Bio-inspired synthesis of highly crystallized hexagonal boron nitride

Ceramics International xxx (xxxx) xxx–xxx

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Ceramics International journal homepage: www.elsevier.com/locate/ceramint

Bio-inspired synthesis of highly crystallized hexagonal boron nitride nanosheets Renjie Genga,b,1, Yancui Xub,1, Songfeng Eb, Chaowei Lib, Cuiping Yub, Taotao Lib, ⁎ ⁎ Xiaoyang Longa, Wenbin Gongb, Jun Wua, , Yagang Yaob, a

Wuhan Univ Sci & Technol, State Key Lab Refractories & Met, Wuhan 430081, Hubei, PR China Division of Advanced Nanomaterials, Key Laboratory of Nanodevices and Applications, CAS Center for Excellence in Nanoscience, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, University of Chinese Academy of Sciences, Suzhou 215123, PR China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Bio-inspired Boron nitride nanosheets Composite Chemical vapor deposition

Boron nitride nanosheets (BNNSs) exhibit great potential as a two-dimensional dielectric material due to superior thermal and chemical stability. However, compared with mature processing techniques for graphene, mass production of BNNSs mainly depends on mechanical exfoliation routes until now, which do not control the morphology and crystallinity of the products. In this study, we demonstrate a biomass-inspired strategy for synthesizing highly crystalline and purified hexagonal BNNSs using ambient pressure chemical vapor deposition. Using Bellamya quadrate shells as both the catalyst and template not only led to a reliable, non-toxic, and lowcost synthetic route, but also used food waste as raw material. The BNNSs produced were characterized to be atomically flat, with few layers, a single crystalline hexagonal morphology, and with a classic band gap of 6.05 eV. In addition, the BNNSs could be utilized as fillers to fabricate thermoplastic polyurethane composites, presenting a 3.6 time and 3.7 time increase in thermal conductivity and elastic modulus than pure thermoplastic polyurethanes, respectively.

1. Introduction The novel structure and unique properties of graphene have raised increasing enthusiasm for two-dimensional (2D) nanomaterials [1–5]. Boron nitride nanosheets (BNNSs), which are a sp2-hybridized honeycomb structure of B and N atoms, are one of the most important III–V group materials and are called “white graphene” [1,6–9]. BNNSs exhibit several properties that rival their carbon counterparts, such as high chemical and thermal stability, strong piezoelectricity, excellent thermal neutron radiation shield ability, robust lubricity, and super hydrophobicity [10–16]. Moreover, due to the large band gap of 5–6 eV, BNNSs are electrically insulating, which differs from metallic or semi-conductive graphite carbon [17,18]. These properties provide a unique potential for diverse applications of BNNSs, including drug delivery, pollution remediation, anti-corrosive coatings, insulating substrates, mechanical or thermal reinforcements, self-cleaning materials, and use in more extreme conditions, such as aerospace applications [11–13,19]. As a result, numerous efforts have been devoted to developing versatile BNNS production techniques. BNNSs with a small number of

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layers were first successfully fabricated via a micromechanical cleavage technique [20], opening the door for BNNS synthesis. Since then, many strategies have been developed to obtain better BNNSs on a large scale [21–29]. Chemical exfoliation [21,22], particularly ultrasonication, of bulk BN can be used to obtain BNNSs with a large area and small thickness by breaking weak Van der Waals interactions between the hBN layers. The number of layers and lateral size can be controlled to some degree by introducing some specific polar solvents. In comparison, ball milling [30,31] is more environmentally friendly because nontoxic solvents are used. The mechanical shearing and compressive forces make it possible to collect many BNNSs, but quality is uncontrolled. The most promising and economical strategy to synthesize 2D BN materials with high quality is the chemical vapor deposition (CVD) method [24,25,28]. It is possible to manipulate the formation process of a nanosheet, and prepare large scale, low defect BNNSs with a small number of layers by controlling the quantity and variety of precursors [28,32], the temperature [26,33], the flow and kind of reaction gas [34] and combining other techniques such as plasma [35]. However, it remains difficult to industrialize the preparation of BNNSs considering the unsatisfactory production rate and high cost of CVD.

