Piezoelectric nanogenerator based on ZnO nanorods

1 Introduction

Zinc oxide nanostructures with a wide direct band gap (3.37 eV) and an efficient excitation emission at room temperature due to large exciton bonding energy (60 MeV) (Wang, 2004; Tang and Zhou, 2004 and Look and Reynolds, 1998) make it proper for opto-electric applications for short wavelengths. Other properties like fast photo-response for ultraviolet (UV) light in the photo-detectors and transparent to visible light was reported by (Tang and Zhou, 2004). Now many applications depend on ZnO nanostructures such as optical pumped laser, light emitting diodes, UV photoelectric devices, biosensors, solar cells, and piezoelectric nanogenerators (Youfan and Chang, 2010; Dhara and Giri, 2013; Zhuo and Feng, 2008; Dagdeviren, 2010 and Dagdeviren and Hwang, 2013). The structure of ZnO crystal consists of alternating planes in which each atom is tetrahedrally coordinated, with the O⁻² and Zn²⁺ ions stacked alternatively along the c-axis, and the center of gravity of the charges is at the center of the tetrahedron where positive and negative charges cancel each other. The lack of center of symmetry combined with the large electromechanical coupling results in a strong piezoelectric response (Wang, 2004 and Özgür and Aliviv, 2005).

ZnO piezoelectric nano-generator (PZG) transfers mechanical energy to kinetic energy and vice versa and hence is utilized for many applications such as transducers, sensors and actuators (Wei and Pan, 2011; Arya and Saha, 2012 and Chen and Choe, 2012). But the challenge here is to produce ZnO nanostructures with the same morphology in relatively large quantity in order to build portable PZG devices. Several morphologies of ZnO such as nanorods, nanocombs, nanobelts, nanowires, nanosheet and nanorings (Consonni and Sarigiannidou, 2014; Lee and Minegishi, 2008 and Elias et al., 2008) have been reported. The variety of nanostructures can be synthesized by various techniques including gel–sol process (Tay and Li, n.d; Shan and Xiao, 2006), hydrothermal growth and chemical vapor deposition (CVD) (Sun and Liu, 2006; Patil and Pawar, 2012; Zhang and Ram, 2012; Umar and Kim, 2005 and Wu and Yang, 2001).

The most important drawbacks of various ZnO nanostructures synthesis methods are their low quantity and physical stability. Unstable produced nanostructures in the hydrothermal aqueous solution for instance may cause the nanostructures to recombine, aggregate and accumulate to form bigger structure instead, while CVD solves this problem and produces stable, high crystalline quality nanostructures with defined size and shape and it can be controlled for relative large mass production. Formation of ZnO nanomaterial in the CVD technique from a mixture of ZnO and graphite was used as source of growth precursors in long horizontal quartz tube. Therefore, the amount of evaporated gases delivered to growth sites such as silicon wafers at cooler locations could allow nanostructures nucleation growth to begin. But due to the limitation sizes of the silicon wafers, growth platforms, inside the horizontal quartz tube the total harvested nanostructures will not be suitable to build working piezoelectric device. Therefore, in this study an alternative solution was achieved by utilizing the inner side of horizontal quartz tube as the favorite nanostructure growth platform for large production and hence a piezoelectric device was designed and built.

2 Experimental setup

In this process we design tube in tube chemical vapor deposition (CVD) system to produce large scale, mass in grams, production of ZnO NRs. Firstly, powders from Sigma Aldrich of graphite (99.99%, <45 μm) and ZnO (99.9%, <5 μm), with mass ratio of (1:1) were mixed and grained well. Then an amount of 20 g of the mixture was added each time into a combustion boat and used as source material. The source material was loaded into a 3.8 cm-inner diameter quartz tube (large tube), which was placed at the center of a 45 cm long horizontal tube furnace as shown in Fig. 1. And another small quartz tube (D = 2.6 cm and L = 12 cm) was loaded inside the horizontal quartz tube to work as growth platform or locations near the boat directly. The inner sides of both large and small quartz tubes are the locations where expected to grow large scale ZnO nanowires. The horizontal quartz tube was connected to argon (99.999%) gas supply and a flow rate control system at one end while the other end kept opened as it is shown in Fig. 1. The Ar gas was then flushed inside the quartz tube to get rid of all other gases and kept at 10 sccm, standard centimeters cubic per minutes. After that, the system is connected to normal ventilation vacuum pump, 10⁻¹ mbar. Then the furnace was switched on and the temperature was raised up to (1000 ± 15 °C) at a heating rate of 1.2 °C/s. The temperature is constant at the middle area of the furnace but gradually decreases near its edges, as illustrated in temperature distribution curve shown in Fig. 1c. After the source material was completely evaporated, the furnace was turned off and kept to cool down to room temperature under same Ar flow rate. The inner side of the short quartz tube was covered with a white-gray color thin layer at locations near to the source material place. Then the thin layer was scratched out for five trials. Finally, the nanostructures in powder form were taken for further analysis and characterizations.

