Functionalization and Dispersion of Hexagonal Boron Nitride (h‐BN) Nanosheets Treated with Inorganic Reagents

A mixture of bulk hexagonal boron nitride (h‐BN) with hydrazine, 30 % H₂O₂, HNO₃/H₂SO₄, or oleum was heated in an autoclave at 100 °C to produce functionalized h‐BN. The product formed stable colloid solutions in water (0.26–0.32 g L ^?1) and N,N‐dimethylformamide (0.34–0.52 g L ^?1) upon mild ultrasonication. The yield of “soluble” h‐BN reached about 70 wt %. The dispersions contained few‐layered h‐BN nanosheets with lateral dimensions in the order of several hundred nanometers. The functionalized dispersible h‐BN was characterized by IR spectroscopy, X‐ray photoelectron spectroscopy (XPS), Raman spectroscopy, UV/Vis spectroscopy, X‐ray diffraction (XRD), dynamic light scattering (DLS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). It is shown that h‐BN preserves its hexagonal structure throughout the functionalization procedure. Its exfoliation into thin platelets upon contact with solvents is probably owing to the attachment of hydrophilic functionalities.     

Fabrication of Hexagonal Boron Nitride Nanosheets by Using a Simple Thermal Exfoliation Process

A simple and inexpensive method to exfoliate boron nitride powder to form boron nitride nanosheets (BNNSs) with few layers was achieved by using a physically thermal process. The obtained BNNSs were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), IR spectroscopy, and Raman spectroscopy. The size distribution of the sheets and average sheet size is in the range of 80–380 nm and 200±62 nm, respectively, and the pure phase h‐BN products were confirmed. XPS result showed the B/N atomic ratio to be 0.99. In addition, the BNNSs can well disperse in aqueous solution to form a cloudy suspension and importantly, can remain suspended for 1 month without precipitate, which would have good potential in a wide range of applications.     

Fabrication of Boron Nitride Nanosheets by Exfoliation

Nanomaterials with layered structures, with their intriguing properties, are of great research interest nowadays. As one of the primary two‐dimensional nanomaterials, the hexagonal boron nitride nanosheet (BNNS, also called white graphene), which is an analogue of graphene, possesses various attractive properties, such as high intrinsic thermal conductivity, excellent chemical and thermal stability, and electrical insulation properties. After being discovered, it has been one of the most intensively studied two‐dimensional non‐carbon nanomaterials and has been applied in a wide range of applications. To support the exploration of applications of BNNSs, exfoliation, as one of the most promising approaches to realize large‐scale production of BNNSs, has been intensively investigated. In this review, methods to yield BNNSs by exfoliation will be summarized and compared with other potential fabrication methods of BNNSs. In addition, the future prospects of the exfoliation of h‐BN will also be discussed.     

Seamless Rim‐Functionalization of h‐BN with Silica—Experiment and Theoretical Modeling

Boron nitride contains six‐ring layers, which are isostructural to graphene, and it exhibits similar extraordinary mechanical strength. Unlike graphene, hexagonal boron nitride (h‐BN) is an insulator and has some polar features that make it a perfect material for those applications graphene is not suitable for, for example, purely ionic conductors, insulating membranes, transparent coatings, composite ceramics, high oxidation resistance materials. We report here a selective rim‐functionalization of h‐BN with SiO₂ by using the Stöber process. A closed, protruding ring of SiO₂ is formed covering all edges perpendicular to the [001] zones of the h‐BN stacks and thus shield the most reactive centers of BN layers. SEM and HAADF‐STEM images, X‐ray spectroscopy, and atomic force microscopy confirm the rim‐functionalization by SiO₂. XRD demonstrates the absence of any intercalation phenomenon of BN and reveals the glassy nature of the SiO₂ rims. Selected variations of synthesis and theoretical modeling both confirm that rim activation by water prior to the Stöber condensation is crucial. First‐principles calculations also confirm that dangling bonds of clean BN edges merge to give interlayer bonds that make further functionalization much more difficult. The reported reaction pathway should allow for other new functionalizations of pure BN and of the rimmed SiO₂/h‐BN composites.     

