Dielectric Properties of Graphene/Titania/Polyvinylidene Fluoride (G/Tio2/PVDF) Nanocomposites

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Article Dielectric Properties of Graphene/Titania/Polyvinylidene Fluoride (G/TiO2/PVDF) Nanocomposites

Saira Ishaq 1,2,3 , Farah Kanwal 1,*, Shahid Atiq 4 , Mahmoud Moussa 2,3, Umar Azhar 2 and Dusan Losic 2,3,*

1 Institute of Chemistry, University of the Punjab, Lahore 54590, Pakistan; [email protected] 2 School of Chemical Engineering, The University of Adelaide, Adelaide, SA 5005, Australia; [email protected] (M.M.); [email protected] (U.A.) 3 The ARC Research Hub for Graphene Enabled Industry Transformation, The University of Adelaide, Adelaide, SA 5005, Australia 4 Centre of Excellence in Solid State Physics, University of the Punjab, Lahore 54590, Pakistan; [email protected] * Correspondence: [email protected] (F.K.); [email protected] (D.L.); Tel.: +92-300-420-5680 (F.K.); +61-88-313-4648 (D.L.) Received: 24 November 2019; Accepted: 31 December 2019; Published: 3 January 2020

Abstract: Flexible electronics have gained eminent importance in recent years due to their high mechanical strength and resistance to environmental conditions, along with their effective energy storage and energy generating abilities. In this work, graphene/ceramic/polymer based flexible dielectric nanocomposites have been prepared and their dielectric properties were characterized. The composite was formulated by combining graphene with rutile and anatase titania, and polyvinylidene fluoride in different weight ratios to achieve optimized dielectric properties and flexibility. After preparation, composites were characterized for their morphologies, structures, functional groups, thermal stability and dielectric characterizations by using scanning electron microscopy, X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, thermal gravimetric analysis and impedance spectroscopy. Dielectric results showed that prepared flexible composite exhibited dielectric constant of 70.4 with minor leakage current (tanδ) i.e., 0.39 at 100 Hz. These results were further confirmed by calculating alternating current (AC) conductivity and electric modulus which ensured that prepared material is efficient dielectric material which may be employed in electronic industry for development of next generation flexible energy storage devices.

Keywords: graphene; titania; polyvinylidene ﬂuoride; nanocomposite; dielectric properties

1. Introduction Flexible electronics are attracting increased attention in the electronic industry day by day due to their compact nature and ease of handling. Along with other electronics, flexible dielectric materials are also getting popular for many applications like in field effect transistors (FET), gate dielectrics, pulse power systems, sensors and energy storage devices, etc [1–4]. Polymers are attractive to synthesize flexible materials but they have limitations of low capacitance and low dielectric constants [4]. In last few years efforts have been made to enhance dielectric constant (ε0) of polymers either by using dielectric or conducting fillers [5,6]. Conducting fillers can elevate ε0 to a much higher value, but the amount of conducting filler plays a critical role in this increase. The dielectric response of conducting filler/polymer composites depends upon the percolation phenomenon. In such composites if the concentration of conducting filler is

Materials 2020, 13, 205; doi:10.3390/ma13010205 www.mdpi.com/journal/materials Materials 2020, 13, 205 2 of 11

