Advances in the study of nano-structured Co/MCM-41 materials: surface and magnetic characterization

Abstract

Co-modified mesoporous supports with MCM-41 structure and several metal loadings were successfully synthesized by a fast wet impregnation method. The nature and location of different Co species formed on the solids were inferred by TEM/SEM, XPS, XANES/EXAFS, adsorption of pyridine coupled to FT-IR spectroscopy and temperature dependence of magnetization. The presence of Co oxide species (clusters and Co3O4 nanoparticles) inside the channels of all the samples could be evidenced by TEM, XPS and XANES/EXAFS. In this sense, the surface Co/Si atomic ratios obtained by XPS were notably lower than the corresponding bulk Co/Si ratios obtained by ICP, indicating that the Co atoms are mostly incorporated inside the mesopores channels of the silica matrix. Nevertheless, by TEM, Co3O4 nanoparticles of small size segregated on the external surface of the silicate were also observed for the higher metal loadings. The temperature dependence of the magnetization performed for the Co/M(2.5) sample allowed to assign its superparamagnetic behavior to the presence of clusters and Co3O4 nanoparticles of very small size that grow inside the MCM-41 mesopores. Therefore, the analyses presented in this work indicate that a Co theoretical loading of 2.5 wt% leads to the formation of Co oxide nanospecies in the MCM-41 support with a particular superparamagnetic behavior. This sample with improved structural and magnetic properties result an attractive porous solid for drug hosting, to be applied in the field of the controlled release of medicaments.