COC Piezoelectret Nanogenerator for Wide-range

Subscriber access provided by READING UNIV

Energy, Environmental, and Catalysis Applications

Flexible THV/COC Piezoelectret Nanogenerator for Wide-range Pressure Sensing Wenbo Li, Jiangjiang Duan, Junwen Zhong, Nan Wu, Shizhe Lin, Zisheng Xu, Shuwen Chen, Yuan Pan, Liang Huang, Bin Hu, and Jun Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11121 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 15, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28 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

ACS Applied Materials & Interfaces

Flexible THV/COC Piezoelectret Nanogenerator for Wide-range Pressure Sensing Wenbo Li‡, Jiangjiang Duan‡, Junwen Zhong, Nan Wu, Shizhe Lin, Zisheng Xu, Shuwen Chen, Yuan Pan, Liang Huang, Bin Hu and Jun Zhou*

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, 430074, China

KEYWORDS: Piezoelectret, nanogenerator, pressure sensor, active sensor, large linear region

ABSTRACT: Flexible pressure sensors possess promising applications in artificial electronic skin (e-skin), intelligence robot, wearable health monitoring, flexible physiological signal sensing, etc. Herein, we design a flexible pressure sensor with robust stability, high sensitivity

and

large

linear

pressure

region

based

on

tetrafluoroethylene-hexafluoropropylene-vinylide (THV) / cyclic olefin copolymer (COC) piezoelectret nanogenerator. According to the theoretical analysis for piezoelectret nanogenerators with imbalanced charge distribution, THV and COC are utilized to promote the electric field inside the piezoelectret for output voltage enhancement. Meanwhile, the compression property of the piezoelectret nanogenerator is facilely tuned. Owing to high inner electric field and optimized compression property, the THV/COC piezoelectret nanogenerator exhibits high sensitivity of 30 mV/kPa, which is ten times higher than the 1

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

traditional cellular polypropylene piezoelectret. Simultaneously, the linear pressure region reaches 150 kPa with excellent linearity (R2=0.99963). The device is demonstrated to realize wearable pressure sensing with wide pressure range from finger typing to fist hammering. This study presents a fabrication strategy for piezoelectret nanogenerators with high sensitivity and large linear pressure region, paving the way for the development of wearable and flexible pressure sensing networks. INTRODUCTION Flexible pressure sensors are of great importance in constructing the wearable sensing network in the internet of things (IoT).1-4 Nowadays, wearable pressure sensors, which are widely used to collect physiological information or imitate human body function, have received extensive attention. Various devices have been designed and applied in the fields like artificial electronic skin (e-skin),5-7 intelligence robot,8-9 wearable health monitoring,10-12 physiological signal sensing,13-14 etc. For all these wearable pressure sensors, the basic requirements locates at good flexibility, simple structure, light weight, nontoxicity, stability, high sensitivity, large linear pressure region, etc. Among them, sensitivity and linear pressure region are the two most important performance indexes for a pressure sensor. In previous works, sensitivity is particularly concerned and it is utilized to evaluate the performance of a pressure sensor. Researchers mostly focus on optimizing the devices and promoting the sensitivity.15-18 Whereas, the linear pressure region is always neglected and rarely works are devoted to widen the linear pressure region, although it determines the actual application range of the device.19 It has been demonstrated that the active flexible pressure sensors based on piezoelectric effect,3, 20-22 triboelectric effect23-25 or electrostatic effect15, 26-27 2


Page 2 of 28

Page 3 of 28 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


possess the linear pressure regions ranging from ~1.2 kPa (PTFE NWs, triboelectric effect)24 to ~50 kPa (PDMS/PTFE, electrostatic effect).26 However, these pressure sensors have limited linear pressure region to cover all the pressure ranges around human body from finger typing (~5 kPa) to plantar walking (~200 kPa).28 Generally, there is a tradeoff between sensitivity and linear pressure region for pressure sensors. Therefore, fabricating a pressure sensor with both large linear pressure region and high sensitivity, while the corresponding output signal is at an easily-measurable magnitude, is still confronted with challenges. In this work, we introduce a flexible pressure sensor based on THV/COC piezoelectret nanogenerator. THV with excellent negative charge storage ability and COC with outstanding positive charge storage ability are bounded by a PDMS array. According to the theoretical analysis for piezoelectret nanogenerators with imbalanced charge distribution, the employment of THV/COC components enables high electric field inside the piezoelectret. On the other side, the compression property of the film is optimized by adjusting the braced PDMS array. The THV/COC piezoelectret nanogenerator with high electric field and small Young’s module exhibits high sensitivity of 30 mV/kPa and large linear pressure region of 1 ~ 150 kPa with excellent linearity (R2=0.99963). The device is demonstrated to realize wearable pressure sensing with wide pressure range from finger typing to fist hammering. This work presents a new fabrication strategy for piezoelectret nanogenerator which can achieve high sensitivity and large linear pressure region simultaneously, promoting the development of wearable and flexible pressure sensing networks. RESULTS AND DISCUSSION Since wearable devices will suffer serious deformations caused by human body motions 3



