Simple Fabrication and Characterization of Piezoelectric Nanogenerators from Cobalt-Doped Zinc Oxide Nanofibers

Corresponding Author: Suyitno Department of Mechanical Engineering, Universitas Sebelas Maret, Surakarta, Indonesia E-mail: suyitno@uns.ac.id Abstract: The ferromagnetic cobalt doped into Zinc Oxide (ZnO) crystals is a promising method for enhancing the performance of PiezoelectricBased Nano Generators (PENGs). The study reports the characterization and testing PENGs which simply fabricated from cobalt-doped ZnO nanofibers (NFs) synthesized by electro spinning machine. The electro spinning processes were conducted at a flow rate of 4 μL/min and at various cobalt concentrations followed by sintering at a temperature of 500°C. The X-Ray Diffraction (XRD) spectra demonstrate that the crystalline diameter of Co-doped ZnO NFs increased as the content of cobalt increased, up to a concentration of 9%. Meanwhile, the maximum output voltage and power density of PENGs with Co-doped ZnO NFs were 221.6 mV and 148.8 nW/cm 2 , respectively when the PENGs were subjected to a cyclic mechanical load of 0.5 kg. Therefore, PENGs fabricated from Co-doped ZnO NFs are challenging for the next generation of self-powered devices.


Introduction
Piezoelectric Nanogenerators (PENGs) have been investigated by many studies because of their capability to harvest mechanical energy from human motions, finger taps and environment movements into electrical energy via piezoelectric materials. PENGs have also been employed in various devices including wireless sensors, self-powered equipments and electromechanical systems (Cho et al., 2017). The key issues in PENGs are the design and the materials used (Dagdeviren et al., 2016).
There are various materials having a noncentrosymmetric property exploited for PENGs. Among them, ZnO materials are very promising because of its abundant availability, environmental friendliness and high Curie temperature. Above the Curie temperature, a non-centrosymmetric crystal transforms into a centrosymmetric structure and both ferroelectric city and piezoelectricity disappear (Ubaidillah et al., 2013;Xing et al., 2015).
The piezoelectricity of materials was represented by the piezoelectric constant, d 33 . The higher the piezoelectricity, the bigger the voltage is generated from the PENGs. ZnO Nanowires (NWs) have a slightly bigger piezoelectric constant d 33 of 15 pm/V (Christian et al., 2016) than that of the bulk ZnO with d 33 of 9.93 pm/V (Zhao et al., 2004). ZnO NWs have a piezoelectric constant ranging from 1 to 75 pm/V (Lee et al., 2012) depending on their morphology. Meanwhile, ZnO Nanobelts (NBs) have bigger d 33 (14.3-26.7 pm/V) than that of ZnO Thin Films (TFs) synthesized by sputtering deposition (14 pm/V) (Dagdeviren et al., 2013). However, the ZnO TFs deposited onto a Si substrate by pulsed laser ablation has a high piezoelectric constant of 49.7 pm/V (Qin et al., 2016a;2016b) because it exhibited a preferred c-axis orientation. In addition, ZnO Nanorods (NRs) were reported have the d 33 ranging from 4.41 to 11.8 pm/V (Tamvakos et al., 2015) for single crystal and they further reached a highest value of 26.7 pm/V when measured for ZnO NBs (Zhao et al., 2004). Therefore, the morphology of ZnO significantly affects to the piezoelectric property, d 33 .
It has also been revealed that doping has an important effect on both a structural and a piezoelectric constant of ZnO. Piezoelectric constant evaluated for ZnO TFs doped with Ni, Cu, Fe and Co shows a variation in the d 33 values ranging from 5 to 14 pm/V (Zamiri et al., 2016). A significant increase in d 33 values up to 110 pm/V has been reported for V-doped ZnO TFs (Zamiri et al., 2016). Meanwhile, Cr-doped ZnO NRs have a very small d 33 value of 1.68 pm/V (Sinha et al., 2014). Besides, Europium-doped ZnO NRs have the mean d 33 of 43.38 pm/V (Yadav et al., 2016). Therefore, the morphology and doping play important role for enhancing the performance of PENGs based on ZnO nanomaterials. However, to the best of our knowledge, there are only few studies on the piezoelectric constant of cobaltdoped ZnO Nano Fibers (NFs) and used for PENGs.
PENGs with a hetero-junction material of Li-doped Cu 2 O/ZnO produced a power of 52 µW (Cho et al., 2017). The Li-doped Cu 2 O/ZnO synthesized by radio frequency magnetron sputtering was flexible and transparent. Moreover, PENGs with the ZnO TFs deposited onto a Si substrate by pulsed laser ablation at a substrate temperature of 500°C generated a voltage, current and power of 95 mV, 35 µA/cm 2 and 5.1 mW/cm², respectively (Qin et al., 2016a;2016b). The PENGs with Ga-doped ZnO NWs produced a higher voltage ranging from 120 to 560 mV (Zhao et al., 2015b) if compared with undoped ZnO NWs generated a lower voltage of up to 25 mV (Wang et al., 2008). Meanwhile, PENGs fabricated from Al-doped ZnO NRs and NFs capable of generating a voltage up to 60 mV (Fang and Kang, 2010) and 265.5 mV (Suyitno et al., 2014b), respectively. However, the voltage and power density of PENGs with ZnO nanomaterials are still low (Ren et al., 2009;Kanjwal et al., 2011).
Among the aforementioned materials used for PENGs, the ZnO NFs are very interesting because NFs can be simply synthesized using a low cost electrospinning machine (He et al., 2008) with a good durability (Suyitno et al., 2014a). Unfortunately, ZnO NFs exploited for PENGs are still limited. Besides Al, Ga, Li, Ni, Cu, Fe and Cr doping; Co-doped ZnO NFs synthesized by electrospinning method is a promising for enhancing the properties of ZnO-based piezoelectric materials fabricated for PENGs. There is a study reported that cobalt doping does not significantly affect their piezoelectric performance, although it induces the paramagnetic state and gives rise to new photoluminescence emission peaks because of small changes in the lattice parameters and no changes to crystal structures (Zhao et al., 2015a). However, cobalt doping into ZnO nanoparticles (NP) synthesized by a coprecipitation method decreased a crystallite size and changed the diamagnetic behavior of ZnO into ferromagnetic nature (Gandhi et al., 2014).
Meanwhile, another study reported that the lattice constants of the c-axis increased with increasing cobalt content up to 25% and decreased above 25% because of the solubility of cobalt exceeds a thermal equilibrium limit below 25% due to a no-equilibrium sol-gel process (Lee et al., 2002).
The aforementioned studies show that the effect of cobalt doping into ZnO materials is still controversy and need to be deeply investigated particularly for ZnO NFs synthesized by electrospinning method. Furthermore, the optimal concentration of Co doping into ZnO NFs applied for PENGs has not been entirely studied and characterized. Therefore, we report here the simple fabrication and the characterization of PENGs from cobalt-doped zinc oxide fibres synthesized by electrospinning method. In addition, we evaluate also the influence of the cobalt content on the performance of PENGs simply fabricated from cobalt-doped ZnO NFs.

