Hybrid Density Functional and Molecular Dynamics Study of Promising Hydrogen Storage Materials: Double Metal Amidoboranes and Metal Amidoborane Ammoniates

In this paper, the recently synthesized materials, Na₂Mg­(NH₂BH₃)₄, NaLi­(NH₂BH₃)₂, Mg­(NH₂BH₃)₂·NH₃, and Ca­(NH₂BH₃)₂·2NH₃, are studied, which were found suitable for hydrogen storage applications. The hybrid density exchange-correlation functional is employed to explore the structural and electronic properties of these materials using the first-principles method on the basis of density functional theory calculations. From the detailed study of electronic structures, it is found that the mixed cation amidoboranes and [NH₃] molecules play an important role in the dehydrogenation process. Bader charge analysis is performed to show the charge distribution between the constituent atoms. The corresponding electron densities and related properties of these compounds are also calculated. Furthermore, ab initio molecular dynamics simulations are performed to study the diffusion of hydrogen in these compounds, which consist of boron, nitrogen, and hydrogen atoms. The common features of these compounds are also revealed by explaining the electronic properties. Finally, it is shown that the mobility of hydrogen in Na₂Mg­(NH₂BH₃)₄ and NaLi­(NH₂BH₃)₂ is slightly higher than that in Mg­(NH₂BH₃)₂·NH₃ and Ca­(NH₂BH₃)₂·2NH₃ at the same temperatures

Divulging the Hidden Capacity and Sodiation Kinetics of Na_<i>x</i>C₆Cl₄O₂: A High Voltage Organic Cathode for Sodium Rechargeable Batteries

In the current emerging sustainable organic battery field, quinones are seen as one of the prime candidates for application in rechargeable battery electrodes. Recently, C₆Cl₄O₂, a modified quinone, has been proposed as a high voltage organic cathode. However, the sodium insertion mechanism behind the cell reaction remained unclear due to the nescience of the right crystal structure. Here, the framework of the density functional theory (DFT) together with an evolutionary algorithm was employed to elucidate the crystal structures of the compounds Na_<i>x</i>C₆Cl₄O₂ (<i>x</i> = 0.5, 1.0, 1.5 and 2). Along with the usefulness of PBE functional to reflect the experimental potential, also the importance of the hybrid functional to divulge the hidden theoretical capacity is evaluated. We showed that the experimentally observed lower specific capacity is a result of the great stabilization of the intermediate phase Na_1.5C₆Cl₄O₂. The calculated activation barriers for the ionic hops are 0.68, 0.40, and 0.31 eV, respectively, for NaC₆Cl₄O₂, Na_1.5C₆Cl₄O₂, and Na₂C₆Cl₄O₂. These results indicate that the kinetic process must not be a limiting factor upon Na insertion. Finally, the correct prediction of the specific capacity has confirmed that the theoretical strategy used, employing evolutionary simulations together with the hybrid functional framework, can rightly model the thermodynamic process in organic electrode compounds

Superhard Semiconducting Phase of Osmium Tetraboride Stabilizing at 11 GPa

Employing a systematic first-principles investigation with crystal structure searching based on an evolutionary algorithm, we have uncovered the novel phase (<i>P</i>4₂/<i>nmc</i>) of OsB₄ with a novel superhardness and semiconducting state. In this investigation, metal-to-semiconductor phase transition is predicted at only a few gigapascals above ambient pressure, i.e., 11 GPa. As a result, the <i>P</i>4₂/<i>nmc</i> phase should potentially become a metastable phase at ambient pressure. The Vickers (polycrystalline) hardness and the band gap of the semiconducting phase are calculated to be 60 GPa and 2.90 eV, respectively. These findings indicate that the <i>P</i>4₂/<i>nmc</i> phase might be a promising superhard-semiconducting material which could be used in cutting and drilling tools, material coating, and other advanced optical technologies. Moreover, under further compression up to 300 GPa, the semiconducting phase transforms into a metallic <i>P</i>6₃/<i>mmc</i> phase at 134 GPa, and then another predicted metallic phase with a <i>Cmca</i> symmetry emerges beyond 270 GPa. Both dynamic and elastic stabilities are fully investigated to ensure the existence of the predicted phases

