Fabrication of silver-doped zinc oxide nanorods piezoelectric nanogenerator on cotton fabric to utilize and optimize the charging system

Abstract

Textile-based piezoelectric nanogenerator generates electrical energy from human motion. Here a novel type of textile-based piezoelectric nanogenerator is reported which is fabricated using the growth of silver-doped zinc oxide on carton fabric. Along with the optical and structural characterization of silver-doped zinc oxide nanorods, the electrical characterization was also performed for silver-doped zinc oxide piezoelectric nanogenerator. The silver-doped zinc oxide piezoelectric nanogenerator was found to generate three times greater power compared to undoped zinc oxide piezoelectric nanogenerator. By applying external mechanical force of 3 kgf and 31 MΩ of load resistance, the silver-doped zinc oxide piezoelectric nanogenerator generated an output power density of 1.45 mW cm⁻². The effect of load resistance and load capacitor was determined and optimum values were calculated. The maximum output power was observed at a load resistance of 31 MΩ. The silver-doped zinc oxide piezoelectric nanogenerator was utilized to charge load capacitors and found that maximum energy could be stored at optimum load capacitance of 22 nF in 600 s (1800 cycles). This research may provide the opportunity to design high-output textile-based nanogenerators for practical applications like powering portable devices and sensors.

Keywords

Cotton fabrics Ag-doped ZnO nanorods piezoelectric nanogenerator charging system optimization mechanical energy harvesting

Introduction

Energy harvesting technology obtains electrical energy from the ambient environment such as solar, mechanical, and thermal energy. Energy harvesting has emerged as one of the most promising solutions for the rapidly increasing energy demand.¹ Self-powered energy system is becoming ubiquitous and is receiving great interest, because they reduce the size and weight of electronics devices. Piezoelectric energy is generated by mechanical and vibrational sources which is used to charge portable devices and wireless sensors.² Energy harvesting devices can be fabricated by using piezoelectric materials such as zinc oxide (ZnO), indium nitride, gallium nitride (GaN), and cadmium sulfide which has lower output power density for practical application.^3

–6 ZnO is one of the best nanomaterials which has unique piezoelectric, semiconducting, and optical properties due to which it has been used in different piezoelectric devices such as piezoelectric gate diode, piezoelectric field effect transistors, and piezoelectric nanogenerators (PENGs).^7

–10 On the other hand, the synthesis of nanomaterials is at a new stage of development, where it is common requirement to control the properties of the obtained nanostructures, and enables their repeatable application, especially for energy harvesting technology. There are number of reports for controlling size, density, growth temperature, and crystal quality of ZnO which enhanced its applicability in fabrication of PENG.^11

–14 Different approaches have been used for the fabrication of PENGs on nonflexible substrate such as GaN,¹⁵ glass,^16,17 titanium foil,¹⁸ and silicon.^19

–22 However, these materials exhibit poor flexibility and deformation due to which PENGs are facing a challenge in practical applications. In addition, there has been rapid progress on fabricating stretchable or flexible PENGs based on ZnO nanorods. The frequently used flexible substrates are sponge,²³ flexible plastic,²⁴ polyethylene terephthalate,^25
–27 paper,^28
–30 and textile fabric.³¹ Among these, textile materials possess mechanical flexibility, low cost, lightweight, and used for integration into different areas such as clothing accessories.³²

Some researchers have used these smart functional textiles for the fabrication of PENGs. Khan et al. reported ID PENG which was fabricated on conductive textile fabric using hydrothermal method.³¹ The output performance of ID PENG has reduced performance due to screening of free charge and parasitic capacitor effect. There is a need to enhance the efficiency of one-dimensional ZnO nanogenerator by modifying its various properties, such as optical, electrical, and piezoelectric,^33,34 which can be accomplished by doping the ZnO with noble metals such as gold,³⁵ palladium,³⁶ and silver (Ag).³⁷ Ag is considered as a good dopant for ZnO due to high solubility, large ionic radii, and less orbital energy,³⁸ since Ag doping increases the surface defects and optical property that shifts the optical absorption toward visible region.³⁹ Ag element acts as acceptors and provides a high concentration of charge carriers in ZnO host materials.⁴⁰ Fan and Freer reported that Ag act as an amphoteric dopant existing both on substitutional Zn sites and in the interstitial sites.⁴¹ However, there is no report till now on the PENG of Ag-doped ZnO nanorods on cotton fabric.

In this research, a novel method for fabrication of textile-based PENG is reported in which Ag-doped ZnO nanorods on cotton fabric substrate are grown using hydrothermal method and utilized to make PENG. The fabricated PENG was employed to store charge in load capacitor. It was found that reactance of capacitor affects the charging capability of PENG. The load capacitance was optimized at 22 nF for maximum stored energy of 0.5819 nJ which gives a reactance of 31 MΩ.

