Gold-Nanoparticle-Deposited Tio2 Nanorod/Poly(Vinylidene Fluoride) Composites with Enhanced Dielectric Performance

Article Gold-Nanoparticle-Deposited TiO2 Nanorod/Poly(Vinylidene Fluoride) Composites with Enhanced Dielectric Performance

Pornsawan Kum-onsa 1, Narong Chanlek 2, Jedsada Manyam 3 , Prasit Thongbai 4,5,* , Viyada Harnchana 4,5 , Nutthakritta Phromviyo 6 and Prinya Chindaprasirt 6

1 Materials Science and Nanotechnology Program, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand; [email protected] 2 Synchrotron Light Research Institute (Public Organization), 111 University Avenue, Muang District, Nakhon Ratchasima 30000, Thailand; [email protected] 3 Nanotechnology Center (NANOTEC), National Science and Technology Development Agency (NSTDA), Pathum Thani 12120, Thailand; [email protected] 4 Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand; [email protected] 5 Institute of Nanomaterials Research and Innovation for Energy (IN-RIE), NANOTEC-KKU RNN on Nanomaterials Research and Innovation for Energy, Khon Kaen University, Khon Kaen 40002, Thailand 6 Sustainable Infrastructure Research and Development Center, Department of Civil Engineering, Faculty of Engineering, Khon Kaen University, Khon Kaen 40002, Thailand; [email protected] (N.P.); [email protected] (P.C.) * Correspondence: [email protected]

Abstract: Flexible dielectric polymer composites have been of great interest as embedded capacitor materials in the electronic industry. However, a polymer composite has a low relative dielectric Citation: Kum-onsa, P.; Chanlek, N.; permittivity (ε0 < 100), while its dielectric loss tangent is generally large (tanδ > 0.1). In this study, we Manyam, J.; Thongbai, P.; Harnchana, fabricate a novel, high-permittivity polymer nanocomposite system with a low tanδ. The nanocom- V.; Phromviyo, N.; Chindaprasirt, P. posite system comprises poly(vinylidene fluoride) (PVDF) co-filled with Au nanoparticles and Gold-Nanoparticle-Deposited TiO2 3+ semiconducting TiO2 nanorods (TNRs) that contain Ti ions. To homogeneously disperse the con- Nanorod/Poly(Vinylidene Fluoride) ductive Au phase, the TNR surface was decorated with Au-NPs ~10–20 nm in size (Au-TNRs) using Composites with Enhanced Dielectric β Performance. Polymers 2021, 13, 2064. a modified Turkevich method. The polar -PVDF phase was enhanced by the incorporation of the 0 https://doi.org/10.3390/polym Au nanoparticles, partially contributing to the enhanced ε value. The introduction of the Au-TNRs 13132064 in the PVDF matrix provided three-phase Au-TNR/PVDF nanocomposites with excellent dielectric properties (i.e., high ε0 ≈ 157 and low tanδ ≈ 0.05 at 1.8 vol% of Au and 47.4 vol% of TNRs). The Academic Editor: Jung-Chang Wang ε0 of the three-phase Au-TNR/PVDF composite is ~2.4-times higher than that of the two-phase TNR/PVDF composite, clearly highlighting the primary contribution of the Au nanoparticles at Received: 18 June 2021 similar filler loadings. The volume fraction dependence of ε0 is in close agreement with the effective Accepted: 21 June 2021 medium percolation theory model. The significant enhancement in ε0 was primarily caused by Published: 23 June 2021 interfacial polarization at the PVDF–conducting Au nanoparticle and PVDF–semiconducting TNR interfaces, as well as by the induced β-PVDF phase. A low tanδ was achieved due to the inhibited Publisher’s Note: MDPI stays neutral conducting pathway formed by direct Au nanoparticle contact. with regard to jurisdictional claims in published maps and institutional affil- Keywords: gold nanoparticle; titanium dioxide nanorod; poly(vinylidene fluoride); heat treatment; iations. hybrid nanoparticle; modified Turkevich method

Copyright: © 2021 by the authors. 1. Introduction Licensee MDPI, Basel, Switzerland. This article is an open access article With recent developments in the electronic industry, dielectric polymer compos- distributed under the terms and ite materials have attracted increasing interest for a wide range of applications, such conditions of the Creative Commons as energy storage devices, dielectric capacitors, and electromechanical actuators [1,2]. Attribution (CC BY) license (https:// Poly(vinylidene fluoride) (PVDF) has been used as a dielectric polymer material due to creativecommons.org/licenses/by/ its high energy density, high electric break down field, and flexibility [3,4]. However, the 0 4.0/). relative dielectric permittivity (ε ) of PVDF is too low (≈10 [3]) for electronic applications.

Polymers 2021, 13, 2064. https://doi.org/10.3390/polym13132064 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 2064 2 of 14

Many studies have attempted to fabricate polymer composites with high ε0 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], 0 Ba0.5Sr0.5TiO3/P(VDF-CTFE) [14], and Ba0.6Sr0.4TiO3/PVDF [15], have high ε values (~50–80 at 1 kHz). The ε0 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 ε0 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 ε0 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 ε0 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 0 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 0 0 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 ε0 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 ε0 [34–37]. Although these composites can exhibit high ε0 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 ε0 (~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], 0 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. Polymers 2021, 13, 2064 3 of 14

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 ε0. Herein, Au-TNR/PVDF nanocomposites with enhanced ε0 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.

2. Experimental Section 2.1. Preparation of Heat-Treated TNRs TNRs (99.5% purity) with particle size <100 nm were purchased from Sigma-Aldrich. Heat treatment at 500 ◦C for 3 h in air was performed on the TNRs to evaporate the moisture.

2.2. Preparation of Au-TNR Hybrid Nanoparticles Au-TNR hybrid nanoparticles were prepared through a modiﬁed Turkevich method. The corresponding procedure is described as follows: heat treatment of TNR powder was carried out by ultrasonically dispersing the powder in deionized water for 30 min. Then, the white TNR suspension was stirred using a magnetic stirrer at ~25 ◦C for 30 min, after which 1 mM HAuCl4·3H2O was dissolved in the TNR solution under constant stirring. After the solution was heated to 300 ◦C, 38.8 mM of sodium citrate (>99.0%, Sigma-Aldrich) solution was dissolved in the TNR solution. To ensure a complete reaction, the suspension was stirred until its color changed from white to purple. The purple suspension was sequentially cooled to room temperature, centrifuged at 8500 rpm, and washed several times with deionized water. Finally, Au-TNR hybrid nanoparticles were obtained without agglomeration by freeze-drying.