lower than its percolation threshold (φc), an insulative-dielectric environment exists. However with the increase in concentration of conducting fillers, ε0 increases and reaches a maximum limit when φc is reached, after which it starts decreasing [7]. A number of composites which consist of conducting fillers and polymers have been reported, but a high ε0 is also associated with high energy losses in the case of conducting fillers. Feng et al. [8] reported a silver/poly(vinylidene fluoride) (Ag/PVDF) composite exhibiting ε0 of 31 and loss tangent (tanδ) of 0.02 at 1 kHz. Zhao and group [9] synthesized a carbon nanotubes/poly(vinylidene fluoride) (CNT/PVDF) composite with ε0 of 80.6 and tanδ of 3.51 at 100 Hz. They also reported ε0 of 112.1 and low tanδ of 0.032 for graphene nanoplatelet/PVDF in the same paper. Dielectric fillers increase ε0 when added to polymers. However, in this case the final dielectric properties depend upon many factors like size, shape, distribution, concentration and connectivity of dielectric fillers in the polymer [7]. Pan et al., [10] used barium titanate nanotubes modified with dopamine in poly(vinylidene fluoride (PVDF), and the resultant composite had ε0 of 47.05 at 1 kHz with very low dielectric loss (ε”). Yang et al. reported that nanosize calcium copper titanate particles are more effective in enhancing ε0 than their microsize equivalents. Nanosize particles increased ε0 of PVDF from 10.5 to 2.49 106 at 100 Hz with tanδ of 48. However, the microsize version resulted in ε × 0 of 49 with tanδ of 0.23 at 100 Hz [11]. The ε0 of such composites is increased with an increase in filler content but when such fillers are added in excessive amount, the flexibility and mechanical strength of the composite is decreased [12]. Another approach is to incorporate conducting and ceramic fillers simultaneously in the polymer matrix. Wang et al., [13] synthesized a copper calcium titanate/PPO composite with ε0 of 7.08 which was increased when copper calcium titanate was coated with graphene and then its composite was made with poly(phenylene oxide) (PPO). The resultant composite had ε0 of 8.60 and tanδ of 0.017 at 100 Hz. Qi and coworkers [14] reported ternary composites consisting of carbon nanotubes loaded with barium titanate/polyamide11. The calculated ε0 of the composite at 1 kHz was 16.2. Also they measured tanδ of the same composite at 100 Hz and it was 0.15. Wan and group reported synthesis of a ternary composite based on functionalized graphene/barium titanate/ polyvinylidene fluoride (frGO/BaTiO3/PVDF) which has ε0 of 65 at 1 MHz and tanδ of 0.35 at same frequency [15]. The aim of present research work was the preparation of graphene/titania/polyvinylidene fluoride (G/TiO2/PVDF) composite films with different dosages and ratios of their components and to investigate their influence on dielectric properties. To formulate these three phase composites, we selected every component considering their specific properties and entire properties of the final composite. The PVDF is semicrystalline in nature, having piezoelectric characteristics, high ε0 and tanδ. These characteristics render it most appropriate candidate for dielectric applications [16]. Graphene was used as rational filler to exalt properties of composites as it elevates high mechanical strength, electrical properties and reasonable thermal stability originating from oxygen functional groups on its surface [17]. Among ceramics, TiO2 was selected because of its semiconductor nature and optical properties. Due to its electrical properties it has replaced silica in dielectric devices. It exists in three crystalline forms viz; brooklite, anatase and rutile. Among them anatase and rutile forms are reported to possess dielectric properties [18]. In the present research work, mixture of anatase and rutile forms of TiO2 was used. Dielectric analysis of the composites confirmed excellent dielectric properties of the prepared composites that render them as effective flexible dielectric materials.

2. Materials and Methods

2.1. Materials Commercial graphite rocks were purchased from Uley (Eyre Peninsula, SA, Australia). Polyvinylideneﬂuroride pellets and potassium permanganate (99%) were bought from Sigma-Aldrich (Gillman, Australia). Hydrochloric acid (37%), hydrogen peroxide (35%), sulfuric acid and N,N–dimethylformamide were purchased from Chem-supply (Gillman, Australia). Potassium Materials 2020, 13, 205 3 of 11 3 of 11

titanium(IV) oxide (purity > 99.5% having specific surface area of 9.6 m2 g−1) were taken from Riedel- persulphate (K2S2O8) was purchased from Scharlau (Germany). Phosphorus pentoxide (P2O5) and de Haen (Germany). All reagents were used without any treatment and2 purification.1 titanium(IV) oxide (purity > 99.5% having specific surface area of 9.6 m g− ) were taken from Riedel-de Haen (Germany). All reagents were used without any treatment and purification. 2.2. Preparation of Graphene/Titania/Polyvinylidene Fluoride Nanocomposite Films 2.2. PreparationGraphite ofpowder Graphene was/Titania oxidized/Polyvinylidene to graphene Fluoride oxide (GO) Nanocomposite by a previously Films reported method [19]. Graphene was synthesized by reduction of GO by using hydrazine hydrate [20]. Firstly, 1 g of PVDF Graphite powder was oxidized to graphene oxide (GO) by a previously reported method [19]. was completely mixed in N,N–dimethylforamide (DMF, 20 mL) while heating and stirring at 80 °C. Graphene was synthesized by reduction of GO by using hydrazine hydrate [20]. Firstly, 1 g of PVDF was Specific amounts of TiO2 and graphene were dispersed separately in 10 mL of DMF by using a probe completely mixed in N,N–dimethylforamide (DMF, 20 mL) while heating and stirring at 80 C. Specific sonicator (Branson Digital Sonifier 450, Branson Ultrasonics Corporation, Danbury, CT, USA).◦ Both amounts of TiO2 and graphene were dispersed separately in 10 mL of DMF by using a probe sonicator solutions were mixed while stirring at 80 °C for 45 min. Film of G/TiO2/PVDF composite was casted (Branson Digital Sonifier 450, Branson Ultrasonics Corporation, Danbury, CT, USA). Both solutions on petri dish by drying the solution at 60 °C. Firstly few composite films of G/TiO2/PVDF were were mixed while stirring at 80 C for 45 min. Film of G/TiO /PVDF composite was casted on petri dish prepared by varying amounts◦ of graphene while amounts2 of other components were kept constant. byAfter drying measuring the solution their atε′, 60 graphene◦C. Firstly concentration few composite giving films maximum of G/TiO ε′2/ PVDFwas fixed were while prepared few more by varying films amounts of graphene while amounts of other components were kept constant. After measuring their were fabricated with varying amounts of TiO2. GxTyPVDF, G2xTyPVDF, G3xTyPVDF, G4xTyPVDF, ε , graphene concentration giving maximum ε was fixed while few more films were fabricated with 0G5xTyPVDF, G4xT2yPVDF and G4xT3yPVDF (where0 x = 0.01, y = 0.1) were symbolized as G1T10PVDF, varyingG2T10PVDF, amounts G3T10PVDF-, of TiO2.GxT G4T10PVDF,yPVDF, G2xT yG5T10PVDF,PVDF, G3xTy PVDF,G4T20PVDF G4xTy PVDF,and G4T30PVDF, G5xTyPVDF, respectively. G4xT2yPVDF andAmount G4xT3y ofPVDF PVDF (where in all samples x = 0.01, was y = 0.1)fixed were i.e., symbolized1 g. A scheme as of G1T10PVDF, the preparation G2T10PVDF, of the films G3T10PVDF-, is shown G4T10PVDF,in Figure 1. G5T10PVDF, G4T20PVDF and G4T30PVDF, respectively. Amount of PVDF in all samples was fixed i.e., 1 g. A scheme of the preparation of the films is shown in Figure1.

