- Email: [email protected]
Self-Powered Piezoelectric Nanogenerator Based on Wurtzite ZnO Nanoparticles for Energy Harvesting Application
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 9826–9830
www.materialstoday.com/proceedings
IC-FNM 2016
Self-Powered Piezoelectric Nanogenerator Based on Wurtzite ZnO Nanoparticles for Energy Harvesting Application Wahida Rahman, Samiran Garain, Ayesha Sultana, Tapas Ranjan Middya, Dipankar Mandal* Organic Nano-Piezoelectric Device Laboratory, Department of Physics, Jadavpur University, Kolkata 700032, India
Abstract We report the synthesis procedure of hexagonal wurtzite structure of zinc oxide (ZnO) nanoparticles via hydrothermal route to use as a main component of piezoelectric nanogenerator. The ZnO nanoparticles are dispersed into polydimethylsiloxane (PDMS) to fabricate the high performance flexible piezoelectric nanogenerator (FPNG).The output voltage and power generated from the FPNG is around 20 V and 20 μW respectively. It is also capable to charge up the capacitors within very short span of time (for example, about 2V is reached at 96s). It is demonstrated that the output power generated from the FPNG can directly drive several commercial blue light emitting diodes (LEDs)that ensuring the applicability as a self-powered energy harvester. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Functional Nano-Materials, 2016.
Keywords:Hydrothermal
synthesis; ZnO nanoparticles; flexible piezoelectric nanogenerator; capacitor charging; self-powered mechanical
energy harvester.
1. Introduction Energy harvesting from renewable and green energy resources, such as wind, solar, biomass, hydro and biomechanical energyhas attracted considerable interest due to the energy crisis and global warming [1–5].However these energy harvesting methods are influenced by environmental factors, thus often not treated as sustainable renewable energy sources. Harvesting different kinds of wasted tiny mechanical energies such as small vibration, sound-wave and body motion energy draws increasing attention because of its huge potential in powering more and more widely used micro/nano devices [3–6]. * Corresponding author. E-mail address:[email protected]
2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Functional Nano-Materials, 2016.
Rahman et al. / Materials Today: Proceedings 5 (2018) 9826–9830
9827
Nanogenerator (NG) has been attracted a great deal of attention since its first report made of ZnO nanowires (NWs) arrays [6].Outstanding progress has been made in developing NGs with different materials such as ZnO nanoparticles [7], BaTiO3 thin film [8],PZT nanofibers [9], ZnO nanorod arrays [10]. However the current and output voltage from this type of nanogenerator is very low and this insufficient power output probably limits their practical application. Inadequate flexibility is another drawback. Therefore, improvement of the output current and power with flexible geometry is a significant and challenging task. ZnO nanoparticles (NPs) attracted great attention due to their easy fabrication process, as compared to ZnO nanowires [11,12]. Recently, ZnO NPs are embedded into a SU-8 matrix that possesses the piezoelectric coefficients in the range between 15 and 23 pm V-1, which is quite promising [13]. In this work, we demonstrate the synthesis of ZnO NPs and dispersed into PDMS to fabricate the FPNG. FPNG is capable to charge up the capacitor under repeated finger tapping process and power up several commercial blue LED’s instantly where the storage device is not necessary. It indicates that FPNG has futuristic application as selfpowered devices. 2. Experimental section 2.1. Materials Zinc acetate dihydrate (Zn (CH3COO)2. 