3d 4p Research Articles

Tuning d–p Orbital Hybridization of NiMoO4@Mo15Se19/NiSe2 Core‐Shell Nanomaterials via Asymmetric Coordination Interaction Enables the Water Oxidation Process

AbstractThe electrocatalytic performance of MoNi‐based nanomaterials undergo selenization has garnered significant interest due to their modified electronic structure, while still posses certain challenges for obtained bimetallic selenides. Here, a novel MoNi‐based electrocatalyst of NiMoO4@Mo15Se19/NiSe2 core‐shell nanomaterials is constructed to promote the desorption of OOH* which can facilitate the water oxidation process. NiMoO4@Mo15Se19/NiSe2 core‐shell nanoarrays show that the “cores” are mainly NiMoO4 nanorods while the “shells” are the bimetallic selenides nanoflakes, which are super architectures that can expand more active sites and accelerate the electron transfer. Moreover, the hybridization interaction between Ni 3d, Mo 4d, and Se 4p orbitals leads to an asymmetric distribution of electric clouds, which decreases the adsorption energy and transformation of oxygen‐containing species. Electrochemical data displays that the overpotentials of NiMoO4@Mo15Se19/NiSe2 core‐shell nanomaterials are only 195 mV, 220 mV, and 224 mV for oxygen evolution reaction (OER) in alkaline freshwater, alkaline simulated seawater, and alkaline natural seawater. The current density decay is negligible after 100 h stability at about 1.46 V with a three‐electrode system in alkaline natural seawater. The low cost and the unique MoNi‐based bimetallic selenides in this work provide a more constructive solution for designing efficient and stable OER catalysts in the future.

Energy levels determination of Zn(O,Se) thin films

Ternary Zn(O,Se) films were grown onto glass and p-type Si substrates by pulsed laser deposition at substrate temperature of 500 °C, to investigate their optical properties and energy band structure using UV–Vis and UPS spectroscopic methods. It was found that the 500 nm thick Zn(O,Se) film is nearly transparent in the visible region, with an optical band gap value of 2.93 eV. The calculated valence band is positioned at −7.60 eV below the vacuum level, and it consist of O 2p and Se 4p levels or the highest bonding sp3 hybrid states with the hybridization of Zn 3d, O 2p and Se 4p orbitals, and also the coupling of 3d–3d electrons among different Zn2+ cations. The findings show that the conduction band of Zn(O,Se) layers can be tuned for alignment with the conduction band of the absorbers layer which will decrease the cliff and spike barriers in the complete solar cell structures.

Chemical bonding of termination species in 2D carbides investigated through valence band UPS/XPS of Ti3C2T x MXene

MXenes are technologically interesting 2D materials that show potential in numerous applications. The properties of the MXenes depend at large extent on the selection of elements that build the 2D MX-layer. Another key parameter for tuning the attractive material properties is the species that terminate the surfaces of the MX-layers. Although being an important parameter, experimental studies on the bonding between the MX-layers and the termination species are few and thus an interesting subject of investigation. Here we show that the termination species fluorine (F) bonds to the Ti3C2-surface mainly through Ti 3p—F 2p hybridization and that oxygen (O) bonds through Ti 3p—O 2p hybridization with a significant contribution of Ti 3d and Ti 4p. The study further shows that the Ti3C2-surface is not only terminated by F and O on the threefold hollow face-centered-cubic site. A significant amount of O sits on a bridge site bonded to two Ti surface atoms on the Ti3C2-surface. In addition, the results provide no support for hydroxide (OH) termination on the Ti3C2-surface. On the contrary, the comparison of the valence band intensity distribution obtained through ultraviolet- and x-ray photoelectron spectroscopy with computed spectra by density of states, weighed by matrix elements and sensitivity factors, reveals that OH cannot be considered as an inherent termination species in Ti3C2T x . The results from this study have implications for correct modeling of the structure of MXenes and the corresponding materials properties. Especially in applications where surface composition and charge are important, such as supercapacitors, Li-ion batteries, electrocatalysis, and fuel- and solar cells, where intercalation processes are essential.

Correlation between electronic and structural orders in 1T−TiSe2

The correlation between electronic and crystal structures of 1T-TiSe2 in the charge density wave (CDW) state is studied by x-ray diffraction. Three families of reflections are used to probe atomic displacements and the orbital asymmetry in Se. Two distinct onset temperatures are found, TCDW and a lower T* indicative for an onset of Se out-of-plane atomic displacements. T* coincides with a DC resistivity maximum and the onset of the proposed gyrotropic electronic structure. However, no indication for chirality is found. The relation between the atomic displacements and the transport properties is discussed in terms of Ti 3d and Se 4p states that only weakly couple to the CDW order.

