Nickel Nanoclusters Research Articles

Crystallization of nickel nanoclusters by molecular dynamics

We investigated the melting properties of bulk nickel and the crystallization of nickel nanocrystals via molecular dynamics using a potential in the framework of the second moment approximation of tight-binding theory. The melting behavior was simulated with the hysteresis approach by subsequently heating and cooling gradually the system over a wide range of temperatures. The crystallization of nickel nanoclusters consisting of 55, 147 and 309 atoms was achieved after repeatedly annealing and quenching the corresponding quasicrystals several times to avoid being trapped in a local energy minimum. The time over which the global minimum was reached was found to increase with the cluster size.

A direct synthesis of nickel nanoclusters embedded on multicomponent mesoporous metal oxides and their catalytic properties for glycerol steam reforming to hydrogen

Nickel nanoclusters embedded in multicomponent mesoporous metal oxides (Ni–MMOs) are obtained at various support compositions by a single-step synthesis of Ni ion incorporated mesoporous metal oxides (NiO–MMOs) followed by selective reduction of the NiO to Ni metal clusters. The resultant Ni–MMOs catalysts displayed enhanced Ni dispersion with well-developed mesopore structures at various support composition, exhibiting superior catalytic properties when compared to a siliceous SBA-16-supported Ni catalyst prepared by a conventional impregnation method. Glycerol steam reforming conducted at 873 K on 1Ni–2Al2O2–2ZrO2 and 1Ni–2SiO2–2ZrO2 catalysts exhibited considerably higher glycerol conversions over the 10 wt%-Ni/SBA-16 catalyst with similar Ni loading amount. This was primarily due to the enhanced Ni dispersion resulting from the direct synthesis process. The multicomponent mesoporous supports also significantly affect product selectivity, favoring higher hydrogen concentration in the product stream. The water–gas shift reaction appears to be positively affected by the 2Al2O2–2ZrO2 and 2SiO2–2ZrO2 multicomponent metal oxide matrices, which facilitated the conversion of the CO produced by the glycerol reforming further to additional hydrogen. Direct single-step synthesis of Ni–MMO catalysts was effective in enhancing the dispersion of Ni nanoclusters, as well as variation of the support components of the mesoporous catalyst systems.

Formation and properties of nickel catalysts for hydrogenation under the action of lithium Di- and tris(tert-butoxy)hydroaluminates

It was demonstrated that systems based on Ni(II) compounds and the alkoxyhydride derivatives LiAlH(tert-BuO)3 and LiAlH2(tert-BuO)2, in contrast to LiAlH4, exhibit high catalytic activity in the reaction of styrene hydrogenation in the absence of activators. The inhibiting effect of an excess of LiAlH2(tert-BuO)2 was found. The analysis of the composition of reaction mixtures by NMR, EPR, and UV spectroscopy and TEM showed that side reactions of the conversion of aluminum-containing components occurred together with the formation of nickel nanoclusters. The most probable reaction schemes of LiAlH(tert-BuO)3 and LiAlH2(tert-BuO)2 disproportionation were proposed. The nature of the stabilizers of nickel nanoparticles was considered.

Molecular-dynamics simulation of the heat capacity for nickel and copper clusters: Shape and size effects

