Research Article www.acsami.org
Ultrasensitive Pressure Sensor Based on an Ultralight Sparkling Graphene Block Lingxiao Lv, Panpan Zhang, Tong Xu, and Liangti Qu* Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Key Laboratory of Cluster Science, Ministry of Education, School of Chemistry, Beijing Institute of Technology, Beijng 100081, P. R. China S Supporting Information *
ABSTRACT: Herein, we develop a supersensitive pressure sensor based on a fully air-bubbled ultralight graphene block through a simple sparkling strategy. The obtained sparkling graphene block (SGB) exhibits excellent elasticity even at 95% compressive strain and rebounds a steel ball with an ultrafast recovery speed (∼1085 mm s−1). Particularly, the SGB-based sensor reveals a record pressure sensitivity of 229.8 kPa−1, much higher than other graphene materials, because of the highly cavity-branched internal structure. Impressively, the pressure sensor can detect the extremely gentle pressures even beyond the real human skin and hence are promising for ultrasensitive sensing applications.
KEYWORDS: ultrasensitive, ultralight, graphene block, compressible, pressure sensor
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INTRODUCTION Flexible and highly sensitive sensors are key elements in advanced wearable or implantable sensing devices.1−5 With the ability to transduce external stimuli (e.g., pressure6−9 and temperature10) into electronic signals, sensors have attracted a great deal of attention because of its wide potential applications. In pressure sensors, sensitivity is a crucial parameter for accurate monitoring. There are mainly four types of pressure sensors based on piezoresistive,10−14 piezoelectric,15−17 capacitive,18,19 and field-effect sensing mechanisms.6,20,21 Among them, piezoresistive sensors that can convert the pressure signal into resistance or current variation have been widely employed because of their advantages such as low cost, feasible fabrication, and easy signal collection.22,23 Until now, a lot of efforts have been devoted to constructing piezoresistive sensors with high performance. However, the sensitivities of most known pressure sensors have rarely surpassed 5 kPa−1, making them unqualified for precisely sensing ultralow pressure.11,12,24 Challenges remain on the development of ultrasensitive pressure sensors that can detect ultragentle pressures beyond the real human skin. Owing to its unique electrical and mechanical characteristics,25−29 graphene is promising as the building block for assembled architectures approaching application in piezoresistive sensors.11,30 Herein, we demonstrate an ultrasensitive pressure sensor based on a fully air-bubbled ultralight graphene block, using a simple sparkling approach toward large-scale production. The specifically prepared sparkling graphene block (SGB) has excellent elasticity, which can rebound a steel ball with ultrafast recovery speed (∼1085 mm s−1) and features the © XXXX American Chemical Society
particular well-connected hierarchical bubble cavities with a low density of 3.7 mg cm−3. It exhibits an extreme low compressive stress of 2 kPa even at 95% compressive strain, a small Young’s modulus of 1.1 kPa, and a high stability for 100 000 compress/ release cycles at 50% compressive strain, indicating extraordinary deformability and excellent elasticity. The SGB pressure sensor possesses a superior pressure sensitivity of 229.8 kPa−1 in the low-pressure regime (0−0.12 kPa), which is the highest among graphene-based piezoresistive pressure sensors reported previously and other microgeometry-designed high-performance sensors,13,31 and is hence promising for ultrasensitive detection.
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RESULTS AND DISCUSSION As illustrated in Scheme 1, graphene oxide (GO) dispersion (5 mg mL−1) and polyethylene glycol sorbitan monooleate (TWEEN 80, sparkling agent, 100 mg mL−1) were mixed under stirring using an automatic egg beater (Scheme 1a). A large number of micron-sized air bubbles (100−300 μm, Figure S1) were spontaneously formed (Scheme 1b), with GO sheets settled around them (Scheme 1c). After freeze-drying and lowtemperature annealing, the SGB with bubbled cavities maintained well due to the assembly of the surrounding graphene sheets (Scheme 1d). GO dispersion presents about 4fold increment in volume after stirring (Figure S2a,b), suggesting that large amounts of air bubbles are involved in Received: May 21, 2017 Accepted: June 20, 2017 Published: June 20, 2017 A
DOI: 10.1021/acsami.7b07153 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Scheme 1. Illustrations for the Preparation of SGB: (a) 50 mL of GO Dispersion (5 mg mL−1) and 7 mL of TWEEN 80 (100 mg mL−1, Dissolved in Ethanol) Mixed under Stirring Using an Automatic Egg Beater at 1300 rpm for 10 min, (b) Enlarged View of the Gas Bubbles in the Mixture (Scale Bar, 1 mm), (c) Homogeneous Mixture of Air Bubbles and GO, and (d) SGB Obtained after Freeze-Drying and Heated at 200 °C
Figure 1. (a) Digital image of SGB. (b−d) SEM images of SGB at different magnifications. (e) Photograph of SGB bent to 180°. (f) Real-time images from a high-speed camera showing that the SGB can rapidly rebound a steel ball (71 times heavier than SGB, Movie S2).
