Cellular Carbon-Film-Based Flexible Sensor and Waterproof

Jun 26, 2019 - We took the sensor based on graphene materials as an example in ... as the building unit would show strong potential to advance flexibl...
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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Cellular Carbon-Film-Based Flexible Sensor and Waterproof Supercapacitors Libo Gao,*,†,‡ Yuejiao Wang,§ Xinkang Hu,†,‡ Wenzhao Zhou,‡,∥ Ke Cao,§ Yongkun Wang,† Weidong Wang,*,†,‡ and Yang Lu*,‡,§,∥ †

School of Mechano-Electronic Engineering, Xidian University, Xian 710071, China CityU-Xidian Joint Laboratory of Micro/Nano-Manufacturing, Shenzhen 518057, China § Department of Mechanical Engineering, City University of Hong Kong, Kowloon 999077, Hong Kong SAR ∥ Nano-Manufacturing Laboratory (NML), Shenzhen Research Institute of City University of Hong Kong, Shenzhen 518057, China

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S Supporting Information *

ABSTRACT: A highly sensitive portable piezoresistive sensor with a fast response time in an extended linear working range is urgently needed to meet the rapid development of artificial intelligence, interactive human−machine interfaces, and ubiquitous flexible electronics. However, it is a challenge to rationally couple these figures of merit (sensitivity, response time, and working range) together as they typically show functionally correlative behavior in the sensor. Here, we aim at introducing the hierarchical pores across several size orders from micro- to larger scale into the intrinsically flexible graphene-based electrode materials that overcome this limitation of the sensor. We achieved a flexible sensor with a prominent sensitivity of 11.9 kPa−1 in the linear range of 3 Pa to ∼21 kPa and a rapid response time of 20 ms to positively monitor the pulse rate, voice recognition, and true force value for biomedical and interactive human−machine interface application assisted by an analog-digital converter. More interesting is the carbon-nanotube-doped graphene that also served as the electrode in the waterproof supercapacitor to actively drive the sensor as a whole flexible system. We believe our findings not only offer a general strategy for the graphene-based platform in flexible electronics but also possess other intriguing potential in functional application such as the heat dissipation component in electron devices or seawater filtration in environment application. KEYWORDS: piezoresistive sensor, hierarchical structure, graphene, supercapacitor, pulse rate

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INTRODUCTION With the rapid steps of coming into the age of artificial intelligence and the interactive human−machine interfaces as well as ubiquitous flexible electronics, effectively transferring the analog signal information into sensible digital information is thus imperative for the timely feedback to the outside stimulation.1−10 Therefore, it is urgently needed to develop a flexible sensor device with the figures of merit of superior sensitivity, extended working range, and fast response time. We took the sensor based on graphene materials as an example in our study because of its excellent electrical property, chemical stability, and mechanical strength.11−13 As the sensitivity of the pressure sensor is determined by S = (ΔI/ I0)/ΔP (where ΔI is the current change, I0 is the initial current, and ΔP is corresponding pressurized variation), it is theoretically to enhance the sensitivity through following the three items: (1) minimizing the initial current; (2) increasing the resistance change at a specific pressure variation; and (3) suppressing the pressure variation at a given ΔI/I0.14 Introducing the porous structure into the electrode materials of the sensor is indeed beneficial to decreasing the initial current due to the discontinuous percolation pathways © XXXX American Chemical Society

compared to its bulk counterpart. Meanwhile, the existence of the pores enables a larger conductive change at a constant applied force by greatly enhancing the contact area due to the smaller stiffness according to Young’s modulus E scale with the average density (E ∼ρ2.73±0.09) estimated by Qin et al.15 However, this is in conflict with the third aspect of keeping a higher stiffness for the lower pressure change variation at a given fixed value of ΔI/I0. As such, it is a challenge and difficult to deal with the tradeoff between the ΔI/I0 and ΔP for enhancing the sensitivity of the sensor, especially the flexible sensor. These days, Zhao et al. proposed the closed-cell porous graphene structure to significantly decouple the above attributes, which showed high sensitivities of 0.051 to 11.1 kPa−1 without changing the elastic modulus.16 Therefore, it is promising to increase the sensitivity through building closed cells as microscale building units in electrode materials. In addition to the sensitivity, the working range of the sensor is also closely related to its structural configuration. As previously Received: May 30, 2019 Accepted: June 26, 2019 Published: June 26, 2019 A