Fabrication PENGs
PENGs were simply fabricated from Co-doped ZnO NFs, two electrodes and Polydimethylsiloxane (PDMS) polymer. The NFs were synthesized by electro spinning method (Suyitno et al., 2014b). To avoid the damage of NFs, the collector used in electro spinning method was aluminum plate and then subsequently used for electrode. Both electrodes were glued together with PDMS as show in Fig. 1.
We first synthesized the PVA solution. The PVA was dissolved in purified water at a mass ratio of 1:10, stirred at a temperature of 70°C for 4 h and allowed to settle for 24 h at a temperature of 25°C.
To synthesize the CoAc/ZnAc solution with a cobalt content of 8%, 0.32 g of CoAc, 4.0 g of ZnAc and 16 g of H 2 O were mixed and stirred by magnetic stirrer at a temperature of 70°C for 2 h. Next, synthesizing the CoAc/ZnAc/PVA solution was conducted by mixing 1 g of the CoAc/ZnAc solution with 5 g of PVA solution, stirred at a temperature of 70°C for 2 h and followed by settling for 24 h at a temperature of 25°C. The aforementioned procedures were repeated for synthesizing the CoAc/ZnAc/PVA solution at a cobalt content of 4, 6, 8, 10, 12 and 14%. In addition, 0.5 mL −1 of CoAc/ZnAc/PVA solution was poured in the syringe pump and electrospun in a horizontal electro spinning machine to produce the green fibers consisting of cobalt-doped ZnAc/PVA. The needle diameter of the syringe pump is 0.6 mm. The distance between the tip needle of the syringe pump and the collector of an aluminum plate is 8 cm. When a high voltage of approximately 15 kV was applied, the solution was pulled out by electrostatic fields, attached on the surface of an aluminum collector plate and finally produced the non-woven green fibers. The green fibers were then sintered for 4 h at a temperature of 500°C (Yang et al., 2004) to remove PVA or organic materials and to growth the crystal of Co-doped ZnO fibers.

