Direct observation of surface structure of mesoporous silica with low acceleration voltage FE-SEM

Recent progress of SEM technology enabled us to observe the ordered structure of mesoporous materials. Direct SEM observation of mesoporous materials can provide detail information on the external and internal structures, though it often faces charging problems of insulating frameworks and the structural collapse due to the electron beam irradiation. Low voltage SEM observation has become common both for top-surface imaging and reducing beam damage. In this work, high-resolution, direct SEM imaging of mesoporous silica powders and films was carried out under the condition of low voltages to characterize the surface structure that cannot be evaluated by TEM, XRD and N2 adsorption/desorption measurements. We synthesized SBA-15 powders using different aging temperatures, and observed the differences in surface structure. We also observed the surface structure of dip-coated SBA-16 films prepared with different template concentrations. The SBA-15 powder, which was aged at a lower temperature, was extremely beam sensitive, and the SBA-16 films prepared on glass substrates were heavily charging. For the SEM observation, we used beam deceleration and an improved scanning method to reduce the charging phenomenon and beam damage. We achieved this using a Hitachi S-4800 or an SU8000 FE-SEM, which both employ a snorkel type objective lens, a beam deceleration function and newly designed beam scan settings that combine slow and fast scanning techniques.