Boron nitride-based materials for water purification: Progress and outlook

Analogous to the carbon family, boron nitride (BN)-based materials have gained considerable attention in recent times for applications in various fields. Owing to their extraordinary characteristics, i.e., high surface area, low density, superior thermal stability, mechanical strength, and conductivity, excellent corrosion, and oxidation resistance, the BN nanomaterials have been explored in water remediation. This article critically evaluates the latest development in applications of BN-based materials in water purification with focus on adsorption, synthesis of novel membranes and photocatalytic degradation of pollutants. The adsorption of various noxious pollutants, i.e., dyes, organic compounds, antibiotics, and heavy metals from aqueous medium BN-based materials are described in detail by illustrating the adsorption mechanism and regeneration potential. The major hurdles and opportunities related to the synthesis and water purification applications of BN-based materials are underscored. Finally, a roadmap is suggested for future research to assure the effective applications of BN-based materials in water purification. This review is beneficial in understanding the current status of these unique materials in water purification and accelerating the research focusing their future water remediation applications.

Two-Dimensional Van der Waals Heterostructures for Synergistically Improved Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is a precise and noninvasive analytical technique that is widely used in chemical analysis, environmental protection, food processing, pharmaceutics, and diagnostic biology. However, it is still a challenge to produce highly sensitive and reusable SERS substrates with a minimum fluorescence background. In this work, we propose the use of van der Waals heterostructures of two-dimensional materials to cover plasmonic metal nanoparticles to solve this challenge. The heterostructures of atomically thin boron nitride (BN) and graphene provide synergistic effects: (1) electrons could tunnel through the atomically thin BN, allowing the charge transfer between graphene and probe molecules to suppress the fluorescence background; (2) the SERS sensitivity is enhanced by graphene via a chemical enhancement mechanism in addition to an electromagnetic field mechanism; and (3) the atomically thin BN protects the underlying graphene and Ag nanoparticles from oxidation during heating for regeneration at 360 °C in the air so that the SERS substrates could be reused. These advances will facilitate wider applications of SERS especially on the detection of fluorescent molecules with higher sensitivity.

Reduction chemistry of hexagonal boron nitride sheets and graphene: a comparative study on the effect of alkali atom doping on their chemical reactivity

Functionalization of 2D BN dramatically increases the charge donated by lithium and 2D BN is no longer inert!

Younis W, Saroka VA, Teleb NH, Yunoki S, Zhang Q. Interaction of hydrated metals with chemically modified hexagonal boron nitride quantum dots: wastewater treatment and water splitting

The electronic and adsorption properties of chemically modified square hexagonal boron nitride quantum dots are investigated using density functional theory calculations.

The geometry of hexagonal boron nitride clusters in the initial stages of chemical vapor deposition growth on a Cu(111) surface

To understand the nucleation process in the growth of hexagonal boron nitride (h-BN) on transition metal substrates by chemical vapor deposition (CVD), the energy of formation and stability of h-BN clusters of different geometries on a pristine Cu(111) surface were systematically investigated using density functional theory calculations. We find that unlike carbon clusters, h-BN clusters on Cu supports can undergo two possible transformations of the minimum-energy structure at a critical size of 13. Different from freestanding h-BN clusters, on a Cu(111) surface, h-BN chains are more stable than h-BN rings and thus dominate the minimum-energy structure for cluster sizes lower than the critical size. Thus, depending on the experimental conditions of CVD, one-dimensional Bn-1Nn (N-rich environment) or BnNn-1 (B-rich) chains are first created, and they transform to two-dimensional sp2 networks or h-BN islands, but for a BnNn chain, the transformation to a two-dimensional sp2 network h-BN island does not occur. In contrast to carbon islands where pentagons are readily formed, odd-membered rings are extremely rare in h-BN islands, where the transformation to the most stable structure occurs through a combination of trapeziums and hexagons at the edges, so as to avoid B-B and N-N bonds. Moreover, on a Cu(111) surface, trapeziums are destabilized when the four edges are connected to other hexagons because of additional curvature energy, thus favoring the nucleation of planar nuclei. A deep insight into h-BN cluster formation on a Cu support is vital to understanding the growth mechanism of h-BN on a transition metal surface in CVD experiments to further improve experimental designs in the CVD growth of h-BN.

