Highly Efficient Mass Production of Boron Nitride Nanosheets via a

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Highly Efficient Mass Production of Boron Nitride Nanosheets via a Borate Nitridation Method Taotao Li, Chaowei Li, Yongqing Cai, Junhao Lin, Xiaoyang Long, Liangjie Wang, Yancui Xu, Juan Sun, Lei Tang, Yong-Wei Zhang, Kazu Suenaga, Zheng Liu, and Yagang Yao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05702 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 14, 2018

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The Journal of Physical Chemistry

Highly Efficient Mass Production of Boron Nitride Nanosheets via a Borate Nitridation Method Taotao Li,1,3‡ Chaowei Li,2,6‡ Yongqing Cai,4 Junhao Lin, 5† Xiaoyang Long,2 Liangjie Wang,2 Yancui Xu,2 Juan Sun,2Lei Tang,2 Yong-Wei Zhang,4 Kazu Suenaga,5 Zheng Liu,3* and Yagang Yao 1,2,6 * 1 National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China 2 Division of Advanced Nanomaterials, Key Laboratory of Nanodevices and Applications, Joint Key Laboratory of Functional Nanomaterials and Devices, CAS Center for Excellence in Nanoscience, Suzhou Institute of Anon-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, China 3 School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore 4 Institute of High Performance Computing, A*STAR, Singapore 138632, Singapore 5 National Institute of Advanced Industrial Science and Technology (AIST), AIST Central 5, Tsukuba 305-8565, Japan 6 Division of Nanomaterials, Suzhou Institute of Nano-Tech and Nano-Bionics, Nanchang, Chinese Academy of Sciences, Nanchang 330200, China

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ABSTRACT: Boron nitride nanosheets (BNNSs) have attracted intensive attention because of their fantastic properties, including excellent electrical insulating ability, splendid thermal conductivity, and outstanding oxidation resistance. However, facing the rising demand for versatile applications, the cost-effective mass production of BNNSs, similar to graphene, remains a huge challenge. Here, we provide a highly effective strategy for BNNS synthesis via a borate nitridation method utilizing solid borate precursors, producing gram-scale yields with efficiencies up to 88%. Combined with density functional theory (DFT) calculations, a vaporsolid-solid (VSS) mechanism was proposed, in which ammonia vapor reacts with the solid borates, producing solid BNNSs at the vapor-solid interfaces. The strategy proposed herein, together with the diversity of borate compounds, allows numerous choices for the facile mass production of BNNSs at low cost. In addition, the remarkably enhanced thermal conductivity in composite materials demonstrated the good quality and huge potential of these BNNSs in thermal management. This work reveals a cost-efficient method for the large-scale production of BNNSs, which should promote practical applications in various fields.

INTRODUCTION As a structural analogue of graphene, two-dimensional boron nitride (BN) sheets with alternating boron and nitrogen atoms in a honeycomb lattice, have attracted much attention because of their favorable mechanical strength, thermal conductivity, chemical stability, and electrical insulating ability. BN nanosheets (BNNSs) have numerous promising applications, such as composite fillers, ultraviolet-light emitters, oxidation-resistant coatings, and ideal substrates for enhancing the performance of 2D-material-based electronic devices.1-7 However, compared to the thriving developments of graphene, studies on BNNSs have proliferated much slower, to some extent owing to the extreme difficulty of BNNS synthesis. Although several exfoliation methods,


