Piezoresistive Sensor with High Elasticity Based ... - ACS Publications

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Piezoresistive Sensor with High Elasticity Based on 3D Hybrid Network of Sponge@CNTs@Ag NPs Hui Zhang,† Nishuang Liu,*,† Yuling Shi,† Weijie Liu,† Yang Yue,† Siliang Wang,† Yanan Ma,† Li Wen,† Luying Li,† Fei Long,‡ Zhengguang Zou,‡ and Yihua Gao*,† †

Center for Nanoscale Characterization & Devices (CNCD), Wuhan National Laboratory for Optoelectronics (WNLO) and School of Physics, Huazhong University of Science and Technology (HUST), LuoyuRoad 1037, Wuhan 430074, P.R. China ‡ School of Material Science & Engineering, Guangxi Nonferrous Metals Mineral and Materials, Collaborative Innovation Center, Guilin University of Technology, Jian’ganRoad 12, Guilin 541004, Guangxi P.R. China S Supporting Information *

ABSTRACT: Pressure sensors with high elasticity are in great demand for the realization of intelligent sensing, but there is a need to develope a simple, inexpensive, and scalable method for the manufacture of the sensors. Here, we reported an efficient, simple, facile, and repeatable “dipping and coating” process to manufacture a piezoresistive sensor with high elasticity, based on homogeneous 3D hybrid network of carbon nanotubes@silver nanoparticles (CNTs@Ag NPs) anchored on a skeleton sponge. Highly elastic, sensitive, and wearable sensors are obtained using the porous structure of sponge and the synergy effect of CNTs/Ag NPs. Our sensor was also tested for over 2000 compression−release cycles, exhibiting excellent elasticity and cycling stability. Sensors with high performance and a simple fabrication process are promising devices for commercial production in various electronic devices, for example, sport performance monitoring and man−machine interfaces. KEYWORDS: piezoresistive sensor, sponge, CNTs, Ag NPs, man−machine interaction

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INTRODUCTION Superelastic electronics, as smart devices, are needed for broad applications, such as intelligent transportation, sport performance monitoring, 1−4 smart robotics, 5−7 wearable electronics,5,8−12 and man−machine interaction.13−17 In working mechanisms, pressure sensors are divided into three main types: piezoelectric,18−20 capacitive,21−26 and piezoresistive.11,27−30 Piezoresistive sensors can transform pressure message to a resistance signal and can be widely applied and rapidly developed because of their advantages, for example, fast response, high sensitivity, simple techniques, and easy signal gathering.4,12,31−39 Because of its advantages, the piezoresistive sensor has become a hot research topic. Recently, the synergy effect of hybrid sensing materials40,41 has attracted a lot of attention because it compensates for the limitaions of each material, while maintaining each material’s merits. Among the various sensing materials, carbon nanotubes (CNTs),27,42−48 with one-dimensional nanostructures of covalently bonded carbon atoms, have superior mechanical flexibility and stable electrochemical behavior. However, the sensitivity of sensors based on CNTs is usually low, and therefore, this material is unable to satisfy the growing demand for high-performance sensor. Silver nanoparticles (Ag NPs)48−51 are also an important sensing material. Because of the conductive mechanism of the electron tunneling process, © 2016 American Chemical Society

which is highly sensitive to the distance of neighboring NPs, metal NP-based12,13,52−54 strain sensors are highly sensitive in most cases. This feature can also result in the unfavorable situation of irreversible deformation between the NPs under a strong mechanical loading, which restricts its stability and working range. If Ag NPs are embedded into the conductive CNT matrix, the NP loading would further enhance the conductivity and strain sensitivity of the CNT matrix, while the CNT matrix would prevent the irreversible deformation between the NPs. Hence, the synergy effect, which compensates for the respective shortcomings of the materials, is essential for the success of the hybrid CNTs@Ag NPs as the sensing elements. For hybrid CNTs@Ag NPs, a suitable substrate is very important in pressure sensing. Here, we made use of flexible sponge48,55,56 to meet the demand of highly elastic piezoresistive sensors. Sponge,57 with a hierarchical macroporous nature, can be obtained easily and is used as a cleaning tool in all aspects of our daily life. In addition, sponge possesses the characteristics of fine hydroscopicity, high porosity, and significant internal surface area. The above features of sponge Received: April 28, 2016 Accepted: August 2, 2016 Published: August 2, 2016 22374

