First-principles study of molecular hydrogen binding to heme in competition with O2, NO and CO.

Molecular hydrogen shows antioxidant activity and distinct efficacy towards vascular diseases, but the understanding of this is not yet satisfactory at the atomic level. In this work, we study the binding properties of H2 to the heme group in relation with other diatomic molecules (DMs), including O2, NO and CO, and their displacement reactions, using first-principles calculations. We carry out molecular modeling of the heme group, using iron-porphyrin with the imidazole ligand, i.e., FePIm, and smaller models of Fe(CnHn+2N2)2NH3 with n = 3 and 1, and of molecular complexes of heme-DM and -H. Through analysis of optimized geometries and energetics, it is found that the order of binding strength of DMs or H to the Fe of heme is NO > O2 > CO > H > H2 for FePIm-based systems, while it is H > O2 > NO > CO > H2 for model-based systems. We calculate the activation energies for displacement reactions of H2 and H by other DMs, revealing that the H2 displacements occur spontaneously while the H displacements require a large amount of energy. Finally, our calculations corroborate that the rate constants increase with increasing temperature according to the Arrhenius relation.

First-principles study of mercaptoundecanoic acid molecule adsorption and gas molecule penetration onto silver surface: an insight for corrosion protection.

Recently, 11-mercaptoundecanoic acid (MUA) molecule has attracted attention as a promising passivation agent of Ag nanowire (NW) network electrode for corrosion inhibition, but the underneath mechanism has not been elaborated. In this work, we investigate adsorption of MUA molecule on Ag(1 0 0) and Ag(1 1 1) surface, adsorption of air gas molecules of H2O, H2S and O2 on MUA molecular end surface, and their penetrations into the Ag surface using the first-principles calculations. Our calculations reveal that the MUA molecule is strongly bound to the Ag surface with the binding energies ranging from -0.47 to -2.06 eV and the Ag-S bond lengths of 2.68-2.97 Å by Lewis acid-base reaction. Furthermore, we find attractive interactions between the gas molecules and the MUA@Ag complexes upon their adsorptions and calculate activation barriers for their migrations from the outermost end of the complexes to the top of Ag surface. It is found that the penetrations of H2O and H2S are more difficult than the O2 penetration due to their higher activation barriers, while the O2 penetration is still difficult, confirming the corrosion protection of Ag NW network by adsorbing the uniform monolayer of MUA. With these findings, this work can contribute to finding a better passivation agent in the strategy of corrosion protection of Ag NW network electrode.

First-principles study on structural, electronic and optical properties of halide double perovskite Cs2AgBX6 (B = In, Sb; X = F, Cl, Br, I).

All-inorganic halide double perovskites (HDPs) attract significant attention in the field of perovskite solar cells (PSCs) and light-emitting diodes. In this work, we present a first-principles study on structural, elastic, electronic and optical properties of all-inorganic HDPs Cs2AgBX6 (B = In, Sb; X = F, Cl, Br, I), aiming at finding the possibility of using them as photoabsorbers for PSCs. Confirming that the cubic perovskite structure can be formed safely thanks to the proper geometric factors, we find that the lattice constants are gradually increased on increasing the atomic number of the halogen atom from F to I, indicating the weakening of Ag-X and B-X interactions. Our calculations reveal that all the perovskite compounds are mechanically stable due to their elastic constants satisfying the stability criteria, whereas only the Cl-based compounds are dynamically stable in the cubic phase by observing their phonon dispersions without soft modes. The electronic band structures are calculated with the Heyd-Scuseria-Ernzerhof hybrid functional, demonstrating that the In (Sb)-based HDPs show direct (indirect) transition of electrons and the band gaps are decreased from 4.94 to 0.06 eV on going from X = F to I. Finally, we investigate the macroscopic dielectric functions, photo-absorption coefficients, reflectivity and exciton properties, predicting that the exciton binding strength becomes weaker on going from F to I.

Revealing the effect of Nb or V doping on anode performance in Na2Ti3O7 for sodium-ion batteries: a first-principles study.

