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3D Synergistical MXene/Reduced Graphene Oxide Aerogel for a Piezoresistive Sensor Yanan Ma, Yang Yue, Hang Zhang, Feng Cheng, Wanqiu Zhao, Jiangyu Rao, Shijun Luo, Jie Wang, Xueliang Jiang, Zhitian Liu, Nishuang Liu, and Yihua Gao ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b06909 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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3D Synergistical MXene/Reduced Graphene Oxide Aerogel for a Piezoresistive Sensor

Yanan Ma,†,‡,¶ Yang Yue,†,¶ Hang Zhang,† Feng Cheng,† Wanqiu Zhao,† Jiangyu Rao, † Shijun Luo,‡ Jie Wang,‖ Xueliang Jiang,§ Zhitian Liu,§ Nishuang Liu† & Yihua Gao†,*

†

Center for Nanoscale Characterization & Devices (CNCD), Wuhan National Laboratory for Optoelectronics (WNLO), School of Physics, Huazhong University of Science and Technology (HUST), Luoyu Road 1037, Wuhan 430074, P.R. China

‡

School of Sciences, Hubei University of Automotive Technology, Shiyan 442002, P.R. China

ǁ

Britton Chance Center for Biomedical Photonics, Wuhan National Laboratory for Optoelectronics, Hubei Bioinformatics & Molecular Imaging Key Laboratory, Systems Biology Theme, Department of Biomedical Engineering, College of Life Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, China.

§

School of Material Science and Engineering, Wuhan Institute of Technology, Wuhan 430205, P. R. China *Corresponding author. E-mail: [email protected];

¶

These authors contributed equally to this work.

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Abstract A piezoresistive sensor based on the ultralight and superelastic aerogel is reported to fabricate MXene/Reduced graphene oxide (MX/rGO) hybrid 3D structure and utilize their pressure-sensitive characteristics. The MX/rGO aerogel not only combines the rGO’s large specific surface area and the MXene’s (Ti3C2Tx) high conductivity but also exhibits rich porous structure, which leads to better performance than single component rGO or MXene in terms of the pressure sensor. And the large nanosheets of rGO can prevent the poor oxidization of MXene by wrapping MXene inside the aerogel. More importantly, the piezoresistive sensor based on MX/rGO aerogel shows extremely high sensitivity (22.56 kPa-1), fast response time (< 200 ms) and good stability over 10,000 cycles. The piezoresistive sensor based on MX/rGO hybrid 3D aerogel can easily capture the signal below 10 Pa, thus clearly test the pulse of an adult at random. Based on its superior performance, it also demonstrates potential application in measuring pressure distribution, distinguishing subtle strain, monitoring healthy activity and so on.

Keywords: MXene, rGO, synergistic effect, aerogel, piezoresistance sensor

Pressure sensor system, which can transduce strain and its change into resistance, electrical potential, capacitor and so on, recently attracted considerable interest with the rapid process of the wearable functional electronics.1-4 There are mainly three kinds of pressure sensors: piezoresistive,2 piezoelectric,3 and capacitive-type sensors.4 Among them, the piezoresistive sensors show not only high sensitivity, fast frequency 2


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response, excellent stability,5 but also have simple and low-cost fabrication techniques, such as the dip-coating, spin-coating, and laser-induced methods,5-7 which guarantee its wide application in biomimetic robot technology. The sensitivity of piezoresistive sensor depends on geometric effects and resistivity effects of the relative system based on the resistance change formula