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Figure 1

Tube in tube CVD growth process for large scale production of ZnO nanowires.

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3 Results and discussions

The experiments were carried using one large quartz tube or large tube embedded with smaller one, see Fig. 1. In all large tube trails high yield grown nanostructures with different sizes and morphologies were obtained in the non-catalytic quartz tube inner side surface as it can be revealed in Fig. 2(a). The various obtained nanostructures morphologies can be attributed to different growth temperatures due to deposition locations far from the source material. The other thinner rods were grown at close distance to the source materials. The growth precursor concentration and temperature profile are playing a more vital role in the morphology control. In Fig. 2(b), EDX spectrum reveals that these nanostructures consist of major elements of Zn and O and a small amount of platinum, which is normally due to the imaging process requirement. Because in order to produce clearer images without any disruption from interface between incident electron beam and secondary reflected electrons the nanostructures were sprayed with Pt layer to absorb the extra electrons and reduce the charging effects.

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Figure 2

FESEM image of ZnO nanostructures with different morphologies grown at various locations inside the large quartz tube, (a) EDX spectrum for Fig. 2(a) image, (c) low magnification of grown ZnO NRs inside the small quartz tube and (d) magnified image.

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To produce ZnO NRs only, which is preferable shape of ZnO nanostructures in piezoelectric devices (Khan and Abbasi, 2012 and Soomro and Hussain, 2012), smaller quartz tube was placed inside the lager tube as illustrated in Fig. 1. The small tube will accumulate more growth precursors, near the source material, under almost 1000 °C heating temperature. In Fig. 2 (c), the scratched ZnO NRs from the small quartz tube were randomly distributed with uniform shape. The base of the NR has hexagonal shape with direction growth along crystal c-axis as shown in Fig. 2(c and d) and the rod diameter decreases to form narrow needle as can be seen in Fig. 2d. The rods have typically (3.9 ± 0.8)μm. average length and (57 ± 11)nm. average diameter, measured at middle point of rod base. Fig. 3 shows the Gaussian distribution of the length and diameter of the obtained nano-rods.

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Figure 3

The Gaussian distribution of (a) the length and (b) diameter of ZnO NRs.

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The XRD pattern of synthesized NRs in Fig. 4 shows a poly crystalline orientation with wurtzite ZnO structure. The grown ZnO NRs have 9 diffraction planes, (100), (002), (101), (102), (110), (103), (200), (112) and (201) at different 2θs with dominant diffraction peak. Also, the ZnO’s XRD pattern indicates the pure phases with no characteristic peaks for other impurities. The strong intensity and narrow width of ZnO diffraction peaks indicate that the resulting products were of high purity and high crystallinity.

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Figure 4

The XRD spectrum of ZnO NRs.

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We believe that the vapor-solid (VS) growth mechanism can be used to explain the nucleation and evolving of ZnO NRs at the surface of quartz tube. The self-seeding of ZnO material is taken place in different orientations. Zn atoms were evaporated from the source material and were nuclei with oxygen. The increase in the concentration of deposition layers of Zn and O causes the upward growth of ZnO crystal along (0001). ZnO crystal is a polar crystal due to the Zn⁺² and O⁻² ions. The positive plane (0001) of the crystal contains the Zn atoms that the growth rate in this plate is the fastest than the negative plate (000 $\overline{1}$ ) where it has the slowest growth rate. This forces the growth along the c-axis direction. Different crystal faces have a different rate of growth as follows: (0001) > (01 1) > (01 0) > (000 $\overline{1}$ ).

The piezoelectric phenomenon is described as the ability of the material to convert the mechanical energy into electrical energy. We have demonstrated an approach for converting mechanical energy into electric power using our fabricated ZnO NRs. The net charge of ZnO crystal is balanced and each positive charge cancels nearby negative charge. When the piezoelectric crystal squeezes, the structure will deform and as a result net positive and negative charges will appear on opposite crystal faces. This produces a potential difference across the material. In-house built-in device was designed with a conductive material of cupper (Cu) disc, 0.05 mm thick aluminum (Al) disc, paper (insulator) and the synthesized ZnO NRs as shown in Fig. 5(a and b). The insulated paper was shaped in ring shape and was placed between Al and Cu disk boundaries. The ZnO NRs in powder form were inserted in the inner area centered between two electrodes of Al and Cu discs. Then, special glow was used to stick the materials together. The circular contact area between two electrodes and ZnO NRs is about 4.52 cm².

