Thermodynamics of the CoO-ZnO system at bulk and nanoscale

Enthalpies of formation of Co _x Zn _1-x O solid solutions (both bulk and nanophase materials) at 298 K have been determined using high-temperature oxide melt solution calorimetry in molten sodium molybdate (3Na ₂O•4MoO ₃) solvent at 973 K. Both the rocksalt and wurtzite phases show an approximately linear dependence of enthalpy of solution on composition, implying a zero heat of mixing in each phase, consistent with negligible lattice parameter changes on substitution of Co ²⁺ for Zn ²⁺. The surface energy of wurtzite Zn _0.88Co _0.12O solid solution was determined to be 2.33 ± 0.30 J/m ² (anhydrous surface) and 1.65 ± 0.25 J/m ² (hydrous surface), which are very close to values for ZnO. The wurtzite CoO surface energy was estimated to be similar. Here, we argue that, because of the lower surface energies of wurtzite phases than of rocksalt phases, the phase field of the wurtzite solid solution expands to higher CoO content at the nanoscale, suggesting that the reported extended solubility of CoO in ZnO nanoparticles represents thermodynamic stabilization and free energy minimization at the nanoscale. Conversely, the rocksalt Co _1-xZn _xO phase shows thermodynamic destabilization, lower zinc content, and easier oxidation (to Co _3-xZn _xO ₄ spinel phase) at the nanoscale than in the bulk.