Electron Density Functional Theory Research Articles

Crystal-Phase-Mediated Restructuring of Pt on TiO2 with Tunable Reactivity: Redispersion versus Reshaping

Restructuring of supported metal nanoparticles (NPs) (e.g., reshaping and redispersion) is of tremendous interest for the rational design of well-defined catalyst materials, but the underlying mechanism to tune their dynamic behaviors and thus reactivity is still unspecified. Here, we show a crystal-phase-mediated redispersion/reshaping of Pt NPs on TiO2, boosting opposite reactivities in hydrogenation/oxidation reactions. Utilizing a variety of state-of-the-art characterization methods, we unraveled that rutile TiO2 favors the reshaping of Pt NPs into two-dimensional planar geometry, whereas the anatase surface facilitates the redispersion of Pt NPs to single atoms (SAs) upon the same calcination procedure. Environmental transmission electron microscopy and density functional theory calculations were employed to directly visualize the dynamic transformation of Pt NPs and reveal the specific role of TiO2 supports in promoting the stability and diffusion of Pt SAs. As a result, the opposite reactivity was achieved by tuning their distinct restructuring behaviors. Thus, the redispersion of Pt on anatase TiO2 facilitates the selective hydrogenation of phenylacetylene with a high styrene yield of 21.22 × 10–2 s–1, whereas the reshaping on the rutile phase favors the combustion of methane with a turnover frequency as high as 3.11 × 10–2 s–1. Our results therefore open up an effective route for tuning the restructuring behavior of supported metal catalysts and designing catalysts with controlled catalytic structures and reactivities.

CdSe cluster-modified biogenic α-FeOOH based on macroporous biochar for Fenton-like reaction of As(III)

In this study, CdSe cluster-modified biogenic α − FeOOH based on macroporous biochar was constructed from rhizosphere iron plaque via pyrolysis. An ultrahigh H2O2 production rate of 655.75 μmol/L was achieved by the photocatalysis-self-Fenton system within 30 min under visible light irradiation. α-FeOOH, with abundant iron ions, can directly activate H2O2 to produce abundant ⋅OH radicals. It was found that As(III) (10 mg/L) could be completely degraded within 20 min. Electron spin resonance spectroscopy (ESR) and density functional theory (DFT) revealed that a suitable energy band structure was achieved by the introduction of CdSe clusters; thus, O2 was mainly reduced to H2O2 through a two-electron reduction route in the reaction system. The photogenerated electrons promoted the conversion of Fe(III)/Fe(II) by reacting with Fe(III) in α-FeOOH. The presence of H2O2 and Fe2+ in this system accelerates the oxidation of As(III) by a Fenton-like reaction.

Molecular Dynamics with Constrained Nuclear Electronic Orbital Density Functional Theory: Accurate Vibrational Spectra from Efficient Incorporation of Nuclear Quantum Effects.

Nuclear quantum effects play a crucial role in many chemical and biological systems involving hydrogen atoms yet are difficult to include in practical molecular simulations. In this paper, we combine our recently developed methods of constrained nuclear-electronic orbital density functional theory (cNEO-DFT) and constrained minimized energy surface molecular dynamics (CMES-MD) to create a new method for accurately and efficiently describing nuclear quantum effects in molecular simulations. By use of this new method, dubbed cNEO-MD, the vibrational spectra of a set of small molecules are calculated and compared with those from conventional ab initio molecular dynamics (AIMD) as well as from experiments. With the same formal scaling, cNEO-MD greatly outperforms AIMD in describing the vibrational modes with significant hydrogen motion characters, demonstrating the promise of cNEO-MD for simulating chemical and biological systems with significant nuclear quantum effects.

Second-order magnetoelastic effects: From the Dirac equation to the magnetic properties of ultrathin epitaxial films for magnetic thin-film applications

Abstract It has always been the endeavour of Professor Kronmüller to combine various methods in order to attain a comprehensive understanding of a physical phenomenon on all relevant scales. In this spirit, we combine the phenomenological nonlinear theory of magnetoelasticity with the relativistic density functional electron theory to describe the magnetoelastic behaviour of epitaxial films. We explain the two already performed cantilever bending beam experiments on the magnetoelasticity and underpin the assumption that second-order magnetoelastic effects are very important for epitaxial Fe films. A complete set of six cantilever experiments is introduced which allows to calculate the intrinsic first- and second-order magnetoelastic constants for cubic materials, and these constants are calculated ab initio for Fe, face-centred cubic Co, Ni, Ni3Fe and CsCl-type CoFe.

