Changes In Electronic Structure Research Articles

A New High-Valent Fe Redox Couple in Intercalation Electrodes

The overwhelming majority of layered oxide cathode materials are based on the redox activity of expensive and scarce transition metals like Co and Ni. While the Fe-redox based material LiFePO4 has become increasingly attractive due to its low cost, its low gravimetric energy density and relatively low operating voltage limit its use cases. A high-voltage Fe-based layered oxide cathode would provide high energy density utilizing the redox-activity of the most abundant transition metal in Earth’s crust, but such a material that provides stable and reversible electrochemical performance remains elusive. Indeed, high-voltage Fe-redox in oxides is plagued by cation disorder and oxygen dimerization, which leads to capacity fading and large voltage hysteresis (> 1 V).[1]In contrast, the layered oxide cathode Li4FeSbO6 (equivalently, Li[Li1/3Fe1/3Sb1/3]O2, abbreviated LFSO) exhibits <0.2 V of hysteresis, and a discharge plateau at 4.2 V versus Li/Li+ with two-electron capacity.[2] Both the exceptional electrochemical reversibility of this material and its phase separating nature upon (de)intercalation raise questions about the origin of these properties that are unusual for layered Fe oxide cathodes. In this work, we will discuss our efforts to probe the electronic structure of LFSO with Fe L–edge x–ray absorption (XAS) and 57Fe Mössbauer spectroscopy, as well as O K–edge resonant inelastic x–ray scattering spectroscopy (RIXS). The combination of these element-specific and valence-state sensitive techniques, in conjunction with charge-transfer multiplet and density functional theory calculations, allows us to reveal the electronic structure changes at Fe and O as a function of state of charge for the first time. Correlating these electronic structure changes with synchrotron diffraction measurements, we reveal a unique redox compensation mechanism that readily rationalizes the exceptional electrochemical reversibility, unlocking a new direction for high-voltage, Fe-based redox systems.

Pressure-induced photocurrent enhancement and metallization in van der Waals compound SiTe2

Layered van der Waals (vdW) dichalcogenides are distinguished by their unique crystal structures and high structural tunability, rendering them suitable for applications in optics and optoelectronics. Despite significant processes, some fundamental questions remain in two-dimensional (2D) vdW dichalcogenides, such as clarifying detailed structure–property relationship and further improving the optoelectronic performance. Herein, by applying pressure to tune the crystal structure in 2D vdW dichalcogenide SiTe2, we realized a five orders of magnitude boost in photocurrent at 8 GPa. Such an enhancement is attributed to bandgap narrowing and an increased carrier concentration. Furthermore, bandgap closing and metallization were observed at 15.4 GPa, further suggesting the significant change of electronic structure upon compression. This study not only elucidates the intriguing pressure-induced behavior of SiTe2 but also paves the way for harnessing the unique pressure-responsive properties of 2D vdW dichalcogenides in advanced optoelectronic systems.

Transfer learning relaxation, electronic structure and continuum model for twisted bilayer MoTe2

Large-scale moiré systems are extraordinarily sensitive, with even minute atomic shifts leading to significant changes in electronic structures. Here, we investigate the lattice relaxation effect on moiré band structures in twisted bilayer MoTe2 with two approaches: (a) large-scale plane-wave basis first principle calculation down to 2.88°, (b) transfer learning structure relaxation + local-basis first principles calculation down to 1.1°. We use two types of van der Waals corrections: the D2 method of Grimme and the density-dependent energy correction, and find that the density-dependent energy correction yields a continuous evolution of bandwidth with twist angles. Based on the above results. we develop a complete continuum model with a single set of parameters for a wide range of twist angles, and perform many-body simulations at ν = −1, −2/3, −1/3.

