Graphene Coating on Fabrics

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Applications of Polymer, Composite, and Coating Materials

Phase Separation Induced PVDF/Graphene Coating on Fabrics towards Flexible Piezoelectric Sensors Tao Huang, Siwei Yang, Peng He, Jing Sun, Shuai Zhang, Dongdong Li, Yan Meng, Jiushun Zhou, Huixia Tang, Junrui Liang, Guqiao Ding, and Xiaoming Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10552 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 21, 2018

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ACS Applied Materials & Interfaces

Phase Separation Induced PVDF/Graphene Coating on Fabrics towards Flexible Piezoelectric Sensors Tao 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 of Sciences, Shanghai 201210, P. R. China. §

Mechatronics and Energy Transformation Laboratory, School of Information Science

and Technology, ShanghaiTech University, Shanghai, 201210, P. R. China.

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

Clothing-integrated piezoelectric sensors possess great potential for future wearable electronics. In this paper, we reported a phase separation approach to fabricate flexible piezoelectric sensors based on PVDF/Graphene composite coating on commercial available fabrics (PVDF/graphene@F). The structural units of -CH2- and -CF2- of PVDF chains were arranged directionally due to the structural induction of graphene and water during phase separation, which is the key of the electroactive phase enrichment. In optimized case, integrating into fabric substrates endows the phase-out PVDF/graphene composite coating four times higher voltage output than its film counterpart. 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) which can be applied to distinguish the voices or monitor the motion of body. This simple and effective approach towards PVDF/graphene@F with excellent flexibility provides a promising route toward the development of wearable piezoelectric sensors. KEYWORDS: PVDF, Graphene, Piezoelectric materials, Fabric, Sensors.


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INTRODUCTION

In situ, real-time biomedical monitoring and human motion sensing require wearable electronic devices that can transform mechanical to electrical signals.1,2 Compared with traditional resistance-based testing system sensors,3,4 piezoelectric sensors show more precise response to outside strength because of the nature of piezoelectricity. Furthermore, piezoelectric materials generate voltage signals themselves without outside energy source.2 In this regard, clothing-integrated piezoelectric sensors for body monitoring have become a current thrust of research in the field of piezoelectric materials.5 Flexible piezoelectric polymers, especially polyvinylidene fluoride (PVDF), have been extensively used to fabricate flexible motion sensors due to the nature of lightweight, high processability and the piezoelectric property.2, 6, 7 Graphene is a kind of 2D material with unique properties. Due to its superior mechanical strength, high specific area and unique electrons structure,8, 9 it has been combined with PVDF to prepare flexible piezoelectric device with excellent mechanical strength and enhanced piezoelectric efficiency.10-12 PVDF/graphene composite with different kinds of graphene like graphene oxide (GO), pristine graphene (reduced GO: rGO) have been widely studied in past decades. GO can boost the β-PVDF content for the interaction between negative sheets and fluorine group (-CF2-) in PVDF.13, 14 Latterly, Xia Liu et al. utilized electrospinning to fabricate PVDF/GO core-shell fibres.15 They increased the piezoelectric constant (d33) of PVDF to 93.75 pm V−1 (the highest d33 of pure electrospinning nanofibre was 58.5 pm



