Enhanced dielectric properties of PVDF polymer nanocomposites: A study on gold−decorated, surface−modified multiwalled carbon nanotubes

Abstract

The integration of surface-modified multiwalled carbon nanotubes (fMWCNTs) into polymer nanocomposites has been extensively studied for their potential to enhance dielectric properties. This study, however, pioneers the use of a novel hybrid filler comprising fMWCNTs coated with metal nanoparticles, specifically aimed at augmenting the dielectric performance of polymers. In our research, poly(vinylidene fluoride) (PVDF) nanocomposite films were synthesized using fMWCNTs with a diameter of ∼6–9 nm and a length of 5 μm, adorned with gold nanoparticles (nAu) of ∼5.4 ± 0.9 nm via an adapted Turkevich method. Comprehensive analyses were conducted on nAu−fMWCNTs hybrid powder and their nanocomposites in PVDF with varying filler concentrations, confirming the formation of nAu−fMWCNTs with a weight ratio of 1.1 : 98.9. Three−phase percolative nanocomposites were produced by dispersing the hybrid filler in N,N−dimethylformamide, facilitated by interactions between the negative charge of nAu−fMWCNTs (zeta potential of ∼ −40.43 ± 0.46 mV) and polar phases of PVDF. This was verified through zeta potential and Fourier−transform infrared spectroscopy analyses. The dielectric permittivity (ε′) of the nanocomposites significantly increased from 17.8 to 524.8 (at 1 kHz) with filler loadings from 0.005 to 0.01 vol%, while the dielectric loss tangent (tanδ) showed a minor increase from 0.05 to 1.18. These enhancements are attributed to the elevated permittivity of nAu−fMWCNTs hybrid powder, PVDF's transition to the β−phase, and interfacial polarization effects. The restrained growth of nAu on fMWCNTs and the inhibition of conductive pathways in the polymer matrix contributed to the low tanδ values.

1. Introduction

Polymer nanocomposites have garnered significant attention as electronic materials due to their lightweight, exceptional electrical breakdown strength, and superior flexibility [[1], [2], [3], [4]]. While ceramic materials typically exhibit superior dielectric properties [5,6], polymers tend to have dielectric constants (ε′) lower than 10 [2,3]. Consequently, there was a focus on enhancing the properties of polymer materials to increase their ε′ value, without compromising flexibility [7]. This enhancement is achieved by blending polymers with fillers, resulting in modified polymer composites suitable for electronic devices, charge storage capacitors, and transducers [1].

Poly(vinylidene fluoride) (PVDF) is a semicrystalline polymer renowned for its excellent chemical resistance, thermal stability, and a straightforward production process that requires lower temperatures compared to ceramic preparations. While PVDF possesses a ε′ of around 10 and is extensively researched compared to other polymers [8,9], its ε′ is insufficient for practical applications in electronic devices like capacitors. To enhance the ε′ of PVDF, filler materials are often employed. A common approach to bolster the ε′ of polymers involves combining them with high ε′ ceramic materials such as BaTiO₃ [10], BFeO₃ (B Bi and La) [11,12], CaCu₃Ti₄O₁₂ [13], and ACu₃Ti₄O₁₂ (A = Ca, Na_0.5Bi_0.5, Na_0.5Y_0.5, and Na_0.33Ca_0.33Bi_0.33) [[14], [15], [16], [17]]. Regrettably, achieving high ε′ values in polymer composite systems often requires a high ceramic material content of about 50 vol% [18,19]. This high content compromises the mechanical integrity of the polymer, resulting in reduced flexibility [20], compromised processability, and an elevated defect density.

To address the issues associated with traditional fillers, conductive fillers like metal particles, carbon nanotubes [1], carbon black, and carbon fiber [21] have been extensively incorporated into polymer composites to elevate ε′ values [22,23]. Through this method, a high ε′ can be attained. Furthermore, the dielectric properties of composites quickly change as the content of conductive fillers increases, often achieving a threshold well below 50 vol%. Multiwall carbon nanotubes (MWCNTs) in particular have been integrated into polymer matrices like percolative composites [21] due to their lightweight properties, substantial aspect ratios, and vast internal surface areas [24]. This results in lower percolation thresholds in composites, enhancing the ε′ value. Notably, MWCNTs not only elevate the ε′ but also allow the polymer to retain its flexibility. When formulating two-phase MWCNTs/PVDF composites with a random system, the percolation threshold for MWCNTs aligns with a volume fraction (f) of 0.0161 [25]. In this case, the ε′ value of the MWCNTs/PVDF composite was ∼50 and the loss tangent (tanδ) was ∼0.3 at 1 kHz. Notably, on increasing the f_MWCNTs fron 0.016 to 0.02, although the ε′ value rapidly increased to ∼300. Concurrently, there was a rapid increase in conductivity by two orders of magnitude, leading to a substantial rise in the tanδ value. Similarly, other previous studies have also demonstrated that while MWCNTs in polymer composites can raise the ε′ value, it also leads to a significant increase in the tanδ of >10 [[26], [27], [28]]. This is because MWCNTs, with their carbon chain structure, can create conductive pathways in the polymer matrix, thereby enhancing conductivity and tanδ, which might not be ideal for all applications. Additionally, a significant barrier with MWCNTs is their poor solubility and processability due to the van der Waals reactions [29,30], and their high surface area aspect ratio often results in self−aggregation [31]. To overcome the self−aggregation of MWCNTs, surfactants [32] were utilized to enhance their dispersibility. Additionally, chemical modifications, particularly the introduction of carboxyl groups on the surfaces of MWCNTs [33], have been shown to improve their distribution within slurries. Such modifications often involve acidic treatments, as extensively documented in the literature [30,[33], [34], [35]]. Furthermore, functionalization stands out as a pivotal strategy to inhibit the self−aggregation of carbon nanotubes. This can be achieved through both noncovalent and covalent means, each significantly enhancing the solubility and dispersal of carbon nanotubes [36]. Sheih et al. [37] observed that the dispersion of carbon nanotubes in solvents like water is markedly improved when they are covalently functionalized, attributing this to the ionization of carboxylic (COOH) groups on the treated nanotubes. In cases where strong acid treatments are employed, such as with nitric [38], sulfuric acid, and nitric acid [30], the attachment of carboxylic groups imparts hydrophilic characteristics to the nanotubes, leading to a markedly more effective dispersion.