2.3. Preparation of Au-TNR/PVDF Nanocomposites

Au-TNR/PVDF nanocomposites containing Au-TNR ﬁllers with different f Au and f TNRs values were prepared through liquid-phase-assisted dispersion and hot-pressing methods. First, Au-TNR hybrid nanoparticles and the PVDF powder (Mw ~ 534,000, Sigma- Aldrich) were mixed by ball-milling with ZrO2 in ethanol for 3 h. Second, the mixture was dried at 80 ◦C to evaporate ethanol, after which the mixed powder was pressed at 200 ◦C for 30 min at 10 MPa. Finally, the Au-TNR/PVDF nanocomposite sample, with a diameter and thickness of ~12 mm and ~0.5–1 mm, respectively, was obtained at room temperature. Au-TNR/PVDF nanocomposite samples with f Au-TNRs = 0.094, 0.216, 0.294, 0.383, 0.492, and 0.624 are referred to as Au-TNRs/PVDF-1, Au-TNRs/PVDF-2, Au-TNRs/PVDF-3, Au- TNRs/PVDF-4, Au-TNRs/PVDF-5, and Au-TNRs/PVDF-6, respectively. The separated volume fractions of Au and TNRs for each composite sample are listed in Table1. Polymers 2021, 13, 2064 4 of 14

Polymers 2021, 13, x FOR PEER REVIEW 4 of 15

0 Table 1. Volume fraction of Au (f Au), TNRs (f TNRs), Au-TNRs (f Au-TNRs), ε , tanδ, and σac at 1 kHz and room temperature for nanocomposites with varying filler amounts. Table 1. Volume fraction of Au (fAu), TNRs (fTNRs), Au-TNRs (fAu-TNRs), ε′, tanδ, and σac at 1 kHz and room temperature for nanocomposites with varying filler amounts. 0 −11 −1 Sample f Au f TNRs f Au-TNRs ε tanδ σac (10 S·cm ) Sample fPVDFAu fTNRs 0fAu 0‒TNRs 0ε′ 10.8tanδ 0.020σac (10−11 S·cm 4.1 −1) PVDF Au-TNR/PVDF-10 0 0.005 0.0890 0.09410.8 29.10.020 0.0124.1 20.3 Au-TNR/PVDF-1 Au-TNR/PVDF-20.005 0.089 0.010 0.2060.094 0.21629.1 37.10.012 0.028 20.3 59.1 Au-TNR/PVDF-2 Au-TNR/PVDF-30.010 0.206 0.013 0.2810.216 0.29437.1 53.80.028 0.062 59.1 188.6 Au-TNR/PVDF-3 Au-TNR/PVDF-40.013 0.281 0.016 0.3670.294 0.38353.8 57.70.062 0.075 188.6 242.8 Au-TNR/PVDF-4 Au-TNR/PVDF-50.016 0.367 0.018 0.4740.383 0.49257.7 156.70.075 0.048 242.8 427.7 Au-TNR/PVDF-5 Au-TNR/PVDF-60.018 0.474 0.021 0.6030.492 0.624156.7 226.30.048 0.052 427.7 657.6 Au-TNR/PVDF-6 TNR/PVDF0.021 0.603 0 0.50.624 0226.3 65.90.052 0.028 657.6 103.5 TNR/PVDF 0 0.5 0 65.9 0.028 103.5 2.4. Characterization 2.4. Characterization The phase structures of the PVDF filler and Au-TNR/PVDF nanocomposites were characterizedThe phase by structures X-ray diffractometry of the PVDF (XRD, filler PANalytical,and Au-TNR/PVDF EMPYREAN). nanocomposites The surface were mor- phologiescharacterized of Au, by TNRs,X-ray diffractometry and the Au-TNR (XRD, nanoparticles PANalytical, were EMPYREAN). revealed usingThe surface transmission mor- electronphologies microscopy of Au, TNRs, (TEM, and FEI the Tecnai Au-TNR G2 nanoparticles 20). The chemical were composition revealed using of eachtransmission element in theelectron Au-TNR microscopy hybrid nanoparticles (TEM, FEI Tecnai was analyzedG2 20). The by chemical X-ray photoelectron composition spectroscopyof each element (XPS, PHI5000in the Au-TNR VersaProbe hybrid II, ULVAC-PHI,nanoparticles Japan)was anal atyzed the SUT-NANOTEC-SLRI by X-ray photoelectron Joint spectroscopy Research Fa- cility,(XPS, Synchrotron PHI5000 VersaProbe Light Research II, ULVAC-PHI, Institute (SLRI),Japan) at Thailand. the SUT-NANOTEC-SLRI The fracture microstructures, Joint Re- search Facility, Synchrotron Light Research Institute (SLRI), Thailand. The fracture micro- distributions, and percentages of each element in the Au-TNR/PVDF nanocomposites structures, distributions, and percentages of each element in the Au-TNR/PVDF nano- were investigated by focused ion beam–field emission scanning electron microscopy (FIB– composites were investigated by focused ion beam–field emission scanning electron mi- FESEM, FEI Helios Nanolab G3 CX). The samples were fractured using liquid N and their croscopy (FIB–FESEM, FEI Helios Nanolab G3 CX). The samples were fractured using2 liq- surfaces were sputtered with Au before SEM characterization. The crystalline phases of the uid N2 and their surfaces were sputtered with Au before SEM characterization. The crys- nanocomposites were determined using Fourier-transform infrared spectroscopy (FTIR, talline phases of the nanocomposites were determined using Fourier-transform infrared Bruker, TENSOR27) in the 700–1800 cm−1 wavelength range. The dielectric properties spectroscopy (FTIR, Bruker, TENSOR27) in the 700–1800 cm−1 wavelength range. The di- ofelectric the samples properties were analyzedof the samples using anwere impedance analyzed analyzer using (KEYSIGHTan impedance E4990A) analyzer in the 2 6 − ◦ 10(KEYSIGHT–10 Hz and E4990A)60–150 in theC 10 frequency2–106 Hz and and −60–150 temperature °C frequency ranges, and respectively, temperature with ranges, an os- cillationrespectively, voltage with of an 0.5 oscillation V. Before voltage any dielectric of 0.5 V. measurement,Before any dielectric both sidesmeasurement, of each circularboth samplesides of were each coated circular with sample Ag towere form coated electrodes. with Ag to form electrodes.

