Phase-Separation-Induced PVDF/Graphene Coating on Fabricstoward Flexible Piezoelectric SensorsTao Huang,†,‡,⊥ Siwei Yang,†,‡,⊥ Peng He,*,†,‡ Jing Sun,‡,§ Shuai Zhang,∥ Dongdong Li,‡,§

Yan Meng,‡,§ Jiushun Zhou,†,‡ Huixia Tang,†,‡ Junrui Liang,*,∥ Guqiao Ding,*,†,‡ and Xiaoming Xie†,‡

†Center for Excellence in Superconducting Electronics (CENSE), State Key Laboratory of Functional Materials for Informatics,Shanghai Institute of Microsystem and Information Technology (SIMIT), Chinese Academy of Sciences, Shanghai 200050, P. R.China‡University of Chinese Academy of Sciences, Beijing 100049, P. R. China§Research Center of Quantum Macro-Phenomenon and Application, Shanghai Advanced Research Institute, Chinese Academy ofSciences, Shanghai 201210, P. R. China∥Mechatronics and Energy Transformation Laboratory, School of Information Science and Technology, ShanghaiTech University,Shanghai 201210, P. R. China

*S Supporting Information

ABSTRACT: Clothing-integrated piezoelectric sensors pos-sess great potential for future wearable electronics. In thispaper, we reported a phase-separation approach to fabricateflexible piezoelectric sensors based on poly(vinylidenefluoride) (PVDF)/graphene composite coating on commer-cially available fabrics (PVDF/graphene@F). The structuralunits of −CH2− and −CF2− of PVDF chains were arrangeddirectionally due to the structural induction of graphene andwater during phase separation, which is the key forelectroactive phase enrichment. In optimized case, integratinginto fabric substrates endows the phase-out PVDF/graphenecomposite coating 4 times higher voltage output than its filmcounterpart. Piezoelectric sensor based on PVDF/graphene@F exhibits a sensitivity of 34 V N−1, which is higher than many reports. It also shows low detecting threshold (0.6 mN), whichcan be applied to distinguish the voices or monitor the motion of body. This simple and effective approach toward PVDF/graphene@F with excellent flexibility provides a promising route toward the development of wearable piezoelectric sensors.

KEYWORDS: PVDF, graphene, piezoelectric materials, fabric, sensors

■ INTRODUCTION

In situ, real-time biomedical monitoring and human motionsensing require wearable electronic devices that can transformmechanical to electrical signals.1,2 Compared with traditionalresistance-based testing system sensors,3,4 piezoelectric sensors(PSs) show more precise response to outside strength becauseof the nature of piezoelectricity. Furthermore, piezoelectricmaterials generate voltage signals themselves without outsideenergy source.2 In this regard, clothing-integrated piezoelectricsensors for body monitoring have become a current thrust ofresearch in the field of piezoelectric materials.5 Flexiblepiezoelectric polymers, especially poly(vinylidene fluoride)(PVDF), have been extensively used to fabricate flexiblemotion sensors due to the nature of lightweight, highprocessability, and the piezoelectric property.2,6,7

Graphene is a kind of two-dimensional material with uniqueproperties. Due to its superior mechanical strength, highspecific area, and unique electrons structure,8,9 it has beencombined with PVDF to prepare flexible piezoelectric device

with excellent mechanical strength and enhanced piezoelectricefficiency.10−12 PVDF/graphene composites have been widelystudied in past decades, and different types of graphene, suchgraphene oxide (GO) and reduced GO (rGO), will bringdifferent enhanced results. GO can boost the β-PVDF contentfor the interaction between negative sheets and fluorine group(−CF2−) in PVDF.13,14 Recently, Liu et al. utilized electro-spinning to fabricate PVDF/GO core−shell fibers.15 Theyincreased the piezoelectric constant (d33) of PVDF to 93.75pm V−1 (the highest d33 of pure electrospinning nanofiber was58.5 pm V−1),16 which testified the superiority of GOenhancement. In comparison, pristine graphene (or rGO)could provide higher mechanical property enhancement whileinducing the formation of β-PVDF.17−20 Karan et al. designedhighly durable piezoelectric nanogenerator by integrating