Sodium titanate Na2Ti3O7 (NTO) has superior electrochemical properties as an anode material in sodium-ion batteries (SIBs), and Nb or V doping is suggested to enhance the electrode performance. In this work, we carry out systematic first-principles calculations of the structural, electronic and electrochemical properties of NTO and Na2Ti2.75M0.25O7 (M = Nb, V), using supercells to reveal the effect of Nb or V NTO-doping on its anode performance. It is found that Nb doping gives rise to the expansion of cell volume but V doping induces the shrinkage of cell volume due to the larger and smaller ionic radius of the Nb and V ions, respectively, compared to that of the Ti ion. We perform structural optimization of the intermediate phases of Na2+xM3O7 with increasing Na content x from 0 to 2, revealing that the overall relative volume expansion rate is slightly increased by Nb and V doping but remains lower than 3%. Our calculations demonstrate that the electrode potential of NTO is slightly raised and the specific capacity is reduced, but the electronic and ionic conductivities are improved by Nb or V doping. With the revealed understanding and mechanisms, our work will contribute to the search for advanced electrode materials for SIBs.

Insight into the structural and electrochemical properties of the interface between a Na6SOI2 solid electrolyte and a metallic Na anode.

Solid electrolytes (SE) have attracted a great deal of interest as they can not only mitigate the safety issues related to currently used liquid organic electrolytes but also enable the introduction of a metallic Na anode with extreme energy density in sodium-ion batteries. For such application, SE should exhibit high interfacial stability against metallic Na as well as high ionic conductivity, and Na6SOI2 with a Na-rich double anti-perovskite structure was recently identified as a promising SE candidate. In this work, we performed first principles calculations to investigate the structural and electrochemical properties of the interface between Na6SOI2 and a metallic Na anode. Our calculations revealed that interfaces could be formed safely, keeping the ultra-fast ionic conductivity of the bulk phase near the interface. Through the electronic structure analysis of the interface models, we found the change of upward valence band bending at the surface to downward band bending at the interface, being accompanied by electronic charge transfer from a metallic Na anode to Na6SOI2 SE at the interface. This work provides valuable atomistic insight into the formation and properties of the interface between SE and alkali metal for enhancing battery performance.

First-principles study of sodium adsorption on defective graphene under propylene carbonate electrolyte conditions.

Hard carbon (HC) has been predominantly used as a typical anode material of sodium-ion batteries (SIBs) but its sodiation mechanism has been debated. In this work, we investigate the adsorption of Na atoms on defective graphene under propylene carbonate (PC) and water solvent as well as vacuum conditions to clarify the sodiation mechanism of HC. Within the joint density functional theory framework, we use the nonlinear polarizable continuum model for PC and the charge-asymmetric nonlocally-determined local electric solvation model for water. Our calculations reveal that the centre of each point defect such as mono-vacancy (MV), di-vacancy (DV) and Stone-Wales is a preferable adsorption site and the electrolyte enhances the Na adsorption through implicit interaction. Furthermore, we calculate the formation energies of multiple Na atom arrangements on the defective graphene and estimate the electrode potential versus Na/Na+, verifying that the multiple Na adsorption on the MV and DV defective graphene under the PC electrolyte conditions is related to the slope region of the discharge curve in HC. This reveals new prospects for optimizing anodes and electrolytes for high performance SIBs.

Defect formation and ambivalent effects on electrochemical performance in layered sodium titanate Na2Ti3O7.