Experimental details

Materials

All chemical reagents were purchased from Sigma Aldrich (USA, St Louis, Missouri), except zinc acetate dehydrate and zinc nitrate hexahydrate which were purchased from Ajax Fine Chem Pty Ltd (Australia). The chemical reagents used in the fabrication process of the Ag-doped ZnO nanorods were of analytical grade and used without further purification.

Synthesis of pure ZnO and Ag-doped ZnO nanorods

The cotton fabric of 1 cm × 1 cm was ultrasonically cleaned in acetone and deionized (DI) water for 10 min, respectively, and dried in air. Then, the cotton fabric was immersed in 1% 1-dodecanethiol for 2 h to facilitate the subsequent seeding of ZnO nuclei. The ZnO seed layer was deposited using sol–gel process. The sol–gel was prepared using zinc acetate dehydrate (Zn(CH ₃COO)₂).2H₂O) (99.5%) and sodium hydroxide (NAOH; 97%) in methanol. Initially, 0.01 g of zinc acetate dehydrate was dissolved in 50 ml methanol at 60°C and 0.03 g of NAOH was dissolved in 25 ml methanol at 60°, separately. The NaOH-based solution was added drop wise into zinc acetate solution with constant stirring at 60° for 2 h to maintain its transparency. Finally, fabric samples were inserted in sol–gel for few seconds and then annealed at 80°C for 10 min. This process was repeated three times. The nutrient solution was prepared using 0.03 M zinc nitrate (Zn(NO₃)₂.6H₂O; 98%), 0.03 M hexamethylenetetramine (C₆H₁₂N₄; 94%), and 0.03 M silver nitrate (99.7%) in 1000 ml of DI water. The prepared fabric samples were stuck on an unsinkable substrate and put upside down in nutrient solution. The growth process was carried out for 6 h at 85°C. After 6 h, the fabric samples were dried and annealed for 1 h at 80°C to obtain the fine crystal structure of Ag-doped ZnO nanorods.

Characterizations of pure and Ag-doped ZnO nanorods

The morphological properties of undoped and Ag-doped ZnO nanorods on cotton fabric were studied by scanning electron microscopy (SEM; Hitachi, S-3400N). The compositional studies of the nanostructures were carried out by energy-dispersive spectroscopy (EDS) attached to the SEM. For SEM analysis, cotton fabrics samples were prepared by making its surface conductive with gold sputtering, whereas no preparation step was done before X-ray diffractometer (XRD) analysis. The crystalline structure of the undoped and Ag-doped ZnO was determined by using a X’Pert PRO 3040/60 Philips XRD at an accelerating voltage of 40 kV using copper K_α line (λ = 0.1542 nm). XRD was performed in the 2θ range of 20–70° with a step size of 0.01° and a scanning rate of 0.02 steps s⁻¹. For optical characteristics, the diffuse reflectance spectra (DRS) of the undoped and Ag-doped ZnO nanorods were investigated in the wavelength range of 200–850 nm on a Perkin-Elmer ultraviolet–visible (UV-Vis) diffuse spectrophotometer (National Centre of Physics, Islamabad, Pakistan). The functional group analysis of the nanostructure was performed in the transmittance mode on a Thermo Scientific (model Nicolet 6700) Fourier-transform infrared spectrophotometer (FTIR), in a spectral range of 400–4000 cm⁻¹. The thermal properties of the sample were investigated by using a thermal analyzer (thermogravimetric analysis (TGA); SDT Q600 (TA Instruments, New Castle, Delaware, USA). The sample was heated from approximately 30°C to 800°C at the rate of 10°C min⁻¹ under a nitrogen flow rate of 200 ml min⁻¹.

Device fabrication and its characterization

The fabrication of PENG was achieved by placing thin sheets of copper (Cu; 0.7 µm), one on the top of Ag-doped ZnO nanorods film grown on cotton fabric and other on the bottom of cotton fabric. The fabrication steps of Ag-doped ZnO nanorods PENG are shown in Figure 1. In order to protect the Cu electrode and PENG during measurement, an adhesive tape was applied on the bottom and top sides of the PENG. The size of the Ag-doped ZnO nanorods PENG is 1 cm². The output voltage and current of the PENG were detected by an oscilloscope (Wave Pro 735Zi; Lecroy, USA).

Figure 1.

The schematic diagram of fabrication steps of Ag-doped ZnO nanorods PENG. Ag: silver; ZnO: zinc oxide; PENG: piezoelectric nanogenerator.

Result and discussion

Morphological analysis

The morphology of cotton fibers with and without ZnO growth is shown in Figure 2. Figure 2(a) shows the SEM image of the original cotton fibers without ZnO. Figure 2(b) clearly shows the dense growth of nanorods on cotton fibers. It was observed from SEM image that Ag-doped ZnO nanorods have homogenous growth on the entire surface of cotton fibers. The good adhesion between Ag-doped ZnO nanorods and cotton fiber is due to the hydroxyl (OH) group presented in the chemical formula of the cotton. Ag-doped ZnO nanorods can easily bind to OH groups through possibly hydrogen bonding.⁴² The average diameter of the Ag-doped ZnO nanorods was found in the range of 85–90 nm, as shown in Figure 2(c).

