First-principles study of 2H-Mo2C-based MXenes under biaxial strain as Li-battery anodes

MXenes, a family of superior 2D materials, have been intensively investigated because they have many promising properties, particularly high-performance energy storage and high flexibility. To approach the expected critical benchmarks of such materials, the strain dependence of the atomic structure is widely considered for tuning the related properties. In this work, by means of density functional theory, we demonstrate the potential application of the strained 2H phase of Mo₂C-based MXenes (Mo₂C and Mo₂CO₂) as anode materials for lithium-ion batteries (LIBs). Adsorption and diffusion of Li on the surfaces of both materials and the impact of biaxial strain (ε_b) in the range of -4% to 4% are insightfully investigated. The lowest adsorption energy of Mo₂C is -0.96 eV, and that of Mo₂CO₂ is -3.13 eV at ε_b = 0%. The diffusion of Li ions, considering the pathway between the first two most favorable adsorption sites, reveals that the biaxial strain refinement under compressive strain decreases the energy barrier, but the induction of tensile strain increases it in both MXenes. The ranges of the energy barriers of Li-ion adsorption on the surfaces of Mo₂C and Mo₂CO₂ are 31-57 meV and 177-229 meV, respectively. Interestingly, the storage capacity of Li can reach three layers corresponding to a comparably high theoretical capacity of 788.61 mA h g^-1 for Mo₂C and 681.64 mA h g^-1 for Mo₂CO₂. The atomic configurations are stable, as verified by the negative adsorption energy as well as the slightly distorted structures, by using ab initio molecular dynamics (AIMD) simulations at 400 K. Moreover, average open circuit voltages (OCVs) of 0.35 V and 0.63 V (at ε_b = 0%) are reported for Mo₂C and Mo₂CO₂, respectively. Furthermore, the tensile strain results in an increase in the OCVs, while compression has the opposite effect. These computational results provide some basic information on the behaviors of Li-ion adsorption and diffusion on Mo₂C-based MXenes upon tuning biaxial strain. They also give a guideline on what conditions are appropriate for practically implementing these MXenes as electrode materials in LIBs.

Improved Thermoelectric Properties of SrTiO3 via (La, Dy and N) Co-Doping: DFT Approach

This work considers the enhancement of the thermoelectric figure of merit, ZT, of SrTiO₃ (STO) semiconductors by (La, Dy and N) co-doping. We have focused on SrTiO₃ because it is a semiconductor with a high Seebeck coefficient compared to that of metals. It is expected that SrTiO₃ can provide a high power factor, because the capability of converting heat into electricity is proportional to the Seebeck coefficient squared. This research aims to improve the thermoelectric performance of SrTiO₃ by replacing host atoms by La, Dy and N atoms based on a theoretical approach performed with the Vienna Ab Initio Simulation Package (VASP) code. Here, undoped SrTiO₃, Sr_0.875La_0.125TiO₃, Sr_0.875Dy_0.125TiO₃, SrTiO_2.958N_0.042, Sr_0.750La_0.125Dy_0.125TiO₃ and Sr_0.875La_0.125TiO_2.958N_0.042 are studied to investigate the influence of La, Dy and N doping on the thermoelectric properties of the SrTiO₃ semiconductor. The undoped and La-, Dy- and N-doped STO structures are optimized. Next, the density of states (DOS), band structures, Seebeck coefficient, electrical conductivity per relaxation time, thermal conductivity per relaxation time and figure of merit (ZT) of all the doped systems are studied. From first-principles calculations, STO exhibits a high Seebeck coefficient and high figure of merit. However, metal and nonmetal doping, i.e., (La, N) co-doping, can generate a figure of merit higher than that of undoped STO. Interestingly, La, Dy and N doping can significantly shift the Fermi level and change the DOS of SrTiO₃ around the Fermi level, leading to very different thermoelectric properties than those of undoped SrTiO₃. All doped systems considered here show greater electrical conductivity per relaxation time than undoped STO. In particular, (La, N) co-doped STO exhibits the highest ZT of 0.79 at 300 K, and still a high value of 0.77 at 1000 K, as well as high electrical conductivity per relaxation time. This renders it a viable candidate for high-temperature applications.

