Boosting Sensing Performance of Flexible Piezoelectric Pressure Sensors by Sb Nanosheets and BaTiO3 Nanoparticles Co‐Doping in P(VDF‐TrFE) Nanofibers Mat

Flexible piezoelectric pressure sensors (FPPS) received a lot of attention because of their prominent performance in body motion detection and energy recovery. However, it has faced a huge challenge in manufacturing FPPS with high sensitivity, low cost, and high flexibility. Herein, a ternary flexible nanofiber composite with Sb nanosheets and BaTiO3 nanoparticles as the fillers of P(VDF‐TrFE) is prepared by electrospinning. An impressive open‐circuit voltage of 17.1 V and short‐circuit current of 4.4 µA is obtained based on the composite nanofiber mat at a pressure of 128 kPa with 2 Hz frequency. The FPPS not only exhibit a high sensitivity of 96 mV kPa−1 but also an ultrafast response time of 2 ms. Excellent flexibility and reliability are demonstrated by the unchanged open‐circuit voltage after 2400 cycles. The enhanced FPPS performance is owed to the interfacial polarization effect at the inorganic filler‐organic matrix interface, which also enhances the ferroelectric and dielectric properties. The present FPPS is further confirmed to be a real‐time motion monitor and voice recognizer, which can distinguish various actions and sounds according to distinct output voltage signals and is expected to be used in virtual reality devices, robotic electronic skin, haptic simulation, artificial throat, etc.


Introduction
Recently, various high-performance flexible pressure sensors with nano-scale precision structures have been designed and DOI: 10.1002/aelm.202300718fabricated one after another. [1]The current mainstream flexible pressure sensors can be categorized by capacitive sensors, [2] piezoresistive sensors, [3] and piezoelectric sensors according to their functional characteristics. [4]arious pressure sensors are widely used in integrated wearable devices, robotic haptics, and smart clothing.However, high flexibility, good adhesion, and high sensing sensitivity is still the main challenge for the flexible pressure sensor. [5]Flexible piezoelectric pressure sensors (FPPS) have been broadly applied to the fields of wearable wireless electronic devices, health testing, etc. because of their self-powered, low-cost, and simple structure. [6]arious piezoelectric materials were prepared and applied in FPPS.[9] And in order to achieve a good skin-fitting properties, recently, polyvinylidene fluoride (PVDF) and its copolymers have found a large number of applications in self-powered wearable devices, electronic integrated devices, etc. [10,11] PVDF-based polymers have five phases of crystallization, , , , , and , among which the  phase has an all-trans structure (TTTT), while the H and F atoms on the chain segment show a 180°flip to form a dipole, and the external pressure causes a change in the dipole moment, thus exhibiting the maximum piezoelectric properties. [12]Therefore, it is important to enhance the piezoelectric output for enlarging the self-powered efficiency by increasing the -phase content.15][16] 2D conductive materials are some of the most promising fillers for improving the piezoelectric behavior of the ferroelectric polymer due to their broad relative surface area and outstanding physical strength as well as high carrier mobility. [16,17]Sb nanosheets (SbNSs) are a single-atom 2D material whose structure is viewed as a "Z" on the side and hexagonal on the top. [18]heoretical calculations on a set of 2D semiconductors predicted that the independent Sb monolayer structure maintains high carrier mobility and a direct band gap. [19]The introduction of SbNSs into the ferroelectric polymers is expected to strengthen the dielectric characteristics of composites owing to the interfacial polarization effect.Furthermore, the conductive nature of the SbNSs also helps to construct a 3D network in the matrix of poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] and form a conductive path, which will speed up the transport of the generated piezoelectric charges and thus enhancing the sensing and self-powered performance of the sensors.To the best of our knowledge, there are no reports on the integration of BTO nanoparticles and co-doping of Sb-NSs in piezoelectric nanocomposite fibers, which the combined advantages of the components in the nanocomposites are hoped to play an essential role to enhance the performance of FPPS.
In the present work, we prepared SbNSs/BTO/P(VDF-TrFE) ternary flexible composite nanofiber films by electrostatic spinning.It was found that the co-doping of BTO and SbNSs resulted in a significant enhancement of the polar -phase content of the polymer; it also synergistically enhanced the piezoelectric output of the composites as well as the sensitivity of the FPPS.A voltage of ≈17.1 V can be generated at 128 kPa, 2 Hz external pressure application, with a sensitivity of 96 mV kPa −1 and an ultrafast response time of 2 ms.It also exhibits different signals under different applied pressure for recognizing human motion and sounds such as finger bending, chewing, and others.Besides, the FPPS has potential applications in virtual re-ality devices, robotic electronic skin, haptic simulation, artificial throat, etc.

