Changes In Electronic Structure Research Articles

Enhanced Understanding of Structure-Function Relationships for Oxomanganese(IV) Complexes.

A series of manganese(II) and oxomanganese(IV) complexes supported by neutral, pentadentate ligands with varied equatorial ligand-field strength (N3pyQ, N2py2I, and N4pyMe2) were synthesized and then characterized using structural and spectroscopic methods. On the basis of electronic absorption spectroscopy, the [MnIV(O)(N4pyMe2)]2+ complex has the weakest equatorial ligand field among a set of similar MnIV-oxo species. In contrast, [MnIV(O)(N2py2I)]2+ shows the strongest equatorial ligand-field strength for this same series. We examined the influence of these changes in electronic structure on the reactivity of the oxomanganese(IV) complexes using hydrocarbons and thioanisole as substrates. The [MnIV(O)(N3pyQ)]2+ complex, which contains one quinoline and three pyridine donors in the equatorial plane, ranks among the fastest MnIV-oxo complexes in C-H bond and thioanisole oxidation. While a weak equatorial ligand field has been associated with high reactivity, the [MnIV(O)(N4pyMe2)]2+ complex is only a modest oxidant. Buried volume plots suggest that steric factors dampen the reactivity of this complex. Trends in reactivity were examined using density functional theory (DFT)-computed bond dissociation free energies (BDFEs) of the MnIIIO-H and MnIV ═ O bonds. We observe an excellent correlation between MnIV═O BDFEs and rates of thioanisole oxidation, but more scatter is observed between hydrocarbon oxidation rates and the MnIIIO-H BDFEs.

N‐doped Mesoporous Carbon Loaded Co Nanoparticles as Highly Effective Non‐noble Metal Electrocatalysts Towards Oxygen Reduction Reaction in Alkaline Solution

AbstractDeveloping non‐noble metal electrocatalysts with low cost and advanced catalytic performance for oxygen reduction reaction (ORR) is of great significance for the large‐scale commercial application of fuel cells. In this work, an N‐doped mesoporous carbon supported Co nanoparticle (Co/NC) has been innovatively designed by a facile pyrolysis method with a nitrogen‐containing organic ligand as nitrogen and carbon sources by forming a complex with Co2+ to prevent the aggregation of Co nanoparticles during the carbonization process. The Co/NC exhibits excellent electrocatalytic ORR activity, including a positive redox peak potential (0.78 V), onset potential (0.90 V), and half‐wave potential (0.78 V) with electron transfer number approximately of 4.02. Co/NC performs long‐term stability and has a stronger tolerance to methanol than 20% Pt/C. Furthermore, the calculated Bader charges and the charge density differences clearly demonstrate the strong metal‐support interaction (SMSI) for Co/NC catalyst, which induces the change of electron structure on the Co/NC surface and is conducive to enhancing its catalytic performance. The outstanding ORR activity of Co/NC is ascribed to its unique property of prompt electron transfer, large specific surface area, and synergistic effect between Co nanoparticles and NC substrates.

Ultrafast dynamics in photo-excited Mott insulator Sr3Ir2O7 at high pressure

High-pressure ultrafast dynamics, as a new crossed research direction, are sensitive to subtle non-equilibrium state changes that might be unresolved by equilibrium states measurements, providing crucial information for studying delicate phase transitions caused by complex interactions in Mott insulators. With time-resolved transient reflectivity measurements, we identified the new phases in the spin–orbit Mott insulator Sr3Ir2O7 at 300 K that was previously unidentified using conventional approaches such as x-ray diffraction. Significant pressure-dependent variation of the amplitude and lifetime obtained by fitting the reflectivity ΔR/R reveal the changes of electronic structure caused by lattice distortions, and reflect the critical phenomena of phase transitions. Our findings demonstrate the importance of ultrafast nonequilibrium dynamics under extreme conditions for understanding the phase transition of Mott insulators.

