Porous Boron Nitride with Tunable Pore Size

Boosting the Cycle Performance of Iron Trifluoride Based Solid State Batteries at Elevated Temperatures by Engineering the Cathode Solid Electrolyte Interface

Iron trifluoride (FeF3) is attracting tremendous interest due to its lower cost and the possibility to enable higher energy density in lithium-ion batteries. However, its cycle performance deteriorates rapidly in less than 50 cycles at elevated temperatures due to cracking of the unstable cathode solid electrolyte interface (CEI) followed by active materials dissolution in liquid electrolyte. Herein, by engineering the salt composition, the Fe3O4-type CEI with the doping of boron (B) atoms in a polymer electrolyte at 60 °C is successfully stabilized. The cycle life of the well-designed FeF3-based composite cathode exceeds an unprecedented 1000 cycles and utilizes up to 70% of its theoretical capacities. Advanced electron microscopy combined with density functional theory (DFT) calculations reveal that the B in lithium salt migrates into the cathode and promotes the formation of an elastic and mechanic robust boron-contained CEI (BOR-CEI) during cycling, by which the durability of the CEI to frequent cyclic large volume changes is significantly enhanced. To this end, the notorious active materials dissolution is largely prohibited, resulting in a superior cycle life. The results suggest that engineering the CEI such as tuning its composition is a viable approach to achieving FeF3 cathode-based batteries with enhanced performance.

Nanostructured Carbon-Doped BN for CO2 Capture Applications

Carbon-doped boron nitride (denoted by BN/C) was prepared through the pyrolysis at 1100 °C of a nanostructured mixture of an alkyl amine borane adduct and ammonia borane. The alkyl amine borane adduct acts as a soft template to obtain nanospheres. This bottom-up approach for the synthesis of nanostructured BN/C is relatively simple and compelling. It allows the structure obtained during the emulsion process to be kept. The final BN/C materials are microporous, with interconnected pores in the nanometer range (0.8 nm), a large specific surface area of up to 767 m2·g-1 and a pore volume of 0.32 cm3·g-1. The gas sorption studied with CO2 demonstrated an appealing uptake of 3.43 mmol·g-1 at 0 °C, a high CO2/N2 selectivity (21) and 99% recyclability after up to five adsorption-desorption cycles.

How to Tailor Porous Boron Nitride Properties for Applications in Interfacial Processes

The research of new porous materials for applications in interfacial processes is key to addressing global energy and sustainability challenges. For example, porous materials can be used to store fuels such as hydrogen or methane or to separate chemical mixtures reducing the energy currently required by thermal separation processes. Their catalytic properties can be exploited to convert adsorbed molecules into valuable or less hazardous chemicals, thereby reducing energy consumption or pollutants emissions. Porous boron nitride (BN) has appeared as a promising material for applications in molecular separations, gas storage, and catalysis owing to its high surface area and thermal stability, as well as its tunable physical properties and chemistry. However, the production of porous BN is still limited to the laboratory scale, and its formation mechanism, as well as ways to control porosity and chemistry, are yet to be fully understood. In addition, studies have pointed toward the instability of porous BN materials when exposed to humidity, which could significantly impact performance in industrial applications. Studies on porous BN performance and recyclability when employed in adsorption, gas storage, and catalysis remain limited, despite encouraging preliminary studies. Moreover, porous BN powder must be shaped into macrostructures (e.g., pellets) to be used commercially. However, common methods to shape porous materials into macrostructures often cause a reduction in the surface area and/or mechanical strength. In recent years, research groups, including ours, have started addressing the challenges discussed above. Herein, we summarize our collective findings through a selection of key studies. First, we discuss the chemistry and structure of BN, clarifying confusion around terminology and discussing the hydrolytic instability of the material in relation to its structure and chemistry. We demonstrate a way to reduce the instability in water while still maintaining high specific surface area. We propose a mechanism for the formation of porous BN and discuss the effects of different synthesis parameters on the structure and chemistry of porous BN, therefore providing a way to tune its properties for selected applications. While the syntheses covered often lead to a powder product, we also present ways to shape porous BN powders into macrostructures while still maintaining high accessible surface area for interfacial processes. Finally, we evaluate porous BN performance for chemical separations, gas storage, and catalysis. While the above highlights key advances in the field, further work is needed to allow deployment of porous BN. Specifically, we suggest evaluating its hydrolytic stability, refining the ways to shape the material into stable and reproducible macrostructures, establishing clear design rules to produce BN with specific chemistry and porosity, and, finally, providing standardized test procedures to evaluate porous BN catalytic and sorptive properties to facilitate comparison.