To enhance the ε′ while maintaining a low tanδ in polymer nanocomposites, one strategy is to incorporate a hybrid filler. This hybrid filler combines a primary component of carbon nanotubes with a high ε′ value, with metal nanoparticles serving as a secondary filler. The synergy between MWCNTs and nanoparticles has garnered considerable interest due to the novel properties that these hybrid nanostructures exhibit, representing the best of both constituent materials. For the integration of MWCNTs with metal nanoparticles, noble metals such as Pt, Pd, Ag, and Au are particularly promising for enhancing the performance of nanocomposites [39]. These metal nanoparticles are not only easy to synthesize but can also be functionalized with various molecules or groups. Notable developments include the formation of gold−decorated multi−walled carbon nanotubes (Au−MWCNTs) [[40], [41], [42]], silver nanoparticle decoration on multiwalled carbon nanotubes (Ag/CNT) [38], and MWCNTs adorned with Ag nanoparticles (MWCNTs−Ag) [30]. Such hybrid fillers are beneficial as they disrupt the formation of conductive pathways, which in turn hinders the development of a conducting network within composites like MWCNTs/PVDF and Ag nanoparticles (Ag−NPs), potentially inducing the Coulomb blockade effect [1].

Recent studies have indicated that the dielectric properties of PVDF polymer nanocomposites can be substantially improved by incorporating such hybrid particles, as seen in Au−MWCNT [43], and Ag–Cu/MWCNTs/reduced graphene oxide (rGO) nanocomposites [44]. However, the dielectric property variation with temperature in Au−MWCNT/PVDF nanocomposites remains unreported [43], which is a crucial aspect for their practical application. For dielectric polymer composites, Au nanoparticles are suitably selected to decorate MWCNTs for their suitable electrical properties and high thermal stability (318 W/mK). Compared to SiO₂, which has a thermal stability of 1.3 W/mK, Au serves as a more effective filler for the composites in electronics due to its crucial role in effective heat management in electronic devices. Au nanoparticles with sizes of 10–50 nm exhibit decreased conductivity (1.5–2.2 × 10⁻² S/m) [45] due to the Coulomb blocking effect, in contrast to SiO₂ at approximately 10⁻¹⁰ S/m. This difference in conductivity between PVDF and Au nanoparticles enables interfacial polarization, unachievable with SiO₂.

Using the concept of modified MWCNT surface, functionalized MWCNTs (fMWCNTs) treated with 3,4,5-trifluorobromobenzene were employed as a filler to augment the dielectric properties of PVDF matrix composites. Dang et al. [46] demonstrated that the ε′ value of the fMWCNTs/PVDF composite, with f_fMWCNTs = 0.08, was ∼550 at 1 kHz, while the tanδ was ∼2.2. Additionally, it has been reported that PVDF composites filled with surface-modified MWNTs, featuring a core/shell structure, exhibited an increased ε′ of ∼600 at 100 Hz [26]. Despite the surface modification of MWCNTs, maintaining control over the conductive pathway remains challenging, often resulting in high conductivity and tanδ.

According to the Coulomb blocking effect observed in Au nanoparticles, it is hypothesized that these nanoparticles could impede the formation of conductive pathways, thereby restraining increases in conductivity and tanδ. Integrating Au with MWCNTs is expected to significantly enhance the dielectric and thermal properties of the nanocomposite, optimizing the performance of PVDF composites. The stability and compatibility of Au with the PVDF matrix, coupled with easier surface decoration on modified MWCNTs, ensure consistent nanocomposite properties. The study aims to reduce dielectric loss and minimize loss tangent, considering the moderate conductivity of Au.

