Solution Processed Boron Nitride Nanosheets: Synthesis, Assemblies and Emerging Applications
Wei Luo, Yanbin Wang, Emily Hitz, Yi Lin,* Bao Yang,* and Liangbing Hu*
In the last decade, few-layer boron nitride nanosheets and monolayer BN nanosheet (BNNS) have gained much attention due to their unique physical and chemical properties. To date, BNNS can be produced by micromechan-ical cleavage of BN crystal, unzipping BN nanotubes, chemical vapor deposi-tion (CVD), and solution processed exfoliation from bulk BN powder. Due to the low cost and abundance of bulk BN powder and its simple processing for potentially scalable production, great efforts have been devoted toward solu-tion processed exfoliation. In this feature article, recent significant advances in solution processed synthesis of BNNS are summarized. In particular, the solvent choice for one-step BN exfoliation is highlighted. Multi-dimensional assemblies consisting of BNNS are discussed, such as BN fiber, BN paper, and BN aerogel. The emerging applications of BNNS in different fields are then focused on, especially in barrier materials, dielectrics, catalysts, and thermal management.
DOI: 10.1002/adfm.201701450
For a long time, BN has been utilized in lubricants, cosmetics, paints, cements and high-temperature equipment due to its unique properties, including high thermal conductivity and great chemical stability.[4] With the discovery of monolayer graphite (graphene) and its extraordinary physical properties,[5] the past decade has witnessed a large growth of research interest in few layer or monolayer 2D layered materials,[6] such as MoS2,[7] Bi2Se3,[8] and black phos-phorous.[9] Due to their wider energy band gaps and electrically insulating proper-ties compared to graphene, few layer and monolayer BNNS have stimulated great research interest,[10] allowing a much wider range of applications than their bulk counterparts. For example, BNNS can be an appealing substrate for graphene devices due to the similar lattice constant
and atomically smooth surface, where graphene exhibi ted a very high mobility[10c] and a rotation-dependent moiré pattern.[11]
To date, strategies for synthesizing BNNS can be generally classified into two methods: bottom-up and top-down methods. In the bottom-up strategy, BNNS are fabricated from atoms, ions, or molecules, where the chemical vapor deposition (CVD) method is usually adopted. In this area, a significant progress for the technology was made by Kong’s group, who has devel-oped a relatively mature recipe for synthesizing BNNS via CVD.[10a] On the other hand, top-town methods for synthe-sizing BNNS typically involve the micromechanical cleavage of bulk BN crystals[12] or unzipping of BNNT.[13] Although these methods produce relatively high quality BNNS, the low yield and high cost have significantly limited their scalability. In con-trast, BNNS made from bulk BN powder by solution processed exfoliation exhibit three important advantages.[14] (i) As raw material, bulk BN powder is abundant and cost-effective. (ii) A relatively high yield can be obtained due to its simple process. (iii) Functional groups on BNNS by solution processes facili-tate the use of BNNS in various fields. All these advantages are essential to the large-scale preparation and application of BNNS.
In this feature article, we focus on the recent development of BNNS synthesized from bulk BN powder by solution pro-cessing, their assemblies, and emerging applications. Since the major achievements in this exciting field mainly began in 2010, this article mainly summarizes studies in the most recent five years. In the first section, we discuss the different solution processed methods for producing BNNS, where the choice of solvent for one-step sonication-based methods is critical. We
Nanosheets
Prof. W. Luo, Y. Wang, E. Hitz, Prof. L. HuDepartment of Materials Science and EngineeringUniversity of MarylandCollege Park, Maryland 20742, USAE-mail: [email protected]. W. Luo, Y. Wang, Prof. B. YangDepartment of Mechanical EngineeringUniversity of MarylandCollege Park, Maryland 20742, USAE-mail: [email protected]. Y. LinNational Institute of Aerospace100 Exploration Way, Hampton, Virginia 23666, USAE-mail: [email protected]
1. Introduction
Boron nitride (BN), formed by equal numbers of boron (B) and nitrogen (N) atoms, has several different polymorphs, such as hexagonal BN (h-BN), cubic BN, and BN nanotube (BNNT) according to its structure.[1] Among them, h-BN is the most used polymorph and is known as the isoelectric analogue of graphite. The structure of h-BN is similar to graphite, where B and N atoms are positioned alternatingly in planar, hexagonally conjugated BN layers, as displayed in Figure 1.[2] The h-BN layers stack to form a two-dimensional (2D) layered structure with van der Waals interactions between the layers and elec-trostatic interactions due to the electronegativity difference between the B and N atoms.[3] In this article, BN will be used to represent h-BN.
then highlight the different assemblies fabricated by BNNS, especially the three-dimensional (3D) BN aerogel. Emerging applications of the novel BNNS-related materials and assem-blies are presented last, in which forming polymer/BNNS com-posite materials is the research focus.
2. Solution Processed Synthesis of BNNS
The solution processed synthesis was first reported by Han and co-workers,[12] where they obtained BNNS from BN single-crystal within 1,2-dichloroethane solution of poly(m-phe-nylenevinylene-co-2,5-dictoxy-p-phenylenevinylene) through a sonication-assisted method. Shortly after, Zhi et al. sonicated BN micro-sized particles in an organic solvent, N,N-dimethyl-formamide (DMF), which gave a relatively large-scale fabrica-tion.[15] Their studies indicated that BNNS can be produced in liquid environments using a one-step sonication-assisted exfoli-ation, which is very similar to the preparation of graphene from graphite by sonication.[16] Since then, this method has been regarded as the most efficient choice because it is versatile and potentially up-scalable. Besides the one-step sonication-assisted exfoliation, surface-modification-assisted exfoliation, ball-milling-assisted exfoliation and some other solution processed methods have also been developed.
2.1. One-Step Sonication
One-step sonication-assisted exfoliation in liquid environments is widely used to produce few-layer nanomaterials, such as graphene,[16,17] MoS2,[18] phosphorene,[9c] considering its low cost, high yield, and simple set-up. In the last few years, many attempts have been made to try different solvents for exfoliating bulk BN, where the surface tension plays a critical role. Gener-ally, the surface tension of a good solvent should match that of BN (≈35 mJ m−2).[19] The resulting BNNS should be able to resist re-aggregation upon long-term storage in the solvent. In contrast, a poor solvent will allow for re-aggregation and sedi-mentation (Figure 2a).[20] Combining surface tension with the Hansen solubility parameter theory, Coleman and co-workers pointed out that good solvents need to match the dispersive (D), polar (P), and hydrogen bonding (H) components of the layered materials and a formula was proposed:[19,21]
χ δ δ δ δ δ δ≈ − + − + − ( ) ( ) ( )0, ,
2, ,
2, ,
2v
k TBD S D M P S P M H S H M
where χ is the Flory-Huggins parameter, “M” stands for the material, and v0 is the solvent molecular volume. To be a suc-cessful solvent, minimizing the χ is highly desired, which cor-responds to a minimized exfoliation energy. Obviously, the closeness of δi,S to δi,M (where i = D, P, H) will lead to a small finite value of χ. A summary of the Hansen solubility para-meters of some typical solvents used for exfoliating BN is pre-sented in Table 1.
