Atomic Charge Transfer Research Articles

Structural, electronic and adsorptive characteristics of phosphorated holey graphene (PHG): First principles calculations

Two-dimensional (2D) materials have attracted attention since the discovery of graphene in 2004. However, the zero band gap limits its application in the field of electronic devices. Opening graphene's band gap has become one of the most important subjects. Nitrogenated holey graphene (NHG) has been successfully synthesized in 2015. It has attracted much attention because of semiconducting properties. However, there is still a lack of detailed study on phosphorated holey graphene (PHG). In this paper, the structural, electronic and adsorptive characteristics of PHG are investigated by first principles calculations. First, we investigate the structural characteristics of PHG, and compare it with the previous discoveries of NHG and graphene. The stacking behavior of PHG is studied, and the most stable stacking order is obtained. Then, we study the band structure characteristics of PHG. Impact of the biaxial strain and vacancy defects on the band structure is discussed. Finally, we investigate atomic adsorption on PHG. Atomic adsorption energy, equilibrium distance, and charge transfer are analyzed. The effect of atomic adsorption on band structure is discussed. Our investigation can provide useful information for insight into the novel 2D materials of PHG.

Catalytic hydrolysis of COS over CeO2 (110) surface: A density functional theory study

Density functional theory (DFT) calculations were performed to investigate the reaction pathways for catalytic hydrolysis of COS over CeO2 (110) surface using Dmol3 model. The thermodynamic stability analysis for the suggested routes of COS hydrolysis to CO2 and H2S was evaluated. The absolute values of adsorption energy of H2O-CeO2 are higher than that of COS-CeO2. Meanwhile, the adsorption energy and geometries show that H2O is easier adsorbed on the surface of CeO2 (110) than COS. H2O plays a role as a bridge in the process of joint adsorption. H2O forms more CeOH groups on the CeO2 (110) surface. CeO2 decreases the maximum energy barrier by 76.15kcal/mol. The migration of H from H2O to COS is the key for the hydrolysis reaction. CO channel is easier to occur than CS channel. Experimental result shows that adding of CeO2 can increase COS removal rate and prolong the 100% COS removal rate from 180min to 210min. The difference between Fe2O3 and CeO2 for the hydrolysis of COS is characterized in the atomic charge transfer and the formation of HO bond and HS bond. The transfer effect of H in H2O to S in COS over CeO2 decreases the energy barriers of hydrolysis reaction, and enhances the reaction activity of COS hydrolysis.

Chlorine gas reaction with ZnO wurtzoid nanocrystals as a function of temperature: a DFT study.

In the present work, we applied density functional theory and temperature-dependent Gibbs free energy calculations to wurtzoid structures to explain the sensitivity of ZnO nanocrystals towards chlorine molecules. In agreement with experimental findings, our results revealed that chlorine sensing under ambient conditions is feasible. Higher temperatures increased the sensitivity of ZnO nanocrystals towards chlorine gas molecules. Peak calculated sensitivities were in the temperature ranges (167-220°C), (447-578°C) and (952-1159°C), which is in good agreement with experimentally determined temperatures. According to the calculated Gibbs free energy, these three ranges correspond to the van der Waals attachment of Cl2 molecules on Zn-polar sites, van der Waals attachment of Cl2 molecules on O sites, and dissociation of Cl2 molecules on ZnO nanocrystal surfaces, respectively. The removal of chlorine atoms from the surface of ZnO nanocrystals is difficult at low temperatures because of the high electron affinity of chlorine gas atoms, which results in a long recovery time and accumulation of chlorine atoms and molecules on the ZnO surface. Atomic charges and charge transfer are depicted using natural bond orbital analysis to explain the present mechanisms.

First principles study of structural, electronic and magnetic properties of graphene adsorbed on the O-terminated MnO(111) surface

In this work, structural, electronic and magnetic properties of graphene adsorbed on MnO(111) with O-terminated surface depending on the number of vacancies in graphene/MnOx(111) interface are investigated using density functional theory. Local atomic reconstructions of the graphene/MnOx(111) interface, and their thermodynamic, electronic and magnetic properties were also studied. Bond lengths and adsorption energy were obtained for various reconstructions of graphene/MnOx(111) interface. Influence of graphene adsorption on the electronic structure of graphene/MnOx(111) interface for various reconstructions is also discussed. The charge transfer and local magnetic moments of carbon atoms and nearest-neighbor atoms were determined for these adsorption models. The charge transfer from carbon to nearest-neighbor atoms is due to the reconstruction of the local atomic and electronic structures, correlating with the number of oxygen vacancies in the interfaces. Magnetism of undefected graphene adsorbed on insulator MnOx(111) substrate with oxygen vacancies is also focused on discussing.