Deciphering structural changes in LiNixMnyCozO2 cathodes of Li ion batteries during cycling: Experimental and theoretical investigations

Layered oxide material LiNixMnyCozO2 is emerging out as promising cathode for new generation of Li-ion batteries, especially as power sources for electric vehicles and plug-in hybrid vehicles. In this work, structural and electronic changes occurring in LiNixMnyCozO2 (LNMC) cathode materials during charge/discharge cycle of the battery is studied over a wide composition range using in-situ X-ray diffraction (XRD) and in-situ X-ray absorption spectroscopy (XAS) techniques. While in-situ XRD measurements decipher the overall structural changes during the charging/discharging cycles, in-situ element specific XAS measurements reveal site specific information on electronic and local structural changes. It has been found that for the LNMC samples the major charge compensation at the metal site during charging or Li ion de-intercalation is achieved by oxidation of Ni2+ ions without significant change in the oxidation state of Co or Mn ions. It is also found that local structure around Ni undergoes reversible change during charging and discharging cycles whereas local structure around Co or Mn sites show no significant change. Moreover, it has been found that local structural changes at Ni sites in the samples with higher Ni concentrations are much less compared to that in the samples with lower Ni concentrations during charging/discharging cycles, the result being corroborated by ab-initio theoretical simulations also. This is a new finding and would have significant implications in determining the optimum composition of LNMC electrodes of Li ion batteries for commercial applications.

Interfacial Topochemical Fluoridation of MAPbI3 by Fluoropolymers

Herein it is demonstrated that, under conditions relevant to perovskite synthesis (>140 °C in air), fluoride can topochemically react across the interface between a halide perovskite and a fluoropolymer when in close contact, thereby creating a small quantity of strongly bonded lead fluoride species. The quantity increases with temperature and processing duration. Photoinduced charge carrier lifetime provides a metric for the resulting changes in electronic structure of the perovskite. Under short-duration and/or moderate temperature processing, fluoride transfer to the perovskite yields increased carrier lifetimes, up to 3-fold longer than control samples, which is attributed to passivation of surface defects. Under more forcing conditions, the trend reverses: excessive fluoridation leads to shortened carrier lifetimes, which is ascribed to substantial interfacial formation of PbF2. It is demonstrated that an interface with bulk crystalline PbF2 quenches perovskite photoluminescence, likely due to PbF2 serving as an electron acceptor for the conduction band of MAPbI3.

Ca/Co-based composites with improved cyclic stability and optical absorption for advanced thermochemical energy storage systems

Thermochemical energy storage (TCES) is a promising technology to overcome solar intermittency and volatility. However, weak solar absorption, poor cyclic stability for calcium carbonates, and cost issues for metal oxides hinder the applicability of these materials for thermochemical energy storage. Herein, an advanced, affordable, and effective TCES system is proposed, and the corresponding calcium carbonate-metal oxide synergistic material is designed with improved solar absorption and cyclic stability. The optimum molar ratio is Ca:Co:Mn:Mg = 100:20:7.5:5 for the synergistic material fabricated by the modified sol–gel method, where Ca element plays the primary role in heat storage, Co element serves as the primary optical performance enhancer and secondary heat storage component, and Mn/Mg elements act as the secondary performance modifiers. The solar absorptivity is boosted to ∼79.3%, and the heat storage density is 1295 kJ/kg after 30 high-temperature cycles, which are 6 times and 2.2 times higher than those of commercial calcium carbonate, respectively. Homogeneous mixing of modifiers at the microscale and stable, well-developed pore structure are critical factors for cyclic stability enhancement. The change of electronic structure on the Fermi level caused by Co/Mn ion doping is the key to optical absorption improvement, and the promoting amplitude exhibits wavelength dependence. This work expands the limited list of currently available TCES materials and the comprehensive description of optical properties offers guidance for future implementation of TCES materials with high solar absorption in specific wavelengths.

Surface-Mediated Spin Locking and Thermal Unlocking in a 2D Molecular Array.

