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chitosan aerogel for piezoresistive pressure sensor
Journal Pre-proofs Conductive and superhydrophobic F-rGO@CNTs/chitosan aerogel for piezoresistive pressure sensor Jingjing Wu, Hongqiang Li, Xuejun Lai, Zhonghua Chen, Xingrong Zeng PII: DOI: Reference:
S1385-8947(19)33413-8 https://doi.org/10.1016/j.cej.2019.123998 CEJ 123998
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
26 August 2019 12 December 2019 30 December 2019
Please cite this article as: J. Wu, H. Li, X. Lai, Z. Chen, X. Zeng, Conductive and superhydrophobic F-rGO@CNTs/ chitosan aerogel for piezoresistive pressure sensor, Chemical Engineering Journal (2019), doi: https://doi.org/ 10.1016/j.cej.2019.123998
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Conductive and superhydrophobic F-rGO@CNTs/chitosan aerogel for piezoresistive pressure sensor Jingjing Wu, Hongqiang Li,* Xuejun Lai, Zhonghua Chen,* Xingrong Zeng School of Materials Science and Engineering, Key Lab of Guangdong Province for High Property and Functional Polymer Materials, South China University of Technology, Guangzhou 510640, China
Corresponding authors: [email protected]; [email protected] ABSTRACT: Piezoresistive pressure sensors with high sensitivity, fast response, and simplified signal collection play an important role in a wide variety of fields. However, most of them are water-sensitive and easily attacked by water, leading to serious signal distortion in practical application. Herein, we fabricated a conductive and superhydrophobic 1H,1H,2H,2H-perfluorooctyltriethoxysilane
(FAS)
modified
reduced
graphene
oxide@carbon nanotubes/chitosan (F-rGO@CNTs/CS) aerogel for piezoresistive pressure sensor. Benefiting from the porous structure of aerogel and the synergy of CNTs and rGO, the aerogel sensor achieved high sensitivity and fast response. Moreover, the sensor maintained a stable electrical resistance response after 1000 loading-unloading cycles. Importantly, owing to the rough structure constructed by CNTs and multi-pores and the low surface energy of FAS, the sensor possessed superhydrophobic property with a high water contact angle of 154o, and exhibited remarkable water repellency even during compression process. In addition, the sensor was successfully applied for detecting human behaviors from small-scale muscle movements to large-scale body motions. Our findings provide a new direction to fabricate functional and high-performance piezoresistive pressure sensor for various applications even 1
under water or wet environment. KEYWORDS: Piezoresistive pressure sensor; Superhydrophobic; Carbon nanotubes; Chitosan; Aerogel; Human motion
1. Introduction Flexible electronics have attracted extensive attention because of their broad applications in electronic skin [1,2], human healthcare [3,4], sport motion monitoring [5,6], smart robotics [7,8], and interactive wearable devices [9-11]. As a typical flexible electronic device, piezoresistive pressure sensor with the capability of converting pressure into resistance signal has become one of the current research hotspots for its high sensitivity, fast response, cost-effective process and simplified signal collection [12-15]. For example, Zhang et al. [16] prepared a piezoresistive pressure sensor based on sponge@carbon nanotubes@silver nanoparticles with good stability and high gauge factor of resistance signal to strain via dip-coating method. Similarly, Ge et al. [17] developed a flexible piezoresistive pressure sensor with tunable sensitivity based on a highly compressible reduced graphene oxide (rGO)/polyaniline wrapped sponge. After mixing chitosan and cellulose nanocrystal in FeCl3 aqueous solution, directional freeze-casting and carbonization, Hu et al. [18] obtained a compressible carbon aerogel with ultrahigh mechanical performance and superior sensitivity. Our group [19] also constructed a multilayer-structure piezoresistive pressure sensor with an in situ generated thiolated graphene@polyester fabric. However, limited by the lack of hydrophobicity, these sensors were easily attacked by water to cause a short circuit or a 2
decrease in electrical conductivity. To date, superhydrophobic surfaces have been widely applied in self-cleaning [20], anti-icing [21-23], anticorrosion [24,25], microfluidic devices [26], and oil-water separation [27-29] for their special wettability. In recent years, with the rapid development of modern artificial intelligence, superhydrophobic surfaces have been proposed to endow conductive materials with water repellency [30-33]. For instance, Li et al. [34] prepared a superhydrophobic and piezoresistive coating with high flexibility by spray-coating carbon nanotubes (CNTs) dispersed in a thermoplastic elastomer solution, followed by treatment with
ethanol.
Taking
advantage
of
the
elasticity
of
polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS) and the hydrophobicity and conductivity of 1-octadecanethiol-modified silver nanoparticles, Su et al. [35] achieved a highly stretchable and conductive superhydrophobic coating via spraying method. Zhu et al. [36] adopted polydimethylsiloxane (PDMS) and CNTs to hydrophobize the surface of a commonly used glassy carbon electrode to achieve self-cleaning ability. Nevertheless, the mentioned conductive and superhydrophobic materials were only limited to coating layer, leading to the relatively low sensitivity and narrow working range, and the application was seriously restricted. Recently, superhydrophobic aerogels for pressure sensing were proposed and prepared by some researcher [37-41], whereas the preservation of the water repellency of the aerogels is still a big challenge. Herein, we present a simple freeze-drying method to fabricate a carbon nanotubes/chitosan (CNTs/CS) aerogel, followed by dip-coating of graphene oxide (GO), reduction by ascorbic acid and modification with 1H,1H,2H,2H-perfluorooctyltriethoxysilane (FAS) on the surface 3
to finally obtain a conductive and superhydrophobic aerogel for piezoresistive pressure sensor. The schematic illustration of the fabrication process is displayed in Fig. 1. Due to the advantages of biocompatibility, nontoxicity, low cost and great flexibility after crosslinking [42], the CS from natural chitin through deacetylation was selected as the skeleton of the aerogel. Meanwhile, to enhance the mechanical property of the porous aerogel and increase the conductive contact points, one-dimensional carboxylated CNTs (CNTs-COOH) were incorporated into the CS skeleton to form crosslinking structure in the presence of glutaraldehyde (GA). Two-dimensional rGO wrapped on the surface of the skeleton further increased the conductivity and improve the sensitivity of the aerogel, and FAS endowed the aerogel
with
low
surface
energy
through
grafting
modification.
The
obtained
F-rGO@CNTs/CS aerogel possessed superhydrophobicity with a water contact angle (WCA) of 154o, and exhibited excellent compressibility and resilience. The aerogel based piezoresistive pressure sensor showed high sensitivity, fast response and good repeatability. The sensor was successfully applied as a wearable electronic device to monitor various human motions. Importantly, the sensor consistently maintained excellent water repellency during compression process, and the WCA was still above 150o even after 50 compression cycles. The approach in this work for fabricating conductive and superhydrophobic aerogel is facile and cost-effective, and the aerogel has great potential in wide application fields such as wearable devices, electronic skin, and artificial intelligence even under water or wet environment.
2. Experimental section
4
2.1. Materials Multiwalled carbon nanotubes (CNTs, >90.0%, the length was 10−30 nm, the internal and outer diameters were 5−10 and 20−40 nm, respectively), N,N-dimethylformamide (DMF, >99.9%) and ascorbic acid (AR) were bought from Aladdin Reagent Co., Ltd. (China). Chitosan (CS, deacetylation degree ≥85%) was purchased from Zhejiang Aoxing Biotechnology Co., Ltd. (China). Acetic acid (HAc, ≥99.5%, AR), glutaraldehyde (GA, 50%) and anhydrous ethanol (AR) were supplied by Guangzhou Chemical Reagent Factory (China). 1H,1H,2H,2H-perfluorooctyltriethoxysilane (FAS) was obtained from Evonik Co., Ltd. (Germany). All chemicals were used as received without further purification, and deionized water was used for all the experiments and tests. 2.2. Preparation of CNTs/CS aerogel First, CNTs-COOH were synthesized according to the procedure reported by our group [42] (Supporting Information). Various amount of CNTs-COOH were dispersed in HAc aqueous solution (1%, v/v) by ultrasonication. Next, 0.1 g of CS was dissolved in 5 mL of CNTs-COOH dispersion under magnetic stirring at room temperature for 6 h to obtain a homogenous mixture. Subsequently, GA aqueous solution (10 wt% with respect to the amount of CS) was added dropwise into the mixture under magnetic stirring, and CNTs/CS hydrogel was formed after standing for 1 h. Finally, the hydrogel was frozen at −15 oC for 12 h and treated via a freeze-drying process with a condenser temperature of −50 oC and inside pressure of 10 Pa in a lyophilizer (FD-1A-50+, Beijing boyikang instruments Co., Ltd., China) to obtain CNTs/CS aerogel. In this case, the CNTs-COOH content of the aerogel was varied 5
at 10, 20, 30, 40 and 50 wt%, respectively. For comparison, the pure CS aerogel was also prepared via the same procedure by directly dissolving CS powder into HAc aqueous solution instead of CNTs-COOH dispersion. 2.3. Fabrication of conductive and superhydrophobic F-rGO@CNTs/CS aerogel GO was synthesized according to the modified Hummers’ method [44] (Supporting Information), and dispersed in DMF/water solution with a volume ratio of 3:1 by ultrasonic treatment to form GO suspension with a concentration of 3 mg mL-1. The as-fabricated CNTs/CS aerogel was immersed into GO suspension and compressed for several times to draw GO into the aerogel. Then, the sample was squeezed to remove excess GO dispersion and dried at 60 oC for 30 min to obtain a GO-wrapped aerogel. Afterwards, the wrapped GO was reduced into rGO by ascorbic acid ethanol solution (30 mg mL-1) at 70 oC for 3 h to achieve a rGO@CNTs/CS aerogel. Finally, the rGO@CNTs/CS aerogel was immersed into a FAS ethanol solution (1 wt%) at room temperature for 24 h and rinsed by ethanol to remove the residual on the surface of the aerogel skeleton, and the conductive and superhydrophobic F-rGO@CNTs/CS aerogel was obtained after drying at 80 oC for 24 h. 2.4 Characterizations Fourier transform infrared spectroscopy (FT-IR) was collected on a Bruker Tensor 27 spectrometer (Bruker Optics, Germany) from 4000 to 400 cm-1 with 16 scans at a resolution of 4 cm-1. Thermogravimetric (TG) analysis was performed on a thermal analyzer (TG209, Netzsch, Germany) under N2 atmosphere at a heating rate of 20 oC min-1 from 30 to 800 oC. The mechanical compressibility of the aerogel was evaluated on a universal testing machine 6
(ESM303, Mark-10, USA) equipped with a 50 N load cell. Surface morphologies of the aerogel were characterized with a scanning electron microscope (SEM, EVO 18, Carl Zeiss Jena, Germany) at an acceleration voltage of 10.0 kV. Element composition analysis was carried out on an energy dispersion spectroscopy (EDS, INCA250, Oxford Instruments, UK) accompanied by SEM and X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD, UK) with a monochromated Al Kα source. Water contact angle (WCA) was measured by a contact angle meter (DSA100, Kruss, Germany) using a 6 µL water droplet as probe liquid at room temperature. Each WCA was tested for at least three different locations to calculate the average value. The variation of electrical resistance was recorded on a two-point probe multimeter (DMM6500, Keithley, USA).
