Piezoelectric nanogenerator based on ZnO nanorods

Accepted Manuscript Original article Piezoelectric Nanogenerator Based on ZnO Nanorods Majid S. Al-Ruqeishi, Tariq Mohiuddin, Butheina Al-Habsi, Fatma AlRuqeishi, Ahmed Al-Fahdi, Ahmed Al-Khusaibi PII: DOI: Reference:

S1878-5352(16)30247-7 http://dx.doi.org/10.1016/j.arabjc.2016.12.010 ARABJC 2022

To appear in:

Arabian Journal of Chemistry

Received Date: Revised Date: Accepted Date:

27 September 2016 13 December 2016 15 December 2016

Please cite this article as: M.S. Al-Ruqeishi, T. Mohiuddin, B. Al-Habsi, F. Al-Ruqeishi, A. Al-Fahdi, A. AlKhusaibi, Piezoelectric Nanogenerator Based on ZnO Nanorods, Arabian Journal of Chemistry (2016), doi: http:// dx.doi.org/10.1016/j.arabjc.2016.12.010

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Piezoelectric Nanogenerator Based on ZnO Nanorods Majid S. Al-Ruqeishi1*, Tariq Mohiuddin1, Butheina Al-Habsi1, Fatma AlRuqeishi1, Ahmed Al-Fahdi1, Ahmed Al-Khusaibi1 1 Department of Physics, College of Science, Sultan Qaboos University, P.O. Box 36 P.C. 123, Al-Khoudh, Sultanate of Oman. *Corresponding author Tel : +968 24141496, Fax : +968 24414228 email: [email protected]

Abstract A piezoelectric nano-generator (PZG) based on in-house ZnO nanorods (ZnO NRs) was constructed and utilized. ZnO NRs were synthesized by tube-in-tube chemical vapor deposition (CVD) technique for large production. To produce large harvested rods, the inner side of a horizontal quartz tube was used as growth platform directly without the aid of substrates or catalysts. The production is about 3-5 grams each trail, which is considered as a large scale production in nano-field synthesis. The fabricated nano-rods are polycrystalline in structure and it has (57±11)nm and (3.9±0.8)μm in average diameter and length, respectively. Piezoelectric properties of ZnO NRs were studied by building a real piezoelectric nano-generator, which shows the proportional relation between exerted mechanical forces and their outcome voltages. It was found that as the stress force increases more current will flow and the maximum voltage has reached 0.7 V. The nano-generator exhibited Schottky-like I-V characteristics and constructively generated harvesting currents. Current jumps 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. This device can be utilized to generate electricity while walking to charge mobile electronic devices like hand phones for instance. Keywords: Piezoelectric device, ZnO nanorods, CVD

1. Introduction Zinc oxide nanostructures with a wide direct band gap (3.37eV) and an efficient excitation emission at room temperature due to large exciton bonding energy (60 MeV) (Wang, 2004, Qun Tang, 2004 and D.C. Look, 1998) make it proper for optoelectric applications for short wavelengths. Other properties like fast photo-response for ultraviolet (UV) light in the photo-detectors and transparent to visible light (Qun Tang, 2004). Now many applications depend on ZnO nanostructures like optical pumped laser, light emitting diodes, UV photoelectric devices, biosensors, solar cells, and piezoelectric nanogenerators (Youfan Hu, 2010, Soumen, 2013, R F Zhuo, 2008, Canan Dagdeviren, 2010 and Canan Dagdeviren S.-W. H., 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 Ü. Özgür, 2005). ZnO piezoelectric nano-generator (PZG) transfers mechanical energy to kinetic energy and vice versa and hence utilized for many applications like transducers, sensors and actuators (Ang Wei, 2011, Sunil K. Arya, 2012 and Shih-Jui Chen, 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 like nanorods, nanocombs, nanobelts, nanowires, nanosheet and nanorings (Consonni, 2014, Lee S. H., 2008 and Jamil Elias, 2008) have been reported. The variety of nanostructures can be synthesized by various techniques including gel–sol process (Y.Y. Tay and Shan GY, 2006), hydrothermal growth and chemical vapor deposition (CVD) (Z.P. Sun, 2006, S.L. Patil, 2012, Yangyang Zhang, 2012, Umar A., 2005 and Yiying Wu, 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 like 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 nanostructures 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 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. 1(c). 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. Large quartz tube Pump

Ar 38 mm

26 mm

Small quartz Source Material tube Figure 1. Tube in tube CVD growth process for large scale production of ZnO nanowires.

