Calorimetric study of maghemite nanoparticles synthesized by laser-induced pyrolysis

The thermodynamic properties of maghemite (gamma-Fe2O3) nanoparticles with size smaller than 10 nm had not been studied previously because of particle size limitations for samples synthesized by wet chemical methods. Laser-induced pyrolysis is a well-established method of producing maghemite with particle sizes smaller than 10 nm. Maghemite nanoparticles, obtained by this method and having a size range of 2-40 nm, were fully characterized and studied by solution calorimetry. The enthalpy of water adsorption was also measured. The surface enthalpy obtained from calorimetric data for the hydrated maghemite surface is 0.57 +/- 0.10 J/m(2) and is in good agreement with previously reported values. The surface enthalpy for the dry, water-free surface is 0.71 +/- 0.13 J/m(2) and is reported for the first time. The difference in the surface enthalpy for the dry surface between alpha- and gamma-polymorphs of Fe2O3 is similar to that between alpha- and gamma-Al2O3. This large difference in surface enthalpy (similar to 1.2 J/m(2)) creates an energy crossover so that fine-grained hematite is metastable relative to fine-grained maghemite at particle size < 15 nm.