2H2O) and sodium hydroxide (NaOH) were purchased from Merck Chemical, India. Polydimethylsiloxane (PDMS) (Sylgard 184) were supplied from, Dow Corning Corp., USA. Deionized water (DI) is used from Millipore®. 2.2. ZnO NPs synthesis The synthesis of ZnONPs was carried out via hydrothermal route. First,a precursor solution was prepared by dissolving Zn(CH3COO)2.2H2O (0.1 M) in 50 ml deionized water and stirred for 10 m.Afterwards, 20 ml of NaOH (0.5M) solution prepared with deionised water and added drop wise under continuous stirring to maintain the pH value of reactants is 12. The mixturesolution was transferred into Teflon-lined autoclave and placed into the oven at 120 oC for 12 h under autogenous pressure. It was then allowed to cool naturally to room temperature. After the reaction was complete, the white solid productwas deposited on the bottom of the autoclave. The final product was centrifuged and washed several times with deionised water and dried in a laboratory oven at 100 oC for 6 h. 2.3. FPNGfabrication The synthesized ZnO NPs and PDMS solution were taken in a petri dish in the mass ratio of 1:40 and mixed by mechanical agitation. Then the mixture was placed into a vacuum desiccator to remove the air bubbles. The bubble free ZnO NPs-PDMS mixture was placed on a hot plate (~60 oC for 1h) to get a composite film. After that, Al foilswereattached on both side of the composite film.Finally, the complete device was further encapsulated by PDMS to prevent from external mechanical damage. Primarily three different weight percent (w/v) (0.25,1 and 2 wt%) of ZnONPs were utilized for the device fabrication and finally we choose 1wt% of ZnO (named as FPNG) due to the better performance in comparison to other two concentrations. 2.4. Characterisation Crystallographic structure, shape and size of synthesized ZnO NPs were recorded by X-ray diffraction (XRD, Bruker, D 8 Advance) with a Cu Kα (λinc. ~1.54 Å) radiation under an operating voltage and current of 40 kV and 40 mA, respectively and field emission scanning electron microscope (FE-SEM) (FEI, INSPECT F 50) operated at an acceleration voltage of 20 kV. Optical properties were conducted by UV−visible (UV−vis) absorption spectra (Shimadzu, 3110PC). Open-circuit output voltages were recorded using a digital storage oscilloscope (Agilent, DSO3102A). The capacitor charging performance was employed via a typical rectifier bridge circuit unit.
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Rahman et al. / Materials Today: Proceedings 5 (2018) 9826–9830
3. Results and discussion 3.1. Crystal structure and surface morphological analysis In a typical hydrothermal based growth of ZnO nanostructure, the formation of Zn(OH)2 is an essential requirement [14]. The basic reactions mechanisms of the formation of ZnO nuclei are given below -
-
Zn(OH)2 + 2H2O = Zn2+ + 2OH + 2H2O = [Zn(OH)4]2 + 2H+ 2-
(1)
-
[Zn(OH)4] = ZnO + H2O +2OH (2) The XRD pattern of wurtzite ZnO NPsis shown in Fig. 1a. All the diffraction peaks are indexed as the wurtzite ZnO (JCPDS 36-1451).The absences of additional peaks indicate that the ZnO is free from any major impurities[15]. (101)
(a)
(b)
Frequency
20
(002)
Intensity (a.u.)
(100)
30
20
30
40
50
60
0
(112)
(103)
(110)
(102)
10
70
40
60 80 100 120 140 Diameter(nm)
80
2 (degree) Fig. 1. (a) XRD spectra and (b) FE-SEM image of the as prepared ZnO NPs. The diameter distribution histogram is placed in the inset of Fig. 1b. 3.2. Piezoelectric performance
(a)
30
Voltage (V)
20
(b)
10 0
-10 0.0
0.5
1.0
Time (s)
1.5
2.0
Fig.2.(a) The unit cell of wurtzite ZnO structure and (b) output voltage signal from FPNG by repeating finger imparting.