3. Results and discussion 3.1. Preparation and characterizations of CNTs/CS aerogel via freeze-drying method In this work, pristine CNTs were firstly grafted with carboxyl groups through acid treatment to improve the dispersion in water. (Fig. S1 and S2). Compared with pristine CNTs, the FT-IR spectrum of CNTs-COOH presented some new peaks at 3417 and 1716 cm-1, corresponding to the stretching vibrations of hydroxyl and carbonyl bonds of carboxyl groups [42] (Fig. 2a). Besides, the TG curve of pristine CNTs kept stable until 800 oC, while the curve of CNTs-COOH exhibited a weight loss of 14.4% for the thermal decomposition of grafted carboxyl groups (Fig. 2b), further confirmed the successful modification of CNTs. Next, the CNTs/CS aerogel was prepared by stirring CS powders in the HAc aqueous dispersion of CNTs-COOH until homogenous, followed by adding GA to initiate chemical 7
crosslinking at room temperature, and then freeze drying for 48 h. Fig. 2c showed the FT-IR spectra of pristine CS, crosslinked CS and crosslinked CNTs/CS composite. In the spectrum of pristine CS, the typical stretching vibrational peaks of hydroxyl and amino groups at 3440 cm-1, methyl groups at 2920 cm-1 and methylene groups at 2878 cm-1 were all clearly observed [45]. For crosslinked CS and CNTs/CS aerogel, a new peak at 1654 cm-1 and a shoulder at 1560 cm-1 were ascribed to the formation of C=N and C=C bonds, respectively, demonstrating the occurrence of the Schiff base reaction between the amino groups on CS and the aldehyde groups of GA [46]. In addition, the reaction was also confirmed by XPS analysis of CNTs/CS aerogel (Fig. S3). Thanks to three-dimensional porous structure by freeze drying method, the CNTs/CS aerogel was superlight with a density of 0.057 g cm-3 and could be supported by two very thin leaf stalks (Fig. 2d). As can be seen from the SEM image (Fig. 2f), the CNTs/CS aerogel appeared multi-pore structure. The pores were relatively regular, the pore diameter was 200−400 μm and the thickness of the skeleton was about 1 μm. Meanwhile, the SEM image of the inner wall of the pores revealed the presence of CNTs, which was very beneficial for the formation of conductive contact points and the construction of hierarchical roughness (Fig. 2g). Moreover, the magnified view of the cross-section of the skeleton showed that the CNTs were regularly arranged inside the skeleton to form the conductive pathways (Fig. 2h and i). With CNTs content increasing, the pore sizes of the aerogels were similar, while the surface roughness of the skeleton obviously increased (Fig. S4). Mechanical property is critical for the CNTs/CS aerogel, especially the reversible compressibility, which is crucial to ensure its reliability [47]. To assess the compressibility of 8
the CNTs/CS aerogels, the compression tests were carried out. As shown in Fig. 2e, the aerogel was easily compressed to a strain of 60% under external force. After removing the force, the aerogel quickly recovered its initial shape. To further investigate the mechanical compressibility of the CNTs/CS aerogel, the compressive stress-strain curves were measured and presented in Fig. 2j-l and Fig. S5. The stress returned to its premier value after unloading for each strain at 10−60%, and it could be clearly seen from the loading process that the curve was characterized by a linear elastic region for bending of frameworks, then a plateau region for buckling phenomena, followed by a densification region ascribed to the closure of the pores [48,49] (Fig. 2j). The compressibility of the CNTs/CS aerogel was mainly attributed to its connected porous structure, the crosslinking network of CS and the strong interaction between CNTs-COOH and CS. In the skeleton of the aerogel, the entangled CNTs tightly bonded with chemically crosslinked CS, which ensured the deformation of pore walls rather than the falling out of the CNTs from CS matrix during the compression [49,50]. After 10 loading-unloading cycles with a strain of 60%, the aerogel remained above 84% of the initial maximum stress (Fig. 2k). The energy loss coefficient (defined as the loop area relative to the area under the loading curve) decreased from 0.76 at the first cycle to 0.71 at the 10th cycle, which was due to the slight damage of the skeleton structure during compression process (Fig. S6). Fig. 2l showed the effect of CNTs contents on the compression property of the CNTs/CS aerogel. Obviously, the aerogels with different CNTs contents all exhibited the similar compressive stress-strain curves with three-segment regions. Without CNTs, the maximum stress value of the aerogel was only 22.2 kPa. With CNTs content increasing to 10, 20, 30, 40 and 50 wt%, the value reached 42.3, 47.0, 61.5, 74.1 and 85.0 kPa, respectively, fully 9
demonstrating the significant reinforcement role of CNTs. Here, the CNTs/CS aerogel fabricated with CNTs content of 40% was selected for the subsequent studies.
Fig.
1.
Schematic
illustration
for
fabricating
conductive
and
superhydrophobic
F-rGO@CNTs/CS aerogel.
10
Fig. 2. (a) FT-IR spectra of CNTs and CNTs-COOH. (b) TG curves of CNTs and CNTs-COOH. (c) FT-IR spectra of pristine CS, crosslinked CS and crosslinked CNTs/CS composite. (d) Photograph of the CNTs/CS aerogel with a diameter of 2 cm and a thickness of 1 cm standing on two thin leaf stalks. (e) Photographs of the loading-unloading process of the CNTs/CS aerogel at 60% strain. SEM images of (f) the CNTs/CS aerogel, (g) the inner wall of the pores and (h,i) cross-section of the skeleton with different magnifications. (j) Compressive stress-strain curves of the CNTs/CS aerogel at strains from 10 to 60%. (k) Cyclic compressive stress-strain curves of the CNTs/CS aerogel at 60% strain. (l) 11
Compressive stress-strain curves of the aerogel with different CNTs contents at 60% strain. 3.2.
Fabrication
and
characterizations
of
conductive
and
superhydrophobic
F-rGO@CNTs/CS aerogel To further construct the conductive pathways of the CNTs/CS and endow it with superhydrophobicity, the surface of the CNTs/CS skeleton was wrapped with a layer of GO under the role of hydrogen bonds by simple dipping. Subsequently, the wrapped GO layer was in situ reduced into rGO layer in the presence of ascorbic acid. After that, low-surface-energy FAS was grafted onto rGO layer through dehydration condensation to obtain conductive and superhydrophobic F-rGO@CNTs/CS aerogel. Fig. 3 presented the SEM images and corresponding WCAs optical images of the GO@CNTs/CS, rGO@CNTs/CS and F-rGO@CNTs/CS aerogels, respectively. It was obvious to note that the morphology of the CNTs/CS aerogel was unaffected by GO and rGO, and the GO@CNTs/CS and rGO@CNTs/CS aerogels also showed the similar morphology of the CNTs/CS aerogel (Fig. 3a-c). From the magnified image of rGO@CNTs/CS aerogel, many tiny curled wrinkles were clearly observed (Fig. 3c1). In addition, the modification had little effect on the mechanical property of the aerogel (Fig. S7). Furthermore, the wettability of the CNTs/CS, GO@CNTs/CS, rGO@CNTs/CS and F-rGO@CNTs/CS aerogels were compared. It can be seen that the CNTs/CS aerogel appeared superhydrophilic state with a WCA of 0o, which was due to the existence of the hydrophilic groups of CS and CNTs-COOH and the porous structure of the aerogel (Fig. 3a). The wettability of the GO@CNTs/CS aerogel had no change because of the polar groups of the GO, and the WCA still remained 0o (Fig. 3b).
12
Comparatively, owing to the removal of the oxygen-containing groups after reduction, the WCA of the rGO@CNTs/CS aerogel sharply increased to 122o (Fig. 3c). Furthermore, after modification by FAS, the obtained F-rGO@CNTs/CS aerogel realized superhydrophobicity with a high WCA of 154o (Fig. 3d). From the SEM elemental mapping images, fluorine was confirmed to be uniformly dispersed on the surface of the F-rGO@CNTs/CS aerogel (Fig. 3d1-d3).
Fig. 3. SEM images of (a) CNTs/CS aerogel, (b) GO@CNTs/CS aerogel and (c) rGO@CNTs/CS aerogel. (c1) Magnified SEM image of rGO. (d) SEM image of F-rGO@CNTs/CS aerogel for EDS analysis and the corresponding mappings of (d1) carbon, (d2) oxygen and (d3) fluorine elements. Inset on the top right were the corresponding WCA optical images. The surface chemical composition of the GO@CNTs/CS, rGO@CNTs/CS and F-rGO@CNTs/CS aerogels were further analyzed by XPS. From the XPS survey scans as shown in Fig. 4a, there were two peaks of C 1s at 284 eV and O 1s at 532 eV for the GO@CNTs/CS and rGO@CNTs/CS aerogels, respectively. Differently, several new peaks at 102, 153 and 688 eV corresponding to Si 2p, Si 2s and F 1s were observed in the spectrum of 13
F-rGO@CNTs/CS aerogel. Additionally, the C/O atomic ratio of the GO@CNTs/CS aerogel was 4.83, while that of rGO@CNTs/CS obviously decreased to 2.99. It was in accordance with the high-resolution C 1s core-level spectra. Furthermore, the C 1s core lever spectra of GO@CNTs/CS and rGO@CNTs/CS aerogels were divided into five peaks at 284.6, 285.5, 286.9, 288.1 and 289.2 eV, which assigned to C=C, C−C, C−O, C=O and O−C=O bonds, respectively [19] (Fig. 4b and c). Compared with GO@CNTs/CS aerogel, the peak intensities of C−O, C=O and O−C=O peaks of the rGO@CNTs/CS significantly decreased. In Fig. 4d, the appearance of the new peaks of −CF2 and −CF3 at 291.6 and 293.8 eV implied the occurrence of the reaction between the silanol groups of FAS and the residual hydroxyl groups of rGO [51].
Fig. 4. (a) XPS survey scans of the GO@CNTs/CS, rGO@CNTs/CS and F-rGO@CNTs/CS 14
aerogels. XPS C 1s spectra of the (b) GO@CNTs/CS aerogel, (c) rGO@CNTs/CS aerogel and (d) F-rGO@CNTs/CS aerogel, respectively. 3.3.