Figure 2.

SEM micrographs of (a) magnified view of original cotton fiber without ZnO growth, (b) magnified image of Ag-doped ZnO nanorods grown on cotton fiber, and (c) Ag-doped ZnO nanorods grown on the cotton fiber at higher magnification. SEM: scanning electron microscopy; Ag: silver; ZnO: zinc oxide.

Compositional analysis

EDS was used to determine the chemical composition of the undoped ZnO and Ag-doped ZnO samples, as shown in Figure 3(a) and (b). The EDS analysis confirms the presence of zinc (Zn), carbon (C), oxygen (O), and Ag elements. There are no detectable traces of any impurity element in EDS profile of pure and Ag-doped ZnO nanorods. The strong signals of O and C in EDS spectra of Figure 3(a) and (b) are due to cotton fabric which consists of a long chain of glucose molecules.^43,44 The EDS spectral peaks of Ag appeared at 3.1 KeV and Zn appeared at 1.0, 8.3, and 8.8 KeV, as shown in Figure 3(b). Also, the strong signal relevant to C at 0.3 KeV and O peak at 0.5 KeV is due to cotton fabric.

Figure 3.

EDS analysis of (a) undoped ZnO nanorods and (b) Ag-doped ZnO nanorods. EDS: energy-dispersive spectroscopy; Ag: silver; ZnO: zinc oxide.

Optical analysis

The optical properties of pure ZnO and Ag-doped ZnO were determined by UV-Vis DRS. Figure 4(a) shows the diffuse reflectance spectrum of nanomaterials measured at room temperature in the wavelength range of 245–800 nm. The pure ZnO spectra showed high reflectance about 92% in visible region. Reflectance of Ag-doped ZnO samples decreased by approximately 39% after incorporation of Ag with ZnO.⁴⁵

Figure 4.

(a) UV-DRS spectrum of ZnO and Ag-doped ZnO nanorods on cotton fabric; (b) Kubelka–Munk diffusive reflectance; and (c) Tauc’s plot for band gap energy estimation of ZnO and Ag-doped ZnO nanorods on cotton fabric. UV: ultraviolet; DRS: diffuse reflectance spectra; Ag: silver; ZnO: zinc oxide.

The Kubelka–Munk function was used to determine the value of optical band gap of pure ZnO and Ag-doped ZnO nanorods, estimated optical absorption edge of nanostructures⁴⁶

F (R) = \frac{{(1 - R)}^{2}}{2 R}

Where F(R) is the Kubelka–Munk function which corresponds to the absorbance⁴⁷ and R is the absolute value of reflectance. The absorption edge was calculated by plotting a graph between F(R) and wavelength (λ) and then extrapolating F(R)-λ of curve to λ-axis at F(R) = 0 corresponds to the absorption edge of pure ZnO and Ag-doped ZnO at 398 and 338 nm, respectively. Alternatively, the band gap energy (E _g) of samples was also determined (see Figure 4(c)) via Tauc’s plot by employing the linear correlation between Kubelka–Munk function F(R) and E _g ⁴⁸

{[F (R) . h v]}^{\frac{1}{n}} = A (h v - E_{g})

where $n = 1 / 2$ for direct band gap material like ZnO. Tauc’s plot was obtained by plotting a graph between ${[F (R) . h v]}^{2}$ and photon energy $(h v)$ . The fitting of linear portion of curve in the range of 2.8–3.90 eV and its extrapolation intercept F(R) = 0 line is at 3.12 and 3.67 eV for pure ZnO and Ag-doped ZnO, respectively. There is shift of band gap for Ag-doped ZnO nanorods toward higher value of E _g compared to the pure ZnO nanorods. It may be attributed to the fact that most Ag atoms may substitute Zn atoms lattice sites in the crystal structure because of the electronegativity and ionic radius difference. In addition, the electro negativity difference between Zn and Ag may also be responsible for the increase in the band gap.⁴⁹