In this study, we introduce a novel approach whereby gold nanoparticles (nAu) are systematically assembled on the surface of functionalized multi-walled carbon nanotubes (fMWCNTs) using a modified Turkevich method. The resulting nAu−decorated fMWCNTs (abbreviated as nAu−fMWCNTs) hybrid powder was synthesized by first functionalizing MWCNTs with carboxylate groups (COO⁻) to create fMWCNTs, which then attract Au³⁺ ions through electrostatic interactions. Although the functionalization of MWCNTs using nitric acid is less common compared to the dual treatment with sulfuric and nitric acids [33,43,44,47], research by Datsyuk et al. [35] has demonstrated that nitric acid treatment alone can result in a higher relative concentration of carboxylic groups, as determined by titration, in comparison to other acids. This finding underscores the potential of nitric acid modification to significantly enhance the functionality of MWCNTs for the synthesis of complex nanocomposites. nAu nanoparticles, averaging approximately 5.4 ± 0.9 nm in size, were intermittently affixed to the surface of fMWCNTs. These nAu−fMWCNT hybrid powders were subsequently incorporated as fillers into a PVDF polymer matrix. The resulting composites underwent extensive characterization using various analytical techniques. Additionally, their dielectric properties were meticulously examined with respect to both frequency and temperature.

2. Experimental details

2.1. Materials

Gold (III) chloride trihydrate (purity: ≥99.9%; HAuCl₄⋅3H₂O) was bought from Sigma Aldrich (Sigma–Aldrich Co., (St. Louis, MO, USA)). Multiwall carbon nanotubes (MWCNTs) (6 − 9 nm in diameter, 5 μm in length) (purity: ≥95%) were bought from Sigma Aldrich (Sigma–Aldrich Co., (St. Louis, MO, USA). Poly (vinylidene fluoride) (PVDF) average MW∼ 534,000 was bought from Sigma Aldrich (Sigma–Aldrich Co., (St. Louis, MO, France)). Sodium citrate tribasic sesquihydrate (purity: ≥99%: C₆H₅Na₃O₇ ⋅2H₂O) was bought from Sigma Aldrich (Sigma–Aldrich Co, Hamburg, Germany). N, N−dimethylformamide (purity: >99%, HCON(CH₃)₂; DMF), and Hydrochloric acid 37% (HCl) were bought from RCI Labscan (RCI LABSCAN LTD., (24 Rama 1 Rd. Rong Mueang, Pathumwan, Bangkok, Thailand)). Nitric acid 65% (HNO₃) and Sodium hydroxide (purity: ≥99; NaOH) were bought from Merck (Merck Chemicals, Darmstadt, Germany).

2.2. Functionalized of MWCNTs

To produce oxidized MWCNTs or MWCNTs−COOH, 3 g of pristine MWCNTs were suspended in 250 mL of nitric acid and sonicated for 2 h then stirred the suspension for 48 h at ∼25 °C. The resulting suspension was centrifuged with deionized water until the neutral pH was obtained. At last, the MWCNTs−COOH was obtained and dried at 80 °C overnight [35,38].

MWCNTs−COOH were suspended in 250 mL of 0.04 N of NaOH and sonicated for 2 h then the suspension was stirred for 48 h at ∼25 °C. The resulting suspension was centrifuged with deionized water until the neutral pH was obtained. At last, the carboxyl group into carboxylate (MWCNTs−COO⁻ or fMWCNTs) was obtained. After that the product was dried at 80 °C overnight [35,38].

2.3. Preparation of nAu−fMWCNTs hybrid powder

The nAu−fMWCNTs hybrid powder was prepared by the modified Turkevich method [48]. To produce nAu−fMWCNTs, first, 0.5g fMWCNTs were suspended in 25 mL deionized water, and the suspension was stirred for 30 min at ∼25 °C and sonicated for 30 min with dispersing fMWCNTs well. Second, 500 μL of HAuCl₄⋅3H₂O (1 mM) were added to the solution and sonicated for 1 h then the suspension was stirred and heated to boiling while stirring. Third, 4 mL of C₆H₅Na₃O₇⋅2H₂O (38.8 mM) was added to the suspended solution. The mixture was further stirred until it became red. The solution was cool at ∼25 °C. The resulting suspension was centrifuged and washed several times (10 × 30 mL) with deionized water. At last, the obtained nAu−fMWCNTs hybrid powder was dried at 80 °C overnight [49]. Moreover, the prepared nAu−fMWCNTs hybrid powder was examined for a surface charge of particles by the Zetasizer technique. Fig. 1 shows the schematic illustration synthesis of nAu−fMWCNTs. UV–vis spectroscopy and TEM techniques confirm that all nAu nanoparticles had been decorated on the surface of fMWCNTs.

Fig. 1.

Schematic diagram of the formation of nAu−fMWCNTs polymer nanocomposites.

2.4. Preparation of nAu−fMWCNTs/PVDF polymer nanocomposites films

The volume fractions of the PVDF matrix and fillers were calculated by the following equation:

$$\%\left(n\text{Au}‐\mathbf{f}\text{MWCNTs}\right)\text{by}\text{weight}=\frac{{\mathrm{V}}_{\text{nAu}‐\text{fMWCNTs}}{\mathrm{\rho }}_{\text{nAu}‐\text{fMWCNTs}}}{{\mathrm{V}}_{\text{nAu}‐\text{fMWCNTs}}{\mathrm{\rho }}_{\text{nAu}‐\text{fMWCNTs}}+\left(1{-\mathrm{V}}_{\text{PVDF}}\right){\mathrm{\rho }}_{\text{PVDF}}}\times 100\text{,}$$ (1)

where V_{nAu−fMWCNTs} is the percentage of nAu−fMWCNTs by volume, ρ_{nAu−fMWCNTS} is the theoretical density of nAu−fMWCNTs, ρ_PVDF is the density of the PVDF polymer (1.74 g/cm³). The ρ_{nAu−fMWCNTs} were calculated from the following Equation:

$${\rho }_{nAu-fMWCNTs}={\rho }_{fMWCNTs}{V}_{fMWCNTs}+{\rho }_{nAu}{V}_{nAu}\text{,}$$ (2)

where ρ_fMWCNTs and ρ_nAu are the fMWCNTs (2.1 g/cm³) and nAu (19.3 g/cm³), respectively. V_fMWCNTs and V_nAu are the volume fractions of the fMWCNTs and nAu, respectively. We showed that the percentage ratio of nAu: fMWCNTs was obtained to be 1.1: 98.9 wt%. By using Equations (5.1) and (5.2), the ρ_{nAu−fMWCNTs} were calculated and found to be 2.12 g/cm³. Thus, the nanocomposites with different volume fractions of nAu−fMWCNTs hybrid powder can be designed in the experimental using Equation (5.1). The nAu−fMWCNTs/PVDF polymer nanocomposites were prepared by suspending an appropriate amount of PVDF powder in DMF with stirring for 30 min. Similarly, the nAu−fMWCNTs powder was also dissolved in DMF with ultrasonic treatment for 1 h. The mixture solution was stirred further using ultrasonic bath for 1 h and further stirring for 6 h with a dispersion of nAu−fMWCNTs nanoparticles. The mixture was then dried in an oven at 70 °C [50]. Polymer nanocomposites films were obtained by hot−pressing at 200 °C for 30 min with compression of 10 MPa to eliminate defects (i.e., Porous) to obtain a nAu−fMWCNTs/PVDF polymer nanocomposites films.

2.5. Characterization techniques and dielectric measurements

The morphologies of MWCNTs, fMWCNTs, and nAu−fMWCNTs hybrid powders were characterized using a transmission electron microscope (TEM, TECNCI G² 20) operating at an accelerating voltage of 200 kV. For sample preparation, these powders were suspended in ethanol and sonicated for 2 h to ensure a homogeneous dispersion. The resulting suspension was then deposited onto a copper grid for TEM analysis. The amount of nAu nanoparticles attached on surface carbon nanotube nanoparticles was determined using UV–vis absorption spectroscopy (Shimadzu, UV−1800). Stability and surface charge of MWCNTs, fMWCNTs, and nAu−fMWCNTs hybrid powder were studied using Zeta potential estimation from Zetasizer (ZS−90 Malvern Instruments). The phase ingredient of MWCNTs, fMWCNTs, nAu−fMWCNTs hybrid powder, and nAu−fMWCNTs/PVDF nanocomposites were examined using an X−ray Diffractometer (XRD; PANalytical, EMPYREAN). The phase of PVDF matrix and functional groups on the surfaces of fMWCNTs were verified by Fourier transform infrared spectroscopy (FTIR, TENSOR27) in the wavenumber range of 600–4000 cm⁻¹. The morphology of nAu−fMWCNTs/PVDF nanocomposites was studied by focus ion beam–field emission scanning electron microscopy (FIB−FESEM, FEI Helios NanoLab G3 CX). Both surfaces of disk sample were painted with silver glue and dried at 100 °C for 30 min for dielectric measurements. Dielectric properties were studied using Impedance Analyzer (KEYSIGHT, E4990A, Santa Rosa, CA, USA) in the frequency range of 100 Hz to 10 MHz at ∼25 °C. The oscillation voltage was 0.5 V.

3. Result and discussion

TEM has been utilized to characterize the morphologies of MWCNTs, fMWCNTs, and nAu−fMWCNTs hybrid fillers. Fig. 2(a) presents the pristine MWCNTs, which display entangled, fibrous structures with a notable aspect ratio, free from any additional particles or discernible surface modifications, signifying their unmodified state. These pristine MWCNTs have a diameter ranging from 6 to 9 nm, which emphasizes the retention of their intrinsic morphology following functionalization. Fig. 2(b) illustrates the morphology of fMWCNTs. In comparison to the pristine MWCNTs, the fMWCNTs may exhibit a slightly altered contrast owing to the presence of functional groups or other molecular entities on their surface, though such details may not be distinguishable at the given scale or resolution. This suggests that acid treatment predominantly affects the surface chemistry and charge of the MWCNTs without altering their overall structure, as evidenced by the consistent diameter range of 6–9 nm for both variants. Fig. 2(c) shows MWCNTs that have been functionalized and subsequently embellished with nAu particles (nAu−fMWCNTs), with the dark spots marked by arrows corresponding to the nAu particles, averaging approximately 5.4 ± 0.9 nm in size, as determined in Fig. 2(d). The nAu particles were discontinuously attached to the surface of the fMWCNTs. The size of these particles is subject to a variety of factors, including the type of surfactant used, preparation temperature, concentration of HAuCl₄⋅3H₂O [51], type of reducing agent, presence of capping ligand [52], concentration of reducing agents [53], and the methodologies and pH levels applied during the preparation process [54,55]. The attachment of Au nanoparticles to the surface of fMWCNTs, which is noteworthy, could significantly influence the functional properties of the resultant hybrid materials.

Fig. 2.

