Localization Of Excess Electron Research Articles

Wettability, electronic structure and optical properties of intrinsic, doped, and oxygen-deficient CeO2: A study using DFT+U and DFT+U-D3

Using density functional theory plus U (DFT + U) and considering dispersion correction with the DFT + U-D3 method, the wettability, electronic structure, and optical properties of MO2 (M = Ce, Ce0.75La0.25, Ce0.75Zr0.25, Ce0.75Y0.25) and CeO1.75 were investigated. The results showed that dispersion correction had a significant impact on the wetting behavior. Through comparison, we identified a calculation method for the water contact angle (WCA) that closely matched experimental values. All structures exhibited hydrophilic properties, but Y doping significantly increased the WCA. La and Y doping transformed CeO2 from semiconductor to exhibits metallic characteristics. Oxygen vacancies localized excess electrons by Ce, forming Ce-f gap states between the conduction and valence bands. Zr doping increased the transparency of CeO2. La doping, Y doping, and oxygen vacancies enhanced the extinction coefficients of CeO2 in infrared light and resulted in higher static refractive indices and static reflectivities.

Roles of Oxygen Vacancies and Excess Electron Localization on Ceria Surfaces: First Principles Study

Roles of Oxygen Vacancies and Excess Electron Localization on Ceria Surfaces: First Principles Study

Stoichiometric and reduced ceria surfaces: Atomic structure, energetics and electron localization

We examine the (111), (110) and (100) surfaces of the stoichiometric and reduced ceria (CeO2) using density functional theory (DFT) within LSDA + U. we explore the formation mechanism of oxygen vacancy sites on CeO2 (111), (110) and (100) surfaces and their stability near the ceria surface regions. Local ordering surface defects and excess electron localization were reported. Both surface and subsurface oxygen vacancies induce the electron localization of reduced CeO2 on all these three terminations of the surface, leading to the appearance of Ce3+ sites. In the case of CeO2 (111) surface, oxygen vacancy at the surface and subsurface forms Ce3+ at next nearest neighbor to the vacancy. This is consistent with the experimental finding that the ratios of these two types of oxygen vacancy are nearly the same. In the case of CeO2 (100) and (110) surfaces, Ce3+ is formed at the sites nearest to the oxygen vacancy sites. The reduced ceria expects to catalyze the dissociation of molecules on it surface. The dissociation is assisted by the oxidation of Ce3+ that generated on reduced ceria surfaces.

E-beam-enhanced solid-state mechanical amorphization of α-quartz: Reduced deformation barrier via localized excess electrons as network modifiers

Under hydrostatic pressure, α-quartz (α-SiO2) undergoes solid-state mechanical amorphization wherein the interpenetration of [SiO4]2− tetrahedra occurs and the material loses crystallinity. This phase transformation requires a high hydrostatic pressure of 14 GPa because the repulsive forces resulting from the ionic nature of the Si–O bonds prevent the severe distortion of the atomic configuration. Herein, we experimentally and computationally demonstrate that e-beam irradiation changes the nature of the interatomic bonds in α-quartz and enhances the solid-state mechanical amorphization at nanoscale. Specifically, during in situ uniaxial compression, a larger permanent deformation occurs in α-quartz submicron pillars compressed during e-beam irradiation than in those without e-beam irradiation. Microstructural analysis reveals that the large permanent deformation under e-beam irradiation originates from the enhanced mechanical amorphization of α-quartz and the subsequent viscoplastic deformation of the amorphized region. Further, atomic-scale simulations suggest that the delocalized excess electrons introduced by e-beam irradiation move to highly distorted atomic configurations and alleviate the repulsive force, thus reducing the barrier to the solid-state mechanical amorphization. These findings deepen our understanding of electron–matter interactions and can be extended to new glass forming and processing technologies at nano- and microscale.

Toward a Consistent Prediction of Defect Chemistry in CeO2.

Polarizable shell-model potentials are widely used for atomic-scale modeling of charged defects in solids using the Mott-Littleton approach and hybrid Quantum Mechanical/Molecular Mechanical (QM/MM) embedded-cluster techniques. However, at the pure MM level of theory, the calculated defect energetics may not satisfy the requirement of quantitative predictions and are limited to only certain charged states. Here, we proposed a novel interatomic potential development scheme that unifies the predictions of all relevant charged defects in CeO2 based on the Mott-Littleton approach and QM/MM electronic-structure calculations. The predicted formation energies of oxygen vacancies accompanied by different excess electron localization patterns at the MM level of theory reach the accuracy of density functional theory (DFT) calculations using hybrid functionals. The new potential also accurately reproduces a wide range of physical properties of CeO2, showing excellent agreement with experimental and other computational studies. These findings provide opportunities for accurate large-scale modeling of the partial reduction and nonstoichiometry in CeO2, as well as a prototype for developing robust interatomic potentials for other defective crystals.