The Hansen solubility parameters of BN were revealed by col-lecting and comparing the BNNS concentrations in a range of solvents (Figure 2b): δD,BN (17–19 MPa1/2), δP,BN (4–10 MPa1/2),
Wei Luo is currently an Assistant Research Professor at University of Maryland (UMD). He received his B.E. and M.E. degrees from Northwestern Polytechnical University (with Prof. Shibin Liu) and Ph.D. from Huazhong University of Science and Technology in 2012 (with Prof. Xianluo Hu and Prof. Yunhui Huang). After two years’ postdoctoral research
with Prof. Xiulei Ji (2012–2014) at Oregon State University, he worked with Prof. Liangbing Hu and Prof. Bao Yang at UMD as a postdoctoral researcher until being promoted to Assistant Research Professor in 2016. His research interests include energy storage and conversion devices, biomass materials, and low-dimensional nanomaterials.
Yi Lin is an associate research fellow at the National Institute of Aerospace (NIA) and residents in the Advanced Materials and Processing Branch at the NASA Langley Research Center. He received his B.S. (1996) and M.S. (1999) degrees at the University of Science and Technology of China, and Ph.D. (2004) at
Clemson University. He was a research assistant professor at Clemson University in 2004–2006 and became a NASA postdoctoral fellow in 2007. He joined NIA in 2009. His main research interest is in the chemistry and processing of low-dimensional nanomaterials and nanohybrids for aeronautics and aerospace applications.
Liangbing Hu received his B.S. in physics from the University of Science and Technology of China in 2002. He did his Ph.D. at the University of California, Los Angeles, focusing on CNT based nano-electronics (2002–2007). In 2006, he joined Unidym Inc. as a co-founding scientist. He worked at Stanford University from 2009–2011, investi-
gating various energy devices based on nanomaterials and nanostructures. Currently, he is an Associate Professor at University of Maryland College Park. His research inter-ests include nanomaterials and nanostructures, roll-to-roll nanomanufacturing, energy storage focusing on solid-state batteries and Na ion batteries, and printed electronics.
and δH,BN (4–10 MPa1/2).[19] According to the above theory, iso-propyl alcohol (IPA) was selected as one of the best solvents for exfoliating BN in Coleman’s study. As shown in Figure 2c, a stable BNNS/IPA dispersion (up to hundreds of hours) with a concentration of 0.06 mg mL−1 had been obtained. Trans-mission electron microscope (TEM) observations revealed the typical morphology with extremely thin sheets and well-defined hexagonal structure (Figure 2d,e). Besides IPA, DMF is also in the range of matched Hansen solubility parameters of BN, where a similar BNNS/DMF solution was presented.[15a]
From Table 1, we may make a prediction that water should be a poor solvent due to the unmatched surface tension. How-ever, Lin and co-workers discovered that water can also be a “good solvent”, where water molecular can break down the sheet surface of BN by decorating hydroxyl groups on the BN edge (Figure 2f).[22] The “cutting” begins at the intrinsic defects on BN that the defects propagate quickly as the B-N bonds become more vulnerable around the defects. The exfoliation of BN occurred along the defects due to the input sonication energy and solvent polarity effect. Since water is low cost, abun-dant and environmental-friendly, it is extremely attractive to prepare BNNS in water. Most recently, Kim et al. discovered that elevating temperature leads to better exfoliation and disper-sion stability of BNNS in water.[23] With this technique, the ini-tial concentration of BNNS in water can reach ≈0.25 mg mL−1 and remain ≈0.2 mg mL−1 after storing for a month at 60 °C. The molecular dynamics (MD) model revealed that the strong polarity across the B atom and N atom termination edges inter-acts with the water molecules (Figure 2g). The water molecules would like to firmly attach to BN surface and create a water–BN interaction.
Besides organic solvents and water, ionic liquids (ILs) are expected to match the surface tension of BN as well. The fully
separated cations and anions of ILs have a stronger solvent polarity effect than other solvents. Based on those properties, a properly selected IL can minimize the exfoliation time and maintain a large lateral size of BNNS.[24] For example, a BNNS ink with concentration of ≈1.9 mg mL−1 and yield of ≈50% can be achieved when [bmim][PF6] was used. A comprehensive density functional theory (DFT) simulation, done by Aparicio and co-workers, proposed that an ideal IL for BN exfoliation should maximize the interaction strength by π–π stacking and charge transfer processes.[25]
Although a single solvent system has been successfully developed to exfoliate BN by a one-step sonication,[26] exploring mixture solvents for combining their individual advantages is also pursued. Among various mixture solvents, researchers paid a great deal of attention to mixing organic solvents with water to lower the cost and improve safety.[27] In 2015, Kaner’s group screened a series of mixed solvents where a mixture of tert-butanol in water (60 wt%) was found as the most effec-tive water based co-solvent.[28] According to the experimental results, they believed that a combinatory consideration of sur-face tension, molecular weight, and chemical structure is the key for choosing good solvents. For example, molecular weight plays a critical role in the yield that a larger molecule enables a larger steric repulsion.[29] In summary, the key for exfoliating BN by one-step sonication-assisted exfoliation is to choose a good solvent, in terms of matching the surface tension (energy) or minimizing the exfoliation energy.
The one-step sonication process typically takes a long time (up to tens of hours) and gives a low concentration and yield.[19] To
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Figure 1. Structural basics of two-dimensional h-BN nanostructures, where the B–N bonds have a length of 0.145 nm, the distance between two adja-cent borazine rings is 0.250 nm, and the interlayer distance is 0.333 nm. The edge of a BN nanosheet can be either zigzag (B- or N-edged) or armchair (BN pair-edged). Reproduced with permission.[2] Copyright 2012, the Royal Society of Chemistry.
address these issues, prior to exfoliation, surface functionaliza-tion has been studied intensively.[30] BN materials in general are highly chemically inert, making covalent modification quite challenging. Many initial efforts have thus been focused on noncovalent approaches. Previously, there were reports on the modification of BNNT using Lewis base molecules to interact with electron deficient B atoms that can be considered as Lewis acids.[31] Inspired by this, in 2010, Lin et al. reported the use of both aliphatic and hydrophilic long-chain amine molecules as Lewis bases to modify BN surfaces, resulting in effective exfo-liation of BN and solubilization of the nanosheets in organic solvents and/or water.[30a] Microscopy data suggested that the exfoliated mono- and few-layered BNNS surfaces were covered with organic moieties. Nuclear magnetic resonance (NMR) results indicated that it was the amine end of the long chain
molecules that interacted with the BNNS surface, thus strongly supporting the Lewis acid-base interaction mechanism. The solubilization efficiency could be significantly enhanced with a more effective attachment between Lewis base molecules and defect sites formed by ball-milling, which was discussed in a later report.[32] Similar Lewis acid-base modification strategies were also reported by other groups.[33]
Exfoliating BN using Lewis bases or, more broadly speaking, polar molecules is in essence similar to liquid exfoliation methods. In these cases, polar-polar interactions between exfoli-ation molecules and the BN surfaces dominate the mechanism. During exfoliation, functional moieties could be simultane-ously introduced to the nanosheet edges. For example, as discussed in Section 2.1, hydroxyl functional groups were found in the exfoliated BNNS using pure water, which was
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Figure 2. a) Bulk layered materials are exfoliated into nanosheets, where the exfoliated nanosheets can remain stable against re-aggregation in good solvents but in poor solvents, re-aggregation and sedimentation will occur. Reproduced with permission.[20] Copyright 2013, the American Association for the Advancement of Science. b) The stable concentration of BNNS ink (plotted as A/l) as a function of surface tension using various solvents; c) A vial of BNNS/IPA dispersion with a concentration of 0.06 mg mL−1; d,e) TEM and HRTEM images of BNNS obtained from the BNNS/IPA dispersion. Reproduced with permission.[19] Copyright 2011, the American Association for the Advancement of Science. f) The “cutting” mechanism of exfoliating BN in water. Reproduced with permission.[22] Copyright 2011, American Chemical Society. g) Molecular simulation studies of BNNS in water: charge density of water around BNNS and water distributions around BNNS. Reproduced with permission.[23] Copyright 2015, Nature Publishing Group.