Defects investigation of bipolar exfoliated phosphorene nanosheets

Two-dimensional (2D) phosphorene has gained attention due to its exceptional chemical, physical, and optoelectronic properties. However, defects analysis in exfoliated phosphorene through experimental techniques is still largely missing. In this paper, a combination of high-resolution transmission electron microscopy (HRTEM) imaging and density functional theory calculations were provided to study the point defects, grain boundaries (GBs), and amorphization phenomenon in exfoliated phosphorene nanosheets via bipolar electrochemistry method. The HRTEM results demonstrate that the single vacancies (SV) and di-vacancies (DV), ad-atoms, and GBs defects are formed in phosphorene nanosheets. However, the exfoliated black phosphorus nanosheets maintained its orthorhombic crystal structure. In addition, amorphization on the edges and surface of nanosheets is unavoidable in the presence of oxygen. Our first-principles simulation confirms the breakage of P-P bonds of phosphorene upon surface oxidation, which results in amorphization. The defect analysis of phosphorene nanosheets obtained from this study could benefit both fundamental research and technological applications.

Phase transition induced by synchroshear in Al-Zn-Mg-Cu alloy

Phase transition of the η-MgZn2 Laves phase from C14 to C15 structure in peak aged Al-Zn-Mg-Cu alloy were studied by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and density functional theory (DFT) calculations. The formation of a well ordered C15 structural MgZn2 as independent precipitate is for the first time observed in Al-Zn-Mg-Cu alloy. The C15 structure is confirmed to be transformed from the C14 by the activation of synchroshear in every other glide plane, which involves the shear induced by two Schockley partial dislocations in the triple-layer of the Laves phase. Besides, the twin-like structure and five-fold symmetry twin are formed for composing C15 structure due to multiple synchroshear. These results expand the structural understanding of the η-MgZn2 precipitates in Al-Zn-Mg-Cu alloy.

Tabernaecorymine A, an 18-normonoterpenoid indole alkaloid with antibacterial activity from Tabernaemontana corymbosa

Tabernaecorymine A, an 18-normonoterpenoid indole alkaloid with conjugated (E)-3-aminoacrylaldehyde fragment was obtained from the stem bark of Tabernaemontana corymbosa. Its structure was elucidated by extensive spectroscopic data analyses, and further verified by ACD/structure elucidator, electronic circular dichroism (ECD) analyses and density functional theory (DFT) chemical shift predictions. The compound exhibited significant antibacterial bioactivity against Streptococcus dysgalactiae with an MIC value of 3.12 μg/mL, which is better than the plant drug berberine.

Interfacial water asymmetry at ideal electrochemical interfaces.

Controlling electrochemical reactivity requires a detailed understanding of the charging behavior and thermodynamics of the electrochemical interface. Experiments can independently probe the overall charge response of the electrochemical double layer by capacitance measurements and the thermodynamics of the inner layer with potential of maximum entropy measurements. Relating these properties by computational modeling of the electrochemical interface has so far been challenging due to the low accuracy of classical molecular dynamics (MD) for capacitance and the limited time and length scales of ab initio MD. Here, we combine large ensembles of long-time-scale classical MD simulations with charge response from electronic density functional theory to predict the potential-dependent capacitance of a family of ideal aqueous electrochemical interfaces with different peak capacitances. We show that while the potential of maximum capacitance varies, this entire family exhibits an electrode charge of maximum capacitance (CMC) between -2.9 and -2.2 μC/cm2, regardless of the details in the electronic response. Simulated heating of the same interfaces reveals that the entropy peaks at a charge of maximum entropy (CME) of -5.1 ± 0.6μC/cm2, in agreement with experimental findings for metallic electrodes. The CME and CMC both indicate asymmetric response of interfacial water that is stronger for negatively charged electrodes, while the difference between CME and CMC illustrates the richness in behavior of even the ideal electrochemical interface.

Monomeric (VO2+) and dimeric mixed valence (V2O33+) vanadium species at the surface of shape controlled TiO2 anatase nano crystals

Metal atoms and ions at well-defined anatase TiO2 crystals with exposed (101) and (001) facets represent a promising platform for fundamental studies in catalysis using model systems of high complexity for the development of novel catalytic systems exhibiting higher than usual activities. Herein, we report the geometric and electronic structures of supported paramagnetic vanadium catalysts obtained by reaction of VCl4 vapours with shape controlled anatase TiO2 supports with preferential (101) and (001) facets. Electron Paramagnetic Resonance (EPR) spectroscopy and Density Functional theory (DFT) calculations reveal the presence specific monomeric (VO2+) and dimeric mixed valence (V2O33+) species with molecular structures dependent on the TiO2 surface termination.

