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Improved performance of a polymer nanogenerator based on silver nanoparticles doped electrospun P(VDF–HFP) nanofibers†
Received 11th December 2013, Accepted 27th February 2014
Dipankar Mandal,*a Karsten Henkelb and Dieter Schmeißerb
DOI: 10.1039/c3cp55238a www.rsc.org/pccp
We report on the electrospinning of poly(vinylidene fluoride– hexafluoropropylene) [P(VDF–HFP)] nanofibers doped with silver nanoparticles for the preparation of a polymer based nanogenerator (PNG). It has been found that the yield of the piezoelectric phase is increased by the addition of silver nanoparticles. Furthermore, defects in the P(VDF–HFP) electrospun fibers are removed resulting in a significant enhancement in the output power of the PNG. A maximum generated PNG output voltage of 3 V with a current density of 0.9 lA cm2 is achieved.
1 Introduction The rising demand for energy and the increasing mobility of human society have driven the development of new alternative power sources. Due to its piezo- and pyroelectric behaviors polyvinylidene fluoride (PVDF) is a good candidate for mechanical energy harvesting due to the possibility of developing tiny electronic devices consuming ultralow electric power. In this work an electrospinning method has been chosen to fabricate a polymer based nanogenerator (PNG) realized by the use of poly(vinylidene fluoride–hexafluoropropylene) [P(VDF–HFP): a copolymer of PVDF] nanofibers. This simple scale-up processing technique has many advantages in generator based applications, because a single-step electrospinning process can eliminate the need for direct contact or corona poling and is able to induce the piezoelectric b-crystal phase and spontaneous dipolar orientation at the same time.1,2 To the best of our knowledge, there have been only a few reports about electrospun PNGs mainly based on PVDF and its copolymer poly(vinylidene fluoride–trifluoroethylene) a
Organic Nano-Piezoelectric Device Laboratory, Department of Physics, Jadavpur University, Kolkata 700032, India. E-mail: [email protected] b ¨t Cottbus-Senftenberg, Brandenburgische Technische Universita Angewandte Physik-Sensorik, K.-Wachsmann-Allee 17, 03046 Cottbus, Germany. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: FE-SEM image of neat PVDF nanofibers, dipole reversibility test and schematic of electrospinning set-up where the direction of the preferential orientation of molecular dipoles is shown. See DOI: 10.1039/c3cp55238a
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[P(VDF–TrFE)].1–5 Recently a few attempts have been undertaken to improve the output voltage and current density of PNGs by the addition of carbon nanotubes (CNTs) and metal nanoparticles in electrospun PVDF nanofibers but no one has tried yet with the costeffective copolymer, P(VDF–HFP).6,7 The electrospun P(VDF–HFP) fibers were utilized as electrolyte membranes in dye-sensitized solar cells, exhibiting a good light to electricity conversion efficiency and improved long term stability.8 However, electrospun PNGs produced with P(VDF–HFP) have not been reported to date. The recent report about a self charging power cell,9 where an electrically poled PVDF film is used as a battery separator, motivated us to choose P(VDF–HFP) because its nanofiber membranes carry a tremendous potential for separators in lithium ion coin cells due to its relatively lower crystallinity, good ionic conductivity and higher electrochemical stability in comparison to PVDF.10–12 In addition, better flexibility and costeffectiveness may aid its importance in industrial large scale production.12 The concept of nanogenerators was initiated with zinc oxide (ZnO) nanorods or nanowire arrays,13 since then a remarkable growth in this research field has been noticed. Currently, several inorganic nanostructures such as lead zirconate titanate (PZT), barium titanate (BaTiO3), zinc sulphide (ZnS), etc. also offer adequate feasibility for their usage as power sources for portable electronic devices and biomedical sensors.14–17 However, limitations are more prominent in inorganic nanostructure based piezoelectric sensors and generators in practical applications due to their rigidity, heaviness, brittleness, poor biocompatibility and cytotoxicity.17–19 In contrast, piezoelectric polymer nanofibers are more promising because of their flexibility, light weight, excellent durability, easy large area fabrication, scope of functionalization, cost-effectiveness and biocompatibility. In order to enhance the piezoelectric b-crystalline phase content and to improve the quality of the P(VDF–HFP) electrospun fibers, silver nanoparticles (Ag-NPs) were doped into the polymer and also acted as piezoelectric phase stabilizers. It has been found that Ag-NPs play a significant role in improving the output power of the PNG. Recently, we have also initiated a
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similar study with other metallic nanoparticles (i.e. gold, palladium, etc.), however, due to easy processing, cost effectiveness and biocompatibility, Ag-NPs were more advantageous. In this communication, we report on a PNG comprising electrospun P(VDF–HFP) nanofibers doped with Ag-NPs which can generate an output voltage of 3 V with a current density of 0.9 mA cm2 opening up possibilities for the use of this system in energy harvesting based device applications.