First-principles calculations of solute–vacancy interactions in aluminum**Project supported by the National Natural Science Foundation of China (Grant Nos. 51701095 and 51771185) and the Natural Science Foundation of Jiangsu Province, China (Grant No. BK20170798).

The interactions of solute atoms with vacancies play a key role in diffusion and precipitation of alloying elements, ultimately influencing the mechanical properties of aluminum alloys. In this study, first-principles calculations are systematically performed to quantify the solute–vacancy interactions for the 3d–4p series and the 4d–5p series. The solute–vacancy interaction gradually transforms from repulsion to attraction from left to right. The solute–vacancy binding energy is sensitive to the supercell size for elements at the beginning. These behaviors of the solute–vacancy binding energy can be understood in terms of the combination and competition between the elastic and electronic interactions. Overall, the electronic binding energy follows a similar trend to the total binding energy and plays a major role in the solute–vacancy interactions.

Effect of Fe2O3 coating on ZnO nanowires in photoelectrochemical water splitting: A synchrotron x-ray spectroscopic and spectromicroscopic investigation

An array of ZnO/Fe2O3 core-shell nanowires (NW) for the highly efficient absorption of light and carrier collection is developed for use in photoelectrochemical (PEC) water-splitting. The oriented NW architecture favors physical matching, providing a direct electron conduction pathway and reducing the diffusion length of photogenerated holes. This work involves a combination of spectral imaging, spectromicroscopy and in situ x-ray absorption spectroscopy; spectra are obtained under operando conditions. Direct investigation of oriented nanowires using polarization-dependent x-ray spectromicroscopy enables the determination of the relationship between anisotropic electronic orbitals and charge carrier water-splitting efficiency. The results of O K-edge STXM demonstrated that the ZnO/Fe2O3 core-shell NW exhibits strong anisotropy and thus provides higher electron-hole transport efficiency than bare ZnO. In situ XAS revealed that interfacial charge transfer between Fe 3d and Zn 4p states enhances the photoelectrochemical reaction in the ZnO/Fe2O3 core-shell NW. The photogenerated electrons of Fe2O3 are transferred from Fe 3d states to the Zn 4p state under photoelectrochemical conditions.

Soft x-ray emission from laser-produced strontium ions

Soft x-ray spectra, in the range from 2 nm to 9 nm, were recorded from strontium plasmas formed by pulses from 20 ps, 170 ps and 5.5 ns Nd:YAG lasers operating at the fundamental wavelength of 1064 nm. Features due to 3d–4p and 3d–4f transitions were identified by comparison with spectra from adjacent ions and atomic structure calculations with both the Cowan code and the Flexible Atomic Code. As in the spectra of ions of other elements in the fifth row of the periodic table, resonant lines 3dn–3dn−14p1, 3dn–3dn−14f1 and satellite lines 3dn−14s1–3dn−24s14p1, 3dn−14s1–3dn−24s14f1 of Δn = 1 were observed over the 3.0–8.5 nm region, emitted by 10+ to 19+ ions. These Δn = 1 transitions provide a range of narrow band emission features which may match to specific multi layer combinations for reflective optics in the extreme ultraviolet region of the spectrum.

Evolution of superconductivity in K2−xFe4+ySe5: Spectroscopic studies of X-ray absorption and emission

This study investigates the evolution of superconductivity in K2-xFe4+ySe5 using temperature-dependent X-ray absorption and resonant inelastic X-ray scattering techniques. Magnetization measurements show that polycrystalline superconducting (SC) K1.9Fe4.2Se5 has a critical temperature (T c) of ∼31 K with a varying superconducting volume fraction, which strongly depends on its synthesis temperature. An increase in Fe-structural/vacancy disorder in SC samples with more Fe atoms occupying vacant 4d sites is found to be closely related to the decrease in the spin magnetic moment of Fe. Moreover, the nearest-neighbor Fe-Se bond length in SC samples exceeds that in the non-SC (NS) sample, K2Fe4Se5, which indicates a weaker hybridization between the Fe 3d and Se 4p states in SC samples. These results clearly demonstrate the correlations among the local electronic and atomic structures and the magnetic properties of K2-xFe4+ySe5 superconductors, providing deeper insight into the electron pairing mechanisms of superconductivity.