FigureFigure 1. Scheme1. Scheme of preparation of preparation of three phaseof three graphene phase/titania graphene/titania/polyvinylidene/polyvinylidene fluoride nanocomposite fluoride films.nanocomposite films. 2.3. Characterizations 2.3. Characterizations Pure PVDF and ternary nanocomposites films of size 0.5 cm 0.5 cm (width length) were Pure PVDF and ternary nanocomposites films of size 0.5 cm × 0.5 cm (width ×× length) were characterized by field emission scanning electron microscopy (FESEM, Quanta 450, FEI, Hillsboro, OR, characterized by field emission scanning electron microscopy (FESEM, Quanta 450, FEI, Hillsboro, USA), X-ray diffraction (XRD, 600 Miniflex, Rigaku, Akishima, Japan) (sample film size: width length = OR, USA), X-ray diffraction (XRD, 600 Miniflex, Rigaku, Akishima, Japan) (sample film size:× width × 1.5 cm 1.5 cm) raman spectroscopy (LabRam HR Evolution, Horiba-Yvon Jobin Technology, Kyoto, length× = 1.5 cm × 1.5 cm) raman spectroscopy (LabRam HR Evolution, Horiba-Yvon Jobin Japan) (sample size: width length = 0.5 cm 0.5 cm) and Fourier transform infrared (FTIR) Technology, Kyoto, Japan) (sample× size: width ×× length = 0.5 cm × 0.5 cm) and Fourier transform spectroscopy (Nicolet 6700, Thermo Fisher, Waltham, MA, USA) (sample size: width length = infrared (FTIR) spectroscopy (Nicolet 6700, Thermo Fisher, Waltham, MA, USA) (sample size:× width 0.5× length cm 0.5 = 0.5 cm). cm Thermal × 0.5 cm). gravimetric Thermal gravimetric analyzer (TGA, analyzer Q500, (TGA, TA Instruments, Q500, TA Instruments, New Castle, New DE, USA)Castle, for × 1 7DE, mg ofUSA) each for sample 7 mg of was each operated sample was at 30–900 operated◦C atat heating30–900 °C rate at heating of 10 ◦C rate min of− 10A °C precision min−1 A impedanceprecision analyzerimpedance (6500B, analyzer Wayer (6500B, Kerr, Wayer West Sussex, Kerr, West UK) Sussex, was used UK) at was 20 Hz–2 used MHz. at 20 Hz–2 MHz.

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3. Results and Discussion 3. Results and Discussion 3.1. Characterization of Graphene/Titania/Polyvinylidene Fluoride Nanocomposite Films 3.1. Characterization of Graphene/Titania/Polyvinylidene Fluoride Nanocomposite Films The FESEM images of nanocomposites shown in Figure 2 were recorded to study their The FESEM images of nanocomposites shown in Figure2 were recorded to study their morphology. morphology. In the FESEM images graphene sheets, TiO2 particles and PVDF are clearly visible. Both In the FESEM images graphene sheets, TiO particles and PVDF are clearly visible. Both ﬁllers are fillers are evenly distributed in the polymer.2 It is evident that graphene sheets are segregated rather evenly distributed in the polymer. It is evident that graphene sheets are segregated rather than being than being aligned due to occurrence of TiO2 and PVDF. Nanoparticles of TiO2 lie on graphene layers aligned due to occurrence of TiO2 and PVDF. Nanoparticles of TiO2 lie on graphene layers and the and the polymer. With increase in weight of TiO2, big aggregates start to form which affect dielectric polymer. With increase in weight of TiO , big aggregates start to form which aﬀect dielectric properties properties of the nanocomposites which2 would be discussed in detail later in dielectric plots. of the nanocomposites which would be discussed in detail later in dielectric plots.

