Sandwich type piezoelectric nanogenerators based on one- and two- dimensional ZnO nanostructures with improved output performance

doi:10.21203/rs.3.rs-2070142/v1

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Sandwich type piezoelectric nanogenerators based on one- and two- dimensional ZnO nanostructures with improved output performance

https://doi.org/10.21203/rs.3.rs-2070142/v1

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The piezoelectric nanogenerators (PENGs) have attracted a great deal of interest owing to their broad application in environmental mechanical energy harvesting to power small electronics devices. Here, the novel flexible and high performance sandwich-type piezoelectric nanogenerators based on one-dimensional (1-D) and two-dimensional (2-D) Zinc Oxide (ZnO) nanostructures and Ni foam as the middle layer have been developed. The morphology and structure of 1- and 2-D ZnO nanostructures have been studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). To study the effect of structural design on the piezoelectric performance, the single layer PENGs were also fabricated. The piezoelectric output of all prepared PENGs were evaluated under cyclic human impacts at various forces and frequencies. The output performance of the double-layer designed PENGs showed a two times larger voltage output compared to the single layer PENGs. The working mechanism of the prepared PENGs is also discussed. Design of nanogenerators as double-layer sandwich structure instead of single layer reduces the overall preparation time and processing steps as well as enhancement of their output performance, enhancing their capability for using in practical applications.

Piezoelectric nanogenerators

ZnO

sandwich structure

voltage output

self-powered systems

Today, portable electronic devices have become an integral part of our daily life and play an important role in several areas such as health care, communication and environmental monitoring [1]. Growing demand along with development of portable and wearable systems in recent years, have caused the interest in the development of renewable and independent energy sources for powering these devices [2]. Thereby self-powered technology based on energy harvesting from the environmental sources, such as solar, mechanical and thermal have garnered a lot of attentions[3].

Harvesting the thermal and mechanical energies from the ambient and converting them into electrical energy, nanogenerators have been introduced to meet the need for self-powered devices[4]. Nanogenerators are mainly based on three effects including piezoelectric and triboelectric effects for mechanical energy harvesting, and pyroelectric effect for thermal energy harvesting[5, 6]. Owing to the abundant availability of ambient mechanical energy sources such as wave, wind, vibration, human motion etc, mechanical energy harvesting is a qualified candidate for transformation to electricity [7–9].

The first piezoelectric nanogenerator, based on ZnO, was introduced in 2006[10]. With the rapid growth of energy harvesting technology, various piezoelectric nanogenerator (PENG) devices based on piezoelectric materials such as PZT[11], PVDF and BaTiO₃ [12] have been studied. Among the various piezoelectric materials, ZnO has been widely studied due to its unique piezoelectric, semiconducting, and optical properties [13–17]. Moreover, ZnO can be easily synthesized in the different nanostructures using a variety of synthesis techniques[18]. Many investigations on PENGs have been focused on enhancement of output power of systems which is one of the main challenges in development of practical piezoelectric devices [19, 20].

The various approaches have been ever developed for improving the output performance of ZnO based PENGs[18]. Recently, some work has demonstrated that double structural design of ZnO based PENGs can improve their energy harvesting performance[21, 22]. Shin et al [21], introduced a double heterostructure of ZnO nanorods/graphene/ZnO nanorods in which increment of the number density of NRs resulted in improvement of piezoelectric output up to two times compared to single heterostructure. This was because the piezoelectric output from both up and down grown nanorods were added. Jung et al[22], fabricated a PENG based on ZnO nanorods on double-sided stainless steel foil. It was found that the voltage output from the fabricated PNG was larger than that from the PNGs on one side substrate. It was also greater than the sum of the output voltage from both sides of stainless steel substrates. They declared that such enhancement may be owing to an electric field induced synergetic effect between the two PNGs. The multilayer design of PENGs consisting a conductive intermediate layer has been demonstrated to improve the output performance [23–26]. Yoon et al [24], developed a sandwich-type ZnO/Ag/ZnO-stacked PENG, generating a relatively high output voltage compared to the single ZnO-layered device. However, they found that this device is structurally weak under the applied mechanical forces owing to hardening and brittleness of the Ag paste layer over time[26]. Therefore, they demonstrated a sandwich-type ZnO piezoelectric nanogenerator, composed of stacked layers of ZnO/carbon tape/ZnO. It was shown that, adopting a conductive carbon tape lead to significant increase of output voltage[26]. Jung et al [25] incorporated a Cu interlayer between the ZnO thin films to form a multilayered structure. They showed that piezoelectric output enhanced by intervention of the Cu layer, associating with the induced interfacial polarization between the ZnO film and Cu inclusion.