due to the simultaneous sonication-enhanced hydrolysis of the BN framework.[22] In another sample, Wang et al. dispersed BN in methanesulfonic acid (MSA), where the interaction between BNNS and MSA molecules can aid in the dispersion of BNNS.[34] Meanwhile, the resultant MSA-modified BNNS can easily re-disperse in other solvents including NMP, DMF, and dimethyl sulfoxide (DMSO). Besides acids, the interac-tions between BN and a range of inorganic reagents (hydrazine, H2O2, HNO3/H2SO4 or oleum) can also lead to a high yield (70 wt%) of BNNS in a subsequent sonication step using water or DMF as solvent.[35]
Another typical noncovalent functionalization strategy is to use large aromatic ring molecules to modify BN surfaces via π–π interactions. For example, BN was sonicated in a NMP solution of nickel phthalocyanine (Ni-Pc), resulting in nonco-valent attachment of Pc to the BN surface that enhanced exfo-liation and solubilization of the nanosheets.[36] Such surface functionalization could help tune the electronic and optical properties of BNNS, which may be promising candidates for photonic and optoelectronic applications. Even milder sur-face modification and exfoliation methods using surfactant-like mole cules can assist in the dispersion of BNNS. In 2011, Coleman and co-workers suggested that surfactants can play an important role in stabilizing exfoliated BNNS by electrostatic repulsion.[37] In their experiments, BN was exfoliated by sonica-tion in water with sodium cholate (SC). The exfoliated BNNS were coated by SC and exhibited great stabilization in water. Inspired by this, different surfactants have been employed. Copolymer surfactant pluronic F68, composed of a hydro-phobic poly (propylene oxide) (PPO) central block and two hydrophilic poly (ethylene oxide) (PEO) blocks, was employed. Mechanistically, the PPO blocks of the surfactant were attracted to the BN basal plane by hydrophobic interactions while the hydrophilic PEO blocks preferentially extended into the water (Figure 3a).[38] Effective steric hindrance was provided by such a copolymer and its interactions, which led to successful exfo-liation and stabilization. This idea is similar to the surfactant-assisted dispersion of carbon nanotubes (CNT) in water.[39] An interesting extension of the surfactant concept is the use of a diblock copolymer P(S-b-MMA) as a “Janus” modifier to tune the solubility of BNNS in different organic solvents.[40] The interaction of BNNS with the diblock copolymer could be either with the polystyrene (PS) block via π–π interactions or with
the poly(methyl methacrylate) (PMMA) block via coordination chemistry in the presence of a small amount of copper (Cu) ions. As a result, the external functional blocks in the dispersed BNNS-containing micelles were PMMA and PS, respectively. Due to the different dissolution properties of the two blocks, a range of organic solvents could thus be used for stabilizing exfoliated BNNS, providing large flexibility in solvent systems for wet fabrication of polymeric composites.
Most recently, our group developed a green, low-cost method by using abundant nanofibrillated cellulose (NFC) as an effec-tive dispersant for exfoliating BN in water.[41] We found that NFC can attach to BNNS through the interaction between NFC’s hydrophobic sites and BNNS’s hydrophobic plane, sim-ilar to the surfactant-BNNS interactions. In addition, hydrogen bonding also exists between the hydroxyl groups of NFC and the edges of BNNS (Figure 3b). Compared to the BNNS dis-persion without using NFC, the dispersion with the assistance of NFC is much more stable. It can be attributed to the steric hindrance and the electrostatic repulsive forces (Figure 3c). All of these interactions enhanced the dispersion stability by preventing the dispersed BNNS from re-aggregation, which indicates NFC is an efficient dispersant for BN exfoliation and stabilization. Covalent modification of the highly inert BN surfaces was also shown to be possible, but only with very reactive species. For example, Du et al. reported a fluorination-assisted exfoliation, where bulk BN was first fluorinated by reacting with NH4F at 180 °C and exfoliated subsequently.[42] The authors proposed an exfoliation mechanism comprised of three steps (Figure 3d). (i) F ions chemically bond to B atoms, leading to sp3-hybridization, where the resulting sp3-hybridized B atoms tend to protrude out from the basal plane. (ii) Buckling driven by the sp3-hybridized B atoms enlarges the interlayer spacing. (iii) Intercalation of NH4 ions triggers the exfoliation of BN. The fluorinated BNNS (F-BNNS) obtained from such a one-step fluorination-assisted exfoliation exhibited a typical nanosheet morphology, where the number of layers deter-mined by AFM was less than three and the lateral size was as large as 4 µm. More interestingly, the fluorinated BNNS exhib-ited ferromagnetic characteristics, indicating a good candidate for spintronic devices (Figure 3e). Sainsbury, Coleman, and co-workers also reported a few covalent reactions of BN with highly reactive species such as oxygen radicals, nitrenes, and dibromocarbene.[30b-d] These reactions were directly conducted
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Table 1. Hansen solubility parameter and surface tension of solvents for exfoliating BN.
with liquid exfoliated BNNS, and the attached functional moie-ties modified the dispersibility of BNNS in different solvents. Various spectroscopic results supported the covalent nature of the attachment of the organic moieties to the BNNS surfaces. Such modifications resulted in modified bandgap properties of BNNS and were shown to be highly beneficial for solvent-processed nanocomposites fabrication to enhance interface compatibility with the targeted polymer matrices.
2.3. Alternative Methods
Besides one-step sonication and surface-modification-assisted exfoliation, some alternative solution processed methods have also been developed. Among them, ball milling BN in solution showed great promise.[32,43] Most recently, Lee and co-workers used an aqueous NaOH solution as a ball milling agent so that the mechanical shear forces and additional chemical peeling helped to exfoliate BN, followed by a short-time sonication (1 hr).[44] Their research discovered that the reaction between BN and the OH ions can cut BN edges, which facilitated exfo-liation, reduced the shear force and resulted in the hydroxyl-functionalized BNNS. Compared to NaOH, when urea was used as the ball milling agent, BNNS can disperse in water without sonication.[45] Importantly, the yield of this method reached as high as 85%. Experimental results and DFT cal-culations demon strated that the combination of mechanical and chemical process leads to the decomposition of urea and the attachment of NH2 to BN edges and defects. Taking the
advantage of the hydrophilic nature of NH2 groups, BNNS can stabilize in water up to several months. Moreover, 2D or 3D BN assemblies can be further fabricated using the as-obtained BNNS dispersion, which will be discussed in the next sec-tion. In addition to ball milling, stress generated by expansion via freezing water or electrochemical ions intercalation has also been reported for exfoliating bulk BN.[46] To date, a large number of solution-processed methods have been successfully developed to produce BNNS, which are significantly versatile, up-scalable and critical for applications.
3. Assemblies
Taking advantage of the solution processed preparation of BNNS, various BN assemblies have been designed and fabricated using BNNS as the building block. The BN assemblies range from one to three dimensions, where one-dimensional (1D) assemblies include BN nanofiber, 2D assemblies refer to BN nanofilm/ nanopaper, and BN aerogel belongs to the 3D category.