Figure 2. FESEM images of (a) G1T10PVDF (b) G2T10PVDF (c) G3T10PVDF (d) G4T10PVDF Figure 2. FESEM images of (a) G1T10PVDF (b) G2T10PVDF (c) G3T10PVDF (d) G4T10PVDF (e) (e) G5T10PVDF (f) G4T20PVDF (g) G4T30PVDF. G5T10PVDF (f) G4T20PVDF (g) G4T30PVDF. XRD patterns of neat polymer and nanocomposite are shown in Figure3A. In XRD pattern of XRD patterns of neat polymer and nanocomposite are shown in Figure 3A. In XRD pattern of pure PVDF, peaks at 20.4◦ and 29.7◦ are indicative of the beta (β) and alpha (α) phases of PVDF, pure PVDF, peaks at 20.4° and 29.7° are indicative of the beta (β) and alpha (α) phases of PVDF, respectively [21]. In XRD patterns of nanocomposites, the peaks located at 25.7◦, 37.9◦, 54.1◦, 69.1◦ and respectively [21]. In XRD patterns of nanocomposites, the peaks located at 25.7°, 37.9°, 54.1°, 69.1° 75.3◦ correspond to (101), (004), (211), (220) and (215) are diﬀraction peaks of anatase TiO2. These peaks and 75.3° correspond to (101), (004), (211), (220) and (215) are diffraction peaks of anatase TiO2. These are exactly according to JCPDS No. 21-1272. X-ray diﬀraction peaks of rutile TiO2 according to JCPDS peaks are exactly according to JCPDS No. 21-1272. X-ray diffraction peaks of rutile TiO2 according to No. 21-1276 are observed at 55.3◦ (220), 62.8◦ (200), and 70.5◦ (112) [22]. Intensity of peak of α-PVDF at JCPDS No. 21-1276 are observed at 55.3° (220), 62.8° (200), and 70.5° (112) [22]. Intensity of peak of α-

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PVDF29.7◦ decreasesat 29.7° decreases due to interaction due to interaction of ﬁllers withof fillers PVDF. with Peak PVDF. for graphene Peak for isgraphene not visible is not in XRD visible peaks in XRDof nanocomposites. peaks of nanocomposites. It may be due It may to presence be due ofto peakpresence for anataseof peak TiO for2 anatasewhich lieTiO at2 25.7which◦ and lie alsoat 25.7° due andto the also reason due to that the graphenereason that is graphene exfoliated is and exfoliated segregated and duringsegregated sonication during process.sonication This process. isolation This of isolationgraphene of layers graphene is also layers shown is inalso FESEM shown images in FESEM in Figure images2. Raman in Figure spectra 2. Raman further spectra elucidating further the elucidatingstructure of the pure structure PVDF and of two pure selected PVDF nanocomposites and two selected i.e., G4T10PVDFnanocomposites and G4T20PVDFi.e., G4T10PVDF are shown and G4T20PVDFin Figure3B. are shown in Figure 3B.

Figure 3. (A) XRD patterns (a) PVDF (b) G1T10PVDF (c) G2T10PVDF (d) G3T10PVDF (e) G4T10PVDF Figure 3. (A) XRD patterns (a) PVDF (b) G1T10PVDF (c) G2T10PVDF (d) G3T10PVDF (e) G4T10PVDF (f) G5T10PVDF (g) G4T20PVDF (h) G4T30PVDF (B) Raman spectra of (a) PVDF (b) G4T10PVDF (f) G5T10PVDF (g) G4T20PVDF (h) G4T30PVDF (B) Raman spectra of (a) PVDF (b) G4T10PVDF (c) (c) G4T20PVDF (C) FTIR spectra of (a) PVDF (b) G1T10PVDF (c) G2T10PVDF (d) G3T10PVDF G4T20PVDF (C) FTIR spectra of (a) PVDF (b) G1T10PVDF (c) G2T10PVDF (d) G3T10PVDF (e) (e) G4T10PVDF (f) G5T10PVDF (g) G4T20PVDF (h) G4T30PVDF (D) TG curves of (a) PVDF (b) graphene G4T10PVDF (f) G5T10PVDF (g) G4T20PVDF (h) G4T30PVDF (D) TG curves of (a) PVDF (b) graphene (c) TiO2 (d) G4T10PVDF (e) G4T20PVDF. (c) TiO2 (d) G4T10PVDF (e) G4T20PVDF.

