Optimization of ZnO Nanorods Concentration in a Micro-Structured Polymeric Composite for Nanogenerators

: The growing use of wearable devices has been stimulating research efforts in the de-velopment of energy harvesters as more portable and practical energy sources alternatives. The ﬁeld of piezoelectric nanogenerators (PENGs) and triboelectric nanogenerators (TENGs), especially employing zinc oxide (ZnO) nanowires (NWs), has greatly ﬂourished in recent years. Despite its modest piezoelectric coefﬁcient, ZnO is very attractive due to its sustainable raw materials and the facility to obtain distinct morphologies, which increases its multifunctionality. The integration of ZnO nanostructures into polymeric matrices to overcome their fragility has already been proven to be fruitful, nevertheless, their concentration in the composite should be optimized to maximize the harvesters’ output, an aspect that has not been properly addressed. This work studies a composite with variable concentrations of ZnO nanorods (NRs), grown by microwave radiation assisted hydrothermal synthesis, and polydimethylsiloxane (PDMS). With a 25 wt % ZnO NRs concentration in a composite that was further micro-structured through laser engraving for output enhancement, a nanogenerator (NG) was fabricated with an output of 6 V at a pushing force of 2.3 N. The energy generated by the NG could be stored and later employed to power small electronic devices, ultimately illustrating its potential as an energy harvesting device.


Introduction
The establishment of wearables has created new energy supply challenges. This supply is mainly provided by batteries and capacitors, which despite delivering a high output, commonly in a mW scale, are only able to do it for a limited time and with the inconvenience of their bulkiness. Given that low power devices do not require such high outputs, energy scavenged from the environment through energy harvesters can be a viable alternative to batteries and similar devices. In fact, energy harvesters can scavenge energy almost at any time without interruptions and through small devices that are more compatible with wearables [1].
In the case of the triboelectric effect, which results from the combination of triboelectrification (surface charges generation through the rubbing of two different materials) and electrostatic induction, almost any pair of dissimilar materials can be employed to induce charges mainly through friction [24]. At the nanoscale, these materials can be stimulated by tiny physical motion and the excitation frequency can vary from a few Hz to thousands of Hz, which is ideal for harvesting random energy from the environment [1].
Nanogenerators (NGs) revolutionized the field of energy harvesting and sensing, while contributing to a sharp growth of publications and research collaborations in these areas of research [15,[24][25][26][27][28][29][30]. In 2006, Wang and co-workers demonstrated the use of materials at the nanoscale, such as a ZnO nanowires (NW) array, to fabricate piezoelectric nanogenerators (PENGs) [31]. This was a turning point for piezotronics, and 6 years later a triboelectric nanogenerator (TENG) was developed [32]. The high potential and capability of PENGs and TENGs can lead to many new instruments in different systems. For example, NGs can be used in biomedical and rehabilitation devices as smart sensors, or as a sustainable energy source [24,25,[33][34][35][36].
Regarding the performance of NGs based on ZnO nanostructures, a wide range of outputs has been reported [37][38][39][40]. The NGs' performance is deeply related to the morphology of the nanostructures, and structure of the electrodes and their material, since one of the drawbacks of these devices is the difficulty in extracting the generated voltages [41]. For NGs based on composites of nanostructures and polymers, the nanostructures concentration should also be considered for the optimization of electric polarization, since an excessively high NPs concentration may lead to partial cancelling of the electrical dipoles, which reduces the output of the device [42,43]. For a NG configuration based on ZnO NWs vertically grown on both sides of a substrate, with electrodes made of ZnO NWs coated with gold and a matrix of polydimethylsiloxane (PDMS) surrounding the NWs, output values of 6 V/4 nA/0.39 nW cm −2 were achieved with a finger bending [41]. For another NG using ZnO nanoflowers mixed with PDMS and multiwalled carbon nanotubes (MWCNTs) sandwiched between two aluminum electrodes, an output of 75 V/3.2 µA/260 mW cm −2 was obtained by a person walking with slippers that had the depicted device inbuilt in the soles [43]. These two examples illustrate the versatility of ZnO nanostructures and the advantages of a composite in the overall device performance.
A common approach to produce robust devices using NRs or NWs is their mixture with a polymer matrix, such as polyvinylidene fluoride (PVDF), PDMS, and poly(methyl methacrylate) (PMMA), allowing the preservation of their original structure [40,73,74]. There are several examples of this approach in which the use of a composite showed good results [37,42,75]. In TENGs, the micro-structuring of the film leads to an increase of the devices triboelectric output, allowing a performance improvement [43,76]. The same type of approach was recently employed in PENGs based on ZnSnO 3 nanowires, which also showed an enhancement of the piezoelectric properties [13]. Beyond the performance improvement, a polymer matrix increases flexibility and fatigue resistance to mechanical strain [1].
With the previous research as a starting point [13], this work presents the fabrication of a NG based on a composite of ZnO NRs embedded in a PDMS matrix. With the aim of enhancing the devices' performance, the concentration of the NRs in the PDMS film was studied and the composite was further micro-structured through laser engraving technique [77]. The device showed a high stability under stress and over time and, as a proof of concept, a high intensity light-emitting diode (LED) and a commercial household thermometer were powered using the optimized device, testifying the NG's capability as an energy harvester. Furthermore, this work demonstrates that ZnO NRs synthesized through microwave radiation assisted hydrothermal synthesis, instead of the conventional synthesis methods, undoubtedly show a great potential for energy harvesting applications, which proves that it is feasible to obtain functional NGs through low-cost, low-temperature, and fast fabrication processes.

