Boron Nitride Nanosheets: novel Syntheses and Applications in polymeric Composites

The Influence of the Size of BN NSs on Silkworm Development and Tissue Microstructure

Boron nitride nanosheets (BN NSs) have emerged as promising materials in a wide range of biomedical applications. Despite the extensive studies on these bio-nano interfacial systems, one critical concern is their toxicity, which is affected by a variety of factors, including size. This study aimed at assessing the relationship between BN NSs size and toxicity. Two silkworm strains (qiufeng × baiyu and Nistari 7019) were used as model organisms to investigate the effect of different sizes of BN NSs (BN NSs-1, thickness of 41.5 nm and diameter of 270.7 nm; BN NSs-2, thickness of 48.2 nm and diameter of 562.2 nm) on silkworm mortality, growth, cocoon weight, and tissue microstructure. The findings show that exposure to BN NSs in this work has no lethal adverse effects on silkworm growth or tissue microstructure. BN NSs have a higher effect on the growth rate of qiufeng × baiyu compared to Nistari 7019, demonstrating that the same treatment does not favorably affect the Nistari 7019 strain, as there is no significant increase in cocoon weight. Overall, the study suggests that the sizes of BN NSs employed in this study are relatively safe and have less negative impact on silkworms. This offers significant insights into the effect of BN NSs size, a crucial factor to consider for their safe use in biomedical applications.

Constructing Gas Transmission Pathways in Two-Dimensional Composite Material ZIF-8@BNNS Mixed-Matrix Membranes to Enhance CO2/N2 Separation Performance

Two-dimensional (2D) nanomaterials, due to their high aspect ratio and high specific surface area, which provide a more tortuous pathway for larger gas molecules, are frequently used in membrane separation. However, in mixed-matrix membranes (MMMs), the high aspect ratio and high specific surface area of 2D fillers can increase transport resistance, thereby reducing the permeability of gas molecules. In this work, we combine boron nitride nanosheets (BNNS) with ZIF-8 nanoparticles to develop a novel material, ZIF-8@BNNS, to improve both CO₂ permeability and CO₂/N₂ selectivity. Growth of ZIF-8 nanoparticles on the BNNS surface is achieved using an in-situ growth method where the amino groups of BNNS are complexed with Zn²⁺, creating gas transmission pathways that accelerate CO₂ transmission. The 2D-BNNS material acts as a barrier in MMMs to improve CO₂/N₂ selectivity. The MMMs with a 20 wt.% ZIF-8@BNNS loading achieved a CO₂ permeability of 106.5 Barrer and CO₂/N₂ selectivity of 83.2, surpassing the Robeson upper bound (2008) and demonstrating that MOF layers can efficiently reduce mass transfer resistance and enhance gas separation performance.

Electrochemical Preparation of Hydroxylated Boron Nitride Nanosheets for Solid-State Flexible Supercapacitors Using Deep Eutectic Solvent and Water Mixture as Electrolytes

One-step and efficient preparation of few-layer hydroxylated boron nitride nanosheets (OH-BNNSs) in electrochemical methods is still challenging. Here, we developed an electrolyte composed of a mixture of deep eutectic solvent (DES, choline chloride-urea) and water for electrochemical methods to enhance the exfoliation and oxidation processes, enabling the one-step preparation of OH-BNNSs. It was found that the obtained OH-BNNSs were an average lateral size of 625 nm and thickness of six layers. Furthermore, the OH-BNNSs and DES were added to the poly(vinyl alcohol) (PVA) substrate to prepare composite gel polymer electrolyte (PVA/DES/OH-BNNSs GPE) for solid-state flexible supercapacitor. The OH-BNNSs can effectively shorten the ionic transport distance and enhance ion conductivity. In addition, their excellent mechanical properties can significantly prevent the electrolyte structure from collapsing during reuse. In the meantime, DES was introduced to improve ionic conductivity and broaden the operating voltage window of supercapacitor. As a result, the symmetric solid-state flexible supercapacitor consisting of activated carbon electrodes and PVA/DES/OH-BNNSs GPE appeared a wide voltage window of 2.3 V, high specific capacitance of 151.22 F g^-1 at 0.1 A g^-1 and remained 98% capacitance after 1500 charge-discharge cycles. This study not only opened a new pathway to efficient exfoliation of insulating layered materials but also found a novel gel polymer electrolyte for solid-state flexible supercapacitors.

