Valence Electron Distribution Research Articles

Revitalizing CO2 photoreduction: Fine-tuning electronic synergy in ultrathin g-C3N4 with amorphous (CoFeNiMnCu)S2 high-entropy sulfide nanoparticles for enhanced sustainability

Sustainability is a key issue in developing stable and efficient catalysts for solar-catalyzed CO2 conversion. The concept of structural stabilization by quenching the Gibbs free energy, which is achieved by increasing the configurational entropy level of the system, introduced high-entropy materials. However, because alloying immiscible multimetallic atoms into a unit system is still an arduous process, the potential of high-entropy materials has not yet been fully exploited. Surprisingly, although metal sulfide photocatalysts have an enviable reputation for CO2 photoreduction (CO2-PR) owing to their optical/electronic merits and more negative redox potential than other materials, the capability of high-entropy metal sulfides has been completely disregarded. Herein, an adaptable, self-templating metal alkoxide appropriately addresses the immiscibility of metallic constituents by the well-blended metal ions with polyalcohol in the metal glycerate. Consequently, (CoFeNiMnCu)S2 high-entropy metal sulfide nanoparticles (HEMS-Nps) were grown in situ on crossed ultrathin g-C3N4 (UCN) monolayers during the sulfidation of the metal glycerate. The as-synthesized HEMS-Np has a crystalline–amorphous structure, which is conducive to the acceleration of charge transfer to/from reaction sites, and a convincing orientation for CO2 adsorption. These features are combined with synergistic electronic multimetallic interactions, facilitating a heterogeneous valence electron distribution and atomic disorder. The CO2-PR power of the (CoFeNiMnCu)S2@UCN nanostructure was noticeable in realizing added-value products in both the liquid–solid (1063 µmol/g h of syngas and 250 µmol/g h of methane) and gas–solid phases (1883 µmol/cm2 h of syngas and 321 µmol/cm2 h of methane). To prove its utility, long-term and corresponding time-equivalent cycling experiments were conducted, demonstrating a highly stable system that endured for a long time. Overall, this approach simultaneously exploits the advantages of sulfide-based materials and high-entropy materials in stable structures to generate sustainable CO2-PR materials.

“Capture-activation-recapture” mechanism-guided design of double-atom catalysts for electrocatalytic nitrogen reduction

Compared with the traditional industrial nitrogen fixation, electrocatalytic methods, especially those utilizing double-atom catalysts containing nonmetals, can give good consideration to the economy and environmental protection. However, the existing “acceptance-donation” mechanism is only applicable to bimetallic catalysts and nonmetallic double-atom catalysts containing boron atoms. Herein, a novel “capture-activation-recapture” mechanism for metal-nonmetal double-atom catalyst is proposed to solve the problem by adjusting the coordination environments of nonmetallic atoms and utilizing the activation effect of metal atoms on nitrogen. Based on this mechanism, the nitrogen reduction reaction (NRR) activity of 48 structures is calculated by density functional theory calculation, and four candidates are selected as outstanding electrocatalytic nitrogen reduction catalysts: Si-Fe@NG (UL = –0.14 V), Si-Co@NG (UL = –0.15 V), Si-Mo@BP1 (UL = 0 V), and Si-Re@BP1 (UL = –0.02 V). The analyses of electronic properties further confirm “capture-activation-recapture” mechanism and suggest that the difference in valence electron distribution between metal and Si atoms triggers the activation of N≡N bonds. In addition, a machine learning approach is utilized to generate an expression and an intrinsic descriptor that considers the coordination environment to predict the limiting potential. This study offers profound insight into the synergistic mechanism of TM and Si for NRR and guidance in the design of novel double-atom nitrogen fixation catalysts.

