Stretchable Ti3C2Tx MXene/Carbon Nanotube Composite based Strain sensor with Ultrahigh Sensitivity and Tunable Sensing Range

Transcription

1 Supporting Information Stretchable Ti3C2Tx MXene/Carbon Nanotube Composite based Strain sensor with Ultrahigh Sensitivity and Tunable Sensing Range Yichen Cai, a Jie Shen, c Gang Ge, a Yizhou Zhang, a Wanqin Jin, c Wei Huang, a Jinjun Shao, a* Jian Yang, b* Xiaochen Dong a,d* a Key Laboratory of Flexible Electronics (KLOFE) and Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (Nanjing Tech), Nanjing , China. b College of Materials Science and Engineering, Nanjing Tech University (Nanjing Tech), Nanjing China. c State Key Laboratory of Materials-Oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (Nanjing Tech), Nanjing , China d School of Physical and Mathematical Sciences, Nanjing Tech University (Nanjing Tech), Nanjing , China. To whom correspondence should be addressed: (X. C. Dong) iamxcdong@njtech.edu.cn, (J. Yang) yangjian1976@163.com, (J. J. Shao) iamjjshao@njteche.edu.cn. 1

2 Normalized absorbance mg/ml Wavelength (nm) Figure S1. UV-vis spectra of Ti3C2Tx dispersion with concentration of 0.02 mg/ml. 2

3 Current ( A) % 1% 2% 3% 4% 5% 10% 20% 30% Voltage (V) Figure S2. Current-voltage curves of Ti3C2Tx MXene/CNT strain sensor at different strains. 3

4 (R-R 0 )/R MXene 1 -CNT 1 RGO 1 -CNT Strain (%) Figure S3. Typical relative resistance-strain curves of RGO1-CNT1 and Ti3C2Tx MXene1-CNT1 with 10 spray coating cycles at a stretching rate of 5% min -1. The inset shows the curve within 50% strain. As shown in Figure S3, RGO 1-CNT1 based strain sensor has higher stretchability and lower sensitivity (GF~30.54) than Ti3C2Tx MXene1-CNT1 (GF~64.60) based sensor. While most of the deformations caused by human-motion are not more than 50%. The sensitivity within 50% is more valuable. Thus, Ti3C2Tx MXene1-CNT1 with higher GF based strain sensor is more favorable for monitoring human physiological activities. (a) (b) 500 nm 500 nm Figure S4. Cross-sectional SEM images of sandwich-like Ti3C2Tx MXene1-CNT1 layers (thickness < 1μm) and RGO1-CNT1 layers (thickness >1μm). 4

5 (a) x 0 x 1 y 0 y 1 (b) Initial state a 0 MXene RGO Stretched state a 1 b 0 b 1 Initial state Stretched state Figure S5. The mechanism diagram of pure Ti3C2Tx MXene and RGO sensing layers during stretching (x and a are the length of sensing layers; y and b are the thickness of the sensing layers). Different stretchabilities and GFs depend on two factors: the intrinsic mechanical rigid structure of the sensing material and the structural engineering of the strain sensor mainly caused by the difference in the thickness of the sensing materials. RGO nanosheets have higher sheet resistance than Ti3C2Tx MXene. 1,2 However, the Young s modulus of Ti3C2Tx (3.52 GPa) 3 and RGO (35 GPa) 1 films are different, which indicates MXene nanosheets are easier to crack leading larger resistance changes. More importantly, the density of RGO is smaller than that of Ti3C2Tx MXene and the device fabricated with the same concentration is thicker than that based on Ti3C2Tx MXene (Figure S4). Especially, the epidermal sensors need ultra-thin thickness that can be perfectly amounted on human skins, otherwise the output signal will be distorted. The mechanism diagram of single sensing materials during stretching indicates that RGO nanosheets slide between each other, extend longer, and connect the conduction pathways, resulting in a lower sensitivity and larger stretchability (Figure S5). 5