DOI: 10.1021/acsami.9b09438 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Flexible and porous ordered rGO film for sensor. (a) Schematic illustration of the porous rGO and (b) corresponding digital optical images of 2D-film and 3D wave- and spiral-like rGO structures. (c−g) FESEM characterizations of the hierarchically porous rGO. (h) The ultralight rGO film stands freely on the flower. (i) Loading−unloading mechanically compression test of the rGO film via a Microtest. (j) Tensile test and flexibility of the PDMS@rGO.

reported,17 with the applied force inducing the stress concentration and the subsequent subtle deformation of the structure, the linear pressure-sensing behavior over an extended pressure range is relatively limited and the sensitivity would significantly decrease when the pressure is further enhanced. Building the hierarchical configuration or multilayer structures is practically available to enhance the structural integrity through relieving the local stress and thus significantly increase the linearity of pressure sensing and maintain the constant pressure sensitivity over a range of enlarged working ranges.18 Besides, compared to the traditional stochastic structures used in the sensor, the periodic structure with regularly controlled pores would greatly boost its extraordinary mechanical properties.18,19 Therefore, it is of great importance to fabricate the hierarchically periodic structure in improving the working range of the high-performance flexible pressure sensor.20 Very recently, Peng et al. adapted 3D printing to create a piezoresistive sensor with hierarchical porosity, which showed a large measurement range of 10 Pa to 800 kPa.21

However, the development of 3D printing for fabrication of those 3D structures is still in its early stages, and the requirement for rigid preparation of ink materials seriously suppresses its sensitivity and tunability. On the other hand, to achieve a short response time of the sensor, the intrinsic flexibility is desirable for the fast reconnection of the conductive pathways upon release of the applied pressure to ensure complete restoration of the electrical resistance.22 Based on the above comprehensive analysis, the ordered and hierarchically porous graphene bulk with intrinsic flexibility composed of closed-cell graphene as the building unit would show strong potential to advance flexible sensors with high sensitivity, larger working range, and fast response time, whereas it is rare to see examples of this or similar device reported before. To this end, we here realize a graphene-based flexible sensor with high sensitivity, extended working range, and fast response time. We rationally employed the “hard template”, namely, copper mesh and ice crystal, and “soft template”, B

DOI: 10.1021/acsami.9b09438 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. Sensor preparation and characterization. (a) Schematic illustration of the flexible piezoresistive sensor. (b,c) Sensing mechanism of the as-synthesized sensor device. (d) Illustration of the setup for the measurements of the current as a function of applied force. (e) Normalized current change as a function of applied pressure. (f) Comparison of the sensitivity and working range of our work with other reported carbon-based piezoresistive flexible sensors. (g) Consistently repeatable amperometric change for various pressure changes. (h,i) In situ SEM observation and corresponding illustration of the rGO film during the compression process.

namely, hydrogen bubbles, as the multiscale templates to create the hierarchically closed-cell porous reduced graphene oxide (rGO) film for the state-of-the-art flexible sensor. Benefitting from its intrinsic flexibility, our sensor not only showed a fast response time in a practical application of wearable bioelectronics for monitoring of the pulse rate but also demonstrated superior sensitivity in voice recognition and true force value in the interactive human−machine interface. What makes it more exciting is that the carbon nanotube (CNT)-doped hybrid carbon film can also be directly regarded as the waterproof supercapacitor to power the abovementioned flexible sensor with an extended broad application. Our findings offer a general graphene-based platform for next-

generation sensors and energy electronics with intriguing prospects.

