The spontaneous oxidation of crystallized ZrAu at room temperature leads to the formation of ultrafine monoclinic zirconia (MZ) particles (about 3.6 nm in diameter). The presence of tetragonal phase (TZ), generally observed in very small zirconia particles, was investigated by X-ray diffraction and Raman experiments but was not observed. Thermodynamical calculations considering the finite size effects in zirconia were introduced to explain the formation of pure MZ considering experimental and theoretical surface energies. The results of these calculations associated with the near equilibrium conditions used for synthesis demonstrate that the monoclinic structure is the stable crystallographic structure of zirconia nanocrystals at room temperature.

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In this study we first report the elaboration of gold and zirconia clusters (ranging from 3.5 to 7 nm in size) obtained by air oxidation of crystallized ZrAu at 25 °C within a few hours. This transformation was observed using X-ray diffraction, infrared spectroscopy (transmission and DRIFTS), and electron microscopy (SEM and HREM) techniques. This solid-state nanostructuration is interpreted as a consequence of combined mechanisms: adsorption and diffusion of oxygen, segregation of gold atoms from ZrAu alloy onto Au clusters, and acceleration of the process by internal residual stress and percolation of the electronic current via gold-cluster chains. The gaseous species adsorption by very small gold clusters supported by zirconia formed at the initial step of this oxidation could induce "catalytically assisted nanostructuring oxidation". Large amounts of gold and zirconia nanoparticles are obtained at the end of this process.

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