Bio-Inspired Fluoro-polydopamine Meets Barium Titanate Nanowires

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Bio-Inspired Fluoro-polydopamine Meets Barium Titanate Nanowires: A Perfect Combination to Enhance Energy Storage Capability of Polymer Nanocomposites Guanyao Wang, Xingyi Huang,* and Pingkai Jiang Department of Polymer Science and Engineering, Shanghai Key Laboratory of Electrical Insulation and Thermal Aging, Shanghai Jiao Tong University, Shanghai 200240, China S Supporting Information *

ABSTRACT: Rapid evolution of energy storage devices expedites the development of high-energy-density materials with excellent flexibility and easy processing. The search for such materials has triggered the development of high-dielectricconstant (high-k) polymer nanocomposites. However, the enhancement of k usually suffers from sharp reduction of breakdown strength, which is detrimental to substantial increase of energy storage capability. Herein, the combination of bio-inspired fluoro-polydopamine functionalized BaTiO3 nanowires (NWs) and a fluoropolymer matrix offers a new thought to prepare polymer nanocomposites. The elaborate functionalization of BaTiO3 NWs with fluoro-polydopamine has guaranteed both the increase of k and the maintenance of breakdown strength, resulting in significantly enhanced energy storage capability. The nanocomposite with 5 vol % functionalized BaTiO3 NWs discharges an ultrahigh energy density of 12.87 J cm−3 at a relatively low electric field of 480 MV m−1, more than three and a half times that of biaxial-oriented polypropylene (BOPP, 3.56 J cm−3 at 600 MV m−1). This superior energy storage capability seems to rival or exceed some reported advanced nanoceramics-based materials at 500 MV m−1. This new strategy permits insights into the construction of polymer nanocomposites with high energy storage capability. KEYWORDS: bio-inspired, nanowire, nanocomposite, dielectric constant, energy storage

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INTRODUCTION In recent years, numerous efforts have been devoted to fabricating energy storage devices to meet the ever-increasing requirements of modern compact electronic devices and electric power systems.1−3 Nevertheless, as the best commercially available film capacitors to date, biaxial-oriented polypropylene (BOPP) possess rather low electric energy storage capability because of their low dielectric constant (usually < 3). The conventional inorganic ceramic materials, such as BaTiO3, TiO2, and (Pb,La)(Zr,Ti)O3, possess a high dielectric constant (high k) but rather low breakdown strength and weak processability.4−11 Therefore, dielectric polymer nanocomposites, consisting of high-k ceramics and an easily processed polymer matrix, have attracted tremendous interest for their potential applications due to their intrinsic charge− discharge capability to store and release the electrical energy through dielectric polarization and depolarization.4−9,12−19 Moreover, the flexible dielectric films are also attaining growing attention for their potential utilization as novel gate dielectrics in transistors due to their superior processability to those conventional ones.20−24 Among various kinds of polymers, PVDF and its copolymer are subjected to keen interest due to their relative high k and breakdown strength in comparison with most common polymers.4−7 Previous literatures have © 2017 American Chemical Society

demonstrated that the one-dimensional (1D) high-k nanoceramics with large aspect ratio are preferred to enhance energy density at low concentration in polymer nanocomposites compared with their particle counterparts.8,25−32 Meanwhile, the 1D nanoceramics possess a smaller specific surface, which might facilitate the reduction of surface energy and alleviate the agglomeration of the nanofillers in the polymer matrixes.13,33−36 However, the permittivity increase usually comes at the expense of breakdown strength, which is detrimental to substantial increase of energy storage capability. Therefore, rational incorporation of high-k ceramics into a polymer matrix might be crucial for the energy storage improvement to address this issue. Surface modification with polymeric coatings affords favorable control over the interfacial properties of nanomaterials irrespective of their intrinsic properties, since the coatings can be designed to diminish undesirable interactions, or enhance desired interactions, and often do both simultaneously, to the host environment.37−39 Over the past decades, a variety of strategies have been employed in surface modification by Received: November 11, 2016 Accepted: February 2, 2017 Published: February 2, 2017 7547

