Introduction

Nowadays with the rapid development of electronic products toward miniaturization, as well as intelligent and wearable devices, a lot of higher requirements are being drawn to the electronic components. Dielectric polymer-based composites have attracted much attention in recent years due to excellent flexibility and plasticity, as well as rapid charge/discharge efficiency, stable chemical properties and high energy storage density [1,2,3]. The key to the performance of dielectric polymer-based composites is the energy storage density, which can be calculated by the formula \(U = \frac{1}{2}\varepsilon_{r} \varepsilon_{0} E^{2}\) (linear dielectrics) and \(U = \int\limits_{0}^{Dm} {EdD}\) (nonlinear dielectrics). It is closely related to the parameter of breakdown strength, dielectric loss and dielectric constant [4,5]. One efficient approach to fabricate the composites is to introduce a high dielectric filler into a piezoelectric polymer, but it is hard to adjust all the parameters of the composite to optimal values [6]. Therefore, how to ensure the composites stays at a low loss, and then exhibit a good breakdown strength and large dielectric constant is a really big challenge.

PVDF (polyvinylidene fluoride) is a common dielectric substrate material with an electrical dipole moment of monomer unit (5–8 × 10−30 cm) which varies with the crystal phase of PVDF [7]. Generally speaking, the β and γ phase usually exhibits superior polarities as it has a more regular polar orientation alignment relative to the α phases, which makes PVDF a good candidate for dielectric materials [8]. The polarity phase of PVDF can be obtained by some processing methods, such as adding some polar fillers (TiO2, BaTiO3) [9,10], copolymerizing with polar groups (poly(methyl methacrylate) (PMMA)) [11], aromatic polythiourea (ArPTU) [12], stretching the molten state of PVDF, polarization at high voltage, etc. It is worth noting, for the approach of adding fillers, that, in addition to the polarization phase transition of the matrix, composites can be specifically modified by different functional fillers through the physical and chemical combination between the filler and the matrix. For example, Zhang et al. [13] and Wang et al. group [14] reported an application of boron nitride nanosheet (BNNS)-modified PVDF in performance energy storage and heat conduction. Bhattacharya et al. [15] reported a high energy storage PVDF flexible device, through the combination of graphene and red-mud nanoparticles, which is an environmentally friendly material with high energy storage.

Moreover, some researchers also reported that the dielectric anisotropy between fillers and polymers could affect the polarization behavior of polymer-based composites, then further affect its’ energy storage performance [16,17]. This dielectric anisotropy highly depends on the structure and morphology of inorganic filler phases. Compared to spherical particles, doping a small amount of 1D or 2D fillers can effectively optimize the macroscopic properties of the composites medium [18,19,20,21]. For example, the dielectric constant of a composite doped with BaTiO3 nanowires is three times higher than that of the composite doped with the same amount of BaTiO3 particles [22]. However, the introduction of large amounts of inorganic nanoparticles usually leads to a serious agglomeration of fillers and the easy formation of internal conductive paths, resulting in a greater increase in dielectric loss of composites [23]. Growing a shell on the desired conductive nanoparticle is a common practice in nanotechnology to enhance the compatibility between matrix and filler, isolate the agglomeration of central particles and suppress dielectric loss under electric field. For example, Pan et al. [24], modified BaTiO3 by Al2O3 to synthesize new one-dimensional core–shell nanofibers and then dropped them into a PVDF matrix. Results showed that the composites with 5 vol% core–shell nanofibers exhibited an excellent energy density of 12.18 J/cm3 at 400 MV/m. Huang et al. [25] reported a good review on the impact of core–shell structure on energy storage performance. Furthermore, the results of previous research in our group indicate that the insulating shell Al2O3 promotes the polarization of the matrix [26].

