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Highly Stretchable and Sensitive Strain Sensor Based on Facilely Prepared Three-Dimensional Graphene Foam Composite Jinhui Li, Songfang Zhao, Xiaoliang Zeng, Wangping Huang, Zhengyu Gong, Guoping Zhang, Rong Sun, and Ching-Ping Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05088 • Publication Date (Web): 06 Jul 2016 Downloaded from http://pubs.acs.org on July 13, 2016

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ACS Applied Materials & Interfaces

Highly Stretchable and Sensitive Strain Sensor Based on Facilely Prepared Three-Dimensional Graphene Foam Composite Jinhui Li,†,‡ Songfang Zhao,║ Xiaoliang Zeng,†,‡ Wangping Huang,† Zhengyu Gong,†,‡ Guoping Zhang,†*, Rong Sun†*,Ching-Ping Wong,§ †

Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen, China

‡

Shenzhen College of Advanced Technology, University of Chinese Academy of Sciences,

Shenzhen 518055, China ║

School of Material Science and Engineering, University of Jinan, Jinan 250022, China

§

School of Materials Science and Engineering, Georgia Institute of Technology, USA

ABSTRACT: Wearable strain sensors with excellent stretchability and sensitivity have emerged as a very promising field which could be used for human motion detection, biomechanical systems, etc. Three-dimensional (3D) graphene foam (GF) has been reported before for high performance strain sensors, however, some problems such as high cost preparation of GF, low sensitivity and stretchability still remain. In this paper, we report a highly stretchable and sensitive strain sensor based on 3D GF and polydimethylsiloxane (PDMS) composite. The GF is

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prepared by assembly process from graphene oxide via a facile and scalable method and possesses excellent mechanical property which facilitates the infiltration of PDMS prepolymer into the graphene framework. The as-prepared strain sensor can be stretched as high as 30% of its original length and the gauge factor of this sensor is as high as 98.66 under 5% of applied strain. Moreover, the strain sensor shows long-term stability in the 200 cycles of stretching-relaxing. Implementation of the device for monitoring the bending of elbow and finger results in reproducibility and various responses in the form of resistance change. Thus, the developed strain sensors exhibit great application potential in fields of biomechanical systems and human-interactive applications.

KEYWORDS: strain sensor, graphene foam, assembly, stretchability, sensitivity, biomechanical systems

INTRODUCTION The past decade has witnessed a growing demand for real-time healthcare monitoring, light-weight and foldable consumer electronics as well as wearable displays, which has increased the practical importance of the flexible and soft devices1-4. In this case, flexible conductor5-6 and various flexible sensors such as organic field-effect transistor (OFET)7, piezoelectric sensor8, capacitive sensor9, resistance-type sensor10 and so forth have been developed. Among these mentioned types, stretchable and wearable strain sensors based on the resistance change have gained much attention due to the advantages such as low-cost, easy fabrication as well as wide working strain which could detect both large and subtle human motions when attached to the human body11. Material approaches are mainly performed by developing conductive stretchable materials, which combines stretchable polymers with conductive nanofillers which are embedded


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or

distributed

spatially

in

polymer

matrices.