∆R / R = (1 + 2v ) ε + ∆ρ ρ , where the first term,

(1 + 2v ) / ε

, refers to the

geometric effect of the device, and the second term, ∆ρ ρ refers to the resistivity effect.8 Most of the researches on the sensing materials have focused on metal nanoparticles, conducting polymers and carbon materials, etc.9, 10 Among them, the carbon materials, especially such as one-dimensional (1D) carbon nanotubes, 2D graphene, possess rich physicochemical properties playing a vital role in pressure sensor.10-13 However, only on their own structure and properties without additives, the obtained 3D nanocarbon-based foams are hard to achieve high sensitivity as a piezoresistive sensor. Graphene oxide (GO) displays large surface area and good hydrophilic surface suitable for assembling into versatile structure.14-16 But the low conductivity hinders its practical application in the piezoresistive sensor. Recently, a new family of 2D materials (MXenes) with unique structure and electronic properties,17-19

have

already

displayed

great

promise

for

applications

in

electromagnetic interference shielding,20 water purification,21 field-effect transistors,22 and energy storage.23, 24 The MXene nanosheets with metal conductivity and good dispersibility in water can combine with GO to solve its conductivity problem. On the other hand, with the help of the large surface area of GO, it can be designed and

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constructed complicate MX/GO elastomer to develop the intelligent bionic device. The Ti3C2Tx, a typical one of 20 kinds of confirmed MXenes, is fabricated by selectively etching out the Al layers from metallically conductive layered Ti3AlC2 phase.16-18 During the etching process, the Al layer between Ti3AlC2 are replaced by the various functional group to yield hydrophily Ti3C2Tx nanosheets, where Tx represents hydroxyl, oxygen or fluorine terminated group.25,26 According to the similarity-intermiscibility theory, the GO terminated with a hydroxyl group can mix well with Ti3C2Tx. The mixture solution fully takes advantage of the synergistic effect of the large surface area of GO and good conductivity of Ti3C2Tx, leading to improving the resistive effect of the sensor.27 In addition, the rGO with a larger surface will partially coat the nanosheet Ti3C2Tx (less than 1µm) and avoid the poor oxidization of Ti3C2Tx to some extent.28 On the other hand, the 3D aerogel with rich porous structure can enhance its geometric effect,29-31 endowing the piezoresistive sensor better resolution capability and wider pressure range than that of the filtrated film from the mixed solution. Thus, the MX/rGO 3D composite can be the appropriate candidate to meet both the geometric and resistive effects of the piezoresistive sensor by fully utilizing the synergistically interaction between the excellent component MXene and rGO. Here, a piezoresistive sensor based on highly ordered hierarchical architectures of hybrid 3D MX/rGO aerogel was fabricated by applying a simple ice-template freezing technique. To further improve the mechanical property of the hybrid sensing materials (MX/GO), it is carried out by a mild annealing under Ar/H2 at 200 ℃ for 2 h. The final

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product was MX/rGO. The obtained piezoresistive sensor based on MX/rGO aerogel shows extremely high sensitivity (22.56 kPa-1), fast response time (< 200 ms) and good stability over 10,000 cycles. Compared with the pure rGO aerogel, its sensitivity and mechanical properties have been greatly improved. When the ratio of GO and MXene is 10:1, its sensing performance achieves an optimal effect. The MX/rGO aerogel-based piezoresistive sensor shows the potential application in measuring pressure distribution and distinguishing subtle strain. It can monitor healthy activity, such as the pulse beats of an adult under the peaceful condition and so on.

Result and Discussion Fabrication and Characterization of the MX/rGO aerogel sensor The MX/rGO 3D aerogel was fabricated by ice-template freezing technique and low-temperature annealing under Ar/H2, which is a green, low-cost and large-scale method to design complex 3D structure. The fabrication process of MX/rGO aerogel and the corresponding sensor were displayed in Figure 1a and 1b. The mechanism of the sensor is due to compress-dependent contact within MX/rGO aerogel, as shown in Figure 1c. Once applying an external force, a compressive deformation of the sensor leads to more contact in MX/rGO aerogel, resulting in more conductive paths. Spontaneously, the resistance related to the MXene (RMX) and rGO (RrGO) naturally reduced, in turn reducing the total resistance. The equation of the total resistance can be simplified as RTotal = Raerogel + RMX RrGO ( RMX + RrGO ) , where Raerogel was the intrinsic resistance of the aerogel, depended on the compressed state.

