Flexible high dielectric thin films based on cellulose nanofibrils and acid oxidized multi-walled carbon nanotubes

Flexible high dielectric materials are of prime importance for advanced portable, foldable and wearable devices. A series of flexible high dielectric thin films based on cellulose nanofibrils (CNF) and acid oxidized multi-walled carbon nanotubes (o-MWCNT) was prepared in aqueous solution. Though no organic solvent was involved during the preparation, the SEM images showed that o-MWCNTs have good distribution within the CNF matrix. The dielectric constant of CNF/o-MWCNT (6.2 wt%) composite films was greatly increased from 25.24 for pure CNF to 73.88, while the loss tangent slightly decreased from 0.70 to 0.68, and the AC conductivity decreased from 3.15 × 10−7 S cm−1 for CNF to 1.77 × 10−7 S cm−1 (at 1 kHz). The abnormal decrease of loss tangent and AC conductivity were attributed to the introduction of oxide-containing groups on the surface of MWCNTs. The nanocomposite films showed excellent flexibility such that they could be bent a thousand times without visible damage. The presence of MWCNTs also helped to improve the thermal stability of the composite films. The excellent dielectric and mechanical properties of the CNF/o-MWCNT composite film demonstrate its great potential to be utilized in the field of energy storage.


Introduction
Nowadays, multifunctional materials have become the key to breakthroughs in many elds. Among them are so high dielectric materials, which are in hugely demand in high-tech applications such as foldable touch screens, antennae, biosensors, inverters, organic transistors, and hybrid electrical vehicles. [1][2][3] Materials with a high dielectric constant can considerably increase the energy density and power, accelerate the charge/discharge speed, and reduce the size of devices. So high dielectric materials are more preferred in portable, foldable and wearable electronic devices. 4 Recently, nanocellulose has been proposed to be a replacement for traditional high dielectric polymer matrices, for their high dielectric constant, excellent mechanical properties, high transparency, lightweight and low coefficient thermal expansion. [5][6][7][8][9][10][11] More attractively, cellulose is an almost inexhaustible bioresource, and nanocellulose can be prepared simply by mechanical grinding without toxic solvent, meeting the request for sustainable development.
Commonly used llers for nanocellulose based high dielectrics are ceramic llers (e.g., barium titanate, 12 titanate dioxide 13 ) and conductive llers (e.g., silver nanowires, 3,14 graphene, [15][16][17][18] triglycine sulfate (TGS), [19][20][21][22] polyaniline (PANI), 23,24 carbon nanotubes 25 ). High dielectric nanocellulose/ceramic composites usually demand high ller loading (>50 wt%), leading to severe agglomerations, poor mechanical properties, and low breakdown strength. 26 On the contrary, a small addition of conductive ller can bring in much higher dielectric constant. According to the percolation theory, 27,28 the dielectric constant grows dramatically near the threshold, which is usually around 5% for raw llers and differs aer different treatments. Beeran et al. 18 used ammonia-functionalized graphene oxide (NGO) nanoplatelets as the ller and incorporated it into CNF and 2,2,6,6-tetramethylpiperidine-1-oxyl oxidized cellulose (TCNF), respectively. With an NGO loading of 3 wt%, the relative dielectric constant (3 r ) of NGO/CNF (thickness 50 AE 2 mm) and NGO/TCNF (thickness 70 AE 2 mm) lms are 50 and 158 (at 1 kHz), respectively. Inui et al. 14 prepared a highdielectric nanocomposite (thickness 50 mm) based on CNF and conductive silver nanowires (AgNWs, diameter z 100 nm, length z 10 mm). A small addition of AgNWs (2.54 vol%) results in an extremely high 3 r of 726.5 with a loss tangent (tan d) of 0.26, while the 3 r of pure CNF is 5.3 with a tan d of 0.2 (at 1.1 GHz). Anju and Narayanankutty 24 coated CNF with conductive polyaniline (PANI) by an in situ polymerization technique in an aqueous medium and mixed the PANI/CNF ller with polyvinyl alcohol (PVA). The 3 r of the PVA/PANI/CNF composite reaches 4759 with a tan d of 12 (at 1 kHz) near the percolation threshold (20 wt%). Zeng et al. 25 employed TCNF as the matrix and multiwalled carbon nanotubes (MWCNT) as llers. Three types of MWCNT llers with different aspect ratios were compared. The results show that MWCNTs with a high aspect ratio have a larger dipole moment and lower percolation threshold, leading to higher 3 r . The maximum 3 r of 3198 (at 1 kHz) with a tan d of about 0.9 is obtained at a low CNT loading of 4.5 wt% (diameter < 8 nm), while the 3 r of pure TCNF is 15 with a tan d of about 0.5.
The increase of 3 r is always accompanied by much-amplied tan d, which is the ratio of dissipated energy and stored energy in each period when sine-wave voltage is applied. The dissipated energy converts to heat during polarization and depolarization process, leading to high working temperature, accelerated deterioration, and early material failure. Thus, many kinds of research have been made to achieve high 3 r while maintaining low tan d. It is reported that the interface between the ller and the matrix is of vital importance. 29 With improved compatibilities between the ller and the matrix, the ller can distribute more uniformly in the composite. The effective methods include applying ultrasonication, modifying the surface of llers, and preparing matrix by in situ polymerizations. However, in most cases, complicated chemical reagents and treatments are involved. 15,18,30,31 In this study, a series of high dielectric thin lms with relatively low tan d was prepared based on MWCNT and CNF. MWCNTs have excellent conductivity, large surface area and high aspect ratio, which are benecial to superior dielectric properties of composites. Their one-dimensional structure makes it easy for them to align with each other and form a network at low loading, resulting in better mechanical properties. In addition, the dispersibility of MWCNT in aqueous can be greatly improved by simply acid oxidation, owing to the introducing of hydrophilic groups (e.g., hydroxyl group, carboxyl group). 32,33 Thus, CNF and MWVNT can be homogeneously mixed without organic solvent. Our previous study shows that CNF is a better choice than TCNF, considering its lower loss tangent and higher energy efficiency. 13 Thus, CNF was used as the matrix. The results show that acid oxidized MWCNTs (o-MWCNTs) have good distribution in the CNF matrix, and the CNF/o-MWCNT exhibits excellent dielectric properties and thermal stabilities.