The band at 839 cm−1 is indicative of β-PVDF. Bands at 397 and 630 cm−1 in the composites correspond to anatase TiO2. Bands at 1352 and 1338 cm−1 in nanocomposites belong to D band and 1574 cm−1 correspond to G band of graphene [21,23].

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1 1 The band at 839 cm− is indicative of β-PVDF. Bands at 397 and 630 cm− in the composites 1 correspond to anatase TiO2. Bands at 1352 and 1338 cm− in nanocomposites belong to D band and 1 1574 cm− correspond to G band of graphene [21,23]. FTIR spectra of pure polymer and it’s all synthesized nanocomposite are shown in Figure3C. 1 1 Peaks at 1077, 1156 and 1406 cm− conﬁrm α-PVDF. Peaks at 827, 879 and 1234 cm− show presence 1 of β-PVDF [24]. Peaks for graphene at 1077 and 1156 cm− are merged with peaks of PVDF. N–H stretching peak at 1406 cm–1 is due to DMF residues in the nanocomposites [21]. Peak for stretching 1 vibrations of Ti-O-Ti bonds of anatase TiO2 is visible at 472 cm− [25]. TG curves of graphene, TiO2, pure polymer, and two selected samples i.e., G4T10PVDF and G4T20PVDF nanocomposites recoded at 30–900 ◦C are shown in Figure3D. TG curve of graphene show that degradation of graphene starts at 380 ◦C and at 590 ◦C it completely degrades. TiO2 does not degrade even at high temperature of 900 ◦C. At 380 ◦C degradation of PVDF starts and at 565 ◦C, 96% of it degrades. G4T10PVDF and G4T20PVDF starts to degrade at 327 ◦C and 334 ◦C, and lose about 93% and 82% of their weights at 510 ◦C and 500 ◦C, respectively. More TiO2 in G4T20PDMS is responsible for increased thermally stability.

3.2. Dielectric Studies of Graphene/Titania/Polyvinylidene Fluoride Nanocomposite Films A Wayer Kerr 6500B precision impedance analyzer was used at 20 Hz–2 MHz to study the dielectric properties of the samples. Films were placed between copper electrodes and measurements were made. Dielectric constant of films was measured by using relation; ε0 = Ct/εoA where C, t, εo and A represent capacitance, thickness of film, vaccum permittivity and area of the film. ε0 vs log f plots of pure PVDF and G/TiO2/PVDF nanocomposites are shown in Figure4A. ε0 is high at lower frequencies due to fact that induced and permanent dipoles can accommodate themselves in proper alignment in this region, a phenomenon called the Maxwell Wagner Sillar (MWS) effect, also termed as interfacial polarization (IP). However, when frequency is increased, dipoles have less time to line up themselves in proper orientations, thus decreasing MWS and IP. This process is called polarization relaxation phenomenon [26]. At very high frequency range, ε0 becomes constant being frequency independent, a phenomenon called microcapacitor structure model [27]. ε0 increases when w/w of graphene is raised till G4T10PVDF which has value of 42.1 at 20 Hz. ε0 decreases with rising frequency and reaches and maintains to 11.6 at 2 MHz. But still it is greater than that of pure PVDF. Increase of ε0 with amount of graphene is due to reason that graphene is conductive and when its amount is increased it causes rise of IP. As conducting graphene is increased in the nanocomposite, more and more free charges accumulate at interfaces of the components thus forming more dipoles. Both increased interfacial area and homogeneous distribution of fillers contribute in rise of IP [26,28]. It was reported earlier that when conducting and non-conducting additives are embedded in the composites simultaneously, microcapacitors are formed where conducting part act as electrodes and non-conducting part acts as dielectric medium. In presently prepared nanocomposites, graphene sheets acts as electrodes while ceramic and polymers act as dielectric medium of the microcapacitors. As the amount of graphene is increased the number of microcapacitors also multiplies till G4T10PVDF. However after G4T10PVDF when the graphene concentration is further raised in G5T10PVDF, ε0 starts to decline. It is due to the fact that graphene sheets become associated to make flow of charges possible, thus the leakage current is raised to depress ε0 [29]. To further enhance ε0 of the nanocomposites the amount of ceramic was raised by keeping all other concentrations constant. ε0 again raised till the percolation level of ceramic was reached in G4T20PVDF. G4T20PVDF possesses highest ε0 among all nanocomposites i.e., 159.8 at 20 Hz which declined and was maintained to 34.8 at 2 MHz. The rise in ε0 after increasing amounts of ceramic is attributed to expansion of effective charge transport network to improve the conductivity within the nanocomposites [30,31]. However after reaching the percolation level when the amount of ceramic was further raised ε0 was decreased as rising amounts of ceramic result in aggregation of ceramic nanoparticles which not only effects the morphology, but also the efficiency of the charge transport network and the dielectric properties as well. 7 of 11 increases as well aligned conducting graphene sheets cause an increase of the leakage current density. Upon application of an electrical field current starts to flow through the conducting graphene as result of which electrical energy is converted into thermal energy [29]. At 20 Hz, tanδ of G4T10PVDF is 0.5 which reduces to 0.25 at 2 MHz. Tanδ decreases even more with addition of TiO2 and G4T20PVDF has tanδ of 0.5 at 20 Hz which further reduces o 0.03 at 2 MHz. Comparative plots of ε′ and Materialstanδ of2020 pure, 13 ,polymer 205 and nanocomposites at various frequencies are shown in Figure 5a–d.7 of 11

