Carbon Nanotube Composite Based

Subscriber access provided by READING UNIV

Article

Stretchable Ti3C2Tx MXene/Carbon Nanotubes Composite Based Strain Sensor with Ultrahigh Sensitivity and Tunable Sensing Range Yichen Cai, Jie Shen, Gang Ge, Yizhou Zhang, Wanqin Jin, Wei Huang, Jinjun Shao, Jian Yang, and Xiaochen Dong ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b06251 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 5, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Nano is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Stretchable Ti3C2Tx MXene/Carbon Nanotubes 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 Donga,d* a

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials

(IAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211800, China. b

College of Materials Science and Engineering, Nanjing Tech University

(NanjingTech), Nanjing 211800. China. c

State Key Laboratory of Materials-Oriented Chemical Engineering, Jiangsu National

Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), Nanjing 211800, China d

School of Physical and Mathematical Sciences, Nanjing Tech University

(NanjingTech), Nanjing 211800, China. KEYWORDS: MXene; carbon nanotubes; ultrasensitivity; artificial electronic skin; controllable sensing range ABSTRACT:

1 ACS Paragon Plus Environment

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 30

It remains challenging to fabricate strain-sensing materials and exquisite geometric constructions for integrating extraordinary sensitivity, low strain detectability, high stretchability, tunable sensing range, thin device dimensions into a single type of strain sensor. A percolation network based on Ti3C2Tx MXene/carbon nanotubes (CNTs) composites was rationally designed and fabricated into versatile strain sensors. This weaving architecture with excellent electric properties combined the sensitive two-dimensional (2D) Ti3C2Tx MXene nanostacks with conductive and stretchable one-dimensional (1D) CNTs crossing. The resulted strain sensor can be used to detect both tiny and large deformations with ultralow detection limit of 0.1% strain, high stretchability (up to 130%), high sensitivity (gauge factor up to 772.6), tunable sensing range (30% to 130% strain), thin device dimension (< 2 µm) as well as excellent reliability and stability (>5000 cycles). The versatile and scalable Ti3C2Tx MXene/CNTs strain sensors provide a promising route to future wearable artificial intelligence with comprehensive tracking ability of real-time and in situ physiological signals for health and sporting applications.

Stretchable and wearable strain sensing microelectronics have attracted a huge surge of interest with the diverse applications, ranging from epidermal sensors to health monitoring systems.1-11 To date, much ongoing effort has been made to develop large-area and high-performance stretchable sensing devices to capture and monitor various physical stimuli and physiological signals. Unfortunately, conventional strain gauge sensors based on constituent metal and semiconductor materials only can detect 2 ACS Paragon Plus Environment

Page 3 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

a narrow range of strain (ε800%,15 GF >3000, and low sensing range down to 0.008%

34

have been

demonstrated by a combination of innovation in nanomaterials, device fabrication technology and mechanism optimization. Despite impressive advances recently, the integrate capability of a single type versatile sensor for low strain detectability as small as artery pulses, high stretchability as large as entire body motions, ultrahigh sensitivity, tunable sensing ranges, and thin device dimensions still remains a challenge. Therefore, combination of rational sensing materials and design of the geometric structures are expected to be effective approaches to achieve these goals. MXenes, a new class of two-dimensional (2D) transition metal carbides and carbonitrides with metallic conductivity, excellent mechanical properties, and hydrophilic surface, show great promise in electrochemical energy field, such as supercapacitors,35 Li(Na)-ion batteries,36-38 and electro-catalytic.39 These 2D materials also exhibit great potential in developing next-generation high-performance sensing devices.40 However, colloidal Ti3C2Tx MXene solution upon delamination contains around 1 nm-thick 2D flakes with lateral sizes up to several micrometers with high aspect ratios. It is difficult to assemble individual Ti3C2Tx MXene sheets into ordered 3 ACS Paragon Plus Environment

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 30

macroscopic geometric structures and excellent connection types without sacrificing their excellent mechanical and electrical properties.41,

42

Thus, a well-designed

structure is of paramount importance, and a simple, scalable, and effective hierarchical structure is also required to fully utilize the advantages of these materials, for the purpose of practical sensing devices.38 In this work, Ti3C2Tx MXenes43 combined with CNTs are presented to the best of our knowledge in manufacturing versatile and scalable strain sensors. Sandwich-like Ti3C2Tx MXene/CNT sensing layer (1~2 µm) were fabricated by using delaminated Ti3C2Tx MXene flakes and hydrophilic single walled carbon nanotubes (SWNTs) through layer-by-layer (LBL) spray coating technique. The water-based assembled sandwich-like microstructure provides the ultrathin devices with a low limit of detection as small as 0.1%, high sensitivity (a gauge factor up to 772.60), tunable sensing range (30% to 130% strain), and thin device dimension (< 2 µm). After 5000 cycles, negligible loading-unloading signal changes can be observed. The extraordinary sensing performances enabled successful detecting of both tiny deformations like phonations and substantial movements like walking, running, and jumping. RESULTS AND DISCUSSION Ti3C2Tx MXene Collidal Ti3C2Tx MXene suspensions were prepared using a method described in our previous work.44 The Ti3AlC2 precursor was in situ exfoliated by HCl-LiF etchant and then the exfoliation products, multilayered Ti3C2Tx MXene (m-Ti3C2Tx) was 4 ACS Paragon Plus Environment