Materials and Methods
Acrylic molds were produced as described in a previous work [77], and used to microstructure the ZnO@PDMS composite, having aligned micro-cones cavities with a cavity gap <100 µm. Briefly, a laser engraving machine (VLS3.50, 50 W, Universal Laser System, Katy, TX, USA) with a CO 2 laser beam, a lens' focus length of 2.0 in, a focal spot diameter of 127 µm, a power of 25 W and a speed of 0.1524 m·s −1 was employed to engrave acrylic plates with arrays of symmetrical aligned crosses (100 µm) with a cross gap of 300 µm in an area of 2 × 2 cm 2 .
The ZnO NRs were produced by hydrothermal synthesis assisted by microwave irradiation, as described in reference [78]. Essentially, 3.3 g of zinc acetate dihydrate (ACS, 98-101.0%, Fisher Scientific, Hampton, NH, USA) were dissolved in 30 mL of de-ionized water, and the solution was put in agitation. 9.6 g of sodium hydroxide (Labkem, Sevilla, Spain) were added to the previous solution. The surfactant solution was produced by mixing 300 mL of deionized water with 0.09 g of sodium lauryl sulfate (SLS, 95%, extra pure, from Scharlau, Barcelona, Spain). Then 6 mL of the first solution were mixed with 15 mL of the surfactant solution and 30 mL of 2-ethoxyethanol (Honeywell Research Chemicals, Porto Salvo, Portugal). The final solution was transferred to 3 Teflon ® vessels of 20 mL. The vessels were then placed in a microwave (CEM-MarsOne, from CEM, Charlotte, NC, USA) to be heated at 110 • C for 40 min (with a temperature ramp of 7 min and under a power of 600 W). The vessels were afterwards cooled down to room temperature and the ZnO NRs were then washed by successive centrifugation at 4000 rpm for 5 min with propan-2-ol (ALLRESIST GmbH, Strausberg, Germany) and deionized water, for about 10 times. The NRs were finally dried at 84 • C for 6 h.
For the fabrication of the NGs, ZnO NRs were mixed with the PDMS elastomer (Dow Corning, Midland, MI, USA) in a ratio of 15, 20, 25 or 30 wt %, and ethyl acetate from Fluka-Honeywell (Portugal) (enough volume to promote the mixing of elastomer and NRs). After mixing until evaporating most of the solvent volume, the curing agent (Sylgard 184, from Dow Corning, Midland, MI, USA) was added in a weight ratio to elastomer of 1:10, and the mixture was mixed thoroughly before spin-coating at 250 rpm for 90 s, with an acceleration of 100 rpm s −1 , on two different substrates: commercial substrates of polyethylene terephthalate (PET) with an indium tin oxide (ITO) thin film deposited on top, PET/ITO (Kintec Company, Hung Hom, Kowloon, Hong Kong), resulting in unstructured NGs; or acrylic molds (5 mm thick, from Dagol, Zambujal, Sesimbra, Portugal), resulting in micro-structured NGs. The ZnO@PDMS films were partially cured at 60 • C for 10 min to 15 min before placing the PET/ITO substrate directly over the composite. After completing the cure at 60 • C for 45 min, the micro-structured ZnO@PDMS film with the PET/ITO substrate was peeled off from the mold and assembled with another electrode of PET/ITO with kapton tape (DuPont, Wilmington, DE, USA). In both types of NGs, copper tape (3M, Saint Paul, MN, USA) was used as an extension of each electrode. The main steps to fabricate one micro-structured NG are illustrated in Figure 1a. Figure 1b illustrates a final micro-structured NG with a micro-structuring area of 4 cm 2 .
a final micro-structured NG with a micro-structuring area of 4 cm 2 .