Wetting
behavior
and
electromechanical
performance
of
conductive
and
superhydrophobic F-rGO@CNTs/CS aerogel Water repellency is very important for conductive materials in practical application especially under water or wet environment. Here, the fabricated F-rGO@CNTs/CS aerogel possessed superhydrophobicity, and the water droplets could stand on the surface with a nearly spherical shape (Fig. 5a). When the CNTs/CS and F-rGO@CNTs/CS aerogels were immersed into water simultaneously, the F-rGO@CNTs/CS aerogel quickly floated on water surface, while the CNTs/CS aerogel sank into water (Fig. 5b). Interestingly, when the F-rGO@CNTs/CS aerogel was pressed into water by a tweezer, a bright mirror-like surface appeared for an air layer trapped by the rough structure of the aerogel to prevent the surface from contacting water [52] (Fig. 5c). Additionally, continuous water droplets were able to quickly roll off the inclined surface of the aerogel at an angle of 60o (Fig. 5d and Video S1). Moreover, the F-rGO@CNTs/CS aerogel exhibited stable superhydrophobicity under various environments (Fig. S8). Two copper sheets were respectively pasted on the top and bottom of the aerogel as electrodes by silver paste for piezoresistive pressure sensor. A simple piezoresistive test was conducted by connecting the sensor with several light-emitting diode (LED) bulbs in a circuit of 10 V (Fig. S9). The electromechanical property of the sensor was evaluated using an experimental test platform consisting of a computer collector, a universal testing machine and
15
a multimeter. To collect electrical response signal, the sensor was fixed on the chuck of the testing machine, and the two electrodes were respectively connected to two probes of the multimeter through copper wires. When the hold down applied a load on the sensor, the electrical resistance change was recorded by the computer (Fig. S10). Sensitivity is a key parameter of piezoresistive pressure sensor, which is defined as the ratio of relative resistance (ΔR/R0, R0 is the initial resistance value of the sensor without loading and ΔR is the absolute value of the resistance change with the applied pressure) changes to applied pressure (ΔP) [53,54]. Fig. 6b displayed the sensitivities of the CNTs/CS aerogel and F-rGO@CNTs/CS aerogel sensors, it can be seen that the sensitivity of the F-rGO@CNTs/CS aerogel sensor was obviously higher than that of the CNTs/CS aerogel sensor. The curve presented two linear intervals including a small pressure range of 0−3 kPa and a large pressure range of 40−80 kPa. The corresponding sensitivities were 4.97 and 0.05 kPa-1, respectively. To explain the sensing mechanism, the schematic illustration for the structural change of the aerogel under pressure was depicted in Fig. 6a. With the exertion of a small pressure, the number of conductive pathways increased for the mutual contacts of the inner walls of the pores, resulting in a rapid decrease of electrical resistance. As pressure increased, the contact area further increased due to the elastic deformation of the skeleton, and the resistance slightly decreased [19]. Additionally, from the real-time curves of ΔR/R0 change of the sensor at different applied strains, the ΔR/R0 appeared periodic changes with continuous loading-unloading cycles, and increased with strain increasing (Fig. 6c). Moreover, dynamic frequency had almost no effect on the electrical cycling response of the sensor, for the values of ΔR/R0 were similar to the different frequencies (Fig. 6d). Notably, 16
the response time of the sensor was only 170 ms, displaying a fast response to external stimuli (Fig. 6e). Importantly, the sensor showed a negligible change of ΔR/R0 even after 1000 loading-unloading cycles, demonstrating the excellent repeatability (Fig. 6f and Fig. S11). It is well known that a large deformation of superhydrophobic materials will lead to the destruction of rough structure or the detachment of low-surface-energy substance, resulting in the loss of its superhydrophobicity [35,55]. Different from the reported superhydrophobic materials, the F-rGO@CNTs/CS aerogel sensor revealed good superhydrophobic stability at large deformations. Fig. 5e presents the WCA variations of the aerogel sensor under different strains. Visibly, the sensor still kept superhydrophobic at a large strain of 60%. Besides, the WCA of the sensor remained above 150o even after 50 loading-unloading cycles (Fig. 5f). Furthermore, the effect of water on the electrical response of the sensor was evaluated and presented in Fig. 5g and Video S2. When a jet of water was sprayed on the flank of the sensor during compression, the response curve was unchanged at all, for the quick rolling off of the water droplets upon contacting the superhydrophobic surface. It indicated that the sensor can normally work under a humid or rainy environment.
17
Fig. 5. Photographs of (a) water droplets standing on the conductive and superhydrophobic F-rGO@CNTs/CS aerogel, (b) the CNTs/CS and F-rGO@CNTs/CS aerogels placed into water, and (c) the F-rGO@CNTs/CS aerogel immersed into water via an external force. (d) Rolling processes of water droplets falling on the surface of the F-rGO@CNTs/CS aerogel. (e) WCAs of the F-rGO@CNTs/CS aerogel at different compressive strains. (f) WCA change of the F-rGO@CNTs/CS aerogel with compression cycles. (g) Real-time electrical resistance variation with water injecting on the aerogel during compression process.
Fig. 6. (a) Schematic illustration of the sensor with the magnified image of its individual skeleton without pressure, under small and large pressures. (b) Relative resistance changes of the CNTs/CS aerogel and the F-rGO@CNTs/CS aerogel with pressure increasing. The response test of the sensor at different (c) strains and (d) frequencies. (e) Response time of the sensor. (f) Repeatability performance of the sensor for 1000 loading-unloading cycles with 30% strain. Inset was the enlarged view of the response curve. 3.4. Application of conductive and superhydrophobic F-rGO@CNTs/CS aerogel sensor in 18
human motion detection To investigate the feasibility of the F-rGO@CNTs/CS aerogel sensor in practical application, a series of tests to detect various human motions were carried out. First, an adhesive tape was utilized to fix the sensor on the cheek of a 24-year-old healthy female tester with a weight of 40 kg and a height of 160 cm. As the tester repeated the mouth-opening process, the tiny pressure change caused by the movement of the facial muscles was acutely detected by the sensor, thereby outputting a series of stable and ordered real-time curves of the ΔR/R0 change (Fig. 7a). Besides, the sensor attached on the tester’s neck could detect throat vibrations as shown in Fig. 7b and c. When the tester swallowed saliva, the sensor presented a regular ΔR/R0 response signal, which was generated by the movements of the muscle around the throat of the tester. Furthermore, when the word “Hello” was pronounced by the tester, a distinct periodic response signal corresponding to the tone of the word was observed, demonstrating the outstanding voice detection capability. Interestingly, the sensor displayed three different signal patterns for the above three motions, exhibiting its high accuracy of pressure response [17, 19]. In addition to the weak motions by small pressures, the sensor was also applied to monitor strong actions. As shown in Fig. 7d-f, the sensor was fixed on the joint of the tester’s finger, wrist, and elbow, respectively, where the large deformations occurred upon bending. It can be seen that the real-time curves were continuous, repetitive and responsive in all flexing-recovery processes. When the finger was at a straight state, the response curve was stable with a ΔR/R0 of 0. However, when the finger was bent, the sensor mounted on the arthrosis was squeezed, leading to the rapid increase of ΔR/R0 to form an upward steep peak. 19
Subsequently, with the extension of the finger, ΔR/R0 recovered to the original value. The results for bending the wrist and elbow were also compliance with this, exhibiting the good stability and repeatability of the sensor. To further monitor human sports, the sensor was placed on the insole to record the real-time response signals of walking, running, and jumping. Compared with the response waveform of walking, the frequency was significantly increased when running (Fig. 7g and h). Moreover, the variation amplitude of ΔR/R0 was enhanced during jumping (Fig. 7i). It was mainly due to the increase of the pressure on the sensor caused by a larger acceleration when landing.
Fig. 7. Real-time electrical resistance response curves of the piezoresistive pressure sensor
20
when the tester (a) opening mouth, (b) swallowing, (c) speaking “Hello”, (d) bending finger, (e) bending wrist, (f) bending elbow, (g) walking, (h) running, and (i) jumping, respectively.
4. Conclusions In
summary,
we
successfully
fabricated
a
conductive
and
superhydrophobic
F-rGO@CNTs/CS aerogel for piezoresistive pressure sensor. Owing to the reinforcement of CNTs, the CNTs/CS aerogel obtained by freeze-drying method possessed excellent mechanical property and compressibility. The F-rGO@CNTs/CS aerogel realized superhydrophobicity with a high WCA of 154o, and maintained superhydrophobic state during compression process. Taking advantage of the porous structure and the synergy of CNTs and rGO, the F-rGO@CNTs/CS aerogel sensor achieved the sensitivities of 4.97 kPa-1 in 0−3 kPa and 0.05 kPa-1 in 40−80 kPa, and a fast response time of 170 ms. Meanwhile, the sensor exhibited good stability and repeatability. Importantly, the sensor demonstrated the outstanding performance for detecting human motions including tiny muscle movements such as mouth opening, swallowing and speaking, and large body activities such as arthrosis bending (finger, wrist, and elbow), walking, running and jumping. The conductive and superhydrophobic F-rGO@CNTs/CS aerogel sensor are highly promising to be utilized in wearable devices, electronic skin, and other electronics, even under rainy or sweaty conditions.
Acknowledgements
21
The work was financially supported by the Science and Technology Planning Project of Guangdong Province, China (2018A030313884) and the Science and Technology Planning Project of Guangzhou City, China (201804010381).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version.
References [1] J. Kang, D. Son, G.N. Wang, Y. Liu, J. Lopez, Y. Kim, J.Y. Oh, T. Katsumata, J. Mun, Y. Lee, L. Jin, J.B. Tok, Z. Bao, Tough and water-insensitive self-healing elastomer for robust electronic skin, Adv. Mater. 30 (2018) 1706846. [2] Z. Ma, S. Li, H. Wang, W. Cheng, Y. Li, L. Pan, Y. Shi, Advanced electronic skin devices for healthcare applications, J. Mater. Chem. B 7 (2019) 173-197. [3] T. Wang, H. Yang, D. Qi, Z. Liu, P. Cai, H. Zhang, X. Chen, Mechano-based transductive sensing for wearable healthcare, Small 14 (2018) 1702933. [4] T.Q. Trung, N.E. Lee, Flexible and stretchable physical sensor integrated platforms for wearable human-activity monitoringand personal healthcare, Adv. Mater. 28 (2016) 4338-4372. [5] X. Jing, H.Y. Mi, Y.J. Lin, E. Enriquez, X.F. Peng, L.S. Turng, Highly stretchable and biocompatible strain sensors based on mussel-inspired super-adhesive self-healing hydrogels for human motion monitoring, ACS Appl. Mater. Interfaces 10 (2018) 20897-20909. [6] S. Ryu, P. Lee, J.B. Chou, R. Xu, R. Zhao, A.J. Hart, S.-G. Kim, Extremely elastic wearable carbon nanotube fiber strain sensor for monitoring of human motion, ACS Nano 9 22