TEM images of (a) MWCNTs, (b) fMWCNTs, and (c) nAu−fMWCNTs particles. (d) Size distribution of nAu nanoparticles deposited on fMWCNTs.

Zeta potential represents the potential difference between the charged surfaces of carbon nanotubes and the surrounding liquid. This value is associated with the stability of carbon nanotubes in a dispersion medium, indicating the repulsive forces present in the suspension environment. Factors such as size, shape, and surface charge significantly affect the dispersion of colloidal nanoparticles. The stability of these nanoparticles in solution is generally indicated by a zeta potential value greater than ±30 mV [56], suggesting stable dispersion without particle aggregation. Conversely, colloids with low zeta potential are prone to coagulation. Table 1 presents the zeta potential values for pristine MWCNTs, functionalized MWCNTs, acid−treated MWCNTs, and nAu−fMWCNTs in an aqueous medium. The zeta potential of fMWCNTs is influenced by the carboxyl groups (COOH) on the MWCNTs surface. In the case of nAu−fMWCNTs, a high negative zeta potential is observed, attributable to the combined effects of carboxyl groups on the carbon nanotubes and the negatively charged nAu.

Table 1.

Zeta potential of MWCNTs, fMWCNTs, and nAu−fMWCNTs.

Specimen	Average zeta potential (mV)	Solution
MWCNTs	−22.17 ± 0.76	DI water
fMWCNTs	−38.10 ± 0.66	DI water
nAu−fMWCNTs	−40.43 ± 0.46	DI water

Fig. 3(a)−(d) illustrate the dispersibility behavior of pristine MWCNTs, fMWCNTs, and nAu-fMWCNTs at a concentration of 0.05 mg/mL in water, observed after sonication for time periods of 1 h, 1 day, 14 days, and 28 days, respectively. It was found that MWCNTs, which possess a low concentration of COOH groups, exhibit poor dispersion and stability in water over time, ranging from 1 h to 28 days. This behavior is further analyzed and described in the context of zeta potential measurements and will be elaborated upon in the FTIR analysis section. Conversely, fMWCNTs and nAu-fMWCNTs, which contain a higher concentration of COOH groups due to the presence of COO⁻ and H⁺ ions, demonstrate significantly different repulsion forces and enhanced stability in water over the same periods. This increased stability is attributed to their higher zeta potential values.

Fig. 3.

Dispersibility behavior of the 0.05 mg/mL of nanofluid, (a) 1 h after sonication, (b) 1 day after sonication, (c) 14 days after sonication, and 28 days after sonication. (Pristine MWCNTs (left), acid−treated MWCNTs (middle) and nAu−fMWCNTs (right)).

Fig. 4 displays the successful attachment of nAu particles on the surface of fMWCNTs, as verified by UV–vis spectroscopy to confirm the presence of nAu. The maximum absorption peak is observed at a wavelength of approximately 520 nm [57], aligning with the absorption peak reported in the literature [58,59]. This is illustrated by the red−colored solution in inset (1) of Fig. 4. Following centrifugation, which sediments the nAu−fMWCNTs powder, the resultant clear solution is again analyzed using UV–vis spectroscopy. Inset (2) in Fig. 4 demonstrates that almost all nAu particles are attached to the surface of fMWCNTs, indicated by the absence of nAu absorption. This finding is in agreement with observations from the TEM image shown in Fig. 2(c). Therefore, the ratio of nAu to fMWCNTs in the nAu−fMWCNTs hybrid powder is deduced to be 1.1 : 98.9 wt%.

Fig. 4.

UV–vis spectra of AuNPs and nAu−fMWCNTs hybrid powder synthesized by a Turkevich method.

Fig. 5 presents the XRD patterns within the 2θ range of 10°–80° for three types of powders: MWCNTs, fMWCNTs, and nAu−fMWCNTs. For MWCNTs, distinct diffraction peaks are observed at 2θ values of 26.5° and 42.7°. These peaks correspond to the (002) and (100) crystallographic planes of graphite, as identified in the JCPDS file 01−0646 [60]. The XRD patterns of fMWCNTs are similar to those of MWCNTs, indicating no significant alteration in the crystalline structure upon functionalization. Additionally, the XRD spectrum of nAu-fMWCNTs shows characteristic peaks corresponding to the (111), (200), (220), and (311) planes of gold (Au), as referenced in the JCPDS file 00−001−1172. This confirms the successful incorporation of nAu in the nAu−fMWCNTs hybrid powder.

Fig. 5.

XRD patterns of MWCNTs powder, fMWCNTs powder, and nAu−fMWCNTs hybrid powder.

FTIR was utilized to identify the functional groups on the surfaces of MWCNTs, treated in various ways, within the range of 700–4000 cm⁻¹, as depicted in Fig. 6. The FTIR spectra of MWCNTs reveal a broad peak at 1640 cm⁻¹, which is indicative of the carbonyl group of the quinone type on the surface of MWCNTs. This could result from oxidation during fabrication. The stretching band at 1460 cm⁻¹, corresponding to the methylene (CH₂) group [38], is observed. Additionally, peaks at 2920 and 2850 cm⁻¹ are also evident in the MWCNTs spectra [34,38]. In the case of fMWCNTs, the carboxylic stretching of C O at 1720 cm⁻¹ indicates the formation of carboxylic groups by oxidation of partial carbon atoms on the MWCNT surface due to nitric acid treatment. The C C stretching peak observed at 1460 cm⁻¹ is diminished due to this nitric acid exposure. Signals between 1050 and 1300 cm⁻¹ may be associated with C–O stretching, originating from lactones, carboxylic acids, alcohols, and other functional groups [38].