3.3. Results Results andand DiscussionDiscussion FigureFigure1 1displays displays TEM TEM imagesimages showing th thee morphologies morphologies of of the the Au, Au, TNRs, TNRs, and and Au- Au- TNRTNR hybrid hybrid nanoparticles. nanoparticles. The The Au Au nanoparticles nanoparticles are are spherical spherical with with diameters diameters of of 10–20 10–20 nm. Meanwhile,nm. Meanwhile, the heat-treated the heat-treated TNRs TNRs are are rod-shaped rod-shaped with with slightly slightly different different aspect aspect ratios,ra- whiletios, somewhile Ausome clusters Au clusters are dotted are dotted on the on TNR the surfacesTNR surfaces of the of Au-TNR the Au-TNR hybrid hybrid nanoparticles, nano- revealingparticles, thatrevealing the Au that nanoparticles the Au nanoparticles successfully successfully formed onformed the TNR on the surfaces. TNR surfaces.

FigureFigure 1. 1.TEM TEM images images of of (a(a)) Au,Au, ((bb)) TNRs,TNRs, and ( (cc)) Au-TNR Au-TNR hybrid hybrid nanoparticles. nanoparticles. Polymers 2021, 13, 2064 5 of 14

Figure2 shows XPS spectra of the Au-TNR powder. As shown in Figure2a, Au 4f peaks were observed at 83.33 and 86.98 eV, which are assigned to Au 4f 7/2 and Au Polymers 2021, 13, x FOR PEER REVIEW4f 5/2 , respectively [41,42]. This confirmed the existence of Au in the prepared Au-TNR5 of 15 powder. As shown in Figure2b, small Ti 2 p peaks were observed at binding energies of 457.69 and 461.34 eV, respectively, corresponding to the presence of Ti3+. Ti 2p signals was observed at binding energies of 458.75 and 464.41 eV, indicating the presence of Ti4+ [43]. Figure 2 shows XPS spectra of the Au-TNR powder. As shown in Figure 2a, Au 4f The Ti3+/Ti4+ ratio was found to be 7.52%. Figure2c shows three of O 1 s XPS peaks; the peaks were observed at 83.33 and 86.98 eV, which are assigned to Au 4f7/2 and Au 4f5/2, peakrespectively at 529.99 [41,42]. eV can This be attributedconfirmed tothe the exis oxygentence of lattice Au in (Ti–O)the prepared [28,43 Au-TNR]. Additional powder. peaks wereAs shown observed in Figure at 531.29 2b, small and Ti 532.32 2p peaks eV, were which observed can be at attributed binding energies to the oxygen of 457.69 vacancy and 3+ in461.34 the rutile eV, respectively, structure [28 corresponding] and hydroxyl to the groups presence [43], of respectively. Ti3+. Ti 2p signals The detectedwas observed Ti in theat Au-TNRbinding energies powder of is 458.75 likely toand have 464.4 originated1 eV, indicating from oxygenthe presence vacancies, of Ti4+ which [43]. canThe be explainedTi3+/Ti4+ ratio byEquations was found (1)to be and 7.52%. (2). Figure 2c shows three of O 1s XPS peaks; the peak at 529.99 eV can be attributed to the oxygen lattice (Ti–O) [28,43]. Additional peaks were x 1 •• − observed at 531.29 and 532.32 eV,O Owhich→ Ocan2 + beV Oattributed+ 2e to the oxygen vacancy in the (1) rutile structure [28] and hydroxyl groups2 [43], respectively. The detected Ti3+ in the Au- TNR powder is likely to have originatedTi4+ +frome− oxygen→ Ti3+ vacancies, which can be explained (2) by Equations (1) and (2). 3+ The presence of the Ti ions can cause1 a significant increase in conductivity, thereby leading to electron hopping between the→ Ti3++and•• Ti+24+ ions under an applied electric(1) field. 2 The XPS results confirmed the existence of Au, Ti3+, and oxygen vacancies, which affected ε0 enhancement in the Au-TNR/PVDF nanocomposites.+ → (2)

FigureFigure 2. 2.XPS XPS spectra spectra of of Au-TNR Au-TNR hybridhybrid nanoparticles; ( (aa)) Au Au 4 4f,f ,((bb) )Ti Ti 2p 2,p and, and (c ()c O) O1s 1. s.

TheThe XRD presence patterns of the of Ti Au,3+ ions PVDF, can cause TNRs, a Au-TNRsignificant nanoparticles, increase in conductivity, and Au-TNR/PVDF thereby nanocompositesleading to electron were hopping obtained between in the the 10–80 Ti3+ and◦ 2θ Tirange,4+ ions asunder shown an applied in Figure electric3. The field. XRD patternThe XPS of results the PVDF confirmed polymer the corresponds existence of Au, to the Ti3+ (100),, and (020),oxygen (110), vacancies, and (021) which planes affected of the αε′-phase enhancement [4]. The in XRD the Au-TNR/PVDF pattern of the TNRsnanocomposites. showed peaks similar to those of the tetragonal structureThe XRD ofpatterns the rutile of Au, phase PVDF, according TNRs, Au-TNR to the standard nanoparticles, reported and inAu-TNR/PVDF JCPDS 21-1276; nonanocomposites impurity phase were was obtained detected. in the In the10–80° case 2θ of range, the Au-TNR as shown hybrid in Figure nanoparticles 3. The XRD and Au-TNR/PVDFpattern of the PVDF nanocomposites, polymer corresponds the XRD to the peak (100), for (020), Au can (110), be and observed (021) planes at 2θ of≈ the38.11 andα-phase assigned [4]. The as aXRD (111) pattern plane of (JCPDS the TNRs 00-00-1172), showed peaks conﬁrming similar to the those existence of the tetragonal of Au in the hybridstructure particles of the rutile and Au-TNR/PVDF phase according nanocomposites.to the standard reported Therefore, in JCPDS the Au21-1276; nanoparticles no im- werepurity conﬁrmed phase was to exist detected. in the In Au-TNR the case nanoparticles of the Au-TNR and Au-TNR/PVDFhybrid nanoparticles nanocomposites. and Au- ≈ Meanwhile,TNR/PVDF nonanocomposites, PVDF diffraction the XRD peaks peak were for observed Au can be in observed the Au-TNR/PVDF at 2θ 38.11 nanocom-and as- positesigned sample, as a (111) which plane can (JCPDS be attributed 00-00-1172), to theconfirming semicrystalline the existence nature of Au of PVDF,in the hybrid which is shieldedparticles by and the Au-TNR/PVDF stronger crystalline nanocomposites. diffraction Th intensityerefore, ofthe the Au TNRs nanoparticles compared were to PVDF.confirmed to exist in the Au-TNR nanoparticles and Au-TNR/PVDF nanocomposites. Mean- while, no PVDF diffraction peaks were observed in the Au-TNR/PVDF nanocomposite sample, which can be attributed to the semicrystalline nature of PVDF, which is shielded by the stronger crystalline diffraction intensity of the TNRs compared to PVDF. PolymersPolymers 20212021, 13, 13, ,x 2064 FOR PEER REVIEW 6 of 14 6 of 15

