Synthesis, phonon and optical properties of nanosized CoCr2O4

Nanoparticles of cobalt(II) chromite, CoCr2O4, have been prepared by hydrothermal synthesis at 230 °C and firing of the obtained nanosized sample at various temperatures. The effect of particle size on the structure and properties of CoCr2O4 nanoparticles has been investigated using the XRD, TEM, diffuse reflectivity, Raman and infrared spectroscopy. The results show that simple hydrothermal synthesis yields material composed of very small crystalline particles with the average size of about 7 nm. Annealing of this sample allows obtaining particles with different sizes. Raman and IR studies show that in addition to the bands characteristic for normal spinel structure, a few additional broad bands appear due to the cation redistribution among the tetrahedral and octahedral sites. Intensity of these bands significantly increases for the samples annealed at lower temperatures. This result shows that with decreasing particle size the cation redistribution is tending towards more inverse spinel configuration compared to bulk, which is predominantly normal spinel. This conclusion is supported by the optical spectra of the obtained samples. Raman and IR spectra also indicate weak phonon confinement effect in CoCr2O4. Significantly larger phonon confinement effect is observed only for the 379 cm−1 IR-active mode involving motions of Cr(III) atoms located at the octahedral sites.