Rahman et al. / Materials Today: Proceedings 5 (2018) 9826–9830
9829
The surface morphology of ZnO NPsis shown in Fig.1b, which display a uniform and densely packed array of sphere like particles. Inset of Fig. 2b shows the diameter distribution of ZnO NPs which specifies the diameter range of 31 nm to 140 nm and the average diameter is 84 nm. The wurtzite structure of ZnO consists of Zn and O pairs which are triangularly arranged through alternating biatomic close-packed and the top face (0001) consist of tetrahedral zinc. ZnO is a polar crystal, O2− is in hexagonal closed packing, and each Zn2+ lies within a tetrahedral group of four oxygen ions where number of alternating planes composed of tetrahedrally coordinated O2− and Zn2+ stacked alternately along the c-axis, as shown in Fig. 2a.The tetrahedral coordination in ZnO results in piezoelectric and pyroelectric properties due to the absence of inversion symmetry [6]. The output performance of FPNG under repeating finger tapping process is shown in Fig. 2b.It indicates that the open circuit output voltage due to repeated finger tapping process is 20V. Asymmetry of the positive and negative peaks arises due to the type of mechanical impacts,i.e., under fast pressing and slow releasing [16]. It is known that piezoelectric charges can be generated under the externalapplied tensile or compressive strain to the wurtzite ZnO crystal structure [6]. The relative displacement of the Zn2+ cations with respect to the O2– anions, resulting the formation of piezoelectric effect. Thus, these ionic charges are not able to freely move and recombine without releasing the strain [6]. 25
25
20
20
15
15
10
10 5
5
0
0 10
(c)
2
Voltage (V)
5
10
15 10 5 0 5
10
10
7
Resistance ()
(d)
4.7 F
1F
6
10
7
10 Resistance (
2.2 F
1
0
6
(b)
20
Power (W)
(a)
Current (A)
Voltage (V)
30
PDMS 0
30
60
Time (s)
90
FPNG
Fig.3. (a) Output voltage and current amplitude, (b) power output from FPNGas a function of external load and (c) transient response of capacitor charging by repeating human finger imparting. The photograph of glowing LED is shown in the inset of Fig. 3c. (d)The schematic of circuit diagram for capacitor charging.
9830
Rahman et al. / Materials Today: Proceedings 5 (2018) 9826–9830
The variation of output voltage and current with different load is shown in Fig. 3a.The output voltage increases with increasing load resistance and reached a peak value at theoretically infinite high resistance (for example, 40 MΩ) similar to the open circuit voltage. Whereas, the current gradually decreases with increasing load resistance. The instantaneous generated power is reached to a maximum value of 20 μW at load resistance of about 40 MΩ,shown in Fig. 3b.To convert the ac output to a dc form, generated from the FPNG, a typical bridge rectifier circuit is employed [17]. By repeating finger imparting process, three different capacitors (1, 2.2 and 4.7μF) are charged up by the FPNG and it reaches the value of 1.7, 1.2 and 2.0 V within 42, 50 and 96 s, respectively. The corresponding transient response is shown in Fig. 3c.The output power generated from FPNG is sufficient to power up several commercial blue LED’s instantly by repeated finger tapping process (inset of Fig. 3c). The schematic circuit diagram of a typical bridge rectifier is demonstrated in Fig. 3d. Conclusion In summary, we synthesized wurtzite structure of ZnO NPs via hydrothermal route and fabricated FPNG by dispersing the ZnO into PDMS matrix. Under simple repeating finger imparting process, the open circuit output voltage generated from the FPNG is ∼ 20V whereas the short circuit current is ∼ 20 μA which correspond to the power of 20 μW. FPNG is enable to charge up different capacitors (1, 2.2 and 4.7 μF) by repeating finger tapping process and light up several commercial LED’s instantlywhich corresponds to the practical application as a selfpowered devices. Acknowledgements Authors are grateful to SERB, Govt. of India, for financial support (SERB/1759/2014-15). Wahida Rahman and Ayesha Sultana are thankful to the UGC for providing the fellowship under the UGC-MANF. Samiran Garain is supported by UGC-BSR fellowship (No. P-1/RS/79/13). References [1] C. K. Xu, P. Shin, L. L. Cao, D. J. Gao, J. Phys. Chem. C 114 (2010) 125–129. [2] W. L. Ma, C. Y. Yang, X. Gong, K. Lee, A. J. Heeger, Adv. Funct. Mater. 