CuCo/BNNSs纳米催化剂对固态储氢材料氨硼烷水解的催化性能

采用共还原法将CuCo双金属负载到通过聚乙烯吡咯烷酮(PVP)辅助离子插层法制备的少层氮化硼纳米片(BNNSs)上,获得了平均粒径为2.7 nm且高度分散的铜钴/氮化硼纳米片(CuCo/BNNSs)纳米催化剂。通过原子力显微镜(AFM)、X射线衍射(XRD)、傅里叶转换红外光谱(FTIR)、拉曼光谱(Raman)、X射线光电子能谱(XPS)、扫描电子显微镜(SEM)和高分辨率透射电镜(HRTEM)对载体及催化剂的结构及形貌进行表征,并研究了CuCo/BNNSs的催化性能。研究发现,由于Cu、Co、BNNSs和OH-之间高效的四重效应协同使得Cu0.5Co0.5/BNNSs纳米催化剂在室温、pH=14条件下对氨硼烷(AB,NH3BH3)水解释氢具有极高的催化活性。转化频率(TOF)值达到104.52 molH2·molmetal^-1·min^-1,且CuCo/BNNSs纳米催化剂具有良好的稳定性,6次循环利用后仍保持较高催化活性。     

2D/2D h‐BN/N‐doped MoS2 Heterostructure Catalyst with Enhanced Peroxidase‐like Performance for Visual Colorimetric Determination of H2O2

Recently, nanozymes have attracted extensive attention because of their advantages of combining nanomaterials with enzymes. Herein, hexagonal boron nitride (h‐BN) and nitride‐doped molybdenum disulfide (N?MoS₂) nano‐composites (h‐BN/N?MoS₂) were synthesized by facile and cost‐effective liquid exfoliation with a solvothermal method in nontoxic ethanol solution. The results show that h‐BN, as a co‐catalyst, can not only dope into the lattice of MoS₂ but also form a heterogeneous structure with MoS₂NSs. It expanded the layer spacing and specific surface area of MoS₂NSs, which was beneficial to the contact between the catalyst and the substrate, and resulted in a synergistic enhancement of the catalytic activity of hydrogen peroxide (H₂O₂) with MoS₂. A colorimetric determination platform of h‐BN/N?MoS₂‐TMB‐H₂O₂ was constructed. It exhibited a wide linear range of 1–1000 μM with a low limit of detection (LOD) of 0.4 μM under optimal conditions, high sensitivity and stability, as well as good reliability (99.4–110.0%) in practice, making the measurement system more widely applicable.1. Introduction     

United in Nitride: The Highly Condensed Boron Phosphorus Nitride BP3N6

Owing to intriguing materials properties non‐metal nitrides are of special interest for both, solid‐state chemistry and materials science. Mixed ternary non‐metal nitrides, however, have only been sparsely investigated, as preparative chemistry lacks a systematic access, yet. Herein, we report on the highly condensed boron phosphorus nitride BP₃N₆, which was synthesized from (PNCl₂)₃, NH₄N₃ and h‐BN in a high‐pressure high‐temperature reaction. By increasing partial pressure of HCl during synthesis using NH₄Cl, single‐crystals of BP₃N₆ up to 80 μm in length were obtained. The unprecedented framework‐type structure determined by single‐crystal XRD blends structural motifs of both, α‐P₃N₅ and c‐BN, rendering BP₃N₆ a double nitride. The compound was further investigated by Rietveld refinement, EDX, temperature‐dependent PXRD, FTIR and solid‐state NMR spectroscopy. The formation of BP₃N₆ through use of reactive precursors exemplifies an innovative access to mixed non‐metal nitrides.     

Radical Chemistry and Reaction Mechanisms of Propane Oxidative Dehydrogenation over Hexagonal Boron Nitride Catalysts

Although hexagonal boron nitride (h‐BN) has recently been identified as a highly efficient catalyst for the oxidative dehydrogenation of propane (ODHP) reaction, the reaction mechanisms, especially regarding radical chemistry of this system, remain elusive. Now, the first direct experimental evidence of gas‐phase methyl radicals (CH₃^.) in the ODHP reaction over boron‐based catalysts is achieved by using online synchrotron vacuum ultraviolet photoionization mass spectroscopy (SVUV‐PIMS), which uncovers the existence of gas‐phase radical pathways. Combined with density functional theory (DFT) calculations, the results demonstrate that propene is mainly generated on the catalyst surface from the C?H activation of propane, while C₂ and C₁ products can be formed via both surface‐mediated and gas‐phase pathways. These observations provide new insights towards understanding the ODHP reaction mechanisms over boron‐based catalysts.     