Many scholars use inorganic insulators such as SiO2 to modify particle surfaces to help suppress leakage current [27]. However, relative to the insulators, semiconductors have the effect of buffering the difference in dielectric constant between the matrix and the functional particles. Moreover, the interface regulation of the semiconductor can also avoid the leakage conduction current and help the polarization of the substrate. In the semiconductor modification, there are inorganic and organic materials. Among them, as typical organic and inorganic materials, zinc oxide (ZnO) [28,29,30] and polydopamine(PDA) [31,32] are widely used in the modification of composites for better optical and electrical properties. This modified composites by ZnO and PDA could be used in electrical storage [33,34,35,36], liquid crystal displays, thin-film transistors, light-emitting diodes, photocatalytic, photoelectrochemical and photoelectrochemical devices, etc. [37,38,39,40]. However, for the same semiconductor, the difference between the surface modification of inorganic and organic semiconductors for the same material is rarely reported.

Silver rods as a functional filler can increase the response rate of dielectric composites, so that the material can obtain a higher polarization ability at low voltage, thereby improving energy storage density at low voltage. Adding silver alone will increase the conductive loss of composites; therefore, semiconductors are used to modify the surface. In this paper, two pristine core–shell nanorods, Ag@ZnO and Ag@PDA, and their PVDF matrix composites were prepared. The microstructure, dielectric and electrical storage performance of composites were tested. The effects of the ZnO and PDA shells on the polarization mechanism of PVDF were investigated.

Experiments

Material

Silver nitrate (AgNO3), polyvinylpyrrolidone (PVP), ethylene glycol ((CH2OH)2) and N, N'-dimethylformamide (DMF) were offered by Nanjing Runsheng Petrochemical Co., Ltd.; zinc acetate dihydrate (C4H10O6Zn) and dopamine hydrochloride were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Polyvinylidene fluoride (PVDF) (Mv = 30,000) were supplied by Guangzhou Xingfeng Chemical Material Co., Ltd.; All chemicals were used without further purification.

Preparation of core–shell nanorods and its composites

The synthesis methods of Ag nanorods can be found in our reported paper [26]. The nucleation of core–shell nanorods is shown in Fig. 1. The preparation process of Ag@PDA: A buffer solution with a pH of 8.5 was prepared by mixing hydrochloric acid and acetic acid, and then the Ag nanorods and dopamine hydrochloride were placed into the solution with a mass ratio of 1.5:1, wherein the concentration of dopamine in the solution was 1 g/L. The reaction solution was reacted at 50 °C for 4 h under reflux. After the reaction, nanorods were harvested by centrifugation, then washing and drying.

The preparation process of Ag@ZnO: 12 ml of distilled water containing 0.15 g of Ag nanorods and 0.61 g of zinc acetate were ultrasonic-processed for 20 min. Then, the reaction solution was transferred into a 50 ml hydrothermal reaction autoclave; 12 ml of NaOH aqueous solution was added, sealed and put it in a 170 °C oven for 12 h. After the reaction, nanorods were harvested by centrifugation, then washing and drying.

Figure 1
figure 1

Process illustration of core–shell nanorods synthesis and the preparation of composites

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The preparation of Ag@PDA/PVDF and Ag@ZnO/PVDF composites: The composites were prepared by dispersing the nanorods into the PVDF matrix with different weight contents (5 wt%, 10 wt%, 15 wt%, 20 wt%). Firstly, the weighed nanorods were added into 50 ml DMF to ultrasonic dispersion for 1 h; then, 10.25 g PVDF powder was added into the mixed solution and continuously ultrasonic-dispersed for 3 h. After the dispersion, the mixture was cast onto a clean glass plate and subsequently dried in an oven for 4 h at 80 °C. Finally, the obtained thin films were hot-pressed at 180 °C by 10 MPa to prepare the composite films.

Results and discussion

To observe the synthesis of core–shell particles, their micro-morphology was tested. Figure 2 shows the morphology results of Ag nanorods before and after ZnO coating. As shown in Fig. 2a–c, the core–shell nanorods exhibit a uniform and rod-like morphology, the average length of the nanorods is 2 μm and their effective diameter is about 120 nm. All surfaces of core–shell nanorods are very rough, coated by condensed elliptical ZnO particles. The formation of these rough surfaces should be attributed to the supercritical state of a hydrothermal reaction during the formation of the ZnO shell, which provided high temperature and large pressure which promoted the adsorption and growth of crystalline ZnO nanoparticles on the surface of the Ag nanorod. The TEM image in Fig. 2d, e accurately proves the existence of the core–shell structure in which the middle dark part is the nano-Ag core, while the outer relatively light part is the ZnO shell. Since the growth of ZnO is formed by the accumulation of microparticles, the effective thickness of the ZnO shell is about 35 nm. Figure 2g shows the XRD spectra of Ag nanorods before and after ZnO coating. As shown in Fig. 2g, the Ag@ZnO nanorods showed a new peak which is the standard diffraction peak of ZnO (see in the PDF#36–1451 standard card). The occurrence of this diffraction peak also proved the formation of the ZnO shell. Figure 2f shows a partially magnified HRTEM image of the ZnO shell, the distance of the ZnO atom lattice planes is 0.247 nm, which corresponds to the ZnO (101) peak at a relatively strong absorption angle of 36.2°.