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including mechanical exfoliation, liquid exfoliation, or a combination of both, which exhibit relatively high efficiencies (up to 18% for hydroxide-assisted ball milling and 85% for ureaassisted ball-milling),8-10 have been proposed to synthesize BNNSs and have encouraged fundamental studies,1,5,11,12 these approaches require long production periods, resulting in deteriorated flakes sizes and poor thickness control. Via chemical vapor deposition (CVD), BNNS flakes or films were attempted to be grown on metal substrates with relatively high quality, which are suitable for electronic applications but not favorable for large-scale applications, owing to the difficulty of large-scale synthesis.2,12,13 Other prevailing efficient methods for graphene fabrication, such as oxidation-reduction methods and intercalation, are not applicable to BNNS fabrication,14-16 as the in-plane covalent B-N bonds and the van der Waals forces between the layers intrinsically exhibit ionic characteristics, which grant BN a higher resistance to oxidization, exfoliation and intercalation than graphene.9 Thus far, other innovative methods for BNNS synthesis have been explored. A chemical blowing method produced large-area BNNSs, which derived efficiencies as high as 40%.17,18 However, borane ammonia, the precursor used in this fabrication, is highly expensive, which limits the industrial applicability of this approach. Another method, a so-called “carbon substitution reaction”, utilizing carbon-based materials including biomass and graphite powder as precursors, is a cheap and gram-scale fabrication method for BNNSs by substituting C atoms with B and N. However, the efficiency is relatively low, approximately 15-26%, under optimum conditions.19-20 In this work, we report a novel method—a borate nitridation method—to synthesize BNNSs with a high efficiency up to 88%, enabling the mass production of BNNSs on the gram level in a single batch depending on the precursor amount and furnace size. This approach innovatively


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employs a series of solid borate precursors with high melting points, including (but not limited to) magnesium borate (MgB4O7, Mg2B2O5, and Mg3B2O6), calcium borate (CaB2O4), lithium borate (Li2B4O7), zinc borate (ZnB2O4) and aluminum borate (Al18B4O33). By nitridation of the solid precursors under an ammonia atmosphere at certain temperatures, BNNSs with thicknesses ranging from few to multi-layers form at the solid surfaces. Together with first-principles calculations, a vapor-solid-solid (VSS) mechanism is proposed, showing that the NH3 gas molecules adsorb above the solid borate surfaces. A strong electric field in close proximity to the surface owing to charge transfer at the vapor-solid interface facilitates the outward diffusion of positively charged defects, such as oxygen vacancies and interstitial boron atoms, from the bulk interior to the surfaces, producing BNNSs at the solid-vapor interfaces. As the solid precursors are thermostable and non-volatile at high temperature, the B element could be solidified until the beginning of the nitridation process, giving rise to a very high production efficiency. The key advantage of this approach is that the utilized borate precursors can be fabricated from very cheap raw materials, resulting in a low-cost method. In addition, we show that the as-synthesized BNNSs in polyvinyl alcohol (PVA)-based composite materials show good thermal conductivity up to 1.98 Wm-1K-1 and 8.85 Wm-1K-1 in the vertical and in-plane directions, respectively, at a filling ratio of 36.8 wt%, demonstrating their potential application in thermal conductive materials. This work reveals a high-efficiency, low-cost and mass-produced method for synthesizing BNNSs and should promote an increased demand for BNNSs in various applications. RESULTS AND DISCUSSION Design of precursors


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For the synthesis of BN nanomaterials, including BNNSs and BN nanotubes (BNNTs), the choice and design of appropriate B sources is critical. For example, for the CVD synthesis of BNNSs and BNNTs, the main difficulty stems from the limited number of gaseous B species that stably exist at room temperature, excluding toxic and corrosive gases, such as BF3 and BCl3. Thus, if an ideal B source would be discovered, BN nanomaterial synthesis would be much easier. The diffusion and segregation method is a powerful technique for the growth of highquality BNNS films by directly annealing solid (B,N)-source under an vacuum atmosphere at high temperature, avoiding the use of an external B source.21 In this work, we employ a new type of solid precursors to fabricate BNNSs. Borates, the salts of boric acid, consisting of linked oxyanion units of BO3 or BO4, usually have very high melting points. For example, Mg3B2O6 (space group Pnmn), an orthorhombic crystal well known as the mineral Kotoite, has a high melting point of 1410 °C (Table S1). In the Mg3B2O6 lattice, each B atom is surrounded by three oxygen atoms forming a triangular BO3 configuration, while each Mg atom is surrounded by an octahedron of oxygen atoms. The single BO3 triangles link chains of oxygen octahedra, as shown in Figure S1.22,23 The high melting points of the precursors ensure the highly efficient utilization of B species by preventing their volatilization loss in the heating process. The borate precursors used in this work were synthesized by high-temperature sintering of powdered mixtures of a metal oxide (or hydroxide) and boron oxide (or boric acid) with fixed ratios corresponding to their chemical formulas. After the designed calcination process, pure and single-phase borate powders were obtained. Notably, pure and single-phase borate precursors are essential for the fabrication of pure BNNSs. If the precursor contains different components or other phases, various mixed BN materials with mixed, tangled morphologies, including