6 The strain sensor prepared by directly spraying mixing materials was defined as M-S and the one fabricated by LBL method as L-S. Electrical resistance response curves as a function of the applied strain for L-S and M-S (MXene1 - CNT1) spray coated for 10 cycles are showed below: (R-R 0 )/R L-S M-S Strain (%) Figure S6. Typical relative resistance-strain curves of M-S and L-S at a stretching rate of 5% min -1. The inset shows the curves within 50% strain. Figure S6 shows both M-S and L-S have corresponding responses and output signals. Noted that the electrical responses of the M-S exhibits a larger reliable stretchability than L-S. The M-S based strain sensor has a lower sensitivity (GF~12.83) than L-S based sensor (GF~64.60) within 50% strain. 6

7 Electrode Electrode Electrode Electrode (a) M-S (b) CNT MXene Tunnelling gap L-S Figure S7. Sensing mechanism (tunneling effect) of (a) M-S and (b) L-S based strain sensors under stretched state (cross-sectional view). MXene is similar to graphene, aggregation and face-to-face self-restacking are usually inevitable during drying and sensor fabrication processes owing to the strong van der Waals interaction between adjacent nanosheets. Self-restacking of MXene nanosheets has been demonstrated to significantly decrease the intrinsic performances. 6 Therefore, the LBL structural engineering was introduced making one-dimensional (1D) CNTs an important role to act as electrically conductive bridges, increasing the interlayer spacing of MXene, facilitating sliding between MXene sheets and greatly shortening the transportation and diffusion distance of electronics. The self-restacking of MXene nanosheets were prevented resulting high GFs of strain sensors. While random physical mixing between MXene and CNTs did not efficiently prevent their self-restacking. What is more, directly mixed composites commonly display irregular and irreversible structural changes during the fast strain-releasing processes, resulting in overall decrease in cyclability and durability. 7

8 A schematic diagram of the strain sensors based on M-S and L-S is shown in Figure S7. Physically mixed MXene nanosheets lead to self-stacking bulked nucleation, resulting in smaller tunneling distance (Figure S 7a). The measure of device sensitivity, GF can be defined with a formula as follows: (1) Where ε is the strain, R0 is electrical resistance under no strain, and d is the tunneling distance. 5 When the same strain is applied, the higher sensitivity corresponds to a more significant tunneling distance change (Δd). Since the R0 of M-S is similar to L-S and ΔR of M-S is smaller than L-S at the same strain (Figure S6). Moreover, the strain ε = Δd/d0, the initial distance between the physical mixed MXene nanosheets (d0) was smaller, which is in accordance with the schematic diagram in Figure S7. 8

9 (a) (b) (c) 500 nm 1 µm 2 µm Figure S8. SEM images of sandwich-like Ti3C2Tx/CNT layers upon stretching. 9

10 (a) MXene 0% 100% Gap CNT (b) R M&C (MXene & CNT) R C (Contact resistance) R M&C (MXene & CNT) Figure S9. (a) Schematic illustration of sensing mechanism of Ti3C2Tx MXene/CNT strain sensor upon stretching. (b) Schematic illustration showing the resistance model of a sensing unit. 10

11 References for Supporting Information: 1. Li, D.; Müller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Processable aqueous dispersions of graphene nanosheets. Nat. Nanotech. 2008, 3, Ling, Z.; Ren, C. E.; Zhao, M.-Q.; Yang, J.; Giammarco, J. M.; Qiu, J.; Barsoum, M. W.; Gogotsi, Y. Flexible and conductive MXene films and nanocomposites with high capacitance. PNAS. 2014, 111, Ling, Z.; Ren, C. E.; Zhao, M.-Q.; Yang, J.; Giammarco, J. M.; Qiu, J.; Barsoum, M. W.; Gogotsi, Y. Flexible and conductive MXene films and nanocomposites with high capacitance. PNAS. 2014, 111, Zhao, M. Q.; Ren, C. E.; Ling, Z.; Lukatskaya, M. R.; Zhang, C.; Van Aken, K. L.; Barsoum, M. W.; Gogotsi, Y. Flexible MXene/carbon nanotube composite paper with high volumetric capacitance. Adv. Mater. 2015, 27, Zhao, J.; Wang, G.; Yang, R.; Lu, X.; Cheng, M.; He, C.; Xie, G.; Meng, J.; Shi, D.; Zhang, G. Tunable piezoresistivity of nanographene films for strain sensing. ACS Nano 2015, 9,