First-principles Calculations Research Articles

Experimental and ab initio studies of enhance photocatalytic efficiency of La-Doped ZnO/g-C3N4 nanocomposites for bromothymol blue dye degradation

The ability of photocatalysis to clean up environmental contamination, notably in the treatment of water and air, has received extensive research. The composite materials synthesized by the coprecipitation method were made of ZnO and graphitic carbon nitride (g-C3N4) with various concentrations of lanthanum doping (La-ZnO). It was discovered that La metal ions were evenly distributed throughout the layered stacking structures of ZnO nanosheet, which had an impact on the band structure and increased the rate of electron-hole separation and visible light absorption. First-principles calculations show that La-doped ZnO-gC3N4 possesses a direct band gap and a type-II band structure with the conduction band minimum and the valence band maximum located in separate layers. A built-in electric field from the gC3N4 to the La-doped ZnO has been established through an analysis of charge density difference, Bader charge, work function, and band alignment. The electron-hole pair becomes spatially separated as a result, which increases the photodegradation activity. High interfacial charge transfer, efficient charge separation and migration, and high visible light optical absorbance were caused by the direct coating of ZnO on an ultra-thin g-C3N4 layer. The 0.6% La-doped ZnO-gC3N4 degraded 97% of the bromothymol blue (BB) dye, which was four times higher than the bulk ZnO, which could remove only about 21% of the dye at the same time. The combined effects of lower photo corrosion, higher solar light usage owing to a hollow structure and heterostructured semiconductor photocatalysis contribute to this increased photocatalytic activity. The capacity of La-doped ZnO/g-C3N4 nanocomposite to effectively break down BB dye via photocatalysis suggests that it has the potential to address the problems associated with dye pollution in water systems.

Disorder induced band gap lowering in kesterite type Cu2ZnSnSe4 and Ag2ZnSnSe4: a first-principles and special quasirandom structures investigation

Quaternary chalcogenides, i.e. Cu2ZnSnS4, crystallising in the kesterite crystal structure have already been demonstrated as potential building blocks of thin film solar cells, containing only abundant elements and exhibiting power conversion efficiencies of about 14.9% so far. However, due to the potential presence of several structurally similar polymorphs, the unequivocal identification of their ground state crystal structures required the application of more elaborate neutron diffraction experiments. One particular complication arose from the later identified Cu–Zn disorder, present in virtually all thin film samples. Subsequently, it has been shown experimentally that this unavoidable Cu–Zn disorder leads to a band gap lowering in the respective samples. Additional theoretical investigations, mostly based on Monte-Carlo methods, tried to understand the atomistic origin of this disorder induced band gap lowering. Here, we present theoretical results from first-principles calculations based on density functional theory for the disorder induced band gap lowering in kesterite Cu2ZnSnSe4 and Ag2ZnSnSe4, where the Cu–Zn and Ag–Zn disorder is modelled via a supercell approach and special quasirandom structures. Results of subsequent analyses of structural, electronic, and optical properties are discussed with respect to available experimental results, and will provide additional insight and knowledge towards the atomistic origin of the observed disorder induced band gap lowering in kesterite type materials.

Deep-Ultraviolet Bialkali-Rare Earth Metal Anhydrous Sulfate Birefringent Crystal.

Birefringence is an important linear optical property of anisotropic crystals that plays a significant role in regulating light polarization. A new bialkali-rare earth metal sulfate, NaRbY2(SO4)4 compound, consisting of non-π-conjugated alkali metals and rare earth metal-centered dodecahedral YO8 has been synthesized. The structure analysis suggests that the three-dimensional (3D) structure of the compound is found to be attributable to the combination of dodecahedral YO8 and tetrahedral SO4 groups with Na+ and Rb+ located in the cavities. The ultraviolet, visible, and near-infrared (UV-vis-NIR) spectra reveal that the compound exhibits transparency at a wavelength of less than 200 nm. The observed birefringence of the compound is 0.045@550 nm, which is comparatively larger than that of most deep-ultraviolet (DUV) birefringent crystals. The birefringence mainly originated from the YO8 dodecahedron, which is suggested by first-principles calculations. This research work can provide a useful perspective to explore new DUV sulfates with excellent birefringence.

