Hydrothermal synthesis and characterization of nanosized transition metal chromite spinels

Homogeneous crystalline transition metal chromite spinels (MCr 2O4, where M = Co, Mn, and Ni) were prepared by hydrothermal reaction of aqueous solutions containing the respective metal nitrate, chromium(III) nitrate, and sodium hydroxide in stoichiometric amounts at 180-200 ° C and pH 10.5-11.5 for 11-13 h. The crystalline structure, microstructure, and thermal stability of the synthesized chromite products were analyzed by X-ray diffraction (XRD), scanning electron microscopy, thermogravimetry, and differential thermal analysis. The analysis results revealed that the CoCr 2O4 and MnCr 2O4 crystalline phases were thermally stable in the range of 750-800 ° C. UV-visible studies showed that higher characteristic bands corresponded to the region of the visible spectrum of cobalt (CoCr 2O4 , bluish-green) and manganese-doped cobalt chromites (Co0.5Mn0.5 Cr2O4 , dark green). XRD powder diffraction data of specimens revealed the formation of a well-crystallized spinel structure of transition metal chromite (approximately 98%) after calcination at 550-750 ° C. The nanocrystallinity of synthesized CoCr2O4 was found to be in the range of approximately 100-120 nm.