Fabrication of Core-Shell Nanoparticles

Abstract

Metallic Cu and Au shells were fabricated around cobalt nanoparticles. A new technique to coat nanoparticles with carbon coatings and poly(methyl methacrylate) (PMMA) was developed. The copper shell formation is a self-limiting process. A thin copper shell (0.82 nm) around the cobalt nanoparticle (1.56 nm) enhanced the magnetic property by increasing the blocking temperature from 124 K to 235 K for nanoparticles with a copper shell. The formed gold shell (0.67 nm) enhanced the cobalt nanoparticle magnetic property by increasing the blocking temperature above room temperature. The magnetic moment in the Co-Cu and Co-Au core-shell nanoparticle is much higher than that of the pure cobalt nanoparticle. However, the copper shell (2.88 nm) around the FeCo alloy nanoparticle (0.87 nm) was fabricated and found to decrease the blocking temperature to 126 K. Complete displacement of Fe nanoparticle by copper ions was observed with a loss of the magnetic property. The electrochemical reaction rate was used to estimate the reaction rate in aqueous solutions between the cobalt, or iron nanoparticle, and the copper ions and was found to be similar: 0.0015 A/cm2 and 0.0022 A/cm2 for the Co-Cu and Fe-Cu systems. The annealing process has a dramatic effect on the behavior of the nanoparticles. The sizes were increased in all the nanoparticles, as expected. A phase change (from fcc to hcp) was found in the cobalt nanoparticles as the annealing temperature increased. Phase segregation and partial oxidation of the FeCo alloy nanoparticle under the annealing process was observed. A tight carbon shell was formed around the iron nanoparticle and protected the iron nanoparticle from oxidation in acid. Fe-C core-shell nanoparticles retained the magnetic property (i.e. saturation magnetization and coercivity) after exposure to acid. A resistance change with a variation of magnetic field is referred to as magnetoresistance (MR). Magnetoresistance was observed in the pressed Co-Au core-shell pellet. Both the fresh Co-Au and annealed core-shell nanoparticle followed the metallic conduction behavior. No MR was observed in the Co-Cu and FeCo-Cu core-shell nanoparticles.