Spin Density Functional Theory Research Articles

Combining muon spin relaxation and DFT simulations of hydrogen trapping in Al6Mn

Hydrogen at the mass ppm level causes hydrogen embrittlement in metallic materials, but experimentally elucidating the hydrogen trapping sites is extremely difficult. We exploit the fact that positive muons can act as light isotopes of hydrogen to study the trapping state of hydrogen in matter. Zero-field muon spin relaxation experiments and density functional theory (DFT) calculations of the hydrogen trapping energy are carried out for Al6Mn. The DFT calculations reveal four possible trapping sites for hydrogen in Al6Mn, at which the hydrogen trapping energies are 0.168 (site 1), 0.312 (site 2), 0.364 (site 3), and 0.495 (site 4) in units of eV/atom. The variations in the deduced dipole field width (Δ) with temperature indicate noticeable changes at 94, 193, and 236 K. Considering the site densities, the observed Δ change temperatures are interpreted as muon trapping at sites 1, 3, and 4.

Identification and quantification of electron-generated atomic hydrogen through in-situ electron spin resonance and density functional theory

Electro-generated atomic hydrogen (H*) plays a crucial role in the electrochemical reduction process, serving as the key species that mediates the electrocatalytic hydrogenation reduction of stubborn contaminants in wastewater treatment. However, precisely identifying and quantifying transient atomic H* presents a significant challenge due to its limited lifespan and its existence solely within the boundary layer at the electrode/solution interface. Herein, we developed the electrodeposition of palladium nanoparticles onto carbon cloth and assessed its effectiveness as a cathode for generating and stabilizing atomic H*. The environmental application of atomic H* was validated through the dechlorination of 2, 4-dichlorophenol wastewater and the reduction of antimonite wastewater. Additionally, the identification of atomic H* was verified by electrochemical measurements, high-resolution mass spectra, and density functional theory. Moreover, introducing an excess of a spin trapping agent (5,5-dimethyl-1-pyrroline-N-oxide) and fast in-situ spin trapping facilitated creating favorable conditions for efficient trapping of atomic H* and subsequent electron spin resonance (ESR) spectroscopy quantification analysis. Subsequently, the quantification of atomic H* was achieved by double integration of the ESR signal of spin adduct and comparison with the external standard agent (4-hydroxy-2,2,6,6-tetramethyl-1-piperidine 1-oxyl). This study introduces a novel method for in-situ spin trapping and quantification of atomic H*, facilitating the advancement of electrochemical reduction technology and its application in wastewater treatment.

Continuous ammonia synthesis from water and nitrogen via contact electrification

We synthesized ammonia (NH3) by bubbling nitrogen (N2) gas into bulk liquid water (200 mL) containing 50 mg polytetrafluoroethylene (PTFE) particles (~5 µm in diameter) suspended with the help of a surfactant (Tween 20, ~0.05 vol.%) at room temperature (25 °C). Electron spin resonance spectroscopy and density functional theory calculations reveal that water acts as the proton donor for the reduction of N2. Moreover, isotopic labeling of the N2 gas shows that it is the source of nitrogen in the ammonia. We propose a mechanism for ammonia generation based on the activation of N2 caused by electron transfer and reduction processes driven by contact electrification. We optimized the pH of the PTFE suspension at 6.5 to 7.0 and employed ultrasonic mixing. We found an ammonia production rate of ~420 μmol L-1 h-1 per gram of PTFE particles for the conditions described above. This rate did not change more than 10% over an 8-h period of sustained reaction.