All of sandwich-designed PENGs in the literature survey report the ZnO thin film, while, the PENGs based on ZnO nanostructures such as nanorods (NRs) and nanosheets (NSs) have been demonstrated as high performance nanogenerators owing to their high surface-to-volume ratio. Therefore, it is important to investigate sandwich design of these nanogenerators that has not been reported yet.

In the present work, novel sandwich designed flexible PENGs based on one- dimensional (1-D) ZnO nanorods (ZnO NRs) and two-dimensional (2-D) ZnO nanosheets (ZnO NSs) and Ni foam has been developed. The Ni foam was used owing to its porous surface that can efficiently increase the output of PENGs. The morphology and structure of ZnO NRs were characterized by SEM and XRD. The sandwich-type devices were fabricated in a cost-effective way by inserting a Ni foam between two layers of vertically grown ZnO nanostructures on flexible substrates. To investigate the effect of double layer structure on the output performance, the single layer PENGs were also prepared. Piezoelectric output of all nanogenerators were measured under periodic impacts. Origin of the piezoelectric output improvement of multilayer devices were also discussed.

2.1. ZnO nanostructures synthesis

The 1- and 2D ZnO nanostructures were hydrothermally synthesized on seeded substrates according to our previous paper[27]. For NRs growth, flexible PET/ITO was selected as substrate. The substrate was cleaned by acetone, ethanol and deionized water to remove any impurities on the surfaces. It then was deposited by room temperature radio frequency (RF) magnetron sputtering. The sputtering was carried out in an argon atmosphere of 3.5 Pa at room temperature, and the RF power was 180 W. The seeded substrates were immersed in aqueous solution of zinc nitrate hexahydrate [Zn (NO₃)₂·6H₂O] and hexamethylenetetramine [(CH₂)₆N₄] (HMTA) in deionized water with the mole ratio of 1:1. The growth solution was transferred into a flask and refluxed for 3 h at 90 ̊C with continuous magnetic stirring. Finally, the substrates were removed from the solution, rinsed with deionized water, and dried at the room temperature. Figure 1(a) shows the schematic diagrams of the ZnO NRs growth on PET/ITO substrate.

For NSs growth, the Al foil was selected as substrate and seeded by chemical method as described following. The substrate was coated by 60 mM of zinc acetate in ethanol solution through spin coating and heated at 100°C. This stage was repeated three times to ensure the uniform formation of the seed layers. Finally, we put the substrate in the oven for 1 h at 150 ° C to form a thick layer of zinc acetate. The growth stage of ZnO nanosheets carried out in same method as described for NRs. The schematic diagrams of the ZnO NSs growth on Al substrate is shown in Fig. 1(a).

2.1. PENGs preparation

Two set of devices include single layer PENGs and multilayer PENGs based on ZnO NRs and NSs were prepared as schematically shown in Fig. 1 (b,c). For fabricating the single layer ZnO NRs based PENG, PET/ITO was used as top electrode. This sample with the structure of PET/ITO-ZnO NRs-ITO/PET, was named as S-PENG1. In the case of single layer PENG based on ZnO NSs, the PVDF/Au was used as top electrode. This nanogenerator with the structure of PET/ITO-ZnO NSs-Au/PVDF, was named as S-PENG2.