3.1. 1D Assemblies
The most famous 1D BN nanostructure must be the BNNT, which is isoelectronic to CNT. Since BNNT was predicted in 1994 and synthesized in 1995, it has attracted great atten-tion due to its large bandgap (5.5–5.8 eV), chemical inertness, high thermal conductivity and thermal stabilities. There are
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Figure 3. a) Schematic process of the copolymer surfactant Pluronic F68 assisted BN exfoliation. Reproduced with permission.[38] Copyright 2015, American Chemical Society. b) Schematic process of exfoliating BN using NFC as a dispersant and c) image of BNNS water dispersion without NFC and with 10 wt% NFC after 10 days sitting. Reproduced with permission.[41] Copyright 2015, Elsevier. d) The mechanism of exfoliating BN by a one-state fluorination-assisted method; and e) hysteresis loop measurements of BN, NH4F and F-BNNS, which demonstrated clear ferromagnetic characteristics of F-BNNS. Reproduced with permission.[42] Copyright 2014, John Wiley and Sons.
several important review or perspective papers systematically describing the history of BNNT and its properties and applica-tions.[47] In this part, we are going to discuss 1D BN fibers fab-ricated with BNNS based building blocks.
As discussed in the previous section, our group discovered that NFC improved the dispersion of BNNS in water and a highly concentrated BNNS/NFC ink (up to 17 mg mL−1) was achieved.[41] By wet spinning, BN fiber can be success-fully fabricated, as shown in Figure 4a. The polarized optical microscopy (POM) image of the BN fiber obtained when the fiber is rotated to an angle of 45° with respect to the polar-izer demonstrated the high level of alignment of the building block along the fiber (Figure 4b). The BN fiber containing 40 wt% BNNS exhibited a tensile strength of 244 MPa (Figure 4c). Note that the mechanical strength of the fiber was higher than the film under the same BNNS loading. This
was mainly due to the increased alignment of the building blocks within the fiber.
Most recently, Coleman’s group also reported fabrication of BN-based fibers where coagulation-spinning method was used.[48] In their study, polyvinyl alcohol (PVA)-BNNS fibers were fabricated using PVA as the fiber matrix and highly concentrated BNNS as the filler. As shown in Figure 4d, the PVA-BNNS fiber is straight with a uniform diameter. Zoomed-in scanning electron microscopy (SEM) images in Figure 4e and 4f showed the fracture surfaces with clear nanosheets. However, tensile mechanical properties of the fiber failed at strains of typically <1% (Figure 4g), which should be attributed to solvent being removed upon drying. The strength and stiffness of the fiber increased with the volume fraction up to ≈20%, which indicated that aggregation may be suppressed during the fiber fabrication process (Figure 4h and 4i). A similarly interesting
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Figure 4. a) BN fibers with different diameters and tied in knots; b) a POM image of the BN fiber, demonstrating alignment along the fiber direction; and c) typical stress–strain curve of the BN fiber with 40 wt% BNNS. Reproduced with permission.[41] Copyright 2015, Elsevier. d–f) SEM images; g) stress strain curves; h) Young’s modulus and i) tensile strength of PVA-BNNS fiber and PVA-graphene fibers as a function of volume fraction. Reproduced with permission.[48] Copyright 2016, Elsevier.
phenomenon was also observed when preparing PVA-graphene fiber, where a maximum modulus and strength of ≈30 GPa and 260 MPa can be achieved.
3.2. 2D Assemblies
2D substrates, such as films and papers, are essential for many applications. Among the various properties of substrates, flexi-bility and thermal conductivity are two critical factors for elec-tronic devices.[49] Considering their high thermal conductivity, solution processed BNNS have been widely used to make thermally conductive composite films.[50] In 2011, Shi et al. reported a poly [2,20-(p-oxydiphenylene)-5,50-bibenzimidazole] (OPBI)/BNNS film, where the incorporation of BNNS not only exhibited higher reinforcement effect but also enhanced the thermal stability (Figure 5a).[34] Later on, Sun’s group syn-thesized a PVA/BNNS film, where the in-plane thermal dif-fusivity (TD) increases monotonically with the increase of BNNS loading.[51] For example, TD reached 6 mm2 s−1 when the BNNS loading was 50 vol%, (Figure 5b). Interestingly, when replacing PVA by epoxy, the resulted epoxy/BNNS film exhibited an even higher TD of 19 mm2 s−1 and a high thermal conductivity of 30 W m−1 K−1 at 50 vol% BN. They also discov-ered that TD of the PVA/BNNS film increased dramatically by mechanical stretching and reached 9 mm2 s−1 at 15 vol% BN after stretching. This interesting phenomenon was attributed to the alignment of BNNS upon mechanical stretching, where researchers also found the alignment induced by magnetic field can improve the thermal conductivity of polysiloxane/BNNS film.[52]
Recently, our group reported a thermally conductive paper with BNNS connected by 1D NFC, where BNNS enabled the thermal transport path and NFC enhances the mechanical strength (Figure 5c–5e).[4b] NFC/BNNS papers with different contents of BNNS were synthesized via a simple and scalable paper-making process and exhibited many interesting and useful properties. For example, the 5% paper (5 wt% BNNS) was highly transparent and can be a potential substrate for flex-ible electronics. When the content of BNNS reached 50 wt%, the paper became opaque but with a high whiteness of 86%. Moreover, the thermal conductivity of NFC/BNNS paper increased sharply with the higher content of BNNS, where the 5% paper gives a thermal conductivity of 26.2 W m−1 K−1 and the 50% one can be as high as 145.7 W/m·K. Considering the thermal conductivity of pure cellulose is only 0.035 W m−1 K−1, the introduction of BNNS into paper was the key to the signifi-cant thermal conductivity improvement.[53] Compared to other polymer/BNNS films, thermally insulating contacts between the BNNS can be avoided using our NFC-based design, as con-firmed by modeling via the commercial finite element analysis software ANSYS.
Inspired by natural nacre structure, a PVA/BNNS paper consisting of ordered “brick-and-mortar” arrangement of BNNS and PVA was also synthesized, where PVA molecules play a role as the bridge for linking BNNS via hydrogen bonds (Figure 5f).[54] Such a structure resulted in a PVA/BNNS paper with an excellent tensile strength of 125.2 MPa and toughness of 2.37 MJ m−3. Most recently, a free-standing BN membrane was fabricated using ball-milling-assisted exfoliated BNNS, which exhibited an interesting fire-resistant properties due to the absence of any polymers or carbons (Figure 5g).[45]
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Figure 5. a) Photo of OPBI/BNNS film. Reproduced with permission.[34] Copyright 2011, Royal Society of Chemistry. b) Photo of PVA/BNNS film. Reproduced with permission.[51] Copyright 2012, John Wiley and Sons. NFC/BNNS papers with c) 5 wt%, d) 50 wt% BNNS, and e) the thermal con-ductivity vs BNNS content in NFC/BNNS papers. Reproduced with permission.[4b] Copyright 2014, American Chemical Society. f) Photo of PVA/BNNS paper. Reproduced with permission.[54] Copyright 2015, the Royal Society of Chemistry. g) A photo of a freestanding BN membrane. Reproduced with permission.[45] Copyright 2015, Nature Publishing Group.
3D aerogel structured materials have extensively impart attrac-tive properties for a wide range of applications due to their high surface area, large pore volume, and low density. In the last few years, a number of methods have been developed to fabricate BN aerogels, including CVD on Ni foam template,[55] CVD on carbon aerogel template,[56] and solid-state reaction[57] with or without graphene template. Here, BN aerogel made by solution processed BNNS is our focus.