during operation process, structure stability and mechanical robustness are significant for continuous work of a flexible pressure sensor. Piezoelectret film is consist of an air gap sandwiched between two electret membranes, and the low interfacial adhesion in laminated structure will cause performance instability or even device malfunction. Template method is widely used to form the structural support in fabricating piezoelectret films.29-30 However, the laminated films are always easy to be delaminated and the structure stability is worrying. So the support and adhesion of the two electret membranes is vitally important and a chemical bonding method is introduced in this work to fabricate the piezoelectret with good structure stability. The fabrication process of THV/COC piezoelectret nanogenerator is schematically shown in Figure 1a. The tetrafluoroethylene-hexafluoropropylene-vinylide fluoride terpolymer (THV) and cyclic olefin copolymer (COC) electret films with good charge storage stability are superior candidates for producing piezoelectret materials. Note that these electret materials are usually chemical inertness with low surface energy, and thus are difficult to toughly bond together. Here, by surface plasma activation and coupling reagent modification treatment in sequence, we successfully fabricated the piezoelectret supported by the polydimethylsiloxane (PDMS) array with good structure stability. In detail, THV and COC films are prefabricated from particles through a hot-press process, followed by the plasma treatment to generate –OH groups on the surface. Simultaneously, the PDMS array is fabricated through template method, using the through-hole templates (Figure S1) to control the parameters of the array. Subsequently, the PDMS array is successively treated by plasma and modified by aminopropyltriethoxysilane (APTES, a coupling reagent). The APTES 4


Page 4 of 28

Page 5 of 28 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


anchored on the surface of PMDS array can react with active –OH groups on the surfaces of THV and COC, by removing ethanol molecules (dealcoholization). Because APTES has three active site and can bond with three –OH groups, it as a crosslinker enables strong adhesion between PDMS and THV/COC. The detailed mechanism is shown in Scheme S1 in the Supporting Information. Finally the PDMS array is sandwiched between the prepared THV and COC films to form the laminated cellular THV/COC piezoelectret film. The inset of Figure 1a shows the parameters of the braced structure and the charge densities on the inner surfaces of the film. The detailed fabrication process is schematically diagramed in Figure S2. The energy dispersed X-ray (EDX) spectra and Fourier transform infrared (FT-IR) spectra (Figure S3) exhibit the modification of APTES to THV, PDMS and COC, separately. As shown in Scheme S1, APTES molecule contains a –NH2 group, which is a characteristic functional group and can be used to prove that APTES is anchored on the surfaces of PDMS and THV/COC. Compared to their pristine films, a weak N1s peak about 400 eV is observed in the PDMS and THV/COC films after treating by APTES, indicating APTES is successfully anchored on their surfaces (Figure S3a-c). Moreover, the peak about 1560 cm-1 for the in-plane bending vibration of –NH2 groups is observed in the FTIR spectra of APTES treated films (Figure S3d-f), which is consistent to the results of EDX. The peeling strength of PDMS/THV and PDMS/COC composites are enhanced for an order of magnitude after APTES treatment (Figure S4). Owing to the strong chemical bond of APTES and –OH groups, THV and COC are tightly bonded to PDMS. The digital picture (Figure 1b) shows the flexibility of the piezoelectret film. The inset shows the cross-section view scanning 5



Page 6 of 28

electron microscope (SEM) image of a PDMS brace, indicating the tightly bonded THV-PDMS-COC structure. The charge storage property of THV and COC are characterized by surface potential measurement. According to the definition of surface potential:

=

×

(1)

where VS is the surface potential, σ is the surface charge density, de is the electret thickness, ε0 is the vacuum permittivity, εr is the relative permittivity. Equation 1 indicates that VS is proportional to σ, so surface potential is measured to reflect the real-time surface charge density of electret films. Before measurement, corona charging process is employed to inject charges onto electret films. THV and COC are negatively (-25 kV) and positively (25 kV) corona charged for 3 min (Figure S5a). The gate voltage (±6 kV) is utilized to enhance the charging effect. In the measurement of 30 days, the surface potential of THV and COC stay stable at -2.6 kV and +1.76 kV (Figure 1c), respectively. THV exhibits excellent property in storing negative charges and COC is outstanding in storing positive charges. We then coat Ag on the both sides of THV/COC piezoelectret by using magnetron sputtering to assemble the electrodes. Subsequently, a contact charging process (Figure S5b) is employed to polarize the piezoelectret. The detailed contact charging process is given in the Experimental Section. The actual charge distribution in THV/COC piezoelectret is measured by open circuit (OC) thermally stimulated discharge (TSD) as shown in Figure S6. Due to the relatively poor thermo-stability, positive charges escape from COC first and a negative peak is generated at about 110 oC (Figure 1d). With the rise of temperature, negative charges on THV are thermally activated and a positive peak is generated at about 6