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The MXene was fabricated by etching out the precursor Ti3AlC2 by HCl+LiF. The final obtained dark-green Ti3C2Tx nanosheets exhibited good dispersion in aqueous solutions, as shown in Figure 2a. Typical transmission electron microscopy (TEM) image of individual Ti3C2Tx nanosheet with diverse lateral sizes is shown in Figure. 2b. From the electron diffraction pattern in the inset of Figure 2b, the hexagonal characteristics of the Ti3C2Tx nanosheets can be confirmed. And the average lateral size distribution of Ti3C2Tx is 584.5 nm, smaller than that of GO (1276.1 nm), as shown in Figure 2b and Figure S2. Based on the similarity-intermiscibility theory, the Ti3C2Tx nanosheet colloid solution mixes well with hydroxy-terminated GO, and the obtained mixtures were stable for a long time, which was proved by the Tyndall effect in Figure S1 (After two weeks later, the mixed solution still presents obvious Tyndall effect). The MXene and graphene nanosheet with the negatively charged surface has a physically mixed interaction between them in our work .32 And the larger rGO sheet wrapped the Ti3C2Tx into its inner surface, thus better avoiding the oxidization of Ti3C2Tx. To construct the 3D hierarchical ordered structure, the above MX/rGO hybrid materials were firstly carried out by the freeze casting step, subsequently by the freeze-drying technique. The size and shape of the 3D foams are mainly depended on the mold employing in the freezing process. To enhance the mechanical strength of the MX/rGO aerogel, low temperature annealing under Ar/H2 atmosphere was carried out, which led to an aerogel darker than that of MX/GO aerogel, as shown in Figure S3a.33 It is worth noting that the excessively high temperature will lead to partial

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oxidation of the MXene.34 And the large electrical conductivity of rGO resulting from full reduction under high temperature will decrease the sensitivity of pressure sensor instead of increase. The final sensing materials MX/rGO aerogel is flexible and ultralight enough that it can be lifted by a petal, as shown in Figure S3a. With the increase of the amount MXene, the color of the aerogel samples become dark, as shown in Figure S3b. In order to measure the sensing performance of MX/rGO aerogel, the flexible interdigital electrodes made by magnetron sputtering Ag/Ni onto the surface of polyimide (PI) substrate35 were used as a conductive path, and then a piece of polyethylene (PE) film was coated with the outer layer to fix and protect the whole device. From the SEM images in Figure 2d-e, the MX/rGO aerogel is in a network-like hierarchical 3D structure, combining micro-mesopores together, in which the stacked nanosheets interconnected with each other. With the increase of the amount MXene in GO system, the network structure contacts more closely as shown in Figure S4. It is observed from the magnified image in Figure 2f that the hybrid nanosheets still remained 2D flat structure, on this basis building the 3D model. The energy-dispersive spectrometer (EDS) elemental mapping of MX/rGO aerogel confirmed that C, Ti, and O were evenly dispersed in the whole system, as shown in Figure S5. After ultrasonic treatment of the MX/rGO aerogel, its high-magnification TEM and corresponding EDS mapping images (in Figure 2g) demonstrated that the smaller nanosheet in the composite aerogel is MXene Ti3C2Tx. On the other hand, the stability and integrity of hybrid composite after experiencing freeze-drying and

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annealing were also confirmed. And the X-ray diffraction (XRD) of the MX/rGO aerogel revealed the respective characteristic peaks of rGO and MXene, where the peaks located at ~7º and ~18º ascribe to Ti3C2Tx, and the peak at ~25º belongs to rGO as shown in Figure 2h.18, 36 The mechanical properties of MX/rGO aerogel were systematically investigated by the compression tests. Figure 2i shows the compressive stress-strain curves for MX/rGO and pure rGO aerogel at a set strain (ε) of 60%. Although MX/rGO and pure rGO aerogel can both sustain large strain deformations (over 60%), the MX/rGO aerogel owns stronger mechanical properties. It is because Young's modulus of MXene is larger19, thus adding MXene into rGO system can improve its anti-pressure ability and robust performance. As shown the inset of Figure 2i, the aerogel can be compressed to over 60%, and the original shape after releasing can be easily recovered, which proves the stability in the continuous compression deformation process helpful for recycling sensing materials. To further study the deformation of the microstructure (mainly the micro-, mesopores), the in-situ SEM images of the MX/rGO aerogel were observed under the loading and unloading operations. Compared with the original state without an external force, the size of the mesopores of the MX/rGO aerogel during the compression process obviously decreases, as well as the micropores as shown in lowand high-magnification SEM images in Figure 3a-b, d-e, g-h. Actually, the change of macroscopic structure was visible to the naked eye as shown the inset of Figure 2i. During the compression process, the pores in the MX/rGO aerogel squashed, and the