In 2012, Jung et al. reported a methodology to make various aerogels from their gel precursors (Figure 6a).[58] They discov-ered that anisotropic nanostructures, mainly nanowires, nano-tubes, and nanosheets, can assemble into cross-linked networks from their colloidal suspensions. The cross-linked networks, or gels, can be facilely transformed to 3D aerogel by extracting the liquid in gels via critical point drying (CPD). In the case of preparing BN aerogel, a dilute BNNS suspension was obtained by sonicating BN powder with planar sodium cholate (SC) surfactant. After evaporating a large amount of solvent at low
temperature, concentrated BNNS gel was given and through surfactant removal, solvent exchange, and CPD formed the BN aerogel (Figure 6b). The two key factors affecting successful formation of BN aerogel are: (i) preventing the bundling or agglomeration of nanomaterials when making the starting dilute dispersion and (ii) evaporating the solvent slowly to avoid aggregation of the nanomaterials. Besides BN aerogel, many other aerogels were demonstrated in the study, such as Ag, Si, MnO2 nanowire, single wall nanotube (SWNT), MoS2 and graphene nanosheet. Theoretically, any type of material can be used to prepare aerogels by this method.
Three years later, Lei and co-workers developed another inter-esting method for producing ultralight BN aerogel from urea-assisted ball milled BNNS, where the density of BN aerogel can be as low as 1.4 mg cm−3 and specific surface area as high as 273 m2 g−1.[45] After dehydrating the BNNS aqueous disper-sion via freeze-drying, BN aerogels with different densities were obtained by tuning the concentration of BNNS solution before drying (Figure 6c and 6d). As shown in Figure 6e and Figure 6f, the low-density BN aerogel exhibited a loose porous
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Figure 6. a) Schematic representation of aerogel production by a critical point drying method and b) the as-obtained BN, MoS2, and graphene aerogels. Reproduced with permission.[58] Copyright 2012, Nature Publishing Group. Photos and SEM images of BN aerogels with density of c,e) 1.4 mg cm−3 and d,f) 20 mg cm−3, respectively. Reproduced with permission.[45] Copyright 2015, Nature Publishing Group. g) A photo of BN aerogels with various shapes; h) A BN aerogel with a density of 21 mg cm−3 standing on a flower-like cattail; i) Out-of-plane and j) in-plane SEM images of BN aerogel. Reproduced with permission.[59b] Copyright 2015, American Chemical Society.
structure, while the denser one gave a “sheet-like” structure with thicker walls. Most recently, BN aerogel was also fabri-cated via a polymer-assisted strategy, where SC and 1,4-butan-ediol diglycidyl ether (BDGE) were used to link BNNS by covalent bonding.[59] In detail, an epoxy ring opening reaction between BNNS and BDGE occurred when mixing SC-grafted BNNS solution with BDGE, where the solution became viscous and transformed into hydrogels in several hours. After a simple freeze-casting technique, BN aerogels with arbitrary shapes and various densities can be synthesized (Figure 6g,h). SEM obser-vation confirmed that the BN aerogel has very high porosity (up to 99%), honeycomb-like structure along the out-of-plane direction and aligned in the in-plane direction (Figure 6i,j). Such an interesting anisotropic microstructure was enabled by the anisotropic direction of ice growth during freeze-drying, which resulted in anisotropic mechanical properties in which the maximum stress of the out-of-plane deformation is much higher than that of the in-plane compression.
Most recently, a concept of van der Waals (vdW) super-structures, or vdW solids, has been proposed, where various 2D nanosheets were stacked and showed unusual physical or electronic properties.[6a,60] For example, Ajayan and co-workers mixed BNNS with graphene nanosheets and fabricated stacked BN/graphene solids, which introduced large number of BN/graphene interfaces.[61] Overall, multidimensional BN assemblies made by BNNS have been successfully demonstrated in the last few years, with numerous applications having ben-efitted or been enabled by their high surface area, low density, and mechanical properties. In the next section, we will focus on the applications of solution processed BNNS in various fields.
4. Applications
Since the scalable production by solution processed synthesis became possible, BNNS have been applied in many fields due to their attractive properties.[62] For example, BNNS have less penetrative channels for O2, making BNNS or polymer/BNNS composites ideal barrier materials. Oxygen reduction reaction (ORR) reactivity was observed on BNNS, which may open a new route for low cost catalysts. To date, fabricating polymer/BNNS composites has attracted great attention, where poly-mers are used as the matrix and BNNS as the additive.[63] The introduction of BNNS significantly improves various properties of polymers, in particular the thermal properties.[64]
4.1. Barrier Material
The issues of metal deterioration by oxidation or corrosion have seriously decreased the life of metal and led to huge economic losses. The well-accepted strategy for metal protection is sur-face coating using a barrier material, such as alloy, ceramic, or polymer. Recently, researchers started to explore 2D materials as novel barrier materials, where graphene has exhibited great promise due its light weight, stability, and high impermeability to gases and moisture. However, graphene is conductive, which may introduce a galvanic cell coupling with the metal.[65] Thus, the oxi-dation or corrosion rate will accelerate in the long term. In sharp
contrast, BNNS can outperform graphene as a barrier mate-rial because BN is thermally/chemically stable, electrically insu-lating and would not result in galvanic corrosion. Several years ago, Ajayan’s group reported that thin layer BNNS synthesized by CVD can be an effective oxidation-resistant coating for metals under oxidizing conditions.[66] Similar results have also been observed by Li and co-workers.[67] Here we are going to discuss the applications of solution processed BNNS as barrier materials.
Various methods were attempted to improve the ability of corrosion protection by coating metal with BNNS/polymer paints. In 2013, Husain et al. reported a BNNS/PVA mixture paint to protect stainless steel (AISI 316LSS), where they dis-covered that a BNNS/PVA coating resulted in a small corrosion current density of 5.14 × 10−8 A cm−2 and a slow corrosion rate of 1.19 × 10−3 mm year−1.[68] Such an impressive corrosion pro-tection is attributed to the addition of BNNS, which is hydro-phobic, chemically stable and dielectric. Inspired by this result, Liu’s group developed a BNNS/polyvinyl butyral (PVB) paint for protecting Cu from corrosion.[69] In their study, BNNS/PVB paints with different mass fractions of BNNS were prepared by dissolving PVB in a BNNS dispersion and coated onto Cu via a dip-coating method. Corrosion tests demonstrated that Cu was corroded in a NaCl aqueous solution with pure PVB coating and the corrosion products were mainly Cu2O (Figure 7a,b). However, after coating with BNNS/PVB0.1 (0.1 wt% BNNS in paint), much less corrosion occurred. When a higher con-tent of BNNS was used, no corrosion took place and Cu kept its metallic luster (Figure 7c,d). In contrast, when graphene/PVB1.0 paint was used, more serious corrosion on Cu was observed than BNNS/PVB0.1 and even pure PVB, confirming that graphene accelerated the corrosion rather than protecting.
Besides metal corrosion protection, BNNS have also been applied to protect materials from atomic oxygen (AO) cor-rosion, which is critical for materials used in high-energy AO environment, such as spacecraft in low earth orbit.[70] Shen et al. reported two kinds of BNNS/polymer composite that exhibited a significantly enhanced AO corrosion resistance. Initially, BN was dispersed in cellulose acetate (CA) solution, where CA acted as the polymer matrix and stabilizer for BNNS as well.[70b] The homogeneous dispersion of BNNS in CA ena-bled the preparation of a uniform BNNS/CA composite film, which exhibited a significant improvement in the AO corro-sion resistance due to the high surface area and excellent sta-bility of BNNS. The same team also discovered that a higher content of BNNS in BNNS/PVA composite can give a better AO corrosion resistance (Figure 7e–h).[70d] The composite with larger-size BNNS showed a much better barrier effect, which can be attributed to its fewer edges and less penetrative chan-nels (Figure 7i). Recently, Lee and co-workers reported that the addition of BNNS into polyethylene can effectively slow down the transport of O2 and water vapor molecules.[44] All the results suggested that pure BNNS or BNNS/polymer composites can be promising barrier materials.