2 Experimental 2.1
2.3
Characterization of electrospun nanofibers
Fourier Transform Infrared (FT-IR) spectra were recorded (Shimadzu, FTIR-8400S spectrophotometer) to characterize the crystalline phases within the nanofibers. The samples reported here are labeled in the diagrams as follows: ‘neat HFP’ for neat P(VDF– HFP) and ‘HFP–Ag’ for Ag-NPs containing P(VDF–HFP) nanofibers. The morphology and orientations of the electrospun fibers were visualized by field emission scanning electron microscopy (FE-SEM) images (JEOL, JSM-7610F). UV-vis spectra (Shimadzu, UV-3600) were recorded to check the existence of the Ag-NPs within the electrospun nanofibers.
Polymer solution preparation
P(VDF–HFP) (chemical structure shown in Fig. 1a) pellets (Sigma-Aldrich) were dissolved in dimethylformamide (DMF) and kept at 60 1C under continuous stirring until a clear polymer solution was formed. Afterwards, 0.1 g (Experimental, ESI†) silver nitrate (Sigma-Aldrich) was added into 6 ml of the polymer solution, and the stirring was continued for 6 h to achieve Ag-NP formation (eqn (S1), ESI†). Prior to the electrospinning process, acetone was added into the solution to improve the solvent evaporation rate. Two sets of solutions were prepared: a pure polymer (neat P(VDF–HFP)) solution and a polymer solution containing Ag-NPs to adjust the P(VDF– HFP) solution to 12 wt% (w/v) where a co-solvent ratio of 6 : 4 (DMF to acetone) was used. 2.2
PCCP
Electrospinning process
A homemade typical electrospinning set-up (Fig. 1b) was used to produce the electrospun nanofibers. A 19 kV DC bias was applied to a metallic needle with a tip diameter of 23 G. The electrospun fibers were collected on a cylindrical collector (grounded electrode) wrapped with a nickel coated flexible fabric, which was placed about 10 cm apart from the needle tip. A constant flow rate (0.6 ml h1) of the polymer solution and a collector rotation speed of 200 rpm were applied throughout the fiber collection process.
Fig. 1 (a) The chemical structure of P(VDF–HFP); (b) schematic diagram of the electrospinning set-up used in this work; (c) simplified equivalent circuit diagram (Ro = 1 MO) for the detection of the output voltage (Vo) from the PNG. In the enlarged diagram of the PNG (left hand side) the pressure imparting probe (PIP), top and bottom electrodes and two surfaces (fibers–air and fibers–collector interface) of the electrospun fibers are illustrated.
10404 | Phys. Chem. Chem. Phys., 2014, 16, 10403--10407
2.4
Device fabrication and testing
Sandwich structure based PNGs were fabricated by arranging the layer of electrospun nanofibers (layer thickness B150 20 mm) in between two flexible electrodes. The electrodes were connected to a bridge diode to rectify the generated piezoelectric signal, as shown in Fig. 1c. The device performance was characterized by imparting periodic pressure (peak amplitude B15 kPa) onto the PNGs using a metallic probe (PIP: pressure imparting probe) with a tip diameter of 1.2 cm (active area) at different frequencies. The typical pressure imparting system was similar to one reported recently,5 except a pressure gauge was introduced for this work.
3 Results and discussion The FE-SEM micrograph (Fig. 2) illustrates that the Ag-NP doped P(VDF–HFP) electrospun fibers are defect free (no bead effects are observed) and the average diameter of the fibers is found to be around 83 nm, as shown in the histogram of the nanofibers in the top inset of Fig. 2. In contrast, neat P(VDF– HFP) electrospun fibers (average diameter B 71 nm) consist of bead defects with undesirable voids (Fig. S1, ESI†). The absence of defects in the doped fibers is due to an increase in the charge density of the polymer solution which causes a stronger elongation force on the ejected jets under the same electric field, resulting in the formation of straighter, homogeneous, dense and defect free nanofibers. The existence of Ag-NPs within the electrospun fibers are evident from the typical surface plasmon characteristic peak around 450 nm in the UV-vis spectra (bottom inset of Fig. 2).20,21
Fig. 2 FE-SEM image of the Ag-NP doped P(VDF–HFP) nanofibers. The distribution of the fiber diameters (top inset) and the surface plasmon bands due to the Ag-NPs (bottom inset, UV-vis spectra) are shown.
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(and CF2) dipoles (interfacial interactions). Recently, we have also demonstrated the evidence of interfacial interactions in Ag-NP doped spin coated and solvent casted P(VDF–HFP) films by means of X-ray photoelectron spectroscopy.21 Due to the interfacial interactions the effective mass of the CH2 dipoles is increased. As a result, a reduction in the vibrational frequency (i.e., wavenumber) of the CH2 stretching vibration is evident due to damping. The angular frequency (oint) of the CH2 stretching vibration (n-CH2) due to damping (interactions) is related to the damping constant (2rdc),22 oint2 = oo2 rdc2,
(1)
where oo is the angular frequency of n-CH2 in damping free oscillation occuring in neat P(VDF–HFP). In terms of wavenumber ( n ), eqn (1) can be rewritten as: r 2 1=2 dc 2 nint ¼ no ; (2) 2pc
Fig. 3 FT-IR spectra of electrospun neat (labeled as neat HFP) and Ag-NP doped (labeled as HFP–Ag) P(VDF–HFP) fibers in the 1600 to 400 cm1 region (a) and in the 3070 to 2930 cm1 region (b). The crystallographic phase assignment is given in part (a) and in the inset the relative proportion of a and b phases are shown. In part (b) the wavenumbers ( n o) of the CH2 asymmetric (nas) and symmetric (ns) stretching vibration modes for neat P(VDF–HFP) nanofibers are marked by dotted lines whereas the shifted wavenumbers ( n int) of the Ag-NP doped P(VDF–HFP) nanofibers are marked by dashed lines.
