FIGURE 6 XRD patterns of the tubular MXene/SS membranes with different electrophoresis time.
In this work, the MXene/SS2.5 membrane was chosen for EDS characterization in order to confirm the element distribution. The SEM image of a cross-section of the MXene/SS2.5 membrane was shown in Figure 7A. Figure 7B-E represented the EDS images of C, Ti, O, and F, respectively, indicating the uniform distribution of each element. There was no obvious cross-sectional distribution in Figure 7B because the electric conductive adhesive sticking to the MXene membrane also contained C element. Subsequently, XPS and FTIR analysis of the tubular MXene/SS membranes were performed to investigate the chemical bonds in the membranes. As shown in Figure 8A, the MXene membrane was primarily consisted of four elements (C, Ti, O, and F), while there were no peaks belonging to Al element, indicating that it had been etched off thoroughly in the MXene membrane, which was also consistent with the XRD characterization results. A more detailed high-resolution XPS spectra analysis of the four elements is shown in Figure 8B-E. Five different chemical groups appeared in the spectrum of Ti element, namely Ti-C (454.8 eV), Ti2+ (455.7 eV), Ti3+(456.5 eV), Ti-O (458.1 eV), and Ti-F (459.6 eV). It should be noted that the chemical bond of Ti-O accounted for about 5.5% of the total Ti element. That’s because the MXene nanosheets were prepared by chemical etching, it was inevitable that a small amount of Ti atoms would be oxidized.61,62 The high-resolution XPS spectra of O1s could be resolved into four components as 529.4, 530.0, 531.3, and 532.5 eV, and their corresponding chemical bonds were O-Ti, O-Ti/OH, O-C/OH, H2Oad, respectively. The C1s spectrum also consisted of four sub-peaks, representing chemical bonds of C-Ti (281.7 eV), C-C (284.7 eV), C-O(285.8 eV), and C-O=C/C-F (288.5 eV), which were consistent with the literatures.63-65 The fitted curves in the F1s spectrum correspond to two chemical bonds of F-Ti (684.7 eV) and F-C (686.4 eV), respectively. In this work, the functional groups that combined with oxygen on the surface of the prepared membranes were further characterized to use FTIR. As shown in Figure 8F, there were two stretching vibration peaks in the FTIR spectra of the tubular MXene/SS membrane, which belonged to -OH at 3457 cm-1 and C=O at 1641 cm-1. -OH denoted the -OH functional group on the membrane surface or the adsorbed water between the interlayers, while C=O represented the terminal functional groups at the edge of the MXene nanosheets.49