Judging phase purity of hematite (α-Fe2O3) nanoparticles through structural and magnetic studies

The effect of post-growth annealing on the phase transformation leading to phase pure hematite (alpha-Fe2O3) nanoparticles is reported in this work. Co-precipitation technique was used to synthesise iron oxide nanoparticles by adjusting the pH of the solution. The xerogel was dried at 80 degrees C and the obtained powder was calcined in the temperature range 400-800 degrees C for 3 h. in air. Annealing temperature was prefixed on the basis of the thermal degradation pattern of the starting precursor. X-ray diffraction (XRD) study endorsed the presence of mixed phases FeO, FeO2 and alpha-Fe2O3 in the as-prepared sample. On annealing, the minor phases transform gradually to alpha-Fe2O3, as confirmed through thermal studies. Performed Rietveld analysis confirms the presence of impurity phases in samples annealed at low temperature. Phase pure sample crystallizes in the rhombohedral corundum structure (space group, R-3c) with a = 5.04044 angstrom, c = 13.7628 angstrom and c/a = 2.73048. FT-Raman spectral investigations allowed for a clear assignment of the alpha-Fe2O3 phase. Chemical analysis advocated the only presence of Fe3+ ions that octahedrally coordinated with hexagonally close packed O2- ions that constitute corundum structure. Electron microscopy (TEM) images proved the non-spherical particle distribution in the range 80-100 nm with mean particle size of 93 nm. Structural phase transition with annealing temperature was further confirmed through the fluctuations in the magnetic structure. In analogues to XRD, magnetic study also served as a judging tool to identify purity and the presence of mixed phases in hematite.

Automated summary

This work investigates the effect of post-growth annealing on the phase transformation leading to phase pure hematite nanoparticles. X-ray diffraction studies and thermal analysis confirm the purity and structural changes in the samples.