Introduction

Due to their various properties, zinc oxide nanoparticles in general and nanowires (ZnO NWs: linear particles with high aspect ratio) in particular are promising materials for a broad range of applications, e.g., piezoelectric sensors (Jeong et al. 2015) and piezoelectric energy harvesting (Lu et al. 2012; Wang 2008; Wang et al. 2015; Wang and Song 2006; Yang et al. 2009; Zhu et al. 2010), gas sensors (e.g., NOx, CO, O2) (Zhang et al. 2012), superhydrophobic surfaces (Mardosaitè et al. 2021), photocatalysis (Rackauskas et al. 2017), antibacterial activity (Sirelkhatim et al. 2015), spintronic devices, biosensors, etc.

ZnO is characterized by a wide band gap (3.37 eV) and a large exciton-binding energy (60 meV). Due to the wurtzite-type crystal structure (zincite), the asymmetric displacement of the ions Zn2+ and O2− upon pressure creates a piezoelectric current. Additionally, ZnO is photostable, temperature resistant and nontoxic. Further, it is a semiconductor of high potential for applications in optoelectronic devices, such as light-emitting diodes (LEDs), electrically pumped lasers, electron collection in photovoltaic cells and as transparent conducting oxide.

In order to explore the potential of zinc oxide nanoparticle-based technologies and to make them industrially viable, cost-efficient and scalable processes have to be developed, which allow tailoring the properties of the particles, such as their shape and size. Typical state-of-the-art processes to synthesize zinc oxide nanowires are wet-chemical methods (sol–gel processes, hydrothermal growth) (Arslan et al. 2022; Broitman et al. 2012; Hu et al. 2007; Maity et al. 2005; Wu et al. Evaluation of the potential for piezoelectric sensors

Test sensors were developed based on so-called interdigit electrode structures (Fig. 10) to evaluate the potential of the plasma-synthesized ZnO nanowires for piezoelectric sensors. The electrodes were printed with an OPTOMEC aerosol jet using PARU nano-silver ink on dielectric substrates (self-adhesive Kapton tape on FR4).

Fig. 10
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From left: Design (a: dimensions in mm), photo (b) and microscope image (c) of interdigit electrodes made from silver ink printed onto dielectric substrates

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ZnO nanowires suspended in an acrylic resin were milled, mixed with methyl methacrylate and a UV initiator and applicated as a thick film (~ 200 µm) onto the electrodes, so that the space between the electrodes was filled with the nanowires containing matrix. To increase the piezoelectric effect of the nanowires, a poling voltage was applied (hysteresis poling) while the matrix was hardened under UV irradiation.

In most samples, the poling led to short circuits and arcing after some seconds, which irreversibly damaged the devices. The observed delay between poling start and short/arcing indicates that the nanoparticles started to align within the electric field until a conducting bridge between the electrodes was built. In a further test with acrylic resin without nanowires, a significantly increased dielectric strength and a significantly increased poling voltage up to arcing were observed. The alignment of the ZnO nanowires as chains during poling is thus likely to play a significant role in arcing. To avoid chain forming the concentration of the ZnO NWs in the matrix was reduced, and the poling voltage and time were varied.

Eventually, the successfully poled devices were tested.

The samples tested are listed in Table 1. ZnO nanopowders with nanowire-like structures were chosen for the piezoelectric experiments comparable to those in Fig. 4 and Fig. 5 that where synthesized with 250 A and 20% O2. Figure 11 shows the signals of the samples and the references (a) during bending, and a picture of the experimental setup (b) with a sample in front (yellow device). For these proof-of-concept experiments, the electrodes of the devices were contacted, and the electric charge was recorded as a function of time while the devices were stimulated mechanically. The films were bent by an operator using a ruler made of non-conductive material. Two thirds of the samples were fixed on a table, one third was free to be bent on the edge downwards. The reason for the different responses in Fig. 11 is that the sensors were not stimulated continuously. The pressure and pauses were varied by the operator on purpose, but were not quantified.