The

stretchable

polymers,

mainly

polydimethylsiloxane (PDMS), and various conductive fillers such as metal nanowires12, carbon black13-14, carbon nanotubes15-17, graphene18-20 and so forth have been investigated to manufacture the strain sensors to satisfy these requirements. Three-dimensional (3D) graphene foam (GF) with special porous network structure has achieved intense attention and has been used in many areas such as absorbents21-22, energy storage23-24 as well as flexible electronics25，etc. For example, Cheng’s group26 reported the 3D graphene foam/PDMS composite for flexible, foldable and stretchable conductors which showed high electrical conductivity and good electrical resistance stability during the stretch. Meanwhile, the compressible 3D GF had been employed as high performance pressure sensor27-31. Despite the progress of the 3D GF for flexible conductors and pressure sensors, it is still of great challenge to prepare the 3D GF for high performance strain sensor. For example, Yarjan Abdul Samad et al. had reported the GF-PDMS composites for their ability of sensing both compressible and bending strains in the form of change in electrical resistance32. Rongqing Xu et al.33 reported the GF-poly(ethylene terephthalate) (PET)/PDMS strain sensor which was fabricated by infiltrating PDMS into 3D GF and a thin layer of PET was introduced as substrate to improve the bending sensitivity of the GF/PDMS composite. The resulted strain sensor exhibited a gauge factor of 6.24 and the maximum strain was only about 16%. After that, Jeong Sook Ha’s group reported the tensile strain sensors composed of fragmentized GF and PDMS with the gauge factor of 15 to 2934. In despite of the low gauge factor, most of the above mentioned 3D graphene strain sensors were prepared by a chemical vapor deposition (CVD) method which is of high cost and time-consuming. Therefore, it is still a great challenge of developing a facile approach to prepare a high performance GF-based flexible strain sensor with high stretchability, sensitivity as well as



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excellent stability during the stretching-relaxing cycles. In this study, we employ a facile approach to prepare GF via a simple reduction and self-assembly process under mild conditions. The as-prepared three-dimensional graphene foam possesses ultralow density (as low as 4.5 mg cm -3), high porosity (as much as 99.8%) and great mechanical stability which facilitates the incorporation of PDMS elastomer into the graphene framework. The desired graphene/PDMS strain sensor could be obtained easily after connecting electrodes and curing. The as-prepared strain sensor also shows good electromechanical stability and high sensitivity which is quite important for the strain sensor. Furthermore, the sensor demonstrates great stability during the tensile strain processes. With unique electrical feature and high sensitivity as well as robust mechanical strength, the as-fabricated 3D graphene foam/PDMS composite as elastic strain sensor materials would possess a great potential application to wearable electronics.

MATERIALS AND METHODS Materials. Graphite powder (CP) and ammonium sulfide solution (20% w/w aq.) were supplied by Aladdin (Shanghai, China) and used as received. The chemicals included potassium permanganate (KMnO4), sodium nitrate (NaNO3), concentrated sulfuric acid (H2SO4, 98%), hydrochloric acid (HCl) and ammonia solution were all reagent grade purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. PDMS prepolymer and the curing agent were supplied as two-part liquid component kits from Dow Corning (Sylgard 184). Synthesis of graphene oxide and graphene foam. The graphene oxide (GO) was prepared by modified Hummers' method and the 3D graphene foam was produced as our previously report with some modification35. In brief, 20 mL of GO solution (1 mg mL-1) was sealed in a glass bottle with


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the addition of 1 mL ammonium sulfide solution. After the reduction at 95 oC for 3 h, the bottle was cooled down naturally to the room temperature resulting in the graphene hydrogel (GH). The GH was washed by deionized water and then enhanced in ammonia solution (14 v/v %) at 90 oC for 1 h in the sealed vessel. The 3D GF was obtained after freeze-drying at last. The preparation of the 3D GF/PDMS strain sensor. A piece of GF with a size of 20 mm in length and 2 mm in width was carefully cut and used for the fabrication of the strain sensor. The slice of GF was firstly connected with copper wires using silver paste and cured at 80 °C for 1h. After the conductive silver adhesives were completely cured, the GF was infiltrated with PDMS in a 10:1 (base: curing agent) ratio in a polytetrafluoroethylene (PTFE) mold and degassed in a vacuum oven for 1 h followed by curing at 80 oC for 3 h. After the curing process of PDMS the 3D GF/PDMS strain sensor was obtained with the thickness of 2 mm which is determined by the PTFE mold.