Fig. 6.

FTIR spectra of pristine MWCNTs, acid−treated MWCNTs, and nAu−fMWCNTs.

The nAu–fMWCNTs hybrid powder was characterized through elemental mapping and EDS using FIB–FESEM, conducted without an Au sputter coating. Fig. 7(a) displays the FE–SEM images of the nAu–fMWCNTs hybrid powder. Fig. 7(b) and (c) present the mapping images, illustrating the homogenous dispersion of C and Au in the nAu–fMWCNTs hybrid powder.

Fig. 7.

(a) FE−SEM images of nAu−fMWCNTs hybrid powder. (b) and (c) FE−SEM mapping elements of C and Au, respectively.

The FE−SEM cross−sectional image of nAu−fMWCNTs/PVDF nanocomposite, with a f_nAu−fMWCTs = 0.008, is depicted in Fig. 8. The surface morphologies are characterized as dense and smooth, attributable to the homogeneous dispersion of fillers within the polymer matrix. Additionally, the fMWCNTs are randomly dispersed throughout the polymer matrix. The dielectric properties of the nanocomposites are influenced by the distribution of the fillers and the presence of defects, such as pores. The agglomeration of fillers tends to increase the tanδ value, leading to higher current leakage.

Fig. 8.

FE−SEM images of fractured surface for nAu−fMWCNTs/PVDF nanocomposite with f_nAu−fMWCTs = 0.008 (a and b).

fMWCNTs/PVDF nanocomposites films with f_fMWCTs = 0.005, 0.01,0.015 and 0.02, as well as nAu−fMWCNTsPVDF nanocomposite films with f_nAu−fMWCTs = 0.005, 0.006, 0.007, 0.008, 0.009, 0.01, 0.015 and 0.02, demonstrate notable flexibility. This characteristic of the polymer nanocomposite films is clearly exhibited in Fig. 9.

Fig. 9.

Photo images of (a–d) fMWCNTs/PVDF and (e–l) nAu−fMWCNTs/PVDF nanocomposite films.

The phase structures of pure PVDF polymer and nAu−fMWCNTs/PVDF nanocomposites with f_{nAu−fMWCNTs} = 0.5, 1.0, and 1.5 vol% were examined using XRD, as shown in Fig. 10. The nAu−fMWCNTs/PVDF nanocomposites retain the characteristic patterns of the base polymer. In the γ−phase of PVDF polymer films, diffraction peaks are observed at a 2θ angle of 18.5°, corresponding to the (020) plane. The polar β−phase is characterized by diffraction peaks at 20.8° and 36.6°, aligning with the (110) and (020) planes, respectively [2,3,61,62]. The XRD results lead to the conclusion that the structure of polymer remains unchanged by the chemical treatment.

Fig. 10.

XRD patterns of PVDF and nAu−fMWCNTs/PVDF nanocomposites with various contents of nAu−fMWCNTs.

Fig. 11(a) illustrates the frequency dependence of the ε′ for both PVDF and nAu−fMWCNTs/PVDF nanocomposites at varying levels of f_{nAu−fMWCNTs}, in comparison to MWCNTs/PVDF nanocomposites. As detailed in Table 2 and depicted in Fig. 11(a), it is observed that the ε′ value of the nAu−fMWCNTs/PVDF nanocomposites exhibits an increase with the rising f_{nAu−fMWCNTs} content. This suggests that the nAu−fMWCNTs hybrid fillers are capable of enhancing the dielectric response in the PVDF polymer. However, it is observed that the ε′ values of the nAu−fMWCNTs/PVDF nanocomposites with filler volume fractions of 0.005 and 0.010 are lower than those of the MWCNTs/PVDF nanocomposites at the same volume fractions. Additionally, as detailed in Table 2 and shown in Fig. 11(b), the tanδ values of the nAu−fMWCNTs/PVDF nanocomposites are markedly lower than those of the MWCNTs/PVDF nanocomposites, exhibiting a difference of 2–3 orders of magnitude across all filler volume fractions. It is important to recognize that low−dimensional systems in nanomaterials, such as Au nanoparticles, exhibit electronic discontinuity and possess quantized values. The quantification of electronic status in these materials leads to the emergence of novel characteristics and phenomena, including the effects of quantum confinement and Coulomb blockade, as detailed in Ref. [63]. Significant blocked charges occur at the interfaces between fMWCNTs−PVDF and nAu−PVDF, which can be attributed to interfacial polarization or the Maxwell−Wagner−Sillars (MWS) effect, particularly at the interfaces of PVDF−nAu and fMWCNTs−nAu [64]. The fMWCNTs, composed of long−chain carbon, can create random arrangements and conductivity paths. This leads to increased tanδ values due to the change in conductivity as the proportion of fillers increases. The MWS effect involves independent charges located on discrete surfaces between insulated phases, enhancing conductivity. Under an applied electric field, dipoles can form, significantly improving the dielectric response of three−phase nanocomposites. In a random two−phase system, the percolation threshold for conducting spherical particle fillers is around 16 vol% [24,25]. However, with non−spherical conducting fillers, there may be a reduction in the percolation threshold of MWCNTs/PVDF to 0.0161 [25]. Notably, in the nAu−fMWCNTs/PVDF composite with an f_nAu−fMWCTs of 0.008, an ε′ value of 65.3 and a tanδ of approximately 0.216 were achieved, as shown in Fig. 11(b). This represents a substantial increase in dielectric response within the PVDF matrix composite, especially when compared with ceramic/polymer composites [10,13,14,16,17].

