3.1 Characterizations of MXene nanosheets
The gas separation performance of the membrane depends on the quality of the building blocks of MXene nanosheets. In this work, the most classical acid etching synthesis method was used to synthesize MXene nanosheets. The microstructure of the MAX bulk phase as raw material, acid-etched Ti3C2TX(MXene) powder and dispersive MXene nanosheet could be observed in Figure 3 A-C. The MXene powder exhibited an obvious furrow structure after acid etching, while the AAO substrate could be seen clearly through the electronic transparent MXene nanosheet upon it, indicating that the nanosheet is very thin. The AFM result of Figure 3D showed the thickness of the MXene nanosheet was about 1.5 nm, which was close to the theoretical thickness of a monolayer MXene nanosheet (1.0 nm).55 Since there might be adsorbed impurities such as water on the nanosheet, the nanosheet with the such thickness could be considered a monolayer.56 In this work, TEM and SAED analysis of selected electron diffraction were also carried out on the MXene nanosheets. Figure 3E showed that the nanosheets were extremely transparent under the irradiation of an electron beam, which also indicated that the nanosheets were extremely thin. As can be seen in Figure 3F, the synthesized nanosheets had a hexagonal structure on the basal plane with high crystallinity, indicating that the nanosheets were of good quality. The XRD patterns of the Ti3AlC2 MAX powder and synthesized MXene nanosheets are shown in Figure 3G. In contrast to the Ti3AlC2 MAX, the diffraction peak in the (104) plane located at 39° did not exist in that of Ti3C2TX MXene, indicating that the Al layer was successfully removed by etching.49,57 More importantly, the main diffraction peak located at crystal plane 002 shifted from 9.44° of Ti3AlC2 to 6.58° of etched Ti3C2TX after acid etching treatment, and the shape of the peak was extremely sharp, showing excellent crystallinity. To understand the lateral size of the synthesized nanosheet and the stability of the nanosheet solution, the synthesized MXene nanosheet solution was characterized by a Malvern nanoparticle analyzer. As shown in Figure 3H, the lateral size of the nanosheet is roughly distributed from 1 to 6 μm, and the predominant size was about 3 μm, indicating a relatively large lateral size. In addition, Figure 3I showed that the Zeta potential of the MXene nanosheet solution was -47 mV. It is generally believed that if the absolute value of Zeta potential is greater than 30 mV, the nanosheet solution system is relatively stable.58 Therefore, the MXene nanosheet solution prepared in this work was exceptionally stable and could be well used for subsequent electrophoretic membrane preparation.