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.