Page 7 of 28 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


150 oC. The OC TSD current curve suggests the storage of positive charges on COC and negative charges on THV inside the THV/COC piezoelectret. The OC voltage is usually used as the sensing signal for the piezoelectret nanogenerators. In previous theoretical study, the output voltage is calculated in the case of balanced charge distribution.31 However, the positive and negative charge distributions are actually imbalanced because of different material properties. A theoretical analysis is performed in the case of imbalanced charge distribution (Figure S7) and the detailed derivation is given in the section entitled “Theoretical derivation for the OC voltage” in the Supporting Information. According to the derivation, the OC voltage is obtained: = × ∆ℎ

(2)

where ∆h=h0–h(t) is the deformation of the air gap, h(t) is the thickness of the air gap, h0=h(t=0). E0 is the corresponding original electric field which is given as follows:

∝ =

+

(3)

where d1 and d2 are the thickness, σ1 and σ2 are the charge densities, ε1 and ε2 are the relative permittivities of the two electret films, respectively. According to Equation 2 and 3, the OC voltage is proportional to the original electric field E0 and E0 is proportional to a parameter M= d1σ1/ε1+d2σ2/ε2, which is related to the electret material directly. Therefore, an effective way to obtain high OC voltage is promoting E0 by enhancing the charge densities. A qualitative simulation is performed at the same device model with different surface charge densities, utilizing COMSOL Multiphysics simulation software. The device deformation is set to be the same cosine form and the OC voltage is simulated at different 7



charge distributions (σ1, -σ2), where σ1 and -σ2 indicate the inner surface charge densities of the piezoelectret (the inset of Figure 1a). The other simulation parameters are shown in Table S1. As shown in Figure 2a, the increase of σ1 and σ2 can both lead to OC voltage promotion. However, the fundamental mechanism is that the increase of parameter M at different charge distributions leads to higher electric field E0 which promotes the OC voltage (Figure 2b). The simulation results are agreed with the theoretical analysis before. In order to experimentally realize higher electric field, we employ a quenching method to promote the charge storage property of THV material. During hot working processes of polymers, the quenching method is often used to reduce the size of crystals. For electret films, it has been observed that reducing the size of crystals can enhance their charge storage property, which is attributed to more grain boundaries for charge storage.32 The quenching process are schematically diagramed in Figure S8a. After hot-press process, the THV film is immediately immersed into water before solidification. The Differential Scanning calorimeter (DSC) is employed to analyze the crystallinity of THV. The heat flow curves for natural cooling and quenching THV are given in Figure S8b, and the corresponding enthalpy change curves are shown in Figure 2c. The enthalpy change of the quenched sample decreases from 14.69 mJ to 13.26 mJ, reflecting the crystallinity reduction of 9.7%. Besides, the relative permittivity is also changed (Figure S8c). Moreover, we employ the X-Ray Diffraction spectrum and polarizing microscope to analyze the crystallinity. The intensity of crystalline peak (Figure S8d) at 2θ=17.8° for the quenching sample significantly weakens compared to that of the natural cooling sample, reflecting the crystallization reduction of the quenching sample, which accords with the results of DSC. The polarization microscope is a powerful 8


Page 8 of 28

Page 9 of 28 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


tool to intuitionally observe the crystallization of materials. In general, the bright field indicates the crystalline domain while dark field indicates the amorphous domain. As shown in Figure S8e, the image of natural cooling THV shows a homogeneously bright field, indicating that THV is highly crystallized during the slow cooling process. Whereas, the image of quenching THV shows the alternation of bright and dark fields, meaning that the crystallinity of quenching THV is reduced. More importantly, the grain boundaries are obviously increased, which might be the reason for the promotion of charge storage property. Owing to the quenching treatment, the negative charge storage property of THV is successfully promoted for 25% (Figure 2d). Acting as the comparative material, polyimide (PI) films are positively and negatively corona charged and the corresponding surface potential curves are given in Figure S9. After the measurement of 30 days, the surface potential of positively and negatively charged samples drop to -156 V and +121 V, separately. The comparison of charge storage property for THV, COC and PI is shown in Figure 2e. In comparison with PI, THV is much better at storing negative charge and COC is much better at storing positive charge. Piezoelectret nanogenerators made from PI/PI, PI/COC and THV/COC are fabricated and the OC voltage curves are measured (Figure 2f). The stimulation of 5 N force and 5 Hz frequency is given at the area of 1 cm2. The OC voltage of THV/COC piezoelectret nanogenerator reaches 1.5 V, but the OC voltage of PI/PI piezoelectret nanogenerator is only 0.13 V and the OC voltage of PI/COC piezoelectret nanogenerator is 0.87 V. The experimental results show that more charge storage and larger electric field inside piezoelectret is beneficial to promote the voltage output. 9



Page 10 of 28

According to Equation 2, the deformation ∆h is another key factor to influence the output performance of the piezoelectret nanogenerator. The output of piezoelectret nanogenerators with different deformation is simulated. In the simulation, the same device models with the fixed charge distribution (σ1, -σ2) are used. The deformation is set to be cosine form with the same frequency but different amplitude. The simulation results (Figure 3a) show that the OC voltage is linear to the deformation. But in the actual measurements and applications, we always control the pressure F but not the deformation ∆h. So, we attribute the influence of OC voltage to the Young’s module Y=(∆F/S)/(∆h/h0) of the piezoelectret. Combine the definition of the Young’s module and Equation 2, we can derive the sensitivity of the pressure sensor: ∆