Revaling the mechanism of microscopic kinking phenomena in Hf6Ta2O17 superstructures

As a promising candidate for a new-generation thermal barrier coating material, Hf6Ta2O17 is characterized by its unique long-period superstructures and excellent mechanical properties. In this study, the unique "kinking" phenomenon of the atomic arrangement in Hf6Ta2O17 crystals is directly visualized via high-resolution transmission electron microscopy for the first time. The evolution of the microstructure, and particularly the micro-mechanism of the "kinking" phenomenon, have been further explored by first-principles calculations. The study suggests that this phenomenon is primarily triggered by the rotation of the polyhedral structure, resulting from the partial elongation or shortening of the Ta-O bond length. These structural alterations are believed to directly influence the macroscopic mechanical properties and can improve the tensile properties of the Hf6Ta2O17 crystals. The findings of this study provide an important microscopic perspective for understanding the strain-induced crystal structure transformation. The quantitative analysis of the strengths of Ta-O and Hf-O bonds in different coordination polyhedra reveals the electronic structure changes behind the crystal structure transition, thus providing a new theoretical basis for an in-depth understanding of the behavior of crystals under the external stress.

Imaging Anisotropic Proton Intercalation in Photochromic MoO3.

Protonation represents a fundamental chemical process with promising applications in the fields of energy, environment, and memory devices. Probing the protonation mechanism, however, presents a formidable challenge owing to the elusiveness of intercalated protons. In this work, we utilize the atomic and electronic structure changes associated with protonation to directly image the proton intercalation pathways in α-MoO3 induced by UV illumination. We reveal the anisotropic intercalation behavior which is initiated by photocatalyzed water dissociation preferentially at the (001) edges and then propagates along the c axis, transforming α-MoO3 into HxMoO3 to realize photochromism. This photochromic process can be reversed via heating in air, leading to anisotropic proton deintercalation, also preferentially along the c axis. The observed anisotropic behavior can be attributed to the intrinsically low energy barriers for both proton migration along the c axis and water dissociation/formation at (001) edges.

Structural and electronic properties of Ag-, Ni-, and Zn-doped copper chalcogenides – computational studies

Copper chalcogenides are among the most extensively studied materials for their application as thermoelectric materials and narrow-bandgap semiconductors for electrocatalytic purposes. The state-of-the-art literature predominantly adopts an experimental approach, often overlooking issues of stability and subtle changes in electronic structure arising from defect formation or doping in these systems. This study focuses on a computational approach to copper(I) chalcogenides doped with the most common impurities, namely Ag, Ni, and Zn. By considering formation energies and defect formation energies, the study offers a discussion with literature data concerning long-term stability of these systems, particularly under chemical/temperature gradient conditions and related ion migration processes. In subsequent sections, the impact of the presence of chosen impurities, as well as vacancies in the cationic sublattice, on structural parameters and electronic structure is presented. This includes selected electronic structure descriptors, providing insights into the influence of dopants and vacancies on the potential thermoelectric and electrocatalytic properties of copper chalcogenides.

Phase Transition Control in Molecular Solids via Complementarity of Hydrogen-Bond Strength.

Phase transitions in molecular solids involve synergistic changes in chemical and electronic structures, leading to diversification in physical and chemical properties. Despite the pivotal role of hydrogen bonds (H-bonds) in many phase-transition materials, it is rare and challenging to chemically regulate the dynamics and to elucidate the structure-property relationship. Here, four high-spin CoII compounds were isolated and systematically investigated by modifying the ligand terminal groups (X=S, Se) and substituents (Y=Cl, Br). S-Cl and Se-Br undergo a reversible structural phase transition near room temperature, triggering the rotation of 15-crown-5 guests and the swing between syn- and anti-conformation of NCX- ligands, accompanied by switchable magnetism. Conversely, S-Br and Se-Cl retain stability in ordered and disordered phases, respectively. H-bonds geometric analysis and ab initio calculations reveal that the electronegativity of X and Y affects the strength of NY-ap-H⋅⋅⋅X interactions. Entropy-driven structural phase transitions occur when the H-bond strength is appropriate; otherwise, the phase stays unchanged if it is too strong or weak. This work highlights a phase transition driven by H-bond strength complementarity - pairing strong acceptor with weak donor and vice versa, which offers a straightforward and effective approach for designing phase-transition molecular solids from a chemical perspective.

