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Porous ZnO Nanostructures Synthesized by Microwave Hydrothermal Method for Energy Harvesting Applications

The ever-growing global market for smart wearable technologies and Internet of Things (IoT) has increased the demand for sustainable and multifunctional nanomaterials synthesized by low-cost and energy-efficient processing technologies. Zinc oxide (ZnO) is a key material for this purpose due to the variety of facile methods that exist to produced ZnO nanostructures with tailored sizes, morphologies, and optical and electrical properties. In particular, ZnO nanostructures with a porous structure are advantageous over other morphologies for many applications because of their high specific surface area. In this chapter, a literature review on the latest progress regarding the synthesis and applications of ZnO with a porous morphology will be provided, with special focus on the synthesis by microwave hydrothermal method of these nanomaterials and their potential for application in energy harvesting devices. Nanogenerators of a composite made by polydimethylsiloxane (PDMS) and porous ZnO nanostructures were explored and optimized, with an output voltage of (4.5 ± 0.3) V being achieved for the best conditions. The daily life applicability of these devices was demonstrated by lighting up a commercial LED, by manually stimulating the nanogenerator directly connected to the LED.


Introduction
Zinc oxide (ZnO) is an inorganic semiconductor material that has been applied in a wide range of applications over the last centuries [1]. The attraction to ZnO can be attributed to its remarkable optical and electronic characteristics. With a direct and wide bandgap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature [2], ZnO has the potential to be applied in advanced electronic and optoelectronic devices with promising results, such as UV sensors [3,4], transparent electrodes [5,6], gas sensors [7], thin film transistors [8,9], and solar cells [10][11][12].

Synthesis and applications of porous ZnO nanostructures
Despite all the established applications of ZnO, the research involving this semiconductor has not yet diminished, mostly due to the continuing development of new synthesis technologies and applications. For instance, ZnO nanomaterials can be easily synthesized into tailored sizes and morphologies at low temperatures (< 200 °C) by a variety of methods, including chemical bath deposition [38], electrodeposition [39], chemical vapor deposition [40], electrospinning [41], laser assisted flow deposition [42], and solvothermal [16] or hydrothermal [43,44] synthesis, either by conventional or microwave-assisted heating [4,45].
Porous oxide semiconductor nanomaterials, particularly two-dimensional (2D) materials with nanoscale thickness, are promising candidates due to their usually large specific surface areas that can improve their performance in several applications [46][47][48][49][50][51]. These nanomaterials can inclusively assemble into three-dimensional (3D) hierarchically structures with controlled morphology and dimensions which can lead to novel properties and applications [52]. The self-assemble technique is a facile method to produce 3D hierarchical structures where low-dimension building units aggregate spontaneously into high-dimensional architectures. This technique Porous ZnO Nanostructures Synthesized by Microwave Hydrothermal Method for Energy… DOI: http://dx.doi.org /10.5772/intechopen.97060 offers many advantages over other methods as it can be performed at low temperatures using low-cost materials while having high yield for scale production [52].
An indirect way to produce porous 3D ZnO structures has been recently developed by thermal decomposition of layered zinc hydroxide (LZH) precursors [52]. LZHs are usually composed of positively charged zinc hydroxyl layers intercalated by anions that balance the overall charge and water molecules [53]. The anions in LZH generally include CO 3 [50,[53][54][55][56][57][58]. These LZH precursors are fabricated with the desired morphology and then converted into porous ZnO nanomaterials by a calcination process at high temperatures [59]. During calcination, the precursors release gaseous molecules and, consequently, the original structure contracts and pores are formed throughout the structures [52].
LZHs are typically obtained via solution techniques, mainly hydrothermal methods where the materials' synthesis occurs in a basic medium that results from the addition of certain reagents, such as hexamethylenetetramine [60], ammonia [58,61], and urea [52,[62][63][64][65]. Although the basic structure is similar in all LZHs, the sites occupied by the anions and water molecules are different and, as a result, the final morphology, crystal structure, interlayer distances, and thermal decomposition temperature differ depending on the anion type [66]. In particular, the LZH carbonate (LZHC) is composed of zinc hydroxide layers combined with carbonate ions and water molecules. During the synthesis of this material, a well-crystallized phase is typically obtained with an invariable distance between the LZH [53]. The resulting morphology of LZHC usually consists of 2D structures stacked in a hierarchical 3D arrangement. However, the synthesis of uniform LZHC 3D morphologies through a simple and fast hydrothermal method has not yet been fully explored.
For this purpose, hydrothermal synthesis assisted by microwave irradiation offers many advantages over conventional heating. In a synthesis assisted by conventional heating, the heat transfer occurs through a combination of conductive and convective mechanisms that result in a low heating rate and, consequently, long synthesis time [67]. Conventional heating method is also dependent on the thermal conduction of the material of which the reaction vessel walls are made. Moreover, the temperature maximum occurs on the vessel wall surface, as shown in Figure 1. All these factors can lead to a non-uniform heating of the reaction medium and, subsequently, originate a heterogeneity in the obtained products [1]. On the other hand, hydrothermal synthesis assisted by microwave irradiation allows for rapid and uniform heating since the heat transfer occurs directly from the microwaves to the molecules of the reaction's materials, as illustrated in Figure 1. This results in high reaction rates and a homogeneous and volumetric heating [68,69].
This work aims to demonstrate the potential of high surface area porous ZnO nanostructures for energy harvesting devices, showing original and novel results regarding the characterization of nanogenerators based on these structures. For that, 3D hierarchically structures composed of LZHC nanoplates were successfully synthesized through a facile, low-cost, and low temperature hydrothermal process assisted by microwave irradiation. Porous ZnO nanostructures were obtained by calcination of the LZHC at 700 °C for 2 h in air while maintaining the LZHC hierarchical 3D structure. Porous ZnO nanostructures were then embedded in PDMS and deposited by spin-coating technique on flexible substrates. Energy harvesting based on a micro-structured composite of porous ZnO nanostructures embedded in PDMS was investigated. The combination of using the porous ZnO nanostructures, which have piezoelectric properties, and triboelectricity resultant from the microstructuring leads to a performance improvement of the nanogenerators [37,77]. To the best of our knowledge, porous ZnO nanostructures were for the first time used to fabricate a micro-structured PDMS/ZnO composite for energy harvesting devices.

