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Cite this: Chem. Commun., 2013, 49, 11047 Received 6th September 2013, Accepted 3rd October 2013

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Enhanced frequency response of a highly transparent PVDF–graphene based thin film acoustic actuator† James S. Lee, Keun-Young Shin, Chanhoi Kim and Jyongsik Jang*

DOI: 10.1039/c3cc46807k www.rsc.org/chemcomm

A high-performance polyvinylidene fluoride (PVDF) based thin film acoustic actuator with chemical vapor deposition (CVD) graphene electrodes was successfully fabricated. Importantly, it showed 60, 19, and 22% enhancement in the bass, middle and treble frequency response, respectively.

A highly transparent thin-film acoustic actuator with a piezoelectric material such as poly(vinylidene fluoride) (PVDF) has attracted a great deal of interest as a next-generation flexible device due to its low power consumption, light weight, and compact size. As a representative semi-crystalline polymer, PVDF has three possible polymorphs i.e., alpha (a), beta (b), and gamma (g) phases according to the chain conformation of trans and gauche linkages.1 Especially, the piezoelectric property strongly dependent on its b phase content owing to high net dipole moment originates from the all-trans structure. In spite of large effort to improve the b phase content, it is still a challenge to use pristine PVDF for thin film acoustic actuators because it has poor bass sound below 500 Hz. Martins et al. recently introduced ceramic nanofillers into a PVDF matrix to enhance the piezoelectric property.2 However, incorporating a high concentration of nanofillers to the matrix caused particle–particle aggregation that resulted in limited permittivity.3 For this reason, the ability to control the morphology of the nanofiller has been focused in order to fabricate nanoparticles with low density, such as hollow nanoparticles. Using a highly conductive electrode to deliver distortion-free input signals from a sound generator to the surface of the film is also important for a high quality thin film acoustic actuator. Graphene is considered a potential breakthrough for carbon based electrodes because of its high electron mobility, transmittance, and mechanical strength with flexibility.4 In our previous work, World Class University (WCU) program of Chemical Convergence for Energy & Environment (C2E2), School of Chemical and Biological Engineering, College of Engineering, Seoul National University (SNU), Seoul, Korea. E-mail: [email protected]; Fax: +82 2 888 1604; Tel: +82 2 880 7069 † Electronic supplementary information (ESI) available: A detailed experimental procedure, characterization of Ba-HNPs and CVD graphene, mathematical equations, crystallization behavior and permittivity factors, POM images, and THD performance of an acoustic actuator. See DOI: 10.1039/c3cc46807k

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reduced graphene oxide (rGO) was modified as electrodes for flexible acoustic actuators.5 However, there are still possibility remained to enhance frequency response with highly conductive electrode. In this regard, the high conductive chemical vapor deposition (CVD) graphene can make it possible to enhance the performance of film acoustic actuators. Herein, we report the behavior of a PVDF thin film acoustic actuator whose performance in every frequency range has been enhanced by incorporation of Ba-doped SiO2–TiO2 hollow nanoparticles (Ba-HNPs) as nanofillers and graphene electrodes grown by CVD. Importantly, a high concentration of monodisperse Ba-HNPs in the PVDF matrix (Ba-HNPs/PVDF) was successfully obtained as nucleation agents, and improved piezoelectricity of PVDF was easily achieved at a large scale via electrospinning.6 To gain insight into PVDF properties according to contents of fillers, the investigation was divided into two respects, crystalline behavior and parameters of permittivities. Furthermore, we applied a PVDF based thin film to practical acoustic actuators to demonstrate acoustic response as a function of concentration of nanofillers and various electrode types. The overall procedure for the fabrication of the graphene based Ba-HNPs/PVDF acoustic actuator is illustrated in Fig. 1. Electrospinning was used to introduce the Ba-HNPs into the ¨ber’s PVDF. First, the Ba-HNPs were synthesized according to Sto method and energy-dispersive X-ray (EDX) analysis showed the presence of Ba, Ti, Si, and O in the nanofiller (ESI†). The Ba-HNPs were reasonably monodisperse and their average diameter was ca. 20 nm (Fig. 1 inset, top-left transmission electron microscopy (TEM) image).7 The prepared Ba-HNPs were mixed in a solution of PVDF, which was then electrospun to form a Ba-HNPs/PVDF mat. The mat was placed on a hot press and pressed into a homogeneous film. Ba-HNPs could be seen on a spun PVDF fiber (Fig. 1, top-right TEM image). Polling and drawing provided high transparency and polarization. Subsequently, an acoustic actuator electrode was constructed from a large (7  7 cm2) film of 1.9 nm thick CVD graphene that consisted of a few layers of low-defect graphene (ESI†). The surface of the fabricated thin film was treated with 3-aminopropyltriethoxysilane (APS) as a coupling agent to improve adhesion to the graphene electrode. Hence, the acoustic actuator consisted of a sandwich of a Chem. Commun., 2013, 49, 11047--11049

