Porous silica structures have many technical applications, like in heterogeneous catalysis and size-exclusion chromatography. To understand and improve the material’s properties, a precise characterization of the pore network is indispensable. Common approaches to investigate porous materials on the nm- and µm-scale are averaging sorption/intrusion techniques or tomography. In this study, we uniquely combine Ga intrusion with electron tomography (ET) of mesoporous silica to obtain optimized pore characteristics and to compare the different methods.

ET allows for precise 3D reconstruction from tilt series with nm-sized spatial resolution of porous samples to access important pore characteristics such as size distribution, tortuosity, and connectivity. By utilizing 360° ET of a pillar-shaped specimen cut with a focused ion beam (FIB), no missing wedge artifacts in the 3D reconstruction are present. But using electrons for imaging of porous silica structures can be challenging. Silica is quite electron-beam sensitive and non-conductive, which leads to charging effects. During FIB pillar preparation, charging leads to drift of the sample and uneven milling by Ga ions. During ET experiment, charging caused by the e-beam results in bending and structure deformation, which strongly affects 3D reconstruction. Moreover, the build-up of carbon contamination during tilt series acquisition worsens the image contrast over time, due to the low atomic number difference of Si, O and the C contamination in high-angle annular dark-field (HAADF) scanning transmission electron microcopy (STEM) imaging mode.

To avoid these drawbacks, the silica samples were intruded with Ga to fill the mesopores of roughly 20 nm in size. Ga increases the sample integrity, mechanically stabilizes the pillar, and forms a conductive path through the sample, eliminating charging effects during preparation and ET. Additionally, the image contrast is enhanced, which significantly improves the quality of 3D reconstruction, also reducing the influence of contamination. Therefore, Ga intrusion can improve ET investigation, and vice versa, ET helps to understand the process of Ga intrusion. For a benchmark comparison, the mesopores of the silica structures were additionally investigated with N2 physisorption, evaluated with the nonlocal density functional theory (NLDFT) method.