DOI: 10.1021/acsami.6b04971 ACS Appl. Mater. Interfaces 2016, 8, 22374−22381

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the fabrication of a flexible pressure sensor.

capacitive pressure sensors are more or less affected by outside environments. Piezoresistive sensors can overcome these weak points, and its working principle can be drawn as shown in Figure S1c, where pressure drives the resistance of the rheostat, and in turn, the pressure can be measured by the resistance value. Here, we designed and fabricated high-performance piezoresistive sensors based on sponge@CNTs@Ag NPs. Figure S1c1 is the working schematic of the sensor under an external pressure by using a constant voltage of 1.0 V. The sensing mechanism of the piezoresistive sensor can be described as follows: a higher pressure (Figure S1c1−2) will make the contacts tighter between Ag NPs and CNTs (or Ag NPs, CNTs) embedded in sponge and lead to more conductive pathways and decreased resistance. The sensor fabrication process had three steps (Figure 1): first, the washed sponge was dipped in the well-dispersed solution, removed from the solution, and subsequently dried in the drying oven at 80 °C for 24 h. We conducted the above dipping and coating method for 3 cycles, so that the hybrid CNTs@Ag NPs were uniformly deposited in the sponge. The main reason that CNTs@Ag NPs could firmly anchor on the sponge was owing to the van der Waals forces of CNTs, which made them entangled in the macroporous sponge substrate, meanwhile the CNTs offered interlocked networks for Ag NPs. In our work, we further optimized the ratio of CNTs to Ag NPs in CNTs@Ag NPs solution, that is, 1:0, 1:10, 1:15, and 1:20 and found that the best ratio for sensing is 1:20. With the increase in the ratio of Ag NPs to CNTs from 0:1 to 20:1, the sensor’s performance was enhanced. We expected to enhance the sensor’s performance by further increasing the ratio of Ag NPs, but this failed. We found that the Ag NPs reached saturation concentration in the solution and started to precipitate beyond the ratio of 1:20 (CNTs to Ag NPs). Second, the sponge with conductivity and elasticity was cut into small patches of 4 × 4 × 4 mm2. Finally, silver paste was adhered on the up-bottom surfaces of the sponge as two electrodes connecting copper wires (about 0.151 mm in diameter). Two plastic flakes were used as the two protective layers on the up-bottom surfaces by a sandwiching assembly way, which were convenient for assembling sensor, and also prevented the sensor from deformation under a certain loading. Such fabricated piezoresistive sensor not only endowed device based on sponge@CNTs@Ag NPs with high stability but also offered them additional structural elasticity, meeting requirements for future generations of wearable, portable, compressive and flexible devices. In addition, we also tested the mechanical strength of the sponge@CNTs@Ag NPs under fold, twist, and stretch conditions (as Figure S2a-c). No any irreversible deformation occurs on the sponge substrate since it could restore to its original shape under repeating compressingreleasing cycles, which revealed the excellent mechanical stability of sponge@CNTs@Ag NPs. Additionly, because of the the simple and readily available fabrication process, sponge, sponge@CNTs, and sponge@CNTs@Ag NPs with diverse

allow good contact with the hydrophilic of solution and hybrid CNTs@Ag NPs sensing materials and provides hybrid CNTs@ Ag NPs with structural stability under fold, twist, and stretch conditions, leading to high performance of the sensor device. Herein, we demonstrated a simple and repeatable dipping− coating method to fabricate a sensor based on sponge@ CNTs@Ag NPs that realizes above requirements with high stability and a high gauge factor of resistance signal to strain. In this work, we selected hybrid materials, CNTs@ Ag NPs, as the sensing material, making the piezoresistive sensor possess a higher sensitivity, shorter response time, and more stable compression−release characteristics, superior to that of a sensor based on sponge@CNTs. It is worth noting that a sensor with such a simple fabrication process and high performance could be used as a promising device for commercial production in various electronic monitoring devices, for example, traffic monitoring devices, human performance monitoring devices, etc. This process makes a tremendous contribution toward the development of wearable sensors.

