Positronium study of porous structure of sol–gel prepared SiO2: influence of pH

Positron annihilation lifetime and Doppler broadening of annihilation line techniques have been used to obtain information about the small-pore structure of SiO2 prepared by the alkoxide method in different experimental conditions. Samples prepared in strong acidic environment (pH = 2) contain only small pores with mean radius R∼5 Å, while those prepared at pH = 6 and pH = 9 contain pores of two sizes, R∼5 and R∼17–26 Å. The influence of pH, water/alkoxide molar ratio and temperature of heat-treatment of the samples on their pore structure has been studied.