Rietveld Refinement, μ-Raman, X-ray Photoelectron, and Mössbauer Studies of Metal Oxide-Nanoparticles Growth on Multiwall Carbon Nanotubes and Graphene Oxide

Applying a modified coprecipitation method, maghemite and anatase nanoparticles embedded in graphene oxide and multiwall carbon nanotube frameworks were prepared, and a detailed structural characterization is presented. Transmission electron images have revealed that the multiwall carbon nanotubes and graphene oxide act as substrates to reduce the nanoparticle agglomeration with narrow sizes of ca. 9-20 nm, in agreement with the results of the Rietveld refinement, which have also indicated their crystallite apparent size and shapes using the spherical harmonics approach. In structural studies of maghemite nanoparticles by Raman spectroscopy, it was found that the effect of optical density and laser power intensity plays a significant role. When no optical filter was located between the powder sample and the laser source, a transformation from the γ-Fe2O3 to the α-Fe2O3 phase was observed, as demonstrated by the disappearance of the characteristic broad Raman peak (A1g) of the γ-Fe2O3 phase when increasing the laser power. X-ray photoelectron spectroscopy has also brought insights into the functionalization mechanism, suggesting that the one-pot reduction of the graphene oxide is favored by the alkaline γ-Fe2O3 nanoparticle growth. The temperature dependence of the 57Fe Mössbauer spectra has indicated that the effective anisotropy constant of Fe oxide-based nanoparticles is similar to that of bulk maghemite, and magnetic relaxation of Fe3+ spins depends on particle sizes.