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EXPERIMENTAL SECTION

Reagents. All the reagents were of analytical grade without further purification. The sponge was purchased directly from the market. CNTs (1−2 nm in diameter, up to 1−3 μm in length, purity >95 wt %, and specific surface area >380 m2/g) were bought from Beijing Boyu High-tech New Material Technology Co., LTD.; Ag NPs (∼100 nm in diameter) were purchased from Nanjing Yoshikura Nano Technology Co., Ltd. Dodecyl benzenesulfonic acid sodium (SDBS) (Mw ≈ 348.48) was bought from Sinopharm Chemical Reagent Co., Ltd. Preparation of CNTs@Ag NPs Hybrid Solution. Both of the concentrations of CNTs and dodecyl benzenesulfonic acid sodium (SDBS) were designed to be 0.75 mg/mL by addition of the compounds into 20 mL of deionized water. Then, the CNTs were dispersed homogeneously using a cell disruptor ultrasonic grinder for 45 min under a power of 300 W. Consequently, 300 mg of Ag NPs were into the CNTs solution. The as-prepared solution was stirred about 15 min and ultrasonicated for 30 min to mix the Ag NPs uniformly into the CNTs solution. All of above operations were conducted at room temperature. Preparation of Sponge. A commercially available, macroporous sponge was washed several times with deionized water, ethanol, and acetone, dried in the oven at 80 °C for 24 h, and cut into small, 4 × 4 cm2 (length × width) patches, which are ∼4 mm thick. Microstructure Characterization. The microstructures of the sponge, sponge@CNTs and sponge@CNTs@Ag NPs were investigated by thermal field scanning electron microscopy SEM (Nova NanoSEM 450, 10 kV) and TEM (Titan G260−300). The photographs were taken by a Cannon A6000 digital camera.

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RESULTS AND DISCUSSION We designed and fabricated high-performance piezoresistive sensors based on sponge@CNTs@Ag NPs by using a facile and fast dipping-coating process method. Figure S1a−c shows the basic sensing mechanisms of piezoelectric, capacitive, and piezoresistive sensors, respectively. Usually, piezoelectric pressure sensors are unstable for low pressure sensing, and 22375


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The signal changes of our piezoresistive sensors were tested by using electrical signal test system under loading and unloading cycles. The test system consists of a computer collector, single-axe motion controller and an electrical signal processing device (Aglient B2901A), as shown in Figure 3a. The above sensor was placed at one end of the uniaxial motion controller, and its electrode was linked to electrical signal processing device. When applying load for the sensor, signal processing device will convert a mechanical signal into an electrical signal output under 1 V. Figure 3b shows the compressive and recovering processes of the sensor. The sponge@CNTs@Ag NPs recovered to its original shape upon unloading. Figure 3c shows SEM images of the sponge@ CNTs@Ag NPs (at ratio of 1:20) with and without loading. As shown in Figure 3d, for all the strains with a ratio of 10−90%, the I−V curves are linear in the scanning voltage ranging from −1 to +1 V, indicating the Ohmic contact characteristics. The slope of the current−voltage curves increases as the applied load increases, demonstrating the resistance decrease when a certain pressure was applied on the sensor. Figure 3e shows current varies with time under the various applied strain from 50% to 90%. It is obvious that current repeatedly changes with the strain loading−unloading and increases with the strain increase. Meanwhile, the output current signals were compared with the pressure inputs at a certain frequency to examine the response of the sensors to external forces and showed a short response time (Figure S6a). And, the relation of the pressure versus the strain of sponge@CNTs@Ag NPs piezoresistive sensor was plotted in Figure S6b. The ratio of relative resistance changes (ΔR/Roff) to the compressive strain ε(the compressive distance divided by the thickness of the sponge) is usually defined as the gauge factor (GF)13of the sensor. The GF is 6.13 in the small-scale strain of 0−10% and 0.32 in the large-scale strain of 10−90% (Figure 3f). The relatively high GF may be attributed to embed Ag NPs in the piezoresistive sensor, which enhances the sensor performance in the small-scale strain regime. Without Ag NPs incorporation, the CNTs show minimal GF when applying a certain loading. Below the ratio 1:20 of CNTs and Ag NPs, the GF is enhanced as the weight ratio of silver particles increased. Thus, the additive of Ag NPs contributes to the improvement of the performance of our sensor. The other important parameter of a pressure sensor is defined as S = (ΔI/Ioff)/ΔP, where ΔP is the pressure change, Ioff is the current of the sensor without loading, and ΔI is its current change before and after applying the pressure.13 Figure 3g shows that the sensitivity of our sensor is enhanced dramatically after adding more Ag NPs. For the ratio of 1:20, the sensitivity is 2.12 kPa−1 at 2.24−11 kPa and about 9.08 kPa−1 at 11−61.81 kPa, where the detectable minimum pressure value is limited by the accuracy of the instrument. With higher Ag NPs loadings, the sensitivity further increases. Table S1 compares the results of our sensing performance and other reported results. Along with the enhanced sensitivity, other performances of our sensor have not been influenced, that is, the fast response and stable compressing-releasing cycles. We think that the change of contact distance between Ag NPs and CNTs (Ag NPs and Ag NPs, CNTs and CNTs) under pressure can affect the electron tunneling process and then the sensitivity of the CNTs@Ag NPs composite materials. Next, the stability of our piezoresistive sensor was further studied. Figures 3h and S7a-c show that the sensor based on sponge@CNTs@Ag NPs have negligible change of electrical