Molecule-based functional devices may take advantage of surface-mediated spin state bistability. Whereas different spin states in conventional spin crossover complexes are only accessible at temperatures well below room temperature, and the lifetimes of the high-spin state are relatively short, a different behavior exhibited by prototypical nickel phthalocyanine is shown here. Direct interaction of the organometallic complex with a copper metal electrode mediates the coexistence of a high spin and a low spin state within the 2D molecular array. The spin state bistability is extremely non-volatile, since no external stimuli are required to preserve it. It originates from the surface-induced axial displacement of the functional nickel cores, which generates two stable local minima. Spin state unlocking and the full conversion to the low spin state are only possible by a high temperature stimulus. This spin state transition is accompanied by distinct changes in the molecular electronic structure that might facilitate the state readout at room temperature, as evidenced by valence spectroscopy. The non-volatility of the high spin state up to elevated temperatures and the controllable spin bistability render the system extremely intriguing for applications in molecule-based information storage devices.

An X-ray spectroscopy study of structural stability of superhydrogenated pyrene derivatives

ABSTRACT The stability of polycyclic aromatic hydrocarbons (PAHs) upon soft X-ray absorption is of crucial relevance for PAH survival in X-ray dominated regions. PAH stability depends on molecular size but also on the degree of hydrogenation that is related to H2 formation in the interstellar medium. In this project, we intend to reveal the changes of electronic structure caused by hydrogenation and the impact of hydrogenation on the stability of the carbon backbone for cationic pyrene and its hydrogenated derivatives by analysis of near C K-edge soft X-ray photoions. In our experiments, the PAH cations were trapped in a cryogenic radiofrequency linear ion trap and exposed to monochromatic X-rays with energies from 279 to 300 eV. The photoproducts were mass-analysed by means of time-of-flight spectroscopy. Partial ion yields were then studied as a function of photon energy. X-ray absorption spectra computed by time-dependent density functional theory (TD-DFT) aided the interpretation of the experimental results. A very good agreement between experimental data and TD-DFT with short-range corrected functionals for all PAH ions was reached. The near-edge X-ray absorption mass spectra exhibit clear peaks due to C 1s transitions to singly occupied molecular orbitals and to low-lying unoccupied molecular orbitals. In contrast to coronene cations, where hydrogen attachment drastically increases photostability of coronene, the influence of hydrogenation on photostability is substantially weaker for pyrene cations. Here, hydrogen attachment even destabilizes the molecular structure. An astrophysical model describes the half-life of PAH ions in interstellar environments.

A new antimicrobial PVC-based polymeric material incorporating bisacylthiourea complexes

A new antimicrobial material incorporating Cu(I) and Cd(II) complexes of bisacylthiourea derivatives in a PVC film was successfully synthesized and characterized by IR, UV, NMR, SEM, and thermal analyses. The results revealed that on coordination, the electronic structure change of the ligand affects practically all their spectral vibrational pattern; however, within the complex pattern, some vibrations indicated that the thiourea derivative behaves as a neutral ligand, which coordinates the metal ion through the sulfur atom of the thiocarbonyl group. The greater affinity of the S atom for Cu+ 1 played a role in Cu(II)→Cu(I) reduction, and the intramolecular hydrogen bonds of the type of (NH···Cl) further stabilized the obtained Cu(I) complex in dioxane. The antimicrobial activity shows that all investigated compounds exhibit excellent activity compared to standard antibiotics. The antibacterial power of the PVC/Cd composite is significantly superior against the most resistant species to both disinfectants and antibiotics compared to its PVC/Cu analogue; nevertheless, the latter exhibited activity equal to an average halo diameter of 29 ± 0.33 mm against pathogenic E. coli ATCC 25,922, indicating excellent G (-) activity. Interestingly, the PVC/Cd composite exhibited excellent activity against pathogenic C. albicans RCMB 005003 (1) ATCC 10,231, while its PVC/Cu analogue was inactive. These materials may be used to reduce infection in wounds either as a composite film or coated barrier dressings, and in addition, the results should open a new direction in antimicrobial surface engineering within the biomedical field. Further challenges are the development of reusable and broad-range antimicrobial polymers..