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nanosheets, nanotubes and thick wires,24 will be produced. The precursors used in this work are all single phase to obtain pure BNNSs and unambiguously demonstrate their mechanisms. Detailed information on the synthesis process for the various single-phase precursors is available in the experimental section. Synthesis of BNNSs In detail, the as-synthesized single-phase borate precursor powders were placed into an alumina crucible and then moved to the centre of a tube furnace. After purging with Ar, 200 standard cubic centimetres per minute (sccm) NH3 was introduced into the tube and heated to the reaction temperature of 1200-1400 °C, depending on the precursor used. At the reaction temperature, the solid borate reacted with NH3 and produced BNNSs and the corresponding metal oxide, as illustrated in Equation 1 using Mg3B2O6 as an example:

Mg 3 B 2 O 6 + 2NH

3

→ 2BN + 3MgO + 3H 2 O

(1)

After nitridation, an acid pickling process was conducted to remove MgO, leaving the pure BNNSs. A schematic of the synthesis process is demonstrated in Figure 1a.


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Figure 1. BNNSs fabricated via the borate nitridation method. (a) Illustration of the BNNS fabrication process via the borate nitridation method. SEM images of (b) the Mg3B2O6 precursor, (c) corresponding products reacted at 1300 °C for 3 h and (d) final BNNS products after acid pickling. (e) XRD spectra of the Mg3B2O6 precursor (purple line), the products reacted at 1300 °C for 3 h (red line) and the pure BNNSs after purification (blue line). (f) The typical Raman spectra of the BNNSs. (g) Photograph of ~2 g of the BNNS products produced in a single batch. Figure 1b and c display the typical scanning electron microscopy (SEM) images of the pristine Mg3B2O6 precursor and its corresponding products reacted at 1300 °C for 3 h, respectively. The pristine Mg3B2O6 particles clearly exhibit smooth surfaces with diameters of approximately 1-4 µm, while after the reaction, the surfaces are coated with graphene-like materials. After the acid pickling process, the graphene-like material remains without an obvious


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morphology change (Figure 1d). X-ray diffraction (XRD) reveals the composition of BN and MgO in the as-grown products and pure BNNSs after acid pickling (Figure 1e), confirming the reaction illustrated in Equation 1. The Raman spectrum further confirms the existence of BNNSs, with the characteristic E2g peak located at 1369.5 cm-1 (Figure 1f). Figure 1g shows the pure BNNS product produced in approximately 2 g in a single batch. The efficiency is as high as 80.1-83.0% according to Equation 1. A more detailed characterization was performed using transmission electron microscopy (TEM), scanning TEM (STEM) and electron energy loss spectroscopy (EELS) to assess the structure, crystallinity and the elemental content of the as-synthesized BNNSs. As shown in Figure 2a, the as-grown products have MgO@BNNS core-shell structures, in which MgO is smoothly encapsulated by the BNNSs. Some MgO cores were also observed to escape out of the shell, leaving only hollow BNNSs. Figure 2b displays the high-resolution TEM image revealing the lattice fringes of the (002) facet of the standing BN flakes in the BNNSs, corresponding to a layer distance of 0.33 nm. Statistics on the BNNS layer number based on the TEM characterization were determined. Most nanosheets were multi-layered with 6-10 layers (Figure S2). Figure 2c shows a low-magnification annular dark-field (ADF) image of the thin regions near the edge of the BNNS shell, showing the intrinsic layered morphology. The chemical purity of the as-synthesized BNNSs was confirmed by EELS (Figure 2d), displaying the boron K-edge (~190 eV) and nitrogen K-edge (~401 eV) features, which correspond to the characteristic Kshell ionization edges of B and N, respectively. The weak peak of C may come from the organic residual during TEM sample preparation. A zoomed-in image of the monolayer region (Figure 2e) at the edge further reveals the hexagonal honeycomb ring structure of the h-BN layers, as


