Largely enhanced dielectric properties of TiO2-nanorods/poly(vinylidene fluoride) nanocomposites driven by enhanced interfacial areas

Abstract In this work, nanocomposites consisting of TiO2-nanorods (TiO2-NRs) with less than 100 nm in size and poly(vinylidene fluoride) (PVDF) were prepared using a liquid-phase assisted dispersion and hot-pressing methods. At 1 kHz and 25 °C, the high dielectric permittivity of ∼66 and loss tangent of ∼0.03 can be obtained in the nanocomposite with a filler volume fraction of 0.5, which was higher than that of a neat PVDF matrix by a factor of 6. Dielectric permittivity of TiO2-NRs/PVDF nanocomposites not only highly increased with TiO2-NRs, but also almost independent of the frequency range of 102–106 Hz. The significant enhancement in dielectric permittivity is mainly attributed to the interfacial polarization at the interfaces of TiO2-NRs and PVDF, and semiconducting properties of TiO2-NRs. Among the various models used for rationalizing the dielectric behavior, the experimental dielectric data is in close agreement with EMT (n = 0.11) and Yamada models (n = 8). Graphical Abstract

To meet the requirement of high e 0 , low tand, e 0 stable ability and environmental friendly, TiO 2 has received much attention thanks to its stability, abundant, environmental friendly, and low toxicity character [27][28][29][30]. Therefore, it is widely used in many applications such as sensors, catalysis, and photocatalyst battery. It is worth noting that high e 0 were found in rutile-TiO 2 ceramic bulks due to the existence of polaron-like electron hopping between Ti 3þ and Ti 4þ ions and oxygen vacancies in bulk ceramic by pentavalent ions [29,30]. Hence, rutile-TiO 2 particles have been used as fillers in a polymer matrix to create composites (e.g. TiO 2 /PVDF [31], TiO 2 / PENs [32], (Er þ Nb) co-doped TiO 2 /P(VDF-TrFE) [33], and TiO 2 /PDMS [34]. One of the most significant effects that enhanced the dielectric properties of these TiO 2 /polymer composites is interfacial polarization. Accordingly, the interfacial area is one of the most important factors contributing to the intensity of interfacial polarization. The surface area of any filler nanoparticle with a spherical shape (aspect ratio ¼ 1) can be increased by using this filler material in other shapes with an aspect ratio of more than 1 such as in nanorods or nanofibers. It was also reported that the dielectric properties of the TiO 2 nanorod array/PVDF composites could be improved. However, the e 0 value was $32.5 at 1 kHz [35]. Furthermore, the TiO 2 nanorod array/PVDF composite system was fabricated by a complex method using multistage processing. To the best of our knowledge, there is no report on the fabrication and dielectric properties of TiO 2 nanorod/PVDF composites with random dispersion of TiO 2 nanorods. Thus, the aim of this work is to improve the dielectric properties of the TiO 2 /PVDF composite system using TiO 2 nanorods as a filler. It is expected that the dielectric response in the composite can be enhanced due to the increased interfacial areas of the filler.
In this work, we prepared two-phase polymer nanocomposites comprising a new rutile-TiO 2 with nanorod shape and PVDF polymer. TiO 2 -nanorods (TiO 2 -NR)/PVDF nanocomposites were fabricated using liquid-phase assisted dispersion and hot-pressing methods. The phase composition, morphologies, microstructures, oxidation stages, and dielectric properties of nanocomposite are investigated. Various theoretical models, including the Maxwell À Garnett model, logarithmic model, effective medium theory model, and Yamada model, were used to predict the dielectric behavior of the TiO 2 -NRs/PVDF nanocomposite.

Preparation of heat treatment of TiO 2 -NRs
TiO 2 -NRs with particle size <100 nm (99.5%, Sigma-Aldrich) were heat treatment at 500 C for 3 h in air to evaporate the moisture. After that, TiO 2 -NRs were obtained to cool down at 25 C.

Preparation of TiO 2 -NRs/PVDF nanocomposites
TiO 2 -NRs were used as filler into commercial PVDF powder (M w $ 534,000, Sigma-Aldrich). Polymer nanocomposites with different volume fraction of TiO 2 -NRs (f TiO2-NRs ¼ 0.1, 0.2, 0.3, 0.4 and 0.5) were prepared by a liquid-phase assisted dispersion method. First, heat treatment of TiO 2 -NRs and PVDF powder were mixed by ball milling with ZrO 2 balls in absolute ethanol for 3 h. Second, the mixture was dried at 80 C for 24 h to evaporate the absolute ethanol. Then, the dry mixed powder was molded by hot-pressing at 200 C for 30 mins with a pressure of 10 MPa. Finally, the sample of polymer nanocomposites with a diameter of about 12 mm and a thickness of about 0.6-1 mm was obtained. Note that, for a pure PVDF polymer sample, a PVDF powder was mixed by ball milling with ZrO 2 balls in absolute ethanol for 3 h. Then, the dried powder of PVDF particles was molded by hot-pressing at 200 C for 30 mins with a pressure of 10 MPa.