= ∆ =

×

(4)

Therefore, the smaller Young’s module is beneficial to promote the sensitivity. Moreover, the simulation results indicate that the OC voltage is negatively linear to the Young’s module (Figure 3b) which means the smaller Young’s module is beneficial to higher voltage outputs. In experiment, we employ a hyperelastic material (PDMS), which enables large deformation and wide pressure range of the piezoelectret nanogenerator, to fabricate the braced array. Two method is introduced to optimize the compression property of the THV/COC piezoelectret. Firstly, we adjust the compression property of the braced array by changing the mixing ratio of PDMS. The recipe of PDMS at different mixing ratio is given in Table S2. With the increase of PDMS mixing ratio, the proportion of crosslinker decreases which leads to lower crosslinking density. So the PDMS is softer and the Young’s module of the piezoelectret decreases (Figure 3c). The second method is adjusting the height (h, unit: 10


Page 11 of 28 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


mm) and diameter (d, unit: mm) of the braces by using different templates. The Young’s module of the piezoelectret results from the synergy of braced PDMS and the air. The increase of h and decrease of d increases the proportion of air, which reduces the Young’s module of the piezoelectret (Figure 3d). The pressure response of piezoelectret nanogenerators with different compression property is experimentally measured. The stimulation of 5 Hz frequency is given at the area of 1 cm2 and the peak OC voltage is recorded at different stimulating pressures. With the increase of PDMS mixing ratio, the peak OC voltage increases (Figure 3e). The linear pressure region increases from 80 kPa to 150 kPa and the corresponding sensitivity increases from 3.5 mV/kPa to 30 mV/kPa. On the other side, with the increase of h and decrease of d, the peak OC voltage increases (Figure 3f). The linear pressure region increases from 90 kPa to 150 kPa and the corresponding sensitivity increases from 3 mV/kPa to 30 mV/kPa. The experimental results indicate that smaller Young’s module is beneficial to promote the sensitivity. Simultaneously, a wide linear pressure region is obtained after optimizing the structure of the piezoelectret. The final piezoelectret nanogenerator is fabricated using THV and COC as the electret material. The PDMS mixing ratio is 20:1, the brace height is 1 mm and the brace diameter is 1.5 mm. The THV/COC piezoelectret nanogenerator is based on electrostatic induction effect. The working mechanism is shown in Figure S10 for better understanding of the signal generating process. We use the OC voltage to present the features of the piezoelectret nanogenerator, as indicated in Figure 4. The frequency response of the THV/COC piezoelectret nanogenerator is shown in Figure 4a. The device is stimulated under the force 11



of 5 N in the area of 1*1 cm2. When the stimulating frequency changes, the constant pressure generates the same deformation. So OC voltage remains unchanged and the frequency response is stable. The linear pressure region and sensitivity are the two most important parameters of a pressure sensor. The linear pressure region is the actual working pressure range of a pressure sensor and a larger linear pressure region enables a pressure sensor with more application scenarios. Figure 4b depicts the pressure response of the THV/COC piezoelectret nanogenerator. The device exhibits ultra large range for pressure sensing (>500 kPa) and the linear pressure region reaches 150 kPa. In the linear pressure region, THV/COC piezoelectret nanogenerator exhibits a high sensitivity of Se=30 mV/kPa, which is ten times larger than the traditional cellular polypropylene piezoelectret (3.15 mV/kPa).33 The pressure response of THV/COC piezoelectret nanogenerator reveals excellent linearity (R2=0.99963) from 1 kPa to 150 kPa. In addition to large linearity and high sensitivity, the THV/COC piezoelectret nanogenerator also features uniformity and stability. In the uniformity measurement, six different positions on a device is pressed by a square plate (area: 1*1 cm2), and the OC voltage remains stable with negligible fluctuation (Figure 4c). To further investigate the stability, the nanogenerator is repeatedly stimulated for ten thousand cycles under the above condition. The OC voltage with no decay (Figure 4d) indicates that the pressure sensing property is repeatable and stable. The response time of the pressure sensor is supplied in Figure S11. The stimulating force reaches the peak at t = 0.1 s and the corresponding output voltage reaches the peak at t = 0.1015 s. Therefore, we evaluate the response time of the pressure sensor is about 1.5 ms. 12


Page 12 of 28

Page 13 of 28 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


Based on the as-fabricated THV/COC piezoelectret nanogenerator, we perform a sequence of demonstrations to verify the functions and potential applications of the device. As shown in Figure 5a, the THV/COC piezoelectret nanogenerators (~1 cm2) are coupled on the hand to detect the skin pressure in various palm movements. The OC voltage signal is collected by an analog-digital conversion (ADC) module directly. Then, the corresponding pressure is calculated by the following formula: = ⁄