Non-collinear magnetic atomic cluster expansion for iron

The Atomic Cluster Expansion (ACE) provides a formally complete basis for the local atomic environment. ACE is not limited to representing energies as a function of atomic positions and chemical species, but can be generalized to vectorial or tensorial properties and to incorporate further degrees of freedom (DOF). This is crucial for magnetic materials with potential energy surfaces that depend on atomic positions and atomic magnetic moments simultaneously. In this work, we employ the ACE formalism to develop a non-collinear magnetic ACE parametrization for the prototypical magnetic element Fe. The model is trained on a broad range of collinear and non-collinear magnetic structures calculated using spin density functional theory. We demonstrate that the non-collinear magnetic ACE is able to reproduce not only ground state properties of various magnetic phases of Fe but also the magnetic and lattice excitations that are essential for a correct description of finite temperature behavior and properties of crystal defects.

Spectral approximation scheme for a hybrid, spin-density Kohn–Sham density-functional theory in an external (nonuniform) magnetic field and a collinear exchange-correlation energy

AbstractWe provide a mathematical justification of a spectral approximation scheme known as spectral binning for the Kohn–Sham spin density-functional theory in the presence of an external (nonuniform) magnetic field and a collinear exchange-correlation energy term. We use an extended density-only formulation for modeling the magnetic system. No current densities enter the description in this formulation, but the particle density is split into different spin components. By restricting the exchange-correlation energy functional to be of a collinear LSDA form, we prove a series of results which enable us to mathematically justify the spectral binning scheme using the method of Gamma-convergence, in conjunction with auxiliary steps involving recasting the electrostatic potentials, justifying the spectral approximation by making a spectral decomposition of the Hamiltonian and “linearizing" the latter Hamiltonian.

Cobalt Doping Effects in Zinc Oxide: A Combined Experimental and Ab Initio Approach

In this paper, we investigate the solubility effects of Co in ZnO (Zn1−xCoxO, where x = 0, 0.03, 0.05, 0.1, 0.2, 0.25, 0.4, 0.8, and 1) by combining the results of perturbed angular correlation (PAC) spectroscopy using highly diluted 111Cd as probe nuclei and ab initio calculations based on spin-density functional theory (SDFT). This combined approach enables us to characterize the local structure around Cd ions, where, through PAC technique, it was possible to measure the EFG as a function of temperature and Co concentration and thereby monitor the changes in the structure and the Co solubility threshold in ZnO and the ZnO/CoO/Co3O4 mixed phase. The full-potential linear augmented plane wave plus local orbital (APW+lo) formalism were used here to describe the electronic structure of the supercells, including the atomic relaxations. These Ab initio calculations show an interesting behavior of the Cd and Co impurity levels in the band structure of ZnO, which explains the experimental results in terms of the origin of EFG and the evidence of ferromagnetic response.

Design and Fabrication of Biomass Derived Black Carbon Modified g‐C3N4/FeIn2S4 Heterojunction as Highly Efficient Photocatalyst for Wastewater Treatment

AbstractThe environmental deterioration caused by dye wastewater discharge has received considerable attention in recent decades. One of the most promising approaches to addressing the aforementioned environmental issue is the development of photocatalysts with high solar energy consumption efficiency for the treatment of dye‐contaminated water. In this study, a novel low‐cost π–π biomass‐derived black carbon modified g‐C3N4 coupled FeIn2S4 composite (i.e., FeInS/BC‐CN) photocatalyst is successfully designed and fabricated that reveals significantly improved photocatalytic performance for the degradation of Eosin Yellow (EY) dye in aqueous solution. Under dark and subsequent visible light irradiation, the amount optimized composite reveals 99% removal performance for EY dye, almost three‐fold compared to that of the pristine FeInS and BC‐CN counterparts. Further, it is confirmed by means of the electron spin resonance spectrometry, quenching experiments, and density functional theory (DFT) calculations, that the hydroxyl radicals (•OH) and superoxide radicals (•O2−) are the dominant oxidation species involved in the degradation process of EY dye. In addition, a systematic photocatalytic degradation route is proposed based on the resultant degradation intermediates detectedduring liquid chromatography tandem mass spectrometry (LC‐MS/MS) analysis. This work provides an innovative idea for the development of advanced photocatalysts to mitigate water pollution.