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RESULTS AND DISCUSSION Structural and Mechanical Characterization. The schematic illustration of preparing the hierarchically porous rGO is shown in Figure 1a. To pattern the desired first-order pores (FOPs) (as indicated by the green and blue arrows), we used the 150 μm sized copper-wire mesh with a pore size of 600 μm as the hard sacrificial template to grow the rGO film. To introduce the second-order pores (SOPs) into the rGO (as indicated by the red arrows), hydrogen bubbles were rationally employed. Note that fewer reports before directly used the hydrogen bubbles as the soft templates derived from C

DOI: 10.1021/acsami.9b09438 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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from cycle 10 to 100, almost recovered to its original state. On the other hand, the stretching ability is needed for the porous graphene film to cater for the seamless manufacture with other broad nonplanar facets when applied as a flexible sensor.23 The PDMS@rGO exemplified a superior tensile strain ability of 40% (Figure 1j) and fully mechanical adaptability such as bending and twisting for better environment accommodation (inset in Figure 1j and Movie S1). In conclusion, we have demonstrated the controllable hierarchically porous architecture and outstanding mechanical performance of the rGO film, indicating its great underlying merits for the piezoresistive sensor since its sensitivity is indeed related to the thickness of pore walls, pore size and distribution, and weight density of the graphene skeleton.24 Therefore, the corresponding sensor application will be well demonstrated next. Sensor Preparation and Application. To prepare a piezoresistive flexible pressure sensor, a corresponding schematic is illustrated in Figure 2a. The rGO film with a thickness of 0.2 cm was carefully mounted on the Kapton film by the strong bonding of conductive silver paste at its two ends of the sample, assembled with silver wire with a diameter of 100 μm serving as the conductive wire. The whole device configuration was encapsulated within the PDMS, thereby forming a mechanically stable structure. As the sensing mechanism for such a device is shown in Figure 2b,c, with continuous force applied on the graphene lattice, the stacking areas between two neighboring graphene sheets are increased gradually to raise various contact modes such as “point-topoint”, “point-to-face”, and “face-to-face”,25 thus shortening the electron transportation pathway and enhancing the conductivity. Further, similar phenomena happened at the near nanoscale in individual graphene sheets, which assembled the wall of a cell as marked in Figure 2b and Figure S4. Therefore, the significant change of contacting areas under loading leads to high sensitivity of the flexible sensor by dramatically increasing the conductivity, and the intrinsic hierarchical mechanically recoverable deformation of the rGO film enables a large force range. Spontaneously, indeed, the contribution of the intrinsic resistance of the graphene sheets deformation to the total resistance would be lower considering the significant mismatch of Youngs’ modulus and weak interfacial bonding strength between rGO and PDMS, resulting in negligible intrinsic deformations of rGO at a device level.22 Therefore, in our study,