V−1)16, which testified the superiority of GO enhancement. In comparison, pristine graphene (or rGO) could provide higher mechanical property enhancement while inducing the formation of β-PVDF.17-20 Sumanta Kumar Karan et al. designed highly durable piezoelectric nanogenerator by integrating PVDF with rGO/Al2O3, which provided higher voltage output of 36 V and ten weeks’ usage without damping.21 It is indicated that graphene provides PVDF with higher endurance owning to its mechanical strength. In summary, pristine graphene can be the most promising nano-filler for both mechanical and electrical enhancement of PVDF based piezoelectric sensors. However, it remains challenging to fabricate PVDF/graphene composite with high flexibility, low energy cost, time-saving merits. Soft PVDF/graphene films can be fabricated through solution casting method.17, 19, 20 But the solidifying process of film shows low efficiency to volatilize out the high-boiling-point organic solvent (N-Methyl-2-pyrrolidinone, for example). Electrospinning method shows great hope for the batch preparation of flexible PVDF/graphene fibers.15, 18 And the piezoelectric performance of electrospinning nanofibres can satisfy the demands of flexibility. Nevertheless, the strength of fabric based on PVDF fibres is hardly satisfying compared with other existing fabric like polyester, and electrospinning method relies on the expensive equipment. In comparison, to achieve remarkable flexibility of PVDF/graphene composite, existing fabric can be optimum substrates for PVDF/graphene coating, which are safe to the body, flexible to the motion and sewable to integration with electronics.22-24 But few reports have focused on the


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combination of the existing fabrics and PVDF. Here we report a fast phase-separation method to fabricate flexible piezoelectric device based on PVDF/graphene coating on fabrics (PVDF/graphene@F). Due to the interactions between PVDF 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 the existing fabric as the substrates would provide both scalability and better reliability in practical applications. Motion sensors based on the PVDF/graphene@F are demonstrated when integrated into day-to-day garments. EXPERIMENTAL SECTION Materials. Sodium persulfate (Na2S2O8, ≥ 99 wt. %, Sinopharm), sulphuric acid (H2SO4, 95-98 wt. %, Sinopharm), Polyvinylidene Fluoride powder (PVDF, molecular weight: 900000, Sigma) and N-Methyl pyrrolidone (NMP, AR, Sigma) were purchased and used as received without further purification. Deionized water obtained through a Milli-Q system was used throughout all experiments. Graphite powders (325 mesh) were purchased from Qingdao Xinghe graphite Co., Ltd. (Qingdao, China). Fabrication of PVDF/graphene/NMP. Graphene was fabricated as we reported before and the detailed process can be found in Supporting Information. The corresponding graphene characterization 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 ultra-sonicated (50 oC, 200 W) for 5 h to get the final homogeneous dispersion. The solution will be cooled to room temperature for the next usage.



Fabrication of PVDF/Graphene@F and PVDF/Graphene membrane. Fabric was immersed in the PVDF/graphene/NMP solution firstly (coating process). After dipping completely, the fabric coated with PVDF/graphene/NMP went through a tank of water to conduct phase separation process. After that, the wet PVDF/graphene@F was dried in the air (60 oC) The dried PVDF/graphene@F was collected for the next usage. The membrane was fabricated through the same process. PVDF/NMP or PVDF/graphene/NMP (about 2 ml) was cast onto surface dish firstly. Water was then poured in the dish slowly to conduct the phase separation process. After 1-2 minutes, the wet membrane was dried in the 60 oC air for the further test. Fabrication of the PVDF/graphene@F based sensors. Aluminum foils were cut into square shapes firstly. Two pieces of foils were then put on the two sides of the fabric and sealed with tapes. Such a sensors can be sewed into pants or marks for the further testing. Characterization. Field emission scanning electron microscopy (FE-SEM) was performed on a JEOL JSM-4700S equipped with an EDSINCA 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 by Thermo Fisher DXR. Differential scanning calorimetry (DSC, TAQ200) was conducted in 100-200 oC temperature range (10 oC min-1) under the N2 atmosphere. X-ray diffraction (XRD) patterns were obtained from an X-ray Diffractometer (Bruker D8 ADVANCE) with a monochromatic source of Cu Kα1