Scrupulous Probing of Bifunctional Catalytic Activity of Borophene Monolayer: Mapping Reaction Coordinate with Charge Transfer

We have envisaged the hydrogen evolution and oxygen evolution reactions (HER and OER) on two-dimensional (2D) noble metal free borophene monolayer based on first-principles electronic structure calculations. We have investigated the effect of Ti functionalization on borophene monolayer from the perspective of HER and OER activities enhancement. We have probed the activities based on the reaction coordinate, which is conceptually related to the adsorption free energies of the intermediates of HER and OER, as well as from the vibrational frequency analysis with the corresponding charge transfer mechanism between the surface and the adsorbate. Ti-functionalized borophene has emerged as a promising material for HER and OER mechanisms. We believe that our probing method, based on reaction coordinate coupled with vibrational analysis that has been validated by the charge transfer mechanism, would certainly become as a robust prediction route for HER and OER mechanisms in coming days

Borophane as a Benchmate of Graphene: A Potential 2D Material for Anode of Li and Na-Ion Batteries

Borophene, single atomic-layer sheet of boron (Science 2015, 350, 1513), is a rather new entrant into the burgeoning class of 2D materials. Borophene exhibits anisotropic metallic properties whereas its hydrogenated counterpart borophane is reported to be a gapless Dirac material lying on the same bench with the celebrated graphene. Interestingly, this transition of borophane also rendered stability to it considering the fact that borophene was synthesized under ultrahigh vacuum conditions on a metallic (Ag) substrate. On the basis of first-principles density functional theory computations, we have investigated the possibilities of borophane as a potential Li/Na-ion battery anode material. We obtained a binding energy of −2.58 (−1.08 eV) eV for Li (Na)-adatom on borophane and Bader charge analysis revealed that Li­(Na) atom exists in Li⁺(Na⁺) state. Further, on binding with Li/Na, borophane exhibited metallic properties as evidenced by the electronic band structure. We found that diffusion pathways for Li/Na on the borophane surface are anisotropic with <i>x</i> direction being the favorable one with a barrier of 0.27 and 0.09 eV, respectively. While assessing the Li-ion anode performance, we estimated that the maximum Li content is Li_0.445B₂H₂, which gives rises to a material with a maximum theoretical specific capacity of 504 mAh/g together with an average voltage of 0.43 V versus Li/Li⁺. Likewise, for Na-ion the maximum theoretical capacity and average voltage were estimated to be 504 mAh/g and 0.03 V versus Na/Na⁺, respectively. These findings unambiguously suggest that borophane can be a potential addition to the map of Li and Na-ion anode materials and can rival some of the recently reported 2D materials including graphene

B–N@Graphene: Highly Sensitive and Selective Gas Sensor

We have performed density functional theory (DFT) calculations to study the gas (CO, CO₂, NO, and NO₂) sensing mechanism of pure and doped (B@, N@, and B–N@) graphene surfaces. The calculated adsorption energies of the various toxic gases (CO, CO₂, NO, and NO₂) on the pure and doped graphene surfaces show, doping improves adsorption energy and selectivity. The electronic properties of the B–N@graphene surfaces change significantly compared to pure and B@ and N@graphene surfaces, while selective gas molecules are adsorbed. So, we report B–N codoping on graphene can be highly sensitive and selective for semiconductor-based gas sensor

Design of High-Efficiency Visible-Light Photocatalysts for Water Splitting: MoS₂/AlN(GaN) Heterostructures

Hydrogen fuel produced from water splitting using solar energy and a catalyst is a clean and renewable future energy source. Great efforts in searching for photocatalysts that are highly efficient, inexpensive, and capable of harvesting sunlight have been made for the last decade, which, however, have not yet been achieved in a single material system so far. Here, we predict that MoS₂/AlN­(GaN) van der Waals (vdW) heterostructures are sufficiently efficient photocatalysts for water splitting under visible-light irradiation based on ab initio calculations. Contrary to other investigated photocatalysts, MoS₂/AlN­(GaN) vdW heterostructures can separately produce hydrogen and oxygen at the opposite surfaces, where the photoexcited electrons transfer from AlN­(GaN) to MoS₂ during the photocatalysis process. Meanwhile, these vdW heterostructures exhibit significantly improved photocatalytic properties under visible-light irradiation by the calculated optical absorption spectra. Our findings pave a new way to facilitate the design of photocatalysts for water splitting

