Effect of low temperature calcination on micro structure of hematite nanoparticles synthesized from waste iron source

Hematite (α-Fe₂O₃) nanoparticles have been synthesized from waste source of iron which contains a prominent amount of iron (93.2 %) and investigated the effect of low temperature calcination. The two-step synthesis method involved preparing ferrous sulfate through acid leaching process followed by oxidation and calcination at temperatures ranging from 200 to 400 °C to produce the desired α-Fe₂O₃ in nano form. The structure, size and morphology of the hematite nanoparticles were characterized using various instrumental techniques including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Fourier transformed infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-visible diffuse reflectance spectroscopy (DRS), and a nanoparticle size analyzer. Hematite single phase was confirmed by XRD and the phase occurred at 200 °C might indicate the stability range of hematite under certain condition. The average crystal sizes were determined using Debye Scherer formula, modified Scherer formula, size strain plot equation and Halder-Wagner-Langford's method and the results show that crystallite sizes decreased with increasing calcination temperature. XPS analysis confirmed the chemical state (Fe³⁺) and surface chemistry of the hematite nanoparticles calcined at 300 °C. Raman spectrum also supported that the nanoparticles were complete hematite phase and the intensity of all the features decreased with increasing calcination temperature which are consistence with the result obtained from XRD pattern. FTIR spectra of the samples also confirms the XRD results. Morphological analysis obtained from SEM and TEM images suggested the agglomerated irregular spherical nanoparticles with grain size 13.49 nm calcined at 300 °C. Band gap energy of the samples were calculated from DRS data and the values ranging from 2.30 to 2.42 eV which are slightly higher than the bulk (∼2.1eV). Particles size analysis have been carried out using DLS and Z-average particle size and poly dispersity index (PDI) were measured which indicate the particles are nearly same size (220-226 nm).