Insights into the Mechanism Related to the Phase Transition from γ-Fe2O3 to α-Fe2O3 Nanoparticles Induced by Thermal Treatment and Laser Irradiation

The nature of the physical mechanisms related to the gamma-Fe2O3 to alpha-Fe2O3 phase transition under laser irradiation and heat treatment has been investigated using in situ micro-Raman spectroscopy and X-ray powder diffraction (XRPD) analysis. Measurements were carried out on as-prepared gamma-Fe2O3 nanoparticles of about 4 nm in size as a function of laser power and on annealed gamma-Fe2O3 particles. Annealing temperature affects the relative fractions of the gamma-Fe2O3 and alpha-Fe2O3 phases, and at 450 degrees C, the phase transition into alpha-Fe2O3 becomes complete with apparent crystallite size < D > of about 30 nm. The hematite nanoparticles increase then up to more than 180 nm at 1400 degrees C. The excellent agreement between evolution of the wavenumbers and bandwidths confirms that the heat treatment and laser irradiation produces the same effects on nanoparticles. Correlations between structure modifications occurring at the nanometric scale during grain coalescence and the evolution of Raman vibrational spectra were quantitatively examined, and a physical mechanism for the gamma -> alpha-Fe2O3 phase transition was proposed.