Polymeric Nanofibers with Ultrahigh Piezoelectricity - ACS Publications

Polymeric Nanofibers with Ultrahigh Piezoelectricity via Self-Orientation of Nanocrystals Xia Liu,†,‡ Jing Ma,§ Xiaoming Wu,†,‡ Liwei Lin,∥ and Xiaohong Wang*,†,‡ †

Institute of Microelectronics, Tsinghua University, Beijing 100084, PR China Tsinghua National Laboratory for Information Science and Technology, Tsinghua University, Beijing 100084, PR China § School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China ∥ Department of Mechanical Engineering, University of California, Berkeley, California 94720, United States ‡

S Supporting Information *

ABSTRACT: Piezoelectricity in macromolecule polymers has been gaining immense attention, particularly for applications in biocompatible, implantable, and flexible electronic devices. This paper introduces core−shell-structured piezoelectric polyvinylidene fluoride (PVDF) nanofibers chemically wrapped by graphene oxide (GO) lamellae (PVDF/GO nanofibers), in which the polar β-phase nanocrystals are formed and uniaxially self-oriented by the synergistic effect of mechanical stretching, high-voltage alignment, and chemical interactions. The β-phase orientation of the PVDF/GO nanofibers along their axes is observed at atomic scale through high resolution transmission electron microscopy, and the β-phase content is found to be 88.5%. The piezoelectric properties of the PVDF/GO nanofibers are investigated in terms of piezoresponse mapping, local hysteresis loops, and polarization reversal by advanced piezoresponse force microscopy. The PVDF/GO nanofibers show a desirable out-of-plane piezoelectric constant (d33) of −93.75 pm V−1 (at 1.0 wt % GO addition), which is 426% higher than that of the conventional pure PVDF nanofibers. The mechanism behind this dramatic enhancement in piezoelectricity is elucidated by three-dimensional molecular modeling. KEYWORDS: PVDF/GO nanofiber, core−shell, piezoelectricity, self-orientation, β-phase, piezoresponse force microscopy

P

polarization per unit cell, which in turn, results in the highest piezoelectric response. The content and orientation of the β phase and the degree of crystallinity (DoC) are the key factors affecting the macroscopic piezoelectric response of PVDF. DoC is a characteristic of semicrystalline polymers and is usually accompanied by the α- to β-phase transformation. The β-phase content can be increased by various techniques, among which, the uniaxial or biaxial stretching of polymer films or fibers at a given temperature is the most widely used technique.11−13 Electrospinning is effective in producing self-poled piezoelectric nanofibers because of the high stretching forces exerted on electrified solution jets.14 However, PVDF films or fibers partially depolarize after mechanical stretching and even electrospinning via thermal motion back to the stable curled state. Recently, the addition of nanofillers such as organically

iezoelectric materials can convert the mechanical energy produced by various activities into electrical energy and vice versa. Piezoelectric micro/nanofilms or fibers1,2 are sensitive even to mild mechanical stress/strain and thus find applications in implantable biosensors and wearable and portable electronic devices.3−6 Although conventional piezoelectric ceramics such as barium titanate,7 zinc oxide,8 and lead zirconate titanate9 exhibit high piezoelectric constants, they are not suitable for biomedical and flexible electronics applications owing to their toxicity and brittleness, respectively. In contrast, piezoelectric polymers such as polyvinylidene fluoride (PVDF) and its copolymers are lightweight, flexible, biocompatible, and electroactive. However, they are semicrystalline and the enhancement of their piezoelectric properties is still a challenge.10 The piezoelectricity of PVDF is mainly contributed by polar crystalline phases such as β phase (TTTT conformation) and γ phase (T3GT3G′ conformation), rather than the nonpolar α phase (TGTG′ conformation). Among these polar phases, the β phase confers to the polymer the largest spontaneous © 2017 American Chemical Society

Received: November 27, 2016 Accepted: January 20, 2017 Published: January 20, 2017 1901

DOI: 10.1021/acsnano.6b07961 ACS Nano 2017, 11, 1901−1910

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Figure 1. Morphology, core−shell structure, and elemental mapping of the PVDF/GO nanofibers. (a) The SEM image showing the rough morphology of the PVDF/GO nanofiber. (b) The nanofiber structure consisting of a PVDF core and PVDF/GO shell is visualized by bright field TEM. (c) The zoomed-in image of (b) showing that the GO lamellae were oriented along the fiber axis. The yellow arrows point to the GO lamellae. (d,e,f) Elemental mapping exhibiting oxygen (blue), carbon (red), and fluorine (green) concentrations of the top-right area in (c), respectively, by EELS. (g) EDS spectrum of the nanofiber giving the atom fraction distribution. (h) Elemental mapping cross sections showing the distributions of carbon and fluorine (along the convex curve) and oxygen (along the concave curve).