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Ba-HNPs/PVDF thin film and CVD graphene electrodes, which was connected to a sound source and amplifier. Fig. 2a shows the changes in Fourier transform infrared (FTIR) spectra of the crystalline a and b phases of the Ba-HNPs/PVDF thin film for different nanofiller concentrations.8 The representative a phase, trans–gauche configuration by skeletal bending vibrations, appeared at 766 cm1. The b crystalline phase has an all trans conformation, which is a planar zigzag configuration whose CH2–CF2 rocking and asymmetric stretching modes are absorbed at 840 and 1275 cm1. The ratio of Ib/Ia increased upon addition of

the nanofiller and the b phase content could be obtained by the Lambert–Beer law (ESI†). As a result, adding 10 wt% of the Ba-HNPs led to an Ib/Ia of ca. 2.37 with 87% b phase, and incorporating 15 wt% Ba-HNPs led to an Ib/Ia of ca. 2.56 with 95% b phase. However, for 20 wt% loading, the Ib/Ia value decreased to ca. 2.38 or 89% b content because the high content of saturated Ba-HNPs limited the dispersion of fillers. Judging from these data, electrospinning can be a useful method used to provide polling and drawing effects to enhance the b phase content. To achieve an in-depth insight into crystallization behavior with different nanofiller concentrations, differential scanning calorimetry (DSC) analysis was conducted (Fig. 2b). It was confirmed that the role of Ba-HNPs as nucleation agents of PVDF can be ascribed to the increase of crystallization temperature (Tc) (ESI†).9 The relative degree of crystallinity as a function of time, X(t), can be calculated (Fig. 2b, inset).10 It indicates that the crystallinity of PVDF is attained most rapidly with 15 wt% of Ba-HNPs. Especially, the high crystallization rate constant (k) of 0.05448, values of Ozawa exponent (n), 2.016, and crystallization half-time (t1/2), 1.694 ms, were obtained at 15 wt% of nanofillers (ESI†). Moreover, the morphology of the crystal determined using polarized optical microscopy (POM) shows that the amounts of spherulites were significantly increased upon loading 15 wt% of Ba-HNPs (ESI†). It means that added Ba-HNPs act as nuclei for PVDF crystallization. Owing to the charge–charge interaction of Ba-HNPs with the –CF2 dipole, a crystalline PVDF chains converted to the trans–trans conformation. As a result, reinforced crystallization behavior is intimately proportional to the b phase content.

Fig. 2 Representative (a) FTIR spectra and (b) DSC thermograms of a pristine PVDF film and a Ba-HNPs/PVDF film containing various amounts of Ba-HNPs (inset: the data represent relative crystallinity as a function of time). The films were cooled from 190 1C at a cooling rate of 10 1C min1.

Fig. 3 (a) Permittivities and loss factors of Ba-HNPs/PVDF thin films containing various amounts of Ba-HNPs. The loss factor was obtained by differentiation of the permittivity values. (b) Schematic representation of the interfacial effect of porous Ba-HNPs upon enhancing the b phase content.

Fig. 1 Fabrication of the Ba-HNPs/PVDF thin film acoustic actuator via electrospinning, hot-pressing, mechanical stretching under high voltage, and transferring CVD graphene onto the silane-treated thin film. The Ba-HNPs/PVDF-based thin film was synthesized at 220 1C and 800 psi, and the polling and drawing processes were carried out at 30 kV cm1.

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Fig. 4 The frequency responses of Ba-HNPs/PVDF-based thin film acoustic actuators (a) with graphene electrodes as a function of Ba-HNPs content and (b) four different electrodes at 15 wt% of nanofillers.

In general, the b phase PVDF is highly ferroelectric and piezoelectric because every dipole points in the same direction. Therefore, the permittivity and loss factor of the fabricated Ba-HNPs/PVDF based thin film were also determined (Fig. 3a). The measured permittivity at lower frequencies was always greater than at higher frequencies, and increasing the loading of Ba-HNPs significantly improved the permittivity (e0 ). Additionally, a range of relaxation slopes (aa) are mainly overlapped at 1 kHz; it represents the reversible conformational rearrangements and chain-folding of the crystalline lamellae in the interior of the PVDF.11 Furthermore, analysis of electrical response demonstrated that the value of dielectric strengths (De) was reinforced by loading increased concentrations of Ba-HNPs, and relaxation times (l) were shortened (ESI†). The permittivity values were slightly enhanced in spite of lower b phase contents than 15 wt%. Considering these results, it could be inferred that the permittivity was enhanced because of the space charge interaction in PVDF at the interface with the Ba-HNPs. Namely, the porous structure of the Ba-HNPs and PVDF can facilitate transportation of charges by modifying the arrangement of the charges (Fig. 3b). The ferroelectric phase is strongly influenced by the geometry of the fillers through the interfacial interaction between the local electric field of the filler and the PVDF dipoles. Consequently, increasing the filler content enhanced the piezoelectric potentials, and decreasing the filler particle size increased the surface area, which also contributed to elevating the permittivity. Fig. 4a illustrates the acoustic performance of fabricated Ba-HNPs/PVDF based thin film acoustic actuators as a function of nanofiller content. As a result, loading the Ba-HNPs in our This journal is