Corresponding authors. E-mail addresses: [email protected] (J. Wu), [email protected] (Y. Yao). These authors contribute equally to this work.

https://doi.org/10.1016/j.ceramint.2018.05.026 Received 23 November 2017; Received in revised form 4 May 2018; Accepted 4 May 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Geng, R., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.05.026


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in the inset of Fig. 1(b) and can be successfully synthesized with a BQS template in a large scale by a single experimental run (Fig. 1(b)). The main constituent of BQS is leaf aragonite, which was demonstrated to be calcium carbonate (CaCO3) by XRD analysis (Fig. 1(d)) (according to JCPDS card 03–1067), with a multi-slice topography structure in the micro chambers shown in the scanning electron microscopic (SEM) images (Fig. 1(c)). CaCO3 can be decomposed in air above 900 °C. After the BQS organic matrix was oxidized into carbon dioxide, the main residue is calcium oxide (CaO) (Fig. 1(f)) (according to JCPDS card 481467), which inherits the multi-slice structure of BQS, as shown by the scanning electron microscopy (SEM) image presented in Fig. 1(e). Similar to other oxide (e.g., MgO and FeO) catalysts [26], CaO can also act as a smart catalyst to facilitate the chemical reaction between B2O3 and NH3 at high temperatures. Nevertheless, only a few studies have been performed with respect to the formation mechanism, due to the complexity of the solid phase chemical reactions. Here, we discuss the synthetic mechanism of BNNSs, which is depicted in Fig. 2(a). The reaction process includes two stages. First, Ca3B2O6 is produced by the interaction between B2O3 and CaO powder, which are stable at room temperature, as presented in the phase diagram [36] (Fig. 2(b)). Then, the BNNSs are formed via nitridation of Ca3B2O6 in NH3. Due to the high melting point of CaO, which is 2845 K, it is easy to retain the laminated structure of the original BQS, resulting in a strong role for the template during growth of the BNNSs. The laminated CaO has high

To overcome these challenges, nature offers us a remarkable and simple paradigm to exploit new materials by obtaining the same structure from a bio-template. Here, for the first time, we report scalable growth of high-crystal-quality BNNSs with bellamya quadrata shells (BQSs), as both a catalyst and template during the CVD process. It is easy to obtain BQSs at a low cost, and the use of a common horizontal tube furnace increases simplicity of the reaction. Moreover, no harmful or toxic substances are introduced in this reaction. The BQS template can be easily removed compared to other template-based methods [24–26]. The as-prepared highly crystalline, and thickness-controllable BNNSs can be replicated from the lamellar morphology of the template. In addition, the cost of the BNNSs will be significantly reduced using this ubiquitous biomass as the raw material. Furthermore, the templateprepared BNNSs have high thermal conductivity and wide applications in electronic packaging as demonstrated by the highly thermal conductive composites (8.34 W/mK) and high elastic modulus (7.14 MPa) of the thermoplastic polyurethanes and as-made BNNSs. 2. Results and discussion BQS is a kind of shelled mollusk and a common food in China. As shown in Fig. 1(a), the BQS shell has a chambered structure, just like other shells, with high hardness and low density. The BNNSs X-ray diffraction (XRD) analysis (according to JCPDS card 34-0412) is given

Fig. 1. Optical photographs of (a) BQS and (b) the BNNSs obtained from annealing the mixture of BQS and B2O3 at 1100 °C for 2 h. The inset of (b) gives the XRD pattern of the BNNSs. SEM images of the original BQS (c) before and (e) after annealing in air at 900 °C for 8 h. The (d) and (f) are the XRD spectra of the products shown in (c) and (e), respectively. 2


