Crystal evolution of nano-sized CoCr2O4 synthesized by a modified sol-gel method

The present study provides a comprehensive assessment of the synthesis, crystal evolution, and degree of inversion of the spinel CoCr2O4 (i.e., cochromite) by a modified sol-gel method using propylene oxide and ultrasonication. Ultrasonic assistance promotes the dissociation and dispersibility of Co2+ and Cr3+ precursors, contributing to the hydrolysis reaction to form hydrated ions. Propylene oxide serves as a proton scavenger to provide the conditions needed for homogeneous gelation. Heat treatments at different temperatures (400, 550, and 700 degrees C) for different durations (1, 4, and 8 h) have been applied to follow the crystal evolution and the inversion of the spinel system. The formation of CoCr2O4 particles was observed at approximately 400 degrees C. An increase in calcination temperature induced particle size growth (from 12 to 42 nm) and sintering effects. A significant decrease in the specific surface area (from 144 to 33 m(2) g(-1)) was observed with changes in the lattice parameter (8.304-8.334 angstrom). Raman, XPS, XRD, and Rietveld analyses indicated the presence of a pure cochro-mite phase. The Bertaut method has been applied to monitor the inversion process of CoCr2O4 during post-synthesis heat treatment. The spinel conformation became a regular type beyond 700 degrees C, whereas a fully inverted structure was obtained at lower temperatures. The modified sol-gel method may be convenient for large-scale production to synthesize other spinel-type compounds.