Biaxial stress and functional groups (T = O, F, and Cl) tuning the structural, mechanical, and electronic properties of monolayer molybdenum carbide

MXenes are a family of novel two-dimensional (2D) materials attracting intensive interest because of the rich chemistry rooted from the highly diversified surface functional groups. This enables the chemical optimization suitable for versatile applications, including energy conversion and storage, sensors, and catalysis. This work reports the ab initio study of the crystal energetics, electronic properties, and mechanical properties, and the impacts of strain on the electronic properties of tetragonal (1T) and hexagonal (2H) phases of Mo₂C as well as the surface-terminated Mo₂CT₂ (T = O, F, and Cl). Our findings indicate that 2H-Mo₂C is energetically more stabilized than the 1T counterpart, and the 1T-to-2H transition requires a substantial energy of 210 meV per atom. The presence of surface termination T atoms on Mo₂C intrinsically induces variations in the atomic structure. The calculated structures were selected based on the energetic and thermodynamic stabilities (400 K). The O atom prefers to be terminated on 2H-Mo₂C, whereas the Cl atom energetically stabilizes on 1T-Mo₂C. Meanwhile, with certain configurations, 2H-Mo₂CF₂ and 1T-Mo₂CF₂ with slightly different energies could exist simultaneously. The Mo₂CO₂ possesses the highest mechanical strength and elastic modulus (σ_max = 52 GPa at ε_b = 20% and E = 507 GPa). The nature of the ordered centrosymmetric layer and the strong bonding between 4 d-Mo and 2 p-O of 2H-Mo₂CO₂ are responsible for its promising mechanical properties. Interestingly, the topological properties of 2H-Mo₂CO₂ at a wide range of strains (-10% to 12%) are reported. Moreover, 2H-Mo₂CF₂ is metallic through the range of calculation. Meanwhile, originally semiconducting 1T-Mo₂CF₂ and 1T-Mo₂CCl₂ preserve their features under the ranges of the strain of -2% to 10% and -1% to 5%, respectively, beyond which they undergo the semiconductor-to-metal transitions. These findings would guide the potential applications in modern 2D straintronic devices.

Elucidating Synergistic Mechanisms of Adsorption and Electrocatalysis of Polysulfides on Double-Transition Metal MXenes for Na-S Batteries

Multiple unfavorable features, such as poor electronic conductivity of sulfur cathodes, the dissolution and shuttling of sodium polysulfides (Na₂S_n) in electrolytes, and the slower kinetics for the decomposition of solid Na₂S, make sodium-sulfur batteries (NaSBs) impractical. To overcome these obstacles, novel double-transition metal (DTM) MXenes, Mo₂TiC₂T₂, (T = O and S) are studied as an anchoring material (AM) to immobilize higher-order polysulfides and to expedite the otherwise slower kinetics of insoluble short-chain polysulfides. Density functional theory (DFT) calculations are carried out to justify and compare the effectiveness of Mo₂TiC₂S₂ and Mo₂TiC₂O₂ as AMs by analyzing their interactions with S₈/Na₂S_n (n = 1, 2, 4, 6, and 8). Mo₂TiC₂S₂ provides moderate adsorption strength compared to Mo₂TiC₂O₂, therefore, it is expected to effectively inhibit Na₂S_n dissolution and shuttling without causing decomposition of Na₂S_n. The calculated Gibbs free energies of the rate-determining step for sulfur reduction reactions (SRR) are found to be significantly lower (0.791 eV for S and 0.628 eV for O functionalization) than that in vacuum (1.442 eV), suggesting that the SRR is more thermodynamically favorable on Mo₂TiC₂T₂ during discharge. Additionally, both Mo₂TiC₂S₂ and Mo₂TiC₂O₂ demonstrated effective electrocatalytic activity for the decomposition of Na₂S, with a substantial reduction in the energy barrier to 1.59 eV for Mo₂TiC₂S₂ and 1.67 eV for Mo₂TiC₂O₂. While Mo₂TiC₂O₂ had superior binding properties, structural distortion is observed in Na₂S_n, which may adversely affect cyclability. On the other hand, because of its moderate binding energy, enhanced electronic conductivity, and significantly faster oxidative decomposition kinetics of polysulfides, Mo₂TiC₂S₂ can be considered as an effective AM for suppressing the shuttle effect and improving the performance of NaSBs.

Two-Dimensional Titanium Carbide (Ti3C2Tx) MXenes to Inhibit the Shuttle Effect in Sodium Sulfur Batteries

Room-temperature sodium sulfur batteries (RT-NSBs) are among the promising candidates for large-scale energy storage applications because of the natural abundance of the electrode materials and impressive energy density. However, one...