Figure 2
figure 2

(a) SEM overview of Ag rod; (b, c) SEM overview of the Ag@ZnO nanorods; (d, e) TEM of the Ag@ZnO nanorods; (f) HRTEM of the ZnO shell; (g) Comparison XRD pattern of the nanorods before and after coating

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Figure 3 shows the morphology results of Ag nanorods before and after PDA coating. As shown in Fig. 3a–c, after coating, the core–shell Ag@PDA nanorods exhibit a smooth and uniform rod-like structure, which is much different from the Ag@ZnO nanorods. The length of the nanorods is about 2 μm and the width is about 120 nm. Compared to the ZnO shell, the polymerization of PDA is a mild reaction, so the resulting shell is smooth and evenly distributed around the Ag particles. The TEM image in Fig. 3d, e also successfully demonstrates the generation of core–shell structures. In general, increasing the molar ratio of the PDA precursor to the Ag nanorods can gradually increase the thickness of the shell. The calculated average thickness of the PDA shell obtained in this experiment is 35 nm, which is almost the same as that of the ZnO shell.

Figure 3
figure 3

(a) SEM overview of Ag rod; (b, c) SEM overview of the Ag@PDA nanorods; (d, e) TEM of the Ag@PDA nanorods

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In order to observe the dispersibility of the nanorods in the matrix, we observed the SEM images of the cross section of the composite material. As shown in Fig. 4, with the mass percentage of the two kinds of nanorods increases, the number of particles in the cross section of the composites increases obviously. It is worth noting that within the scope of this experimental design, there was no obvious agglomeration for both Ag@ZnO and Ag@PDA nanorods in the PVDF matrix. Moreover, the nanorods still maintain a rod-like structure, indicating that the core–shell particles are relatively stable, and there are no obvious voids or cracks in the interfaces between nanorods and the PVDF matrix. Moreover, for composites with a relatively low filler's loading (below 15 wt%), the cross section of the Ag@PDA/PVDF composites is much smoother than the Ag@ZnO/PVDF ones. This can be attributed to the fact that the PDA shell is also a polymer and can be better combined with a polymer matrix with a similar structure. In contrast, the ZnO shell exhibits more occlusal bonding with the PVDF matrix, then resulting in the fracture of Ag@ZnO/PVDF in a brittle fracture mode.

Figure 4
figure 4

Cross-sectional SEM of (a) Ag@PDA/PVDF and (b) Ag@ZnO/PVDF composites with different addition amounts (0 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%)

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To observe the effect of nanoparticle addition on the crystallization behavior of the PVDF matrix, we performed XRD on the composite. As shown in Fig. 5, after the introduction of the nanorods, there appeared some new peaks and the main phase of the PVDF matrix shifted. For example, for all the composites, there are some common peaks of the Ag crystal at 38.15°, 44.32°, 64.28° and 77.53o which corresponds to the typical crystal planes of Ag at (111), (200), (220) and (311), as proved by PDF#36 1451. Moreover, for the Ag@ZnO/PVDF composites, there also appeared some new peaks at 31.78°, 34.48°, 36.20° corresponding to the crystal plane of ZnO at (100) (002) (101) in PDF#36–1451 card. However, for the Ag@PDA/PVDF composite, there is no new diffraction peak appearing in the XRD diffractogram as its amorphous. Besides, the characteristic peaks intensity of the PVDF matrix changed greatly. For example, in both of composites, the absorption peaks also appear at 17.6°, 18.3°, 19.9° and 26.5° which corresponds to the α(100), α(020), α(110) and α(021) phases of PVDF crystals, respectively [41], but the intensity is significantly weakened compared to the pure one. Also the peak intensity of composite material around 20.0° is slightly higher and wider than that of 18.0°, which is opposite to the trend of the pure film. This is credited with that the peak of β(110)(200) at 20.26° and the peak of γ(110) at 20.04° of PVDF matrix in composite also appeared near 20.0°, not only the peak of α(110) at 19.9°.

