Boron Sheets Research Articles

High‐Entropy‐Stabilized Platinum Diborides for Poison‐Resistant Catalysis

AbstractAlloying and solid‐solution formation is a powerful technique that enhances and adds properties through elemental mixing, but unfortunately, some elements simply cannot mix as their chemical nature prevents a thermodynamically stable structure. For example, the inherent nobility of platinum group metals does not favor bond formation and precludes their incorporation into higher (boron‐rich) metal borides. However, we demonstrate that when using five or more constituents, the higher mixing entropy will overcome these chemical limitations and form a stable high‐entropy alloy, demonstrating the formation of new compounds with substituents that are seemingly impossible with a traditional metal alloying approach. The high‐entropy boride (HEB) Al0.2Nb0.2Pt0.2Ta0.2Ti0.2B2 was synthesized, where platinum was forced to occupy a 12‐coordinate site, sandwiched between honeycomb borophene sheets. In addition to the unusual coordination, the boron serves as a poison panacea. Pure platinum is strongly susceptible to sulfur poisoning by adsorption, rendering a platinum catalyst ineffective. Boron is known to be resistant to sulfur poisoning. The boron sheets present in the HEB shield the platinum from sulfur while maintaining high catalytic activity. This is confirmed with the facile hydrogenation of thiol‐containing nitro compounds, where the HEB resists sulfur poisoning while retaining its high catalytic activity.

Bilayer Borophenes Exhibit Silicon‐Like Bandgap and Carrier Mobilities

AbstractBorophene, a boron analogue of graphene, is typically metallic, while all bulk boron phases are insulating. Here, we predict by first‐principles calculations that recently synthesized bilayer borophene, being suggested to be composed of two stacked v1/12 sheets, is a semiconductor with a bandgap of 1.13 eV, almost the same as that of silicon. It is shown that the stacking mode between two boron sheets as well as the density and pattern of the interlayer boron‐boron (B─B) bonds are the key factors for opening the bandgap in the otherwise metallic boron sheet. Moreover, the bilayer borophene exhibits electron mobility of 878.6 cm2 V−1 s−1 and a significantly higher optical absorption coefficient in the visible region than silicon. These excellent electronic and optical properties hosted in a two atom‐thick space, together with high thermal and mechanical stability, position the bilayer borophene as a promising nanomaterial for maintaining the miniaturization of electronics.

Ion Current Rectification Activity Induced by Boron Hydride Nanosheets to Enhance Magnesium Analgesia.

The limited analgesic efficiency of magnesium restricts its application in pain management. Here, we report boron hydride (BH) with ion currents rectification activity that can enhance the analgesic efficiency of magnesium without the risks of drug tolerance or addiction. We synthesize MgB2, comprising hexagonal boron sheets alternating with Mg2+. In pathological environment, Mg2+ is exchanged by H+, forming two-dimensional borophene-analogue BH sheets. BH interacts with the charged cations via cation-pi interaction, leading to dynamic modulation of sodium and potassium ion currents around neurons. Additionally, released Mg2+ competes Ca2+ to inhibit its influx and neuronal excitation. In vitro cultured dorsal root neurons show a remarkable increase in threshold potential from the normal -35.9 mV to -5.9 mV after the addition of MgB2, indicating potent analgesic effect. In three typical pain models, including CFA-induced inflammatory pain, CINP- or CCI-induced neuropathic pain, MgB2 exhibits analgesic efficiency approximately 2.23, 3.20, and 2.0 times higher than clinical MgSO4, respectively, and even about 1.04, 1.66, and 1.95 times higher than morphine, respectively. The development of magnesium based intermetallic compounds holds promise in addressing the non-opioid medical need for pain relief.

