Impact annealing temperature process on oxidation state of iron ions and structural phase transition in magnetite nanoparticles

This investigation is focused on the effect of annealing temperature (T-ann) on the structural, vibrational, optical, and magnetic properties of the magnetite nano-ferrites at several temperatures in ambient air. The wet-chemical co-precipitation route was used to synthesize the samples which were examined by XRD, EDX, XPS, FT-IR, UV-VIS, and VSM techniques. This study revealed that the crystal structure of the samples transferred from magnetite into maghemite (gamma-Fe2O3) by sintering at 250 degrees C for 1 h and into hematite (alpha-Fe2O3) by annealing at temperatures T-ann >= 500 degrees C. According to this transformation process, the oxidation process leads to different equivalents of iron ions. The XPS intensity percentage (%) of Fe+4, Fe+3, and Fe+2 at B-site approximately remained stable after the transformation process that takes place at T-ann >= 500 degrees C. However, the intensity of Fe+3 and Fe+2 at A-site decreases during heating. The values of energy band gab (E-g) decrease with T-ann before the transformation process then thereafter increase, this behavior shows a strong dependence on the crystalline size (R) so that R can control their values. While the saturation magnetization (M-S) increases with T-ann before the transformation process then thereafter decreases, this behavior confirms the transformation process where the magnetite and maghemite have higher M-S than hematite.