Received: June 25, 2018Accepted: August 20, 2018Published: August 20, 2018

Research Article

www.acsami.orgCite This: ACS Appl. Mater. Interfaces 2018, 10, 30732−30740

© 2018 American Chemical Society 30732 DOI: 10.1021/acsami.8b10552ACS Appl. Mater. Interfaces 2018, 10, 30732−30740

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PVDF with rGO/Al2O3, which provided higher voltage outputof 36 V and 10 weeks’ usage without damping.21 It is indicatedthat graphene provides PVDF with higher endurance owing toits mechanical strength. In summary, pristine graphene can bethe most promising nanofiller for both mechanical andelectrical enhancement of PVDF-based piezoelectric sensors.However, it remains challenging to fabricate PVDF/

graphene composite with high flexibility, low energy cost,and time-saving merits. Soft PVDF/graphene films can befabricated through solution-casting method.17,19,20 But, thesolidifying process of film shows low efficiency to volatilize outthe high-boiling-point organic solvent (N-methyl-2-pyrrolidi-none, for example). Electrospinning method shows great hopefor the batch preparation of flexible PVDF/graphenefibers.15,18 And, the piezoelectric performance of electro-spinning nanofibers can satisfy the demands of flexibility.Nevertheless, the strength of fabric based on PVDF fibers ishardly satisfying compared with other existing fabric-likepolyester, and electrospinning method relies on expensiveequipment. In comparison, to achieve remarkable flexibility ofPVDF/graphene composite, existing fabrics can be optimumsubstrates for PVDF/graphene coating, which are safe for thebody, flexible in motion, and sewable for integration withelectronics.22−24 But, few reports have focused on thecombination of the existing fabrics and PVDF.Here, we report a fast phase-separation method to fabricate

flexible piezoelectric device based on PVDF/graphene coatingon fabrics (PVDF/graphene@F). Due to interactions betweenPVDF and graphene, as well as water, the electroactive phase(β/γ phase) crystallinity in PVDF can be increased to 87%.Compared with traditional methods, our approach using theexisting fabric as substrates would provide both scalability andbetter reliability in practical applications. Motion sensors basedon PVDF/graphene@F are demonstrated when integrated intoday-to-day garments.

■ EXPERIMENTAL SECTIONMaterials. Sodium persulfate (Na2S2O8, ≥99 wt %, Sinopharm),

sulfuric acid (H2SO4, 95−98 wt %, Sinopharm), poly(vinylidenefluoride) powder (PVDF, molecular weight: 900 000, Sigma), and N-methyl pyrrolidone (NMP, AR, Sigma) were purchased and used asreceived without further purification. Deionized water obtained

through a Milli-Q system was used throughout all experiments.Graphite powders (325 mesh) were purchased from Qingdao Xinghegraphite Co., Ltd. (Qingdao, China).

Fabrication of PVDF/Graphene/NMP. Graphene was fabricatedas we reported before and the detailed process can be found in theSupporting Information. The corresponding graphene character-ization can be found in Figure S1. To obtain the graphene/PVDF/NMP dispersions, prepared graphene powder, PVDF powder (5 g),and NMP (100 mL) are mixed and ultrasonicated (50 °C, 200 W) for5 h to get the final homogeneous dispersion. The solution will becooled to room temperature for next usage.

Fabrication of PVDF/Graphene@F and PVDF/GrapheneMembrane. First, fabric was immersed in the PVDF/graphene/NMP solution (coating process). After dipping completely, the fabriccoated with PVDF/graphene/NMP went through a tank of water toconduct phase-separation process. After that, the wet PVDF/graphene@F was dried in the air (60 °C). The dried PVDF/graphene@F was collected for the next usage.

