Changes on structural and magnetic properties of maghemite nanoparticles during their coverage with MCM-41

Abstract

This work reports a simple synthesis of a mesoporous ordered silica structure (MCM-41) grown up around pre-synthesized maghemite nanoparticles. A composite with magnetic response and high specific surface area was obtained adding maghemite nanoparticles coated with oleic acid in the synthesis gel of MCM-41 and finally calcining in air. Different characterizations techniques such as magnetic measurements, Mössbauer spectroscopy at 25 and -260 °C and transmission electron microscopy were used to study the magnetic and structural changes of the initial maghemite nanoparticles produced during its coverage with the mesoporous silica structure. The presence of the mesoporous structure inhibits the expected transformation from maghemite to hematite when the system was calcined in air at 510 °C. It was established that a sintering of the nanoparticles during the calcination step occurs, leading to a magnetization saturation value similar to that of the bulk compound. In order to evaluate the capability as adsorbent, the magnetic composite was subjected to a surface amino-functionalization and then tested for Cr(VI) and Cu(II) adsorption from water.