Contact Electrification as an Emerging Strategy for Controlling the Performance of Metal Nanoparticle Catalysts

Contact electrification (CE) is an emerging strategy for controlling the performance of metal nanoparticle (NP) catalysts. The underlying physical principle of this control is the sensitivity of the Fermi level to metal‐metal contacts. This change in electronic structure has a direct impact on surface properties and chemical reactivity. The concept article briefly introduces the basic theory of CE and its relationship to catalytic performance. We then highlight selected recent examples of advances in the preparation of hybrid metal NP assemblies, experimental techniques for characterizing CE, and finally applications of CE for altering catalytic performance.

Improved Urea Oxidation Performance via Interface Electron Redistributions of the NiFe(OH)x/MnO2/NF p-p Heterojunction.

The development of highly efficient urea oxidation reaction (UOR) electrocatalysts is the key to simultaneously achieving green hydrogen production and the treatment of urea-containing wastewater. Ni-based electrocatalysts are expected to replace precious metal catalysts for UOR because of their high activity and low cost. However, the construction of Ni-based electrocatalysts that can synergistically enhance UOR still needs further in-depth study. In this study, highly active electrocatalysts of NiFe(OH)x/MnO2 p-p heterostructures are constructed on nickelfoam (NF) by electrodeposition (NiFe(OH)x/MnO2/NF), illustrating the effect of electronic structure changes at heterogeneous interfaces on UOR and revealing the catalytic mechanism of UOR. The NiFe(OH)x/MnO2/NF only needs 1.364V (vs Reversible Hydrogen Electrode, RHE) to reach 10mA cm-2 for UOR. Structural characterizations and theoretical calculations indicate that energy gap leads to directed charge transfer and redistribution at the heterojunction interface, forming electron-rich (MnO2) and electron-poor (NiFe(OH)x) regions. This enhances the catalyst's adsorption of urea and reaction intermediates, reduces thermodynamic barriers during the UOR process, promotes the formation of Ni3+ phases at lower potentials, and thus improves UOR performance. This work provides a new idea for the development of Ni-based high-efficiency UOR electrocatalysts.

The mechanism of enhanced electrocatalytic water splitting on S-doped NiFe2O4/Ni–Fe alloy@copper foams

The development of bifunctional catalysts with subtle structures, high efficiencies, and good durabilities for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is crucial for overall water splitting. In this work, a multicomponent S-doped NiFe2O4/Ni–Fe micro nano flower electrocatalyst was synthesized rapidly on foam copper using a simple one-step constant current electrodeposition method. The introduction of S leads to the transformation of the microsphere structure of the Ni–Fe alloy into a cauliflower-like morphology and induces changes in the surface electronic structure, significantly enhancing the catalytic performance for the HER and OER. The S-NiFe2O4/Ni–Fe alloy/CF showed low overpotentials of 220 and 66 mV at 10 mA cm−2 in 1.0 M KOH for the OER and HER, respectively. High durability OER and HER performances were demonstrated through 60 h of chronopotentiometry and 6000 CV cycles test. Excellent overall water splitting electrocatalytic activity was observed in the S-NiFe2O4/Ni–Fe alloy/CF‖S-NiFe2O4/Ni–Fe alloy/CF two-electrode system. In particular, active-phase NiOOH, a highly active substance for OER, can be controllably formed in the reaction process owing to the nanoflower structure of multi-layer sulfur which slows down the dissolution of NiFe2O4/Ni–Fe alloy. These results suggest that this composite structure is a promising bifunctional electrocatalyst.

Primary processes of the archetypal model complex azido(porphinato)iron(III) from ultrafast vibrational-electronic spectroscopy.