The Synergistic Promotion Effect of In‐situ Formed Metal Cationic Vacancies and Interstitial Metals on Photocatalytic Performance of WO3 in CO2 Reduction

AbstractMicrostructure modulation of photocatalysts is an effective way to improve photocatalytic performances. The synergistic effect of metal cationic vacancies and interstitial metals on photocatalysts is rarely studied. In‐situ formations of metal cationic vacancies and interstitial metals are achieved in WO3 via acid treatments, resulting from the migration of metal impurities in WO3. Characterizations of structures, components, photo‐responsive and photoelectric properties identify the differences of WO3 photocatalysts via different treatments. The presence of both metal cationic vacancies and interstitial metal impurities exhibits the positive synergistic effect in photocatalytic reaction. Photocatalytic activities in CO2 reduction are enhanced with the high production rates of CO and CH4. Experimental characterizations of WO3 photocatalysts indicate the increased photoinduced electrons and photocurrent intensities, and the decreased electrochemical impedances. Theoretical simulations confirm the changes of electronic structures of WO3. The calculated band gaps of WO3 after acid treatment decrease and the defect energy levels form, which favor the separation and the transfer of photoinduced carriers for the promotion of photocatalytic activities. To utilize the dissolution difference of metals is feasible to fabricate metal cationic vacancies and interstitial metals simultaneously, an option for other metal‐containing photocatalysts with the in‐situ introduced impurities.

Electronic structure alteration in MgO monolayer through alkali metal doping

We have studied the structural and electronic properties of MgO monolayer (ML) by substituting one of its Mg sites with alkali metal atoms (Li, Na, K) using DFT calculations in Quantum ESPRESSO. All the planar-doped monolayers are structurally stable shown by the computed binding energy per atom values. Formation energies suggest that Li exhibits the strongest binding with MgO ML. Doping turns the non-magnetic semiconducting MgO MLs into half-metallic magnets with magnetic moment of ∼ 1μB. The corresponding magnetic states are contributed by the valence orbitals of O atoms near the doping sites. Thus, Li, Na, K substitution introduces huge changes in electronic structure and properties of MgO ML.

Imaging the complex interactions in CuAg-PEO nanoparticles ensemble for enhanced CO2 reduction

CO2 electrochemical reduction (CO2ER) is a promising technology that can convert CO2 into useful chemicals and fuels with simultaneously reducing greenhouse gas emissions. Cu nanoparticles-based catalysts are an optimal choice of CO2ER due to its excellent conductivity and catalytic activity. In this study, we combined scanning transmission X-ray microscopy (STXM) and spectro-ptychography to investigate the chemical and electronic structural changes of the CuAg nanocatalysts before and after CO2ER. We observed that Cu species with the same valence state self-aggregate into separated phases. For the spent CuAg nanocatalysts after CO2ER, most Cu(I) and Cu(0) species are oxidized to Cu(II) together with CuO transforming into undetermined Cu(II) species. We believe these mechanisms are the key reason why the CuAg nanocatalysts lost catalytic activity. Such information will be highly valuable for understanding Cu nanoparticles catalyzed CO2ER.

Correlation between Local Structure and Reaction Mechanism in Disordered Rock-Salt-Type Lithium–Manganese Oxides

First-principles (FP) calculations of disordered rock-salt-type lithium–manganese oxides were performed by assuming a quasi-random structure model with the composition of Li16Mn16O32. The effects of randomness in the crystal structure and the changes in local atomic configurations during the Li extraction process on electronic structures, phase stability, and electrochemical properties were investigated by the FP calculations. The calculations indicate that the randomness of local atomic configurations, which originates from the disordered structure, makes it possible for Mn and O sites to take on a variety of electronic states. Analyses of densities of states, magnetic moments of Mn atoms, and Bader charges indicate the possibility that not only Mn but also oxygen atoms contribute to the redox reaction. The migration of Li and Mn is found to appear from octahedral to tetrahedral sites at a high state of charge. The FP calculations suggest that the change in local atomic configurations and electronic structures causes the transition in voltage profiles from sloping to 3 V plateau shape as the cycle progresses, which is characteristic of spinel-related positive electrode materials.

