Freeze-drying and hot-pressing strategy to embed two-dimensional Ti0.87O2 monolayers in commercial polypropylene films with enhanced dielectric properties

The dielectric capacitor has been widely used in advanced electronic and electrical power systems due to its capability of ultrafast charging-discharging and ultrahigh power density. Nevertheless, its energy density is still limited by the low dielectric constant(≈ 2.2) of the commercial dielectric polypropylene (PP). The conventional enhancement strategy by embedding inorganic fillers in PP matrix is still difficult and challenging due to that PP hardly dissolves in any inorganic/organic solvent. In this work, we develop a new strategy including freeze-drying, surface functionalization, and hot-pressing to incorporate Ti0.87O2 monolayers in PP film. A series of uniform composited Ti0.87O2@PP film has been successfully fabricated with Ti0.87O2 content range of 0–15 wt%. The maximum dielectric constant of the as-prepared Ti0.87O2@PP film is 3.27 when the Ti0.87O2 content is 9 wt%, which is about 1.5 times higher than that of pure PP. Our study provides a feasible strategy to embed two-dimensional material into commercial PP thin-film with superior dielectric performance for practical application.


Introduction 
Recently, electrical energy storage devices have been attracting immense research interest with the worldly growing demand for energy requirement [1][2][3]. Dielectric capacitors play an important role in ultrafast charge-discharge capability, which is desired for a In principle, the energy density (W) of dielectric capacitors is determined by the applied electric field (E) and dielectric constant (ε r ) as where ε 0 is the vacuum permittivity (8.85×10 −12 F/m). Theoretically, improving dielectric constant (ε r ) is a very effective route to gain high energy density. Note that ceramics show high dielectric constant but low breakdown strength while polymers exhibit high breakdown strength but low dielectric constant. During the past decades, a lot of researches have been carried out on introducing ceramics nanofillers into polymer matrix for polymer/inorganic composites to realize the optimized dielectric properties. Two-dimensional (2D) oxide nanosheets, which possess atomic or molecular thickness and infinite planar dimensions, have been attracting remarkable interests on energy storage fields due to their ultrahigh specific surface area, excellent mechanical flexibility, and even quantum confinement. Typically, Ti 0.87 O 2 atomic monolayer has been considered as a novel high-κ compound with a layered crystallographic structure in which TiO 6 octahedra are edge-linked in a lepidocrocitetype 2D lattice. Previous experimental investigations demonstrate that a multilayer thin-film constructed from Ti 0.87 O 2 nanosheets as building blocks on solidstate SrRuO 3 substrate exhibits a high dielectric constant of ~125 when the thicknesses are down to 10 nm [10]. Such a dielectric constant is much larger than that of anatase (ε r = 30-40) and rutile TiO 2 (ε r = 80-100) [11]. The excellent high-κ behavior should be originated from the existence of abundant Ti vacancies rather than oxygen vacancies which act as carrier traps and high-leakage paths.
Motivated by these intrinsic merits, incorporation of oxide nanosheets into polymer matrix to form polymer nanocomposites has recently emerged as a very promising strategy to realize the dielectric thin-film with high dielectric constant and enhanced energy density. For example, Wen et al. [12] revealed that Ti 0.87 O 2 nanosheets are desirable inorganic fillers in poly(vinylidene fluoride) (PVDF) for developing flexible thin-film based capacitor. They successfully incorporated Ti 0.87 O 2 nanosheets into the PVDF matrix through a facile solution casting strategy using NMP as the solvent, which delivers an energy density enhancement of 190% over the bare PVDF. Furthermore, Bao et al. [13] reported the successful incorporation of Ca 2 Nb 3 O 10 nanosheets into ferroelectric PVDF matrix to realize a high energy density of 36.2 J/cm. However, compared with commercial PP, PVDF exhibits high dielectric loss as well as the ferroelectric hysteresis resulting in an energy loss at alternating voltage [14,15], which significantly restricts its application in high frequency circuits. It is highly desired to incorporate monolayer oxide nanosheets in commercial PP system toward practical application. Unfortunately, the conventional liquid-casting strategy is not viable in commercial PP system due to that PP hardly dissolves in any inorganic/ organic solvent. The main challenge remains the thinfilm fabrication of PP-based polymer/inorganic filler composites.
Our previous study has realized the application of Ti 0.87 O 2 nanosheets on lithium-ion storage, resistive random access memory (RRAM), and CO 2 electroreduction [16][17][18]. In this work, we develop a new strategy including freeze-drying, surface functionalization, and hot-pressing to incorporate Ti 0.87 O 2 monolayers in commercial PP film. High-quality colloidal solution consisted of Ti 0.87 O 2 nanosheets with atomic thickness was obtained by a soft-chemical exfoliation route from its layered, protonic precursor. Subsequently, aerogel-like, solid-state Ti 0.87 O 2 nanosheets were freezedried from the corresponding single-layer colloidal solution and then surface-modified by coupling agent KH550. Then such an aerogel was successfully embedded in PP matrix with well controllable Ti 0.87 O 2 nanosheet concentration through a hot-pressing strategy (Fig. 1). The resulting Ti 0.87 O 2 @PP composited thin-film displayed a remarkable enhancement of 134% on dielectric constant with an optimized Ti 0.87 O 2 concentration of 9 wt%. Our study provides a feasible strategy to embed two-dimensional material into commercial PP thin-film with superior dielectric performance for practical energy-storage application. at a solution-to-solid ratio of 100 g/mL and then shaken for 7 days, which also led to the osmotic swelling. After exfoliation by ultrasonic, well-dispersed unilamellar nanosheets of Ti 0.87 O 2 were obtained as a stable colloidal suspension.

