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Research Papers

Fabrication and Characterization of Zinc Oxide-Based Electrospun Nanofibers for Mechanical Energy Harvesting OPEN ACCESS

[+] Author and Article Information
Suyitno Suyitno

Mem. ASME
Department of Mechanical Engineering,
Sebelas Maret University,
Jl. Ir. Sutami 36 A, Surakarta 57126, Indonesia
e-mails: suyitno@uns.ac.id;
suyitno@gmail.com

Agus Purwanto

Department of Chemical Engineering,
Sebelas Maret University,
Jl. Ir. Sutami 36 A, Surakarta 57126, Indonesia
e-mail: aguspur@yahoo.com

R. Lullus Lambang G. Hidayat

Department of Mechanical Engineering,
Sebelas Maret University,
Jl. Ir. Sutami 36 A, Surakarta 57126, Indonesia
e-mail: lulus_l@yahoo.com

Imam Sholahudin

Postgraduate Program in Mechanical Engineering,
Sebelas Maret University,
Jl. Ir. Sutami 36 A, Surakarta 57126, Indonesia
e-mail: slatem25@gmail.com

Mirza Yusuf

Postgraduate Program in Mechanical Engineering,
Sebelas Maret University,
Jl. Ir. Sutami 36 A, Surakarta 57126, Indonesia
e-mail: langkahsiguci@gmail.com

Sholiehul Huda

Department of Mechanical Engineering,
Sebelas Maret University
Jl. Ir. Sutami 36 A, Surakarta 57126, Indonesia
e-mail: sholiehulhuda@gmail.com

Zainal Arifin

Department of Mechanical Engineering,
Sebelas Maret University,
Jl. Ir. Sutami 36 A, Surakarta 57126, Indonesia
e-mails: zainal_a@uns.ac.id; zainal_mp@yahoo.co.id

Manuscript received September 24, 2013; final manuscript received April 12, 2014; published online May 2, 2014. Assoc. Editor: Roger Narayan.

J. Nanotechnol. Eng. Med 5(1), 011002 (May 02, 2014) (6 pages) Paper No: NANO-13-1068; doi: 10.1115/1.4027447 History: Received September 24, 2013; Revised April 12, 2014

Doped and undoped zinc oxide fibers were fabricated by electrospinning at various solution flow rates of 2, 4, and 6 μl/min followed by sintering at 550 °C. The nanogenerators (NGs) fabricated from the fibers were examined for their performance by applying loads (0.25–1.5 kg) representing fingers taps on the keyboard. A higher solution flow rate resulted in a larger fiber diameter, thus reducing nanogenerator voltage. The maximum power density for undoped zinc oxide-based and doped zinc oxide-based nanogenerators was 17.6 and 51.7 nW/cm2, respectively, under a load of 1.25 kg. Enhancing nanogenerator stability is a topic that should be investigated further.

FIGURES IN THIS ARTICLE
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The application of piezoelectric devices is an interesting topic, particularly with regard to self-powered NGs in keyboards. Piezoelectric material-based NGs may play an important role because they can scavenge mechanical energy and transform it into a more flexible form, such as electrical energy. To date, piezoelectric materials based on PZT (lead zirconate titanate) [1-3], BaTiO3 [4,5], polyvinylidene difluoride (PVDF) [6,7], BNT (bismuth sodium titanate) [8], and zinc oxide (ZnO) [9-20] have been the most widely investigated ones for use in transducers, actuators, transformers, photonics, optoelectronics, and sensors. However, fiber-based piezoelectric NGs [21-23] have not been extensively developed for keyboards because they must be resistant to large mechanical loads. The load applied through a finger tap on a keyboard is larger than that used in atomic force microscopy (AFM) commonly used to investigate the piezoelectric effect. Some materials exhibiting good characteristics according to AFM loading do not necessarily meet the requirements for NGs on keyboards under 0.5 kg loading.