Characterization
The phase structures and crystal composition of fillers, PVDF, and polymer nanocomposites were characterized by an X-ray diffractometer (XRD, PANalytical, EMPYREAN, Netherlands). Surface morphologies of TiO 2 -NRs were revealed using transmission electron microscopy (TEM, FEI Tecnai G 2 , Netherlands). The oxidation stages of TiO 2 -NRs/PVDF nanocomposites were analyzed by X-ray photoelectron spectroscopy (XPS, PHI5000 VersaProbe II, ULVAC À PHI, Japan) at the SUT-NANOTEC À SLRI Joint Research Facility, Synchrotron Light Research Institute (SLRI), Thailand. The crystalline phase of PVDF and nanocomposites was determined using Fourier transform infrared spectroscopy (FTIR, Bruker, TENSOR27, Germany) in the range of wave number 600-2000 cm À1 . The fractured microstructure of the nanocomposites was characterized using a focus ion beam-field emission scanning electron microscope (FIB À FESEM, FEI Helios Nanolab G3 CX, USA). Before FESEM characterization, nanocomposite samples were fractured by liquid N 2, and then their surfaces were coated using gold using the sputtering technique. Capacitance (C p ) and tand (D) were measured using an Impedance analyzer (KEYSIGHT E4990A, USA) over the frequency range of 10 2 -10 6 Hz with an oscillation voltage of 0.5 volts. Before dielectric measurements, the top and bottom surfaces of the samples were painted with an Ag paste as the electrodes and dried at 150 C for 2 h.  (112). It is confirmed that the main phases can be indexed to the rutile phase of TiO 2 with a tetragonal structure (JCPDS 21-1276). In this case, the characteristic peaks were observed at the same position compared to the rutile-TiO 2 standard data, which indicates the temperature of heat treatment at 500 C has no effect on the phase and crystallinity of the TiO 2 -NRs. The impurity phase was not observed in TiO 2 -NRs. Surface morphologies of TiO 2 -NRs were revealed by TEM technique, as illustrated in the inset of Figure  1. The size of submicron TiO 2 -NRs is less than approximately 100 nm with rod shape and smooth surface.