A New BCN Compound with Monoclinic Symmetry: First-Principle Calculations

In this study, we predicted and investigated a new light-element compound B-C-N in Pm phase, denoted as Pm-BCN, using density functional theory. Pm-BCN is mechanically, dynamically, and thermodynamically stable. The elastic moduli of Pm-BCN are larger than those of other B-C-N and light-element compounds, such as P2₁3 BN, B₂C₃, P4/m BN, Pnc2 BN, and dz4 BN. By studying the mechanical anisotropy of elastic moduli, we proved that Pm-BCN is a mechanically anisotropic material. In addition, the shear anisotropy factors A₂ and A_Ba of Pm-BCN are smaller than those of the seven B-C-N compounds mentioned in this paper. Pm-BCN is a semiconductor material with an indirect and wide band gap, suggesting that Pm-BCN can be applied in microelectronic devices.

3-X Structural Model and Common Characteristics of Anomalous Thermal Transport: The Case of Two-Dimensional Boron Carbides

Improving the reliability of electronic devices requires effective heat management, and the key is the relationship between the thermal transport and temperature. Inspired by synthesized T-carbon and H-boron, the 3-X structural models are proposed to unify the two-dimensional (2D) multitriangle materials. Employing structural searches, we identify the stability of the 3-X configuration in 2D boron carbides as 3-9 BC₃ monolayer, which, unexpectedly, exhibits a linear thermal conductivity versus temperature, not the traditional ∼1/T trend. We summarize the common characteristics and explore why this behavior is absent in 3-9 AlC₃ and graphene via investigating the optical modes. We show that the linear behavior is a direct consequence of the special oscillation modes by the 3-X model associated with the largest group velocity. We find that 2D materials with such behavior usually share a relatively low thermal conductivity. Our work paves the way to deeply understand the lattice thermal transport and to widen nanoelectronic applications.

New boron nitride monolith phases from high-pressure compression of double-walled boron nitride nanotubes

Pressure-induced phase transition of boron nitride nanotubes (BNNTs) provides an effective approach to develop new boron nitride nanostructures with more desirable functions than those of carbon nanotubes, owing to the unique polar B-N bonds. However, the synthetic BNNTs usually comprise double- or multi-walls, whose structural evolution under pressure is complicated and remains largely elusive. Here, we unveil the complete phase transition behavior of hexagonal bundles of double-walled (DW) BNNTs of different chirality and diameters under hydrostatic pressures of up to 60 GPa. A series of new monolith phases are obtained from the compressed DW-BNNT bundles, whose structures can be well retained even after releasing the pressure. The bonding characters; electronic, optical, and mechanical properties; and Raman signature of these monolith phases are elucidated, which provide essential guidance for synthesis of new boron nitride materials with unprecedented properties for technological applications.

Novel III-V Nitride Polymorphs in the P42/mnm and Pbca Phases

In this work, the elastic anisotropy, mechanical stability, and electronic properties for P4₂/mnm XN (XN = BN, AlN, GaN, and InN) and Pbca XN are researched based on density functional theory. Here, the XN in the P4₂/mnm and Pbca phases have a mechanic stability and dynamic stability. Compared with the Pnma phase and Pm-3n phase, the P4₂/mnm and Pbca phases have greater values of bulk modulus and shear modulus. The ratio of the bulk modulus (B), shear modulus (G), and Poisson’s ratio (v) of XN in the P4₂/mnm and Pbca phases are smaller than those for Pnma XN and Pm-3n XN, and larger than those for c-XN, indicating that Pnma XN and Pm-3n XN are more ductile than P4₂/mnm XN and Pbca XN, and that c-XN is more brittle than P4₂/mnm XN and Pbca XN. In addition, in the Pbca phases, XN can be considered a semiconductor material, while in the P4₂/mnm phase, GaN and InN have direct band-gap, and BN and AlN are indirect wide band gap materials. The novel III-V nitride polymorphs in the P4₂/mnm and Pbca phases may have great potential for application in visible light detectors, ultraviolet detectors, infrared detectors, and light-emitting diodes.

Novel porous aluminum nitride monolayer: a first-principles study

Using ab initio calculations within the density functional theory, we explored the possible structures and properties of porous AlN monolayer materials. Two kinds of porous AlN monolayers (H- and T-) are identified. The phonon dispersion spectra together with the ab initio molecular dynamics simulations demonstrate that these structures are stable. We further show that the H- and T-AlN porous monolayers have well-defined porous nanostructures and high specific surface areas of 2863 m² g^-1 and 2615 m² g^-1 respectively, which is comparable to graphene (2630 m² g^-1), and can be maintained stably at high temperatures (>1300 K). Furthermore, both porous monolayers exhibit semiconductor properties, with indirect band gaps of 2.89 eV and 2.86 eV respectively. In addition, the electronic structures of the porous monolayers can be modulated by strain. The band gap of porous T-AlN monolayer experiences an indirect-direct transition when biaxial strain is applied. A moderate  -9% compression can trigger this gap transition. These results indicate that porous AlN monolayers may potentially be used in future optoelectronic and catalyst applications.