In addition, the performance of piezoelectric-based NGs is strongly dependent on the structural and electrical properties of the piezoelectric materials used. A study of these properties and their dependence is very important to realize better NGs. One way to improve the properties of piezoelectric-based devices is by changing their morphology, e.g., microcantilevers [13], functionally graded materials [24], thin films [15,25-28], nanorods [10,29], and nanobelts [14]. Nanofiber structures are significantly different from microscale or macroscale structures, particularly with regard to the high active areas of the former [30]. The pattern of a nanofiber structure increases mechanical energy absorption in fiber-based NGs, making it higher than that in the case of nearly spherical particle-based NGs. Moreover, nanofibers can gradually distribute the mechanical force from one fiber to another over a long period, thereby resulting in a better effect for piezoelectric-based NGs. Nanofibers also possess several remarkable characteristics, such as high porosity, flexibility in surface functionality, and superior mechanical performance. Therefore, nanofiber structures are interesting to study, as they have many advantageous properties and can be produced using a simple low-cost electrospinning machine [16,17,31-34].

Furthermore, the structural and electrical properties of fiber-based NGs are also determined by the process parameters during fiber growth in electrospinning, as well as by the presence of impurities and defects in the fibers. In this study, we used an AlCl3 solution for doping ZnO because the incorporation of aluminum into ZnO increases its electrical conductivity [35]. However, the flow rate is considered to a key parameter controlling the fiber diameter and its distribution, initiating droplet shape, controlling the jet trajectory, and maintaining the Taylor cone and deposition area. The amount of precursor solution for electrospinning was varied in accordance with the flow rate. However, there is a close relationship between applied voltage, precursor properties, and precursor flow rate in maintaining a stable Taylor cone. A solution containing polysulfone/dimethylacetamide was electrospun at 10 kV at a flow rate of 11.0 μl/min and thick fibers with beads were produced [36]. Furthermore, solutions of polyvinylpyrrolidone and Ti(OCH(CH3)2)4 were electrospun at flow rates ranging from 0.1 to 1.2 ml/h. The smallest anatase TiO2 fibers were 120 nm ± 10 nm in diameter and were produced at a flow rate of 0.1 ml/h (1.7 μl/min). Smooth fibers without beads were produced when the flow rate of the solution was 0.5 ml/h (8.3 μl/min) [37]. Too high flow rates resulted in beading since fibers did not get enough time to dry before reaching the collector [38]. The higher the solution flow rate, the bigger the fiber diameter or bead size [39]. Generally, a lower flow rate is more desirable as the precursor solution would get enough time for drying [40] and would yield smaller-diameter fibers [38].

Unfortunately, there are only a few reports regarding the application of ZnO-based electrospun fibers for the fabrication of piezoelectric NGs. Thus, the objectives of this work are to fabricate ZnO-based electrospun fibers at various precursor solution flow rates and to characterize the fabricated NGs to assess their suitability for harvesting mechanical energy over a reasonable finger tap load range.

Preparation of Precursor Material.

First, a polyvinyl alcohol (PVA) solution was prepared in advance using 2 g of polyvinyl alcohol (MW = 70,000, Merck Ltd., Germany) and then mixing with 20 ml of distilled water. The solution was stirred at 70 °C for 4 h and was allowed to settle at room temperature for 8 h. To produce a zinc acetate (ZnAc) solution, 2 g of zinc acetate dehydrate Zn(CH3COO)2·2H2O (Merck Ltd., Germany) was mixed with 8 ml of water and stirred for 1 h at 70 °C. To produce a ZnAc/AlCl3 solution, 4 wt. % of aluminum chloride hexahydrate (AlCl3·6H2O, Merck) was mixed with 4 g of the ZnAc solution in 16 ml of distilled water at 70 °C and stirred for 1 h.

Both ZnAc and ZnAc/AlCl3 solutions were mixed separately with the PVA solution at a weight ratio of 1:4, stirred at 70 °C for 8 h, and allowed to settle at room temperature for 24 h. In this way, the ZnAc/PVA and ZnAc/AlCl3/PVA solutions used for electrospinning were produced.

Fabrication of Fibers by Electrospinning.