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Figure 5

(a) The device setup, (b) scope photo of the device and (c) the connection view for taking measurements.

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The piezoelectric device design depends on an unique coupling between piezoelectric and semiconducting properties of the ZnO NRs (Song and Zhou, 2006 and Wang and Zhou, 2006). The asymmetric piezoelectric potential and the Schottky contact between the metal electrode and the NRs are the two key factors for creating, separating, preserving, accumulating, and outputting the charges (Wang and Song, 2006). The Al (ϕ = 4.08 eV) disk not only enhanced the conductivity of the electrode, but also created an ohmic contact at the interface with n-type ZnO NRs, which has electron affinity Ea = 4.5 eV (Hasegawa and Nishida, 2005). Therefore, there is no barrier at the interface of Al-ZnO contact and electrons can move freely both sides, i.e. from electrode to ZnO NRs and vice versa (Mead, 1965). At the bottom electrode, Cu has ϕ = 4.53–5.10 eV (Xu and Shin, 2010); therefore, Cu-ZnO contact is a Schottky barrier and dominates the entire transport process. Because the compressed side of the semiconductor ZnO NRs has negative potential and the stretched side has positive potential, two distinct transport processes will occur across the Schottky barrier.

In practice, the device becomes like a battery with a positive charge on one face and a negative charge on the opposite face when pressed and relaxed. The circuit was completed by connecting the device with the voltmeter as illustrated in Fig. 5c, and the measured voltage was recorded at various stress forces. The force was obtained by measuring and estimating finger pressed force over a weigh balance and then multiplied with the gravity acceleration g = 9.8 m/s². The mechanical force per circular area was measured and all results are illustrated in Table 1.

It is noticed that as the stress force increases more current will flow and the maximum voltage has reached $0.7\text{V.}$ . Fig. 6a shows current-voltage characteristics (I–V) of the piezoelectric device or nano-generator was measured twice, directly after preparing the piezoelectric device (t = 5 min) and at time (t = 2 h), to make sure of the device work stability. These measurements were conducted under compressed mode, the mechanical force being turned on, and relaxation mode regularly. The nano-generator exhibited Schottky-like I–V characteristics and constructively generated harvesting currents. Current jumps were by 4.14 μA when the applied force was increased by about 20 N. Correspondingly, the voltage signal exhibited a similar output of ∼0.25 V. Similarly, at nano-generator relaxation mode voltage output increased in opposite direction correspondingly. The energy harvester generated 0.74 V of the maximum output voltage and 1.2 × 10⁻⁰⁵ A/cm² of the maximum current during compression and relaxation periodic motion. Therefore, our device maximum power is about 8.97 μW/cm². This efficiency considered to be in the middle domain of piezoelectric devices when compared with others groups’ published work (Gu and Cui, 2013; Chen and Xu, 2010; Chun and Kang, 2015 and Xu and Hansen, 2010).

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Figure 6

Output I–V characteristic lines of the nano-generator at t = 5 min and 2 h.

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In Fig. 7 the physical principal behind the discharge energy in the piezoelectric device arises from how the piezoelectric and semiconducting properties of ZnO are coupled. The ZnO NRs deformation create strain field along the nano-rod, its outer surface being stretched (positive strain ε) and the inner surface compressed (negative ε), so while positive strain (+ε) causes positive electric field (+E). When this happened for each nano-rod the collective electric fields among all nano-rods will create common electric field, which depends on the alignment of these nano-rods.

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Figure 7

(a) Schematic drawing of a piezoelectric device with compression and relaxation modes, (b) output voltage in both compressed and relaxation modes due to an external applied mechanical stress forces and (c) the same voltage fluctuations at both modes and mechanical forces.

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The collective potential (V) is created by the relative displacement of the Zn⁺² cations with respect to the O⁻² anions, a result of the piezoelectric effect in the wurtzite crystal structure; thus, these ionic charges will cause a potential difference (ΔV) across the nano-rods, see Fig. 7(a and b), and this is because these ions cannot move or recombine without releasing the strain. In Fig. 7(b) the output voltage measured after compression and relaxation modes is directly proportional to the applied mechanical press force. This voltage will fluctuate depending on the force applied and which mode is measured, see Fig. 7(c).

4 Conclusion

An in-house piezoelectric nano-generator (PZG) based on ZnO NRs was built and utilized successfully as an alternating electric current producer. It was found that tube-in-tube CVD technique produces 3–5 g of ZnO NRs each cycle, which are required for construction of PZG. The PZG responds to mechanical stress force by producing direct current on one direction in compression mode and in opposite direction in the relaxation mode and the maximum voltage has reached 0.7 V. This voltage will fluctuate depending on the force applied and which mode is measured.