Modulating electron distributions by integrating ligands with metal molecules in THF

While the complex structures and dynamic behavior of alkali metal cations in solvents have been fully understood, insights into the structures, properties and dynamic behaviors of metal dimers in aprotic solvents (e.g. tetrahydrofuran/THF) are quite scarce, and especially the bonding characteristics, valence electron distribution and effect of co-solvents (e.g. crown ether/18-crown-6 (18C6)) are poorly understood. We here perform ab initio molecular dynamics simulation characterization on K2 molecular solute in THF with and without co-solvent molecule (18C6). Our findings indicate that K2 can be integrated by different numbers of THF molecules and 18C6 (if available), forming various chemical identities with unique structures and electron distributions in THF solvent ((THF)xK-K(THF)y, x + y ≤ 9; (THF)x(18C6)K-K(THF)y, x + y ≤ 4). These identities have considerable lifetimes in time evolution and the structure fluctuation dynamics of solution can realize interconversion among these identities through removing and/or adding THFs from/to two K subcluster moieties. In general, compared with the gaseous K2, integration of THFs considerably expands the K2 electron cloud and exhibit much more active states featuring distorted diffuse distributions which can be viewed as electron-expanded chemical entities. Further 18C6 coordination can tightly bind an end K and keep one THF coordinating a K at the axis direction and another K keeps a low coordination status. Such a coordination mode largely forces valence electrons to THF bulk acting as the electron-expanded complexes. In particular, the coordination of 18C6 greatly polarizes the bonding electron, changing the K-K covalent bond into highly ionic one with a Kδ+-Kδ− core structure. All these identities present lower ionization potentials than that of the gaseous K2, and thus have highly reductive reactivity, in agreement with the experiments. This work provides a promising strategy for how to modulate the bonding electrons around the electron-flexible metal–metal bonds.

Microwaves affect the formation of volatile compounds in peper powder by changing the nucleophilic addition reactions in Maillard reactions.

We explored the effect of microwave heating (MWH) and electric heating (ETH) on the volatile compounds (VCs) of pepper (Capsicum annuum L.). The spectral of the produced melanoidins by baking were used to screen samples with similar baking degrees. Mass spectrometry was used to detect the differences of VCs in samples. The results showed a dose-dependent effect between the intensity of absorption and fluorescence of melanoidins, which can be utilized as indicators for assessment baking degree. MWH samples produced larger variety of VCs than ETH. Changes in the variety and content of VCs infer changes in the flavor of pepper. According to the mechanism of Maillard reaction (MR) and MWH, it was deduced that MWH changes the type of chemical reaction in MR by affecting the distribution of valence electrons in the compounds. Therefore, MWH can be used as a novel method to modify the VCs and flavor of peppers.

Real-Space Observation of Ligand Hole State in Cubic Perovskite SrFeO3.

An anomalously high valence state sometimes shows up in transition-metal oxide compounds. In such systems, holes tend to occupy mainly the ligand p orbitals, giving rise to interesting physical properties such as superconductivity in cuprates and rich magnetic phases in ferrates. However, no one has ever observed the distribution of ligand holes in real space. Here, a successful observation of the spatial distribution of valence electrons in cubic perovskite SrFeO3 by high-energy X-ray diffraction experiments and precise electron density analysis using a core differential Fourier synthesis method is reported. A real-space picture of ligand holes formed by the orbital hybridization of Fe 3d and O 2p is revealed. The anomalous valence state in Fe is attributed to the considerable contribution of the ligand hole, which is related to the metallic nature and the absence of Jahn-Teller distortions in this system.

Regulating electron distribution of P2-type layered oxide cathodes for practical sodium-ion batteries

For transition metal oxide materials, high Ni content is an effective method to obtain a high specific capacity. However, the theoretical capacity is determined due to the certain amount of variable charges of transition metal ions. The increased capacity in specific voltage window may attribute to the easier transport of alkali ions, instead of more active elements. Borrowing the theory of Ni-rich materials in LIB, excess Ni elements were added into P2-type layered oxide material to form the Jahn-Teller active Ni3+ ions. About 25%-61% Ni3+ ions can effectively promote de-/intercalation of Na+ ions due to the decreased diffusion energy barrier and increased adsorption energy of Na+. The preferred “Ni-rich” material Na0.67Mn0.45Ni0.22Co0.33O2 (Ni-R1) shows a reversible specific capacity of 114 mA h g−1 in the voltage range of 2.0–4.25 V. In addition, it shows an excellent cycle stability, the capacity retention ration is 80% after 1000 cycles at a current density of 1 A g−1. The in-depth study proves that, Jahn-Teller active Ni3+ ions can effectively regulate the valence electron distribution of surrounding ions in synthesis stage. However, it will promote Jahn-Teller distortion when the Ni3+ content is increased to 74%, which makes the rate performance deteriorate dramatically. The present work provides a simple and efficient way to increase the capacity in suitable voltage range for application.