DOI: 10.1021/acsami.6b14454 ACS Appl. Mater. Interfaces 2017, 9, 7547−7555

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ethyl acetate solution was further washed with brine for several times. Then, the solution was dried over anhydrous MgSO4 for several hours and filtered. After evaporation, the raw product was dissolved in a small amount of hot ethyl acetate and crystallized from petroleum ether (60−90 °C). Yield: 8.17 g, colorless solid (63.52%). Surface Modification of Nanowires. The BaTiO3 NWs were synthesized by employing the hydrothermal method described in our previous work.56 Then, the dopamine derivative f-DOPA was utilized to modify the surface of the pristine BaTiO3 NWs. In a typical process, 3g of BaTiO3 NWs was dispersed in 80 mL of Tris-HCl buffered solution (pH = 8.5) and ultrasonicated for 1 h. Meanwhile, 4 mmol of f-DOPA was dissolved in 40 mL of 2-propanol. Then, the 2-propanol solution of f-DOPA was added dropwise into the aforementioned aqueous solution of BaTiO3 NWs under stirring at 60 °C. After stirring for another 48 h, the color of the mixture turned black, as the adherent polydopamine derivative shell layers spontaneously deposited upon the nanowires. Then, the modified nanowires were washed with deionized water and water for several times until the supernatant was nearly colorless. These surface modified nanowires were denoted as fDOPA@BaTiO3. Fabrication of P(VDF-HFP)-Based Nanocomposite Films. The preparation of P(VDF-HFP)-based nanocomposite films is described as follows: After grinding thoroughly, those functionalized BaTiO3 nanowires were homogeneously dispersed in DMF by ultrasonication for 1 h. After the addition of a given amount of P(VDF-HFP), the mixture was then stirred vigorously for another 24 h. The mixture was cast into films on a glass plate with a doctor blade, and then heated at 40 °C to accelerate the evaporation of DMF. After being dried in vacuum at 40 °C for 12 h to remove the remaining trace solvent, the cast films were heated at 200 °C for several minutes and then quenched in ice water immediately. The quenched films were peeled from the glass substrates and dried at 40 °C for another 12 h. The typical thickness of these nanocomposite films is about 15 μm. Nanocomposites with different volume fractions (2.5, 5, 10, and 15%) of f-DOPA modified BaTiO3 NWs were prepared. Characterization. The 1H, 13C, and 19F nuclear magnetic resonance (NMR) spectra were recorded on an AVANCE III HD 400 spectrometer (Bruker, USA). The scanning electron microscopy (SEM, Nova NanoSEM 450, FEI, USA) and transmission electron microscopy (TEM, JEM-2010, JEOL, Japan) were carried out to characterize the morphology of the nanowires and nanocomposite samples. The cross-sectional SEM images of nanocomposite films were prepared by fracturing the films in liquid nitrogen. The TEM samples were prepared by dropping the sample solution on carbon-coated copper grids and air-dried before measurement. X-ray photoelectron spectra (XPS) were performed using an Axis UltraDLD spectrometer (Shimadzu-Kratos Analytical, UK) with a monochromated Al Kα source. The Fourier-transform infrared spectroscopy (FT-IR) of the nanowires before and after modification was performed on a PerkinElmer Paragon 1000 spectrometer with the range of 4000− 400 cm−1. Thermogravimetric analysis (TGA) was conducted using a NETZSCH TG209 F3 with a 20 °C min−1 heating rate in a nitrogen flow (20 mL min−1). Copper electrodes with a diameter of 12 mm were sputtered on both sides of the nanocomposite films for the measurements of dielectric properties. The dielectric spectra of the samples were acquired by using a Novocontrol Alpha-A high resolution dielectric analyzer (GmbH Concept 40) from 10−1 to 107 Hz at room temperature and various temperatures (−50 to 150 °C). The breakdown strength was measured with a dielectric strength tester under a DC ramping rate of 500 V s−1 (Shanghai Juter High Voltage Electrical & Equipment Co., Ltd., China). The thicknesses of the samples for breakdown strength tests were around 15 μm. Electric displacement−electric field (D−E) loops and current−voltage (I−V) spectra were collected using a Precision Multiferroic Materials Analyzer equipped with Precision 10 kV HVI-SC and Trek MODEL 609B (Radiant Inc.).