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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 a Hitachi H-8100 EM (Hitachi, Tokyo, Japan) with an accelerating voltage of 300 kV. 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 the fabric, the polyester was immersed in the PVDF/graphene/NMP solution for continuous impregnation. The impregnation time in this solution was 9-10 s which allowed sufficient coating. For the continuity of fabrication, the stability of PVDF/graphene/NMP solution should be considered. After 3 weeks’ settling down, no clear delamination or sediment in the solution was observed as shown in Figure 1b, indicating the good dispersibility of graphene. Actually, graphene is very stable in NMP solution as reported before. 25 The dipped polyester fabric was further immersed in the water to separate out the solid PVDF/graphene composite from the liquid NMP system (a typical phase separation process). Long immersion time (> 1 minute) in water was necessary to ensure the removal of NMP and sufficient polarisation of the PVDF molecules. After air-drying, polyester coated with PVDF/graphene (PVDF/graphene@P) was acquired (the average mass uptake of PVDF/graphene was about 0.5 g m-2). The polyester becomes darker like Figure 1c showing for the addition of graphene. Noticeably, after coating, the polyester was stiffer than original state. The fabric with once coating still exhibits good flexibility, but after the fifth



coating, it becomes rigid as shown in Figure S2. The more times the coating perform, the more rigid crystal PVDF fabric were coated with, which leads to the higher rigidity. Fortunately, PVDF/graphene@P with one time of coating remains the flexibility and can be folded as the uncoated fabric substrate. The folded PVDF/graphene@P generates strong electrostatic attraction with butyronitrile gloves, which prevents it from falling (Figure S3) and indicates a strong piezoelectricity. Moreover, the mechanical property of PVDF/graphene@P and polyester was tested. The maximum load of PVDF/graphene@P is about 1.55 MPa, close to the 1.51 MPa of raw polyester fabric (Figure S4), which means that a short immersion time in NMP does not cause significant degradation of the polyester’s strength. Moreover, this approach can also be applied to various fabrics: as shown in Figure S5, various types of fabric (nylon, tissue, cotton, bamboo fibres and flax) can also be uniformly coated with PVDF/graphene after similar treatment. This approach was convenient and time-saving for the mass-production of soft piezoelectric fabrics (productivity exceeding 36 m2 h-1). Besides, the water triggered phase separation process greatly improves the elimination efficiency of the high-boiling-point solvent (such as NMP) and decreases the pollution of the organic solvent steam.


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Figure 1. (a) Schematic illustration of the preparation process of soft piezoelectric fabric and its micro-structure. (b) PVDF/graphene/NMP solution standing still for 3 weeks. (c) Digital photo comparison of polyester fabric and PVDF/graphene coated fabric. To judge the uniformity of coating, the morphological of PVDF/graphene coated polyester fabric (PVDF/graphene@P) was observed through FE-SEM in Figure 2. Figure 2 a-b indicates that the interwoven structure of PVDF/graphene@P is well retained during the coating processes. Figure 2a shows the polyester fibres with an average diameter of 13 µm and the insert image shows the smooth surface; in contrast, the average diameter increases to about 16 µm with much rougher surface after PVDF/graphene coating (Figure 2b, the diameter statistic results see Figure S6). The increased diameter indicates that the average coating thickness is approximately 1.5 µm, because the PVDF/graphene tends to form shell covering the whole polyester fibre. And the rougher surface was induced by the contraction of PVDF during water triggered phase-separation process. The folding state of PVDF/graphene@P is also



observed in SEM and there is no obvious crack on the surface of folding place (Figure S7), which would ensure the integrity of PVDF/graphene@P during practice applications. X-ray energy dispersive spectroscopy (EDS) mapping (Figure 2c) reveals the presence of well-spread F element (blue) along with the C element (red), which indicates the uniform coating of PVDF.

