Fig. 3. SEM and AFM characterization of different membranes. SEM images of the COF-deposited PES substrate: (a) surface and (b) cross section. SEM images of the PA composite membrane: (c) surface, (d) cross section at low magnification and (e) cross section at high magnification. (f) Schematic structure of the PA composite membrane. AFM topography images of (g) the COF-deposited PES substrate and (h) the PA composite membrane (scanning area was 5 μm × 5 μm). (i) The root mean square roughness and the surface area increase of the different membranes (the results were obtained from AFM measurements).