chemical bond formation or physisorption, such as layer-bylayer assembly,40 and grafting from or grafting to methods to generate covalently adherent polymer chains.41−48 Nevertheless, these conventional methods are either materialdependent or cumbersome. As a mussel-inspired protein, polydopamine (PDA) has attracted growing attention due to its astonishing adhesion offered by catechol compounds via simple chemistry.49−52 The robust grafting of PDA on various substrates has enlightened increasing exploitation for its utilization in surface coatings.53 Recently, PDA has been employed to adjust the compatibility between nanoceramics and a ferroelectric polymer matrix in the fabrication of dielectric nanocomposites.31,54−56 However, compared with other exploited catechols anchoring versatile end groups or polymer chains, the direct introduction of PDA upon the modification of nanoceramics is still at the primary stage. Herein, in order to unlock the full potential of dopamine modification and thus improve the inclusion of nanoceramics into the polymer matrix, a fluoro-dopamine derivate is utilized to generate corresponding thin layers upon BaTiO3 nanowires (NWs). The fluoro-polydopamine shell layers were designed to possess the following features: (1) a surface layer preventing nanofillers from agglomerating in the polymer matrix more effectively than simple dopamine functionalization due to the better affinity with the ferroelectric polymer matrix; (2) a surface layer confining the movement of the charge carriers in the interface between polymer and nanofillers, which might lead to lower dielectric loss and smaller leakage current densities in comparison with the nanocomposites consisting of raw nanowires and a polymer matrix. By anchoring an elaborately designed long fluoro-chain upon the dopamine, the thus-fabricated fluoro-polydopamine pave the way for preferable inclusion of BaTiO3 NWs into the fluoro-polymer matrix. The nanocomposite films with 2.5 and 5 vol % proposed nanowires possess the ultrahigh discharged energy densities of 12.68 and 12.87 J cm−3, respectively, far more than that of commercial biaxial-oriented polypropylene (BOPP, 3.56 J cm−3 at 600 MV m−1).57 Moreover, the nanocomposite with 15 vol % loading also discharges a gratifying energy density of 8.28 J cm−3 at a rather low electric field of 340 MV m−1, which is significantly enhanced compared to that of neat polymer (4.38 J cm−3). Compared with those traditional intricate methods, our straightforward and modularized approach turns out to be adjustable to meet different demands for energyrelated polymer nanocomposites.

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EXPERIMENTAL SECTION

Materials. Poly(vinylidene fluoride-co-hexafluoropylene) (P(VDFHFP)) with 15% HFP was kindly supplied by Solvay Plastics (Shanghai, China). Titanium dioxide nanopowder (TiO2, P25, ≥99.5%) and 1H,1H,2H,2H-perfluoro-1-decanol (TCI, 96%) were provided by Sigma-Aldrich and Tansoole (China), respectively. 3,4Dihydroxy-L-phenylalanine (L-DOPA, 99%) and p-toluenesulfonic acid monohydrate (98%) were purchased from Aladdin (China). Other reagents were supplied by Sinopharm Chemical Reagent Co., Ltd. (China) and Tansoole (China). Synthesis of 1H,1H,2H,2H-Heptadecafluorodecyl 2-Amino3-(3,4-dihydroxyphenyl)propanoate (f-DOPA). A suspension of L-DOPA (3.94 g, 20 mmol), 1H,1H,2H,2H-perfluoro-1-decanol (9.28 g, 20 mmol), and p-toluenesulfonic acid monohydrate (3.80 g, 20 mmol) in toluene (100 mL) was refluxed under a N2 atmosphere for 48 h, using a Dean−Stark trap to azeotropically remove water. After cooling to room temperature, the toluene was evaporated under vacuum. The gel-like solid residue was washed with saturated NaHCO3 aqueous solution and extracted with ethyl acetate. The 7548

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Figure 1. Schematic illustration of the preparation process for f-DOPA@BaTiO3 NWs. Inset is a photograph of a mussel.

Figure 2. (a, b) TEM images of f-DOPA@BaTiO3 NWs. (c) XRD patterns of BaTiO3 NWs before and after surface modification. Cross-sectional SEM images of P(VDF-HFP)-based nanocomposite films with (d) BaTiO3 NWs and (e) f-DOPA@BaTiO3 NWs volume fraction of 15%.