Figure 2. SEM images of (a) polyester and (b) PVDF/graphene@P, the insert implying the surface morphology changes. (c) EDS mapping of carbon (red) and fluorine (blue) element. (d, e) TEM images of PVDF/graphene@P. (f, g) HR-TEM images showing lattice fringe of crystal PVDF and graphene. 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 2d presents the microstructure of PVDF/graphene composite. Between the Pt. coating (protecting the structure from destruction of FIB) and polyester substrates,


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graphene dominates the black area and is surrounded by transparent PVDF. More details of interfaces in the PVDF/graphene are shown in higher resolution image (Figure 2e). In the PVDF layer, the crystal area is clear with a lattice spacing of 0.256 nm (Figure 2f), corresponding to the typical lattice spacing along c axis in the orthorhombic β-PVDF crystal.26 Among three main phases (α, β, γ) of PVDF, polar β/γ-PVDF are highly desirable electroactive phases and α-PVDF is non-electroactive phase.27 And we observed that the majority of PVDF areas in TEM images are the crystal β-PVDF which illustrates the strong piezoelectric properties. It should be pointed out that the presence of significant crystal particles of β-PVDF has not been reported yet by previous work to our best knowledge, which emphasizes the importance of phase separation strategy.15, 16 Selected area electron diffraction (SAED) patterns of the crystal area in PVDF also shows the diffraction spots corresponding to the crystallographic plane of β (1 0 0) (Figure S8). And the lattice fringe of graphene (d=0.248 nm) 9, 28 can be seen in Figure 2g, which proves the high quality of graphene (see discuss of graphene in Supporting Information). The above results demonstrate uniform PVDF/graphene coating layer with significant presence of piezoelectric PVDF phase. How the crystal piezoelectric PVDF formed should be investigated furthermore. FT-IR, XRD and DSC measurements are commonly used to assess the phase composition. However, it is not suitable to test PVDF/graphene@P directly with these analyses because the strong signals of textile substrates like polyester will blur that of PVDF/graphene. Therefore, PVDF

and

PVDF/graphene

membranes

were

prepared


through

similar


phase-separation process (Figure S9) and used for FT-IR, XRD and DSC measurement. FT-IR spectra can be applied to detect the structure information in PVDF chains.27 From the black curve in Figure 3a (raw PVDF), the vibration bands at 766, 795 and 976 cm-1 can be observed, which correspond to the -CF2- bending vibration and -CH2- rocking or out of plane vibration of α-PVDF.27 In the red curve (representing the PVDF membrane fabricated by phase separation method in water), vibration bands at 840, 1234, and 1279 cm-1 are enhanced intensively. These peaks can be attributed to the -CH2- wagging vibration and -CF2- symmetric stretching of β/γ-PVDF, respectively.27, 29 This indicates the enriched β/γ-PVDF phases through the phase-separation process. By contrast, the FT-IR spectrum of PVDF/graphene (blue curve) shows that the significant decrease of α-PVDF signals at 766, 795 and 976 cm-1 wavenumbers. It indicates that graphene sheets interact with PVDF chains which induce amorphous macromolecular chains into crystal electroactive PVDF instead of α-phase. These FT-IR results showed that phase separation and graphene sheets were very effective to form electroactive phase. Through Raman investigation, we found that, compared to the G band at 1600 cm-1, it splits to two distinct peaks at 1580 and 1605 cm-1 in PVDF/graphene. The G band splitting of PVDF/graphene hybrid may indicate an interaction between PVDF and graphene surface as shown in Figure S10. 30

Additionally, DSC is a thermo-analytical technique which is complementary to the

other identification techniques.29 As shown in Figure 3b, the raw PVDF powder (the black curve) shows the melting temperature of 148-177 oC.29 After phase separation,


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PVDF (red curve) and PVDF/graphene (blue curve) membranes show the higher melting temperature (about 172 oC). However, higher melting temperature of γ-PVDF at 179 oC cannot be recognized which means the β-PVDF is dominant. DSC is also used to calculate the crystallinity (χc) of PVDF through equation (1).31 ∆‫݂ܪ‬

߯௖ = ∆‫ܪ‬ 100

(1)

Where ∆Hf represents the melting enthalpy of the composite and ∆H100 (taken as 104.6 J g-1: the melting enthalpy for a 100% crystalline sample of pure PVDF). 32 According to the calculation result, the χc of PVDF/graphene films reaches about 89%, which is much higher than pure processed PVDF film (70%). The above results illustrate that water induced phase separation and graphene sheets help the crystallization of electroactive phase.