Figure 2. (a) Comparisons of the recovery speed of the SGB with relevant compressible foams listed in Table S1.32−38 (b,c) Strain−stress curves of SGB at different compressive strains at a strain rate of 0.1 Hz. (d) Comparison of the maximum stress normalized by the corresponding maximum compressive strain of different materials listed in Table S2.39−46 (e) Corresponding compressive stress (squares), Young’s modulus (circles), and total strain loss (triangles, the right axis) at the compressive strain of 50% for 100 000 cycles in air.
from 100 to 300 μm. Enlarged views clearly demonstrate that graphene sheets settle around the bubbles and assemble into graphene branches, forming the interconnected microporous graphene skeleton (Figure 1c). This graphene monolith is stabilized by numerous branches constructed by few layers of graphene (Figure 1d) that are strong enough to support the
the GO assembly. Attributed to the self-supported macroporous framework, the produced SGB (Figure 1a) shows ultraelastic compressibility even at a very low density of 3.7 mg cm−3. The scanning electron microscopy (SEM) image of the SGB in Figure 1b shows interconnected big bubble cavities ranging B
DOI: 10.1021/acsami.7b07153 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 3. (a) Multiple-cycle tests of current changes under different compressive strains with 0.1 V voltage input. From bottom to top, the curves correspond to 0.1, 0.3, 0.5, 1, 3, 5, 10, 30, 50, 70, and 90% compressive strains. (b) Plot of current variation versus strain of the SGB for a typical compress (squares line) and release (circles line) cycle. (c) Strain sensitivity of the SGB in comparison with other pressure sensors. Strain sensitivity is the current variation ratio normalized by the corresponding compressive strain (see Table S3 for details).47−54 (d) Pressure−response curve for the SGB showing the pressure sensitivity with 229.8 kPa−1 in the low-pressure regime (0−0.1 kPa) and about 26.9 kPa−1 in the high-pressure regime (0.4−1 kPa). (e) Pressure sensitivity of the SGB in comparison with other pressure sensors (see Table S4 for details).55,56 (f) Microstructure deformation of the graphene network within the SGB under a small compressive strain (10%).
characteristic peak located at 1100 cm−1, which is attributed to the C−O−C stretching vibration of TWEEN 80, and disappear after annealing at 1000 °C (Figure S8a). The Raman spectra patterns of the SGB in Figure S8b show two remarkable bands at around 1345 and 1585 cm−1, which are the D-band and Gband peaks of graphitic carbon, respectively. The X-ray diffraction (XRD) pattern shows a broad and intense peak around 22°, corresponding to graphene (Figure S8c). Real-time images from a high-speed camera present the outstanding collision resistance property of the SGB (Figure 1f). Remarkably, it is able to fully rebound a steel ball (71 times heavier than the SGB itself, Movie S2) with a high recovery speed (∼1084.6 mm s−1), which is much faster than other relevant compressive foams (Figure 2a and Table S1), revealing excellent elastic performance with instantaneous recovery. SGB possesses a remarkable low compressive stress of 2 kPa at 95% strain and a small Young’s modulus of 1.1 kPa (Figure 2b,c). As listed in Table S2, SGB is the lowest among carbonbased foams reported previously in terms of Young’s modulus and normalized stress (defined as the maximum compressive stress divided by the corresponding maximum compressive strain), suggesting high deformability, whereas the same pressure value could generate a larger deformation on the
bubbled framework. With this unique microstructure, SGB shows excellent compressive elasticity and completely recovers to its original shape upon large-strain compression without yielding deformation (Figure S2c−e and Movie S1). With this remarkable elasticity, the SGB can easily bend to 180° without any damage (Figure 1e). The stirring speed has effects on the final characters of SGB. As shown in Figure S3, when the stirring speed is decreased to 900 rpm, the GO dispersion presented only 2-fold increment in volume, the density of the as-prepared graphene bulk increased to 6.3 mg cm−3, and large bubbles around 500 μm were observed in the internal framework (Figure S3b). Because of the remaining TWEEN 80 on the graphene sheets (Figure S2f−i), the SGB has a small specific surface area of 7.644 m2/g (Figure S4). The thermogravimetric analysis (TGA) of SGB further confirmed that TWEEN 80 can survive at the annealing temperature of 200 °C for 2 h and can be completely removed after heating at 1000 °C (Figure S5). Herein, TWEEN 80 is used not only as the bubbling agent to create homogeneous air bubbles but also as the bonding agent between graphene sheets to support the whole structure for high compressibility (Figures S6 and S7). The Fourier transform infrared (FT-IR) spectra of SGB show a prominent C
DOI: 10.1021/acsami.7b07153 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 4. (a) Real-time I−t curves of SGB for over 1000 loading/unloading cycles with an applied pressure of 20 Pa, 4 s for each cycle. (b) Curves of current (black squares, the left axis) and applied stress (red circles, the right axis) as functions of time for two cycles and enlarged views of the curves during loading and unloading processes.