Figure 5
figure 5

XRD diffractogram of Ag@PDA/PVDF and Ag@ZnO/PVDF composites with 10 wt% addition

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To better observe this phase transition, we conducted FTIR experiments on the composite material. As Fig. 6 shows, there is also a huge change around the 840 cm−1 absorption range; all the composites represent a stronger absorption than the pure one. The Ag@ZnO/PVDF composites present a broad mixture of β and γ absorption at 840 cm−1 and a new γ absorption peak at 812 cm−1 [41]. These changes in peaks suggest that the polarity of PVDF has changed, which would take a large effect on the dielectric properties of the composites. Usually, these changes of β and γ phase of PVDF molecules would improve the dielectric constant of the composite than before.

Figure 6
figure 6

FTIR spectrogram of Ag@PDA/PVDF and Ag@ZnO/PVDF composites with 10 wt% addition

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To better characterize the dielectric properties of the composite materials, we tested the dielectric properties as a function of frequency. As shown in Figs. 7a-1, b-1 and 8d, for these two types of particles, the dielectric constant of both Ag@PDA/PVDF and Ag@ZnO/PVDF composites increased with the addition of nanorods, they showed almost the same ability to modify the dielectric constant of the matrix, and the dielectric constant of the PDA modification component is slightly higher than that of ZnO. For the entire dielectric material, the magnitude of the vector dipole moment is proportional to the charge. So the major improvement is attributed to a better in-store charge ability of Ag nanorods under the action of an alternating electric field than for the polymer, at the same time, 1D nanoparticles have a larger electric dipole moment than 0D nanoparticles, which could amplify the electron displacement polarization in the matrix. The difference in the degree of polarization caused by PDA and ZnO is mainly due to the difference in polarization behavior. PDA is a polar dielectric, under the action of an electric field, electronic polarization and steering polarization will occur. However, ZnO belongs to the ionic dielectrics, which mainly generates electronic polarization and ionic polarization. Usually, to establish steering polarization spend more time than electronic polarization and ionic polarization behaviors. Moreover, the PDA itself has an electric dipole moment, so the polarization degree of PDA is higher than for ZnO in the low-frequency range, when the frequency increases, this polarization gap is weakened.

Figure 7
figure 7

Dielectric constant, dielectric loss and conductivity of (a) Ag@PDA/PVDF and (b) Ag@ZnO/PVDF as a function of testing frequency

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Figure 8
figure 8

Weibull breakdown field of (a) Ag@PDA/PVDF composites, (b) Ag@ZnO/PVDF composites and (c) Weibull breakdown strength of composites, (d) dielectric constant of Ag@PDA/PVDF and Ag@ZnO/PVDF composites with different contents at 10 Hz

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Furthermore, as shown in Fig. 7a-2, b-2, after the modification by adding core–shell nanorods, the overall dielectric loss of the composites remains at a low level. For the dielectric loss of Ag@PDA component, only at 5 wt% filler amount it is lower than that of the pure film at low frequency. The others composites dielectric loss increases with different fillers in different frequency ranges, but the overall loss is still lower than 0.25. On the other hand, below a frequency of 103 Hz, the loss of composites of Ag@ZnO is all lower than the pure film, then the dielectric loss slowly increases as the frequency increases. At low frequencies, the difference in dielectric loss between the Ag@PDA and Ag@ZnO composites is closely related to the polarization behavior. Due to the inherent electric dipole moment of the PDA, it undergoes steering polarization under an electric field. This behavior takes a longer time than the ZnO ion polarization, and the energy cannot be fully recovered. The addition of an Ag core will amplify this effect, so the dielectric loss will be greater. At high frequencies, the dielectric loss is mainly due to the relaxation loss of the polar base. The introduction of these two core–shell particles in PVDF is equivalent to the introduction of impurities in the matrix crystal, so the loss of both composite materials is higher than that of the pure film [42].

