From Coated to Uncoated: Scanning Electron Microscopy Corrections to Estimate the True Surface Pore Size in Nanoporous Membranes

Scanning electron microscopy (SEM) is the premier method for characterizing the nanoscale surface pores in ultrafiltration (UF) membranes and the support layers of reverse osmosis (RO) membranes. Based on SEM, the conventional understanding is that membranes typically have low surface porosities of <10%. We demonstrated and quantified how the high acceleration voltage during SEM imaging and the sputtered-metal coating thickness required for SEM systematically underestimate membrane surface porosity and pore size. We showed that imaging a commercial UF membrane at 1, 5, and 10 kV reduced the measured surface porosity from 10.3 ± 0.3% (1 kV) to 6.3 ± 0.4% (10 kV), while increasing the Pt coating thickness from 1.5 to 5 nm reduced the porosity by 54% for the UF membrane (12.9 ± 0.9% to 5.8 ± 0.6%) and 46% for an RO support (13.1 ± 0.6% to 7.0 ± 0.2%). To account for the coating thickness, we then developed a digital correction method that simulates pore dilation, enabling the surface pore structure to be estimated for uncoated membranes. Pore dilation yielded uncoated surface porosity values of 23% for the UF membrane and 20% for the RO support, which are approximately 3-fold greater than the directly observed values for a typical coating thickness of 4 nm. Similarly, mean pore diameters for uncoated membranes were 2-fold greater for the UF membrane and 1.5-fold greater for the RO support than directly observed. Critically, the dilation-derived pore-size distributions agreed with low-flux dextran-retention measurements fitted with the Bungay-Brenner model. Our results suggest that the surface porosities and pore sizes of nanoporous membranes are much larger than previously understood, which has major implications for structure/transport relationships. For future nanoscale pore analysis of membranes (and other nanoporous materials), we recommend low acceleration voltage (1 kV), minimal coatings (1-2 nm), and digital dilation to account for coating-induced artifacts.