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confirmed by the three-fold symmetric fast Fourier transformation (FFT) pattern of the image (Figure 2f), which confirms the low defect concentration in the sample.

Figure 2. Structural characterization of the BNNSs. (a) Low-magnification TEM image showing the MgO@BNNS core-shell structure. (b) High-resolution TEM image showing the (002) lattice fringes of the BNNSs. The inset shows the interplanar spacing of 0.33 nm for the (002) lattice fringes. (c) An ADF image of the thin regions near the edge of the BNNS. (d) EELS spectrum of the as-grown BNNSs. (e) Atomic-resolution TEM image of a monolayer region. (f) Corresponding FFT pattern showing a set of hexagonal patterns from monolayer h-BN


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We also examined other types of magnesium borates as precursors, such as MgB4O7 and triclinic Mg2B2O5 (t-Mg2B2O5), and obtained similar results (Figure S3 and S4). Notably, MgB4O7 produced the thickest BNNS layers due to the highest B content among the three precursors, while t-Mg2B2O5 yielded medium thickness layers. The specific surface area data indirectly confirmed the thickness difference, as thinner layers would provide a higher surface area when given the same weight (Figure S5). Pure BNNSs could be obtained via an ultrasonic acid pickling process. In this process, the MgO cores were removed, and the BNNS shells remained owing to their high chemical stability. After drying, gram-scale BNNSs were obtained in a single batch, depending on the precursor amount and the tube furnace volume. The efficiencies, based on the boron contents, were calculated to be as high as 80.0-83.2% for MgB4O7, 85.0-88.2% for t-Mg2B2O5 and 80.1-83.0% for Mg3B2O6, demonstrating that the borate nitridation method is the most effective way to synthesize BNNSs among previously reported procedures. In addition, the by-products, such as MgCl2·6H2O, which were extracted from the hydrochloric acid purification process, could be reutilized for the borate synthesis,25,26 thus representing a green and recyclable BNNS synthesis method. Mechanism To carefully investigate the growth mechanism of the BNNSs via the borate nitridation process, STEM and the corresponding energy dispersive X-ray spectroscopy (EDX) mapping images were obtained. As illustrated in Figure 3, homogeneous distributions of B and N elements are found on the shells, while Mg and O elements exist in the core, demonstrating the MgO@BNNS core-shell structure.


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Figure 3. The spatial distribution of elements demonstrating the structural and component information. (a) STEM and (b-e) corresponding EDX mapping images, revealing the MgO@BNNS core-shell structure.

We speculate that the fabrication process involves a VSS reaction, in which the gaseous NH3 species reacts with the solid-state borate precursor to form solid BNNSs at the vapor-solid interface. As Mg3B2O6 has a high melting point of 1410 °C, beyond the reaction temperature of 1300 °C, the precursor sustains its solid state at the reaction temperature, as confirmed by the product morphology showing no signs of fusion or consolidation. More explicitly, for the monoclinic Mg2B2O5 whiskers (m-Mg2B2O5, melting point = 1353 °C) as precursors, the product sustains the pristine whisker morphology with tubular structures, as illustrated in Figure S6.