hydrochloric acid, although electroplating rGO on the conductive substrate is a typical way. The honeycomb-like cells of the third-order pores (TOPs) were then created in a freeze-casting manner using ice crystals as hard templates after etching the metal template (as indicated by the yellow arrows). Accordingly, the hierarchically structural rGO film was obtained as shown in Figure 1b. Importantly, our rationally designed route allows one to fabricate an arbitrary shape of the rGO aerogel from 2D to 3D while keeping the structural integrity (Figure 1b and Figure S1). The phase structure was characterized as shown in Figures S2 and S3. To further explore our concept for the unique arrangements of the graphene pores, the microstructure of the rGO film was carefully characterized by a field-emission scanning electron microscope (FESEM) (Figure 1c−g). The ordered pores were uniformly patterned throughout the whole rGO film without any obvious flaws (Figure 1c). The FOP with a mean size of 400 μm was circularly surrounded by the 100 μm sized SOP (Figure 1d) along the vertical growth direction. More importantly, the continuous SOP was densely distributed throughout the FOP’s wall (Figure 1e), forming an ant-nestlike morphology. This kind of special architecture will benefit the electrochemical performance with fast electrolyte ions penetrating, which will be discussed later. Further, the cross section view of the rGO film with interesting hierarchical patterns is shown in Figure 1f. The FOP (as indicated by the green arrows) formed perforative channels after the etching of copper mesh, and its size was easily controlled by the copper wire’s size. The FOP as indicated by the blue arrows showed the hemisphere-like framework with a semiclosed wall between the adjacent channels based on the special configuration of the copper mesh, which will enhance the interface bonding of the graphene film with the soft polydimethylsiloxane (PDMS) polymer due to the increased surface area. The SOP grew along the perpendicular direction nearly across half thickness of the film (red arrows), surrounded by the multi-TOP closed cells spanning from 5 to 100 μm created during the freezecasting process (Figure 1f,g). Specially, the building unit of the film was composed of multilayered graphene sheets with crumpled features as shown Figure S4, and these pores were mainly ascribed to the gas or water escaping during the postheating treatment of the rGO film. Thanks to the above multiscale and hierarchical configuration of the porous structure, the rGO exhibited an ultralight mass density of 2.5 mg cm−3 to stand freely on the Ixora chinensis flower (Figure 1h). Additionally, the pores were able to be fully controlled as shown in Figure S5. Note that the electroplating time and applied voltage both played important roles in the fully control fabrication of the rGO film. To explore the potential of the rGO film as a candidate for the pressure sensor, the mechanical compression test was performed via a Microtest since the flexibility of the electrode material itself plays an imperative role in flexible electronics’ performance due to the strong electromechanical coupling attributes. The as-synthesized rGO film exhibited outstanding resilience when compressed at 90% strain (Figure 1i). As regard to the first compression cycles, the linear elastic approached 45% compressive strain. More importantly, the pure rGO achieved high compressive stress of 20 KPa at 90% strain, demonstrating its great potential in extending the working range of the sensor device. Also, we can clearly find that each compression causes a degree of permanent plastic deformation, but the recoverability of the film remains stable

ΔR /R = (1 + 2v)ε + Δρ /ρ

(1)

where the conductive variation (ΔR/R) is primarily determined by the former expression related to the geometrical change (v is Poisson’s ratio, andε is the strain ratio, and Δρ/ρ is the variation of intrinsic resistance). In view of the hierarchical structure, our device possibly realized a promising sensitive ability. To demonstrate this concept, the setup of measurement configuration for the sensor is well established in Figure 2d. As expected, our sensor displayed relatively high sensitivities of 5.23 and 11.9 kPa−1 at working ranges of 3 Pa to 9.25 kPa and 9.25 kPa to 21.25 kPa, respectively (Figure 2e). Obviously, two different linear regions were shown for the flexible sensor that exhibited a higher sensitivity under increased applied pressure, which is also always observed in other reports.26,27 This is because the evident deformation of the relatively large scaled pores of the hierarchical rGO only occurred when pressure reached a certain value. Further, correlation coefficients of 0.996 and 0.998 were obtained, D

DOI: 10.1021/acsami.9b09438 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. Practical application of the sensor. (a,b) Detected signals when small objects including barley and water drop fell from a height of 5 cm. (c) Corresponding response time of the sensor. (d) Digital optical image of the sensor for monitoring the pulse rate. (e,f) Measured pulse rate of 29 year old male subject in good health conditions before and after 5 min of exercise, indicating heart rates of 56 and 89 beats min−1, respectively.