High-Yield Production of Selected 2D Materials by Understanding Their Sonication-Assisted Liquid-Phase Exfoliation

Liquid-phase exfoliation (LPE) is a widely used and promising method for the production of 2D nanomaterials because it can be scaled up relatively easily. Nevertheless, the yields achieved by this process are still low, ranging between 2% and 5%, which makes the large-scale production of these materials difficult. In this report, we investigate the cause of these low yields by examining the sonication-assisted LPE of graphene, boron nitride nanosheets (BNNSs), and molybdenum disulfide nanosheets (MoS₂ NS). Our results show that the low yields are caused by an equilibrium that is formed between the exfoliated nanosheets and the flocculated ones during the sonication process. This study provides an understanding of this behaviour, which prevents further exfoliation of nanosheets. By avoiding this equilibrium, we were able to increase the total yields of graphene, BNNSs, and MoS₂ NS up to 14%, 44%, and 29%, respectively. Here, we demonstrate a modified LPE process that leads to the high-yield production of 2D nanomaterials.

Dielectric and Electro-Optic Effects in a Nematic Liquid Crystal Doped with h-BN Flakes

A small quantity of hexagonal boron nitride (h-BN) flakes is doped into a nematic liquid crystal (LC). The epitaxial interaction between the LC molecules and the h-BN flakes rising from the π−π electron stacking between the LC’s benzene rings and the h-BN’s honeycomb structure stabilizes pseudo-nematic domains surrounding the h-BN flakes. Electric field-dependent dielectric studies reveal that the LC-jacketed h-BN flakes follow the nematic director reorientation upon increasing the applied electric field. These anisotropic pseudo-nematic domains exist in the isotropic phase of the LC+h-BN system as well, and interact with the external electric field, giving rise to a nonzero dielectric anisotropy in the isotropic phase. Further investigations reveal that the presence of the h-BN flakes at a low concentration in the nematic LC enhances the elastic constants, reduces the rotation viscosity, and lowers the pre-tilt angle of the LC. However, the Fréedericksz threshold voltage stays mostly unaffected in the presence of the h-BN flakes. Additional studies show that the presence of the h-BN flakes enhances the effective polar anchoring strength in the cell. The enhanced polar anchoring strength and the reduced rotational viscosity result in faster electro-optic switching in the h-BN-doped LC cell.

Poly(l-lactic acid)/Boron Nitride Nanocomposites: Influence of Boron Nitride Functionalization on the Properties of Poly(l-lactic acid)

Differently functionalized boron nitride nanosheets (BNNSs) [hydroxyl (OH_BNNSs), amine (NH₂_BNNSs), and poly(ethylene glycol) (PEG) (PEG_BNNSs)] were synthesized, and their effects on the structure and thermal properties of poly(l-lactic acid) (PLLA) along with those of the pristine BNNSs were studied. Highly dispersed nanocomposites were prepared using PLLA and 0.5 wt % of pristine/functionalized BNNSs via a solvent blending method. Homogeneous dispersion of BNNSs in the polymer matrix was confirmed using X-ray diffraction and scanning electron microscopy. Pristine BNNSs and OH_BNNSs accelerated the crystallization of PLLA as effective nucleating agents and favored the formation of the α form in melt-crystallized samples. On the other hand, NH₂_BNNSs and PEG_BNNSs incorporated samples result in the moderate crystallization rate of PLLA and lead to the formation of a mixture of α and α' forms similar to the PLLA. It is also found that thermal stability and thermal conductivity of PLLA nanocomposites significantly depend on the type of functionalization of BNNSs. At 0.5 wt % loading, the thermal conductivity enhancement is maximum for PEG_BNNSs incorporated PLLA (∼62%), and that is only 9% for pristine BNNSs incorporated PLLA. The thermal stability of PLLA nanocomposites was significantly improved by 32-41 °C depending on the type of functionalized BNNSs compared to PLLA. It is proposed that the strong interaction between functionalized BNNSs and PLLA matrix is responsible for the improved thermal management properties.

High-Quality Boron Nitride Nanosheets and Their Bioinspired Thermally Conductive Papers

Hexagonal boron nitride has displayed increased potential in heat dissipation applications due to its desirable high thermal conductivity and remarkable thermal stability. However, the large-yield and high-quality preparation of boron nitride nanosheets (BNNSs) has been still an enormous challenge. In present work, we developed a universal exfoliation strategy to synthesize few-layer and defect-free BNNSs, which involved the intercalation of hexafluorosilicates/sodium hydroxide into BN crystals followed by exfoliation through a mild stirring process. The yield and concentration of as-obtained BNNS reached up to 78.5% and 12.78 mg/mL, respectively. More importantly, this method has been proven to exfoliate other layered materials like graphene (G), MoS₂, and WS₂. These as-obtained BNNSs can be directly used for constructing freestanding papers with high thermal conductivities. Typically, the thermal conductivities of the BNNS-G hybrid paper were up to 63.5 W/mK along the in-plane direction and 7.4 W/mK along the through-plane direction. According to the thermal interface materials performance measures, BNNS-G hybrid paper shows great promising applications for heat transfer in integrated circuit packaging.