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Fig. 2. (a) Formation process for biomass-inspired synthesis of BNNSs. (b) Schematic diagram of binary phase diagram for B2O3 and CaO, (c) XRD spectra of products before pickling produced at 800, 1000, 1100, 1200, and 1400 °C.

second H+ is separated from NH2- and combines with OH- to form H2O. 4) NH- becomes an active N atom when the final H+ is separated, and the active N atom reacts with B atom to generate BN, while the only H+ combines with another O atom to form a new OH-. The decomposition barrier of the NH3 molecule is not very high due to the surface exposed Ca atoms. All B-O bonds are broken when the outermost layer B atoms are exhausted, and the NH3 diffuses into the Ca3B2O6 bulk and reacts with the next layer atoms. Fig. 4 displays the SEM images of samples produced at 1100 °C (Fig. 4(a) and (b)) and 1400 °C (Fig. 4(c) and (d)) after acid pickling. Different from the curly nanosheets synthesized on normal substrates or templates, such as silicon [26] and carbon [24], the samples inherited the flat topography characteristic of the BQS template because the reaction occurred on the biomass surface. The morphologies of the synthesized BNNSs were irregular hexagons or other polygons without byproducts, such as wires, rods, or tubes, which is shown in Fig. S1, and the average size and thickness of the nanosheets were estimated to be 0.5–3 µm and no more than 20 nm, respectively. We used commercial CaO (Fig. S2(a) and (b)) as another starting material to make BNNSs as a comparison. It was clear that the amorphous morphology of commercial CaO was inherited by the resulting BN (Fig. S2(c) and (d)). By comparison, the BNNSs obtained from the BQSs had a layered-like appearance, suggesting that their morphologies were replicated. The morphologies and fine structures of the as-prepared BNNSs were also carefully determined by transmission electron microscopy (TEM), which showed that the BNNSs represented a classic graphenelike 2D structure (Fig. 5(a), (b)). In Fig. 5(a), several layered nanosheets are piled up together. Single-layered BNNSs were detected on the edge of these nanosheets in the magnified image (Fig. 5(b)). Fig. 5(c) and (d) show the typical high resolution TEM images of the synthesized BNNSs,

chemical activity to absorb B2O3 onto their surfaces and generate lamellar Ca3B2O6, which reserve B2O3 in the solid phase. Then, the uniform and laminated Ca3B2O6 reacts with NH3 to form the BNNSs. When the temperature was maintained at 1400 °C for more than 4 h, the conversion rate of B2O3 to BN was 90%. The chemical reactions for the two stages are shown as follows: 3CaO (s) + 2B2O3 (s) + 2NH3 (g) = Ca3B2O6 (s) + 2BN (s) + 3H2O (g) (1) Ca3B2O6 (s) + 2NH3 (g) = 2BN (s) + 3H2O (g) + 3CaO (s)

(2)

Fig. 2(c) shows the XRD patterns of the target products at different temperatures. The Ca3B2O6 (according to JCPDS card 73-1203) and CaO (according to JCPDS card 48-1467) patterns appeared at 800 °C, indicating that the first stage occurred partially at 800 °C. The CaO was entirely transferred into Ca3B2O6 until the temperature was above 1000 °C. Furthermore, the peak of the (002) h-BN crystal face (according to JCPDS card 34-0412) weakly appeared at 1100 °C, and became stronger at 1200 and 1400 °C, which suggests that the second stage occurred at 1100 °C. When the reaction temperature reached 1400 °C, the second stage proceeded more completely; the h-BN peaks were clear and strong, whereas the peaks representing Ca3B2O6 were much weaker than those observed at 1200 °C. The second stage was clearly more effortless with the increase in temperature, and the crystallinity of the BNNSs increased. To uncover the reaction mechanism, we performed first-principles density functional theory calculations (Fig. 3), based on the Ca3B2O6 precursor, and revealed that the reaction was comprised of four steps. 1) NH3 molecules are adsorbed onto the Ca sites on the Ca3B2O6 surface. 2) NH3 molecules decompose into NH2-, and H+, while the first H+ combines with the O atom site on Ca3B2O6 to form OH-. 3) The 3


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Fig. 3. The density functional theory calculations of the boron nitride formation mechanisms. Kinetics of the chemical decomposition process of NH3 above the surface of Ca3B2O6. IS, MS, and FS represent the initial, middle, and final states of the reaction, respectively.