FigureFigure 3. 3.XRD XRD patterns patterns of theof the Au Au standard standard data Audata standard Au standard (JCPDS (JCPDS 00-00-1172), 00-00-1172), TNRs, fabricated TNRs, fabricated Au-TNRAu-TNR hybrid hybrid nanoparticles, nanoparticles, and and Au-TNR/PVDF-4 Au-TNR/PVDF-4 nanocomposite. nanocomposite.

TheThe FTIR FTIR spectra spectra of theof the PVDF PVDF polymer polymer nanocomposites nanocomposites ﬁlled with filled the with TNRs the and TNRs Au- and Au- TNRs are shown in Figure4. Both nanocomposite systems consisted of α-, β-, and γ-PVDF TNRs are shown in Figure 4. Both nanocomposite systems consisted of α-, β-, and γ-PVDF phases. Weak transmittance bands observed at 766 and 976 cm−1 are attributed to the −1 nonpolarphases. Weakα-phase transmittance [4], consistent bands with the observed XRD result at (Figure766 and3). 976 As thecm characteristic are attributed bands to the non- ofpolar the β α--phase and γ-phase [4], consistent overlapped with at 840 the cm XRD−1, they result were (Figure difﬁcult 3). to As distinguish. the characteristic However, bands of thethe characteristicβ- and γ-phase band overlapped at 1279 cm− at1 only840 correspondscm−1, they were to the difficultβ-phase to [4 ].distinguish. As shown inHowever, Figurethe characteristic4, the transmittance band at intensity 1279 cm of−1 the onlyβ-phase corresponds for the three-phaseto the β-phase Au-TNR/PVDF-5 [4]. As shown in Fig- compositeure 4, the transmittance is stronger than intensity that of the of two-phase the β-phase TNR/PVDF for the three-phase composite, Au-TNR/PVDF-5 particularly at com- −1 1279posite cm is .stronger To estimate than the that %β -phasesof the two-phase in the nanocomposites, TNR/PVDF the comp absorptionosite, particularly ratios of the at 1279 βcm- and−1. Toα-phase estimate were the compared. %β-phases Equation in the (3)nanocomposites, was used to quantify the absorption the relative ratios fraction of ofthe β- and the β-phase (F(β)) [4], assuming that only the β- and α-phase exist: α-phase were compared. Equation (3) was used to quantify the relative fraction of the β-

phase (F(β)) [4], assuming that only the β- Aandβ α-phase exist: F(β) = (3) (Kβ/Kα)Aα + Aβ A F(β) = (3) (K /K )A +A−1 where Aα and Aβ are the absorption bands at 766 and 840 cm , respectively, and Kα and 4 Kβ are the absorption coefficients of the respective bands (Kα = 6.1 × 10 and Kβ = 7.7 × where Aα and Aβ are the absorption bands at 766 and 840 cm−1, respectively, and Kα and 104 cm2·mol−1). The calculated F(β) of the two-phase and three-phase nanocomposites Kβ are the absorption coefficients of the respective bands (Kα = 6.1 × 104 and Kβ = 7.7 × 104 were 0.220 and 0.331, respectively. The negative charge of the Au nanoparticles causes an 2 −1 increasecm ·mol in). amount The calculated of the polar F(β)-phase of the of two-phase the PVDF nanocompositesand three-phase [44 nanocomposites], leading to a were Au-TNR/PVDF0.220 and 0.331, nanocomposite respectively. with The a negative significantly charge enhanced of theε0 Au[45]. nanoparticles causes an in- creaseThe in fracture amount cross-sectional of the polar images β-phase ofthe of nanocompositesthe PVDF nanocomposites containing various [44], Au-TNRleading to a Au- hybridTNR/PVDF particles nanocomposite are shown in Figure with5. a The significantly microstructure enhanced of the PVDF ε′ [45]. polymer is shown in Figure5a and reveals that the PVDF molecules form a continuous phase. Figure5b,c show the microstructures of the Au-TNRs/PVDF-2 and Au-TNRs/PVDF-4 nanocomposites. The Au-TNR hybrid nanoparticles are dispersed homogeneously in the PVDF matrix without aggregation. Some air voids and Au-TNR nanoparticle aggregation were observed with increasing Au-TNR hybrid particle content, as exemplified by Au-TNR/PVDF-6, as shown in Figure5d. Polymers 2021, 13, x FOR PEER REVIEW 7 of 15

Polymers 2021, 13, 2064 7 of 14 Polymers 2021, 13, x FOR PEER REVIEW 7 of 15

Figure 4. FTIR spectra of the TNR/PVDF and Au-TNR/PVDF-5 nanocomposites.

The fracture cross-sectional images of the nanocomposites containing various Au- TNR hybrid particles are shown in Figure 5. The microstructure of the PVDF polymer is shown in Figure 5a and reveals that the PVDF molecules form a continuous phase. Figure 5b,c show the microstructures of the Au-TNRs/PVDF-2 and Au-TNRs/PVDF-4 nanocomposites. The Au-TNR hybrid nanoparticles are dispersed homogeneously in the PVDF matrix without aggregation. Some air voids and Au-TNR nanoparticle aggregation were observed with increasing Au-TNR hybrid particle content, as exemplified by Au- FigureFigureTNR/PVDF-6, 4.4. FTIR spectra as shown ofof thethe in TNR/PVDFTNR/PVDF Figure 5d. an andd Au-TNR/PVDF-5 Au-TNR/PVDF-5 nanocomposites. nanocomposites.

