Calorimetric Study of Maghemite Nanoparticles Synthesized by Laser-Induced Pyrolysis

The thermodynamic properties of maghemite (γ-Fe 2 O 3 ) 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 α- and γ-polymorphs of Fe 2 O 3 is similar to that between α- and γ-Al 2 O 3 . This large difference in surface enthalpy (∼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.