Three metallic BN polymorphs: 1D multi-threaded conduction in a 3D network

In this paper, three novel metallic sp2/sp3-hybridized boron nitride (BN) polymorphs are proposed by first-principles calculations. One of them, denoted as tP-BN, is predicted based on the evolutionary particle swarm structural search. tP-BN is composed of two interlocked rings forming a tube-like 3D network. The stability and band structure calculations show that tP-BN is metastable and metallic at zero pressure. Calculations for the density of states and electron orbitals confirm that the metallicity originates from the sp2-hybridized B and N atoms, forming 1D linear conductive channels in the 3D network. According to the relationship between the atomic structure and electronic properties, another two 3D metastable metallic sp2/sp3-hybridized BN structures are constructed manually. Electronic property calculations show that both of these structures have 1D conductive channels along different axes. The polymorphs predicted in this study enrich the structures and provide a different picture of the conductive mechanism of BN compounds.

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).

Low-energy 3D sp2 carbons with versatile properties beyond graphite and graphene

Low-energy sp²-carbons with metallicity, graphene-like Dirac cone, rubber-like ultra-stretchability, and negative Poisson's ratio are theoretically designed from graphene nanoribbons.

Multiporous sp2-hybridized boron nitride (d-BN): stability, mechanical properties, lattice thermal conductivity and promising application in energy storage

sp²-hybridized multiporous d-BN is used for solid electrolytes in lithium–sulfur batteries.

Pnma-BN: Another Boron Nitride Polymorph with Interesting Physical Properties

Structural, mechanical, electronic properties, and stability of boron nitride (BN) in Pnma structure were studied using first-principles calculations by Cambridge Serial Total Energy Package (CASTEP) plane-wave code, and the calculations were performed with the local density approximation and generalized gradient approximation in the form of Perdew-Burke-Ernzerhof. This BN, called Pnma-BN, contains four boron atoms and four nitrogen atoms buckled through sp³-hybridized bonds in an orthorhombic symmetry unit cell with Space group of Pnma. Pnma-BN is energetically stable, mechanically stable, and dynamically stable at ambient pressure and high pressure. The calculated Pugh ratio and Poisson's ratio revealed that Pnma-BN is brittle, and Pnma-BN is found to turn brittle to ductile (~94 GPa) in this pressure range. It shows a higher mechanical anisotropy in Poisson's ratio, shear modulus, Young's modulus, and the universal elastic anisotropy index A^U. Band structure calculations indicate that Pnma-BN is an insulator with indirect band gap of 7.18 eV. The most extraordinary thing is that the band gap increases first and then decreases with the increase of pressure from 0 to 60 GPa, and from 60 to 100 GPa, the band gap increases first and then decreases again.

Evidencing the existence of exciting half-metallicity in two-dimensional TiCl3 and VCl3 sheets

Half-metallicity combined with wide half-metallic gap, unique ferromagnetic character and high Curie temperature has become a key driving force to develop next-generation spintronic devices. In previous studies, such half-metallicity always occurred under certain manipulation. Here, we, via examining a series of two-dimensional transition-metal trichlorides, evidenced that TiCl₃ and VCl₃ sheets could display exciting half-metallicity without involving any external modification. Calculated half-metallic band-gaps for TiCl₃ and VCl₃ sheets are about 0.60 and 1.10 eV, respectively. Magnetic coupled calculation shows that both sheets favor the ferromagnetic order with a substantial collective character. Estimated Curie temperatures can be up to 376 and 425 K for TiCl₃ and VCl₃ sheets, respectively. All of these results successfully disclose two new promising two-dimensional half-metallic materials toward the application of next-generation paper-like spintronic devices.

Functionalized hexagonal boron nitride nanomaterials: emerging properties and applications

Chemical and physical functionalization of hexagonal boron nitride materials breeds new properties and applications.

A novel two-dimensional MgB6 crystal: metal-layer stabilized boron kagome lattice

Based on first-principles calculations, we designed for the first time a boron-kagome-based two-dimensional MgB₆ crystal, in which two boron kagome layers sandwich a triangular magnesium layer.

AlxC Monolayer Sheets: Two-Dimensional Networks with Planar Tetracoordinate Carbon and Potential Applications as Donor Materials in Solar Cell

We perform a global search of the most stable structures of 2D stoichiometric AlxC (x = 1/3, 1, 2, and 3) monolayer sheets. In the most stable 2D planar AlC network, every carbon atom is tetracoordinated. In addition to the structure of AlC, structures of the most stable Al2C and Al3C monolayer sheets are also predicted for the first time. AlC and Al2C monolayers are semiconducting, while Al3C monolayer is metallic. In particular, Al2C monolayer possesses a bandgap of 1.05 eV (based on HSE06 calculation), a value suitable for photovoltaic applications. Moreover, three Al2C/WSe2, Al2C/MoTe2, and AlC/ZnO van der Waals heterobilayers are investigated, and their power conversion efficiencies are estimated to be in the range of 12-18%. The near-perfect match in lattice constants between the Al2C monolayer and PdO (100) surface suggests strong likelihood of experimental realization of the Al2C monolayer on the PdO (100) substrate.