Influence Of The Thermodynamics And Coalescence Processes On The Microstructural Evolutions In Bimetallic Nanoclusters

Particular physical and chemical properties are exhibited in many bimetallic nanoclusters and can be tuned by varying their composition, atomic distribution and size. Furthermore, new materials with exceptional structure and functionality may be created by controlling their synthesis processes and have extensive applications in heterogeneous catalysts and biodiagnostics, as well as optoelectronic and magnetic devices. Therefore, the research on their thermodynamics and interaction processes has become a hot topic recently. However, the understanding and predicting of these processes by using experimental studies are very difficult. Computer simulation methods, in particular molecular dynamics (MD), have become a powerful means of investigating the structural evolutions.An alloy MD was constructed to apply in bimetallic clusters with a wide range of sizes and compositions. Then, the thermodynamics and coalescence processes of different clusters were studied by using this model. The influence mechanism of the synthesis processes on the structural evolutions of the bimetallic clusters was explored. The results showed as follows:1. Construction of the alloy MD and the test of its accuracyIn MD simulation, the validity of the simulated results is strongly related to the accuracy of the interatomic potential. A general embedded atom method (EAM), which is based on the density functional theory, was used. This EAM can force the potentials to go smoothly to zero at the same cutoff distance for 16 metals and make the calculations of alloys able with their any combinations. The simulated liquidus curves of the Cu-Ni bulk and clusters with different sizes agree well with the experimental and theoretical phase diagrams. This indicates the model can be used to describe the atomic interactions in the pure and bimetallic clusters.2. Influence of the thermodynamics on the structures of bimetallic clustersThe microstructural evolutions of the bimetallic clusters (constructed by the elements with different physical parameters) during the thermodynamics processes were studied. The results showed that:(1) The structural evolutions of the Ag-Co clusters are strongly related to the temperature and Co compositions. The most stable position for the Co atoms is the subsurface layer at lower temperature and changes to the core layer with the increase in temperature. But there is an energy barrier in the middle layer makes the cluster form an Ag-Co-Ag onion-like configuration. When the temperature is high enough, Co atoms can obtain enough energy to overcome the energy barrier to form a Co-Ag core-shell configuration. (2) The structural transformation of the symmetric icosahedron is strongly related to the initial configuration, atomic number and atomic distribution, and can be controlled by doping hetero atoms and changing their compositions and distributions. (3) The size effects of the cooling clusters are obvious. Icosahedron was formed during the freezing of the Co clusters with small size, while HCP structure was formed for the large-size Co clusters. This is because the released energy during the formation of the icosahedron decreases with the increase of the cluster size. The freezing structures are different after the Co clusters doping with Cu or Ni atoms.3. Influence of the coalescence on the structural evolutions of the clustersThe structural evolutions are different for different pure metal clusters and can be changed by the coalescence. The coalescences of the clusters with different states (liquid and solid) were firstly studied. Then, the structural evolutions were investigated by simulating the freezing of the pure cluster, homocluster and heterocluster coalescences. The results indicate that the structural evolutions are different and can be induced by the coalescences. Especially for the liquid-solid coalescence at a low temperature, a shell layer will be formed by the atoms with low surface energy. And it can inhibit the structural transformation of the solid cluster. Otherwise, the icosahedral transformation will be induced in the coalesced cluster by the solid cluster, which can easily transform to the icosahedron. In addition, it also found that the freezing structure of the homocluster coalescence is same to that of the pure cluster. But, there are some differences for the freezing structures of the heterocluster coalescence. Co-Ni and Co-Cu coalesced to form an icosahedron because the Co cluster easily froze to form an icosahedron. However, the freezing structures of the Cu-Ni are decahedron and similar to those of Ni and Ni-Ni. Therefore, nanoparticles with special structure and morphology can be fabricated by controlling the doping of the hetero atoms and coalescence processes.4. Structural evolutions induced by controlling the cooling and coalescenceSince the structures of the bimetallic clusters will be influenced by both the thermodynamics and coalescence processes. It is significant to understand the structural evolutions of the bimetallic clusters in the cooling and coalesced cooling processes. The structural evolutions of the Cu-Co clusters under different synthesis processes were studied. For the high temperature cooling cluster, there were obvious differences in the structures of the cooling (HCP) cluster and coalesced cooling (onion-like FCC) cluster. For the low temperature cooling cluster, the cluster formed an FCC structure. However, the coalesced cluster formed a Co-Cu core-shell morphology under the same condition. A twinned crystalline of the FCC and HCP structures formed. It is reasonable to conclude that nanoparticle with different structures and morphologies can be obtained by controlling the synthesis process.In conclusion, the nanoparticles with special structure and morphology can be fabricated by controlling their synthesis processes. The model can be used to explore the functionality-structure relationship. It also can be used to design new materials and analyze the experimental results.