Results and discussion
XRD patterns of PVDF, TiO 2 -NRs, and TiO 2 -NR/PVDF nanocomposites with various content loading of TiO 2 -NRs are presented in Figure 2. The diffraction peaks of PVDF corresponding to the (100), (020), (110), and (021) planes were observed, which were assigned to the a-phase [10]. The nanocomposites show high diffraction intensity of PVDF at only low f TiO2-NRs . With increasing f TiO2-NRs , the intensity of PVDF decreased while the intensity of TiO 2 -NRs increased. It is due to the crystalline structure of TiO 2 -NRs more dominant than the semi-crystalline structure of PVDF. All nanocomposites show intensity peaks of TiO 2, and no impurity was observed.
Among the five known crystalline forms of PVDF, namely a-, c-, e-, d-, and b [31], the b-phase results in good piezoelectric, ferroelectric and dielectric properties. Unfortunately, the b-phase cannot be observed by XRD. To further investigate the b-phase, FTIR characterization was collected. Figure  3 shows the FTIR spectra of PVDF and TiO 2 -NR/ PVDF nanocomposites with different f TiO2-NRs . PVDF and nanocomposites exhibit the characteristic bands at 840 cm À1 , corresponding to the polar cand b-phases [10] at the same time the characteristic band at 1279 cm À1 , assigned to the only polar b-phase [10]. The transmittance bands at 614, 766, 795 and 976 cm À1 , are attributed to the non-polar a-phase [10]. This result demonstrated that the TiO 2 -NRs/PVDF consist of the polar phase of c-, b-PVDF and nonpolar phase of a-PVDF. Importantly, b-phase of PVDF has dielectric, piezoelectric, and ferroelectric properties, leading to enhancement of e 0 of nanocomposites [36]. When the volume fraction of TiO 2 -NRs increased, the transmittance band became very weak because of the reducing PVDF, corresponding to the result shown in the XRD of Figure 2. It is important to note that the observed b-phase of PVDF in the FTIR spectra may be associated with effect of hotpress process, giving rise to very well oriented crystallites in the perpendicular direction of the applied pressure. Thus, it can be sensitive by the FTIR technique due to the vibration of the b-PVDF molecules. The b-phase of PVDF was not sensitive by the XRD technique. Figure 4 presents the FESEM images of the fracture surface of TiO 2 -NRs/PVDF with f TiO2-NRs ¼ 0, 0.3, and 0.5. It can be clearly observed that the f TiO2-NRs ¼ 0 (pure PVDF) is smooth and self-connected into a continuous network (see Figure 4(a)).
The slightly partial aggregation of TiO 2 -NRs can be seen in the nanocomposites at high content of filler, as shown in Figure 4(b,c). The overall results indicated that homogeneous TiO 2 -NR/PVDF nanocomposites were achieved by using a liquid-phase assisted dispersion method. TiO 2 -NRs were found to become connected to form a continuous cluster, but only a small number of pores were observed in the nanocomposites.
X-ray photoelectron spectroscopy (XPS) analysis was collected to investigate the chemical states and compositions. Figure 5 shows XPS results of TiO 2 -NR/PVDF nanocomposites with f TiO2-NRs ¼ 0.5 using Gaussian-Lorentzian profile fitting. As was shown in Figure 5(a), the peak positions of the Ti 2p 3/2 were observed at 457.50 and 458.82 eV, corresponding to the presence of Ti 3þ and Ti 4þ , respectively [29,30]. The Ti 3þ /Ti 4þ ratio was found to be 3.9%. This result demonstrated the existence of Ti 3þ in the samples by electron hopping (Ti 4þ þ e -$ Ti 3þ ). The XPS spectra of O 1 s profiles were measured and shown in Figure 5(b). Three prominent peaks were ascribed to an oxygen lattice (Ti-O), oxygen vacancies, and hydroxyl group [29,30]. The result confirmed the semiconducting properties of TiO 2 -NRs, leading to enhancement of the e 0 TiO 2 -NRs. Figure 6 illustrates the frequency dependence of e 0 and tand at 25 C for TiO 2 -NRs/PVDF nanocomposites various content loading of TiO 2 -NRs. As can be seen from Figure 6(a), it was observed that e 0 of the nanocomposites increased with increasing concentrations of TiO 2 -NRs fillers. The e 0 showed a relatively stable frequency dependence for all samples in the wide range from 10 2 Hz to 10 6 Hz. The e 0 values of nanocomposites were achieved to be 81.9, 65.9, and 63.1 at the frequency of 10 2 , 10 3, and 10 4 Hz, respectively, with f TiO2-NRs ¼ 0.5. Interestingly, it shows that the tand remained very low with increasing TiO 2 -NRs, even though the volume fraction of filler was as high as 50 vol% (f TiO2-NRs ¼ 0.5), as illustrated in Figure 6(b). The tand value remained 0.029 at 1 kHz, which value is   similar to tand of pure PVDF [2]. Especially in the high-frequency range of 10 5 -10 6 Hz, the tand of TiO 2 -NR/PVDF increased with the increase of TiO 2 -NRs loading. The tand of nanocomposites at low frequency was relatively low, indicating that the contribution of the interface was rather low, and then the tand greatly increased at high frequency, mainly determined by the PVDF polymer matrix. It is important to note that tand increases with decreasing the frequency from 10 3 to 10 2 Hz. Nevertheless, tand values at 10 2 Hz of the TiO 2 -NR/ PVDF composites were lower than those reported in CaCu 3 Ti 4 O 12 /PVDF and Ba(Fe 0.5 Nb 0.5 )O 