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Surfaces, Interfaces, and Applications

Acid-Interface Engineering of Carbon Nanotube/Elastomer with Enhanced Sensitivity for Stretchable Strain Sensor Sijia Chen, Rongyao Wu, Pei Li, Qi Li, Yang Gao, Bo Qian, and Fuzhen Xuan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16591 • Publication Date (Web): 04 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018

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ACS Applied Materials & Interfaces

Acid-Interface Engineering of Carbon Nanotube/Elastomer with Enhanced Sensitivity for Stretchable Strain Sensor Sijia Chena,†, Rongyao Wua,†, Pei Lia,†, Qi Li, Yang Gao a,*, Bo Qian a, Fuzhen Xuana aSchool

of Mechanical and Power Engineering, East China University of Science and

Technology, Shanghai 200237 (China) †Authors

are contributed equally

* Correspondence to Y. G. ([email protected])

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Abstract Stretchable strain sensors with high sensitivity or gauge factor (GF), large stretchability, and long-term durability are highly demanded in human motion detection, artificial intelligence, and electronic skins. Nevertheless, to develop high sensitive sensors without sacrifice of stretchability cannot be realized using simple device configurations. In this work, an acid-interface engineering (AIE) method was proposed to develop a stretchable strain sensor with high GF and large stretchability. The AIE generates a layer of SiOx at the interface between carbon nanotube film and Ecoflex, playing a key role in enhancing the sensor’s GF. Compared to devices without AIE (GF=2.4), the ones with AIE are significantly improved. At an AIE time of 10 min, GF up to 1665.9 is achieved without sacrificing the stretchability (>100%). The AIE-generated cracks are found to modulate the electrical behaviors and enhance the GFs of sensors with AIE through the crack-induced rapid reduction in the electrical conduction pathway, which is manipulated by the CNTs bridging over the cracks. The device with AIE proves its high mechanical durability through a cycling test (>10,000 cycles) at a high strain up to ~80%, further paving its practical applications in various human motion detections.

Keywords: stretchable strain sensor, acid-interface engineering, surface cracks, human motion detection, electronic skin


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Introduction Recent years, electronics1-4 endowed with stretchability have opened up diverse applications that include artificial intelligence,5 in-situ health condition surveillance,6 and structural health condition monitoring,7,8 among others. Strain sensor is an indispensable component in stretchable electronics,7,9 working as the mechanical-electrical transducer with the following necessitate characteristics for the aforementioned applications: 1) A large stretchability for the applications where a wide strain (ε) detection range more than 50% is required;10 2) A high sensitivity or gauge factor (GF) larger than 100 for measuring the subtle mechanical deformations such as pulsing, eye blinking, and phonation;7 3) A high stability for long-term repeated usage.11,12 Strain sensors are able to own large stretchability through using elastomers to accommodate

active

materials

including

carbon

black

nanoparticles,13

metal

nanoparticle,14 polyaniline microparticles,15 carbon nanotube (CNT),16-19 graphene,20-22 nanowires (NWs), 23-25 and ionic liquid.26,27 Nevertheless, it is difficult for these devices to deliver high GFs with simple device configurations.7 A series of processing methods have been proposed to improve the sensitivity by generating large relative resistance change at a certain strain change. In particular, Park’s group demonstrated that the GF of a sensor based on Ag NWs could be increased by reducing the density of Ag NWs.28 Shi et al. reported a graphene-based device with a fish-scale configuration, which owns high GF and large stretchability attributed to the sliding among graphene flakes.29 The fragmentation of graphene,30 carbonized foam,31 and CNT/elastomer composites32 is reported to increase the



sensors’ GFs, without losing their large stretchability. Choi and Kim’s group produce cutthrough cracks on a metal film-based strain sensor, having a high GF of 2,000 that results from the significant reduction in conductive pathway among cracked metal films during mechanical deformation.33 However, the detectable strain of the device is limited to 2%.34,35 Zhou et al successfully generated cracks on CNT-based films,36 elastomerwrapped CNT fibers,37 and CNT-microwires38 by pre-stretching to gain high GF and large stretchability at the same time. A record sensitivity of 107 has been achieved for the CNTbased films with a stretchability of 50%.36 In this research, to simultaneously achieve high GF and large stretchability, acidinterface engineering (AIE) was developed to enhance the CNT-based sensor’s sensitivity while maintaining the stretchability more than 100%. Different from the mechanical methods such as bending34,35 and pre-stretching,36-38 the interface between elastomer and active material was engineered by strong acid to enhance the sensitivity of the device. The AIE generates a thin layer of SiOx on Ecoflex surface by directly dipping elastomer into a strong acid, which is simple, highly efficient and cost-effective. When the device is stretched, cracks perpendicular to the strain-applied direction are produced, which is ascribed to the residual stress existing at the SiOx-Ecoflex interface. Through the controlling of AIE time, the performance of the sensors is successfully engineered. For the sensors with an AIE time of 10 min, GFs of 35.2 at ε97%) and sodium dodecyl sulfate were brought from Shenzhen Nanotubes Port and Sigma-Aldrich, respectively. Ecoflex was purchased from Smooth-On. Acid-interface engineering of Ecoflex substrate. The Ecoflex was prepared according to the standard procedure reported in our previous studies.39 The cured Ecoflex was cut into strips for the AIE process. H2SO4 (content 70%) was firstly prepared. Then, Ecoflex strips were immersed into the H2SO4 for the AIE with time of 10, 20, and 30 min. Finally, the AIE-processed Ecoflex strips were cleaned by deionized (DI) water to remove the residual acid. Fabrication of the stretchable AIE sensors. The AIE-processed Ecoflex strips with a size of 40 mm ×6 mm were employed as substrates to prepare the devices. 1 g/L CNT solution was prepared using DI water and sodium dodecyl sulfate as the solvent and surfactant. The concentration of sodium dodecyl sulfate is 1.5 g/L. The solution was first dispersed by ultrasonication (DS-5510DTH,



