Molecular dynamics simulation of the growth and diffusion mechanisms of Fe–Cu bimetallic nanoparticles

Bimetallic core-shell nanoparticles such as NiCu are of great interest not only due to their excellent stability, selectivity, and magnetic and catalytic properties, but also because they are tunable by changing the morphology, surface element distribution, and particle size of the nanoparticles. The surface segregation and structural features of NiCu bimetallic nanoparticles, the deposition growth and the surface diffusion of Cu adsorbed atoms on the Ni substrate surface are studied by using molecular dynamics and the Montero method combined with embedded atomic potential. The results show that the Cu atom has a strong tendency of surface segregation. With the increase of concentration of Cu atoms, Cu atoms preferentially occupy the vertex, edge, (100), and (111) facet of nanoparticles due to the difference in configuration energy between Cu atoms and surface Ni atoms with different coordination numbers after the exchange, and finally form perfect Ni-core/Cu-shell nanoparticles. When growth temperature <i>T</i> = 400 K, the Ni-core/Cu-shell structure formed is the most stable. By observing the NiCu core-shell structure’s growth sequence, it is found that a few Ni atoms are replaced by Cu atoms on the step edge of the Ni substrate. The diffusion energy barrier of Cu atoms adsorbed on a Ni substrate surface is calculated by using the nudged elastic band method. The results show that Cu atoms adsorbed need to overcome a large ES barrier for both exchange and diffusion, making it difficult to diffuse between the facets of Ni substrate surface in a temperature range of 200–800 K. The lowest energy barrier for the diffusion of Cu atoms between facets of Ni substrate surface is 0.43 eV, and the diffusion path is from (111) facet to (100) facet. In contrast to Ni substrate, Ni atoms deposited on Cu substrate can easily migrate from the (111) facet to the (100) facet with a diffusion energy barrier of only about 0.12 eV, and at the present simulated temperature, Ni adsorbed atoms are unable to migrate on the (100) facet, resulting in a growth configuration toward an octahedral shape with its eight apex angles almost occupied by Ni atoms. In this paper, a new idea and method are provided for the preliminary design of NiCu nano-catalysts from atoms.

A new approach to enhance the thermoelectric performance of quaternary chalcogenides copper zinc tin sulfide thin films by varying copper molar concentration

Diffusion of single adatom Cu on Cu (0 0 1) and (1 1 0) surfaces

The effect of surface oxygen vacancies on the adsorption and clustering of the Pt adatoms over the defective anatase TiO2(101) surface has been studied using density functional theory slab calculations. The surface oxygen vacancy site was found to be the most active site for a single Pt adatom with an adsorption energy of 4.87 eV. As such, this site may act as a nucleation center for particle growth on the defective anatase TiO2(101) surface. The pathways for forming the two stable Pt2 adsorption configurations from a single Pt adatom in the oxygen vacancy site and another from the neighboring bridging 2cO sites involve the diffusion of the second Pt adatom out of the bridging 2cO site. The transition states for the dimer formation were located at Pt adatom diffusing out of the bridging 2cO site. Furthermore, we found that the bond-breaking step determines the barrier height for the Pt adatom diffusion, which is ∼1 eV. Among five stable Pt3 adsorption structures at the oxygen vacancy site on the defective anatase TiO2(101), the most stable structure is triangular with all three Pt atoms interacting directly with the surface atoms. The possible pathways for Pt adatom diffusion to form the most stable Pt3 configuration were analyzed. The barriers for Pt adatom diffusion in Pt3 formation were predicted to be similar to that of the Pt2 formation due to similar transition state structures. The barrier height indicates that clustering will be kinetically hindered at low temperature.

