​Boron Nitride Nanoparticles

​Boron Nitride Nanoparticles

Nanoparticles being the tiniest particles yet doing wonders in the industry are considered as one of the most used and profound particles. Boron nitride nanoparticles are one of the types of these nanoparticles. BNNs are highly exceptional materials that are vastly being used in various fields. Their properties and characteristics make them unique and visibly excellent in the field of work that they are a part of. All these applications are paving way for the BNNs towards success as they are helping the industries flourish and enabling people to have an easy life.

Introduction

Nanoscale Boron Nitride Particles are typically 10 - 100 nanometers (nm). They are particles that have a spherical high surface area. Being spherical high surface area particles, boron nitride (BN) nanoparticles, nanopowder, or nanodots, has SSA (specific surface area) of 10-75 m2/g range. Nano Boron Nitride Particles are available in many forms, for instance, dispersed forms, coated forms, high purity forms, and ultra-high purity forms. As a dispersion, their availability is through the AE Nanofluid production group.

When the nanoparticles are suspended in solution by using surface technology or surfactant, they are defined as nanofluids. Coating selection and nanofluid dispersion technical guidances are available too. Nanopyramids, nano-horns, nano-whiskers, nano-rods, and other nano-composites are included in the other nanostructures. Chemically bound polymers are used by surface-functionalized nanoparticles for absorbing the particles preferentially at the surface interface.

Formation of boron nitride nanoparticles

There have been no reports of the natural formation of the boron nitride (BN)-based nanomaterials, however, in China and Tibet, there are some rare mineral sites, suggesting the natural occurrence of the boron nitride (BN)-based nanomaterials. Nowadays, most of the synthesis of BN nanomaterials occurs in laboratories. They are made up of an equal number of atoms of both nitrogen (N) and boron (B) that have particular conformations resulting in different structure crystallinity. Strong covalent bonds are possessed by hexagonal boron nitrides (hBNs) between B-N atoms with a structure that’s like Graphene as hexagonal boron nitrides are BNs’ most stable form. Van der Waals interactions are used to hold the 2D BN layers together.

Binding of the nanoparticles

Also, in wurtzite boron nitrides (wBNs), diamond-like (cBNs), and rhombohedral (rBNs), B and N atoms bind to the neighboring BN3 tetrahedrons at different angles as they are sp3 hybridized and they restrain N and B atom’s identical pattern. Under different experimental conditions like pressure and temperature, one can obtain the varying crystallinity in the BN structures. hBNs are formed by wBNs at room temperature under high pressure whereas rBNs and hBNs are formed at high temperature and ambient pressure. hBNs are then used to prepare cBNs at high temperatures under high pressure.

Simulations of the synthesis of boron-nitride nanostructures in a hot, high  pressure gas volume - Chemical Science (RSC Publishing)  DOI:10.1039/C8SC00667A

The structural formation of boron nitride nanoparticles

BN nanomaterials are graphene's structural analogs. The bonding nature between the atoms is the major difference between the BN-based nanomaterials and the C counterparts of the BN-based nanomaterials. In carbon nanomaterials, the C-C bond possesses a pure covalent character, whereas a partially ionic character is displayed by the B-N bonds because of the e− pairs in sp2 hybridized B-N. The confinement of e- pairs is more with the N atoms because of their high electronegativity in the BN-based nanomaterials as it influences the electronic, optical, and mechanical characteristics strongly. Thus, the antioxidant and mechanical characteristics of various composites along with their thermal conductivity can be improved by the BN-based nanomaterials.

Thermal conductivity

When it comes to the formation of tissue-mimicking biomaterials for being utilized in transportation, BN-based nanomaterials are highly needed as they have improved mechanical characteristics like high elasticity and tensile strength along with high thermal conductivity. Piezoelectric characteristics are possessed by BN-based nanomaterials as they allow the production of novel biomaterials for being utilized in tissue engineering processing. Electrical insulation is another major property of BN-based nanomaterials than their C-counterparts, resulting in the BN-based nanomaterials being more favorable for additives.

Possible uses of boron nitride nanomaterials

Recently, a huge amount of attention has been gained by the BN-based nanomaterials because of their potential usages in medical applications because of their high mechanical and chemical stability, and their excellent biocompatibility. Moreover, researchers were enabled by their cellular internalization to investigate the possible usage of the BN-based nanomaterials as gene and drug carriers. In the treatment of cancer, the boron compound's therapeutic effects turned the BN-based nanomaterials into interesting structures. Their usage has been seen broadly in microwave-transparent materials, insulators, lubricators, and cosmetics. Due to their high Young modulus than diamonds, cBNs are also known as white diamonds. Just like a diamond, they also don't react with the related alloys and they can cut various industrial ferrous materials.