Page 5 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

delaminated in deionized water by sonication. After centrifugation, stable suspension of delaminated Ti3C2Tx nanosheets (d-Ti3C2Tx) were obtained for the fabrication of active sensing layers, whose concentration were further adjusted in the range of 0.1-1 mg/ml by further dilution or concentration. In addition, aqueous dispersions of hydrophilic single walled carbon nanotubes (SWNTs) with different concentrations (0.1-1 mg/ml) were also prepared by ultrasonic processing. Figure 1a schematically illustrates the procedure of spray coating Ti3C2Tx MXene suspension and CNTs suspension alternatively to fabricate sandwich-like Ti3C2Tx MXene/CNT sensing layers. First, a thin continuous layer of MXene flakes was formed by spray coating of delaminated Ti3C2Tx flake suspension on latex, followed by drying with nitrogen gas gun. Then a CNT layer was deposited on the top of it by the same method. Afterwards, the process was repeated several times to produce the whole sensing layers. For comparison, individual Ti3C2Tx MXene and CNT sensing layers were also prepared by using the same approach. Figure 1b shows the Tyntall effect of the Ti3C2Tx MXene and CNT colloidal suspensions, indicating their well-dispersion property which is beneficial for preparing uniform films. Figure 1c presents a typical transmission electron microscope (TEM) image of single to a few layered Ti3C2Tx ﬂakes obtained from colloidal solution. The lateral dimension of the ﬂakes is in the range of hundreds of nano-meters up to several micro-meters. Water-soluble SWNTs were employed (Figure 1d) to minimize the chemical residues and enhance the connectivity between the Ti3C2Tx layers. Finally, two electrodes were connected to both ends of the Ti3C2Tx MXene/CNT/elastic latex rubber film, and a skin-attachable 5 ACS Paragon Plus Environment

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 30

and highly stretchable strain sensor was obtained (Figure 1e). The strategy of fabricating Ti3C2Tx MXene/CNT strain sensor without toxic organic solvents or expensive instruments involved, possesses large-scale/low cost manufacturing potential. Figure 2a shows the XRD patterns of the Ti3C2Tx MXene/CNT films obtained through the layer by layer (LBL) spray coating method. The Ti3C2Tx MXene/CNT composites show primary diffraction peaks of Ti3C2Tx and additional peaks for CNTs, which indicates the prospective assembly of Ti3C2Tx and CNTs. XPS spectrum (Figure 2b) was used to characterize the surface chemical environment of the synthesized Ti3C2Tx MXene nanosheets, proving that the MXene surfaces are terminated by O-, OH-, and/or F- groups, which is in accordance with the previous report.40 The optical properties of Ti3C2Tx MXene suspension were evaluated by ultraviolet-visible (UV-vis) spectrophotometry (Figure S1). The absorption peaks at 270 nm and 700-800 nm are also in accordance with the previous report.40 The top view SEM image (Figure 2c) shows that pure Ti3C2Tx film presents non-oriented stacking manner. The cross-sectional SEM image demonstrates that the Ti3C2Tx film fabricated via 20 cycles of spray coating (Figure 2d) is 1-2 µm in thickness. In contrast, Ti3C2Tx MXene/CNT films possess an enhanced and more ordered connection, and the sandwich-like structure obtained via layer-by-layer (LBL) assembly can be clearly seen in Figure 2e and f. The long hairy CNTs knitted the loosened Ti3C2Tx sheets into an excellent textile, which greatly improved the orderliness of the layer structures and electronic pathways. Therefore, such structure 6 ACS Paragon Plus Environment

Page 7 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

endowed MXene/CNT film with high elasticity, typical of highly stretchable polymeric films. Electromechanical Behavior Analysis Piezoresistivity of Ti3C2Tx/CNT films for strain sensing can be tuned by adjusting the cycles (1, 3, 5, 10, 15 and 20) of layer-by-layer spray coating and concentrations (1 mg/ml, 0.2 mg/ml and 0.1 mg/ml) of Ti3C2Tx and CNTs suspensions, respectively. Hence, different sensing films denoted as MXene1 - CNT1, MXene1 - CNT0.2, MXene1 - CNT0.1, CNT1 - MXene1, CNT1 - MXene0.2, and CNT1 - MXene0.1 have been designed and prepared, respectively. The subscript numbers represent the concentration of MXene or CNTs dispersion, for example, “1” means the concentration is 1 mg/ml, “0.1” means the concentration is 0.1 mg/ml and so on. Figure 3a shows the typical piezoresistive properties of a stretchable MXene1 - CNT1 film spray coated for 10 cycles as the applied strain was varied from 1% to 80%. The figure of merit to measure the sensitivity of the device, the gauge factor (GF) is defined as GF = (R – R0)/R0ε

(1)

Where ε is the strain, R0 is electrical resistance under no strain, and R is the electrical resistance under various strains. It can be seen that the relative resistance (R-R0)/R0 experienced a slow decline ascent in the strain range of 0 - 30% with a GF of 64.6 (Figure 3a), revealing a decrease in electrical conductance. This could be understood by the disconnection of mechanical microcrack junctions under tensile stress in Ti3C2Tx layers, followed by the increasing of crack-gap and crack-density under 7 ACS Paragon Plus Environment

