Enhanced Cross-Sectional Imaging of the Selective Layer of Reverse Osmosis Membranes

The integrity and morphology of the selective layer of RO membranes significantly influence transport phenomena and membrane performance, motivating its cross-sectional imaging. Focused ion beam scanning electron microscopy (FIB-SEM) is a less complex technique for cross-sectional imaging that offers a broader field of view and a greater depth of focus, albeit with less resolution than transmission electron microscopy (TEM). In this study, we introduce and demonstrate a new surface-milling approach to improve the FIB-SEM imaging of RO membranes. We develop a bilayer conductive Pt coating topped with a protective carbon deposit; this approach avoids the need to cryo-fracture the membranes, which overcomes drawbacks associated with TEM and conventional FIB-SEM cross-sectional milling (e.g., curtaining and polymer melting). Applied to a commercial RO membrane, the method enabled robust quantification of key structural features and clear visualization of void spaces within the ridge-and-valley structure of the selective layer and pores in the uppermost portion of the support layer. The bilayer platinum coating enhances the delineation of the upper boundary of the selective layer, enabling rapid and accurate identification. With a field of view more than 20 times larger than TEM, FIB-SEM imaging captures a substantially greater cross-sectional area, yielding more representative measurements.