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distance between the pores decreased, thus leading to the increase of conductive path. Naturally, this cause increase of the corresponding current under the compressed state when a fixed voltage is applied, which is the core physical mechanism in the piezoresistive sensor. Most importantly, after releasing the stress, the MX/rGO aerogel nearly recovered its original shape as shown in Figure 3c, f, i, which was in agreement with the compression texts. In the high magnification SEM images, we measured and analyzed the volume change of MX/rGO aerogel after loading; it is found that the volume can return to 97.8% of its original volume as shown in Figure S6. Pressure-Sensing Properties of the MX/rGO aerogel sensor Apparently, the mechanical property of MX/rGO aerogel is superior to pure rGO aerogel, but how about the electrical properties? So a series of the static and dynamic electrical texts were carried out as follows. Our study found that various ratios of MXene and GO cause various electrical response under the same pressure value as shown in Figure 4a. When the ratio of GO and MXene is ~10:1, the current intensity reached the maximum, and the MX/rGO hybrid aerogel tends to higher current intensity than that of pure rGO, as shown in Figure 4a. Figure 4b shows the relative current change of this MX/rGO aerogel-based sensor with applying various pressure. It is observed that with continuously increasing pressure, the current intensity gradually increases, making sure the sensors to distinguish different levels of pressure. Furthermore, the linear relation of the I–V curves (Figure. 4c) from -0.1 V to 0.1 V suggested that ohmic contacts were formed between MX/rGO and interdigital

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electrodes. With the increase of the pressure, the slope of I-V curves increased, indicating the continuous decrease of the sensor’s resistivity. Our MX/rGO aerogel-based sensor showed a steady and constant response to dynamic pressure with various rates controlled by the stepping motor, while the number of loading-unloading periods is varied, as shown in Figure S7. The sensitivity of the pressure sensor is defined as S = ( ∆I I off

)

∆P , where ∆I is

the relative change in current, I off is current of the sensor without loading and ∆P is the change of the applied force. Figure 4d presents the electrical response of the pressure sensors with various ratio of GO and MXene, indicating that the middle ratio (10:1) has the highest sensitivity compared to the large one (5:1) and the small one (20:1), let alone the pure rGO. The high sensitivity of MX/rGO aerogel-based sensor can be explained as follows. On the one hand, as the aerogel is pressed, the pores constituted by nanosheets will closely contact each other so that the contact area is increased, leading to the increase of current. On the other hand, when adding MXene (Ti3C2Tx) with metallic conductivity into the rGO system, the MX/rGO sample tends to exhibit larger current variation than that of pure GO. Under a constant concentration of rGO, with increasing the addition amount of MXene, the sensitivity of the hybrid aerogel progressively increases due to the increase of the conductive path. However, the excess addition of Ti3C2Tx will significantly increase conductivity (when the ratio of GO and MXene exceeds 10:1), making it become a good conductor, in return affecting its further increase of corresponding conductive pathways. When the conductive path approaches saturation, the resistance variation tends to decrease,