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Figure 4. DFT calculations of the BNNS formation mechanism. (a) Adsorption of NH3 at the Mg site of the Mg3B2O6 surface. (left panel) Isosurface plot of the charge difference indicates charge transfer between NH3 and Mg3B2O6. The yellow (purple) color represents the loss (gain) of electrons after the adsorption of NH3. (right panel) The amount of charge transfer across the interface. (b) Kinetics of the chemical decomposition process of NH3 above the surface of Mg3B2O6. The IS and FS represent the initial and final states of the reaction, respectively. Eb is the barrier calculated by the nudged elastic band (NEB) method. (c) Kinetics of the bulk defects, VO and Bi, within bulk Mg3B2O6.


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To uncover the reaction mechanism clearly, we performed first-principles density functional theory (DFT) calculations based on the Mg3B2O6 precursor, revealing that the reaction comprises three steps. 1) First, chemical adsorption of the NH3 molecules occurs at the Mg sites on the Mg3B2O6 surface, accompanying with a strong charge transfer and effective electric field between the gas-solid interfaces (Figure 4a). Charge transfer analysis reveals that the electrons are favourably transferred from the Mg3B2O6 surface to the NH3 molecule (approximately 0.08 e per molecule). 2) Decomposition of the chemically adsorbed NH3 molecules occurs, forming embedding N atoms reacting with the surface O atoms by the following reactions: I) NH3* -> NH2* + OH*; II) NH2* + OH* -> NH* + H2 (gas); III) NH* -> N* + OH* (Figure 4b), where the superscripts in NH3*, NH2*, NH*, N*, and OH* indicate surface adsorbed species. Interestingly, the decomposition barrier of the NH3 molecule, which is activated by surface-exposed Mg atoms, is very low (0.92 eV). 3) Positively charged defects, such as oxygen vacancies (VO) and interstitial B atoms (Bi), within the bulk Mg3B2O6 diffuse outwardly to the surface, which can be accelerated by the strong electric field formed at the surface due to the negatively charged NH3 species. This step is critical for the supplying of the B atoms for BN formation at the surface and reducing Mg3B2O6. The activation energy of an oxygen vacancy (VO, 4.80 eV) is much larger than that of an interstitial B atom (Bi, 1.76 eV) under zero electric field (Figure 4c). According to the Cabrera-Mott theory which explains the low-temperature oxidation of metals27, the built-in electric field due to adsorbed chemical species can dramatically lower the diffusion barrier and promote the outward diffusion of nuclei. The real activation energy is believed to be even lower. Notably, the BNNS shell layers very loosely attached to the MgO cores, with certain spaces between them (Figure 2a and 4a), and even some MgO cores escaped and left with only BNNS shells, which would facilitate the purification of BNNSs. This phenomenon is quite different


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from CVD growth of BNNSs on metal substrates, in which the BNNSs tightly stick to the substrate.12,13 This behaviour is reasonable considering that some solid-state phase transformations can occur during the fabrication process, in which the resultant core shrinks and peels off the BNNS shells. Upon the diffusion and segregation of the B atoms and a portion of the O atoms, the resultant core gradually becomes low-boron-containing borate and eventually MgO. Therefore, the VSS reaction is intrinsically accompanied by a solid-state phase transformation from orthorhombic Mg3B2O6 to cubic MgO when the solubility of B atoms reaches a critical value. An approximately 63.58% volume reduction was calculated to occur upon the transformation of orthorhombic Mg3B2O6 to MgO. Likewise, volume reductions of 36.37% for MgB4O7 and 56.44% for t-Mg2B2O5 were calculated with a constant Mg content (supporting information). Thus, the BNNSs would peel off the MgO cores during this shrinkage. In addition, H2O vapor produced at the surface may also contribute to the peeling off BNNSs. As the magnesium borate precursor (MgB4O7, Mg2B2O5 and Mg3B2O6) can be produced by the reaction of MgO and B2O3 (Equation 2) and MgO and the BNNSs are produced by the nitridation process (Equation 3), the borate nitridation method can be considered an indirect reaction of B2O3 with NH3 via the involvement of borates, as demonstrated in Equation 4, i.e., a sum of Equation 2 and 3. 3MgO + B 2 O 3 → Mg 3 B 2 O 6