15 (2005) 1617–1622. [3] S. Thorsell, F. M. Epplin, R. L. Huhnke, C. M. Taliaferro, Biomass Bioenergy 27 (2004) 327–337. [4] C. H. Williams, A. M. Schwarz, V. Reid, Hydrobiologia 340 (1996) 229–234. [5] J. Donelan, Q. Li, V. Naing, J. Hoffer, D. Weber, A. Kuo, Science 319 (2008) 807–810. [6] Z. L. Wang, J. H. Song, Science 312 (2006) 242–246. [7] H. Sun, H. Tian, Y. Yang, D. Xie, Y.C. Zhang, X. Liu, S. Ma, H. M. Zhao and T. L. Ren, Nanoscale 5 (2013) 6117–6123. [8] K. I. Park, S. Xu, Y. Liu, G. T. Hwang, S. J. L. Kang, Z.L. Wang, K. J. Lee, Nano Lett. 10 (2010) 4939–4943. [9] X. Chen, S. Y. Xu, N. Yao, Y. Shi, Nano Lett. 10 (2010) 2133–2137. [10] Y. H. Ko, G. Nagaraju, S. H. Lee, J. S. Yu, ACS Appl. Mater. Interfaces 6 (2014) 6631–6637. [11] M. Kandpal, C. Sharan, P. Poddar, K. Prashanthi, P. R. Apte, V. R. Rao, Appl. Phys. Lett. 101 (2012) 104102–104106. [12] S. Dai, M. Gharbi, P. Sharma, H. S. Park, J. Appl. Phys. 110 (2011) 104305–104311. [13] M. Majdoub, P. Sharma, T. Cagin, Phys. Rev. B: Condens. Matter Mater. Phys. 77 (2008) 125424–125433. [14] H. Zhang, D. Yang, X. Ma, Y. Ji, J. Xu, D. Que, Nanotechnology 15 (2004) 622–626. [15] S. Baruah, J. Dutta,Sci. Technol. Adv. Mater. 10(2009) 013001–013018. [16] A. Sultana, M. M. Alam. S. Garain, T. K. Sinha, T. R. Middya, D. Mandal, ACS Appl. Mater. Interfaces 7 (2015) 19091–19097. [17] S. Garain, S. Jana, T. K. Sinha, D. Mandal, ACS Appl. Mater. Interfaces 8 (2016) 4532–4540.
ScienceDirect Materials Today: Proceedings 5 (2018) 9826–9830
www.materialstoday.com/proceedings
IC-FNM 2016
Self-Powered Piezoelectric Nanogenerator Based on Wurtzite ZnO Nanoparticles for Energy Harvesting Application Wahida Rahman, Samiran Garain, Ayesha Sultana, Tapas Ranjan Middya, Dipankar Mandal* Organic Nano-Piezoelectric Device Laboratory, Department of Physics, Jadavpur University, Kolkata 700032, India
Abstract We report the synthesis procedure of hexagonal wurtzite structure of zinc oxide (ZnO) nanoparticles via hydrothermal route to use as a main component of piezoelectric nanogenerator. The ZnO nanoparticles are dispersed into polydimethylsiloxane (PDMS) to fabricate the high performance flexible piezoelectric nanogenerator (FPNG).The output voltage and power generated from the FPNG is around 20 V and 20 μW respectively. It is also capable to charge up the capacitors within very short span of time (for example, about 2V is reached at 96s). It is demonstrated that the output power generated from the FPNG can directly drive several commercial blue light emitting diodes (LEDs)that ensuring the applicability as a self-powered energy harvester. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Functional Nano-Materials, 2016.
Keywords:Hydrothermal
synthesis; ZnO nanoparticles; flexible piezoelectric nanogenerator; capacitor charging; self-powered mechanical
energy harvester.
1. Introduction Energy harvesting from renewable and green energy resources, such as wind, solar, biomass, hydro and biomechanical energyhas attracted considerable interest due to the energy crisis and global warming [1–5].However these energy harvesting methods are influenced by environmental factors, thus often not treated as sustainable renewable energy sources. Harvesting different kinds of wasted tiny mechanical energies such as small vibration, sound-wave and body motion energy draws increasing attention because of its huge potential in powering more and more widely used micro/nano devices [3–6]. * Corresponding author. E-mail address:[email protected]
2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Functional Nano-Materials, 2016.