SGB (Figure 2d). Its internal structure could fully recover to the original state after 90% deformation (Figure S9), ensuring superior elasticity. SGB nearly maintains the high compressive stability in 100 000 consecutive cycles to 50% compressive strain in air (Figures 2e and S10). The outstanding flexibility, elasticity, and deformability make the SGB a potential candidate for a pressure sensor. To investigate the pressure-sensitive property of the SGB, the current variation versus the compressive strain was scrutinized with an applied constant voltage of 0.1 V. Obviously, the current variation synchronizes with the compressive strain (Figure 3a), and it is supersensitive to trigger a high 12% current variation ratio even to 0.1% strain change (∼10 Pa), which is much better than that of about 10% variation responding to 10% deformation of reduced GO and polyimide (rGO/PI) nanocomposite.11 The current variation ratio of the SGB reached 200% at only 3% compressive strain (∼30 Pa), confirming the outstanding sensitivity. Meanwhile, about 1000, 2000, and 4000% current variation ratios occur at 30, 70, and 90% deformations, respectively. In addition, the current variation shows stable signals under 10% compressive strain, and the baseline presents no significant change before and after compression (Figure S11). The current responses present the same tendency under the loading and unloading processes and recover well during compression (Figure 3b), with a larger current increasing rate within 10% deformation (circles in Figure 3b). The strain sensitivity, which is defined as the current variation ratio under per compressive strain unit, achieves a high value of 109.7 for a small deformation, much higher than other carbon-based compressive foams (Figure 3c and Table S3). Furthermore, with loading frequencies of 0.1, 0.5, 1, and 2 Hz (Figure S12), no obvious frequency dependence hysteresis is observed for the
SGB under 5% deformation, indicating good reversible deformation at a high recovery rate. The pressure sensitivity (S) of the SGB can be defined as the slope of the curve in Figure 3d. S = δ(ΔI /I0)/δP
(1)
ΔI = I − I0
(2)
where I and I0 denote the currents with and without pressure, respectively. ΔI is the current variation; P denotes the applied pressure. As shown in Figure 3d, the curve shows two significant stages corresponding to different sensitivities. During compression, the current response rate gradually decreases (the slope of the curve in Figure 3b), whereas the stress increasing rate progressively increases (Figure 2b). Under the combined effect of the current and the compressive stress, the pressure sensitivity presents a large value at first and gradually decreases with the increasing compressive strain (Figure 3d). The sensitivity reaches an extraordinary high value of 229.8 kPa−1 in the low-pressure regime (0−0.1 kPa) and gradually stabilizes to about 26.9 kPa−1 in the high-pressure regime (0.4−1 kPa), but it is still much higher than that of many other piezoresistive pressure sensors such as polymer-based flexible pressure sensors31 (Figure 3e and Table S4). With this high sensitivity, the SGB pressure sensor is suitable for low-pressure accurate detection. The sensitivity of SGB depends on the deformability. With the compressive stress as low as 2 kPa at a large compressive strain of 95%, SGB is easy to be deformed. Compared with other compressive materials such as ultralight graphene aerogel (ULGA), the same pressure value (δP) could generate a larger deformation on the SGB (Figure 2d and Table S2), which is vital to the outstanding sensitivity. D
DOI: 10.1021/acsami.7b07153 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 5. (a) Schematic illustration of the ultrahigh sensitive pressure sensor of SGB. (b−d) Plot of the current ratio variation vs time of the SGB pressure sensor under different kinds of pressures such as (b) dandelion, (c) feather (the feather slipping over the upper surface), (d) hair, and (e) breeze (provided by swinging a feather).