(2)

Mg 3 B 2 O 6 + 2NH 3 → 2BN + 3MgO + 3H 2 O

(3)

B 2 O 3 + 2NH 3   MgO   → 2BN + 3H 2 O

(4)

B2O3 is normally an amorphous solid with a low “melting point” of approximately 450 °C and a boiling point of approximately 1500 °C. In the direct reaction of B2O3 with NH3, B2O3 would melt prior to their reaction. A thin-layer BNNS film would form on the melted surface, stopping


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the reaction progress by the isolation of NH3 with B2O3.28 Therefore, the use of borates as precursors greatly enhanced the efficiency and yield, owing to the introduction of sufficient solid-vapor interfaces to host the reactions. The universality of the borate nitridation method Due to the diversity of the borate family, we have a wide range of precursor options to synthesize BNNSs. In addition to magnesium borate (MgB4O7, Mg2B2O5 and Mg3B2O6), many other borates, such as lithium borate (Li2B4O7), calcium borate (CaB2O4), zinc borate (ZnB2O4) and aluminium borate (Al18B4O33), have proved to be effective precursors. Figure S7 shows the SEM images and Raman spectra of the BNNSs synthesized from Li2B4O7, CaB2O4, ZnB2O4 and Al18B4O33, manifesting the universality of the borate nitridation method. Notably, for precursors with lower melting points, such as Li2B4O7 (917 °C) and ZnB2O4 (982 °C), the BNNS shell surrounded the precursor particles prior to melting, protecting the particles against consolidation to maintain solid powders after the reaction. All these borates can be easily synthesized via solid-phase sintering of their corresponding metal oxides (or hydroxides, salts, or elementary substances) with boron oxide (or boric acid or borax)29,30 or by liquid-phase precipitation25,26 in the forms of nanowires, 26,31 nanobelts, 32 and nanotubes.33 This wealth of structural and morphological features would deduce abundant BN species. All the raw materials involved are common industrial products and extremely cheap. Therefore, the borate nitridation method is a low-cost and highly productive way to fabricate BNNSs, enabling mass production and practical application. Moreover, more than 100 types of natural mineral borates with different components and structures are vastly deposited around the world. These borate minerals have been classified into six subclasses according to their configurations and bonding natures, including neso-, soro-, cyclo-, inco-, phyllo- and


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tectoborates.34 All these borates minerals are suitable precursor candidates for BNNS fabrication. This method explores a new application direction of these natural minerals and paves an avenue to the industrial production of BNNSs. Thermal application of BNNSs The high productivity of this method facilely enables us to perform research on the applications of BNNSs. BNNSs have been reported to possess ultrahigh thermal conductivity in the range of 300-2000 W·m-1·k-1,4 suggesting their potential application as thermal conductive composite materials. Here, we demonstrate enhanced thermal conductivity of our product in polyethylene glycol (PVA)/BNNS composite materials. A series of PVA/BNNS composite materials with BNNS contents up to 36.8 wt% were fabricated. The thermal characterization shows that the thermal conductivity monotonically increases with increased BNNS loading and reaches 1.98 Wm-1K-1 in the vertical direction and 8.85 Wm-1K-1 in the in-plane direction at a filling ratio of 36.8 wt% (Figure 5). The SEM images (Figure S8) show that there’re somewhat pores in the composite materials, which may be derived from the hollow structure of the asprepared BNNSs and produced during the solvent evaporation process. The thermal conductivity should be higher if these pores have been filled and eliminated during the fabrication process. This enhancement implies the good quality of the as-synthesized BNNSs and suggests their potential application in thermal management.