Structural analysis

The crystalline properties of pure ZnO and Ag-doped ZnO were analyzed by XRD spectroscopy at room temperature in the range of 2 $θ$ , as shown in Figure 5. The peaks at 2θ values of 22.59° and 25.59° are the diffractive peaks of cotton fiber.⁵⁰ Figure 5(a) shows that all the diffraction peaks are clearly exhibiting the crystalline nature with peaks corresponding to miller indices $(100),$ $(002)$ , $(101), (102)$ , $(103)$ , $(110)$ , and $(112)$ planes. The standard diffraction peaks reveal that the crystal structure of pure ZnO and Ag-doped ZnO nanorods is of hexagonal wurtzite structure (space group p63mc and JCPDS card no: 36–1451). In the XRD spectrum, the main diffraction peaks of ZnO are (100), (002), and (101), indicating that grown undoped and doped ZnO nanorods are vertically aligned along and off c-axis. In order to investigate the effect of Ag doping on the structure, an enlarged version of XRD profile between 31° and 37° is shown in Figure 5(b). It is observed from XRD profile that Ag-doped ZnO has much wider line broadening than the undoped ZnO. It is worth to mention the diffraction peaks of Zn shift toward higher angle with Ag doping due to difference in ionic radii of ${Zn}^{+ 2}$ ions (0.074 nm) and Ag⁺ ions (0.126 nm). This shift indicates that the Ag ions replaced the Zn sites in the ZnO nanorods crystal matrix.^37,51
–53

Figure 5.

XRD pattern of (a) pure ZnO and Ag-doped ZnO nanorods prepared by hydrothermal method. (b) The enlarged view of (100), (002), and (101) peaks of pure ZnO and Ag-doped ZnO nanorods. XRD: X-ray diffractometer; Ag: silver; ZnO: zinc oxide.

Vibrational analysis

As-synthesized pure ZnO and Ag-doped ZnO nanorods on cotton fabric were analyzed by FTIR, as shown in Figure 6. In the FTIR spectrum, metal oxides exhibit absorption bands below 1000 cm⁻¹, which arise from interatomic vibrations.⁵⁴ It was found that the absorption band at around 470 and 719.45 cm⁻¹ was indicated the stretching mode of Zn–O and Ag–O bonds.^55,56 The peak near to 1380.84 and 1450.21 was observed in sample that corresponds to the stretching vibrations of C–O group, which may be due to zinc acetate used in reaction. The peak located at 1721 cm⁻¹ was assigned to (C=O) stretching vibration of cellulose. There is an absorption band located at 2240 cm⁻¹ due to the stretching modes of vibration which formed as a result of binding of O–H group present in the cotton fabric. The absorption peak in the range of 2924.09 cm⁻¹ indicates the stretching mode of C–H in the methyl group. The absorption bands in the range of 3431 cm⁻¹ were observed which indicate the stretching mode of OH group of cotton fabric.⁵⁰

Figure 6.

FTIR spectrum of (a) pure ZnO and (b) Ag-doped ZnO nanorods. FTIR: Fourier-transform infrared spectrophotometer; Ag: silver; ZnO: zinc oxide.

Thermal analysis

The thermal properties of pure ZnO and Ag-doped ZnO nanorods on cotton fabric were determined by TGA and derivative thermogravimetry analysis, as shown in Figure 7(a) and (b). As-synthesized fabric sample was heated in a ceramic pan up to 800°C at the rate of 10°C min⁻¹ under a nitrogen atmosphere. In TGA analysis, three main decomposition steps can be observed. In the first step (35–103°C), a small weight loss (5%) and (9%) was observed in pure ZnO fabric and Ag-doped ZnO fabric samples, which was ascribed to the evaporation of moisture from the surfaces of the samples.⁵⁷ In the second step (103–500°C), the main weight loss was occurred due to the pyrolysis of cellulose.^58,59 The maximum pyrolysis rate occurred at 430°C and 400°C, with weight losses reaching 63% for the pure ZnO fabric and 41% for Ag-doped ZnO fabric, respectively. In the third step (500–800°C), the weight loss percentages of pyrolysis residue were approximately 7 and 13 mass % for the pure ZnO fabric and Ag-doped ZnO fabric, respectively.

Figure 7.

TGA-DTG curves of (a) pure ZnO and (b) Ag-doped ZnO nanorods on cotton fabric. TGA: thermogravimetric analysis; DTG: derivative thermogravimetric; Ag: silver; ZnO: zinc oxide.

Electrical output of a PENG

By pressing the top electrode, undoped ZnO and Ag-doped ZnO nanorods are deformed and create piezoelectric potential along the growth direction. The potential is generated by the relative dislocation of Zn⁺² cations and O⁻² anions due to the piezoelectric effect in the wurtzite crystal structure. Thus, these ions can neither move nor recombine without releasing the strain. The pure and Ag-doped ZnO nanorods could provide well order synchronization that maximum of the nanorods generate piezoelectric potential at the same time and in the same direction, which ultimately decreases the screening effect and enhances the output of PENG.

When top electrode is pressed by some external force, equal amount of positive and negative piezoelectric potential is produced on the top and bottom side of ZnO nanorods. At this time, electrons will flow from the top electrode to the bottom electrode through external circuit and accumulate at bottom electrode. As the external force is released from the Ag-doped ZnO nanorods, which results in the diminution of piezoelectric potential, at the same time, the electrons flow reversely. Figure 8 shows a schematic illustration of the fabricated PENG.

Figure 8.