Characterizing ZnO NFs and Testing PENGs
The crystalline and morphological structures of Codoped ZnO NFs were characterized using X-Ray Diffraction (XRD) and scanning electron microscopy (SEM, FEI: Inspect-S50). The analysis of SEM images was conducted by image software from Image J.
The piezoelectric constant d 33 of Co-doped ZnO NFs was measured by d 33 Piezometer Systems from Piezotest Pte Ltd. Meanwhile, the PENGs were measured their output electric energy using an ADAM Advantech (series 4018) and software PCLS-Adam View 32. When measuring the output electric energy from NGs, the cyclic mechanical load of 0.5 kg was applied to represent the condition of a light finger tapping (Choi et al., 2010). The power of piezoelectric-based NGs was subsequently calculated from the output voltage generated by the variation of load resistance (Ω) (Chen et al., 2010), as following:

Results and Discussion
Structural Analysis Figure 2 shows the spectra of X-ray diffraction of the fibers of Co-doped ZnO NFs with cobalt content from 0 to 14%. The sharp diffraction peaks show the good crystalline quality. The NFs tested showed similar diffraction patterns with the highest peak between 35.88 and 36.15°, which is the typical peak of zincite ZnO crystal (101). The diffraction patterns of Co-doped ZnO NFs corresponded to the data from JC-PDS standard card (PDF#36-1451, space group P63 mc (186)).
Meanwhile, the shifts in the angle of diffraction peaks indicated the impurities of ZnO crystals caused by cobalt doping. Moreover, Fig. 2 shows also that clear sharp peaks coinciding with other impurities did not appear, suggesting that new phases did not arise and cobalt atoms were substituted within ZnO crystal lattices (Feng et al., 2009). At higher cobalt content, the peaks of diffraction appeared sharper which is in agreement with previous study (Nirmala and Anukaliani, 2011). In contrast, at a cobalt content of 14%, the diffraction peaks were smaller and no peaks of cobalt compounds or cobalt existed, indicating the crystallization process was inhibited by means of impurity atoms.
Meanwhile, the crystalline diameter of cobalt-doped ZnO fibers was calculated using the Debye-Scherrer equation (Pongwan et al., 2012;Asikuzun et al., 2015): Where: D = The crystalline diameter λ = The wavelength of X-ray (CuKα = 0.15406 nm) k = The constant (0.9) θ = The diffraction angle B = The full width at half maximum. Table 1 shows that the crystalline diameter of Codoped ZnO NFs were influenced by the concentration of cobalt. The corresponding crystalline diameter of Co-doped ZnO NFs increased by almost twice at a cobalt concentration of 9%, which declined as the cobalt content further increased. The increase of grain size at a certain concentration of doping cobalt was also revealed from previous studies (Lee et al. 2002;Bahadur et al., 2010;Benramache et al., 2013;Gandhi et al., 2014;Asikuzun et al., 2015). Therefore, the previous study from (Zhao et al., 2015a) stating that cobalt doping does not significantly affect the lattice parameters and no changes to crystal structures is misleading within the synthesis of Co-doped ZnO NFs by the electrospinning machine. Furthermore, the crystalline diameter of Co-doped ZnO NFs varied ranging from 9.0 to 17.7 nm. Doping cobalt at a concentration higher than 9% affected the decrease of the crystalline diameter because more cores were formed during the crystallization process (Lee et al., 2008).
Meanwhile, Fig. 3 shows the SEM images of the Codoped ZnO NFs at a magnification of 50,000×. It can be observed that the fibers are randomly arranged. As shown in Table 2, the diameter of the fibres increased as the cobalt content increased up to a concentration of 9%. The average diameter of the Co-doped ZnO NFs ranged from 87 to 133 nm.