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Figure 5. The thermal conductivity of the PVA/BNNS composite materials with different filling contents. The inset shows a representative PVA/BNNS sample with a 5 cm diameter. The error bars represent the standard deviation calculated from three measurements. CONCLUSION In summary, we explored a low-cost and high-efficiency strategy for the bulk synthesis of BNNSs via a borate nitridation method. The method utilized solid-state borates as precursors, which effectively prevents the vaporization loss of the B source and enables the bulk synthesis of BNNSs on the gram scale with high productivity up to 88%. DFT calculations revealed that the process involves a VSS reaction, in which NH3 molecules chemically adsorb onto the exposed Mg sites of the precursor surface, building a strongly negative electric field. This electric field facilitates diffusion and segregation of the B atoms from the surface, reacting with N species to form BNNSs. In addition, the diverse synthetic borates, especially the plentiful mineral borates, pave the way for the industrial production of BNNSs. The application of these BNNSs in composite materials was demonstrated by the dramatic enhancement of thermal conductivity, suggesting potential applications in thermal management fields.


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ASSOCIATED CONTENT Supporting Information. Experimental Section Figure S1. The crystal structure of orthorhombic Mg3B2O6. Table S1. The melting point of several typical borates. Figure S2. The statistics on BNNS layer number produced from Mg3B2O6 based on TEM characterization. Figure S3. Morphological and structural information of pristine MgB4O7 precursor and the corresponding products. Figure S4. Morphological and structural information of pristine t-Mg2B2O5 precursor and its corresponding BNNS products. Figure S5. Nitrogen adsorption-desorption isotherm and specific surface areas of BNNSs produced by the three magnesium borates, MgB4O7, t-Mg2B2O5, and Mg3B2O6. Figure S6. The characterization of pristine m-Mg2B2O5 whiskers and the corresponding resultants. Figure S7. The SEM and Raman characterization of BNNSs derived from (a) Li2B2O7, (b) CaB2O4, (c) ZnB2O4, and (d) Al18B4O33. Figure S8. SEM images of BNNS/PVA composite materials with different BNNSs content. Table S2. The crystal parameters of magnesium borates and their volume shrinkage in a complete nitridation reaction. Figure S9. The crystal structure of m-Mg2B2O5. The calculations about crystal volume shrinkage

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], [email protected] Present Addresses † Department of Physics, Southern University of Science and Technology, Shenzhen 518055, People’s Republic of China. Author Contributions


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Y. Yao and Z. Liu supervised the research work. T. Li conceived and designed the experiments, performed the BN growth. C. Li, X. Long, Y. Xu and L. Wang performed the synthesis of various single-phase borate precursors and purification process. C. Li fabricated the PVA/BNNSs composite materials and performed the test of thermal properties. J. Sun and L. Tang carried out the spectral characterization. Y. Cai and Y. Zhang performed the DFT calculations. J. Lin and K. Suenaga performed the TEM and EELS characterizations. T. Li analyzed the experimental data, designed the figures and wrote the manuscript. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by supported by the National Key R&D Program of China (No. 2017YFB0406000), the Key Research Program of Frontier Science of Chinese Academy of Sciences (No. QYZDB-SSW-SLH031), the Thousand Youth Talents Plan, the Postdoctoral Foundation of China (Nos. 2016M601905 and 2017M621855), the Natural Science Foundation of Jiangsu Province, China (Nos. BK20160399), and the Postdoctoral Foundation of Jiangsu Province (No. 1601065B).

J. Lin and K. Suenaga acknowledge JST-ACCEL and JSPS

KAKENHI (JP16H06333 and P16823) for financial support. REFERENCES (1) Zhi, C.; Bando, Y.; Tang, C.; Kuwahara, H.; Golberg, D. Large-Scale Fabrication of Boron Nitride Nanosheets and Their Utilization in Polymeric Composites with Improved Thermal and Mechanical Properties. Adv. Mater. 2009, 21, 2889-2893.


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Highly Efficient Mass Production of Boron Nitride Nanosheets via a

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