The role of sp2 and sp3 hybridized bonds on the structural, mechanical, and electronic properties in a hard BN framework

A first-principles approach is used to systematically investigate the role of sp² and sp³ hybridized bonds on the structural, mechanical, and electronic properties in a new BN phase (denoted Hex-(BN)₁₂). Hex-(BN)₁₂ has the same number of sp² and sp³ hybridized atoms. The calculated cohesion energy, phonon frequencies, and elastic constants unambiguously confirm the structural stability of this compound. Due to the different types of hybridization and B–N covalent bonds with ionic characteristics, Hex-(BN)₁₂ has unequal bond lengths and bond angles in these hybrid orbitals. These cause the relative energetic stability to be slightly lower than c-BN and w-BN. The hardness of Hex-(BN)₁₂ is estimated to range from 33 to 40 GPa. The bond-breaking order under stress is sp³–sp³, sp²–sp³, and sp²–sp². DFT calculations with the gradient approximation (GGA) and HSE06 functional indicate the electronic structure contains an indirect band gap at 3.21 and 4.42 eV, respectively. The electronic states in the region near the Fermi level primarily arise from the 2p orbitals in sp²-hybridized atoms. In general, sp³ bonded B and N atoms guarantee higher mechanical properties, and sp² bonded atoms ensure ductility and even conductivity, although all changes vary with spatial structure. Hex-(BN)₁₂ can be obtained from multilayer yne-BN, and BN nanosheets, nanotubes and nanoribbons under pressure.

A first-principles approach is used to systematically investigate the role of sp² and sp³ hybridized bonds on the structural, mechanical, and electronic properties in a new BN phase (denoted Hex-(BN)₁₂).

A theoretical study on the structures and electronic and magnetic properties of new boron nitride composite nanosystems by depositing superhalogen Al13 on the surface of nanosheets/nanoribbons

Inorganic boron nitride (BN) nanomaterials possess outstanding physical and chemical characteristics, and can be considered as an excellent building block to construct new composite nanomaterials. In this work, on the basis of the first-principles computations, a new type of composite nanostructure can be constructed by depositing superhalogen Al13 on the surface of low-dimensional BN monolayer or nanoribbons (BNML/BNNRs). All these Al13-modified BN nanosystems can possess large adsorption energies, indicating that superhalogen Al13 can be stably adsorbed on the surface of these BN materials. In particular, it is revealed that independent of the chirality, ribbon width and adsorption site, introducing superhalogen Al13 can endow the BN-based composite systems with a magnetic ground state with a magnetic moment of about 1.00 μB, and effectively narrow their robust wide band gaps. These new superhalogen-Al13@BN composite nanostructures, with magnetism and an appropriate band gap, can be very promising to be applied in multifunctional nanodevices in the near future.

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.

Mechanical properties of atomically thin boron nitride and the role of interlayer interactions

Atomically thin boron nitride (BN) nanosheets are important two-dimensional nanomaterials with many unique properties distinct from those of graphene, but investigation into their mechanical properties remains incomplete. Here we report that high-quality single-crystalline mono- and few-layer BN nanosheets are one of the strongest electrically insulating materials. More intriguingly, few-layer BN shows mechanical behaviours quite different from those of few-layer graphene under indentation. In striking contrast to graphene, whose strength decreases by more than 30% when the number of layers increases from 1 to 8, the mechanical strength of BN nanosheets is not sensitive to increasing thickness. We attribute this difference to the distinct interlayer interactions and hence sliding tendencies in these two materials under indentation. The significantly better interlayer integrity of BN nanosheets makes them a more attractive candidate than graphene for several applications, for example, as mechanical reinforcements.

Catalytic Directional Cutting of Hexagonal Boron Nitride: The Roles of Interface and Etching Agents

Transition-metal (TM) nanoparticle catalyzed cutting has been proven to be an efficient approach to carve out straight channels in graphene to produce high-quality nanoribbons. However, the applicability of such a catalytic approach to hexagonal boron nitride (h-BN) is still an open question due to binary element compositions. Here, our ab initio study indicates that long and straight channels along either the zigzag or the armchair direction of the BN sheet can be carved out, driven by the energetically favored TM-zigzag or TM-armchair BN interface, regardless of roughness of the TM particle surface. Optimal experimental conditions for the catalytic cutting of either BN or BN/graphene hybrid sheet across the domain boundary are proposed via the analysis of the competition between TM-BN (or TM-graphene) interface and H-terminated BN (or graphene) edge. The computation results can serve to guide the experimental design for the production of highly uniform BN (or hybrid BN/graphene) nanoribbons with atomically smooth edges.

Hexagonal boron nitride supported mesoSiO2-confined Ni catalysts for dry reforming of methane

Hexagonal boron nitride supported mesoSiO₂-confined Ni catalysts were designed and developed for dry reforming of methane.