orientation of the β-phase nanocrystals was examined by transmission electron microscopy (TEM). The β-phase content was determined by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. We experimentally investigated the piezoelectric domain distribution within the nanofibers by advanced piezoresponse force microscopy (PFM). For intensive investigation of the piezoelectricity of the PVDF/GO nanofibers, local hysteresis loops were obtained to calculate the piezoelectric constant. The polarization reversal images were captured to analyze the domain switching behavior by PFM. Finally, we developed a three-dimensional molecular model for the PVDF/GO nanofibers to investigate the underlying mechanism for their enhanced piezoelectricity by examining the variation and dynamics of the chemical bonds.

modified nanoclays,15 metal nanoparticles,16,17 functionalized multiwalled carbon nanotubes (MWNTs),18,19 and graphene and its derivatives20,21 has emerged as an alternative chemical method by introducing specific interactions to increase the βphase content and stabilize the β-phase nanocrystals. In addition to the β-phase content and DoC, the uniaxial orientation of the β phase also makes a crucial contribution to piezoelectricity of the PVDF.22 The above-mentioned methods cause reorientation of the β-phase nanocrystals during the βphase formation process. This is desirable from piezoelectric perspective. Because of the small energy gap between the TGTG′ and TTTT conformations, mechanically or electrically treated PVDF can relax back to the α phase via thermal motion. It means that the β-phase orientation is dominant during the early stage of rapid jetting under a high electric field, while relaxation occurs during the downstream electrospinning.23 When a nanofiller is added, layers of oriented PVDF chains are formed at these additional interfaces; however, a postprocess is needed for the overall orientation of the β-phase nanocrystals.24 Thus, controlling the β-phase orientation is necessary. In order to achieve a high content of the β-phase and its uniaxial orientation, graphene oxide (GO) lamellae, in other words, graphene nanosheets functionalized with oxygen groups are used. They play the role of heterogeneous nucleating agents in the β-phase crystallization process and form reinforcements to maintain the residual orientation of the β phase. In this paper, we report the results of reinforced structure and piezoelectricity of PVDF/GO nanofibers. The atomic scale

RESULTS AND DISCUSSION The morphology and elemental structure of the electrospun PVDF composite nanofibers (seen in Experimental Section) with 1.0 wt % GO lamellae are shown in Figure 1. The SEM image reveals that the PVDF/GO nanofibers had rough morphology (Figure 1a), while the pure PVDF nanofibers have quite smooth surfaces (Figure S1 in Supporting Information). It implies that the GO lamellae probably distribute near the nanofiber surface, thus giving rise to wrinkled morphology. The hierarchical structure of the nanofiber was directly visualized through bright field TEM, as shown in Figure 1b. A magnified view of Figure 1b reveals that the GO lamellae wrap the PVDF molecules along the fiber axis 1902


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Figure 2. Crystallinity characteristics of the PVDF/GO nanofibers. (a) High resolution TEM image of the PVDF/GO nanofiber with 1.0 wt % GO lamellae. (b) The SAED pattern demonstrates the orientation of the polymer chains. (c) The image is produced from the inverse FFT with the filter mask (the inset). (d) XRD analysis of PVDF/GO and pure PVDF samples. (e) Variation of the DoC and β-crystallinity as a function of the GO content. (f) FTIR spectra of PVDF/GO and pure PVDF samples. (g) Variation of the fraction of the polar-phase (Fpolar) and β-phase (Fβ) contents as a function of the GO content. (▼, ●, and ★ represent α, β, and γ phases, respectively.).

in the same way as corn leaves wrap a corn (Figure 1c). The shell region of the composite nanofiber consists of layers of GO lamellae in parallel to the fiber axis. Moreover, the GO lamellae distribute uniformly in the PVDF matrix with no signs of aggregation or agglomeration. To investigate the distribution of the GO lamellae in the hierarchical structure PVDF/GO nanofibers we carried out several elemental mapping experiments. Figures 1d−f show the corresponding electron energy-loss spectroscopy (EELS) images (blue, red, and green mapping) of the upper part of the nanofiber shown in Figure 1c. The corresponding EELS spectra confirm the presence of O, C, and F elements. This is further confirmed by the energy-dispersive X-ray spectroscopy (EDS) line scanning (Figure 1g), which reveals that the nanofiber is composed of 46.7 at% carbon, 6.7 at% oxygen, and 42.3 at% fluorine (as well as hydrogen which could not be detected). It also suggests that all of the solvent evaporated because of detection of no nitrogen element. Further, the elemental mapping cross-section images reveal that carbon and fluorine distribute along the convex curves, while oxygen

