Valence Electron Configuration Research Articles

Doping behaviors of adatoms adsorbed on phosphorene

Density functional theory calculations were performed to assess the electronic structures of single‐layer phosphorene upon adsorption of various adatoms. Structural relaxations showed that adatoms belonging to a particular group exhibit similar behavior. The favorable configuration for adsorption of the adatoms is mainly determined by the unsaturated lone pairs on the phosphorene surface and the valence electron configuration of the adatoms. The unsaturated lone pairs enhanced the binding energies of the adatoms, indicating that phosphorene exhibits better adsorption capability for foreign atoms than graphene. Band structure calculations demonstrated that Li and Ag adatoms act as n‐type dopants with low ionization energies. Pd adatom introduced p‐type impurity states, thus acting as p‐type dopants for phosphorene. Furthermore, the calculated energy barriers demonstrate that adatoms diffusion on phosphorene monolayer is quite anisotropic because of the puckered structure of the pristine phosphorene. Extra biaxial strain will not significantly influence the adsorption stability of adatoms and it seems impossible for us to release these adatoms by applying strain. This approach therefore offers a potential route for tailoring the electronic properties of phosphorene via adatom functionalization and should prove helpful for evaluating the performance of phosphorene‐based electronic devices.

FIRST PRINCIPLES STUDY OF HCN ADSORPTION ON GRAPHENE DOPED WITH 5D TRANSITION METAL

Hydrogen cyanide (HCN) adsorption on graphene doped with 5d transition metal (TM) is investigated by the first principles based on density functional theory. It is observed that Hg atom cannot be doped into graphene due to saturated valence electron configurations of 5d106s2. Three kinds of HCN adsorption configurations are investigated, in which H, C and N in HCN are close to the adsorption site, respectively. The most stable adsorption configuration is obtained by total energy optimization. HCN adsorption can be studied by adsorption energy and electron density difference. HCN can only be physisorbed on Ir, Pt and Au-doped graphenes, while chemisorption is observed for Lu, Hf, Ta, W, Re and Os-doped graphenes. The band structure is calculated by B3LYP and Generalized gradient approximation (GGA) functionals. It is observed from B3LYP method that the conductivity of Lu, Hf, Re and Os-doped graphenes does not obviously change before and after HCN adsorption. Ta and W-doped graphenes change from semiconductor to metal after adsorption of HCN molecule. The results indicate that Ta and W-doped graphenes may be a promising sensor for detecting HCN. This study provides a useful basis for understanding of a wide variety of physical properties on graphene.

Dislocation locking versus easy glide in titanium and zirconium.

The ease of a metal to deform plastically in selected crystallographic planes depends on the core structure of its dislocations. As the latter is controlled by electronic interactions, metals with the same valence electron configuration usually exhibit a similar plastic behaviour. For this reason, titanium and zirconium, two transition metals of technological importance from the same column of the periodic table, have so far been assumed to deform in a similar fashion. However, we show here, using in situtransmission electron microscopy straining experiments, that plasticity proceeds very differently in these two metals, being intermittent in Ti and continuous in Zr. This observation is rationalized using first-principles calculations, which reveal that, in both metals, dislocations may adopt the same set of different cores that are either glissile or sessile. An inversion of stability of these cores between Zr and Ti is shown to be at the origin of the profoundly different plastic behaviours.

Electrochemical Corrosion of Bulk Nanocrystalline Aluminum in Acidic Sodium Sulfate Solutions at Room Temperature

The corrosion properties of metallic materials are usually associated with the compactness of the oxide film and electrochemical parameters, such as corrosion potential, corrosion current density, pitting potential, etc. The valence electron configurations of metallic materials can also affect their corrosion properties because the nature of their corrosion processes involves the exchange and transportation of valence electrons between metallic atoms and ions. In the present work, the electrochemical corrosion of bulk nanocrystalline aluminum (BN-Al) produced by severe rolling technique and its conventional polycrystalline aluminum (CP-Al) counterpart in 0.01, 0.05, and 0.1 M HCl + 0.25 M Na2SO4 solutions at room temperature was studied by potentiodynamic polarization, electrochemical impedance spectroscopy, and scanning electron microscopy. The uniform and pitting corrosion resistances of BN-Al were simultaneously enhanced in comparison with those of CP-Al. The ion adsorption in oxide films along depth p...