Point defects can be formed readily in layered transition metal oxides used as electrode materials for alkali-ion batteries but their influence on the electrode performance is yet obscure. In this work, we report a systematic first-principles study of intrinsic point defects and defect complexes in sodium titanate Na2Ti3O7, a low-voltage anode material for sodium-ion batteries. Within the density functional theory framework, we calculate the defect formation energies with a set of atomic chemical potentials, which define the synthesis conditions for the stable Na2Ti3O7 compound. Given the atomic chemical potential landscape and defect formation energies, we find that Na interstitials (Nai+), Na antisites (NaTi3-), and Na vacancies (VNa-) are dominant defects depending on the synthesis conditions. Furthermore, our calculations reveal that O vacancies (VO) and Ti antisites (TiNa) lower the electrode potential compared with the perfect system, whereas Ti vacancies (VTi) and NaTi increase the voltage. Finally, we evaluate the activation barriers for vacancy-mediated Na diffusion in the defective systems, finding that the intrinsic point defects improve the Na ion conduction. Our results provide a profound understanding of defect formation and influences on electrode performance, paving a way to designing high-performance anode materials.

Antioxidation activity of molecular hydrogen via protoheme catalysis in vivo: an insight from ab initio calculations.

Recently, molecular hydrogen has been found to exhibit antioxidation activity through many clinical experiments, but the mechanism has not been fully understandable at atomic level. In this work, we perform systematic ab initio calculations of protoheme-hydrogen complexes to clarify the antioxidation mechanism of molecular hydrogen. We make molecular modeling of iron-protoporphyrin coordinated by imidazole, FeP(Im), and its hydrogen as well as dihydrogen complexes, together with reactive oxygen/nitrogen species (RONS). We carry out structural optimization and Mulliken charge analysis, revealing the two kinds of bonding characteristics between FeP(Im) and H[Formula: see text]: dihydrogen bonding in the end-on asymmetric configuration and Kubas bonding in the side-on symmetric configuration of H[Formula: see text] molecule. The activation barriers for adsorption and dissociation of H[Formula: see text] on and further desorption of H atom from FeP(Im) are found to be below 2.78 eV at most, which is remarkably lower than the H-H bond breaking energy of 4.64 eV in free H[Formula: see text] molecule. We find that the hydrogen bond dissociation energies of FeP(Im)-H[Formula: see text] and -H complexes are lower than those of RONS-H complexes, indicating the decisive role of protoheme as an effective catalyst in RONS antioxidation by molecular hydrogen in vivo.

High thermoelectric performance in metal phosphides MP2 (M = Co, Rh and Ir): a theoretical prediction from first-principles calculations.

Although metal phosphides have good electronic properties and high stabilities, they have been overlooked in general as thermoelectrics based on expectation of high thermal conductivity. Here we propose the metal phosphides MP2 (M = Co, Rh and Ir) as promising thermoelectrics through first-principles calculations of their thermoelectric properties. By using lattice dynamics calculations within unified theory of thermal transport in crystal and glass, we obtain the lattice thermal conductivities κ l of MP2 as 0.63, 1.21 and 1.81 W m-1 K at 700 K for M = Co, Rh and Ir, respio ectively. Our calculations for crystalline structure, phonon dispersion, Grüneisen parameters and cumulative κ l reveal that such low κ l originates from strong rattling vibrations of M atoms and lattice anharmonicity, which significantly suppress heat-carrying acoustic phonon modes coupled with low-lying optical modes. Using mBJ exchange-correlation functional, we further calculate the electronic structures and transport properties, which are in good agreement with available experimental data, evaluating the relaxation time of charge carrier within deformation potential theory. We predict ultrahigh thermopower factors as 10.2, 7.1 and 6.4 mW m-1 K2 at 700 K for M = Co, Rh and Ir, being superior to the conventional thermoelectrics GeTe. Finally, we estimate their thermoelectric performance by computing figure of merit ZT, finding that upon n-type doping ZT can reach ∼1.7 at 700 K specially for CoP2. We believe that our work offers a novel materials platform to search for high-performance thermoelectrics using metal phosphides.

Point defects and their impact on electrochemical performance in Na0.44MnO2 for sodium-ion battery cathode application.

