3.2 Characterizations of tubular MXene/SS membranes
The microstructures of the tubular stainless steel substrates and the MXene membranes prepared by electrophoresis were then characterized by SEM. Figure 4A shows the bare commercial stainless steel substrate (SSbare) without pretreatment, where the surface was rough with amounts of large holes (diameter greater than 10 μm). It was difficult to prepare continuous MXene membranes on such substrate without any pretreatment. In order to reduce the surface roughness, the substrate was polished and the corresponding microstructure is shown in Figure 4B. Additionally, the polished stainless steel substrate was modified with CNTs (SSCNTs) by vacuum suction to cover the large surface pores. As shown in Figure 4C, the filamentous CNTs were filled in the big pores of the surface of the porous substrate without blocking the ventilation channels of the substrate itself nor reducing the gas permeability (see following data in section 3.3). From the characterization of the pore size distribution, as shown in Figure 4D, the average aperture of both the substrates before and after modification was around 2 μm. After modification with CNTs, most pore sizes did not change significantly. That is to say, only the surface pores of the substrate were filled by CNTs, while most pores of the bulk substrate were not blocked, indicating that the modification with filamentous CNTs showed a successful pretreatment for the substrate aiming for consequent membrane assembly. Figure 4E-F compared the surface structure of the MXene membrane assembled on the substrate before and after modification. It could be clearly seen that a large number of defects existed on the MXene membrane/SSbaredue to the rough substrate with large pores (Figure 4E). On the contrary, the MXene/SSCNTs membrane had a comparatively flat surface, and there were no obvious large holes or pinhole defects (Figure 4F). Moreover, the surface view and cross-section of the MXene/SS membranes grown with various electrophoresis time were shown in Figure 5, all the MXene membranes were relatively flat without obvious defects, and the membrane thickness was positively correlated with the electrophoresis time, where the thickness increased from ~100 nm to ~330 nm when the assembly time increased from 1.5 min to 4 min. The fast preparation process of MXene membrane by electrophoretic deposition had the advantages of high efficiency, as well as favorable conditions for membrane scale-up production.