Surface self-diffusion of adatom on Pt cluster with truncated octahedron structure

Self-diffusion behaviors of Pd adatom and dimer on Pd(001) surface

As energy demand increases and global warming progresses, expectations for fuel cells, which generate electricity through chemical reactions between hydrogen and oxygen, are rising. There are various types of fuel cells, which are classified according to the electrolyte. Polymer electrolyte fuel cells (PEFCs) are expected to be used in stationary power sources for home use and fuel cell vehicles because of their low operating temperature, short start-up time, and ease of miniaturization. The cost has been an issue in the widespread use of fuel cells. The amount of platinum used in the catalyst layer (CL) must be reduced to reduce costs. However, it increases the current density per unit area of platinum. There are three factors that contribute to voltage drop: ohmic polarization, activation polarization, and diffusion polarization. At high current densities, the voltage drop due to diffusion polarization is dominant. The main cause of diffusion polarization is the transport resistance of oxygen molecules in the CL, and improving the oxygen transport properties in the CL will lead to lower cost PEFCs. In this study, we analyzed the transport properties of oxygen molecules in the CLs of PEFCs using the Monte Carlo (MC) method. The effect of oxygen scattering on the overall oxygen transport properties was investigated by considering the behavior on the surface of the ionomer film (surface diffusion) into the MC method, which simulates the overall transport in the PEFC CL. The objective of this study is to reproduce accurate oxygen transport by comparing the results of MC and molecular dynamics (MD) methods. First, we used the MC method to analyze oxygen scattering phenomena. In this study, we used the 3D structure data of CL in the MC simulations. The three-dimensional structure of the sample was reconstructed from sequential slice images. Molecules were assumed to reflect specularly on the boundaries of the simulation domain. Oxygen molecules were randomly placed in the simulation system. The time step was set at 5 ps and the simulation time was set at 4 μs. In order to examine the effect of surface diffusion on the transport properties of oxygen molecules, the simulation was performed using various conditions of surface diffusion. We calculated the effective diffusion coefficient of oxygen in the CL as the transport property of oxygen molecules in the MC method. As the surface diffusion coefficient increases, the effective diffusion coefficient of oxygen increases. When the surface diffusion coefficient is small, the effective diffusion coefficient of oxygen increases due to a decrease in the time constant, which determines the probability distribution of the surface residence time. However, when the surface diffusion coefficient is large, a different trend emerges. As the time constant increases, the effective diffusion coefficient has a peak value and then decreases. Next, to verify the model of surface diffusion in the MC method, we analyzed the scattering phenomenon of oxygen molecules in a simulation system that simulates the pores in the CL using the MD method. We modeled the nano pores in the CL as a slit between the ionomer walls in the MD method. Nafion with the equivalent weight (EW) of 1146 was used as polymer model. The number of Nafion chains is set at 4, and the water content λ is set at 7. Three-dimensional periodic boundary conditions were applied to the simulation domain. After the system was equilibrated, 1 ns of NVT simulation was performed for production. The temperature was set at 297 K. The behavior of oxygen molecules impacting on the ionomer wall was analyzed. We analyzed the behavior of oxygen molecules colliding with the ionomer membrane using the MD method. The average residence time of surface diffusion on the ionomer thin film was of the order of 10-10s. When the calculation results were compared with the MC results, it was found that the average residence time obtained by the MD method was the value where the surface diffusion coefficient affects much on the effective diffusion coefficient. Comparing the results of the MD method with the surface diffusion model used in the MC method, there are some discrepancies between the results and the model. For example, the MC method used a model in which oxygen molecules reflect diffusively on the ionomer surface. However, in the MD method, it was found that oxygen molecules tend to reflect in the direction of travel (Fig. 1). Therefore, a more accurate model of the MC method needs to be developed in the future. Figure 1