A schematic illustration of the fabricated PENG. PENG: piezoelectric nanogenerator.

The insulating fabric layer plays an important role in the working mechanism of the PENG by creating a high potential barrier between the top and bottom electrodes. Moreover, its insulating characteristic avoids the electron leakage through internal ZnO nanorods film. The electrical output of PENG was measured by applying periodic external mechanical pulses. In order to confirm that the measured signal originated from the device rather than noise, a switching polarity test of the induced output voltage was performed by reversing the connection.

To investigate the output performance of PENG, a mechanical force was periodically applied. At an applied stress of 3 kgf, the obtained output voltage (peak to peak) and output current (peak to peak) values for undoped ZnO are estimated at 2.28 V and 1.16 µA, respectively, as shown in Figure 9(a) and (b). Similarly at an applied stress of 3 kgf, the Ag-doped ZnO PENG gives 6.85 V and 3.42 µA of output voltage (peak to peak) and output current (peak to peak), respectively, as shown in Figure 9(a) and (b). It is revealed from Figure 9(a) and (b) that the value of positive peaks is much larger than the corresponding negative peaks. This difference in the magnitude of the output peaks occurs due to difference in strain rate during pressing and releasing process.^60,61 From Figure 9(a) and (b), it is clear that the output voltage and current of Ag-doped ZnO nanorods is about three times higher than that of undoped ZnO nanorods. A noticeably enhanced piezoelectric output voltage after Ag doping was mainly associated with the efficient reduction of free charge carrier concentration. Figure 9(c) shows the behavior of output current and voltage as a function of R_L . With increasing R_L , the output voltage increases while the current decreases. Therefore, the output power of the device is related to R_L . The maximum output power of 1.45 mW cm⁻² was achieved at 31 MΩ of R_L , as shown in Figure 9(d).

Figure 9.

Electrical output performance with (a) an output voltage and (b) an output current of the undoped ZnO nanorods and Ag-doped ZnO nanorods PENG. (c) The behavior of output current and voltage against R_L (Ag-doped ZnO PENG) and (d) The output power as a function of R_L (Ag-doped ZnO PENG). PENG: piezoelectric nanogenerator; Ag: silver; ZnO: zinc oxide.

Charging characteristics of load capacitors using Ag-doped ZnO nanorods PENG

The PENG was utilized to charge a load capacitor $(C_{L})$ through bridge rectifier under unidirectional mechanical motion. Moreover, the bridge diode rectifier was used to obtain unidirectional signal and control the charge leakage back from C_L to PENG. In this process, inherent capacitance of device was assumed to be constant. The schematic of the charging system of Ag-doped ZnO nanorods PENG is shown in Figure 10.

Figure 10.

A schematic illustration of the PENG with bridge rectifier to charge the load capacitor. PENG: piezoelectric nanogenerator.

The results of charging different load capacitors are shown in Figure 11. For all the load capacitor values, the voltage obtains the same saturation value, which is similar to resistor–capacitor charging curve, as shown in Figure 11(a). The charging speed is higher for smaller load capacitor, and it reaches to saturation voltage quickly. On the other hand, the stored charges on different load capacitors were calculated by using this relation, $Q = C_{L} V$ , as shown in Figure 11(b). However, various trends are observed for the stored charges. The load capacitor of 1 nF stores minute amount of charge and quickly reaches the saturation value. As the value of load capacitor is increased, then their storage capacity is also increased, and the curve is elevated until it becomes linear.^62,63 In addition, the load capacitor of 400 nF stored the maximum charges of 17.2 nC in a given time of 600 s.

Figure 11.

(a) Output saturation voltage of PENG at different load capacitances. (b) Stored charge at different load capacitances. (c) The behavior of output voltage and stored charges against the load capacitance and (d) the maximum stored energy at an optimum C_L of 22 nf. PENG: piezoelectric nanogenerator.

The stored charge and output voltage were analyzed as function of load capacitor for 1800 charging cycles, as shown in Figure 11(c). The results indicate that there is inverse relation between output voltage and stored charge. When load capacitor is 1 nF, then voltage on load capacitor is at its saturation value of 369 mV, while the stored charges are equal to 0.369 nC. In contrast, when load resistance is 400 nF, the output voltage is equal to 43 mV, while the stored charges reach at maximum value of 17.2 nC. Figure 11(d) illustrates that maximum stored energy is 0.5819 nJ at the optimum capacitance of 22 nF at 1800 cycles. Figure 11(d) shows the maximum stored energy of 0.5819 nJ at the optimum of 22 nF at 1800 cycles for time duration of 600 s. The numerical calculations show that the reactance of 22 nF capacitance is approximately 31 MΩ at 6 Hz. With reference to Figure 11(d), maximum power was achieved at 31 MΩ which resembles the reactance of at 22 nF.

