Investigation of thermal stability of 2D and 3D CoAl2O4 particles in core-shell nanostructures by Raman spectroscopy

Abstract

We are studying the formation of a nanostructured spinel CoAl2O4 layer on α-Al2O3 giving rise to a core-shell composite. In the final product, two mechanisms of CoAl2O4 crystallization onto a α-Al2O3 microparticle surface are observed, depending on the dispersion grade of Co3O4: a 3D nanostructure from the arrangement of Co3O4 agglomerated nanoparticles; 2D nanoparticles from the diffusion mechanism of isolated Co3O4 nanoparticles. As a consequence, two different crystallization pathways may occur during the thermal treatment. In order to understand the formation mechanisms a Raman Confocal Study is performed. The features of the Raman spectra of the samples depend strongly on the morphology of the nanoparticles located in the shell of the microparticle. Average spectra of the samples show a variation in the Raman shift between the different samples. The differences between 3D or 2D structures is associated with the transmission of phonons among the nanoparticles. The high absorbance of the cobalt species could mask the Raman shift displacement by local heating, so both the temperature and the laser source power are considered in the Raman study. The evolution of the Raman spectra with applied temperature indicates a decoupling of the Raman modes. This fact is more relevant for the 3D aggregates in which nanoparticles produced a larger scattering of phonons and a higher sensitivity to temperature variation. These results clearly indicate that extrinsic parameters such as the size of the crystals and their aggregation state affects their Raman properties.