With the increase in energy demand and the progression of global warming, fuel cells, which generate electricity through the chemical reaction between hydrogen and oxygen, have been attracting growing attention. Especially, polymer electrolyte fuel cells (PEFCs) are expected to be applied in stationary power sources for home use and fuel cell vehicles due to their advantages such as low operating temperature, short startup time, and ease of miniaturization. However, at high current density conditions, the voltage drop due to diffusion polarization becomes dominant, primarily caused by the transport resistance of oxygen molecules in the catalyst layer (CL) [1]. Therefore, enhancing oxygen transport in the CL is crucial for cost reduction. It has been pointed out that the scattering behavior of oxygen molecules on the ionomer surface within the CL can be classified into surface scattering and surface diffusion. However, since the pore size within the CL ranges from a few nanometers to approximately 100 nm, detailed experimental analysis remains challenging. To address this, this study performs a multiscale approach by incorporating surface diffusion parameters obtained from molecular dynamics (MD) simulations, which analyze the behavior of oxygen molecules on ionomer thin films, into the direct simulation Monte Carlo (DSMC) method, which examines the overall behavior of oxygen molecules within the CL. The objective of this research is to appropriately evaluate the oxygen transport properties in the entire CL of a PEFC.In the MD simulations conducted in this study, the simulation system consists of carbon walls, Nafion, water molecules, and oxygen molecules. The simulation conditions include a two-dimensional periodic boundary condition and a sampling time of 10 ns. For the DSMC method, a three-dimensional structural model was generated from continuous cross-sectional images of a CL sample obtained using a focused ion beam–scanning electron microscope. The simulation conditions include a three-dimensional periodic boundary condition, a time step of 1 ps, and a sampling time of 10 ns.The behavior of surface diffusion was modeled by assuming that oxygen molecules colliding with the wall surface remain adsorbed for a certain period, move isotopically along the surface, and undergo diffuse reflection. First, time constants and surface diffusion coefficients were defined for the surface residence time and surface travel distance using the MD simulation. The surface residence time obeyed an exponential distribution, with its time constant, or the mean residence time, determined as τ=20.5×10-15 s. The probability density distribution of the surface migration distance at residence times of 10 ps, 20 ps, and 30 ps was fitted to the Rayleigh distribution derived from the diffusion equation using the least squares method, thereby defining the comprehensive surface diffusion coefficient as Ds =24.0×10-9 m2/s. Next, the effect of surface diffusion on oxygen transport properties was evaluated using the DSMC method. The effective diffusion coefficient of oxygen molecules was calculated as De =8.30×10-6 m2/s in the absence of surface diffusion and De =1.83×10-6 m2/s when surface diffusion was considered. The effective diffusion coefficient was lower in the system with surface diffusion, indicating that the applied surface diffusion model successfully reproduced the experimental tendency of the suppression of diffusion due to energy loss upon collisions between oxygen molecules and the ionomer thin film. The oxygen transport resistance was calculated as R=5.52 s/m without surface diffusion and R=25.1 s/m with surface diffusion. The latter value was approximately twice than the experimentally obtained value of R=12.1 s/m. This suggests that the current surface diffusion modeling overestimates the transport resistance caused by collisions with the ionomer thin film. One possible reason for this overestimation is the assumption that oxygen molecules retained on the wall surface move isotopically and undergo diffuse reflection. To properly evaluate the transport resistance of oxygen molecules, the scattering behavior at the ionomer surface must be further analyzed to better reproduce the movement of oxygen molecules along the surface.From the above analysis, incorporating surface diffusion reproduced the suppression of oxygen diffusion by collision with the ionomer thin film. However, since the simulation overestimated the experimental values, improvements in the surface diffusion modeling approach are necessary.Reference[1] S. J. Peighambardoust, et al., International Journal of Hydrogen Energy, volume 35, issue 17, pp. 9349–9384, Sep. 2010.