Synthesis of Sb Nanosheets
SbNSs were prepared by the solution phase synthesis method, [20] as shown in Figure 1a.In this process, 0.389 g of octylphosphonic acid (OPA) and 0.137 g of SbCl 3 were added to a beaker and reacted at 90 °C in air until the color of the solvent changed to brown to obtain the SbCl 3 -OPA precursor.Immediately afterward, 4.5 ml oleylamine (OA) was added into the flask.The reaction complex was deflated for 30 min at 110 °C under vacuum and purged with argon.The flask was then heated to 300 °C and the SbCl 3 -DDT precursor solution was rapidly injected into the reaction system.After 10 s of reaction, the flask was spontaneously cooled to room temperature, and then the final SbNS was obtained by centrifugation at 8000 rpm for 5 min and washed with chloroform.

Preparation of SbNSs/BTO/P(VDF-TrFE) Electrostatic Spinning Fibers
A mixture of DMF and acetone solvent (3:2, m/m) was used to prepare the electrospinning solution.First, disperse the P(VDF-TrFE) power in the mixed solvent with stirring for 2 h.Disperse the BTO nanoparticles and add SbNSs in the mixed solvent with stirring for 12 h.In the P(VDF-TrFE)/BTO/SbNSs (SBP) series solution, the content of the addictive BTO nanoparticles was fixed at 15wt.%, while that of SBNSs was set at 0.00, 0.05, 0.2, 0.5, and 1wt.%, respectively.Correspondingly, the pure P(VDF-TrFE) solution and the BTO/P(VDF-TrFE) solution were made as comparative experiments.In the BTO/P(VDF-TrFE) series, the proportion of BTO nanoparticles in the additive was selected at 5, 10, 15, 20, and 25wt.%,respectively, where no SbNSs were added.These series solutions were prepared in a similar way as P(VDF-TrFE)/BTO/SbNSs solution with a fixed SbNSs concentration of 20wt.%.
The electrostatic spinning process involves filling a 10 ml syringe with the prepared solution and then injecting 1 mL through a syringe pump at a flow rate of 0.3 mL h −1 into a needle with a diameter of 0.63 mm.Electrostatic spinning was carried out at a voltage of 15 kV with a distance of 10 cm between the tip of the needle and the roll collector covered with aluminum foil.All fibers were collected at a roll speed of 1500 rpm for 3 h.

Flexible Piezoelectric Pressure Sensor (FPPS) Fabrication
To fabricate the FPPS, an electrostatically spun fiber film was cut to a square shape with the size of 2.5 × 2.5 cm 2 and then sandwiched between the top and bottom aluminum foil electrodes.The copper tape was then affixed to the electrodes and wrapped inside the outermost polyimide (PI) film.Finally, the sandwiched FPPS pieces were laminated with a certain pressure and pressed tightly to avoid creating voids that cause frictional electrical effects and it also prevents the FPPS from being damaged by the surrounding environment.

Measurement of Characteristics and Electrical Outputs
A field emission scanning electron microscope (FESEM, Sigma 500, Carl, Germany) was used to observe the microscopic morphology of the nanofibers.Energy Dispersive Spectroscopy (EDS, Sigma 500, Carl, Germany) was used to demonstrate the presence of BTO and SbNSs in the composite nanofibers.A transmission electron microscope (G20, FEI, USA), and atomic force microscopy (AFM, MFP-3D, Oxford Instruments) were used to observe the surface morphology of a single fiber, the morphology of SbNSs, surface potential analysis, and microarea conductivity measurements of a single fiber, respectively.The crystal structure of the composite fibers was analyzed by X-ray diffraction (D8 Advance, Bruker) and Fourier transform infrared spectroscopy (FTIR, Nicolet iS50, USA).The ferroelectric material test system (Precision Mutiferroic II, Radiant, USA) was used to test the polarization-electric field (P-E) loops of the material and to analyze the electrical properties.