Electrocatalytic degradation of p-nitrophenol on metal-free cathode: Superoxide radical (O2•−) production via molecular oxygen activation

Although metal-free electrodes in molecular oxygen-activated Fenton-like wastewater treatment technologies have been developed, the reactive oxygen species (ROS) generation mechanisms are still not sufficiently clear. As a typical example of refractory phenolic wastewater, p-nitrophenol (PNP) has been widely studied. This study demonstrated the critical role of superoxide radicals (O2•−) in PNP degradation by metal-free electrodes through electron spin resonance (ESR), ROS quenching, and density functional theory (DFT) tests. The most superior metal-free electrode exhibited a mass activity of approximately 133.5 h−1 gcatalyst−1. Experimental and theoretical studies revealed the mechanism of O2•− generation via oxygen activation, including one- and three-electron transfer pathways, and found that O2•− mainly attacked the nitro group of PNP to degrade and transform the pollutant. This study enhances the mechanistic understanding of metal-free materials in the electrochemical degradation of refractory pollutants.

Application of on-site hybrid exchange and correlation functional in the study of multiferroic R3c BiFeO3 compound

In this work, we report calculations of the structural, electronic, magnetic, optical and ferroelectric properties of multiferroic R3c BiFeO3 compound using an on-site PBE0 hybrid and the generalized gradient approximation (GGA) exchange and correlation (XC) functional within the framework of spin density functional theory (SDFT). R3c BiFeO3 is a prototypical example of a strongly correlated and magnetoelectric material, which poses a challenge for a standard XC functional (such as GGA). We compare our results with available experimental data and those calculated using the GGA, as well as with previous SDFT calculations employing a different XC functional. We verify that PBE0 calculations of R3c BiFeO3 can predict the lattice parameters, Fe-O distances, energy band gap, spin magnetic moment of the Fe atom, magnetic order, and optical spectra, with results that are in good agreement with the experimental data. This study offers perspectives for further investigations of other multiferroic compounds and other crystalline phases of BiFeO3 using the on-site PBE0 hybrid XC functional.

Magnetic interactions in intercalated transition metal dichalcogenides: A study based on ab initio model construction

Transition metal dichalcogenides (TMDs) are known to have a wide variety of magnetic structures by hosting other transition metal atoms in the van der Waals gaps. To understand the chemical trend of the magnetic properties of the intercalated TMDs, we perform a systematic first-principles study for 48 compounds with different hosts, guests, and composition ratios. Starting with calculations based on spin density functional theory, we derive classical spin models by applying the Liechtenstein method to the ab initio Wannier-based tight-binding model. We show that the calculated exchange couplings are overall consistent with the experiments. In particular, when the composition rate is 1/3, the chemical trend can be understood in terms of the occupation of the 3d-orbital in the intercalated transition metal. The present results give us a useful guiding principle to predict the magnetic structure of compounds that are yet to be synthesized.

Linker Independent Regioselective Protonation Triggered Detoxification of Sulfur Mustards with Smart Porous Organic Photopolymer.

The development of efficient metal-free photocatalysts for the generation of reactive oxygen species (ROS) for sulfur mustard (HD) decontamination can play a vital role against the stockpiling of chemical warfare agents (CWAs). Herein, one novel concept is conceived by smartly choosing a specific ionic monomer and a donor tritopic aldehyde, which can trigger linker-independent regioselective protonation/deprotonation in the polymeric backbone. In this context, the newly developed vinylene-linked ionic polymers (TPA/TPD-Ionic) are further explored for visible-light-assisted detoxification of HD simulants. Time-resolved-photoluminescence (TRPL) study reveals the protonation effect in the polymeric backbone by significantly enhancing the life span of photoexcited electrons. In terms of catalytic performance, TPA-Ionic outperformed TPD-Ionic because of its enhanced excitons formation and charge carrier abilities caused by the donor-acceptor (D-A) backbone and protonation effects. Moreover, the formation of singlet oxygen (1 O2 ) species is confirmed via in-situ Electron Spin Resonance (ESR) spectroscopy and density functional theory (DFT) analysis, which explained the crucial role of solvents in the reaction medium to regulate the (1 O2 ) formation. This study creates a new avenue for developing novel porous photocatalysts and highlights the crucial roles of sacrificial electron donors and solvents in the reaction medium to establish the structure-activity relationship.