the first test (Figure 3a,b). As a strong and sharp signal was produced by the gravity and impact force of the object, the sensor exhibited a rapid response time of 20 ms during the contact process (Figure 3c). This high value allows the sensor to precisely serve as a flexible device in medical and smart robotics for human−computer interactions. As a typical example, our sensor was able to be easily mounted on skin to investigate the physiological signals such as the pulse rate (Figure 3d). The three featured peaks corresponding to percussion (P), tidal (T), and diastolic (D) for the pulse waveforms of a 29 year old male were obviously observed, indicating a pulse rate of 56 beats min−1 at a rest state while 89 beats min−1 after 5 min of exercise (Figure 3e). Additionally, the enlarged views of the signal after 5 min of exercise exhibited a diverse spectrum in which the T wave increased gradually and the peak migrated toward the D wave compared to that of the rest one (Figure 3f). These results to some extent demonstrated that the exercise would help to relieve the vessels’ stiffness through dilation and simultaneously enhance the diastolic and systolic functions of vessels for our better health.38 Besides, the performance of our sensor can be even higher than the commercial ones as shown in Figure S6. The acoustic sound typically shows a low pressure regime from 1 to 10 Pa and high -frequency transportation in air; therefore, it is typically a challenge to correctly monitor the sound waveform.17 We accordingly assembled the monitoring system based on our pressure sensor to detect the sound and music fluctuations in real time due to its superior sensitivity and fast response time (Figure 4a). For instance, we recorded the spectrum of the word “Hello” multiple times and then fit a characteristic data F(t) that only belongs to the person with the sensor device (Figure 4b). Then we can easily use the fitted curve to identify the man’s voice through the featured peak location (Figure 4c). Further, our flexible sensor can detect different words from the same speaker as shown in Figure S7.

demonstrating its superior linear performance, and were comparable to or even higher than those in other reports.28−30 The sensitivity of the sensor as a function of linear working range is plotted in Figure 2f. Our flexible rGO film simultaneously showed relatively superior sensitivities (5.23 and 11.9 kPa−1) and extended linear working range (21.1 kPa) compared to other recent pressure sensors made of PDMS/ SWNT,31 RGO-PU-HT-P,32 LSG/PDMS,33 graphene/silver,34 double-layered graphene,35 PDMS/silver,36 and silver/cotton.37 Further, even when suffering from a wide range of compressive loading−unloading stress cycles at various applied pressures of 100 to 600 Pa, the sensor showed a reversible behavior with nearly negligible hysteresis with a constant applied voltage of 0.1 V only, evidenced by Figure 2g; this not only evidenced its superior sensitivity but also demonstrated its linear broad working range. To further deeply explore the underlying deformation mechanism of the sensor, an in situ SEM compression was employed. During the compression process, the tubular graphene wall tends to be buckling under applied stress, thereby resulting in the contact mode of face-toface with increasing the conductivity (Figure 2h). While the small-sized pores, already shown in Figure 1g, assembled by graphene sheets with wrinkled features (Figure S4) occurred point-to-point and point-to-face contacting behaviors (Figure 2i) under applied compression. Specifically, the nearly parallel graphene sheets tend to contact with each other, and the wallto-wall densification would be produced to release the externally applied force during this compression process. This behavior would lead to the current increase owing to the decreased contact resistance. As already demonstrated by the superior sensitivity and wide linear working range of the flexible sensor, it is assumed that the device held advanced potential in practical application in daily life. To validate its capability in detecting force of small objects, ∼5 mg of barley and one drop of water (∼0.1 mL) were dropped on the sensor’s surface from a height of 5 cm in E

