Improving Piezoelectric Nanogenerator Comprises ZnO Nanowires by

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Improving Piezoelectric Nanogenerator Comprises ZnO Nanowires by Bending the Flexible PET Substrate at Low Vibration Frequency Cheng-Liang Hsu* and Kuan-Chao Chen Department of Electrical Engineering, National University of Tainan, Tainan 700, Taiwan (Republic of China) ABSTRACT: The well-aligned and vertical ZnO nanowires were grown on PET substrates by the hydrothermal method. The novel piezoelectric nanogenerator was fabricated from ZnO nanowires and a Pt/ZnO nanowire electrode on a flexible PET substrate. A sample was compressed and bent generating internal stress in the PET substrate, which output a current of approximately 5 × 10−10 A without a source of vibration. The sample with 2% bending was also measured with vibration at a low frequency, yielding a maximum piezoelectric current of about 2.5 × 10−7 A, which is 4 times the current of a nonbending sample. These results demonstrate that a little bending of a flexible substrate improves piezoelectric performance.

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effect of various flexible substrates, such as paper, zinc foil, and others.8,12 However, these substrates tend to suffer from cracking. This study suppresses cracking using a PET (polyethylene terephthalate) substrate, which is transparent, can be formed into large-area substrates, is flexible, and has a low cost. A PET substrate has a high chemical resistance and is not very effective at high temperatures. Although ZnO nanowires can be synthesized by various methods, including chemical vapor deposition (CVD),9,13 template-assisted growth,14 catalyst-driven molecular-beam-epitaxy (MBE),15 metalorganic chemical vapor deposition (MOCVD),16 both vapor−liquid−solid (VLS),2−4 sol−gel systhesis8 and hydrothermal methods.17,18 The hydrothermal method is better than other methods because the growth temperature is very low, making the method suited to the chemical properties of the PET substrate and eliminating the problem concerning the PET temperature. Furthermore, the hydrothermal method has low cost and can be adopted in mass production. Onedimensional ZnO nanowire arrays with larger length-todiameter and surface-to-volume than ratios ZnO nanowires than ZnO bulk and films are well-known to have better physical and chemical properties. A piezotronic device was fabricated herein using ZnO nanowires on a PET substrate. Details of the growth of ZnO nanostructures and the characteristics of the fabricated piezotronic devices are also discussed.

INTRODUCTION Zinc oxide (ZnO) is one of most promising II−VI compound semiconductor materials, because it has a direct band gap of 3.37 eV, a high-exciton binding energy of 60 meV, and a high electron mobility of around 100 cm2 V−1 s−1 at room temperature. One-dimensional (1D) ZnO nanostructures can now be formed with favorable physical and chemical properties. The ZnO nanostructure has a hexagonal wurtzite lattice and exhibits favorable piezoelectricity as a result of the built-in polarization along its c-axis. Numerous exciting ZnO nanowire applications are currently available. They include the ultraviolet (UV) nanolaser,1 field-effect transistor,2 solar cell,3 gas sensor,4 UV photodetector,5 light emitting diode,6 and nanogenerator.7−11 The nanogenerator is attracting substantial attention from researchers because ZnO exhibits highly efficient piezoelectricity. The function of the nanogenerator is to convert mechanical energy to electricity coupled semiconducting and piezoelectric properties. Electronic products are being miniaturized to improve portability. In nanoscale devices, the sizes of traditional rechargeable and replaceable batteries are limited. The development of a nanogenerator to convert the available form of energy from the environment into electric energy would facilitate the development of nanodevices. Such a nanogenerator can be used to develop a battery-less system for future applications. In recent years, various articles have reported upon the piezoelectric properties of ZnO nanowires, but most have focused on single ZnO nanowires or nanorods.9,10 A few investigations have discussed ZnO nanowire arrays with glass, sapphire, Si, or other hard substrates.9−11 However, such hard substrates suffer from the fact that it is difficult to enhance the piezoelectric effect by transferring the mechanical energy into electrical energy. Therefore, research groups have exploited the piezoelectric © 2012 American Chemical Society

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EXPERIMENTAL SECTION Figure 1 schematically depicts the growth and processing steps that were utilized in this work. Before sputtering, the PET Received: February 15, 2012 Revised: March 24, 2012 Published: April 9, 2012 9351