Azidoiron complexes serve as valuable photochemical precursors for catalytically active species containing high-valent iron. In bioinorganic chemistry, azido(tetraphenylporphinato)iron(III), i.e., [FeIII(tpp)(N3)] with tpp = 5, 10, 15, 20-tetraphenylporphyrin-21, 23-diido, constitutes the archetypal model system that was used to access for the first time the terminal nitridoiron core, FeV ≡ N, in the biomimetic redox-non-innocent ligand environment. So far, the light-induced dynamics leading to the oxidation of the metal and the release of dinitrogen from the N3-ligand have only been studied for precursors featuring redox-innocent auxiliary ligands that simplify the electronic structure change accompanying the photo-transformation. Here, we monitored the primary events of the above paradigmatic complex, following its optical excitation in the ultraviolet-to-visible spectral range using femtosecond spectroscopy with probing in both the UV-vis and mid-infrared regions. Following ultrafast Soret-excitation at 400nm, the complex relaxes to the lowest excited sextet state by a first internal conversion in less than 200fs. The excited state then undergoes vibrational relaxation on a time scale of roughly 2ps before internally converting yet again to recover the sextet electronic ground state within 19.5ps. Spectroscopic evidence is obtained neither for a transient occupation of the energetically lowest metal-centered state, 41A1, nor for vibrational relaxation in the ground-state. The primary processes seen here are thus in contrast to those previously derived from ultrafast UV-pump/vis-probe and UV-pump/XANES-probe spectroscopies for the halide congener [FeIII(tpp)(Cl)]. Any photochemical transformation of the complex arises from two-photon-induced dynamics.

Constant potential energetics of metallic and semiconducting electrodes: A benchmark study on 2D materials.

Grand-canonical (GC) constant-potential methods within an implicit solvent environment provide a general approach to compute the potential-dependent energetics at electrified solid-liquid interfaces with first-principles density-functional theory. Here, we use a mindfully chosen set of 27 isostructural 2D metal halides MX2 to analyze the variation of this energetics when the electronic structure changes from metallic to semiconducting and insulating state. Apart from expectable changes due to the opening up of the electronic bandgap, the calculations also show an increasing sensitivity to the numerical Brillouin zone integration and electronic smearing, which imposes computational burdens in practice. We rationalize these findings within the picture of the total interfacial capacitance arising from a series connection of the electrochemical double-layer capacitance and the so-called quantum capacitance resulting from the filling of electronic states inside the electrode. For metals, the electrochemical double-layer capacitance dominates at all potentials, and the entire potential drop takes place in the electrolyte. For semiconductors, the potential drop occurs instead fully or partially inside the electrode at potentials within or just outside the bandgap. For 2D semiconductors, the increased sensitivity to numerical parameters then results from the concomitantly increased contribution of the quantum capacitance that is harder to converge. Fortunately, this understanding motivates a simple extension of the CHE + DL approximation for metals, which provides the approximate GC energetics of 2D semiconductors using only quantities that can be obtained from computationally undemanding calculations at the point of zero charge and a generic double-layer capacitance.

Probing Electronic Structure Changes in Cobalt Oxalate Anode for Lithium‐Ion Batteries

AbstractTransition metal‐based materials exhibit a broad range of catalytic activities in numerous electrochemical processes. Their displayed catalytic functions rely essentially on their distinctive electronic structures, changes of which play pivotal roles in dictating reaction dynamics within many electrochemical devices. Nevertheless, accurately probing electronic structure changes, especially in real‐time, of active materials remains a formidable challenge. In this work, a viable approach to achieve it within a CoC2O4/Li model system by employing a combined microscopic is demonstrated, spectroscopic analysis, plus a unique operando magnetometry technique. The findings reveal that upon completion of the conventional conversion of CoC2O4, a surface‐dominated capacitance emerges at the Co/Li2C2O4 interface owing to the injection of spin‐polarized electrons. Subsequently, the decomposition of Li2C2O4 proceeds through the releasing of the spin‐polarized electrons from Co, which, therefore serve as a catalyst leading to further discharge products. Such real‐time monitoring of electron transfer is realized by in situ monitoring electronic structure changes of Co, manifested by its intriguing magnetization alterations during the process. This work highlights a novel characterization tool that provides a solid explanation for the commonly observed large capacities in this type of materials and sheds light on design rules and selection guidance of materials for high‐performance electrochemical energy storage systems.

Flavonoid Oxidation Potentials and Antioxidant Activities-Theoretical Models Based on Oxidation Mechanisms and Related Changes in Electronic Structure.