Sodium manganese oxide Na0.44MnO2 (NMO) in an open structure with large tunnels is of great interest for sodium-ion battery cathode materials due to its high electrode voltage and capacity. However, its practical application is limited by poor rate performance, which can be tuned and improved by controlling point defects. We herein present a comprehensive study of intrinsic point defects in NMO using density functional theory (DFT) calculations in combination with thermodynamics. Using the DFT+U approach, we determine the formation energies of elementary defects and defect complexes depending on the sets of atomic chemical potentials, corresponding to a certain thermodynamic condition for the synthesis of stable NMO. Sodium interstitials are found to have the lowest formation energies in the relevant ranges of temperature and pressure. Other intrinsic point defects such as oxygen vacancies, sodium vacancies and manganese antisites can also be formed with proper formation energies and have an impact on the cathode performance. Compared to the perfect system, oxygen vacancies lower the electrode voltage, whereas manganese vacancies and antisites increase the voltage. We find that most point defects and defect complexes improve the sodium ion diffusivity, highlighting a proper control of defect formation for enhancing the performance of sodium-ion batteries.

Improving the stability of hybrid perovskite FAPbI3 by forming 3D/2D interfaces with organic spacers.

Interfaces composed of three-dimensional (3D) and 2D organic-inorganic hybrid formamidinium lead iodide (FAPbI3) linked by organic spacers (OSs) are studied using first-principles calculations. The OS cations with aromatic rings, like phenylethylammonium and anilinium (AN), are found to be more favourable for enhancing the stability of the 3D/2D interface than butylammonium with aliphatic chains. The AN-based interface shows the highest resistance to penetration of water molecules.

First-principles study on structural, electronic, magnetic and thermodynamic properties of lithium ferrite LiFe5O8.

Lithium ferrite, LiFe5O8 (LFO), has attracted great attention for various applications, and there has been extensive experimental studies on its material properties and applications. However, no systematic theoretical study has yet been reported, so understanding of its material properties at the atomic scale is still required. In this work, we present a comprehensive investigation into the structural, electronic, magnetic and thermodynamic properties of LFO using first-principles calculations. We demonstrate that the ordered α-phase with ferrimagnetic spin configuration is energetically favourable among various crystalline phases with different magnetic configurations. By applying the DFT + U approach with U = 4 eV, we reproduce the lattice constant, band gap energy, and total magnetization in good agreement with experiments, emphasizing the importance of considering strong correlation and spin-polarization effects originating from the 3d states of Fe atoms. We calculated the phonon dispersions of LFO with ferrimagnetic and non-magnetic states, and subsequently evaluated the Gibbs free energy differences between the two states, plotting the P-T diagram for thermodynamic stability of the ferrimagnetic against non-magnetic state. From the P-T diagram, the Curie temperature is found to be ∼925 K at the normal condition and gradually increase with increasing pressure. Our calculations explain the experimental observations for material properties of LFO, providing a comprehensive understanding of the underlying mechanism and useful guidance for enhancing performance of LFO-based devices.

First-principles study on the elastic, electronic and optical properties of all-inorganic halide perovskite solid solutions of CsPb(Br1-x Cl x )3 within the virtual crystal approximation.

All-inorganic halide perovskites have drawn significant attention for optoelectronic applications such as solar cells and light-emitting diodes due to their excellent optoelectronic properties and high stabilities. In this work, we report a systematic study on the material properties of all-inorganic bromide and chloride perovskite solid solutions, CsPb(Br1-x Cl x )3, varying the Cl content x from 0 to 1 with an interval of 0.1 by applying the first-principles method within the virtual crystal approximation. The lattice constants of the cubic phase are shown to follow the linear function of mixing ratio x, verifying that Vegard's law is satisfied and the pseudopotentials of the virtual atoms are reliable. We calculate the band structures with the HSE06 hybrid functional with and without spin-orbit coupling, yielding band gaps in good agreement with experimental results, and find that the band gap increases along the quadratic function of the Cl content x. With increasing Cl content x, the elastic constants and moduli increase linearly, the effective mass of the electron and hole increase, while mobilities decrease linearly, the static dielectric constant decreases linearly, and exciton binding energy increases quadratically. We calculate the photo-absorption coefficients and reflectivity, predicting the absorption peaks shift to the ultraviolet region from bromide to chloride.