Rahman et al. / Materials Today: Proceedings 5 (2018) 9826–9830
9827
Nanogenerator (NG) has been attracted a great deal of attention since its first report made of ZnO nanowires (NWs) arrays [6].Outstanding progress has been made in developing NGs with different materials such as ZnO nanoparticles [7], BaTiO3 thin film [8],PZT nanofibers [9], ZnO nanorod arrays [10]. However the current and output voltage from this type of nanogenerator is very low and this insufficient power output probably limits their practical application. Inadequate flexibility is another drawback. Therefore, improvement of the output current and power with flexible geometry is a significant and challenging task. ZnO nanoparticles (NPs) attracted great attention due to their easy fabrication process, as compared to ZnO nanowires [11,12]. Recently, ZnO NPs are embedded into a SU-8 matrix that possesses the piezoelectric coefficients in the range between 15 and 23 pm V-1, which is quite promising [13]. In this work, we demonstrate the synthesis of ZnO NPs and dispersed into PDMS to fabricate the FPNG. FPNG is capable to charge up the capacitor under repeated finger tapping process and power up several commercial blue LED’s instantly where the storage device is not necessary. It indicates that FPNG has futuristic application as selfpowered devices. 2. Experimental section 2.1. Materials Zinc acetate dihydrate (Zn (CH3COO)2. 2H2O) and sodium hydroxide (NaOH) were purchased from Merck Chemical, India. Polydimethylsiloxane (PDMS) (Sylgard 184) were supplied from, Dow Corning Corp., USA. Deionized water (DI) is used from Millipore®. 2.2. ZnO NPs synthesis The synthesis of ZnONPs was carried out via hydrothermal route. First,a precursor solution was prepared by dissolving Zn(CH3COO)2.2H2O (0.1 M) in 50 ml deionized water and stirred for 10 m.Afterwards, 20 ml of NaOH (0.5M) solution prepared with deionised water and added drop wise under continuous stirring to maintain the pH value of reactants is 12. The mixturesolution was transferred into Teflon-lined autoclave and placed into the oven at 120 oC for 12 h under autogenous pressure. It was then allowed to cool naturally to room temperature. After the reaction was complete, the white solid productwas deposited on the bottom of the autoclave. The final product was centrifuged and washed several times with deionised water and dried in a laboratory oven at 100 oC for 6 h. 2.3. FPNGfabrication The synthesized ZnO NPs and PDMS solution were taken in a petri dish in the mass ratio of 1:40 and mixed by mechanical agitation. Then the mixture was placed into a vacuum desiccator to remove the air bubbles. The bubble free ZnO NPs-PDMS mixture was placed on a hot plate (~60 oC for 1h) to get a composite film. After that, Al foilswereattached on both side of the composite film.Finally, the complete device was further encapsulated by PDMS to prevent from external mechanical damage. Primarily three different weight percent (w/v) (0.25,1 and 2 wt%) of ZnONPs were utilized for the device fabrication and finally we choose 1wt% of ZnO (named as FPNG) due to the better performance in comparison to other two concentrations. 2.4. Characterisation Crystallographic structure, shape and size of synthesized ZnO NPs were recorded by X-ray diffraction (XRD, Bruker, D 8 Advance) with a Cu Kα (λinc. ~1.54 Å) radiation under an operating voltage and current of 40 kV and 40 mA, respectively and field emission scanning electron microscope (FE-SEM) (FEI, INSPECT F 50) operated at an acceleration voltage of 20 kV. Optical properties were conducted by UV−visible (UV−vis) absorption spectra (Shimadzu, 3110PC). Open-circuit output voltages were recorded using a digital storage oscilloscope (Agilent, DSO3102A). The capacitor charging performance was employed via a typical rectifier bridge circuit unit.
