Facile route for preparation of nanochristalline ZnMn2O4 ; effect of preparation conditions on structure and microstructure

Traditional synthesis of spinel materials such as solid-state route involving grinding and firing of a mixture of oxides, nitrates or carbonates which requires elevated temperatures and prolonged process times have been abandoned. Indeed, majority of papers on manganese spinels are focused on low cost preparation methods which proceed at moderate temperatures (600°C) with enhanced reaction kinetics [1, 2]. Interestingly, Liu et al. reported a room temperature route for preparation of AMn2O4 (A=Zn, Co, Cd) from metal acetates [3]. Although synthetic route proposed by Liu et al. represents a facile and very efficient route for low temperature synthesis of spinel materials, in order to utilize this route for the targeted design of nanomaterials it is necessary to establish, very precisely, correlations between specific preparation conditions(concentration of NaOH, aging period, and temperature of additional heat treatments), structure, microstructure and properties. Detailed structural investigation using X-ray powder diffraction (XRPD) and Raman spectrocopy have been carried out in order to correlate specific structural and microstructural features with changes in preparation conditions. ZnMn2O4 was prepared by precipitation with NaOH (c=0.25-0.8 M) from solution of Zn and Mn acetates. Also, sample obtained by 0.8 M NaOH was additionaly heat treated at T=300, 400 and 500 °C. Pronounced difference in crystallinity was observed with increase in concentration of NaOH. Based on the results of Raman spectroscopy a model described by the formula: [Zn2+1-xMn2+ x]tetra[Zn2+xMn3+2-2xMn4+x]octaO4 has been proposed and tested upon structural data. It was shown, based on Rietveld structure refinement, that unit-cell constants as well as the inversion parameter of spinel lattice increase with the increase in temperature of thermal treatment. 1. L. Hu et al., Sci. Rep., 2, 217-226, 2012. Y. 2. Li et al., Nanoscale, 5, 2045-2054, 2013. 3. Y. Liu et al. RSC Advances, 4, 4727-4731, 2014.