(2015) 5929-5936. [7] T. Li, G. Li, Y. Liang, T. Cheng, J. Dai, X. Yang, B. Liu, Z. Zeng, Z. Huang, Y. Luo, Fast-moving soft electronic fish, Sci. Adv. 3 (2017) 1602045. [8] C. Wu, S. Zeng, Z. Wang, F. Wang, H. Zhou, J. Zhang, Z. Ci, L. Sun, Efficient mechanoluminescent
elastomers
for
dual-responsive
anticounterfeiting
device
and
stretching/strain sensor with multimode sensibility, Adv. Funct. Mater. 28 (2018) 1803168. [9] J. Cao, C. Lu, J. Zhuang, M. Liu, X. Zhang, Y. Yu, Q. Tao, Multiple hydrogen bonding enables the self-healing of sensors for human-machine interactions, Angew. Chem. Int. Ed. 56 (2017) 8795-8800. [10] A. Qiu, P. Li, Z. Yang, Y. Yao, I. Lee, J. Ma, A path beyond metal and silicon: polymer/nanomaterial composites for stretchable strain sensors, Adv. Funct. Mater. 29 (2019) 1806306. [11] J. Lee, H. Kwon, J. Seo, S. Shin, J.H. Koo, C. Pang, S. Son, J.H. Kim, Y.H. Jang, D.E. Kim, T. Lee, Conductive fiber-based ultrasensitive textile pressure sensor for wearable electronics, Adv. Mater. 27 (2015) 2433-2439. [12] S. Gong, W. Schwalb, Y. Wang, Y. Chen, Y. Tang, J. Si, B. Shirinzadeh, W. Cheng, A wearable and highly sensitive pressure sensor with ultrathin gold nanowires, Nat. Commun. 5 (2014) 3132. [13] J. Li, S. Orrego, J. Pan, P. He, S.H. Kang, Ultrasensitive, flexible, and low-cost nanoporous piezoresistive composites for tactile pressure sensing, Nanoscale 11 (2019) 2779-2786. [14] M. Cao, M. Wang, L. Li, H. Qiu, M.A. Padhiar, Z. Yang, Wearable rGO-Ag NW@cotton 23
fiber piezoresistive sensor based on the fast charge transport channel provided by Ag nanowire, Nano Energy 50 (2018) 528-535. [15] W. Huang, K. Dai, Y. Zhai, H. Liu, P. Zhan, J. Gao, G. Zheng, C. Liu, C. Shen, Flexible and
lightweight
pressure
sensor
based
on
carbon
nanotube/thermoplastic
polyurethane-aligned conductive foam with superior compressibility and stability, ACS Appl. Mater. Interfaces 9 (2017) 42266-42277. [16] H. Zhang, N. Liu, Y. Shi, W. Liu, Y. Yue, S. Wang, Y. Ma, L. Wen, L. Li, F. Long, Z. Zou, Y. Gao, Piezoresistive sensor with high elasticity based on 3D hybrid network of sponge@CNTs@Ag NPs, ACS Appl. Mater. Interfaces 8 (2016) 22374-22381. [17] G. Ge, Y. Cai, Q. Dong, Y. Zhang, J. Shao, W. Huang, X. Dong, A flexible pressure sensor based on rGO/polyaniline wrapped sponge with tunable sensitivity for human motion detection, Nanoscale 10 (2018) 10033-10040. [18] Y. Hu, H. Zhuo, Z. Chen, K. Wu, Q. Luo, Q. Liu, S. Jing, C. Liu, L. Zhong, R. Sun, X. Peng, Superelastic carbon aerogel with ultrahigh and wide-range linear sensitivity, ACS Appl. Mater. Interfaces 10 (2018) 40641-40650. [19] L. Zhang, H. Li, X. Lai, T. Gao, J. Yang, X. Zeng, Thiolated graphene@polyester fabric-based multilayer piezoresistive pressure sensors for detecting human motion, ACS Appl. Mater. Interfaces 10 (2018) 41784-41792. [20] T. Ren, J. He, Substrate-versatile approach to robust antireflective and superhydrophobic coatings with excellent self-cleaning property in varied environments, ACS Appl. Mater. Interfaces 9 (2017) 34367-34376. [21] L. Wang, Q. Gong, S. Zhan, L. Jiang, Y. Zheng, Robust anti-icing performance of a 24
flexible superhydrophobic surface, Adv. Mater. 28 (2016) 7729-7735. [22] T. Wang, Y. Zheng, A.R. Raji, Y. Li, W.K. Sikkema, J.M. Tour, Passive anti-icing and active deicing films, ACS Appl. Mater. Interfaces 8 (2016) 14169-14173. [23] T. Cheng, R. He, Q. Zhang, X. Zhan, F. Chen, Magnetic particle-based super-hydrophobic coatings with excellent anti-icing and thermoresponsive deicing performance, J. Mater. Chem. A 3 (2015) 21637-21646. [24] Y. Zhu, F. Sun, H. Qian, H. Wang, L. Mu, J. Zhu, A biomimetic spherical cactus superhydrophobic coating with durable and multiple anti-corrosion effects, Chem. Eng. J. 338 (2018) 670-679. [25] D. Zang, R. Zhu, W. Zhang, X. Yu, L. Lin, X. Guo, M. Liu, L. Jiang, Corrosion-resistant superhydrophobic coatings on Mg alloy surfaces inspired by lotus seedpod, Adv. Funct. Mater. 27 (2017) 1605446. [26] C. Li, M. Boban, S.A. Snyder, S.P.R. Kobaku, G. Kwon, G. Mehta, A. Tuteja, Paper-based surfaces with extreme wettabilities for novel, open-channel microfluidic devices, Adv. Funct. Mater. 26 (2016) 6121-6131. [27] Q. Ma, H. Cheng, A.G. Fane, R. Wang, H. Zhang, Recent development of advanced materials with special wettability for selective oil/water separation, Small 12 (2016) 2186-2202. [28] C. Cao, M. Ge, J. Huang, S. Li, S. Deng, S. Zhang, Z. Chen, K. Zhang, S.S. Al-Deyab, Y. Lai, Robust fluorine-free superhydrophobic PDMS-ormosil@fabrics for highly effective self-cleaning and efficient oil–water separation, J. Mater. Chem. A 4 (2016) 12179-12187. [29] J. Li, R. Kang, X. Tang, H. She, Y. Yang, F. Zha, Superhydrophobic meshes that can 25
repel hot water and strong corrosive liquids used for efficient gravity-driven oil/water separation, Nanoscale 8 (2016) 7638-7645. [30] Q. Li, H. Liu, S. Zhang, D. Zhang, X. Liu, Y. He, L. Mi, J. Zhang, C. Liu, C. Shen, Z. Guo, Superhydrophobic electrically conductive paper for ultrasensitive strain sensor with excellent anticorrosion and self-cleaning property, ACS Appl. Mater. Interfaces 11 (2019) 21904-21914. [31] L. Wang, J. Luo, Y. Chen, L. Lin, X. Huang, H. Xue, J.F. Gao, Fluorine-free superhydrophobic and conductive rubber composite with outstanding deicing performance for highly sensitive and stretchable strain sensors, ACS Appl. Mater. Interfaces (2019) 17774-17783. [32] J.N. Wang, Y. Q. Liu, Y.L. Zhang, J. Feng, H. Wang, Y.H. Yu, H.B. Sun, Wearable superhydrophobic elastomer skin with switchable wettability, Adv. Funct. Mater. 28 (2018) 1800625. [33] G. Ding, W. Jiao, L. Chen, M. Yan, L. Hao, R. Wang, A self-sensing, superhydrophobic, heterogeneous graphene network with controllable adhesion behavior, J. Mater. Chem. A 6 (2018) 16992-17000. [34] L. Li, Y. Bai, L. Li, S. Wang, T. Zhang, A superhydrophobic smart coating for flexible and wearable sensing electronics, Adv. Mater. 29 (2017) 1702517. [35] X. Su, H. Li, X. Lai, Z. Chen, X. Zeng, Highly stretchable and conductive superhydrophobic coating for flexible electronics, ACS Appl. Mater. Interfaces 10 (2018) 10587-10597. [36] X. Zhu, Y. Chen, C. Feng, W. Wang, B. Bo, R. Ren, G. Li, Assembly of self-cleaning 26
electrode surface for the development of refreshable biosensors, Anal. Chem. 89 (2017) 4131-4138. [37] J. Xiao, Y. Tan, Y. Song, Q. Zheng, A flyweight and superelastic graphene aerogel as a high-capacity adsorbent and highly sensitive pressure sensor, J. Mater. Chem. A 6 (2018) 9074-9080. [38] L.Z. Guan, J.F. Gao, Y.B. Pei, L. Zhao, L.X. Gong, Y.J. Wan, H. Zhou, N. Zheng, X.S. Du, L.B. Wu, J.X. Jiang, H.Y. Liu, L.C. Tang, Y.W. Mai, Silane bonded graphene aerogels with tunable functionality and reversible compressibility, Carbon 107 (2016) 573-582. [39] Y. Lin, G.J. Ehlert, C. Bukowsky, H.A. Sodano, Superhydrophobic functionalized graphene aerogels, ACS Appl. Mater. Interfaces 3 (2011) 2200-2203. [40] G. Zu, K. Kanamori, K. Nakanishi, J. Huang, Superhydrophobic ultraflexible triple-network graphene/polyorganosiloxane aerogels for a high-performance multifunctional temperature/strain/pressure sensing array, Chem. Mater. 31 (2019) 6276-6285. [41] G. Zu, K. Kanamori, K. Nakanishi, X. Lu, K. Yu, J. Huang, H. Sugimura, Superelastic multifunctional aminosilane-crosslinked graphene aerogels for high thermal insulation, three-component separation, and strain/pressure-sensing arrays, ACS Appl. Mater. Interfaces 11 (2019) 43533-43542. [42] C. Su, H. Yang, H. Zhao, Y. Liu, R. Chen, Recyclable and biodegradable superhydrophobic and superoleophilic chitosan sponge for the effective removal of oily pollutants from water, Chem. Eng. J. 330 (2017) 423-432. [43] X. Su, H. Li, X. Lai, Z. Yang, Z. Chen, W. Wu, X. Zeng, Vacuum-assisted layer-by-layer superhydrophobic carbon nanotube films with electrothermal and photothermal effects for 27
deicing and controllable manipulation, J. Mater. Chem. A 6 (2018) 16910-16919. [44] L. Zhang, H. Li, X. Lai, X. Su, T. Liang, X. Zeng, Thiolated graphene-based superhydrophobic sponges for oil-water separation, Chem. Eng. J. 316 (2017) 736-743. [45] J. Yang, L. Yi, X. Fang, Y. Song, L. Zhao, J. Wu, H. Wu, Self-healing and recyclable biomass aerogel formed by electrostatic interaction, Chem. Eng. J. 371 (2019) 213-221. [46] M. Salzano de Luna, C. Ascione, C. Santillo, L. Verdolotti, M. Lavorgna, G.G. Buonocore, R. Castaldo, G. Filippone, H. Xia, L. Ambrosio, Optimization of dye adsorption capacity and mechanical strength of chitosan aerogels through crosslinking strategy and graphene oxide addition, Carbohyd. polym. 211 (2019) 195-203. [47] Y.J. Wan, P.L. Zhu, S.H. Yu, R. Sun, C.P. Wong, W.H. Liao, Ultralight, super-elastic and volume-preserving cellulose fiber/graphene aerogel for high-performance electromagnetic interference shielding, Carbon 115 (2017) 629-639. [48] X. Su, H. Li, X. Lai, Z. Chen, X. Zeng, 3D porous superhydrophobic CNT/EVA composites for recoverable shape reconfiguration and underwater vibration detection, Adv. Funct. Mater. 29 (2019) 1900554. [49] W. He, G. Li, S. Zhang, Y. Wei, J. Wang, Q. Li, X. Zhang, Polypyrrole/silver coaxial nanowire aero-sponges for temperature-independent stress sensing and stress-triggered joule heating, ACS Nano 9 (2015) 4244-4251. [50] P. Lv, X.W. Tan, K.H. Yu, R.L. Zheng, J.J. Zheng, W. Wei, Super-elastic graphene/carbon nanotube aerogel: A novel thermal interface material with highly thermal transport properties, Carbon 99 (2016) 222-228. [51] F. Qiang, L.L. Hu, L.X. Gong, L. Zhao, S.-N. Li, L.C. Tang, Facile synthesis of 28
super-hydrophobic, electrically conductive and mechanically flexible functionalized graphene nanoribbon/polyurethane sponge for efficient oil/water separation at static and dynamic states, Chem. Eng. J. 334 (2018) 2154-2166. [52] X. Su, H. Li, X. Lai, L. Zhang, J. Wang, X. Liao, X. Zeng, Vapor-liquid sol-gel approach to fabricating highly durable and robust superhydrophobic polydimethylsiloxane@silica surface on polyester textile for oil-water separation, ACS Appl. Mater. Interfaces 9 (2017) 28089-28099. [53] H. Li, K. Wu, Z. Xu, Z. Wang, Y. Meng, L. Li, Ultrahigh-sensitivity piezoresistive pressure sensors for detection of tiny pressure, ACS Appl. Mater. Interfaces 10 (2018) 20826-20834. [54] A. Tewari, S. Gandla, S. Bohm, C.R. McNeill, D. Gupta, Highly exfoliated MWCNT-rGO ink-wrapped polyurethane foam for piezoresistive pressure sensor applications, ACS Appl. Mater. Interfaces 10 (2018) 5185-5195. [55] J. Ju, X. Yao, X. Hou, Q. Liu, Y.S. Zhang, A. Khademhosseini, A highly stretchable and robust non-fluorinated superhydrophobic surface, J. Mater. Chem. A 5 (2017) 16273-16280.
29
F-rGO@CNTs/CS aerogel was simply prepared by freeze-drying and dip-coating.
F-rGO@CNTs/CS
aerogel
possessed
superhydrophobicity,
compressibility
and
resilience.