Nanostructured materials

Nanostructured materials are those materials that have a nanometer-scale size in one, two, or three dimensions and 1-100 nm is the range of one dimension. The interest of many scientists is gained by these materials over the past years due to their excellent biological, electrical, and physicochemical characteristics as they allow them to be applicable in a broad range in various fields (for instance, health, environment, and energy). The characteristics of the bulk material can be improved by their addition to a matrix.

Properties

There has been a progressive increase in the scientist's interest in the boron nitride (BN)–based materials among the nanostructured materials, due to the high electrical insulation, thermal conductivity, resistance to oxidation, mechanical strength, and chemical stability of the BN-based nanomaterials. An equal amount of atoms of nitrogen (N) and boron (B) is possessed by BN. B atoms and N atomic nucleus combine an sp2 orbital for forming a strong σ bone. BN is isostructural to graphite. Also, a partially ionic character is exhibited by them due to the weak van der Waals forces between adjacent layer’s N and B atoms and the electron pairs in sp2 hybridized B–N, therefore giving it its anisotropic characteristics.

Boron nitride’s crystalline forms

Wurtzite BN (w-BN), rhombohedral BN (r-BN), cubic BN (c-BN), and hexagonal BN (h-BN), are the four crystalline forms in which BN exists. Hybridization is the major difference between these phases. w-BN and c-BN are low-density phases with sp3 hybridized B–N bonds whereas with sp2 hybridization, r-BN and h-BN are dense phases. h-BN is more interesting among them due to h-BN’s structural analogy with graphite. BN nanomaterials can be three-dimensional (for instance, nanostructured porous materials), two-dimensional (for instance, nanosheets and thin films), one-dimensional (for instance, nanoribbons, nanofibers, and nanotubes), and zero-dimensional (for instance, nanospheres).

As compared to macro-and micro-scale materials, interfacial effects and quantum confinement is shown by the low-dimensional materials. When BN's surface effects and low-dimensional quantum confinement combines, remarkable chemical and physical characteristics are attained, for instance, excellent electric insulation, high thermal and chemical stability, and wide bandgap (~5.5 eV). Thus, for many potential applications and functional materials, h-BN is a promising scaffold in gas and water separation, health, hydrogen storage, sensors, and electronics.

Functionalization methods

Numerous reviews are reported for 10 years on the promising applications, functionalization methods, and synthesis of the low-dimensional BN-based materials. There has been a publication on low-dimensional BN nanomaterial's history by Goldberg et al. It is specifically on three things, on the methods of synthesis like chemical exfoliation, ball milling, and chemical vapor deposition, for 1D and 2D BN-based materials, on the applications of those materials, and their morphology. Advances have been reported by Miele et al. in manufacturing nanostructured BN from polymeric precursors containing hydrogen, nitrogen, and boron. Their future perspectives and potential applications for their applications and synthesis have also been discussed.

Chemical, physical, and structural characteristics of the functionalized h-BN nanomaterials are described by Weng et al. Different approaches for functionalizing BN, for instance, their applications and characteristics, have also been discussed. The applications, characteristics, structure, and synthesis of h-BN materials and graphene have been described by Wang et al. Their main focus was on the magnetic, electric, thermal, optical, and mechanical properties of these 2-dimensional materials. A general review was published by Wang et al. On the restoration of the environment by utilizing the BN-based materials for eliminating the pollutants.

Recent advances

They presented the developments on how to use the BN-based materials to remove inorganic/organic pollutants and described the underlying interaction mechanisms. Good removal performances of organic pollutants and heavy metal ions from aqueous solutions are shown by the BN-based materials along with their high sorption capacity due to their chemical inertness and high surface areas.

Nanocomposite synthesis methods

The new composite materials having at least one constituent with nanometric dimensions are the Nanocomposites. Generally, the matrix is massiveand the strengthening nanometric. The physicochemical, thermal, and mechanical characteristics of the resulting materials can be enhanced by adding nanofillers. Nanofillers’ uniform dispersion in the ceramic matrix or polymer is the ideal situation for nanocomposite synthesis even if the metallic matrices are being utilized.

Large interface areas can occur between these components of nanocomposite because of the uniform dispersion. The classification of nanocomposites is done according to the types of matrix materials and nanofillers that are used. Ceramic- and polymer-based nanocomposites are the BN nanocomposite’s 2 main classes that will be discussed in chosen matrix material’s function.

