Pore structure of mesoporous silica (KIT-6) synthesized at different temperatures using positron as a nondestructive probe

Abstract

Ordered mesoporous SiO2 (KIT-6) was synthesized using triblock copolymer P123 as the structure template and tetraethyl orthosilicate (TEOS, C8H20O4Si) as silica source. Small-angle X-ray scattering and high resolution electron microscope measurements indicate the 3d cubic Ia3d symmetry of the pore structure of KIT-6 synthesized at 30–120 °C. When the synthesis temperature increases to 180 °C, the order of pores was deteriorated. The pore size was estimated by nitrogen adsorption/desorption measurements, which increases from 3.8 nm to 18.5 nm as the synthesis temperature increases from 30 °C to 180 °C. With the increase of mesopore size, the pore wall thickness shows continuous decrease. Positron annihilation lifetime spectra was measured for the synthesized KIT-6. The lifetime spectra can be resolved to two short and two long lifetime components. The two long lifetimes τ3 and τ4 correspond to o-Ps lifetime in micropore and mesopores, respectively. The size of mesopores was estimated from the o-Ps lifetime by using Goworeck’s model for cylindrical pores, which shows continuous increase with synthesis temperature, and is consistent with the N2 adsorption/desorption measurements. The size of micropore has no change with increasing synthesis temperature. However due to the decrease in wall thickness, the number of micropores in the wall shows subsequent decrease. In addition, the second lifetime component τ2 is also found to be a sensitive parameter for the pore size, which shows the same trend as lifetime τ4 with increasing synthesis temperature.