distributes along the concave curve (Figure 1h). This implies that oxygen elements distribute along the nanofiber periphery, as well as part of carbon and fluorine elements. The solid fitting lines can interpret a core−shell structure (Figure S2 in Supporting Information), in which the nanofiber consists of a core with a diameter (Rcore) of 27 nm and a shell with a thickness (Rshell) of 9 nm. In addition, the line profile of the elemental mapping along the fiber axis gives information on conformability of the core−shell structure along the fiber axis, particularly the uniformity of the shell structure along the extended length of the nanofiber (Figure S3 in Supporting Information). As the fiber diameter increases, the Rshell/Rcore ratio increases (Figure S4 in Supporting Information). The PVDF/GO nanofibers also exhibit core−shell structures when 0.5 or 2.0 wt % of GO lamellae are added and not when the GO content is 0.1 or 4.0 wt % (Figure S5 in Supporting Information). In contrast, the TEM images of the pure PVDF nanofibers with different diameters show uniform structures (Figure S6 in Supporting Information). When the PVDF nanocrystals nucleate and grow during the electro1903


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compared. The intensity of the peak at 840 cm−1 in the case of the PVDF/GO nanofibers is higher than that in the case of the pure PVDF nanofibers, implying that the GO lamellae contribute to the formation of the polar phases (β and γ phases). From Figure 2g, it can be observed that the proportion of the polar phases (Fpolar) increases rapidly up to 93.6% at 1.0 wt % of GO addition (Figure S11 in Supporting Information). Quantification of the individual β and γ phases was also performed by deconvolution of the 840 cm−1 bands, where the broadening contribution of the β phase reaches the highest value of 88.5% (Figure 2g, calculation details are given in Text S2). The α- to β-phase transformation occurs gradually with an increase in the nanofiller content. The conversion to the β phase is the highest when 1.0 wt % GO lamellae is added (Figure S12 in Supporting Information). Complementary information was also obtained by performing differential scanning calorimetry (DSC) studies of the same samples. The DSC testing results of the nanofiber samples are shown in Figure S13 of Supporting Information. Comparing to the pure PVDF nanofibers, the DSC thermograms of the PVDF/GO nanofibers present a peak melting temperature shifting toward higher temperatures and narrowing of the melting point features with GO addition. These thermograms prove that the GO nanofiller induces an increase in the total amount of the β phase in the composite nanofibers. Apart from the sharp peak responding to the β phase, a small peak at 179° assigned to the γ phase appears in the PVDF/GO nanofibers.27 The positive charges present in the GO sheets can interact with the −CF2− dipoles of the PVDF chains, which assists in the stabilization of the γ phase by aggregating all locally ordered conformations. The above studies show that both the electrospinning process and the GO addition with the proper content can greatly enhance the content and orientation of the β and γ phases.28 The directional alignment can be assigned to the strong elongation forces exerted by both the polymer backbones and the layered-structure GO during the electrospinning process. Particularly, the higher voltage and smaller needle tip are advantageous to obtain thinner nanofibers with high-oriented backbones.29 The subsequent fast solvent evaporation and crystallization suppress some relaxation effects, so that both PVDF chains and GO networks retain largely their self-oriented conformations. It has been pointed out that in certain instances the embedded GO lamellae maintain a higher degree of orientation compared to the polymer matrix, as a direct consequence of their slower relaxation. It is worth mentioning that uniaxial deformation and electric poling can further promote the α- to β-phase transformation. Moreover, the formation of the crystalline β phase is irreversible since GO lamellae would provide a large energy barrier preventing them from transforming back to the amorphous or other crystalline phases. The increased β-phase content and uniaxial orientation can predict the enhanced piezoelectricity of the composite nanofibers. Subsequently, we measured the vector (including out-of-plane (OP) and in-plane (IP)) PFM images and obtained the local hysteresis loops for both the PVDF/GO and pure PVDF nanofibers using the dual AC resonance tracking (DART) mode of PFM.30,31 The nanodomain switching properties of the nanofibers were resolved in real space with nanometric resolution by switching spectroscopy PFM (SS-PFM).32 The advances lead to the generation of large