Ab initio exploration of short-pitch skyrmion materials: Role of orbital frustration

In recent years, the skyrmion lattice phase with a short lattice constant has attracted attention due to its high skyrmion density, making it a promising option for achieving high-density storage memory and for observing novel phenomena like the quantized topological Hall effect. Unlike conventional non-centrosymmetric systems where the Dzyaloshinsky–Moriya interaction plays a crucial role, the short pitch skyrmion phase requires a quadratic magnetic interaction J ( q ) with a peak at finite- Q, and weak easy-axis magnetic anisotropy is also critical. Thus, conducting first-principles evaluations is essential for understanding the formation mechanism as well as for promoting the discovery of new skyrmion materials. In this Perspective, we focus on recent developments of the first-principles evaluations of these properties and apply them to the prototype systems Gd T 2 X 2 and Eu T 2 X 2, where T denotes a transition metal and X represents Si or Ge. In particular, based on the spin density functional theory with the Hubbard correction combined with the Liechtenstein method in the Wannier tight-binding model formalism, we first show that the Hubbard U and Hund’s coupling is essential to stabilize a skyrmion lattice state by enhancing the easy-axis anisotropy. We then discuss mechanisms of finite- Q instability and show that competition among Gd-5 d orbitals determines whether ferromagnetism or a finite- Q structure is favored in Gd T 2Si 2 with T = Fe and Ru. Our systematic calculations reveal that GdRu 2 X 2, GdOs 2 X 2, and GdRe 2 X 2 are promising, while GdAg 2 X 2, GdAu 2 X 2, and EuAg 2 X 2 are possible candidates as the skyrmion host materials. Analysis based on a spin spiral calculation for the candidate materials is also presented.

Constructing semilocal approximations for noncollinear spin density functional theory featuring exchange-correlation torques

We present a semilocal exchange-correlation energy functional for noncollinear spin density functional theory based on short-range expansions of the spin-resolved exchange hole and the two-body density matrix. Our functional is explicitly derived for noncollinear magnetism, is U(1) and SU(2) gauge invariant, and gives rise to nonvanishing exchange-correlation torques. Testing the functional for frustrated antiferromagnetic chromium clusters, the exchange part is shown to perform favorably compared with the far more expensive Slater and optimized effective potentials, and a delicate interplay between exchange and correlation torques is uncovered.

Low-rank approximations for accelerating plane-wave hybrid functional calculations in unrestricted and noncollinear spin density functional theory.

The adaptively compressed exchange (ACE) operator combined with interpolative separable density fitting (ISDF) decomposition has been utilized to accelerate plane-wave hybrid functional calculations for restricted Kohn-Sham density functional theory (DFT), but the neglect of spin degree of freedom has limited its application in the exploration of systems where the spin property of the electron is critical. Herein, we derive the ACE-ISDF formulation for hybrid functional calculations in both unrestricted and noncollinear spin DFT with plane waves and periodic boundary conditions. We proposed an improved ISDF algorithm for the sum of Kohn-Sham orbital pairs to further reduce the computational cost for the spin-noncollinear case. Numerical results demonstrate that these improved ACE-ISDF low-rank approximations can not only significantly reduce the computational time by two orders of magnitude compared with conventional plane-wave hybrid functional calculations but also lead to a good convergence behavior when a moderate rank parameter is set, even for complex periodic magnetic systems. By using these ACE-ISDF approximations, we investigate the electronic and magnetic properties of two-dimensional periodic ferromagnetic semiconductors consisting of triangular zigzag graphene quantum dots and transition metal atoms. Our computational results showcase that hybrid functional calculations in spin DFT can provide not only accurate electronic structures but also accurate magnetic order temperature of ferromagnetic semiconductors compared to local or semilocal functional calculations.