FigureFigure 5.5. SEM cross-section images images of of ( (aa)) PVDF, PVDF, ( (bb)) Au-TNR/PVDF-2, Au-TNR/PVDF-2, (c ()c Au-TNR/PVDF-4,) Au-TNR/PVDF-4, and and (d) (Au-TNR/PVDF-6.d) Au-TNR/PVDF-6.

SEM element maps and EDS were employed to further conﬁrm the existence of Au in the three-phase nanocomposites. As shown in Figure6, the microstructure of Au- TNR/PVDF-4 exhibited Au clusters dispersed on the TNR surfaces that are surrounded by the PVDF matrix. EDS was used to determine that Au, Ti, O, C, and F are present in the nanocomposite at levels of 1.3, 57, 24.5, 14.3, and 2.9 wt%, respectively.

Figure 5. SEM cross-section images of (a) PVDF, (b) Au-TNR/PVDF-2, (c) Au-TNR/PVDF-4, and (d) Au-TNR/PVDF-6. Polymers 2021, 13, x FOR PEER REVIEW 8 of 15

SEM element maps and EDS were employed to further confirm the existence of Au in the three-phase nanocomposites. As shown in Figure 6, the microstructure of Au- Polymers 2021, 13, 2064 TNR/PVDF-4 exhibited Au clusters dispersed on the TNR surfaces that are surrounded8 of 14 by the PVDF matrix. EDS was used to determine that Au, Ti, O, C, and F are present in the nanocomposite at levels of 1.3, 57, 24.5, 14.3, and 2.9 wt%, respectively.

FigureFigure 6.6. ElementElement mappingmapping andand EDS-FESEMEDS‒FESEM characterization characterization of of Au-TNR/PVDF-4. Au-TNR/PVDF-4.

0 TheThe frequency dependences of of ε′ε, ,tan tanδ,δ and, and σacσ ofac nanocompositesof nanocomposites with with different different vol- volumeume fractions fractions of Au-TNRs of Au-TNRs (fAu-TNRs (f Au-TNRs) at room) at roomtemperature temperature are shown are shown in Figure in Figure 7. As shown7. As 0 shownin Figure in Figure7a, the7 ε′a, increased the ε increased with increasing with increasing fAu-TNRs.f AAu-TNRs significant. A significant enhancement enhancement in ε′ was inachievedε0 was achievedby incorporating by incorporating small amounts small of amounts Au and of TNR Au andnanoparticles TNR nanoparticles in the nanocom- in the nanocomposite.posite. The enhanced The enhancedε′ value ofε the0 value Au-TNR/PVDF-6 of the Au-TNR/PVDF-6 composite was composite ~226 at 1 was kHz, ~226 which at 1is kHz, ~20 times which larger is ~20 than times thatlarger of a pure than PVDF that polymer of a pure (ε′ PVDF ≈ 10.78). polymer The increase (ε0 ≈ 10.78). in ε′ for The the increasethree-phase in ε 0Au-TNR/PVDFfor the three-phase nanocomposites Au-TNR/PVDF can be nanocomposites ascribed to the canformation be ascribed of Au-TNR to the formationhybrid nanoparticles. of Au-TNR A hybrid large nanoparticles.amount of blocked A large charges amount at the of interface blocked chargesbetween at TNR- the interfacePVDF and between Au-PVDF TNR-PVDF can enhance and Au-PVDFinterfacial canpolarization, enhance interfacial which is known polarization, as Maxwell– which isWagner–Sillars known as Maxwell–Wagner–Sillars (MWS) polarization [6,46]. (MWS) Theref polarizationore, in an [ 6electric,46]. Therefore, field, the inenhanced an electric in- 0 field,terfacial the polarization enhanced interfacial enhances polarization the ε′ of the enhances Au-TNR/PVDF the ε of nanocomposites. the Au-TNR/PVDF Another nanocom- fac- posites.tor is the Another semiconductor factor is nature the semiconductor of the TNR nanoparticles, nature of the TNR which nanoparticles, can producewhich interfacial can 0 producepolarization interfacial over a polarization wide range overof frequencies. a wide range Moreover, of frequencies. the ε′ behavior Moreover, of theeachε samplebehav- 2 6 iorexhibits of each a similar sample trend exhibits in athe similar 102–10 trend6 Hz inrange. the 10Meanwhile,–10 Hz range. the tan Meanwhile,δ values of the the tan Au-δ valuesTNR/PVDF of the nanocomposites Au-TNR/PVDF decreased nanocomposites as the frequency decreased was as the increased frequency to wasapproximately increased 4 to104 approximately kHz and gradually 10 kHz increased and gradually at higher increased frequencies, at higher as shown frequencies, in Figure as 7b. shown This in- in Figurecrease7 inb. tan Thisδ is increase generally in tan consistentδ is generally with the consistent dielectric with relaxation the dielectric of the relaxation pure PVDF of pol- the pureymer PVDF [6]. Considering polymer [6 ].a low-frequency Considering a low-frequencyrange, tanδ of the range, Au-TNR/PVDF tanδ of the Au-TNR/PVDFnanocomposites nanocomposites increased with increasing f . The increased tanδ value as a result of increased with increasing fAu-TNRs. The increasedAu-TNRs tanδ value as a result of increased filler increasedloading is filler attributed loading to isthe attributed conduction to the of free conduction charge carriers of free charge[6,47], which carriers corresponds [6,47], which to corresponds to the increase in f . Furthermore, for the composites with high filler the increase in fAu-TNRs. Furthermore,Au-TNRs for the composites with high filler loading, it is ob- 3 loading, it is observed that tanδ continuously increases with decreasing frequency3 2 from 10 served2 that tanδ continuously increases with decreasing frequency from 10 to 10 Hz. This toobservation 10 Hz. This was observation resulted from was the resulted conduction from the of free conduction charge carriers, of free charge which carriers, is more whichprom- is more prominent in a low-frequency range. The increase in tanδ in the high-frequency inent in a low-frequency range. The increase in tanδ in the high-frequency range is at- range is attributed to the αa relaxation from the glass transition in the PVDF polymer [6,48]. tributed to the αa relaxation from the glass transition in the PVDF polymer [6,48]. Th tanδ Th tanδ of the nanocomposite increases slowly with increasing Au-TNR content. Interest- of the nanocomposite increases slowly with increasing Au-TNR content. Interestingly, ingly, tanδ is exceptionally low for all nanocomposites at 1 kHz. The maximum value of tanδ is exceptionally low for all nanocomposites at 1 kHz. The maximum value of tanδ is tanδ is less than 0.08 at a frequency of 1 kHz. The tanδ value of Au-TNR/PVDF-6 is 0.05, less than 0.08 at a frequency of 1 kHz. The tanδ value of Au-TNR/PVDF-6 is 0.05, which is which is much lower than values obtained in other work (tanδ > 0.1) that used Ag@TiO2 as much lower than values obtained in other work (tanδ > 0.1) that used Ag@TiO2 as fillers fillers [34,35,37,49]. As shown in Figure7c, the σac value of the Au-TNR/PVDF nanocomposite increased slightly with increasing Au-TNR content. At f Au-TNRs = 0.624, the σac value of the nanocomposite was only 6.58 × 10−9 S·cm−1 at 1 kHz, which is lower than that of the other three-phase composite systems (>10−7 S·cm−1)[34,35]. These results confirm that no conducting network is formed, indicating that the Au-TNR-PVDF nanocomposites exhibit good insulation properties. Polymers 2021, 13, x FOR PEER REVIEW 9 of 14