(5)

where Voc is the measured OC voltage and Se=30 mV/kPa is the sensitivity of the sensor, which is valid at the pressure of 1~150 kPa. The force is simultaneously measured by a commercial forcemeter to verify the accuracy of the device’s sensing property. The pressure sensor is firstly demonstrated at low pressure during index finger typing and a slightly peak pressure for about 10 kPa is measured by the THV/COC piezoelectret nanogenerator, which is in accordance with the signal from the force meter (Figure 5b). When the pressure reaches about 60 kPa, e.g. thumb pressing, it is beyond the testing range of many previous reported pressure sensors. Whereas, the THV/COC piezoelectret nanogenerator keeps proper functioning and a peak pressure for about 60 kPa is accurately displayed (Figure 5c). The THV/COC piezoelectret nanogenerator can even work at a high pressure over 100 kPa. When someone is thumping the desktop, the device at the bottom of the palm detects a high peak pressure for about 110 kPa, which still matches the result of the commercial forcemeter (Figure 5d). The whole demonstration process is shown in Supporting Video 1. The THV/COC piezoelectret nanogenerator exhibits wide pressure range with high accuracy, which possess extensive applications in wearable and flexible sensing networks. 13



CONCLUSIONS In summary, the laminated THV/COC piezoelectret nanogenerator with a robust structure is proposed via hot-press and chemical bonding process. According to the theoretical analysis for piezoelectret nanogenerators with imbalanced charge distribution, THV with excellent negative charge storage property and COC with outstanding positive charge storage property are employed to promote the electric field inside the piezoelectret which leads to high output voltage. The sensitivity and linear pressure region are further promoted by optimizing the compression property of the piezoelectret. The optimized THV/COC piezoelectret nanogenerator exhibits high sensitivity of 30 mV/kPa, which is ten times higher than the traditional cellular polypropylene piezoelectret. Simultaneously, the linear pressure region reaches 150 kPa with excellent linearity (R2=0.99963). Moreover, a wearable pressure sensor is demonstrated based on the THV/COC piezoelectret nanogenerator and a wide pressure range from finger typing to fist hammering is accurately detected. Our study offers a new and controllable method to fabricate piezoelectret nanogenerators with high sensitivity and large linear pressure region, which is significant in constructing the wearable pressure sensing network. EXPERIMENTAL SECTION Fabrication of THV/COC piezoelectret nanogenerator: Firstly, the THV particles (815GZ, 3M Dyneon Fluoropolymers) and the COC particles (6013, TOPAS Advanced Polymers GmbH) are hot-pressed at 10 MPa (260 oC) and 15 MPa (220 oC) to fabricate electret films (~50 µm), separately. The prepared THV and COC samples (5*5 cm2) are washed with pure water and isopropyl alcohol, and dried with N2 gas. Then, the PDMS array together with the 14


Page 14 of 28

Page 15 of 28 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


steel template is treated with O2 plasma (PDC-MG, Ming Heng) for 30 min and immediately soaked into 10% wt APTES (99%, aladdin) aqueous solution for 2 h. Simultaneously, the prepared THV film is treated with O2 plasma for 30 min to modify the surface with –OH groups and then attached to the APTES treated PDMS array (dried by N2 gas). The two components are kept at 80 oC under the pressure of about 4 kPa. After dealcoholization reaction for 24 h, the PDMS array and THV film are tightly bonded through APTES. Then the steel template is removed, followed by the same process to bond COC on the other side of the PDMS array. The laminated THV-PDMS-COC structure constitute the piezoelectret film. The detailed fabrication process is schematic diagramed in Figure S2. Polarization: After the fabrication of the THV/COC piezoelectret, both sides of the film are covered with silver (Ag) by magnetron sputtering as electrodes. Subsequently, a contact charging (Figure S5b) process is employed to polarize the piezoelectret. The high potential terminal of high voltage source (10 kV) is connected to the Ag electrode on THV and the low potential terminal is connected to the Ag electrode on COC, generating the electric field from THV to COC. The applied high voltage induces air breakdown inside the piezoelectret and generate positive and negative charges. The charges will move along with the electric field force and finally be captured by COC (positive charges) and THV (negative charges), forming the dipole-like charge distribution. Characterization: The morphology and energy dispersed X-ray spectra of the samples are characterized by a high-resolution field emission scanning electron microscope (FEI Nova NanoSEM 450). The Young’s modules and peeling strength are measured by universal testing machine (RWT10, Reger). The high voltage in the charging process is provided by a high 15