A PANalytical's X'Pert PRO MRD diffractometer (PANalytical B.V., Almelo, The Netherlands) with Cu Kα radiation (10-60° 2θ range, step size of 0.033°) was employed for the structural characterization of ZnO NRs and ZnO@PDMS composites. The morphology analysis of the ZnO NRs was performed with a Carl Zeiss AURIGA CrossBeam (FIB-SEM) workstation (Carl Zeiss Microscopy GmbH, Oberkochen, Germany), while the morphology and EDX analysis of the composite was performed in standard observation mode and EDX mode, respectively, at 15 kV, using a tabletop SEM Hitachi TM3030Plus.
A home-made machine with a linear motor and a contact area of 0.3 cm 2 was used to deliver a mechanical stimulus of variable force (0.8 N to 4.1 N) at a frequency of 0.5, 1, 1.5, and 2 pushes per second ( Figure S1). A digital oscilloscope (Tektronix TDS 2001C, 10 MΩ input impedance, from Tektronix, Beaverton, OR, USA) was used for voltage measurements, while a potentiostat (Gamry 600, from Gamry Instruments, Philadelphia, PA, USA) was employed for current measurements. The force applied by the home-made machine was measured with a commercial force sensing resistor (Ref. SEN05003) from Interlink Electronics (Camarillo, CA, USA).   Figure 2a shows SEM images of a micro-structured ZnO@PDMS film. The film is composed of an array of aligned cones with an average diameter of 300 µm, an average height of 380 µm, and a gap inferior to 100 µm. The close-up views of the SEM images clearly reveal the ZnO NRs homogenously dispersed throughout the PDMS matrix. To verify that the structures were indeed ZnO NRs, the samples were analyzed through EDX, as presented in Figure 2b, where the zinc element is more concentrated in the referred structures, as expected. Figure 2c shows the EDX data, highlighting the main elements present in the ZnO@PDMS film: carbon and silicon (which integrate the PDMS polymeric chains), zinc (which constitutes the ZnO NRs), and oxygen (present in both the PDMS matrix and the NRs). Figure 2d,e show the SEM images of ZnO NRs, which have an average length and diameter of (2.3 ± 0.7) µm and (0.4 ± 0.2) µm, respectively. The distribution of lengths and diameters of the NRs can be found in the respective histograms from Figure S2. Figure 2f presents the XRD diffractogram of ZnO NRs, ZnO@PDMS, and pure PDMS. The peaks observable for the NRs are assigned to the hexagonal wurtzite ZnO structure, with lattice constants of a = 0.3296 nm and c = 0.52065 nm. As expected, those peaks are also present in the ZnO@PDMS composite, yet absent in pure PDMS. Figure 2a shows SEM images of a micro-structured ZnO@PDMS film. The film composed of an array of aligned cones with an average diameter of 300 µm, an avera height of 380 µm, and a gap inferior to 100 µm. The close-up views of the SEM imag clearly reveal the ZnO NRs homogenously dispersed throughout the PDMS matrix. verify that the structures were indeed ZnO NRs, the samples were analyzed through ED as presented in Figure 2b, where the zinc element is more concentrated in the referr structures, as expected. Figure 2c shows the EDX data, highlighting the main elemen present in the ZnO@PDMS film: carbon and silicon (which integrate the PDMS polyme chains), zinc (which constitutes the ZnO NRs), and oxygen (present in both the PDM matrix and the NRs). Figure 2d,e show the SEM images of ZnO NRs, which have an av age length and diameter of (2.3 ± 0.7) µm and (0.4 ± 0.2) µm, respectively. The distributi of lengths and diameters of the NRs can be found in the respective histograms from Figu S2. Figure 2f presents the XRD diffractogram of ZnO NRs, ZnO@PDMS, and pure PDM The peaks observable for the NRs are assigned to the hexagonal wurtzite ZnO structu with lattice constants of a = 0.3296 nm and c = 0.52065 nm. As expected, those peaks a also present in the ZnO@PDMS composite, yet absent in pure PDMS.