Shanghai Sonxi Ultrasonic Instrument Company, Power: 300W) for 25 min. Then it was centrifuged by an automatic balancing centrifuge (TDZ6B-WS, Shanghai Lu Xiangyi Centrifuge Instrument Co.,Ltd) at 3500 r/min for 4 min. The obtained CNT supernatant was drop-coated onto the AIE-processed substrates and dried at 50 oC for 2.5 hr. Finally, the devices were encapsulated by applying fresh Ecoflex on the samples’ surfaces, with a curing step of 15 min at 65 oC. Characterization of AIE sensors. The morphologies of AIE-processed Ecoflex substrates were characterized using a field-emission scanning electron microscope (FE-SEM, Hitachi S-4800). The surface chemical structure of AIE-processed Ecoflex substrates was investigated by Fourier Transform Infrared Spectroscopy (FTIR, Nicolet 5700). An optical microscope (Zeiss HAL100) was employed to study the surface crack evolution on the AIE devices. The Derjaguin-Muller-Toporov (DMT) modulus maps were obtained using a Dimension Icon AFM (Bruker AXS Inc.) at the Peak-Force QNM mode. Electrical signals of the AIE sensors under stretching was measured by an electrochemical workstation (CHI760E, Shanghai Chenhua Inc.). The AIE sensors were clipped onto a motorized stand for the application of strains. Results and discussion AIE was used to develop the stretchable strain sensors with cut-through cracks (Figure1a and b), with detailed procedures exhibited in Figure 1c. Briefly, strips of prepared Ecoflex were immersed into 70% H2SO4 for a certain of time from 10 to 30 min.


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After removal of residual H2SO4 by DI water, one of the AIE-treated Ecoflex surface was coated with a layer of CNT film. Then, the samples were packaged by Ecoflex. Figure 1d shows a strip of Ecoflex with an AIE time of 10 min. After it was stretched, cracks are observed (Figure S1) on the AIE-treated surface. Figure 1e is a photograph of an AIE device, with carbon fiber as the current collectors. Cracks perpendicular to the stretching direction are identified upon stretching (Figure 1f). The possible mechanism for the crack formation was investigated. Figure 2a and b shows the XPS spectra of the samples with different AIE time. As demonstrated in Figure 2b, the AIE processing obviously decreases the C/O ratio, suggesting the oxidation of the Ecoflex by acid treatment. The AIE induced oxidation of Ecoflex was further confirmed by the FTIR characterization of the samples. Figure 2c and d presents FTIR spectra of the Ecoflex substrates with and without AIE. For the sample without AIE (Figure 2c), the bands at 793, 1259, 1412, 2906, and 2962 cm−1 are ascribed to the CH3 rocking, symmetric bending, asymmetric bending, symmetric stretching, and asymmetric stretching, respectively (Table S1).40-42 The bands at 1081 and 1016 cm−1 attribute to siloxane bond (Si−O−Si) stretching.42 After the AIE process (here for AIE time of 30 min), OH band from 2600 to 3900 cm-1 is observed and the intensity of the Si−O−Si increases.41 Since the AIE process can generate SiOx on the Ecoflex surface, it could harden the AIE-treated samples. Figure 2e and f shows the DMT modulus maps of the AIE-0 and AIE-30 samples. The sample after AIE treatment has larger Young’s moduli than the pristine one. As suggested by Watanabe et al,42 acid treatment can cleave the methyl group through oxidation reaction,



resulting in the formation of silanol groups. The yield silanol groups generate siloxane bonds through self-condensation reactions and thus forming SiOx with greater stiffness than the original substrate. Figure S2 illustrates the cross-sectional FE-SEM images of the AIE samples. SiOx layer was observed to form on the Ecoflex surface. Additionally, the thickness of the SiOx increases as the AIE time increases. Due to the strain mismatch between the AIE-generated SiOx and Ecoflex, residual stress exists at the SiOx-Ecoflex interface. When the AIE-treated Ecoflex is stretched, cracks normal to the stretching axis are formed due to the residual stress and applied unidirectional stress.39,43 The cracks resulting from AIE play important roles in improving the device sensitivity. Figure 3 compares the electromechanical performance of the devices with and without AIE. The samples with different AIE time are named with AIE-0, AIE-10, AIE-20, and AIE-30, respectively. Table S2 shows the maximum strain and sensitivity of the devices. As demonstrated in Figure 3a, a GF of 2.4 and a stretchability of 200% are obtained for the sensor of AIE-0. The AIE significantly improves the devices’ GFs (Figure 3b-d). As the AIE time increases, the GF increases. The AIE-30 has the highest GF over 8000, higher than the pioneer work using surface cracking to enhance device sensitivity (GF=2000).32 Although the sensitivity of the samples with AIE is significantly improved, the stretchability degrades with the increased AIE time. Only the sample of AIE-10 has a stretchability larger than 100%, but still provides a larger sensitivity (GF=35.2 at ε

Elastomers with

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