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so the ratio of 10:1 is higher than that a ratio of 5:1. In addition, the plots of the current change versus pressure can be divided into two parts, i.e., the low and high-pressure regions, similar to most of the reported pressure sensors. In the low-pressure region under 1 kPa, the sensitivity is 4.05 kPa-1, while in the high-pressure regions over 1 kPa, the sensitivity is 22.56 kPa-1, which is apparently higher than Ag nanowires, graphene, carbon nanotube and other based pressure sensor as shown in Figure S8 .7, 37-39 The reason why the sensitivity under a larger applied force is higher is that the large deformation of the many micropores ranging from several micrometers to dozens of micrometers in our aerogel sample happens only in relatively high pressure. The MX/rGO aerogel-based sensor offers rapid response (245 ms) and recovery abilities (212 ms), as shown in Figure 4e. Taken into consideration of 3D network aerogel structure, the rapid response and recovery time are so fast to meet the practical application. And compared to MX/rGO aerogel, the pure rGO aerogel displays slower recovery rate while the response time is almost the same. To evaluate the durability of the MX/rGO aerogel-based sensors, we conducted 10,000 cycle tests by applying and releasing pressure of 0.81 kPa as shown in Figure 4f. It is observed that after 10,000 cycles, the sensor signal shows little attenuation and maintaines nearly identical sharp current amplitude after each loading-unloading cycle, except for a current shift due to the relatively small sliding displacement, indicating that the pressure sensors have a long working life and high stability. The practical application of the MX/rGO aerogel sensor

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The MX/rGO aerogel-based sensor shares the advantages of highly conductive network, high sensitivity and robust mechanical properties, which is favorable for the potential applications. Firstly, an RGB-LED light is applied to the sensor to test its electrical response to varieties of pressures by forming a series circuit under a power supply of 3V. As shown in Figure 5, the luminance of the LED light gets brighter as the external force increases from 0 to 4.90 kPa, and the predominant color of the light converts from red to green then to blue, reflecting an increase of current stemming from the decreased resistance when suffered force continually. The current threshold for operation of the individual red, green and blue channels of the RGB-LED under the constant voltage is gradually increased, so the final color of the LED light is thus a mixed combination of RGB and tends to become blue for the strongest applied force (4.90 kPa). Afterwards, the limit of detection (LOD) of our sensor was also measured to be ~10 Pa, which is lower than that of Ag nanowires (13 Pa)7 and conductive composite arrays (23 Pa)40. And it is found that even placing a flower bud onto the MX/rGO aerogel, the electrical response is obvious as shown in Figure 6a. Subsequently, our sensor was attached to throat of a person, it recognized the different word “one,” “two”, “three” and “four” as Figure 6b, indicating the high sensitivity. Monitoring the jugular venous pulse (JVP) is important for human to diagnose some disease. The relevant waveforms of the sensor to measure JVP was obtained in Figure 6c, where the fluctuant change is obvious. Except for the above application, the MX/rGO aerogel-based sensor was attached to a wrist of an adult man as a pulse detector. As

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shown in Figure 6d, the characteristic peaks of the typical human pulse waveform corresponding to “P” (percussion), and “D” (diastolic) are clearly distinguishable

41

,

demonstrating the high sensitivity of the sensor. As a consequence, the MX/rGO aerogel-based sensor performed well in the real-time detection of human health.

Conclusion In summary, a hybrid MX/rGO aerogel was fabricated by the simple ice-template freezing technique and applied in the piezoresistive sensor. Based on the synergistic effect between MXene and rGO, this 3D aerogel displayed good performance in sensing device, demonstrating high sensitivity (22.56 kPa-1), fast response time (< 200 ms) and good stability over 10,000 cycles. The limit of detection (LOD) of the sensor was 10 Pa, which is meaningful for its practical application. And compared to the pure rGO aerogel, the mechanical properties and sensitivity of MX/rGO aerogel have been greatly improved. The sensor not only recognizes a wide range of pressure but also detects subtle healthy activities, even the wrist pulse beating. We provide a low-cost, large-scale method to fabricate sensor based on MXene hybrid materials for excellent performances.