Boron Nitride Nanosheets (BNNSs) Chemically Modified by “Grafting‐From” Polymerization of Poly(caprolactone) for Thermally Conductive Polymer Composites

To meet the growing demand for rapid heat dissipation in electronic devices to ensure their reliable performance with a high level of safety, many polymer composites with thermally conductive but electrically insulating 2D boron nitride nanosheets (BNNSs) are being developed. Here we present an efficient way to enhance the thermal conductivity (TC) of a polymer composite by means of “grafting‐from” polymerization of a poly(caprolactone) (PCL) onto BNNSs. The BNNSs, which were exfoliated from bulk BN by means of ultra‐sonication, were prepared by means of radical oxidation. These oxidized BNNSs (oxi‐BNNSs) were employed as initiators for subsequent ring‐opening polymerization of PCL, which successfully resulted in PCL chemically grafted onto BNNSs (PCL‐g‐BNNSs). The excellent dispersion of PCL‐g‐BNNSs in common solvents allowed us to readily fabricate a polymer composite that contained PCL‐g‐BNNSs embedded in a PCL matrix, and the composite showed TC values that were five and nine times greater in the out‐of‐plane and in‐plane mode, respectively, than those of pristine PCL.     

Controlled Gas Exfoliation of Boron Nitride into Few‐Layered Nanosheets

The controlled exfoliation of hexagonal boron nitride (h‐BN) into single‐ or few‐layered nanosheets remains a grand challenge and becomes the bottleneck to essential studies and applications of h‐BN. Here, we present an efficient strategy for the scalable synthesis of few‐layered h‐BN nanosheets (BNNS) using a novel gas exfoliation of bulk h‐BN in liquid N₂ (L‐N₂). The essence of this strategy lies in the combination of a high temperature triggered expansion of bulk h‐BN and the cryogenic L ‐N₂ gasification to exfoliate the h‐BN. The produced BNNS after ten cycles (BNNS‐10) consisted primarily of fewer than five atomic layers with a high mass yield of 16–20 %. N₂ sorption and desorption isotherms show that the BNNS‐10 exhibited a much higher specific surface area of 278 m² g^?1 than that of bulk BN (10 m² g^?1). Through the investigation of the exfoliated intermediates combined with a theoretical calculation, we found that the huge temperature variation initiates the expansion and curling of the bulk h‐BN. Subseqently, the L ‐N₂ penetrates into the interlayers of h‐BN along the curling edge, followed by an immediate drastic gasification of L ‐N₂, further peeling off h‐BN. This novel gas exfoliation of high surface area BNNS not only opens up potential opportunities for wide applications, but also can be extended to produce other layered materials in high yields.     

Nanocrystalline‐Graphene‐Tailored Hexagonal Boron Nitride Thin Films

Unintentionally formed nanocrystalline graphene (nc‐G) can act as a useful seed for the large‐area synthesis of a hexagonal boron nitride (h‐BN) thin film with an atomically flat surface that is comparable to that of exfoliated single‐crystal h‐BN. A wafer‐scale dielectric h‐BN thin film was successfully synthesized on a bare sapphire substrate by assistance of nc‐G, which prevented structural deformations in a chemical vapor deposition process. The growth mechanism of this nc‐G‐tailored h‐BN thin film was systematically analyzed. This approach provides a novel method for preparing high‐quality two‐dimensional materials on a large surface.     

Facile Green Synthesis of BCN Nanosheets as High‐Performance Electrode Material for Electrochemical Energy Storage

Two‐dimensional hexagonal boron carbon nitride (BCN) nanosheets (NSs) were synthesized by new approach in which a mixture of glucose and an adduct of boric acid (H₃BO₃) and urea (NH₂CONH₂) is heated at 900 °C. The method is green, scalable and gives a high yield of BCN NSs with average size of about 1 μm and thickness of about 13 nm. Structural characterization of the as‐synthesized material was carried out by several techniques, and its energy‐storage properties were evaluated electrochemically. The material showed excellent capacitive behaviour with a specific capacitance as high as 244 F g^?1 at a current density of 1 A g^?1. The material retains up to 96 % of its initial capacity after 3000 cycles at a current density of 5 A g^?1.     