Table 1 Samples prepared for the piezoelectric experiments
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Fig. 11
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a Recorded charge upon bending of the samples and b setup of the piezoelectric experiments (the yellow sample is bent with a pen)

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The "Dummy" and the "Matrix" sample show some responses to the mechanical stimulation, which have to be attributed to the electrostatic charging of the polymer stick with which the samples were bent. Similarly, the unpoled ZnO-loaded sample (“ZnO, 0 V”) did not respond to the mechanical stimulation. Only with poling, the ZnO-loaded samples react with signal peaks to the stimulation, which can be attributed to the piezoelectric effect. Because of the low particle loading and, thus, low charge displacements the piezoelectric effect ranges in the order of magnitude of the electrostatic charging. The values are, therefore, to be understood as semi-quantitative. From the poled ZnO-loaded samples, the sample “ZnO, 600 V, 30 s” shows the most significant charge peaks, which may be attributed to the four times higher ZnO concentration in the matrix. In terms of amplitude with ~ 100 pC, however, it is still orders of magnitude below the piezoelectric effect of polymer-based sensors (e.g., PyzoFlex® from JOANNEUM RESEARCH). One approach to improve the sensors would be to use a piezoelectric matrix material such as poly(vinylidene fluoride) (PVDF) or a PVDF co-polymer. Dodds and co-workers showed that the voltages generated can be increased with the ZnO content (Dodds et al. 2013), in their study up to a factor of 1.7 with 10–20% ZnO NPs. In a comparative study of PVDF composites with different nanoparticles (SiO2, TiO2 and ZnO), Arjun Hari and co-workers found the most improvement of the output voltage when PVDF composites with 3% ZnO nanoparticles were tested.

Furthermore, the inner polarizability of the ZnO nanowires was investigated. For this purpose, sample “ZnO, 0 V”, in which the matrix curing took place without applying a polarization voltage and in which the nanowires were thus immobilized without a preferred direction, was subjected to hysteresis polarization. In the case of polarizability, charge shifts and, thus, currents would have to be measurable in the course of repeated repolarization. Such charge shifts and currents could not be observed, not even with a gradual increase in the voltage amplitude up to the arcing at 1500 V.

To improve the polarizability of the ZnO nanoparticles and, thus, the piezoelectric responses, it could be beneficial to dope the nanoparticles during the plasma synthesis with shallow donors (group III/VII) or shallow acceptors (group I, V) (Albertsson et al. 1989). Lithium, for example, has proven to enhance the output voltage of piezoelectric devices based on ZnO nanowires after poling (Consonni and Lord 2021; Shin et al. 2014).

However, the fact that in the case of previously immobilized nanowires breakdown only occurs at ~ 1500 V, while in the case of an acrylate matrix that has not yet been cured arcing already occurs at 600–700 V, may be seen as a further strong indication that the ZnO nanowires themselves strongly favour the formation of short circuits as they orient themselves to chains.

In conclusion, although minor piezoelectric signals are discernible, the ZnO nanowires are not yet qualitatively sufficient to fabricate usable sensors. The signals are too low to allow quantitative measurements of the piezoelectric coefficients. Therefore, future research should focus on implementing dopants like Li in the plasma flight-thru synthesis (e.g., as aerosol) in combination with piezoelectric matrices based on PVDF.

Conclusion

In the present work, we demonstrated the successful development of an atmospheric pressure plasma jet reactor for the morphology-controlled zinc oxide nanoparticle synthesis with zinc powder and oxygen. The particle morphology was found to depend on the oxygen rate in the carrier gas and the plasma, the discharge current and the energy inside the reactor, respectively. Zinc oxide nanoparticles with a high aspect ratio (nanorods and nanowires) grew, when 20% O2 was present in the carrier gas (10% in the plasma) and discharge currents of 250–350 A were applied. Piezoelectric signals could be detected upon mechanical stimulation when these nanoparticles were dispersed in a matrix of acrylic resin and fixed between finger-electrodes while poling. The signals were significantly greater than the responses assigned to electrostatic charging of the untreated reference, a ZnO-free and a non-poled sample. Hence, the signals of the poled ZnO sample prove that the synthesized ZnO nanoparticles (grown in the wurtzite structure) have the potential to animate piezoelectric sensors and devices. However, a further improvement of the particle morphology control in combination with high-yield harvesting and homogenization techniques will be necessary to advance towards scale-up for industrial purposes. To improve the polarizability of the ZnO NPs and, thus, the piezoelectric responses, on the other hand, the plasma flight-thru process should be adopted to enable the incorporation of dopants like Li into the ZnO NPs.