9828
Rahman et al. / Materials Today: Proceedings 5 (2018) 9826–9830
3. Results and discussion 3.1. Crystal structure and surface morphological analysis In a typical hydrothermal based growth of ZnO nanostructure, the formation of Zn(OH)2 is an essential requirement [14]. The basic reactions mechanisms of the formation of ZnO nuclei are given below -
-
Zn(OH)2 + 2H2O = Zn2+ + 2OH + 2H2O = [Zn(OH)4]2 + 2H+ 2-
(1)
-
[Zn(OH)4] = ZnO + H2O +2OH (2) The XRD pattern of wurtzite ZnO NPsis shown in Fig. 1a. All the diffraction peaks are indexed as the wurtzite ZnO (JCPDS 36-1451).The absences of additional peaks indicate that the ZnO is free from any major impurities[15]. (101)
(a)
(b)
Frequency
20
(002)
Intensity (a.u.)
(100)
30
20
30
40
50
60
0
(112)
(103)
(110)
(102)
10
70
40
60 80 100 120 140 Diameter(nm)
80
2 (degree) Fig. 1. (a) XRD spectra and (b) FE-SEM image of the as prepared ZnO NPs. The diameter distribution histogram is placed in the inset of Fig. 1b. 3.2. Piezoelectric performance
(a)
30
Voltage (V)
20
(b)
10 0
-10 0.0
0.5
1.0
Time (s)
1.5
2.0
Fig.2.(a) The unit cell of wurtzite ZnO structure and (b) output voltage signal from FPNG by repeating finger imparting.
Rahman et al. / Materials Today: Proceedings 5 (2018) 9826–9830
9829
The surface morphology of ZnO NPsis shown in Fig.1b, which display a uniform and densely packed array of sphere like particles. Inset of Fig. 2b shows the diameter distribution of ZnO NPs which specifies the diameter range of 31 nm to 140 nm and the average diameter is 84 nm. The wurtzite structure of ZnO consists of Zn and O pairs which are triangularly arranged through alternating biatomic close-packed and the top face (0001) consist of tetrahedral zinc. ZnO is a polar crystal, O2− is in hexagonal closed packing, and each Zn2+ lies within a tetrahedral group of four oxygen ions where number of alternating planes composed of tetrahedrally coordinated O2− and Zn2+ stacked alternately along the c-axis, as shown in Fig. 2a.The tetrahedral coordination in ZnO results in piezoelectric and pyroelectric properties due to the absence of inversion symmetry [6]. The output performance of FPNG under repeating finger tapping process is shown in Fig. 2b.It indicates that the open circuit output voltage due to repeated finger tapping process is 20V. Asymmetry of the positive and negative peaks arises due to the type of mechanical impacts,i.e., under fast pressing and slow releasing [16]. It is known that piezoelectric charges can be generated under the externalapplied tensile or compressive strain to the wurtzite ZnO crystal structure [6]. The relative displacement of the Zn2+ cations with respect to the O2– anions, resulting the formation of piezoelectric effect. Thus, these ionic charges are not able to freely move and recombine without releasing the strain [6]. 25
25
20
20
15
15
10
10 5
5
0
0 10
(c)
2
Voltage (V)
5
10
15 10 5 0 5
10
10
7
Resistance ()
(d)
4.7 F
1F
6
10
7
10 Resistance (
2.2 F
1
0
6
(b)
20
Power (W)
(a)
Current (A)
Voltage (V)
30
PDMS 0
30
60
Time (s)
90
FPNG
Fig.3. (a) Output voltage and current amplitude, (b) power output from FPNGas a function of external load and (c) transient response of capacitor charging by repeating human finger imparting. The photograph of glowing LED is shown in the inset of Fig. 3c. (d)The schematic of circuit diagram for capacitor charging.