The FT-IR spectra of neat and Ag-NP doped P(VDF–HFP) electrospun nanofibers are shown in Fig. 3a and b. The vibrational bands at 1212, 1152, 976, 854, 796, 763, 613, 532 cm1 in neat P(VDF–HFP) nanofibers are attributed to the non-polar a-phase, whereas the characteristic peaks at 1276 and 841 cm1 correspond to the electroactive b-phase.20 It should be also noted that the absence of any 1232 cm1 peak (g-phase) indicates that the 841 cm1 peak is mainly due to the b-phase. Thus, the neat P(VDF–HFP) fibers are mainly composed of the b- and a-phases. It is a clear suggestion that the electrospinning process can induce the polar b-phase (Fig. S2, ESI†) in neat P(VDF–HFP) fibers. The doping with Ag-NPs leads to a substantial improvement (inset of Fig. 3a) in the contribution of the b-phase whereas the a-phase almost diminishes, evident from the intense vibrational bands at 1276 and 841 cm1 and the reduced bands intensity at 763 cm1 respectively. This is due to the interaction of the surface charges of the Ag-NPs with the molecular dipoles (CH2 or CF2) of P(VDF–HFP) which assist in the improvement of the b-phase content.20,21 Distinct frequency shifts of the CH2 asymmetric (nas) and symmetric (ns) stretching vibration modes are observed (Fig. 3b) in the NP doped fibers indicating clearly the interaction of surface charges of the Ag-NPs with CH2
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where c is the velocity of the light. From eqn (2), it is easily understood that due to the damping term (where, rdc B 2.4 1013 s1) the energy ( n int) of the n-CH2 vibration is effectively reduced. Therefore, the FT-IR spectra in that region, shown in Fig. 3b, are extremely useful to check the interfacial interactions between the surface charges of the Ag-NPs and the CH2 dipoles. As the vibrational modes existing in this region are not coupled with any other vibrational modes, a clear shift of the n-CH2 peaks is expected if any perturbation of the vibrations happens. The relative proportion of the b-phase within the electrospun P(VDF–HFP) fibers is calculated by the formula: FðbÞ ¼
Ab ; Kb =Ka Aa þ Ab
(3)
where Ab and Aa are the absorption intensities at 841 and 763 cm1 respectively, and Kb and Ka are the corresponding absorption coefficients assuming that the absorption coefficients are similar to those in pure PVDF.23 The relative proportion of the b- and a-phase in neat and Ag-NP doped P(VDF–HFP) nanofibers calculated by eqn (3) are shown in the inset of Fig. 3a. It reveals that Ag-NP doped P(VDF–HFP) fibers contain more than 85% of the relative proportion of electroactive b-phase, whereas neat (PVDF–HFP) fibers have less than 80%. A dipole reversibility test was performed on the PNGs to check whether the piezoelectric signal could really be attributed to the molecular dipoles. Therefore, measurements where the PIP was applied on the fibers–air interface side (let’s say top electrode side) were compared with those on the fibers–collector interface side (bottom electrode side) – refer to Fig. S3 (ESI†). The results pointed out that the alternating (AC) output signals from the PNG are reversible. Furthermore, a similar observation was also noticed upon switching the connection, i.e. interchanging the positive terminal to the negative terminal and vice versa. This also underlines the fact that the preferential alignment of the molecular dipoles existing in P(VDF–HFP) nanofibers is along the thickness (z-) direction (Fig. S4, ESI†) as previously proposed.5 The performance of the PNGs was further investigated by different PIP imparting frequencies but at the same pressure amplitude, the
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Therefore, this preparation procedure combined with a dedicated material system might be suitable for an anisotropic pressure sensor as well. In contrast, a recently reported PVDF–CNT based PNG might be not suitable for such applications as its working principle is based on the bending and release state of the electrospun fibers.6 The output voltage (B3 V) and current density (B0.9 mA cm2) indicate the feasibility of Ag-NP doped P(VDF–HFP) nanofiber based nanogenerators for self powering systems in portable electronic devices.