Enhancing the Photocatalytic Hydrogen Evolution Performance of the CsPbI3/MoS2 Heterostructure with Interfacial Defect Engineering.

Developing highly efficient photocatalysts for the hydrogen evolution reaction (HER) by solar-driven water splitting is a great challenge. Here, we study the atomistic origin of interface properties and the HER performance of all-inorganic iodide perovskite ß-CsPbI3/2H-MoS2 heterostructures with interfacial vacancy defects using first-principles calculations. Both CsI/MoS2 and PbI2/MoS2 heterostructures have strong binding and dipole moment, which are enhanced by interfacial iodine vacancies (VI). Because of the nature of type II heterojunctions, photogenerated electrons on the CsPbI3 side are promptly transferred to the MoS2 side where HER occurs, and sulfur vacancies (VS) spoil this process, acting as surface traps. The formation energies of various defects are calculated by applying atomistic thermodynamics, identifying the growth conditions for promoting VI and suppressing VS formation. The HER performance is enhanced by forming interfaces with lower ΔGH values for hydrogen adsorption on the MoS2 side, suggesting PbI2/MoS2 with VI to be the most promising photocatalyst.

Twofold rattling mode-induced ultralow thermal conductivity in vacancy-ordered double perovskite Cs2SnI6.

We report a first-principles study of lattice vibrations and thermal transport in Cs2SnI6, the vacancy-ordered double perovskite. Twofold rattlers of Cs atoms and SnI6 clusters in Cs2SnI6, being different from CsSnI3 with only Cs atom rattlers, largely scatter heat-carrying acoustic phonons strongly coupled with low-lying optical phonons and lower phonon group velocity. Using renormalized phonon dispersions at finite temperatures, we reveal that anharmonicity and twofold rattling modes induce an ultralow thermal conductivity at room temperature.

Superior thermoelectric properties of ternary chalcogenides CsAg5Q3 (Q = Te, Se) predicted using first-principles calculations.

Tailoring novel thermoelectric materials (TEMs) with a high efficiency is challenging due to the difficulty in realizing both low thermal conductivity and high thermopower factor. In this work, we propose ternary chalcogenides CsAg5Q3 (Q = Te, Se) as promising TEMs based on first-principles calculations of their thermoelectric properties. Using lattice dynamics calculations within self-consistent phonon theory, we predict their ultralow lattice thermal conductivities below 0.27 W m-1 K-1, revealing the strong lattice anharmonicity and rattling vibrations of Ag atoms as the main origination. By using the mBJ exchange-correlation functional, we calculate the electronic structures with the direct band gaps in good agreement with experiments, and evaluate the charge carrier lifetime as a function of temperature within the deformation potential theory. Our calculations to solve Boltzmann transport equations demonstrate high thermopower factors of 2.5 mW m-1 K-2 upon p-type doping at 300 K, comparable to the conventional dichalcogenide thermoelectric GeTe. With these ultralow thermal conductivities and high thermopower factors, we determine a relatively high thermoelectric figure of merit ZT along the z-axis, finding the maximum value of ZTz to be 2.5 at 700 K for CsAg5Se3 by optimizing the hole concentration. Our computational results highlight the great potentiality of CsAg5Q3 (Q = Te, Se) for high-performance thermoelectric devices operating at room temperature.

First-principles study of the structural and electrochemical properties of NaxTi2O4 (0 ≤x≤ 1) with tunnel structure for anode applications in alkali-ion batteries.

Due to their low cost and easy synthesis method, several kinds of sodium titanates have been explored as anode materials for sodium ion batteries (SIBs). However, some of them have not yet been considered as electrode materials for SIBs, and here we have carried out a first-principles study on NaxTi2O4 compounds with two different tunnel structures, denoted as single and double phases, to demonstrate their structural and electrochemical properties upon Na or Li insertion. Our calculation results reveal that these compounds exhibit structural stability during sodiation/desodiation and a moderate electrode voltage of ∼0.82 V vs. Na+/Na with a specific capacity of ∼150 mA h g-1. In particular, the activation energy of Na+ ion migration in the double phase is estimated to be as low as 0.28 eV, which is the lowest value among the SIB electrodes developed so far, and this can be attributed to the wide tunnel structure. In addition, we verify their potentiality for use as anode materials in lithium ion batteries (LIBs) by exploring their properties upon Li insertion. Since these compounds are predicted to be promising anode materials for SIBs or LIBs by our calculations, we believe that our findings will promote further experimental studies.