Fig. 4. SEM images of BNNSs inspired by BQS at (a, b) 1100 °C and (c, d) 1400 °C. 4


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(110), and (112) planes, in agreement with the reported standard h-BN, including lattice constants of a = 0.2504 nm and c = 0.6656 nm (according to JCPDS card 34-0421). The BNNSs were mainly AB stacked and the P63/mmc space group, which is identical with the bulk structured BN. No other diffraction peaks were detected, indicating the high purity of the BNNSs obtained under the current conditions. Raman spectra provide information about the lattice vibration modes of the samples as well. Fig. 6(c) displays the Raman spectrum of the BNNSs prepared on the Si substance, and the dominant peak centered at 1367 cm−1 was assigned to the E2g vibration mode of h-BN [23,39]. UV–vis absorption spectroscopy is widely used to measure the band gap of materials. Previously reported BN band gap energy values are widely dispersed in the range of 3.6–7.1 eV. Moreover, the absorption lines at 6.15 eV corresponded to classic h-BN nanosheets [17,18]. We obtained the UV–vis absorption spectrum of the BNNSs (Fig. 6(d)). The BNNSs were highly transparent in the range of 250–800 nm, while the UV absorbance peak at 200 nm tended to be out of the measuring range of the equipment. The inset plot in Fig. 6(d) was obtained from the wavelength data according to Eg = hc/λg [42]. Thus, the optical band gap obtained from the UV–vis absorption spectrum was ~ 6.05 eV, which is close to the standard h-BN nanosheets [28,43]. X-ray photoelectron spectroscopy (Fig. S3) was applied to analyze the electronic structure and chemical composition of the sample. The peaks of elements C, O, B, and N are displayed on the survey spectrum (Fig. S3(c)). The intensity of the C and O signals was very weak, which may have originated from the CO2, O2, or H2O adsorbed on the surface of the BNNSs. The high-resolution B 1s peak is displayed in the inset of Fig. S3(a), and the binding energies were centered at 190.5 eV, which was very close to the previously reported value of 190.1 eV [39]. Our sample presents an N 1s-core level at 398.1 eV, which is in good agreement with literature values [23]. The atomic ratio of B to N was about 1.22 by quantifying the B 1s and N 1s peaks, which also approximated the literature values [41]. Both the B 1s and the N 1s spectra indicate that the configuration for B and N atoms is the B-N bond, suggesting that the hexagonal phase existed in our BNNSs. The BNNSs were added to a polymeric matrix to build a thermal conductivity network, using four designed fractions of 4, 8, 12, and 16 wt%. We used N, N-dimethylformamide (DMF) to disperse the mixture of BNNSs and TPU, as DMF has greater dispersibility than water for BN. The effect of BNNSs concentration on the mechanical properties of the BNNSs/polyurethane composites is presented in Fig. 7(a). The elastic modulus increased from 0% to 16% of the BNNSs fillers due to π-π interactions, while there tended to be linear growth after 4%. As the elastic modulus of pure TPU film is 2.0 MPa, a 16 wt% BNNSs-filled TPU composite possessed an elastic modulus of 7.1 MPa, which was a three-fold increase compared to pure TPU. Aggregated fillers often result in a decrease in tensile strength, but these results demonstrate that the BNNSs filler dispersed well in the TPU matrix, and the interfacial adhesion between the TPU matrix and BNNSs filler improved the tensile properties of the composite films. The thermal performance of the TPU/BNNS composite films was evaluated in Fig. 7(b) and (c). The parallel thermal conductivity of all investigated composites increased considerably as BNNS filler content increased. The thermal conductivity values of the TPU/BNNSs composite films were 1.3, 1.6, 3.1, and 3.7 times that of TPU film which was 1.79 W/mK. We attributed the reinforcement in the parallel thermal conductivity to be the high thermal conductivity of the BNNSs and the BNNSs conductive network. The infrared images (Fig. 7(c)) demonstrated that the BNNSs filler effectively enhanced heat dissipation of the TPU matrix. During the heat accumulation phase (1–19 s), the films were heated with a lighted light emitting diode (LED) (5 W). A larger yellow patch was observed as more BNNS filler was introduced, indicating that the heat from LED diffused more quickly than those of smaller patches. Then, after turning off the LED (25–40 s), the films with higher BNNSs content cooled faster, as the yellow patch vanished faster (Fig. 7(c)). Both heat accumulation and dissipation experiments revealed that the BNNSs