voltage source (DW-N503-4ACDE, Tianjin Dongwen High Voltage Power Supply Co., Ltd). The surface potential is measured by an electrostatic voltmeter (Model 347, Trek Inc.) on a scanning platform (PG201W, Pegasus Instrument Inc.). The thermally stimulated discharging current is measured by an electrometer (Keithley 6517B) together with a TSC tester (ZC1655, Zesheng Instrument Inc.). The FT-IR spectra is characterized by a Fourier transform infrared spectrometer (VERTEX 70, Bruker Co.). The DSC curves are characterized by a differential scanning calorimeter (Diamond DSC, PerkinElmer Instruments). A resonator (JZK, Sinocera) controlled by a signal generator (YE 1311-D, Sinocera) is utilized to periodically trigger the devices attached to a force meter (ZPS-DPU-50N, Imada Inc.). The output performance of the device is measured using a data acquisition card (PCI-6259, NI), a low-noise current preamplifier (Model SR570, Stanford) and a programmable electrometer (Keithley 6514). ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge at http://pubs.acs.org. Theoretical derivation for the OC voltage. Parameters used in the simulation. Recipe of PDMS at different mixing ratio. Digital picture of the steel templates. Detailed schematic diagram for the fabrication process of the piezoelectret film. Energy dispersed X-ray spectrums and Fourier transform infrared spectrums for APTES treatment. Peeling strength of APTES treated PDMS/THV and PDMS/COC composites. Schematic diagrams indicating the corona charging method and the contact charging method. Schematic diagrams indicating the open-circuit thermally stimulated discharge. The theoretical model of a piezoelectret

16


Page 16 of 28

Page 17 of 28 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


generator in the case of imbalanced charge distribution. Quenching process for THV films. Surface potential of positively and negatively polarized PI. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (51322210, 61434001, 51672097), the National Program for Support of Top-notch Young Professionals, the Program for HUST Academic Frontier Youth Team and Director Fund of WNLO. The authors thank the facility support of the Center for Nanoscale Characterization & Devices, WNLO-HUST and the Analysis and Testing Center of Huazhong University of Science and Technology. REFERENCES (1)

Gao, W.; Emaminejad, S.; Nyein, H. Y. Y.; Challa, S.; Chen, K.; Peck, A.; Fahad, H. M.; Ota, H.; Shiraki, H.; Kiriya, D.; Lien, D. H.; Brooks, G. A.; Davis, R. W.; Javey, A., Fully Integrated Wearable Sensor Arrays for Multiplexed in situ Perspiration Analysis. Nature 2016, 529, 509-514.

17



(2)

Son, D.; Lee, J.; Qiao, S.; Ghaffari, R.; Kim, J.; Lee, J. E.; Song, C.; Kim, S. J.; Lee, D. J.; Jun, S. W.; Yang, S.; Park, M.; Shin, J.; Do, K.; Lee, M.; Kang, K.; Hwang, C. S.; Lu, N.; Hyeon, T.; Kim, D. H., Multifunctional Wearable Devices for Diagnosis and Therapy of Movement Disorders. Nat. Nanotechnol. 2014, 9, 397-404.

(3)

Wu, W.; Wen, X.; Wang, Z. L., Taxel-Addressable Matrix of Vertical-Nanowire Piezotronic Transistors for Active and Adaptive Tactile Imaging. Sci. 2013, 340, 952-957.

(4)

Zhong, J.; Zhang, Y.; Zhong, Q.; Hu, Q.; Hu, B.; Wang, Z. L.; Zhou, J., Fiber-Based Generator for Wearable Electronics and Mobile Medication. ACS Nano 2014, 8, 6273-6280.

(5)

Schwartz, G.; Tee, B. C.; Mei, J.; Appleton, A. L.; Kim, D. H.; Wang, H.; Bao, Z., Flexible Polymer Transistors with High Pressure Sensitivity for Application in Electronic Skin and Health Monitoring. Nat. Commun. 2013, 4, 1859-1865.

(6)

Wang, S.; Xu, J.; Wang, W.; Wang, G. J. N.; Rastak, R.; Lopez, F. M.; Chung, J. W.; Niu, S.; Feig, V. R.; Lopez, J.; Lei, T.; Kwon, S. K.; Kim, Y.; Foudeh, A. M.; Ehrlich, A.; Gasperini, A.; Yun, Y.; Murmann, B.; Tok, J. B. H.; Bao, Z., Skin Electronics from Scalable Fabrication of an Intrinsically Stretchable Transistor Array. Nature 2018, 555, 83-88.

(7)

Zhang, Y.; Fang, Y.; Li, J.; Zhou, Q.; Xiao, Y.; Zhang, K.; Luo, B.; Zhou, J.; Hu, B., Dual-Mode Electronic Skin with Integrated Tactile Sensing and Visualized Injury Warning. ACS Appl. Mater. Interfaces 2017, 9, 37493-37500.

18


Page 18 of 28

Page 19 of 28 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


(8)

Bartolozzi, C.; Natale, L.; Nori, F.; Metta, G., Robots with a Sense of Touch. Nat. Mater. 2016, 15, 921-925.

(9)

Li, S.; Zhao, H.; Shepherd, R. F., Flexible and Stretchable Sensors for Fluidic Elastomer Actuated Soft Robots. MRS Bulletin 2017, 42, 138-142.