4 Conclusions
Fig. 4 (a) The performance of the PNGs (fabricated with Ag-NP doped P(VDF–HFP) nanofibers) as a function of different PIP imparting frequencies when the rectifying bridge circuit is not employed and (b) corresponding output voltage and current density. (c) The output voltage from the PNGs fabricated with neat (neat HFP) and Ag-NP doped (HFP–Ag) P(VDF–HFP) nanofibers when PIP with 2 Hz frequency and rectifying bridge is used.
corresponding results are shown in Fig. 4a. We notice an increasing trend in the generated output voltage amplitude up to 2 Hz of the PIP imparting frequency (Fig. 4b). The current density dependence on the PIP imparting frequency of the PNGs (Fig. 4b) indicates that a frequency of 2 Hz is optimal to achieve a stable performance of the PNGs. Furthermore, Fig. 4c illustrates that the AC output can be transferred into a pulsed rectified output signal by a simple bridge circuit (see Fig. 1c). Here, the output voltages generated on the PNGs fabricated with neat P(VDF–HFP) and with Ag-NP doped nanofibers are shown. We emphasize that the PNGs fabricated with neat P(VDF–HFP) fibers can give rise to an output voltage of more than 1 V and the current density is about 0.3 mA cm2 which is quite promising when compared to state of the art PNG research.6,7,24 In addition, we emphasize that the output voltage can be increased up to 3 V by Ag-NP doping as clearly shown in Fig. 4c (the comparative results are shown in Fig. S5, ESI†). It should be also noted that CNT added PVDF nanofiber based PNGs exhibit an output voltage of about 8 V, however further improvement of the current density is needed for realistic applications.6 This underlines that our PNGs can be extremely useful for powering many portable low powered personal electronic devices and other self-powered systems. The improvement of the output voltage is mainly caused by the larger content of the electroactive b-phase present in the Ag-NP doped P(VDF–HFP) nanofibers and in particular by the cooperative integrated poling due to Ag-NP presence as discussed in the FT-IR results (Fig. 3a). @P The piezoelectric charge coefficient, d3j ¼ (change in polariza@s j tion P due to the applied pressure s along the z-direction, j = 3) is much more effective due to the alignment of the molecular dipoles (i.e., CH2/CF2) along the z-direction, i.e., parallel to the applied electric DC field (E) in the electrospinning set-up (Fig. 1b).5
10406 | Phys. Chem. Chem. Phys., 2014, 16, 10403--10407
In this work, we have demonstrated the possibility to use P(VDF–HFP) nanofibers instead of PVDF in PNG fabrication. The incorporation of Ag-NPs into P(VDF–HFP) fibers led to a further improvement in the performance of the PNGs. An output voltage of 3 V was achieved accompanied by a current density of 0.9 mA cm2. This suggests that successful implementation of this material system into power sources (piezoelectric energy harvesters) in portable electronic devices, wireless sensors and implantable biomedical devices is feasible. It also offers the possibility to replace the commercially poled PVDF film separator by a P(VDF–HFP) nanofiber membrane in self-charging power cells.
Acknowledgements We would like to thank the authority of Jadavpur University (JU) for providing the financial support from the JU innovative research fund and DST, Govt. of India for developing instrumental facilities under FIST-II programme.
Notes and references 1 D. Dhakras, V. Borkar, S. Ogale and J. Jog, Nanoscale, 2012, 4, 752. 2 J. Fang, H. Niu, H. Wang, X. Wang and T. Lin, Energy Environ. Sci., 2013, 6, 2196. 3 B. J. Hansen, Y. Liu, R. Yang and Z. L. Wang, ACS Nano, 2010, 4, 3647. 4 C. Chang, V. H. Tran, J. Wang, Y.-K. Fuh and L. Lin, Nano Lett., 2010, 10, 726. 5 D. Mandal, S. Yoon and K. J. Kim, Macromol. Rapid Commun., 2011, 32, 831. 6 H. Yu, T. Huang, M. Lu, M. Mao, Q. Zhang and H. Wang, Nanotechnology, 2013, 24, 405401. 7 C. Xu, K. J. Kim, Y. Wang, S. Yoon, G. Ren, D. Mandal and B. Li, in Flexible Nanogenerator and Nano-Pressure Sensor Based on Nanofiber Web of PVDF and its Copolymers, in Soft Fibrillar Materials: Fabrication and Applications, ed. X. Y. Liu and J.-L. Li, Wiley-VCH, Weinheim, Germany, 2013, pp. 233–264, DOI: 10.1002/9783527648047.ch8. 8 A. R. Sathiya Priya, A. Subramania, Y. S. Jung and K. J. Kim, Langmuir, 2008, 24, 9816. 9 X. Xue, S. Wang, W. Guo, Y. Zhang and Z. L. Wang, Nano Lett., 2012, 12, 5048.