spinning process, the GO lamellae move to the surfaces of the nanofibers because of the internal radial orientation of the electrostatic field and the rapid evaporation of the solvents. Figure 2a shows the high resolution TEM image of the core− shell-structured PVDF/GO nanofiber, revealing that the PVDF chains are elongated and self-oriented side by side along the fiber axis and that the straight molecular segments in these chains are at least 5−20 nm long. The inset shows the straight chain arrays with gray and white dots separated by short distances. The atom-to-atom distance in the same chain is 0.245 nm, while the chain-to-chain distance is 0.608 nm. The selected area electron diffraction (SAED) pattern shows the orientation of the individual polymer chains (Figure 2b). Also the SAED pattern of the PVDF/GO nanofibers implies much higher crystallinity when comparing to the pattern of the pure PVDF nanofiber in Figure S7 of Supporting Information. Fast Fourier transform (FFT) of the image data (the inset of Figure 2c) produces diffraction patterns, which indicates that the PVDF/ GO nanofiber has crystalline phases. In the case of the thin PVDF/GO nanofiber (Figure S8 in Supporting Information), the PVDF molecules are extended in parallel and straightened at least up to tens of nanometers, resulting in the higher orientation of the nanocrystals along the fiber axis and even reproducible crystalline faceting. A filter mask for the reverse FFT was chosen to include the strong diffraction spots that are well oriented along the direction from the bottom left to the top right (Figure 2c). In contrast, the high resolution TEM images of the pure PVDF nanofibers show the same orientation trend but with poor orientation and less crystallinity (Figure S7 in Supporting Information). The crystallization behavior of the PVDF/GO nanofibers was investigated to identify their crystalline phases and quantify the DoC by XRD (Figure 2d). In the XRD spectrum of the PVDF/GO nanofibers, the peaks at 20.8 and 36.5° correspond to the β phase. The sharp peak at 20.8° indicates that the β phase is predominant in these nanofibers. The peak at 39.5° corresponds to the γ phase. These peaks are also observed in the XRD spectrum of the PVDF/GO casting membrane. So it is implied that the nucleation of the electroactive β and γ phases is contributed by the interaction between the negatively charged GO lamellae and the dipolar moments of the PVDF chains.25,26 In other words, the presence of electrostatic interaction and/or hydrogen bonding between the oxygen functionalities of GO and PVDF leads to the α- to β/γ-phase transformation. In the case of the pure PVDF nanofibers, the peak at 18.8° corresponds to the α phase, while the other peaks at 20.8° and 36.5° are assigned to the β phase. Thus, the XRD results indicate that the addition of the GO lamellae significantly promotes the β-phase formation in the PVDF/ GO nanofibers. The intensity of the β-phase peaks increases and then decreases with the increasing GO content (Figure S9 in Supporting Information). The crystallinity of the PVDF/GO and pure PVDF nanofibers was evaluated from the XRD patterns by the curve deconvolution technique (Figure S10 in Supporting Information). The DoC of the PVDF/GO nanofibers is dependent on the GO content. The highest DoC value is 72.6% (at 1.0 wt % GO addition), which is almost 1.3 times higher than that of the pure PVDF nanofibers (Figure 2e, calculation details are given in Text S1). Particularly, the βphase crystallinity varies remarkably with the GO content. FTIR spectroscopy was used to further evaluate the contents of the individual crystalline phases (Figure 2f). The FTIR spectra of the PVDF/GO and pure PVDF nanofibers are 1904


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Figure 3. OP piezoelectric response images of the PVDF/GO and pure PVDF nanofibers at a harmonic AC voltage of 1.6 V. (a) Surface topography, (b) OP PFM phase, and (c) OP PFM amplitude images of the PVDF/GO nanofiber showing rough topography, clear 180° switching domain wall, and strong piezoelectric contrast, respectively. (d) Surface topography, (e) OP PFM phase, and (f) OP PFM amplitude images of the pure PVDF nanofiber showing smooth topography, randomly distributed granular-type nanodomains with 180° switching, and light piezoelectric contrast, respectively.

Figure 4. SS-PFM phase and amplitude responses for the PVDF/GO and pure PVDF nanofibers show the local hysteresis loop behavior and the polarization reversal. Square-shaped phase hysteresis and butterfly shaped amplitude loops of (a) the PVDF/GO nanofiber and (b) the pure PVDF nanofiber, obtained with a DC voltage varying from −30 to 30 V. (c) Diameter-dependent characteristics of the PVDF/GO and pure PVDF nanofibers. (d) Comparative d33 studies on the 50 nm-thick PVDF/GO nanofibers varying with different GO contents, which is consistent with the results of crystallinity characteristics shown in Figure 2h.