To verify the above anticipations, the stored energy was determined as a function of load capacitance $(C_{L})$ for different number of cycles (n). The analysis shows that when the number of charging cycles increases, optimum capacitance and the maximum energy stored both increase and an accurately linear trend between optimum capacitance and the charging cycle numbers is observed.⁶⁴ A decline behavior is observed if capacitance value is increased or decreased beyond optimum value, as shown in Figure 12(a). Further, the energy storage characteristics of all selected load capacitors were analyzed as function of n, as shown in Figure 12(b). The capacitor with smaller value of 1 nf has little capacity of storage and quickly gains the saturation value. Relatively, larger load capacitors of 5, 10, and 22 nf exhibit exponentially increasing behavior as function of number of cycles (n). The load capacitor of 22 nf shows larger capacity to store maximum energy from Ag-doped ZnO nanorods PENG which reveals the matching of impedance between load capacitor and PENG, as shown in Figure 12(b).

Figure 12.

Analysis of the optimum load capacitance. (a) Relationship between load capacitance $(C_{L})$ and stored energy at different number of cycles. (b) Relation between energy storage and number of cycles (n) for different load capacitance $(C_{L})$ .

Conclusion

In summary, the undoped and Ag-doped ZnO nanorods were grown on cotton fabric by hydrothermal method and utilized to fabricate textile-based PENG. Elemental and optical analysis revealed the fine crystal quality and piezoelectric property of Ag-doped ZnO nanorods.

Ag-doped ZnO was found to be a good candidate for PENG compared to undoped ZnO. By applying the same conditions, the Ag-doped ZnO PENG power output was three times greater than undoped ZnO PENG. The maximum output power density of 1.45 mW cm⁻² was achieved from Ag-doped PENG at load resistance of 31 MΩ. The effect of load resistance was observed on the output power of PENG and found that at 31MΩ the Ag-doped PENG gives maximum power.

The fabricated PENG was utilized to store charge in load capacitors and found that capacitive reactance also affects the charging capability of PENG. It was observed that the maximum energy could be stored at optimum load capacitance of 22 nF at 1800 cycles. This research work shows the unique PENG charging characteristics, which may provide a pathway to design an integrated PENG energy harvesting system for practical applications.

Footnotes

Acknowledgment

The authors are very grateful to the chairperson, Department of Physics, University of Balochistan and National Centre for physics, Islamabad, Pakistan, for support in the characterization.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Sumera Rafique

References

Jalali

Woolliams

Stewart

, et al. Improved performance of p–n junction-based ZnO nanogenerators through CuSCN-passivation of ZnO nanorods. J Mater Chem A 2014; 2(28): 10945.

Qin

, et al. Self-powered nanowire devices. Nat Nanotechnol 2010; 5(5): 366–373.

Song

, et al. Piezoelectric nanogenerator using p-type ZnO nanowire arrays. Nano Lett 2009; 9(3): 1223–1227.

Huang

Song

Tsai

, et al. Single-InN-nanowire nanogenerator with up to 1 V output voltage. Adv Mater 2010; 22(36): 4008–4013.

Chen

Zhu

, et al. Gallium nitride nanowire based nanogenerators and light-emitting diodes. ACS Nano 2012; 6(6): 5687–5692.

Lin

Song

Ding

, et al. Piezoelectric nanogenerator using CdS nanowires. Appl Phys Lett 2008; 92(2): 1–4.

Wang

Kong

Ding

, et al. Semiconducting and piezoelectric oxide nanostructures induced by polar surfaces. Adv Function Mater 2004; 14(10): 943–956.

Hsin

Liu

, et al. Piezoelectric gated diode of a single ZnO nanowire. Adv Mater 2007; 19(6): 781–784.

Fei

Yeh

Zhou

, et al. Piezoelectric potential gated field-effect transistor based on a free-standing ZnO wire. Nano Lett 2009; 9(10): 3435–3439.

10.

Wang

Song

. Piezoelectric nanogenerators based on zinc oxide nanowire arrays. Science 2006; 312(5771): 242–246.

11.

Duraimurugan

Kumar

Venkatesh

, et al. Morphology and size controlled synthesis of zinc oxide nanostructures and their optical properties. J Mater Sci: Mater Electron 2018; 29: 9339–9346.

12.

Wojnarowicz

Chudoba

Koltsov

, et al. Size control mechanism of ZnO nanoparticles obtained in microwave solvothermal synthesis. Nanotechnology 2018; 29(6): 065601.

13.

Lee

Leem

. Size control of ZnO nanorods using the hydrothermal method in conjunction with substrate rotation. J Nanosci Nanotechnol 2017; 17(11): 7952–7956.

14.

Wang

. One-dimensional ZnO nanostructures: solution growth and functional properties. Nano Res 2011; 4(11): 1013–1098.

15.

Song

, et al. Growth of ZnO nanotube arrays and nanotube based piezoelectric nanogenerators. J Mater Chem 2009; 19(48): 9260–9264.