[34,35,37,49]. As shown in Figure 7c, the σac value of the Au-TNR/PVDF nanocomposite increased slightly with increasing Au-TNR content. At fAu-TNRs = 0.624, the σac value of the nanocomposite was only 6.58 × 10−9 S·cm−1 at 1 kHz, which is lower than that of the other three-phase composite systems (>10−7 S·cm−1) [34,35]. These results confirm that no con- Polymers 2021, 13, 2064 ducting network is formed, indicating that the Au-TNR-PVDF nanocomposites exhibit 9 of 14 good insulation properties.

(a) PVDF Au-TNR/PVDF-4 103 Au-TNR/PVDF-1 Au-TNR/PVDF-5 Au-TNR/PVDF-2 Au-TNR/PVDF-6 Au-TNR/PVDF-3

' 2

ε 10

1 10

0.2 PVDF (b) Au-TNR/PVDF-4 Au-TNR/PVDF-1 Au-TNR/PVDF-5 Au-TNR/PVDF-2 Au-TNR/PVDF-6 Au-TNR/PVDF-3

0.1

tan

0.0 (c) PVDF Au-TNR/PVDF-4 -5 10 Au-TNR/PVDF-1 Au-TNR/PVDF-5 Au-TNR/PVDF-2 Au-TNR/PVDF-6 -6 10 Au-TNR/PVDF-3

) -7

-1 10 cm

. -8 10 (S 10-9 σ 10-10 10-11

-12 10 102 103 104 105 106 Frequency (Hz) 0 Figure 7. FrequencyFigure 7. Frequencydependence dependence of (a) ε′, ( ofb) (tana) εδ,,( andb) tan(c)δ ,σ and for (nanocompositesc) σ for nanocomposites with varying with varying Commented [M1]: Please provide a better quality of this amounts ofamounts Au-TNRs. of Au-TNRs. figure in order to convert to a clear PDF document with high quality. Figure8 shows the ε0 and tanδ of Au-TNR/PVDF at 1 kHz as functions of temperature. Figure 8 shows the ε′ and tanδ of Au-TNR/PVDF at 1 kHz as functions of tempera- Commented [PT2R1]: A change has been made. ε0 ture. As shownAs shown in Figure in Figure 8a, steady8a, steady values values of ε′ of werewere observed observed for almost for almost all nanocompo- all nanocomposites ε0 sites with withincreasing increasing temperature. temperature. Only Only Au-TNR/PVDF-5 Au-TNR/PVDF-5 and Au-TNR/PVDF-6 and Au-TNR/PVDF-6 exhibited exhibited values that were slightly temperature dependent. Figure8b shows the tan δ relaxation ε′ values that were slightly temperature dependent. Figure 8b shows the tanδ relaxation peaks in the pure PVDF polymer. The ﬁrst relaxation was observed between −40 and 0 ◦C, peaks in the pure PVDF polymer. The first relaxation was observed between −40 and 0 °C, which can be attributed to the β-relaxation of PVDF. The second relaxation was observed which can be attributed to the β-relaxation of PVDF. The second relaxation was observed at a temperature above 40 ◦C, which can be attributed to the α-relaxation [50]. at a temperature above 40 °C, which can be attributed to the α-relaxation [50]. Polymers 2021, 13, x FOR PEER REVIEW 10 of 15

Figure 8 shows the ε′ and tanδ of Au-TNR/PVDF at 1 kHz as functions of temperature. As shown in Figure 8a, steady values of ε′ were observed for almost all nanocomposites with increasing temperature. Only Au-TNR/PVDF-5 and Au-TNR/PVDF-6 exhibited ε′ values that were slightly temperature dependent. Figure 8b shows the tanδ relaxation Polymers 2021, 13, 2064 peaks in the pure PVDF polymer. The first relaxation was observed between −40 and 0 °C,10 of 14 which can be attributed to the β-relaxation of PVDF. The second relaxation was observed at a temperature above 40 °C, which can be attributed to the α-relaxation [50].

FigureFigure 8. Temperature 8. Temperature dependence dependence of of (a )(aε)0 ε′and and ( b(b)) tantanδ for nanocomposites nanocomposites with with varying varying amounts amounts of Au-TNRs. of Au-TNRs.