Chemical potential, derivative discontinuity, fractional electrons, jump of the Kohn-Sham potential, atoms as thermodynamic open systems, and other (mis)conceptions of the density functional theory of electrons in molecules.

Many references exist in the density functional theory (DFT) literature to the chemical potential of the electrons in an atom or a molecule. The origin of this notion has been the identification of the Lagrange multiplier μ = ∂E/∂N in the Euler-Lagrange variational equation for the ground state density as the chemical potential of the electrons. We first discuss why the Lagrange multiplier in this case is an arbitrary constant and therefore cannot be a physical characteristic of an atom or molecule. The switching of the energy derivative ("chemical potential") from -I to -A when the electron number crosses the integer, called integer discontinuity or derivative discontinuity, is not physical but only occurs when the nonphysical noninteger electron systems and the corresponding energy and derivative ∂E/∂N are chosen in a specific discontinuous way. The question is discussed whether in fact the thermodynamical concept of a chemical potential can be defined for the electrons in such few-electron systems as atoms and molecules. The conclusion is that such systems lack important characteristics of thermodynamic systems and do not afford the definition of a chemical potential. They also cannot be considered as analogues of the open systems of thermodynamics that can exchange particles with an environment (a particle bath or other members of a Gibbsian ensemble). Thermodynamical (statistical mechanical) concepts like chemical potential, open systems, grand canonical ensemble etc. are not applicable to a few electron system like an atom or molecule. A number of topics in DFT are critically reviewed in light of these findings: jumps in the Kohn-Sham potential when crossing an integer number of electrons, the band gap problem, the deviation-from-straight-lines error, and the role of ensembles in DFT.

Электронная зонная структура и термоэлектрические характеристики SrTiO-=SUB=-3-=/SUB=-, BaTiO-=SUB=-3-=/SUB=- и CaTiO-=SUB=-3-=/SUB=-: ab initio подход

The calculations of Seebeck’s coefficient, conductivity and power functions for the electron-doped SrTiO_3, BaTiO_3 and CaTiO_3 compounds have been performed depending on temperature and current carrier concentration by employing ab initio method based on the electron density functional theory, on the Frohlich’s approach for the electron-phonon interaction and on the theory of Boltzmann–Onsager for the thermoelectric properties. The calculated Seebeck’s coefficient and conductivity correspond to experimental data. It is shown that for SrTiO_3 and BaTiO_3 the dependencies of power functions on the carrier concentration have maxima in the range of (200–250) × 10^{19} cm^{–3} at any temperature, while for CaTiO_3 the maxima are typical only at temperatures below 500 K. The temperature dependencies of the power function also confirm that such carrier concentration range is favorable for achieving high values of the SrTiO_3 figure of merit, while the maximally possible carrier concentration is necessary for optimal CaTiO_3 figure of merit.

Reassignment of magic numbers for icosahedral Au clusters: 310, 564, 928 and 1426.

Icosahedral Au clusters with three and four shells of atoms are found to deviate significantly from the commonly assumed Mackay structures. By introducing additional atoms in the surface shell and creating a vacancy in the center of the cluster, the calculated energy per atom can be lowered significantly, according to several different descriptions of the interatomic interaction. Analogous icosahedral structures with five and six shells of atoms are generated using the same structural motifs and are similarly found to be more stable than Mackay icosahedra. The lowest energy per atom obtained here is for clusters containing 310, 564, 928 and 1426 atoms, as compared with the commonly assumed magic numbers of 309, 561, 923 and 1415. Some of the vertices in the optimized clusters have a hexagonal ring of atoms, rather than a pentagon, with the vertex atom missing. An inner shell atom in some cases moves outwards by more than an Ångström into the surface shell at such a vertex site. This feature, as well as the wide distribution of nearest-neighbor distances in the surface layer, can strongly influence the properties of icosahedral clusters, for example catalytic activity. The structural optimization is initially carried out using the GOUST method with atomic forces estimated with the EMT empirical potential function, but the atomic coordinates are then refined by minimization using electron density functional theory (DFT) or Gaussian approximation potential (GAP). A single energy barrier is found to separate the Mackay icosahedron from a lower energy structure where a string of atoms moves outwards in a concerted manner from the center so as to create a central vacancy while placing an additional atom in the surface shell.