F-rGO@CNTs/CS aerogel based sensor showed high sensitivity, fast response and good repeatability.
The sensor was successfully applied for detecting human motions even under wet environment.
30
31
S1385-8947(19)33413-8 https://doi.org/10.1016/j.cej.2019.123998 CEJ 123998
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
26 August 2019 12 December 2019 30 December 2019
Please cite this article as: J. Wu, H. Li, X. Lai, Z. Chen, X. Zeng, Conductive and superhydrophobic F-rGO@CNTs/ chitosan aerogel for piezoresistive pressure sensor, Chemical Engineering Journal (2019), doi: https://doi.org/ 10.1016/j.cej.2019.123998
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Conductive and superhydrophobic F-rGO@CNTs/chitosan aerogel for piezoresistive pressure sensor Jingjing Wu, Hongqiang Li,* Xuejun Lai, Zhonghua Chen,* Xingrong Zeng School of Materials Science and Engineering, Key Lab of Guangdong Province for High Property and Functional Polymer Materials, South China University of Technology, Guangzhou 510640, China
Corresponding authors: [email protected]; [email protected] ABSTRACT: Piezoresistive pressure sensors with high sensitivity, fast response, and simplified signal collection play an important role in a wide variety of fields. However, most of them are water-sensitive and easily attacked by water, leading to serious signal distortion in practical application. Herein, we fabricated a conductive and superhydrophobic 1H,1H,2H,2H-perfluorooctyltriethoxysilane
(FAS)
modified
reduced
graphene
oxide@carbon nanotubes/chitosan (F-rGO@CNTs/CS) aerogel for piezoresistive pressure sensor. Benefiting from the porous structure of aerogel and the synergy of CNTs and rGO, the aerogel sensor achieved high sensitivity and fast response. Moreover, the sensor maintained a stable electrical resistance response after 1000 loading-unloading cycles. Importantly, owing to the rough structure constructed by CNTs and multi-pores and the low surface energy of FAS, the sensor possessed superhydrophobic property with a high water contact angle of 154o, and exhibited remarkable water repellency even during compression process. In addition, the sensor was successfully applied for detecting human behaviors from small-scale muscle movements to large-scale body motions. Our findings provide a new direction to fabricate functional and high-performance piezoresistive pressure sensor for various applications even 1
under water or wet environment. KEYWORDS: Piezoresistive pressure sensor; Superhydrophobic; Carbon nanotubes; Chitosan; Aerogel; Human motion
1. Introduction Flexible electronics have attracted extensive attention because of their broad applications in electronic skin [1,2], human healthcare [3,4], sport motion monitoring [5,6], smart robotics [7,8], and interactive wearable devices [9-11]. As a typical flexible electronic device, piezoresistive pressure sensor with the capability of converting pressure into resistance signal has become one of the current research hotspots for its high sensitivity, fast response, cost-effective process and simplified signal collection [12-15]. For example, Zhang et al. [16] prepared a piezoresistive pressure sensor based on sponge@carbon nanotubes@silver nanoparticles with good stability and high gauge factor of resistance signal to strain via dip-coating method. Similarly, Ge et al. [17] developed a flexible piezoresistive pressure sensor with tunable sensitivity based on a highly compressible reduced graphene oxide (rGO)/polyaniline wrapped sponge. After mixing chitosan and cellulose nanocrystal in FeCl3 aqueous solution, directional freeze-casting and carbonization, Hu et al. [18] obtained a compressible carbon aerogel with ultrahigh mechanical performance and superior sensitivity. Our group [19] also constructed a multilayer-structure piezoresistive pressure sensor with an in situ generated thiolated graphene@polyester fabric. However, limited by the lack of hydrophobicity, these sensors were easily attacked by water to cause a short circuit or a 2
decrease in electrical conductivity. To date, superhydrophobic surfaces have been widely applied in self-cleaning [20], anti-icing [21-23], anticorrosion [24,25], microfluidic devices [26], and oil-water separation [27-29] for their special wettability. In recent years, with the rapid development of modern artificial intelligence, superhydrophobic surfaces have been proposed to endow conductive materials with water repellency [30-33]. For instance, Li et al. [34] prepared a superhydrophobic and piezoresistive coating with high flexibility by spray-coating carbon nanotubes (CNTs) dispersed in a thermoplastic elastomer solution, followed by treatment with
ethanol.
Taking
advantage
of
the
elasticity
of
polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS) and the hydrophobicity and conductivity of 1-octadecanethiol-modified silver nanoparticles, Su et al. [35] achieved a highly stretchable and conductive superhydrophobic coating via spraying method. Zhu et al. [36] adopted polydimethylsiloxane (PDMS) and CNTs to hydrophobize the surface of a commonly used glassy carbon electrode to achieve self-cleaning ability. Nevertheless, the mentioned conductive and superhydrophobic materials were only limited to coating layer, leading to the relatively low sensitivity and narrow working range, and the application was seriously restricted. Recently, superhydrophobic aerogels for pressure sensing were proposed and prepared by some researcher [37-41], whereas the preservation of the water repellency of the aerogels is still a big challenge. Herein, we present a simple freeze-drying method to fabricate a carbon nanotubes/chitosan (CNTs/CS) aerogel, followed by dip-coating of graphene oxide (GO), reduction by ascorbic acid and modification with 1H,1H,2H,2H-perfluorooctyltriethoxysilane (FAS) on the surface 3
to finally obtain a conductive and superhydrophobic aerogel for piezoresistive pressure sensor. The schematic illustration of the fabrication process is displayed in Fig. 1. Due to the advantages of biocompatibility, nontoxicity, low cost and great flexibility after crosslinking [42], the CS from natural chitin through deacetylation was selected as the skeleton of the aerogel. Meanwhile, to enhance the mechanical property of the porous aerogel and increase the conductive contact points, one-dimensional carboxylated CNTs (CNTs-COOH) were incorporated into the CS skeleton to form crosslinking structure in the presence of glutaraldehyde (GA). Two-dimensional rGO wrapped on the surface of the skeleton further increased the conductivity and improve the sensitivity of the aerogel, and FAS endowed the aerogel
with
low
surface
energy
through
grafting
modification.
The
obtained
F-rGO@CNTs/CS aerogel possessed superhydrophobicity with a water contact angle (WCA) of 154o, and exhibited excellent compressibility and resilience. The aerogel based piezoresistive pressure sensor showed high sensitivity, fast response and good repeatability. The sensor was successfully applied as a wearable electronic device to monitor various human motions. Importantly, the sensor consistently maintained excellent water repellency during compression process, and the WCA was still above 150o even after 50 compression cycles. The approach in this work for fabricating conductive and superhydrophobic aerogel is facile and cost-effective, and the aerogel has great potential in wide application fields such as wearable devices, electronic skin, and artificial intelligence even under water or wet environment.
2. Experimental section
4
2.1. Materials Multiwalled carbon nanotubes (CNTs, >90.0%, the length was 10−30 nm, the internal and outer diameters were 5−10 and 20−40 nm, respectively), N,N-dimethylformamide (DMF, >99.9%) and ascorbic acid (AR) were bought from Aladdin Reagent Co., Ltd. (China). Chitosan (CS, deacetylation degree ≥85%) was purchased from Zhejiang Aoxing Biotechnology Co., Ltd. (China). Acetic acid (HAc, ≥99.5%, AR), glutaraldehyde (GA, 50%) and anhydrous ethanol (AR) were supplied by Guangzhou Chemical Reagent Factory (China). 1H,1H,2H,2H-perfluorooctyltriethoxysilane (FAS) was obtained from Evonik Co., Ltd. (Germany). All chemicals were used as received without further purification, and deionized water was used for all the experiments and tests. 2.2. Preparation of CNTs/CS aerogel First, CNTs-COOH were synthesized according to the procedure reported by our group [42] (Supporting Information). Various amount of CNTs-COOH were dispersed in HAc aqueous solution (1%, v/v) by ultrasonication. Next, 0.1 g of CS was dissolved in 5 mL of CNTs-COOH dispersion under magnetic stirring at room temperature for 6 h to obtain a homogenous mixture. Subsequently, GA aqueous solution (10 wt% with respect to the amount of CS) was added dropwise into the mixture under magnetic stirring, and CNTs/CS hydrogel was formed after standing for 1 h. Finally, the hydrogel was frozen at −15 oC for 12 h and treated via a freeze-drying process with a condenser temperature of −50 oC and inside pressure of 10 Pa in a lyophilizer (FD-1A-50+, Beijing boyikang instruments Co., Ltd., China) to obtain CNTs/CS aerogel. In this case, the CNTs-COOH content of the aerogel was varied 5
at 10, 20, 30, 40 and 50 wt%, respectively. For comparison, the pure CS aerogel was also prepared via the same procedure by directly dissolving CS powder into HAc aqueous solution instead of CNTs-COOH dispersion. 2.3. Fabrication of conductive and superhydrophobic F-rGO@CNTs/CS aerogel GO was synthesized according to the modified Hummers’ method [44] (Supporting Information), and dispersed in DMF/water solution with a volume ratio of 3:1 by ultrasonic treatment to form GO suspension with a concentration of 3 mg mL-1. The as-fabricated CNTs/CS aerogel was immersed into GO suspension and compressed for several times to draw GO into the aerogel. Then, the sample was squeezed to remove excess GO dispersion and dried at 60 oC for 30 min to obtain a GO-wrapped aerogel. Afterwards, the wrapped GO was reduced into rGO by ascorbic acid ethanol solution (30 mg mL-1) at 70 oC for 3 h to achieve a rGO@CNTs/CS aerogel. Finally, the rGO@CNTs/CS aerogel was immersed into a FAS ethanol solution (1 wt%) at room temperature for 24 h and rinsed by ethanol to remove the residual on the surface of the aerogel skeleton, and the conductive and superhydrophobic F-rGO@CNTs/CS aerogel was obtained after drying at 80 oC for 24 h. 2.4 Characterizations Fourier transform infrared spectroscopy (FT-IR) was collected on a Bruker Tensor 27 spectrometer (Bruker Optics, Germany) from 4000 to 400 cm-1 with 16 scans at a resolution of 4 cm-1. Thermogravimetric (TG) analysis was performed on a thermal analyzer (TG209, Netzsch, Germany) under N2 atmosphere at a heating rate of 20 oC min-1 from 30 to 800 oC. The mechanical compressibility of the aerogel was evaluated on a universal testing machine 6
(ESM303, Mark-10, USA) equipped with a 50 N load cell. Surface morphologies of the aerogel were characterized with a scanning electron microscope (SEM, EVO 18, Carl Zeiss Jena, Germany) at an acceleration voltage of 10.0 kV. Element composition analysis was carried out on an energy dispersion spectroscopy (EDS, INCA250, Oxford Instruments, UK) accompanied by SEM and X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD, UK) with a monochromated Al Kα source. Water contact angle (WCA) was measured by a contact angle meter (DSA100, Kruss, Germany) using a 6 µL water droplet as probe liquid at room temperature. Each WCA was tested for at least three different locations to calculate the average value. The variation of electrical resistance was recorded on a two-point probe multimeter (DMM6500, Keithley, USA).