Synthesis and characterization of porous ZnO nanostructures
Porous ZnO nanostructures were synthesized by hydrothermal method assisted by microwave irradiation. Zinc nitrate hexahydrate (Zn(NO 3 ) 2 ·6H 2 O, Sigma-Aldrich 98%) and urea (CH 4 N 2 O, Sigma-Aldrich 99.0-100.5%) were used without further purification. In a typical synthesis, 0.05 M of zinc nitrate was first dissolved in de-ionized water, and after its total dissolution, urea was added to the aqueous solution. The molar ratio of zinc to urea was kept at 1:5. Then, 25 mL of the obtained solution was transferred to a 35 mL Pyrex vessel which was placed in a CEM Discovery SP microwave. The synthesis was carried out at 140 °C for 15 min under a power of 100 W.
After the synthesis, the resulting white precipitates were washed with deionized water followed by isopropanol and centrifuged at 4500 rpm for 5 min. This washing process was repeated three times. The powders were dried in air at room temperature for 48 h and then calcinated in air in a Nabertherm muffle furnace at 700 °C for 2 h with a heating rate of 250 °C h −1 .
The crystallinity of the produced nanostructures was analyzed by X-ray diffraction (XRD) using a PANalytical's X'Pert PRO MRD X-ray diffractometer, with a monochromatic Cu Kα radiation source with wavelength 1.540598 Å. XRD measurements were carried out from 10 to 90° (2θ), with a scanning step size of 0.016°. The morphology of the LZHC precursor and porous ZnO nanostructures was evaluated by scanning electron microscopy (SEM) using a Carl Zeiss AURIGA CrossBeam FIB-SEM workstation equipped with an Oxford X-ray Energy Dispersive Spectrometer.
Differential scanning calorimetric (DSC) and thermogravimetry (TG) measurements of the synthesized product without any temperature treatment were carried out with a simultaneous thermal analyzer NETZSCH STA 449 F3 Jupiter. Approximately 20 mg of the synthesized powder was loaded into an open platinumrhodium crucible and heated in air from room temperature to 850 °C with a heating rate of 10 °C min −1 . Diffuse reflectance measurements of the porous ZnO nanostructures were performed at room temperature using a PerkinElmer lambda 950 UV/VIS/NIR spectrophotometer with a diffuse reflectance module with a 150 mm diameter integrating sphere, internally coated with Spectralon. The calibration of the system was achieved by using a standard Spectralon reflector sample as reference. The reflectance spectra were obtained from 350 to 800 nm.