Efficient electrochemical CO2 reduction via CuAg doped CeO2

CO2 electrocatalysis reduction (CO2RR) is considered to be one of the most efficient methods to achieve carbon neutralization and realize renewable energy conversion. Unfortunately, at present, CO2 electrocatalysts confront the challenges of high overpotential, low selectivity, and poor stability. Ag based electrocatalysts are considered to be the most promising catalysts for CO production via CO2RR on a large scale, due to their relatively higher catalytic performance and more abundant reserves compared with other noble metals. Whereas, they still suffer from high cost. In this paper, we design bimetallic CuAg/CeO2-6 catalysts by the simple impregnation method for CO2RR, exhibiting the highest FECO of 84% at –1.1 V vs. RHE compared with other bimetallic catalysts with different Cu/Ag ratio and monometal catalysts. XPS results demonstrate the electron delocalization effect not only between Cu and Ag bimetals but also among CeO2 support. This effect results in the electronic structure change and the formation of the highest concentration of oxygen vacancies, promoting the CO2 adsorption and activation as well as facilitating the formation of COOH* intermediates as evidenced by the in situ ATR-SEIRAS measurements. Additionally, the highest active surface for CO2RR and the smallest charge resistance of CuAg/CeO2-6 also contributes to its good CO2RR performance. This work sheds light on the design of other efficient CO2RR catalysts through electronic structure tuning by bimetallic strategy.

Enhancing power factor and ZT in non-toxic Bi2S3 bulk materials via band engineering and electronic structure modulation

Bismuth sulfide-based thermoelectric materials have attracted considerable interest owing to their abundance of raw materials and non-toxicity. However, their poor electrical properties, particularly their conductivity, have limited their practical applications. In this work, a melting method and spark plasma sintering were employed to synthesize bulk Bi2S3 materials. The dopant MoCl5 was used to regulate the electrical properties of Bi2S3. The results showed that Mo can significantly enhance the conductivity of Bi2S3 compared to other transition metals. The doped sample exhibited an almost two-orders-of-magnitude rise in conductivity relative to the Bi2S3 sample in its pure form. Due to its superior conductivity, the Bi2S3+1 wt% MoCl5 sample achieved an excellent power factor of 550 μW m−1K−1 which is the maximum ever reported in the Bi2S3 system. Simultaneously, the ZT value of this sample approached 0.70 at 773 K, which is a six-fold improvement compared to the pristine sample. Density functional theory was applied to understand the electronic structure changes of the Mo-doped sample, which revealed that trace amounts of MoCl5 doping could significantly boost the density of states in the vicinity of the conduction band and shift the Fermi level to the valence band, leading to a reduction in the bandgap and an increase in free electrons. This work offers new insights into the role of Mo elements in thermoelectric materials and provides a good strategy for augmenting the thermoelectric figures of merit of other n-type thermoelectric materials with inadequate electrical conductivity in the future.

Defect engineering of PdMo metallene for sensitive electrochemical detection of dopamine

Reasonable design and construction of high-performance catalysts are desirable for improving the sensitivity of electrochemical sensors, but still remain a great challenge. Herein, ultrathin PdMo metallene with defects (D-PdMo) is fabricated and employed as an electrocatalyst to construct electrochemical sensors for the sensitive determination of small biomolecules. Taking advantage of the change of electronic structure, conductivity and electrochemical active surface areas caused by the defect engineering, the resultant D-PdMo metallene exhibits a much better electrocatalytic ability to dopamine (DA) than PdMo metallene. Furthermore, the proposed electrochemical sensor realizes sensitive DA detection in human blood serum and cells. Our work not only verifies the feasibility of introducing defects into metallene to enhance catalytic performance but also broadens the application of metallene in the electrochemical sensors for monitoring biomolecules in living organisms.