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10 V. Aravindan, A. Sundaramurthy, P. S. Kumar, N. Shubha, W. C. Ling, S. Ramakrishna and S. Madhavi, Nanoscale, 2013, 5, 10636. 11 Y. Ding, P. Zhang, Z. Long, Y. Jiang, F. Xu and W. Di, Sci. Technol. Adv. Mater., 2008, 1, 015005. 12 G. Moggi, P. Bonardelli and J. C. J. Bart, Polym. Bull., 1982, 7, 115. 13 Z. L. Wang and J. Song, Science, 2006, 312, 242. 14 X. Chen, S. Xu, N. Yao and Y. Shi, Nano Lett., 2010, 10, 2133. 15 K. I. Park, S. Xu, Y. Liu, G. T. Hwang, S. J. L. Kang and Z. L. Wang, Nano Lett., 2010, 10, 4939. 16 M. Y. Lu, J. Song, M. P. Lu, C. Y. Lee, L. J. Chen and Z. L. Wang, ACS Nano, 2009, 3, 357. 17 G. Zhu, R. Yang, S. Wang and Z. L. Wang, Nano Lett., 2010, 10, 3151. 18 S. Y. Xu, Y. Shi and S. G. Kim, Nanotechnology, 2006, 17, 4497.
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19 G. T. Zhang, S. Y. Xu and Y. Shi, Micro Nano Lett., 2011, 6, 59. 20 D. Mandal, K. Henkel and D. Schmeisser, Proceedings of International Conference on Nanoscience, Technology, and Societal Implications, 08–10, December 2011, Bhubaneswar, India, DOI: 10.1109/NSTSI.2011.6111801. 21 D. Mandal, K. Henkel, S. Das and D. Schmeisser, in Frontiers in Electronic Materials: A Collection of Extended Abstracts of the Nature Conference Frontiers in Electronic Materials June 17 to 20 2012, Aachen, Germany, ed. J. Heber, D. Schlom, Y. Tokura, R. Waser and M. Wuttig, Wiley-VCH, Weinheim, Germany, 2013, ch. 69. 22 G. C. King, Vibrations and Waves, Wiley-Sons, UK, 2009. 23 P. Martins, A. C. Lopes and S. Lanceros-Mendez, Prog. Polym. Sci., 2014, 39, 683. 24 J. Chang, M. Dommer, C. Chang and L. Lin, Nano Energy, 2012, 1, 356.
Phys. Chem. Chem. Phys., 2014, 16, 10403--10407 | 10407
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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 10403
Improved performance of a polymer nanogenerator based on silver nanoparticles doped electrospun P(VDF–HFP) nanofibers†
Received 11th December 2013, Accepted 27th February 2014
Dipankar Mandal,*a Karsten Henkelb and Dieter Schmeißerb
DOI: 10.1039/c3cp55238a www.rsc.org/pccp
We report on the electrospinning of poly(vinylidene fluoride– hexafluoropropylene) [P(VDF–HFP)] nanofibers doped with silver nanoparticles for the preparation of a polymer based nanogenerator (PNG). It has been found that the yield of the piezoelectric phase is increased by the addition of silver nanoparticles. Furthermore, defects in the P(VDF–HFP) electrospun fibers are removed resulting in a significant enhancement in the output power of the PNG. A maximum generated PNG output voltage of 3 V with a current density of 0.9 lA cm2 is achieved.
1 Introduction The rising demand for energy and the increasing mobility of human society have driven the development of new alternative power sources. Due to its piezo- and pyroelectric behaviors polyvinylidene fluoride (PVDF) is a good candidate for mechanical energy harvesting due to the possibility of developing tiny electronic devices consuming ultralow electric power. In this work an electrospinning method has been chosen to fabricate a polymer based nanogenerator (PNG) realized by the use of poly(vinylidene fluoride–hexafluoropropylene) [P(VDF–HFP): a copolymer of PVDF] nanofibers. This simple scale-up processing technique has many advantages in generator based applications, because a single-step electrospinning process can eliminate the need for direct contact or corona poling and is able to induce the piezoelectric b-crystal phase and spontaneous dipolar orientation at the same time.1,2 To the best of our knowledge, there have been only a few reports about electrospun PNGs mainly based on PVDF and its copolymer poly(vinylidene fluoride–trifluoroethylene) a
Organic Nano-Piezoelectric Device Laboratory, Department of Physics, Jadavpur University, Kolkata 700032, India. E-mail: [email protected] b ¨t Cottbus-Senftenberg, Brandenburgische Technische Universita Angewandte Physik-Sensorik, K.-Wachsmann-Allee 17, 03046 Cottbus, Germany. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: FE-SEM image of neat PVDF nanofibers, dipole reversibility test and schematic of electrospinning set-up where the direction of the preferential orientation of molecular dipoles is shown. See DOI: 10.1039/c3cp55238a
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[P(VDF–TrFE)].1–5 Recently a few attempts have been undertaken to improve the output voltage and current density of PNGs by the addition of carbon nanotubes (CNTs) and metal nanoparticles in electrospun PVDF nanofibers but no one has tried yet with the costeffective copolymer, P(VDF–HFP).6,7 The electrospun P(VDF–HFP) fibers were utilized as electrolyte membranes in dye-sensitized solar cells, exhibiting a good light to electricity conversion efficiency and improved long term stability.8 However, electrospun PNGs produced with P(VDF–HFP) have not been reported to date. The recent report about a self charging power cell,9 where an electrically poled PVDF film is used as a battery separator, motivated us to choose P(VDF–HFP) because its nanofiber membranes carry a tremendous potential for separators in lithium ion coin cells due to its relatively lower crystallinity, good ionic conductivity and higher