cycles (Figure 4a, 4 s for each cycle). The stable current variation with tiny fluctuations indicates that the SGB pressure sensor has high repeatability, stability, and durability. Enlarged views in Figure 4b show that the current perfectly synchronizes with the applied pressure with no hysteresis, confirming the remarkable instantaneous response (Figure 2a). With an ultrahigh sensitivity, the SGB pressure sensor can detect extremely tiny disturbance (Figures 5a and S13). For example, the current rapidly increases by about 20% on contact with a dandelion (about 1.4 mN) and decreases to a lower value when taking away the pressure because the SGB rebounds to a higher position probably because of the inertia effect, and then, it quickly goes back to the original state (Figure 5b and Movie S3). A gentle touch with a feather (approximately 0.5 mN, Figure 5c and Movie S4) and a hair ( approximately 0.03 mN, Figure 5d and Movie S5) can trigger about 1.8 and 0.3% current variation, respectively. Natural perturbation of breeze by swinging a feather (∼0.009 mN, Figure 5e and Movie S6), which is about 3 orders of magnitude smaller than a gentle touch by a human hand, which is hardly detected by the human
During a small compression of only 10%, the highly branched graphene bundles deform conformably and tend to physically contact with each other closely (Figure 3f). As a result, a high current response rate is achieved at an early compressing stage (Figure 3b), which is crucial for low-pressure sensing.12 With this sensitive structure deformation, the current variation ratio δ(ΔI/I0) in the low-pressure regime of the SGB became much higher. According to S = δ(ΔI/I0)/δP, the higher δ(ΔI/I0) and the lower δP led to a larger sensitivity.10−12 As shown in Figure 3e, the pressure sensor of SGB has the best sensitivity among the typical piezoresistive pressure sensors (Table S4). In addition, the current increment is mainly caused by the physical contacts of highly branched graphene bundles. Under the same compressive strains, the SGB has a larger current variation ratio than SGB-1000 (Figures 3a and S7d) because the electric conductivity of the SGB increased from 0.0039 to 0.351 S m after annealing at 1000 °C. To check the stability of the SGB pressure sensor, the current changes are measured for the repeated loading/unloading process of an applied pressure of 20 Pa for more than 1000 E
DOI: 10.1021/acsami.7b07153 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
ACS Applied Materials & Interfaces
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EXPERIMENTAL SECTION
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ASSOCIATED CONTENT
Preparation of the SGB. GO was prepared by the oxidation of natural graphite powder using a modified Hummers method, as reported in our previous work.43 GO dispersion (50 mL, 5 mg mL−1) and 7 mL of polyethylene glycol sorbitan monooleate (TWEEN 80 was purchased from Aladdin, 100 mg mL−1 dissolved in ethanol) were mixed using an automatic egg beater at 1300 rpm (the maximum stirring speed of the automatic egg beater) for at least 5 min. After freeze-drying, the bubbled GO monolith was heated at 200 °C for 2 h under an Ar atmosphere. The SGB-1000 was prepared by annealing the SGB at 1000 °C for 2 h under an Ar atmosphere. Characterization. The morphology of the as-prepared samples was investigated using the scanning electron microscope (JSM-7500F) and transmission electron microscope (7650B, Hitachi). Brunauer− Emmett−Teller (BET) specific surface area was determined by nitrogen adsorption−desorption isotherm measurements at 77 K (NOVA 2200e). Raman spectra were recorded on a Horiba JY HR800 Raman spectrometer with an excitation wavelength of 633 nm. XRD patterns were obtained using a Netherlands 1710 diffractometer with a Cu Kα irradiation source (λ = 1.54 Å). FT-IR spectra were recorded on a Bruker VERTEX 700 spectrometer. Electrical conductivity was measured by using a four-probe electrical resistivity tester (KDY-1) at room temperature. TGA was performed on a NETZSCH STA 449C instrument. The collision resistance process of a steel ball by SGB was captured by a high-speed video camera (Phantom V611). The mean recovery speed of the SGB was calculated via the displacement of the steel ball divided by the time for the motion between the minimum height of the ball and the time the ball leaves the monolith. Compressive tests were carried out using a Shimadzu AGS-X instrument equipped with two flat-surface compression stages. All compressive samples were cut to a cubic shape using a laser, and their stress−strain curves in the second