Characterization. Atomic force micrographs (AFM) were recorded with a Dimension Icon (Bruker, USA) instrument and operated in air in AC mode. Scanning electronic micrographs (SEM) were recorded with a Nova NanoSEM 450. Powder X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (RiGSku D/Max 2500) with monochromated Cu Ka radiation (λ=1.54 Å). The Raman spectra were measured by LabRAM HR Raman Spectrometer (HORIBA Jobin-Yvon, France) with a laser at the excitation wavelength of 632.8 nm and 15.7 mW power irradiation. The tensile test was carried out on a stretching machine (AG-X Plus 100N) at room temperature with a speed of 5-10 mm min-1. The electrical properties were recorded by a digital source meter (Keithley 2000 and Keithley 4200-SCS).



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RESULTS and DISCUSSION Fabrication of the strain sensor

Figure 1. (a) The schematic of the fabrication process of the GF/PDMS composite stain sensor. (b) An optical image of the as-prepared strain sensor. (c) Optical images of the strain senor before (top) and after (bottom) being stretched by 30%.

The scalable fabrication processes of the GF and the GF/PDMS strain sensor are schematically illustrated in Figure 1a. Firstly, GF was synthesized via the self-assembly process at mild conditions. Secondly, the GF was cut into slice and connected with two copper wires at both ends. At last, PDMS prepolymer and curing agent were mixed and infiltrated in GF which was degassed and cured in a PTFE mold. So far, several methods have been developed to prepared 3D GF such as CVD method, template-assisted method and self-assembly process. The CVD is one of the most common methods for the preparation of high-quality graphene foam which exhibit superior conductivity, however it’s costly and time-consuming33. Another widely employed method is using porous templates such as polyurethane36 or nickel foam32. This method is easy


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fabrication, however, the templates are need to be removed in order to get the neat 3D GF for further applications31, 33, 37-38 such as high performance strain sensor or flexible energy storage device. Self-assembly process usually starts from GO which combines with abundant oxygen-containing groups on the GO sheets surface. During the reduction and self-assembly process GO sheets change from hydrophilicity towards the hydrophobicity because of the reduction of the oxygen-containing groups by the reducing agent. Also, the π–π conjugated structures of the reduced GO sheets increased gradually which resulted in porous hydrogel and after freeze-drying the 3D GF could be obtained. The self-assembly process of 3D GF is of great potential for various fields for its low cost, easy fabrication for large scale, ultralow density, high porosity as well as high electrical conductivity. However, most of the GF are poor of the mechanical property. The 3D GF here is produced as our previously report35 with some modification and exhibits high mechanical property (which would be discussed in the following part). Importantly, compared with other strategies to fabricate highly stretchable conductive composites, such as metal ion implantation39, filling microchannels with liquid metals40 and direct mixing approach14, the whole process in our work is at room or mild condition which means it can be scaled up. Firstly, the 3D GF could be facilely prepared at mild condition and easy to scale up because of the only restriction is the shape of the vessels. Secondly, benefit from the great mechanical property, the GF could be cut in to slice about 20 mm in length and 2 mm in width and connected with copper wires without any destruction. At last, the infiltration and curing of PDMS prepolymer and curing agent could be easily processed. The as-prepared strain sensor is exhibited in Figure 1b and could be stretched as much as 30% which could meet the demands of the stretchable sensor41 (Figure 1c). The thickness of the strain sensor is about 2 mm which is determined by the PTFE mold. The sandwich-structure of the as-prepared strain sensor can be



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easily handled with a high reliability by complete encapsulation. They can be directly adhered to the skin of human body and easily attached to complex surfaces without any damage to the inner network of 3D GF.

Figure 2. (a) XRD patterns of GO and GF. (b) Raman spectra of GO and GF. (c) The compressive stress-strain curves of 10 cycles of graphene foam under the strain of 50%. (d) SEM image of GF. The SEM images of the fracture surfaces of the composite at different magnification (e and f).