shapes such as cubes and ribbons were facilely prepared (Figure S3). Microstructures and morphologies of sponge@CNTs and sponge@CNTs@Ag NPs were imaged at different magnifications using SEM (Figure S4). The 3D hierarchical macroporous open-pore structure indicates that sponge is a framework for firm coating the hybrid CNTs@Ag NPs (Figure S4). The sensor is expected to have a high elasticity and performance. As shown in Figure S4a, the microscopic sponge possess threedimensional network structure and a large number of pores, which provides the basic skeleton for the hybrid sensing materials and ensure enough CNTs@Ag NPs to anchor on the microscopic sponge. Under a certain pressure on the sensor, the distance between CNTs@CNTs, Ag@Ag NPs, and Ag NPs@CNTs will decrease. Then, the more compact contacts lead to more conductive pathways of sponge@CNTs@Ag NPs than that of the initial state of the sensor under no pressure, similar to the case that more conductive pathways form during mechanical deformation.48 We also studied the dispersion of CNTs@Ag NPs at ratios of 1:0, 1:10, 1:15 (Figure S5a-c). We presented a collaborative design concept by imparting appropriate sensitive sensing materials, a substrate with high elasticity and a simple, inexpensive and scalable method to fabricate such a sensor based on sponge@CNTs@Ag NPs. The working mechanism is that more contacts between CNTs@ CNTs, Ag@Ag NPs, and Ag NPs@CNTs form under loading. Under no loading, there are also some tight contacts between them, as depicted the images of the transmission electron microscopy in Figure 2. Figure 2a shows that Ag NPs are well-

Figure 2. TEM images of CNTs@Ag NPs. (a) Ag NPs are dispersed in CNTs matrix. (b−d) The CNTs@CNTs, Ag@Ag NPs, and Ag NPs@CNTs contact well.

dispersed in CNTs matrix, while CNTs matrix provides a network for Ag NPs. Figure 2b−d shows that the contacts between CNTs@CNTs, Ag@Ag NPs, and Ag NPs@CNTs are tight, which can explain the Ohmic characteristics (Figure 3d) in a macroscopic state. The CNTs forming the conductive matrix provide a network for Ag NPs adhesion (Figure 2b). Figure 2d shows that Ag NPs are embedded into the conductive CNTs matrix. 22376


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Figure 3. Pressure response of sponge@CNTs@Ag NPs piezoresistive sensor. (a) Schematic of the pressure sensor. (b) Real-time optical photographs of a sponge @CNTs@Ag NPs showing the compressing and recovering process. (c) SEM images of sponge@CNTs@Ag NPs during compression and relaxation process. (d) I−V characteristics under various strains. (e) I−T curves at various strains (50%, 60%, 70%, 80%, and 90%, the current are magnified by 10, 10, 10, 2, and 1 times, respectively). (f) Resistance responses to various strains. (g) Current change ratios under various load. (h) I−T curves over 80 cycles. (i) Enlarged view of the I−T curve in panel h of several loading−unloading cycles. In panels d, e, h, and i, the sensor has a ratio of CNTs:Ag NPs = 1:20.

signals after 2000 compression−release cycles tests, which demonstrates the high stability and durability of the sensor. In addition, our sensor also shows excellent performance with moderate hysteresis (Figure S7d). This high reliability also indicates the importance of porous sponge substrate, which provides a skeleton for firmly anchoring hybrid CNTs@Ag NPs sensing materials. The presented sensors with a simple fabricated process and high performances can be applied in wearable devices for monitoring human movement behavior. We made a wearable sensor by adhereing sponge@CNTs@Ag NPs on a plastic substrate. Here, the articular and tiny muscle motion behavior was monitored by applying the wearable sensor. The flexible sensor pasting on the skin can produce deformation when we repeatedly bended the knee, arm or finger, and thus caused the repeated change of the current signals (as shown in Figure 4a, b, and c). Additionally, we can also monitor tiny changes in muscle contraction, such as the forearm (Figure 4d), cheek (Figure 4e), and throat muscle movement (Figure 4f). It demonstrates different peak intensities on current−time curve, which correspond to various articular or muscular movement