2 Preparation of freeze-dried aerogel and surface-modified Ti 0.87 O 2 nanosheets
100 mL Ti 0.87 O 2 nanosheet colloidal solution with a concentration of 4.0 g/L was added in a 300 mL beaker and then freeze-dried in the freeze dryer for 4 days. Afterwards, loose gel-like sample with a cotton-like shape can be directly obtained. The KH550 modification process of freeze-dried Ti 0.87 O 2 was as follows. First, 2 mL silane coupling agent KH550 was dispersed in 5 mL ethanol. Then 3 g freeze-dried Ti 0.87 O 2 nanosheets and KH550/ethanol mixture were added together into 10 mL ethanol. After being stirred for 24 h, the mixture was centrifuged at 10,000 rpm for 5 min to collect the white slurry at the bottom of the tube and washed by ethanol three times to remove the excess KH550. The product was then dried in the oven at 60 ℃ for 12 h and the white KH550 modified freeze-dried Ti 0.87 O 2 powder can be obtained.

3 Preparation of Ti 0.87 O 2 @PP composite film
PP particles (melt flow rate 3.2 g/10 min) were purchased from Borealis and the xylene (AR) was purchased from Aladdin. To homogeneously blend the as-prepared Ti 0.87 O 2 and PP particles, firstly, PP particles were ball milled into powder form as follows: 10 g PP commercial particles and 100 mL xylene were added in a 400 mL beaker and heated in oil bath at 140 ℃. Until PP particles dissolved completely, the uniform solution was placed at room temperature for 2 days to volatilize xylene. The resulting white PP block by this treatment was cut to small pieces. 3 g PP pieces and grinding medium (steel balls of different diameters, 5, 10, and 20 mm, with a weight ratio of 7:2:1 in sequence) were put into a 50 mL sealed agate jar and milled in a highenergy planetary ball mill system with an autorotation speed of 400 rpm at room temperature for 2 h. Then the ball-milled powder was passed through an 80-mesh sieve. The screened PP powder and as-prepared Ti 0.87 O 2 were mixed in different weight ratio of Ti 0.87 O 2 (0%, 3%, 6%, 9%, 12%, 15%) and milled with an autorotation speed of 600 rpm at room temperature for 4 h. The well mixed powder was heated at 180 ℃ for 15 min without pressure and then kept at 180 ℃ under 12 MPa for 15 min in the press vulcanizer. Upon cooling to room temperature, the Ti 0.87 O 2 @PP composite film was removed from the plate for further analysis.

4 Material characterization
The phase structure was characterized by a Bruker D8-A25 diffractometer using Cu Kα radiation (λ = 1.5406 Å). Sample morphologies were characterized using a FEI Navo Nano SEM 450 field-emission scanning electron microscope (SEM) and a Bruker Dimension Icon atomic force microscope (AFM). Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) were performed using a Philips CM 200 FEG field emission microscope at an acceleration voltage of 300 kV. The film thickness test was carried out via a step profiler (SD-LX).

5 Dielectric measurement
The dielectric properties of the composite samples were measured using LRC digital bridge (HIOKI IM3533) in the frequency range of 10 Hz to 100 kHz. Prior to measurement, a thin cover of the silver paste was coated on the two sides of all samples. The electrical breakdown test was carried out via high voltage DC generator (ZGF-120/2). The generator applied an increasing voltage on the sample until the sample breakdown. Multimeter (MASTECH MY65) is used to measure the voltage of the sample. We carried out the electrical breakdown test three times on each sample and the real data was the average value of three measurements.