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 30

higher strains. As the applied strain increased progressively (40 - 70%), the resistance experienced a relatively rapid increase with a GF of 772.6, indicating a distinctly decrease in electrical conductance. This phenomenon is due to the increased disconnection between CNT conductive pathways, which escalated the effect of microcracks. It can be concluded that Ti3C2Tx/CNT composite based strain sensor outperformed the conventional brittle metal foil or semiconductor strain sensors (GF~2, ε < 5%) in terms of both sensing strain range and GF.12 Although previous fiber-based strain sensors reached a higher strain range, it typically demonstrated a low sensitivity and limited GF (GF < 5%).11 Meanwhile, graphene-based strain sensor reached a higher GF with a limited strain range and stretchability (< 30%).14, 45 When the applied strain reached 70%, the resistance began to increase quickly due to the increasing disconnection of mechanical crack junctions in Ti3C2Tx layers and reduced conductive pathways in CNTs. The linear I-V curves indicate that the relative resistance was constant for each specific static strain (0 - 30%, Figure S2). At the large repeated strain region, the variations in relative resistance upon stretching to maximum strains of 5%, 10%, 20%, and 50% were measured to be 2.8, 3.5, 10.3, and 67.1, respectively (Figure 3b). These signal peaks varied with each other at different strains, which is in good agreement with those shown in Figure 3a. The lower strain limit of the Ti3C2Tx/CNT based sensor is also estimated. As shown in Figure 3c, it presents the relative resistance response as a function of time under a tiny strain of around 0.1% with minimum displacement of 10 µm. It indicates that Ti3C2Tx/CNT


Page 9 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

based sensor can be used to detect the tiny strain with a relative resistance increase of 0.0034. Figure 3d shows the frequency response and output signals of Ti3C2Tx MXene/CNT based strain sensors. Noted that the electrical responses of the strain sensors exhibit stable durability with 20% strain at frequencies ranging from 0.1 to 2 Hz. The peak variations of electrical signals were almost the same as the frequency increase, which maybe attributed to the strong adhesion between Ti3C2Tx MXene/CNT and latex rubber. Besides, the output current signals were compared with the dynamic strain inputs at a frequency of 0.5 Hz (Figure 3e). And the current waves were almost the same as the input strain waves, indicating the excellent response of the sensors to external forces. Figure 3f shows the tests under a cyclic strain of 0% - 20% - 0% at a frequency of 1 Hz. As the figure depicts, the Ti3C2Tx MXene/CNT based strain sensors responded to the loading-unloading cycles with an excellent stability and reproducibility and high signal-to-noise ratios for more than 5000 cycles. And the electrical current of the strain sensor showed negligible changes after the cyclic test, demonstrating its high durability and stability. A comparison with reduced graphene oxide RGO1-CNT1 produced with the same procedure used here for MXene1-CNT1 is demonstrated (Figure S3-S5). The RGO1-CNT1 based strain sensors show lower GF and higher stretchability mainly due to the intrinsic mechanical rigid structure and lower density of RGO nanosheets than Ti3C2Tx MXene. The detailed mechanism is shown in Figure S5. In addition, a comparison of direct physical mixing MXene and CNT is also provided (Figure S6-S7). 9 ACS Paragon Plus Environment

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 30

To characterize the influence of layer-by-layer spray coating cycles (SCC) and concentrations of sensing materials (COSM) (Ti3C2Tx and CNT) on the stretchability and sensing of the device, a series of devices (MXene1 - CNT1, MXene1 - CNT0.2, MXene1 - CNT0.1, CNT1 - MXene1, CNT1 - MXene0.2, and CNT1 - MXene0.1) were fabricated and measured. Notably, the relative resistance-strain curves demonstrate that the Ti3C2Tx MXene/CNT/latex strain sensors have broad and tunable sensing ranges, which can be tuned through adjusting the SCC and COSM, respectively (Figure 4a, b). Larger relative resistance changes were observed for MXene1 - CNT0.1 and CNT1 - MXene0.1 with the same spray coating cycles when they were stretched to the same strains. In addition, the sensing range increased gradually as the COSM improved. This phenomenon can be explained by the increased conductive pathways between 2D MXene nanosheets and 1D CNTs. More importantly, for samples with single sensing material (Ti3C2Tx or SWNT) fabricated with same SCC, sensing range were much different from those based on Ti3C2Tx and CNTs. A smaller sensing range but a larger GF can be achieved by a Ti3C2Tx based samples than CNT based samples, arising from the fact that Ti3C2Tx nanosheets are more likely to form cracks or wrinkles instead of sliding between CNTs. Thus, ultrathin flexible and sandwich-like Ti3C2Tx MXene/CNT sensing layers demonstrate combined advantages of both materials (Figure 4c). Furthermore, the relative resistance change and sensing range of CNT1 - MXene1 films with various SCC values were investigated (Figure 4d). As expected, the sensing range become larger with the increase of SCC values, which


