Journal Article

Study on the effect of atmospheric gases adsorbed in MnFe 2 O 4 /MCM-41 nanocomposite on ortho -positronium annihilation

Budzyński, Mieczysław, Gac, Wojciech, Surowiec, Zbigniew, Wiertel, Marek

2015

Overview

In this paper, results of positron annihilation lifetime spectroscopy (PALS) studies of MnFe 2 O 4 /MCM-41 nanocomposites in N 2 and O 2 atmosphere have been presented. In particular, the influence of manganese ferrite loading and gas filling on pick-off ortho -positronium ( o -Ps) annihilation processes in the investigated samples was a point of interest. Disappearance of the longest-lived o -Ps component with τ 5 present in the PAL spectrum of initial MCM-41 mesoporous material in the PAL spectra of MnFe 2 O 4 -impregnated MCM-41 measured in vacuum is a result of either a strong chemical o -Ps quenching or the Ps inhibition effects. The intensity I 4 of the medium-lived component initially increases, reaching a maximum value for the sample with minimum manganese ferrite content, and then decreases monotonically. Analogous dependence for the intensity I 3 of the shortest-lived component shows a maximum at higher MnFe 2 O 4 content. Filling of open pores present in the studied nanocomposites by N 2 or O 2 at ambient pressure causes partial reappearance of the τ 4 and τ 5 components, except a sample with maximum ferrite content. The lifetimes of these components measured in O 2 are shortened in comparison to that observed in N 2 because of paramagnetic quenching. Anti-inhibition and anti-quenching effects of atmospheric gases observed in the MnFe 2 O 4 /MCM-41 samples are a result of neutralization of some surface active centers acting as inhibitors and weakening of pick-off annihilation mechanism, respectively.