An electrospinning machine was employed to produce two types of green fibers. One milliliter each of the ZnAc/PVA and ZnAc/AlCl3/PVA solutions was placed in the syringe pump of the electrospinning machine. The needle in the electrospinning machine was horizontally connected to the positive terminal, while the aluminum plate was connected to the negative terminal. The distance between the terminals was 15 cm. A voltage of 15 kV was applied to generate solution flow rates of 2, 4, and 6 μl/min, and the effects of the nanofiber diameter on the NG performance were then investigated. Solution flow rates lower than 2 μl/min were not possible because at such low flow rates, the solution dried and clogged the hole of the syringe of the electrospinning machine. In previous studies, the electrospinning machine has commonly been used at flow rates of 5 μl/min [16], 16 μl/min [41], and 16.7 μl/min [16]. Larger solution flow rates require stronger electrostatic forces.

The fibers were attracted by the electrostatic field, and they attached themselves to the surface of the negative collector plate. The green fibers were further sintered at 550 °C for 4 h to remove any organic materials and to produce undoped ZnO and doped ZnO nanofibers. It is known that organic materials in PVA solutions decompose completely at temperatures of 440 °C [42].

Characterization of Fibers for NGs.

The morphology of the nanofibers was studied by scanning electron microscopy (SEM, FEI: Inspect-S50), and the crystal structure of the fibers was determined by X-ray diffraction (XRD). Because ZnO fibers were brittle [21,43] and firmly attached to the collector, a conductive aluminum collector material can be used as a substrate as well as electrodes on both sides. Both electrodes were glued together using polydimethylsiloxane, as shown in Fig. 1, and were used to produce undoped ZnO-based and doped ZnO-based NGs.

The performance of the synthesized NGs was studied by measuring the output voltage (V) with real-time data-acquisition hardware (Advantech USB-4716) and software (PCLS-Adam View 32). The NGs were subjected to a compressive cyclic load (loading and unloading) of 0.5 kg for 3 s per cycle, representing a load corresponding to a weak finger tap [27]. For determining power, the output voltage was measured for various external resistances (R) ranging from 0 to 10 MΩ during the excitation process. The equipment used for this purpose is shown in Fig. 2. The generated power (P) for each load cycle of NGs was calculated using the following formula:

Again, to determine the maximum output power, NGs produced at a flow rate of 2, 4, and 6 μl/min were subjected to a load of 0.25, 0.5, 0.75, 1.0, 1.25, and 1.5 kg for 3 s per cycle. These loads represent reasonable finger tap loads.

Structure Analysis.

The solution flow rates of 2 and 6 μl/min increased the diameter of the undoped ZnO and doped ZnO nanofibers by 3.4 and 2.7 times, respectively. A higher solution flow rate produced a larger fiber diameter, as shown in Fig. 3, which led to a smaller deformation of the undoped ZnO and doped ZnO nanofibers under the same load, thus reducing the output voltage. For the undoped ZnO nanofibers, increasing the solution flow rate to above 4 μl/min caused dramatic growth in the fiber diameter as can be seen by comparing Figs. 3(b) and 3(c). When a larger solution flow rate was employed, a larger bubble size was generated at the tip of the needle in the electrospinning machine. Larger bubbles caused larger fibers to be drawn without any sufficient stretching for the same magnitude of the electrostatic field. In addition, the fibers with a greater diameter because of the increased flow rate required a longer time to dry. Thus, the solvents stored in the fibers did not get enough time to evaporate and the residual solvents led to bead formation or caused the fibers to fuse together when they reached the collector [44]. In contrast, a small amount of solution was ejected from the capillary tips that led to a smaller droplet size at lower flow rates. At a solution flow rate of 4 μl/min, the respective average diameter of the undoped ZnO and doped ZnO nanofibers was 82.8 nm [Fig. 3(b)] and 65.3 nm [Fig. 3(e)], respectively. The smallest average diameter of 61.0 nm was measured for the doped ZnO fibers fabricated by electrospinning the ZnAc/AlCl3/PVA solution at a solution flow rate of 2 μl/min.