The characterization of the structure of GF and as-prepared strain sensor are shown in Figure 2. Firstly, GO solution was synthesized by modified Hummers' method and the as prepared GO sheets in the aqueous dispersion ranges in size from hundreds nanometers to several micrometers with the thickness about 1 nm was measured by AFM and shown in Figure S1 which means GO sheets used in this work were mostly single layer. Secondly, the GF was prepared by assembly process and by the reduction of GO. The organic groups of GO was reduced gradually and the hydrophobicity and the π–π conjugated interaction increased at the same time which resulted in a rod-shaped hydrogel at the end. The GH was freeze-dried and the GF (Figure S2) was obtained which was examined by XRD and Raman spectra (Figures 2a and b). The diffraction peak of the


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graphite is around 2θ = 26.46 o and the peak of GO is at 2θ = 11.02 o. After the reduction by ammonium sulfide, a new diffraction peak at 2θ = 24.08 o appeared in the GF, which is quite close to that of graphite indicating the chemical reduction happened successfully during the assembly process. The Raman spectra of GO and GF showed two remarkable bands at around 1328 and 1586 cm−1 which are assigned to the D- and G-bands of carbon materials. GO shows an ID /IG ratio of 0.79 while the corresponding value of GF has been increased up to 1.47. The ID /IG ratio of our graphene foam with more defects in the plane of the carbon is quite different from the graphene foam prepared by CVD method33-34. The graphene foam was prepared by 1 mg mL-1 GO solution and the as-prepared graphene foam showed an ultralow density of 4.5 mg cm-3 and a conductivity of up to 0.20 S m−1 which was recorded by a digital source meter. Furthermore, the porosity is achieved as high as 99.8% which is calculated based on the followed equation35, 42, ɛ=1-ρ/ρo (where ε represent the porosity, ρ is the density of the as-prepared graphene foam while ρo is the density of graphite which is assumed to be 2.2 g cm-3). Therefore, the graphene foam obtained here is of great electrical conductivity, ultralight as well as high porosity. Besides all of these, the graphene foam shows great mechanical stability which facilitates the incorporation of PDMS elastomer into the graphene framework. As shown in Figure 2c, the repeated compression rebound test shows that the graphene foam could be compressed as much as 50% strain and after 10 cycles compression the graphene foam shows reproducible results with the maximum stress of 2.8 KPa. The inner structure of the GF was observed by SEM (Figure 2d and Figure S3). GF exhibits a cellular structure with continuous 3D network. The walls of the pores are made up of assembled graphene sheets during the reduction and self-assembly process. When the liquid PDMS prepolymer and the curing agent diffuse into the GF, the liquid polymer penetrates into the



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interconnected pores of the 3D GF network, owing to the low viscosity and low surface energy of the liquid PDMS. Figures 2e and f exhibit the SEM images of the fracture surface of the composite which exhibited that PDMS was uniformly dispersed in the most parts of the GF and the continuous 3D structure of GF has been kept and unbroken during the PDMS infiltration process as shown by the white cycles in the Figure 2f. The intact structure of 3D GF is benefit from the excellent mechanical property of GF. In addition, the sandwich-structure also benefits the stability of the as-prepared strain sensor as shown in the following parts.

Strain sensing of 3D GF/PDMS

Figure 3. (a) I−V curves of the strain sensor under various strains. (b) The relative resistance variation of the strain sensor in the stretching process. (c) The relative change in resistance during 0% to 20% strain in the stretching-relaxing cycles. (d) The gauge factor distribution of the strain sensor under various strains.