NiSe2 as Co-Catalyst with CdS: Nanocomposites for High-Performance Photodriven Hydrogen Evolution under Visible-Light Irradiation.

One-dimensional NiSe2 /CdS nanocomposites (NCs), with NiSe2 nanoparticles (NPs) integrated onto the surface of CdS nanowires (NWs) were designed and fabricated through a two-step in situ solvothermal method. Density functional theory (DFT) calculations showed that NiSe2 plays an important role in enhancing photocatalystic hydrogen (H2 ) production when coupled with CdS catalyst, which arises from 1) the contribution of Ni 3d and Se 4p orbitals to the electronic structures and 2) the charge redistribution at the interface region of the heterostructure results in efficient promotion of the charge transfer and separation of the photogenerated electron-hole pairs. Under visible-light illumination (λ≥400 nm), the NCs showed improved photocatalytic H2 -generation activity in the presence of lactic acid. The peak value of 7610.1 μmol h-1 g-1 for H2 production was achieved with an apparent quantum efficiency (AQE) of 41.68 % when the Ni/Cd ratio was set to 10 %, which was approximately 48 times higher than that of bare CdS NWs. Photoelectrochemical measurements also confirmed the pivotal role of NiSe2 in the improvement of the H2 -evolution performance of NiSe2 /CdS NCs, which is consistent with the DFT investigations.

Enhanced interfacial perpendicular magnetic anisotropy in Fe/MgO heterostructure via interfacial engineering

Interfacial perpendicular magnetic anisotropy in the Fe/MgO heterostructure has attracted increasing attention because of its potential usage in building out-of-plane magnetized magnetic tunnel junctions for spin-transfer-torque magnetic random-access memory devices. A large interfacial anisotropy constant (Ki) is required to achieve a high thermal stability that is critical for large-capacity magnetic tunnel junctions. Here, we show that inserting one layer of appropriate heavy elements (X) at the interface of Fe/MgO can significantly enhance Ki using first-principles electronic structure calculations. A total number of 33 X metal elements including 3d, 4d, and 5d transition metals and 4p, 5p, and 6p main group elements were considered. By analyzing the interfacial magnetic anisotropy and interfacial bonding strength of Fe/X/MgO, we are able to identify three promising heavy transition-metal elements including W, Re, and Pt for enhancing perpendicular magnetic anisotropy in Fe/MgO, with an improved Ki of 2.43, 2.37, and 9.74 mJ/m2, respectively. This work indicates that interfacial engineering is one effective way to modify the interfacial magnetic anisotropic property.

Electronic structure of CeCo1−xFexGe3 studied by X-ray photoelectron spectroscopy and first-principles calculations

A transformation between the magnetically ordered CeCoGe3 and heavy fermion CeFeGe3 is isostructural but not isoelectronic, therefore the characterization of the electronic structure of the CeCo1−xFexGe3 series is of special importance. We report both the experimental investigation by the X-ray photoelectron spectroscopy (XPS) measurements and the first-principles calculations within the full-potential local-orbital (FPLO) scheme based on the density functional theory. Experimentally, we investigate mainly the valence band and the Ce 3d spectra, both giving the possibility to conclude about the level of the f states localization. Computationally, we investigate the most characteristic CeCoGe3, CeCo0.5Fe0.5Ge3, and CeFeGe3 compositions. Based on the electronic band structure results we calculate the X-ray photoelectron spectra of the valence band. We consider the effects of the spin-orbit coupling and intra-atomic Hubbard U repulsion. Furthermore, we discuss the charge distribution and occupation of valence-band orbitals. The experimentally observed evolution of the 3d band with the Fe concentration is related to the decrease of the number of electrons and reduction of the 3d photoionization cross-sections. Calculations indicate that charge is transferred mainly from the Ce to Ge sites and the bondings are formed mainly by the Ce 5d, Fe/Co 3d, and Ge 4p and Ge 4s orbitals.