Quantum crystallographic method improves the accuracy of screening of inhibitors: A set of Alzheimer’s disease responsible protein‐ligand complexes

AbstractBackgroundIn drug design and development, identification of lead structure is a crucial step to predict the ligand binding affinity and functional potency with any of target enzyme/protein. The recently developed quantum crystallographic method (fragment‐based approach) has been used as a substitutional way to calculate the interaction energies. This quantum mechanical level information gives a deeper understanding of structural facts with the position of hydrogen atoms and valence electron distributions. It is very much essential to understanding the electronic properties of drug‐receptor interactions. Therefore, the quantum crystallographic based interaction energy calculation is a better alternative method to docking score‐based modelling.MethodA set of AD responsible protein‐ligand complexes were selected for the quantitative analysis. Molecular docking of selected compounds with four different enzymes (AChE/BChE/BACE1/GSK3β) was carried out and then, all the selected complexes have been subjected to MD simulation. Later, the 3D coordinates of ligand with active site residues (around 4Å from the ligand) were used for quantum crystallographic studies.ResultThe screened molecules are forming strong hydrogen bonding and hydrophobic interactions with active site residues of all four enzymes; particularly, π‐π stacking, π‐cationic, π‐alkyl and π‐σ types of interactions, which are highly stable during dynamic behavior. After the quantum crystallographic studies, the interaction energies of selected compounds with different active site residues of all four enzymes were analyzed; it is ranged from ‐20.83 to ‐3.92 kcal/mol and the total interaction energies of each complex are higher than binding energy gained from MD simulation. And, the RMSD of each molecule was compared with the corresponding geometry of solid‐state and dynamic state; it is found to be significantly low (>0.5Å) and similar. Interestingly, the correlation between experimental IC50 and averaged interaction energies of selected compounds exhibits good agreement.ConclusionThe in‐silico results revealed conformational flexibility, high stability and compactness, binding affinity and orientation of the target molecules in the active site residues of all enzymes in the physiological nature (water‐medium). Quantum crystallographic method is a novel approach that opens new window to delineate the different type of interactions at electronic level. And, it may allow us to design new drug against target disease.

Enhanced reduction of smelting gypsum by H2S and in-situ DRIFTs investigated on the mechanisms of S-O bond activation

The gypsum from flue gas desulfurization or the neutralization (D/N gypsum) process of acid wastewater, is an industrial by-product in nonferrous metal smelters. The S and Ca sources of D/N gypsum have significant application prospects. However, the S(IV) reduction is difficult due to the electronic inactivity of CaSO4 and the stability of the S-O band. Our experimental results indicated that the start-up temperature of CaSO4 reduced by CH4 is 900 ℃. However, adding H2S gas with a H2S/CaSO4(S/Ca) ratio of 0.2, the start-up temperature of CaSO4 reacted with CH4 decreases to 650 ℃. With the increase of the S/Ca ratio from 0.04 to 0.2, the yield of CaS increased to 94.97%, and the selectivity toward CO2 formation increases to 71.67%. In-situ diffuse reflectance Fourier transformations spectroscopy (in-situ DRIFTs) results indicate that adding H2S generates the new sulfide groups on the surface of CaSO4, resulted from the S(-II) in H2S changing the distribution of valence electrons of O and activating O. Moreover, the density functional theory (DFT) calculation results exhibit that the activation energy for the first O dissociation decreases to 2.89 eV in the presence of H2S. Furthermore, H2S optimized the reaction path of CH4 and improved the selectivity toward CO2 formation. Comparing to 1000 ℃ without H2S, under 800 ℃ with H2S, the consumption of CH4 reduces to 45%, and the CO2 selectivity increases to 71.67%. Thus, we have developed a method of CaSO4 low-temperature reduction and provide a theoretical basis for its application in engineering.

Embedding Double Transition Metal Atoms in B-Modified Two-Dimensional Carbon-Rich Conjugated Frameworks for Efficient Ammonia Synthesis.