Fabrication and characterization of energy harvesting devices
The devices were fabricated as described in references [27,78] and the fabrication process is illustrated in Figure 2. Briefly, composites of porous ZnO nanostructures embedded in PDMS were produced with concentrations of 20, 25, and 30 wt%. Firstly, the nanostructures were mixed with the PDMS elastomer (from Dow Corning) and a volume of ethyl acetate (from Fluka-Honeywell) enough to ensure a homogeneous mixture of elastomer and nanostructures. The mixture was stirred until the evaporation of the solvent, and then a curing agent (Sylgard 184, from Dow Corning) was added in a weight ratio to the elastomer of 1:10 while stirring to obtain a homogeneous mixture. Two types of devices were produced, unstructured and micro-structured nanogenerators. The former was fabricated by spin-coating the mixture at 250 rpm for 90 s, with an acceleration of 100 rpm·s −1 , on commercial substrates of polyethylene terephthalate (PET) with a layer of indium tin oxide (ITO) deposited on top (PET/ITO, from Kintec Company), whereas the latter was obtained by depositing the mixture in a similar way on acrylic molds (5 mm thick, from Dagol). The acrylic molds used were produced as described in reference [79].
The composites were then cured at 60 °C for 1 h. After the curing process, PET/ITO electrodes were placed on top of the composite films, as shown in Figure 2. The electrical characterization of the produced nanogenerators was performed by applying a mechanical stimulus in a contact area of 0.3 cm 2 with a pushing force of 2.3 N at different frequencies (0.5, 1, 1.5, and 2 pushes per second) with a home-made machine with a linear motor.   Figure 3(a) presents the X-ray diffractogram obtained from the final product of the hydrothermal synthesis prior to the calcination process. All the peaks from the diffractogram can be indexed to zinc hydroxide carbonate hydrate (Zn 4 (CO 3 ) (OH) 6 ·H 2 O) (ICDD 11-0287). The morphology of the precursor was observed in SEM and it is shown in Figure 3(b). The SEM image reveals that the LZHC precursor obtained after only 15 min of microwave hydrothermal synthesis consists of many flower-like structures, with a few micrometers of diameter, composed of densely packed LZHC nanoplates with a few nanometers of thickness.

Characterization of the LZHC precursor
Differential scanning calorimetric (DSC) measurements were carried out in air from room temperature to 800 °C to analyze the conversion process of LZHC into ZnO. The DSC curve in Figure 3(c) shows two endothermic peaks at 64 °C and 266 °C. The peak at 64 °C corresponds to the removal of water that is weakly adsorbed to the LZHC nanostructures [80], resulting in a weight loss of 38.12%. The second peak at 266 °C results in a weight loss of 14.87% and it is associated with the release of water and carbon dioxide from the thermal decomposition of LZHC precursor [50,59].