electrochemical stability in comparison to PVDF.10–12 In addition, better flexibility and costeffectiveness may aid its importance in industrial large scale production.12 The concept of nanogenerators was initiated with zinc oxide (ZnO) nanorods or nanowire arrays,13 since then a remarkable growth in this research field has been noticed. Currently, several inorganic nanostructures such as lead zirconate titanate (PZT), barium titanate (BaTiO3), zinc sulphide (ZnS), etc. also offer adequate feasibility for their usage as power sources for portable electronic devices and biomedical sensors.14–17 However, limitations are more prominent in inorganic nanostructure based piezoelectric sensors and generators in practical applications due to their rigidity, heaviness, brittleness, poor biocompatibility and cytotoxicity.17–19 In contrast, piezoelectric polymer nanofibers are more promising because of their flexibility, light weight, excellent durability, easy large area fabrication, scope of functionalization, cost-effectiveness and biocompatibility. In order to enhance the piezoelectric b-crystalline phase content and to improve the quality of the P(VDF–HFP) electrospun fibers, silver nanoparticles (Ag-NPs) were doped into the polymer and also acted as piezoelectric phase stabilizers. It has been found that Ag-NPs play a significant role in improving the output power of the PNG. Recently, we have also initiated a
Phys. Chem. Chem. Phys., 2014, 16, 10403--10407 | 10403
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Published on 27 February 2014. Downloaded by Louisiana State University on 28/08/2014 13:32:36.
similar study with other metallic nanoparticles (i.e. gold, palladium, etc.), however, due to easy processing, cost effectiveness and biocompatibility, Ag-NPs were more advantageous. In this communication, we report on a PNG comprising electrospun P(VDF–HFP) nanofibers doped with Ag-NPs which can generate an output voltage of 3 V with a current density of 0.9 mA cm2 opening up possibilities for the use of this system in energy harvesting based device applications.
2 Experimental 2.1
2.3
Characterization of electrospun nanofibers
Fourier Transform Infrared (FT-IR) spectra were recorded (Shimadzu, FTIR-8400S spectrophotometer) to characterize the crystalline phases within the nanofibers. The samples reported here are labeled in the diagrams as follows: ‘neat HFP’ for neat P(VDF– HFP) and ‘HFP–Ag’ for Ag-NPs containing P(VDF–HFP) nanofibers. The morphology and orientations of the electrospun fibers were visualized by field emission scanning electron microscopy (FE-SEM) images (JEOL, JSM-7610F). UV-vis spectra (Shimadzu, UV-3600) were recorded to check the existence of the Ag-NPs within the electrospun nanofibers.
Polymer solution preparation
P(VDF–HFP) (chemical structure shown in Fig. 1a) pellets (Sigma-Aldrich) were dissolved in dimethylformamide (DMF) and kept at 60 1C under continuous stirring until a clear polymer solution was formed. Afterwards, 0.1 g (Experimental, ESI†) silver nitrate (Sigma-Aldrich) was added into 6 ml of the polymer solution, and the stirring was continued for 6 h to achieve Ag-NP formation (eqn (S1), ESI†). Prior to the electrospinning process, acetone was added into the solution to improve the solvent evaporation rate. Two sets of solutions were prepared: a pure polymer (neat P(VDF–HFP)) solution and a polymer solution containing Ag-NPs to adjust the P(VDF– HFP) solution to 12 wt% (w/v) where a co-solvent ratio of 6 : 4 (DMF to acetone) was used. 2.2
PCCP
Electrospinning process
A homemade typical electrospinning set-up (Fig. 1b) was used to produce the electrospun nanofibers. A 19 kV DC bias was applied to a metallic needle with a tip diameter of 23 G. The electrospun fibers were collected on a cylindrical collector (grounded electrode) wrapped with a nickel coated flexible fabric, which was placed about 10 cm apart from the needle tip. A constant flow rate (0.6 ml h1) of the polymer solution and a collector rotation speed of 200 rpm were applied throughout the fiber collection process.
Fig. 1 (a) The chemical structure of P(VDF–HFP); (b) schematic diagram of the electrospinning set-up used in this work; (c) simplified equivalent circuit diagram (Ro = 1 MO) for the detection of the output voltage (Vo) from the PNG. In the enlarged diagram of the PNG (left hand side) the pressure imparting probe (PIP), top and bottom electrodes and two surfaces (fibers–air and fibers–collector interface) of the electrospun fibers are illustrated.
10404 | Phys. Chem. Chem. Phys., 2014, 16, 10403--10407
2.4
Device fabrication and testing
Sandwich structure based PNGs were fabricated by arranging the layer of electrospun nanofibers (layer thickness B150 20 mm) in between two flexible electrodes. The electrodes were connected to a bridge diode to rectify the generated piezoelectric signal, as shown in Fig. 1c. The device performance was characterized by imparting periodic pressure (peak amplitude B15 kPa) onto the PNGs using a metallic probe (PIP: pressure imparting probe) with a tip diameter of 1.2 cm (active area) at different frequencies. The typical pressure imparting system was similar to one reported recently,5 except a pressure gauge was introduced for this work.