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Figure 1. Schematic diagram of the growth and processing steps of the device.

was coated to a thickness of 20 nm on ZnO nanowires by sputtering to form a nanostructured contact electrode. The novel nanogenerator thus fabricated consisted of ZnO nanowires and a Pt/ZnO nanwire electrode on the flexible PET substrate. Figure 2 presents the structure of what was used

substrates were cleaned with ethanol and deionized water in an ultrasonic cleaner, and then baked at 100 °C for 10 min to remove moisture. AZO (Al-doped ZnO) seed layers were deposited on PET using a RF magnetron sputtering method. The thickness of the AZO film was approximately 50 nm. The ZnO nanowires were hydrothermally grown at 95 °C in a sealed beaker by immersing the PET substrates for 6 h in the aqueous solution of 0.06 M zinc nitrate hexhydrate [Zn(NO3)2] and 0.06 M hexamethylenetetramine (C6H12N4). The Zn(NO3)2/C6H12N4 molar ratio was always 1:1. Hexamethylenetetramine (HMTA) is a highly water-soluble, nonionic tetradentate cyclic tertiary amine. Thermal degradation of HMT releases hydroxyl ions, which react with Zn2+ ions to form ZnO. The crystal growth of ZnO nanowires involves the following chemical reactions.17,18 (CH 2)6 N4 + 6H 2O ↔ 6CH 2O + 4NH3 (CH 2)6 N4 + Zn 2 + → [Zn(CH 2)6 N4]2 +

NH3 + H 2O → NH4 + + OH− Zn 2 + + 4NH3 → Zn(NH3)4 2 + Figure 2. Package structure used to make piezoelectric measurements.

Zn 2 + + 4OH− → Zn(OH)4 2 −

for making the piezoelectric measurements. The inset image is a photograph of the package sample. Vibrations were induced using a cooling fan whose frequency was approximately 120 Hz. The sample was placed on the furnace and vibrated by the cooling fan. Most environmental vibrations are low-frequency, but most related investigations have focused on ultrasonic vibration,7 whose frequency is too for use in application to a nanogenerator. This ZnO nanowire piezoelectric device receives a low vibration frequency and converts mechanical energy into electrical energy.

Zn(NH3)4 2 + + 2OH− → ZnO + 4NH3 + H 2O Zn(OH)4 2 − → ZnO + 2H 2O + 2OH− [Zn(CH 2)6 N4]2 + + 2OH− → ZnO + H 2O + (CH 2)6 N4

A MAC MXP18 X-ray diffractometer (XRD) was adopted to characterize the crystallographic and structural properties of the as-grown ZnO nanowires. The surface morphologies of the samples and size distribution of the nanowires were characterized using a JEOL JSM-7000F field emission scanning electron microscope (FESEM) at 10 keV. The piezoelectric current properties of ZnO nanowires were measured using a Keithley 237 instrument at room temperature. Platinum (Pt)

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RESULTS AND DISCUSSION Figure 3 shows the cross-sectional FESEM images obtained at a tilt angle of 20° of ZnO nanowires that were grown on PET 9352

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Figure 5. Current−voltage (I−V) curves of ZnO nanowires of variously bent static samples.

Figure 3. Cross-sectional FE-SEM images obtained at a tilt angle of 20° of ZnO nanowires grown on PET substrate. The inset presents a top-view SEM image of ZnO nanowires.

compression increases, suggesting that the area of contact between the nanowire and Pt/nanowire increases with the compression as the sample is deformed. The left and right sides of the wires are differently compressed,7 enhancing the current. The change in the internal stress of the soft substrate slightly increases the current. Figure 6 presents simple bending of a sample that is isolated from the environmental source of vibration. The inset displays

substrate. The inset displays top-view SEM images of the ZnO nanowires. The images indicate the uniform growth of highdensity and well-aligned nanowires on PET substrate with a thin AZO buffer layer by the hydrothermal method. As presented in Figure 3, the average length and diameter of the pure ZnO nanowires were about 1 μm and 100 nm, respectively. The diameter of the nanowire exceeds the distance between the nanowires. The above conditions contribute to the piezoelectric effect of the nanowire. Figure 4 shows the 2θ-scan

Figure 4. 2θ-scan XRD spectra of the result from the deposition of zinc oxide nanowires on PET substrate.