The magnetism and work function of pure Ni(001) and Ni-Cu slab alloys were investigated using first-principles methods based on density functional theory. The calculated results reveal that both magnetic moments and work functions of the alloys depend strongly on the surface orientation, but hardly on the distribution of doped Cu atoms for a given surface orientation. It is found that the doped Cu atoms have evident influence on the magnetic moment of Ni-Cu slabs, and the average magnetic moment of Ni atoms for Ni-Cu alloys decreases with increasing concentration of Cu atoms. Moreover, it is observed that the work function of Ni(001) is insensitive to the supercell thickness and the inner concentration of Cu atoms. In the meantime, the spin polarization is found to have an obvious role on the work function of the Ni-Cu alloys, which may give a new way to modulate the work function of the metal gate.

This paper investigates the effect of copper (Cu) doping on the thermoelectric properties of Tin Oxide (SnO2) nanoparticles. The Cu doped samples were prepared with the help of hydrothermal method using different concentration of Cu atoms. Various characterization techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, Hall effect and Seebeck measurements are performed to analyze the structural, morphological, electrical and thermoelectric of grown nanomaterials. Based upon the XRD and Raman measurements, it was concluded that the crystal quality of SnO2 samples degraded with increasing the doping concentration of Cu atoms. SEM images reveal that with the increase of Cu concentration, the porosity of the samples decreases, and the distribution of nanoparticle sizes becomes more uniform. The electrical conductivity of the Cu-doped SnO2 nanoparticles increases with increasing the Cu concentration because Cu atoms are supposed to act as donor defects in SnO2 crystal. However, the Seebeck coefficient decreases, indicating a reduced ability to generate a thermoelectric voltage in response to a temperature gradient. The power factor, initially increases with Cu doping, however, beyond an optimal value, the power factor starts to decrease, indicating a saturation or detrimental effect caused by excessive Cu doping.

Investigating the thermoelectric power generation performance of ZnCuO: A p-type mixed-metal oxide system

Experimental measurement of the intensity of the spectral line CuI 324.7 nm and the concentration of Cu atoms in a hollow cathode discharge under pulsed and steady-state conditions

Nanometer copper (Cu)-doped poly-4-methyl-1-pentene (PMP) foam is prepared by thermally induced phase inversion and ultrasonic dispersal. The nanometer Cu powders are dispersed uniformly in the horizontal direction of the foam and sink a bit in the vertical direction. The practical concentration of Cu atoms in the foams is lower than the theoretical concentration. Experiments reveal that a higher concentration of Cu atoms results in a finer foam structure.

The paper studies the mechanisms of plastic relaxation and mechanical response depending on the concentration of Cu atoms at grain boundaries (GBs) in nanocrystalline aluminum with molecular dynamics simulations. A nonmonotonic dependence of the critical resolved shear stress on the Cu content at GBs is shown. This nonmonotonic dependence is related to the change in plastic relaxation mechanisms at GBs. At a low Cu content, GBs slip as dislocation walls, whereas an increase in Cu content involves a dislocation emission from GBs and grain rotation with GB sliding.

The role of the Cu atoms sputtered from the cathode material in a cylindrical hollow cathode discharge (HCD) and the corresponding Cu+ ions are studied with a self-consistent model based on the principle of Monte Carlo (MC) and fluid simulations. In order to obtain a more realistic view of the discharge processes, this model is coupled with other submodels, which describe the behavior of electrons, fast Ar atoms, Ar+ ions, and Ar metastable atoms, also based on the principles of MC and fluid simulations. Typical results are, among others, the thermalization profile of the Cu atoms, the fast Cu atom, the thermal Cu atom and Cu+ ion fluxes and densities, and the energy distribution of the Cu+ ions. It was found that the contribution of the Ar+ ions to the sputtering was the most significant, followed by the fast Ar atoms. At the cathode bottom, there was no net sputtered flux but a net amount of redeposition. Throughout the discharge volume, at all the conditions investigated, the largest concentration of Cu atoms was found in the lower half of the HCD, close to the bottom. Penning ionization was found the main ionization mechanism for the Cu atoms. The ionization degree of copper atoms was found to be in the same order as for the argon atoms (10−4).