Page 11 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

indicates that CNT microyarns exhibit a significant advantage for endowing sensor skin with a wide sensing range of human activities. The sensing mechanism of the Ti3C2Tx MXene/CNT/latex strain sensor during the first stretching cycle was further investigated by surface SEM images. As shown in Figure 5, before stretching, the 2D Ti3C2Tx sheets and the 1D CNTs closely weaved into a smooth film without crack or gap. Within low-strain region (< 5%), some minor cracks appeared on the film (Figure 5b). As the strain further increased, cracks grew gradually (Figure 5 c-d). Despite larger cracks under larger strains, most of the Ti3C2Tx islands were still inter-connected with CNTs, giving rise to the apparent conductivity. With further increased strain, conductivity reduced rapidly as a result of less CNT inter-connection (Figure 5e). Remarkably, the cracks were self-repaired after the full strain recovery, only leaving some tiny cracks (Figures 5f). Therefore, the

cracks during

the

second

stretching and

releasing

of

the

Ti3C2Tx

MXene/CNT/latex strain sensor tend to expand along the original cracks. Figure S8 shows the SEM images of cracked sandwich-like Ti3C2Tx/CNT sensing layers, with hair like CNTs bridging two ends of the gap upon stretching. After the first stretching, the cracking under strains and crack-repairing after strain release were fully reversible,

indicating

the

high

stability

and

durability

of

the

Ti3C2Tx

MXene/CNT/latex strain sensors (Figure 3f). The fracturing mechanism of the Ti3C2Tx MXene/CNT/latex strain sensors is similar to previously reported gold nanowires (AuNWs)13 and CNT sensors.26 The morphology changes of Ti3C2Tx MXene/CNT film before and after stretching have been simulated by a simple model 11 ACS Paragon Plus Environment

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 30

(Figure S9). Islands and gaps induced by stretching decreased conductive pathways, which is responsible for the increasing of the overall electrical resistance of Ti3C2Tx MXene/CNT film. The outstanding performance of the Ti3C2Tx MXene/CNT/latex strain sensors allowed their use as skin-attachable wearable devices for real-time human slight physiological signal and large-scale motion monitoring. For example, it is able to reliably identify complex skin stretching over human throat associated with speaking “Carbon”, “Sensor”, and “MXene” repeatably (Figure 6 a-d). It appears that for the two-syllable word “Carbon” and “Sensor”, two little peaks with different intensity appeared. As expected, the word “MXene” with three consecutive syllables corresponds to three little peaks. The excellent phonation recognition ability enables this sensor to have great potential in phonation rehabilitation exercise and human/machine interaction.1, 46 The Ti3C2Tx MXene/CNT/latex strain sensor can also be used for less subtle human motions. By mounting the sensors on the knee joint, the fore knee muscle movement when walking, running and jumping from squatting could be easily reflected by the changes in the relative resistance of sensor with high signal-to-noise ratios in a highly repeatable manner (Figure 6 e-h). Therefore, the skin-mountable Ti3C2Tx MXene/CNT/latex strain sensors have the potential in monitoring human physiological signals and body motions, which provides important information for feedback control in robotic systems and prosthetic devices.47 CONCLUSIONS


Page 13 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

In summary, micron-thick skin-mounted strain sensors with extraordinary sensitivities, low limit of detections, high stretchability, and tunable sensing ranges have been developed using delaminated Ti3C2Tx MXene flakes and CNTs. The piezoresistive dependence of various sandwich-like quasi-continuous Ti3C2Tx MXene/CNT films was systematically explored. This configuration allows various initial tunneling distances between neighboring overlapped Ti3C2Tx MXene layers and interconnected CNTs to readily change their overlapping areas and interconnecting pathways via reversible slipping and consequently change their contact resistances. The obtained ultrathin sensor featured ultrahigh GFs of 4.4 772.6, low limit of detections as small as 0.1%, tunable sensing range of 30 - 130%, and high durability of > 5000 cycles. These extraordinary attributes enabled us to detect broad sensing signals from tiny deformations (phonation) to large-scale muscle movement associated with a variety of human body motions (walking, running, and jumping). The designed Ti3C2Tx MXene/CNT strain sensor provides a promising platform for future interactive processing, prosthetic feedback, and wearable sensing for comprehensive monitoring in health and human motion applications. MATERIALS AND METHODS Synthesis of Ti3C2Tx MXene Ti3C2Tx aqueous dispersions were prepared by a method described in our previous work in which Ti3AlC2 powder (~38 µm) was chemically exfoliated by HCl+LiF etchant.44 Ti3AlC2 powder (1 g) was slowly added to a premixed solution of 6 M HCl (10 mL) and LiF (0.666 g) with magnetic stirring (250 rpm) to avoid vigorous 13 ACS Paragon Plus Environment

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 30

reaction. Then the mixture was stirred at 250 rpm for 24 h at 35 °C. The resulting suspension was washed with ethanol and centrifuged at 3500 rpm for several times until its pH was about 6. The sediment was then filtrated on nylon membrane filters and dried in air at room temperature, producing multilayer Ti3C2Tx MXene (m-Ti3C2Tx). The delaminated Ti3C2Tx MXene (d-Ti3C2Tx), m-Ti3C2Tx (0.1 g) was added into 50 ml of DI water, which was followed by sonication and centrifugation at 3500 rpm. The stable colloidal suspension of 2D d-Ti3C2Tx nanosheets with a concentration of 0.4 mg/ml was obtained. By further dilution or concentration via vacuum evaporation at room temperature, concentration of Ti3C2Tx MXene suspension was regulated in the range of 0.1-1 mg/ml. Fabrication of Ti3C2Tx/CNT/latex Strain Sensors Hydrophilic CNT (Carbon Solutions, Inc) dispersion with different concentrations (0.1, 0.2, and 1 mg/ml) were prepared by sonicating functionalized CNTs for two hours in DI water until CNTs are perfectly dispersed in water. A soft latex rubber substrate was attached on a glass slide and patterned with polyimide masks (20 × 5 mm2 rectangular pattern size). Then layer-by-layer assembly air-spray coating was utilized as a facile and scalable method for the alternately deposition of the Ti3C2Tx and CNT into sandwiched Ti3C2Tx/CNT thin films (spray pressure ~ 2 bars). After the films were dried in fume cupboard, polyimide masks and glass side were removed. The CNT solution was coated on the patterned polyimide (PI) films patterned with a plotter (GRAPHTEC, CE2000-120) at 100 °C to obtain the CNT percolation thin films. 14 ACS Paragon Plus Environment