Highly charged ions for atomic clocks and search for variation of the fine structure constant

We review a number of highly charged ions which have optical transitions suitable for building extremely accurate atomic clocks. This includes ions from Hf$^{12+}$ to U$^{34+}$, which have the $4f^{12}$ configuration of valence electrons, the Ir$^{17+}$ ion, which has a hole in almost filled $4f$ subshell, the Ho$^{14+}$, Cf$^{15+}$, Es$^{17+}$ and Es$^{16+}$ ions. Clock transitions in most of these ions are sensitive to variation of the fine structure constant, $\alpha$ ($\alpha = e^2/\hbar c$). E.g., californium and einsteinium ions have largest known sensitivity to $\alpha$-variation while holmium ion looks as the most suitable ion for experimental study. We study the spectra of the ions and their features relevant to the use as frequency standards.

Low-valent iron(i) amido olefin complexes as promotors for dehydrogenation reactions.

Fe(I) compounds including hydrogenases show remarkable properties and reactivities. Several iron(I) complexes have been established in stoichiometric reactions as model compounds for N2 or CO2 activation. The development of well-defined iron(I) complexes for catalytic transformations remains a challenge. The few examples include cross-coupling reactions, hydrogenations of terminal olefins, and azide functionalizations. Here the syntheses and properties of bimetallic complexes [MFe(I) (trop2 dae)(solv)] (M=Na, solv=3 thf; M=Li, solv=2 Et2 O; trop=5H-dibenzo[a,d]cyclo-hepten-5-yl, dae=(N-CH2 -CH2 -N) with a d(7) Fe low-spin valence-electron configuration are reported. Both compounds promote the dehydrogenation of N,N-dimethylaminoborane, and the former is a precatalyst for the dehydrogenative alcoholysis of silanes. No indications for heterogeneous catalyses were found. High activities and complete conversions were observed particularly with [NaFe(I) (trop2 dae)(thf)3 ].

Identification of an iridium-containing compound with a formal oxidation state of IX.

One of the most important classifications in chemistry and within the periodic table is the concept of formal oxidation states. The preparation and characterization of compounds containing elements with unusual oxidation states is of great interest to chemists. The highest experimentally known formal oxidation state of any chemical element is at present VIII, although higher oxidation states have been postulated. Compounds with oxidation state VIII include several xenon compounds (for example XeO4 and XeO3F2) and the well-characterized species RuO4 and OsO4 (refs 2-4). Iridium, which has nine valence electrons, is predicted to have the greatest chance of being oxidized beyond the VIII oxidation state. In recent matrix-isolation experiments, the IrO4 molecule was characterized as an isolated molecule in rare-gas matrices. The valence electron configuration of iridium in IrO4 is 5d(1), with a formal oxidation state of VIII. Removal of the remaining d electron from IrO4 would lead to the iridium tetroxide cation ([IrO4](+)), which was recently predicted to be stable and in which iridium is in a formal oxidation state of IX. There has been some speculation about the formation of [IrO4](+) species, but these experimental observations have not been structurally confirmed. Here we report the formation of [IrO4](+) and its identification by infrared photodissociation spectroscopy. Quantum-chemical calculations were carried out at the highest level of theory that is available today, and predict that the iridium tetroxide cation, with a Td-symmetrical structure and a d(0) electron configuration, is the most stable of all possible [IrO4](+) isomers.

Potassium under pressure: Electronic origin of complex structures

Recent high-pressure X-ray diffraction studies of alkali metals revealed unusual complex structures that follow the body-centred and face-centred cubic structures on compression. The structural sequence of potassium under compression to 1 Mbar is as follows:bcc–fcc–h-g (tI19*), hP4–oP8–tI4–oC16.We consider configurations of Brillouin-Jones zones and the Fermi surface within a nearly-free-electron model in order to analyze the importance of these configurations for the crystal structure stability. Formation of Brillouin zone planes close to the Fermi surface is related to opening an energy gap at these planes and reduction of crystal energy. Under pressure, this mechanism becomes more important leading to appearance of complex low-symmetry structures. The stability of the post-fcc phases in K is attributed to the changes in the valence electron configuration under strong compression.

Influence of the d orbital occupation on the structures and sequential binding energies of pyridine to the late first-row divalent transition metal cations: a DFT study.