Piezoelectric Characterization of ZnO Nanorods
In a previous work [13], piezoelectric force microscopy was employed in untreated individual ZnO NRs for the characterization of their electromechanical properties, as shown in Figure S3. The response is slightly above the noise level and leads to an effective Chemosensors 2021, 9, 27 6 of 13 piezoelectric constant of d 33 = (9 ± 2) pm·V −1 , in agreement with the values reported for ZnO nanostructures [20]. Moreover, such work proved that the relaxed NRs do not exhibit static surface charges, which could impact on their electromechanical response [13]. Thus, the findings demonstrate the piezoelectric properties of the individual NRs. The fact that the NRs were obtained through microwave radiation assisted hydrothermal synthesis and still present piezoelectric properties proves that this synthesis technique is feasible to produce functional structures, with the advantages of being faster, low-cost, and using lower temperatures.

Optimization of the ZnO Concentration in the ZnO@PDMS Composite
In order to produce NGs with the highest potential, it is important to evaluate their performance with the ZnO concentration. In fact, several groups verified that with an increasing mass of piezoelectric particles dispersed in a polymeric matrix there is an output increase, yet beyond a certain mass value, the output starts decreasing [7,10,79,80]. Such occurrence may be explained by a possible electrical breakdown due to an excessive nanoparticles concentration, an inefficient dispersion of the nanoparticles in the polymeric matrix, which then prevents the alignment of electric dipoles in a large proportion [10,79], or due to a degradation of electromechanical coupling effects, such as the deformability, with increasing concentration [7]. Figure 3a presents the peak-to-peak voltage with the concentration of ZnO NRs in the ZnO@PDMS composite. The composites were mechanically stimulated with a home-made bending machine, adapted to exert a pushing force of 2.3 N at a frequency of 2 pushes per second. There seems to be a trend of increasing voltage with the increase of ZnO NRs concentration until 25 wt %, after which the output decreases for greater concentrations. This observation is therefore in accordance to what was previously reported for composites with other types of materials [7,10,79,80], with the current work being the first to verify this effect in PDMS composites with ZnO NRs, to the best of the author's knowledge.