Enhancement of effective polar anchoring strength and accelerated electro-optic switching in a two-dimensional hexagonal boron nitride/polyimide hybrid liquid crystal device

The monolayer hexagonal boron nitride (h-BN) nanosheet is transferred onto an indium tin oxide (ITO)-coated glass substrate. This h-BN slide is placed together with a conventional planar-aligning polyimide (PI) slide to fabricate a liquid crystal (LC) cell, in which the LC achieves uniform planar alignment. The effective polar anchoring strength coefficient of this h-BN-based hybrid LC device is found to increase significantly compared to that of a standard PI/PI LC device. The presence of the monolayer h-BN nanosheet as an alignment agent increases the planar anchoring energy through the epitaxial interaction between the LC and the honeycomb structure of the two-dimensional h-BN lattice in this hybrid device. The amplified polar anchoring energy is found to accelerate the electro-optic response time of the LC in this h-BN-based hybrid device.

Two-dimensional hexagonal boron nitride nanosheet as the planar-alignment agent in a liquid crystal-based electro-optic device

The planar-alignment agent in an electro-optic liquid crystal (LC) device plays an essential role for the LC's electro-optical characteristics. Rubbed polyimide (PI) layers are conventionally used as the planar-alignment agent in traditional liquid crystal displays (LCDs). Here we experimentally demonstrate that the 2D hexagonal boron nitride (h-BN) nanosheet can serve as the planar-alignment agent in an LC cell. This h-BN has higher chemical stability and more optical transparency than the PI layer. Two h-BN-covered indium tin oxide (ITO) glass slides (without any conventional PI layers) are placed together to fabricate an LC cell. A nematic LC inside this h-BN-based cell exhibits uniform planar-alignment-which is probed by a crossed polarized optical microscope. This planar-alignment at the molecular scale is achieved due to the coherent overlay of the benzene rings of the LC molecules on the hexagonal BN lattice. This h-BN-based LC cell shows the typical electro-optical effect when an electric field is applied via ITO electrodes. The dielectric measurement across this h-BN-based electro-optic cell shows a standard Fréedericksz transition of the LC, confirming that the 2D h-BN, as the planar-alignment agent, supplies adequate anchoring energy-which can be overcome by the Fréedericksz threshold voltage. Finally, we show that the h-BN-based LC cell exhibits more optical transparency than a regular PI alignment layer-based LC cell.

Magnetotransport Study on Iron Doped Novel 2D Nanoribbons via Electron – Acoustical Phonon Interactions

The electron transport parameters such as electron energy relaxation rate and phonon limited electron resistivity for iron (transition metal) doped 2D nanoribbons of armchair graphene nanoribbon (aGNR) and h-boron nitride nanoribbon (h-BNNR) have been calculated via hot electron acoustical phonon interactions on the basis of acoustical deformation potential (ADP) coupling mechanism. We have performed the investigation for the lower concentration ([Formula: see text]%) of iron doping under the influence of externally applied magnetic field at low temperature to room temperature regime. The hot electron acoustical phonon relaxation rates are observed with electric field and under constant applied magnetic field. The doping of iron increases the electron energy relaxation rate with respect to their pristine counter parts. Moreover, the pristine h-BNNR exhibits less electron energy relaxation rate with respect to pristine aGNR. Upon applying magnetic field on Fe doped armchair GNR as well as Fe-doped h-BNNR the electron energy relaxation rate reduces down to a considerable extent with respect to their pristine counterparts. Moreover, under the impact of magnetic field, the acoustical phonon restricted electrical resistivity of Fe-doped GNR is considerably low compared to pristine GNR.

Boron Nitride Nanosheets/PNIPAM Hydrogels with Improved Thermo-Responsive Performance

Thermo-responsive hydrogel is an important smart material. However, its slow thermal response rate limits the scope of its applications. Boron nitride nanosheet-reinforced thermos-responsive hydrogels, which can be controlled by heating, were fabricated by in situ polymerization of N-isopropylacrylamide in the presence of boron nitride nanosheets. The hydrogels exhibit excellent thermo-responsiveness and much enhanced thermal response rate than that of pure poly(N-isopropylacrylamide) hydrogels. Interestingly, the hydrogels can be driven to move in aqueous solution by heating. Importantly, the composite hydrogel is hydrophilic at a temperature below lower critical solution temperature (LCST), while it is hydrophobic at a temperature above LCST. Therefore, it can be used for quick absorption and release of dyes and oils from water. All these properties demonstrate the potential of hydrogel composites for water purification and treatment.

