Surface Effects Under Visible Irradiation and Heat Treatment on the Phase Stability of γ-Fe2O3 Nanoparticles and γ-Fe2O3 -SiO2 Core-Shell Nanostructures

The structural evolution of gamma-Fe2O3 (maghemite) in bare nanoparticles and in core/shell gamma-Fe2O3/SiO2 systems was studied as a function of laser irradiation and heat treatment by the combined use of Raman spectroscopy, transmission electron microscopy, X-ray diffraction. The study was addressed to deepen understanding the driving mechanisms at the basis of the maghemite to hematite (alpha-Fe2O3) phase transition and to understand the possible correlation with surface/defects states. In the bare system, phase transformation was obtained with very low beam density powers (less than 2 mW at 632.8 nm focalized with a conventional 10x microscope objective) and the threshold for transition further decreases in vacuum conditions, suggesting that the phase transformation can be achieved even without thermal assistance. On the contrary, phase transformation cannot be obtained by light irradiation in a gamma-Fe2O3/SiO2 core/shell system, but it can be induced by heat treatment at very high temperature (1100 degrees C). Fe2O3 nanoparticles at high temperature can diffuse inside the silica matrix forming aggregates with the alpha phase and increased size. The key role of the particle surface is discussed and a physical mechanism for the nucleation of hematite crystallites from the bonding of neighboring maghemite nanoparticles through hydrate defect states is proposed.