The double-layer PENGs based on ZnO NRs and NSs were prepared as described below. For multilayer layer ZnO NRs based PENG fabrication, a Ni foam was sandwiched between two PET/ITO-ZnO NRs layers. The volumetric porosity of Ni foam is 90–98% with the density of 0.15–0.45 g/cm³.The double-layer device with the structure of PET/ITO-ZnO NRs-Ni foam-ZnO NRs-ITO/PET named as D-PENG1. For the double-layer PENG based on ZnO NSs design, the ZnO NRs have grown on Au in same way as described for PET/ITO. A Ni foam sandwiched between Al-ZnO NSs and PVDF/Au-ZnO NRs. This double-layer sample with the structure of Al-ZnO NSs-Ni foam-ZnO NSs-Au/PVDF, was named as D-PENG2. The dimensions of all devices were 1.5 × 2 cm².

To complete the devices, Cu wires were connected to the electrodes. An image of prepared D-PENG2 is shown in Fig. 1 (d), illustrating the flexibility of fabricated nanogenerator devices, making them to be an appropriate choice for wearable electronic devices. For the single layer PENGs, the Cu wires were connected to the top and bottom electrodes, which is shown as an example for the S-PENG1 in Fig. 1(e). For the double-layer PENGs, the Cu wires were connected to the top, middle and bottom electrodes as shown in Fig. 1 (f) for the D-PENG1. The output voltage of all prepared devices were measured as depicted in Fig. 1 (g).

2.1. Characterization

The surface morphology of the ZnO nanostructures were investigated by a scanning electron microscope with the TESCAN MIRA3-XMU, Brno-Kohoutovice, Czech Republic) and the XL-30E Philips Co., Holland. To analyze the crystal structures of ZnO nanostructures, the X-ray diffraction analysis were carried out by an ITALSTRUCTURE, ADP 200 in the 2θ range of 30–60 at the scanning speed of 1.8 ̊ min⁻¹. The output voltage of the prepared PENG devices was measured with an oscilloscope (Agilent Technologies, DSO3062A, digital storage oscilloscope).

2.2. SEM

Figure 2 (a-c) show the SEM image of ZnO nanostructures on the different substrate. The surface SEM images of ZnO NRs grown on PET/ITO and PVDF/Au are shown in Fig. 2 (a,b), respectively. As evidence, the rather uniform and vertically well-aligned nanorods with high density were obtained. The mean diameter of nanorods grown on ITO and Au substrates were about 90 and 60 nm, respectively. Figure 2 (c) illustrates the SEM image of ZnO NSs grown on Al foil. It can be observed that ZnO NSs are also uniformly and densely grown on the Al substrate with the thickness of about 50 nm. The corresponding SEM image of Ni foam surface is also illustrated in Fig. 2 (d).

2.3. XRD

The X-ray diffraction analysis was also carried out to further characterize the crystalline structure of the as-grown ZnO nanostructures. The corresponding results for ZnO NRs and NSs are illustrated in Fig. 3(a) and (b), respectively. The peaks located at 2θ of 32.3 ̊, 35̊, 36.8 ̊, 48.1̊ and 57 ̊ are correspond to diffraction from (100), (002), (101), (102) and (110) planes of the hexagonal structure of ZnO[28]. Orientation of grown ZnO nanostructured can be examined from the ratio of intensity of (002) peak to other peaks such as (101) and (100). As evidence from Fig. 3(a), the intensity of the peak related to (002) plane is much higher than other peaks, indicating the well aligned growth of ZnO NRs along the c-axis direction. The XRD pattern of ZnO NRs on PVDF/Au was similar and has not been shown here to avoid duplication. From Fig. 3(b), it was observed that the intensity of the (100) and (101) peaks were higher than the (002) one. This result shows that the NSs are relatively tilted and not all are grow along the c-axis. These results are consistent with SEM. The sharp peak at about 44.3̊ is related to diffraction from the Al substrate[29].

2.4. Piezoelectric output

The voltage output of all single and double layer prepared nanogenerators has been measured under cyclic impacts. To evaluate the effect of working frequency on the piezoelectric output of PENGS, an impact experimental setup has been used to trigger PENGs at different frequencies. The employed setup is consisting of an impactor, a load cell and an oscilloscope. The samples were placed on impact stage and the vertical cyclic forces at desired frequencies were applied. The frequencies of the impacts were selected in the range of 1–5 Hz which is the normal frequency range for the human motions. To ensure repeatability of results, all the experiments were repeated three times and the median was considered as the response of the device.

