Mild aqueous synthesis of octahedral Mn3O4 nanocrystals with varied oxidation states

Mn3O4 nanocrystals were prepared by a simple aqueous precipitation method using Mn(CH3COO)2·4H2O and NaOH as precursors at low temperature of 30 °C. The as-prepared Mn3O4 nanocrystals were characterized by means of XRD, SEM, UV–vis and FT-IR analyses. Mn3O4 nanoparticles prepared at 30 °C extended octahedral, spherical and rod-like structures with the sizes in the range of 60–100 nm. The size and shape could be manipulated by reaction time. The as-prepared Mn3O4 was so sensitive to hydrochloric acid (HCl) that the sample color changed obviously from brown to black. Analyses on the HCl treated and untreated samples gave results that varied ratio of Mn2+, Mn3+ and Mn4+ in Mn3O4 crystals led to color changes of the samples. Optical properties of the samples were discussed via the UV–vis and FT-IR results.