Synthesis of nickel nanoparticles in water-in-oil microemulsions

The synthesis of nickel nanoparticles by the reduction of nickel chloride wit hydrazine in the cationic water-in-oil microemulsions of water/CTAB (cetyltrimethylammonium bromide)/n-hexanol at 73 degrees C has been studied. By the analyses of electron diffraction pattern and X-ray diffraction, the resultant particles were characterized to be the pure nickel crystalline with a face-centered cubic (fcc) structure. The investigation on the composition of microemulsion solution indicated that the average diameter of nickel nanoparticles was affected mainly by the ratio of CTAB to n-hexanol instead of the size of microemulsion droplets. Smaller particles could be obtained at higher ratios of CTAB to n-hexanol. At a constant nickel chloride concentration, the average diameter of nickel nanoparticles decreased with the increase of hydrazine concentration and then approached to a constant value when the concentration ratio of hydrazine to nickel chloride was above 10. At a sufficient high hydrazine concentration, the average diameter of nickel nanoparticles was independent of nickel chloride concentration. The effects of microemulsion composition and the concentrations of hydrazine and nickel chloride on the particle size are discussed in detail. The magnetic measurements for a typical sample with an average diameter of 4.6 nm showed that the resultant nickel nanoparticles were superparamagnetic due to extremely small size. The saturation magnetization (26.2 emu/g), remanent magnetization (0.67 emu/g), and the coercivity (7.5 Oe) were significantly smaller than those of the bulb nickel, reflecting the nanoparticle nature. In addition, the magnetization was observed to increase with the decrease of temperature due to the decrease in ther mal energy.