Oxidation of MWCNT
When mixed directly with CNF, MWCNTs tended to aggregate with each other, form into large particles, and deposit on the bottom of the beaker. Therefore, MWCNTs were oxidized by concentrated HNO 3 to improve their dispersity in water. In a typical experiment, 0.2 g of MWCNT was added into 20 mL of concentrated HNO 3 , and the mixture was subjected to an ultrasonic bath for 30 min. Subsequently, the mixture was stirred vigorously while heated up to 120 C for 6 h. Aer being cooled to room temperature, the mixture was diluted with deionized water and centrifuged at 8000 rpm for 10 min. Then the supernatant was removed. The dilution and centrifugation processes were repeated until the pH value of the supernatant turned nearly neutral. Finally, o-MWCNT was obtained by drying the black sediment at 70 C for about 12 h.

Preparation of MWCNT/CNT nanocomposite lms
The purchased CNF slurry was diluted into 0.3 wt% with deionized water and was stirred by a homogenizer (RCD-1A, Changzhouyuexin, China) for 5 min. Aer the dilution, a certain amount of o-MWCNT was added according to the dry solid content. Then the mixture was subjected to an ultrasonic bath for 30 min and homogenized for 5 min again. CNF have good dispersity in water owing to its abundant hydroxyl groups. And the oxidation of MWCNTs greatly improves their dispersity in water. Thus, these two components could be mixed well in water, and no organic solvent is needed. Aer being poured into a Petri dish, the mixture was dried in a fume hood for several days. Finally, the obtained lms were hot-pressed under 80 C for 3 hours. The thickness of the exible lms was in the range of 60-90 mm. The schematic of the preparation of CNF/o-MWCNT lms is shown in Fig. 1. The paperlike lms with a size of 2 cm Â 2 cm were bent manually to make the top edge meet the bottom edge and then released. Aer repeating the process for a thousand times, no visible change was made (as shown in Fig. 1), which demonstrates its exibility over other fragile or stiff dielectric composites.

