Nonlinear Size-Dependent Melting of Silica-Encapsulated Ag-Cu Alloy Nanoparticles

We report on the size dependence of the Ag-Cu nano-alloy melting temperatures via differential scanning calorimetry (DSC) measurements of silica-coated Ag-Cu nanoparticles (NPs) and the thermodynamic modeling. Four Ag-Cu alloy NP samples with compositions ranging from Ag8Cu2 to Ag5Cu5 were prepared via NaBH4 co-reduction and silica-coating. The experimental size dependence of melting temperature was obtained via UV visible absorption spectra, transmission electron microscopy, and DSC. On the basis of the measured melting temperatures of the Ag-Cu nano-alloys in this work as well as the thermodynamic properties of the pure Ag and Cu NPs and Ag-Cu systems in literature, the theoretically calculated size dependence of Ag-Cu NP melting temperatures was then obtained via thermodynamic modeling of the Ag-Cu nano-system. Thermodynamically calculated size dependence of the melting temperatures for Ag-Cu alloy NPs shows a nonlinear function with respect to the inverse of the particle size and satisfactory agreement with experimental observations. This work establishes a methodology for determining the melting temperature as a function of size for different nano-alloy systems.