Investigation of structural and electrochemical properties of LaSrCo1−xSbxO4 (0≤x≤0.20) as potential cathode materials in intermediate-temperature solid oxide fuel cells

The structural and electrochemical properties of the layered perovskite oxides LaSrCo1−xSbxO4 (0≤x≤0.20) were investigated to study the effects of substituting Sb for Co for application as cathode materials in intermediate temperature solid oxide fuel cells (IT-SOFCs). The results of crystal structure analyses show the maximum content of Sb in LaSrCo1−xSbxO4 to be 0.05 as a pure single phase. XPS shows that Co and Sb in LaSrCo0.95Sb0.05O4 may possess mixed-oxidation states. The electrical conductivity increased greatly after Sb substitution. An improvement in the cathode polarization (Rp) values is observed from the Sb-doped sample with respect to the undoped samples. For example, Rp of LaSrCo0.95Sb0.05O4 on LSGM was observed to be 0.16Ωcm2 at 800°C in air. The main rate-limiting step for LaSrCo0.95Sb0.05O4 cathode is charge transfer of oxygen atoms. These results indicate that Sb can be incorporated into LaSrCo1−xSbxO4 based materials and can have a beneficial effect on the performance, making them potentially suitable for use as cathode materials in IT-SOFCs.

Designing IrO2 Nanoparticles for Oxygen Evolution

IrO2 is one of the most efficient electrocatalyst for the oxygen evolution reaction (OER) and water splitting process for the efficient solar energy into fuel (H2) generation. In solar fuels, IrO2 is used in the form of supported nanoparticles and its catalytic activity strongly depends on the size and shape of nanoparticles. Atomistic modeling of IrO2 nanoparticles with different sizes and shapes can enable fundamental understanding of catalytic processes govern at nanoparticle surfaces. Here, we used density functional theory (DFT) and variable charge force field calculations to study catalytic activity changes at the surfaces, edges and corners on the nanoparticle. We used O adsorption energy as a descriptor for the catalytic activity for water splitting reaction and determined the activity changes with respect to atomic coordination and charge transfer at different active sites on the nanoparticle. Calculations with the developed variable charge force field enable investigation of thermodynamic stability of relatively large (up to 2 nm) IrO2 nanoparticles with different shapes. We construct a metric to describe the overall catalytic activity of a nanoparticle and describe the effects of nanoparticle shape and size on the catalytic activity in terms of surface coordination changes, charge transfer and structural stability. Our results will shed light on the design and development of stable nanoscale IrO2 nanoparticle electrocatalytsts that efficiently utilize solar energy for water splitting reaction. ACKNOWLEDGEMENT: Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The submitted abstract has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

Resonant charge transfer of hydrogen Rydberg atoms incident at a metallic sphere

A wavepacket propagation study is reported for the charge transfer of low principal quantum number (n = 2) hydrogen Rydberg atoms incident at an isolated metallic sphere. Such a sphere acts as a model for a nanoparticle. The three-dimensional confinement of the sphere yields discrete surface-localized ‘well-image’ states, the energies of which vary with sphere radius. When the Rydberg atom energy is degenerate with one of the quantized nanoparticle states, charge transfer is enhanced, whereas for off-resonant cases little to no charge transfer is observed. Greater variation in charge-transfer probability is seen between the resonant and off-resonant examples in this system than for any other Rydberg-surface system theoretically investigated thus far. The results presented here indicate that it may be possible to use Rydberg-surface ionization as a probe of the surface electronic structure of a nanoparticle, and nanostructures in general.

Is Nanostructuring Sufficient To Get Catalytically Active Au?

Gold nanoparticles on transition-metal oxides were synthesized by two different methods: precipitation and photoinduced decomposition of an intermediate gold–azido complex. Only samples prepared by the precipitation method showed significant CO conversion at low temperature. XPS shows the formation of two Au species (Au0 and Auδ+) on the surface of active Au/TiO2 and Au/Fe2O3 samples. The energy shift of the Auδ+ peak depends on the support and is 0.6 and 0.9 eV for Au/TiO2 and Au/Fe2O3, respectively. TEM images indicate the formation of overlayer on Au particles. These results prove Au activation via a strong metal–support interaction, on the basis of the strong influence of the support on the electronic structure of the gold through charge transfer and stabilization of low-coordinated Au atoms.