3 /PVDF composites [37,38]. The significant enhancement in e 0 of TiO 2 -NR/PVDF nanocomposites are influenced by the inherent semiconducting properties of TiO 2 -NRs, the polar b-phase, and the interfacial polarization. Generally, interfacial polarization induced by the interface between semiconducting TiO 2 -NRs and insulating PVDF, would lead to an improvement of electron mobility and Maxwell-Wagner-Sillars (MWS) effect [3], resulting in larger e 0 of the TiO 2 -NR/PVDF nanocomposites. Figure 7 shows the dependence of e 0 and tand of TiO 2 -NR/PVDF nanocomposites on the f TiO2-NRs (measured at 1 kHz). The e 0 of nanocomposites slightly increased with increasing loading of TiO 2 -NRs. As the volume fraction of TiO 2 -NRs was    The comparison of e 0 and tand for PVDF-based polymer composites with other filler (f filler ¼ 0.5 at 1 kHz) is illustrated in Figure 8 and Table 1. It shows that most of all composites exhibit an e 0 less than 100. Although the e 0 of Na 0.5 Bi 0.5 Cu 3 Ti 4 O 12 / PVDF, Na 0.5 Y 0.5 Cu 3 Ti 4 O 12 /PVDF, and BiFeO 3 / PVDF are higher than TiO 2 -NR/PVDF, their tand are too high (>0.1) [17,18,39]. Interestingly, the e 0 values of CaCu 3 Ti 4 O 12 /PVDF, Ba 0.6 Sr 0.4 TiO 3 /PVDF, and Ba(Fe 0.5 Nb 0.5 )O 3 /PVDF are similarly to TiO 2 -NR/PVDF, but their tand values are higher than TiO 2 -NR/PVDF [37,38,40]. Compared with TiO 2 -NR/PVDF to BaTiO 3 /PVDF and LaFeO 3 /PVDF, the e 0 of TiO 2 -NR/PVDF is higher than BaTiO 3 /PVDF and LaFeO 3 /PVDF, while tand remains quite low compared to BaTiO 3 /P VDF and LaFeO 3 /PVDF [25,41]. It indicates that both satisfactory high e 0 and very low tand are accomplished in the PVDFbased polymer composites incorporating TiO 2 -NRs. Polymer nanocomposites are a key material in energy harvesting and storage devices due to their fast charge-discharge capability [35,42,43]. The energy storage performance is influenced by the e 0 and tand values. Although the TiO 2 -NR/PVDF nanocomposites can exhibit a high e 0 with low tand, the energy density of the nanocomposites cannot be obtained due to the limitations of the dielectric data in this current study, which was measured at a low electric field. Thus, the high field properties such as dielectric displacement, ferroelectric hysteresis loops, and breakdown strength of the TiO 2 -NR/PVDF nanocomposites need to be tested before consideration for application in energy storage devices [42,43].
Various models, including the effective medium theory (EMT) model, Yamada model, the logarithmic model, and the Maxwell À Garnett (M À G) model. were put forward to predict the e 0 of the TiO 2 -NR/PVDF nanocomposites [38,[44][45][46]. These four models can be expressed as follows: Effective medium theory (EMT) model: Yamada model: Logarithmic model: Maxwell-Garnett model: As followed in Equations (1)-(4), e 0 is the effective dielectric permittivity of the TiO 2 -NR/PVDF nanocomposite. f T is the volume fraction of TiO 2 -NRs (f TiO2-NRs ). f P is the volume fraction of PVDF. e 0 T and e 0 P are dielectric permittivities of the rutile-TiO 2 and PVDF were found to be 150 [29] and 10.78 (an experimental value), respectively. The e 0 P data was measured at 1 kHz and $25 C. n is a ceramic morphology fitting factor. Figure 9 presents the experimental e 0 of TiO 2 -NR/PVDF nanocomposite and the e 0 calculated based on various models for different volume fractions of TiO 2 -NRs. All models fitted well with the experimental e 0 values at lower filler loading (f TiO2-NRs < 0.3). Large deviations from the experimentally observed values in the Logarithmic and Maxwell-Garnet models indicate that these two models are not suitable to describe   [18]. Using the Yamada model, the experimental value fitted well with the morphology parameter n ¼ 8, similarly to that of reported (9.3) [37]. However, the experimental dielectric data of nanocomposites at a high concentration of filler (f TiO2-NRs ¼ 0.5) deviated from all models. The possible reason is that interfacial physical and chemical properties of composites have not been taken into consideration in the models [33]. The deviation of the experimental data at f TiO2-NRs ¼ 0.5 is due to the dominant effect of the interfacial polarization. As the filler loading increased, the interparticle distance is very short, giving rise to the strong interfacial polarization.

Conclusion
Dielectric nanocomposites were successfully prepared by introducing the semiconducting TiO 2 -NRs fillers into a PVDF matrix. It was clearly demonstrated that TiO 2 -NRs can cause a significant increase in e 0 , while a very low tand was suppressed.
The high e 0 of TiO 2 -NR/PVDF nanocomposites is mainly attributed to the formation of interfacial polarization at the interface between TiO 2 -NRs and PVDF. The variation in e 0 of TiO 2 -NR/PVDF nanocomposites was well described by the EMT and Yamada models. To further study the possible application of the TiO 2 -NR/PVDF nanocomposites in energy harvesting and storage devices, ferroelectric hysteresis loop and breakdown strength that are used to calculate the energy density in dielectric capacitor need be studied.

Disclosure statement
No potential conflict of interest was reported by the authors.