Fig. 11.

Frequency dependence at ∼25 °C of (a) ε′ and (b) tanδ for nAu−fMWCNTs/PVDF nanocomposites with various contents of nAu−fMWCNTs hybrids fillers compared to MWCNTs/PVDF nanocomposites.

Table 2.

β−phase fraction (F(β)), ε′, tanδ, and σ_ac of MWCNTs/PVDF, fMWCNTs/PVDF and nAu−fMWCNTs/PVDF polymer nanocomposites films at 1 kHz and 25 °C.

Volume fraction	F(β) (%)	ε′	tanδ	σac (S.cm−1)
PVDF	66	12.5	0.021	1.50 × 10−8
MWCNTs
0.005	62	1991.7	6.97	7.82 × 10−4
0.010	60	940.4	113.91	6.03 × 10−3
0.015	–	1487.2	1054.42	8.83 × 10−2
0.020	52	1343.5	1636.15	1.24 × 10−1
fMWCNTs
0.005	62	902.4	0.834	4.23 × 10−5
0.010	58	911.8	2.813	1.44 × 10−4
0.015	52	1709.3	2.163	2.08 × 10−4
0.020	52	2571.4	79.61	1.15 × 10−2
nAu−fMWCNTs
0.005	63	17.8	0.057	5.74 × 10−8
0.006	–	37.6	0.068	1.43 × 10−7
0.007	–	47.6	0.126	3.37 × 10−7
0.008	–	65.3	0.216	7.95 × 10−7
0.009	–	250.8	1.070	1.51 × 10−5
0.010	57	524.8	1.175	3.47 × 10−5
0.015	53	1621.8	1.950	1.78 × 10−4
0.020	51	2477.4	51.345	7.16 × 10−3

Fig. 12 illustrates the ac conductivity (σ_ac) of nAu−fMWCNTs/PVDF nanocomposites, in comparison to MWCNTs/PVDF nanocomposites. The σ_ac values at 1 kHz and 25 °C are summarized in Table 2. The σ_ac of MWCNTs/PVDF nanocomposites is significantly higher than that of nAu–fMWCNTs/PVDF nanocomposites by 2–4 orders of magnitude at the same filler volume fraction, corresponding to the notable changes observed in their tanδ values. As shown in Fig. 12 and Table 2, σ_ac increases with increasing filler volume fraction for all nanocomposite systems. This observation is similar to those reported in previous works [[65], [66], [67]]. The attachment of Au nanoparticles to the surface of fMWCNTs aids in filler dispersion within the polymer nanocomposites. This is attributed to the charge–induced repulsion, which increases the surface area of the composite, thereby reducing the tanδ. Additionally, the decrease in σ_ac is also a result of the Coulomb blockade effect. Surface modification of MWCNTs with Au nanoparticles has been effective in significantly reducing both tanδ and σ_ac. The increase in ε′ should be associated with the number of free charges, which can induce interfacial polarization [1,24]. Furthermore, the increase in ε′ can also be explained by the formation of micro–capacitors and percolation effect [43,44,63]. According to percolation theory, the properties transition from insulating to conducting [25,68,69]. This transition occurs when the concentration of the conductive phase becomes sufficiently high to form continuous pathways, leading to an immediate change in ε′, tanδ, and σ_ac values [68,70].

Fig. 12.

Frequency dependence of σ_ac of nAu−fMWCNTs/PVDF polymer nanocomposites with various contents of nAu−fMWCNTs at ∼25 °C compared to MWCNTs/PVDF nanocomposites.

Fig. 13 illustrates the ε′, σ_ac, and tanδ values at 1 kHz for nAu−fMWCNTs/PVDF nanocomposites compared to MWCNTs/PVDF nanocomposites, as a function of filler volume fractions. For nAu−fMWCNTs/PVDF nanocomposites, these three parameters increase rapidly when the f_nAu-fMWCNTs value exceeds 0.008. The electrical conductivity of nAu−fMWCNTs/PVDF nanocomposites, particularly near−percolation composite materials, has been studied due to the presence of conductive fillers. The conductivity is calculated using Equation (3):

$$\mathrm{\sigma }={\mathrm{\sigma }}_0{\left(f-f_C\right)}^q\text{,}$$ (3)

Fig. 13.

(a) ε′ and (b) tanδ at ∼25 °C and 10³ Hz for nAu−fMWCNTs/PVDF polymer nanocomposites with various contents of nAu−fMWCNTs compared to MWCNTs/PVDF nanocomposites; insert shows σ_AC as a function of f_{nAu−fMWCNTs}.

