Surface functionalization of BaTiO₃ nanoparticles and improved electrical properties of BaTiO₃/polyvinylidene fluoride composite

Abstract

Surface functionalization of BaTiO₃ nanoparticles with dopamine was carried out using a reflux method to strongly bind dopamine on the BaTiO₃ nanoparticle surface and improve its compatibility with the polyvinylidene fluoride (PVDF) polymer matrix. Fourier transform infrared spectra confirm the successful surface functionalization of BaTiO₃ nanoparticles after immobilization with dopamine. Electrical properties of the resultant nanocomposite show that the dielectric constant can be enhanced up to 56.8 with low dielectric loss.

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Ferroelectric ceramics possess very high dielectric constants but are brittle and have low dielectric strength. On the other hand, polymers are flexible, easy to process with low processing temperatures, and possess high dielectric breakdown fields. Nanocomposites of electroactive ceramics and a ferroelectric polymer are very attractive for many applications such as embedded capacitors, multilayer capacitors, high energy density capacitors, memories, and gate insulators in organic field effect transistors.^1–5 Recent work has demonstrated that the addition of ferroelectric ceramics as fillers can increase the dielectric constant of the resultant composites.^6,7 Because of the differences in surface characteristics between the inorganic reinforcement fillers and the organic matrix, it was found to be difficult to naturally disperse the reinforcement fillers uniformly, which greatly affects the electrical performance of the composite.⁶ Hence, improvement of the interface between the inorganic phase and the polymer matrix is very critical to the development of high performance nanocomposites.

Addition of surfactants such as phosphate esters and oligomers thereof, can improve the dispersion of BaTiO₃ nanoparticles for composites.^4,7 Thus, much attention has been paid recently to the use of coupling agents owing to their special structures, which have two different functional groups: one attached to the matrix, and the other attached to the surface of the filler, leading to a link between the reinforcement filler and the matrix. Surface functionalization of BaTiO₃ nanoparticles with phosphoric acid and ethylene diamine on BaTiO₃ has been reported to improve its compatibility with PVDF based copolymers.^8,9 However, those surfactants have not been shown to bind strongly with the BaTiO₃ nanoparticles surface. Hence, Dang et al. have reported the chemical modification of BaTiO₃ nanoparticles surfaces by silane coupling agents (KH550).¹⁰ In such systems, residual free surfactant can lead to high leakage current and dielectric loss.¹¹ The dielectric loss value of the binary composite increases with the size of BaTiO₃.¹² Although widely explored in research, many of these coupling agents have limitations in widespread practical use; specifically, their compliance with the well-established industrial processes for film casting is not evident as the resulting nanocomposites in the previous work have only been assessed by making disks using hot press technology. As a result, it is of great significance to investigate surfactants which allow the casting of defect-free films.

In addition, most of these common dispersants react with the BaTiO₃ surface only by the stirring method, leading to the ineffective binding of the dispersant onto the BaTiO₃ surface and reduced compatibility of the nanocomposite. In this work, we have employed a refluxing method to promote effective chemical binding. Refluxing has been widely used in applications such as synthesis of nanoparticles,^13,14 grafting polymers¹⁵ and surface modification of nanoparticles.¹⁶ The advantage of refluxing is that it can provide effective stirring for an extended period without the need to add more solvent or the fear of boiling dry in the reaction vessel as any vapor is immediately condensed in the condenser.

Dopamine is a bioinspired building block for surface coatings or adhesion purposes and can be biosynthesized in a wide variety of living species.¹⁷Dopamine has been used to coat MnO₂ nanowires for energy storage applications.¹⁸ In addition, it has also been reported that dopamine and other catechol compounds perform very well as binding agents for coating inorganic surfaces.^19,20 In this paper, we investigate the use of dopamine as a coupling agent in order to improve the compatibility of BaTiO₃ with PVDF. This is based on the fact that dopamine interacts strongly with a variety of metals and metal oxides by hydrogen bonding. In addition, the NH₂ group of dopamine reacts with C–F groups of PVDF which can enhance the compatibility between fillers and polymer as well. Using this approach, we have successfully created the covalent bonds for the first time through the refluxing method and the resultant composites exhibit high dielectric constant of 56.8 with low dielectric loss of 0.04.

The dopamine chemical structure contains OH groups and NH₂. Dopamine is believed to interact strongly with a variety of metal and metal oxides by hydrogen bonding. We have created the covalent bonds by a refluxing method. The reaction mechanism of surface functionalization is shown in Fig. 1. It has been reported that the dielectric loss value of the binary composite increases with the size of BaTiO₃.¹² In our studies, the BaTiO₃ nanoparticles with sub-100 nm size were successful synthesized via a simple hydrothermal method.²¹ The FTIR results after surface functionalization show that –OH groups can be detected on the BaTiO₃ nanoparticle surfaces. Based on covalent bonding, the –OH groups of dopamine are expected to bind to the BaTiO₃ surface. The –NH₂group of dopamine is expected to react with the molecules of PVDF based on the formation of hydrogen bonds due to the possible formation of F–H bonds.

Fig. 1 The reaction scheme for surface functionalization of BaTiO₃ nanopowders with dopamine.