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RESULTS AND DISCUSSION

the two methylene groups from 1H,1H,2H,2H-perfluoro-1decanol. Besides, the peaks at 4.29 ppm in the 1H NMR spectrum of f-DOPA could be ascribed to the methylene group on the dopamine moiety that originally lay in the range of 2.5 and 3.0 ppm in comparison with 1H NMR spectrum of LDOPA (Figure S1, Supporting Information). The 13C and 19F

Figure 1 shows a schematic illustration of the preparation of fDOPA@BaTiO3 NWs. The successful synthesis of f-DOPA was verified by NMR spectra (Figures S1−S6, Supporting Information). The multiplet peaks between 2.50 and 2.75 ppm in Figure S3 (Supporting Information) can be assigned to 7549

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Figure 3. (a) Dielectric constant and dielectric loss (tan δ) as a function of frequency at room temperature for nanocomposites with different contents of f-DOPA@BaTiO3 NWs and 15 vol % raw BaTiO3 NWs. Dielectric constant and dielectric loss (tan δ) as a function of temperature for nanocomposites with (b) 2.5 vol %, (c) 5 vol %, (d) 10 vol %, (e) 15 vol % f-DOPA@BaTiO3 NWs, and (f) 15 vol % raw BaTiO3 NWs.

sharp peaks that lay in the range of 1500−1000 cm−1 in FT-IR spectra for the modified nanowires (Figure S9, Supporting Information), further confirming the incorporation of f-DOPA upon BaTiO3 NWs. The composition and thermal stability for the BaTiO3 NWs before and after surface modification were investigated by TGA. As shown in Figure S10 (Supporting Information), after washing with water and ethanol, the deposition of fluoro-polydopamine layers was 8.93 wt %. However, the weight ratio of fluoro-polydopamine layers washed with water only came to 27.88 wt % (Figure S10), far more than the aforementioned. The extra loss between 100 and 200 °C from the DTG curve might be assigned to the existence of a discrete monomer of f-DOPA, which could be washed thoroughly by ethanol. The assumption was further validated by TEM images of f-DOPA@BaTiO3 NWs washed with water only (Figure S11, Supporting Information), in which ∼18 nm of fluoro-polydopamine layers could be observed. P(VDF-HFP)-based nanocomposite films with given contents (2.5, 5, 10, and 15 vol %) of f-DOPA@BaTiO3 NWs and 15 vol % bare BaTiO3 NWs were prepared by a solution blending method. As shown in the freeze-fractured crosssectional SEM images (Figure 2d,e), the comparison between nanocomposites with 15 vol % BaTiO3 NWs before and after

NMR spectra of f-DOPA gave a more conclusive evidence for the existence of fluorine atoms (Figures S5 and S6, Supporting Information).58 These above-mentioned results demonstrate that the long fluoro-chain was successfully anchored upon LDOPA by esterification. The morphology of bare and modified BaTiO3 NWs was characterized by SEM and TEM. From Figure 2a,b, the observed thin amorphous layers with ∼10 nm thickness confirmed the successful spontaneous self-polymerization of fDOPA upon the surface of BaTiO3 NWs. The composition of fDOPA@BaTiO3 NWs has been identified by the appearance of Ba, Ti, O, C, N, and F in energy-dispersive X-ray (EDX) elemental mapping images (Figure S7b, Supporting Information). The phase structure of BaTiO3 NWs remained unchanged after modification from XRD patterns (Figure 2c). XPS and FT-IR are employed to further demonstrate the successful deposition of the fluoro-polydopamine shell layer upon the raw BaTiO3 NWs. The presence of the N 1s peak at 400 eV and F 1s peak at 689 eV in the XPS spectra shown in Figure S8 (Supporting Information) gave a solid evidence of the nitrogen-containing dopamine moiety and fluorinated end chain for f-DOPA, respectively. Moreover, the specific bending vibrations of methylene C-H might account for the multiple 7550