4.2. Catalyst
It is well known that catalysts play an important role in many chemical reactions. Take ORR as an example: Pt based catalysts
are used to overcome the large overpotential and improve the kinetics of certain chemical reactions. Since ORR is essen-tial for many applications such as fuel cells and Li-O2 bat-teries, considerable efforts have been devoted to developing alternative catalysts for replacing expensive Pt. Among dif-ferent candidates, N- and B-doped carbons have been widely investigated,[71] where the catalytic activity was attributed to the electron accepting nature of N- or B-species so that O2 can be adsorbed on B atom. For example, Hu’s group investigated two kinds of B and N co-doped CNTs. One was bonded B and N co-doped CNTs, which exhibited little ORR activity. On the other hand, CNTs co-doped by separated B and N can became excel-lent ORR electrocatalysts.[72] Interestingly, when substituting all the C atoms by B and N atoms, BN is given. Our question is whether BN can exhibit electrocatalytic activity for ORR. To answer this question, Lyalin et al. predicted that monolayer BNNS can become catalytically active for the ORR by N-doping based on DFT calculations.[73] However, since BN is an insu-lator, an electron transport support was needed.[74] It is believed that bare BNNS without doping can also be catalytically active with the support of Ni (111), where Ni (111) substrate can
greatly enhance the adsorption of O2, OOH, OH, and O on BN/Ni (111).[75] Similar first-principles DFT modeling also indicated that BNNS on Cu (111) can be an efficient catalyst for CO oxidation.[76]
To prove the concept, Uosaki et al. reported that BNNS on Au substrate can allow a large reduction in the overpotential for ORR.[77] Using DFT calculations, they first demonstrated that the calculated partial density of states (PDOS) of BNNS was slightly modified and the unoccupied BN states shifted slightly toward the Fermi level when BNNS were deposited on Au (111), due to the interaction between BNNS and Au (Figure 8a,b). When testing bare Au and BNNS on Au electrodes using linear sweep voltammograms (LSVs), an about 0.27 V decrease in overpotential was found with the deposition of BNNS. The plausible ORR product was H2O2, corresponding to a two-elec-tron reduction process (Figure 8c). Furthermore, the authors suggested that the edges of BNNS may be the preferred ORR sites due to their highly activated states. Although the current performance of BNNS as ORR catalyst is still poorer than that of Pt, the new findings are meaningful in replacing Pt with low cost materials.
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Figure 7. Photos of Cu foils protected by the BNNS/PVB coatings: a) newly polished Cu, with b) BNNS/PVB0 coating, c) BNNS/PVB3.0 coating, and d) BNNS/PVB5.0 coating. Reproduced with permission.[69] Copyright 2016, The Electrochemical Society. SEM images of PVA films with different BNNS content: e) 0%, f) 0.7 wt%, g) 0.5 wt%, and h) 1.0 wt% after exposure to atomic oxygen; i) The proposed mechanism that BNNS can enhance oxygen atom corrosion resistance of PVA film. Reproduced with permission.[70d] Copyright 2015, the Royal Society of Chemistry.
Recently, researchers also discovered that BNNS can be used as oxidative dehydrogenation of propane reaction catalysts,[78] phase-transfer catalysts,[30b] Fenton catalysts,[79] or act as a good support for noble metals,[80] metal oxides,[81] metal–organic frameworks[82] and semiconductors catalysts.[83] Particularly, BNNS/Ag nanocomposites have drawn great attention.[84] For example, Ag nanoparticles with a relatively uniform size (below 10 nm) have been in situ grown on BNNS (Figure 8d), which exhibited great catalytic activity for reducing 4-nitrophenol due to the conjugate structure of the BNNS support and the small size of Ag nanoparticles (Figure 8e,f).[80b]
4.3. Thermal Management
As we discussed above, BNNS are analogous to graphene, which share many similar material properties, like high thermal conductivity and great mechanical strength. But unlike graphene, BN is an electrical insulator in the absence of free movement of electrons, where phonons are the dominant heat carriers in BN. This unique decoupling of the electrical and thermal transport properties enables the possibility of BNNS for many applications where electrical insulating is required. Since polymers typically have low thermal conductivity, solution processed BNNS have been widely studied as fillers in polymeric matrices, significantly increasing the thermal conductivity, thermal stability and mechanical strength of the pure materials as well for their composites.[15a,30c,d,85]
4.3.1. Thermal Energy Storage and Heat Transfer
Thermal energy storage is critical for improving the energy efficiency of many systems.[86] For example, thermal energy storage in solar cells using molten salt can effectively improve the cell efficiency due to the intermittent nature of solar energy. Currently, phase change materials (PCMs) have been widely studied to store thermal energy due to their high latent heat (heat of fusion).[87] However, the low thermal conductivity of PCMs significantly suppressed the energy storage rate. To address this issue, introducing materials with high thermal conductivity into PCMs was reported, where various materials have been investigated, such as CNT, graphene, metal, and metal oxide nanoparticles.[88]
Recently, Fang and co-workers added BNNS into paraffin-based PCMs for improving their thermal energy storage per-formance.[89] As expected, the addition of BNNS improved the thermal conductivity of PCM over a wide temperature range. Below 60 °C, the improvement was more obvious, indicating that BNNS affected the thermal conductivity of the composite more strongly in the solid phase (data points on the left side of the vertical dashed line in Figure 9a). In contrast, when the PCMs were transferred into liquid phase at higher tempera-ture, the improvement as a result of BNNS addition became less significant. Due to the increased thermal conductivity, the melting and solidification rates were accelerated by up to 25%, suggestive of a better thermal response.
High-performance heat-transfer liquids are also highly desirable for heat-transfer systems, such as cooling systems
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Figure 8. a) Spin-polarized DOS calculated of the defect-free monolayer BNNS; b) PDOS projected on B and N atoms of BNNS on Au (111); c) LSV curves of (i) bare Au, (ii) BNNS/Au, (iii) bare glassy carbon (GC), and (iv) BNNS/GC. Reproduced with permission.[77] Copyright 2012, American Chemical Society. d) TEM image of Ag nanoparticles on BNNS; e) UV-vis absorption spectra for reduction of 4-nitrophenol using BNNS/Ag catalyst with different reaction time; f) Plots of ln(Ct/C0) against reaction time for the reduction of 4-nitrophenol with different BNNS based catalysts. Reproduced with permission.[80b] Copyright 2015, the Royal Society of Chemistry.
in vehicles and electronics. The state-of-the-art heat-transfer fluids (water, ethylene glycol (EG), and engine/transformer oils) exhibit low-efficiency due to their low thermal conduc-tivity. Among various additive choices, Ajayan’s group adopted solution processed BNNS to prepare a novel heat-transfer fluid based on mineral oil (Figure 9b).[90] They discovered that min-eral oil (MO) based heat-transfer liquids with small amounts of BNNS (0.01–0.1 wt%) can be very stable due to a high zeta-potential of ≈22 mV without any surfactant. The effect of BNNS on viscosity was also limited due to the oleophilic nature of BNNS. As shown in Figure 9c, the thermal conductivity increased with the BNNS concentration in MO. Due to the high surface area of the flakes, the percentage of enhancement in thermal conductivity at 323 K reached ≈80% when only 0.1 wt% BNNS was added. Compared to graphene additive, BNNS also increased the electrical resistance of MO (Figure 9d), which is very important in practical applications. Meanwhile, the addi-tion of BNNS resulted in a lower freezing point of MO. All the above improvements indicated great promise of MO/BNNS in many applications, such as thermal oils for lubrication and effi-cient thermal management for transformers.