Electrical Characterization of ZnO@PDMS NGs
Given the best performance obtained with a micro-structured NG, this type of device was further characterized regarding its performance for mechanical stimuli of different frequencies, force magnitudes, and applied for long periods for fatigue tests. Figure 4a illustrates the performance of the referred NG as a function of the stimulus frequency for a constant force of 2.3 N. As expected, there is an increase in voltage output with the frequency of the mechanical stimulus, as observed by other groups [81][82][83][84][85]. This behavior may be explained by the fact that when the pushing stimuli is faster, the system is unable to efficiently neutralize the induced charges, thus leading to accumulation of Considering 25 wt % as the optimum concentration of ZnO NRs in the composite, two types of NGs were fabricated to further explore and enhance the potential of the composite: an unstructured one and a micro-structured one, with its micro-structuring being based on micro-cones of 380 µm height, 300 µm diameter, and a gap inferior to 100 µm, as shown Chemosensors 2021, 9, 27 7 of 13 in Figure 2a. The output voltage for these two NGs in the same conditions as before is shown in Figure 3b. The micro-structured NG shows a better performance, achieving a peak-to-peak voltage of (6.00 ± 0.7) V, against only (0.87 ± 0.2) V for the unstructured counterpart. Such enhancement in voltage, about 6 times, may be explained by two effects: an increased efficiency in force delivery to the ZnO NRs due to the micro-structuring of the composite, reinforcing the piezoelectric effect of the ZnO NRs (as it was verified for a composite of ZnSnO 3 @PDMS in [13]), and an additional triboelectric effect that arises due to the introduction of air gaps between the ZnO@PDMS composite and the ITO electrode with the micro-structuring. However, it is not trivial to clearly distinguish the isolated contribution of piezoelectric or triboelectric effects to the performance of the NG.

Electrical Characterization of ZnO@PDMS NGs
Given the best performance obtained with a micro-structured NG, this type of device was further characterized regarding its performance for mechanical stimuli of different frequencies, force magnitudes, and applied for long periods for fatigue tests. Figure 4a illustrates the performance of the referred NG as a function of the stimulus frequency for a constant force of 2.3 N. As expected, there is an increase in voltage output with the frequency of the mechanical stimulus, as observed by other groups [81][82][83][84][85]. This behavior may be explained by the fact that when the pushing stimuli is faster, the system is unable to efficiently neutralize the induced charges, thus leading to accumulation of residual charges [81].

Proof-of-Concept of the NG
The micro-structured ZnO@PDMS NG was connected to several load resistances (10 MΩ to 100 MΩ) to evaluate the output voltage and power density with a fixed pushing force of 2.3 N at 2 pushes per second. As shown in Figure 5a, the peak-to-peak voltage generated by the NG increases with an increasing load resistance. Regarding the power density, despite reaching an apparent peak at 40 MΩ due to a slight power density decrease for 50 MΩ and 60 MΩ, which is mainly explained by experimental errors, it hits a maximum peak approximately at 70 MΩ, steadily decreasing afterwards. Therefore, 70 MΩ is considered to be the load that maximizes the power density at 4.8 µW cm −2 .
To test the practicality of this NG, the device was connected to a full-rectifier bridge and its DC output (for a pushing force of 2.3 N applied at a frequency of 2 pushes per second) was used to charge a 10 µF capacitor. After charging for approximately 1 h, the capacitor (roughly reaching 4 V and a stored energy of 80 µJ) was connected to a blue LED (Ref. 017-1059 from Robert Mauser, Portugal; Video S1) or a digital thermometer (Video S2) to power them up, as illustrated in Figure 5b,c. These results show that the NG can generate enough energy to be stored and later used to power up small electronic devices, When the frequency of the stimuli is fixed at 2 pushes per second, the output voltage of the NG increases with force, as shown in Figure 4b, which highlights the potential of these devices not only for energy harvesting but also for force/pressure sensing. This effect is also widely documented in other works [81,83,[85][86][87][88] and may be explained by two factors. Firstly, the piezoelectric effect that is present in the NGs. With an increase of the intensity of the mechanical stimulus, the deformation applied to the crystalline structure of the ZnO NRs increases as well, which induces a larger charge rearrangement and thus translates into a greater piezoelectric signal. Secondly, due to triboelectric effect, mechanical stimuli of increasing force induce a greater rubbing and increase the contact area between the ZnO@PDMS micro-cones and the ITO electrode, thus inducing more charges at their interface. Figure 4c shows the peak-to-peak voltage for a fixed pushing force of 2.3 N applied at 2 pushes per second for 15,000 cycles. The micro-structured NG seems to have a greater voltage with the number of pushing cycles, possibly due to charges accumulation throughout the cycles. After being subjected to 15,000 pushing cycles, the micro-structured NG shows a peak-to-peak voltage of (7.7 ± 0.1) V. This value proves the robustness of the NG and its potential use in a daily life situation, where it is necessary to have a device able to withstand a great number of cycles without a significant deterioration of its performance.