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ACS Applied Materials & Interfaces modification demonstrated that the obvious agglomeration of the former nanowires was significantly restrained by adopting the latter ones. The fluoro-polydopamine coated nanowires possessed a homogeneous distribution in the polymer matrix, suggesting their excellent compatibility with the host matrix. The enhanced dielectric constant and restrained dielectric loss of the proposed nanocomposites in comparison with the neat polymer are shown in Figure 3a. The nanocomposite films possess a gradual increase of dielectric constant with the increased loading of nanowires, due to the higher dielectric constant of BaTiO3 NWs than that of neat polymer. At the same loading, the elaborately modified f-DOPA@BaTiO3 NWs show inferior elevation of dielectric constant compared with raw BaTiO3 NWs, resulting from the robust shell to hamper the role of inorganic BaTiO3 inside the host matrix, which is consistent with the mixing model for the dielectric constant. Due to the interfacial polarization, the nanocomposites show a slight increase of dielectric loss over the polymer matrix at low frequencies.59−61 By contrastively evaluating with the nanocomposites with 15 vol % raw BaTiO3 NWs as the reference, the dielectric loss of the fabricated nanocomposite is drastically suppressed, especially at lower frequencies (Figure 3a). This is owing to the fact that, by introducing the fluoro-polydopamine shell on the BaTiO3 NWs, the interfacial polarization and leakage current density discussed below are effectively curbed. Notably, lower dielectric loss of the nanocomposites at high frequencies was observed compared with the neat polymer because of restriction of the dipolar polarization of the P(VDFHFP) macromolecular chains. Overall, the dielectric loss of the proposed nanocomposites shows little variation in comparison to the pristine P(VDF-HFP), further indicating the well dispersion of f-DOPA@BaTiO3 NWs in the polymer matrix. To further study these properties of the proposed nanocomposites, temperature-dependent dielectric spectroscopy was employed. The intuitional variation tendencies of the dielectric constant and dielectric loss are shown Figure 3b−f. The elaborately selected f-DOPA@BaTiO3 NWs show inferior elevation of dielectric constant to raw BaTiO3 NWs (Figure 3e,f), due to the hindering effect of the choreographed fluoropolydopamine shell to high-k BaTiO3 NWs inside the host matrix. The major peak at −30 °C in the dielectric loss spectra at 100 Hz, aroused from the glass-transition (Tg) of the neat polymer,62 gradually shifts to higher temperature and becomes broader along with increasing frequency. Moreover, another peak in the dielectric constant plot located at 80 °C (100 Hz) could be assigned to the ferroelectric−paraelectric (FE-PE) Curie transition,63 resulting in the reduction of dielectric constant at higher temperature. The energy storage capability of the dielectric materials can be evaluated from D−E loops by a modified Sawyer−Tower circuit, in which D is the electric displacement, and E the applied electric field. Typical D−E loops presented in Figure 4a and Figure S12 (Supporting Information) indicate clearly that the relatively higher dielectric constant of f-DOPA@BaTiO3 NWs in comparison with neat polymer gives rise to the increased electric displacement. Meanwhile, the remnant polarizations also confront a slight increase with the loading of the nanowires, due to the higher remnant polarization of BaTiO3 than that of pure polymer. Generally, high remnant polarization would lead to the reduction of the discharged energy density because of the decreased integrated area of D−E loops. Thus, as depicted evidently in the Figure 4a and Figure S12, the nanocomposites with lower loadings of modified

Figure 4. (a) D−E loops under unipolar electric fields of 100 Hz for nanocomposites with different contents of f-DOPA@BaTiO3 NWs and 15 vol % raw BaTiO3 NWs. (b) Discharged energy densities of nanocomposites with different contents of f-DOPA@BaTiO3 NWs and 15 vol % raw BaTiO3 NWs under varied applied fields. (c) Discharged energy densities of 5 vol % f-DOPA@BaTiO3 NWs/ P(VDF-HFP) and other dielectric materials reported in previous literatures in the range of 400 to 500 MV m−1 (sandwich film with 2 vol % BaTiO3 NFs as center layer and 10 vol % BaTiO3 NPs as out layers;25 3 vol % BaTiO3 NPs/PVDF and 3 vol % TiO2 NFs/PVDF;35 3 vol % BaTiO3 NPs/P(VDF-HFP) and 3 vol % TiO2 NFs/P(VDFHFP);36 metl-stretched P(VDF-TrFE-CFE);64 3 vol % Ba0.6Sr0.4TiO3 NFs/PVDF;65 P(VDF-BTFE) (PVB2 and PVB0.5)66).

nanowires, as well as the pristine polymer, possess much slimmer D−E hysteresis loops in comparison with the nanocomposite with bare BaTiO3 NWs, denoting their relatively high energy efficiencies discussed below. The energy densities (Ue) of the proposed nanocomposites derived from the D−E loops and equation Ue = ∫ EdD are summarized in Figure S13a (Supporting Information) and Figure 4b. The total stored energy densities of the proposed 7551