Figure 4(a,b) show output voltages measured from S-PENG1 and S-PENG2, respectively, under the force of 4N at the frequencies of 1, 3 and 5 Hz. For both of the devices, the positive and negative peaks have been observed under compression and release owing to charging and discharging, respectively. As the frequency increased from 1 to 5 Hz, the average peak-to-peak voltage (V_pp) of 1 and 1.8 V were increased up to 7.2 and 10.3 V for S-PENG1 and S-PENG2, respectively. As results show, the voltage from both single layer nanogenerator devices increased with increasing frequency. This enhancement in the output voltage with applied frequency originates from the effect of initial impact speed[30].

To investigate the effect of sandwich design on the performance of nanogerator devices, the output of upper part, lower part and total of double layer PENGs were measured and compared. The upper part refer to when the output measurement is made between electrodes 1 and 2, depicted in Fig. 1 (f). As the same way, the lower part means when the output is measured between the electrodes of 2 and 3. For total double layer measurement, the electrodes of 1 and 3 are connected and the output has been measured between 1 + 3 and 2. Figure 5 shows the output performance of PENG1 and PENG2 at the three state of up, down and total under the forces of 4N at the frequency of about 1.5 Hz. From Fig. 5 (a), the output voltage of PENG1-up and PENG1-down were observed to be 1.7 and 1.5 V, respectively. For PENG1-total, the output voltages from the up and down parts were successfully collected and a total voltage of 3.1 V was obtained. The output voltage of PENG2-total has been enhanced almost 2-fold compared to those related to the up and down parts, owing to the integration of the piezoelectric output from both ZnO nanorods and nanosheets.

Figure 5 (b) illustrate that the output voltage of the upper and lower parts of PENG2 were 1.8 and 2.2 V, respectively. The PENG2-total showed a total output voltage of 4 V. It is clear that the output of nanogenerators in the state of total is almost two times higher than those for up and down parts. The results imply that each nanorod acts as a charging pump when is compressed. As a result, if the polarization of many nanorods under periodic vertical impacts could be synchronized, the output voltage could be constructively integrated.

From the Fig. 5, the effect of the nanostructure of ZnO on the output performance of nanogenerators can also be evaluated. As we know, for PENG1, the structure of the up and down parts are same and are based on ZnO NRs, leading to almost the same output. Whereas, the PENG2 is a hybrid system composed of the upper part based on ZnO nanorods and the lower part based on ZnO nanosheets. As evidence from Fig. 5 (d) and (e), the output of ZnO nanosheets is higher than ZnO nanorods. This is originated from the network structure of nanosheets (Fig. 2 (c)) along with the existence of vacant spaces results in the stress applied on a small area being transferred to the entire network, leading to deformation in relatively large area. Hence, in comparison to 1-D structure, higher surface polarization occurs in a 2-D networked structure which results in greater voltage output owing to enhancement of electromechanical coupling efficiency[31].

Another consequence that can be concluded from the Fig. 4 and Fig. 5 and is related to the effect of structure and material of the electrodes on the output performance. In this study there are three types of PENGs based on ZnO NRs with different electrodes including ITO/ZnO/ITO, ITO/ZnO/Ni and Au/ZnO/Ni corresponding to S-PENG1, D-PENG1-up (or down) and D-PENG2-up, respectively. In ITO/ZnO/ITO and ITO/ZnO/Ni, the top electrodes are different. From Fig. 4 and Fig. 5 it can be concluded that the output performance of ITO/ZnO/Ni is higher than ITO/ZnO/ITO which may be owing to porous surface of Ni foam that can efficiently increase the output performance[32]. From comparing the output voltage of ITO/ZnO/Ni and Au/ZnO/Ni, which are different in bottom electrode, it was observed that the output are not too much different.