Development and Applications of an eReaxFF Force Field for Graphitic Anodes of Lithium-Ion Batteries

Graphene is one of the most promising materials for lithium-ion battery anodes due to its superior electronic conductivity, high surface area for lithium intercalation, fast ionic diffusivity and enhanced specific capacity. A reliable description of many battery processes requires an explicit description of electrochemical interactions involving electrons. A detailed atomistic modeling of electronic conduction and non-zero voltage simulations of graphitic materials require the inclusion of an explicit electronic degree of freedom. To enable large length- and time-scale simulations of electron conduction in graphitic anodes, we developed an eReaxFF force field concept describing graphitic materials with an explicit electron. The newly developed force field, verified against quantum chemistry-based data describing, amongst others, electron affinities and equation of states, reproduces the qualitative behavior of electron conductivity in pristine and imperfect graphitic materials at different applied temperatures and voltages. In addition, excess electron localization near a defect site estimated from eReaxFF simulations agree quite well with the corresponding density functional theory calculations. Our eReaxFF simulations show the initiation of lithium-metal-plating driven by electron transfer from the graphene surface to the exposed lithium ions demonstrating the method’s potential for studying lithium-graphene interactions with explicit electrons and explain many unresolved electrode and electrode-electrolyte interface processes.

Local polarization in oxygen-deficient LaMnO3 induced by charge localization in the Jahn-Teller distorted structure

The functional properties of transition metal perovskite oxides are known to result from a complex interplay of magnetism, polarization, strain, and stoichiometry. Here, we show that for materials with a cooperative Jahn-Teller distortion, such as LaMnO$_3$ (LMO), the orbital order can also couple to the defect chemistry and induce novel material properties. At low temperatures, LMO exhibits a strong Jahn-Teller distortion that splits the $e_g$ orbitals of the high-spin Mn$^{3+}$ ions and leads to alternating long, short, and intermediate Mn--O bonds. Our DFT+$U$ calculations show that, as a result of this orbital order, the charge localization in LMO upon oxygen vacancy formation differs from other manganites, like SrMnO$_3$, where the two extra electrons reduce the two Mn sites adjacent to the vacancy. In LMO, relaxations around the defect depend on which type of Mn--O bond is broken, affecting the $d$-orbital energies and leading to asymmetric and hence polar excess-electron localization with respect to the vacancy. Moreover, we show that the Mn--O bond lengths, orbital order and consequently the charge localization and polarity are tunable via strain.

La-doping induced localized excess electrons on (BiO)2CO3 for efficient photocatalytic NO removal and toxic intermediates suppression

Photocatalysis technology has been extensively adopted to abate typical air pollutants. Nevertheless, it is a challenge to develop photocatalysts aiming to simultaneously improve photocatalytic selectivity and efficiency. In this study, to improve the photocatalytic selectivity and the performance of (BiO)2CO3 in the oxidation of NO to target products (NO2− /NO3−), we developed a novel method to construct La-doped (BiO)2CO3 (La-BOC) for forming localized excess electrons (Ex) on (BiO)2CO3 surface. The results indicate that the Ex could effectively accelerate the activation of reactants and promote charge separation and transfer. Under visible light, the gas molecules could capture the Ex and get activated to produce reactive oxygen species (ROS) with high oxidation ability, which enables complete oxidation of NO to target products instead of producing other toxic by-products. Due to the functionality of the Ex, the photocatalytic selectivity and efficiency of La-BOC have been synchronously improved. Combining experimental and theoretical methods, this work unravels the pathway of charge carriers transportation/transformation and elucidates the photocatalytic NO oxidation mechanism. The present work could provide a novel method to improve photocatalytic selectivity and activity for safe air pollutant abatement.