Page 15 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Characterization The morphologies of samples were characterized by field emission scanning electron microscope (FESEM, S4800, Hitachi, Japan) at an accelerating voltage of 10.0 kV, transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan) at an accelerating voltage of 200 kV. The crystalline structures of prepared Ti3C2Tx MXene and Ti3C2Tx MXene/CNTs films were analyzed by X-ray diffraction (XRD, Bruker, D8 Advance) using Cu Kα radiation (λ = 1.54 Å) at 40 kV and 15 mA. The chemistry attributes of samples were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250, USA) using monochromatized Al Kα radiation (1486.6 eV). The optical properties of Ti3C2Tx MXene suspension were evaluated by UV-vis spectrophotometry (UV-3600 Plus). To test the strain-sensing characteristics, two ends of the devices were connected to a force gauge (M4-2, Mark-10) and a stretchable station (ESM302, Mark-10). A standard semiconductor parametric tester (Keithley-4200) was used to record the I-V characteristics for the strain sensor. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxx. UV-Vis spectra of Ti3C2Tx dispersion with concentration of 0.02 mg/ml (Figure S1). Current-voltage curves of Ti3C2Tx MXene/CNT strain sensor at different strains (Figure S2). Detailed comparison experiment of RGO-CNT produced with the same procedure used for MXene-CNT (Figure S3-S5). Detailed discussion on the 15 ACS Paragon Plus Environment

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 30

experiment of spraying a mixture of MXene-CNT (Figure S6-S7). SEM images of sandwich-like Ti3C2Tx/CNT sensing layers upon stretching (Figure S8). Schematic illustration of sensing mechanism of Ti3C2Tx MXene/CNT strain sensor upon stretching and the resistance model of a sensing unit (Figure S9). This material is available free of charge (PDF). AUTHOR INFORMATION Corresponding Author *E-mail (J. J. Shao): [email protected] * E-mail (J. Yang): [email protected] * E-mail (X.C. Dong): [email protected] ORCID Xiaochen Dong: 0000-0003-4837-9059 Author Contributions J.J.S, J. Y, X. C. D. designed and supervised the overall research. The manuscript was written through contributions of Y. C. C. and J. S. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The work was supported by the NNSF of China (61525402, 61604071), Natural Science Foundation of Jiangsu Province (BK20161012), the Priority Academic 16 ACS Paragon Plus Environment

Page 17 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Program Development of Jiangsu Higher Education Institutions (PAPD), and the Program for Chang jiang Scholars and Innovative Research Team in University (PCSIRT), IRT1146 and IRT15R35. REFERENCES (1) Li, X.; Yang, T.; Yang, Y.; Zhu, J.; Li, L.; Alam, F. E.; Li, X.; Wang, K.; Cheng, H.; Lin, C. T. Large‐Area Ultrathin Graphene Films by Single‐Step Marangoni Self‐Assembly for Highly Sensitive Strain Sensing Application. Adv. Funct. Mater. 2016, 26, 1322-1329. (2) Willner, I.; Katz, E. Integration of Layered Redox Proteins and Conductive Supports for Bioelectronic Applications. Angew. Chem., Int. Ed. 2000, 39, 1180-1218. (3) Zang, Y.; Zhang, F.; Huang, D.; Gao, X.; Di, C.-a.; Zhu, D. Flexible Suspended Gate Organic Thin-Film Transistors for Ultra-Sensitive Pressure Detection. Nat. Commun. 2015, 6, 6269. (4) Choong, C. L.; Shim, M. B.; Lee, B. S.; Jeon, S.; Ko, D. S.; Kang, T. H.; Bae, J.; Lee, S. H.; Byun, K. E.; Im, J. Highly Stretchable Resistive Pressure Sensors Using a Conductive Elastomeric Composite on a Micropyramid Array. Adv. Mater. 2014, 26, 3451-3458. (5) Cai, Y.; Shen, J.; Dai, Z.; Zang, X.; Dong, Q.; Guan, G.; Li, L. J.; Huang, W.; Dong, X. Extraordinarily Stretchable All‐Carbon Collaborative Nanoarchitectures for Epidermal Sensors. Adv. Mater. 2017, 29, 1606411. (6) Yang, T.; Wang, W.; Zhang, H.; Li, X.; Shi, J.; He, Y.; Zheng, Q.-s.; Li, Z.; Zhu, H. Tactile Sensing System Based on Arrays of Graphene Woven Microfabrics: 17 ACS Paragon Plus Environment