9830
Rahman et al. / Materials Today: Proceedings 5 (2018) 9826–9830
The variation of output voltage and current with different load is shown in Fig. 3a.The output voltage increases with increasing load resistance and reached a peak value at theoretically infinite high resistance (for example, 40 MΩ) similar to the open circuit voltage. Whereas, the current gradually decreases with increasing load resistance. The instantaneous generated power is reached to a maximum value of 20 μW at load resistance of about 40 MΩ,shown in Fig. 3b.To convert the ac output to a dc form, generated from the FPNG, a typical bridge rectifier circuit is employed [17]. By repeating finger imparting process, three different capacitors (1, 2.2 and 4.7μF) are charged up by the FPNG and it reaches the value of 1.7, 1.2 and 2.0 V within 42, 50 and 96 s, respectively. The corresponding transient response is shown in Fig. 3c.The output power generated from FPNG is sufficient to power up several commercial blue LED’s instantly by repeated finger tapping process (inset of Fig. 3c). The schematic circuit diagram of a typical bridge rectifier is demonstrated in Fig. 3d. Conclusion In summary, we synthesized wurtzite structure of ZnO NPs via hydrothermal route and fabricated FPNG by dispersing the ZnO into PDMS matrix. Under simple repeating finger imparting process, the open circuit output voltage generated from the FPNG is ∼ 20V whereas the short circuit current is ∼ 20 μA which correspond to the power of 20 μW. FPNG is enable to charge up different capacitors (1, 2.2 and 4.7 μF) by repeating finger tapping process and light up several commercial LED’s instantlywhich corresponds to the practical application as a selfpowered devices. Acknowledgements Authors are grateful to SERB, Govt. of India, for financial support (SERB/1759/2014-15). Wahida Rahman and Ayesha Sultana are thankful to the UGC for providing the fellowship under the UGC-MANF. Samiran Garain is supported by UGC-BSR fellowship (No. P-1/RS/79/13). References [1] C. K. Xu, P. Shin, L. L. Cao, D. J. Gao, J. Phys. Chem. C 114 (2010) 125–129. [2] W. L. Ma, C. Y. Yang, X. Gong, K. Lee, A. J. Heeger, Adv. Funct. Mater. 15 (2005) 1617–1622. [3] S. Thorsell, F. M. Epplin, R. L. Huhnke, C. M. Taliaferro, Biomass Bioenergy 27 (2004) 327–337. [4] C. H. Williams, A. M. Schwarz, V. Reid, Hydrobiologia 340 (1996) 229–234. [5] J. Donelan, Q. Li, V. Naing, J. Hoffer, D. Weber, A. Kuo, Science 319 (2008) 807–810. [6] Z. L. Wang, J. H. Song, Science 312 (2006) 242–246. [7] H. Sun, H. Tian, Y. Yang, D. Xie, Y.C. Zhang, X. Liu, S. Ma, H. M. Zhao and T. L. Ren, Nanoscale 5 (2013) 6117–6123. [8] K. I. Park, S. Xu, Y. Liu, G. T. Hwang, S. J. L. Kang, Z.L. Wang, K. J. Lee, Nano Lett. 10 (2010) 4939–4943. [9] X. Chen, S. Y. Xu, N. Yao, Y. Shi, Nano Lett. 10 (2010) 2133–2137. [10] Y. H. Ko, G. Nagaraju, S. H. Lee, J. S. Yu, ACS Appl. Mater. Interfaces 6 (2014) 6631–6637. [11] M. Kandpal, C. Sharan, P. Poddar, K. Prashanthi, P. R. Apte, V. R. Rao, Appl. Phys. Lett. 101 (2012) 104102–104106. [12] S. Dai, M. Gharbi, P. Sharma, H. S. Park, J. Appl. Phys. 110 (2011) 104305–104311. [13] M. Majdoub, P. Sharma, T. Cagin, Phys. Rev. B: Condens. Matter Mater. Phys. 77 (2008) 125424–125433. [14] H. Zhang, D. Yang, X. Ma, Y. Ji, J. Xu, D. Que, Nanotechnology 15 (2004) 622–626. [15] S. Baruah, J. Dutta,Sci. Technol. Adv. Mater. 10(2009) 013001–013018. [16] A. Sultana, M. M. Alam. S. Garain, T. K. Sinha, T. R. Middya, D. Mandal, ACS Appl. Mater. Interfaces 7 (2015) 19091–19097. [17] S. Garain, S. Jana, T. K. Sinha, D. Mandal, ACS Appl. Mater. Interfaces 8 (2016) 4532–4540.