Geometrical and Electronic Structure Analysis of Mn‐Doped CaMO3 (M = Ti, Zr, and Sn)

The geometrical and electronic structures of Mn‐doped CaTiO3, CaZrO3, and CaSnO3 deep‐red emission phosphors are investigated both experimentally and theoretically. All the samples are synthesized with solid‐state reaction method by changing the concentration of the doped Mn ions. Crystal structures of the fabricated samples are examined with the powder X‐ray diffraction technique and valence states of the doped Mn ions are determined by measuring the Mn‐L3 X‐ray absorption near‐edge structure (XANES) spectra. Diffuse reflectance spectra are observed to study the change in electronic structures due to the Mn doping. First‐principles calculations are also carried out to investigate geometrical and electronic structures of Mn‐doped CaMO3, in which different types of electron–electron correlation functionals are employed.

Observation of Electronic Structure Modification in the Hidden Order Phase of CeCoSi

CeCoSi with no local inversion symmetric crystal structure (P4/nmm) exhibits a phase transition of unknown origin (Hidden Order: HO) at about 12 K (T0) above the antiferromagnetic transition temperature (TN = 9.4 K). The electronic structure change across T0 was investigated with high-precision optical reflection spectroscopy. The optical spectrum changed from a typical metallic behavior above T0 to a gap-like structure at around 15 meV below T0. The gap-like structure was unchanged across TN except for the narrowing of the Drude component of carriers due to the suppression of magnetic fluctuations. This result suggests a slight change from the typical metallic electronic structure above T0 to that with an energy gap near the Fermi level in the HO phase. The change in electronic structure in the HO phase was concluded to be due to electron/valence instability.

Direct spectroscopic observation of the reversible redox mechanism in A3V2(PO4)3(A=Li,Na) cathode materials for Li-ion batteries

Alkali Vanadium (III) phosphates are a promising class of positive electrode materials for Li-, Na- and K-ion battery applications. Herein, the reversible redox mechanism of Li3V2(PO4)3 and Na3V2(PO4)3 cathodes during charge/discharge vs. Li+/Li0 is directly observed by Soft X-ray Absorption Spectroscopy. For both Li3V2(PO4)3 and Na3V2(PO4)3 spontaneous surface Li + insertion is observed after exposure to Li + electrolyte. Furthermore, for Li3V2(PO4)3 a reversible charge-discharge process is observed from 3.0 V–4.8 V with only slight increase in O contributions to O 2p - V 3d unoccupied hybridised states after charge/discharge vs. Li+/Li0. For Na3V2(PO4)3, persistent VO6 tetrahedral distortion and an accompanying V5+ state is observed, particularly at the surface, after discharge vs. Li+/Li. These observations further elucidate the particular atomic and electronic structure changes and charge-discharge mechanisms of high voltage NASICON and anti-NASICON cathode materials for Li-ion battery applications.

Dielectric Saturation in Water from a Long-Range Machine Learning Model.

Machine learning-based neural network potentials have the ability to provide ab initio-level predictions while reaching large length and time scales often limited to empirical force fields. Traditionally, neural network potentials rely on a local description of atomic environments to achieve this scalability. These local descriptions result in short-range models that neglect long-range interactions necessary for processes like dielectric screening in polar liquids. Several approaches to including long-range electrostatic interactions within neural network models have appeared recently, and here we investigate the transferability of one such model, the self-consistent field neural network (SCFNN), which focuses on learning the physics associated with long-range response. By learning the essential physics, one can expect that such a neural network model should exhibit at least partial transferability. We illustrate this transferability by modeling dielectric saturation in a SCFNN model of water. We show that the SCFNN model can predict nonlinear response at high electric fields, including saturation of the dielectric constant, without training the model on these high field strengths and the resulting liquid configurations. We then use these simulations to examine the nuclear and electronic structure changes underlying dielectric saturation. Our results suggest that neural network models can exhibit transferability beyond the linear response regime and make genuine predictions when the relevant physics is properly learned.