First-Principles Study of Properties of Strained PbTiO3/KTaO3 Superlattice**Supported by the National Natural Science Foundation of China under Grant Nos 11372085 and 10902029, and the Shenzhen Science and Technology Project under Grant No JCYJ20150625142543461.

The impacts of strain and polar discontinuities on the performance of superlattices have attracted widespread attention. Using first-principles calculation, we study the polarization and piezoelectricity of PbTiO3/KTaO3 (PTO/KTO) superlattices with strain and polar discontinuities. The strain caused by lattice mismatch between the superlattice and the substrate induces lattice distortion, the displacement of each atom and dynamical charge transfer between the Ti atom or Ta atom and the O atoms in the PTO/KTO superlattice. With more compressive or less tensile strain, the polarization value increases linearly, piezoelectric tensor e31 (e32) increases while e33 and e25 (e16) increase negatively. Polarity discontinuity caused by the interfacial charge will produce large irreversible polarization. Proved by Γ-point phonons of PTO/KTO superlattices of different strain values, the polar discontinuity and the piezoelectric properties are just weakly dependent on temperature as found in PTO/KTO superlattices.

Charge-dependent many-body exchange and dispersion interactions in combined QM/MM simulations.

Accurate modeling of the molecular environment is critical in condensed phase simulations of chemical reactions. Conventional quantum mechanical/molecular mechanical (QM/MM) simulations traditionally model non-electrostatic non-bonded interactions through an empirical Lennard-Jones (LJ) potential which, in violation of intuitive chemical principles, is bereft of any explicit coupling to an atom's local electronic structure. This oversight results in a model whereby short-ranged exchange-repulsion and long-ranged dispersion interactions are invariant to changes in the local atomic charge, leading to accuracy limitations for chemical reactions where significant atomic charge transfer can occur along the reaction coordinate. The present work presents a variational, charge-dependent exchange-repulsion and dispersion model, referred to as the charge-dependent exchange and dispersion (QXD) model, for hybrid QM/MM simulations. Analytic expressions for the energy and gradients are provided, as well as a description of the integration of the model into existing QM/MM frameworks, allowing QXD to replace traditional LJ interactions in simulations of reactive condensed phase systems. After initial validation against QM data, the method is demonstrated by capturing the solvation free energies of a series of small, chlorine-containing compounds that have varying charge on the chlorine atom. The model is further tested on the SN2 attack of a chloride anion on methylchloride. Results suggest that the QXD model, unlike the traditional LJ model, is able to simultaneously obtain accurate solvation free energies for a range of compounds while at the same time closely reproducing the experimental reaction free energy barrier. The QXD interaction model allows explicit coupling of atomic charge with many-body exchange and dispersion interactions that are related to atomic size and provides a more accurate and robust representation of non-electrostatic non-bonded QM/MM interactions.

A survey of ab-initio calculations shows that segregation-induced grain boundary embrittlement is predicted by bond-breaking arguments

The segregation of solute atoms to grain boundaries can have a large influence on the mechanical behavior of polycrystals, particularly in metallic alloys. An overview of 400 calculations of solute-induced changes in grain boundary cohesion from 81 separate studies quantitatively demonstrates that the majority of the variation in solute-induced changes in boundary cohesion is explained by simple bond-breaking arguments. This trend is robust to changes in crystal structure and computational methods. The secondary contributions to embrittlement from other mechanisms, such as atomic size effects and charge transfer, are quantified and discussed as well.

Noncomparative scaling of aromaticity through electron itinerancy

Aromaticity is a multidimensional concept and not a directly observable. These facts have always stood in the way of developing an appropriate theoretical framework for scaling of aromaticity. In the present work, a quantitative account of aromaticity is developed on the basis of cyclic delocalization of π-electrons, which is the phenomenon leading to unique features of aromatic molecules. The stabilization in molecular energy, caused by delocalization of π-electrons is obtained as a second order perturbation energy for archetypal aromatic systems. The final expression parameterizes the aromatic stabilization energy in terms of atom to atom charge transfer integral, onsite repulsion energy and the population of spin orbitals at each site in the delocalized π-electrons. An appropriate computational platform is framed to compute each and individual parameter in the derived equation. The numerical values of aromatic stabilization energies obtained for various aromatic molecules are found to be in close agreement with available theoretical and experimental reports. Thus the reliable estimate of aromaticity through the proposed formalism renders it as a useful tool for the direct assessment of aromaticity, which has been a long standing problem in chemistry.