In addition to changes in the solution composition for electrospinning, the fiber diameter was significantly influenced by doping. The doped ZnO-based NGs had smaller fibers than undoped ZnO-based NGs, as shown in Fig. 3, which indicates that doping effectively and significantly reduced the fiber diameter. This is consistent with the XRD analysis results. As shown in Fig. 4, the undoped ZnO crystalline structure did not exhibit any significant changes after doping. All the diffraction peaks have indices of (100), (002), (101), (102), (110), and (103) corresponding to the typical hexagonal structure of ZnO (JCPDS Nr. 36-1451). Although doping was not reflected in the XRD results for undoped and doped ZnO, the XRF (X-ray fluorescence) results in Table 1 clearly show that approximately 3% aluminum was present in the doped ZnO samples.

Next, the respective full widths at half maximum of undoped and doped ZnO were 0.264 and 0.338, corresponding to the respective crystallite diameters of 34.2 nm and 25.9 nm, respectively. As a consequence, the green fibers produced from the ZnAc/PVA solution were more uniform than those prepared from the ZnAc/AlCl3/PVA solution, which is consistent with a previous research [38]. The fiber diameter of the undoped and doped ZnO-based NGs could be appropriately controlled by tuning the solution flow rate and doping; this provides enough evidence for the suitability of the synthesis of size-controlled nanofibers by an electrospinning machine.

Figure 3 shows SEM images of nanofibers. The fibers electrospun using the ZnAc/PVA and ZnAc/AlCl3/PVA solutions as precursors showed different morphological features. Wires or dendrites grew on the undoped ZnO fibers at all flow rates of the ZnAc/PVA solution. The dendrite growth can be attributed to the rapid and random crystallization of zinc acetate [45]. Notably, no wires appeared on the doped ZnO fibers as more nucleation centers were present as a result of the doping [30]. However, wires or dendrites growing on the undoped ZnO fibers may have inhibited deformation, and the undoped ZnO-based NGs produced a lower output voltage than the doped ZnO-based NGs did, as shown in Fig. 5. Furthermore, by introducing doping into ZnO, it became possible to enhance the morphology of fibers as well as the output voltage of ZnO-based NGs.

Performance Analysis of NGs at 0.5 kg Load.

In Fig. 5, the maximum open circuit voltages (Voc) for the undoped ZnO-based and doped ZnO-based NGs are 121.1 and 265.5 mV, respectively. The output voltages obtained from the NGs are larger than those obtained from ZnO-PSS/PVA thin films that range from 10 to 100 mV [46], and from PZT nanofibers with diameters of 50–150 nm that are approximately 170 mV [47]. However, these output voltages vary slightly. The maximum output voltage measured in the case of the undoped ZnO-based and doped ZnO-based NGs was considerably lower than that in the case of PVDF-TrFE nanofibers with a diameter of 60–120 nm. The NGs from PVDF-TrFE nanofibers have a maximum output voltage of 400 mV [47].

Furthermore, the solution flow rate has a significant effect on the output voltage of NGs, as shown in Fig. 5. A solution flow rate of 2 μl/min resulted in the highest output voltage for both the undoped ZnO-based and doped ZnO-based NGs. A higher solution flow rate produced a lower output voltage for the NGs. The output voltage can be assumed to depend on the deformation of the piezoelectric fibers on loading. Smaller fibers are more easily deformed than larger ones.

NGs that produced a higher output voltage possessed a greater output power. The output voltage steadily increased with the external resistance, and then it plateaued above an external resistance of approximately 2 MΩ, as shown in Fig. 5. Piezoelectric transducers operated with a resistive load are generally unable to provide a sufficient damping force under such circumstances. This limitation is primarily caused by the intrinsic shunt capacitance, which limits the real power transfer capability of the transducer into a resistive load [48]. Therefore, there exists a maximum output power for the undoped ZnO-based and doped ZnO-based NGs when the external resistance ranges from 50 kΩ to 0.5 MΩ. A maximum output power of 38.8 nW was realized for the doped ZnO-based NGs at a resistance of 0.1 MΩ, as shown in Fig. 6(b). This output power was higher than that of a PZT fiber-based NG with a power of 30 nW at a resistance of 6 MΩ [2]. It is recommended that an external resistance ranging from 0.1 to 0.5 MΩ be applied to these NGs to achieve the maximum output power when they are used in a keyboard with average finger tap loads of 0.5 kg.