Morphological Analysis
The obtained SbNSs were hexagonal and triangular nanosheets of different sizes, as shown in the SEM image in Figure 1b.
The thickness of both the triangular SbNSs and hexagonal Sb-NSs was determined to be ≈10 nm by using AFM, as shown in Figure S1 (Supporting Information) in the supporting information.Figure 1c shows the schematic diagram of the electrostatic spinning process of the composite fibers, and the top right inset is an optical photograph of the spun fiber membrane.shows the TEM image of an individual SBP fiber, the nanoparticles agglomerate in small amounts and occasionally have nanoclusters distributed on the fiber surface.The hexagonal SbNSs and BTO spherical nanoparticles can be clearly distinguished in morphology and size.The BTO nanoparticles and SbNSs were identifiable and uniformly distributed in P(VDF-TrFE) and a large portion of them were embedded in the fibers and spread homogeneously along the fiber axis.Figure 1e,f shows the microscopic morphology of P(VDF-TrFE) and SBP composite fibers.One may note that the undoped P(VDF-TrFE) nanofibers have a smoother surface compared with that of the SBP fibers.Protrusions caused by nanofillers could be observed on the fiber surface.When the nanofiller fills the interior of the fiber, it causes the nanofiber diameter to become larger, which corresponds to the protrusion and unsmooth feature of the fiber surface in Figure 1d.As shown in the images of Figure S1e,f (Supporting Information), the fibers showed a radially compliant arrangement, and the diameters were statistic to be ≈250 nm for P(VDF-TrFE) and ≈150 nm for the SBP composite fibers.The reduction of fiber diameter was due to the improved electrical conductivity of the fibers with the addition of conductive SbNSs, which would be subjected to more coulombic forces, thus achieving a reduction in fiber diameter. [21]EDS mappings of SBP composite fibers are shown in Figures S2,S3 (Supporting Information).Comparing the distribution of F, Ba, Ti, and Sb elements in the spun fiber membrane, it can be found that F elements are mainly present in P(VDF-TrFE) fibers, and Ba and Ti elements are uniformly distributed in the fiber membrane at a macroscopic level and locally produce agglomerations.

Crystal Structure and Electrical Properties of the Nanofibers
Figure 2a shows the XRD patterns for different doping cases.By co-doping of BTO nanoparticles and SbNSs, distinct diffraction peaks appear at 18.3 °, 19.9 °, and 26.6 °, corresponding to the reflections of (020), ( 110) and (021) for -phase P(VDF-TrFE).Moreover, the peak intensities related to the reflections of (200) for -phase at 20.4 °are greatly enhanced with the doping of inorganic nanofiller, indicating the increased content of the -phase. [22,23]Without nanofillers, the diffraction pattern of P(VDF-TrFE) film is relatively smooth, and the peak intensities of  and -phase crystallization peaks are relatively low.One may also note that there are two additional peaks at ≈22.2 °, corresponding to the (001) and (100) reflections from the ferroelectric tetragonal BTO nanoparticles. [24]he peak at ≈28.7°corresponding to the (012) reflection from the SbNSs (JCPDS No.35-0732), confirming the presence of SbNSs in the fibers.-phase is believed to be generated from the stretching of the electric field and in situ polarization during the electrostatic spinning process.Furthermore, the nucleating agent nanofillers also speed up the polymer crystallization and the formation of the polar phase.
The results of the IR spectroscopy tests are presented in Figure 2b.The vibrational bands at 763 and 976 cm −1 belong to the unpolarized -phase, while the characteristic peaks at 841 and 1276 cm −1 belong to the electroactive -phase.Note that the negligible peak at 1234 cm −1 (-phase) indicates that the sample is mainly composed of -phase and -phase.The relative fraction Responsive curves when wearer spoke g) "WOW"; h) "Sensor"; i) "Qianjin"; j) "Houtui"; k) Photograph of FPPS attached to the toy; l) Responsive curves when toy works.
of -phase in the electrostatically spun fibers was calculated according to the Formula (1), where A  and A  are the absorbance at 841 and 763 cm −1 , respectively, and K  and K  are 6.1 × 10 4 and 7.7 × 10 4 cm 2 mol −1 , respectively. [25]Figure 2c shows the -phase content in the composite fibers with inorganic nanofillers, especially with is significantly higher than that in the pure P(VDF-TrFE) fibers, and the -phase content varied with the increase of SbNSs, reaching a maximum of 80.75% at 0.2wt.% of SbNSs doping and 15wt.% of BTO doping.This result further supports the idea that the nanofiller acts as a nucleating agent in the P(VDF-TrFE) nanofibers.The -phase crystallization of P(VDF-TrFE) was inhibited with the further SbNSs doping because the excessive addition of nanofillers led to the aggregation of PVDF chains, [26] which restricted the chain segment movement and thus prohibiting the -phase nucleation.
[28] Figure 2d shows the mechanism diagram of BTO nanoparticles and SbNSs assisting P(VDF-TrFE) nanocomposite fibers to form -phase.The F atoms in the -CH 2 -CF 2 dipole at the BTO/P(VDF-TrFE) interface have a tendency to approach the BTO nanoparticles due to hydrogen bonding interactions between the hydroxyl H atoms on the BTO surface. [29]Furthermore, with the addition of SbNSs, the additional electrostatic interactions arising from the electronegativity difference between Sb-NSs and H-atoms attract H-atoms to approach SbNSs and promote the formation of -phase. [30]Thus, the co-doping of BTO nanoparticles and SbNSs increases the content of the polar phase of the polymer and further enhances the piezoelectric performance of the composites.This has been proved by the XRD patterns and IR spectra results.
The higher -phase content the composite contains, the better ferroelectric property it expects.The P-E loop of the SBP fibers film resembles that of a typical ferroelectric, as shown in Figure 3a, the figure shows that maximum polarization and remnant polarization (P r ) increase along with the amount of SbNSs filler until loading up to 0.2wt.%and are followed by a notable decrease at 0.5wt.%loading of SbNSs in the nanofibers film.The hysteresis loop becomes more enlightened in the polarization direction reaching a maximum at 0.2wt.%SbNSs incorporation and P r increased up to 1.54 from 0.35 μC cm −2 of pristine P (VDF-TrFE).On the one hand, these results may be attributed to the increased formation of the electroactive -phase after the incorporation of BTO nanoparticles and SbNS in the composites.On the other hand, the organic-inorganic interface formed due to the addition of inorganic fillers leads to the accumulation of space charges on the 2D conducting material, which results in interfacial polarization and leads to a higher polarization rate.This accumulation of space charges at the interface can be expressed using the change in dielectric constant. [24]Figure 3b,c demonstrates the change in the dielectric constant and dielectric loss of the composites.Starting from 10 2 Hz, the dielectric constant decreases with increasing frequency, showing a typical frequencydependent property as traditional ferroelectrics.However, with further doping of SbNSs, the dielectric constant shows a decreasing trend, which is probably due to the conductive nature of the SbNSs nanosheets, which could generate conductive pathways and also increase the dielectric loss, as shown in Figure 3c. [23,31]igure 3d,e shows the PFM amplitude and phase curves of P(VDF-TrFE), BTO/P(VDF-TrFE) and SBP fibers.The piezoelectric amplitudes of all three fibers exhibit good butterfly lines, and the effective piezoelectric coefficient d 33,eff was calculated from the butterfly lines as d 33,eff = u/V, where V is the applied AC voltage that induces the piezoelectric vibration, and u is the maximum amplitude value of the nanofibers. [32]Notice that the d 33,eff of the nanofiber calculated from the butterfly loop is gradually increased from ≈4.82 pm V −1 for P(VDF-TrFE) to ≈13.34 pm V −1 for SBP fibers, confirming the enhanced piezoelectric property of the inorganic filler doped nanofibers. [33]The shift in the center of the butterfly curve of the SBP fiber indicates that the charges accumulated at the interface generate an internal electric field, which could enhance the polarization value.The piezoelectric phase curves also exhibit 180 o reversal confirming the switching ability of the nanofibers.The transportation properties of the SBP, BTO/P(VDF-TrFE), and SbNSs/P(VDF-TrFE) composite nanofiber doping with 0.2wt.%SbNSs were measured by CAFM to expose the fiber's internal SbNSs function.As shown in Figure 3f, the nanofibers incorporating with SbNSs alone exhibited higher leakage current due to the high conductivity of SbNSs.

Pressure Sensing Performance
The structure of FPPS is shown in Figure 4a. Figure 4b,c shows the open-circuit voltage (V oc ) and short-circuit current (I sc ) of the FPPS with different SbNSs content.The BTO content has been optimized (as summarized in Figure S4, Supporting Information) and has been set to be 15wt.% in the SbNSs/BTO/P(VDF-TrFE) nanofibers.As the content of the SbNSs increases, both V oc and I sc exhibit an increasing and then decreasing trend, reaching peaks of 17.1 V and 4.4 μA at the SbNSs and BTO contents of 0.2wt.%and 15wt.%,respectively.It can be clearly seen that when the working frequency remains constant, V oc is proportional to the applied force.The output performances of the as-prepared FPPS show the identical variation trends with the ferroelectric, and piezoelectric characteristics of composite fibers.According to the output mechanism of the FPPS, as shown in Figure S5 (Supporting Information), in the pristine state, the internal polarization of the material is not modulated, the dipoles are randomly arranged and the positive and negative charge centers overlap.There is no piezoelectric charge in the composites (Figure S5 stage i, Supporting Information).After polarization, the dipoles reach a coherent arrangement driven by the electric field so that a large number of positive and negative charges accumulate on the upper and lower surfaces of the material (Figure S5 stage ii, Supporting Information), respectively, and the device remains in a balanced state.When external pressure is applied to the FFPS, the positive and negative charge centers of the dipoles inside the piezoelectric material are displaced, resulting in an increase in the dipole moment and a piezoelectric potential on the upper and lower surfaces.Free electrons at the electrodes flow in response to the potential to counteract the external potential difference.Therefore, the pulsed voltage and current signals are generated in the closed circuit (Figure S5 stage iii, Supporting Information).When the external pressure is withdrawn, the dipole deforms back to its state before the application of the force forming a reverse piezoelectric potential and leading to a reverse flow of carriers.As a result, the device generates periodic AC pulses under stress.A widely used "switching polarity" test method was applied to confirm that the output signals arise from the piezoelectric effect.As shown in Figure 4d, the reversible voltages were observed by reversely connecting the device, indicating that the pulse signals are purely generated by the piezoelectric effect.While the imbalance in the electrical signals of the positive and negative charge outputs is due to the positive and negative charge flux differences. [34]s displayed in Figure 4e, the response and recovery time are 2 and 11 ms with 128 kPa applied pressure in a frequency of 2 Hz in the FPPS with 2wt.%SbNSs/15wt.%BTO.More importantly, the response time of the as-prepared FPPSs has been greatly enhanced by SbNSs doping, as shown in Figure S6 (Supporting Information).This may be due to the conductive network constructed by SbNSs in the ferroelectric polymer matrix, which might speed up the transportation of the inductive charges.
These experimental results show that the FPPS can be used as a pressure sensor when voltage response is used as a detecting signal.The variation of device output with applied stress from 0-128 kPa is shown in Figure 4f,g.The output voltages and currents increase with the increase of the applied stress.The improvement in output performance is mainly due to the strong deformation of the composite film under higher stresses.For FPPS, sensitivity is an important parameter.The pressure sensitivity is calculated using the following Equation ( 2): Where U is the open circuit voltage and  is the applied force.Different slopes were obtained by linearly fitting the positive peak value of V oc to the applied force.The effect of different SbNSs doping on the sensitivity is shown in Figure S7 (Supporting Information) and Figure 4h.A maximum S value of 96 mV kPa −1 is achieved in the medium external pressure range (0-128 kPa), together with the response time of the FPPS suggesting an ultrafast responsiveness and high sensitivity compared to other piezoelectric sensors of the same type as shown in Table S1 (Supporting Information).The variation of the piezoelectric output signals shows the variation trends with that of the ferroelectric and dielectric properties, which shows an optimized BTO and SbNSs contents of 15 and 0.2wt.%,respectively.The enhanced piezoelectric output mechanism can be illustrated in the schematic shown in Figure 5.
First of all, the electrospinning process for the nanofibers increases the crystalline as well as the content of the polar -phase of the P(VDF-TrFE) polymer due to the stretching effect of the electric field during electrospinning and small change in dipole moment inside the fiber when pressure is applied, as described in Figure 5(I).Second, the introduction of the inorganic piezoelectric BTO and SbSNs further enhances the content of the polar -phase.The boosting crystalline nature and -phase content of the P(VDF-TrFE) polymer will inevitably enhance the piezoelectric as well as the dielectric properties of the composites, as demonstrated in Figure 3. Except the intrinsic orientation polarization arising from both BTO and P(VDF-TrFE), the interfacial polarization due to the accumulation of space charge at the interfaces between the inorganic fillers and the P(VDF-TrFE) polymer is also expected to contribute to the enhanced electrical outputs. [35]The inorganic BTO particles with bound charges due to the spontaneous polarization on both sides can enhance the organization of dipole moments in P(VDF-TrFE), which will expand directed polarization.Furthermore, the introduction of conductive single-layered SbSNs also results in the accumula-tion of induced charges around dipoles with high mobility in both P(VDF-TrFE) matrix and BTO nanoparticles, inducing additional interfacial polarization between interfaces such as BTO-SbSNs and SbSNs-P(VDF-TrFE) interfaces, [36] as depicted in Figure 5(II).Both boosted intrinsic and interfacial polarization then enhance the electrical outputs.
The increase of the inductive charges was also confirmed by the increased surface potential of the composites with inorganic fillers, as shown in Figure 6d,f.Corresponding AFM images of the fiber surface are shown in Figure 6a-c, as well as the protruding on the surface of the fiber in Figure 6b,c illustrate the existence of filler inside the fibers, which is consistent with the results in Figure 1f. Figure 6g-i shows the cross-sectional profiles of the surface potential distributions for PVDF-TrFE, BTO/PVDF-TrFE, and SBP fibers, respectively.With the doping of BTO and SbNSs, the negative surface potential increases from −164 to −1400 mV compared with that of composites without any doping.Furthermore, the conductive nature of the SbNSs may also benefit for constructing a 3D network in the matrix of P(VDF-TrFE) and forming a conductive path, which will speed up the transport of the inductive charges and thus enhance the sensing and self-powered performance of the sensors.In contrast, in the composite nanofibers containing more SbNSs (> 0.2wt.%), the aggregation of the nanoparticles and nanosheets can weaken or even hinder the crystallization in P (VDF-TrFE) electrospinning nanofibers, and shown in Figure 6h and Figure S8 (Supporting Information), when the filler breaks through the fiber surface, the charge at the interface between the filler and the P(VDF-TrFE) matrix will be dissipated and caused a decrease in the negative surface potential.This also explains why the increase of filler leads to an increase in dielectric loss and a decrease in piezoelectric performance.In addition, aggregation of SbNSs can also gather and trap charges, and the opposite inductive charges neutralize with each other, reducing the inductive charges on the outside surfaces of the composites, thus decreasing the electrical outputs.
To further explore the overall sensing output capability of the FPPS, we measured the output power at different external loads (10 6 -10 8 Ω), and the measurement circuit is shown in Figure 7a.The peak power (P) transferred to the load R can be assessed as Equation (3).
where R is the external load imposed as impedance and I (t) is the average value of the peak current on the load. [34]The plots of current/voltage outputs changing with the load resistance are shown in Figure 7b.As the load resistance increases, the output voltage increases from 1.6 to 15.1 V, while the current shows a decreasing trend from 4.95 to 1.72 μA.As shown in Figure 7c, the maximum peak power reaches 8.12 μW with the external resistance at 80 MΩ (a peak power density of 1.3 μW cm −2 ).Table S2 (Supporting Information) summarizes the piezoelectric output of SBP fibers FPPS compared to previous reports on PVDF-based piezoelectric materials.It is noted that the electrical output performance of this work, especially the power density, exhibits relatively high levels in most PVDF-based FPPSs.The cycling stability of the electrical output performance of the asprepared devices is demonstrated in Figure 7d.The outputs are stable within 2400 bending-releasing cycles, proving good durability and stability of the present FPPS.The charging capability of the FPPS is analyzed by charging the commercial capacitors of 33 and 47 F using a rectifier (Figure 7e,f).The rectifier can convert the AC signal generated by the FPPS into a DC signal in the same direction.During charging the capacitor, a gradual increase in the charging voltage of the capacitor can be observed.The inset graph clearly shows the gradual increase of the charging voltage with the charging time, which indicates the completion of the self-powered charging of the sensor and the good energy storage capacity of the capacitor.Finally, the potential applications of FPPS as the real human motion signal detection were also studied.In order to utilize the high sensitivity and ultra-fast response ability, the FPPS was used as a voice recognizer which should be very sensitive and fast response to the throat variation during talking. [10,37,38]The output signals are shown in Figure 8g-j.For capturing the throat variation during speech, the pressure sensor was attached onto the neck by a polyurethane film dressing (set in Figure 8g).The FPPS shows good recognition when the tester said different words such as "WOW", "Sensor", "Qianjin" and "Houtui" respectively.In addition, as shown in Figure R5 and Figure 8k,l, FPPS combined with toys can recognize the signal of a toy beating a drum.The output waveforms for various words can be well recognized, showing the potential application in the sensing of the slight mechanical vibration.

Conclusion
In summary, we utilized the synergistic enhancement effect of BTO nanoparticles and SbNSs on the piezoelectric properties of electrospinning P(VDF-TrFE) nanocomposite fibers to successfully prepare a novel flexible and highly sensitive FPPS.Electrostatic interactions between inorganic fillers and polymers effectively increase the content of P(VDF-TrFE) polar -phase in the nanocomposite fibers.The piezoelectric output of FPPS was significantly enhanced by the addition of BTO nanoparticles and Sb-NSs, and the open-circuit voltage and the short-circuit current of the nanocomposite fiber FPPS with 0.2wt.%SbNSs and 15wt.%BTO can reach 17.1 V and 4.4 μA under the pressure of 128 kPa at 2 Hz, with a sensitivity of up to 96 mV kPa −1 .The obtained FPPS exhibits excellent flexibility and reliability, and no significant decrease in the open-circuit voltage was observed after 2400 cycles.After filling with nanofillers, more polarized interfaces were generated, which enhanced the interfacial polarization effect, and creation of new dipoles, and resulted in an increase in the dielectric constant.The enhanced piezoelectric and dielectric properties synergistically affect the piezoelectric output of the fibers at the macroscopic and microscopic scales by the organic-inorganic structure.It also exhibits different signal characteristics under different force application conditions to achieve recognition of real motion monitoring and has potential for application in virtual reality devices, robotic electronic skin, haptic simulation, etc.

Figure 1 .
Figure 1.a) Schematic diagram of the synthesis of SbNSs; b) SEM image of SbNSs; c) Schematic diagram of electrospinning SbNSs/BTO/P(VDF-TrFE) (SBP) composite fibers and optical images of the fiber film (inset); d) TEM image of SBP composite fibers; e,f) SEM images of PVDF-TrFE and SBP composite fibers, respectively.

Figure 2 .
Figure 2. a) XRD patterns and b) IR spectra of electrospinning SBP composite fibers with different doping cases; c) Variation of -phase content fraction with SbNSs doping contents; d) Simplified diagram of the mechanism of -phase forming by co-doping of BTO nanoparticles and SbNSs in nanocomposite fibers.

Figure 3 .
Figure 3. a) Room temperature P-E hysteresis loops; b) dielectric constants; c) dielectric losses of electrospinning SBP composite fibers with different doping cases; d) PFM amplitude; e) PFM phase; f) CAFM I-V curves of electrospinning SBP composite fibers with different doping cases.

Figure 4 .
Figure 4. a) Schematic diagram of FPPS device structure; b) and c) The output voltages and short circuit currents of SBP composite fibers with various doping contents as a function of dynamic compressive pressure at 2 Hz; d) Piezoelectric signals from the forward and reverse connected FPPS with 2wt.%SbNSs/15wt.%BTO at 64 kPa and 2 Hz; e) Response and recovery time of the FPPS with 2wt.%SbNSs/15wt.%BTO at 128 kPa and 2 Hz; f) and g) Time-dependent open-circuit voltages and currents of FPPS with 2wt.%SbNSs/15wt.%BTO under various pressures at 2 Hz; h) Output voltages and currents with different applied pressure.

Figure 5 .
Figure 5. Schematic illustration of the formation mechanism of interfacial polarization in the fibers.

Figure 7 .
Figure 7. a) Illustration of the rectifying bridge circuit and LED lighting schematic; b) Voltages and currents variation with different resistors; c) Peak power of the SBP fibers FPPS with respect to the load resistances ranging from 10 6 Ω to 10 8 Ω. d) Output stability of FPPS with 0.2wt.%SbNSs and 15wt.%BTOdoped in (2 Hz, 128 kPa), inset: amplifying view of V oc from 890 s to 895 s; e) rectified voltage signals under 128 kPa, 2 Hz applied pressure; f) Charging curves of 33 and 47 F commercial capacitors.
Figure 8a-f shows the sensing signals of different body motions such as finger bending, clicking, walking, arm bending, and chewing.It can be found that different motions exhibit signals of different amplitudes and waveforms, which demonstrates the recognition of the corresponding motions.The results reveal an appealing possibility of the SBP fibers FPPS in the applications of body motion recognition devices and human/machine interaction interface.