16.

Shao

. Direct-current piezoelectric nanogenerator based on p-Si/n-ZnO heterojunction. Phys E: Low-Dimen Syst Nanostruct 2016; 77: 44–47.

17.

Liao

, et al. The enhanced performance of piezoelectric nanogenerator via suppressing screening effect with Au particles/ZnO nanoarrays Schottky junction. Nano Res 2016; 9(2): 372–379.

18.

Zhao

, et al. Ga-doped ZnO nanowire nanogenerator as self-powered/active humidity sensor with high sensitivity and fast response. J Alloys Comp 2015; 648: 571–576.

19.

Zhu

Yang

Wang

, et al. Flexible high-output nanogenerator based on lateral ZnO nanowire array. Nano Lett 2010; 10(8): 3151–3155.

20.

Oshman

Opoku

Dahiya

, et al. Measurement of spurious voltages in ZnO piezoelectric nanogenerators. J Microelectromech Syst 2016; 25(3): 533–541.

21.

Baek

Hasan

Park

. Output power enhancement from ZnO nanorods piezoelectric nanogenerators by Si microhole arrays. Nanotechnology 2016; 27(6): 065401.

22.

Qin

, et al. A high power ZnO thin film piezoelectric generator. Appl Surf Sci 2016; 364, 670–675.

23.

Chen

Kumar

, et al. Sponge-templated macroporous graphene network for piezoelectric ZnO nanogenerator. ACS Appl Mater Interf 2015; 7(37): 20753–20760.

24.

Park

Son

Hwang

, et al. Highly-efficient, flexible piezoelectric PZT thin film nanogenerator on plastic substrates. Adv Mater 2014; 26(16): 2514–2520.

25.

Promsawat

Wichaiwong

Pimpawat

, et al. A study of flexible piezoelectric generators by sputtering ZnO thin film on PET substrate. Integrat Ferroelectric 2019; 195(1): 220–229.

26.

Wang

Yang

Qiu

, et al. Two-dimensional ZnO nanosheets grown on flexible ITO-PET substrate for self-powered energy-harvesting nanodevices. Appl Phys Lett 2018; 112(6): 063906.

27.

Aleksandrova

Kolev

Vucheva

, et al. Characterization of piezoelectric microgenerator with nanobranched ZnO grown on a polymer coated flexible substrate. Appl Sci 2017; 7(9): 890.

28.

Lei

Qiu

Yang

, et al. A vibration-driven nanogenerator fabricated on common paper substrate for harvesting energy from environment. J Renew Sustain Energy 2015; 7(3): 033115.

29.

Chansaengsri

Onlaor

Tunhoo

, et al. Based flexible piezoelectric nanogenerator using fibrous polymer/piezoelectric nanoparticle composite material. Electron Lett 2018; 54(12): 772–773.

30.

Nour

Bondarevs

Huss

, et al. Low-frequency self-powered footstep sensor based on ZnO nanowires on paper substrate. Nano Res Lett 2016; 11(1): 156.

31.

Khan

Abbasi

Hussain

, et al. Piezoelectric nanogenerators based on zinc oxide nanorods grown on textile cotton fabric. Appl Phys Lett 2012; 101:193506.

32.

Huang

Wang

Zheng

. Textile-based electrochemical energy storage devices. Adv Energy Mater 2016; 6(22): 1600783.

33.

Sohn

Cha

Song

, et al. Engineering of efficiency limiting free carriers and an interfacial energy barrier for an enhancing piezoelectric generation. Energy Environ Sci 2013; 6(1): 97–104.

34.

Chang

Chen

Yang

, et al. Excellent piezoelectric and electrical properties of lithium-doped ZnO nanowires for nanogenerator applications. Nano Energy 2014; 8: 291–296.

35.

Hongsith

Viriyaworasakul

Mangkorntong

, et al. Ethanol sensor based on ZnO and Au-doped ZnO nanowires. Ceramics Int 2008; 34(4): 823–826.

36.

Kashif

Ali

SMU

, et al. Sol–gel synthesis of Pd doped ZnO nanorods for room temperature hydrogen sensing applications. Ceramics Int 2013; 39(6): 6461–6466.

37.

Lee

, et al. Solution-processed Ag-doped ZnO nanowires grown on flexible polyester for nanogenerator applications. Nanoscale 2013; 5(20), 9609–9614.

38.

Yan

Al-Jassim

Wei

. Doping of ZnO by group-IB elements. Appl Phys Lett 2006; 89(18): 181912.

39.

Nour

Echresh

Liu

, et al. Piezoelectric and opto-electrical properties of silver-doped ZnO nanorods synthesized by low temperature aqueous chemical method. Aip Adv 2015; 5(7): 077163.

40.

Volnianska

Boguslawski

Kaczkowski

, et al. Theory of doping properties of Ag acceptors in ZnO. Phys Rev B 2009; 80(24): 245212.