DOI: 10.1021/acsami.9b09438 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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layer capacitive performance (Figure 5c,d). An areal capacitance of 30.94 mF cm−2 was calculated at a current density of 2.5 mA cm−2 and still kept a high value of 26.5 mF cm−2 even at an enlarged current density of 20 mA cm−2. This high areal capacitance is superior to existing carbon-based supercapacitors such as vertically aligned graphene (VG, 7.3 mF cm−2),41 laser rGO (0.5−2.5 mF cm−2),42 rGO (2.47 mF cm−2),43 3D cellular graphene (∼6 mF cm−2),44 B-3D-PCP MSC (2.95 mF cm−2),45 and TiN@C (19.4 mF cm−2)46 (Figure 5e). Such excellent areal capacitance and rate capability are primarily derived from the following items: (I) the 3D hierarchically porous architecture serves as the reservoir for the sufficient impregnation of the electrolyte with fast electrolyte ion transportation. (II) The porous structure increases the activated surface area with corresponding enlarged areal capacitance. (III) Superior conductivity is achieved as highly conductive pathways are built by the twodimensional graphene and one-dimensional carbon nanotubes (inset in Figure 5e and Figure S10). (IV) CNTs also supply extra capacitive contribution to the whole electrode’s performance. Besides, energy density and power density are two respectively imperative considerations for practical application of energy storage devices. Our solid-state supercapacitor showed the highest areal energy density of 4.3 μW h cm−2 and power density of 1.25 mW cm−2 (Figure 5f), which are higher or comparable to other supercapacitors of similar materials such as MSC (0.22 μW h cm−2, 0.37 mW cm−2),43 WSSs (0.01 μW h cm−2, 1 mW cm−2),47 3D GP-MSC (0.38 μW h cm−2, 0.86 mW cm−2),48 3D-graphene/graphite (1.24 μW h cm−2, 24.5 μW cm−2),49 and GFSC (3.4 μW h cm−2, 0.27 mW cm−2).50 Importantly, our device exhibited durability with 96% capacitance retention after 10000 charge/discharge cycles, demonstrating its potential in long-time practical application (Figure 5g). To demonstrate its potential in backup function in energy storage, the supercapacitor charged by the solar cell was able to drive a commercial watch successfully as well. Even removing the irradiation of the sunlight, the watch was in working status powered by the supercapacitor (Figure 5h). Since the supercapacitor typically works in harsh environments, the device’s capacitive performance was carefully tested under bending deformation and in static as well as under the flowing water (Movies S3 and S4). Amazingly, our supercapacitor was capable of driving the electronics in these rigorous environments for long-time working (Figure 5i,j). Similarly, our sensor that was put in water capable of measuring the water wave also showed waterproof performance as shown in Figure S11. Most importantly, as we highly expected, the sensor was successfully powered by the portable supercapacitor mounted on the elbow due to its planar features, with the demonstration of the monitoring of the pulse rate in Figure 5k. These above practical applications demonstrated the smart concept in designing and manufacturing the hierarchically porous graphene structures for sensors and supercapacitors, which should also inspire others to dig more functional application or further improvement of this unique architecture.

Figure 4. Sensor application in acoustic vibration and force detection. (a) Setup for acoustic vibration sensing. (b) Recorded sound spectrum of a man aged 29 (saying the word “Hello”) six times and the corresponding fitted characteristic sound vibration. (c) Using the fitted data to identify the speaker. (d) Current responses to the acoustic vibrations of the music “Star Sky” by Two Steps from Hell. (e) Consistent current change of the sensor in response of the word “Hello”. (f) Illustration of the setups for the sensor to directly measure the applied force and (g,h) corresponding experimental demonstrations.

Such high sensitivity of our sensor combined with big data would potentially provide a strategy for personal security, which is further confirmed by the accurate recognition of the music “Star Sky” by Two Steps from Hell played by a mobile phone 2 cm away from the sensor (Figure 4d). Additionally, the sensor remained its original working status without any obvious decay after several repeated tests with the demonstration of the long durable ability (Figure 4e and Figure S8). By contrast, the force value should be obtained rather than a mere current signal considering the practical application of the pressure sensor to the artificial skin or smart device.39 The sensor device integrated with the analog-digital conversion module (Figure 4f) was capable of successfully transferring the analog signal of the current to the digital data of force as the successful demonstration of 0.01 N of a 1 g weight and gentle tapping test in Figure 4g,h and Movie S2. The detailed design for the circuit can be found in Figure S9. Supercapacitor-Driven Sensor. As a strong backup for the flexible electronics, supercapacitors have recently been considered as the power sources integrated in the self-powered flexible solar-cell-based systems in our studies (Figure 5a).8,40 We directly used the highly conductive porous rGO film with doping of CNTs to assemble the symmetrical waterproof supercapacitor, which was carefully encapsulated within Ecoflex as shown in Figure 5b. The CV curves at various scan rates from 10 to 500 mV s−1 and symmetrical GCD curves indicated the excellent rate capability with an ideal double-

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CONCLUSIONS In summary, we have successfully fabricated the hierarchically porous carbon (graphene and graphene/CNT) film with ordered channels using the metallic mesh (copper or nickel), hydrogen bubbles, and ice crystals as the first-, second-, and F