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Experimental section Materials TiH2, Al, and C powders were respectively purchased from Aladdin Reagent Company. The crystalline flake graphite was purchased from the Sinopharm Chemical Reagent Company. Milli-Q water (18.2 MΩ, resistivity) was used for all solution preparations. Preparation of Ti3C2TX colloidal solution The Ti3AlC2 MAX phase was synthesized by mixed TiH2, Al, and C powders in a molar ratio of 3:1.1:2. The powders were ball-milled for 18 h. Then the obtained mixtures were placed in a graphite crucible in a furnace and heated to 1400 °C at 10 °C /min for 2 h under flowing argon, Ar. Ti3C2TX MXene nanosheet was synthesized by the following etching procedures: 1g of Ti3AlC2 powder was slowly added into a solution of 1 g lithium fluoride (LiF, Alfa Aesar, 98+%) in 20 ml 9 M hydrochloric acid (HCl, Fisher, technical grade, 35-38%), and followed by magnetic stirring at 35 °C for 24 h. The acidic suspension was washed by centrifuged for 6 times until a stable dark green supernatant of Ti3C2TX was obtained with pH ≥ 6. The above Ti3C2TX flakes were then sonicated in a cold bath for 60 min, followed by centrifugation at 3500 rpm for 60 min. The concentration of MXene solution was quantified by filtrating, drying and weighing the MXene film. Preparation of GO colloidal solution The GO is synthesized by the modified Hummers method using flake graphite.42

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The final obtained concentration of GO solution is 10 M. The concentration of GO solution was quantified by filtrating, drying and weighing the GO film. Preparation of MX/rGO aerogel and the MX/rGO-based pressure sensor The homogeneous solution was obtained by simple mixing GO solution with MXenes solution, and then sonicated for 10 min. To optimize the best properties of the MX/rGO aerogel, the various ratio of GO: MXene (20:1, 10:1, 5:1) were synthesized and investigated. Then the above-mixed solution was transferred into any container and freeze-dried (−60 °C, 1 Pa) to form the aerogel. The final MX/rGO aerogel was got by further annealing at 200 °C in the flow of 10% H2 and 90% Ar gas for 2 h. The pure rGO aerogel was prepared by the same technique as the contrast experiment. And MX/rGO-based pressure sensor was fabricated by fixed suitable size sample (MX/rGO or rGO) in the flexible polyimide (PI) interdigital electrodes, then fixed and encapsulated with a piece of plastic polyethylene (PE) film. Finally, the sensing element was finally connected with copper wire by silver paste. The preparation process of PI interdigital electrodes was described in our previous paper in detail.35 Characterization and Measurement. The morphology and structure, and composition of the MXene and MX/rGO aerogel were investigated by high-resolution field emission scanning electron microscope (SEM, FEI Nova NanoSEM 450, 10 kV), a transmission electron microscope (TEM, FEI Titan G2 60-300), and a X-ray diﬀractometer (XRD Rigaku X-ray diﬀractometer with Cu Kα-radiation and Ni filter). The in-situ tests of Figure 3

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were carried out by a focused ion beam (FIB) microscope (FEI Quanta 3D FEG) equipped with a nanomanipulator (Oxford Instruments OmniProbe 100). The particle size distribution of MXene solution was measured and analyzed by Nano-ZEN 3600 (Malvern Instruments, UK) equipped with a He-Ne laser (633 nm). To test the response of our MX/rGO-based sensors under static and dynamic forces, a system with a computer controlled stepping motor, a force sensor and an electrochemical workstation (Agilent B2901A) were used. The input voltage was set to 0.1 V during the tests.

Acknowledgments: Authors Y. M. and Y. Y. contributed equally to this work. This work was supported by the National Natural Science Foundations of China (11374110, 11674113 and 51371085) and the authors gratefully acknowledge financial supported by Key Laboratory of Automotive Power Train and Electronics （Hubei University of Automotive Technology）. The authors thank the Analysis and Testing Center of HUST for support and Dr. Qing Wu Huang for the XPS testing.