Measurements and characterization
The surface and cross-section morphologies of sample lms were studied by eld-emission scanning electron microscopy (FE-SEM, TESCAN MIRA3, Czech Republic) with an accelerating voltage of 5 kHz. Prior to the measurement, the samples were coated with a layer of gold using a sputter coater (Quorum SC7620, UK) in a vacuum to reduce charge interruptions. X-ray diffraction (XRD) patterns of the sample lms were determined by an X-ray diffraction spectrometer (XRD, PANalytical XPert Pro, Netherlands) with Cu K a radiation (l ¼ 1.542Å) at 40 kV and 40 mA in the 2 theta (2q) value range from 5 to 80 . Fourier Transform Infrared (FTIR) spectra of MWCNTs and CNF/o-MWCNT composite lms were recorded with an FTIR spectrophotometer (Nicolet Impact-5700, USA) at a resolution of 2 cm À1 within the range of 4000-400 cm À1 . The thermal stabilities of the nanocomposites were tested by thermogravimetric analysis and differential scanning calorimetry (TGA-DSC, Netzsch STA449F3, Germany) at a heating rate of 10 C min À1 from 35 C to 790 C under a nitrogen atmosphere. The dielectric constant, loss tangent and AC electrical conductivity of sample lms were measured using an LCR meter (Keysight E4980A, USA). The results were recorded in the frequency range from 40 Hz to 1 MHz with an oscillation signal of 1 V at ambient temperature.
Prior to the measurement, gold electrodes were sputtered on both sides of the specimens, and the test was repeated four times for each composition. The relative dielectric constants (3 r ) of the sample lms were calculated by eqn (1): where, C is the capacitance; 3 0 is the absolute dielectric constant of vacuum, 3 0 ¼ 8.854 Â 10 À12 F m À1 ; A is the electrode area, A ¼ 4.52 Â 10 À6 m 2 ; d is the thickness of the sample lm. The polarization-electric eld (P-E) curves of the sample lms, with a ferroelectric tester (Radiant Precision Multiferroic II, USA) at 100 Hz.

Morphology
The surface images of the pure CNF and CNF/o-MWCNT (6.2 wt%) composite lms were investigated by FE-SEM (as shown in Fig. 2). Pure CNF lm was compact and had a smooth surface (as shown in Fig. 2(a)). CNF/MWCNT composite lms showed distinctly different morphologies. Aer the addition of o-MWCNT, the surface of the lm became rough, and many pores were observed, revealing a loose lm structure. The o-MWCNTs were uniformly dispersed throughout the CNF matrix when the ller content is lower than 6.2 wt%. With the further addition of o-MWCNTs, many bigger o-MWCNTs rstly aggregated with each other and formed small clusters. Then these clusters started to connect with each other and formed a network. This result shows that uniform CNF/o-MWCNT nanocomposite lms can be obtained by simple mechanical mixing and casting methods when ller content below 6.2 wt%.

Acid oxidation of MWCNT
Acid oxidation is conducive to the dispersion of MWCNTs in water due to the introduction of hydrophilic groups, e.g., hydroxyl and carboxyl groups. FTIR spectra were analyzed to identify these molecular structural differences of MWCNTs before and aer the acid oxidation, as shown in Fig. 3. The FTIR spectrum of pristine MWCNT displays not only C]C peaks at 1383 cm À1 and 1122 cm À1 , but also many other small peaks, namely, O-H peak at 3434 cm À1 , C-H peaks at 2925 cm À1 and 2850 cm À1 , and C]O peak at 1624 cm À1 , indicating the presence of hydroxyl and carboxyl groups. [38][39][40] These hydroxyl and carboxyl groups in as-received MWCNT could be attributed to the purication process by the manufacturer. 41 On acid treatment, two characteristic peaks of the carboxyl group are seen at 1705 cm À1 and 1579 cm À1 , respectively (as shown in the dashed circle of Fig. 3), demonstrating that more COOH groups are introduced by the acid oxidation. 39 Additionally, the peaks at 1624 cm À1 , 1383 cm À1 and 1122 cm À1 shis to 1633 cm À1 , 1384 cm À1 , and 1125 cm À1 , respectively, it also reveals the structure changes of MWCNT upon carboxylation.

