Synthesis and long-term phase stability of Mn3O4 nanoparticles

The synthesis and long-term phase stability of hausmannite (Mn3O4) nanoparticles were investigated. The experimental techniques used were XRD, FT-IR, Raman, UV/Vis/NIR and FE-SEM. Mn3O4 nanoparticles were synthesized by (a) precipitation from Mn(NO3)2 and NH4OH solutions and ageing of suspensions obtained at 160 or 20 °C and (b) precipitation from MnCl2 solution with addition of H2O2 and NH4OH solutions and ageing of suspensions obtained at 90 or 20 °C. The order of adding H2O2 or NH4OH influenced the phase composition of the precipitate. A combination of XRD, Raman and FT-IR techniques was useful in investigating the long-term phase stability of Mn3O4 and changes in the nonstoichiometry of these particles. The IR bands recorded in the range 623–635 cm−1 can be assigned to the nonstoichiometry of these particles. It was shown that the IR bands at 1085, 1115 and 1150 cm−1 corresponding to OH− vibrations can be used in the interpretation of a very early stage of the phase transformation Mn3O4 → γ-MnOOH. The appearance of an IR band (shoulder) in FT-IR spectra in the range 594–599 cm−1 was discussed in terms of a possible presence of very fine γ-MnOOH particles in traces and restricted to the surface of Mn3O4 nanoparticles. The water (moisture) adsorbed on Mn3O4 particles plays a key role in the phase transformation Mn3O4 → γ-MnOOH. Other parameters related to the synthesis route also play a certain role in this phase transformation. UV/Vis spectra were briefly discussed.

• Sample procedures for the preparation of Mn3O4 nanoparticles were proposed. • Long-term stability of Mn3O4 particles was determined. • FT-IR and Raman were applied in the monitoring of Mn3O4 → γ-MnOOH transition.