Surface modification of boron nitride via poly (dopamine) coating and preparation of acrylonitrile‐butadiene‐styrene copolymer/boron nitride composites with enhanced thermal conductivity

A biology‐inspired approach was utilized to functionalize hexagonal boron nitride (h‐BN), to enhance the interfacial interactions in acrylonitrile‐butadiene‐styrene copolymer/boron nitride (ABS/BN) composites. The poly (dopamine), poly (DOPA) layer, was formed on the surface of BN platelets via spontaneously oxidative self‐polymerization of DOPA in aqueous solution. The modified BN (named as mBN) coated with poly (DOPA) was mixed with ABS resin by melting. The strong interfacial interactions via π‐π stacking plus Van der Waals, both derived from by poly (DOPA), significantly promoted not only the homogeneous dispersion of h‐BN in the matrix, but also the effective interfacial stress transfer, leading to improve the impact strength of ABS/mBN even at slight mBN loadings. A high thermal conductivity of 0.501 W/(m·K) was obtained at 20 wt% mBN content, reaching 2.63 times of the value for pure ABS (0.176 W/(m·K)). Meanwhile, the ABS/mBN composites also exhibited an excellent electrical insulation property, which can be expected to be applied in the fields of thermal management and electrical enclosure.     

Cleaning of pyrolytic hexagonal boron nitride surfaces

Hexagonal boron nitride (h‐BN) has recently garnered significant interest as a substrate and dielectric for two‐dimensional materials and devices based on graphene or transition metal dichalcogenides such as molybdenum disulfide (MoS₂). As substrate surface impurities and defects can negatively impact the structure and properties of two‐dimensional materials, h‐BN surface preparation and cleaning are a critical consideration. In this regard, we have utilized X‐ray photoelectron spectroscopy to investigate the influence of several ex situ wet chemical and in situ thermal desorption cleaning procedures on pyrolytic h‐BN surfaces. Of the various wet chemistries investigated, a 10 : 1 buffered HF solution was found to produce surfaces with the lowest amount of oxygen and carbon contamination. Ultraviolet/ozone oxidation was found to be the most effective ex situ treatment for reducing carbon contamination. Annealing at 1050 °C in vacuum or 10^?5 Torr NH₃ was found to further reduce oxygen and carbon contamination to the XPS detection limits. Copyright © 2015 John Wiley & Sons, Ltd.     

Synthesis and morphology control of nanocrystalline boron nitride

Nanocrystalline boron nitride (BN) with needle-like and hollow spherical morphology has been synthesized by nitriding of MgB₂ with NH₄Cl and NH₄Cl-NaN₃, respectively. The amount of NaN₃ has an obvious effect on the size of the hollow spheres. The samples were characterized by X-ray powder diffraction, Fourier transformation infrared spectroscopy, X-ray photoelectron spectra, and transmission electron microscopy. The possible mechanism of morphology control is also discussed.     

Zur Kenntnis von Tripraseodym‐hexanitridotriborat Pr3B3N6: Neue Synthese und Verfeinerung der Kristallstruktur

On Tripraseodymium Hexanitridotriborate Pr₃B₃N₆: New Synthesis and Crystal Structure Refinement Single‐crystalline Pr₃B₃N₆ was obtained by the reaction of praseodymium and BN_x(NH)_y(NH₂)_z in a NaCl melt under N₂ atmosphere in a high‐frequency furnace at 1250 °C. Contrary to literature data, Pr₃B₃N₆ crystallizes in the centrosymmetric space group R 3 c as revealed by single‐crystal X‐ray diffraction (a = 1211.95(9), c = 701.53(7) pm, Z = 6, R1 = 0.0258, wR2 = 0.0658). In the solid, Pr₃B₃N₆ contains Pr³⁺ and planar cyclotrinitridoborate units B₃N₆^9–. The anions represent motifs from the structure of hexagonal boron nitride (h‐BN) and they are stacked analogously along [001]. Both the bond lengths B–N (average value 147.8 pm) and the interionic distances between the anions (350.8 pm) are comparable with the values in h‐BN.     