Synergistic Effect of Singly Charged Oxygen Vacancies and Ligand Field for Regulating Transport Properties of Resistive Switching Memories

The controlled incorporation of defect species in amorphous oxide (AO) for regulating charge transport properties is highly demanding and challenging, especially for resistive switching (RS) memories. Here, we use pulsed electron beam deposition technique to engineer the growth of AO films, which show a large decrease (∼33%) in the t2g–eg gap. Our collective experiments and density functional theory (DFT) based ab initio simulations reveal the presence of undercoordinated TiO5 units, causing a decrease in the t2g–eg gap. Further, the importance of TiO5 in facilitating excess electron localization is demonstrated, showing an evolution of a broad defect band within the energy gap. The origin of this band (singly charged oxygen vacancy) is resolutely established by X-ray photoelectron spectroscopy and electron paramagnetic resonance measurements and is also supported by calculated partial charge density and Bader charge analysis. The preeminence of singly charged oxygen vacancies cause a low-current level (u...

Long-range electron-electron interaction and charge transfer in protein complexes: a numerical approach.

With application to the nitrite reductase hexameric protein complex of Desulfovibrio vulgaris, NrfH2A4, we suggest a strategy to compute the energy landscape of electron transfer in large systems of biochemical interest. For small complexes, the energy of all electronic configurations can be scanned completely on the level of a numerical solution of the Poisson-Boltzmann equation. In contrast, larger systems have to be treated using a pair approximation, which is verified here. Effective Coulomb interactions between neighbouring sites of excess electron localization may become as large as 200 meV, and they depend in a nontrivial manner on the intersite distance. We discuss the implications of strong Coulomb interactions on the thermodynamics and kinetics of charging and decharging a protein complex. Finally, we turn to the effect of embedding the system into a biomembrane.

Tailoring the rate-determining step in photocatalysis via localized excess electrons for efficient and safe air cleaning

Regulating the rate-determining step in photocatalysis is crucial for advancing its application in environmental remediation. However, approaches for tailoring the rate-determining step have been largely overlooked. Herein, Ca-intercalated g-C3N4 is designed as a model photocatalyst to deeply understand the electron transportation behavior and the mechanisms of photocatalytic NO removal. The intercalation of Ca builds an interlayer channel for electron migration between g-C3N4 layers, which extends the sp2 hybridized planes and enables the electrons to transform from a delocalized state to a localized state around Ca, leading to the formation of localized excess electrons (e−ex). Under visible light irradiation, these e−ex are subsequently captured by gas molecules for more efficient reactive oxygen species (ROS) generation and reactant activation. The ROS generated by Ca-intercalated g-C3N4 demonstrate stronger oxidation capability than those generated by pure CN. The ROS directly participate in photocatalytic NO oxidation and tailor the rate-determining step by decreasing the reaction activation energies, resulting in an overall increase in NO removal efficiency and a reduction in NO2 production. The photocatalytic efficiency and selectivity have been significantly improved owing to the functionality of the e−ex. Using closely combined experimental and theoretical methods, this work provides a new approach for understanding the behaviors of e−ex in environmental photocatalysis and tailoring the rate-determining step to enhance reaction efficiency, achieving efficient and safe air purification.

The activation of reactants and intermediates promotes the selective photocatalytic NO conversion on electron-localized Sr-intercalated g-C3N4

Photocatalysis technology has been widely adapted to address air pollution. However, the photocatalysis efficiency and selectivity should be optimized to achieve efficient and safe air purification. Take the g-C3N4 (CN) as a case study, in order to promote the photocatalytic performance and the selectivity of g-C3N4 in oxidizing NO into target products of NO2− and NO3−, the electron localization has been built in Sr-intercalated g-C3N4 (CN-Sr) to promote the activation of reactants and intermediates, as well as the charge separation and transfer. The intercalated Sr atom causes the uneven electron distribution on the plane of CN. The O2 could capture the localized excess electrons and be activated producing O2− radicals. The NO and the reaction intermediates could deplete the electrons more easily and be activated on the surface of CN-Sr. The activated species possess longer bond length and have higher reactivity during photocatalysis, making them easier to be destroyed by active radicals and transforming to target products of NO2− and NO3− rather than other toxic byproducts. With the pivotal effect of localized electrons in CN-Sr, the photocatalytic activity and selectivity can be simultaneously promoted. With the in situ DRIFTS investigation and theoretical calculation, the present work specified the transportation and transformation of photogenerated carriers and revealed the mechanism of photocatalytic NO oxidation. This work could provide a new approach to enhance the photocatalytic activity and selectivity for efficient and safe air pollution control.

