Search

Ordered mesoporous SiO 2 (KIT-6) was synthesized using triblock copolymer P123 as the structure template and tetraethyl orthosilicate (TEOS, C 8 H 20 O 4 Si) 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 τ 3 and τ 4 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 N 2 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 τ 2 is also found to be a sensitive parameter for the pore size, which shows the same trend as lifetime τ 4 with increasing synthesis temperature. [ABSTRACT FROM AUTHOR]

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 SiO 2 . 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 SiO 2 , and the two long lifetimes τ 3 and τ 4 correspond to the annihilation of o-Ps in the micropores and large pores of the material, respectively. The longest lifetime τ 4 tends to increase slightly with increasing synthesis temperature. However, its intensity I 4 shows an overall decrease with exception at 150 °C. At the synthesis temperature of 180 °C, the intensity I 4 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 N 2 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. [ABSTRACT FROM AUTHOR]