Zirconia coating for enhanced thermal stability of gold nanoparticles

This paper describes a rapid, simple and one-step method for the preparation of 2–4 nm diameter zirconia-coated gold nanoparticles at room temperature. These nanoparticles were synthesized by two simultaneous processes: the chemical reduction of tetrachloroauric acid with sodium borohydride and the formation of zirconia sol–gel matrices. All the gold nanoparticle sols were characterized by UV–visible absorption and transmission electron microscopy to determine the nanoparticle size and shape. The synthesis method is a combination of a polymeric structure of the amorphous zirconia and the use of a strong reducing agent, and it yields to very small quasi-spherical gold nanoparticles at room temperature. The thermal stability up to 1200 °C of the coated nanoparticles was studied by x-ray diffraction. The metastable tetragonal phase of the zirconia coating was obtained at 400 °C, and a progressive transformation from tetragonal to monoclinic phases of the zirconia coating was observed up to 1100 °C. After the heat treatment at 400 °C, the crystallite size of the gold nanoparticles was about 29 nm, and it remained unchanged from 400 °C to 1200 °C. These results are promising for the development of such materials as doping elements for optical fiber applications.