The membrane was fabricated through the same process. First,PVDF/NMP or PVDF/graphene/NMP (about 2 mL) was cast ontosurface dish. Then, water was poured in the dish slowly to conduct thephase-separation process. After 1−2 min, the wet membrane wasdried in 60 °C air for further test.

Fabrication of the PVDF/Graphene@F-Based Sensors. First,aluminum foils were cut into squares. Two pieces of foils were thenput on the two sides of the fabric and sealed with tapes. Such sensorscan be sewed into pants or marks for further testing.

Characterization. Field emission scanning electron microscopy(FE-SEM) was performed on a JEOL JSM-4700S equipped with anEDSINCA X-Max at 30 kV. Fourier transform infrared spectroscopy(FT-IR) was recorded by a Bruker Vertex 70v FT-IR spectrometer.Raman spectra excited by Ar+ laser (λ = 532 nm) were collected byThermo Fisher DXR. Differential scanning calorimetry (DSC,TAQ200) was conducted at 100−200 °C temperature range (10°C min−1) under N2 atmosphere. X-ray diffraction (XRD) patternswere obtained from an X-ray diffractometer (Bruker D8 ADVANCE)with a monochromatic source of Cu Kα1 radiation (λ = 0.15405 nm)at 1.6 kW (40 kV, 40 mA). X-ray photoelectron spectroscopy (XPS)was recorded on a PHI Quantera II system (Ulvac-PHI, INC, Japan).Transmission electron microscope (TEM) was carried out on aHitachi H-8100 EM (Hitachi, Tokyo, Japan) with an acceleratingvoltage of 300 kV.

Figure 1. (a) Schematic illustration of the preparation process of soft piezoelectric fabric and its microstructure. (b) PVDF/graphene/NMPsolution standing still for 3 weeks. (c) Digital photo comparison of polyester fabric and PVDF/graphene-coated fabric.

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■ RESULT AND DISCUSSION

Figure 1a shows the facile fabrication process of PVDF/graphene@F including three steps: continuous impregnation,phase separation, and drying. When using polyester as thefabric, the polyester was immersed in the PVDF/graphene/NMP solution for continuous impregnation. The impregnationtime in this solution was 9−10 s, which allowed sufficientcoating. For the continuity of fabrication, the stability ofPVDF/graphene/NMP solution should be considered. After 3weeks’ settling down, no clear delamination or sediment in thesolution was observed, as shown in Figure 1b, indicating gooddispersibility of graphene. Actually, graphene is very stable inNMP solution, as reported before.25 The dipped polyesterfabric was further immersed in water to separate out the solidPVDF/graphene composite from the liquid NMP system (atypical phase-separation process). Long immersion time (>1min) in water was necessary to ensure the removal of NMPand sufficient polarization of the PVDF molecules. After air-drying, polyester coated with PVDF/graphene (PVDF/graphene@P) was acquired (the average mass uptake ofPVDF/graphene was about 0.5 g m−2). The polyester becomesdarker, as shown in Figure 1c, with the addition of graphene.Noticeably, after coating, the polyester was stiffer than theoriginal state. The fabric with one coating still exhibits goodflexibility but becomes rigid after fifth coating, as shown inFigure S2. The more coatings, the more rigid the crystal PVDFfabric, which leads to greater rigidity. Fortunately, PVDF/graphene@P with one coat retains the flexibility and can befolded as an uncoated fabric substrate. The folded PVDF/graphene@P generates a strong electrostatic attraction withbutyronitrile gloves, which prevents it from falling (Figure S3),and indicates a strong piezoelectricity. Moreover, the

mechanical property of PVDF/graphene@P and polyesterwas tested. The maximum load of PVDF/graphene@P is about1.55 MPa, close to the 1.51 MPa of raw polyester fabric(Figure S4), which means that a short immersion time in NMPdoes not cause significant degradation of the polyester’sstrength. Moreover, this approach can also be applied tovarious fabrics: as shown in Figure S5, various types of fabric(nylon, tissue, cotton, bamboo fibers, and flax) can also beuniformly coated with PVDF/graphene after similar treatment.This approach was convenient and time-saving for the mass-production of soft piezoelectric fabrics (productivity exceeding36 m2 h−1). Besides, the water-triggered phase-separationprocess greatly improves the elimination efficiency of the high-boiling-point solvent (such as NMP) and decreases thepollution of the organic solvent steam.To judge the uniformity of coating, the morphology of the