DOI: 10.1021/acsami.9b09438 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. Self-powered sensor system coupled with the flexible solid high-performance supercapacitor. (a) Schematic illustration of the flexible selfpowered system including energy harvesting and energy storage as well as the sensor. (b) Digital optical image and schematic illustration of the symmetrical solid-state supercapacitor. (c) CV and (d) GCD curves as well as (e) areal capacitance of the supercapacitor device curves at various current densities. (f) Ragone plots and (g) long cycling times of the supercapacitor. (h) Successful demonstration of the supercapacitor charged by the flexible solar cell. (i) Electromechanical stability of the supercapacitor under bending deformation. (j) Waterproof demonstration of the supercapacitor. (k) The sensor driven by the supercapacitor was able to monitor the pulse rate.

in building structurally controlled carbon films for flexible electronics in the future of building the age of artificial intelligence. We also highly anticipate collaborations with other researchers in potential application of such a carbon film for heat dissipation components and seawater filtration due to its interesting and unique aligned porous configuration.

third-order templates, respectively. With this versatile methodology, the graphene-based highly sensitive flexible sensor device with a larger linear working range combined with a graphene/CNT-based waterproof solid supercapacitor with high energy density was realized. The pressure sensor was proven to be practical and effective in application of the pulse monitoring for the healthcare system, acoustic sound discrimination for authentication, and true force acquisition for practical artificial skin. As a powerful energy source for the flexible sensor, the supercapacitor showed a reliable performance in harsh environments with superior capacitance retention after 10000 charge/discharge cycles. We believe that our research provides an innovative and general advance

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EXPERIMENTAL

Preparation of the Porous Graphene. The copper (or nickel) mesh (1 × 3 cm) was used as the template for electroplating the porous graphene hydrogel after being washed sufficiently by ultrasonication with alcohol and deionized (DI) water to remove its outer organic impurities. A constant voltage of 30 V was applied on the copper mesh as the working electrode and Pt foil as the counter G

DOI: 10.1021/acsami.9b09438 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces electrode into a solution of 3 mg mL−1 GO (Hengqiu Tech. Inc.) mixed with 1 mM HCl. The sample was collected carefully and washed using DI water after deposition for 5 min. Then the whole sample was put into the etching solution consisting of 0.1 mol L−1 K2S2O8 (Sigma-Aldrich Corp., Hong Kong) solution to remove the copper mesh with repeated washing by DI water. Then the sample was kept at −80 °C for 1 h and freeze-dried for 2 days. Before using, the sample was dried at 350 °C for 24 h to obtain the flexible graphene film. Preparation of the Sensor. To prepare the sensor, the silver paste was mounted on the two edge of the cleaned Kapton film with silver wire. The rGO film was then carefully encapsulated within the PDMS (base resin and curing agent in a 10:1 (w/w) ratio) via vacuum infiltration and cured at 80 °C for 2 h. Note that the mass density of the PDMS@rGO was calculated to be 81.4 mg cm−3 and its mechanical performance was characterized as shown in Figure S12. Preparation of the Supercapacitor. The as-synthesized symmetrical supercapacitor device was assembled according to our previous report.7 Briefly, the gel-like electrolyte was prepared by adding 3 g of poly(vinyl alcohol) (PVA,1799) into 20 mL of DI water with magnetic stirring at 90 °C until it became transparent. Then 10 mL of KOH (0.3 g mL−1) solution was carefully added dropwise slowly until the solution changed to transparent again. The prepared rGO film (2.5 mg cm−2) was immersed into the above solution five times and then pressed them together when the gel electrolyte dried. Finally, the symmetrical electrodes were sealed within the gel electrolyte again using the poly(ethylene terephthalate) (PET) membrane and Kapton film as the protective layers. Microstructural and Electromechanical Characterization. The morphology of the products was investigated via a field-emission scanning electron microscope equipped with energy dispersive spectroscopy (EDS) (FESEM, Quanta 450) and transmission electron microscope (TEM, JEOL JEM 2100). Structural information was acquired by an X-ray powder diffractometer (XRD, Rigaku SmartLab) with monochromatic Cu Kα radiation (1.5418 Å). Compressive strain and strength were measured by a Microtest machine (DEBEN) with an elongation speed of 0.1 mm min−1. The current change of the sensor was conducted on the electrochemical workstation in twoelectrode mode. The electrochemical tests were performed also on the electrochemical workstation (CHI 760E, Chenhua). X-ray photoelectron spectroscopy (XPS) was employed by a VG ESCALAB 250 spectrometer with Al Kα X-ray radiation. The electrochemical tests associated with the cyclic voltammetry (CV) and galvanostatic charge−discharge (GCD) curves were calculated below. The areal capacitance (C, F cm−2) of the electrode was calculated by the following equation C=