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Table 1. Comparison of the Piezoelectric Constant (d33) As Derived from PFM for PVDF-Based Piezoelectric Nanostructures and Some Ceramic Micro/Nanofibers PVDF-based piezoelectric nanostructures

ceramic micro/nanofibers

sample

d33 [−pm V−1]

reference

sample

d33 [pm V−1]

reference

PVDF nanoribbon Ultrathin PVDF film PVDF film PVDF/BaTiO3 nanofiber Poled melt-spun PVDF nanofiber Core−shell-structured PVDF/GO nanofiber

58.5 47.7 49.6 48 17.1 110

11 39 40 41 42 This work

BaTiO3 nanofiber PZT-5A microfiber BiFeO3 nanofiber 0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 nanofiber (K,Na)NbO3−LiTaO3−LiSbO3 ceramic microfiber BTZ-0.5BCT nanofiber

40 260 2 50 ± 2 140 180

7 9 43 44 45 46

multidimensional data sets to evaluate the nanofiber piezoelectricity (Figure S14 in Supporting Information). Figures 3a−c show the topography, OP PFM phase, and the corresponding amplitude images of the PVDF/GO nanofiber, respectively, as well as Figures 3d−f for the pure PVDF nanofiber. The PVDF/GO nanofiber with an area of 50 nm × 50 nm was scanned with a harmonic Vac of 1.6 V applied to the cantilever tip (Figure 3a). The OP PFM phase image (Figure 3b) of the PVDF/GO nanofiber clearly displays both the negative (purple) and positive (yellow) values indicating antiparallel ferroelectric nanodomains with 180° domain walls. The OP PFM amplitude image (Figure 3c) shows a strong piezoelectric contrast because of the deflection caused by the applied AC field. On the other hand, the IP PFM signals are almost nonpolar (Figure S15 in Supporting Information), which implies that the polar-phase nanocrystals exhibit much higher piezoelectricity in the direction perpendicular to the fiber axis. It is evident that the polarization direction of the nanodomains is closely normal to the nanofiber and bottom electrode. Also, this observation is in good agreement with the β-phase orientation results obtained by TEM and XRD. On the contrary, though the pure PVDF nanofibers exhibit the smooth topography (Figure 3d), the phase image (Figure 3e) shows the presence of granular-type nanodomains with a random distribution of positive (up) and negative (down) piezoresponse vectors. The OP piezoelectric response (Figure 3f) of the pure PVDF nanofiber is weaker than that of the PVDF/GO nanofiber. The IP piezoelectric response also shows the same trend (Figure S16 in Supporting Information). Further, in order to get insight into the local hysteresis loops under the effect of an electric field, the ferroelectric switching and piezoelectric strain were evaluated by the PFM phase and amplitude loops, respectively (Figure 4). The PFM phase curve of the PVDF/GO nanofiber (Figure 4a) exhibits standard square-shaped 180° switching hysteresis loops as well as good repeatability. A sharp switching within these hysteresis loops produces no damage to the nanocrystal structures under an electric field, which is favorable for electronic device applications. The PVDF/GO nanofiber shows butterfly shaped amplitude versus Vdc loops (Figure 4a). From these loops it can be observed that the highest amplitude of 10 nm is obtained at Vdc = −30 V. The forward and reverse coercive fields are 18 and −14 V, respectively. The asymmetries in the amplitude and phase loops also point to the existence of defects since asymmetries are known to be associated with the internal field created by nonuniformly distributed charged defects.33 The stability over time of the signal was also demonstrated as shown in Figure S17 of Supporting Information. Owing to the carbon network of hexagonal rings of the GO lamellae, the interlayer charges and interfaces of the PVDF/GO composite has less effect on the efficiency of the piezoelectric response.18,34

Further, the hysteresis loops were measured at different places of the samples, so that the features presented in the following figures are representative of the behavior of the samples. In the case of the pure PVDF nanofiber (Figure 4b), the phase curve also shows standard 180° switching loops and the amplitude curve shows butterfly shaped loops. However, in this case, the maximum amplitude is 2.5 nm, which is obtained at Vdc = 30 V (Figure 4b). It is well-known that electrostatic and electrochemical factors also give rise to hysteretic effects in the PFM phase and amplitude. The above local PFM measurements were performed the SS-PFM method reported by Jesse et al.35 In this method, Vdc was applied in a sequence of pulses instead of sweeping continuously, and the phase and amplitude measurements were carried out in the “off state” of the pulses (Figure S14 in Supporting Information). Hence, the electrostatic effect was eliminated from the hysteresis exhibited by the nanofibers. The effective values of the piezoelectric constant (d33) were calculated by fitting the linear portion of the amplitude (A) curves as A = d33VdcQ, where Q is a proportionality factor that varies with the tip−sample contact resonance. The d33 of the PVDF/GO nanofibers is −94 ± 5 pm V−1, the absolute value of which is much higher than that for the pure PVDF nanofibers (−21 ± 2 pm V−1) as reported by Sencadas et al.36 Moreover, the energy-harvesting device based on the PVDF/GO nanofibers yields more than four times voltage output than that based on the pure PVDF nanofibers (Figure S18 in Supporting Information). It is worth noting that the thinner PVDF/GO and pure PVDF nanofibers both show higher d33 than the thicker ones (Figure 4c). This suggests that piezoelectricity is diameter-dependent because of polarization and strong coupling of the electromechanical strain gradient.37,38 This finding is consistent with the increase in the polar-phase content and orientation with a decrease in the dimension of the fibers as observed from the high resolution TEM analysis. The well-oriented GO lamellae cause the PVDF chains to swell and form an extended TTTT conformation around them. In the case of the PVDF/GO nanofibers, the highest |d33| is obtained when 1.0 wt % GO is added (Figure 4d). However, the |d33| decreases as the GO content increases beyond 1.0 wt %. This indicates that the GO lamellae are partially oriented along the fiber axis during the electrospinning process, thus decreasing the degree of the β-phase orientation. The |d33| value of the PVDF/GO nanofibers is found to be significantly greater than those of various PVDF-based nanostructures (i.e., film,39,40 nanoribbon,11 nanofibers41,42) and comparable to some ceramic micro/nanofibers7,9,43−46 (Table 1). In addition, the polarization reversal of the PVDF/GO nanofibers was investigated by applying a DC voltage to the cantilever tip to switch the polarization. Figure 5a shows that a selected region (550 nm × 550 nm) of the PVDF/GO nanofiber has almost the same polarization direction before 1906