Efficient and Scalable Production of 2D Material Dispersions using Hexahydroxytriphenylene as a Versatile Exfoliant and Dispersant

Thin‐layer 2D materials have been attracting enormous interest, and various processes have been investigated to obtain these materials efficiently. In view of their practical applications, the most desirable source for the preparation of these thin‐layer materials is the pristine bulk materials with stacked layers, such as pristine graphite. There are many options in terms of conditions for the exfoliation of thin‐layer materials, and these include wet and dry processes, with or without additives, and the kind of solvent. In this context, we found that the versatile exfoliant hexahydroxytriphenylene works efficiently for the exfoliation of typical 2D materials such as graphene, MoS₂, and hexagonal boron nitride (h‐BN) by both wet and dry processes by using sonication and ball milling, respectively, in aqueous and organic solvents. As for graphene, stable dispersions with relatively high concentrations (up to 0.28 mg mL^?1) in water and tetrahydrofuran were obtained from graphite in the presence of hexahydroxytriphenylene by a wet process with the use of bath sonication and by a dry process involving ball milling. Especially, most of the graphite was exfoliated and dispersed as thin‐layer graphene in both aqueous and organic solvents through ball milling, even on a large scale (47–86 % yield). In addition, the exfoliant was easily removed from the precipitated composite by heat treatment without disturbing the graphene structure. Bulk MoS₂ and h‐BN were also exfoliated by both wet and dry processes. Similar to graphene, dispersions of MoS₂ and h‐BN of high concentrations in water and DMF were produced in high yields through ball milling.     

Lanthanide Nitrido Borates with Six-Membered B3N6 Rings: Ln3B3N6

Metal compounds with heteroatomic ring systems of main group elements are a domain of coordination chemistry. However, lanthanide nitrido borates Ln₃B₃N₆ (Ln=La or Ce; see structure) are synthesized by the reaction of hexagonal boron nitride with LnN. The compounds contain the six-membered B₃N₆ ring, which can be seen as a fragment from one layer of the hexagonal BN structure.     

Structural and electronic properties of fluorinated boron nitride nanotubes

The effects of F doping on the structural and electronic properties of the (5, 5) single-walled boron nitride nanotube (BNNT) are investigated by using the density functional theory method. The chemiadsorption of F maintains the hexagonal BN network, increases the lattice constant, and introduces acceptor impurity states. On the other hand, substitutional doping of F destroys the hexagonal BN network, decreases the lattice constant, but does not alter the insulating feature of the BNNT. The observed insulator-to-semiconducting transition, a lattice contraction, and a highly disordered atom arrangement in the sidewall of BNNTs upon F doping appear to be most reasonably attributed to a codoping of dominating substitutional F over chemiabsorbed F, which can induce deep donor impurity states, a lattice contraction, and a destruction of the hexagonal BN network simultaneously.     

Enhancement of the thermal and mechanical properties of a surface‐modified boron nitride–polyurethane composite

Thermoplastic polyurethane (PU) elastomer, prepared from poly(tetramethylene glycol) and methyl diphenyl diisocyanate, was blended with boron nitride (BN) to fabricate a thermally conductive interface material. BN treated by a silane coupling agent (BN―NH₂) and PU‐grafted BN were prepared to fabricate a composite that has better thermal conductivity and mechanical strength. The surface‐modified filler showed enhanced dispersibility and affinity because of the surface treatment with functional groups that affected the surface free energy, along with the structural similarity of the doped crystallized diisocyanate molecule with the matrix. The thermal conductivity increased from 0.349 to 0.467 W mk^?1 on 20 wt% PU‐grafted BN loading that is a 1.34‐fold higher value than in the case of pristine BN loading at the same weight fraction. Moreover, the number of BN particles acting as defects, thereby reducing the mechanical strength, is decreased because of strong adhesion. We can conclude that these composite materials may be promising materials for a significant performance improvement in terms of both the thermal and mechanical properties of PU‐based polymers. Copyright © 2014 John Wiley & Sons, Ltd.