Thermodynamic Research On The Order-disorder Transition And Structural Stability Of Bimetallic Nanoparticles

ABSTRACT:Based on our previous work, we propose the Gibbs energy model and Gibbs energy of surface atom for bimetallic nanosolids. Then a research is carried on about the order-disorder transition and surface order-disorder transition of bimetallic nanosolids. Furthermore, we generalize the cohesive energy model for bimetallic core shell nanoparicles on the basis of Bond Energy model. Then we investigate the thermodynamic stability of bimetallic core shell nanoparicles. The main contents of our research are as follows:1). On the basis of Debye model, we take into account the configurational entropy, the vibrational energy and entropy of surface atoms as well as the shape factor and build the size-and shape-dependent Gibbs energy and Gibbs energy of surface atoms. The order-disorder transition temperature corresponds to the value when the Gibbs energy of the ordered is equal to that of the disordered. Specifically, the size and shape dependent order-disorder transtion of CoPt and FePt nanoparticles is investigated. Then we compare the research results of the surface order-disorder transition temperature with the experimental annealing temperature, so that we can choose the maximum temperature for annealing to induce ordering. As a result, we obtain deep insights into the mechanism of order-disorder transition of bimetallic nanoparticles that the ordering starts from the surface.2). Based on the model proposed in part1, we generalize the Gibbs energy model and Gibbs energy of surface atoms to bimetallic nanosolids, i.e. nanoparticles, nanowires, nanofilms and nano-disks. Then we investigate the case of CoPt nanowires, nano-disks and FePt nanofilms. At size larger than2nm, the relative transition temperature difference is identical to the relative ordering enthalpy difference of nanosolids. And the ratio of the relative transition temperature difference for nanoparticles, nanowires, and nanofilms follows3:2:1.3). In this work, the bond energy (BE) model, originally for cohesive energy of pure NPs, has been generalized to core-shell nanosolids. In the generalized BE model, the core-shell NPs can be divided into three regions:the core, the interface and the shell. By summation the atomic contributions of the three regions, the relation for the cohesive energy of core-shell NPs can be derived. Using this model, we discuss the cohesive energy and the phase stability of Pd-Pt and Au-Ag core-shell and alloy NPs. Comparing with Pt@Pd core shell and PdPt nanoalloy NPs, the Pd@Pt core shell NPs are most unstable. Comparing with Au@Ag core shell and AuAg nanoalloy NPs, the Ag@Au core shell NPs are most unstable. There exists critical composition for the stability of Au@Ag core shell and AuAg nanoalloy NPs. Furthermore the alloy stable region of AuAg bimetallic NPs is wider than that of PdPt system, indicating that AuAg nanoalloy is more stable. The generalized BE model has also been used to predict the melting of core shell NPs.The consistency between our model predictions with the experiment and simulation results verifies the validity of our model. Our research deepens the insights into the mechanism of order-disorder transition and can well serve as a guideline for the choosing of the annealing temperature and the material design of bimetallic core shell nanoparticles.