Fig. 5. (a, b) Transmission electron microscopic (TEM) image of the BNNS products. (c) High resolution (HR) TEM image of the product (inset is the enlarged image of the BNNS). (d) Enlarged HRTEM image of the BNNSs, revealing the peculiar honeycomb lattice with basis vectors marked. (e) Electron diffraction pattern taken from a region covering the 100 nm scale domain around the two corners, oriented along the [001] zone axis. (f) Energy-dispersive X-ray spectroscopy spectrum of the BNNSs.

which revealed that the BNNSs were a perfect single crystal. The thickness of the nanosheets was less than 10 nm based on the turned-up edges. The inset in Fig. 5(c) is the enlarged image of the rectangular domain, where highly ordered lattice fringes are clearly observed. The interplane distance was 0.34 nm [8,37], corresponding to the distance of (002) the hexagonal BN plane lattice. The periodic dots in Fig. 5(d) correspond to the honeycomb lattice, with a distance of 0.22 nm between two atoms, approaching the (100) spacing of h-BN. Furthermore, the (100) spacing was measured to be 0.217 nm by electron diffraction, which was in agreement with the standard h-BN lattice [8,20,29,38–40], and electron diffraction was also used to determine the single-crystal structure of the BNNSs. According to the periodic bond chain theory, the zigzag N-B-N chains paralleling the < 100 > orientation was strong and occupied a leading position in the growth of the BNNSs [8,39,40]. These strong chains keep the BN atomically flat, which is the reason why the morphology of the as-prepared BNNSs tended to be flat rather than scale-like and wrinkled. Fig. 5(f) reveals the energy dispersive X-ray (EDX) spectrum of the BNNSs. According to the integral calculation, the atomic ratio of boron and nitrogen was approximately 1. The carbon and copper peaks in the EDX spectrum were derived from the microgrid used as the substrate for preparing the TEM samples. The BNNSs prepared after acid pickling were characterized by Fourier transform-infrared (FTIR) and Raman spectra as well as XRD and UV–visible spectra, as shown in Fig. 6. The FTIR spectrum of the product (Fig. 6(a)) shows two strong vibrations at 794 cm−1, corresponding to the B–N–B bending vibration (E1u) mode out of the plane, and at 1371 cm−1, resulting from the in-plane B–N stretching vibration (A2u) mode [38,41]. These two vibrations indicate the good crystallinity of the BNNSs. It has been reported that single-crystalline h-BN vibrates at 1365 cm−1, while the phonon modes shifts to 792 and 1372 cm−1 for multi-layered h-BN compared with our samples [8]. The weak absorption peak near 3420 cm−1 are O–H bonds due to water in the atmosphere. According to this FTIR spectrum, no other absorption peaks associated with the starting materials and impurities were observed. The XRD shown in Fig. 6(b) was conducted to investigate the crystalline structure and phase components of the BNNSs. All diffraction peaks were indexed as the h-BN (002), (100), (101), (102), (004),

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Fig. 6. Typical (a) FTIR, (b) XRD, (c) Raman, and (d) UV–vis spectra of the as-grown BNNSs.