cycle were taken. The small-strain Young’s modulus for different samples was calculated from the initial slope of the loading curves at an engineering compression rate of 0.1 Hz. The current response during the compression was recorded in real time using a Keithley 2612 SourceMeter controlled by a LabVIEWbased data acquisition system. During compression, the applied constant voltage was 0.1 V, and the top and bottom of the sample were silver pasted to two flat metal stages with the sample size of 7.18 mm × 12.33 mm × 2.54 mm. The pressure sensor device was constructed by a well-cut SGB (2 mm × 7 mm × 7 mm), whereas the top and bottom were connected to aluminum wires with silver paint; the computer-controlled Keithley 2612 SourceMeter was used to record the current variation, and the pressure was applied on the upper surface. The flexible strain/pressure sensor using an SGB with a rectangular shape (2.5 mm × 5 mm × 25 mm), which was pasted onto a transparent polyester (PET) film (10 mm × 40 mm), with two copper films acting as the current collector connected with an electrochemical workstation.
Figure 6. Time retention curve of the current variation ratios of the sensor in response to different bending (a) and twisting (b) states.
skin, can be sensed by the SGB device, highlighting the excellent sensitivity of the SGB. The SGB shows great potential as a multifunctional flexible sensor, owing to its excellent elasticity and remarkable current variation ratio under a wide strain range.57 Figure 6a shows that the current responses get higher along with the increasing bending angle and maintain a distinctly stable plateau regime for each state. The current variation ratio reaches about 50% at 30° bending angle, which gradually increases to 145% at 140°. SGB sensors exhibit a reliable current response during the repetitive bending−unbending process (Figure S14a). Accordingly, the slice of the SGB can also be twisted, and the response value is in sync with the twisting degree (Figure 6b, the twisting degree is the number of radians from twisting). During the twisting-untwisting process, it maintains a stable current signal at each state (Figure S14b). About 50, 110, and 210% current variation ratios occur at π/2, π, and 2π radians, respectively. These results suggest that the SGB is a viable candidate for use in multifunctional strain/pressure sensors under different kinds of deformations.
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CONCLUSIONS In summary, an ultrasensitive pressure sensor has been developed on the basis of the unique sparkling graphene block. The fully air-bubbled graphene block is easily prepared in a large scale, which exhibits excellent elasticity with an ultralow density of 3.7 mg cm−3. The SGB shows a small-strain Young’s modulus of 1.1 kPa and a remarkable low maximum compressive stress of 2 kPa at 95% compressive strain. The pressure sensor based on the SGB demonstrates a superior pressure sensitivity of 229.8 kPa−1 to extremely low pressure (0−0.1 kPa), which is the highest among other graphene-based piezoresistive pressure sensors. As a result, the SGB pressure sensor can detect extremely tiny disturbance such as breeze beyond the touch with a dandelion, a feather, and a hair. With the sensitive performance in response to the bending and twisting process, the SGB exhibits great potential for multifunctional sensing applications.
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07153. Experimental details, data, and supplementary explanations (PDF) Compressible nature of SGB (AVI) Rebound of a steel ball (71 times heavier than SGB itself) from SGB (AVI) Current response of SGB to a gentle contact with a dandelion (AVI) Current response of SGB to a gentle touch with a feather (AVI) Current response of SGB to a gentle contact with a hair (AVI) F
DOI: 10.1021/acsami.7b07153 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Current response of SGB to a natural perturbation of breeze by swinging a feather (AVI)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Liangti Qu: 0000-0002-7320-2071 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the NSFC (nos. 21325415, 51673026), the National Key R&D Program of China (2017YFB1104300), the Beijing Natural Science Foundation (2152028), and the Beijing Municipal Science and Technology Commission (Z161100002116022).
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DOI: 10.1021/acsami.7b07153 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
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DOI: 10.1021/acsami.7b07153 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
