Particle size effects on the thermal behaviour of maghemite synthesised by flame pyrolysis

Thermal treatment of pure nanoscale maghemite (γ-Fe₂O₃) powders under argon yields different products depending on particle size. Particles with a mean diameter above 15 nm transform to hematite (-Fe₂O₃). Smaller particles, which were synthesised under identical conditions but from solutions with lower Fe³⁺ concentrations, transform into magnetite (Fe₃O₄) during thermal treatment. This phenomenon can be explained by adsorbed or incorporated reducing carboxylate groups. The origin of the reducing species and possible reaction mechanisms with the maghemite particles are discussed.