Resonant Charge Transfer of Hydrogen Rydberg Atoms Incident on a Cu(100) Projected Band-Gap Surface.

The charge transfer (ionization) of hydrogen Rydberg atoms (n=25-34) incident on a Cu(100) surface is investigated. Unlike fully metallic surfaces, where the Rydberg electron energy is degenerate with the conduction band of the metal, the Cu(100) surface has a projected band gap at these energies, and only discrete image states are available through which charge transfer can take place. Resonant enhancement of charge transfer is observed for Rydberg states whose energy matches one of the image states, and the integrated surface ionization signals (signal versus applied field) show clear periodicity as a function of n as the energies come in and out of resonance with the image states. The surface ionization dynamics show a velocity dependence; decreased velocity of the incident H atom leads to a greater mean distance of ionization and a lower field required to extract the ion. The surface ionization profiles for "on resonance" n values show a changing shape as the velocity is changed, reflecting the finite field range over which resonance occurs.

Unraveling photoluminescence quenching pathways in semiconductor nanocrystals

Photoluminescence quenching studies have been used as a means to study different surface attributes of semiconductor nanocrystals. Here we compare the quenching characteristics of iodine and iodobenzene on different sizes of nanocrystals. Iodine quenches statically via the external heavy atom effect and charge transfer, whereas iodobenzene quenches dynamically and only by way of the external heavy atom effect. This qualitative study shows that in both cases quenching efficiency increases with decreasing size and that in the case of charge transfer quenching the core excitonic state is more efficiently quenched than the surface emission.

Atomic structure and polarity compensation of BaTiO3 (1 1 1) surface

Surfaces of perovskite-type oxides have been attracting increasing interest for their primary importance in various potential applications such as multiferroic thin films, interface electronics and catalysis. However, the (1 1 1) surface of BaTiO3, the most typical ferroelectric, is far from well understood. In this work, the atomic structure and polarity compensation of BaTiO3 (1 1 1) surface have been investigated combining aberration-corrected transmission electron microscopy and first-principle calculations. Depending on the density of oxygen vacancies, the surface shows different degrees of atomic relaxation and electronic charge transfer, which compensates the surface polarity together with the ionic charges associated with the oxygen vacancies. The atomic relaxation and charge transfer would have a direct impact on the ferroelectric and catalytic properties of low-dimensional BaTiO3.

Effect of cation–π interaction on lithium and halogen bonds: a comparative study

Ab initio calculations have been performed to study the complex of M+–PhX–NCY (M = Li, Na; X = Li, Br and Y = H, OH and NH2). The aim is to compare the mutual cooperative effects between cation–π and lithium/halogen interactions. These effects are studied in terms of geometric and energetic properties, 15N chemical shielding parameters, molecular electrostatic potentials and natural bond orbital analyses. A positive cooperativity is observed on introduction of a cation–π interaction to the PhLi–NCY and PhBr–NCY dyads. The cation–π interaction has an enhancing effect on the halogen bond interaction energy, making it increased by152%–233%, whereas that of lithium bond interaction by 18%–22%. The enhancing mechanism is analysed in views with the atomic charges, charge transfer and electrostatic potentials.

X-ray and neutron scattering: from ideal zeolites to real guest-host systems

Tetrahedral MOFs, microporous metallophosphates and zeolites share common framework topologies despite very different chemical nature of their crystalline skeleton. In the case of zeolites or metallophosphates, the polymeric [TO2]_ framework is based on single tetrahedral atoms T (T= Si4+/Al3+ or Ga3+/B3+/P5+ respectively) connected by corner oxygen atoms O. In the case of MOFs, the skeleton is built from supertetraedra formed by a central metal atom complexed by 4 organic linkers (terephtalate,...); hence homologous structures expanded by a "scale factor". In "molecular sieves", the static porosity but also the "breathing effect" control, in first approximation, the adsorption and the self-diffusion of guest molecules. These latter are finely tuned by the interaction of these molecules with the framework itself (though H-bonds or electrostatic interactions), with charge compensating cations in basic zeolites (through coulombic interaction), or with other adsorbed molecules. However, the interplay between guest molecule adsorption, framework relaxation, cation redistribution and atomic charge transfer leads to an inextricable problem for the modeling and prediction of adsorption properties in these systems in absence of any polarizable and adaptive force field. We will present experimental charge density studies on activated industrial zeolites FAU, MFI and MOR. The electrostatic properties derived on these systems (electric field, atomics charges) are validated by a comparison with those derived, experimentally and theoretically, on a comprehensive set of simpler quartz-type materials (SiO2, AlPO4, GaPO4,...) representative of typical microporous compositions. Finally, we will combine the informations brought by diffraction, diffuse scattering and Monte-Carlo modelling to analyse the adsorption process or the role of the template in the stabilisation of the framework in these systems and in the famous MIL-53 MOF

