Green synthesis of nanocrystalline manganese (II, III) oxide

Nanocrystalline Manganese (II, III) Oxides (Mn3O4) were synthesized through green approach using the leaf extract of Adalodakam (Adhatoda vasica Nees / Justicia adhatoda). The probable mechanism towards the formation of nanoparticles, crystallographic structure, the average and effective crystallite sizes, optical characteristics and thermal behavior of the sample were studied comprehensively. The obtained powder x-ray diffraction pattern was indexed with characteristic peaks corresponding to ICDD PCPDFWIN data for tetragonal body-centered lattice of Mn3O4. Considerably broadened peaks in the XRD pattern indicate diminished size of crystallites and in consistent with Debye-Scherer equation, average crystallite size was estimated to be 44 nm. Based on the uniform deformation stress model as suggested in Hall-Williamson technique, the broadening of XRD peaks contributed from micro strains developed in the nanocrystalline sample was analyzed and accordingly, the effective crystallite size was separated from micro strain and the value is estimated to be 66 nm. Analysis on absorption spectrum recorded for the sample in the UV-Visible-NIR range revealed the direct allowed transition with semiconducting energy band gap equal to 2.50 eV. Further, the thermogravimetric analysis resulted in an insight into the thermal changes inside the sample during the process of heating in air. The broad peak in the curve observed at 498.3 °C was reasonably assigned to the absolute phase transformation from Mn3O4 to Mn5O8 and further decomposition to Mn2O3 which crystallizes at 581.3 °C