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The unique sandwich-structure of the as-prepared GF/PDMS strain sensor would not be affected by the change of the outside environment such as the temperature, humidity as well as slight friction which enables them to maintain certain resistance-change stability during the stretching-relaxing cycles. The porous network of the embedded graphene foam not only provides the electrically conductive channel but also exhibits the resistance change during the mechanical deformation for the change of the contact area of the graphene sheets. The thin structure of the device promotes the strain sensor not only excellent flexibility but also high sensitivity. The electromechanical characteristics are very important for the GF/PDMS strain sensor which are presented in Figure 3. The strain sensor demonstrates linear current-voltage curves under various strain indicating that the I−V curves of the device strictly confirm to the Ohm’s law under various applied strains, as shown in Figure 3a. When strain is applied to the graphene foam/PDMS composite, cracking and breaking of the graphene occurs, which causes the increase in resistance. The resistance change mainly derives from the variation of the contacted area between graphene sheets under strain. Figure 3b shows the relative resistance variation of the strain sensor in the stretching process. The strain sensor was stretched to different strains (5%, 10%, and 20%). The relative resistance increased with the increase of the applied strain and strain sensor exhibited great reproducibility when stretched to larger strains which indicated the great stability of the sandwich-structure of the as-prepared strain sensor. As shown in the Figure 3c, there is no obvious hysteresis in the response of the GF/PDMS strain sensor for over ε= 20% of stretching-relaxing cycle and the original resistance of the sensor could be fully recovered after releasing. By the general calculation formula of gauge factor, GF =(∆R/R0)/ε, where ∆R, R0, and ε represents the change of resistance, the resistance at 0% strain, and the applied strain, respectively, the gauge factors of the strain sensor under 5%, 10% and 20% strain are calculated (Figure 3d). The gauge



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factors ranges from 47.74 to 98.66, which is much higher than the previous results33-34. For example, the R0 here is about 3.5 KΩ and at 5% strain, the ∆R/R0 was as large as 4.96 and the calculation result of gauge factor is 98.66. However, with the increase of the strain, the relative change of resistance slows down and the gauge factor at 20% is about 47.74. The possible reason could be that during the initial stretch process (≤5%) much more cracks occurs because of the breaking of 3D continuous graphene foam. While with the increase of the strain (˃5%), cracks appears as well but not as much as the initial period which results in the slowly increase of ∆R and according to the calculation formula of gauge factor, the gauge factor goes down with the increase of the strain after 5%. According to the literatures33-34, the strain sensors of 3D GF prepared by CVD method usually possess much lower gauge factors, and the possible reason could be that the GF prepared by self-assembly process possesses abundant pores with the pore size ranges from tens of nanometres to hundreds of micrometres, as shown in Figure S3, which means that much more defects could be introduced during the stretch compared with the 3D GF prepared by CVD method. The 3D GF prepared by CVD method usually possesses uniform pore size because of the applied template. Thus, the as-prepared strain sensor here exhibits higher sensitivity than the reported 3D GF strain sensor prepared by CVD method.

Figure 4. (a) The relative change in resistance versus strain. (b) The relative resistance change


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during the stretching-relaxing cycles of the sensor at 10% strain.

The changes in the relative resistance of the sensor at the strain of 10% are shown in Figure 4. The strain sensor was tested at the speed of 5 mm/min and after each cycle the strain sensor was directly stretched for another cycle without any pause. The relative resistance change versus strain demonstrates the linearity and negligible hysteresis as shown in Figure 4a, which enables the strain sensor with excellent stability and shorter response time. Figure 4b showed the relative change in resistance versus time at the strain of 10% for the first 20 cycles. The results exhibits that the strain senor shows some unstable curves at first and after several cycles the relative resistance change becomes constant and exhibits excellent reproducibility. The reason could be that cracks occur in the graphene foam during the stretch from the beginning but the damaged structure of graphene foam is not stable at first and after several stretching-relaxing cycles the damaged structure of GF becomes stable and the resistance of the strain sensor at different applied strain then becomes stable as well. The work mechanism of our strain sensor is shown in Figure 5. At first, the 3D GF is continuous structure and provides the electrically conductive path. The reduced graphene oxide sheets are stacked because of the π–π conjugated interaction. When the external force is applied to the strain sensor, the graphene sheets starts to slip and at last the cracks occur because of the separation of the graphene sheets, which results in the decrease of connected area between graphene sheets and the increase of the resistance. During the relaxing process, microcracks between the GF which is produced in the stretching process reconnected gradually because of the reversion of the PDMS elastomer. However, the break of the π–π conjugated interaction is an irreversible process which resulted in the slight increase in the resistance of the strain sensor at first several cycles as shown in Figure 4b. The strain sensor became stable after several stretching-



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relaxing cycles. So the mechanism of our strain sensor could be concluded in two levels: Level 1, the irreversible damage of π–π conjugated interaction between graphene sheets; level 2, the reversible formation of the microcracks in the GF. The break of π–π conjugated interaction between graphene sheets mainly occurs during the first several stretching-relaxing cycles at a certain strain which results in the microcracks in the GF. The microcracks could be reversibly formed during the stretching-relaxing cycles which mainly contribute for the sensing of the strain sensor.