Fig. 7. (a) Elastic modulus and (b) thermal conductivities of the thermoplastic polyurethane/boron nitride nanosheet (TPU/BNNS) composite films. (c) Infrared images of the TPU/BNNSs composite films upon heating (1–19 s) and cooling (25–40 s). 6


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filler played a significant role in the high reinforcement effect of thermal conductivity.

4.3.1. Instruments and characterization SEM images were obtained on a field-emission SEM (S4800; Hitachi, Tokyo, Japan). TEM was performed on a Tecnai G2 F20S-TWIN (FEI, Hillsboro, OR, USA) with an operating voltage of 200 kV. Raman spectra were collected over the spectral range of 1000–2000 cm−1 using Lab RAM ARAMIS Raman confocal micro-scope (HR800 Raman system; Horiba, Tokyo, Japan) equipped with a 532 nm diode pumped solid state laser. The X-ray diffraction (XRD) data were collected at room temperature, using Cu-Ka (λ = 1.5418 Å) X-rays. Additional acquisition parameters were: 2θ range, 10–90°; scan rate, 0.02°s−1. FTIR spectra were collected over the spectral range of 600–4000 cm−1 using a Fourier transform infrared spectrometer (Thermal Scientific Instruments, model NicoletiN10, Waltham, MA, USA). All tested samples were loaded at the same quantity. Diffraction patterns are referenced against the JCPDS database to identify the samples. Thermal diffusivity (mm2 s−1) of the composites at room temperature was measured on disk samples using the laser flash method (LFA; Nanoflash LFA 447 System; Netzsch Instruments Co., Selb, Bavaria, Germany). The mechanical properties of the TPU elastomer composites were determined by standard tensile stress-strain tests to measure modules. The tensile tests were performed using a universal material testing machine (UTM: Instron 3365, Instron, Norwood, MA, USA) at a crosshead speed of 100 mm min−1.

3. Conclusion In summary, we developed a bio-inspired approach to synthesize high crystallinity and highly pure BNNSs. Using ubiquitous BQS biomass as both the catalyst and template produced a large amount of high quality BNNSs in a simple atmospheric pressure chemical vapor deposition system, which significantly reduced the cost to synthesize mass BNNSs. Very few toxic substances were produced during the synthesis, making it a realistic way to achieve large scale green production of BNNSs. TPU composites with different BNNSs contents as a thermally conductive filler were successfully fabricated. As filler content increased, thermal conductivity also increased with the elastic modulus, and the TPU matrix was reinforced 3.7-fold and 3.6-fold. The reason can be attributed to the dispersibility of the BNNSs in TPU, which led to the formation of an integrated thermally conductive network. Strong ππ stacking interactions between the TPU matrix and BNNSs filler induced favorable mechanical performance. This study demonstrates that BNNSs can be used as a promising thermal conductive filler of polymers to enhance their thermal conductivity and mechanical strength. 4. Experimental section

Acknowledgements

4.1. Materials

This study was supported by the National Natural Science Foundation of China (Nos. 51522211, 51372265, 51528203, and 51602339), the Key Research Program of Frontier Science of Chinese Academy of Sciences (No. QYZDB-SSW-SLH031), the National Key Research and Development Program of China (Nos. 2017YFB0406002 and 2016YFA0203301), the Thousand Youth Talents Plan, the Postdoctoral Foundation of China (No. 2016M601905), the Natural Science Foundation of Jiangsu Province, China (Nos. BK20160399 and BK20140392), the Transformation of Scientific and Technological Achievements in Jiangsu Province (No. BA2016026), the Postdoctoral Foundation of Jiangsu Province (No. 1601065B), and the Science and Technology Project of Suzhou, China (Nos. SZS201508, ZXG201428, and ZXG201401).