4.3.2. Dielectrics
Polymer dielectrics have been widely used in power elec-tronics and energy storage devices, which typically exhibit higher breakdown strengths, lighter weight, and easier for processing compared to their ceramic counterparts. A critical
disadvantage of polymer dielectrics is their low working tem-peratures due to their poor thermal conductivity.[91] However, high temperature performance is essential for many electronics due to the exothermic nature of circuits during operating. Cur-rently, biaxially oriented polypropylene (BOPP), the best com-mercial polymer dielectric, can only be used at temperatures below 105 °C. Thus, a high-temperature polymer dielectric is highly desirable.[92] According to the high thermal conductivity and dielectric nature of BNNS, addition of BNNS into polymer dielectrics can be a promising solution. Indeed, for a long time, few layer or single layer BNNS was believed to be an excellent dielectric substrate for other 2D materials, such as graphene, MoS2, electronic and photonic devices.[38,93] However, these devices require a high degree of quality, since mechanical exfo-liation or CVD methods are usually applied. The applications of solution processed BNNS could be a more scalable and low cost solution for polymer dielectrics.
Recently, Wang and co-workers reported a high tem-perature crosslinked divinyltetramethyldisiloxane-bis(benzocyclobutene)/BNNS (c-BCB/BNNS) nanocomposite, where BNNS was obtained by solution phase exfoliation (Figure 10a).[94] When polymer dielectrics were tested under an applied field of 200 MV m−1 at 150 °C, c-BCB/BNNS exhib-ited an electrical conductivity of 9.2 × 10−14 S m−1, which is two orders of magnitude lower than that of crosslinked pris-tine BCB (c-BCB, 4 × 10−12 S m−1). Such a lower electrical con-ductivity resulted in a much lower conduction loss in c-BCB/BNNS (3%) compared to 18% in c-BCB. The addition of BNNS increased the mechanical strength of the polymer composite,
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Figure 9. a) Thermal conductivity of the paraffin-based composite PCMs as a function of temperature with various amounts of BNNS added. Repro-duced with permission.[89] Copyright 2014, Elsevier. b) Photos of pure MO, MO/BNNS and MO/graphene liquids; c) Thermal conductivity of MO based thermal transfer liquids has been improved with the addition of BNNS (inset shows the variation in TC between 0.01 and 0.05 wt%); d) Electrical resistivity of MO based thermal transfer liquids with BNNS addition or graphene addition. Reproduced with permission.[90] Copyright 2012, American Chemical Society.
where Weibull breakdown strength (Eb) was greatly enhanced. For example, c-BCB/BNNS exhibited Eb of 403 MV m−1 at 250 °C while c-BCB was limited to 262 MV m−1 for the same test conditions. Furthermore, comparing c-BCB/BNNS with state-of-art polymer dielectrics, such as polycarbonate (PC), poly(ether ether ketone) (PEEK), polyetherimide (PEI), flu-orene polyester (FPE) and polyimide (Kapton PI), c-BCB/BNNS had a higher thermal conductivity (1.8 W m−1 K−1 for c-BCB/BNNS vs ≈0.2 W m−1 K−1 for the other polymers). c-BCB/BNNS therefore gave much more uniform temperature distribution at high working temperatures and resulted in much better per-formance in terms of discharged energy density and charge–discharge efficiency at temperatures ranging from 150 °C to 250 °C. As shown in the simulated steady-state temperature dis-tribution image (Figure 10b), the PEI-, FPE- and Kapton based film capacitors overheated at 200 MV m−1 and 200 °C, while c-BCB/BNNS exhibited uniform temperature distribution and a low temperature (204 °C). Considering the above-mentioned advantages, introduction of BNNS into polymer dielectrics is very promising.[95] The same team also added BNNS into poly(vinylidene fluoride)-based ferroelectric terpolymers, which significantly enhanced the breakdown strength, energy density and charge–discharge efficiency of ferroelectric terpolymers.[96]
4.4. Surface-Enhanced Raman Spectroscopy
Raman signal enhancement on noble metal enables the sur-face-enhanced Raman spectroscopy (SERS) technology for tracing organic or biological molecules.[97] Currently, the high cost of noble metal and poor cyclability of SERS are two main challenges that high-performance and reusable SERS devices
are highly desired. To overcome these problems, loading noble metal nanoparticles on 2D materials have attracted attention due to their role in improving sensitivity and signal reproducibility.[98]
To explore the capability of BNNS as a substrate for noble metal nanoparticles using in SERS, Lin and co-workers deco-rated solution processed BNNS surfaces with Ag nanoparticles using a room-temperature reaction.[99] As shown in Figure 11a, a simple vacuum filtration and transfer technique was adopted to prepare Ag/BNNS based thin film SERS devices, which is very convenient for recycling due to the relatively large lateral size of BNNS. When evaluating the Ag/BNNS-based SERS device with a low concentration of rhodamine 6G (R6G), well-resolved Raman signatures can be detected. Comparing the SERS result obtained with the typical Raman spectrum of a bulk R6G sample, the SERS enhancement factor of such an Ag/BNNS-based SERS device was calculated to be on the order of 1 × 105. The Ag SERS device with BNNS substrate was so robust that great response could still be achieved after repeated measurement−ethanol washing cycles. Furthermore, the SERS devices can survive upon a short-duration thermal treatment (400 °C in air) with no significant decrease in performance, due to the thermal oxidation resistant property of BNNS substrate. In 2015, Cai and co-workers discovered that faceted Au nano-particles on BNNS substrate can also exhibit enhanced Raman signal (Figure 11b–e).[100] In addition, no noticeable SERS signal can be detected from BNNS substrate, which is impor-tant for practical applications. The possible mechanism for the better performance was attributed to the much greater adsorp-tion of R6G molecules among Au particles on BNNS substrate compared to that of other substrates and the dipole interactions between R6G and BNNS (Figure 11f).
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Figure 10. a) Schematic for the preparation of crosslinked BCB/BNNS (c-BCB/BNNS), the chemical structure of BCB monomer and the unit of c-BCB; b) Simulated steady-state temperature distribution in spiral-wound film capacitors based on c-BCB/BNNS, PEI, FPE ,and PI (Kapton), respectively. Reproduced with permission.[94] Copyright 2015, Nature Publishing Group.