Magnetization dynamics with time-dependent spin-density functional theory: Significance of exchange-correlation torques

In spin-density functional theory (SDFT) for noncollinear magnetic materials, the Kohn-Sham system features exchange-correlation (xc) scalar potentials and magnetic fields. The significance of the xc magnetic fields is not very well explored; in particular, they can give rise to local torques on the magnetization, which are absent in standard local and semilocal approximations. Exact benchmark solutions for a five-site extended Hubbard lattice at half filling and in the presence of spin-orbit coupling are compared with SDFT results obtained using orbital-dependent exchange-only approximations. The magnetization dynamics following short-pulse excitations is found to be reasonably well described in the exchange-only approximation for weak to moderate interactions. For stronger interactions and near transitions between magnetically ordered and frustrated phases, exchange and correlation torques tend to compensate each other and must both be accounted for.

The enhanced photocatalytic and photothermal effects of Ti3C2 Mxene quantum dot/macroscopic porous graphitic carbon nitride heterojunction for Hydrogen Production

A new heterostructure between Ti3C2 MXene quantum dot and 3D macroscopic porous graphitic carbon nitride (PGCN) was successfully obtained by integrating Ti3C2 quantum dots onto porous graphitized carbon nitride (Ti3C2QDs/PGCN) using in situ electrostatic self-assembly techniques. The photocatalytic H2 evolution rate of optimized 5.5 wt% Ti3C2 QD/PGCN composites is nearly 15.24 and 3.53 times higher than pristine CN, and PGCN, respectively. Ti3C2 quantum dots can significantly enhance the hydrogen production activity of PGCN. In addition, their good photothermal conversion ability accelerates the overall reaction process and enhances the light absorption and carrier density. Furthermore, to elucidate the photocatalytic mechanism, a series of tests involving electron spin resonance (ESR) and density functional theory (DFT) calculations were performed. The results confirmed that the Schottky barrier between PGCN and Ti3C2 QD can effectively promote spatial charge separation and significantly improve the photocatalytic performance. This work provides a new approach for the construction of photocatalytic systems and the application of MXene QD.

Orientated construction of visible-light-assisted peroxymonosulfate activation system for antibiotic removal: Significant enhancing effect of Cl.

Antibiotic contaminants can migrate over long distances in the water, thus possibly causing severe detriment to the environment and even potential harm to human health. Heterogeneous activation of peroxymonosulfate (PMS) assisted by visible light is an emerging and promising technology for the purification of such wastewater. This study designed an ultra-efficient and stable PMS activator (FeCN) to restore the typical antibiotic-polluted water under harsh conditions. About 90.94% of sulfamethoxazole (SMX) was degraded in 35min in the constructed FeCN+PMS/vis system, and the reaction rate constant was nearly 50-fold higher than direct photocatalysis. Electron spin resonance, quenching experiments, LC/MS technique, eco-toxicity assessment, and density functional theory validated that the SMX removal was dominated by the attack of h+, •O2- and 1O2 on the active atoms of SMX molecules with high Fukui index, presenting as a simultaneous degradation and detoxification process. Such a visible-light-assisted PMS activation system also had good resistance to the environmental water bodies and a broad spectrum in the degradation of various pollutants. In particular, Cl- (50mM) could significantly accelerate the removal of SMX with a 32.6-fold increase in catalytic activity, and the mineralization efficiency could reach 56.6% under identical conditions. Moreover, this Cl- containing system excluded the degradation products of disinfection by-products, and such a system was also versatile for different contaminants. This work demonstrates the feasibility of the FeCN+PMS/vis system for the remediation of antibiotic-contaminated wastewater in the presence and absence of Cl-, and also highlights their great potential in WWTPs.