Ab initio study on the size effect of symmetric and asymmetric ferroelectric tunnel junctions: A comprehensive picture with regard to the details of electrode/ferroelectric interfaces

Ferroelectric size effect of BaTiO3 (BTO) tunnel junctions with metal Pt and/or oxide SrRuO3 (SRO) electrodes has been comprehensively investigated by the first-principle calculations. A vacuum layer is included in the supercell calculations, so that full-relaxation is achieved without artificial constraint on the supercell strains. We have constructed all of ten possible types of tunnel junctions with either symmetric or asymmetric geometries to systematically explore the influence of electrode/ferroelectric interfaces. The characteristics of atomic structure, polarization, charge density, and electrostatic potential for different geometries and sizes are revealed. It is found that the ferroelectric stability of a tunnel junction depends significantly on the details of the two electrode/ferroelectric interfaces, which present specific short- and long-range properties, e.g., local bonding environment, electronic screening, built-in field, etc. Result shows that Pt/BTO interfaces have strong coupling with ferroelectric distortion and thus play more dominant roles than the SRO/BTO interfaces in affecting the ferroelectric stability of the tunnel junctions. Particularly, it is found that Pt2/TiO2 interface can induce collective ferroelectric distortion in the initially non-distorted barrier. With a full-relaxation of the strains, an abnormal enhancement of ferroelectricity by Pt2/BaO interface due to Pt-O bonding effect is demonstrated, where a strong interfacial-bonding-related polarizing field is verified. Also importantly, polarization stability of asymmetric tunnel junctions is found dependent on direction, manifested with the appearing of a new critical thickness, below which the tunnel junction loses polarization bistability. Furthermore, it shows that the local features of a specific electrode/ferroelectric interface (e.g., the interfacial atomic structure, local polarization, charge transfer, and potential step) are well kept in different types of tunnel junctions. By analyzing and summarizing the results, our results suggest that traditional phenomenological models need several modifications in order to quantitatively reproduce the size effect of ferroelectric tunnel junctions. Our study provides a comprehensive picture of the ferroelectric size effect in BTO tunnel junctions as a function of electrode/ferroelectric interfaces and should have valuable implications for future studies and applications.

Structure and interaction between the [BMIM][Ala] alanine anion and the 1-butyl-3-methylimidazolium cation in ion pairs

The equilibrium geometries and vibrational frequencies of the ionic liquid 1-butyl-3-methylimidazolium cation and the alanine anion [BMIM][Ala] are studied using density functional theory (DFT) at the B3PW91/6-311+G(d,p) leve1. The most stable structures of the anion, the cation, and the ion pairs are obtained and characterized, and the geometry parameters of the ion pairs confirm the presence of a hydrogen bonding interaction between the anion and the cation. Natural bond orbital (NBO) analysis is also performed to analyze the atomic charge distribution and charge transfer in the [BMIM]+ cation and [BMIM][Ala] ionic liquids. The results show that there are the electrostatic interaction and multiple hydrogen bond interactions between the cation and the anion of the ionic liquids, and the stability of the ground state of the ion pairs mostly results from the hydrogen bonding between the lone pairs of O atoms in the anion and H in the imidazole cycle of the cation. There are some changes in microstructures and the charge distribution during the formation of the ion pairs.

Effects of hydrogen in a vanadium grain boundary: From site occupancy to mechanical properties

We have investigated site occupancy and mechanical properties of a vanadium (V) Σ5(310)/[001] grain boundary (GB) with hydrogen (H) using a first-principles method. The segregation energy is calculated to be −0.29 eV for the energetically favorable V GB interstitial site, indicating that H energetically prefers to segregate into the V GB. We demonstrate that H can largely affect the mechanical properties of the V GB. The tensile strength and the Griffith fracture energy are reduced by approximately 13% (to 18.42 GPa) and 10% (to 1.74 J/m2) because of H segregation in comparison with that of the clean V GB, respectively. Our total energy calculations show that H acts as an embrittler to the V GB based on the Rice-Wang model. The atomic configurations and charge transfer analysis show that the segregated H weakens the surrounding interfacial V-V bonds, leading to the V GB mechanical properties degradation.