PVDF/graphene-coated polyester fabric (PVDF/graphene@P)was observed through FE-SEM in Figure 2. Figure 2a,bindicates that the interwoven structure of PVDF/graphene@Pis well retained during coating processes. Figure 2a showspolyester fibers with an average diameter of 13 μm and theinsert image shows a smooth surface; in contrast, the averagediameter increases to about 16 μm with a much roughersurface after PVDF/graphene coating (Figure 2b shows thediameter; for statistic results, see Figure S6). The increaseddiameter indicates that the average coating thickness isapproximately 1.5 μm because PVDF/graphene tends toform shell covering the whole polyester fiber. And, the roughersurface was induced by the contraction of PVDF during water-triggered phase-separation process. The folding state ofPVDF/graphene@P is also observed in SEM, and there is noobvious crack on the surface of the folding place (Figure S7),

Figure 2. SEM images of (a) polyester and (b) PVDF/graphene@P, the insert implying the surface morphology changes. (c) EDS mapping ofcarbon (red) and fluorine (blue) element. (d, e) TEM images of PVDF/graphene@P. (f, g) High resolution TEM (HR-TEM) images showinglattice fringe of crystal PVDF and graphene.

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which would ensure the integrity of PVDF/graphene@Pduring practice applications. X-ray energy-dispersive spectros-copy (EDS) mapping (Figure 2c) reveals the presence of well-spread F element (blue) along with the C element (red),which indicates uniform coating of PVDF.To observe coated PVDF more carefully, focused ion beam

(FIB) was applied to obtain a little cross section of PVDF/graphene@P, which can be observed in TEM. Figure 2dpresents the microstructure of a PVDF/graphene composite.Between Pt coating (protecting the structure from destructionof FIB) and polyester substrates, graphene dominates the blackarea and is surrounded by transparent PVDF. More details ofthe interfaces in PVDF/graphene are shown in a higher-resolution image (Figure 2e). In the PVDF layer, the crystalarea is clear with a lattice spacing of 0.256 nm (Figure 2f),corresponding to the typical lattice spacing along c-axis in theorthorhombic β-PVDF crystal.26 Among three main phases (α,

β, γ) of PVDF, polar β/γ-PVDF are highly desirableelectroactive phases and α-PVDF is a non-electroactivephase.27 And, we observed that the majority of PVDF areasin the TEM images are crystal β-PVDF, which illustrates strongpiezoelectric properties. It should be pointed out that thepresence of significant crystal particles of β-PVDF has not beenreported yet by previous work, to the best our best knowledge,which emphasizes the importance of phase-separationstrategy.15,16 Selected area electron diffraction (SAED)patterns of the crystal area in PVDF also shows the diffractionspots corresponding to the crystallographic plane of β (100)(Figure S8). And, the lattice fringe of graphene (d = 0.248nm)9,28 can be seen in Figure 2g, which proves the high qualityof graphene (see discussion on graphene in the SupportingInformation).The above results demonstrate a uniform PVDF/graphene

coating layer with significant presence of the piezoelectric

Figure 3. Raw PVDF (black curve), processed PVDF (red curve), and processed PVDF/graphene (blue curve, graphene content: 0.5 wt %)patterns of characterization: (a) FT-IR spectra, (b) DSC thermoanalysis, and (c) XRD. (d) Variation in β/γ-phase content (black axis) and thecorresponding crystallinity (red axis) as a function of graphene content. (e) The three-dimensional (3D) structure models of β-PVDF formation.