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 30

Electromechanical Behavior and Electronic Skin Application. ACS Nano 2015, 9, 10867-10875. (7) Pang, C.; Koo, J. H.; Nguyen, A.; Caves, J. M.; Kim, M. G.; Chortos, A.; Kim, K.; Wang, P. J.; Tok, J. B. H.; Bao, Z. Highly Skin‐Conformal Microhairy Sensor for Pulse Signal Amplification. Adv. Mater. 2015, 27, 634-640. (8) Ryu, S.; Lee, P.; Chou, J. B.; Xu, R.; Zhao, R.; Hart, A. J.; Kim, S.-G. Extremely Elastic Wearable Carbon Nanotube Fiber Strain Sensor for Monitoring of Human Motion. ACS Nano 2015, 9, 5929-5936. (9) Kim, D.-H.; Lu, N.; Ma, R.; Kim, Y.-S.; Kim, R.-H.; Wang, S.; Wu, J.; Won, S. M.; Tao, H.; Islam, A. Epidermal Electronics. Science 2011, 333, 838-843. (10) Yan, C.; Wang, J.; Kang, W.; Cui, M.; Wang, X.; Foo, C. Y.; Chee, K. J.; Lee, P. S. Highly Stretchable Piezoresistive Graphene-Nanocellulose Nanopaper for Strain Sensors. Adv. Mater. 2014, 26, 2022-2027. (11) Yang, T.; Xie, D.; Li, Z.; Zhu, H. Recent advances in wearable tactile sensors: Materials, Sensing Mechanisms, and Device Performance. Mater. Sci. Eng., R. 2017, 115, 1-37. (12) Barlian, A. A.; Park, W.-T.; Mallon, J. R.; Rastegar, A. J.; Pruitt, B. L. Review: Semiconductor Piezoresistance for Microsystems. P. IEEE. 2009, 97, 513-552. (13) Gong, S.; Lai, D. T.; Su, B.; Si, K. J.; Ma, Z.; Yap, L. W.; Guo, P.; Cheng, W. Highly Stretchy Black Gold E‐Skin Nanopatches as Highly Sensitive Wearable Biomedical Sensors. Adv. Electron. Mater. 2015, 1, 1400063.


Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

(14) Wang, Y.; Wang, L.; Yang, T.; Li, X.; Zang, X.; Zhu, M.; Wang, K.; Wu, D.; Zhu, H. Wearable and Highly Sensitive Graphene Strain Sensors for Human Motion Monitoring. Adv. Funct. Mater. 2014, 24, 4666-4670. (15) Boland, C. S.; Khan, U.; Backes, C.; O’Neill, A.; McCauley, J.; Duane, S.; Shanker, R.; Liu, Y.; Jurewicz, I.; Dalton, A. B. Sensitive, High-Strain, High-Rate Bodily Motion Sensors Based on Graphene-Rubber Composites. ACS Nano 2014, 8, 8819-8830. (16) Trung, T. Q.; Tien, N. T.; Kim, D.; Jang, M.; Yoon, O. J.; Lee, N. E. A Flexible Reduced Graphene Oxide Field‐Effect Transistor for Ultrasensitive Strain Sensing. Adv. Funct. Mater. 2014, 24, 117-124. (17) Bae, S.-H.; Lee, Y.; Sharma, B. K.; Lee, H.-J.; Kim, J.-H.; Ahn, J.-H. Graphene-Based Transparent Strain Sensor. Carbon 2013, 51, 236-242. (18) Hempel, M.; Nezich, D.; Kong, J.; Hofmann, M. A Novel Class of Strain Gauges Based on Layered Percolative Films of 2D Materials. Nano Lett. 2012, 12, 5714-5718. (19) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J.-H.; Kim, P.; Choi, J.-Y.; Hong, B. H. Large-Scale Pattern Growth of Graphene Films for Stretchable Transparent Electrodes. Nature 2009, 457, 706-710. (20) Wang, Y.; Yang, R.; Shi, Z.; Zhang, L.; Shi, D.; Wang, E.; Zhang, G. Super-Elastic Graphene Ripples for Flexible Strain Sensors. ACS Nano 2011, 5, 3645-3650.


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 30

(21) Jeong, J. W.; Yeo, W. H.; Akhtar, A.; Norton, J. J.; Kwack, Y. J.; Li, S.; Jung, S. Y.; Su, Y.; Lee, W.; Xia, J. Materials and Optimized Designs for Human‐Machine Interfaces Via Epidermal Electronics. Adv. Mater. 2013, 25, 6839-6846. (22) Cochrane, C.; Koncar, V.; Lewandowski, M.; Dufour, C. Design and Development of A Flexible Strain Sensor for Textile Structures Based on A Conductive Polymer Composite. Sensors 2007, 7, 473-492. (23) Mattmann, C.; Clemens, F.; Tröster, G. Sensor for Measuring Strain in Textile. Sensors 2008, 8, 3719-3732. (24) Wu, X.; Han, Y.; Zhang, X.; Zhou, Z.; Lu, C. Large‐Area Compliant, Low‐Cost,

and

Versatile

Pressure‐Sensing

Platform

Based

on

Microcrack‐Designed Carbon Black@ Polyurethane Sponge for Human–Machine Interfacing. Adv. Funct. Mater. 2016, 26, 6246-6256. (25) Lipomi, D. J.; Vosgueritchian, M.; Tee, B. C.; Hellstrom, S. L.; Lee, J. A.; Fox, C. H.; Bao, Z. Skin-like Pressure and Strain Sensors Based on Transparent Elastic Films of Carbon Nanotubes. Nat. Nanotech. 2011, 6, 788-792. (26) Yamada, T.; Hayamizu, Y.; Yamamoto, Y.; Yomogida, Y.; Izadi-Najafabadi, A.; Futaba, D. N.; Hata, K. A Stretchable Carbon Nanotube Strain Sensor for Human-Motion Detection. Nat. Nanotech. 2011, 6, 296-301. (27) Cohen, D. J.; Mitra, D.; Peterson, K.; Maharbiz, M. M. A Highly Elastic, Capacitive Strain Gauge Based on Percolating Nanotube Networks. Nano Lett. 2012, 12, 1821-1825.


Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

(28) Herrmann, J.; Müller, K.-H.; Reda, T.; Baxter, G.; Raguse, B. d.; De Groot, G.; Chai, R.; Roberts, M.; Wieczorek, L. Nanoparticle Films as Sensitive Strain Gauges. Appl. Phys. Lett. 2007, 91, 183105. (29) Amjadi, M.; Pichitpajongkit, A.; Lee, S.; Ryu, S.; Park, I. Highly Stretchable and Sensitive Strain Sensor Based on Silver Nanowire-Elastomer Nanocomposite. ACS Nano 2014, 8, 5154-5163. (30)

Wu, J. M.; Chen, C.-Y.; Zhang, Y.; Chen, K.-H.; Yang, Y.; Hu, Y.; He, J.-H.;

Wang, Z. L. Ultrahigh Sensitive Piezotronic Strain Sensors Based on a ZnSnO3 Nanowire/Microwire. ACS Nano 2012, 6, 4369-4374. (31) Chen, Z.; Wang, Z.; Li, X.; Lin, Y.; Luo, N.; Long, M.; Zhao, N.; Xu, J.-B. Flexible Piezoelectric-Induced Pressure Sensors for Static Measurements Based on Nanowires/Graphene Heterostructures. ACS Nano 2017, 11, 4507-4513. (32) Miyamoto, A.; Lee, S.; Cooray, N. F.; Lee, S.; Mori, M.; Matsuhisa, N.; Jin, H.; Yoda, L.; Yokota, T.; Itoh, A. Inflammation-Free, Gas-permeable, Lightweight, Stretchable on-Skin Electronics with Nanomeshes. Nat. Nanotech. 2017, 125. (33) Cai, G.; Wang, J.; Qian, K.; Chen, J.; Li, S.; Lee, P. S. Extremely Stretchable Strain Sensors Based on Conductive Self‐Healing Dynamic Cross‐Links Hydrogels for Human‐Motion Detection. Adv. Sci. 2017, 4, 1600190. (34) Sun, Q.; Seung, W.; Kim, B. J.; Seo, S.; Kim, S. W.; Cho, J. H. Active Matrix Electronic Skin Strain Sensor Based on Piezopotential‐Powered Graphene Transistors. Adv. Mater. 2015, 27, 3411-3417.


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 30

(35) Lukatskaya, M. R.; Mashtalir, O.; Ren, C. E.; Dall’Agnese, Y.; Rozier, P.; Taberna, P. L.; Naguib, M.; Simon, P.; Barsoum, M. W.; Gogotsi, Y. Cation Intercalation and High Volumetric Capacitance of Two-Dimensional Titanium Carbide. Science 2013, 341, 1502-1505. (36) Xie, X.; Zhao, M.-Q.; Anasori, B.; Maleski, K.; Ren, C. E.; Li, J.; Byles, B. W.; Pomerantseva, E.; Wang, G.; Gogotsi, Y. Porous Heterostructured MXene/Carbon Nanotube Composite Paper with High Volumetric Capacity for Sodium-Based Energy Storage Devices. Nano Energy 2016, 26, 513-523. (37) Byeon, A.; Glushenkov, A. M.; Anasori, B.; Urbankowski, P.; Li, J.; Byles, B. W.; Blake, B.; Van Aken, K. L.; Kota, S.; Pomerantseva, E. Lithium-Ion Capacitors with 2D Nb2CTx (MXene)-Carbon Nanotube Electrodes. J. Power. Sources. 2016, 326, 686-694. (38) 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, 339-345. (39) Zhao, L.; Dong, B.; Li, S.; Zhou, L.; Lai, L.; Wang, Z.; Zhao, S.; Han, M.; Gao, K.; Lu, M. Inter-Diffusion Reaction Assisted Hybridization of Two-Dimensional Metal-Organic Frameworks and Ti3C2Tx Nanosheets for Electrocatalytic Oxygen Evolution. ACS Nano 2017, 11, 5800-5807. (40) Hantanasirisakul, K.; Zhao, M. Q.; Urbankowski, P.; Halim, J.; Anasori, B.; Kota, S.; Ren, C. E.; Barsoum, M. W.; Gogotsi, Y. Fabrication of Ti3C2Tx MXene


Page 23 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Transparent Thin Films with Tunable Optoelectronic Properties. Adv. Electron. Mater. 2016, 2, 1600050. (41) Mashtalir, O.; Lukatskaya, M. R.; Zhao, M. Q.; Barsoum, M. W.; Gogotsi, Y. Amine‐Assisted Delamination of Nb2C MXene for Li‐Ion Energy Storage Devices. Adv. Mater. 2015, 27, 3501-3506. (42) Mashtalir, O.; Naguib, M.; Mochalin, V. N.; Dall’Agnese, Y.; Heon, M.; Barsoum, M. W.; Gogotsi, Y. Intercalation and Delamination of Layered Carbides and Carbonitrides. Nat. Commun. 2013, 4, 1716. (43)