It is important to develop an efficient green catalyst for electrochemical nitrogen reduction under environmental conditions to replace the Haber-Bosch process with high energy consumption, limited economic scale of production, and high CO2 emissions, but there are still some difficulties in its development and design. In this work, based on 2D CCFs, a B-modified rectangular-shaped expanded phthalocyanine bimetallic atomic catalyst is proposed. Because both the TM-d orbital and the B-sp3 hybrid orbital can accept the lone pair and feedback electrons of N2, and there are significantly different valence electron distributions between the two different metals and boron, this catalyst not only keep the advantages of metal catalysts but also play the role of the B atom to further increase the polarity of nitrogen and activate N2. In order to evaluate the catalytic performance of this catalyst, its stability, activity, selectivity, reactivity, and other aspects are screened and analyzed by density functional theory calculation. Among them, the FeNb-BPc CCF catalyst has better N2 adsorption capacity under ambient conditions, exhibits excellent catalytic activity through a continuous mechanism, and has a lower limiting potential of -0.24 V. We believe that this work opens a new path for the rational design of efficient N2 reduction reaction catalysts and also provides a theoretical basis for experimental development.

Upper limit of the transition temperature of superconducting materials

Why are the transition temperatures (T c) of superconducting materials so different? The answer to this question is not only of great significance in revealing the mechanism of high-T c superconductivity but also can be used as a guide for the design of new superconductors. However, so far, it is still challenging to identify the governing factors affecting the T c. In this work, with the aid of machine learning and first-principles calculations, we found a close relevance between the upper limit of the T c and the energy-level distribution of valence electrons. It implies that some additional inter-orbital electron-electron interaction should be considered in the interpretation of high-T c superconductivity.

Mapping Valence Electron Distribution and Magnetic Field by 4D-STEM

Mapping Valence Electron Distribution and Magnetic Field by 4D-STEM

Functionalization of Quasi-Two-Dimensional Materials: Chemical and Strain-Induced Modifications

Among the family of currently known promising quasi-two-dimensional (2D) materials, the authors of this survey concentrate on the problem of functionalization of the graphene- and phosphorene-based structures. In most cases, the modification of their properties occurs through the covalent or noncovalent surface functionalization and mechanical affects. The atomic structures and some physicochemical features of 2D materials possessing novel properties as compared to their bulk counterparts are analysed. Their main advantages are the thickness of one or more atoms, the absence of surface-broken bonds, high mobility of charge carriers, the flexibility, the ability to be combined artificially into coplanar (lateral) or lamellar heterostructures, as well as the possibility to manipulate widely the band-gap changing from the semi-conducting state even into the semi-metallic one (or vice versa) when needed. In order to reveal new factors affecting the electronic properties of 2D materials by means of the computational experiment using the author’s (self-constructed) software code, a series of studies are carried out. They are the calculations of the spatial distribution of valence electrons’ density, the electron densities of states, the band-gap widths, Coulomb potentials along selected directions, the charge values in regions of different-size material, the dielectric matrices, the macroscopic relative permittivities, and absorption spectra. A series of recent studies, which the authors carried out modelling the electronic and transport properties of single- or multilayer graphene films subjected to deformation or/and magnetic fields and containing different-type (point- or/and linear-acting) defects is reviewed. Analysing the obtained results and revealed effects, it is claimed that the uniaxial tensile deformations or shear deformations along with their combinations as well as the structural imperfections (mainly, the mutually configured defects) can be useful for achieving the new level of functionalization of graphene. So, for modification of its electrotransport properties through tuning the band-gap value as much as it is enough to achieve the graphene transformation from the zero-band-gap semi-metallic state into the semi-conducting state and even reach the gap values, which are substantially higher than that for some materials (including silicon) currently used widely in the nanoelectronic devices. The strain- and defect-induced electron–hole asymmetry and anisotropy of conductivity and its nonmonotony as a function of deformation suggest a confidence in manipulating the electrotransport properties of graphene-like and beyond quasi-2D materials through a variety of both strains and defects. The use of reviewed and analysed results serves as a significant step in improving the properties of the considered materials in order to implement the multifunctional applications of them in the immediate prospect.