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bias. Figure 5b shows the phase image of the same area after the polarization. The image clearly shows the six poling sites with the color contrast to the surrounding; accordingly, the linescanning results in Figure 5c precisely ascribe that the selected regions occurred about 180° switching after the DC polarization. Previously, it was reported that piezoelectric imaging could hardly be done on PVDF nanofibers even with the electroactive β phase.19 However, our results unambiguously suggest that domain switching is possible in even thin PVDF/ GO nanofibers. On the basis of the results obtained so far, we can state that an increase in the content of the β phase and its uniaxial orientation contribute to the excellent piezoelectricity of core− shell-structured PVDF/GO nanofibers. Accordingly, we built a three-dimensional molecular model for the PVDF/GO nanofiber to demonstrate the formation of the core−shell structure and β-phase crystallinity (Figure 6a,b). In the molecular model, the GO lamellae are aligned along the fiber axis and distribute near the surface of the nanofiber with the following effects: (1) The PVDF chains, which usually exist in the α phase, are polarized in alternating directions, resulting in a nonpolar behavior. However, here, the PVDF chains twist into a zigzag structure with CF2 groups in the upward direction and CH2 groups in the downward direction due to the reaction with the oxygen functionalities of the GO lamellae. (2) The mechanical stretching and the electric poling from the electrospinning process result in the extension of the twisted PVDF chains and their reorientation along the fiber axis. (3) A rapid solvent evaporation also contributes to the alignment and distribution of the GO lamellae. So when the PVDF nanocrystals nucleate and grow during the electrospinning process, the GO lamellae move to the surfaces of the nanofibers because of the internal radial orientation of the electrostatic field and the rapid

Figure 5. Polarization reversal of the PVDF/GO nanofibers. (a) PFM phase image of the PVDF/GO nanofiber before polarizing. The six hollow circles in blue represent the poling sites. The blue line represents the scanning path. (b) PFM phase image of the same nanofiber after polarizing, in which the nanodomains in the six poling sites are switched by 180°. (c) The phase curves along the scanning lines in (a) and (b) show that the 180° switching occurs in the three poling sites (L, M, R) by the DC polarization.

polarizing. The polarization reversal measurements were conducted by applying a high DC bias of −25 V for 5 s per site, followed by the scanning of the entire region with an AC

Figure 6. Structure modeling studies to demonstrate the excellent piezoelectricity of core−shell-structured PVDF/GO nanofibers. (a) Threedimensional molecular model for the PVDF/GO nanofiber showing the core−shell structure and the β-phase orientation. (b) The chemical interactions between the GO lamellae and the β-phase nanocrystals. (c) Survey spectra (left) of the PVDF/GO and pure PVDF nanofibers and C 1s spectrum (right) of the PVDF/GO nanofibers probed by XPS. The C 1s spectrum of the pure PVDF nanofibers is shown in Figure S19. (d) FTIR spectra of the PVDF/GO and pure PVDF nanofibers in the range 850−1450 cm−1. 1907


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portend a significant advance in the complexity and functionality of building blocks for nanoscience and nanotechnology.