Evaluation of Non‐covalent Binding Energies and Optoelectronic Properties of New CuBr2(C6H7N)2 Complex: DFT Approaches

For the first time, the structural and optoelectronic properties of a new complex formulated as CuBr2(C6H7N)2 (1) [trans‐dibromidobis(3‐methylpyridine‐κN) copper(II)] were studied by density functional theory (DFT) calculations. They are performed using B3LYP through the Gaussian 09 program and also with full potential linearized augmented plane wave (FP‐LAPW) methods within the Generalized Gradient Approximation (GGA) and Hartree‐Fock (HF) theory by the Wien2k package. The neutral monomeric complex participates in a variety of non‐covalent interactions, including hydrogen bonding and π stacking to create a 2D coordinate plane. The binding energy value of the non‐covalent interactions responsible for the crystalline network formation of 1 were calculated using the method of dispersion corrected density functional theory (DFT‐D). In this method, the independent smallest fragment (monomer) and subsequently the related network, including seven monomers bearing all non‐covalent interactions were optimized. The results demonstrate that hydrogen bonds, especially non‐conventional C–H···Br interactions, govern the network formation along the a and c axes. It can be mentioned because of these directed interactions, increasing of charge transfer along x and z directions results in increasement of the absorption and refractive index along y and z directions, and vice versa. The results of band structure show indirectly and directly the nature of the bandgap within GGA and HF, respectively. The bandgap value of CuBr2(C6H7N)2 is comparable to those of binary semiconductor compounds. DOSs spectra reveal that 3d Cu, 4p Br, and 2p C states play important roles in the optical transitions of the electrons. The calculated electronic absorption of the UV/Vis spectrum shows six major electron‐transition bands derived from d → d (ligand field) n → n, n → π*, π → n, and σ → n MLCT and LMCT transitions. The calculated absorption spectrum of the titled complex through FP‐LAPW within GGA method shows good consistency with the B3LYP/def2‐TZVP/6‐311+G(d,p) method. Our calculated birefringence results show that 1 has capability of nonlinear optical, which can be used in the nonlinear optoelectronic devices.

XUV spectra of 2nd transition row elements: identification of 3d–4p and 3d–4f transition arrays

The use of laser produced plasmas (LPPs) in extreme ultraviolet/soft x-ray lithography and metrology at 13.5 nm has been widely reported and recent research efforts have focused on developing next generation sources for lithography, surface morphology, patterning and microscopy at shorter wavelengths. In this paper, the spectra emitted from LPPs of the 2nd transition row elements from yttrium (Z = 39) to palladium (Z = 46), with the exception of zirconium (Z = 40) and technetium (Z = 43), produced by two Nd:YAG lasers which delivered up to 600 mJ in 7 ns and 230 mJ in 170 ps, respectively, are reported. Intense emission was observed in the 2–8 nm spectral region resulting from unresolved transition arrays (UTAs) due to 3d–4p, 3d–4f and 3p–3d transitions. These transitions in a number of ion stages of yttrium, niobium, ruthenium and rhodium were identified by comparison with results from Cowan code calculations and previous studies. The theoretical data were parameterized using the UTA formalism and the mean wavelength and widths were calculated and compared with experimental results.

Electronic structure of Cu2ZnGeSe4 single crystal: Ab initio FP-LAPW calculations and X-ray spectroscopy measurements

High-quality Cu2ZnGeSe4 single crystal has been successfully grown by a solution-fusion method and the crystal structure of the compound is refined within tetragonal kesterite-type (space group 14¯), with the unit cell parameters a=5.6112(1) Å and c=11.0473(3) Å. X-ray photoelectron core-level and valence-band spectra for pristine and Ar+-ion irradiated surfaces of the Cu2ZnGeSe4 crystal are measured. Our X-ray photoelectron spectroscopy (XPS) data indicate that the Cu2ZnGeSe4 single crystal surface is very rigid with respect to Ar+ ion-irradiation. The electronic structure of the Cu2ZnGeSe4 compound has been theoretically studied using first principles calculation employing the full potential linearized augmented plane wave (FP-LAPW) method. Calculations of the total and partial densities of states of atoms constituting Cu2ZnGeSe4 reveal that the dominant contributors to the valence band of the compound studied are the Cu 3d states, which contribute mainly at the top of the band, while the central portion of the band is composed mainly from the Cu 3d and Se 4p states in almost equal proportion. Additionally, the present FP-LAPW calculations indicate that the bottom of the valence band is dominated by the Zn 3d states, while the bottom of the conduction band is composed mainly from the unoccupied Ge 4s and Se 4p states. The data of the present FP-LAPW calculations are confirmed by comparison on a common energy scale of the X-ray emission bands representing the energy distribution of the valence Cu d, Zn d, Ge p and Se p states and the XPS valence-band spectrum of the Cu2ZnGeSe4 single crystal.

