Calorimetric study of maghemite nanoparticles synthesized by laser-induced pyrolysis

The thermodynamic properties of maghemite (γ-Fe₂O 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/m² and is in good agreement with previously reported values. The surface enthalpy for the dry, water-free surface is 0.71 ± 0.13 J/m² and is reported for the first time. The difference in the surface enthalpy for the dry surface between α- and γ-polymorphs of Fe₂O₃ is similar to that between α-and γ-Al₂O₃. This large difference in surface enthalpy (∼1.2 J/m²) creates an energy crossover so that fine-grained hematite is metastable relative to fine-grained maghemite at particle size <15 nm.