Performance of PENGs from Co-Doped ZnO NFs
The piezoelectric constant of Co-doped ZnO NFs has been measured and shown in Table 3. There is a slight increase of d 33 as increasing concentration of doping Co up to 9-10%. The d 33 values of Co-doped ZnO NFs ranging from 3.7 to 4.1 pm/V which is comparable with that of ZnO NRs ranging from 4.41 to 11.8 pm/V (Tamvakos et al., 2015) and a bit higher than that of Crdoped ZnO NRs (1.68 pm/V) (Sinha et al., 2014).
Meanwhile, testing performance of NGs was conducted by applying the cyclic mechanical load of 0.5 kg every 3 sec. When the c-axis of cobalt-doped ZnO fibers was under the applied deformation, a piezoelectric field was created along fibers, which drive the electrons flowing in the external circuit. Figure 4 shows the output voltage (mV) produced from PENGs. Meanwhile, Fig. 5 shows the output power (nW) of PENGs. The maximum power and the maximum voltage produced by the PENGs can be seen in Table 4.     6  8  9  10  11  12  14  Maximum  283  169  204  208  304  202  199  164  250  Minimum  42  39  37  40  33  50  47  26  37  Mean  91  93  96  110  133  102  88  87  87  Standard deviation  50  40  39  42  77  35  32  31  52   Table 3  Piezoelectric material is materials applied loads produces electricity. At a cyclic mechanical load of 0.5 kg, the output voltage generated by PENGs with Codoped ZnO NFs was influenced by the resistance loads. The output voltage generated from PENGs increased as the concentration of cobalt doping increased. The sharp increase of output voltage was generated when the resistance load increased up to 1 kΩ, followed with slight increase at higher resistance load than 1 kΩ. At doping cobalt about 9%, the output voltage generated increased nearly 3.45 times than that produced by PENGs without doping cobalt.
The maximum output voltage of PENGs was 221.6 mV. The output voltage of the PENGs with Co-doped ZnO NFs was higher when compared with previous study (Choi et al., 2010). PENGs with ZnO TFs generated voltages of 20 and 60 mV at a mechanical load of 0.4 and 1 kg, respectively. In contrast, the PENGs based on NiO/ZnO NWs were capable to generate a voltage of 388 mV (Qu et al., 2016) because ZnO NWs were vertically grown. Meanwhile, by ferrous doping, PENGs with the vertical ZnO NWs generated a voltage up to 850 mV (Zhu et al., 2015).
In addition, the output power generated by PENGs with Co-doped ZnO NFs increased as the cobalt concentration increased up to 9%. As previously discussed, the synthesized NFs demonstrated the highest crystallinity at a cobalt concentration at 9%. The maximum output power of PENGs, 148.9 nW, was generated when the cobalt concentration of 9% as shown in Fig. 5 corresponding to an output current of 1.2 µA. The current generated from the PENGs with Co-doped ZnO NFs was greater than that from the PENGs with ZnO NWs/PZT (lead zirconate titanate) hetero-junction, namely 270 nA (Im-Jun et al., 2013) and with NiO-ZnO hetero-junction, namely 40 nA/cm² (Yin et al., 2015). Moreover, the maximum output power was equivalent to a power density of 148.8 nW/cm². The power density produced by PENGs with Co-doped ZnO NFs was greater than the achieved by a previous study, namely 51.7 nW/cm 2 (Suyitno et al., 2014b).

Conclusion
PENGs were simply fabricated from Co-doped ZnO NFs and successfully tested. The greatest crystallinity was achieved by cobalt doping at a concentration of 9%. Furthermore, the cobalt concentration has doubled the crystalline diameter of ZnO NFs. In contrast, the cobalt content in the ZnO NFs significantly influenced the output voltage and output power of PENGs. The maximum output voltage of PENGs with Co-doped ZnO NFs is 221.6 mV. Meanwhile, the maximum output power, which is equivalent to the power density of 148.8 nW/cm², was achieved when PENGs were subjected to a cyclic mechanical load of 0.5 kg. The PENGs fabricated from Co-doped ZnO NFs generated high power density values and are interesting candidates for employing in self-power devices.