We have investigated the heat capacity of ideal Cu and Ni fcc clusters with diameters from 2 to 6 nm in the temperature range 200–800 K by the molecular-dynamics method using a modified tight-binding potential. Our analysis has shown consistency with the experimental results at temperatures of 200–300 K. The data obtained are also indicative of several regularities that are in agreement with the analytical calculations. We have concluded from the results of our computer simulations that the heat capacity in the case of isolated free clusters can exceed that of a bulk material, with this difference decreasing as the nanoparticle grows proportionally to the reduction in the fraction of surface atoms. The excess of the heat capacity for ideal copper and nickel nanoclusters with D = 6 nm at T = 200 K has been found to be 10% and 13%, respectively. Consequently, the large heat capacities of copper and nickel nanostructures observed in some real experiments cannot be related to the characteristics of free clusters. We hypothesize that these properties of a nanomaterial depend on the degree of agglomeration of its constituent particles, i.e., the surfaces and interphase boundaries of interconnected nanoclusters can have a strong effect. To test this hypothesis, we took nickel and copper clusters of various sizes (4000–7200 atoms) produced through the simulation of condensation from the gas phase. At high temperatures, we failed to adequately assess the role of the interphase boundaries in calculating the heat capacity of nanoparticles. The reason was the mass diffusion of Ni or Cu atoms to impart an energetically more favorable shape and structure to the synthesized clusters. At low temperatures, the heat capacity of such clusters exceeded that of clusters with an ideal shape and structure by a value from 3.2% to 10.6%. We have concluded that the Ni and Cu clusters produced in real experiments cannot be applied in devices using the thermal energy of such clusters without a preliminary optimization stage, because their external shape and interior structure are nonideal.

Analysis of Nickel Nanoclusters Size Distribution Synthesized from the Gas Phase

The process of Ni nanoclusters synthesis from the gas phase had been studied by the moleculardynamics method based on tight-binding potentials. The gas phase was produced after evaporation of melted nickel using high energy electron beam. Experimental procedure originated the nanoparticles of two fractions with average size 100 and 200 nanometers. Computer analysis of system evolution shows the splitting can occur due to consolidation of large enough primary clusters. The simulation shows two stages of system evolution. At the first stage the consolidation occurs for clusters belonging to the range of the largest values of size distribution in accordance with theory predictions. At the second stage the largest probability corresponds to consolidation of biggest particles. Finally it leads to bimodal size distribution which is not observed at usual theoretical analysis of condensation process.

Changing Chirality during Single-Walled Carbon Nanotube Growth: A Reactive Molecular Dynamics/Monte Carlo Study

The growth mechanism and chirality formation of a single-walled carbon nanotube (SWNT) on a surface-bound nickel nanocluster are investigated by hybrid reactive molecular dynamics/force-biased Monte Carlo simulations. The validity of the interatomic potential used, the so-called ReaxFF potential, for simulating catalytic SWNT growth is demonstrated. The SWNT growth process was found to be in agreement with previous studies and observed to proceed through a number of distinct steps, viz., the dissolution of carbon in the metallic particle, the surface segregation of carbon with the formation of aggregated carbon clusters on the surface, the formation of graphitic islands that grow into SWNT caps, and finally continued growth of the SWNT. Moreover, it is clearly illustrated in the present study that during the growth process, the carbon network is continuously restructured by a metal-mediated process, thereby healing many topological defects. It is also found that a cap can nucleate and disappear again, which was not observed in previous simulations. Encapsulation of the nanoparticle is observed to be prevented by the carbon network migrating as a whole over the cluster surface. Finally, for the first time, the chirality of the growing SWNT cap is observed to change from (11,0) over (9,3) to (7,7). It is demonstrated that this change in chirality is due to the metal-mediated restructuring process.

Molecular dynamics simulation of deposition of nickel nanocluster on copper surface

The burrowing of nickel nanocluster deposited onto the copper surface is investigated by molecular dynamics (MD) simulation. The simulation is carried out at different temperatures for 40.1 ns with three different lattice orientations Cu(100), Cu(110), and Cu(111) that have 20 × 20 × 15, 14 × 14 × 15, and 12 × 14 × 9 lattice units, respectively. The Ni(100) nanocluster consists of 249 atoms (five lattices diameter) and the initial kinetic energy is assigned to be 0 eV. The results show that the burrowing process goes extremely slow as temperature is at or under 900 K. There is virtually no burrowing observed when the system temperature is below 500 K. The burrowing processes at different temperatures are discussed in terms of kinetic energy of the cluster exerted by the strong capillary force. It is found that the kinetic energy will play a key role in the acceleration of the burrowing process.