Herein, I will review our efforts to develop a comprehensive and robust model for the estimation of the first oxidation potential, Ep1, and antioxidant activity, AA, of flavonoids that would, besides enabling fast and cheap prediction of Ep1 and AA for a flavonoid of interest, help us explain the relationship between Ep1, AA and electronic structure. The model development went forward with enlarging the set of flavonoids and, that way, we had to learn how to deal with the structural peculiarities of some of the 35 flavonoids from the final calibration set, for which the Ep1 measurements were all made in our laboratory. The developed models were simple quadratic models based either on atomic spin densities or differences in the atomic charges of the species involved in any of the three main oxidation mechanisms. The best model takes into account all three mechanisms of oxidation, single electron transfer-proton transfer (SET-PT), sequential proton loss electron transfer (SPLET) and hydrogen atom transfer (HAT), yielding excellent statistics (R2 = 0.970, S.E. = 0.043).

Photophysical Studies of a Zr(IV) Complex with Two Pyrrolide-Based Tetradentate Schiff Base Ligands.

The reaction of two equivalents of N,N'-bis(2-pyrrolylmethylidene)-1,2-phenylenediamine (H2bppda) with tetrabenzylzirconium provided the air- and moisture-stable eight-coordinate complex Zr(bppda)2. Temperature-dependent steady-state and time-resolved emission spectroscopy established weak photoluminescence (ΦPL = 0.4% at 293 K) by a combination of prompt fluorescence and thermally activated delayed fluorescence (TADF) upon visible light excitation at and around room temperature. TADF emission is strongly quenched by 3O2 and shows highly temperature-sensitive emission lifetimes of hundreds of microseconds. The lifetime of the lowest energy singlet excited state, S1, was established by transient absorption spectroscopy and shows rapid deactivation (τ = 142 ps) by prompt fluorescence and intersystem crossing to the triplet state, T1. Time-dependent density functional theory (TD-DFT) calculations predict moderate ligand-to-metal charge transfer (LMCT) contributions of 25-30% for the S1 and T1 states. A comparison of Zr(bppda)2 to related zirconium pyridine dipyrrolide complexes, Zr(PDP)2, revealed important electronic structure changes due to the eight-coordinate ligand environment in Zr(bppda)2, which were correlated to differences in the photophysical properties between the two compound classes.

Enhanced thermoelectric figure of merit in graphene nanoribbons by creating a distortion and transition-metal doping

Thermoelectric (TE) materials have the exceptional ability to directly convert waste heat into electricity and theoretical studies-based guidance is an effective way to investigate high-performance nanostructured TE materials throughout a range of temperatures. Herein, we studied the influence of distortion and transition metal (TM) atoms (V, Cr, Co and Pt) on the electronic and TE properties of armchair graphene nanoribbons (AGNRs) at various temperatures. The results showed that TM-doping and distortion might significantly alter the electronic structure of AGNRs. The band gap and valence band convergence are reduced as a result of the synergistic effect. A higher TE figure of merit results from a combination of decreased heat conductivity and increased electrical conductivity brought about by the electronic structural change. This study proposes that by doping and distortion-induced TE characteristics engineering of AGNR, high-performance nanostructured TE materials may be produced that have a wide range of TE applications.

Effects of Static Electric Fields on the Excitations of Silver Nanowire Dimers.

We theoretically studied the introduction of static electric fields to Ag10 nanowire dimer systems, including the effects of this field on optical absorption characteristics and the orbitals responsible for these excitations. Linear-response time-dependent density functional theory computations were performed on three distinct dimer systems: end-to-end, parallel, and 90° angle dimer systems separated by a closest interparticle distance of 7.0 Å. The calculations were performed in the presence of a 0.1 V/Å electric field strength applied in the z and y directions. The orientation of the dimer system and the direction of the applied static electric field each play a significant role in the resulting absorption spectra and the electronic structure of the nanowires. As a result, a dimer system can exhibit a blue shift for the longitudinal excitation and a red shift for the transverse excitation in the presence of one direction of the static electric field but not for the other direction. Notably, the electron density shifts from one nanowire to the other in the presence of the static electric field. Changes in optical characteristics and electronic structure suggest that the usage of a static electric field with a particular spatial configuration of nanowires provides a way to tune the optical properties of the system.