3. Results and discussion 3.1. Preparation and characterizations of CNTs/CS aerogel via freeze-drying method In this work, pristine CNTs were firstly grafted with carboxyl groups through acid treatment to improve the dispersion in water. (Fig. S1 and S2). Compared with pristine CNTs, the FT-IR spectrum of CNTs-COOH presented some new peaks at 3417 and 1716 cm-1, corresponding to the stretching vibrations of hydroxyl and carbonyl bonds of carboxyl groups [42] (Fig. 2a). Besides, the TG curve of pristine CNTs kept stable until 800 oC, while the curve of CNTs-COOH exhibited a weight loss of 14.4% for the thermal decomposition of grafted carboxyl groups (Fig. 2b), further confirmed the successful modification of CNTs. Next, the CNTs/CS aerogel was prepared by stirring CS powders in the HAc aqueous dispersion of CNTs-COOH until homogenous, followed by adding GA to initiate chemical 7
crosslinking at room temperature, and then freeze drying for 48 h. Fig. 2c showed the FT-IR spectra of pristine CS, crosslinked CS and crosslinked CNTs/CS composite. In the spectrum of pristine CS, the typical stretching vibrational peaks of hydroxyl and amino groups at 3440 cm-1, methyl groups at 2920 cm-1 and methylene groups at 2878 cm-1 were all clearly observed [45]. For crosslinked CS and CNTs/CS aerogel, a new peak at 1654 cm-1 and a shoulder at 1560 cm-1 were ascribed to the formation of C=N and C=C bonds, respectively, demonstrating the occurrence of the Schiff base reaction between the amino groups on CS and the aldehyde groups of GA [46]. In addition, the reaction was also confirmed by XPS analysis of CNTs/CS aerogel (Fig. S3). Thanks to three-dimensional porous structure by freeze drying method, the CNTs/CS aerogel was superlight with a density of 0.057 g cm-3 and could be supported by two very thin leaf stalks (Fig. 2d). As can be seen from the SEM image (Fig. 2f), the CNTs/CS aerogel appeared multi-pore structure. The pores were relatively regular, the pore diameter was 200−400 μm and the thickness of the skeleton was about 1 μm. Meanwhile, the SEM image of the inner wall of the pores revealed the presence of CNTs, which was very beneficial for the formation of conductive contact points and the construction of hierarchical roughness (Fig. 2g). Moreover, the magnified view of the cross-section of the skeleton showed that the CNTs were regularly arranged inside the skeleton to form the conductive pathways (Fig. 2h and i). With CNTs content increasing, the pore sizes of the aerogels were similar, while the surface roughness of the skeleton obviously increased (Fig. S4). Mechanical property is critical for the CNTs/CS aerogel, especially the reversible compressibility, which is crucial to ensure its reliability [47]. To assess the compressibility of 8
the CNTs/CS aerogels, the compression tests were carried out. As shown in Fig. 2e, the aerogel was easily compressed to a strain of 60% under external force. After removing the force, the aerogel quickly recovered its initial shape. To further investigate the mechanical compressibility of the CNTs/CS aerogel, the compressive stress-strain curves were measured and presented in Fig. 2j-l and Fig. S5. The stress returned to its premier value after unloading for each strain at 10−60%, and it could be clearly seen from the loading process that the curve was characterized by a linear elastic region for bending of frameworks, then a plateau region for buckling phenomena, followed by a densification region ascribed to the closure of the pores [48,49] (Fig. 2j). The compressibility of the CNTs/CS aerogel was mainly attributed to its connected porous structure, the crosslinking network of CS and the strong interaction between CNTs-COOH and CS. In the skeleton of the aerogel, the entangled CNTs tightly bonded with chemically crosslinked CS, which ensured the deformation of pore walls rather than the falling out of the CNTs from CS matrix during the compression [49,50]. After 10 loading-unloading cycles with a strain of 60%, the aerogel remained above 84% of the initial maximum stress (Fig. 2k). The energy loss coefficient (defined as the loop area relative to the area under the loading curve) decreased from 0.76 at the first cycle to 0.71 at the 10th cycle, which was due to the slight damage of the skeleton structure during compression process (Fig. S6). Fig. 2l showed the effect of CNTs contents on the compression property of the CNTs/CS aerogel. Obviously, the aerogels with different CNTs contents all exhibited the similar compressive stress-strain curves with three-segment regions. Without CNTs, the maximum stress value of the aerogel was only 22.2 kPa. With CNTs content increasing to 10, 20, 30, 40 and 50 wt%, the value reached 42.3, 47.0, 61.5, 74.1 and 85.0 kPa, respectively, fully 9
demonstrating the significant reinforcement role of CNTs. Here, the CNTs/CS aerogel fabricated with CNTs content of 40% was selected for the subsequent studies.
Fig.
1.
Schematic
illustration
for
fabricating
conductive
and
superhydrophobic
F-rGO@CNTs/CS aerogel.
10
Fig. 2. (a) FT-IR spectra of CNTs and CNTs-COOH. (b) TG curves of CNTs and CNTs-COOH. (c) FT-IR spectra of pristine CS, crosslinked CS and crosslinked CNTs/CS composite. (d) Photograph of the CNTs/CS aerogel with a diameter of 2 cm and a thickness of 1 cm standing on two thin leaf stalks. (e) Photographs of the loading-unloading process of the CNTs/CS aerogel at 60% strain. SEM images of (f) the CNTs/CS aerogel, (g) the inner wall of the pores and (h,i) cross-section of the skeleton with different magnifications. (j) Compressive stress-strain curves of the CNTs/CS aerogel at strains from 10 to 60%. (k) Cyclic compressive stress-strain curves of the CNTs/CS aerogel at 60% strain. (l) 11
Compressive stress-strain curves of the aerogel with different CNTs contents at 60% strain. 3.2.
Fabrication
and
characterizations
of
conductive
and
superhydrophobic
F-rGO@CNTs/CS aerogel To further construct the conductive pathways of the CNTs/CS and endow it with superhydrophobicity, the surface of the CNTs/CS skeleton was wrapped with a layer of GO under the role of hydrogen bonds by simple dipping. Subsequently, the wrapped GO layer was in situ reduced into rGO layer in the presence of ascorbic acid. After that, low-surface-energy FAS was grafted onto rGO layer through dehydration condensation to obtain conductive and superhydrophobic F-rGO@CNTs/CS aerogel. Fig. 3 presented the SEM images and corresponding WCAs optical images of the GO@CNTs/CS, rGO@CNTs/CS and F-rGO@CNTs/CS aerogels, respectively. It was obvious to note that the morphology of the CNTs/CS aerogel was unaffected by GO and rGO, and the GO@CNTs/CS and rGO@CNTs/CS aerogels also showed the similar morphology of the CNTs/CS aerogel (Fig. 3a-c). From the magnified image of rGO@CNTs/CS aerogel, many tiny curled wrinkles were clearly observed (Fig. 3c1). In addition, the modification had little effect on the mechanical property of the aerogel (Fig. S7). Furthermore, the wettability of the CNTs/CS, GO@CNTs/CS, rGO@CNTs/CS and F-rGO@CNTs/CS aerogels were compared. It can be seen that the CNTs/CS aerogel appeared superhydrophilic state with a WCA of 0o, which was due to the existence of the hydrophilic groups of CS and CNTs-COOH and the porous structure of the aerogel (Fig. 3a). The wettability of the GO@CNTs/CS aerogel had no change because of the polar groups of the GO, and the WCA still remained 0o (Fig. 3b).
12
Comparatively, owing to the removal of the oxygen-containing groups after reduction, the WCA of the rGO@CNTs/CS aerogel sharply increased to 122o (Fig. 3c). Furthermore, after modification by FAS, the obtained F-rGO@CNTs/CS aerogel realized superhydrophobicity with a high WCA of 154o (Fig. 3d). From the SEM elemental mapping images, fluorine was confirmed to be uniformly dispersed on the surface of the F-rGO@CNTs/CS aerogel (Fig. 3d1-d3).
Fig. 3. SEM images of (a) CNTs/CS aerogel, (b) GO@CNTs/CS aerogel and (c) rGO@CNTs/CS aerogel. (c1) Magnified SEM image of rGO. (d) SEM image of F-rGO@CNTs/CS aerogel for EDS analysis and the corresponding mappings of (d1) carbon, (d2) oxygen and (d3) fluorine elements. Inset on the top right were the corresponding WCA optical images. The surface chemical composition of the GO@CNTs/CS, rGO@CNTs/CS and F-rGO@CNTs/CS aerogels were further analyzed by XPS. From the XPS survey scans as shown in Fig. 4a, there were two peaks of C 1s at 284 eV and O 1s at 532 eV for the GO@CNTs/CS and rGO@CNTs/CS aerogels, respectively. Differently, several new peaks at 102, 153 and 688 eV corresponding to Si 2p, Si 2s and F 1s were observed in the spectrum of 13
F-rGO@CNTs/CS aerogel. Additionally, the C/O atomic ratio of the GO@CNTs/CS aerogel was 4.83, while that of rGO@CNTs/CS obviously decreased to 2.99. It was in accordance with the high-resolution C 1s core-level spectra. Furthermore, the C 1s core lever spectra of GO@CNTs/CS and rGO@CNTs/CS aerogels were divided into five peaks at 284.6, 285.5, 286.9, 288.1 and 289.2 eV, which assigned to C=C, C−C, C−O, C=O and O−C=O bonds, respectively [19] (Fig. 4b and c). Compared with GO@CNTs/CS aerogel, the peak intensities of C−O, C=O and O−C=O peaks of the rGO@CNTs/CS significantly decreased. In Fig. 4d, the appearance of the new peaks of −CF2 and −CF3 at 291.6 and 293.8 eV implied the occurrence of the reaction between the silanol groups of FAS and the residual hydroxyl groups of rGO [51].
Fig. 4. (a) XPS survey scans of the GO@CNTs/CS, rGO@CNTs/CS and F-rGO@CNTs/CS 14
aerogels. XPS C 1s spectra of the (b) GO@CNTs/CS aerogel, (c) rGO@CNTs/CS aerogel and (d) F-rGO@CNTs/CS aerogel, respectively. 3.3.
