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.

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.

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.

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.

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.

Interface Engineering in Hybrid Iodide CH3NH3PbI3 Perovskites Using Lewis Base and Graphene toward High-Performance Solar Cells.

Photovoltaic solar cells based on organic-inorganic hybrid halide perovskites have achieved a substantial breakthrough via advanced interface engineering. Reports have emphasized that combining the hybrid perovskites with the Lewis base and/or graphene can definitely improve the performance through surface trap passivation and band alignment alteration; the underlying mechanisms are not yet fully understood. Here, using density functional theory calculations, we show that upon the formation of CH3NH3PbI3 interfaces with three different Lewis base molecules and graphene, the binding strength with S-donors thiocarbamide and thioacetamide is higher than with O-donor dimethyl sulfoxide, while the interface dipole and work function reduction tend to increase from S-donors to O-donor. We provide evidences of deep trap state elimination in the S-donor perovskite interfaces through the analysis of defect formation on the CH3NH3PbI3(110) surface and of stability enhancement by estimation of activation barriers for vacancy-mediated iodine atom migrations. These theoretical predictions are in line with the experimental observation of performance enhancement in the perovskites prepared using thiocarbamide.

First-principles study of NaxTiO2 with trigonal bipyramid structures: an insight into sodium-ion battery anode applications.

Developing efficient anode materials with low electrode voltage, high specific capacity and superior rate capability is urgently required on the road to commercially viable sodium-ion batteries (SIBs). Aiming at finding a new SIB anode material, we investigate the electrochemical properties of NaxTiO2 compounds with unprecedented penta-oxygen-coordinated trigonal bipyramid (TB) structures by using first-principles calculations. Identifying the four different TB phases, we perform the optimization of their crystal structures and calculate their energetics such as sodium binding energy, formation energy, electrode potential and activation energy for Na ion migration. The computations reveal that the TB-I phase is the best choice among the four TB phases for a SIB anode material due to a relatively low volume change of under 4% upon Na insertion, low electrode voltage under 1.0 V with a possibility of realizing the highest specific capacity of ∼335 mA h g-1 from full sodiation at x = 1, and reasonably low activation barriers under 0.35 eV at the Na content from x = 0.125 to x = 0.5. Through the analysis of electronic density of states and charge density difference upon sodiation, we find that the NaxTiO2 compounds in TB phases change from electron insulating to electron conducting materials due to the electron transfer from Na atoms to Ti ions, offering the Ti4+/Ti3+ redox couple for SIB operation.

First-Principles Study on Structural, Electronic, and Optical Properties of Inorganic Ge-Based Halide Perovskites.

Using density functional theory calculations, we explore the structural, electronic, and optical properties of the inorganic Ge-based halide perovskites AGeX3 (A = Cs, Rb; X = I, Br, Cl) that can possibly be used as light absorbers. We calculate the lattice parameters of the rhombohedral unit cell with an R3 m space group, frequency-dependent dielectric constants, photoabsorption coefficients, effective masses of charge carriers, exciton binding energies, and electronic band structures by use of PBEsol and HSE06 functionals with and without SOC effect. We also predict the absolute electronic energy levels with respect to the external vacuum level by using the (001) surfaces with AX and GeX2 terminations, demonstrating their strong dependence on the surface terminations. The calculated results are found to be in reasonable agreement with the available experimental data for the cases of CsGeX3, while for the cases of RbGeX3 they are predicted for the first time in this work. We reveal that replacement of Cs with Rb can offer reasonable flexibility in optoelectronic properties matching for solar cell design and optimization, while X anion exchange gives rise to large changes.

Computational prediction of structural, electronic, and optical properties and phase stability of double perovskites K2SnX6 (X = I, Br, Cl).

The vacancy-ordered double perovskites K2SnX6 (X = I, Br, Cl) attract significant research interest due to their potential applications as light absorbing materials in perovskite solar cells. However, deeper insight into their material properties at the atomic scale is currently lacking. Here we present a systematic investigation of the structural, electronic, and optical properties and phase stabilities of the cubic, tetragonal, and monoclinic phases based on density functional theory calculations. Quantitatively reliable predictions of lattice constants, band gaps, effective masses of charge carriers, and exciton binding energies are provided and compared with the available experimental data, revealing the tendency of the band gap and exciton binding energy to increase on lowering the crystallographic symmetry from cubic to monoclinic and on moving from I to Cl. We highlight that cubic K2SnBr6 and monoclinic K2SnI6 are suitable for applications as light absorbers for solar cell devices due to their appropriate band gaps of 1.65 and 1.16 eV and low exciton binding energies of 59.4 and 15.3 meV, respectively. The constant-volume Helmholtz free energies are determined through phonon calculations, which predict phase transition temperatures of 449, 433 and 281 K for cubic-tetragonal and 345, 301 and 210 K for tetragonal-monoclinic transitions for X = I, Br and Cl, respectively. Our calculations provide an understanding of the material properties of the vacancy-ordered double perovskites K2SnX6, which could help in devising a low-cost and high performance perovskite solar cell.

Critical Role of Water in Defect Aggregation and Chemical Degradation of Perovskite Solar Cells.

The chemical stability of methylammonium lead iodide (MAPbI3) under humid conditions remains the primary challenge facing halide perovskite solar cells. We investigate defect processes in the water-intercalated iodide perovskite (MAPbI3_H2O) and monohydrated phase (MAPbI3·H2O) within a first-principles thermodynamic framework. We consider the formation energies of isolated and aggregated vacancy defects with different charge states under I-rich and I-poor conditions. It is found that a PbI2 (partial Schottky) vacancy complex can be formed readily, while the MAI vacancy complex is difficult to form in the hydrous compounds. Vacancies in the hydrous phases create deep charge transition levels, indicating the degradation of the lead halide perovskite upon exposure to moisture. Electronic structure analysis supports a mechanism of water-mediated vacancy pair formation.

Ab initio thermodynamic study of the SnO2(110) surface in an O2 and NO environment: a fundamental understanding of the gas sensing mechanism for NO and NO2.

For the purpose of elucidating the gas sensing mechanism of SnO2 for NO and NO2 gases, we determine the phase diagram of the SnO2(110) surface in contact with an O2 and NO gas environment by means of an ab initio thermodynamic method. Firstly we build a range of surface slab models of oxygen pre-adsorbed SnO2(110) surfaces using (1 × 1) and (2 × 1) surface unit cells and calculate their Gibbs free energies considering only oxygen chemical potential. The fully reduced surface containing the bridging and in-plane oxygen vacancies under oxygen-poor conditions, while the fully oxidized surface containing the bridging oxygen atom and the oxygen dimer under oxygen-rich conditions, and the stoichiometric surface in between, was proved to be most stable. Using the selected plausible NO-adsorbed surfaces, we then determine the surface phase diagram of SnO2(110) surfaces in (ΔµO, ΔµNO) space. Under NO-rich conditions, the most stable surfaces were those formed by NO adsorption on the most stable surfaces in contact with only oxygen gas. Through the analysis of electronic charge transfer and density of states during NOx adsorption on the surface, we provide a meaningful understanding about the gas sensing mechanism.