BNNS has been investigated for fuel cells and batteries as well. The membrane is an important component in poly mer electrolyte membrane fuel cells (PEMFCs). Recently, hydrocarbon-type (HC-type) PEMs have been intensively investigated to replace perfluorinated sulfonic acid (PFSA) polymer based PEMs, the most adopted PEMs, due to the present disadvantages including high cost, small tempera-ture window, and so on. However, HC-type PEMs usually suffer from poor mechanical and dimensional stabilities so that achieving a long cycle life has been challenge. In 2014, Oh and co-workers added solution processed BNNS into a well-known HC-type membrane, sulfonated poly(ether ether ketone) (sPEEK), which significantly improved the mechanical strength of the membrane.[101] As shown in Figure 12a, the BNNS solution obtained by sonicating BN powder in DMF could mix well with sPEEK polymer electrolyte with 1-pyr-enesulfonic acid (PSA) modification. Using a well-established film casting technique, BNNS/sPEEK composite membranes with a thickness of 40 µm were fabricated (Figure 12b). Com-pared to bare sPEEK membrane, BNNS/sPEEK composite membrane with only 0.3 wt% BNNS exhibited 41% increase on tensile strength. Meanwhile, the ion exchange capacities (IEC) and proton conductivity also had higher values. All of these improvements resulted in BNNS modified membranes better ablility to resist chronic edge failure from the volume upon wet/dry cycling so that long-term durability of PEMFC was achieved.
BNNS has also been employed in lithium-ion batteries (LIBs) to improve the performance of electrodes and separa-tors. As a flexible substrate for metal oxide anodes, BNNS can effectively accommodate the volume change of metal oxides during lithiation/delithiation cycling.[102] Currently, micro porous polymeric membranes, such as polyethylene (PE) and polypropylene (PP), have dominated the market of Li-ion batteries for a long time. However, these separators typically exhibit low thermal conductivity, making thermal dissipation within the separator a challenge if a local hot point is occurred. Most recently, solution processed BNNS
was used to coat separators (Figure 12c–e), which improved the thermal conductivity of separator.[103] The improved thermal conductivity made for a more uniform thermal dis-tribution exothermic nature of the Li deposition/stripping reaction and thus suppressed the local hot spots, which allowed more uniform deposition/stripping of Li and better cycling performance (Figure 12f–g). A similar phenomenon has also been achieved by other 2D material coated current collectors.[104]
4.6. Other Applications
Besides the above-mentioned applications, other applica-tions have also been explored in the last several years. For example, due to their good biocompatibility properties, solu-tion processed BNNS were developed as a promising support for gold nanoclusters (GNCs) used in immunoassays.[105] In that study, GNCs were loaded onto poly diallyldimethylam-monium chloride (PDDA) modified BNNS by a layer-by-layer assembly, which exhibited strong orange fluorescence prop-erties. After assembling the PDDA-BNNS with antibody con-jugates (Ab2) and fabricating a labelling system in bioassays, the observed fluorescence signal was much higher than that of the GNC–Ab2 probe without BNNS support. Such a PDDA–BNNS–GNC composite can also be useful for drug delivery, diagnostics and so on. Moreover, Zhi’s group synthesized mon-olayered BN and boron carbon oxynitride (BCNO) quantum dots (QDs) through a hydrothermal method using bulk BN as starting mateirals. They found that BN/BCNO QDs have high biocompatibility and low cytotoxicity, which enables the appli-cation of BN/BCNO QDs for bio-imaging probes.[106]
Recently, Tang and co-workers reported that introducing solution processed BNNS into poly(butylene adipate) (PBA) can change the formation condition of the polymorphic crystals of PBA.[107] They discovered that BNNS could induce α-form crystals under either non-isothermal or isothermal crystalli-zation. For example, α-form crystals disappeared at 28 °C for pure PBA, but with a slight addition of BNNS (0.5 wt%), this value changed to 13 °C. This interesting phenomenon and
Figure 11. a) Schematic of fabricating Ag/BNNS-based SERS devices and a photo of BNNS film and Ag/BNNS films. Reproduced with permission.[99] Copyright 2012, American Chemical Society. AFM images of Au nanoparticles on b) BNNS, c) bulk BN crystal, and d) Si substrates; e) Raman spectra of R6G using different substrates; f) Schematic to show that R6G molecules are easily absorbed on BNNS, which can improve the SERS enhancement. Reproduced with permission.[100] Copyright 2012, the Royal Society of Chemistry.
further enzymatic degradation experiments proved that BNNS can effectively regulate the formation of the polymorphic crys-tals and help to control the degradation rate of PBA. Moreover, BNNS exhibited nonlinear optical (NLO) properties, which indicated potential optical applications for advanced photonic devices.[108] Carbon-doped BNNS showed high-temperature fer-romagnetism, which is also very interesting for fundamental studies and further applications. It is worth noting that BNNS have been widely applied in various fields, so high-efficiency solution processed strategies for scalable production of BNNS are highly desired.
5. Conclusions
In this contribution, we focus on summarizing the recent progress on the synthesis of BNNS via solution processed methods. Through sonication in liquid envi-ronments, surface-functionalization-assisted exfoliation,
ball-milling-assisted exfoliation, or other solution processed methods, BNNS have been successfully prepared using low cost and abundant bulk BN particles as starting materials. Although significant progress has been made in this field, the develop-ment of solution processed BNNS just started recently and there are still many challenges. First of all, increasing the yield and shortening the time of synthesizing BNNS via one-step sonication is highly attractive. Therefore, studying the funda-mental chemistry of choosing a good solvent is of great impor-tance. Secondly, the lateral size of BNNS obtained by solution processed methods is usually smaller than other methods, such as CVD and mechanical cleavage. In future research, it is desir-able to develop effective methods for preparing BNNS with large lateral size. Furthermore, knowledge obtained from devel-oping other 2D materials, such as graphene, can play a critical role in the development of BNNS. For example, the graphene oxide preparation by Hummers method enables graphene for a wide range of applications. Developing effective oxidation methods for bulk BN is also very attractive.
Figure 12. a) Photos of bare sPEEK and BNNS/sPEEK solutions; b) Photo of BNNS/sPEEK membrane using a casting technique and schematic to show the interaction between BNNS and sPEEK. Reproduced with permission.[101] Copyright 2014, American Chemical Society. Photo images of c) BNNS/PVDF suspension and d) BNNS-coated separator, where the BNNS layer is marked by a green circle; e) SEM image of BNNS-coated separator; Cycling perfor-mance of cells using pristine separator (f) and BNNS-coated separator (g). Reproduced with permission.[103] Copyright 2015, American Chemical Society.
To date, multidimensional assemblies including 1D fiber, 2D film/paper, and 3D aerogel made by solution processed BNNS have been demonstrated and exhibited many advantages. For example, BN aerogel’s 3D porous structure gives high sur-face area, high porosity, and low density, which showed great application potential in water treatment. Moreover, thermally conductive BN paper developed by our group can be a great substrate for electronics and 2D material devices. In addi-tion to these assemblies, BNNS have been employed in many emerging applications due to their high thermal conductivity, chemical/thermal stability, and other superior properties of BNNS (Figure 13). It is worth mentioning that the unique decoupling property of electrical and thermal transport makes BNNS more attractive than graphene in some applications. For example, in order to improve the thermal properties of polymer dielectrics, BNNS can be adopted due to its insulating nature while graphene cannot. As barrier materials for protecting metal from corrosion, BNNS can outperform graphene because BN is electrically insulating, while graphene introduces a gal-vanic cell coupling with the metal and accelerates the oxidation or corrosion rate in the long term. Although the great promise of BNNS has been demonstrated in many studies, efforts are still needed to fully exploit their unique properties and develop further applications. Research efforts on the synergistic effect between BNNS and other functional materials, especially dif-ferent polymers and various nanoparticles, are critical in the near future.
AcknowledgementsThe authors thank the support from the Office of Naval Research (ONR) under grant N000141410721 and ONR 2016 Young Investigator Award. B.Y. acknowledges the financial support from National Science Foundation (NSF) under Grants CBET1232949 and CBET 1336778.
Conflict of InterestThe authors declare no conflict of interest.
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