Calorimetric study: Surface energetics and the magnetic transition in nanocrystalline CoO

CoO nanoparticles ranging in size from 7 to 21 nm were prepared via precipitation and thermal decomposition methods. The particles had water contents ranging from 2.18 to 0.19 wt %; water content decreased with increasing particle size. All particles also contained a small amount of Co 3 O 4 impurity. The surface enthalpy was determined to be 2.82 ± 0.20 Jm -2 by acid solution calorimetry at 298 K. Corrections were made for the water and cobalt spinel impurity. Heat capacity measurements using adiabatic (10-320 K) and semi-adiabatic (0.6-40 K) calorimeters were performed on the sample with particle size 7.0 ± 1.0 nm. The nanoparticle heat capacity had a broad anomaly with a rounded maximum at 265 K, a reduction of 23 K from the Neel temperature T N observed as a sharply peaked maximum in the heat capacity of single-crystal CoO. When corrected for water and Co 3 O 4 , the heat capacity of nanophase CoO is greater than that of single-crystal CoO below about 250 K. Above 250 K, the much larger magnetic peak in the heat capacity of the single crystal dominates the heat capacity difference. Thus the excess entropy of the nanoparticles, calculated from the heat capacity difference, has a maximum of 2.4 ± 0.3 J.K -1 .mol -1 at 245 K but drops to 1.5 ± 0.3 J.K -1 .mol -1 at 298 K. The magnetic and surface contributions to the excess entropy cannot be resolved in a definitive manner, but an estimate of the surface entropy, 0.28 ± 0.03 mJ.K -1 .m -2 , is similar to one literature report for MgO, and the excess magnetic entropy is negative, -0.8 ± 0.3 J.K -1 .mol -1 .