Wetting
behavior
and
electromechanical
performance
of
conductive
and
superhydrophobic F-rGO@CNTs/CS aerogel Water repellency is very important for conductive materials in practical application especially under water or wet environment. Here, the fabricated F-rGO@CNTs/CS aerogel possessed superhydrophobicity, and the water droplets could stand on the surface with a nearly spherical shape (Fig. 5a). When the CNTs/CS and F-rGO@CNTs/CS aerogels were immersed into water simultaneously, the F-rGO@CNTs/CS aerogel quickly floated on water surface, while the CNTs/CS aerogel sank into water (Fig. 5b). Interestingly, when the F-rGO@CNTs/CS aerogel was pressed into water by a tweezer, a bright mirror-like surface appeared for an air layer trapped by the rough structure of the aerogel to prevent the surface from contacting water [52] (Fig. 5c). Additionally, continuous water droplets were able to quickly roll off the inclined surface of the aerogel at an angle of 60o (Fig. 5d and Video S1). Moreover, the F-rGO@CNTs/CS aerogel exhibited stable superhydrophobicity under various environments (Fig. S8). Two copper sheets were respectively pasted on the top and bottom of the aerogel as electrodes by silver paste for piezoresistive pressure sensor. A simple piezoresistive test was conducted by connecting the sensor with several light-emitting diode (LED) bulbs in a circuit of 10 V (Fig. S9). The electromechanical property of the sensor was evaluated using an experimental test platform consisting of a computer collector, a universal testing machine and
15
a multimeter. To collect electrical response signal, the sensor was fixed on the chuck of the testing machine, and the two electrodes were respectively connected to two probes of the multimeter through copper wires. When the hold down applied a load on the sensor, the electrical resistance change was recorded by the computer (Fig. S10). Sensitivity is a key parameter of piezoresistive pressure sensor, which is defined as the ratio of relative resistance (ΔR/R0, R0 is the initial resistance value of the sensor without loading and ΔR is the absolute value of the resistance change with the applied pressure) changes to applied pressure (ΔP) [53,54]. Fig. 6b displayed the sensitivities of the CNTs/CS aerogel and F-rGO@CNTs/CS aerogel sensors, it can be seen that the sensitivity of the F-rGO@CNTs/CS aerogel sensor was obviously higher than that of the CNTs/CS aerogel sensor. The curve presented two linear intervals including a small pressure range of 0−3 kPa and a large pressure range of 40−80 kPa. The corresponding sensitivities were 4.97 and 0.05 kPa-1, respectively. To explain the sensing mechanism, the schematic illustration for the structural change of the aerogel under pressure was depicted in Fig. 6a. With the exertion of a small pressure, the number of conductive pathways increased for the mutual contacts of the inner walls of the pores, resulting in a rapid decrease of electrical resistance. As pressure increased, the contact area further increased due to the elastic deformation of the skeleton, and the resistance slightly decreased [19]. Additionally, from the real-time curves of ΔR/R0 change of the sensor at different applied strains, the ΔR/R0 appeared periodic changes with continuous loading-unloading cycles, and increased with strain increasing (Fig. 6c). Moreover, dynamic frequency had almost no effect on the electrical cycling response of the sensor, for the values of ΔR/R0 were similar to the different frequencies (Fig. 6d). Notably, 16
the response time of the sensor was only 170 ms, displaying a fast response to external stimuli (Fig. 6e). Importantly, the sensor showed a negligible change of ΔR/R0 even after 1000 loading-unloading cycles, demonstrating the excellent repeatability (Fig. 6f and Fig. S11). It is well known that a large deformation of superhydrophobic materials will lead to the destruction of rough structure or the detachment of low-surface-energy substance, resulting in the loss of its superhydrophobicity [35,55]. Different from the reported superhydrophobic materials, the F-rGO@CNTs/CS aerogel sensor revealed good superhydrophobic stability at large deformations. Fig. 5e presents the WCA variations of the aerogel sensor under different strains. Visibly, the sensor still kept superhydrophobic at a large strain of 60%. Besides, the WCA of the sensor remained above 150o even after 50 loading-unloading cycles (Fig. 5f). Furthermore, the effect of water on the electrical response of the sensor was evaluated and presented in Fig. 5g and Video S2. When a jet of water was sprayed on the flank of the sensor during compression, the response curve was unchanged at all, for the quick rolling off of the water droplets upon contacting the superhydrophobic surface. It indicated that the sensor can normally work under a humid or rainy environment.
17
Fig. 5. Photographs of (a) water droplets standing on the conductive and superhydrophobic F-rGO@CNTs/CS aerogel, (b) the CNTs/CS and F-rGO@CNTs/CS aerogels placed into water, and (c) the F-rGO@CNTs/CS aerogel immersed into water via an external force. (d) Rolling processes of water droplets falling on the surface of the F-rGO@CNTs/CS aerogel. (e) WCAs of the F-rGO@CNTs/CS aerogel at different compressive strains. (f) WCA change of the F-rGO@CNTs/CS aerogel with compression cycles. (g) Real-time electrical resistance variation with water injecting on the aerogel during compression process.
Fig. 6. (a) Schematic illustration of the sensor with the magnified image of its individual skeleton without pressure, under small and large pressures. (b) Relative resistance changes of the CNTs/CS aerogel and the F-rGO@CNTs/CS aerogel with pressure increasing. The response test of the sensor at different (c) strains and (d) frequencies. (e) Response time of the sensor. (f) Repeatability performance of the sensor for 1000 loading-unloading cycles with 30% strain. Inset was the enlarged view of the response curve. 3.4. Application of conductive and superhydrophobic F-rGO@CNTs/CS aerogel sensor in 18
human motion detection To investigate the feasibility of the F-rGO@CNTs/CS aerogel sensor in practical application, a series of tests to detect various human motions were carried out. First, an adhesive tape was utilized to fix the sensor on the cheek of a 24-year-old healthy female tester with a weight of 40 kg and a height of 160 cm. As the tester repeated the mouth-opening process, the tiny pressure change caused by the movement of the facial muscles was acutely detected by the sensor, thereby outputting a series of stable and ordered real-time curves of the ΔR/R0 change (Fig. 7a). Besides, the sensor attached on the tester’s neck could detect throat vibrations as shown in Fig. 7b and c. When the tester swallowed saliva, the sensor presented a regular ΔR/R0 response signal, which was generated by the movements of the muscle around the throat of the tester. Furthermore, when the word “Hello” was pronounced by the tester, a distinct periodic response signal corresponding to the tone of the word was observed, demonstrating the outstanding voice detection capability. Interestingly, the sensor displayed three different signal patterns for the above three motions, exhibiting its high accuracy of pressure response [17, 19]. In addition to the weak motions by small pressures, the sensor was also applied to monitor strong actions. As shown in Fig. 7d-f, the sensor was fixed on the joint of the tester’s finger, wrist, and elbow, respectively, where the large deformations occurred upon bending. It can be seen that the real-time curves were continuous, repetitive and responsive in all flexing-recovery processes. When the finger was at a straight state, the response curve was stable with a ΔR/R0 of 0. However, when the finger was bent, the sensor mounted on the arthrosis was squeezed, leading to the rapid increase of ΔR/R0 to form an upward steep peak. 19
Subsequently, with the extension of the finger, ΔR/R0 recovered to the original value. The results for bending the wrist and elbow were also compliance with this, exhibiting the good stability and repeatability of the sensor. To further monitor human sports, the sensor was placed on the insole to record the real-time response signals of walking, running, and jumping. Compared with the response waveform of walking, the frequency was significantly increased when running (Fig. 7g and h). Moreover, the variation amplitude of ΔR/R0 was enhanced during jumping (Fig. 7i). It was mainly due to the increase of the pressure on the sensor caused by a larger acceleration when landing.
Fig. 7. Real-time electrical resistance response curves of the piezoresistive pressure sensor
20
when the tester (a) opening mouth, (b) swallowing, (c) speaking “Hello”, (d) bending finger, (e) bending wrist, (f) bending elbow, (g) walking, (h) running, and (i) jumping, respectively.
4. Conclusions In
summary,
we
successfully
fabricated
a
conductive
and
superhydrophobic
F-rGO@CNTs/CS aerogel for piezoresistive pressure sensor. Owing to the reinforcement of CNTs, the CNTs/CS aerogel obtained by freeze-drying method possessed excellent mechanical property and compressibility. The F-rGO@CNTs/CS aerogel realized superhydrophobicity with a high WCA of 154o, and maintained superhydrophobic state during compression process. Taking advantage of the porous structure and the synergy of CNTs and rGO, the F-rGO@CNTs/CS aerogel sensor achieved the sensitivities of 4.97 kPa-1 in 0−3 kPa and 0.05 kPa-1 in 40−80 kPa, and a fast response time of 170 ms. Meanwhile, the sensor exhibited good stability and repeatability. Importantly, the sensor demonstrated the outstanding performance for detecting human motions including tiny muscle movements such as mouth opening, swallowing and speaking, and large body activities such as arthrosis bending (finger, wrist, and elbow), walking, running and jumping. The conductive and superhydrophobic F-rGO@CNTs/CS aerogel sensor are highly promising to be utilized in wearable devices, electronic skin, and other electronics, even under rainy or sweaty conditions.
Acknowledgements
21
The work was financially supported by the Science and Technology Planning Project of Guangdong Province, China (2018A030313884) and the Science and Technology Planning Project of Guangzhou City, China (201804010381).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version.
References [1] J. Kang, D. Son, G.N. Wang, Y. Liu, J. Lopez, Y. Kim, J.Y. Oh, T. Katsumata, J. Mun, Y. Lee, L. Jin, J.B. Tok, Z. Bao, Tough and water-insensitive self-healing elastomer for robust electronic skin, Adv. Mater. 30 (2018) 1706846. [2] Z. Ma, S. Li, H. Wang, W. Cheng, Y. Li, L. Pan, Y. Shi, Advanced electronic skin devices for healthcare applications, J. Mater. Chem. B 7 (2019) 173-197. [3] T. Wang, H. Yang, D. Qi, Z. Liu, P. Cai, H. Zhang, X. Chen, Mechano-based transductive sensing for wearable healthcare, Small 14 (2018) 1702933. [4] T.Q. Trung, N.E. Lee, Flexible and stretchable physical sensor integrated platforms for wearable human-activity monitoringand personal healthcare, Adv. Mater. 28 (2016) 4338-4372. [5] X. Jing, H.Y. Mi, Y.J. Lin, E. Enriquez, X.F. Peng, L.S. Turng, Highly stretchable and biocompatible strain sensors based on mussel-inspired super-adhesive self-healing hydrogels for human motion monitoring, ACS Appl. Mater. Interfaces 10 (2018) 20897-20909. [6] S. Ryu, P. Lee, J.B. Chou, R. Xu, R. Zhao, A.J. Hart, S.-G. Kim, Extremely elastic wearable carbon nanotube fiber strain sensor for monitoring of human motion, ACS Nano 9 22