3 Results and discussion The FE-SEM micrograph (Fig. 2) illustrates that the Ag-NP doped P(VDF–HFP) electrospun fibers are defect free (no bead effects are observed) and the average diameter of the fibers is found to be around 83 nm, as shown in the histogram of the nanofibers in the top inset of Fig. 2. In contrast, neat P(VDF– HFP) electrospun fibers (average diameter B 71 nm) consist of bead defects with undesirable voids (Fig. S1, ESI†). The absence of defects in the doped fibers is due to an increase in the charge density of the polymer solution which causes a stronger elongation force on the ejected jets under the same electric field, resulting in the formation of straighter, homogeneous, dense and defect free nanofibers. The existence of Ag-NPs within the electrospun fibers are evident from the typical surface plasmon characteristic peak around 450 nm in the UV-vis spectra (bottom inset of Fig. 2).20,21
Fig. 2 FE-SEM image of the Ag-NP doped P(VDF–HFP) nanofibers. The distribution of the fiber diameters (top inset) and the surface plasmon bands due to the Ag-NPs (bottom inset, UV-vis spectra) are shown.
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(and CF2) dipoles (interfacial interactions). Recently, we have also demonstrated the evidence of interfacial interactions in Ag-NP doped spin coated and solvent casted P(VDF–HFP) films by means of X-ray photoelectron spectroscopy.21 Due to the interfacial interactions the effective mass of the CH2 dipoles is increased. As a result, a reduction in the vibrational frequency (i.e., wavenumber) of the CH2 stretching vibration is evident due to damping. The angular frequency (oint) of the CH2 stretching vibration (n-CH2) due to damping (interactions) is related to the damping constant (2rdc),22 oint2 = oo2 rdc2,
(1)
where oo is the angular frequency of n-CH2 in damping free oscillation occuring in neat P(VDF–HFP). In terms of wavenumber ( n ), eqn (1) can be rewritten as: r 2 1=2 dc 2 nint ¼ no ; (2) 2pc
Fig. 3 FT-IR spectra of electrospun neat (labeled as neat HFP) and Ag-NP doped (labeled as HFP–Ag) P(VDF–HFP) fibers in the 1600 to 400 cm1 region (a) and in the 3070 to 2930 cm1 region (b). The crystallographic phase assignment is given in part (a) and in the inset the relative proportion of a and b phases are shown. In part (b) the wavenumbers ( n o) of the CH2 asymmetric (nas) and symmetric (ns) stretching vibration modes for neat P(VDF–HFP) nanofibers are marked by dotted lines whereas the shifted wavenumbers ( n int) of the Ag-NP doped P(VDF–HFP) nanofibers are marked by dashed lines.
The FT-IR spectra of neat and Ag-NP doped P(VDF–HFP) electrospun nanofibers are shown in Fig. 3a and b. The vibrational bands at 1212, 1152, 976, 854, 796, 763, 613, 532 cm1 in neat P(VDF–HFP) nanofibers are attributed to the non-polar a-phase, whereas the characteristic peaks at 1276 and 841 cm1 correspond to the electroactive b-phase.20 It should be also noted that the absence of any 1232 cm1 peak (g-phase) indicates that the 841 cm1 peak is mainly due to the b-phase. Thus, the neat P(VDF–HFP) fibers are mainly composed of the b- and a-phases. It is a clear suggestion that the electrospinning process can induce the polar b-phase (Fig. S2, ESI†) in neat P(VDF–HFP) fibers. The doping with Ag-NPs leads to a substantial improvement (inset of Fig. 3a) in the contribution of the b-phase whereas the a-phase almost diminishes, evident from the intense vibrational bands at 1276 and 841 cm1 and the reduced bands intensity at 763 cm1 respectively. This is due to the interaction of the surface charges of the Ag-NPs with the molecular dipoles (CH2 or CF2) of P(VDF–HFP) which assist in the improvement of the b-phase content.20,21 Distinct frequency shifts of the CH2 asymmetric (nas) and symmetric (ns) stretching vibration modes are observed (Fig. 3b) in the NP doped fibers indicating clearly the interaction of surface charges of the Ag-NPs with CH2
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where c is the velocity of the light. From eqn (2), it is easily understood that due to the damping term (where, rdc B 2.4 1013 s1) the energy ( n int) of the n-CH2 vibration is effectively reduced. Therefore, the FT-IR spectra in that region, shown in Fig. 3b, are extremely useful to check the interfacial interactions between the surface charges of the Ag-NPs and the CH2 dipoles. As the vibrational modes existing in this region are not coupled with any other vibrational modes, a clear shift of the n-CH2 peaks is expected if any perturbation of the vibrations happens. The relative proportion of the b-phase within the electrospun P(VDF–HFP) fibers is calculated by the formula: FðbÞ ¼
Ab ; Kb =Ka Aa þ Ab
(3)