First principles design of 2 dimensional Nickel dichalcogenide Janus materials NiXY (X,Y = S,Se,Te)

In this work, we propose novel two-dimensional (2D) Janus Ni dichalcogenide materials and explore their feasibility, stability and evaluate their electronic and optical properties with ab-initio calculations. Three unique Janus materials, namely NiSSe, NiSTe and NiSeTe, with 2H hexagonal structure were proposed. Density functional theory (DFT) calculations, show that among the three proposed NiXY Janus 2D materials, NiSSe had the best energetic and dynamical stability. GGA PBE calculations showed NiSSe to have a metallic bandstructure with the Ni-Se interaction having a dominant role in the band profile near the Fermi energy. Electron localization function (ELF) and total potential plots show a distinguishable asymmetry in terms of valence electron localization and distribution between the S and Se atoms in 2D NiSSe. The presence of large amount of electron gas like feature in the ELF around the chalcogen atoms also indicates their importance in the conduction properties. Optical properties calculated with random phase approximation (RPA) show the 2D NiSSe to have broad spectrum optical response with significant peaks lying in each of the infra-red, visible and the ultraviolet range of the spectra.

The emergence of valency in colloidal crystals through electron equivalents.

Colloidal crystal engineering of complex, low-symmetry architectures is challenging when isotropic building blocks are assembled. Here we describe an approach to generating such structures based upon programmable atom equivalents (nanoparticles functionalized with many DNA strands) and mobile electron equivalents (small particles functionalized with a low number of DNA strands complementary to the programmable atom equivalents). Under appropriate conditions, the spatial distribution of the electron equivalents breaks the symmetry of isotropic programmable atom equivalents, akin to the anisotropic distribution of valence electrons or coordination sites around a metal atom, leading to a set of well-defined coordination geometries and access to three new low-symmetry crystalline phases. All three phases represent the first examples of colloidal crystals, with two of them having elemental analogues (body-centred tetragonal and high-pressure gallium), while the third (triple double-gyroid structure) has no known natural equivalent. This approach enables the creation of complex, low-symmetry colloidal crystals that might find use in various technologies.

Electronic structures and emission properties of typical binary single crystal REB<sub>6</sub>

Binary rare earth hexaborides (REB<sub>6</sub>) have different rare earth elements with different valence electron distributions, which lead to different strange physical properties and different emission properties. However, in the electron emission properties, whether PrB<sub>6</sub>, NdB<sub>6</sub>, SmB<sub>6</sub> and GdB<sub>6</sub> all have excellent emission properties remains to be further studied, and the physical mechanism affecting their emission properties needs investigating. In this paper, the electronic structures, work functions of typical binary single crystal REB<sub>6</sub> (LaB<sub>6</sub>, CeB<sub>6</sub>, PrB<sub>6</sub>, NdB<sub>6</sub>, SmB<sub>6</sub>, GdB<sub>6</sub>) are studied by first principles calculations. The single crystal REB<sub>6</sub> are prepared by optical zone melting method, and their thermionic electron emission properties are tested experimentally. The theoretical calculation results show that the typical binary REB<sub>6</sub> have large densities of states near the Fermi level. The d-orbitals with broad distributions in conduction bands are beneficial to electron emission. The localized f-orbital electrons in valence bands are not conducive to their electron emission. The theoretical calculations of work functions of typical binary single crystal REB<sub>6</sub> (100) surface are consistent with the analyses of their electronic structures. The theoretical calculation values of work functions are ordered as GdB<sub>6</sub> (2.27 eV) < CeB<sub>6</sub> (2.36 eV) < LaB<sub>6</sub> (2.40 eV) < PrB<sub>6</sub> (2.58 eV) < SmB<sub>6</sub> (2.63 eV) < NdB<sub>6</sub> (2.91 eV). The experimental test results of thermionic electron emission of single crystal show that the experimental thermionic electron properties are consistent with the theoretical ones. The LaB<sub>6</sub> and CeB<sub>6</sub> both have good thermionic and field emission properties, and the GdB<sub>6</sub> has excellent field emission properties.

Novel Design Strategy of High Activity Electrocatalysts toward Nitrogen Reduction Reaction via Boron-Transition-Metal Hybrid Double-Atom Catalysts.