Figure 4. (A) Dielectric constant (ε0)(B) loss tangent (tanδ) of (a) PVDF (b) G1T10PVDF (c) G2T10PVDF Figure(d) G3T10PVDF4. (A) Dielectric (e) G4T10PVDF constant ( f)(ε′ G5T10PVDF) (B) loss (gtangent) G4T20PVDF (tanδ ()h )of G4T30PVDF. (a) PVDF (b) G1T10PVDF (c) G2T10PVDF (d) G3T10PVDF (e) G4T10PVDF (f) G5T10PVDF (g) G4T20PVDF (h) G4T30PVDF. Energy dissipation of pure polymer and all nanocomposites was calculated in the form of tanδ by using formula: Tanδ = ε”/ε0 where ε” and ε0 represent the dielectric loss and dielectric constant, AC conductivity (σac) of films calculated by using the relation σac = ɛoε′ωtanδ (angular frequency respectively. Plots of tanδ of PVDF and all nanocomposites against frequency are shown in Figure4B. = ω = 2πf) σac of pure PVDF and nanocomposites plotted against frequency are shown in Figure 6A. It is clear that tanδ decreases with raised frequency. The effect of different amounts of fillers on tanδ was σac risesalso when studied. amount It was foundof conducting that when graphene the amount is ofraised graphene due istoincreased, formation tan ofδ conductingalso increases routes. as well σac is furtheraligned raised conducting when the graphene ceramic sheets concentration cause an increase is raised. ofthe Among leakage all current nanoco density.mposites, Upon G4T20PVDF application has the highestof an electrical σac. Ceramic field current addition starts improves to flow through σac due the to conducting formation graphene of a charge as result transport of which network electrical within the nanocomposites.energy is converted The into results thermal are energy in complete [29]. At 20harmony Hz, tanδ withof G4T10PVDF previously is 0.5literature which reducesfindings to [31]. Comparative0.25 at 2 MHz. values Tan δofdecreases ε′, tan evenδ and more σ withac of addition pure ofpolymer, TiO2 and G4T10PVDF G4T20PVDF has and tan δG4T20PVDFof 0.5 at nanocomposites20 Hz which furtherat diverse reduces frequencies o 0.03 at 2 MHz.are shown Comparative in Table plots 1. ofElectricε0 and modulus tanδ of pure is polymerused to andexplain transportnanocomposites of electrical at variouscharges frequencies within the are dielectric shown in medium. Figure5a–d. Real (M′) and imaginary parts (M″) of σ σ ε ε ω δ electric modulusAC conductivity were (calculatedac) of films calculatedby using byrelations; using the M relation′ = ε′/(acε″=2 +o ε′0 2)tan and(angular M″ = frequencyε″/(ε′2 + ε″2), = ω = 2πf) σ of pure PVDF and nanocomposites plotted against frequency are shown in Figure6A. respectively. M′ac and M″ of pure polymer and all ternary nanocomposites plotted against log f are σ rises when amount of conducting graphene is raised due to formation of conducting routes. σ is shownac in Figure 6B,C. Figure 6B exhibits that M′ rises with rise in frequency till it acquiresac its further raised when the ceramic concentration is raised. Among all nanocomposites, G4T20PVDF has maximum value, known as asymptotic value (M∞). Achieving M∞ is indicative of the capacitive nature the highest σac. Ceramic addition improves σac due to formation of a charge transport network within of thethe ternary nanocomposites. nanocomposites. The results It is arealso in indicative complete harmony of a relaxation with previously process which literature extends findings over [31 a]. long frequencyComparative range. values Figure of ε6C0, tan showsδ and thatσac of M pure″ also polymer, rises G4T10PVDFand reaches and a maximum G4T20PVDF value, nanocomposites M″max, after whichat diverseit starts frequencies to decline arewith shown rise in Tablehigh1 frequency. Electric modulus range [32]. is used At tolow explain frequency transport range of electrical charges and chargecharges carriers within are theable dielectric to move medium. freely according Real (M0) to and applied imaginary electric parts field (M”) and of electric may travel modulus long were distance calculated by using relations; M = ε /(ε”2 + ε 2) and M” = ε”/(ε 2 + ε”2), respectively. M and M” of 0 0 0 0 0 Materials 2020, 13, 205 8 of 11 pure polymer and all ternary nanocomposites plotted against log f are shown in Figure6B,C. Figure6B exhibits that M0 rises with rise in frequency till it acquires its maximum value, known as asymptotic value (M ). Achieving M is indicative of the capacitive nature of the ternary nanocomposites. ∞ ∞ It is also indicative of a relaxation process which extends over a long frequency range. Figure6C shows 8 of 11 that M” also rises and reaches a maximum value, M”max, after which it starts to decline with rise tillin highM″max frequency is achieved. range However [32]. At lowat high frequency frequencies, range charges charges or and charge charge carriers carriers may are move able toto movesmall coursefreely according only as they to applied do not electrichave sufficient field and time may for travel alliance long distanceas there electric till M”max field is is achieved. rapidly changing However [33,34].at high frequencies, charges or charge carriers may move to small course only as they do not have sufficient time for alliance as there electric field is rapidly changing [33,34].