In this equation, σ and σ₀ represent the electrical conductivity of nAu−fMWCNTs/PVDF nanocomposites and constant value, f is the filler volume fraction, f_c is percolation threshold, and q is critical exponent. Our experimental data fits Equation (3), with f_c = 0.008 and q = 1.272, as shown in the inset of Fig. 13. The ε′ value of MWCNTs/PVDF nanocomposites increases rapidly as the fMWCNTs value ranges from 0 to 0.005, indicating that the f_c for MWCNTs/PVDF nanocomposites is ≤ 0.005. Generally, the q value for a two−dimensional (2D) system is approximately 1.1–1.3, and for a three−dimensional (3D) system, it is between 1.6 and 2.0 [24]. This suggests that a 3D conductivity network in nAu−fMWCNTs/PVDF nanocomposites is formed through the contact between nAu−fMWCNTs. The low percolation threshold is likely due to the high aspect ratio of fMWCNTs, whose negative surface charges result in repulsion, assisting in uniform dispersion within the polymer matrix. This leads to a decrease in tanδ when the concentration of nAu−fMWCNTs/PVDF nanocomposites is less than 0.008, as shown in Fig. 13(b).

Fig. 14 presents the ε′ and tanδ values for all the nanocomposite systems studied. The significantly increased tanδ values in the MWCNTs/PVDF polymer nanocomposites can be attributed to the accumulation of mobile charge carriers at the interface between the fillers and the PVDF polymer matrix, as well as the formation of a continuous conducting network. Conversely, the decreased tanδ values observed in the fMWCNTs/PVDF and nAu−fMWCNTs/PVDF nanocomposites can be attributed to improved dispersion resulting from surface modification through acid treatment. Notably, when the nAu−fMWCNTs loading is less than 0.009, a reduction in tanδ values is observed, as previously mentioned, especially when compared to the fMWCNTs and MWCNTs fillers at equivalent volume fractions. Factors contributing to lower tanδ values include charge distributions, dc conduction within the composites, dipole loss [43], quantum confinement effects, and Coulomb blockade [63]. These phenomena are likely a result of nAu particles being attached to the surface of fMWCNTs. The Coulomb blockade effect is notably observed in metallic nanoparticles with low-dimensional systems, particularly in Au nanoparticles. These nanoparticles exhibit electronic discontinuity with quantized values. Quantification of their electronic states reveals the emergence of novel features and phenomena. Studies have shown that the conductivity of Au nanoparticles diminishes as their size reduces to the nanometer scale. Specifically, Au nanoparticles with sizes ranging from 10 to 50 nm exhibited conductivities between 1.5 and 2.2 × 10⁻² S/m [45]. As detailed in Table 2, the tanδ value of the nAu−fMWCNTs/PVDF nanocomposites was significantly lower compared to the fMWCNTs/PVDF composites at the same filler volume fraction. at equivalent filler volume fractions. Consequently, the insulating properties of Au nanoparticles can also impede the formation of conductive pathways, thereby reducing DC conduction and tanδ.

Fig. 14.

Filler dependence of (a) ε′ and (b) tanδ at ∼25 °C and 10³ Hz for all nanocomposite systems.

It is important to acknowledge that the dielectric properties of the nAu−fMWCNTs/PVDF nanocomposites were evaluated using a relatively low oscillation voltage of 0.5 V in our study. It is essential to recognize that in practical applications, these dielectric composites are typically subjected to much higher operating voltages. This disparity underscores the critical importance of high–voltage testing in the context of dielectric composite applications. Therefore, for future research endeavors, it is strongly advised to assess the electrical and dielectric properties of such materials under high field conditions. This approach aligns with methodologies employed in prior studies, as reported in previous works [[71], [72], [73]].

4. Conclusions

In summary, this study successfully synthesized PVDF nanocomposite films using nAu−fMWCNTs. The nanohybrids, produced through an adapted Turkevich method, demonstrated a favorable weight ratio and substantial improvements in dielectric properties. Detailed analyses confirmed the formation of nAu−fMWCNTs and their effective integration into the PVDF matrix at various filler concentrations. The novel three−phase percolative nanocomposites, formed by dispersing the nAu−fMWCNTs in DMF, showed enhanced interaction between the negatively charged nAu−fMWCNTs and the polar phases of PVDF. This interaction was further substantiated by zeta potential and FTIR analyses. A remarkable increase in ε′ from 17.8 to 524.8 at 1 kHz was observed with filler loadings ranging from 0.005 to 0.01 vol%, highlighting the significant influence of the nAu−fMWCNTs. Moreover, despite the high ε′, tanδ remained relatively low, increasing only marginally from 0.05 to 1.18. This balance between high ε′ and low tanδ is attributed to the elevated permittivity of the nAu−fMWCNTs hybrid powder, the transition of PVDF to the β−phase, and the effects of interfacial polarization. Additionally, the restrained growth of nAu on the fMWCNTs and the prevention of conductive pathways within the polymer matrix were key factors in maintaining low tanδ values. The findings of this research provide significant insights into the development of high−performance dielectric materials. The enhanced properties of the PVDF nanocomposites, as demonstrated in this study, have potential applications in electronic and energy storage devices, where high permittivity and low dielectric loss are essential.

Data availability statement

Data will be made available on request.

CRediT authorship contribution statement

Kaniknun Sreejivungsa: Writing – original draft, Visualization, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Prasit Thongbai: Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Resources, Project administration, Funding acquisition, Formal analysis, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.