Polymer-based nanocomposites

They are materials that possess a polymer matrix that serves as the host for nanofillers (BN nanomaterials) and materials that are utilized as reinforcements. Interesting characteristics like fire resistance, heat resistance, gas barrier characteristics, low cost, ductility, resistance to corrosion, easy processing, and lightweight are displayed by the polymers. Their low electrical and thermal conductivities are their main drawback. The characteristics of the fabricated nanocomposite significantly improve when nanofillers are added into the polymer matrix as reinforcing agents.

Classification of nanofillers

2-dimensional (nanosheets), 1-dimensional (fibers and nanotubes), and 0-dimensional (spherical particles) are the three classes into which nanofillers can be classified based on their dimensionality. Many parameters determine the manufacture of novel polymer nanocomposites with improved characteristics, for instance, the external stimulus, the morphology, the dispersion, and the shape of the nanofiller. The targeted application determines the type of reinforcement material to be used. There are many factors that have a significant role in polymer matrix reinforcement. Some are distribution, orientation, average particle size, concentration, the nature of the nanofiller, and the nature of the matrix.

In-situ polymerization

Nanofillers can be effectively dispersed in the polymer matrix through In-situ polymerization. Nanomaterials are generally mixed into neat monomer or monomer solutions. An organic initiator, radiation, or heat is used to polymerize it later. Thus, some benefits are possessed by this method to make BN/polymer-based nanocomposites, specifically the strong interaction between polymer matrix and BN nanoparticles because of the covalent bonds, and particle aggregation's suppression because of these polymer chain's controllable growth.

Although, there needs to be more research regarding the removal of the solvent. A reversible addition-fragmentation chain transfer polymerization method was designed by Huang et al. for preparing thermally conductive polystyrene (PS)/BN nanosphere nanocomposites by initiating styrene macromolecular chains on amino-functionalized BN nanosphere’s surface.

Non-functionalized boron nitride composites

Higher thermal conductivity and dielectric characteristics are displayed by the manufactured BN nanospheres@PS nanocomposites as compared to the non-functionalized BN composites. Self-healing materials with enhanced self-healing efficiency were made by using the in-situ polymerization approach.

During thiol-epoxy elastomer’s polymerization, micron-sized BN (mBN) filler was briefly introduced in the system. Better thermal and mechanical characteristics and higher self-healing performance were seen by the attained mBN/thiol epoxy elastomer nanocomposites with various mBN loads. A 3-dimensional BN network was produced in a PS matrix by Wang et al. by using a double strategy. They made styrene oil droplets first in water and BN stabilized them for forming Pickering emulsions, and then in-situ polymerization was used by them for synthesizing PS microspheres with an ultrathin BN layer at the surface. At last, nanocomposites based on BN networks were formed by hot-pressing PS@BN microspheres.

Ceramic-based nanocomposites

There have been investigations on the ceramics for being the candidate structural materials so that they can be utilized in conditions (for instance, chemical and wear aggression, loading rates, and temperature) that are too much or extreme for metals and polymers. Although, when it comes to ceramics, the main problem is the intrinsic brittleness of ceramics that prevents them from being used in various applications in real life. Thus, designing a new generation of ceramics is the main focus of many types of research especially through the incorporation of secondary phases (whiskers, fibers, or particles) which helps in tolerating flaws by attenuating or deflecting the ceramic stress and cracks.

Producing nanocomposite ceramics has been one of the significant developments in which multiple phases are distributed in the ceramic composite at the nanoscopic scale.

Applications

The toughness and strength of the matrix can be enhanced by using BN nanocomposites. Beneficial characteristics of different components are combined by the obtained materials. Following are the most important applications of nanostructured BN and BN-based nanocomposites in the health, environment, and energy fields.

Energy applications

Thermal conductivity

A lot of research is done on developing novel nanocomposites to use BN as reinforcing nanofiller as remarkable thermal characteristics are possessed by BN. ~ 1,700–2,000 W m−1 K−1 of thermal conductivity is possessed by single-layer h-BN, thus the thermal conductivity of the nanocomposite can be improved on the addition of BN in different matrices as nanofillers. Interfacial thermal resistance is reduced between ceramic or polymer matrics and nanofillers for enhancing thermal conductivity in the nanocomposites. It is a well-known fact that filler loads and polymer matrics determine the thermal conductivity.

Photovoltaic/solar cells

Nanocomposites contribute as solar panel surfaces or as photoactive and protective layers to solar cells’ efficiency and multifunctionality. Despite having low efficiency, graphene-on-silicon (Gr/Si) Schottky junction solar cells are easily fabricated and cheap. Performance of Gr/Si Schottky junction solar cells can be improved by introducing a few-layered h-BN between n-Si and graphene. h-BN suppresses interface recombination to function as an effective electron-blocking/hole-transporting layer. Due to their processability at low temperature and their high absorption coefficients, perovskites are promising materials for solar cells.