Hexagonal Bipyramidal Mn3Ge5 Cluster and Its One-Dimensional Ferrimagnetic Sandwich Nanowire: Mn3Ge3 Rings as Structural Motifs.

Bipyramidal structures featuring planar rings serve as potential building blocks for one-dimensional (1D) nanostructures. Pure Ge atoms typically prefer to form three-dimensional rather than planar structures. Although a few-metal-doped bipyramids with pure Ge planar rings are predicted for constructing Ge-based 1D nanostructures, there is limited knowledge about those with both Ge and doped atoms on the same planar rings. Here, we report a hexagonal bipyramidal Mn3Ge5 cluster containing a Mn3Ge3 six-membered ring with the potential to construct a 1D germanium-based nanostructure. We investigated the structures and properties of Mn3Ge5-/0 using anion photoelectron spectroscopy and theoretical calculations. Mn3Ge5- has a C3v symmetric distorted hexagonal bipyramidal structure, while Mn3Ge5 has a C2v symmetric hexagonal bipyramidal structure. Chemical bonding analyses show that Mn3Ge5- could be considered as a [Mn3]V[Ge5]6- complex. First-principles calculations indicate that Mn3Ge5 may be used to construct a 1D ferrimagnetic [Mn3Ge5]∞ nanostructure.

Narrow-Bandgap Tellurium-Based Chiral Hybrid Perovskite Single Crystals with Rashba-Dresselhaus Effect and Piezoelectricity.

Chiral hybrid perovskites have aroused great interest due to their unique versatile properties. However, designing chiral perovskites with narrow bandgaps is challenging, with their electronic properties such as the Rashba-Dresselhaus effect and piezoelectricity remaining unclear. Herein, single crystals of zero-dimensional (0D) tellurium-based chiral hybrid perovskite, (R-/S-α-PEA)2TeI6 and (R-/S-α-PEA)2TeBr6 (PEA = phenylethylammonium), with sizes of over 5 mm are grown by seed-crystal-assisted solution-temperature-lowering. The optical bandgaps are about 1.60 and 2.18 eV for the iodide and bromide analogues, respectively, which are the lowest among various chiral lead-free hybrid perovskites with the same halide ions in the X-site to the best of our knowledge. First-principles calculations reveal that (R-/S-α-PEA)2TeBr6 shows a larger Rashba-Dresselhaus spin-splitting than (R-/S-α-PEA)2TeI6, probably thanks to the greater distortion of [TeBr6] octahedra. Moreover, the piezoelectric coefficients d33 of (R-/S-α-PEA)2TeI6 and (R-/S-α-PEA)2TeBr6 are about 2.6 and 1.8 pC N-1, respectively. This work deepens the understanding of physical properties of 0D tellurium-based chiral perovskites with potential multifunctionality, including spintronic and piezoelectric performances.

Modelling Photocatalytic N2 Reduction: Where We Stand and Where We Are Going.

Artificial ammonia synthesis via the Haber-Bosch process is environmentally problematic due to the high energy consumption and corresponding CO2 emissions, produced during the reaction and before hand in hydrogen production upon methane steam reforming. Photocatalytic nitrogen fixation as a greener alternative to the conventional Haber-Bosch process enables us to perform nitrogen reduction reaction (NRR) under mild conditions, harnessing light as the energy source. Herein, we systematically review first-principles calculations used to determine the electronic/optical properties of photocatalysts, N2 adsorption and to expound possible NRR mechanisms. The most commonly studied photocatalysts for nitrogen fixation are usually modified with dopants, defects, co-catalysts and Z-scheme heterojunctions to prevent charge carrier recombination, improve charge separation efficiency and adjust a band gap to for utilizing a broader light spectrum. Most studies at the atomistic level of modeling are grounded upon DFT calculations, wholly foregoing excitation effects paramount in photocatalysis. Hence, there is a dire need to consider methods beyond DFT to study the excited state properties more accurately. Furthermore, a few studies have been examined, which include higher level kinetics and macroscale simulations. Ultimately, we show there is still ample room for improvement with regard to first principles calculations and their integration in multiscale models.