The average output power of NGs showed a steady decrease with the solution flow rate, as shown in Fig. 6. The maximum output power of the NGs was significantly influenced by the solution flow rate during electrospinning. However, the maximum output power of the NGs was achieved at a lower external resistance than that in the case of PZT-based NGs because ZnO fibers are more brittle than PZT fibers. Doping of ZnO and controlling the solution flow rate cannot enhance the elasticity of the fibers. However, the maximum output power can be maximized by almost three times by using the doped ZnO-based NGs.

Performance Analysis of NGs at Reasonable Finger Tap Loads.

To represent a reasonable finger tap, the undoped ZnO-based and doped ZnO-based NGs were subjected to loads ranging from 0.25 to 1.5 kg for 3 s for each cycle. Table 2 shows a steady increase in the maximum output power under loads up to 1.25 kg. Furthermore, there was a significant decrease in the output power when the NGs were subjected to a load of 1.5 kg because of the severe failure of fibers, as shown in Fig. 7(b). Figure 7(a) shows that the fibers under a load of 0.5 kg for 12,000 cycles were still stable. In contrast, the severe failure of fibers was visible when they were subjected to a load of 1.5 kg for 12,000 cycles. Moreover, the maximum output power of the undoped ZnO-based and doped ZnO-based NGs for a reasonable finger tap load was 17.6 and 51.7 nW, respectively.

This study shows that the maximum output power of undoped ZnO-based and doped ZnO-based NGs occurred at a solution flow rate of 2 μl/min, which is the minimum flow rate of the electrospinning machine. At a reasonable finger tap load, the NGs showed maximum power densities of approximately 17.6 and 51.7 nW/cm2, respectively. These values can be further enhanced by using an appropriate dopant to develop more flexible ZnO-based NGs.

Undoped ZnO-based and doped ZnO-based NGs were successfully fabricated by electrospinning at various solution flow rates and sintering at 550 °C for 4 h. XRD and XRF results showed that aluminum was successfully incorporated into ZnO nanofibers. Aluminum doping of ZnO nanofibers halted the wire growth and reduced the diameter of the nanofibers at solution flow rates ranging from 2 to 6 μl/min. The effect of the solution flow rate on the morphology of the fibers and the performance of the undoped ZnO-based and doped ZnO-based NGs was very significant. The undoped ZnO-based and doped ZnO-based NGs exhibited maximum open circuit voltages of 121.1 mV and 265.5 mV, respectively, and maximum power densities of approximately 12.9 and 38.8 nW/cm2, respectively, when subjected to a load of 0.5 kg. The doped ZnO-based NGs synthesized at a low solution flow rate of 2 μl/min exhibited an increase in the generated power by almost three times that observed in the case of the undoped ZnO-based NGs. At a reasonable finger tap load, the maximum power density increased at loads up to 1.25 kg. Because of severe failure of fibers, the output power of NGs was very low when subjected to a load of 1.5 kg. Furthermore, at a reasonable finger tap load, the maximum power density of doped ZnO-based and undoped ZnO-based NGs was approximately 17.6 and 51.7 nW/cm2, respectively. The doped ZnO-based NGs showed high output power, and are promising candidates for use in self-powered keyboards. Further investigation is needed to enhance the generated power density of these NGs under higher external loads by using codoping to modulate the mechanical properties of the doped ZnO fibers.

The authors thank the DGHE-Ministry of National Education of Indonesia for financial support (Grant Nos. 165a/UN27.11/PN/2013 and 2341/UN27.16/PN/2012).

 