41.

Fan

Freer

. The roles played by Ag and Al dopants in controlling the electrical properties of ZnO varistors. J Appl Phys 1995; 77(9): 4795–4800.

42.

Gardner

Oporto

Mills

, et al. Adhesion and surface issues in cellulose and nanocellulose. J Adhes Sci Technol 2008; 22: 545–567.

43.

Cai

. Fabrication of a superhydrophobic ZnO nanorod array film on cotton fabrics via a wet chemical route and hydrophobic modification. Appl Surf Sci 2008; 254(18): 5899–5904.

44.

Shaban

Abdallah

Khalek

. Characterization and photocatalytic properties of cotton fibers modified with ZnO nanoparticles using sol–gel spin coating technique. Beni-Suef University J Basic Appl Sci 2016; 5(3): 277–283.

45.

Zeferino

Flores

Pal

. Photoluminescence and Raman scattering in Ag-doped ZnO nanoparticles. J Appl Phys 2011; 109(1): 014308.

46.

Irzh

Genish

Klein

, et al. Synthesis of ZnO and Zn nanoparticles in microwave plasma and their deposition on glass slides. Langmuir 2010; 26(8): 5976–5984.

47.

Yakuphanoglu

. Electrical characterization and device characterization of ZnO microring shaped films by sol–gel method. J Alloys Compounds 2010; 507(1): 184–189.

48.

Parra

Haque

. Aqueous chemical route synthesis and the effect of calcination temperature on the structural and optical properties of ZnO nanoparticles. J Mater Rese Technol 2014; 3(4): 363–369.

49.

Lanjewar

Gohel

. Enhanced performance of Ag-doped ZnO and pure ZnO thin films DSSCs prepared by sol-gel spin coating. Inorganic Nano-Metal Chem 2017; 47(7): 1090–1096.

50.

Shaban

Mohamed

Abdallah

Production and characterization of superhydrophobic and antibacterial coated fabrics utilizing ZnO nanocatalyst. Sci Rep 2018; 8(1): 3925.

51.

Karunakaran

Rajeswari

Gomathisankar

. Combustion synthesis of ZnO and Ag-doped ZnO and their bactericidal and photocatalytic activities. Superlatt Microstruct 2011; 50(3): 234–241.

52.

Ahn

Kang

Kim

, et al. Synthesis and analysis of Ag-doped ZnO. J App Phys 2006; 100(9): 093701.

53.

Kang

Ahn

Kim

, et al. Structural, electrical, and optical properties of p-type ZnO thin films with Ag dopant. Appl Phys Lett 2006; 88(20): 202108.

54.

Khan

Ansari

Hameedullah

, et al. Sol-gel synthesis of thorn-like ZnO nanoparticles endorsing mechanical stirring effect and their antimicrobial activities: potential role as nano-antibiotics. Sci Rep 2016; 6: 27689.

55.

Janet

Navaladian

Viswanathan

, et al. Heterogeneous wet chemical synthesis of superlattice-type hierarchical ZnO architectures for concurrent H2 production and N2 reduction. J Phys Chem C, 2010; 114(6): 2622–2632.

56.

Lamba

Umar

Mehta

, et al. Enhanced visible light driven photocatalytic application of Ag2O decorated ZnO nanorods heterostructures. Separat Purificat Technol 2017; 183: 341–349.

57.

Athauda

Hari

Ozer

. Tuning physical and optical properties of ZnO nanowire arrays grown on cotton fibers. ACS Appl Mater Interf 2013; 5(13): 6237–6246.

58.

Wang

Shen

Wang

, et al. ZnO microstructures as flame-retardant coatings on cotton fabrics. ACS Omega 2018; 3(6): 6330–6338.

59.

Yao

Wang

Fan

, et al. One-step solvothermal deposition of ZnO nanorod arrays on a wood surface for robust superamphiphobic performance and superior ultraviolet resistance. Sci Rep 2016; 6: 35505.

60.

Parida

Bhavanasi

Kumar

, et al. Fast charging self-powered electric double layer capacitor. J Power Source 2917; 342: 70–78.

61.

Chang

Tran

Wang

, et al. Direct-write piezoelectric polymeric nanogenerator with high energy conversion efficiency. Nano Lett 2010; 10(2): 726–731.

62.

Niu

Wang

. Theoretical systems of triboelectric nanogenerators. Nano Energy 2015; 14: 161–192.

63.

Niu

Liu

Zhou

, et al. Optimization of triboelectric nanogenerator charging systems for efficient energy harvesting and storage. IEEE Tran Electron Dev 2014; 62(2): 641–647.

64.

Yao

Jiang

Zhang

, et al. Charging system optimization of triboelectric nanogenerator for water wave energy harvesting and storage. ACS Appl Mater Interf 2016; 8(33): 21398–21406.