ПЕРВОПРИНЦИПНАЯ МОДЕЛЬ ДВУХСЛОЙНОГО ГРАФЕНА

Using the electron density functional theory, numerical modelling of bilayer graphene has been performed. The structure and binding energy of the layers depending on the representation of the wave function of the system have been studied: plane waves (VASP package) and atomic-like orbitals (SIESTA package); and choice of approximation for the exchange-correlation (XC) functional. It has been shown that being in the free form the system creates a stable AB structure of bilayer graphene. The calculation of the layer binding energy has shown that the results of modelling performed with different basis for the wave function are consistent when using XC functionals corresponding to each other and considering the correction to the basis set superposition error in the tight binding method. As expected, the generalized gradient approximation (GGA) has shown underestimated values of the interaction energy of graphene layers. Comparison with experimental data has shown that the energy and geometric characteristics of bilayer graphene are best described by the local electron density approximation (LDA). The 2nd and 3rd generation semi-empirical Grimme corrections for GGA have given estimates of the binding energy higher than LDA, but also close to the experimental results.

Electronic band structure and thermoelectric properties of SrTiO-=SUB=-3-=/SUB=-, BaTiO-=SUB=-3-=/SUB=- and CaTiO-=SUB=-3-=/SUB=-: ab initio approach

The calculations of Seebeck's coefficient, conductivity and power functions for the electron-doped SrTiO3, BaTiO3 and CaTiO3 compounds have been performed depending on temperature and current carrier concentration by employing ab initio method based on the electron density functional theory, on the Frohlich's approach for the electron-phonon interaction and on the theory of Boltzmann--Onsager for the thermoelectric properties. The calculated Seebeck's coefficient and conductivity correspond to experimental data. It is shown that for SrTiO3 and BaTiO3 the dependencies of power functions on the carrier concentration have maxima in the range of (200-250)·1019 cm-3 at any temperature, while for CaTiO3 the maxima are typical only at temperatures below 500 K. The temperature dependencies of the power function also confirm that such carrier concentration range is favorable for achieving high values of the SrTiO3 figure of merit, while the maximally possible carrier concentration is necessary for optimal CaTiO3 figure of merit. Keywords: Ca, Sr, Ba titanates, PAW method, electronic structure, thermoelectric properties.

Theoretical Study of the Electronic and Transport Properties of Lateral 2D–1D–2D Graphene–CNT–Graphene Structures

The electronic and transport properties of new hybrid 2D–1D–2D structures of carbon atoms, which are graphene sheets continuously connected through a fragment of a single-layer carbon nanotube, frequently observed experimentally, are theoretically studied. The evolution of the electronic properties of such systems with “zigzag” carbon nanotubes of various diameters with chirality indices (14, 0), (15, 0), (16, 0), and (18, 0) is studied using the tight coupling method within electron density functional theory. The calculation of the transmission coefficient demonstrates a strong nonlinearity in the behavior of the transport properties of these structures near the Fermi energy as a function of the diameter of carbon nanotubes, which explains discrepancies in the previously obtained experimental data.

On the possibilities of ab initio modeling for electron-phonon relaxation and transport properties by the examples of cadmium oxide and strontium titanate

The calculations of the electron-phonon relaxation time, Seebeck coefficient and conductivity were performed for cadmium oxide with oxygen vacancies and strontium titanate doped with niobium using the first-principle methods based on the electron density functional perturbation theory, Boltzmann theory and many-body theory of electron-phonon interaction. It is shown that the calculations of relaxation time based on the many-body theory lead to significantly more accurate results on transport characteristics than in the case of the standard approximation of a constant relaxation time. It is shown that interaction with defects has a significant effect on conductivity. Keywords: cadmium oxide, strontium titanate, electronic structure, PAW method, Boltzmann theory, transport characteristics.

The antiferromagnetic phase transition in the layered Cu0.15Fe0.85PS3 semiconductor: experiment and DFT modelling

The experimental studies of the paramagnetic-antiferromagnetic phase transition through Mössbauer spectroscopy and measurements of temperature and field dependencies of magnetic susceptibility in the layered Cu0.15Fe0.85PS3 crystal are presented. The peculiar behavior of the magnetization - field dependence at low-temperature region gives evidence of a weak ferromagnetism in the studied alloy. By the ab initio simulation of electronic and spin subsystems, in the framework of electron density functional theory, the peculiarities of spin ordering at low temperature as well as changes in interatomic interactions in the vicinity of the Cu substitutional atoms are analyzed. The calculated components of the electric field gradient tensor and asymmetry parameter for Fe ions are close to the ones found from Mössbauer spectra values. The Mulliken populations show that the main contribution to the ferromagnetic spin density is originated from 3d-copper and 3p-sulfur orbitals. The estimated total magnetic moment of the unit cell (8.543 emu/mol) is in reasonable agreement with the measured experimental value of ∼9 emu/mol.