evaporation of the solvents. Furthermore, Figure 6b shows the interactions which maintain the core−shell structure and βphase orientation. Electrostatic attraction/repulsion between the delocalized π-electrons in GO and the −CH2/−CF2 dipoles of PVDF anchors the PVDF chains to the GO lamellae. The OCO and CO groups in the GO lamellae interact with the −CF2 groups of PVDF leading to an increase in the electrostatic repulsion or Coulombic force.47 Strong interactions such as hydrogen bonds are formed between the CF2 groups of PVDF and the −COOH/−OH bonds in the GO lamellae. Thus, the PVDF chains are transformed into the TTTT conformation and are uniaxially oriented by the GO addition and electrospinning process. Moreover, the studies on oxygen functionalities were conducted to validate the molecular model. The oxygen functionalities were probed by XPS to confirm that the GO lamellae mainly distribute near the surface of the nanofibers. The GO content near the nanofiber surface (the probing depth was about 10 nm) is 8.3%, which is much higher than the original GO content (Figure 6c, left curves). This is consistent with the elemental mapping results of the core−shell structure visualized by TEM. In the C 1s XPS spectrum, the peaks of the carbon binding to the oxygen appear, including the peaks corresponding to C−O and CO groups (Figure 6c, right curves), revealing that the oxygen functionalities interact with the CH2 or CF2 groups of PVDF. This phenomenon suggests that the interlocking of the GO lamellae with the polymer matrix reinforces the specific core−shell structure. The chemical interactions of the core−shell structure were detected by analyzing the vibrational modes by FTIR (Figure 6d). The absorbance intensity of the C−H bending vibration band at 880 cm−1 is strengthened by the electrostatic attraction between the delocalized π-electrons in GO and the −CH2 dipoles of PVDF. We found that the intensity of the C−F stretching vibration band is strengthened and shifts by 14 cm−1 to the lower wavenumbers. Also, the peak at 1167 cm−1 corresponds to the stretching vibration band of −O− in the hydroxyl groups of the GO lamellae. The peak at 1230 cm−1 corresponding to the C− F stretching bond is strengthened, which indicates the presence of the wagging CF2 groups. Eventually, it should be noted that the enhancement in the piezoelectric properties is not proportional to the GO content. Thus, it can be concluded that when the GO content is increased beyond a certain limit, the GO lamellae provide a number of nucleating agents and that the GO lamellae become less oriented along the fiber axis, which indicates a lower degree of the β-phase orientation.

EXPERIMENTAL SECTION Preparation of Nanofibers. Semicrystalline PVDF powder with an average molecular weight of 534 000 g mol−1 was obtained from Sigma-Aldrich (St. Louis, MO, USA). N,N-Dimethylacetamide (DMAC) and acetone were purchased from Fisher Scientific. GO powder (purity: > 99 wt %, oxygen functionalities: 30−40%, average thickness: 0.55−1.2 nm, diameter: 0.5−3 μm, number of layers: 1−5) was purchased from Beijing Dk Nano technology Co., LTD (China). A mixture of DMAC and acetone (4:6 by volume/volume) was used as the solvent. GO lamellae, which were blended with the solvent (by weight/volume), were ultrasonically mixed and dispersed in the solvent to make solutions with GO mass ratios of 0.1, 0.5, 1.0, 2.0, and 4.0 wt %. Subsequently, PVDF (16 wt %) was dissolved in these solutions under stirring for 2 h at 60 °C. The electrospinning process was initiated by loading the PVDF/GO solution into a 1.0 mL plastic syringe tipped with a 25-gauge stainless steel needle. The positive lead from a high voltage supply was connected to the metallic needle at a bias value of 30 kV. The solution was injected into the needle at an injection rate of 0.2 mL h−1 with a syringe pump. The distance between the needle and the collector was 12 cm. Morphology and Structure Characteristics. The morphology of the electrospun PVDF/GO nanofibers was observed using SEM (FEI Quanta 450, Netherlands). All the nanofiber samples were goldcoated by sputter coating prior to the SEM observations. The orientation of the β-phase nanocrystals in the PVDF/GO nanofibers was visualized by TEM (JEOL JEM-2010F FasTEM, Japan) at an acceleration voltage of 140 kV. The TEM samples were prepared by a one-step process, in which a few electrospun nanofibers were directly dropped on a porous carbon membrane supported by the copper grid. Fast Fourier transform (FFT) patterns of the TEM images were analyzed to examine the crystallite structure. Crystallinity Characteristics. The crystallite structures were examined by XRD (D/max2550HB+/PC diffractometer, Rigaku, Japan) with Cu (40 kV, 200 mA) Kα radiation. The nanofiber and membrane samples were scanned in the 2θ range of 10−40° with a scan rate of 2° min−1. The FTIR spectra of the samples were collected in the attenuated total reflectance (ATR) mode using a spectrophotometer (Bruker 13006875, Germany). The samples were placed on top of the ATR set and scanned from 1600 to 400 cm−1. A total of six scans were collected for signal averaging. The oxygen functionalities and C 1s spectra were probed by XPS (250XI, Thermo Scientific, England). Piezoelectric Response Measurements. Single nanofibers were produced by the electrospinning jet and directly loaded on to the Pt/ Ti coated silicon substrate (the bottom electrode) because of its high surface energy. We analyzed the vector (including OP and IP) piezoresponse images and local hysteresis loops in both the PVDF/ GO and pure PVDF nanofibers using the dual AC resonance tracking (DART) mode of PFM (Cypher S, Asylum Research). In this study, a train of DC voltage pulses with a constant duration of 25 ms was applied to the tip. The amplitude of the pulses varied stepwise from −30 to 0 V and then from 0 to 30 V (Figure S14) and back to −30 V. The nanodomain switching properties of the nanofibers were then resolved in real space with a nanometric resolution by SS-PFM. Au/Cr coated tips were used during the measurement (spring constant = 0.11 N m−1, free air resonance frequency = 35 kHz). An AC excitation (Vac = 1.6 V) riding on the DC bias voltage (Vdc) was applied between the tip and the bottom electrode. Electrical Output Measurement of Nanofiber-Based Generators. The generator device consists of a nanofiber mat (area: 4 cm2, thickness: 200 μm) and two aluminum foil electrodes. The nanofiber mat was directly collected on the electrode substrate. The metal electrodes were attached separately to the two sides of the nanofiber mat. Then the sandwich structure was placed between two pieces of glass coverslip. As a control, the pure PVDF nanofiber-based

