Hydrothermal synthesis and characterization of nanosized transition metal chromite spinels

Homogeneous crystalline transition metal chromite spinels (MCr ₂ O ₄, 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 ₂ O ₄ and MnCr ₂ O ₄ 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 ₂ O ₄, bluish-green) and manganese-doped cobalt chromites (Co _0.5 Mn _0.5 Cr ₂ O ₄, 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 CoCr ₂ O ₄ was found to be in the range of approximately 100-120 nm.