Positron annihilation study of iron oxide nanoparticles in mesoporous silica MCM-41 template

The subject of investigation were the samples obtained by impregnation of MCM-41 template with an aqueous solution prepared from Fe(NO3)(3)center dot 9H(2)O. As a result of such a procedure, iron oxides deposits on MCM-41 were formed. The Mossbauer studies revealed an occurrence of Fe3O4 nanocrystallites in the both fern- and superparamagnetic states. Almost 80% small particles exist in a paramagnetic state. The positron annihilation lifetime spectroscopy (PALS) spectra were measured in air or in vacuum. The long-lived ortho-positronium (o-Ps) components and their intensities are time dependent due to air molecules interaction with iron oxide nanocrystallites and silica walls surfaces. The adsorption of air on the nanocrystallites surface causes a total screening of their surface and a raise of the o-Ps lifetime values. Observed anti-quenching effect is a result of competition of two phenomena: practically switching off a pick-off mechanism related to interaction of o-Ps with magnetite nanoparticles and considerably weaker usual quenching by paramagnetic oxygen molecules.