FigureFigure9a 9a shows shows the the εε′0 valuesvalues of of TNR/PVDF TNR/PVDF and andAu-TNR/PVDF-5 Au-TNR/PVDF-5 as a function as a function of of frequency. The The ε′ valueε0 value of the of three-phase the three-phase nanoco nanocompositemposite (Au-TNR/PVDF-5) (Au-TNR/PVDF-5) was found was foundto be to much be much higher higher than that than of thatthe two- of thephase two-phase nanocomposite nanocomposite (TNR/PVDF) (TNR/PVDF) (with nearly (with the same total volume fraction of filler) in the 102–106 Hz frequency range, which indicates nearly the same total volume fraction of filler) in the 102–106 Hz frequency range, which that the addition of a small amount of Au nanoparticles can result in a significant enhance- indicates that the addition of a small amount of Au nanoparticles can result in a significant ment in the ε′ of a polymer0 composite. Interestingly, the tanδ value of the Au-TNR/PVDF- enhancement5 nanocomposite in the atε 1of kHz a polymer was 0.048. composite. These excellent Interestingly, dielectric the properties tanδ value of ofAu- the Au- TNR/PVDF-5TNR/PVDF are nanocomposite not only due to atthe 1 introduction kHz was 0.048. of the These Au-TNR excellent hybrid dielectricnanoparticles, properties but of Au-TNR/PVDFalso due to the are increasing not only polar due β to-phase the introduction in the PVDF ofmatrix, the Au-TNR which was hybrid confirmed nanoparticles, by butFTIR also spectroscopy due to the increasing(Figure 4). The polar largeβ -phaseinterfacial in area the of PVDF the semiconducting matrix, which TNRs was is confirmed one byof FTIR the spectroscopymost important (Figure factors4 that). The significan large interfacialtly increases area the ofdielectric the semiconducting response in the TNRs is onenanocomposite. of the most As important shown in Figure factors 9b, that although significantly tanδ of the increases Au-TNR/PVDF-5 the dielectric nanocom- response in theposite nanocomposite. was increased over As the shown measured in Figure frequency9b, although range compared tan δ of to the that Au-TNR/PVDF-5 of the two- nanocompositephase TNR/PVDF was nanocomposite, increased over the the obtained measured tanδ frequency value was rangelower than compared 0.08 in tothe that fre- of the quency range of 102–106 Hz. Polymers 2021, 13, x FOR PEER REVIEWtwo-phase TNR/PVDF nanocomposite, the obtained tanδ value was lower than 0.0811 in of the 15

frequency range of 102–106 Hz.

Figure 9. Frequency dependence ofof ((aa)) εε′0 andand ((bb)) tantanδδ forfor TNR/PVDFTNR/PVDF and Au-TNR/PVDF-5Au-TNR/PVDF-5 at 20 ◦°C;C; the different εε′0 values (green(green area)area) resultedresulted fromfrom thethe AuAu nanoparticles.nanoparticles.

The ε′ε0 valuesvalues of of the the Au-TNR/PVDF Au-TNR/PVDF nan nanocompositesocomposites could could not be not fitted be ﬁttedto two-phase to two-phase com- positecomposite models models consisting consisting of a ceramic of a ceramic and a andpolymer a polymer (e.g., effective (e.g., effective medium medium theory (EMT), theory Maxwell–Garnett,(EMT), Maxwell–Garnett, Yamada, Yamada, logarithmic logarithmic [5,51]) with [5,51 high]) with Au-TNR high Au-TNR contents, contents, as demon- as strated in the inset of Figure 10. This is due to interfacial polarization at the interface between fillers and PVDF polymer matrix. Moreover, the ε′ values of the Au-TNR/PVDF nanocomposites could not be fitted to the percolation model, which is employed for metal/polymer dual phases. As shown in Figure 10, the dielectric behavior of the Au- TNR/PVDF nanocomposites is in good agreement with the EMPT model [35,52], which combines the EMT model with percolation theory, as shown in Equation (4): (ε −ε) − = 1 + (4) ε +n(1−)(ε −ε)

where εeff is the effective ε′ of the Au-TNR/PVDF composite, fTNRs is the volume fraction of the TNRs, fc is the percolation threshold, εPVDF is the ε′ of PVDF (εPVDF = 10.78), εTNRs is the ε′ of TNRs (εTNRs = 150), n is the morphology fitting factor, and q is the critical exponent. Due to the semiconducting nature of TNRs and conducting nature of Au nanoparticles, f is assigned as the volume fraction of Au-TNR hybrid particles, which can influence the percolation behavior of the composites. For the curve fitted using the EMPT model, the optimum fitting parameters were determined to be: n = 0.11, q = 1.0, and fc = 0.8. It is worth noting that n and q are very close to those reported for the Ag-BaTiO3/PVDF (n = 0.11) [52] and the Ni-BaTiO3/PVDF (q = 1.0) [23], respectively. The percolation threshold is expected to occur at a high filler loading (fc = 0.8), which is much higher than the maximum filler loading used in this current study, and is due to the small amount of conductive Au nanoparticles used and the hybrid structure of the Au-TNR particles. Therefore, the percolation network (or conduction pathway) would not be formed in the Au-TNR/PVDF composite because the hybrid structures of the Au-TNRs prevent the formation of conducting pathways because the randomly grown Au nanoparticles do not continuously coat the TNR surface. The large increase in the ε′ value is primarily attributed to interfacial polarization between the Au–PVDF, Au–TNR, and TNR–PVDF interfaces. Polymers 2021, 13, 2064 11 of 14

demonstrated in the inset of Figure 10. This is due to interfacial polarization at the interface between ﬁllers and PVDF polymer matrix. Moreover, the ε0 values of the Au-TNR/PVDF nanocomposites could not be ﬁtted to the percolation model, which is employed for metal/polymer dual phases. As shown in Figure 10, the dielectric behavior of the Au- TNR/PVDF nanocomposites is in good agreement with the EMPT model [35,52], which combines the EMT model with percolation theory, as shown in Equation (4):

−q fTNRs(εTNRs − εPVDF) fc − f εeff = εPVDF 1 + (4) εPVDF + n(1 − fTNRs)(εTNRs − εPVDF) f

0 where εeff is the effective ε of the Au-TNR/PVDF composite, f TNRs is the volume fraction 0 of the TNRs, f c is the percolation threshold, εPVDF is the ε of PVDF (εPVDF = 10.78), εTNRs is 0 the ε of TNRs (εTNRs = 150), n is the morphology fitting factor, and q is the critical exponent. Due to the semiconducting nature of TNRs and conducting nature of Au nanoparticles, f is assigned as the volume fraction of Au-TNR hybrid particles, which can influence the percolation behavior of the composites. For the curve fitted using the EMPT model, the optimum fitting parameters were determined to be: n = 0.11, q = 1.0, and f c = 0.8. It is worth noting that n and q are very close to those reported for the Ag-BaTiO3/PVDF (n = 0.11) [52] and the Ni-BaTiO3/PVDF (q = 1.0) [23], respectively. The percolation threshold is expected to occur at a high filler loading (f c = 0.8), which is much higher than the maximum filler loading used in this current study, and is due to the small amount of conductive Au nanoparticles used and the hybrid structure of the Au-TNR particles. Therefore, the percolation network (or conduction pathway) would not be formed in the Au-TNR/PVDF composite because the hybrid structures of the Au-TNRs prevent the formation of conducting pathways because the randomly grown Au nanoparticles do not Polymers 2021, 13, x FOR PEER REVIEW 0 12 of 15 continuously coat the TNR surface. The large increase in the ε value is primarily attributed to interfacial polarization between the Au–PVDF, Au–TNR, and TNR–PVDF interfaces.