iΔt SΔV

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*E-mail: [email protected] (L.G.). *E-mail: [email protected] (W.W.). *E-mail: [email protected] (Y.L.). ORCID

Libo Gao: 0000-0002-5964-2337 Ke Cao: 0000-0001-7857-4467 Yang Lu: 0000-0002-9280-2718 Author Contributions

L.G. and Y.W. contributed equally to this work. L.G. designed and fabricated samples, conducted experiments, analyzed data, and wrote the manuscript. Y.W. analyzed data and drafted manuscript partially. W.Z. wrote the manuscript partially, and K.C. performed the TEM experiments. Y.L. and W.W. led this project. All authors have approved the better final version of the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank the Research Grants Council of the Hong Kong Special Administrative Region of China (GRF No. CityU11216515) and City University of Hong Kong (Project Nos. 7005070 and 9667153), as well as Shenzhen Science and Technology Innovation Committee under the grant JCYJ20170818103206501. W.W. greatly thanks the Natural Science Basic Research Plan in Shaanxi Province of China (grant no. 2017JM5003) and the State Administration of Science, Technology and Industry for National Defense (Grant No. HTKJ2019KL510007). Y.W. thanks the funding of Natural Science Foundation of Shaanxi Province (No. 2017JQ5002).

where i (A) is the current, Δt (s) is the discharge time, S is the surface area, and V (V) is the potential range. Note that, for the device testing, the surface area is the total device area. Thus, the energy density (E, W h cm−2) and power density (P, W cm−2) were based on the following equations, respectively (ΔV )2 1 ×C× 2 3600

(3)

P=

E × 3600 t

(4)

where C (F cm−2) is the areal capacitance and ΔV (s) is the potential range.

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AUTHOR INFORMATION

Corresponding Authors

(2)

E=

Digital optical images of the reduced graphene oxide (rGO) film; SEM-EDX of the rGO film; XRD and Raman of the rGO; SEM and TEM of the graphene sheets; morphology of the rGO at various deposition times and applied voltages; comparison of the flexible sensor with commercial one; different words from the same speaker identified by the flexible sensor; repeatability of the sensor; circuit design for the signal output of the flexible sensor; TEM images of the assynthesized graphene-CNT composite; the waterproof sensor that was put in water was able to measure the water wave; mechanical performance of the PDMS@ rGO (PDF) Mechanical flexibility such as bending and twisting of the PDMS@rGO (AVI) Gentle tapping test of the sensor device connected to the computer (AVI) Capacitive performance of the supercapacitor device under bending deformation in static water (with 8× acceleration) (AVI) Capacitive performance of the supercapacitor device under flowing water (MP4)

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ASSOCIATED CONTENT

REFERENCES

(1) Cui, H.; Hensleigh, R.; Yao, D.; Maurya, D.; Kumar, P.; Kang, M. G.; Priya, S.; Zheng, X. Three-Dimensional Printing of Piezoelectric Materials with Designed Anisotropy and Directional Response. Nat. Mater. 2019, 18, 234−241.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b09438. H

DOI: 10.1021/acsami.9b09438 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.9b09438 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX