A novel self-powered sensitive porous ZnO NWs/PDMS sponge capacitive pressure sensor

The demand for flexible sensors has increased due to the explosive expansion of flexible electronic applications, but there are concerns that the development of high-precision measurement is limited. Capacitive pressure sensors are extensively utilized among diverse sensors due to their simple structure, high resolution, and outstanding dynamic response characteristics. Therefore, by sandwiching a porous ZnO NWs PDMS (PZP) sponge dielectric layer between two ITO/PET electrodes, a flexible capacitive pressure sensor with high sensitivity, short response time, and wide working range was prepared. The porous PDMS sponge was prepared utilizing the salt particle as a template, then ZnO NWs in different precise proportions were added to it. The sensitivity of PZP sponge capacitive pressure sensors with different salt particle sizes and ZnO NWs dosages were compared. The research results indicate that when salt particle sizes less than 150 μm, and the amount of ZnO NWs in PZP sponge is increased to 200 mg, the sensor sensitivity is highest in the pressure range of 2.5-7.5 kPa, reaching 0.1279 kPa-1, response time less than 70 ms. When the PZP sponge sensor is used as a self-powered source, it achieves the best output of 1.2 V and 6.8 nA at 5.74 kPa pressure. It provides a new way to tackle the challenge of low sensitivity of flexible sensors to capacitive signal and lays a foundation for expanding the application scenarios of ZnO NWs.


Introduction
Because of its straightforward design, excellent dynamic response, high temperature and radiation resistance, high resolution, and simple construction, capacitive pressure sensors are widely employed in measurements of liquid level, vibration, pressure, displacement, acceleration, and other parameters [1][2][3].In recent years, flexible electronic applications have developed rapidly, and the demand for cost-effective capacitive pressure sensors to tackle the challenge of detecting the input of flexible electronic devices has exploded.Flexible electronic devices are often constructed on flexible polymer substrates or elastomers such as polyvinyl alcohol (PVA), polyester (PET), polyimide (PI), polyethylene naphthalene dimethyl ester (PEN), polydimethylsiloxane (PDMS), and others.As one of the most widely used elastic body materials, PDMS is extensively utilized in wearable devices, bionic materials, flexible electronic devices, and so on [4][5][6][7][8][9].PDMS has unique advantages, researchers usually design new physical structures or add fillers to optimize its mechanical properties, such as filling carbon nanotubes [10], graphite particles [11], silica nanoparticles [12], forming internal microporous structures in the PDMS film itself.Recently, porous PDMS have been widely used in flexible sensors [13], photocatalysis [14], and friction electric nanogenerators [15] because of their relatively large surface area, low density, outstanding water repellence, and ability to compress, they are highly valued.
Tan added highly conductive silver nanowires (AgNWs) to a PDMS sponge to prepare a highly sensitive and flexible capacitive pressure sensor with the highest sensitivity of 0.62 kPa -1 in 0-1 kPa [16].Tang incorporated calcium copper titanate (CCTO) into PDMS to improve the elastomer's dielectric coefficient, built an ultrasensitive capacitive sensor, and confirmed the modeling of the elastomer aperture and volume, as well as stress and strain, through numerical and experimental constructions [17].Masihi used HNO3 to make porous PDMS pressure sensors and discovered the best curing temperature, acid concentration, and PDMS manufacturing ratio [18].Pruvost presented a capacitive sensor based on a composite foam material filled with conductive carbon black particles with a sensitivity more than 35 kPa -1 when the pressure is less than 0.2 kPa [19].
Known 1D ZnO semiconductor nanostructures, such as nanowires [20] and nanoribbons (NBs) [21], are considered to be important multifunctional building blocks for manufacturing various nanodevices.ZnO NWs/NBs have been effectively used in field effect transistors, light emitting diodes, laser diodes, sensors, resonators, and piezoelectric devices due to their unique electrical, optical, and piezoelectric capabilities [22][23][24][25][26]. Furthermore, ZnO possesses both semiconductor and piezoelectric capabilities that can be used to build electromechanically coupled sensors and transducers, and the coupling of the two qualities provides it with unique benefits and novel application scenarios [27].Hossain doped ZnO particles onto the surface of PDMS to fabricate a composite sponge that was employed as a high-efficiency photocatalyst, and it was proved that the composite sponge had a faster kinetic response than the pure sponge under UV light rate [28].He prepared 500 nm polymerized ZnO spheres and mixed them with PDMS to create a flexible and transparent friction electric nanogenerator with ordered nested porous PDMS films and graphene transparent electrodes, which provides a novel and effective way of improving its optical features [29].Researchers add conductive materials to PDMS to obtain better electrical conductivity, there is no case of semiconductor materials embedded in PDMS sponge to make capacitive pressure sensors for experimental research.
A novel porous ZnO NWs PDMS (PZP) sponge capacitive pressure sensor has been prepared.The structure is composed of a gentle PZP sponge dielectric layer along with two ITO/PET electrodes.Porous PDMS sponges were prepared using salt particles as templates, and ZnO NWs were loaded onto the sponges as excellent semiconductor materials with piezoelectric properties.To improve the sensor sensitivity, the method of using different salt particle sizes and adding different ZnO NWs dosages in porous PDMS sponge was tested (the best results were obtained with salt particle sizes less than 150 μm and ZnO NWs dosage of 200 mg).The results indicate that the sensor sensitivity is highest in the pressure range of 2.5-7.5 kPa, reaching 0.1279 kPa -1 , response time less than 70 ms.The PZP sponge is highly elastic and lightweight (0.2497 g/cm 3 ), with excellent mechanical properties and low density.Also, when the PZP sponge sensor is used as a self-powered power supply, it achieves the best output of 1.2 V and 6.8 nA at 5.74 kPa pressure.

Materials
The polydimethylsiloxane (PDMS, SYLGARD 184) elastomer and curing agent used in the experiment were from Dow. ZnO NWs powdery was commercially purchased from Nanjing XFNANO Materials Tech Co., Ltd, China.Other chemicals and reagents were purchased from Sinopharm Chemical Reagent Co., Ltd, China.The standard sampling sieve mesh was conformed to the Chinese national standard GB/T6003.1-2012.

Preparation
Salt with different aperture sizes (<150 μm, 150~300 μm, 300~450 μm, >450 μm) were selected as the template for preparing porous PDMS sponges.After the PDMS and curing agent are fully mixed at a mass ratio of 10:1, then salt is placed into the degassed PDMS prepolymer, and the salt is thoroughly stirred to ensure even mixing.After that, mixture is placed in the vacuum dryer to let the bubble entirely evaporate and cure.After curing, the salt PDMS package is removed and immersed in deionized water to dissolve the salt particles.After dissolving is completed, the porous PDMS sponge template can be obtained by drying, named as PP ＜150 , PP150 ～300 , PP300 ～450 , PP ＞450 .ZnO NWs powder (50 mg, 100 mg, 150 mg, 200 mg) was put into ethanol solution to prepare ZnO NWs ethanol dispersion solution.Using a dropper, the solution was then applied to the porous PDMS sponge and repeatedly squeezed to ensure equal dispersion.It is then dried for 12 hours in the oven heated to 60 °C.After the sponge has been cured, a little PDMS prepolymer is dipped from each side of the sponge, and the required PZP sponge can be obtained.
ITO/PET film was selected as the electrode, conductive silver adhesives was coated on the top and bottom electrodes of the ITO/PET film for assembly, and a sandwich structure sensor was constructed by sandwiching PZP sponge between the two electrodes.

Characterization of the PZP Sponge
After removing the salt template, the PDMS sponge appeared white, but turned a little grey after loading with ZnO NWs as shown in Figure 2(a).The morphology of the sample was tested using a scanning electron microscope (SEM, SU3500, Hitachi, Japan).Figure 2(b) are marked with SEM images and show the PP>450 and PP150~300.Figure 2(c) is under the magnification of 950, and the presence of ZnO NWs in the PZP can be clearly observed.Pressure is applied to the sensor using a digital display thrust meter (DS2-10N, ZHIQU, China).Utilizing an impedance analyzer (E4990A, KEYSIGHT, US), the PZP sponge capacitive pressure sensor's real-time output capacitance was measured.The impedance analyzer is connected to wires that are fastened to both ends of the pressure sensor.As shown in Figure 2(d), the PZP sponge is constantly pressurized, and after compression deformation, the loaded PZP fully recovered.This showed that the sponge's flexibility and compressibility were not affected by the addition of ZnO NWs.The PZP sponge composite demonstrated good elasticity, low density (0.2497 g/cm 3 ), light weight.To demonstrate its extremely light mass, it is placed in water and its ability to float at the water-air interface, as shown in Figure 2(e).

Sensitivity Performance of the PZP Sponge sensor
In order to explore the influence of porous PDMS sponge aperture size on sensing performance, the stress-capacitance curve is plotted as Figure 3(a), and the original data were processed and the pressure-relative capacitance variation curve was drawn.Because the pressure is not fixed during detection, the capacitance value fluctuates, and the relative capacitance changes fluctuate greatly after data processing.Because of the manual control of the propulsion meter, it is not feasible to correctly regulate the maximum pressure delivered to 5 N at the conclusion of the curve.and to ensure the data is sampled several times, when the maximum pressure is maintained at 5 N, the pressure will fluctuate quickly due to the sponge somewhat recovering, so the tail end data is not stable enough.The pressure sensor's sensitivity S is defined as: where C represents the measured capacitance under pressure, C0 represents the initial capacitance without pressure, and P represents the pressure.According to Figure 3(b), the sensitivity S<150>S>450>S300~450>S150~300.When the aperture size <150 μm, the sensor achieves the highest sensitivity 0.1239 kPa -1 in the pressure range 2.5-5 kPa, which was taken as a template for the following experiment.As shown in Figure 3(c), the relation between the aperture size and the relative capacitance shows, the relative capacitance changes gradually with the increase of the aperture.When the aperture is <150μm, the relative capacitance changes to 0.2797.It can be seen from the source data that the three measurement results are basically consistent, and there is no large deviation, which indicates that the porous PDMS sponge capacitive pressure sensor has good repeatability and stability.PZP sponges with different ZnO NWs dosage (50 mg, 100 mg, 150 mg and 200 mg) were prepared and characterized, denoting as PZP50, PZP100, PZP150 and PZP200.A continuous force is applied to each of them, the stress-capacitance curve is showed as Figure 3(d), according to Figure 3(e), the sensor PZP200 has the highest sensitivity of 0.1279 kPa -1 within the pressure range of 2.5-7.5 kPa, which means more ZnO NWs dosage (within a certain range) can improve the sensor's sensitivity effectively.When the sensor is exposed to an external force, more contacts are formed between more nanowires, resulting in the formation of pathways.When the distance between the ZnO nanowires is too wide for the electrons to achieve a quantum tunneling effect, the PDMS sponges form a capacitor structure, giving the PZP an equivalent capacitive property.The pressure in range of 0-1 kPa, the sensitivity of PZP200 is reduced to 0.0507 kPa -1 .Similarly, the PZP100 sensor is 0.0416 kPa -1 , with increasing pressure rising to 0.1092 kPa -1 .At the same time, we also get that the relative capacitance change of PZP200 is 0.3171 in Figure 3(f).Experiments show that the sensor has the greatest sensitivity in the 2.5-7.5 kPa range.However, the sensitivity of PZP50 was not the lowest among the four PZP sponges loaded with different ZnO NWs, indicating that this effect does not scale linearly.Table 1 compares the capacitive pressure sensors with similar structure.PZP has high sensitivity and fast response time in a small pressure response range.

Other performance testing and analysis of sensors
The PZP200 was found to have the highest sensitivity range among the produced sponges, and further research was conducted using it as the flexible capacitive pressure sensor's best dielectric layer.Both the distance between the top and lower electrodes and the spacing between the ZnO NWs adsorbed inside the sponge changed when the sensor was exposed to external pressure.Here, the parallel plate capacitator's capacitance, C is defined as: where, 0 is the dielectric constant of vacuum, is the dielectric layer's relative permittivity, and the value of could be ascertained by the filler material content and the dielectric layer's deformation under pressure.A is the effective region's area, and d is the distance between the upper and lower electrode plates.When pressure is applied to the capacitive pressure sensor, the sensor's , A, or d parameters change, and the capacitive pressure sensor's capacity value changes as well.Based on the porous PDMS capacitive pressure sensor, its main functional component is the porous PDMS of the medial layer, which uses its elastic characteristics to pressurize the median layer, reduce the upper-down electrode distance, and decrease the air ratio, considering the ZnO NWs' median constant number is about ~4 [33], the PDMS's median constant number is about ~2.4 and the air median electricity constant is about ~1 [34], which will increase the median electrolyte layer and cause cavitation.
The sensor has weights of 20 g, 50 g, and 100 g (0.77 kPa, 1.91 kPa, and 3.83 kPa) pressed on it.Different weight codes are used to set pressure, and immediate time changes in the loading and releasing pressure of the appropriate response sensors are explored (see Figure 4(a)).According to Figure 4(b), when the pressure of sensor is 1.91 kPa, its response time is less than 70 ms, and the release time is less than 100 ms, the response speed is faster, the timely output of information can be met.The curves when applying pressure and removing pressure remain basically the same (Figure 4(c)).The PZP200 sensor has small hysteresis.The internal pore structure can well recover its original shape after compression, and there is no internal viscosity.In order to study the reliability and stability of the sensor, the sensitivity of the sensor after 100 deformations was tested, as shown in Figure 4(d).The sensor sensitivity stays almost intact, with 0.1279 kPa -1 before bending and 0.1206 kPa -1 after bending, indicating that the sensor's performance is stable.its application situation.Based on the inherent piezoelectric effect of ZnO NWs, it is used as a self-powered sensor to achieve the best output of 1.2 V and 6.8 nA at 5.74 kPa pressure.The generation of self-power supply highlights the capacitive pressure sensor's potential applicability in flexible and wearable electronics.

Conclusions
By adding the ZnO NWs to the porous PDMS sponge synthesized with salt as a mold and then covering it with another layer of PDMS film, the PZP sponge was prepared.Sponges show the characteristics of great elasticity, low density (0.2497 g/cm 3 ), light weight and excellent mechanical properties.A flexible, wide working range and sensitive capacitive pressure sensor has been fabricated through a simple and low-cost process.Varying the different variables in the experiment to optimize the capacitance response, it was found that the best results (0.1279 kPa -1 ) were obtained by using less than 150 μm salt particle as a template and adding 200 mg ZnO NWs to PZP sponge, response time less than 70 ms.When ZnO NWs were added to sponge, the sensor can be used as a self-powered power supply, it achieves the best output of 1.2 V and 6.8 nA at 5.74 kPa pressure.It provides a new way to tackle the challenge of low sensitivity of flexible sensor to capacitive signal, and lays a foundation for expanding the application scenarios of ZnO NWs.

Figure 3 .
Figure 3. Capacitive pressure sensor with different apertures (a)Stress vs capacitance; (b)Pressure vs relative

Figure 4 .
Figure 4. (a) Compares the capacitor-time curves for loading and unloading different pressures; (b) Fast response time of the sensor (under 1.91kPa pressure); (c) Hysteresis curve; (d) Stability of pressure response after 100-cycle test.

Figure 5 .
Figure 5. (a) The output voltage of the sensor, under the compression of the external force cycle; (b) One cycle of the output voltage generated; (c) The output current of the sensor, under the compression of the external force cycle; (d) One cycle of the output current generated.The ZnO NWs we added exhibit strong piezoelectric characteristics, because of the asymmetry of the ZnO crystal structure, it has a piezoelectric effect.The applied electric field, pressure, temperature, and other variables cause the ZnO NWs crystal to distort, resulting in unequal charge distribution and voltage generation.As a result, the intrinsic piezoelectric effect is being investigated further in light of

Table 1 .
Comparison with the same types of pressure sensors.