Hydrothermal synthesis and characterization of nanosized transition metal chromite spinels

Homogeneous crystalline transition metal chromite spinels (MCr2O4, 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 degrees 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 CoCr2O4 and MnCr2O4 crystalline phases were thermally stable in the range of 750-800 degrees C. UV-visible studies showed that higher characteristic bands corresponded to the region of the visible spectrum of cobalt (CoCr2O4 bluish-green) and manganese-doped cobalt chromites (Co0.5Mn0.5Cr2O4, 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 degrees C. The nanocrystallinity of synthesized CoCr2O4 was found to be in the range of approximately 100-120 nm.