**1. Introduction**

With recent developments in the electronic industry, dielectric polymer composite materials have attracted increasing interest for a wide range of applications, such as energy storage devices, dielectric capacitors, and electromechanical actuators [1,2]. Poly(vinylidene fluoride) (PVDF) has been used as a dielectric polymer material due to its high energy density, high electric break down field, and flexibility [3,4]. However, the relative dielectric permittivity (ε ) of PVDF is too low (≈10 [3]) for electronic applications.

**Citation:** Kum-onsa, P.; Chanlek, N.; Manyam, J.; Thongbai, P.; Harnchana, V.; Phromviyo, N.; Chindaprasirt, P. Gold-Nanoparticle-Deposited TiO2 Nanorod/Poly(Vinylidene Fluoride) Composites with Enhanced Dielectric Performance. *Polymers* **2021**, *13*, 2064. https://doi.org/10.3390/polym 13132064

Academic Editor: Jung-Chang Wang

Received: 18 June 2021 Accepted: 21 June 2021 Published: 23 June 2021

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Many studies have attempted to fabricate polymer composites with high ε values by incorporating fillers into the PVDF matrix. Two-phase ceramic/polymer and metal/polymer composites have been synthesized and widely studied for improving the dielectric performance of polymer composite materials [5–11]. Several ceramic/polymer composites, such as CaCu3Ti4O12/PVDF [5,12], CaCu3Ti4O12/polystyrene [13], BaTiO3/PVDF [6], Ba0.5Sr0.5TiO3/P(VDF-CTFE) [14], and Ba0.6Sr0.4TiO3/PVDF [15], have high ε values (~50–80 at 1 kHz). The ε of a ceramic/polymer composite is generally below 100 even at a high ceramic loading (50 vol%), while its dielectric loss tangent (tanδ) is also elevated (>0.1 at 1 kHz and ~25 ◦C) [5,16]. However, metal/polymer composites, such as Ni/PVDF, Ni/P(VDF-CTFE) [17,18], MWCNT/PVDF [8,19], and Ag/PVDF [7,20], can exhibit significantly higher ε at low concentrations of conducting fillers than ceramic/PVDF composites. It is difficult to maintain the filler loading at the percolation threshold (*f* c) to achieve a high relative permittivity. Metal/polymer composites generally exhibit significantly large tanδ and electrical conductivity (σ) values at *f* c. It should be noted that the metal/polymer composites with extreme ε values also have high tanδ and σ, which limits the practical applications of these metal/polymer composites.

Owing to such challenges, developing polymer composites with high ε and low tanδ values is desirable. Several researchers have studied and reported three-component composites comprising metal, ceramic, and polymer matrices, such as Ba(Fe0.5Nb0.5)O3/Ni/PVDF, Ni/CaCu3Ti4O12/PVDF, Ni/BaTiO3/PVDF, Na0.5Bi0.5Cu3Ti4O12/MWCNTs/PVDF, and Ag/Na0.5Bi0.5Cu3Ti4O12 [21–25]. In particular, a novel composite with structured hybrid fillers has been of great interest. Recently, many studies on PVDF-based composites filled with hybrid nanoparticles have been reported. Luo et al. [26] reported a novel polymer composite filled with Ag-BaTiO3 hybrid nanoparticles. This Ag-BaTiO3/PVDF composite exhibited a high ε (160) with tanδ ≈ 0.11 at a filler volume fraction (*f* Ag-BT) of 0.568. This tanδ value is much lower than those reported in many conventional three-phase polymer composites; unfortunately, it is still much larger than 0.05, which is an acceptable value for capacitor applications. Although incorporating Ag-BaTiO3 hybrid nanoparticles can increase the ε of a composite, the ε of ferroelectric BaTiO3 is generally strongly dependent on its Curie temperature. Furthermore, most ferroelectric oxides are piezoelectric, which results in mechanical resonance in the device during charging and discharging, thereby limiting its reliability [27].

Rutile-TiO2 is one of the most widely used oxides in electronic materials, sensors, and semiconductors [28,29]. Furthermore, rutile-TiO2 can exhibit colossal dielectric properties when a minor portion of Ti4+ is reduced to Ti3+ due to the existence of oxygen vacancies and/or substitution by pentavalent ions (e.g., Nb5+ or Ta5+). Polaron-like electron hopping between Ti3+ and Ti4+ ions can cause a significant increase (by a factor of ~104) in dielectric permittivity [30]. Since TiO2 is not a ferroelectric ceramic, TiO2 nanoparticles were used as a filler in various polymer composites [31–33]. Unfortunately, the ε values of the TiO2/polymer composites are still significantly low owing to the low ε' of the TiO2 nanoparticles. Polymer composites filled with modified TiO2 nanoparticles such as Ag-TiO2 hybrid particles and Ag@TiO2 core–shell structures were developed to enhance ε [34–37]. Although these composites can exhibit high ε values of ~60–150, large tanδ values are generally obtained (~0.1–1) at high filler concentrations (70 vol%) [34,35]. Among various metal nanoparticles, gold nanoparticles are widely used as fillers to improve the insulation properties of polymer materials because they are nontoxic and less likely to be oxidized [38]. A significantly enhanced ε (~54-118) and low tanδ (<0.06) were achieved in Au-BaTiO3/PVDF [39] and Au-BiFeO3/PVDF, with only a small amount of Au in the third phase of each polymer composite (*f* Au < 0.02) [40]. According to previous works [39,40], the Au-BaTiO3/PVDF and Au-BiFeO3/PVDF composites not only exhibited high ε values, but their tanδ and σ were also suppressed due to the incorporation of the Au nanoparticles. Therefore, the conductive Au phase nanoparticle is one of the most interesting conductive phases for use as a filler in three-phase polymer composites.

To the best of our knowledge, there is a lack of substantial information on polymer composites incorporated with Au-TiO2 hybrid nanoparticles. Therefore, in this study we aimed to fabricate a novel nanocomposite comprising a PVDF polymer matrix, Au nanoparticles, and TiO2 nanorods (TNRs). TNRs have higher surface areas than spherical TiO2; therefore, they lead to stronger interfacial polarization and a significantly enhanced ε . Herein, Au-TNR/PVDF nanocomposites with enhanced ε and low tanδ were fabricated. A modified Turkevich method was used to attach Au onto the surfaces of the TNRs. The Au-TNR/PVDF nanocomposites were prepared through liquid-phase-assisted dispersion and hot-pressing methods. Several properties of these nanocomposites such as their morphologies, microstructures, phase structures, chemical stages, and dielectric properties were investigated, and the significantly improved dielectric properties of the nanocomposites are discussed.
