Low temperature heat capacity of bulk and nanophase ZnO and Zn1−xCoxO wurtzite phases

The low temperature heat capacity of the ZnO–CoO solid solution system was measured from 2 to 300 K using the heat capacity option of a Quantum Design Physical Property Measurement System (PPMS). The thermodynamic functions in this temperature range were derived by curve fitting. The standard entropies of bulk ZnO and bulk ZnO–CoO (wurtzite, 18 mol% CoO) at T = 298.15 K were calculated to be (43.1 ± 0.4) J · mol−1 · K−1 and (45.2 ± 0.5) J · mol−1 · K−1, respectively. The surface entropy of ZnO was evaluated to be (0.02 ± 0.01) mJ · K−1 · m−2, which is essentially zero. No sharp magnetic transitions were observed in the solid solution samples. The nanophase solid solution, 12 mol% CoO, appears to bind H2O on its surface more strongly than ZnO.

► Heat capacities of bulk and nanophase ZnO and a ZnO–CoO wurtzite solid solution sample from 2 to 300 K were measured. ► Thermodynamic functions in this temperature range were derived. ► Surface entropy of ZnO was evaluated to be essentially zero (0.02 ± 0.01) mJ · K−1 · m−2. ► Strong size-driven solubility shifts between the wurtzite and rocksalt solid solution phases were confirmed. ► The cobalt-containing wurtzite phase bonds H2O more strongly than pure ZnO.