Ion Current Rectification Activity Induced by Boron Hydride Nanosheets to Enhance Magnesium Analgesia

AbstractThe limited analgesic efficiency of magnesium restricts its application in pain management. Here, we report boron hydride (BH) with ion currents rectification activity that can enhance the analgesic efficiency of magnesium without the risks of drug tolerance or addiction. We synthesize MgB2, comprising hexagonal boron sheets alternating with Mg2+. In pathological environment, Mg2+ is exchanged by H+, forming two‐dimensional borophene‐analogue BH sheets. BH interacts with the charged cations via cation‐pi interaction, leading to dynamic modulation of sodium and potassium ion currents around neurons. Additionally, released Mg2+ competes Ca2+ to inhibit its influx and neuronal excitation. In vitro cultured dorsal root neurons show a remarkable increase in threshold potential from the normal −35.9 mV to −5.9 mV after the addition of MgB2, indicating potent analgesic effect. In three typical pain models, including CFA‐induced inflammatory pain, CINP‐ or CCI‐induced neuropathic pain, MgB2 exhibits analgesic efficiency approximately 2.23, 3.20, and 2.0 times higher than clinical MgSO4, respectively, and even about 1.04, 1.66, and 1.95 times higher than morphine, respectively. The development of magnesium based intermetallic compounds holds promise in addressing the non‐opioid medical need for pain relief.

Structure and exfoliation mechanism of two-dimensional boron nanosheets

Exfoliation of two-dimensional (2D) nanosheets from three-dimensional (3D) non-layered, non-van der Waals crystals represents an emerging strategy for materials engineering that could significantly increase the library of 2D materials. Yet, the exfoliation mechanism in which nanosheets are derived from crystals that are not intrinsically layered remains unclear. Here, we show that planar defects in the starting 3D boron material promote the exfoliation of 2D boron sheets—by combining liquid-phase exfoliation, aberration-corrected scanning transmission electron microscopy, Raman spectroscopy, and density functional theory calculations. We demonstrate that 2D boron nanosheets consist of a planar arrangement of icosahedral sub-units cleaved along the {001} planes of β-rhombohedral boron. Correspondingly, intrinsic stacking faults in 3D boron form parallel layers of faulted planes in the same orientation as the exfoliated nanosheets, reducing the {001} cleavage energy. Planar defects represent a potential engineerable pathway for exfoliating 2D sheets from 3D boron and, more broadly, the other covalently bonded materials.

Computing Molecular Descriptors of Boron Icosahedral sheet

AbstractBoron, the element which is in close proximity with carbon in the periodic table has been hypothesized to produce diverse two‐dimensional structures that differ from well‐studied 2D materials. Icosahedron , the monomers of which form the crystalline boron structure, appears as a recurring pattern in the chemistry of boron compounds, elemental boron and boron‐rich materials, functions as a building block in a majority of boron allotropes. The topological study of the icosahedral sheets is a necessary field to be investigated for the effective analysis and characterization of its physico‐chemical attributes without undertaking time‐consuming experimental research. The topological descriptors being expressed as numerical values enable scientists to compare and correlate data, which can be applied to quantitative structure‐activity relationships and quantitative structure‐property relationships modeling. In this study, various degree based molecular descriptors of the crystal boron icosahedral sheet has been formulated from a two‐dimensional lattice perspective using the ‐polynomial approach. To establish the significance of these descriptors in the study of chemical attributes, an efficient linear regression modeling has also been performed, enabling the prediction of properties of several other boron sheets.

Borophene: Synthesis, Chemistry, and Electronic Properties.

As a neighbor of carbon in the periodic table, boron exhibits versatile structural and electronic configurations, with its allotropes predicted to possess intriguing structures and properties. Since the experimental realization of two-dimensional (2D) boron sheets (borophene) on Ag(111) substrates in 2015, the experimental study of the realization and characteristics of borophene has drawn increasing interest. In this review, we summarize the synthesis and properties of borophene, which are mainly based on experimental results. First, the synthesis of borophene on different substrates, as well as borophane and bilayer borophene, featuring unique phases and properties, are discussed. Next, the chemistry of borophene, such as oxidation, hydrogenation, and its integration into heterostructures with other materials, is summarized. We also mention a few works focused on the physical properties of borophene, specifically its electronic properties. Lastly, the brief outlook addresses challenges toward practical applications of borophene and possible solutions.