Ternary Rotational Polyanion Coupling Enables Fast Li Ion Dynamics in Tetrafluoroborate Ion Doped Antiperovskite Li2OHCl Solid Electrolyte.

Li-rich antiperovskite (LiRAP) hydroxyhalides are emerging as attractive solid electrolyte (SEs) for all-solid-state Li metal batteries (ASSLMBs) due to their low melting point, low cost, and ease of scaling-up. The incorporation of rotational polyanions can reduce the activation energy and thus improve the Li ion conductivity of SEs. Herein, we propose a ternary rotational polyanion coupling strategy to fasten the Li ion conduction in tetrafluoroborate (BF4-) ion doped LiRAP Li2OHCl. Assisted by first-principles calculation, powder X-ray diffraction, solid-state magnetic resonance and electrochemical impedance spectra, it is confirmed that Li ion transport in BF4- ion doped Li2OHCl is strongly associated with the rotational coupling among OH-, BF4- and Li2-O-H octahedrons, which enhances the Li ion conductivity for more than 1.8 times with the activation energy lowering 0.03 eV. This work provides a new perspective to design high-performance superionic conductors with multi-polyanions.

First-principles study of two-dimensional transition metal carbide M n+1 C n O2 (M = Nb,Ta)

In the present work, the three stable MXenes M n+1 C n O2 (M = Nb,Ta) are explored based on first-principles calculations. These materials are important derivatives of 2D materials and exhibit distinctive properties, holding vast potential in nanodevices. All these M n+1 C n O2 (M = Nb,Ta) materials exhibit outstanding superconducting performance, with corresponding superconducting transition temperatures of 23.00 K, 25.00 K, and 29.00 K. Analysis reveals that the high superconducting transition temperatures of MXenes M n+1 C n O2 (M = Nb,Ta) are closely associated with the high value of the logarithmic average of phonon frequencies, ωlog , and the strong electron–phonon coupling, attributed to the crucial contribution of low-frequency phonons. Additionally, we applied strain treatments of 2% and 4% to M n+1 C n O2 (M = Nb,Ta), resulting in varying changes in superconducting transition temperatures under different strains.

‘Fraternal-twin’ ferroelectricity: competing polar states in hydrogen-doped samarium nickelate from first principles

In ferroelectric switching, an applied electric field switches the system between two polar symmetry-equivalent states. In this work, we use first-principles calculations to explore the polar states of hydrogen-doped samarium nickelate (SNO) at a concentration of 1/4 hydrogen per Ni. The inherent tilt pattern of SNO and the presence of the interstitial hydrogen present an insurmountable energy barrier to switch these polar states to their symmetry-equivalent states under inversion. We find a sufficiently low barrier to move the localized electron to a neighboring NiO6 octahedron, a state unrelated by symmetry but equal in energy under a square epitaxial strain (a = b), resulting in a large change in polarization. We term this unconventional ferroelectric a ‘fraternal-twin’ ferroelectric.

First-Principles Study of the Schottky Contact, Tunneling Probability, and Optical Properties of MX/TiB4 Heterojunctions (M = Ge, Sn; X = S, Se, Te): Strain Engineering Tunability.

Designing two-dimensional (2D) heterojunctions with rapid response and minimal energy consumption holds immense significance for the advancement of the next generation of electronic devices. Here, we construct a series of Schottky heterojunctions based on TiB4 monolayer and group-IV monochalcogenide monolayers MX (M = Ge, Sn; X = S, Se, Te). Using first-principles calculations, we investigate the structural stability, Schottky contact barrier, tunneling probability, and optical properties of MX/TiB4 heterojunctions. The calculated binding energies reveal that X-type MX/TiB4 heterojunctions exhibit more stable structures than M- and C-type stacking modes. Schottky barrier heights (SBHs) indicate that X-type GeSe/TiB4 and GeTe/TiB4 form n-type Schottky contacts with SBHs of 0.497 and 0.132 eV, respectively, while SnS/TiB4 and SnSe/TiB4 form p-type Schottky contacts with SBHs of 0.557 and 0.418 eV, respectively. Moreover, X-type MX/TiB4 heterojunctions exhibit high susceptibility to interlayer electron tunneling due to their large tunneling probability and strong interlayer interaction. Meanwhile, enhanced optical absorption capacity in MX/TiB4 heterojunctions is also observed compared with individual TiB4 and MX monolayers. By applying in-plane biaxial strain, the transformation of MX/TiB4 heterojunctions from a Schottky contact to an Ohmic contact can also be realized. Our findings could offer valuable candidate materials and guidance for the design of the next generation of nanodevices with high electronic and optical performances.

