Stabilization of metastable phases in spatially restricted fields: the case of the Fe2O3 polymorphs

In this work we show how the confinement of particles in a silica matrix with pores, acting as nano-vessels, plays an important role in the formation of pure E-Fe2O3 nanoparticles and their thermal stability. In particular, a HRTEM study of a series of Fe2O3-SiO2 xerogels annealed at different temperatures reveals that, at low temperatures, gamma-Fe2O3 nanoparticles are formed and only transform to F-Fe2O3 after subsequent annealing at higher temperatures. These data are complemented by measurements of the SiO2 matrix porosity as well as with calorimetric and structural analysis of the nanoparticles after matrix removal. The gathered data indicate that the SiO2 matrix acts as a barrier, hindering thermal diffusion and particle growth even at the high temperatures where the formation of epsilon-Fe2O3 appears to be favoured, thereby preventing its transformation to alpha-Fe2O3.