Dielectric properties
The inuence of ller loading and frequency on dielectric constant (3 r ) and loss tangent (tan d) were studied, as presented in Fig. 4(a) and (b). In the lower frequency range of 40-10 kHz, both 3 r and tan d decrease rapidly with the increase of frequency. It is ascribed to the electrode effect and Maxwell-Wagner-Sillars interfacial polarization. 24,31,[42][43][44] When an electric eld is applied on the nanocomposite lm, many dipoles and charge carriers accumulate at the interface between the electrode and the sample and between the llers and matrices. With the frequency going up, the high periodic reversal of the Paper RSC Advances electric eld makes it harder for the dipoles and charge carriers to catch up with the change of frequency. Hence, the interfacial polarization is declined and results in lower 3 r and tan d. 45 In higher frequency range, both 3 r and tan d tend to remain unchanged, which demonstrates that electronic polarization, atomic polarization, and orientation polarization starts to replace interfacial polarization to play a predominant role.
The ller loading shows more effects on dielectric properties in the lower frequency range of 40 Hz to 10 kHz. The 3 r rises with the addition of MWCNT when the ller content is lower than 6.2 wt% and falls with the further addition of MWCNT. In the meantime, the tan d uctuates on a narrow scale. It can be explained by percolation theory, 27,28,46 according to which, with the addition of conductive ller, the ller particles rstly aggregate with each other, then form conductive clusters. When the ller content is close to the percolation threshold, the conductive clusters start to connect with each other and result in a conductive path. At the same time, the insulative composite turns into a conductor. The dielectric constant and conductivity of composite obey the following the power law equation. 26,47-51 where, f f is the volume fraction of ller, f c is the volume fraction at the percolation threshold, s m and s f are the electrical conductivities of the polymer matrix and the conductive llers, respectively. s and t are the critical exponents, and u ¼ t/(t + s).
In this case, the percolation threshold is 6.2 wt%. It is noticeable that the tan d remains nearly unchanged, and it is attributed to the functional groups at the surface of MWCNTs. The 3 r of CNF/MWCNT composite lms reaches its maximum value of 73.88 with 6.2 wt% of MWCNT. It is about three times higher than that of pure CNF (25.24, at 1 kHz). Meanwhile, the tan d slightly decrease from 0.70 to 0.68. The outstanding dielectric properties of CNF/o-MWCNT nanocomposite lm make it a promising candidate for capacitor applications.
To further explore the electric properties of CNF/MWCNT (6.2 wt%) nanocomposite, the frequency dependence of AC conductivity (s AC ) was tested and compared with pure CNF (as shown in Fig. 4(c)). The s AC of both samples go up with the increase of frequency. In the lower frequency range of 40 Hz to 100 kHz, the trend was gradual, while it turned rapidly with the frequency further increased. This phenomenon complies well with Dyre's Random Free Energy Barrier Model. 52 The frequency-dependent property of s AC is related to the hopping of the charge carriers in the localized state and upper states in the conduction band. And this process can be accelerated by higher frequency. 12,53 Interestingly, the composite lm with 6.2 wt% of o-MWCNT exhibits lower s AC than that of pure CNF. At 1 kHz, the s AC decreases from 3.15 Â 10 À7 S cm À1 for CNF to 1.77 Â 10 À7 S cm À1 for CNF/MWCNT (6.2 wt%). Cellulose contains many hydrophilic hydroxyl groups, resulting in absorbed moisture and increased conductivity. The addition of o-  MWCNTs brings in many oxide-containing groups (e.g. carboxyl groups), which are more attractive to the hydroxyl groups in CNF to form hydrogen bonds than water molecules, leading to lower absorbed moisture and decreased conductivity.
Polarization-electric eld (P-E) hysteresis curve is an efficient way to evaluate the energy storage properties. The P-E hysteresis curves of pure CNF and the CNF/o-MWCNT (6.2 wt%) were measured, as shown in Fig. 4(d). Both remnant polarization (P r ) and saturated polarization (P s ) of the CNF/MWCNT nanocomposite lm are higher than that of pure CNF. It demonstrates that the addition of o-MWCNT makes it more easily for samples to be polarized, which is also proved by the results of higher 3 r of CNF/o-MWCNT composites than that of pure CNF. However, the P r of the composite lm is close to P s , suggesting a low efficiency of energy storage. 12 The polarization of the CNF/MWCNT lms saturates before the electric eld reaches its maximum value. It may be caused by the leakage current, which is the result of a heterogeneous structure and partial conductive network. With more efforts made on improving the dispersion of MWCNT llers and reducing the remnant polarization, the CNF/MWCNT nanocomposite lm has great potential in energy storage applications.