Figure 7a-3, b-3 shows the spectrum of the conductivity of the composites as a function of frequency. As the amount of nanorods added increases, the conductivity of the two composites shows different changes. The Ag@PDA/PVDF composites showed an overall upward trend in electrical conductivity, while the Ag@ZnO/PVDF composites did not change significantly. Electrical conductivity is an important parameter for evaluating the mobility of carriers in dielectrics. The experimental results indicate that ZnO has a stronger ability to restrict carrier migration than PDA. This may be closely related to the ZnO crystal structure, as the high crystallinity has a higher band gap than the disordered state. Therefore, Ag@ZnO/PVDF has a stronger ability to bind carriers.

The electrical breakdown strength of composite materials is an important parameter in the calculation of energy storage density, and it usually described by the Weibull cumulative distribution function with two parameters (Eqs. 1–3), as in Eq. (1).

$$ P(E) = 1 - \exp \left[ { - \left( \frac{E}{Eo} \right)^{\beta } } \right]. $$
(1)

In Eq. (1), P(E) is the cumulative failure probability; E is the breakdown field strength; β is the shape parameter; E0 is the breakdown field strength at P(E) = 63.28%. Taking the logarithm of both sides of Eq. (1), one obtains Eq. (2):

$$ \ln \left[ { - \ln \left( {1 - p} \right)} \right] = \beta (\ln E - \ln Eo). $$
(2)

For each corresponding field strength E0, a P value can be calculated, which is calculated by Eq. (3), where i indicates that the E values are arranged from small to large; n indicating the number of tests of each sample.

$$ p = \frac{i - 0.5}{{n + 0.25}}. $$
(3)

The correspondence between ln (− ln (1 − P(E))) and ln E is shown in Fig. 8a–c. As shown, with the addition of both Ag@PDA and Ag@ZnO core–shell nanorods, the overall breakdown strength of the composites shows a trend of rising first and then decreasing. The Ag@ZnO/PVDF composites show better resistance to breakdown than the Ag@PDA/PVDF composites. Among them, when the addition amount is 10 wt%, the breakdown strength of Ag@ZnO/PVDF composites is ~ 1799 kV/cm (β = 13.94), and Ag@PDA/PVDF is 1519 kV/cm (β = 6.09), which is 33.35% and 12.6% higher than that of a pure film 1349 kV/cm, respectively. Usually, the breakdown field strength of a material is closely related to its ability to limit the space charge to form a leakage current [43]. In addition, for the leakage current test of materials as shown in Fig. 9d, the leakage currents of ZnO and PDA at 200 kV/cm are 3.37E−8 A/cm2 and 4.153.37E−8 A/cm2, respectively. It shows that under the same conditions, ZnO has a stronger ability to limit space charge than PDA, which is in agreement with the results of the Weibull analysis.

Figure 9
figure 9

D–E curve of (a) Ag@PDA/PVDF composites with different additions; (b) Ag@ZnO/PVDF composites with different additions; (c) Ag@PDA/PVDF and Ag@ZnO/PVDF with 10 wt% addition at 1000 kV/cm; (d) leakage current density for Ag@PDA/PVDF and Ag@ZnO/PVDF composites with 10 wt% addition

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To better demonstrate the polarization behavior of different composites along with the electric field strength, and to calculate the energy storage density and cycle efficiency of the material, the change curves of electric displacement and electrical field strength of two kinds of core–shell (Ag@PDA, Ag@ZnO) nanorods under different additions are tested and the results are shown in Fig. 9a,b. As shown, the 10 wt% Ag@PDA/PVDF composites at 1500 kV/cm obtained the maximum polarization value of 5.60 μc/cm2, while the 10 wt% Ag@ZnO/PVDF composites reached the maximum polarization value of 6.84μc/cm2 in 1800 kV/cm. It is assumed that the maximum polarization degree of the material is related to the breakdown field strength. However, under the same field strength and fillers' amount, as shown in Fig. 9c, the PDA shows better polarization performance, which is consistent with the dielectric constant results.