CONCLUSIONS In summary, we successfully presented the core−shellstructured PVDF/GO nanofibers with over 4-fold enhanced piezoelectric properties. The atomic scale visualization of the nanofibers by high resolution TEM reveals that the 88.5% βphase nanocrystals incline to self-orientate along the fiber axis. The maximum piezoelectric constant (d33) of the PVDF/GO nanofibers is −110 pm V−1 and the average d33 is −93.75 pm V−1 (at 1.0 wt % GO lamellae). The enhancement of the piezoelectric properties of the PVDF/GO nanofibers is consistent with the increase in the content of the β-phase nanocrystals and their orientation, which is contributed by interactions with the GO lamellae and the subsequent mechanical and electrical treatments. The design methods and molecular model proposed in this study can be applied to other nanometer-scaled piezoelectric polymer composites and 1908


Article

ACS Nano

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generator was also fabricated. A nanovoltmeter (2182A, Keithley) was used to measure the open-circuit voltage output of the generator devices, respectively. The Labview software with a program was used to monitor and record the measurement. A DC linear motor was used to provide strain for the measurement.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b07961. Equations, more details on atomic structure, crystalline structure, and IP piezoelectric responses (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Jing Ma: 0000-0003-0103-9858 Xiaohong Wang: 0000-0003-4569-7537 Author Contributions

X.L. and X.H.W. conceived the idea of core−shell-structured PVDF/GO nanofibers. Some of our previous work was started in L.L.’s lab. X.L. and X.H.W. designed the detailed experiments and characterization methods of TEM, SEM, XRD, FTIR, PFM, and XPS. X.L. and X.H.W. conducted the fabrication and characterization of the materials. X.M.W. contributed to the PFM measurements. X.H.W. proposed the structure modeling of the nanofibers. X.L. and X.H.W built a three-dimensional molecular model of the PVDF/GO nanofibers. X.L. and X.H.W. prepared the manuscript and commented on it. J.M. supervised the PFM analysis. X.L., J.M., L.L., and X.H.W. discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the grants from the National Natural Science Foundation of China (No. 61474071, 61531166006) and National Basic Research Program (973 Program, No. 2015CB352106). The authors would like to thank Huihua Zhou from Beijing National Center for Electron Microscopy for her assistance and fruitful discussions on TEM sample preparation and measurements. The authors acknowledge Prof. Xiaoqing Xi from Tsinghua University for providing the PFM measuring platform. The authors also acknowledge Weiqi Wang for providing the PFM cantilevers customized in The State Key Laboratory of Tribology (SKLT) at Tsinghua University. REFERENCES (1) Persano, L.; Dagdeviren, C.; Su, Y.; Zhang, Y.; Girardo, S.; Pisignano, D.; Huang, Y.; Rogers, J. A. High Performance Piezoelectric Devices Based on Aligned Arrays of Nanofibers of Poly(Vinylidenefluoride-co-Trifluoroethylene). Nat. Commun. 2013, 4, 1633. (2) Yang, R.; Qin, Y.; Dai, L.; Wang, Z. L. Power Generation with Laterally Packaged Piezoelectric Fine Wires. Nat. Nanotechnol. 2009, 4, 34−39. (3) Staples, M.; Daniel, K.; Cima, M. J.; Langer, R. Application of Micro- and Nano-Electromechanical Devices to Drug Delivery. Pharm. Res. 2006, 23, 847−863. 1909


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