The ground-state structures and sequential binding energies of the late first-row divalent transition metal cations to pyridine (Pyr) are determined using density functional theory (DFT) methods. Five late first-row transition metal cations in their +2 oxidation states are examined including: Fe(2+), Co(2+), Ni(2+), Cu(2+), and Zn(2+). Calculations at B3LYP, BHandHLYP, and M06 levels of theory using 6-31G* and 6-311+G(2d,2p) basis sets are employed to determine the structures and theoretical estimates for the sequential binding energies of the M(2+)(Pyr)x complexes, where x = 1-6, respectively. Structures of the Ca(2+)(Pyr)x complexes are compared to those for the M(2+)(Pyr)x complexes of Fe(2+), Co(2+), Ni(2+), Cu(2+), and Zn(2+) to further assess the effects of the d-orbital occupation on the preferred binding geometries. The B3LYP, BHandHLYP, and M06 levels of theory yield very similar geometries for the analogous M(2+)(Pyr)x complexes. The overall trends in the sequential BDEs for all five metal cations at all three levels of theory examined are highly parallel, and are determined by a balance of the effects of the valence electronic configuration and hybridization of the metal cation, but are also influenced by repulsive ligand-ligand interactions. Present results for the M(2+)(Pyr)x complexes are compared to the analogous complexes of the late first-row monovalent transition metal cations, Co(+), Ni(+), Cu(+), and Zn(+) previously investigated to assess the effect of the charge/oxidation state on the structures and sequential binding energies. Trends in the sequential binding energies of the M(2+)(Pyr)x complexes are also compared to the analogous M(2+)(water)x, M(2+)(imidazole)x, M(2+)(2,2'-bipyridine)x, and M(2+)(1,10-phenanthroline)x complexes.

The electrochemical corrosion characterization of bulk nanocrystalline aluminium by X-ray photoelectron spectroscopy and ultra-violet photoelectron spectroscopy

The electrochemical corrosion of bulk nanocrystalline aluminium (BN-Al) produced by severe rolling technique and its conventional polycrystalline aluminium counterpart (CP-Al) was studied by weight loss and scanning electron microscope (SEM). The uniform and localized corrosion resistances of BN-Al were simultaneously enhanced in comparison with CP-Al in 0.25mol/L HCl solution at room temperature due to the thinner, more compact and stable oxide film on BN-Al. The valence electron configurations of BN-Al and CP-Al were characterized by ultra-violet photoelectron spectroscopy (UPS). The electronic structures of oxide films on BN-Al and CP-Al were studied by X-ray photoelectron spectroscopy (XPS). The larger work function of BN-Al, the larger binding energies of valence electrons of BN-Al, the weaker Cl− adsorption on BN-Al, and the larger binding energies of Al3+2p3/2 and Cl−2p3/2 in the oxide film on BN-Al could explain the enhanced corrosion resistance of BN-Al and the inconsistent corrosion results of nanocrystalline materials at atomic scale.

Palladium Nanodendrites for Electrocatalytic Applications

Polymer electrolyte membrane fuel cells (PEMFC) are attractive energy conversion devices for portable applications and electric vehicles owing to their high efficiency, high power density and low operating temperature. However, the high price and the limited resources of platinum catalysts are among the main challenges hindering their further commercialization. As palladium (Pd) possesses a similar valence electronic configuration to platinum and also has high methanol-tolerant ability, researchers have devoted to develop palladium-based catalysts as a promising alternative to platinum. A number of wet-chemical routes have been developed to produce Pd nanostructures to improve the catalytic activity and selectivity. However, they have been limited due to the use of organic surfactants, polymeric stabilizers at elevated temperature or template techniques relying on the use of nanometer-sized pores as structure-directing agents, which are technically complicated, due to the requirements for template removal to obtain pure products.In this work, Pd dendrites were prepared by electrochemical deposition directly on carbon-based substrates, which are widely used as support material in fuel cells. Electrodeposition of Pd was carried out on an electrochemically activated carbon black substrate using potentiostatic and cyclic voltammetric techniques. Comparison was also made with Pt dendritesprepared by the electrochemical deposition. The effects of electrochemical activation, deposition potential, time, precursor concentration and carbon loading were studied in relation to the morphology and development of the nanostructures. The results indicated that controlled insertion of defects on the carbon black during electrochemical activation favored a good metal-support interaction and improved the hydrophilicity of the surface, resulting in a dendritic morphology with enhanced catalytic activity towards oxidation and reduction reactions in fuel cells.