Figure 6. Simple bending of sample isolated from environmental vibration. The inset shows measured piezoelectric current under various degrees of compressive bending.

spectra of the XRD measurements, which demonstrate the crystallographic characteristics of the ZnO nanowires and PET substrate. The PET substrate yielded a peak at around 26°, and the ZnO nanowires (002) yielded a strong peak at 34.41°. The ZnO (002) diffraction peak in the spectra reveals the c-axispreferring orientation of the ZnO nanowires. Fairly consistent crystalline growth of ZnO nanowires on the PET substrate is observed. The bending measurement of ZnO nanowires was made from current−voltage (I−V) curves of various static bent samples. These experimental I−V curves plotted in Figure 5 correspond to Schottky junctions.9−11 The current increases with bending indicate that the impedance decreases as the

the measurement of piezoelectric current under various degrees of bending. Figure 6 reveals that the increase in the output current is correlated with the bending, for which the result is consistent with the measurements in Figure 5. The compressive energy of the sample ZnO nanowires was transferred to piezoelectric energy. The sample under 2% bending generated approximately double the current that was generated by the sample under 1% bending. The former sample generated a maximum measured current of around 2.5 × 10−9 A under instantaneous compressive bending, but the compressioninduced piezoelectric current decreased over time. The bent sample was clamped in a holder, which did not supply any 9353

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billions of ZnO nanowires. Such a sample is expected to have a piezoelectric current of mA, but a standard ZnO nanowire sample was packaged with the Pt/ZnO nanowire electrode or a zigzag electrode, yielding a current of only around 1−100 nA. On the basis of the current ratio, the ultralong ZnO nanowires are around 103−105 wires. Figure 8a schematically depicts the

physical energy to the sample, but the bent sample still provided a stable current of approximately 5 × 10−10 A, suggesting that the current is associated with the flexible substrate and deformation energy of the substrate under internal stress. Therefore, the internal stress increased with deformation, increasing the current. Figure 7 plots the piezoelectric current of the bent sample under vibration, which was caused by a rotating cooling fan.

Figure 8. (a) Interface between ZnO nanowires and Pt/ZnO nanowire electrode. (b) Interface of compressively bent sample. (c) I, II, and III: schematic diagram for three types of contact between ultralong ZnO nanowires and the Pt/ZnO nanowire electrode. (d) Equivalent Schottky diodes for the interface between Pt metal and ZnO nanowire. Figure 7. Piezoelectric current of bending sample with low-frequency vibration. The inset schematically depicts the experimental setup for piezoelectric measurement.

interface between the ZnO nanowires and the Pt/ZnO nanowire electrode. Compressive bending of the sample increased its piezoelectric current, suggesting that bending increased the larger area of contact between the ZnO nanowires and the Pt/ZnO nanowire electrode, as presented in Figure 8b. Although the compressive energy causes the sample to bend, little compressive energy was converted to vertical energy in the PET substrate. The energy associated with an effect that operates in the vertical direction causes the ZnO nanowires to press against the Pt/ZnO nanowire electrode with a larger contact area. The sample with 2% bending exhibited the largest piezoelectric current because it exhibited the highest transfer of compressive energy for vertical force. I, II, and III in Figure 8c schematically depict three possible types of contact between ultralong ZnO nanowires and an Pt/ZnO nanowire electrode.7,10,20 On the basis of previous reports of the piezoelectric effect, the stretched side of the ZnO nanowires develops a positive potential (V+) and its compressed side has a negative potential (V−). When the compressed side of the ZnO nanowire is in contact with the electrode, as in types I and II in Figure 8c, the piezoelectric property of the nanowire converts the energy associated with compressive to electrical energy, causing accumulated electrons to flow from the nanowire to the electrode. The type III Pt/ZnO nanowire in Figure 8c is in contact with the stretched side of the ZnO nanowire. The Schottky barrier effect is important to the switch function. Figure 8d presents equivalent Schottky diodes for the interface between Pt metal and ZnO nanowire.9,10,21−25 The side of the Pt metal is the anode, and the n-type ZnO nanowire is the cathode. Types I and II (in OR of) Figure 8c and d are equivalent electrical circuits that are forward-biased under compressive contact. Type III in Figure 8c and d is the equivalent electrical circuit to reverse-biasing under stretched