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PVDF phase. How the crystal piezoelectric PVDF is formedshould be investigated further. FT-IR, XRD, and DSCmeasurements are commonly used to assess the phasecomposition. However, it is not suitable to test PVDF/graphene@P directly with these analyses because the strongsignals of textile substrates like polyester will blur that ofPVDF/graphene. Therefore, PVDF and PVDF/graphenemembranes were prepared through similar phase-separationprocess (Figure S9) and used for FT-IR, XRD, and DSCmeasurement.FT-IR spectra can be applied to detect the structure

information in PVDF chains.27 From the black curve in Figure3a (raw PVDF), the vibration bands at 766, 795, and 976 cm−1

can be observed, which correspond to the −CF2− bendingvibration and −CH2− rocking or out-of-plane vibration of α-PVDF.27 In the red curve (representing the PVDF membranefabricated by phase-separation method in water), vibrationbands at 840, 1234, and 1279 cm−1 are enhanced intensively.These peaks can be attributed to the −CH2− waggingvibration and −CF2− symmetric stretching of β/γ-PVDF,respectively.27,29 This indicates the enriched β/γ-PVDF phasesthrough the phase-separation process. By contrast, the FT-IRspectrum of PVDF/graphene (blue curve) shows a significantdecrease in α-PVDF signals at 766, 795, and 976 cm−1

wavenumbers. It indicates that interaction of graphene sheetswith PVDF chains induces amorphous macromolecular chainsinto the crystal electroactive PVDF instead of α-phase. TheseFT-IR results showed that phase separation and graphenesheets were very effective to form an electroactive phase.Through Raman investigation, we found that, compared to theG band at 1600 cm−1, this G band splits into two distinct peaksat 1580 and 1605 cm−1 in PVDF/graphene. The G bandsplitting of PVDF/graphene hybrid may indicate an interactionbetween PVDF and graphene surface, as shown in FigureS10.30 Additionally, DSC is a thermoanalytical technique thatis complementary to other identification techniques.29 Asshown in Figure 3b, the raw PVDF powder (the black curve)shows the melting temperature of 148−177 °C.29 After phaseseparation, PVDF (red curve) and PVDF/graphene (bluecurve) membranes show higher melting temperature (about172 °C). However, higher melting temperature of γ-PVDF at179 °C cannot be recognized, which means the β-PVDF isdominant. DSC is also used to calculate the crystallinity (χc) ofPVDF through eq 1.31

HHc

f

100χ =

ΔΔ (1)

where ΔHf represents the melting enthalpy of the compositeand ΔH100 (taken as 104.6 J g−1) the melting enthalpy for a100% crystalline sample of pure PVDF.32 According to thecalculation result, χc of PVDF/graphene films reaches about89%, which is much higher than that of pure processed PVDFfilm (70%). The above results illustrate that water-inducedphase separation and graphene sheets help the crystallization ofthe electroactive phase.XRD characterization could determine the phases of PVDF

clearly.29 Figure 3c gives the XRD patterns of the raw PVDFpowder (black curve), phase-separated PVDF (red curve), andPVDF/graphene membranes (blue curve). The obvious peaksat 2θ = 17.6, 18.3, 19.8, and 26.5° in raw PVDF (black curve)corresponded to the (100), (020), (110), and (021) planes ofα-PVDF, respectively.29,33 But, the peaks disappeared after