Figure 5. The schematic diagram of working principle of the stain sensor. Level 1, the irreversible damage of π–π conjugated interaction between graphene sheets; level 2, the reversible formation of the microcracks in the GF.


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Figure 6. (a) Durability test of the strain sensor under 20% strain for 200 cycles. (b) The resistance change curves of the strain sensor extracted from the red part in (a). (c) The resistance-time curve of the strain sensor under wrist bending. (d) The I-T curve of the strain sensor during the finger bending cycles at different bending interval time.

For the strain sensor, durability is a very important indicator in practical applications, which can remarkably reduce its use cost and enlarge its popularity. The strain sensor was tested at the speed of 10 mm/min and relaxed for 20 seconds after each cycle. As shown in Figure 6a, after 200 cycles of stretching−relaxing under 20% strain, the strain sensor remains favorably stable. Figure 6b represents a random stretching-relaxing cycles extracted from Figure 6a (red pane) and the relative change in resistance versus time curves exhibits excellent stability. Besides, the as-prepared stain sensor also shows great stability during the stretching-relaxing cycles without any relax as well (Figure S4). At last, the as-prepared strain was employed for the human motion detection such as elbow bending and finger bending. In the bending process, large deformation of human body could



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be formed, for example, the strain resulting from the human wrist bending was about 30%43-44. Thus, the deformation could be sensed during the motion of the bending in the change of the resistance. Figures 6c and d illustrate the detection of elbow and finger bending, respectively, following the placement of the sensor on the joints of the body. As seen from Figure 6c, the relative change in resistance in several bending cycles demonstrates a favorable reproducibility. In addition, the finger bending has also been detected at different motions states such as quickly bending, slowly bending and quickly-slowly bending cycles which exhibits great sensitive and stability (Figure 6d). The results illustrate that the 3D GF/PDMS composite might be potentially applied as wearable strain sensor materials.

CONCLUSION In summary, the flexible and stretchable 3D GF/PDMS strain sensor has been manufactured and exhibits high flexibility which could be stretched as much as 30% of its original length. The as-prepared strain sensor also shows high sensitivity with a high gauge factor in the range of 47.74-98.66 and has successfully been employed to detect the motion of human body such as the bending of elbows and fingers. In addition, the strain sensor shows long-term stability in the 200 cycles of stretching-relaxing, which is very important in practical applications. Considering the facile synthesis of the high performance graphene foam, the easy fabrication of the special sandwich-structure 3D GF/PDMS sensor as well as the high sensitivity and reproducibility during the stretching-relaxing cycles and detection of the motions of human body, we believe that the reported stain sensor demonstrates its high potential for the application as a wearable strain sensor.

■ASSOCIATED CONTENT


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Supporting Information Additional figures as described in the text (Figure S1-S4) are available free of charge via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially supported by National Natural Science Foundation of China (Grant No. 21201175), Guangdong and Shenzhen Innovative Research Team Program (No.2011D052, KYPT20121228160843692), R&D Funds for basic Research Program of Shenzhen (Grant No. JCYJ20150401145529012) and Key Deployment Project of Chinese Academy of Sciences (Grant No. KFZD-SW-202).

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Highly stretchable and sensitive strain sensor based on 3D GF and PDMS composite was fabricated in which GF was prepared by a facile assembly process. The as-prepared strain sensor exhibited high stretchability (30%), high gauge factor (98.66 under 5% of applied strain) and long-term stability (200 stretching-relaxing cycles). Moreover, the strain sensor was applied for monitoring the bending of elbow and finger. Thus, the developed strain sensors exhibited great application potential in fields of biomechanical systems and human-interactive applications.



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