Nanosizing as a way to enhanced solubility of transition metals in ZnO

The solubility of nickel and cobalt oxides in ZnO nanoparticles was explored using the equilibrium thermodynamic approach. The equilibrium conditions were derived for the binary system ZnO–TMO (TM = Ni or Co) consists of two different spherical nanoparticles, one comprising the wurtzite (Zn1−xTMx)O(wz) solid solution and the second the rocksalt (TM1−xZnx)O(rs) solid solutions. Thermodynamic parameters for both phases were evaluated from bulk phase diagrams and the surface energy terms were included. Calculations of phase boundaries between the wurtzite and the rocksalt solid solutions reveal significant rise of the TM content in wurtzite phase. At the same time, the solubility of ZnO in rocksalt phase decreases. This behavior is more obvious for nanoparticles less than 5 nm in diameter and becomes more evident with increasing temperature. These results are in qualitative agreement with published experimental data.