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 results clearly indicate that extrinsic parameters such as the size of the crystals and their aggregation state affects their Raman properties.

Crystal structure, microwave dielectric properties, and dielectric resonant antenna studies of novel low-permittivity CoAl2O4 spinel ceramics

“…The wavenumbers of the Raman active peaks are obtained via fitting the spectra using a Lorentz function, and the results are consistent with the previous works [ 25 – 27 ]. The F 2 g (1) mode at 202 cm −1 is assigned to the Co/Al-O 4 tetrahedron translation [ 25 ]. The position of E g mode is reported to have a reasonable correlation with the cation radius at the 8a site [ 11 ].…”

Pt-free CoAl2O4 catalyst for soot combustion with NOx/O2

“…CoAl2O4 crystallizations are envisaged on the α-Al2O3 microparticles surface (Figure 2b). These nanocrystals have an irregular shape, with an average equivalent diameter of 427±118 nm, and present a hole in the middle; they expose the lowest-energy (111) facets represented in Figure S1c [20,25]. The morphology of the CoAl2O4 powder before and after ball milling is shown in Figure 2c-f.…”

Performance and Stability of Wet-Milled CoAl2O4, Ni/CoAl2O4, and Pt,Ni/CoAl2O4 for Soot Combustion

“…Such conclusions were drawn from the results of XRD studies ( Figure S1). XPS analyses of the fresh and used samples revealed traces of zirconia contamination, attributed to the milling procedure [55], and indicated that all the samples had a normal spinel structure and Co 2+ cations in tetrahedral sites ( Figure S2, Table S1) [56,57]. Figure S3a shows a STEM-HAADF (Scanning Transmission Electron Microscopy-High Angle Annular Dark Field) illustrative image of a region with bright spots dispersed all over the CoAl 2 O 4 that correspond to Pt and Ni, according to the EDX mapping of these elements in the region.…”