Tantalum and niobium diffusion in single crystalline lithium niobate

Abstract LiNbO3 and LiTaO3 are isomorphous and Nb and Ta have the same valence electron configuration and the same ionic radii. This suggests the use of Ta as a tracer to probe the self-diffusion of Nb in LiNbO3. The diffusion system consisted of a 20 nm layer of LiTaO3 sputter deposited on top of (i) a congruent LiNbO3 single crystal, i.e. (48.3 ± 0.1) mol% Li2O, and on top of (ii) a VTE processed LiNbO3 single crystal with nearly stoichiometric composition, i.e. (49.9 ± 0.1) mol% Li2O. The diffusion anneals (1000 °C ≤ T ≤ 1100 °C) were performed under a constant oxygen partial pressure of 200 mbar. From the resulting SIMS depth profiles of tantalum a constant diffusivity was extracted which can be assumed to reflect the niobium self-diffusivity in LiNbO3. For sub-stoichiometric LiNbO3 the joint discussion of this work and of literature data on the basis of the generally accepted defect model, 4[NbLi4•] = [VLi′], suggests Nb transport in the Li sublattice. For hyper-stoichiometric LiNbO3 the defect model 5 V Nb 5 ′ = L i i • is derived from the Li3NbO4/LiNbO3 solution reaction of the VTE process designed to obtain Li2O-rich LiNbO3 in accordance with the Li2O–Nb2O5 phase diagram. This model is supported by theoretical calculations of the defect formation energy. Interestingly, the migration enthalpy for the Li vacancy mediated transport of the anti-site defect NbLi4• in the Li sublattice of congruent, i.e. sub-stoichiometric, LiNbO3 is similar (within a ± 10% error) to the one derived for the Nb vacancy mediated transport of Nb in the Nb sublattice of hyper-stoichiometric LiNbO3, i.e. about 3 eV. The significant discrepancies between our results and some earlier literature data can be consistently rationalised if the experimental procedures of those studies are carefully analysed.

Tetravalent Doping of CeO2: The Impact of Valence Electron Character on Group IV Dopant Influence

Fluorite CeO2 doped with group IV elements is studied within the density functional theory (DFT) and DFT + U framework. Concentration‐dependent formation energies are calculated for Ce1−xZxO2 (Z = C, Si, Ge, Sn, Pb, Ti, Zr, Hf) with 0 ≤ x ≤ 0.25 and a roughly decreasing trend with ionic radius is observed. The influence of the valence and near valence electronic configuration is discussed, indicating the importance of filled d and f shells near the Fermi level for all properties investigated. A clearly different behavior of group IVa and IVb dopants is observed: the former are more suitable for surface modifications and the latter are more suitable for bulk modifications. For the entire set of group IV dopants, there exists an inverse relation between the change, due to doping, of the bulk modulus, and the thermal expansion coefficients. Hirshfeld‐I atomic charges show that charge‐transfer effects due to doping are limited to the nearest‐neighbor oxygen atoms.

Hydrogen Adsorption on Gallium Nanoclusters

The reactivity of metal nanoclusters can vary significantly as a function of the valence electronic configuration of the cluster. Hydrogen adsorption provides a simple probe to investigate variations in reactivity of nanoclusters and identify factors that contribute to regioselectivity of interactions. In this study density functional theory is used to investigate the structures, stabilities, and electronic properties of doped gallium nanocluster hydrides, Ga12XH (X = B, C, Al, Si, P, Ga, Ge, and As). Hydrogen adsorption is found to be energetically favorable, but there is significant isomerization with respect to adsorption site. Consequently, adsorption energies vary not only with valence electronic configuration, but also with adsorption site. This regioselectivity of hydrogen adsorption is analyzed in terms of cluster bonding and frontier orbital interactions.

A study of the valence shell electronic states of pyridazine by photoabsorption spectroscopy and time-dependent density functional theory calculations

The valence shell electronic states of pyridazine have been studied experimentally, by recording the photoabsorption spectrum, and theoretically, by calculating oscillator strengths and excitation energies. The absolute photoabsorption cross section has been measured between 4 and 40 eV, using synchrotron radiation, and is dominated by prominent bands associated with intravalence transitions. In contrast, structure due to Rydberg excitations is weak. One Rydberg state, belonging to a series converging onto the state limit has been observed and assigned. The accompanying vibrational structure has been characterized by analogy with that in the corresponding photoelectron band. Vibrational progressions associated with Rydberg states belonging to one or more series converging onto the state limit have also been observed. The absorption structure associated with these series is complex and only tentative assignments have been proposed for the Rydberg states. The time-dependent version of density functional theory has been used to calculate oscillator strengths and excitation energies for the optically allowed singlet–singlet valence transitions and also to obtain the excitation energies for electric-dipole-forbidden and/or spin-forbidden transitions. The valence shell photoionization dynamics have been investigated theoretically by calculating photoelectron angular distributions and photoionization partial cross sections of the four outermost orbitals. In addition, the ground state outer valence electronic configuration has been obtained at the complete active space self-consistent field and the N-electron valence state perturbation theory to second-order levels of theory.

