Generalized free volume diagrams from positron annihilation data for the study of nanoporosity in silica gel specimens

Abstract 

Generalized positronium annihilation diagrams are proposed in this paper. In particular, we point out how the free volume plots arising from a very simple model can be utilized to determine the electronic layer thickness ΔR in which the ortho-positronium (o-Ps) annihilation events occur. We comment on the relevant role played by the Positronium Annihilation Lifetime Technique (PALT) as a powerful tool for studying diverse properties of materials and discuss the range of pore size distributions detectable by this technique. These considerations have been applied to the study of different silica gel specimens, prepared by the sol-gel technique. We found that the free volume curves fit well to the experimental data provided the thickness parameter ranges between ΔR_min= 0.71 nm and ΔR_max= 2.3 nm and also observed a bimodal pore size distribution which corresponds to the existence of two types of nanopores present in the silica gel samples. The variation of the mean free volume radius R with the chemical composition and the correlation between the positronium annihilation parameters and the potential catalysis properties of the modified silica gels, are also presented.