Atomic and electronic structure of both perfect and nanostructured Ni(111) surfaces: First-principles calculations

In this study, we perform first principles simulations on both atomically smooth and nanostructured Ni(111) slabs. The latter contains periodically distributed nickel nanoclusters atop a thin metal film gradually growing from adatoms and serving as a promising catalyst. Applying the generalized gradient approximation within the formalism of the density functional theory we compare the atomic and electronic structures of Ni bulk, as well as both perfect and nanostructured (111) surfaces obtained using two different ab initio approaches: (i) the linear combination of atomic orbitals and (ii) the projector augmented plane waves. The most essential inter-atomic forces between the Ni adatoms upon the substrate have been found to be formed via: (i) attractive pair-wise interactions, (ii) repulsive triple-wise interactions within a triangle and (iii) attractive triple-wise interactions within a line between the nearest adatoms. The attractive interactions surmount the repulsive forces, hence resulting in the formation of stable clusters from Ni adatoms. The magnetic moment and the effective charge (within both Mulliken and Bader approaches) of the outer atoms in Ni nanoparticles increase as compared to those for the smooth Ni(111) surface. The calculated electronic charge redistribution in the Ni nanoclusters features them as possible adsorption centers with increasing catalytic activity, e.g., for further synthesis of carbon nanotubes.

Water soluble nickel(0) and cobalt(0) nanoclusters stabilized by poly(4-styrenesulfonic acid-co-maleic acid): Highly active, durable and cost effective catalysts in hydrogen generation from the hydrolysis of ammonia borane

This paper reports the in-situ generation and catalytic activity of nickel(0) and cobalt(0) nanoclusters stabilized by poly(4-styrene sulfonic acid-co-maleic acid), PSSA-co-MA, in the hydrolysis of ammonia borane (AB). PSSA-co-MA stabilized nickel(0) (PSMA-Ni) and cobalt(0) nanoclusters (PSMA-Co) having average particle size of 2.1 ± 0.6 and 5.3 ± 1.6 nm, respectively, were generated by in-situ reduction of nickel(II) chloride or cobalt(II) chloride in an aquoues solution of NaBH 4/H 3NBH 3 in the presence of PSSA-co-MA. The in-situ generated nanoclusters were isolated from the reaction solution and characterized by UV–Vis, TEM, XRD and FT-IR techniques. Compared with the previous catalyst systems, PSMA-Ni and PSMA-Co are found to be highly active catalysts for hydrogen generation from the hydrolysis of AB with the turnover frequency values of 10.1 min −1 for Ni and 25.7 min −1 for Co. They are also very stable during the hydrolysis of AB providing 22450 and 17650 turnovers, respectively. The results of mercury poisoning experiments reveal that PSMA-Ni and PSMA-Co are heterogeneous catalysts in the hydrolysis of AB. Herein, we also report the results of a detailed kinetic study on the hydrogen generation from the hydrolysis of AB catalyzed by PSMA-Ni and PSMA-Co depending on catalyst concentration, substrate concentration, and temperature along with the activation parameters of catalytic hydrolysis of AB calculated from the kinetic data.

Hydrogen Adsorption on Mixed Platinum and Nickel Nanoclusters: The Influence of Cluster Composition and Graphene Support

The physical and chemical properties of transition metal nanoclusters have been extensively investigated. In particular, we study the energetics of the mixed clusters Pt4−nNin, focusing on the binding energy of the clusters Ebind to a graphene support, and the hydrogenation energy Eads in both the gas-phase and the graphene-supported clusters. For each cluster composition, the cluster can bind to graphene in either a face-on or an edge-on configuration, and in each of these orientations, binding can occur through different atoms; we explore these binding configurations comprehensively. We discuss the variation of Ebind and Eads with respect to the composition of the cluster and the binding configuration of the cluster to the graphene support. Our results show that hydrogen is generally chemisorbed at a Pt site and physisorbed at a Ni site, with a dependence of the adsorption energy upon the composition and the adsorption configuration. Compared with the gas-phase cluster, the chemisorption energies are ge...

