Optical Bandgap Tuning of Manganese Ferrite by Nickel Doping and Heat Treatment

Controlled doping is the paradigmatic approach for improving the properties of any pristine material. Ni_xMn_1–xFe₂O₄ (x = 0, 0.01, 0.03, 0.05, 1) were synthesized by the facile sol–gel auto combustion method. The as-prepared samples were further annealed. Formation of impurity levels, structural, morphological, vibrational, optical and electronic variations as a result of Nickel doping is systematically analyzed by structural and optical studies. The average crystallite size shows decremental trend for pre annealed and an incremental trend for annealed samples. Increased particle agglomeration can be seen in SEM micrographs. EDAX confirms the compositional variation of Nickel from 1 to 5%. FTIR and Raman spectra represent band shifting towards higher wavenumber and Raman peak broadening with Nickel substitution. Optical analyses suggests that MnFe₂O₄ which has optical bandgap of ~2.2 eV is reduced to ~1.7 eV (pre annealed) and ~1.4 eV (annealed) and thus can be made as a photo catalyst material. Photoluminescence spectra of pre- and post-annealed samples show presence of peak in visible region with a red shift in PL spectra indicating band narrowing.