Sustainable micro-activation of dissolved oxygen driving pollutant conversion on Mo-enhanced zinc sulfide surface in natural conditions

The activation of inert oxygen (O2) often consumes enormous amounts of energy and resources, which is a global challenge in the field of environmental remediation and fuel cells. Organic pollutants are abundant in electrons and are promising alternative electron donors. Herein, we implement sustainable microactivation of dissolved oxygen (DO) by using the electrons and adsorption energy of pollutants by creating a nonequilibrium microsurface on nanoparticle-integrated molybdenum (Mo) lattice-doped zinc sulfide (ZnS) composites (MZS-1). Organic pollutants were quickly removed by DO microactivation in the MZS-1 system under natural conditions without any additional energy or electron donor. The turnover frequency (TOF, per Mo atom basis) is 5 orders of magnitude higher than those of homogeneous systems. Structural and electronic characterization technologies reveal the change in the crystalline phase (Zn-S-Mo) and the activation of π-electrons on six-membered rings of ZnS after Mo doping, which results in the formation of a nonequilibrium microsurface on MZS-1. This is the key for the strong interfacial interaction and directional electron transfer from pollutants to MZS-1 through the delocalized π-π conjugation effect and from MZS-1 to DO via Zn-S-Mo, as demonstrated by electron paramagnetic resonance (EPR) techniques and density functional theory (DFT) calculations. This process achieves the efficient use of pollutants and the low-energy activation of O2 through the construction of a nonequilibrium microsurface, which shows new significance for water treatment.

Exploring the Solvation Sphere and Spatial Accumulation of Dissolved Transition-Metal Ions in Batteries: A Case Study of Vanadyl Ions Released from V2O5Cathodes

Dissolution of transition-metal ions from Li-ion battery cathodes is a well-known degradation phenomenon. In addition to capacity decrease due to metal loss in the bulk cathode, dissolved transition-metal ions might migrate through the electrolyte and participate in redox reactions resulting in electrolyte decomposition. Aside from the oxidation state and concentration of the transition metal, its ligand sphere and spatial accumulation have to be determined for complete characterization and prediction of possible detrimental impact. Here, pulse electron paramagnetic resonance and density functional theory are exploited to investigate the coordination and location of vanadyl ions, which are a dissolution product of V2O5 cathodes. The solvation sphere is found to be dominated by electrolyte degradation products and also contains contributions from nonelectrolytic sources. Additionally, adjacent lithium and hexafluorophosphate ions are observed.

Interações químicas entre monômero e molécula molde em polímeros com impressão molecular, o EGDMA: 2-VP (4: 1) - estudo de caso MIP lumefantrina

Molecularly Imprinted Polymers (MIP) are synthetic materials used as a tool to enhance the selectivity in different analytical approaches, such as solid-phase extraction, chromatography, and sensing devices. Knowing the mechanism involved in the interaction between the template and monomer is essential for a further successful application. However, studies on this topic are scarce. This work evaluates the involved mechanisms in the template-monomer interaction for a lumefantrine MIP system, an antimalarial drug. Field-emission gun scanning electron microscopy, thermal analysis, X-ray diffraction, and density functional theory were applied to determine the mechanism involved in two MIPs obtained in different conditions. A new parameter, named Molecularly Imprinting Factor (MIF), was proposed to evaluate the contribution of specific interactions in the sorption of the analyte by the MIP structure. MIF allows direct insights into specific binding, non-specific contributions, interaction nature, behavior predictability, system acid-base behavior, pre-screening pairs capability, and binding site affinities evaluation. Two interaction types were observed, covalent and non-covalent, when methacrylic acid and 2-vinyl pyridine were used as monomers, respectively. Therefore, the use of methacrylic acid formed a sorbent inappropriate for solid-phase extraction since the binding is not reversible. On the other hand, 2-vinyl pyridine-lumefantrine binding was reversible, and MIF = 0.59 (59.02% of specific site sorption) indicates that the predominant mechanism in the sorption is specific.