Formation, nature of activity, and hydrogenation catalysis by nickel bis(acetylacetonate)-lithium tetrahydroaluminate systems

A new approach to synthesis of nickel catalysts under the action of lithium tetrahydroaluminate was proposed which allows preparation of high-performance nanosized catalytic systems with well-reproducible properties. The major stages of formation and the nature of catalytically active species and inhibitors formed in the Ni(acac)2-LiAlH4 system were determined. The catalytic properties of the nickel nanoclusters were studied in relation to the nature and concentration of the proton-containing compounds. Factors responsible for the promoting action exhibited by these compounds were analyzed.

The effect of thermal treatment on the organization of copper and nickel nanoclusters synthesized from the gas phase

The condensation of 85000 Cu or Ni atoms from the high-temperature gas phase has been simulated by molecular dynamics with the tight binding potential. The efect of the subsequent thermal treatment on the shape and structure of synthesized particles was studied by simulating their gradual heating in a range of 100–1200 K. Some tendencies are revealed that are characteristic of the influence of heat treatment on the nanoparticles synthesized from the gas phase. It is concluded that short-term heating leads to significant ordering of the internal structure in 70% of agglomerated nanoparticles with the predominant formation of spherical shapes. In order to explain this result, the main mechanisms of cluster formation from the gas phase have been analyzed and it is found that the agglomeration temperature plays the main role in the formation of clusters with unified shape and structure. This opens the fundamental possibility of obtaining Cu and Ni nanoclusters with preset size, shape, and structure and, hence, predictable physical properties.

Pulsed Current Electrochemical Synthesis of Nickel Nanoclusters and Application as Catalyst for Hydrogen and Oxygen Revolution

The main objective of this research is to prepare nickel nanoparticles with more porous structure by the pulsed current electrochemical method. A nickel optimized nanopowder was synthesized by using nickel chloride (0.005 M) as precursor, silver nitrate as a nucleation agent (at 0.5% mole of nickel salt in the starting solution), polyvinyl pyrrolidone (PVP) as structure director (with PVP/Ni = 1.7 g/g), ammonia (2 M), and hydrazine as reduction agent (with Hydrazine/Ni = 16 g/g) by pulsed current of 58 mA cm−2 with a frequency of 12 Hz. The morphology and particle size of each synthesized sample was studied by scanning electron microscopy (SEM). The obtained results showed that temperature has no considerable effect on the morphology and particle size of nickel nanopowder. The nickel nanopowder synthesized in optimum conditions has excellent uniform and a more porous structure including nanoclusters with a particle size of approximately 10–20 nm. The obtained results indicate that the pulsed current electrochemical method can be used as a confident and controllable method for the preparation of nickel nanoparticles. Optimized nickel nanoclusters were used as catalyst for hydrogen and oxygen revolutions. Cyclic Voltammetry results showed that the synthesized nanoclusters can facilitate hydrogen reduction and increase hydrogen and oxygen revolution rates.

Synergism of the catalytic effect of nanosized gold-nickel catalysts in the reaction of selective acetylene hydrogenation to ethylene

Gold and nickel nanoclusters immobilized on aluminum oxide exhibited high activity in the reaction of acetylene hydrogenation to ethylene with molecular hydrogen at temperatures from 20 to 64°C. The reaction selectivity on Au-Ni nanocomposites with metal concentrations from 0.02 to 0.36 wt % was no lower than 99.99%, and the stability of catalysts was retained for at least 12 h. The simultaneous presence of gold and nickel in the systems resulted in the synergism of their catalytic effects: acetylene conversion on layer-by-layer immobilized metal clusters was higher than the total conversion on individual gold and nickel clusters. The dependence of the conversion of acetylene on bimetallic catalysts on the Au : Ni ratio exhibited an extremal character.