Hydrogen storage and production

Despite being a renewable energy resource, the storage and generation of H2 remain an issue for practical applications. According to demonstrations, high H2 uptake capacity is exhibited by the low-dimension BN nanophase materials due to partial H2 chemisorption and stronger interactions with the heteropolar B–N bonds. A one-step template-free reaction was used between ammonia and boric acid-melamine precursors for synthesizing highly porous BN micro-belts at moderate conditions. 1,488 m2 g−1 of a high specific area is possessed by this material.

Electrochemical applications

Using PDCs in electrochemical devices is an interesting option. PDC-based nanocomposites were worked on by Idrees et al. and Wan et al. to convert them into stable electrodes for Li-ion batteries in electrochemical applications. Combining PDCs with other nanomaterials like BN, graphite, and carbon nanotubes, results in their characteristics being structurally modified as they are very important and beneficial in supercapacitors and LIB for potential applications. The charge capacity of free-standing SiCN-based LIB electrodes majorly increases by integrating exfoliated BNNSs in SiCN.

There were investigations by Singh et al. on how is it possible to increase Li+ insertion capacity by modifying SiOC ceramics with numerous and different BN nanotube (BNNT) loads. The composite can be utilized as a material of flexible electrode for electrochemical energy storage devices, like supercapacitors and LIB.

Environment

BN possesses good oxidation resistance and is chemically and thermally stable. Also, the hydrophobic nature of BN can be exploited for producing filtration systems and non-wetting surfaces. Thus, for environmental applications like water technologies, BN has a lot of potential.

Water technologies

There has been the development of novel BN-based materials for water technologies, specifically water purification. BNNTs are best for water purification and the sorption of pollutants like organic solvents and oil from heavy industries, due to their good adsorption characteristics. The obtained nanostructures possess high lipophilicity and hydrophobic behavior that is needed for water/oil separation.

Water purification

BN-based nanocomposites were made by using the templating method for water purification. Novel h-BNNS/PVA-based nanocomposites were made by Gonzalez Ortiz et al. The casted homogeneous h-BNNS/PVA dispersion onto glass support for preparing porous membranes, followed by coagulation in a water bath for creating the porous structure and eliminating the solvent.

Gas separation

In chemical industries, the gas separation technologies are significant as they are clean technologies having high transport selectivity and low energy requirements. There has been an extensive amount of studies on those nanocomposites that are made from polymeric materials as they have some benefits like low cost, high process flexibility, and lightweight. Although, low gas permeation/separation characteristics are displayed by these materials. Selectivity and permeability of the polymer material can be enhanced by the incorporation of the inorganic particles (2-dimensional materials) in the polymer matrix. There have been demonstrations of h-BN potential applications for gas separation in some theoretic calculations, for instance, H2/CH4.

Excellent selectivity

Low adsorption energies (0.1 eV less or more) and remarkable H2/CH4 selectivity (>105 at room temperature) are displayed by h-BN particularly for both CH4 and H2 on monolayer membranes. Polymer membrane's transport characteristics are enhanced when h-BN uses nanofiller, because of its functional group’s specific adsorption effect.

Mechanical characteristics and gas transport of thermally rearranged polyimide (XTR) can be tailored by using functionalized BN nanosheets (FBN) as fillers, therefore forming an FBN-XTR nanocomposite. When it comes to H2 separation, FBN-XTR membranes are the best choice to be used as they have higher and excellent H2/CH4 separation performance as compared to those of state-of-the-art membranes. h-BN (0.55, 0.45, and 0.35 wt%) was incorporated by Kamble et al. into polyvinylidene fluoride (PVDF) matrix for fabricating homogeneous h-BN/PVDF nanocomposites through phase inversion.

Conclusion

Boron nitride nanoparticles are highly compatible as they are being utilized in various fields for various purposes. All these applications serve the purpose of building and taking these particles towards success as they are performing excellently in their said fields. Their characteristics make them one of a kind and enable them to achieve all the required demands of the industries.

References:

https://www.sciencedirect.com/science/article/pii/S2590049820300540

https://www.americanelements.com/boron-nitride-nanoparticles-10043-11-5

https://www.frontiersin.org/articles/10.3389/fbioe.2019.00363/full

https://pubs.acs.org/doi/10.1021/acs.nanolett.9b01913

31st Aug 2021 Arslan Safder

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