Atomic structures and electronic properties of 2H-NbSe2: The impact of Ti doping

Layered transition metal dichalcogenides have aroused renewed interest as electronic materials, yet their electronic performances could be modified by chemical doping. Here, we perform a systematic first-principles calculation to investigate the effect of Ti doping on atomic structure and electronic properties of the 2H-NbSe2. We consider a total of three possible Ti-doping models and find that both the substitution and intercalated models are chemically preferred with the intercalation model being more favorable than the substitution one. Structural analyses reveal a slight lattice distortion triggered by Ti doping, but the original structure of 2H-NbSe2 is maintained. We also observe an expansion of c axis in the substituted model, which is attributed to the reduced van der Waals interaction arising from the increased Se-Se bond length. Our calculations also predict that the electron transport properties can be enhanced by the Ti doping, especially for the Ti-intercalated 2H-NbSe2, which should be beneficial for the realization of superconductivity. Furthermore, the covalence element is found in the Ti-Se bonds, which is ascribed to the hybridization of Ti 3d and Se 4p orbitals. The findings indicate that doping of transition metals can be regarded as a useful way to tailor electronic states so as to improve electron transport properties of 2H-NbSe2.

Atomic data for astrophysics: Fe IX

We present the results of a new large-scale intermediate-coupling frame transformation R-matrix scattering calculation for electron collisional excitation of Fe ix. The target includes all the main configurations up to n = 5, to improve our earlier R-matrix and distorted-wave (DW) calculations for the n = 3,4 levels. Unlike similar calculations which we carried out for the other coronal iron ions, in this case the larger target does not significantly affect the collision strengths of the strongest transitions to the n = 3,4 levels. Some differences are however present for a few transitions, in particular for the 3d–4p line at 197.86 A. For the weaker transitions, significant enhancements due to extra resonances resulting from this much bigger target are found. Several new line identifications are suggested. We find excellent agreement between predicted and observed line intensities in the EUV (Hinode EIS) showing that Fe ix lines provide a reliable temperature diagnostic. We also show that the visible forbidden lines are a good diagnostic to measure electron densities.

Electronic structures of the FeSe superconductor studied by high-energy photoelectron spectroscopy

The bulk electronic structures of an iron-based superconductor FeSe have been investigated by photoelectron and photoabsorption spectroscopies (PES and XAS) with use of high-energy photons (hν ≊ 0.7–0.9 and 8 keV). The core-level PES and XAS combined with the configuration interaction cluster model calculation reveal the highly coherent nature of the Fe 3d electrons which is realized by the hybridization between Fe 3d and Se 4p states.

XUV spectra of laser-produced zirconium plasmas

XUV spectra of laser-produced zirconium plasmas in the 20 to 65 Å wavelength range, where 3d–4p and 3d–4f transitions dominate the observed emission, were studied experimentally and theoretically. Atomic structure calculations were performed with the Cowan and flexible atomic codes. Spectator satellite transitions, especially 3dn-14s–3dn-24s4f, where the 4s electron acts as a ‘real’ spectator, in Mn-, Fe-, Co-, Ni- and Cu-like ions have been found to play an important role. The resulting emission causes arrays resulting from 3dn–3dn4f transitions to broaden and must be included in spectral simulations in order to reproduce the overall spectral profile.

Electronic structure of the new Ni-based superconductor KNi2Se2

The electronic structure of the newly discovered superconductor KNi2Se2 is studied by first-principles calculations. Our results show that its ground state is a quasi-two-dimensional metal and the Fermi surface is contributed by the Ni 3d and Se 4p states. These states determine the significant physical properties of the heavy-fermion superconductor KNi2Se2, such as the heavy-fermion behavior and low superconducting transition temperature. Based on the multi-orbital character of our calculation, we suggest that KNi2Se2 may have a complicated superconducting multi-gap structure. The influence of the Coulomb interaction on the electronic structure and the 3d orbital character in this material is also investigated.

Electronic properties of FeSe1−xTex probed by x-ray emission and absorption spectroscopy

The electronic structure of FeSe1−xTex chalcogenide superconductors has been studied by x-ray emission (XES) and x-ray absorption (XAS) as a function of Te substitution. The Fe Kβ XES spectra reveal a relatively low spin state for Fe in FeSe1−xTex superconductors, persisting in the whole range of Te substitution. The Fe K-edge high-resolution XAS shows systematic spectral changes due to the evolving hybridization between the Fe 3d (4p) and chalcogen p (d) orbitals. The resonant inelastic x-ray scattering (RIXS) spectra hardly show any feature except the one due to charge transfer from occupied to unoccupied bands, that changes substantially from FeSe to FeTe. The results provide important information on the electronic states and their evolution in the FeSe1−xTex chalcogenides.