(10) Chen, L. Y.; Tee, B. C. K.; Chortos, A. L.; Schwartz, G.; Tse, V.; J. Lipomi, D.; Wong, H. S. P.; McConnell, M. V.; Bao, Z., Continuous Wireless Pressure Monitoring and Mapping with Ultra-Small Passive Sensors for Health Monitoring and Critical Care. Nat. Commun. 2014, 5, 5028-5038. (11) Wu, N.; Cheng, X.; Zhong, Q.; Zhong, J.; Li, W.; Wang, B.; Hu, B.; Zhou, J., Cellular Polypropylene Piezoelectret for Human Body Energy Harvesting and Health Monitoring. Adv. Funct. Mater. 2015, 25, 4788-4794. (12) Yang, P.; Lin, L.; Yi, F.; Li, X.; Pradel, K.; Zi, Y.; Wu, C.; He, J.; Zhang, Y.; Wang, Z. L., A Flexible, Stretchable and Shape-Adaptive Approach for Versatile Energy Conversion and Self-Powered Biomedical Monitoring. Adv. Mater. 2015, 27, 3817-3824. (13) Li, W.; Zhao, S.; Wu, N.; Zhong, J.; Wang, B.; Lin, S.; Chen, S.; Yuan, F.; Jiang, H.; Xiao, Y.; Hu, B.; Zhou, J., Sensitivity-Enhanced Wearable Active Voiceprint Sensor Based on Cellular Polypropylene Piezoelectret. ACS Appl. Mater. Interfaces 2017, 9, 23716-23722. (14) Tao, L.; Zhang, K.; Tian, H.; Liu, Y.; Wang, D.; Chen, Y.; Yang, Y.; Ren, T., Graphene-Paper Pressure Sensor for Detecting Human Motions. ACS Nano 2017, 11, 8790-8795. 19



(15) Wang, B.; Liu, C.; Xiao, Y.; Zhong, J.; Li, W.; Cheng, Y.; Hu, B.; Huang, L.; Zhou, J., Ultrasensitive Cellular Fluorocarbon Piezoelectret Pressure Sensor for Self-Powered Human Physiological Monitoring. Nano Energy 2017, 32, 42-49. (16) Mannsfeld, S. C. B.; Tee, B. C. K.; Stoltenberg, R. M.; Chen, C. V. H. H.; Barman, S.; Muir, B. V. O.; Sokolov, A. N.; Reese, C.; Bao, Z., Highly Sensitive Flexible Pressure Sensors with Microstructured Rubber Dielectric Layers. Nat. Mater. 2010, 9, 859-864. (17) Zhang, S. L.; Lai, Y. C.; He, X.; Liu, R.; Zi, Y.; Wang, Z. L., Auxetic Foam‐Based Contact‐Mode Triboelectric Nanogenerator with Highly Sensitive Self‐Powered Strain Sensing Capabilities to Monitor Human Body Movement. Adv. Funct. Mater. 2017, 27, 1606695-1606702. (18) Liu, S.; Wang, L.; Wang, Z.; Cai, Y.; Feng, X.; Qin, Y.; Wang, Z. L., Double-Channel Piezotronic Transistors for Highly Sensitive Pressure Sensing. ACS Nano 2018, 12, 1732-1738. (19) Lin, X.; Seet, B. C., A Linear Wide-Range Textile Pressure Sensor Integrally Embedded in Regular Fabric. IEEE Sensors Journal 2015, 15, 5384-5385. (20) Sharma, T.; Je, S. S.; Gill, B.; Zhang, J. X. J., Patterning Piezoelectric Thin Film PVDF–TrFE Based Pressure Sensor for Catheter Application. Sensors and Actuators A: Physical 2012, 177, 87-92. (21) Pan, C.; Dong, L.; Zhu, G.; Niu, S.; Yu, R.; Yang, Q.; Liu, Y.; Wang, Z. L., High-Resolution Electroluminescent Imaging of Pressure Distribution using a Piezoelectric Nanowire LED Array. Nature Photonics 2013, 7, 752-758.

20


Page 20 of 28

Page 21 of 28 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


(22) Persano, L.; Dagdeviren, C.; Su, Y.; Zhang, Y.; Girardo, S.; Pisignano, D.; Huang, Y.; Rogers, J., High Performance Piezoelectric Devices Based on Aligned Arrays of Nanofibers of Poly(vinylidenefluoride-co-trifluoroethylene). Nat. Commun. 2013, 4, 1633-1643. (23) Lin, Z.; Yang, J.; Li, X.; Wu, Y.; Wei, W.; Liu, J.; Chen, J.; Yang, J., Large‐Scale and Washable Smart Textiles Based on Triboelectric Nanogenerator Arrays for Self‐ Powered Sleeping Monitoring. Adv. Funct. Mater. 2018, 28, 1704112-1704119. (24) Yang, J.; Chen, J.; Su, Y.; Jing, Q.; Li, Z.; Yi, F.; Wen, X.; Wang, Z.; Wang, Z. L., Eardrum-Inspired