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5a, after the slightly increased Eb with 2.5 vol % f-DOPA@ BaTiO3 NWs, a further increase of the loading would give rise

nanocomposites, as well as the discharged energy densities, exhibit a gradually increasing trend along with the increasing loading of f-DOPA@BaTiO3 NWs and applied electric fields. For instance, the total stored energy densities of the nanocomposite films with 2.5 and 5 vol % f-DOPA@BaTiO3 NWs are 21.35 and 22.58 J cm−3, respectively, much higher than that of pure polymer (13.75 J cm−3). However, a further increase of the loading results in the decreased energy densities (20.33 and 16.52 J cm−3 for the nanocomposites with 10 and 15 vol % nanowires, respectively), which is the consequence of reduced breakdown strength discussed below. Similar tendency could be found in the discharged energy densities for those proposed nanocomposites. Due to the high remnant polarization and low breakdown strength discussed below, the 15 vol % raw BaTiO3 NWs/P(VDF-HFP) nanocomposite possesses much inferior energy storage capability to those nanocomposites with modified nanowires, further affirming the importance of elaborate functionalization of the nanofillers. Figure 4c and Figure S14 (Supporting Information) summarize the discharged energy densities of some dielectric materials based on PVDF or its copolymers reported in previous literatures.25,35,36,64−66 The discharged energy density of the proposed nanocomposite with 5 vol % f-DOPA@BaTiO3 NWs at 480 MV m−1 seems to rival or exceed those of previous fabricated dielectric materials (mostly nanoceramics-based) in the range of 400−500 MV m−1. In order to demonstrate the splendid energy storage capabilities of these nanocomposites, the discharged energy densities of these nanocomposites against commercial BOPP at their respective breakdown strength are shown in Figure S15 (Supporting Information). The nanocomposite films with 2.5 vol % f-DOPA@BaTiO3 NWs possess the discharged energy density as high as 12.68 J cm−3 at 550 MV m−1, more than three and a half times that of BOPP (3.56 J cm−3 at 600 MV m−1).57 More notably, the nanocomposite with 5 vol % nanowires also discharges a striking ultrahigh energy density of 12.87 J cm−3 at 480 MV m−1. Besides, at a rather low electric field of 340 MV m−1, a quite high energy density of 8.28 J cm−3 could be achieved with the loading of 15 vol % nanowires, which is significantly enhanced compared to that of neat polymer (4.38 J cm−3 at 340 MV m−1). These amazing results indicate the remarkable potential as energy storage materials at both low and high electric fields by adjusting the loading of modified BaTiO3 NWs. Figure S13b (Supporting Information) shows the progressively decreased energy efficiencies with the increasing nanowire loadings. The energy efficiencies of the nanocomposites with lower loading of modified nanowires seem to rival or exceed that of neat polymer at lower electric fields, further affirming the maintenance effect of the elaborate modification upon the BaTiO3 NWs toward the high energy efficiencies. Even at the highest loading, the nanocomposites with modified nanowires could still bear the efficiencies of ca. 50%, much higher than those of the nanocomposite with bare nanowires. The much improved efficiencies can also be attributed to the synergistic effect of the significantly reduced leakage current densities discussed below. However, the highfield energy loss would probably hinder the potential application of thus-fabricated nanocomposites. Further work needs to be carried out to address this issue. Electric breakdown strength (Eb) is also of tremendous importance for the practical applications of dielectric materials. As seen from Eb of these proposed nanocomposites in Figure

Figure 5. (a) Breakdown strength of nanocomposites with different contents of f-DOPA@BaTiO3 NWs and raw BaTiO3 NWs. (b) Leakage current density of nanocomposites with different contents of f-DOPA@BaTiO3 NWs and 15 vol % raw BaTiO3 NWs at varied applied electric field. (c) Electrical resistivity of nanocomposites with different contents of f-DOPA@BaTiO3 NWs and 15 vol % raw BaTiO3 NWs under an electric field of 100 MV m−1.

to the decrease of Eb due to the introduction of more defects and voids. It is encouraging that these fabricated nanocomposites possess higher breakdown strength than those nanocomposites with the raw nanowires at the same loading, which could be unambiguously assigned to the much improved dispersion of modified nanowires in the polymer matrix. The outstanding electric breakdown strength of these nanocomposite films might be understood from the orientation of high-aspect-ratio nanowires, which facilitates the formation of anisotropic polymer nanocomposite dielectrics, resulting in the anisotropy in the susceptibility of the proposed nanocomposites under the applied electric field and a rather low 7552