Nucleation and growth of nickel nanoclusters on graphene Moiré on Rh(111)

Regularly sized Ni nanoclusters (NCs) have been grown on a graphene Moiré on Rh(111). Using scanning tunneling microscopy, we determine that initial growth of Ni at 150 K leads to preferential nucleation of monodispersed NCs at specific sites of the Moiré superstructure. However, a defined long-range ordering of NCs with increasing coverage is not observed. Room temperature Ni deposition leads to the formation of flat triangular-shaped islands which are well-matched to the Moiré registry.

Bond switching regimes in nickel and nickel–carbon nanoclusters

Understanding the fundamental dynamics in carbon nanotube (CNT) catalysts is of primary importance to understand CNT nucleation. This Letter reports on calculated bond switching (BS) rates in pure and carbon containing nickel nanoclusters. The rates are analyzed in terms of their temperature dependent spatial distribution and the mobility of the cluster atoms. The BS mechanism is found to change from vibrational to diffusional at around 900 K, with a corresponding strong increase in activation energy. Furthermore, the BS activation energy is observed to decrease as the carbon content in the cluster increases, resulting in an effective liquification of the cluster.

Structural, electronic, magnetic and optical properties of icosahedral silver–nickel nanoclusters

We present a systematic study of the structural, electronic, magnetic and optical properties of icosahedral bimetallic Ag((13-p))Ni(p) (p < or = 6) clusters in the gas phase investigated in the framework of density functional theory (DFT and TDDFT). In the most-stable structures, the central position is found to be occupied by a nickel atom. The evolution of spin-multiplicities and local atomic spin densities with Ag/Ni composition are discussed. The evolution of the optical properties with the Ag/Ni composition and the spatial positions of Ni atoms are analysed. An interpretation of spectroscopic patterns in terms of contribution from s- and d-type excitations is also given. In particular the d-electrons of nickel atoms are found to play a crucial role in the optical transitions of Ni-rich systems. Finally, our theoretical spectra are compared to the experimental ones for large Ni-core/Ag-shell clusters (about 2-5 nm in size).

Silicon Nitride Nanopillars and Nanocones Formed by Nickel Nanoclusters and Inductively Coupled Plasma Etching for Solar Cell Application

The external quantum efficiency of solar cells can be improved by using textured surface with minimum reflection. We have fabricated nanopillars and nanocone structures on silicon nitride surface by means of self-assembled nickel nano particle masks with single step inductively coupled plasma (ICP) ion etching and double step ICP etching, respectively. Thus, sub-wavelength nanopillar and nanocone-like structures displaying low reflectance were obtained readily without the need for any lithography equipment. The formation mechanism of nanopillar and nanocone like structures fabricated on silicon nitride surface has been discussed. The relationship of etching time with structure height and average reflectance spectra has been drawn.

Controllable growth and magnetic properties of nickel nanoclusters electrodeposited on theZnO nanorod template

The ZnO nanorods were used as a template to fabricate nickel nanoclusters byelectrodeposition. The ZnO nanorod arrays act as a nano-semiconductor electrodefor depositing metallic and magnetic nickel nanoclusters. The growth sites ofNi nanoclusters could be controlled by adjusting the applied potential. Under−1.15 V the Ni nanoclusters could be grown on the tips of ZnO nanorods. On increasing thepotential to be more negative the ZnO nanorods were covered by Ni nanoclusters. Themagnetic properties of the electrodeposited Ni nanoclusters also evolved with the appliedpotentials.

Adsorption of carbon oxide and nitrogen oxide molecules on the surface of the Ni/MgO(111) system

The coadsorption of carbon oxide (CO) and nitrogen oxide (NO) molecules on the surface of nickel nanoclusters formed on a thin magnesium oxide MgO(111) film grown on the Mo(110) face in an ultrahigh vacuum is studied by reflective infrared spectroscopy and thermodesorption spectroscopy (TDS). The adsorption of NO molecules is found to substantially change the state of the initially adsorbed CO molecules. The TDS and IR spectra suggest that the adsorption of NO molecules stimulates the surface migration of CO molecules from the surface of metallic clusters to the cluster-oxide interface, which is accompanied by a decrease in the angle of inclination of the molecular axis to the surface.