Naguib, M.; Kurtoglu, M.; Presser, V.; Lu, J.; Niu, J.; Heon, M.; Hultman, L.;

Gogotsi, Y.; Barsoum, M. W. Two‐Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2. Adv. Mater. 2011, 23, 4248-4253. (44) Zhang, T.; Pan, L.; Tang, H.; Du, F.; Guo, Y.; Qiu, T.; Yang, J. Synthesis of Two-Dimensional Ti3C2Tx MXene Using HCl+LiF Etchant: Enhanced Exfoliation and Delamination. J. Alloy. Compd. 2017, 695, 818-826. (45) Park, J.; You, I.; Shin, S.; Jeong, U. Material Approaches to Sretchable Strain Sensors. Chem. Phys. Chem. 2015, 16, 1155-1163. (46) Wang, X.; Que, M.; Chen, M.; Han, X.; Li, X.; Pan, C.; Wang, Z. L. Full Dynamic‐Range Pressure Sensor Matrix Based on Optical and Electrical Dual‐Mode Sensing. Adv. Mater. 2017, 29, 1605817. (47) Chin, S. Y.; Poh, Y. C.; Kohler, A.-C.; Compton, J. T.; Hsu, L. L.; Lau, K. M.; Kim, S.; Lee, B. W.; Lee, F. Y.; Sia, S. K. Additive Manufacturing of Hydrogel-Based


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

Materials for Next-Generation Implantable Medical Devices. Sci. Robot. 2017, 2, 6451.


Page 25 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Figure 1. (a) Fabrication process of sandwich-like Ti3C2Tx MXene/CNT layer. (b) Tyndall effect of MXene (Ti3C2Tx) and CNTs suspension. (c, d) TEM images of Ti3C2Tx flakes and SWCNTs, respectively. (e) Photographs of a strain sensor belt clipped on two clamps before and after stretching up to 100%, 200%, respectively.


ACS Nano

(b)

(a) Ti3 C 2 T X - SW N T Ti3 C 2 T X 15

30

45

2 Theta (degree)

60

Intensity (a.u.)

O1s

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

800

Ti2p F1s

C1s

600

400

200

0

Binding Energy (eV)

(c)

(d)

(e)

(f)

Figure 2. (a) XRD patterns of Ti3C2Tx and Ti3C2Tx/CNT film. (b) XPS spectrum of Ti3C2Tx MXene nanosheets. (c) SEM image of pure Ti3C2Tx film. (d) Cross-sectional SEM image of Ti3C2Tx MXene flakes. (e) Top view and (f) cross-sectional SEM images of sandwich-like Ti3C2Tx MXene/CNT layers.


Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Figure 3. (a) Typical relative resistance-strain curve of Ti3C2Tx MXene/CNT/latex strain sensor at a stretching rate of 5% min-1. The inset shows the curve within 0.6% strain. (b) Multicycle tests of relative resistance variation as a function of time (strain frequency 0.1Hz) for four applied strain (5%, 10%, 20%, and 50%). (c) Relative resistance changes as a function of time under a tiny strain of 0.1% with stage moving speed of 0.01 mm s-1. (d) Relative resistance response of the strain sensor at different frequencies under 20% strain. (e) Time retention curve of change in resistance and strain with time. (f) Durability test under a strain of 20% at frequency of 1 Hz. The resistance change curves recorded after each 1000 cycles and 50 cycles were presented in each recording. 27 ACS Paragon Plus Environment

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 30

Figure 4. (a-c) Strain-sensing characterization of Ti3C2Tx MXene/CNT/latex strain sensor with different COSM at 10 SCC. (d) The comparison of the strain response for CNT1 - MXene1 with different SCC.


Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Figure 5. Surface SEM images of Ti3C2Tx MXene/CNT film at various stretching states during the first strain-release cycle: (a) 0%, (b) 5%, (c) 20%, (d) 40%, (e) 80%, and back to (f) 0%.


ACS Nano

(R-R0)/R0

0.3 0.2 0.1 0.0

-0.1

0

2

4

6

8

10 12

(f)

8

0.4

0.2

0.2

0.0

0.0

-0.2

-0.2

(g) 8

Walking

(R-R0)/R0

4 2 0

0

2

4

6

8 10 12 14

6

Running

4 2 0

-2 2

4

6

Time (s)

“ M X ene” ene”

0.8

0

2

4

8

10

6

8 10 12

Time (s)

Time (s)

6

0

(d) 1.0 0.6

0.4

Time (s)

(e)

“ Sen sor”

0.6

(R-R0)/R0

(c) 0.8

“ C arb on”

(h)10

Jumping

8 6 4 2 0 -2

(R-R0)/R0

(b) 0.5 0.4 (R-R0)/R0

(a)

(R-R0)/R0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 30

0.0 0.5 1.0 1.5 2.0 2.5

Time (s)

0

2

4

6

8 10 12 14

Time (s)

Figure 6. (a) Photograph of Ti3C2Tx MXene/CNT/latex strain sensor attached to throat of person. (b-d) Responsive curves recorded during speaking “carbon”, “sensor”, and “MXene”, respectively. (e) Photograph of the sensor attached to human knee. (f-h) Relative resistance responses of the sensor in detecting human leg movement: walking, running, and jumping, respectively.


Carbon Nanotube Composite Based

Recommend Documents