Nair K, Surendran KP. Silver-Decorated Boron Nitride Nanosheets as an Effective Hybrid Filler in PMMA for High-Thermal-Conductivity Electronic Substrates

High-thermal-conductivity and low-dielectric-loss polymer nanocomposites have gained tremendous attention in microelectronics technology. Against this background, the present study deals with the development of a high-thermal-conductivity, low-dielectric-constant, and low-loss polymer nanocomposite based on silver nanoparticle (AgNP)-decorated boron nitride nanosheets (BNNSs) as the filler in poly(methyl methacrylate) (PMMA) matrix. The nanocomposites were prepared through a facile solution-blending process. Elemental mapping of the prepared nanocomposite indicates the uniform distribution of filler particle in PMMA matrix. An impressive high-thermal conductivity (TC) enhancement of around 363% was achieved for nanocomposite of 0.35 V _f of hybrid filler (1.48 W/m K) compared to pristine PMMA (0.32 W/m K). The addition of AgNP reduces the thermal contact resistance (R _c) by bridging individual BNNS, thereby improving thermal transport. Measured TC values were fitted with a theoretical model that showed good agreement. Dielectric measurements performed at radiofrequencies and microwave frequencies revealed that the nanocomposites show a low dielectric constant (<5), low loss (10^-2), and very low alternating current conductivity (10^-7 S/cm). The results suggest that silver-decorated BNNS is a promising hybrid filler for effective thermal management.

Hydrolytic Unzipping of Boron Nitride Nanotubes in Nitric Acid

Boron nitride nanoribbons (BNNRs) have very attractive electrical and optical properties due to their unique edge states and width-related properties. Herein, for the first time, BNNRs were produced by a simple reflux of boron nitride nanotubes (BNNTs) in nitric acid containing water, which had led to unzipped sidewalls through hydrolysis. Their high reactivity that originated from edges was verified via a strong interaction with methylene blue.

Cosolvents as Liquid Surfactants for Boron Nitride Nanosheet (BNNS) Dispersions

Despite a range of promising applications, liquid-phase exfoliation of boron nitride nanosheets (BNNSs) is limited, both by low yield in common solvents as well as the disadvantages of using dissolved surfactants. One recently reported approach is the use of cosolvent systems to increase the as-obtained concentration of BNNS; the role of these solvents in aiding exfoliation and/or aiding colloidal stability of BNNSs is difficult to distinguish. In this paper, we have investigated the use of a t-butanol/water cosolvent to disperse BNNSs. We utilize solvent-exchange experiments to demonstrate that the t-butanol is in fact essential to colloidal stability; we then utilized molecular dynamics simulations to explore the mechanism of t-butanol/BNNS interactions. Taken together, the experimental and simulation results show that the key to the success of t-butanol (as compared to the other alcohols of higher or lower molecular weight) lies in its ability to act as a "liquid dispersant" which allows it to favorably interact with both water and BNNSs. Additionally, we show that the stable dispersions of BNNS in water/t-butanol systems may be freeze-dried to yield nonaggregated, redispersible BNNS powders, which would be useful in an array of industrial processes.

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.

Boron nitride colloidal solutions, ultralight aerogels and freestanding membranes through one-step exfoliation and functionalization

Manufacturing of aerogels and membranes from hexagonal boron nitride (h-BN) is much more difficult than from graphene or graphene oxides because of the poor dispersibility of h-BN in water, which limits its exfoliation and preparation of colloidal solutions. Here, a simple, one-step mechano-chemical process to exfoliate and functionalize h-BN into highly water-dispersible, few-layer h-BN containing amino groups is presented. The colloidal solutions of few-layer h-BN can have unprecedentedly high concentrations, up to 30 mg ml⁻¹, and are stable for up to several months. They can be used to produce ultralight aerogels with a density of 1.4 mg cm⁻³, which is ∼1,500 times less than bulk h-BN, and freestanding membranes simply by cryodrying and filtration, respectively. The material shows strong blue light emission under ultraviolet excitation, in both dispersed and dry state.

The poor dispersibility of 2D hexagonal boron nitride in water currently limits its exfoliation and applications. Here, the authors present a one-step mechano-chemical process to achieve unprecedented colloidal concentrations, which permits fabrication of ultralight aerogels and freestanding membranes.