On the Origin of the Above-Room-Temperature Magnetism in the 2D van der Waals Ferromagnet Fe3GaTe2.

2D magnetic materials have attracted growing interest driven by their unique properties and potential applications. However, the scarcity of systems exhibiting magnetism at room temperature has limited their practical implementation into functional devices. Here we focus on the van der Waals ferromagnet Fe3GaTe2, which exhibits above-room-temperature magnetism (Tc = 350-380 K) and strong perpendicular anisotropy. Through first-principles calculations, we examine the magnetic properties of Fe3GaTe2 and compare them with those of Fe3GeTe2. Our calculations unveil the microscopic mechanisms governing their magnetic behavior, emphasizing the pivotal role of ferromagnetic in-plane couplings in the stabilization of the elevated Tc in Fe3GaTe2. Additionally, we predict the stability, substantial perpendicular anisotropy, and high Tc of the single-layer Fe3GaTe2. We also demonstrate the potential of strain engineering and electrostatic doping to modulate its magnetic properties. Our results incentivize the isolation of the monolayer and pave the way for the future optimization of Fe3GaTe2 in magnetic and spintronic nanodevices.

New Insight into Bulk Structural Degradation of High-Voltage LiCoO2 at 4.55 V.

The aggravated mechanical and structural degradation of layered oxide cathode materials upon high-voltage charging invariably causes fast capacity fading, but the underlying degradation mechanisms remain elusive. Here we report a new type of mechanical degradation through the formation of a kink band in a Mg and Ti co-doped LiCoO2 cathode charged to 4.55 V (vs Li/Li+). The local stress accommodated by the kink band can impede crack propagation, improving the structural integrity in a highly delithiated state. Additionally, machine-learning-aided atomic-resolution imaging reveals that the formation of kink bands is often accompanied by the transformation from the O3 to O1 phase, which is energetically favorable as demonstrated by first-principles calculations. Our results provide new insights into the mechanical degradation mechanism of high-voltage LiCoO2 and the coupling between electrochemically triggered mechanical failures and structural transition, which may provide valuable guidance for enhancing the electrochemical performance of high-voltage layered oxide cathode materials for lithium-ion batteries.

Design of Single-atom Catalysts on C5N2 forNitrogen Fixation at Ambient Conditions : A First-principles Study.

Single atom catalysts (SACs) exhibit the flexible coordinationstructure of the active site and high utilization of activeatoms, making them promising candidates for nitrogenreduction reaction (NRR) under ambient conditions. Bythe aid of first-principles calculations based on DFT, wehavesystematically explored the NRR catalytic behavior ofthirteen 4d- and 5d-transition metal atoms anchored on 2Dporous graphite carbon nitride C5N2. With high selectivityand outstanding activity, Zr, Nb, Mo, Ta, W and Re-dopedC5N2 are identified as potential nominees for NRR. Particularly,Mo@C5N2 possesses an impressive low limiting potentialof -0.39 V (corresponding to a very low temperatureand atmospheric pressure), featuring the potential determiningstep involving *N-N transitions to *N-NH via thedistal path. The catalytic performance of TM@C5N2 canbe well characterized by the adsorption strength of intermediate*N2H. Moreover, there exists a volcanic relationshipbetween the catalytic property UL and the structure descriptorΨ, which validates the robustness and universalityof Ψ, combined with our previous study. This work sheds light on the design of SACs with eminent NRR performance.