Enhanced nonlinear optical responses of doped superalkali metal salts with fluorinated/non-fluorinated planar boron sheets: A theoretical study

Enhanced nonlinear optical responses of doped superalkali metal salts with fluorinated/non-fluorinated planar boron sheets: A theoretical study

Structures and Stabilities of Two-Dimensional Boron Sheets with Point Defects

Structures and Stabilities of Two-Dimensional Boron Sheets with Point Defects

Theoretical Study on the Regioselectivity of Leapfrog B18 and B30 Boron Sheets in Electrophilic and Nucleophilic Reactions Using DFT-Based Reactivity Indices.

A combined computational and interpretational DFT study is performed to investigate the regioselectivity of B18 and B30 leapfrog boron sheets upon reaction with XH3-type electrophiles and nucleophiles (X = N, P, As, B, Al). The M062X, B3LYP, and B3LYP-D3 functionals are used combined with the 6-31+G(d,p) basis. The molecular electrostatic potential (MEP), Fukui functions, and the dual descriptor are employed to predict the local reactivity of B18 and B30. Our results reveal that both clusters are hard and prefer to react with hard bases and acids, such as NH3 and BH3. Further, these leapfrog B6n clusters can play the role of catalysts as they break B-H and Al-H bonds of BH3 and AlH3 in s-BH3-B6n and s-AlH3-B6n complexes, respectively. Leapfrog B6n-XH3 complexes (X = B and Al) can be considered as an interaction between two electron-deficient systems. Therefore, the chemical reactivity between these systems cannot be interpreted in terms of the Hard-Soft-Acid-Base principle.

Structural Characterization of Boron Sheets beyond the Monolayer and Implication for Experimental Synthesis and Identification.

The successful synthesis of quasi-freestanding bilayer borophene has aroused much attention for its superior physical properties and holds great promise for future electronic devices. Herein, we comprehensively explore six boron sheets beyond the monolayer and structurally characterize them via various methods using first-principles calculations for experimental references. On the basis of atomic models of borophenes, simulated scanning tunneling microscope (STM) images show different morphologies at different bias voltages and are explained by the partial densities of states and the height differences in the vertical direction. Simulated transmission electron microscope images further probe the internal atomic arrangement of boron sheets and compensate for the shortcomings of STM images to better distinguish different phases of boron sheets. The interlayer coupling strength is stronger in bilayer borophenes than in the three-layer system via the electron localization function and Mulliken bond population. In addition, simulated X-ray diffraction and infrared spectra show different characteristic peaks and corresponding vibrational modes to further characterize these boron sheets. These theoretical results can decrease the prime cost and provide vital guidance for the experimental synthesis and identification of boron sheets beyond the monolayer.

New insights into the mechanisms of TiB[formula omitted](001) thermal oxidation combining molecular dynamics and density functional theory calculations

Using reactive force field molecular dynamics and Density Functional Theory, this study unravels the early stages of TiB2 oxidation. Overall, it is found that the TiB2 boron sheets hinder the oxidation until a temperature of 800 K is reached, above which a complex sequence of chemical mechanisms leads to the formation of a titanium oxide at the top surface and agglomeration of pure boron underneath, preventing further TiO2 growth below 900 K. It is demonstrated that titanium oxidation is possible after the softening and distortion of boron sheets due to oxygen adsorbate presence; mostly BO radicals, no B2O3-like structure is formed. The local distortions of boron aromatic rings allow oxygen insertion into the subsurface, which is followed by titanium extraction and migration towards the outer surface in contact with the molecular oxygen atmosphere. Mechanistic details of these complex extraction–oxidation processes and their relation to cooperative oxygen atoms and molecules are detailed in terms of pathways and corresponding energetics. Interestingly different reaction stages are identified, that do not exceed 2 eV activation. The proposed hierarchical scenario of oxidation of titanium and boron in TiB2 might benefit to the general understanding of metal diboride oxidation.