FigureFigure 10.10. ExperimentalExperimental data data of of ε′ε for0 for the the Au-TNR/PVDF Au-TNR/PVDF nanocomposites nanocomposites at 1 atkHz 1 kHzand 20 and °C 20 fitted◦C ﬁttedby the by effective the effective medium medium theory theory (EMPT) (EMPT) model; model; inset insetis the is experimental the experimental data dataof ε′of forε0 thefor theAu- Au-TNR/PVDFTNR/PVDF nanocomposites nanocomposites fitted ﬁtted by tw byo-phase two-phase various various theoretical theoretical models. models.

4. Conclusions This study presented a novel method for successfully achieving high ε′ and low tanδ in three-phase PVDF polymer-matrix nanocomposites. The dielectric properties of a PVDF polymer improved significantly by incorporating conductive Au nanoparticles and semiconductive TNRs with enlarged interfacial areas. The Au nanoparticles were discretely attached to the TNR surfaces to enhance interfacial polarization and simultaneously prevent the formation of conducting pathways in the insulative PVDF matrix. As a result, a high ε′ (∼157) and low tanδ (∼0.05) were obtained in the three-phase nanocomposite filled with 1.8 vol% Au and 47.4 vol% TNRs. The dielectric response in the two-phase TNR/PVDF composite increased by more than a factor of two after introducing small amounts of Au nanoparticles. This dielectric behavior is described using the EMPT model. The results indicate that Au nanoparticles significantly contribute to enhancing interfacial polarization and creating a more polar β-PVDF phase, which increases ε′. In contrast, due to the small amount of Au nanoparticles used and their discrete growth on the TNRs, the value of tanδ remained low. To further investigate the possible use of the Au-TNR/PVDF nanocomposites in capacitor applications, fabrication conditions that produce nanocomposite thin films need be studied.

Author Contributions: Conceptualization, P.K.-o., and P.T.; Formal analysis, P.K.-o. and P.T.; Fund- ing acquisition, P.C.; Investigation, P.K.-o., N.C., J.M., V.H., and N.P.; Methodology, W.T.; Visuali- zation, P.K.-o.; Writing–original draft, P.K.-o. and P.T.; Writing–review & editing, P.T. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Research and Graduate Studies and the Basic Research Fund of Khon Kaen University, grant number 1500147. This work was funded by the Synchrotron Light Research Institute, Khon Kaen University, and the Thailand Research Fund (TRF), grant number BRG6180003 and the Post–doctoral Program from Research Affairs and Graduate School, Khon Kaen University, grant number 60170. It was partially supported by the Research Network NANOTEC (RNN) program of the National Nanotechnology Center (NANOTEC), NSTDA, Minis- try of Higher Education, Science, Research, and Innovation (MHESI), grant number P1851882. Institutional Review Board Statement: Not applicable. Polymers 2021, 13, 2064 12 of 14

4. Conclusions This study presented a novel method for successfully achieving high ε0 and low tanδ in three-phase PVDF polymer-matrix nanocomposites. The dielectric properties of a PVDF polymer improved significantly by incorporating conductive Au nanoparticles and semiconductive TNRs with enlarged interfacial areas. The Au nanoparticles were discretely attached to the TNR surfaces to enhance interfacial polarization and simultaneously prevent the formation of conducting pathways in the insulative PVDF matrix. As a result, a high ε0 (~157) and low tanδ (~0.05) were obtained in the three-phase nanocomposite filled with 1.8 vol% Au and 47.4 vol% TNRs. The dielectric response in the two-phase TNR/PVDF composite increased by more than a factor of two after introducing small amounts of Au nanoparticles. This dielectric behavior is described using the EMPT model. The results indicate that Au nanoparticles significantly contribute to enhancing interfacial polarization and creating a more polar β-PVDF phase, which increases ε0. In contrast, due to the small amount of Au nanoparticles used and their discrete growth on the TNRs, the value of tanδ remained low. To further investigate the possible use of the Au-TNR/PVDF nanocomposites in capacitor applications, fabrication conditions that produce nanocomposite thin films need be studied.

Author Contributions: Conceptualization, P.K.-o., and P.T.; Formal analysis, P.K.-o. and P.T.; Funding acquisition, P.C.; Investigation, P.K.-o., N.C., J.M., V.H., and N.P.; Methodology, P.K.-o.; Visualization, P.K.-o.; Writing–original draft, P.K.-o. and P.T.; Writing–review & editing, P.T. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Research and Graduate Studies and the Basic Research Fund of Khon Kaen University, grant number 1500147. This work was funded by the Synchrotron Light Research Institute, Khon Kaen University, and the Thailand Research Fund (TRF), grant number BRG6180003 and the Post–doctoral Program from Research Affairs and Graduate School, Khon Kaen University, grant number 60170. It was partially supported by the Research Network NANOTEC (RNN) program of the National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Higher Education, Science, Research, and Innovation (MHESI), grant number P1851882. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available in article. Acknowledgments: This research was supported by Research and Graduate Studies and the Basic Research Fund of Khon Kaen University (Grant No. 1500147). This work was partially supported by the Synchrotron Light Research Institute, Khon Kaen University, and the Thailand Research Fund (TRF) (Grant No. BRG6180003). This work also received a scholarship under the Post–doctoral Program from Research Affairs and Graduate School, Khon Kaen University (60170). It was partially supported by the Research Network NANOTEC (RNN) program of the National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Higher Education, Science, Research, and Innovation (MHESI) (Grant No. P1851882), and Khon Kaen University, Thailand. P.K. would like to thank the Science Achievement Scholarship of Thailand (SAST). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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