where Ab and Aa are the absorption intensities at 841 and 763 cm1 respectively, and Kb and Ka are the corresponding absorption coefficients assuming that the absorption coefficients are similar to those in pure PVDF.23 The relative proportion of the b- and a-phase in neat and Ag-NP doped P(VDF–HFP) nanofibers calculated by eqn (3) are shown in the inset of Fig. 3a. It reveals that Ag-NP doped P(VDF–HFP) fibers contain more than 85% of the relative proportion of electroactive b-phase, whereas neat (PVDF–HFP) fibers have less than 80%. A dipole reversibility test was performed on the PNGs to check whether the piezoelectric signal could really be attributed to the molecular dipoles. Therefore, measurements where the PIP was applied on the fibers–air interface side (let’s say top electrode side) were compared with those on the fibers–collector interface side (bottom electrode side) – refer to Fig. S3 (ESI†). The results pointed out that the alternating (AC) output signals from the PNG are reversible. Furthermore, a similar observation was also noticed upon switching the connection, i.e. interchanging the positive terminal to the negative terminal and vice versa. This also underlines the fact that the preferential alignment of the molecular dipoles existing in P(VDF–HFP) nanofibers is along the thickness (z-) direction (Fig. S4, ESI†) as previously proposed.5 The performance of the PNGs was further investigated by different PIP imparting frequencies but at the same pressure amplitude, the
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Therefore, this preparation procedure combined with a dedicated material system might be suitable for an anisotropic pressure sensor as well. In contrast, a recently reported PVDF–CNT based PNG might be not suitable for such applications as its working principle is based on the bending and release state of the electrospun fibers.6 The output voltage (B3 V) and current density (B0.9 mA cm2) indicate the feasibility of Ag-NP doped P(VDF–HFP) nanofiber based nanogenerators for self powering systems in portable electronic devices.
4 Conclusions
Fig. 4 (a) The performance of the PNGs (fabricated with Ag-NP doped P(VDF–HFP) nanofibers) as a function of different PIP imparting frequencies when the rectifying bridge circuit is not employed and (b) corresponding output voltage and current density. (c) The output voltage from the PNGs fabricated with neat (neat HFP) and Ag-NP doped (HFP–Ag) P(VDF–HFP) nanofibers when PIP with 2 Hz frequency and rectifying bridge is used.
corresponding results are shown in Fig. 4a. We notice an increasing trend in the generated output voltage amplitude up to 2 Hz of the PIP imparting frequency (Fig. 4b). The current density dependence on the PIP imparting frequency of the PNGs (Fig. 4b) indicates that a frequency of 2 Hz is optimal to achieve a stable performance of the PNGs. Furthermore, Fig. 4c illustrates that the AC output can be transferred into a pulsed rectified output signal by a simple bridge circuit (see Fig. 1c). Here, the output voltages generated on the PNGs fabricated with neat P(VDF–HFP) and with Ag-NP doped nanofibers are shown. We emphasize that the PNGs fabricated with neat P(VDF–HFP) fibers can give rise to an output voltage of more than 1 V and the current density is about 0.3 mA cm2 which is quite promising when compared to state of the art PNG research.6,7,24 In addition, we emphasize that the output voltage can be increased up to 3 V by Ag-NP doping as clearly shown in Fig. 4c (the comparative results are shown in Fig. S5, ESI†). It should be also noted that CNT added PVDF nanofiber based PNGs exhibit an output voltage of about 8 V, however further improvement of the current density is needed for realistic applications.6 This underlines that our PNGs can be extremely useful for powering many portable low powered personal electronic devices and other self-powered systems. The improvement of the output voltage is mainly caused by the larger content of the electroactive b-phase present in the Ag-NP doped P(VDF–HFP) nanofibers and in particular by the cooperative integrated poling due to Ag-NP presence as discussed in the FT-IR results (Fig. 3a). @P The piezoelectric charge coefficient, d3j ¼ (change in polariza@s j tion P due to the applied pressure s along the z-direction, j = 3) is much more effective due to the alignment of the molecular dipoles (i.e., CH2/CF2) along the z-direction, i.e., parallel to the applied electric DC field (E) in the electrospinning set-up (Fig. 1b).5
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In this work, we have demonstrated the possibility to use P(VDF–HFP) nanofibers instead of PVDF in PNG fabrication. The incorporation of Ag-NPs into P(VDF–HFP) fibers led to a further improvement in the performance of the PNGs. An output voltage of 3 V was achieved accompanied by a current density of 0.9 mA cm2. This suggests that successful implementation of this material system into power sources (piezoelectric energy harvesters) in portable electronic devices, wireless sensors and implantable biomedical devices is feasible. It also offers the possibility to replace the commercially poled PVDF film separator by a P(VDF–HFP) nanofiber membrane in self-charging power cells.
Acknowledgements We would like to thank the authority of Jadavpur University (JU) for providing the financial support from the JU innovative research fund and DST, Govt. of India for developing instrumental facilities under FIST-II programme.
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