Exploring the hardness and high-pressure behavior of osmium and ruthenium-doped rhenium diboride solid solutions

Rhenium diboride (ReB2) exhibits high differential strain due to its puckered boron sheets that impede shear deformation. Here, we demonstrate the use of solid solution formation to enhance the Vickers hardness and differential strain of ReB2. ReB2-structured solid solutions (Re0.98Os0.02B2 and Re0.98Ru0.02B2, noted as “ReOsB2” and “ReRuB2”) were synthesized via arc-melting from the pure elements. In-situ high-pressure radial x-ray diffraction was performed in the diamond anvil cell to study the incompressibility and lattice strain of ReOsB2 and ReRuB2 up to ∼56 GPa. Both solid solutions exhibit higher incompressibility and differential strain than pure ReB2. However, while all lattice planes are strengthened by doping osmium (Os) into the ReB2 structure, only the weakest ReB2 lattice plane is enhanced with ruthenium (Ru). These results are in agreement with the Vickers hardness measurements of the two systems, where higher hardness was observed in ReOsB2. The combination of high-pressure studies with experimentally observed hardness data provides lattice specific information about the strengthening mechanisms behind the intrinsic hardness enhancement of the ReB2 system.

Unlocking the potential of hexagonal boron sheets: Giant improvements in thermal conductivity and mechanics through molybdenum intercalation

A new two-dimensional (2D) material, MoB4, known as a Dirac material, has been found to possess exceptional electronic properties and promising thermal and mechanical characteristics. Our calculations reveal that MoB4 has a remarkable Young's modulus of 384 N/m, surpassing that of graphene. Additionally, we predict that MoB4 exhibits a thermal conductivity of 461 W/mK at room temperature. This is a significant improvement when compared to a single boron sheet in a honeycomb structure and MoB2, showing 32 and 8 times higher thermal conductivity, respectively. This enhancement in thermal conductivity is attributed to the suppression of anharmonicity and scattering phase space, along with an increased group velocity in MoB4.

Borophene molecular plasmons

The two-dimensional boron sheet known as borophene is a potential monolayer metal with exceptional optical characteristics. In this work, using time-dependent density functional theory (TDDFT), we examine the optical and plasmonic characteristics of borophene chains. We show that the plasmons in borophene chains are size-dependent and tunable on demand. We identify molecular plasmons based on our analysis of the collectivity of the electron-hole transitions, the transition contribution map, and the dipolar oscillation of the induced charge density. The molecular plasmons in borophene chains open wide possibilities for their applications in next generation optical devices.

Two-dimensional Be2Al and Be2Ga monolayer: anti-van't Hoff/Le Bel planar hexacoordinate bonding and superconductivity.

Because of the electron deficiency of boron, a triangular network with planar hexacoordination is the most common structural and bonding property for isolated boron clusters and two-dimensional (2D) boron sheets. However, this network is a rule-breaking structure and bonding case for all other main-group elements. Herein, the Be2M (M = Al and Ga) 2D monolayer with P6/mmm space group was found to be the lowest-energy structure with planar hexacoordinate Be/Al/Ga motifs. More interestingly, Be2Al and Be2Ga were observed to be intrinsic phonon-mediated superconductors with a superconducting critical temperature (Tc) of 5.9 and 3.6 K, respectively, where compressive strain could further enhance their Tc. The high thermochemical and kinetic stability of Be2M make a promising candidate for experimental realization, considering its high cohesive energy, absence of soft phonon modes, and good resistance to high temperature. Moreover, the feasibility of directly growing Be2M on the electride Ca2N substrate was further demonstrated, where its intriguing electronic and superconducting properties were well maintained in comparison with the freestanding monolayer. The Be2M monolayer with rule-breaking planar hexacoordinate motifs firmly pushes the ultimate connection of the "anti-van't Hoff/Le Bel" structure with promising physical properties.

Enhanced electron transport and self-similarity in quasiperiodic borophene nanoribbons with line defects.