Characterization of the porous ZnO nanostructures
After the calcination process, the LZHC precursor was successfully converted into porous ZnO nanostructures, which can be inferred from the X-ray diffractograms of the samples obtained at 700 °C, depicted in Figure 4(a). All the peaks in the diffractogram correspond to the hexagonal wurtzite ZnO structure (ICDD 36-1451). No characteristic peaks from any other impurities were detected, indicating that the LZHC precursor was completely converted into ZnO. SEM images of the calcinated product are presented in Figure 4(b) with different magnifications. The low magnification image shows that the morphology of the final ZnO product did not suffer significant changes when compared with the LZHC precursor, since ZnO nanoplates are still assembled into flower-like structures. However, when observing the high magnification SEM images, it is possible to see that the ZnO nanoplates present a porous structure with serrate edges and a wide pore size distribution.
The synthesis of LZHC precursor by urea-assisted hydrothermal method, followed by the calcination process to originate porous ZnO nanostructures, has been explained before in the literature [44,65,73,80,81]. Figure 5 shows a simple schematic of the synthesis and transformation process of LZHC precursor into porous ZnO nanostructures. During the hydrothermal synthesis, urea is hydrolyzed leading to the formation of hydroxide (OH − ) and carbonate (CO 3 2− ) ions. Zn 2+ ions from the added zinc salt react with both OH − and CO 3 2− ions forming the LZHC precursor (Zn 4 (CO 3 )(OH) 6 ·H 2 O). It has been reported that the surface of LZHC plates is hydrophobic whereas the lateral sides are hydrophilic, resulting in a vertical growth of this material and consequent plate-like morphology [76,82,83]. The agglomeration of these nanoplates into stable flower-like microstructures occurs to favor the minimization of surface energy by reducing exposed surface areas [83]. Under calcination at high temperature, LZHC is decomposed into ZnO by releasing H 2 O and CO 2 in the form of gas, which leads to a contraction of the original structure which originates pores throughout the nanoplates and a consequent porous ZnO structure, as illustrated in Figure 5 [84].
The UV-Vis diffuse reflectance of the produced ZnO samples is presented in Figure 6. The optical band gap E g was calculated by applying the Kubelka-Munk (K-M) method to the reflectance (R) data [85]. The K-M method is based on the following equation: (1)  The K-M function (F(R)) is proportional to the absorption coefficient (α). Therefore, by considering the Tauc relation, the following expressions can be obtained [86]: where A is a constant and n is equal to 2 for semiconductors with direct allowed transitions [87]. As shown by the inset graph in Figure 6, the value of E g can be determined by extrapolating the linear part of the function curve with the energy axis. The estimated bandgap energy is 3.26 eV for ZnO nanostructures obtained at 700 °C, which is consistent with the values reported in the literature [52,88].

Characterization of the PDMS/ZnO composite films
Composites of porous ZnO nanostructures embedded in PDMS (PDMS/ZnO films) were fabricated. The composites were produced with a micro-structuring and in an unstructured form. SEM images of a micro-structured porous PDMS/ ZnO film are presented in Figure 7(a). The array of aligned cones observed has an average height of 380 μm, an average diameter of 300 μm, and a gap around 100 μm. Figure 7(b) combines the XRD diffractogram of the porous ZnO nanostructures, the PDMS/ZnO composite film, and the pure PDMS film. As expected, even if presenting a much lower intensity, the hexagonal wurtzite ZnO structure (ICDD 36-1451) can be identified in the PDMS/ZnO composite, whereas the PDMS film presents an amorphous structure.