Oxidative Unzipping and Transformation of High Aspect Ratio Boron Nitride Nanotubes into "White Graphene Oxide" Platelets

Morphological and chemical transformations in boron nitride nanotubes under high temperature atmospheric conditions is probed in this study. We report atmospheric oxygen induced cleavage of boron nitride nanotubes at temperatures exceeding 750 °C for the first time. Unzipping is then followed by coalescence of these densely clustered multiple uncurled ribbons to form stacks of 2D sheets. FTIR and EDS analysis suggest these 2D platelets to be Boron Nitride Oxide platelets, with analogous structure to Graphene Oxide, and therefore we term them as “White Graphene Oxide” (WGO). However, not all BNNTs deteriorate even at temperatures as high as 1000 °C. This leads to the formation of a hybrid nanomaterial system comprising of 1D BN nanotubes and 2D BN oxide platelets, potentially having advanced high temperature sensing, radiation shielding, mechanical strengthening, electron emission and thermal management applications due to synergistic improvement of multi-plane transport and mechanical properties. This is the first report on transformation of BNNT bundles to a continuous array of White Graphene Oxide nanoplatelet stacks.

Oxidative Etching of Hexagonal Boron Nitride Toward Nanosheets with Defined Edges and Holes

Lateral surface etching of two-dimensional (2D) nanosheets results in holey 2D nanosheets that have abundant edge atoms. Recent reports on holey graphene showed that holey 2D nanosheets can outperform their intact counterparts in many potential applications such as energy storage, catalysis, sensing, transistors, and molecular transport/separation. From both fundamental and application perspectives, it is desirable to obtain holey 2D nanosheets with defined hole morphology and hole edge structures. This remains a great challenge for graphene and is little explored for other 2D nanomaterials. Here, a facile, controllable, and scalable method is reported to carve geometrically defined pit/hole shapes and edges on hexagonal boron nitride (h-BN) basal plane surfaces via oxidative etching in air using silver nanoparticles as catalysts. The etched h-BN was further purified and exfoliated into nanosheets that inherited the hole/edge structural motifs and, under certain conditions, possess altered optical bandgap properties likely induced by the enriched zigzag edge atoms. This method opens up an exciting approach to further explore the physical and chemical properties of hole- and edge-enriched boron nitride and other 2D nanosheets, paving the way toward applications that can take advantage of their unique structures and performance characteristics.

Dielectric screening in atomically thin boron nitride nanosheets

Two-dimensional (2D) hexagonal boron nitride (BN) nanosheets are excellent dielectric substrate for graphene, molybdenum disulfide, and many other 2D nanomaterial-based electronic and photonic devices. To optimize the performance of these 2D devices, it is essential to understand the dielectric screening properties of BN nanosheets as a function of the thickness. Here, electric force microscopy along with theoretical calculations based on both state-of-the-art first-principles calculations with van der Waals interactions under consideration, and nonlinear Thomas-Fermi theory models are used to investigate the dielectric screening in high-quality BN nanosheets of different thicknesses. It is found that atomically thin BN nanosheets are less effective in electric field screening, but the screening capability of BN shows a relatively weak dependence on the layer thickness.

Boron nitride nanosheets as improved and reusable substrates for gold nanoparticles enabled surface enhanced Raman spectroscopy

Boron nitride nanosheets covered by gold nanoparticles are controllably fabricated for highly-sensitive and reusable substrates for surface enhanced Raman spectroscopy.

High-efficient production of boron nitride nanosheets via an optimized ball milling process for lubrication in oil

Although tailored wet ball milling can be an efficient method to produce a large quantity of two-dimensional nanomaterials, such as boron nitride (BN) nanosheets, milling parameters including milling speed, ball-to-powder ratio, milling ball size and milling agent, are important for optimization of exfoliation efficiency and production yield. In this report, we systematically investigate the effects of different milling parameters on the production of BN nanosheets with benzyl benzoate being used as the milling agent. It is found that small balls of 0.1–0.2 mm in diameter are much more effective in exfoliating BN particles to BN nanosheets. Under the optimum condition, the production yield can be as high as 13.8% and the BN nanosheets are 0.5–1.5 μm in diameter and a few nanometers thick and of relative high crystallinity and chemical purity. The lubrication properties of the BN nanosheets in base oil have also been studied. The tribological tests show that the BN nanosheets can greatly reduce the friction coefficient and wear scar diameter of the base oil.

High-yield synthesis of boron nitride nanoribbons via longitudinal splitting of boron nitride nanotubes by potassium vapor

Boron nitride nanoribbons (BNNRs) are theorized to have interesting electronic and magnetic properties, but their high-yield synthesis remains challenging. Here we demonstrate that potassium-induced splitting of BN nanotubes (BNNTs) is an effective high-yield method to obtain bulk quantities of high-quality BNNRs if a proper precursor material is chosen. The resulting BNNRs are crystalline; many of them have a high aspect ratio and straight parallel edges. We have observed numerous few-layer and monolayer BNNRs; the multilayered ribbons predominantly have an AA' stacking. We present a detailed microscopy study of BNNRs that provides important insights into the mechanism of the formation of BNNRs from BNNTs. We also demonstrate that the BNNTs prepared by different synthetic approaches could exhibit dramatically different reactivities in the potassium splitting reaction, which highlights the need for future comparison studies of BN nanomaterials prepared using different methods to better understand their preparation-dependent physical and chemical properties.

