Determination of the magnetic contribution to the heat capacity of cobalt oxide nanoparticles and the thermodynamic properties of the hydration layers

We present low temperature (11K) inelastic neutron scattering (INS) data on four hydrated nanoparticle systems: 10nm CoO0.10H₂O (1), 16nm Co₃O₄0.40H₂O (2), 25nm Co₃O ₄0.30H₂O (3) and 40nm Co₃O₄0. 026H₂O (4). The vibrational densities of states were obtained for all samples and from these the isochoric heat capacity and vibrational energy for the hydration layers confined to the surfaces of these nanoparticle systems have been elucidated. The results show that water on the surface of CoO nanoparticles is more tightly bound than water confined to the surface of Co₃O₄, and this is reflected in the reduced heat capacity and vibrational entropy for water on CoO relative to water on Co ₃O₄ nanoparticles. This supports the trend, seen previously, for water to be more tightly bound in materials with higher surface energies. The INS spectra for the antiferromagnetic Co₃O₄ particles (2-4) also show sharp and intense magnetic excitation peaks at 5meV, and from this the magnetic contribution to the heat capacity of Co ₃O₄ nanoparticles has been calculated; this represents the first example of use of INS data for determining the magnetic contribution to the heat capacity of any magnetic nanoparticle system.