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 precursors concentration and temperature profile are playing a more vital role in the morphology control. In Fig. 2 (b), EDX spectrum reveals that these nanostructures are consisted 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. To produce ZnO NRs only, which is preferable shape of ZnO nanostructures in piezoelectric devices (Khan A, 2012 and Soomro M. Y., 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 decrease to form narrow needle as can be seen in Fig. 2(d). The rods have typically in average length and in 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. The XRD pattern of synthesized NRs in Fig. 4. shows a poly crystalline orientation with wurtzite ZnO structure. The grown ZnO NRs has 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 indicate 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. X 1900

(b)

(a)

Element Zn O

Wt% 78 22.0

σ 0.7 0.7

10µm X 11000

X 18,000

(d)

(c)

1µm

1µm

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

12

10

10

8

8

Frequency

frequency

6

4

6

4 2

2 0

0 2.0

2.5

3.0

3.5

4.0

4.5

Length (m)

5.0

5.5

6.0

0.04

0.05

0.06

0.07

Diameter (m)

Figure 3 The Gaussian distribution of (a) the length and (b)diameter of ZnO NRs.

0.08

0.09

(101) (100)

(002)

(110) (102)

(103)

(200) (112) (201)

Figure 4 The XRD spectrum of ZnO NRs.

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 increasing of the consternation of deposition layers of Zn and O cause 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 contain the Zn atoms that the growth rate in this plate is the fastest than the negative plate (000 ) where it has the slowest growth rate. This forces the growth along the c-axis direction. Different crystal faces has different rate of growth as follows: (0001) > (01 1) > (01 0) > (000 ). 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 each positive charge cancel 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, 0.05mm thick aluminum (Al) disc, paper (insulator) and the synthesized ZnO NRs as shown in figure 5(a and b). The insulated paper was shaped in ring shape and was placed between, Al and Cu disc boundaries. The ZnO NRs in powder form was 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.

Aluminum Electrode ZnO NRs

(c)

(a) Voltmeter

Device

Insulate (paper) Cupper Electrode

(b)

Figure 5 (a) The device setup, (b) scope photo of the device and (c) the connection view for taking measurements.

The piezoelectric device design depends on a unique coupling between piezoelectric and semiconducting properties of the ZnO NRs (Song J.H., 2006 and X.D. Wang, 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 Z.L., 2006). The Al (ϕ=4.08ev) disc 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 S., 2005). Therefore, there is no barrier at the interface of AlZnO 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 to 5.10 eV (Chengkun Xu, 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 become 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 figure 5(c), 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 multiply with the gravity acceleration . The mechanical force per circular area was measured and all results are illustrated in table 1.

Table 1 (a) The I-V measurements of piezoelectric device.

Mass (g)

Weight,

Mechanical Force/Area (N/m2)

Compression Voltage

Relaxation Voltage

1022 944 824 715 630 521 214

10.016 9.251 8.075 7.007 6.174 5.106 2.097

22140 20449 17849 15489 13647 11286 4635

0.74 0.70 0.66 0.52 0.46 0.38 0.28

- 0.57 - 0.54 - 0.48 - 0.43 - 0.37 - 0.32 - 0.24

Current (µA)

It is noticed that as the stress force increase more current will flow and the maximum voltage has reached In Fig. 6 a current-voltage characteristics (I-V) of the piezoelectric device or nano-generator was measured twice, directly after preparing the piezoelectric device (t=5 minutes) and at time (t= 2h), 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 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 x 10-05A/cm2 of the maximum current during compression and relaxation periodic motion. Therefore, our device maximum power is about 8.97µW/cm2. This efficiency considered to be in the middle domain of piezoelectric devices when compared with others groups published work (Gu L., 2013, Chen X., 2010, Chun J., 2015 and Xu S., 2010).

Voltage (V)

Figure 6 Output I-V characteristic lines of the nano-generator at t=5 mints and 2h.

In Fig7. 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 creates 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. 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 fluctuates depending on the force applied and which mode is measured, see Fig. 7 (c).

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.

4.Acknowledgments Authors wish to express their sincere thanks to Mr. Ibrahim Al-Khosabi from CARUU, Central Analytical and Applied Research Unit, Collage of Science, SQU. 5. 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 tubein-tube CVD technique produces 3-5 grams 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 fluctuates depending on the force applied and which mode is measured.

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