phase separation (red curve) and new peaks are observed at 2θ= 20.2 and 18.5°, which can be attributed to the (200) or(110) plane of β-PVDF and (110) plane of γ-PVDF,respectively.34,35 After adding graphene to PVDF, thesecharacteristic peaks of β-/γ-PVDF are significantly enhanced.To explore the role of graphene, different graphene weightcontents of PVDF/graphene composite, like 0, 0.1, 0.3, 0.5, 1,2, and 3 wt %, were examined with the XRD test. The relativeelectroactive phase content (Cβ+γ) and their contribution tocrystallinity (χβ+γ) can be calculated by deconvolution method,as given in Figure S11 (Supporting Information). We definethe value Cβ+γ to explore the relative content of electroactivephases and χβ+γ to depict the absolute content of crystal β/γ-phase in whole PVDF. As we can see in Figure 3d, when thegraphene content is 0 wt %, the phase-separated PVDF alreadyhas 77% Cβ+γ and 60% χβ+γ. The enrichment of χβ+γ iscontributed by a strong interaction between water and PVDFchains, as reported before.36 With addition of 0.1, 0.3, and 0.5wt % graphene, Cβ+γ increases from 77 to 83, 90, and 97% andχβ+γ increases from 60 to 68, 80, and 87%, respectively,indicating that graphene can induce more crystal-polarizedPVDF during phase separation. But, decrease in Cβ+γ and χβ+γwith higher content of graphene (more than 1 wt %) indicatedthat more graphene obstructs the formation of electroactivephase. We assume that too much graphene would obstruct thecontinuous diffusion of NMP in PVDF during phaseseparation. This leads to less interaction between water andPVDF, which inhibited the bulk formation of β/γ-PVDF.According to Density Function Theory calculations (see theSupporting Information) of the interaction between graphene,water, and PVDF, as shown in Table S1, the action betweenwater and PVDF chains (25.5 kJ mol−1) is much stronger thanthat between graphene and PVDF (19.2 kJ mol−1). Once thewater was blocked, the weaker interaction between grapheneand PVDF would be insufficient to help form a large content ofelectroactive phase.We also observe different samples through TEM to deepen

our understanding. For PVDF without graphene (phaseseparation), β-PVDF crystal particles show up only in theoutmost layer (Figure S12a,b). Compared with the result inFigure 2e, the existence of graphene helps the formation ofelectroactive phases, as discussed above. However, PVDF/graphene sample with graphene content of 2 wt % shows lesscrystal area ratio. As shown in Figure S12c,d, graphene sheetsencase PVDF, which impedes the diffusion of NMP. Moreimportantly, when PVDF was cast onto Si from NMP solution(0.05 g mL−1), no clear crystal area was observed (FigureS12e,f).We build a structure model to help understand the process

of phase separation in Figure 3e. Upon the introduction ofwater, PVDF and graphene in NMP simultaneously separatefrom the liquid phase to form a solid PVDF/graphenecomposite coating on the fibers. The electron injection groups(−CH2−) in PVDF chains exhibiting electropositive propertypromote intense interaction with π-conjugation electrons ofgraphene.21,37 Simultaneously, at the interface of PVDF andwater, electronegative −CF2− units of other chains tend toform hydrogen bonds with H2O.

36 Under the synergistic effectof water and graphene, the dissolved PVDF chains can bepolarized into an electroactive phase instead of a non-electroactive phase.Actually, the coating thickness and the coating substrates

also have influence on the crystallinity of the electroactive

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phases. With increase in the PVDF/graphene membranes’thickness (from 0.15 to 1.50 mm, measured by a thicknessgauge), the crystallinity decreases, as shown in Figure S13a.Despite the inverse relationship between crystallinity andthickness (between 0.15 and 1.50 mm), this influence maybenot as apparent because of a much thinner coating on fabric inthe micrometer range, which allows fast and complete phaseout after water introduction. On the other hand, thehydrophilic substrates will improve the crystallization ofPVDF (see Figure S13b). But, the ester substrates have noobvious improvement or impairment to PVDF. In brief, theresults from film are analogous to those of the PVDF/graphenelayer coated on polyester fibers.According to the above results and discussion, the phase-