Tuning band gaps of BN nanosheets and nanoribbons via interfacial dihalogen bonding and external electric field

Density functional theory computations with dispersion corrections (DFT-D) were performed to investigate the dihalogen interactions and their effect on the electronic band structures of halogenated (fluorinated and chlorinated) BN bilayers and aligned halogen-passivated zigzag BN nanoribbons (BNNRs). Our results reveal the presence of considerable homo-halogen (FF and ClCl) interactions in bilayer fluoro (chloro)-BN sheets and the aligned F (Cl)-ZBNNRs, as well as substantial hetero-halogen (FCl) interactions in hybrid fluoro-BN/chloro-BN bilayer and F-Cl-ZBNNRs. The existence of interfacial dihalogen interactions leads to significant band-gap modifications for the studied BN nanosystems. Compared with the individual fluoro (chloro)-BN monolayers or pristine BNNRs, the gap reduction in bilayer fluoro-BN (B-FF-N array), hybrid fluoro-BN/chloro-BN bilayer (N-FCl-N array), aligned Cl-ZBNNRs (B-ClCl-N alignment), and hybrid F-Cl-ZBNNRs (B-FCl-N alignment) is mainly due to interfacial polarizations, while the gap narrowing in bilayer chloro-BN (N-ClCl-N array) is ascribed to the interfacial nearly-free-electron states. Moreover, the binding strengths and electronic properties of the interactive BN nanosheets and nanoribbons can be controlled by applying an external electric field, and extensive modulation from large-gap to medium-gap semiconductors, or even metals can be realized by adjusting the direction and strength of the applied electric field. This interesting strategy for band gap control based on weak interactions offers unique opportunities for developing BN nanoscale electronic devices.

Strong oxidation resistance of atomically thin boron nitride nanosheets

Investigation of oxidation resistance of two-dimensional (2D) materials is critical for many of their applications because 2D materials could have higher oxidation kinetics than their bulk counterparts due to predominant surface atoms and structural distortions. In this study, the oxidation behavior of high-quality boron nitride (BN) nanosheets of 1-4 layers thick has been examined by heating in air. Atomic force microscopy and Raman spectroscopy analyses reveal that monolayer BN nanosheets can sustain up to 850 °C, and the starting temperature of oxygen doping/oxidation of BN nanosheets only slightly increases with the increase of nanosheet layer and depends on heating conditions. Elongated etch lines are found on the oxidized monolayer BN nanosheets, suggesting that the BN nanosheets are first cut along the chemisorbed oxygen chains and then the oxidative etching grows perpendicularly to these cut lines. The stronger oxidation resistance of BN nanosheets makes them more preferable for high-temperature applications than graphene.

Porous Boron Nitride with Tunable Pore Size

On the basis of a global structural search and first-principles calculations, we predict two types of porous boron-nitride (BN) networks that can be built up with zigzag BN nanoribbons (BNNRs). The BNNRs are either directly connected with puckered B (N) atoms at the edge (type I) or connected with sp(3)-bonded BN chains (type II). Besides mechanical stability, these materials are predicted to be thermally stable at 1000 K. The porous BN materials entail large surface areas, ranging from 2800 to 4800 m(2)/g. In particular, type-II BN material with relatively large pores is highly favorable for hydrogen storage because the computed hydrogen adsorption energy (-0.18 eV) is very close to the optimal adsorption energy (-0.15 eV) suggested for reversible hydrogen storage at room temperature. Moreover, the type-II materials are semiconductors with width-dependent direct bandgaps, rendering the type-II BN materials promising not only for hydrogen storage but also for optoelectronic and photonic applications.

Continuous growth of hexagonal graphene and boron nitride in-plane heterostructures by atmospheric pressure chemical vapor deposition

Graphene-boron nitride monolayer heterostructures contain adjacent electrically active and insulating regions in a continuous, single-atom thick layer. To date structures were grown at low pressure, resulting in irregular shapes and edge direction, so studies of the graphene-boron nitride interface were restricted to the microscopy of nanodomains. Here we report templated growth of single crystalline hexagonal boron nitride directly from the oriented edge of hexagonal graphene flakes by atmospheric pressure chemical vapor deposition, and physical property measurements that inform the design of in-plane hybrid electronics. Ribbons of boron nitride monolayer were grown from the edge of a graphene template and inherited its crystallographic orientation. The relative sharpness of the interface was tuned through control of growth conditions. Frequent tearing at the graphene-boron nitride interface was observed, so density functional theory was used to determine that the nitrogen-terminated interface was prone to instability during cool down. The electronic functionality of monolayer heterostructures was demonstrated through fabrication of field effect transistors with boron nitride as an in-plane gate dielectric.