Figure 3. The 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 thermo-analysis and (c) XRD. (d) Variation of β/γ-phase content (black axis) and corresponding crystallinity (red axis) as a function of the graphene content. (e) The 3D structure models of β-PVDF formation. XRD characterization could determine phases of PVDF clearly.29 Figure 3c gives the XRD patterns of the raw PVDF powder (black curve), phase separated PVDF (red curve) and PVDF/graphene membranes (blue curve). The peaks at 2θ = 17.6°, 18.3°, 19.8° and 26.5° in raw PVDF (black curve) are obvious which corresponded to (1 0 0),


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(0 2 0), (1 1 0) and (0 2 1) of ɑ-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 (2 0 0) or (1 1 0) plane of β-PVDF and (1 1 0) plane of γ-PVDF, respectively.34, 35 After adding graphene to the PVDF, these characteristic peaks of β-/γ-PVDF are significantly enhanced. To explore the role of graphene, different graphene weight contents of PVDF/graphene composite, like 0, 0.1, 0.3, 0.5, 1, 2 and 3 wt. %, were conducted with XRD test. The relative electroactive phase content (Cβ+γ) and their contribution to crystallinity (χβ+γ) can be calculated by deconvolution method as given in Figure S11 (Supporting Information). We define the value Cβ+γ to explore the relative content of electroactive phases and χβ+γ to depict the absolute content of crystal β/γ-phase in whole PVDF. As we can see in Figure 3d, when the graphene content is 0 wt. %, the phase-separated PVDF has already got 77% Cβ+γ and 60% χβ+γ. The enrichment of χβ+γ is contributed by the strong interaction between water and PVDF chains as reported before.36 With 0.1, 0.3 and 0.5 wt. % graphene added, Cβ+γ increases from 77% to 83, 90, 97% and χβ+γ increases from 60% to 68, 80, 87%, respectively, indicating that graphene can induce more crystal polarized PVDF during phase separation. But with higher content of graphene (more than 1 wt. %), the Cβ+γ and χβ+γ decrease which indicated that more graphene obstacle the formation of electroactive phase on the contrary. We assume that too much graphene would obstacle the continuous diffusion of NMP in PVDF during phase separation. This leads to less interaction between water and PVDF, which inhibited the major formation of β/γ-PVDF. Through Density Function Theory calculations (see



Supporting Information) of the interaction between graphene, water and PVDF as shown in Table S1, the action between water and PVDF chains (25.5 kJ mol-1) is much stronger than that between graphene and PVDF (19.2 kJ mol-1). Once the water was blocked, the weaker interaction between graphene and PVDF would be insufficient to help form large content of electroactive phase. We also observe the different samples through TEM to deepen the understanding. For the PVDF without graphene (phase separation), β-PVDF crystal particles show up only in the outmost layer (Figure S12 a-b). Compared with the result in Figure 2e, the existence of graphene helps the formation of electroactive phases, as we discussed above. However, PVDF/graphene sample with graphene content of 2 wt. % shows less crystal area ratio. As shown in Figure S12 c-d, graphene sheets encase PVDF which impedes the diffusion of NMP. More importantly, when PVDF was cast onto Si from NMP solution (0.05 g ml-1), no clear crystal area was observed (Figure S12 e-f). We build a structure model to help understand the process of phase separation in Figure 3e. Upon the introduction of water, PVDF and graphene in NMP will simultaneously separate from the liquid phase to form the solid PVDF/graphene composite coating on the fibres. The electron injection groups (-CH2-) in PVDF chains exhibiting electropositive property promotes the intense interaction with π-conjugation electrons of graphene.21, 37 Simultaneously, at the interface of PVDF and water, electronegative -CF2- units of other chains tend to form hydrogen bonds with H2O.36 Under the synergistic effect of water and graphene, the dissolved PVDF chains can be polarized into electroactive phase instead of non-electroactive phase.