separation approach is convenient and effective to coatelectroactive PVDF/graphene composite on fabrics. Here, weintegrated PVDF/graphene@P with electrodes into piezo-electric sensors to evaluate piezoelectricity. At the same time,to give a constant testing environment, an electromagneticpunch was selected as the strength source for the piezoelectrictest (Figures 4a and S14). The electromagnetic punch couldimpart constant strength at a given frequency and ensure thesame testing conditions for different samples. A piezoelectricsensor (PS) device based on PVDF/graphene@P wasmanufactured to test piezoelectric property, as shown inFigure S15. PSs based on pure PVDF film and PVDF-coatedpolyester (PVDF@P) were also integrated with Al foils (Figure4b; for details see the Supporting Information). As shown inFigure 4c, the pure-PVDF-film-based PS generated less than 10V, whereas PVDF@P PS generated over 15 V. In contrast,PVDF/graphene@P generated the strongest voltage signal ofabout 20 V, which is 4 times higher than that for PVDF filmsand much higher than in many reports (less than 5 V).14,38,39 It

is clear that porous structure can absorb more mechanicalenergy than film, a fact that has been testified in manyreports.40,41 The textile fabric provided a natural porousstructure that contributes to the intense voltage outputenhancement. Furthermore, the graphene’s addition is alsoconfirmed to contribute to the increase in the piezoelectricperformance compared with PVDF@P and PVDF/graphene@P. As a sensor, the higher voltage signals under the samestrength causes lower signal-to-noise ratio, which is useful forwearable sensors. As shown in Figure 4d, we hit the front sideof PS and will generate the voltage signals of hit and release.The voltages were found to reverse when we overturn the PSand hit its back side. The highest voltage of back side wasalmost the same as that of the front side, indicating indirectdipole reversibility due to the alignment of −CH2−/−CF2−dipoles of PVDF.21

To realize human motion monitoring, sensors with ultra-sensitivity to force changes, high stability for practicalapplication, and high flexibility for wearable devices arerequired. To test the sensitivity, we controlled the strengthapplied on the PS by adjusting the duty ratio of relay (TableS2). As shown in Figure 4e, with the increase in input strengthfrom 0.05 to 0.45 N gradually, the voltage signals also show alinear growth from 3 to 18 V, indicating excellent signalcorrespondence. We fitted the relationship between input andoutput signals linearly, as shown in Figure S16, and the slopewas 34 V N−1. We can consider this value as the sensitivity ofour PS based on our fabrication, which is much higher thanthat based on the electrospinning method (0.041 V N−1).42

Table S3 lists the sensitivity comparison of other reports; thehighest value of sensitivity of our PVDF/graphene@Pillustrates the superiority of our method. The highest voltageof PVDF/graphene@P was over 60 V (Figure S17) when we

Figure 4. Three-dimensional (3D) models schematically representing (a) electromagnetic punch and (b) PVDF film, PVDF@P, and PVDF/graphene@P. (c) Voltage output of PVDF film, PVDF@P, and PVDF/graphene@P. (d) The voltage signals of hitting the front (black signals) andback (red signals) side of PVDF/graphene@P. (e) The relationship between voltage output and different strength input (from 0.05 to 0.45 N). (f)More than 125 cycles of voltage output illustrating the durability of PVDF/graphene@P.

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hit the PS with 2 N force, and only few reports could reachsuch a high voltage based on the PVDF composite piezo-electric devices.19 In fact, due to integration with daily cloth,repetitive motion may cause the deformation of such apiezoelectric-based device. To prove the stability of PVDF/graphene@P-based PS, we ran the electromagnetic punchsuccessively; after 125 circles (Figure 4f), the PS generatesstable voltage signals of about 20 V, which promises multipleusages in our daily life.43 All of these results prove the excellentpiezoelectric properties of PVDF/graphene@P.Interestingly, the testing threshold of PS is very low. We put