Theoretical Insights into the Experimental Observation of Stable p-Type Conductivity and Ferromagnetic Ordering in Vacuum-Hydrogenated TiO2

Tuning of electrical and magnetic properties to achieve stable p-type conductivity and room temperature ferromagnetism in undoped TiO2 is quite challenging. Here both are attained simultaneously through a facile method of vacuum-hydrogenation, wherein vacuum annealing as well as hydrogenation play crucial roles. The p-type conductivity in hydrogenated TiO2 is investigated through the Hall measurement studies, which show considerable enhancement in Hall mobility and electrical conductivity. The high and low pressures of hydrogenation show strong and weak ferromagnetic ordering, respectively, whereas the pristine TiO2 NPs manifest paramagnetic behavior. In order to understand the mechanism of these characteristic changes, density functional theory (DFT) calculations are performed. DFT calculations reveal that the smaller amount of hydrogenation leads to gap-states above valence band maximum (VBM) due to the effect of hydrogen atoms 1s orbitals and by the formation of ∼Ti–H and ∼O–H bonds. Further increase in the hydrogenation changes the ∼O–H bond to the ∼H2O bond, and these H2O molecules will be easily detached during the next vacuum annealing step. These processes will lead to the formation of excess oxygen vacancies and cause the localization of excess electrons on Ti atoms. This results in emergence of well pronounced midgap states in the forbidden bandgap. These midgap states are mostly contributed by the 3d orbitals of Ti atoms. DFT studies also disclose that the higher spin polarization for the high hydrogen concentration, which is reflected as the ferromagnetic ordering in the experimental results.

Protonation-modulated localization of excess electrons in histidine aqueous solutions revealed by ab initio molecular dynamics simulations: anion-centered versus cation-centered localization

In this work, we present an ab initio molecular dynamics simulation study on the interaction of an excess electron (EE) with histidine in its aqueous solution. Two different configurations of histidine (imidazole group protonated or not) are considered to reflect its different existing forms in neutral or slightly acidic surroundings. The simulation results indicate that localizations of EEs in different aqueous histidine solutions are quite different and are strongly affected by protonation of the side chain imidazole group and are thus pH-controlled. In neutral aqueous histidine solution, an EE localizes onto the carboxyl anionic group of the amino acid backbone after a relatively lengthy diffuse state, performing just like in an aliphatic amino acid solution. But in weakly acidic solution in which the side chain imidazole group is protonated, an EE undergoes a short lifetime diffuse state and finally localizes on the protonated imidazole group. We carefully examine these two different localization dynamics processes and analyze the competition between different dominating groups in their corresponding electron localization mechanisms. To explain the difference, we investigate the frontier molecular orbitals of these two systems and find that their energy levels and compositions are important to determine these differences. These findings can provide helpful information to understand the interaction mechanisms of low energy EEs with amino acids and even oligopeptides, especially with aromatic rings.

Competition between alkalide characteristics and nonlinear optical properties in OLi3MLi3O (M = Li, Na, and K) complexes

Alkalides possess enhanced nonlinear optical (NLO) responses due to localization of excess electrons on alkali metals. Employing MP2/6-311 + G(d) level, we design novel alkalides by placing alkali atoms (M) between two Li3O superalkalis. We notice a competition between alkalide characteristics and NLO properties in OLi3MLi3O (M = Li, Na, and K) isomers. For instance, the atomic charge on M (qM) in D2h structure is −0.58e for M = Li and its first static mean hyperpolarizablity (βo) is 1 a.u., but in C2v structure, qM = −0.12e and βo= 3.4 × 103 a.u. More interestingly, the βo value for M = K (C2v) increases to 1.9 × 104 a.u. in which qM = 0.24e. These findings may provide new insights into the design of alkalides, an unusual class of salts and consequently, lead to further researches in this direction.

Localized plasmons on a metal surface

This paper considers collective oscillations of electron density localized near a metal surface. Its description involves the wave equation of plasma oscillations. Assuming the axial symmetry of the problem, separation of variables in the equation is carried out and a set of analytical functions providing the variety of solutions is obtained. The solutions provide natural multipole quantization of surface plasmon states. Expressions are obtained for local potential fields formed by multipole plasmon modes at the interface. Their role in physical implementation of mechanisms of localization of excess electrons on the metal surface is discussed.