The inset schematically depicts the experimental setup for piezoelectric measurement. Figure 7 presents the sample in Figure 6 under the influence of an external vibration source. The maximum piezoelectric current of the sample under 2% bending under vibration was measured to be around 2.5 × 10−7 A. The sample with 2% bending generated a higher vibration current than the 1 and 0% samples by factors of 2 and 4, respectively. However, Figures 6 and 7 are not obtained under an applied external voltage (0 V) and only the current of the piezoelectric device generator was measured. These results confirm that the experimental samples generated their own power. Most of the current is converted from the external vibration source. The bending of PET samples enhances the piezoelectric current, whether or not they are vibrating. In the Figure 6 display, the maximum current of 2% bending internal stress is around 2.5 × 10−9A, but the maximum piezoelectric current of the sample under 2% bending under vibration was measured to be around 2.5 × 10−7 A. The current of vibration is higher than internal stress cause current around 2 orders. The internal stress current is too small and should be ignored in the vibrations measured. Suggest that vibration provide an additional energy and the contact area increasing dominate the piezoelectric current rising. The hydrothermal method does not prevent the growth of a few, ultralong ZnO nanowires, which are touched and sustained Pt/ZnO nanowires electrode substrate. Previous investigations have established that a single ZnO nanowire generates a piezoelectric current of about 10−100 pA,10,19 according to AFM measurements. The standard sample is composed of 9354

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ACKNOWLEDGMENTS The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC 100-2221-E-024-011.

contact. The piezoelectric current of Figure 6 was generated in the same direction by the internal stress, but Figure 7 included the positive (+) and negative (−) piezoelectric current. Suggest that vibration source cause that wires continue moving, the type III of Figure 8c and d is potential to generate pulse piezoelectric current due to the contact touching is continuous change. The pulse piezoelectric current contains a high frequency to overcome the Schottky barrier. The ZnO nanowire is an n-type wurtzite crystal structure, which grows along the c-axis. The compressive force is associated with strain s33 along the c-axis. The single ZnO nanowire piezoelectric polarization and current density can be calculated as11 PZ = e33s33 = qρpiezoWpiezo

(1)

J = JD0 e(qe33s33Wpiezo/2εskT )(eqV / kT − 1)

(2)

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where PZ denotes the piezoelectric polarization, J denotes the current density, e33 denotes the piezoelectric tensor, s33 denotes the small uniform mechanical strain, ρpiezo denotes the density of polarization charges, Wpiezo denotes the ZnO nanowire depletion layer widths, and JD0 defines the saturation current density in the absence of piezoelectric charges. According to Wang et al., eqs 1 and 2 are calculated for piezoelectric polarization of a single ZnO nanowire. These piezoelectric parameters were related to the piezoelectric current. The single ZnO nanowire generates a piezoelectric current of about 10− 100 pA, but these experiment samples (billions of ZnO nanowires) only generate current around 100 nA. The touch number between Pt metal and ZnO nanowire can be estimated by the current density and output current, and then, the current ratio and piezoelectric parameter reveal that ultralong ZnO nanowires are around 103−105 wires.

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CONCLUSIONS Well-aligned, vertical ZnO nanowires were grown on PET substrates by the hydrothermal method at 95 °C. FE-SEM and XRD measurements verified that they were highly uniform and highly crystalline. We fabricated a novel piezoelectric nanogenerator consisting of a ZnO nanowire and Pt/ZnO nanowire electrode on the flexible PET substrate. The piezoelectric nanogenerator sample with 2% bending exhibited a piezoelectric current of approximately 2.5 × 10−9 A, suggesting that the 2% bent sample caused internal stress in the PET substrate, which output a current of around 5 × 10−10 A without a vibration source. The maximum piezoelectric current of the sample with 2% compressive bending under vibration at low frequency was measured to be about 2.5 × 10−7 A. Bending of 2% was associated with a higher piezoelectric current under vibration than bending of either 1% or 0%, by a factor of 2 or 4, respectively. Compressive bending can improve the flexibility of substrate design and the performance of a piezoelectric nanogenerator in which the substrate is used.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 9355

dx.doi.org/10.1021/jp301527y | J. Phys. Chem. C 2012, 116, 9351−9355