Photoferroic prospective of multiferroic HoMnO3 compound, evaluated on the base of the DFT study of its magnetic, electronic, and optical properties

In this work, non-collinear spin density functional theory based all-electron full-potential linearized augmented plane wave electronic structure method have been employed to study magnetic, electronic, and optical properties of multiferroic HoMnO3 compound in order to evaluate its potential to be used in photovoltaic applications. Exchange and correlation effects of the Ho 4f- and Mn 3d-electrons were treated on the base of the local spin density approximation including effective Hubbard (Ueff) correction. We concluded that the ground state magnetic structure should be described by Γ3 irreducible representation of the P63cm space group, for both the Mn and the Ho magnetic sub-lattices. The calculated indirect and direct band gaps, corresponding to the transition from the O 2p-to the Mn 3d-states, are found to be 1.25 eV and 1.4 eV, respectively, in very good agreement with the experimental findings. Calculated dielectric tensor spectra are found to be in satisfactory agreement with the experiment as well. On the base of these results, we investigated photoferroic properties of the HoMnO3 by calculating its absorption coefficient spectrum and the effective mass tensor of the photogenerated electrons and holes. By comparing results with those of isostructural LuMnO3 and the YMnO3 multiferroics, we concluded that the HoMnO3 exhibits similar photoferroic properties, a fact that classifies it as a material suitable for photovoltaic applications.

Local Structure and Self‐Defect Evolution under Nonstoichiometric Effect Unlock Zero‐Thermal‐Quenching

AbstractObtaining phosphors with higher quenching temperatures is a key scientific problem in developing high‐quality full‐spectrum white lighting devices. Herein, a near ultraviolet (NUV) activated orange‐yellow‐emitting single‐phase Li2Sr1‐ΔSiO4:Eu2+ phosphor is synthesized based on a nonstoichiometric ratio (Δ), and the relationship between local structure and photoluminescence (PL) is studied by electron spin resonance spectrum (ESR), Raman spectrum and density functional theory (DFT) calculations. Under the nonstoichiometric ratio effect, the local structure maintains stability by varying bond lengths, interatomic bond properties, and symmetry of coordination polyhedrons, which in turn modulates the crystal field and the PL spectra. Moreover, the optimum sample (Δ = 0.1%) specifically remains at 102%, 110%, and 114% of PL intensities at 150, 200, and 225 °C, respectively, compared with the initial at 25 °C, and the zero‐thermal‐quenching is mainly compensated by certain wide‐distributed self‐defect levels (300–550 K). Finally, a white light emitting diode with high luminous efficiency (89.61 lm W−1), low corresponding color temperature (5352 K), and good color rendering index (Ra = 95.8) is successfully fabricated. The strategy of designing self‐defect by nonstoichiometric synthesis provides a new insight for developing novel thermal‐quenching resistant phosphors and applying potential.

Electronic and magnetic properties of MoTe2 monolayer doped with single and double transition metals: spin density functional theory

The spin density functional computations are exploited to determine the electronic and magnetic properties of MoTe2 monolayer doped with single and double transition metal atoms (V, Cr, Mn, Fe and Co). These properties are sensitive with the types and numbers of the doping transition metals. The semiconductors with narrow band gaps are shown in Cr and Co single-doped MoTe2 monolayer. V and Mn single-doped MoTe2 monolayer are metal. Fe single-doped MoTe2 monolayer reveals the half-metallic behaviours with a 100% spin polarization. V and Co provide the first-two lowest formation energies and are used for the co-doping studies. MoTe2 monolayers simultaneously doped with two Co are characterized as half metal, while the others are metal. The p-d and d-d hybridization around the Fermi level introduce the magnetism, called double exchange mechanism.