SEM images of the Ti3AlC2 and MXenes Ti3C2Tx powder are shown in Fig. 1 (a, b). The Ti3AlC2 raw material had the typical structure of a layered MAX phase (Fig. 2a). After removing the Al atomic layers from the Ti3AlC2, the resulting MXene, Ti3C2Tx, had an accordion-like multi-layered structure (Fig. 1b). As seen in the SEM image of the d-Ti3C2Tx (as shown in Fig. 1c), the MXene (Ti3C2Tx) was delaminated into single- or few-layered d-Ti3C2Tx nanosheets. The thickness of the exfoliated MXene nanosheets was characterized by atomic force microscope (AFM) (Fig. 1d). AFM analysis indicated that the nanosheets had a thickness of about 1.5 nm (Fig. 1d inset), confirming the d- Ti3C2Tx was composed of a single or a few-layers of sheets. Moreover, the obtained, dark green MXene nanosheet solution was a stable dispersion in water (Fig. 1e) and an exhibited an obvious Tyndall effect. The XRD patterns of the MAX phase, MXene, and exfoliated MXene nanosheets are shown in Fig. 1f. Compared with MAX phase, the characteristic peak of Ti3AlC2 disappeared in the MXene pattern and a new diffraction peak appeared at 6.9° that belonged to the (002) orientation of MXene Ti3C2Tx [42]. After exfoliation, the (002) diffraction peak significantly shifted from 6.9° to a smaller angle of 5.86°, indicating that Ti3C2 was effectively delaminated into single- or few-layered nanosheets.
Figure 2a shows a schematic of the highly sensitive MXene/Cotton fabric strain sensor fabrication process. First, surface-modified cotton fabric was obtained by immersing clean fabric into a PEI solution for 24 h, followed by drying the fabric in an oven at 60°C (Fig. 2b). Then, the PEI-modified fabric was dipped into the MXene solution for a given time. The MXene was adsorbed onto the fabric fiber due to the electrostatic attraction between the MXene and the positively charged PEI on the fabric (Fig. 2c) during impregnation. Finally, the ends of the obtained MCF were coated with silver paste and attached to copper tape electrodes, and the material packaged within PDMS silicone rubber (Fig. 2d).
Figure 2e shows the FTIR spectra of the fabric, [email protected], and MCF, which gave insights into the functional groups present in the materials. As seen in Fig. 1f, the broad peak around 3200 ~ 3500 cm− 1 was attributed to the O-H/N-H stretching from the PEI and cotton cellulose of the fabric. The peak near 2910 cm− 1 was assigned to the C-H stretching vibration band. The characteristic peaks at 1652, 1430 ~ 1310, and 1017 cm− 1 were assigned to C = O stretching, C-H bending, and C-O stretching [43], respectively. These characteristic absorption peaks weakened obviously after adsorption of the MXene. Therefore, it could be inferred that the fabric was wrapped in MXene nanosheets. To verify this, the sample also was characterized by XRD (Fig. 2f). It can be found that in addition to the characteristic diffraction peak of the cotton fabric, a new peak appeared at 5.86° which was attributed to d-Ti3C2Tx. In summary, the analytical results provided good evidence that the fabric surface was successfully capped with MXene d-Ti3C2Tx nanosheets. As can be seen from the TGA curves in Fig. 2g, all samples completely evaporated at temperatures higher than 400°C, and the MCF had the most ash residue. However, the thermal decomposition temperature was slightly lower due to the surface effect of nanomaterials.
SEM images of the clean, conductive cotton fabric and the MCF strain sensor are shown in Fig. 3. Figure 3a shows the morphology of the clean cotton fabric at different magnifications. It can be seen that the fabric consisted of woven cotton fiber bundles, and the surface of fibers were relatively smooth. Figure 3(c-e) shows the SEM imagess of the conductive MCF from different angles after dipping the fabric in the MXene suspension and drying. The smooth cotton fiber surface became rough after the flexible 2D MXene nanosheets decorated the fiber surface, and the assembled MXene nanosheets were observed on the cotton fibers. Hence, MXene decorated cotton fibers with a core-shell structure were obtained. Figure 3g is an SEM image of a MXene wrapped fiber and the corresponding elemental mapping. It was observed that Ti, C, and O were uniformly distributed on the cotton fiber surface, indicating the fiber was tightly wrapped by a layer of MXene nanosheets. Figure 3f shows that the conductive cotton fibers were well encapsulated by the PDMS layers that play a protective and restrictive role for the inner conductive cotton fibers, and the fabric structure was maintained after the encapsulation process.
Figure 4(a,b) shows the resistance change rates (ΔR/R0) at different strains with repeated loading-unloading cycles under a tensile speed of 4 mm/min. During the stretching process, the tension led to a decrease in the yarn spacing, which resulted in the formation of conductive networks and a decrease in the resistance. The results showed that the greater the applied tension, the greater the measured change in the resistance rates. In addition, the corresponding ΔR/R0 values were almost constant after different loading-unloading cycles, which indicated the high cyclic stability of the MCF strain sensor material. Figure 3c shows the tensile stress-strain curves at different strains with repeated loading-unloading cycles. It could be found that the strain of the sensor returned to the initial value after five cycles under different strains. These results indicated that the MCF strain sensor exhibited excellent cyclic stability performance in mechanics. Figure 4d gives the ΔR/R0 of MCF strain sensor at different stretching frequencies under the same strain of 9%. These data suggested that the sensor also had a steady dynamic response to frequency changes from 0.01 to 0.375 Hz. The durability of the strain sensor under a tensile strain of 6% at a 150 mm/min strain rate is shown in Fig. 4e. The strain sensor had a very stable signal output after 500 cycles of loading-unloading tests, showing excellent repeatability, which revealed that the different components in the MCF strain sensor were highly compatible, structurally stability, and able to stretch and recover these properties during loading and unloading cycles.
Figure 4g is the schematic diagram of the formation of the conductive pathways in the stretching direction (L direction) and vertical direction (T direction). Figure 4g1 is the cross section of fiber bundle in an unstretched state, g2 is a stretched state in the L direction, and g3 is a stretched state in the T direction. Stretching the fabric in the L direction decreased the yarn spacing and made more contacts elsewhere on the surface of parallel fiber, which led to the formation of a conductive network between the conductive strands of yarn (g2) and the measured decrease in resistance. Meanwhile, stretching in the T direction increased the yarn spacing which led to the reconstruction of the conductive networks and the increase in resistance (g3). In both cases, the formation of conductive networks in L direction played a major role in the resistance variation.
As mentioned above, the MCF strain sensor possessed high sensitivity under different strains. A series of tests were carried out to detect different human motions to verify the feasibility of using this strain sensor as a wearable electronic device. As a result, the ΔR/R0 value of MCF strain sensor increased and then returned to its initial state when the wearer tautologically bent his or her finger and leg (Fig. 5a and 5b, respectively), achieving detection from small to large human body movements. Figure 5c demonstrates the detection of eye movements by attaching the MCF strain sensor to the corner of eye. The MCF sensor accurately recorded strain changes and showed regular variations in the resistance due to the repeated eye motions during blinking. When the sensor was attached to the neck, it showed a repeatable electrical signal instantaneously for twisting motions of the neck joint, as shown in Fig. 5d. Furthermore, Fig. 5e and 5f show the stable response of the strain sensor under different pressures ranging from 0.98 to 3.92 kPa. Therefore, the MCF strain sensor may have potential application in wearable devices to monitor joint movements during human motion and health monitoring.