Plasmon Enhanced Heterogeneous Electron Transfer: A Model Study

Plasmon enhancement of photoinduced charge injection from a perylene dye into a large TiO2 cluster is studied theoretically. A system is investigated where a spherical metal nanoparticle (MNP) is placed near the dye at the cluster surface. The simulations account for optical excitation of the dye coupled to the MNP and subsequent electron injection into the rutile TiO2 cluster with (110) surface. The electron motion in the cluster is described in a tight-binding model and focuses on excess electron localization at the Ti atoms and inter Ti charge transfer. Clusters with about 105 atoms can be treated. Charge injection dynamics is described in the framework of the density matrix theory which, however, ignores in this first attempt molecular vibrations. Considering short optical excitations, the overall probability to have the electron injected into the cluster reaches an intermediate steady state. This probability is used to introduce an enhancement factor which rates the influence of the MNP. Values large...

Excess electrons in ice: a density functional theory study

We present a density functional theory study of the localization of excess electrons in the bulk and on the surface of crystalline and amorphous water ice. We analyze the initial stages of electron solvation in crystalline and amorphous ice. In the case of crystalline ice we find that excess electrons favor surface states over bulk states, even when the latter are localized at defect sites. In contrast, in amorphous ice excess electrons find it equally favorable to localize in bulk and in surface states which we attribute to the preexisting precursor states in the disordered structure. In all cases excess electrons are found to occupy the vacuum regions of the molecular network. The electron localization in the bulk of amorphous ice is assisted by its distorted hydrogen bonding network as opposed to the crystalline phase. Although qualitative, our results provide a simple interpretation of the large differences observed in the dynamics and localization of excess electrons in crystalline and amorphous ice films on metals.

Equation of state of CaMnO3: a combined experimental and computational study

Elastic properties of CaMnO3 are of primary importance in the science and technology of CaMnO3-based perovskites. From X-ray diffraction experiments performed at pressures up to 100 kbar using a diamond-anvil cell to hydrostatically compress our sample, a bulk modulus, K 0, of 1734(96) kbar was obtained after fitting parameters to the third-order Birch–Murnaghan equation of state. Mean field, semiclassical simulations predict, for the first time, the third-order equation-of-state parameters and show how the bulk modulus increases with pressure (the zero pressure value being 2062.1 kbar) and decreases with the extent of nonstoichiometry caused by the formation of oxygen vacancies. These trends are amplified for the shear modulus. A more accurate model that allows for the explicit reduction of Mn ions, or localization of excess electrons, yields qualitatively similar results. The experimental and calculated axial ratios show the same trends in their variation with rising pressure.

Dynamics and Reactivity of Trapped Electrons on Supported Ice Crystallites

The solvation dynamics and reactivity of localized excess electrons in aqueous environments have attracted great attention in many areas of physics, chemistry, and biology. This manifold attraction results from the importance of water as a solvent in nature as well as from the key role of low-energy electrons in many chemical reactions. One prominent example is the electron-induced dissociation of chlorofluorocarbons (CFCs). Low-energy electrons are also critical in the radiation chemistry that occurs in nuclear reactors. Excess electrons in an aqueous environment are localized and stabilized by the local rearrangement of the surrounding water dipoles. Such solvated or hydrated electrons are known to play an important role in systems such as biochemical reactions and atmospheric chemistry. Despite numerous studies over many years, little is known about the microscopic details of these electron-induced chemical processes, and interest in the fundamental processes involved in the reactivity of trapped electrons continues. In this Account, we present a surface science study of the dynamics and reactivity of such localized low-energy electrons at D(2)O crystallites that are supported by a Ru(001) single crystal metal surface. This approach enables us to investigate the generation and relaxation dynamics as well as dissociative electron attachment (DEA) reaction of excess electrons under well-defined conditions. They are generated by photoexcitation in the metal template and transferred to trapping sites at the vacuum interface of crystalline D(2)O islands. In these traps, the electrons are effectively decoupled from the electronic states of the metal template, leading to extraordinarily long excited state lifetimes on the order of minutes. Using these long-lived, low-energy electrons, we study the DEA to CFCl(3) that is coadsorbed at very low concentrations (∼10(12) cm(-2)). Using rate equations and direct measurement of the change of surface dipole moment, we estimated the electron surface density for DEA, yielding cross sections that are orders of magnitude higher than the electron density measured in the gas phase.