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Actually, the coating thickness and coating substrates also have influence on the crystallinity of electroactive phases. With the increase of PVDF/graphene membranes’ thickness (from 0.15 to 1.50 mm, measured by thickness gauge), the crystallinity decreases as the thickness of PVDF/graphene increased, as shown in Figure S13a. Despite the anti-proportional relationship between crystallinity and thickness (between 0.15 to 1.50), this influence maybe not as apparent since a much thinner coating thickness on fabric in micrometer range, which allows fast and complete phase out after water introduction. On the other hand, the hydrophilic substrates will improve the crystallization of PVDF (see Figure S13b). But the ester substrates have no obvious improvement or impairment to PVDF. In brief, the coated PVDF/graphene layer on polyester has also high crystallinity of electroactive phases in our method.

Figure 4. 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) and back (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. According to above results and discussion, the phase separation approach is convenient and effective to coat electroactive PVDF/graphene composite on fabrics. Here we integrated PVDF/graphene@P with electrodes into piezoelectric sensors to evaluate the piezoelectricity. At the same time, to give a constant testing environment, an electromagnetic punch was selected as the strength source for the piezoelectric test (Figure 4a, Figure S14). Such an electromagnetic punch, controlled by a frequency control electric relay, could impart constant strength in a given frequency and ensure the same testing condition of different samples. A piezoelectric sensor (PS) device based on PVDF/graphene@P was manufactured for piezoelectric property testing as shown in Figure S15. PS based on pure PVDF film and PVDF coated polyester (PVDF@P) were also integrated with Al foils into PS (Figure 4b, the detail seeing Supporting Information). As shown in Figure 4c, the pure PVDF film-based PS generated less than 10 V voltage while PVDF@P PS generated over 15 V. By contrast, the PVDF/graphene@P created stronger voltage signals about 20 V, which was four times higher than PVDF films and much higher than many reports (less than 5 V).14, 37, 38

It is clearly that porous structure can absorb more mechanical energy than film

which has been testified in many reports.39, 40 The textile fabric provided a natural porous structure which contributes to the intense voltage output enhancement. Furthermore, the graphene’s addition is also confirmed to contribute to the increase of piezoelectric performance by the comparison of PVDF@P and PVDF/graphene@P.


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As a sensor, the higher voltage signals under the same strength brings lower signal to noise ratio, which is meaningful to the wearable sensors. As shown in Figure 4d, we hit the front side of PS and it will generate the voltage signals of hit and release. The voltages were found to reverse when we overturn the PS and hit its back side. The highest voltage of back side was almost the same with the front side, which indicated indirectly dipole 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 practical application and high flexibility for the wearable devices are required. To test the sensitivity, we controlled the strength applied on PS through adjusting the duty ratio of relay (Table S2). As shown in Figure 4e, with the increase of input strength from 0.05 N to 0.45 N gradually, the voltage signals also show linear growth from 3 V to 18 V indicating the excellent signal correspondence. We fitted the relationship between input and output signals linearly as Figure S16 shown, and the slope was 34 V N-1. We can consider this value as the sensitivity of our PS based on our fabrication which is much higher than that based on electrospinning method (0.041 V N-1).42 Table S3 lists the sensitivity comparison of other reports, and our PVDF/graphene@P is the highest illustrating the superiority of our method. The highest voltage of PVDF/graphene@P was over 60 V (Figure S17) when we hit the PS with 2 N force and few reports could reach such a high voltage based on PVDF composite piezoelectric devices.19 In fact, due to the integration with daily cloth, repetitive motion may cause the deformation of such a piezoelectric based device. To prove the stability of PVDF/graphene@P based PS, we ran the



electromagnetic punch successively, and after 125 circles (Figure 4f), the PS generates stable voltage signals about 20 V which promises the multiple times usage in our daily life.43 All these results prove the excellent piezoelectric properties of PVDF/graphene@P.