Confinement Effects on Proton Transfer in TiO2 Nanopores from Machine Learning Potential Molecular Dynamics Simulations.

Improved understanding of proton transfer in nanopores is critical for a wide range of emerging applications, yet experimentally probing mechanisms and energetics of this process remains a significant challenge. To help reveal details of this process, we developed and applied a machine learning potential derived from first-principles calculations to examine water reactivity and proton transfer in TiO2 slit-pores. We find that confinement of water within pores smaller than 0.5 nm imposes strong and complex effects on water reactivity and proton transfer. Although the proton transfer mechanism is similar to that at a TiO2 interface with bulk water, confinement reduces the activation energy of this process, leading to more frequent proton transfer events. This enhanced proton transfer stems from the contraction of oxygen-oxygen distances dictated by the interplay between confinement and hydrophilic interactions. Our simulations also highlight the importance of the surface topology, where faster proton transport is found in the direction where a unique arrangement of surface oxygens enables the formation of an ordered water chain. In a broader context, our study demonstrates that proton transfer in hydrophilic nanopores can be enhanced by controlling pore size, surface chemistry, and topology.

Two-Dimensional Ultrathin Fe3Sn2 Kagome Metal with Defect-Dependent Magnetic Property.

Two-dimensional (2D) Fe3Sn2, which is a room-temperature ferromagnetic kagome metal, has potential applications in spintronic devices. However, the systematic synthesis and magnetic study of 2D Fe3Sn2 single crystals have rarely been reported. Here we have synthesized 2D hexagonal and triangular Fe3Sn2 nanosheets by controlling the amount of FeCl2 precursors in the chemical vapor deposition (CVD) method. It is found that the hexagonal Fe3Sn2 nanosheets exist with Fe vacancy defects and show no obvious coercivity. While the triangular Fe3Sn2 nanosheet has obvious hysteresis loops at room temperature, its coercivity first increases and then remains stable with an increase in temperature, which should result from the competition of the thermal activation mechanism and spin direction rotation mechanism. A first-principles calculation study shows that the Fe vacancy defects in Fe3Sn2 can increase the distances between Fe atoms and weaken the ferromagnetism of Fe3Sn2. The resulting 2D Fe3Sn2 nanosheets provide a new choice for spintronic devices.

First-Principles Study on Polymer Electrolyte Interface Engineering for Lithium Metal Anodes.

Modifying the interface between the lithium metal anode (LMA) and the electrolyte is crucial for achieving high-performance lithium metal batteries (LMBs). Recent research indicates that altering Li-metal interfaces with polymer coatings is an effective approach to extend LMBs' cycling lifespan. However, the physical properties of these polymer-Li interfaces have not yet been fully investigated. Therefore, the structural stability, electronic conductivity, and ionic conductivity of polymer-Li interfaces were examined based on first-principles calculations in this study. Several representative polymer compounds utilized in LMBs were assessed, including polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polyethylene oxide (PEO). Our research revealed that lithium fluoride is formed upon fluoropolymer degradation, explaining previously observed experimental results. Polymers containing nitrile groups exhibit strong adhesion to lithium metal, facilitating the formation of the stable interface layer. Regarding electronic conductivity, the fluoropolymers preserve a good insulating property, which diminished marginally in the presence of lithium, but that of PAN and PEO significantly reduces. Additionally, lithium diffusion on PTFE and PEO demonstrates low diffusion barriers and high coefficients, enabling easy transportation. Overall, our investigation reveals that the interfaces formed between various polymers and LMA have distinct characteristics, providing new fundamental insights for designing composites with tailored interface properties.