Solar-photocatalytic treatment of industrial wastewater using mechanically doped CNS-TiO2 and synergistic TiO2 incorporation: A promising cost-effective approach for industrial wastewater treatment

TiO2 alone demonstrated limited efficiency in degrading industrial wastewater. This study aims to the effectiveness of CNS-doped TiO2 nanoparticles for the degradation of anionic dyes, cationic dyes, and industrial wastewater. For the first time, mechanical milling is employed to synthesize CNS-doped TiO2 nanoparticles using thiourea as a precursor. This results in lattice strains and concomitant crystal and electronic structure changes. We note that these lead to increased photocatalytic activity. Mechanical milling creates a new phase known as brookite, which further improves photocatalytic efficiency through synergistic effects. The physicochemical properties and photocatalytic performance of CNS-TiO2 are evaluated and compared with mspd-TiO2 and commercially available Degussa P-25 TiO2. In photocatalytic degradation tests, remarkably, CNS-TiO2 exhibited beter degradation efficiency for anionic dyes (MO - 65 ± 3 % in 120 min; CR - 100 % in 30 min), cationic dyes (MB - 100 % in 30 min; Rh B 100 % in 120 min) and industrial wastewater (∼100 % in 120 min). The treated wastewater TDS (56 ± 2 ppm), TSS (0.16 g/L), CHNS, and COD (190 ± 10 mg/L) results are closer to drinking water quality. The reusability test revealed lower activity in the mspd-TiO2 and CNS-TiO2 attributed to particle aggregation, the loss of activation sites, and mass loss. This study demonstrates CNS-TiO2 enhanced photocatalytic properties and cost-effectiveness making them highly promising for treating any industrial effluents containing dyes.

High piezoelectric coefficients and rich phase transitions in ternary TlXY (X = S, Se; Y = Cl, Br, I) monolayers

In this work, we investigated the structural, electronic, optical, and piezoelectric properties of TlXY (X = S, Se; Y = Cl, Br, I) monolayers using the first-principles calculations. Five types of TlXY monolayers are structurally stable. The band gap, which ranges from 0.371-2.196 eV, is direct for most TlXY and indirect for TlSeBr. With adequate flexibility, the in-plane and out-of-plane piezoelectric coefficients, d11 and d31, of TlXY can approach 77.05-90.55 pm/V and 0.70-1.08 pm/V, respectively, which is larger than those of reported 2D materials. Strain can cause a direct-to-indirect band gap transition or even a topological phase transition. Furthermore, TlSeBr and TlSI can undergo structural phase transitions, resulting in dramatic changes in electronic structure and optical absorption. Our results suggest 2D TlXY as a series of promising materials for optoelectronic and piezoelectric applications.

Computational investigation of structural, electronic, and spectroscopic properties of Ni and Zn metalloporphyrins with varying anchoring groups.

Porphyrins are prime candidates for a host of molecular electronics applications. Understanding the electronic structure and the role of anchoring groups on porphyrins is a prerequisite for researchers to comprehend their role in molecular devices at the molecular junction interface. Here, we use the density functional theory approach to investigate the influence of anchoring groups on Ni and Zn diphenylporphyrin molecules. The changes in geometry, electronic structure, and electronic descriptors were evaluated. There are minimal changes observed in geometry when changing the metal from Ni to Zn and the anchoring group. However, we find that the distribution of electron density changes when changing the anchoring group in the highest occupied and lowest unoccupied molecular orbitals. This has a direct effect on electronic descriptors such as global hardness, softness, and electrophilicity. Additionally, the optical spectra of both Ni and Zn diphenylporphyrin molecules exhibit either blue or red shifts when changing the anchoring group. These results indicate the importance of the anchoring group on the electronic structure and optical properties of porphyrin molecules.