Figure 5. (a) Loss tangent (tanδ)(b) Real part of permittivity of neat PVDF and G/TiO2/PVDF Figurenanocomposites 5. (a) Loss films tangent with di ff(tanerentδ) graphene(b) Real fractionpart of (permittivityc) Loss tangent of (tan neatδ)( PVDFd) Real and part ofG/TiO permittivity2/PVDF nanocomposites films with different graphene fraction (c) Loss tangent (tanδ) (d) Real part of of G/TiO2/PVDF nanocomposites films with different TiO2 fraction. permittivity of G/TiO2/PVDF nanocomposites films with different TiO2 fraction. Table 1. Dielectric permittivity (ε0), loss tangent (tanδ) and AC conductivity (σac) of neat PVDF,

TableG4T10PVDF 1. Dielectric and G4T20PVDF. permittivity (ε′), loss tangent (tanδ) and AC conductivity (σac) of neat PVDF,

G4T10PVDF andDielectric G4T20PVDF. Permittivity (ε ) Loss Tangent (tanδ) AC Conductivity (σ ) (Sm 1) Frequency (Hz) 0 ac − Neat PVDF G4T10PVDF G4T20PVDF Neat PVDF G4T10PVDF G4T20PVDF Neat PVDF G4T10PVDF G4T20PVDF −1 Dielectric Permittivity (ε′) Loss Tangent (tanδ) AC Conductivity8 9 (σac) (Sm8 ) 20 20.5 42.1 159.8 0.6 0.5 0.5 1.1 10− 8.4 10− 1.8 10− Frequency × 9 × 9 × 8 40Neat 13.1 G4T10PV 36.3 139.8Neat 0.4 G4T10PV 0.1G4T20PV 0.49 Neat6.5 10 − G4T10PV4.1 10− 7.6 10− × 8 × 8 × 8 (Hz)100 9.3 24.4G4T20PVDF 70.4 0.3 0.07 0.39 1.8 10− 1.9 10− G4T20PVDF6.9 10− × × × 103 PVDF7.6 DF 22.9 47.7PVDF 0.07 DF 0.03 DF 0.21 PVDF6.8 10 9 3.5DF10 8 5.7 10 7 × − × − × − 104 7.0 21.7 39.9 0.03 0.06 0.09 1.3 10 7 7.2 10 7 2.1 10 6 × −8 × −−9 × − −8 2010 5 20.56.8 42.1 19.2 159.8 36.8 0.020.6 0.5 0.17 0.5 0.05 1.17.1 × 1010 7 8.41.7 × 1010 5 1.89.6 ×10 106 × − × − × − 106 6.7 12.7 35.0 0.02 0.26 0.03 6.9 10−69 1.9 10 −59 6.6 10 5 −8 40 13.1 36.3 139.8 0.4 0.1 0.49 6.5 ×× 10− 4.1 ×× 10− 7.6× × 10− 2 106 6.7 6.7 11.6 34.8 0.02 0.25 0.03 1.5 10 5 3.5 10 4 100× 9.3 24.4 70.4 0.3 0.07 0.39 1.8 × 10−8 1.9 ×× 10−−8 6.9× × 10− −8 103 7.6 22.9 47.7 0.07 0.03 0.21 6.8 × 10−9 3.5 × 10−8 5.7 × 10−7 104 7.0 21.7 39.9 0.03 0.06 0.09 1.3 × 10−7 7.2 × 10−7 2.1 × 10−6 105 6.8 19.2 36.8 0.02 0.17 0.05 7.1 × 10−7 1.7 × 10−5 9.6 × 10−6 106 6.7 12.7 35.0 0.02 0.26 0.03 6.9 × 10−6 1.9 × 10−5 6.6 × 10−5 2 × 106 6.7 6.7 11.6 34.8 0.02 0.25 0.03 1.5 × 10−5 3.5 × 10−4