Transition-metal decorated graphdiyne monolayer as an efficient sensor toward phosphide (PH3) and arsine (AsH3)

Graphdiyne (GDY), a two-dimensional (2D) carbon, uniquely possesses mixed sp–sp2 hybridization, uniform nano-sized porous structure, semiconducting character, and excellent electrical conductivity.

Conversion of CO2 into Formic Acid on Transition Metal-Porphyrin-like Graphene: First Principles Calculations

Recently, transition metal (TM)-porphyrin-like graphene has been predicted to be a promising material for CO₂ capturing under favorable conditions. Such materials can capture CO₂ at 300 K and release it at 450 K. However, the captured CO₂ gas is mostly stored in oceans. With the aid of first principles calculations, we herein propose a method in which the captured CO₂ is converted into an environmentally friendly product, formic acid. Addition of H₂ to CO₂ molecules adsorbed on Sc- and Ti-porphyrin-like graphene was found to catalyze this conversion. We also performed nudged elastic band calculations and thermodynamic analysis using the first-order Polanyi-Wigner equation and equilibrium statistical mechanics to investigate the chemical reactions involved in this conversion. In addition, we performed Bader charge analysis to obtain insights into the mechanism of charge transfer and adsorption throughout the conversion. Our study presents a novel method in which the captured CO₂ is treated by converting it into an environmentally friendly product. Since this method does not require CO₂ storage, it is expected to be an effective strategy to manage the rising CO₂ level in the environment.

Novel green phosphorene as a superior chemical gas sensing material

Green phosphorus and its monolayer variant, green phosphorene (GreenP), are the recent members of two-dimensional (2D) phosphorus polymorphs. The new polymorph possesses the high stability, tunable direct bandgap, exceptional electronic transport, and directionally anisotropic properties. All these unique features could reinforce it the new contender in a variety of electronic, optical, and sensing devices. Herein, we present gas-sensing characteristics of pristine and defected GreenP towards major environmental gases (i. e., NH₃, NO, NO₂, CO, CO₂, and H₂O) using combination of the density functional theory, statistical thermodynamic modeling, and the non-equilibrium Green's function approach (NEGF). The calculated adsorption energies, density of states (DOS), charge transfer, and Crystal Orbital Hamiltonian Population (COHP) reveal that NO, NO₂, CO, CO₂ are adsorbed on GreenP, stronger than both NH₃ and H₂O, which are weakly physisorbed via van der Waals interactions. Furthermore, substitutional doping by sulfur can selectively intensify the adsorption towards crucial NO₂ gas because of the enhanced charge transfer between p orbitals of the dopant and the analyte. The statistical estimation of macroscopic measurable adsorption densities manifests that the significant amount of NO₂ molecules can be practically adsorbed at ambient temperature even at the ultra-low concentration of part per billion (ppb). In addition, the current-voltage (I-V) characteristics of S-doped GreenP exhibit a variation upon NO₂ exposure, indicating the superior sensitivity in sensing devices. Our work sheds light on the promising application of the novel GreenP as promising chemical gas sensors.

The quantum confined Stark effect in N-doped ZnO/ZnO/N-doped ZnO nanostructures for infrared and terahertz applications

The terahertz (THz) frequency range is very important in various practical applications, such as terahertz imaging, chemical sensing, biological sensing, high-speed telecommunications, security, and medical applications. Based on the density functional theory (DFT), this work presents electronic and optical properties of N-doped ZnO/ZnO/N-doped ZnO quantum well and quantum wire nanostructures. The density of states (DOS), the band structures, effective masses, and the band offsets of ZnO and N-doped ZnO were calculated as the input parameters for the subsequent modeling of the ZnO/N-doped ZnO heterojunctions. The results show that the energy gaps of the component materials are different, and the conduction and valence band offsets at the ZnO/N-doped ZnO heterojunction give type-II alignment. Furthermore, the optical characteristics of N-doped ZnO/ZnO/N-doped ZnO quantum well were studied by calculating the absorption coefficient from transitions between the confined states in the conduction band under the applied electric field (Stark effect). The results indicate that N-doped ZnO/ZnO/N-doped ZnO quantum wells, quantum wires, and quantum cascade structures could offer the absorption spectrum tunable in the THz range by varying the electric field and the quantum system size. Therefore, our work indicates the possibility of using ZnO as a promising candidate for infrared and terahertz applications.