manners. Therefore, our highly senstive sensor will play a role in the development of medical care, such as health diagnoses and analysis of human tissue monitoring in sports. Figure 5 shows that a toy car (weight 0.44 kg) drove to two sensors in series and simultaneously the current−time signal was monitored. By measuring the distance between the two sensors, and recording the time interval that the car goes through the two sensors, we can calculate the speed that the car drives through the two sensors. The sensor may realize intelligent and digital detection in the vehicle overload and the speed limit in the meantime, consequently have potential application prospect in helping to reduce the number of traffic accidents. As shown in Figure 6a, based on sponge@CNTs@Ag NPs, a piezoresistive sensor array of 16 pixels (4 × 4 units) was fabricated with a 16 mm2 area of each sensing pixel. The sensing matrix identified not only the different weight of pieces but also the corresponding position according to the different signal intensity in current when pieces were placed on the sensing array. Figure 6b shows the different current of the sensor with the weight of the chess pieces and we can identify 22377


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Figure 4. Piezoresistive sensor for articular and muscular motion detection. (a−f) The current changes of our strain sensor attached on different position of human skin: bending (a) knee, (b) arm, and (c) finger; (d) forearm muscle movement; (e) cheek movement; and (f) throat muscle movement when swallowing.

Figure 5. Monitoring the speed when car moves to the sensor at a constant speed. (a)The car is running to the first sensor. (b) The car is running off the second sensor. (c) I−T curves detected by the two sensors for the car’s movement.

Figure 6. Piezoresistive sensor array under a Weiqi board. (a) The pieces were placed on the piezoresistive sensor array. (b) Reconstructed map with different column height, that is, current, identifying different weights at the corresponding position.

the pieces with different weight on the corresponding position in accordance with the column height. This raw device of

pressure sensor demonstrates the potential application in manmachine interfaces. 22378



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CONCLUSION In summary, we have developed a facile and scalable approach using “dip and dry” technique to fabricate hybrid CNTs@Ag NPs on a sponge as high-performance sensing materials. The hierarchical porous structure creates good accessibility of the pressure sensor with high performance. The fabricated piezoresistive sensor can work under the fixed voltage of 1.0 V and shows a maximum sensitivity much higher than that of a device based on sponge@CNTs. In short, our sensor shows high performance, for example, a wide pressure range up to 61.81 kPa, excellent cycling stability (>2000 cycles), and low voltage operation characteristics (1 V). We believe that our innovative architectural design based on hybrid CNTs@Ag NPs nanostructured sponge with high performance will provide new insight into the development of piezoresistive sensors and flexible electronic devices, such as intelligent transportation devices, sport performance monitoring devices, and human− machine interfacing devices.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b04971. Working schematics of three types of pressure sensor with and without loading, photographs showing the flexibility of the sponge, photographs of sponge, sponge@CNTs, and sponge@CNTs@Ag NPs with diverse shapes, SEM images the sponge@CNTs an sponge@CNTs@Ag NPs in low, medium, and high magnification, SEM images of the morphology of sponge@CNTs and sponge @CNTs @Ag NPs at different ratio of CNTs@Ag NPs, current responses with time under the pressure, pressure versus the strain of sponge@CNTs@Ag NPs piezoresistive sensor, current changes with time (I−T) curves, resistance of the sponge @CNTs @Ag NPs pressure sensor, and characteristics of the sponge @CNTs @Ag NPs piezoresistive sensor sensors (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +86-15927225564. E-mail: [email protected]. *Tel: +86-15807135274. E-mail: [email protected]. Author Contributions

N.L. and Y.G. devised the original concept, designed the experiments, discussed the interpretation of results and revised the paper; H.Z. performed almost all the experiments and wrote the draft of the manuscript; Y.S., W.L., and Y.Y. participated in the analysis of the experimental results. S.W., Y.M., and L.W. contributed some SEM observations. L.L. contributed the TEM observations. Z.Z. and F.L. contributed the discussion of the manuscript. All authors participated in manuscript revision. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work obtained support of the National Natural Science Foundation of China (11204093, 11374110, 11304106, and 51371085). Y.H.G. thanks Prof. Zhong Lin Wang for the support of experimental facilities in WNLO of HUST. 22379


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