Materials 2020, 13, 205 99 ofof 1111

Figure 6. (A) AC conductivity (σac)(B) Real part of electric modulus (M0)(C) Imaginary part of electricFigure 6. modulus (A) AC conductivity (M”) of (a) PVDF (σac) ( (Bb) Real G1T10PVDF part of electric (c) G2T10PVDF modulus (M (d)′) G3T10PVDF(C) Imaginary (e )part G4T10PVDF of electric (modulusf) G5T10PVDF (M″) of (g )( G4T20PVDFa) PVDF (b) (hG1T10PVDF) G4T30PVDF. (c) G2T10PVDF (d) G3T10PVDF (e) G4T10PVDF (f) G5T10PVDF (g) G4T20PVDF (h) G4T30PVDF. 4. Conclusions 4. ConclusionsIn summary, in this research work, we have demonstrated the successful fabrication of new flexible ternaryIn summary, nanocomposite in this films research by combination work, we have of PVDF demonstrated polymer used the assuccessful base matrix fabrication and to achieveof new flexibilityflexible ternary with titania nanocomposite and graphene films additives by combination providing of their PVDF specific polymer properties. used as Thebase comprehensive matrix and to characterizationachieve flexibility of preparedwith titania composites and graphene with diff erentadditives ratios providing of these three their components specific properties. was performed The tocomprehensive confirm their morphologies,characterization structures, of prepared functional composites groups, with thermal different stability ratios and of their these dielectric three properties.components Results was performed showed thatto confirm nanocomposite their morpho G4T20PVDFlogies, structures, (G:TiO2:PVDF functional= 4:20:100) groups, exhibited thermal astability dielectric and constant their anddielectric energy lossproperties. of 70.4 andResults 0.39 atshowed 100 Hz. Calculationthat nanocomposite of AC conductivity G4T20PVDF and electric modulus further confirmed that prepared nanocomposite films are effective dielectric materials (G:TiO2:PVDF = 4:20:100) exhibited a dielectric constant and energy loss of 70.4 and 0.39 at 100 Hz. whichCalculation are proposed of AC conductivity for use in theand electronics electric modu industrylus further for development confirmed that of prepared next generation nanocomposite flexible energyfilms are storage effective devices. dielectric materials which are proposed for use in the electronics industry for Authordevelopment Contributions: of next Performinggeneration allflexible experiments energy and storage drafting devices. of the manuscript: S.I., performing additional experiments and characterizations: S.A., M.M., U.A. Planning and supervision of the research: F.K., S.A. and D.L. AllAuthor authors Contributions: have read and Performing agreed to all the experiments published version and drafting of the manuscript.of the manuscript: S.I., performing additional Funding:experimentsThis and research characterizations: received no externalS.A., M.M., funding. U.A. Planning and supervision of the research: F.K., S.A. and D.L. Acknowledgments: We are thankful to Higher Education Commission of Pakistan for funding the research project underFunding: International This research Research received Support no external Initiative funding. Program (IRSIP). The authors acknowledge the support of the Australian Research Council (IH 150100003 ARC Research Hub for Graphene Enabled Industry Transformation). WeAcknowledgments: acknowledge the We Institute are thankful of Chemistry, to Higher University Education of the Commissi Punjab,on Lahore, of Pakistan Pakistan for for funding providing the researchresearch facilities.project under We are International thankful to Research Centre of Support Excellence Initiative in Solid Program State Physics, (IRSIP). University The authors of the acknowledge Punjab, Lahore, the Pakistansupport forof providingthe Australian facilities Research for dielectric Council spectroscopy. (IH 150100003 ARC Research Hub for Graphene Enabled Industry ConflictsTransformation). of Interest: We Theacknowledge authors declare the Institute no conflict of Chemis of interest.try, University of the Punjab, Lahore, Pakistan for providing research facilities. We are thankful to Centre of Excellence in Solid State Physics, University of the Punjab, Lahore, Pakistan for providing facilities for dielectric spectroscopy.

Conflicts of Interest: The authors declare no conflict of interest.

Materials 2020, 13, 205 10 of 11

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