Thermal stability
In practice, the life length and reliability of dielectric materials are closely bound up with their thermal stability. Thus, the TGA-DSC curves of pure CNF and CNF/o-MWCNT (6.2 wt%) composite lm were measured, as depicted in Fig. 5. Two stages are presented in the degradation process. The rst degradation stage is the slight weight loss before 100 C, resulting from the evaporation of absorbed water. 13 The weight loss of CNF/o-MWCNT (6.2 wt%) slightly decreased from 2.02% to 1.82%. It indicates that the introduction of oxide-containing groups helps to decrease absorbed moisture. The temperature at 5.0% weight loss (T 5% ) is used as the start signal of degradation. CNF/ o-MWCNT (6.2 wt%) nanocomposite has a T 5% of 271 C, which was a little higher than that of pure CNF (265 C). It means that the incorporation of MWCNTs helps to improve thermal stability. The second degradation stage is in the temperature range from 250 C to 400 C, where there is a sharp drop of weight, as well as a DTG peak at 340 C and a DSC endothermal peak at 325 C. It is due to the decomposition process of CNF. With the addition of MWCNTs, the DTG peak shied slightly from 340 C to 343 C, and the DSC endothermal peak shied from 325 C to 341 C. It also illustrates that the incorporation of MWCNTs helps to improve thermal stability. In a word, the TGA-DSC curves suggest that the CNF/MWCNT composite lms have acceptable water absorption and good thermostability.

Conclusions
A series of exible high dielectric nanocomposite lms based on CNF and acid oxidized MWCNT were prepared by casting method in aqueous solution. Though no organic solvents were involved during the preparation, MWCNT had good distribution within the CNF matrix. The porous structure appears with the addition of o-MWCNT, and its adverse effects are reected in the dielectric properties. The dielectric constant of CNF/o-MWCNT (6.2 wt%) composite lms greatly increases from 25.24 for pure CNF to 73.88, while the loss tangent slightly decreases from 0.70 to 0.68, and the AC conductivity decreases from 3.15 Â 10 À7 S cm À1 for CNF to 1.77 Â 10 À7 S cm À1 (at 1 kHz). The abnormal decrease of loss tangent and AC conductivity are attributed to the introduction of oxide-containing groups on the surface of MWCNTs. The nanocomposite lms show excellent exibility that they can be bent for a thousand times without obvious damage. The presence of MWCNT helps to improve the thermal stability of the composite lms. With more efforts being made to cut down the remanent polarization and improve the compatibility between the MWCNT ller and CNF matrix, the novel CNF/MWCNT composite lms is a promising green candidate for high dielectrics in the eld of energy storage.

Conflicts of interest
There are no conicts to declare.