Properties of Core−Shell Ni−Au Nanoparticles Synthesized through a Redox-Transmetalation Method in Reverse Microemulsion

Core−shell Ni−Au nanoparticles were chemically synthesized through a redox-transmetalation method in reverse microemulsion. The powder X-ray diffraction patterns revealed the presence of crystalline gold and nickel and the absence of any nickel oxides, nickel boride or other byproducts. The core−shell structure could be clearly observed by the transmission electron microscope. In addition, the Ni cores and the gold shells were further verified by the high-resolution transmission electron microscope and the Z-contrast image. The diameter of the nanoparticles ranged from 15 to 30 nm, with 5−10 nm core diameters and 5−10 nm shell thickness. The UV−visible absorption spectra of these nanoparticles showed a red shift (relative to pure gold nanoparticles), also in agreement with the gold shell morphology. For magnetic properties, the zero-field-cooled and field-cooled temperature dependence of the magnetization indicated the blocking temperature was at 16 K. The magnetization curves carried out at 5 K showed that the saturation magnetization, remanent magnetization, and coercivity at this temperature were 9.0 emu/g, 4.1 emu/g, and 2 kOe, respectively. The magnetization curves at 300 K presented the typical superparamagnetic behavior without any remains of remanent magnetization or coercivity, and the saturation magnetization at this temperature was 0.7 emu/g.