Efficient suppression of the shuttle effect in Na-S batteries with an As2S3 anchoring monolayer

Sodium-sulfur batteries (NaSBs) have emerged as a promising energy storage technology for large-scale stationary applications such as smart electrical grids due to their exceptionally high energy density and cost-effectiveness. However, one of the challenging problems impeding their practical applications is the sulfur shuttle effect by which the active redox intermediates are gradually dissolved in electrolytes. In this work, we have employed first-principles density functional theory (DFT) calculations to unravel the suppression of the shuttle effect in NaSBs with a two-dimensional (2D) As2S3 monolayer as the anchoring material. We show that semiconducting As2S3 is a suitable anchoring layer to inhibit the dissolution of the polysulfide intermediates in common electrolytes because of its stronger chemical binding with sodium polysulfides than with the electrolytes. The immense adsorption is attributed to the electron donation from the unfilled S-3p states of the polysulfides to As2S3. These mechanisms increase the carrier population and consequently improve the electrical conductivity of As2S3. Hence, the use of As2S3 can both reduce the shuttle effect and enhance the cathode electron conductivity to enable improved cycling stability and coulombic efficiency of the battery.

Graphitic carbon nitride nano sheets functionalized with selected transition metal dopants: an efficient way to store CO2

Proficient capture of carbon dioxide (CO₂) is considered to be a backbone for environment protection through countering the climate change caused by mounting carbon content. Here we present a comprehensive mechanism to design novel functional nanostructures capable of capturing a large amount of CO₂ efficiently. By means of van der Waals corrected density functional theory calculations, we have studied the structural, electronic and CO₂ storage properties of carbon nitride (g-C₆N₈) nano sheets functionalized with a range of transition metal (TM) dopants ranging from Sc to Zn. The considered TMs bind strongly to the nano sheets with binding energies exceeding their respective cohesive energies, thus abolishing the possibility of metal cluster formation. Uniformly dispersed TMs change the electronic properties of semiconducting g-C₆N₈ through the transfer of valence charges from the former to the latter. This leaves all the TM dopants with significant positive charges, which are beneficial for CO₂ adsorption. We have found that each TM's dopants anchor a maximum of four CO₂ molecules with suitable adsorption energies (-0.15 to -1.0 eV) for ambient condition applications. Thus g-C₆N₈ nano sheets functionalized with selected TMs could serve as an ideal sorbent for CO₂ capture.

Defected and Functionalized Germanene-based Nanosensors under Sulfur Comprising Gas Exposure

Efficient sensing of sulfur containing toxic gases like H₂S and SO₂ is of the utmost importance due to the adverse effects of these noxious gases. Absence of an efficient 2D-based nanosensor capable of anchoring H₂S and SO₂ with feasible binding and an apparent variation in electronic properties upon the exposure of gas molecules has motivated us to explore the promise of a germanene nanosheet (Ge-NS) for this purpose. In the present study, we have performed a comprehensive computational investigation by means of DFT-based first-principles calculations to envisage the structural, electronic, and gas sensing properties of pristine, defected, and metal substituted Ge-NSs. Our initial screening has revealed that although interaction of SO₂ with pristine Ge-NSs is within the desirable range, H₂S binding however falls below the required values to guarantee an effective sensing. To improve the binding characteristics, we have considered the interactions between H₂S and SO₂ with defected and metal substituted Ge-NS. The systematic removals of Ge atoms from a reasonably large super cell lead to monovacancy, divacancies, and trivacancies in Ge-NS. Similarly, different transition metals like As, Co, Cu, Fe, Ga, Ge, Ni, and Zn have been substituted into the monolayer to realize substituted Ge-NS. Our van der Waals corrected DFT calculations have concluded that the vacancy and substitution defects not only improve the binding characteristics but also enhance the sensing propensity of both H₂S and SO₂. The total and projected density of states show significant variations in electronic properties of pristine and defected Ge-NSs before and after the exposure to the gases, which are essential in constituting a signal to be detected by the external circuit of the sensor. We strongly believe that our present work would not only advance the knowledge towards the application of Ge-NS-based sensing but also provide motivation for the synthesis of such efficient nanosensor for H₂S and SO₂ based on Ge monolayer.