Fig. 2 shows the FTIR spectra of BaTiO₃ particles treated with various concentrations of dopamine. Compared with the spectrum of pure BaTiO₃ particles, additional peaks were observed at 1478 (–NH₃⁺ deformation), 1540 (amide band, NH bending), and 1261 cm⁻¹ (aryl oxygen stretching) after immobilization of BaTiO₃ particles treated with 1 mmol dopamine.¹⁴ When the amount of dopamine is less than 1 mmol, it is insufficient to initiate quantifiable chemical attachments with BaTiO₃ nanopowders and PVDF. In addition, the TEM image (ESI,† Fig. S1) confirms that the dopamine is grafted on the surface of the BaTiO₃ particles.

Fig. 2 FTIR spectra of BaTiO₃ particles treated with various concentrations of dopamine (a) 0 (b) 0.1 (c) 0.5 (d)1 mmole.

The composite thin film of BaTiO₃ and PVDF has been prepared by a drop casting method. Fig. 3 presents the FE-SEM images of various BaTiO₃ amounts with PVDF thin film composites without and with dopamine surface functionalization. FE-SEM images clearly show that the dopamine has improved the morphology of the nanocomposite due to the improved compatibility of fillers in the matrix after chemical bonding of interfaces between BaTiO₃ and PVDF. In addition, FE-SEM images show that the nanocomposite without dopamine treatment has pores when loaded at high filler concentration originating from the agglomeration of particles. The aggregate issue causes the pores in the composite films and reduces the breakdown voltage of the nanocomposite.²² However, the nanocomposite of BaTiO₃ treated with dopamine exhibited significantly improved dispersion even at higher filler concentrations.

Fig. 3 FE-SEM images of BaTiO₃/PVDF thin film composites with various weight% of BaTiO₃: (a) 10%, (b) 30%, (c) 50% without dopamine; and (d) 10%, (e) 30%, (f) 50% with dopamine.

Fig. 4 presents the frequency dependences of dielectric constant and dielectric loss of the composite with various percentage weight fillings of untreated and dopamine treated BaTiO₃ at room temperature. At 50 wt% BaTiO₃, the nanocomposite treated with dopamine showed enhanced dielectric constant as well as low dielectric loss in comparison to the composite without dopamine. For the BaTiO₃ composite without dopamine treatment, the dielectric constant increases with increasing weight of the BaTiO₃ nanoparticles up to 30 wt% only; the dielectric constant dropped with further increase of particles to 50 wt%, indicating the non-uniform dispersion caused by high concentration of ceramic fillers. Dielectric loss increases with increasing weight percentage of ceramic fillers. This is attributed to the presence of hydroxyl groups resulted from the synthesis in NaOH.^23,24 A clear peak of dielectric loss could be seen at about 2 × 10⁵ Hz. This indicates an obvious relaxation loss process related to the PVDF polymer. The relaxation loss makes the dielectric permittivity reduce significantly at this frequency as shown in Fig. 4 (a) and (c). Dopamine not only acts as a bridge-link between interfaces of ceramic nanoparticles and polymer to eliminate the aggregate formation, but also reduces the concentration and mobility of ionizable hydroxyl groups on the nanoparticle surface and therefore also minimizes the leakage current. Experimental results showed that the surface functionalization imparts excellent compatibility between the fillers and the polymer matrix and ensures the formation of uniform composite films even at high filler concentrations. This is evidently shown from the experimental results that the dielectric constant of the polymer–matrix composite with 50 wt% dopamine–BaTiO₃ nanoparticle is enhanced to as high as 56.8 with a dielectric loss of 0.04 at 10³ Hz which is similar to the values for pristine PVDF.²⁵ The dielectric constant of PVDF is around 14.²⁵ The dielectric constant of composite treated dopamine is increased by 40% as compared to the highest reported value of 40.74 at 10³ Hz for BaTiO₃/PVDF composites.²⁶

Fig. 4 Dependences of (a) dielectric constant and (b) dielectric loss of BaTiO₃/PVDF composites; (c) dielectric constant and (d) dielectric loss of dopamine–BaTiO₃/PVDF composites on frequency measured at room temperature from 10² to 10⁶ Hz.

In summary, we investigated the surface functionalization of BaTiO₃ nanoparticles for the first time using dopaminevia a reflux method. FTIR spectra confirmed that BaTiO₃ has been successfully surface-functionalized by covalent bonds. We further studied the electrical properties of the BaTiO₃/PVDF nanocomposite films. The presence of organic surface layers on the particles imparted excellent compatibility between the fillers and the polymer matrix and ensured uniform composite films even at higher filler concentrations. Scanning electron microscopy images showed defect-free films after functionalizing with dopamine which is a significant result for practical applications. As a result, the dielectric constant of the resultant composite at 50 wt% filling has been enhanced up to 56.8 at a low loss of 0.04 at 10³ Hz.

Acknowledgements

This work was supported by DSTA-NTU TL/POD0814080/03.

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Footnote

† Electronic Supplementary Information (ESI) available. See DOI: 10.1039/c1ra00210d/

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