Hydrothermal synthesis and characterization of nanosized transition metal chromite spinels

摘要

Homogeneous crystalline transition metal chromite spinels(MCr_2O_4, where M = Co, Mn, and Ni) were prepared by hydrothermalreaction of aqueous solutions containing the respective metal nitrate,chromium(III) nitrate, and sodium hydroxide in stoichiometric amountsat 180-200 °C and pH 10.5-11.5 for 11-13 h. The crystallinestructure, microstructure, and thermal stability of the synthesizedchromite products were analyzed by X-ray diffraction (XRD), scanningelectron microscopy, thermogravimetry, and differential thermalanalysis. The analysis results revealed that the CoCr_2O_4 andMnCr_2O_4 crystalline phases were thermally stable in the range of750-800 °C. UV-visible studies showed that higher characteristic bandscorresponded to the region of the visible spectrum of cobalt(CoCr_2O_4, bluish-green) and manganese-doped cobalt chromites(Co_{0.5}Mn_{0.5}Cr_2O_4, dark green). XRD powder diffraction data ofspecimens revealed the formation of a well-crystallized spinelstructure of transition metal chromite (approximately 98%) aftercalcination at 550-750 °C. The nanocrystallinity of synthesizedCoCr_2O_4 was found to be in the range of approximately 100-120 nm.