B2O3 and metal oxide (CaO) were purchased from Sinopharm Chemical Reagents Co. (Shanghai Shi, China) High purity gases (99.999%, Ar and NH3) were supplied by Linde Gas (Beijing, China). DMF and HCl were purchased from Aladdin Reagent Co. (Shanghai, China) The TPU used was #1180A10 from BASF (Gladden, Germany). Deionized water was used in all experiments. All chemicals were analytical grade and used as received without further treatment. 4.2. Preparation of the BNNSs The BNNSs were directly synthesized from the BQS template using a thermal CVD route. In a typical process, BQS powder was first calcinated at 900 °C in air to completely remove all organic substances. Then, after mixing the B2O3 powder with a mortar and pestle, it was placed into an Al2O3 boat. The Al2O3 boat was placed into a horizontal quartz/Al2O3 tube furnace (Thermo Lindberg/blue, 2 or 3-in. tube). The furnace was heated to different temperatures of 800, 900, 1000, 1100, 1200, 1300, and 1400 °C at a heating rate of 10 °C/min, and each temperature was held for 180 min. A mixture of 200 standard cubic centimeters per minute (sccm) Ar and 100 sccm NH3 was introduced into the tube during the heating process. The thickness and uniformity of the BNNSs layers was controlled by varying the growth temperature. A white powder was obtained after cooling the system to room temperature. To remove the impurities in the products, the BNNS powder was immersed in a dilute HCl solution (1 M) at room temperature overnight to dissolve the oxides and Ca3B2O6 and was dried in a vacuum.

Conflicts of interest There are no conflicts of interest to declare. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ceramint.2018.05.026. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (5696) (2004) 666–669. [2] X. Hong, J. Kim, S.F. Shi, Y. Zhang, C. Jin, Y. Sun, S. Tongay, J. Wu, Y. Zhang, F. Wang, Ultrafast charge transfer in atomically thin MoS2/WS2 heterostructures, Nat. Nanotechnol. 9 (9) (2014) 682–686. [3] L. Su, Y. Yu, L. Cao, Y. Zhang, In-situ monitoring of thermal annealing induced evolution in film morphology and film-substrate bonding in a monolayer MoS2 film, Phys. Rev. Appl. 7 (2016) 034009. [4] H. Zhang, Ultrathin two-dimensional nanomaterials, ACS Nano 9 (10) (2015) 9451–9469. [5] Y. Luo, D. Kong, J. Luo, S. Chen, D. Zhang, K. Qiu, X. Qi, H. Zhang, C.M. Li, T. Yu, Hierarchical TiO2 nanobelts@MnO2 ultrathin nanoflakes core–shell array electrode materials for supercapacitors, RSC Adv. 3 (34) (2013) 14413–14422. [6] L. Song, L. Ci, H. Lu, P.B. Sorokin, C. Jin, J. Ni, A.G. Kvashnin, D.G. Kvashnin, J. Lou, B.I. Yakobson, Large scale growth and characterization of atomic hexagonal boron nitride layers, Nano Lett. 10 (8) (2010) 3209–3215. [7] C. Zhi, Large‐scale fabrication of boron nitride nanosheets and their utilization in polymeric composites with improved thermal and mechanical properties, Adv.

4.3. Preparation of the BN/TPU composites The BNNSs were first dispersed in DMF as a suspension at a concentration of 10 mg/mL to obtain the BNNSs dispersion. The suspension was ultrasonically treated for 8 h and mechanically stirred for 4 h. The TPU was dissolved in DMF after stirring for 2 h, under heating at 80 °C. The TPU solution and ratio-designed BNNSs were fed and stirred for 3 h to ensure dispersal of the BNNS. The resulting mixture was poured into a quartz dish and maintained at 60 °C for 12 h to evaporate the DMF and form the films. 7


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