To confirm the practical applicability of the prepared PENGs for converting the mechanical energy generated from human motions to electricity, the output of all prepared nanogenerator devices were recorded under different human impacts including finger tapping, hand slapping and hand hammering. It should be noted that the force and frequency selected in Fig. 5 are also similar to human walking conditions as explained in following. Since the diameter of the impactor is 9 mm, the applied pressure on the PENg devices is obtained 62.87 kPa which is similar to the applied pressure of a human foot during walking which is considered to be in the range of 60 to 80 kPa, as explained in our previous work[33]. The frequency of walking is also about 1–2 Hz.

Figure 6 (a, b) illustrate the V_pp of single and double layer PENG1 and PENG2 under finger tapping, hand slapping and hand hammering. As evidence from this figure, for all devices, the output voltage generated by hand hammering is obviously greater than those for finger tapping and hand slapping. These results imply that the prepared PENGs can be utilized as a body motion sensor, detecting human body activity. Such lightweight flexible nanogenerator have also potential application to be utilized in self-powered wearable electronic devices as the multifunctional power sources.

The overall effect of various parameters mentioned above include sandwich design, electrode structure and nanostructure of ZnO on the piezoelectric output of PENGs can be clearly seen from Fig. 6 (a, b). As evidence, porous structure of the Ni electrode can enhance the output about 1.8 fold. Also, the ZnO nanosheets show the output 1.4 times higher than ZnO nanorod. Finally, by applying a sandwich design, the output increases up to two times rather than single layer. Overall, the piezoelectric output has been enhanced about five times for all applied forces.

The working mechanism of single and double-layer PENGs can be explained through the schematic diagram shown in Fig. 7. For single layer structure, Fig. 7 (a), as an external compressing stress is applied on a piezoelectric ZnO nanorod, relative separation of O²⁻ anions and Zn²⁺ cations results in generation of a piezoelectric field along the ZnO NR driving the electrons flow from the top electrode to the bottom one through the external circuits. In double layer structures, Fig. 7 (b), the generated electric field enforce the electrons to flow from the both sides of PENGs. These electrons gather together, resulting in the increment of output voltages of the double layer nanogenerator compared to the single layer one.

In this work, we have demonstrated the novel double layer piezoelectric nanogenerators based on 1- and 2-D ZnO nanostructures. The Ni foam was used as middle electrode. The results of SEM and XRD confirmed the growth of uniform and high density well aligned vertical nanostructure. The piezoelectric output of the all prepared PENGs were evaluated under cyclic impacts at various forces and frequencies. The measured output voltage of nanogenerators were compared in various aspect including effect of nanostructure of ZnO, structural design, electrodes structure and applied forces and frequencies. The results showed that the output of PENG-total is almost two times higher than those for up and down parts. Moreover, utilizing the Ni foam as middle electrode increased the output voltage by 1.8 fold compared to flat ITO and Au electrodes. Also, the ZnO nanosheets show the voltage output 1.4 times higher than ZnO nanorod.

Overall, It was found that, design of nanogenerators in a double-layer sandwich structure compared to a single layer reduces the overall preparation time and processing steps and increase their output performance as well, leading to enhancement of their capability for using in practical applications.

Funding

This work was supported by Grant numbers [99- 212] of Bu-Ali Sina University.

Competing Interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author Contributions

Parisa Fakhri: Conceptualization, Methodology, Writing - original draft, Writing - review & editing, Validation, Formal analysis. Naeimeh Eaianli: Methodology, Writing original draft, Writing - review & editing, Validation, Investigation. Roohollah bagherzadeh: Writing - original draft, review & editing. Babak Jaleh: Conceptualization, Supervision, Methodology, Writing - original draft, Writing - review & editing; Mika Sillanpää: Writing - original draft, review & editing

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Sandwich type piezoelectric nanogenerators based on one- and two- dimensional ZnO nanostructures with improved output performance

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Abstract

Figures

1. Introduction

2. Experiments

2.1. ZnO nanostructures synthesis

2.1. PENGs preparation

2.1. Characterization

2.2. SEM

2.3. XRD

2.4. Piezoelectric output

3. Conclusion

Declarations

References

Additional Declarations

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