(2015) 5929-5936. [7] T. Li, G. Li, Y. Liang, T. Cheng, J. Dai, X. Yang, B. Liu, Z. Zeng, Z. Huang, Y. Luo, Fast-moving soft electronic fish, Sci. Adv. 3 (2017) 1602045. [8] C. Wu, S. Zeng, Z. Wang, F. Wang, H. Zhou, J. Zhang, Z. Ci, L. Sun, Efficient mechanoluminescent
elastomers
for
dual-responsive
anticounterfeiting
device
and
stretching/strain sensor with multimode sensibility, Adv. Funct. Mater. 28 (2018) 1803168. [9] J. Cao, C. Lu, J. Zhuang, M. Liu, X. Zhang, Y. Yu, Q. Tao, Multiple hydrogen bonding enables the self-healing of sensors for human-machine interactions, Angew. Chem. Int. Ed. 56 (2017) 8795-8800. [10] A. Qiu, P. Li, Z. Yang, Y. Yao, I. Lee, J. Ma, A path beyond metal and silicon: polymer/nanomaterial composites for stretchable strain sensors, Adv. Funct. Mater. 29 (2019) 1806306. [11] J. Lee, H. Kwon, J. Seo, S. Shin, J.H. Koo, C. Pang, S. Son, J.H. Kim, Y.H. Jang, D.E. Kim, T. Lee, Conductive fiber-based ultrasensitive textile pressure sensor for wearable electronics, Adv. Mater. 27 (2015) 2433-2439. [12] S. Gong, W. Schwalb, Y. Wang, Y. Chen, Y. Tang, J. Si, B. Shirinzadeh, W. Cheng, A wearable and highly sensitive pressure sensor with ultrathin gold nanowires, Nat. Commun. 5 (2014) 3132. [13] J. Li, S. Orrego, J. Pan, P. He, S.H. Kang, Ultrasensitive, flexible, and low-cost nanoporous piezoresistive composites for tactile pressure sensing, Nanoscale 11 (2019) 2779-2786. [14] M. Cao, M. Wang, L. Li, H. Qiu, M.A. Padhiar, Z. Yang, Wearable rGO-Ag NW@cotton 23
fiber piezoresistive sensor based on the fast charge transport channel provided by Ag nanowire, Nano Energy 50 (2018) 528-535. [15] W. Huang, K. Dai, Y. Zhai, H. Liu, P. Zhan, J. Gao, G. Zheng, C. Liu, C. Shen, Flexible and
lightweight
pressure
sensor
based
on
carbon
nanotube/thermoplastic
polyurethane-aligned conductive foam with superior compressibility and stability, ACS Appl. Mater. Interfaces 9 (2017) 42266-42277. [16] H. Zhang, N. Liu, Y. Shi, W. Liu, Y. Yue, S. Wang, Y. Ma, L. Wen, L. Li, F. Long, Z. Zou, Y. Gao, Piezoresistive sensor with high elasticity based on 3D hybrid network of sponge@CNTs@Ag NPs, ACS Appl. Mater. Interfaces 8 (2016) 22374-22381. [17] G. Ge, Y. Cai, Q. Dong, Y. Zhang, J. Shao, W. Huang, X. Dong, A flexible pressure sensor based on rGO/polyaniline wrapped sponge with tunable sensitivity for human motion detection, Nanoscale 10 (2018) 10033-10040. [18] Y. Hu, H. Zhuo, Z. Chen, K. Wu, Q. Luo, Q. Liu, S. Jing, C. Liu, L. Zhong, R. Sun, X. Peng, Superelastic carbon aerogel with ultrahigh and wide-range linear sensitivity, ACS Appl. Mater. Interfaces 10 (2018) 40641-40650. [19] L. Zhang, H. Li, X. Lai, T. Gao, J. Yang, X. Zeng, Thiolated graphene@polyester fabric-based multilayer piezoresistive pressure sensors for detecting human motion, ACS Appl. Mater. Interfaces 10 (2018) 41784-41792. [20] T. Ren, J. He, Substrate-versatile approach to robust antireflective and superhydrophobic coatings with excellent self-cleaning property in varied environments, ACS Appl. Mater. Interfaces 9 (2017) 34367-34376. [21] L. Wang, Q. Gong, S. Zhan, L. Jiang, Y. Zheng, Robust anti-icing performance of a 24
flexible superhydrophobic surface, Adv. Mater. 28 (2016) 7729-7735. [22] T. Wang, Y. Zheng, A.R. Raji, Y. Li, W.K. Sikkema, J.M. Tour, Passive anti-icing and active deicing films, ACS Appl. Mater. Interfaces 8 (2016) 14169-14173. [23] T. Cheng, R. He, Q. Zhang, X. Zhan, F. Chen, Magnetic particle-based super-hydrophobic coatings with excellent anti-icing and thermoresponsive deicing performance, J. Mater. Chem. A 3 (2015) 21637-21646. [24] Y. Zhu, F. Sun, H. Qian, H. Wang, L. Mu, J. Zhu, A biomimetic spherical cactus superhydrophobic coating with durable and multiple anti-corrosion effects, Chem. Eng. J. 338 (2018) 670-679. [25] D. Zang, R. Zhu, W. Zhang, X. Yu, L. Lin, X. Guo, M. Liu, L. Jiang, Corrosion-resistant superhydrophobic coatings on Mg alloy surfaces inspired by lotus seedpod, Adv. Funct. Mater. 27 (2017) 1605446. [26] C. Li, M. Boban, S.A. Snyder, S.P.R. Kobaku, G. Kwon, G. Mehta, A. Tuteja, Paper-based surfaces with extreme wettabilities for novel, open-channel microfluidic devices, Adv. Funct. Mater. 26 (2016) 6121-6131. [27] Q. Ma, H. Cheng, A.G. Fane, R. Wang, H. Zhang, Recent development of advanced materials with special wettability for selective oil/water separation, Small 12 (2016) 2186-2202. [28] C. Cao, M. Ge, J. Huang, S. Li, S. Deng, S. Zhang, Z. Chen, K. Zhang, S.S. Al-Deyab, Y. Lai, Robust fluorine-free superhydrophobic PDMS-ormosil@fabrics for highly effective self-cleaning and efficient oil–water separation, J. Mater. Chem. A 4 (2016) 12179-12187. [29] J. Li, R. Kang, X. Tang, H. She, Y. Yang, F. Zha, Superhydrophobic meshes that can 25
repel hot water and strong corrosive liquids used for efficient gravity-driven oil/water separation, Nanoscale 8 (2016) 7638-7645. [30] Q. Li, H. Liu, S. Zhang, D. Zhang, X. Liu, Y. He, L. Mi, J. Zhang, C. Liu, C. Shen, Z. Guo, Superhydrophobic electrically conductive paper for ultrasensitive strain sensor with excellent anticorrosion and self-cleaning property, ACS Appl. Mater. Interfaces 11 (2019) 21904-21914. [31] L. Wang, J. Luo, Y. Chen, L. Lin, X. Huang, H. Xue, J.F. Gao, Fluorine-free superhydrophobic and conductive rubber composite with outstanding deicing performance for highly sensitive and stretchable strain sensors, ACS Appl. Mater. Interfaces (2019) 17774-17783. [32] J.N. Wang, Y. Q. Liu, Y.L. Zhang, J. Feng, H. Wang, Y.H. Yu, H.B. Sun, Wearable superhydrophobic elastomer skin with switchable wettability, Adv. Funct. Mater. 28 (2018) 1800625. [33] G. Ding, W. Jiao, L. Chen, M. Yan, L. Hao, R. Wang, A self-sensing, superhydrophobic, heterogeneous graphene network with controllable adhesion behavior, J. Mater. Chem. A 6 (2018) 16992-17000. [34] L. Li, Y. Bai, L. Li, S. Wang, T. Zhang, A superhydrophobic smart coating for flexible and wearable sensing electronics, Adv. Mater. 29 (2017) 1702517. [35] X. Su, H. Li, X. Lai, Z. Chen, X. Zeng, Highly stretchable and conductive superhydrophobic coating for flexible electronics, ACS Appl. Mater. Interfaces 10 (2018) 10587-10597. [36] X. Zhu, Y. Chen, C. Feng, W. Wang, B. Bo, R. Ren, G. Li, Assembly of self-cleaning 26
electrode surface for the development of refreshable biosensors, Anal. Chem. 89 (2017) 4131-4138. [37] J. Xiao, Y. Tan, Y. Song, Q. Zheng, A flyweight and superelastic graphene aerogel as a high-capacity adsorbent and highly sensitive pressure sensor, J. Mater. Chem. A 6 (2018) 9074-9080. [38] L.Z. Guan, J.F. Gao, Y.B. Pei, L. Zhao, L.X. Gong, Y.J. Wan, H. Zhou, N. Zheng, X.S. Du, L.B. Wu, J.X. Jiang, H.Y. Liu, L.C. Tang, Y.W. Mai, Silane bonded graphene aerogels with tunable functionality and reversible compressibility, Carbon 107 (2016) 573-582. [39] Y. Lin, G.J. Ehlert, C. Bukowsky, H.A. Sodano, Superhydrophobic functionalized graphene aerogels, ACS Appl. Mater. Interfaces 3 (2011) 2200-2203. [40] G. Zu, K. Kanamori, K. Nakanishi, J. Huang, Superhydrophobic ultraflexible triple-network graphene/polyorganosiloxane aerogels for a high-performance multifunctional temperature/strain/pressure sensing array, Chem. Mater. 31 (2019) 6276-6285. [41] G. Zu, K. Kanamori, K. Nakanishi, X. Lu, K. Yu, J. Huang, H. Sugimura, Superelastic multifunctional aminosilane-crosslinked graphene aerogels for high thermal insulation, three-component separation, and strain/pressure-sensing arrays, ACS Appl. Mater. Interfaces 11 (2019) 43533-43542. [42] C. Su, H. Yang, H. Zhao, Y. Liu, R. Chen, Recyclable and biodegradable superhydrophobic and superoleophilic chitosan sponge for the effective removal of oily pollutants from water, Chem. Eng. J. 330 (2017) 423-432. [43] X. Su, H. Li, X. Lai, Z. Yang, Z. Chen, W. Wu, X. Zeng, Vacuum-assisted layer-by-layer superhydrophobic carbon nanotube films with electrothermal and photothermal effects for 27
deicing and controllable manipulation, J. Mater. Chem. A 6 (2018) 16910-16919. [44] L. Zhang, H. Li, X. Lai, X. Su, T. Liang, X. Zeng, Thiolated graphene-based superhydrophobic sponges for oil-water separation, Chem. Eng. J. 316 (2017) 736-743. [45] J. Yang, L. Yi, X. Fang, Y. Song, L. Zhao, J. Wu, H. Wu, Self-healing and recyclable biomass aerogel formed by electrostatic interaction, Chem. Eng. J. 371 (2019) 213-221. [46] M. Salzano de Luna, C. Ascione, C. Santillo, L. Verdolotti, M. Lavorgna, G.G. Buonocore, R. Castaldo, G. Filippone, H. Xia, L. Ambrosio, Optimization of dye adsorption capacity and mechanical strength of chitosan aerogels through crosslinking strategy and graphene oxide addition, Carbohyd. polym. 211 (2019) 195-203. [47] Y.J. Wan, P.L. Zhu, S.H. Yu, R. Sun, C.P. Wong, W.H. Liao, Ultralight, super-elastic and volume-preserving cellulose fiber/graphene aerogel for high-performance electromagnetic interference shielding, Carbon 115 (2017) 629-639. [48] X. Su, H. Li, X. Lai, Z. Chen, X. Zeng, 3D porous superhydrophobic CNT/EVA composites for recoverable shape reconfiguration and underwater vibration detection, Adv. Funct. Mater. 29 (2019) 1900554. [49] W. He, G. Li, S. Zhang, Y. Wei, J. Wang, Q. Li, X. Zhang, Polypyrrole/silver coaxial nanowire aero-sponges for temperature-independent stress sensing and stress-triggered joule heating, ACS Nano 9 (2015) 4244-4251. [50] P. Lv, X.W. Tan, K.H. Yu, R.L. Zheng, J.J. Zheng, W. Wei, Super-elastic graphene/carbon nanotube aerogel: A novel thermal interface material with highly thermal transport properties, Carbon 99 (2016) 222-228. [51] F. Qiang, L.L. Hu, L.X. Gong, L. Zhao, S.-N. Li, L.C. Tang, Facile synthesis of 28
super-hydrophobic, electrically conductive and mechanically flexible functionalized graphene nanoribbon/polyurethane sponge for efficient oil/water separation at static and dynamic states, Chem. Eng. J. 334 (2018) 2154-2166. [52] X. Su, H. Li, X. Lai, L. Zhang, J. Wang, X. Liao, X. Zeng, Vapor-liquid sol-gel approach to fabricating highly durable and robust superhydrophobic polydimethylsiloxane@silica surface on polyester textile for oil-water separation, ACS Appl. Mater. Interfaces 9 (2017) 28089-28099. [53] H. Li, K. Wu, Z. Xu, Z. Wang, Y. Meng, L. Li, Ultrahigh-sensitivity piezoresistive pressure sensors for detection of tiny pressure, ACS Appl. Mater. Interfaces 10 (2018) 20826-20834. [54] A. Tewari, S. Gandla, S. Bohm, C.R. McNeill, D. Gupta, Highly exfoliated MWCNT-rGO ink-wrapped polyurethane foam for piezoresistive pressure sensor applications, ACS Appl. Mater. Interfaces 10 (2018) 5185-5195. [55] J. Ju, X. Yao, X. Hou, Q. Liu, Y.S. Zhang, A. Khademhosseini, A highly stretchable and robust non-fluorinated superhydrophobic surface, J. Mater. Chem. A 5 (2017) 16273-16280.
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F-rGO@CNTs/CS aerogel was simply prepared by freeze-drying and dip-coating.
F-rGO@CNTs/CS
aerogel
possessed
superhydrophobicity,
compressibility
and
resilience.
F-rGO@CNTs/CS aerogel based sensor showed high sensitivity, fast response and good repeatability.
The sensor was successfully applied for detecting human motions even under wet environment.
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