Room Temperature Synthesis of Thermally Immiscible Ag-Ni Nanoalloys

The properties of bimetallic nanoparticles (NPs) are greatly affected by their structures, that is, alloy versus core-shell. Many binary phases are dictated by their thermodynamically most favorable composition. We recently synthesized stable small Ag-Ni alloy NPs (< 10 nm in diameter) by a radiolytic approach. Normally immiscible in bulk, Ag-Ni NPs are considered a core-shell structure from both experiments and simulations. Big lattice mismatch between Ag and Ni, plus lower surface energy of silver, makes core-shell structure thermodynamically favorable. To prepare kinetically favored Ag-Ni alloy NPs, a high dose rate from gamma irradiation was applied. A number of techniques were employed to fully characterize the Ag-Ni NPs and confirm their alloying. The HRTEM lattice spacing measured from Ag-0.5-Ni-0.5 NPs was found between the lattice spacing of Ag and Ni. Images from high-angle annular dark-field show that the Ag-Ni NPs are not core-shell structure but are homogeneous in composition. Energy filtered transmission electron microscopy maps show the homogeneity of the nanoalloys and that both Ag and Ni are present. Heating studies indicate that these NPs or nanoalloys are stable and did not experience any dealloying process. Our approach may provide a general methodology to synthesize nanoalloys. A mechanism is provided to explain the homogeneous formation of immiscible Ag-Ni nanoalloy.