Electrocatalytic nitrogen reduction reaction (NRR) is a promising method for sustainable production of NH3, which provides an alternative to the traditional Haber-Bosch process. However, the poor Faraday efficiency caused by N≡N triple bond activation and competitive hydrogen evolution reaction (HER) have seriously hindered the application of NRR. In this work, a novel strategy to promote NRR through boron-transition-metal (TM) hybrid double-atom catalysts (HDACs) has been proposed. The excellent catalytic activity of HDACs is attributed to a significant difference of valence electron distribution between boron and TMs, which could better activate N≡N bonds and promote the conversion of NH2 to NH3 compared with boron or metal single-atom catalysts and traditional double-atom catalysts (DACs). Hence, by means of DFT computations, the stability, activity, and selectivity of 29 HDACs are systematically investigated to evaluate their catalytic performance. B-Ti@g-CN and B-Ta@g-CN are screened as excellent nitrogen-fixing catalysts with particularly low limiting potentials of 0.13 and 0.11 V for NRR and rather high potentials of 0.54 and 0.82 V for HER, respectively. This work provides a new idea for the rational design of efficient nitrogen-fixing catalysts and could also be widely used in other catalytic reactions.

Measurements of polarization dependencies in parametric down-conversion of x rays into ultraviolet radiation

We present measurements of the polarization dependencies of the x-ray signal photons generated by the effect of parametric down-conversion of x rays into ultraviolet radiation. The results exhibit pronounced discrepancies with the classical model for the nonlinearity but qualitatively agree with a recently developed quantum mechanical theory for the nonlinear interaction. Our work shows that the reconstruction of the atomic scale charge distribution of valence electrons in crystals by using nonlinear interaction between x rays and longer wavelength radiation, as was suggested in previous works, requires the knowledge of polarization of the generated x-ray signal beam. The results presented in this work indicate a methodology for the study of properties of the Wannier functions in crystals.

Sr7N2Sn3: a layered antiperovskite-type nitride stannide containing zigzag chains of Sn4 polyanions

Abstract Metallic black platelet single crystals of a new ternary compound, Sr7N2Sn3, were obtained by heating Sr and Sn in a Na flux together with NaN3 as a nitrogen source at 1073 K, followed by slow cooling. Single-crystal X-ray analysis revealed that this compound crystallizes in an orthorhombic cell with the cell parameters a = 10.4082(2), b = 18.0737(4), and c = 7.43390(10) Å (space group Pmna, Z = 2), and has a layered (modular) antiperovskite-type structure which could be related to the inverse structure of Ca2Nb2O7 ((Ca2)[Ca2Nb4O14]). Four-membered zigzag [Sn4] chains are situated between slabs comprising four antiperovskite layers cut by the (110) plane of the ideal anitiperovskite structure, and Sr7N2Sn3 can be expressed as [Sn4][Sn2N4Sr14]. Although an electron-precise valence electron distribution according to the formula (Sr2+)14(N3−)4(Sn4−)2([Sn4]8−) is proposed for this ternary compound, yet, there are certain structural peculiarities which cannot be explained by this idealized picture. Therefore, first principles-based means were employed to account for the aforementioned structural features.

Empirical potential profile model for subbands with unusual energy separation in Si(111) p-type inversion layer

To explain the deviation in the valence subband energy levels of the Si(111) inversion layer obtained by experiments and calculations, the band edge profile that has the same subband levels as the experimental results is searched through numerical calculations. The obtained band edge profile is characterized by its flat feature in the subsurface region over 1.2 nm from the surface of the Si substrate. By taking the second derivative of the obtained band edge profile, the carrier distribution in the space charge layer is obtained, and the existence of negative charges in the subsurface region is revealed. The origin of the negative charges is attributed to the inhomogeneous valence electron distribution due to the standing wave formation in the narrow space charge layer.

Identification of the anti-triangular etched MoS2 with comparative activity with commercial Pt for hydrogen evolution reaction

The introduction of anti-triangular dots by etching MoS2 has been recently reported as an efficient way to produce active internal edges. However, it is still ambiguous to identify the configuration of etched MoS2 with comparative hydrogen evolution reaction (HER) activity to commercial Pt. In this work, the HER activity on etched anti-triangular MoS2 are investigated by density functional theory calculations. We found the valence electron distribution of inner edge atoms can be affected by the corner, and accordingly determine their activity. The second-nearest atom to anti-triangular corner possesses the highest activity. The simulated polarization curves show etched anti-triangular Mo-edge MoS2 with moderate size (around 12 Å) maximize the HER performance. Further increasing size of etched MoS2 deactivate their HER activity to a certain degree, validated by available experimental data. This work suggests etched MoS2 catalysts with rational design may be a candidate to substitute Pt electrodes as HER electrocatalyst.