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concentration of electric field in the polymer matrix.25,29−31,54,55 Furthermore, the incorporation of fluoropolydopamine shell layers favors significantly improved dispersion of nanowires and thus improves the affinity between the polymer matrix and the nanowires, leading to the regions where the polymer chains were bound to the nanowires became more robust. Namely, the effective surface modification by fDOPA plays a vital role in achieving the high breakdown strength for those polymer nanocomposites. In order to further validate the influence of the leakage current toward these aforementioned properties, the current− voltage (I−V) curves of neat polymer and the presented nanocomposites are shown in Figure 5b. The restrained leakage current densities of these nanocomposites with lower loading of nanowires rationalize the aforementioned superior Eb and energy storage efficiencies. The leakage current densities of these nanocomposites follow a reasonable rising trend upon the increasing loading, due to defects and voids accompanied by the incorporation of nanowires. The higher leakage current densities of the nancomposite with raw nanowires, resulting from poor dispersion of unmodified nanowires in the polymer matrix, might account for its aforementioned high dielectric loss, low breakdown strength, and low energy storage capability. Furthermore, the electrical resistivity shown in Figure 5c indicates their excellent insulating property qualified for the practical applications.

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b14454. NMR spectra of 1H,1H,2H,2H-perfluoro-1-decanol, LDOPA, h-DOPA, and f-DOPA, SEM images, XPS spectra, FT-IR spectra, and TGA spectra of BaTiO3 NWs before and after surface modification, TEM images of f-DOPA@BaTiO3 NWs washed with water, D−E loops, total stored energy densities, and charge− discharge efficiencies of P(VDF-HFP)-based nanocomposites with different contents of f-DOPA@BaTiO3 NWs and 15 vol % raw BaTiO3 NWs under varied electric fields (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Guanyao Wang: 0000-0002-7151-7782 Author Contributions

X.H. supervised the work. G.W. prepared the samples, performed the measurements, and wrote the manuscript. All authors have given approval to the final version of the manuscript.

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CONCLUSIONS In conclusion, bio-inspired fluoro-polydopamine functionalized BaTiO3 NWs were incorporated into a fluoropolymer to realize flexible polymer nanocomposites with high energy storage capability. The rationally designed fluoro-polydopamine shell layers could not only faciliate the confinement of the movement of charge carriers in the interface between polymer and nanowires but also prevent the nanowires from agglomerating in the polymer matrix, resulting in lower dielectric loss and smaller leakage current densities in comparison with the nanocomposite on the basis of raw nanowires. At a relatively low electric field of 480 MV m−1, the nanocomposite with 5 vol % f-DOPA@BaTiO3 NWs gives the ultrahigh discharged energy densities of 12.87 J cm−3, more than three and a half times that of BOPP (3.56 J cm−3 at 600 MV m−1). These splendid energy storage capabilities of our proposed nanocomposites seem to rival or exceed those of some reported advanced nanoceramics-based materials in the range of 400−500 MV m−1. The elaborate functionalization of BaTiO3 NWs by bio-inspired fluoro-polydopamine has guaranteed both the increase of k and the maintenance of breakdown strength, which is crucial for the substantial increase of energy storage capability. Further investigations need to be focused on the reduction of high-field energy loss since this limitation would restrict the practical application of our proposed nanocomposites. Compared with the conventional tedious methods, our approach is straightforward and versatile. More importantly, the strategy is modularized, in which the anchoring end group or chain is adjustable to meet different demands. Therefore, this work provides an attractive paradigm along with a unique strategy for polymer nanocomposites with high energy storage capability. Moreover, the proposed flexible dielectric nanocomposites would also find their potential application prospects in electrical devices such as gate dielectrics in transistors.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Special Fund of National Basic Research Program of China (Grant no. 2014CB239503), the NSF of China (Grant nos. 51522703, 51277117, 51477096). X.H. thanks the Shanghai Pujiang Program (Grant No. PJ14D018) and the 2013 SMC Excellent Young Faculty Award of Shanghai Jiao Tong University. The authors also thank the researcher from Instrument Analysis Center of Shanghai Jiao Tong University for XPS measurements and material analysis.

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REFERENCES

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