Fig. 4: SEM images and XRD patterns of ZSM-5 zeolite membranes synthesis by one-layer (Series SLS, M2), Dual-layer (Series DLS, M5 and M7) and Triple-layer (Series TLS, M9 and M11) seeding methods, (♦) are from the alumina substrate peaks
This is due to the fact that large seeds accumulate less in density at the top layer, resulting to form larger gaps and lower space limitation suppression. This allows large seeds to grow in different directions with a slower rate than that of small seeds [11]. While, the small seeds, due to their lower inter-crystalline gaps, mainly grow along the direction perpendicular to the support surface with a fast growth rate resulting in the thicker zeolite layer [14]. It can be concluded that the seed size at the top layer determines the final thickness of the membrane. The SEM images in Figures 4 and S2 indicate also that the membranes in TLS series have more uniform and smooth surfaces than those in DLS and SLS series. In fact, intergrown of the penetrated small seeds from the top layer into the spaces and inter-crystalline gaps at the bottom layer densifies the density of the zeolite layer resulting in the more uniform and continuous membrane surface.