Effect of Al and Si content on properties of Ti(1-x-y)AlxSiyN coating materials: First-principles calculation

This paper investigates the effect of Al and Si atomic doping on the Ti(1-x-y)AlxSiyN alloy using density functional theory (DFT). The results indicate that the phase structure stability of the alloy decreases with increasing Al and Si doping content. Particularly, excessive doping leads to the transformation of crystal structure from rocksalt to wurtzite structure with lower mechanical properties. The bulk modulus of the alloy decreases with the increase of doping content. Specifically, Al atom doping initially reduces and then increases the shear modulus and Young's modulus of Ti1-xAlxN, while Si atom doping causes the shear modulus and Young's modulus of Ti0.375-yAl0.625SiyN to increase first and then decrease. In terms of hardness, the hardness of Ti1-xAlxN increases with the increase of Al doping, but the toughness decreases. Si atom doping further enhances the alloy's hardness, reaching a maximum of 29.8 GPa, consistent with the trends observed in experimental results. Electronic structure analysis reveals that Al and Si doping induces orbital hybridization, enhancing the bonding ability and chemical bond strength between atoms. The results confirm that the ionic bonds, metal bonds and covalent bonds in multi-component nitride jointly determine the mechanical properties of the alloy.

Strain-tunable electronic structure, optical and thermoelectric properties of BAs

Strain engineering stands as a reliable method for tailoring the physicochemical properties of materials to achieve desired performance. However, the effects of strain on the physicochemical properties of BAs remain unclear, impeding the comprehensive understanding of its practical performance. Here, employing first-principles calculations coupled with semiclassical Boltzmann transport theory, we investigate the dynamic stability, mechanical stability, electronic structure, and thermoelectric properties of cubic boron arsenide (BAs) under various strains. The results demonstrate that BAs maintains excellent stability throughout the triaxial strain range. The electronic structure of BAs is less affected by strain. Young’s modulus and Poisson’s ratio show a corresponding linear increase when the compression strain increases. The optical absorption coefficient in the visible region of BAs under tensile strain showed an overall increasing trend, and the optical absorption coefficient in the visible wavelength region of BAs under 5% tensile strain was as high as 2 × 105 cm−1. The thermoelectric properties of BAs under tensile strain have been improved, and the ZT value of BAs under 5% tensile strain at 1500 K has been increased to 0.6. The research findings address the gaps in understanding the properties of BAs under strain and provide theoretical support for its applications in the fields of thermoelectrics and optoelectronics.

First-Principles Insights into the Selectivity of CO2 Electroreduction over Heterogeneous Single-Atom Catalysts.

Heterogeneous metal-nitrogen-carbon (M-N-C) single-atom catalysts (SACs) have garnered considerable attention in the two-electron CO2 reduction reaction (2e-CO2RR). Interestingly, almost M-N-C SACs mainly produce CO, while Sb is one of the few SACs reported so far that can produce HCOOH. Nevertheless, the underlying factors for different selectivities on Sb-N-C SAC remain controversial, and the lack of in-depth understanding of limiting factors hampers further regulations. Here, by using constant-potential first-principles calculations, we revealed that the high HCOOH selectivity of Sb-N-C SAC is mainly attributed to their weak charge accumulation ability. Remarkably, considering the highly tunable geometric structure of M-N-C SACs, we provide that Sb-N-C SAC with the SbN3S1 center is a promising candidate for CO production. Our work provides the mechanism insight into 2e-CO2RR selectivity and further paves the way toward electrocatalyst regulation and design.

Hyperbolic response and low-frequency ultra-flat plasmons in inhomogeneous charge-distributed transition-metal monohalides

Plasmon, the collective oscillations of free electron gas in materials, determines the long-wavelength excitation spectrum and optical response, are pivotal in the realm of nanophotonics and optoelectronics. In this study, using the first-principles calculations, we systematically investigated the dielectric response and plasmon properties of bulk transition-metal monohalides MXs (M = Zr, Mo; X = Cl, F). Due to the strong electronic anisotropy, MXs exhibit a broadband type-II hyperbolic response and direction-dependent plasmon modes. Particularly, local field effect (LFE) driven by the charge distribution inhomogeneity, significantly modifies the optical response and excitation spectra in MX along the out-of-plane direction. Taking into account LFE, the energy dissipation along the out-of-plane direction is almost completely suppressed, and an ultra-flat and long-lived plasmon mode with a slow group velocity is introduced. This finding reveals the role of charge density in modifying the optical response and excitation behavior, shedding light on potential applications in plasmonics.