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 photos of pants integrated with PS and (e) the voltage signals of running. Interestingly, the testing threshold of PS is very low. We put melon seeds (0.6 mN), micro-capacitor (1 mN) and scindapsus leaf (1.8 mN) onto the piezoelectric sensors and they can all be tested (Figure 5a). Based on that, the threshold of our device is 0.6 mN which corresponds to the force created by melon seeds. It illustrated that even tiny disturbance outside could be detected if we integrated this PS with our daily garment. When we sewed the PS into the mask (Figure 5b), the air flow came out from our mouth will cause the vibration of PS when we speak. As we can see in


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Figure 5c, the voices of “Graphene” and “PVDF” bring out the different signals output. When we say “Graphene”, the mouth motion during speaking /'gr/ pronunciation brings the first signal at 3 s and the air flow of /fi: n/ voice creates the second signal at 4 s. And the signals of saying “PVDF” is different, the voltage will show up when we say the /pi:/ and /ef/ voices. Also, the /vi/ and /di/ voices can generate some signals fluctuation. We believe that it can distinguish the voices through judging the air flow during speaking. Furthermore, we embedded the PVDF/graphene@P PS in the pants to test the body shake while running (Figure 5d). Because of the intensive shake, the pants cause the intensive deformation of PS devices. The fibres in fabric pull and strike with each other which contributes to the harsh change in voltage signal. From the enlarged part of voltage signals in 3.4-3.7s (Figure 5e), we can distinguish the landing moments of legs. Compared with the conventional piezoelectric ceramic, 1, 5, 7 every pulse signal width is narrower because of the less electric quantity. Less electric quantity but higher voltage signals ensure the safe application and easy collection of signals, the easy fabrication process against other methods gives the promise for the wide usage in future. Compared with the solution casting method, this approach provides much greater work efficiency and scale-up capability. Compared with the electrospinning method to prepare piezoelectric fibers, our approach is energy-efficient and independent of special equipment. CONCLUSIONS In summary, a convenient, continuous and time-saving strategy for the fabrication



of flexible piezoelectric fabric (PVDF/graphene@F) with high voltage output was developed. The PVDF/graphene solid phase formed with enriched electroactive PVDF when the liquid NMP phase dissolved into water. The structural units of -CH2and -CF2- in PVDF chains were arranged directionally due to the induction of graphene and water during phase-separation, which contributed 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 integrated into apparel, which can be applied to monitor the speaking and body motions. This work is helpful, not only for the development of PVDF composites with high β-PVDF content but also for the development of promising wearable piezoelectric sensors. ASSOCIATED CONTENT

Supporting Information. This material is available free of charge via the internet at: http://pubs.acs.org. Fabrication details of graphene and PVDF/graphene@F; SEM images of bended fabrics; XPS, TEM, Raman characterization of graphene; XRD data analysis; piezoelectric voltage testing system; HR-TEM, SAED, and additional data. AUTHOR INFORMATION Corresponding Author *Peng He: [email protected] *Junrui Liang: [email protected] *Guqiao Ding: [email protected]


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ORCID Guqiao Ding: 0000-0003-1674-3477 Peng He: 0000-0003-0606-8762 Author Contributions +

These authors contributed equally to this work. Tao Huang conceived and conducted

the major experiment work. Peng He and Siwei Yang designed part of the experiment and participated in the manuscript writing. Peng He and Guqiao Ding gave vital suggestions and directions during the experiments and paper revision. Jing Sun, Shuai Zhang and Junrui Liang provided the devices designation and test suggestion. All authors attended the discussion and data analysis of this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 11774368 and 11704204) and by a Project funded by the China Postdoctoral Science Foundation (Grant BX201700271). REFERENCES (1) Wang, Z. L.; Song, J. Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays. Science 2006, 312, 242–246.

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Table of Contents (TOC) Graphic


ACS Applied Materials & Interfaces Page 32 of 32 1 2 3 4 5 6 7


Graphene Coating on Fabrics

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