The difference in the polarization strength of the two types of semiconductor core–shell particles is largely due to the different aggregate states of the shell materials. The simplified mechanism is shown in Fig. 10. As a typical amorphous polymer, the PDA layer between Ag nanorods and the PVDF matrix also acts as an amorphous phase with a good fusion. It is easy to pass polarization when an electric field is applied. However, ZnO has a highly regular crystal structure [44], and when it acts as an interfacial layer, it will have a barrier effect during the polarization process of polymeric molecules, which could weaken the polarization of composites and reduce the accumulation of space charge to a certain extent. At the same time, amorphous polymers will also generate more voids during the behavior of turning polarization, causing higher dielectric loss, which will further affect the charge/discharge efficiency of the composites.

Figure 10
figure 10

Illustration of polarization process for composites

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The polarization and depolarization behavior of a dielectric under an external electric field represents the charging and discharging process of a dielectric capacitor, respectively. In principle, the energy storage density of a nonlinear dielectric can be calculated from the applied electric field and electric displacement using Eq. (4).

$$ U = \int {EdD} $$
(4)

where E is the electric field strength and D is the electrical displacement. The charge and discharge efficiency of the dielectric material can be calculated by Eq. (5).

$$ \eta = \frac{{U_{{{\text{dis}}}} }}{{U_{{{\text{dis}}}} + U_{{{\text{loss}}}} }} $$
(5)

where Udis is the discharge energy and Uloss is the loss energy which corresponds to the area integral value in the D–E curve in Fig. 9.

Via calculation, we obtain the relationship spectrum between the material's energy storage density, charge/discharge efficiency and breakdown strength in Fig. 11. In the case of 10 wt% fillers' content and before breakdown strength, the energy storage densities of Ag@PDA/PVDF and Ag@ZnO/PVDF composites are 2.34 J/cm3 and 4.02 J/cm3, which is an increase of 79.53% and 209.2% compared to pure PVDF films, respectively. The energy storage densities compared with other PVDF-based composites are shown in Table 1. It is worth noting that while Ag@ZnO/PVDF increases the energy storage density, it still maintains a good charge and discharge efficiency. However, due to the good compatibility of the PDA and the matrix, the Ag@PDA/PVDF composites perform better in terms of dynamic mechanical behavior. As shown in Fig. 12 at the initial temperature of 50 °C, the Youngs modulus of the Ag@ZnO/PVDF and Ag@PDA/PVDF is 1019.83 MPa and 1103.39 MPa, compared with the one of a pure PVDF film 633.18 MPa, it is increased by 61.06% and 74.26%, respectively.

Figure 11
figure 11

Energy storage density and charge/discharge efficiency of (a) Ag@PDA/PVDF and (b) Ag@ZnO/PVDF composites with different additions

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Table 1 Energy storage densities compared with other PVDF-based composites
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Figure 12
figure 12

Dynamic mechanical analysis of PVDF film, Ag@ZnO/PVDF and Ag@PDA /PVDF composites with 10wt% adding content

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Conclusion

We successfully synthesized two kinds of 1D core–shell nanorods(Ag@PDA, Ag@ZnO) and their PVDF matrix composites. The different effects on the dielectric properties between inorganic ZnO and organic PDA shells have been investigated. Microstructural analysis showed that the Ag@PDA and Ag@ZnO nanorods could be evenly dispersed in a PVDF matrix without obvious aggregation. In the case of 10 wt% fillers' content and before breakdown strength, the energy storage densities of Ag@PDA/PVDF and Ag@ZnO/PVDF composites are 79.53% and 209.2% higher than that of pure PVDF films, respectively. Moreover, the charge/discharge efficiency of Ag@ZnO/PVDF composite is also higher than that of pure PVDF and Ag@PDA/PVDF composite. It is worth to note that at 1800 kV/cm testing electrical strength, the discharge energy storage density of the Ag@ZnO/PVDF composites reaches to 4.02 J/cm3. In summary, this PVDF-based composite doped with Ag nanorods decorated by inorganic ZnO shell exhibits a higher ability to constrain electrons than PDA, which is beneficial to the performance regarding dielectric properties.