Recent experiments have revealed multiple borophene phases of distinct lattice structures, suggesting that the unit cells of ν1/6 and ν1/5 boron sheets, namely α and β chains, serve as building blocks to assemble into novel borophene phases. Motivated by these experiments, we present a theoretical study of electron transport along two-terminal quasiperiodic borophene nanoribbons (BNRs), with the arrangement of the α and β chains following the generalized Fibonacci sequence. Our results indicate that the energy spectrum of these quasiperiodic BNRs is multifractal and characterized by numerous transmission peaks. In contrast to the Fibonacci model that all the electronic states should be critical, both delocalized and critical states appear in the quasiperiodic BNRs, where the averaged resistance saturates at the inverse of one conductance quantum for the delocalized states in the large length limit and contrarily exhibits a power-law dependence on the nanoribbon length for the critical states. Besides, the self-similarity is observed from the transmission spectrum, where the conductance curves overlap at different energy regions of two quasiperiodic BNRs of different Fibonacci indices and the resistance curves are analogous to each other at different energy scales of a single quasiperiodic BNR. These results complement previous studies on quasiperiodic systems where the multifractal energy spectrum and the self-similarity are observed by generating quasiperiodic potential energies, suggesting that borophene may provide an intriguing platform for understanding the structure-property relationships and exploring the physical properties of quasiperiodic systems.

Sodium-Stabilized Hexagonal Borophene: Structure, Stability, and Electronic and Mechanical Properties

The crystalline form of sodium-doped hexagonal borophene (B2Na2) has been studied using DFT calculations. The calculations predict the dynamic stability of B2Na2 whose structure is a flat honeycomb boron sheet sandwiched between two sodium layers. According to estimated electronic and mechanical properties, B2Na2 is a rather soft material with metallic characteristics. Evaluation of thermal stability by the molecular dynamics method indicates sufficient stability of the predicted material, which makes it possible to observe it experimentally at temperatures below 200 K.

Porous-Induced Performance Enhancement of Flat Boron Sheets for Lithium-Ion Batteries

Low-capacity anode materials have been limiting the further take-off of lithium-ion batteries (LIBs), and many researchers are relentlessly pursuing breakthroughs in high-capacity anode materials. Therefore, the properties of materials with a high theoretical capacity and the reasons behind them are worthy of further study. Here, based on the first-principles calculation, we systematically investigate the electrochemical properties of the six flat boron sheets with different hexagonal hole densities η (η = 1/4, 1/5, 1/6, 1/7, 1/8, 1/9), including adsorption, diffusion, theoretical capacity, and open-circuit voltage. We find that due to their own structural and electronic properties, the Li atoms tend to be adsorbed on their hexagonal hole sites, which result in the formation of structures of flat boron sheets with more hexagonal holes that have a higher theoretical capacity for lithium. Besides, their theoretical capacities can be calculated using the empirical equation, with the hexagonal hole densities (η). Finally, we obtain their capacities with increasing hexagonal hole density (η), which are 620, 708, 826, 992, 1240, and 1653 mA h g–1, respectively. Therefore, our results indicate that the porous structure of the anode material is beneficial to improve the storage capacity performance of LIBs.

Substrate-Mediated Borophane Polymorphs through Hydrogenation of Two-Dimensional Boron Sheets.

The two-dimensional boron monolayer (borophene) stands out from the two-dimensional atomic layered materials due to its structural flexibility and tunable electronic and mechanical properties from a large number of allotropic materials. The stability of pristine borophene polymorphs could possibly be improved via hydrogenation with atomic hydrogen (referred to as borophane). However, the precise adsorption structures and the underlying mechanism are still elusive. Employing first-principles calculations, we demonstrate the optimal configurations of freestanding borophanes and the ones grown on metallic substrates. For freestanding borophenes, the energetically favored hydrogen adsorption sites are sensitive to the polymorphs and corresponding coordination numbers of boron atoms. With various metal substrates, the hydrogenation configurations of borophenes are modulated significantly, attributed to the overlap between B pz and H s orbitals. These findings provide a deep insight into the hydrogenating borophenes and facilitate the stabilization of two-dimensional boron polymorphs by engineering hydrogen adsorption sites and concentrations.