Influence of H, C, N and O impurities on the stability of Mg and Al from first-principles calculations

The influence of H, C, N and O impurities on the energetic and mechanical stabilities of Mg and Al were studied by first-principles total energy calculations. The occupation preference and formation energy of impurities in Mg and Al were estimated. H and C decrease the energetic stability of Mg and Al, while N and O significantly increase it. In Mg, H and O prefer to occupy the tetrahedral site, while C and N tend to occupy the octahedral site. In Al, only C prefers the octahedral site, while other elements will occupy the tetrahedral site. Electronic structures were analyzed to clarify the bonding properties and stability mechanisms of H, C, N and O in Mg and Al. The bonding characteristics around the Fermi energy level respond to the influence of impurities on the stability of Mg and Al. This work shows that all the impurities form strong bonds with the host atoms, but H and C weaken the interactions between host atoms, while N and O rarely affect these kinds of interactions. It seems that these bonding characteristics are independent of the crystal structures of matrices and the valence electron configurations of the host atoms. Elastic constants of impurity-containing systems were estimated from curves of energy against strain to evaluate the mechanical stability and elastic properties of the systems considered. All the impurity-containing systems satisfy the mechanical stability criteria. The influence of impurities on the elastic properties of Mg and Al was also studied.

Full field spectroscopic imaging at the ANKA-XAS- and -SUL-X-Beamlines

Full field transmission spectroscopic imaging is a powerful method for screening complex and large systems to map the distribution of valence state and electronic configuration of elements. Here, we describe an optimized hardware set-up and data evaluation tool chain for spectroscopic X-ray imaging. We tested this system successful on a ferruginous quartz (SiO2) sample.

The high-temperature oxidation of bulk nanocrystalline 304 stainless steel in air

The high-temperature oxidation of bulk nanocrystalline 304 stainless steel (BN-SS304) and its conventional polycrystalline counterpart (CP-SS304) in air at 900°C for 24h were studied by thermogravimetric analysis, X-ray photoelectron spectroscopy and scanning electron microscope. We studied the valence electron configurations of BN-SS304, CP-SS304 and their oxide scales by ultra-violet photoelectron spectroscopy. The high-temperature oxidation resistance of BN-SS304 was enhanced in both initial and isothermal oxidation, which was attributed to its larger work function and more chemical stability, its more chemically stable and compact oxide scale, its weaker O2 adsorption and diffusion, its weaker Cr and Mn atoms diffusions.

Structures and Stability of Doped Gallium Nanoclusters

The stability and reactivity of metal nanoclusters are critically dependent on the valence electronic configuration of the cluster. Many magic electronic configurations are inaccessible for pure trivalent metal clusters: However, doping is one method by which the electronic configuration and properties of a cluster can be significantly modified or even tailored. Density functional theory is used in this study to investigate the structures, stabilities and electronic properties of doped gallium nanoclusters, Ga12X (X = B, C, N, Al, Si, P, Ga, Ge, and As). In all cases doping of the cluster leads to increased stability relative to Ga-13. Stabilization is largely due to electronic contributions, although for many of the clusters the dopant also induces a small increase in the stability of the Ga-12 framework Generally, the endohedrally doped isomers are either lower in energy or close in energy to the isomers with the dopant at the surface of the cluster. Endohedral Ga12C is the most stable cluster and exhibits the most jellium-like orbital structure. Trends in vertical ionization potentials and electron affinities can be explained in terms of the interactions in frontier orbitals and generally adhere to the predictions of the jellium model.

Photoelectron imaging spectroscopy and theoretical investigation of ZrSi

The photoelectron spectrum of ZrSi(-) has been measured at two different photon energies: 2.33 eV and 3.49 eV, providing electron binding energy and photoelectron angular distribution information. The obtained vertical detachment energy of ZrSi(-) is 1.584(14) eV. The neutral ground and excited state terms are assigned based on experimental and theoretical results. The ground state of ZrSi is tentatively assigned as a (3)Σ(+) state with a configuration of 1σ(2) 1π(4) 1δ(0) 2σ(1) 3σ(1). A low lying (3)Π(i) neutral excited state is identified to be 0.238 eV (1919 cm(-1)) above the ground state. The anion ground state is designated as a (2)Σ(+) state with a 1σ(2) 1π(4) 1δ(0) 2σ(2) 3σ(1) valence electron configuration. A Franck-Condon (FC) simulation of the photoelectron spectrum has been carried out. For the (3)Σ(+) ← (2)Σ(+) band, theoretically calculated bond lengths and frequencies are used in the FC calculation which give good agreement with experiment, while for the (3)Π(i) ← (2)Σ(+) band, the ZrSi bond length is estimated from the FC spectrum. Comparisons are made with previously published theoretical studies and inconsistencies are pointed out. To the best of our knowledge, this study provides the first spectroscopic information on the transition metal-silicon diatomic, ZrSi.