melon seeds (0.6 mN), microcapacitor (1 mN), andscindapsus leaf (1.8 mN) on the piezoelectric sensors fortesting (Figure 5a). According to the results, the threshold ofour device is 0.6 mN, which corresponds to the force createdby melon seeds. It is illustrated that even tiny disturbanceoutside could be detected if we integrated this PS with ourdaily garment. When we sewed the PS into the mask (Figure5b), the air flow coming out from our mouth will cause thevibration of PS when we speak. As we can see in Figure 5c, thevoices of “graphene” and “PVDF” bring out the differentsignals. When we say graphene, the mouth motion duringspeaking /’gr/ pronunciation brings the first signal at 3 s andthe air flow of/fi: n/voice creates the second signal at 4 s. And,the signals of saying PVDF is different and the voltage willshow up when we say the /pi:/ and /ef/ voices. Also, the /vi/and /di/ voices can generate some signal fluctuation. Webelieve that it can distinguish the voices by evaluating the airflow during speaking. Furthermore, we embedded PVDF/graphene@P PS in the pants to test the body shake whilerunning (Figure 5d). Because of the intensive shake in thepants, there was intensive deformation of the PS devices. Thefibers in fabric pull and strike each other, thus contributing tothe harsh change in voltage signal. From the enlarged part ofvoltage signals in 3.4−3.7 s (Figure 5e), we can distinguish thelanding moments of legs. Compared with the conventionalpiezoelectric ceramic,1,5,7 every pulse signal width is narrowerbecause of the less electric quantity. Less electric quantity but

higher voltage signals ensure the safe application and easycollection of signals. The easy fabrication process compared toother methods gives the promise for the wide usage in future.Compared with the solution casting method, this approachprovides a much greater work efficiency and scale-upcapability. Compared with the electrospinning method toprepare piezoelectric fibers, our approach is energy efficientand independent of special equipment.

■ CONCLUSIONS

In summary, a convenient, continuous, and time-savingstrategy for the fabrication of flexible piezoelectric fabric(PVDF/graphene@F) with high-voltage output was devel-oped. The PVDF/graphene solid phase formed with enrichedelectroactive PVDF when the liquid NMP phase dissolved inwater. The structural units of −CH2− and −CF2− in PVDFchains were arranged directionally due to the induction ofgraphene and water during phase separation, which con-tributed to the highest of 87% electroactive phase crystallinity.Such a PVDF/graphene@F-based sensor with 34 V N−1

sensitivity and 0.6 mN testing threshold could be integratedinto the apparel, which can be applied to monitor the speakingand body motions. This work is helpful, not only for thedevelopment of PVDF composites with high β-PVDF contentbut also for the development of promising wearable piezo-electric sensors.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.8b10552.

Fabrication details of graphene and PVDF/graphene@F;

SEM images of bended fabrics; XPS, TEM, Raman

characterization of graphene; XRD data analysis; piezo-

electric voltage testing system; HR-TEM, SAED, and

additional data (PDF)

Figure 5. (a) Digital photos and voltage signals of melon seeds (a1), microcapacitor (a2), and leaf (a3) demonstrating the sensitivity of PVDF/graphene@P. (b) Digital photo of PS integrating with mask and (c) the corresponding signals when saying graphene and PVDF. (d) Digital photosof pants integrated with PS and (e) the voltage signals of running.

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■ AUTHOR INFORMATIONCorresponding Authors*E-mail: hepeng@mail.sim.ac.cn (P.H.).*E-mail: liangjr@shanghaitech.edu.cn (J.L.).*E-mail: gqding@mail.sim.ac.cn (G.D.).ORCIDSiwei Yang: 0000-0002-5227-8210Guqiao Ding: 0000-0003-1674-3477Author Contributions⊥T.H. and S.Y. contributed equally to this work. T.H.conceived and conducted the major experiment work. P.H.and S.Y. designed part of the experiment and participated inthe manuscript writing. P.H. and G.D. gave vital suggestionsand directions during the experiments and paper revision. J.S.,S.Z., and J.L. provided the devices designation and testsuggestion. All authors attended the discussion and dataanalysis of this work.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the National Natural ScienceFoundation of China (Grants 11774368 and 11704204) andby a Project funded by the China Postdoctoral ScienceFoundation (Grant BX201700271).

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