Interfacial Enhancement of Photovoltaic Performance in MAPbI3/CsPbI3 Superlattice.

Perovskite solar cells have continued to fascinate over the past decade due to fast increasing power conversion efficiency and very low fabrication cost but still suffered from poor stability. Interface engineering is evolved to be one of the most promising solutions to the instability problem. In this work, we perform a first-principles study on the MAPbI3/CsPbI3 interface system, aiming at clarifying the underlying mechanism of interfacial enhancement of solar cell performance. We devise the atomistic modeling of superlattices as increasing the number of included unit cells and carry out structural optimizations, revealing that the binding strength between the perovskite layers becomes stronger while the band gap decreases as the supercell size increases. Using enough large supercells of the interface system, we further estimate the formation energies of the interfacial vacancy defects and activation barriers for vacancy-mediated I atom migrations. Our calculations show the shallow transition states for most of the defects and the higher activation barriers for I atom migrations across the interface, providing an evidence of performance enhancement by the interface formation. By giving an insightful understanding of the MAPbI3/CsPbI3 heterojunction, this work definitely contributes to the design of interface systems for high-performance solar cells.

First-principles study on structural, electronic and optical properties of perovskite solid solutions KB1-x Mg x I3 (B = Ge, Sn) toward water splitting photocatalysis.

Perovskite materials have been recently attracting a great amount of attention as new potential photocatalysts for water splitting hydrogen evolution. Here, we propose lead-free potassium iodide perovskite solid solutions KBI3 with B-site mixing between Ge/Sn and Mg as potential candidates for photocatalysts based on systematic first-principles calculations. Our calculations demonstrate that these solid solutions, with proper Goldschmidt and octahedral factors for the perovskite structure, become stable by configurational entropy at finite temperature and follow Vegard's law in terms of lattice constant, bond length and elastic constants. We calculate their band gaps with different levels of theory with and without spin-orbit coupling, revealing that the hybrid HSE06 method yields band gaps increasing along the quadratic function of Mg content x. Moreover, we show that the solid solutions with 0.25 ≤ x ≤ 0.5 have appropriate band gaps between 1.5 and 2.2 eV, reasonable effective masses of charge carriers, and suitable photoabsorption coefficients for absorbing sunlight. Among the solid solutions, KB0.5Mg0.5I3 (B = Ge, Sn) is found to have the most promising band edge alignment with respect to the water redox potentials with different pH values, motivating experimentalists to synthesize them.

Effect of vacancy concentration on the lattice thermal conductivity of CH3NH3PbI3: a molecular dynamics study.

Hybrid halide perovskites are drawing great interest for photovoltaic and thermoelectric applications, but the relationship of thermal conductivities with vacancy defects remains unresolved. Here, we present a systematic investigation of the thermal conductivity of perfect and defective CH3NH3PbI3, performed using classical molecular dynamics with an ab initio-derived force field. We calculate the lattice thermal conductivity of perfect CH3NH3PbI3 as the temperature increases from 300 K to 420 K, confirming a good agreement with the previous theoretical and experimental data. Our calculations reveal that the thermal conductivities of defective systems at 330 K, containing vacancy defects such as VMA, VPb and VI, decrease overall with some slight rises, as the vacancy concentration increases from 0 to 1%. We show that such vacancies act as phonon scattering centers, thereby reducing the thermal conductivity. Moreover, we determine the elastic moduli and sound velocities of the defective systems, revealing that their slower sound speed is responsible for the lower thermal conductivity. These results could be useful for developing hybrid halide perovskite-based solar cells and thermoelectric devices with high performance.