 Nomenclature
  • P =

    power, W

  • R =

    external load, Ω

  • t =

    time, s

  • T =

    period, s

  • V =

    voltage, V

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Wang, D. W., Cao, M. S., Yuan, J., Zhao, Q. L., Li, H. B., Lin, H. B., and Zhang, D. Q., 2011, “Piezoelectric, Ferroelectric and Mechanical Properties of Lead Zirconate Titanate/Zinc Oxide Nanowhisker Ceramics,” J. Mater. Sci.: Mater. Electron., 22(9), pp. 1393–1399. [CrossRef]
Salem, J. K., Hammad, T. M., and Harrison, R. R., 2012, “Synthesis, Structural and Optical Properties of Ni-Doped ZnO Micro-Spheres,” J. Mater. Sci.: Mater. Electron., 24(5), pp. 1670–1676. [CrossRef]
Rutledge, G. C., Li, Y., Fridrikh, S., Warner, S. B., Kalayci, V. E., and Patra, P., 2000, “Electrostatic Spinning and Properties of Ultrafine Fibers,” National Textile Center, Technical Report, No. M98-D01.
Bahadur, H., Samanta, S. B., Srivastava, A. K., Sood, K. N., Kishore, R., Sharma, R. K., Basu, A., Rashmi, Kar, M., Pal, P., Bhatt, V., and Chandra, S., 2006, “Nano and Micro Structural Studies of Thin Films of ZnO,” J. Mater. Sci., 41(22), pp. 7562–7570. [CrossRef]
Loh, K. J., and Chang, D., 2011, “Zinc Oxide Nanoparticle-Polymeric Thin Films for Dynamic Strain Sensing,” J. Mater. Sci., 46(1), pp. 228–237. [CrossRef]
Chang, J., Dommer, M., Chang, C., and Lin, L., 2012, “Piezoelectric Nanofibers for Energy Scavenging Applications,” Nano Energy, 1(3), pp. 356–371. [CrossRef]
Dicken, J., Mitcheson, P. D., Stoianov, I., and Yeatman, E. M., 2009, “Increased Power Output From Piezoelectric Energy Harvesters by Pre-Biasing,” PowerMEMS 2009, Washington, DC, Dec. 1–4, pp. 75–78.
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Salem, J. K., Hammad, T. M., and Harrison, R. R., 2012, “Synthesis, Structural and Optical Properties of Ni-Doped ZnO Micro-Spheres,” J. Mater. Sci.: Mater. Electron., 24(5), pp. 1670–1676. [CrossRef]
Rutledge, G. C., Li, Y., Fridrikh, S., Warner, S. B., Kalayci, V. E., and Patra, P., 2000, “Electrostatic Spinning and Properties of Ultrafine Fibers,” National Textile Center, Technical Report, No. M98-D01.
Bahadur, H., Samanta, S. B., Srivastava, A. K., Sood, K. N., Kishore, R., Sharma, R. K., Basu, A., Rashmi, Kar, M., Pal, P., Bhatt, V., and Chandra, S., 2006, “Nano and Micro Structural Studies of Thin Films of ZnO,” J. Mater. Sci., 41(22), pp. 7562–7570. [CrossRef]
Loh, K. J., and Chang, D., 2011, “Zinc Oxide Nanoparticle-Polymeric Thin Films for Dynamic Strain Sensing,” J. Mater. Sci., 46(1), pp. 228–237. [CrossRef]
Chang, J., Dommer, M., Chang, C., and Lin, L., 2012, “Piezoelectric Nanofibers for Energy Scavenging Applications,” Nano Energy, 1(3), pp. 356–371. [CrossRef]
Dicken, J., Mitcheson, P. D., Stoianov, I., and Yeatman, E. M., 2009, “Increased Power Output From Piezoelectric Energy Harvesters by Pre-Biasing,” PowerMEMS 2009, Washington, DC, Dec. 1–4, pp. 75–78.

Figures

Grahic Jump Location
Fig. 1

A nanofiber-based NG

Grahic Jump Location
Fig. 2

Equipment for measuring the performance of undoped ZnO-based and doped ZnO-based NGs

Grahic Jump Location
Fig. 3

SEM images of undoped ZnO and doped ZnO nanofibers

Grahic Jump Location
Fig. 4

XRD patterns for (a) doped ZnO and (b) undoped ZnO nanofibers

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Fig. 5

Output voltage of (a) undoped ZnO-based and (b) doped ZnO-based NGs obtained by gradually changing the amount of external resistance

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Fig. 6

Average output power from (a) undoped ZnO-based NGs and (b) doped ZnO-based NGs

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Fig. 7

SEM image of fibers after being subjected to a load of (a) 0.5 kg for 12,000 cycles and (b) 1.5 kg for 12,000 cycles

Tables

Table Grahic Jump Location
Table 1 XRF results
Table Grahic Jump Location
Table 2 Maximum output power of NGs at reasonable fingers tap load

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