Active

Sensors

for

Self-Powered

Cardiovascular

System

Characterization and Throat-Attached Anti-Interference Voice Recognition. Adv. Mater. 2015, 27, 1316-1326. (25) Wang, X.; Zhang, H.; Dong, L.; Han, X.; Du, W.; Zhai, J.; Pan, C.; Wang, Z. L., Self-Powered High-Resolution and Pressure-Sensitive Triboelectric Sensor Matrix for Real-Time Tactile Mapping. Adv. Mater. 2016, 28, 2896-2903. (26) Tsai, J.; Wang, J.; Su, Y., Piezoelectric Rubber Films for Human Physiological Monitoring and Energy Harvesting. IEEE 26th International Conference on Micro Electro Mechanical Systems (MEMS) 2013, 841-844. (27) Chen, S.; Wu, N.; Ma, L.; Lin, S.; Yuan, F.; Xu, Z.; Li, W.; Wang, B.; Zhou, J., Noncontact Heartbeat and Respiration Monitoring Based on a Hollow Microstructured Self-Powered Pressure Sensor. ACS Appl. Mater. Interfaces 2018, 10, 3660-3667.

21



(28) McKay, M. J.; Baldwin, J. N.; Ferreira, P.; Simic, M.; Vanicek, N.; Wojciechowski, E.; Mudge, A.; Burns, J., Spatiotemporal and Plantar Pressure Patterns of 1000 Healthy Individuals Aged 3–101 Years. Gait & Posture 2017, 58, 78-87. (29) Zhang, X.; Sessler, G. M.; Xue, Y.; Ma, X., Audio and Ultrasonic Responses of Laminated Fluoroethylenepropylene and Porous Polytetrafluoroethylene Films with Different Charge Distributions. Journal of Physics D Applied Physics 2016, 49, 205502-205510. (30) Wang, J.; Hsu, T.; Yeh, C.; Tsai, J.; Su, Y., Piezoelectric Polydimethylsiloxane Films for MEMS Transducers. J. Micromech. Microeng. 2012, 22, 015013-015022. (31) Li, W.; Wu, N.; Zhong, J.; Zhong, Q.; Zhao, S.; Wang, B.; Cheng, X.; Li, S.; Liu, K.; Hu, B.; Zhou, J., Theoretical Study of Cellular Piezoelectret Generators. Adv. Funct. Mater. 2016, 26, 1964-1974. (32) Pope, M.; Swenberg, C. E., Electronic Processes in Organic Crystals and Polymers. 2nd ed.; Oxford University Press, Oxford, 1999. (33) Zhong, Q.; Zhong, J.; Cheng, X.; Yao, X.; Wang, B.; Li, W.; Wu, N.; Liu, K.; Hu, B.; Zhou, J., Paper-Based Active Tactile Sensor Array. Adv. Mater. 2015, 27, 7130-7136.

22


Page 22 of 28

Page 23 of 28 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


Figure 1. Fabrication of the THV/COC piezoelectret nanogenerator. (a) Schematic diagram indicating the fabrication process of the piezoelectret film. (b) Digital picture of the flexible piezoelectret film. Inset shows the cross-section view SEM image of the braced structure. (c) Surface potential of THV and COC materials. (d) Open-circuit thermally stimulated discharge (TSD) current curve of THV/COC piezoelectret.

23



Figure 2. Material selection for the piezoelectret nanogenerator. Simulation results of open-circuit voltage at (a) different surface charge densities and (b) corresponding electric fields. V0, E0 and M0 are all constants for qualitative simulation. (c) Enthalpy change of natural cooling and quenching THV. (d) Normalized surface potential of natural cooling and quenching THV and the potential promotion for the quenching method. (e) Surface potential comparison of THV, PI and COC. (f) Experimental results of open-circuit voltage for piezoelectret nanogenerators utilizing different electret materials.

24


Page 24 of 28

Page 25 of 28 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


Figure 3. Structure optimization for the piezoelectret nanogenerator. Simulation results of open-circuit voltage at (a) different deformations and (b) corresponding Young’s modules. V0’, Y0 and ∆h0 are all constants for qualitative simulation. Pressure-deformation rate curves of piezoelectret with (c) different PDMS mixing ratio and (d) different braced structure. Pressure responses of piezoelectret nanogenerators with (e) different PDMS mixing ratio and (f) different braced structure.

25



Figure 4. Sensing property of the THV/COC piezoelectret nanogenerator. (a) Frequency response, (b) pressure response, (c) uniformity and (d) stability of the THV/COC piezoelectret nanogenerator.

26


Page 26 of 28

Page 27 of 28 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


Figure 5. Wide-range pressure sensing based on the THV/COC piezoelectret nanogenerator. (a) Pressure sensing configuration and circuit connection for the THV/COC piezoelectret nanogenerator. Stimulating force (measured by a commercial forcemeter) and measured pressure (measured by the THV/COC piezoelectret nanogenerator) of (b) index finger typing, (c) thumb pressing and (d) fist hammering. (Sensor area: 1 cm2)

27



TOC

28


Page 28 of 28

COC Piezoelectret Nanogenerator for Wide-range

Recommend Documents