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Ba-HNPs/PVDF heterogeneous thin film improved the overall frequency response. Notably, loading with 15 wt% of filler maximized the bass frequency response to 60%, which was approximately a 20 dB full-scale increase in the sound level. In addition, the behavior of the total harmonic distortion (THD) decreased by 25% (ESI†). This provided clear evidence that 15 wt% of Ba-HNPs in PVDF was optimum for the thin film acoustic actuator for maximum sound with stable distortion. Importantly, this is the first report demonstrating that the b phase content plays a key role in the improvement of acoustic performance of PVDF based thin films. To confirm the function of the electrode, various electrodes were applied to 15 wt% Ba-HNPs/PVDF thin films. The sheet resistances of commercial poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), carbon nanotubes (CNT) and synthesized rGO were 72, 11 and 10 kO sq1 with 75, 68 and 84% of transmittance, respectively. The CVD graphene had a sheet resistance of 937 O sq1 and a high transmittance of 95% (ESI†). The performance of the CVD graphene based acoustic actuator was 6 and 17% better in the middle and treble frequency ranges than the modified rGO electrode. Moreover, it had 19 and 22% higher frequency response, and 21 and 17% less distortion in the midrange and treble frequencies compared to a thin-film acoustic actuator with a PEDOT:PSS electrode. We have demonstrated that incorporating Ba-HNPs with PVDF and using a CVD graphene electrode is an effective way to enhance the acoustic performance of a PVDF-based thin-film acoustic actuator. Most of all, it is confirmed that the high degree of crystallinity of PVDF strongly effected low bass sound improvement. This superior performance of the fabricated thin film acoustic actuator has great possibilities for future-oriented applications. This work was supported by WCU (World Class University) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (R31-10013).

Notes and references ¨ zyilmaz 1 (a) S. H. Bea, O. Kahya, B. K. Sharma, J. Kwon, H. J. Cho, B. O and J. H. Ahn, Nano Lett., 2013, 7, 3130; (b) S. Wang, L. Lin and Z. L. Wang, Nano Lett., 2012, 12, 6339; (c) P. Calvert, Nature, 1975, 256, 694; (d) A. J. Lovinger, Science, 1983, 220, 1115. 2 P. Martins, R. Goncalves, M. Benelmekki, G. Botelho and S. LancerosMendez, J. Phys. Chem. C, 2012, 116, 15790. 3 (a) M. Verduyn, J. Sefcik, G. Storti and M. Morbidelli, Polym. Colloids, 2001, 801, 23; (b) A. Y. Kim and J. C. Berg, Langmuir, 2000, 16, 2101; ¨lfen and S. Mann, Angew. Chem., Int. Ed., 2003, 42, 2350; (c) H. Co ´ri and B. Puka ´nszky, Nanoscale, 2012, 4, 1919. (d) G. Keledi, J. Ha 4 (a) A. Majumdar, J. Kim, J. Vuckovic and F. Wang, Nano Lett., 2013, 13, 515; (b) A. Venugopal, L. Colombo and E. M. Vogel, Appl. Phys. Lett., 2010, 96, 013512; (c) L. Xiao, Z. Chen, C. Feng, L. Liu, Z. Q. Bai, Y. Wang, L. Qian, Y. Zhang, Q. Li, K. Jiang and S. Fan, Nano Lett., 2008, 8, 4539; (d) S. A. You, O. S. Kwon and J. Jang, J. Mater. Chem., 2012, 22, 17805. 5 K. Y. Shin, J. Y. Hong and J. Jang, Chem. Commun., 2011, 47, 8527. 6 (a) C. V. Chanmal and J. P. Jog, Int. J. Plast. Technol., 2011, 15, 1; (b) T. Khudiyev, E. Ozgur, M. Yaman and M. Bayindir, Nano Lett., 2011, 11, 4661; (c) N. Lanigan and X. Wang, Chem. Commun., 2013, 49, 8133. ¨ber and A. Fink, J. Colloid Interface Sci., 1968, 26, 62. 7 W. Sto 8 (a) P. Martins, C. Caparros, R. Gonçalves, P. M. Martins, M. Benelmekki, G. Botelho and S. Lanceros-Mendez, J. Phys. Chem. C, 2012, 116, 15790; (b) S. Tan, E. Liu, Q. Zhang and Z. Zhang, Chem. Commun., 2011, 47, 4544. 9 S. Lee, J. Y. Hong and J. Jang, Polym. Int., 2013, 62, 901. 10 J. Cheng, J. Zhang and X. Wang, J. Appl. Polym. Sci., 2013, 127, 3997. 11 (a) J. Y. Hong, E. Lee and J. Jang, J. Mater. Chem. A, 2013, 1, 117; (b) L. K. H. Van Beek, Physica, 1960, 26, 66. Chem. Commun., 2013, 49, 11047--11049

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Enhanced frequency response of a highly transparent PVDF-graphene based thin film acoustic actuator.

A high-performance polyvinylidene fluoride (PVDF) based thin film acoustic actuator with chemical vapor deposition (CVD) graphene electrodes was succe...
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