ASSOCIATED CONTENT Supporting Information Figures giving (1) The Tyndall effect of (a) MXene solution, (b) GO solution, (c) MX/GO solution; (2) The size distribution of GO nanosheets; (3) The photographs of MX/GO and MX/rGO aerogel; (4) The SEM images of MX/rGO aerogel with varous ratios (GO:MXene); (5) Plan-view of MX/rGO aerogel and its elements’ mapping; (6) The microstructure comparison of the MX/rGO aerogel before and after compression; (7) The current response of the MX/rGO aerogel operated at various test speed; (8) Sensitivity comparison of six pressure sensors. The highest sensitivity was chosen under pressure below 30 kPa.

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This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author Tel:+86-15807135274 Email: [email protected] (YHG)

Author Contributions Yihua Gao & Nishuang Liu devised the original concept, designed the experiments, discussed the interpretation of results and revised the paper; Yanan Ma and Yang Yue performed almost all the experiments; Yanan Ma wrote the draft of the manuscript; Hang Zhang, Feng Cheng, Wanqiu Zhao and Jiangyu Rao contributed some SEM and TEM observations. Jie Wang contributed the size distribution of MXene and GO solutions. Shijun Luo, Xueliang Jiang and Zhitian Liu contributed the discussion of the manuscript. ¶These two authors contributed equally to this work. All authors participated in manuscript revision.

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Figure 1. Pressure sensor based on the MX/rGO aerogel. (a) Schematic illustration of fabrication of MX/rGO aerogel. (b) Schematic illustration of fabrication of MX/rGO aerogel-based sensor. (c) Schematic illustration of the sensing mechanism.

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Figure 2. The characterization of the MX/rGO aerogel. (a) The photograph of MXene colloid solution, showing its Tyndall effect in the inset. (b) TEM image of the sheet of monolayer MXene, the inset shows its corresponding diffraction pattern. (c) The size distribution of MXene nanosheets. (d, e) The different sectional view SEM images of MX/rGO aerogel. (f) The high magnification SEM image of the MX/rGO aerogel. (g) Plan-view of MX/rGO nanosheet and its Ti, O elements’ mapping. (h) XRD patterns of MX/rGO and rGO aerogel. (i) Stress-strain curves of the MX/rGO and rGO aerogel, inset showing photographs of the MX/rGO under a compressing-releasing cycle.

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Figure 3. The inner microstructure of the MX/rGO aerogel and its in-situ dynamic process under external pressure. (a, d&g) The various magnification SEM images of the MX/rGO aerogel in the initial state. (b, e&h) The various magnification SEM images of the MX/rGO aerogel in the compressed state, corresponding to (a, d&g), respectively. (c, f&i) The various magnification SEM images of the MX/rGO aerogel in (b, e&h) after releasing.

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Figure 4. The basic sensing performance of the MX/rGO aerogel. (a) The I-T curves of MX/rGO aerogel with various ratios of GO: MXene (5:1, 10:1, 20:1, pure GO) under the same pressures. (b) The current change response under various applied pressures ranging from 115 Pa to 970 Pa. (c) The I-V curves of MX/rGO aerogel device. (d) The relative current change with respect to the applied pressure, the four curves respectively represent the different ratio of GO: MXene (5:1, 10:1, 20:1, 1:0). (e) The responsive time and recovery time of MX/rGO aerogel and rGO aerogel. (f) The current output after more than 10,000 loading and unloading cycles under 510 Pa, indicating well durability.

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Figure 5. The response of the LED luminance varies with the applied pressure on the MX/rGO aerogel sensors. (a) 0 kPa. (b) 0.245 kPa. (c) 0.49 kPa. (d) 0.98 kPa. (e) 2.45 kPa. (f) 4.90 kPa.

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Figure 6. Real-time monitoring tiny strain, human sounds, and motions using MX/rGO aerogel sensors. (a) The current response of the pressure sensor operating with a small flower bud. (b) The current response of the pressure sensor when the wearer spoke “ONE,” “TWO,” and “THREE,” “FOUR,” the inset showing a photograph of the sensors attached to the throat. (c) The current response of the jugular venous pulse (JVP) by attaching the sensor to the neck. (d) The current response of the arterial pulse waves by attaching the sensor to a wrist.

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