Performance of the PDMS/ZnO nanogenerators
To optimize the nanogenerator output, its performance was evaluated by varying the concentration of the porous ZnO nanostructures in the PDMS film. This study was performed with unstructured composites. Three concentrations were considered to produce the devices: 20, 25, and 30 wt%. Figure 8(a) presents the peak-to-peak output voltage of the nanogenerators. The electrical characterization of the nanogenerators was performed by applying a mechanical stimulus with a pushing force of 2.3 N at frequency of 2 pushes per second with a home-made bending machine. The obtained results reveal an increase of the output voltage from 20 to 25 wt%, and then a decrease for 30 wt%. These results are in agreement to what was previously observed using the same approach for ZnO nanorods, where the optimal concentration for the nanogenerators output was also 25 wt% [78]. As such, to further characterize the nanogenerators, the concentration considered was 25 wt%.
In previous studies from our group [27,78], an enhanced response was achieved by micro-structuring the composite, as shown in Figure 7, and, therefore, the same approach was adopted in this study. Figure 8(b) presents the output voltage  for this nanogenerator in comparison with the unstructured one. A peak-to-peak output voltage of (4.5 ± 0.3) V was obtained for the micro-structured nanogenerator against only (0.5 ± 0.2) V for the unstructured one. The micro-structuring can not only improve the force delivery into the nanostructures, leading to an increase of the piezoelectric effect, but it can also induce an extra triboelectric effect, as a consequence of the air gaps between the PDMS/ZnO composite micro-structures and the ITO electrode. These two effects originate an enhanced response of the micro-structured nanogenerator.
Considering the micro-structured nanogenerator with the best performance (25 wt%), the influence of varying the frequency of the stimulus was investigated while maintaining the applied force at 2.3 N. Figure 8(c) shows the peak-to-peak output voltage of the nanogenerator as function of the frequency, where the output voltage increases with increasing frequency. This trend has been observed by other groups, and it can be explained by the eventual accumulation of residual charges due to an inefficient neutralization of the induced charges provoked by a faster stimulation [89].
To study the potential of the nanogenerator in a daily life application, its stability along 12,000 cycles was also investigated. For this study, the stimulus was applied with a pushing force of 2.3 N while maintaining the frequency at 2 pushes per second. Figure 8(d) shows the output voltage along the pushing cycles, and no  deterioration of its performance is observed. Instead, it is possible to detect a slight increase of the output voltage to (7.2 ± 0.1) V along the pushing cycles, which can also be related to charges accumulation.

Proof-of-concept of the PDMS/ZnO nanogenerator
To understand the applicability of the micro-structured PDMS/ZnO nanogenerator, it is important to study its performance when connected to external load resistances with different values (1 to 30 MΩ). This study was performed with a fixed pushing of 2.3 N at 2 pushes per second. Figure 9(a) presents the peak-to-peak output voltage and current, while Figure 9(b) shows the resultant instantaneous power density. An increase of the power density with increasing load resistance is observed until 10 MΩ, reaching a maximum value of 2.7 μW cm −2 , after that a slight decrease is observed. Comparing to the recent results on PDMS/ZnO nanorods nanogenerators [78], the maximum power obtained here is just slightly lower, presenting the same order of magnitude. Its lower output is expected due the absence of a preferential direction for piezoelectric response (c-axes) in these nanostructures. Nevertheless, the synthesis of these porous ZnO nanostructures allows for a faster and low-cost fabrication of nanogenerators, since it is a rapid, simple, and high yield approach to obtain ZnO nanostructures.
Additionally, the nanogenerator output is very satisfactory, proven to be enough to light up a blue LED (2.8-4 V, 20 mA), by directly connecting the nanogenerator to the LED and manually stimulating the energy harvester, as shown in Figures 9(c) and (d) and Video 1 available from (can be viewed at) https:// youtu.be/JCT60ozKCX8. These results prove not only the applicability of these nanogenerators in simple daily life applications but also demonstrate their potential to power wearable sensors or multifunctional platforms where these porous ZnO nanostructures are employed in more than one application.

Conclusions
In summary, porous ZnO nanostructures were successfully synthesized via a facile and fast hydrothermal method assisted by microwave irradiation and calcinated at 700 °C for 2 h in air. The effect of calcination temperature on the morphological, structural, and optical properties of the porous ZnO nanostructures was investigated. Nanogenerators based on a micro-structured composite of PDMS with embedded porous ZnO structures were successfully produced, reaching an output voltage of (4.5 ± 0.3) V. The devices proved to be very robust and stable by presenting no deterioration of their performance after 12,000 pushing cycles. An external load of 10 MΩ optimized the nanogenerators performance, reaching a power density of 2.7 μW cm −2 . The capability of these nanogenerators to lighting up commercial LEDs, through direct connection and with a manual stimulus, was shown, demonstrating their potential for daily life applications.