An appraisal of high-resolution scanning electron microscopy applied to porous materials

Nanoporous materials such as zeolites and mesoporous silica crystals have attracted a lot of attention particularly for the incorporation of various materials such as molecules or metal nanoparticles within the pores in order to develop marvelous functions with great potential for future applications. It is therefore essential to determine their structures and chemical compositions and to understand crystal growth processes for synthesizing crystals with an appropriate function. Recent progress in the performance of SEM is enormous especially in low energy imaging where we can now directly observe fine surface structures of porous materials even those that are electrical insulators. Furthermore, by precise filtration and detection of the energies of the emitted electrons we can obtain selective information of materials such as composition, location of particles inside or outside the pores etc. The physical processes and technologies behind this precise tuning of landing and detection energies for both impact and emitted electrons, respectively, are explained and illustrated using a number of porous materials including zeolite A, SBA-15, SBA-16, zeolite L, FDU-16 and Au@TiO2 ???rattle spheres,??? along with comparisons with other techniques such as atomic force microscopy (AFM) and transmission electron microscopy (TEM). We conclude that HRSEM is now an important technique for study in terms of both their structure and growth mechanisms. By using extremely low landing energies, advanced sample preparation techniques and through a thorough understanding of the physical processes involved, HRSEM is providing new and unique information and perspectives on these industrially important materials.