Results and discussion
Firstly, titanate precursor (K 0  Fig. 2(a)) [19]. Then 2D Ti 0.87 O 2 nanosheets with atomic thickness were obtained by a liquidexfoliation strategy from the layered H 1.07 Ti 1.73 O 4 · nH 2 O precursor with an aqueous solution of tetrabuthylammonium hydroxide (TBAOH) [20]. After the soft-chemical exfoliation, highly stable, transparent colloidal suspension of Ti 0.87 O 2 nanosheets exhibits a clear Tyndall light scattering ( Fig. 2(b)). As demonstrated by the atomic force microscopy (AFM) characterization, the thickness of the Ti 0.87 O 2 nanosheets is approximately 2-3 nm (Figs. 2(c) and 2(d)). The transmission electron microscopy (TEM) images exhibit two-dimensional ultrathin sheets with lateral dimensions up to 4 μm while fragments and folded edges were also observed in small amounts (Figs. 2(e) and 2(f)). The SAED pattern taken from the aggregation of several sheets exhibits polycrystalline diffraction rings ( Fig. 2(g)).
Considering that PP matrix is insoluble in most of organic/inorganic solvent, it is necessary to recover our Ti 0.87 O 2 nanosheets from the colloidal suspension and then embed it into PP through a non-solution based strategy. We used the freeze-drying technique to obtain the aerogel sample of the atomically flat Ti 0.87 O 2 nanosheets. After freeze-drying the colloidal solution with a concentration of 4.0 g/L in vacuum for 4 days, white, loose aerogel-like sample with a cotton-like shape was obtained as shown in Fig. 3(a). A set of sharp diffraction lines indexed as (0k0) (k = 1, 2, 3, 4, 5) diffraction peaks can be detected in the corresponding XRD pattern (Fig. 3(b)). The basic d-spacing of (010) peak is estimated to be 1.73 nm, which is indicative of a lamellar structure with a gallery height of 1.73 nm. SEM observation further confirms that the freeze-dried , suggesting that about 10-15 atomically thin nanosheets were restacked along [00l] direction during the freeze-drying process. Note that such a thickness is also drastically smaller than that of pristine layered H 1.07 Ti 1.73 O 4 ·nH 2 O bulk, leading to a large aspect ratio of more than 100.
Subsequently, the freeze-dried Ti 0.87 O 2 aerogel was used as the inorganic fillers embedded in PP matrix to fabricate composited thin-film through a hot-pressing process. We found that commercial PP particles are hardly ball-milled directly into powder form. However, dissolution of PP particles in xylene at 140 ℃ and cooled down to room temperature results in the recrystallization of PP, which can be ball-milled into powder easily. After the hot-pressing of the freezedried Ti 0.87 O 2 aerogel and the ball-milled PP powder into a composite thin-film, the characteristic (0k0) peaks of Ti 0.87 O 2 disappeared ( Fig. 4(a)). We considered that it should be attributed to the drastic aggregation of the Ti 0.87 O 2 aerogel in PP matrix. Such a problem has been successfully solved by the surface modification using coupling agent KH550 in ethanol. After such a pretreatment, the Ti 0.87 O 2 nanosheets still maintain a layered crystal structure (Fig. 4(b)) and (010) peaks can be clearly detected from the composited thin-film as shown in Figs. 4(c) and 4(d). We further confirm the uniformity of this composited film by recording the XRD patterns from four distinct positions on the surface of this nanocomposite film. As shown in Fig.  4(f), these four positions 1-4 show analogous XRD patterns with the appearance of (010) peaks from our Ti 0.87 O 2 fillers. This result demonstrates that Ti 0.87 O 2 fillers are homogeneously dispersed in PP polymer. Figure 5 show the characteristic scanning electron microscopy (SEM) images of the resulting Ti 0.87 O 2 @PP composite films with various concentration of KH550/Ti 0.87 O 2 nanosheet fillers from 0 to 15 wt%. One can see that KH550/Ti 0.87 O 2 nanosheet fillers are well dispersed in PP matrix without any accumulation until the concentration up to 9% (Figs. 5(a)-5(d)). However, the agglomeration of Ti 0.87 O 2 flakes with size of 500-1000 nm can be observed when its concentration up to 12-15 wt% (Figs. 5(e) and 5(f)). In addition, the thickness of this series of Ti 0.87 O 2 @PP composite films is determined into 230-285 µm via a step profiler (SD-LX) as listed in Table 1.  The dielectric constant (ε r ) of Ti 0.87 O 2 @PP composite films at different frequencies is shown in Fig. 6(a). It can be seen a downward trend of dielectric constant in low frequency (< 10 3 Hz) in all of the Ti 0.87 O 2 @PP  composite films, and such a downward trend generally becomes stable in high frequency range (> 10 3 Hz). This result implies excellent compatibility between KH550 treated Ti 0.87 O 2 and pure PP matrix. The dielectric constant of the hybrid film with different Ti 0.87 O 2 concentrations at 10 3 Hz is given in Fig. 6 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.