Electronic Properties of h-BCN-Blue Phosphorene van der Waals Heterostructures

Van der Waals heterostructures, a new class of materials made of a vertically selective assembly of various 2D monolayers held together by van der Waals forces, have attracted a great deal of attention due to their promise to deliver novel electronic and optoelectronic properties that are not achievable by using individual 2D crystals. Using density functional theory (DFT), it is revealed that van der Waals heterostructures composed of monolayers of hexagonal boron nitride (h-BN) and the latest P allotrope blue phosphorus (blue phosphorene, BlueP) forms a straddling type I band offset for which the band edges exclusively belong to BlueP. This feature enables h-BN to act as a protective coating material to resolve the air instability of BlueP. Furthermore, substitutional doping of C into h-BN (h-BCN) at a suitable concentration induces h-BCN-BlueP into staggered type II band offset. The type II band alignment triggered by the intensified built-in electric field across the sheets implies improved carrier mobility and the suppressed recombination of photogenerated hole pairs. These major benefits can pave the way for the potential functionality of h-BCN-BlueP to be exploited for efficient photovoltaic devices.

Improved sensing characteristics of methane over ZnO nano sheets upon implanting defects and foreign atoms substitution

Thanks to the growing interests of metal oxide sensors in environmental and industrial uses, this study presents the sensing mechanism of methane gas (CH₄) on recently synthesized two-dimensional form of ZnO, ZnO nano sheets (ZnO-NS). The adsorption energy of CH₄ on pristine ZnO-NS, calculated by means of van der Waals corrected first-principles calculations, is found to be insufficient restricting its application as an efficient nano sensor. However, the creation of (O/Zn) vacancies and the substitution of foreign dopants into ZnO-NS considerably intensify the binding energy of CH₄. Through a comprehensive energetic analysis, it is observed that among all the substituents, boron (B), sulphur (S) and gallium (Ga) improves the binding of CH₄ to 2.75, 6.1 and 7.5 times respectively than its values on pristine ZnO-NS. In addition to the CH₄ binding energies falling ideally between physisorption and chemisorption range, a prominent variation in the electronic properties before and after CH₄ exposure indicates the promise of substituted Zn-NS as a useful nano sensors.

Crystal Phase Effects in Si Nanowire Polytypes and Their Homojunctions

Recent experimental investigations have confirmed the possibility to synthesize and exploit polytypism in group IV nanowires. Driven by this promising evidence, we use first-principles methods based on density functional theory and many-body perturbation theory to investigate the electronic and optical properties of hexagonal-diamond and cubic-diamond Si NWs as well as their homojunctions. We show that hexagonal-diamond NWs are characterized by a more pronounced quantum confinement effect than cubic-diamond NWs. Furthermore, they absorb more light in the visible region with respect to cubic-diamond ones and, for most of the studied diameters, they are direct band gap materials. The study of the homojunctions reveals that the diameter has a crucial effect on the band alignment at the interface. In particular, at small diameters the band-offset is type-I whereas at experimentally relevant sizes the offset turns up to be of type-II. These findings highlight intriguing possibilities to modulate electron and hole separations as well as electronic and optical properties by simply modifying the crystal phase and the size of the junction.

Dynamic compression of dense oxide (Gd3Ga5O12) from 0.4 to 2.6 TPa: Universal Hugoniot of fluid metals

Materials at high pressures and temperatures are of great current interest for warm dense matter physics, planetary sciences, and inertial fusion energy research. Shock-compression equation-of-state data and optical reflectivities of the fluid dense oxide, Gd3Ga5O12 (GGG), were measured at extremely high pressures up to 2.6 TPa (26 Mbar) generated by high-power laser irradiation and magnetically-driven hypervelocity impacts. Above 0.75 TPa, the GGG Hugoniot data approach/reach a universal linear line of fluid metals, and the optical reflectivity most likely reaches a constant value indicating that GGG undergoes a crossover from fluid semiconductor to poor metal with minimum metallic conductivity (MMC). These results suggest that most fluid compounds, e.g., strong planetary oxides, reach a common state on the universal Hugoniot of fluid metals (UHFM) with MMC at sufficiently extreme pressures and temperatures. The systematic behaviors of warm dense fluid would be useful benchmarks for developing theoretical equation-of-state and transport models in the warm dense matter regime in determining computational predictions.

A new, layered monoclinic phase of Co3O4 at high pressure

We present the crystal structures and electronic properties of a Co3O4 spinel under high pressure. Co3O4 undergoes a first-order transition from a cubic (CB) Fd3̄m to a lower-symmetry monoclinic (MC) P21/c phase at 35 GPa, occurring after the local high-spin to low-spin phase transition. The high-pressure phase exhibits the octahedral coordination of Co(II) and Co(III), whereas the CB phase contains the fourfold coordination of Co(II) and the sixfold coordination of Co(III). The CB-to-MC transition is attributed to the charge-transfer between the di- and trivalent cations via the enhanced 3d-3d interactions.