‘rotating electrode’ is developed. Rotating the cathode rotates the fluid around it, which provides centrifugal force to the particles to move away from the electrode where they cannot grow. This method has been successfully used for fabrication of nanoparticles from several metal salts.     

Synthesis of metallic magnesium nanoparticles by sonoelectrochemistry

4 nm sized metallic magnesium particles were prepared by the sonoelectrochemical method.     

Relative optical absorption of metallic and semiconducting single-walled carbon nanotubes

While it is well-known that tube-tube interaction causes changes (peak red-shift and suppression) in the optical absorption of single-walled carbon nanotubes (SWNTs), we found in this work that, upon bundling, the optical absorption of metallic SWNTs (M11) is less affected compared to their semiconducting counterparts (S11 or S22), resulting in enhanced absorbance ratio of metallic and semiconducting SWNTs (A(M)/A(S)). Annealing of the SWNTs increases this ratio due to the intensified tube-tube interaction. We have also found that the interaction between SWNTs and the surfactant Triton X-405 has a similar effect. The evaluation of SWNT separation by types (metallic or semiconducting) based on the optical absorption should take these effects into account.     

Low-temperature metallic alloying of copper and silver nanoparticles with gold nanoparticles through digestive ripening

We describe a remarkable and simple alloying procedure in which noble metal intermetallic nanoparticles are produced in gram quantities via digestive ripening. This process involves mixing of separately prepared colloids of pure Au and pure Ag or Cu particles and then heating in the presence of an alkanethiol under reflux. The result after 1 h is alloy nanoparticles. Particles synthesized according to this procedure were characterized by UV-vis spectroscopy, EDX analysis, and high-resolution electron microscopy, the results of which confirm the formation of alloy particles. The particles of 5.6+/-0.5 nm diameter for Au/Ag and 4.8+/-1.0 nm diameter for Cu/Au undergo facile self-assembly to form 3-D superlattice ordering. It appears that during this digestive ripening process, the organic ligands display an extraordinary chemistry in which atom transfer between atomically pure copper, silver, and gold metal nanoparticles yields monodisperse alloy nanoparticles.     

Analytical chemistry of metallic nanoparticles in natural environments

The use of metallic nanoparticles (NPs) has exponentially increased in the past decade due to their unique physical and chemical properties at nano-scales [1]. They are added to a myriad of materials and compositions. The key question is not if NPs will enter environmental compartments but rather when. The fate and the stability of NPs in the environment play important roles in determining their environmental distributions and probably control the risk to human health through exposure. Emissions of nanomaterials (NMs) could be intentional or unintentional but occur in particulate, aggregate or embedded states.Despite environmental transformations and changes in their surrounding environment, metallic NPs (M-NPs) tend to exist as stable colloidal aggregates or dispersions. Characterizing NPs and NMs in environmental samples implies determination of their size, their chemical composition and their bulk concentrations in the matrix. Differential size filtration is the most commonly used method to isolate NPs from aqueous matrices. Micro-filtration, nano-filtration, cross-flow filtration, and ultracentrifugation are usually employed to achieve the highest degree of segregation.Chemical characterization of NPs and NMs has traditionally been done using transmission/scanning electron microscopy (TEM/SEM) followed by energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). However, because of their intrinsic limitations, methods have also been combined and validated [e.g., size exclusion and ion chromatography (SEC and IC) with multi-element detection {inductively-coupled plasma mass spectrometry and optical emission spectroscopy (ICP-MS and ICP-OES)].This review describes the current state and the challenges of isolating, segregating and detecting M-NPs in environmental samples. A simple case study shows a common procedure for the analysis of NPs in complex aqueous matrices.     

Synthesis of metallic copper nanoparticles coated with polypyrrole

This paper describes a method for polypyrrole (PPy) coating of metallic Cu nanoparticles in aqueous solution in atmosphere. Colloid solution of Cu nanoparticles was prepared by reducing Cu ions with the use of hydrazine in an aqueous solution dissolving citric acid and cetyltrimethylammonium bromide as stabilizers. The PPy coating was performed by polymerizing pyrrole with the use of hydrogen peroxide as an initiator in an aqueous colloid solution of the Cu nanoparticles. Ultraviolet–visible extinction measurements, transmission electron microscopy observation, and X-ray diffraction measurements revealed that the metallic Cu nanoparticles with a size of 27.6 ± 11.1 nm were coated with PPy. The obtained PPy-coated Cu particles were chemically stable even in atmosphere.     

A new and specific mode of stabilization of metallic nanoparticles

We report in this paper the stabilization of ruthenium nanoparticles using a very simple ligand (4-(3-phenylpropyl)pyridine), through strong pi-coordination of the phenyl moiety.     

Synthesis and characterization of metallic copper nanoparticles via thermal decomposition

Copper oxalate was used as a precursor to prepare metallic copper nanoparticles by thermal decomposition. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy and UV–Vis spectroscopy. XRD analysis revealed broad pattern for fcc crystal structure of copper metal. The particle size by use of Debye–Scherrer’s equation was calculated to be about 40 nm.     

Iron and cobalt oxide and metallic nanoparticles prepared from ferritin

Metallic Fe and Co and Fe- and Co-based oxide nanoparticles were prepared by a novel method utilizing the biologically relevant protein ferritin. In particular, iron and cobalt oxyhydroxide nanoparticles were assembled within horse spleen and Listeria innocua derived ferritin, respectively, in the aqueous phase. Ferritin containing either Fe or Co oxide was transferred and dried on a SiO2 support where the protein shell was removed during exposure to a highly oxidizing environment. It was also shown that the metal oxide particles could be reduced to the respective metal by heating in hydrogen. X-ray photoelectron spectroscopy was used to characterize the composition of the particles and atomic force microscopy was used to characterize the size of the nanoparticles. Depending on the Fe or Co loading and/or type of ferritin used, metallic and oxide nanoparticles could be produced within a range of 20-60 A.     

Morphological determination of face-centered-cubic metallic nanoparticles by X-ray diffraction

X-ray diffraction studies of face-centered-cubic metallic nanoparticles with different morphologies are discussed based on the experimental data and the calculation of the Debye equation with the truncated and perfect strain models. At least four basic morphologies frequently observed in face-centered-cubic metallic nanoparticles, namely sphere, cube, decahedron, and icosahedron, can be clearly distinguished from their characteristic integrated intensity ratios of the first two X-ray diffraction peaks, that is, (200) to (111).     

Colloid stability of thymine-functionalized gold nanoparticles

Gold nanoparticles surface-coated with thyminethiol derivatives containing long hydrocarbon chains have been prepared. The diameter of the particles is 2.2 and 7.0 nm, respectively, with a relatively narrow size distribution. Thyminethiol derivatives are attached to the gold particle surfaces with thymine moieties as the end groups. The colloid stability of the gold nanoparticles as a function of the type and concentration of monovalent salt, pH, and particle size was investigated in alkaline, aqueous solutions. The gold particles are stable in concentrated NaCl and KCl solutions, but are unstable in concentrated LiCl and CsCl solutions. The larger gold particles are more sensitive to salt concentration and aggregate at lower salt concentrations. The reversible aggregation and dispersion of the gold particles can be controlled by changing the solution pH. The larger gold particles can be dispersed at higher pH and aggregate faster than the smaller particles, due to stronger van der Waals forces between the larger particles. Hydration forces play an important role in stabilizing the particles under conditions where electrostatic forces are negligible. The coagulation of the gold nanoparticles is attributed to van der Waals attraction and reduced hydration repulsion in the presence of LiCl and CsCl.