Atomic Simulations Of The Alloying Behavior Of Au/Ag Nanoparticles

Bimetallic nanoparticles （NPs） occupy an increasingly important position in modernmaterials due to their unique physical and chemical properties. How to control the particlesize, structure and chemical order in the preparation process becomes a basic research projectin the study of alloy NPs. The preparation of alloy NPs is a complicated physical andchemical process, but essentially it is an alloying process at the nanoscale. The study ofalloying behavior and mechanism at nanoscale from the perspective of thermodynamics anddynamics has an important role in guiding and improving the preparation of alloy NPs.In the present paper, we have implemented modified analytic embedded atom method（MAEAM） and molecular dynamics （MD） to investigate the alloying process of Ag-Au NPswith dumbbell shape and core-shell structure from the viewpoint of atomic diffusion,microstructure evolution and alloying extent.The thermodynamics stability of Au/Ag NPs shows that FCC NPs is the most favorableone at large size, but it is almost same for IDEC and ICO NPs. We firstly studied the alloyingbehavior of dumbbell-shaped Au/Ag NPs. The results presented that the temperature has anobvious influence on the alloying rate of Au/Ag NPs. Ag atoms mainly diffuse on the surfaceof Au NPs. The mechanisms of alloying are different with each other for Au_FCC/Ag_FCCandAu_ICO/Ag_ICONPs. It is demonstrated that the growth of grain boundary plays a key role in thealloying process of Au_FCC/Ag_FCCNPs, whereas the recrystallization is predominated forAu_ICO/Ag_ICONPs. The alloying extent of Au_ICO/Ag_ICONPs is much greater than that ofAu_FCC/Ag_FCCNPs. The alloy NPs finally obtained possess FCC structure and ICO-IDECcoexisting phases, respectively.Molecular dynamic simulations are also implemented to investigate the alloying behaviorof Au/Ag NPs with different structures. For Au/Ag NPs with the same size (Au_FCC/Ag_ICOandAu_ICO/Ag_FCC), a solid-to-solid structural transition occurs from ICO to FCC structure. Finally,it obtains a core-shell structured nanoparticle with the core rich in Au atoms. However, theirmechanisms are much different during the alloying process, which induced by thespontaneous disposition of ICO Ag nanoparticle for Au_FCC/Ag_ICOand the recrystalization ofAu atoms due to the diffusion of Ag atoms for Au_ICO/Ag_FCC, respectively. The alloyingbehavior is also investigated for NPs with different sizes. It is shown that almost no phasetransition from ICO to FCC structure occurs during the alloying process for ICO-predominantNPs （AuFCC:AgICO=1:4, AuICO:AgFCC=4:1）。In Au₅₃₁/Ag₂₀₅₇NPs, the alloying behavior is mainly induced by the nucleation and growth of grain owing to the competition between ICOand IDEC structure; whereas in Au₂₀₅₇/Ag₅₃₁NPs, it is almost induced by the recrystalizationof atoms to reach the metastable IDEC phase.We also simulated the alloying process of core-shell structured Ag-Au NPs. Fornanoparticle with a thin shell, there is apparent atomic diffusion in the boundary between Agcore and Ag shell. The Ag core shows a polyhedron shape. For nanoparticle with a thick shell,the atomic diffusion is located to surface. The appearance of the nanoparticle converts topolyhedron structure with Ag core unchanged. The above shape changes are all performedthrough the growth of stacking fault. Compared to the thick shell, the alloying extent ofnanoparticle with a thin shell is much greater.Based on the above investigation, thermodynamic and kinetic effects play an importantrole in the alloying process.