Effect of synthesis temperature on the ordered pore structure in mesoporous silica studied by positron annihilation spectroscopy

Abstract

Mesoporous silica with ordered pore structure was synthesized at various temperatures using TEOS as the silica source and P123 as the template. Both small angle X-ray scattering (SAXS) and high resolution transition electron microscopy (HRTEM) measurements verify the ordered pore structure of the synthesized SiO2. With synthesis temperature increasing, the pore structure has slight damage at 130°C, while it shows ordered structure again at 150°C. When the synthesis temperature increases to 180°C, the ordered pore structure is severely destructed. The change of pore structure is further confirmed by scanning electron microscopy (SEM) measurements. Positron lifetime measurements reveal four lifetime components in the synthesized mesoporous SiO2, and the two long lifetimes τ3 and τ4 correspond to the annihilation of o-Ps in the micropores and large pores of the material, respectively. The longest lifetime τ4 tends to increase slightly with increasing synthesis temperature. However, its intensity I4 shows an overall decrease with exception at 150°C. At the synthesis temperature of 180°C, the intensity I4 decreases drastically to about 17.5%. This indicates variation of the size and fraction of pores with increasing synthesis temperature, and the pore structure is seriously destructed at 180°C. By comparing with the N2 adsorption–desorption measurements, it was found that the Goworek's model is more suitable for the size estimation of cylindrical pores from the o-Ps lifetime, while Dull's and Ito's model is appropriate for the rectangular and spherical pores, respectively.