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

Abstract

The low temperature heat capacity of the ZnO–CoO solid solution system was measured from 2 to 300K 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, 18mol% CoO) at T=298.15K 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, 12mol% CoO, appears to bind H2O on its surface more strongly than ZnO.