Proof-of-Concept of the NG
The micro-structured ZnO@PDMS NG was connected to several load resistances (10 MΩ to 100 MΩ) to evaluate the output voltage and power density with a fixed pushing force of 2.3 N at 2 pushes per second. As shown in Figure 5a, the peak-to-peak voltage generated by the NG increases with an increasing load resistance. Regarding the power density, despite reaching an apparent peak at 40 MΩ due to a slight power density decrease for 50 MΩ and 60 MΩ, which is mainly explained by experimental errors, it hits a maximum peak approximately at 70 MΩ, steadily decreasing afterwards. Therefore, 70 MΩ is considered to be the load that maximizes the power density at 4.8 µW cm −2 .

Conclusions
This work reports the use of ZnO NRs, produced through microwave radiatio sisted hydrothermal synthesis, to fabricate energy harvesters. Specifically, the ZnO were mixed with PDMS to achieve a flexible composite suitable for robust NGs.
Even though the ZnO NRs were produced through a faster and low-cost met they still presented piezoelectric properties, namely a d33 of (9 ± 2) pm V −1 , which ar pectable for these structures when produced through conventional methods, thus val To test the practicality of this NG, the device was connected to a full-rectifier bridge and its DC output (for a pushing force of 2.3 N applied at a frequency of 2 pushes per second) was used to charge a 10 µF capacitor. After charging for approximately 1 h, the capacitor (roughly reaching 4 V and a stored energy of 80 µJ) was connected to a blue LED (Ref. 017-1059 from Robert Mauser, Portugal; Video S1) or a digital thermometer (Video S2) to power them up, as illustrated in Figure 5b,c. These results show that the NG can generate enough energy to be stored and later used to power up small electronic devices, proving its potential as an effective energy harvester.

Conclusions
This work reports the use of ZnO NRs, produced through microwave radiation assisted hydrothermal synthesis, to fabricate energy harvesters. Specifically, the ZnO NRs were mixed with PDMS to achieve a flexible composite suitable for robust NGs.
Even though the ZnO NRs were produced through a faster and low-cost method, they still presented piezoelectric properties, namely a d 33 of (9 ± 2) pm V −1 , which are expectable for these structures when produced through conventional methods, thus validating the synthesis technique as a proper way to obtain piezoelectric and functional structures.
The influence of the ZnO NRs concentration in the devices' performance is clear. After the optimization of the NRs concentration, micro-structuring was added to the composite films, resulting in a great enhancement on the performance of the nanogenerators. The micro-cones contributed to the performance improvement through two aspects: first, the structuring increases the efficiency in force delivery to the ZnO NRs, increasing their piezoelectric output; second, the structuring introduced air gaps between the composite and the ITO electrodes, which is beneficial to potentiate the triboelectric effect. Nanogenerators with a ZnO@PDMS micro-structured into micro-cones and a ZnO NRs concentration of 25 wt % presented a peak-to-peak voltage of approximately 6 V at a pushing frequency of 2 pushes per second with a pushing force of only 2.3 N.
The nanogenerators could withstand 15,000 pushing cycles without a performance deterioration, which shows their robustness, being also able to charge a 10 µF capacitor until storing approximately 80 µJ (after 1 h of pushing) for powering a blue LED or a digital thermometer. With an external load of 70 MΩ, the nanogenerators could deliver a power density of 4.8 µW cm −2 . These results highlight the potential of these flexible nanogenerators, with an easy, low-cost, low-temperature, and scalable fabrication process, for energy harvesting and real-life applications such as powering of small electronic devices.
Supplementary Materials: The following are available online at https://www.mdpi.com/2227-9 040/9/2/27/s1, Figure S1: Photographs of the bending machine employed to exert a controlled force on the NGs; Figure S2: Histograms for the (a) length, and (b) diameter of the ZnO NRs produced synthesized through microwave assisted hydrothermal synthesis; Figure S3: Piezoresponse characterization of ZnO NRs through piezoelectric force microscopy; Video S1: Powering a blue LED; Video S2: Powering a digital thermometer.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.