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.