Investigation of melting and freezing of Ag–Au alloy nanoclusters supported on carbon nanotube using molecular dynamics simulations

Two-Steps Versus One-Step Solidification Pathways of Binary Metallic Nanodroplets

The solidification of AgCo, AgNi, and AgCu nanodroplets is studied by molecular dynamics simulations in the size range of 2-8 nm. All these systems tend to phase separate in the bulk solid with surface segregation of Ag. Despite these similarities, the simulations reveal clear differences in the solidification pathways. AgCo and AgNi already separate in the liquid phase, and they solidify in configurations close to equilibrium. They can show a two-step solidification process in which Co-/Ni-rich parts solidify at higher temperatures than the Ag-rich part. AgCu does not separate in the liquid and solidifies in one step, thereby remaining in a kinetically trapped state down to room temperature. The solidification mechanisms and the size dependence of the solidification temperatures are analyzed, finding qualitatively different behaviors in AgCo/AgNi compared to AgCu. These differences are rationalized by an analytical model.

Influence of elemental composition on structural, thermal and hydration behavior of gold-silver bimetallic nanoparticles

In this study, molecular dynamics simulations (MDs) have been employed to explore the influence of elemental composition on the structural, thermal, and hydration behavior of the core-shell gold-silver bimetallic nanoparticles with three different concentrations Au₁₃₅Ag₁₁₄, Au₈₇Ag₁₆₂, and Au₅₅Ag₁₉₄. The pure gold and silver metal nanoparticles have also been studied for the sake of comparison. The calculated cohesive and formation energy values reveal the enhancement in the stability of gold-silver bimetallic nanoparticles with the increase in the concentration of gold. The specific heat capacity value of the bimetallic gold-silver nanoparticles has been found to increase linearly with the concentration of silver. This suggests that the specific heat capacity value of the gold-silver bimetallic nanoparticles may be improved by raising their silver concentration. The enhancement of specific heat capacity value with respect to the concentration of silver may be attributed to the enhanced phonon density. There is no significant difference in the computed hydration shell and water residence time of the gold-silver bimetallic nanoparticles with respect to their mixing ratio. The obtained stable structure, tunable specific heat capacity values, and the predicted hydration properties of the gold-silver bimetallic nanoparticles may be harnessed for their technological and biological applications.

Competition between stability of icosahedral and cuboctahedral morphologies in bimetallic nanoalloys

In this study, we investigated the heating process for pure (Rh₅₅ and Cu₅₅), single dopant (Rh₁Cu₅₄ and Rh₅₄Cu), core@shell (Rh₁₃@Cu₄₂ and Cu₁₃@Rh₄₂), and alloy (Rh₁₃Cu₄₂, Rh₄₂Cu₁₃) nanoclusters in two structures (cuboctahedral and icosahedral) from 0 to 2000 K using molecular dynamics (MD) simulations. Our aim was to investigate the effects of composition and chemical arrangement on the kinetic and thermodynamic stability of Rh-Cu bimetallic nanoalloys. Our results indicated that Cu₅₅, Ir₅₅, Rh₁Cu₅₄, Rh₅₄Cu, and Cu₁₃@Rh₄₂ in the cuboctahedral and icosahedral structures and Rh₄₂Cu₁₃ in the icosahedral structure did not experience any transformation with the exception of melting. It was also observed that the cuboctahedral Rh₄₂Cu₁₃ shows a solid-solid transformation to the icosahedral structure before the melting point. It is also observed that Rh₁₃@Cu₄₂ and Rh₁₃Cu₄₂ nanoclusters in both structures exhibit a transition to a pseudo-spherical structure before the melting point. Our results also illustrated that the Rh and Cu atoms tend to lie in the inner and outer shells of the nanoclusters, respectively. We have also discussed the changes in the melting points of the doped nanoclusters in the different arrangements.

Phase transition in crown-jewel structured Au-Ir nanoalloys with different shapes: a molecular dynamics study

We have studied the melting process for crown-jewel structured Ir₅₅, Ir₅₄Au, Ir₄₃Au₁₂, Ir₂₅Au₃₀, Ir₁₃Au₄₂, and Au₅₅ nanoclusters in the icosahedral, Ir₅₅, Ir₅₄Au, Ir₄₃Au₁₂, Ir₁₉Au₃₆, Ir₁₃Au₄₂, and Au₅₅ nanoclusters in the cuboctahedral, and Ir₅₄, Ir₅₃Au, Ir₄₇Au₇, Ir₁₇Au₃₇, Ir₇Au₄₇, and Au₅₄ nanoclusters in the decahedral morphologies. We have investigated the different thermodynamic, structural, and dynamical properties for the different nanoclusters in the different structures. Our thermodynamic results indicated that as the concentration of Au atoms in the nanoclusters increases, the absolute value of internal energy, and so the melting points, of the nanoclusters decrease. It is also shown that the Au atoms decrease the melting temperature of the pure cuboctahedral cluster more than that of the other structures. We have also found that the Au atoms were located in favorable positions at the surface sites of nanoalloys. Also, the doping of the Ir nanocluster by Au atoms makes the cluster more stable. It is also found that nanoclusters with different morphologies have almost the same stability. Our structural results indicated that after the melting process, the Au atoms generally tend to lie in the outer shell of the cluster, but the Ir atoms generally tend to lie in the core of the cluster (see the Ir₁₃Au₄₂ and Ir₇Au₄₇ nanoclusters, for example). We have also found the interesting result that the Ir₇Au₄₇ nanocluster shows a solid-solid transition from a decahedral structure to an icosahedral structure before melting. The Ir₄₃Au₁₂ nanocluster also shows a transformation from a cuboctahedral structure to an icosahedral-like structure before melting. Our dynamical results showed that doping of the Ir₅₅ cluster with an Au atom sharply increases the self-diffusion coefficient in the initial state in the solid phase, especially in icosahedral and cuboctahedral structures. It is also shown that the Ir₁₃Au₄₂ cluster in icosahedral and cuboctahedral and the Ir₇Au₄₇ and Ir₁₇Au₃₇ clusters in decahedral morphologies have smaller values of self-diffusion coefficients than other clusters after the melting point and that this could be due to the formation of core-shell structures.

Investigation of thermal, structural and dynamical properties of (Aux–Cuy–Niy)N=32,108,256 ternary nanosystems: effect of Au addition to Cu–Ni bimetallic nanoclusters via MD simulation

In this work, we have investigated the heating and cooling processes for ternary metallic nanoclusters with different Au mole fractions using molecular dynamics simulation.