Toward the Realization of 2D Borophene Based Gas Sensor

To the league of rapidly expanding 2D materials, borophene is a recent addition. Herein, a combination of ab initio density functional theory (DFT) and nonequilibrium Green’s function (NEGF) based methods is used to estimate the prospects of this promising elemental 2D material for gas sensing applications. We note that the binding of target gas molecules such as CO, NO, NO₂, NH₃, and CO₂ is quite strong on the borophene surface. Interestingly, our computed binding energies are far stronger than several other reported 2D materials like graphene, MoS₂, and phosphorene. Further rationalization of stronger binding is made with the help of charge transfer analysis. The sensitivity of the borophene for these gases is also interpreted in terms of computing the vibrational spectra of the adsorbed gases on top of borophene, which show dramatic shift from their gas phase reference values. The metallic nature of borophene enables us to devise a setup considering the same substrate as electrodes. From the computation of the transmission function of system (gas + borophene), appreciable changes in the transmission functions are noted compared to pristine borophene surface. The measurements of current–voltage (<i>I</i>–<i>V</i>) characteristics unambiguously demonstrate the presence and absence of gas molecules (acting as ON and OFF states), strengthening the plausibility of a borophene based gas sensing device. As we extol the extraordinary sensitivity of borophene, we assert that this elemental 2D material is likely to attract subsequent interest

Lithium and Calcium Carbides with Polymeric Carbon Structures

We studied the binary carbide systems Li₂C₂ and CaC₂ at high pressure using an evolutionary and ab initio random structure search methodology for crystal structure prediction. At ambient pressure Li₂C₂ and CaC₂ represent salt-like acetylides consisting of C₂^2– dumbbell anions. The systems develop into semimetals (<i>P</i>3̅<i>m</i>1-Li₂C₂) and metals (<i>Cmcm</i>-Li₂C₂, <i>Cmcm</i>-CaC₂, and <i>Immm</i>-CaC₂) with polymeric anions (chains, layers, strands) at moderate pressures (below 20 GPa). <i>Cmcm</i>-CaC₂ is energetically closely competing with the ground state structure. Polyanionic forms of carbon stabilized by electrostatic interactions with surrounding cations add a new feature to carbon chemistry. Semimetallic <i>P</i>3̅<i>m</i>1-Li₂C₂ displays an electronic structure close to that of graphene. The π* band, however, is hybridized with Li-sp states and changed into a bonding valence band. Metallic forms are predicted to be superconductors. Calculated critical temperatures may exceed 10 K for equilibrium volume structures

Polyfulvenes: Polymers with “Handles” That Enable Extensive Electronic Structure Tuning

The fundamental electronic structure properties of substituted poly­(penta)­fulvenes and pentafulvene-based polymers are analyzed through qualitative molecular orbital (MO) theory combined with calculations at the B3LYP and HSE06 hybrid density functional theory (DFT) levels. We argue that the pentafulvene monomer unit has a unique character because electron density in the exocyclic CC double bond can be polarized into and out of the five-membered ring, a feature that is not available to other more commonly used monomers. It is investigated how the energy gaps between the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively), as approximate band gaps, are influenced by exocyclic substitution, introduction of linker groups, benzannulation, and ring substitution. In particular, the exocyclic positions of the fulvene act as “handles” by which the electronic structure of the polymer can be tuned between the quinoid and fulvenoid valence bond isomers; electron-withdrawing exocyclic substituents lead to polyfulvenes in the quinoid form while those with electron-donating substituents prefer the fulvenoid. Taken together, the HOMO–LUMO gaps of polyfulvenes can be tuned extensively, varying in ranges 0.77–2.44 eV (B3LYP) and 0.35–2.00 eV (HSE06) suggesting that they are a class of polymers with highly interesting, yet nearly unexplored, properties