Changes In Electronic Structure Research Articles

On the Acid Stability of PGM-Free OER Catalyst for H2 Production in PEMWE

Low temperature proton exchange membrane water electrolyzer (PEMWE) represents a critical technology for green hydrogen production. It offers the advantages of significantly higher current density and higher H2 purity, rendering it a preferred technology when high energy efficiency and low footprint are essential.Working in the oxidative and acidic environment under high polarization voltage, however, adds substantial demand to the electrode catalyst and the support. This is particularly the case at anode where the oxygen evolution reaction (OER) takes place. At present, the platinum group metal (PGM) materials such as Ir black or Ir oxide are catalysts of choice. Their high cost and limited reserve, however, adds a significant cost to PEM electrolyzer, which contributes to the overall expense of hydrogen production next only to the cost of electricity. Replacing Ir with earth-abundant transition metal oxides could help to reduce the electrolyzer system cost.Argonne National Laboratory has recently designed and prepared a new family of PGM-free OER catalyst for PEM electrolyzer. The new catalysts have high porosity and unique nanoarchitecture, and have demonstrated very promising activity and durability in both rotating disk electrode (RDE) and in operating PEMWE.The most challenging aspect of PGM-free OER catalyst is its stability in the acidic media. To this end, we conducted extensive structural characterizations as well as computational modeling. Particularly, in situ X-ray absorption spectroscopy combined with high resolution imaging revealed insightful information on the electronic and surface structural changes during the electrochemical reaction. Computational Pourbaix diagram also elucidated the catalyst stability under the acidic OER operating condition. Acknowledgement: This work is supported by U. S. Department of Energy, Hydrogen and Fuel Cell Technologies Office through Office of Energy Efficiency and Renewable Energy and by Office of Science, U.S. Department of Energy under Contract DE-AC02-06CH11357.

Copper-doped layered Fe2VO4 nanorods for aqueous zinc-ion batteries

Zinc-ion batteries (ZIBs) have attracted an increasing attention as a potential low-cost, environmentally friendliness, and high-safety energy storage system. Among them, transition metal vanadates with high oxidation state vanadium have great potential in ZIBs cathode research due to their high theoretical capacity. However, many vanadate particles still inevitably suffer from low ion mobility, low electrical conductivity and stability. Cation doping or compositing is an effective pathway capable of enhancing electrical conductivity. In this work, layered Cu-Fe2VO4 porous nanorods are obtained by introducing Cu2+ into MIL-88A(Fe) (a metal–organic framework; MIL stands for materials from Institute Lavoisier) and further ion-exchanged with NH4VO3, exhibiting excellent zinc storage properties as an cathode. The existence of oxygen vacancies and the change of electronic structure caused by Cu2+ substituting part of Fe2+ enhanced the conductivity and electron transfer rate. It delivers a reversible discharge capacity of 237 mAh/g at 0.3 A/g and a satisfactory high rate capacity of 126 mAh/g after 30 cycles at 5 A/g, and stable cycling performance (198 mAh/g after 1000 cycles at 1 A/g). Furthermore, the energy density can reached to 230.97 Wh kg−1 at 208.6 W kg−1. The assembled quasi-solid-state ZIBs maintain a high capacitance retention of 75% after 8000 cycles at 1 A/g.

Imaging the dynamic influence of functional groups on metal-organic frameworks

Metal-organic frameworks (MOFs) with different functional groups have wide applications, while the understanding of functionalization influences remains insufficient. Previous researches focused on the static changes in electronic structure or chemical environment, while it is unclear in the aspect of dynamic influence, especially in the direct imaging of dynamic changes after functionalization. Here we use integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM) to directly ‘see’ the rotation properties of benzene rings in the linkers of UiO-66, and observe the high correlation between local rigidity and the functional groups on the organic linkers. The rigidity is then correlated to the macroscopic properties of CO2 uptake, indicating that functionalization can change the capability through not only static electronic effects, but also dynamic rotation properties. To the best of our knowledge this is the first example of a technique to directly image the rotation properties of linkers in MOFs, which provides an approach to study the local flexibility and paves the way for potential applications in capturing, separation and molecular machine.

Relaxation effects in twisted bilayer molybdenum disulfide: structure, stability, and electronic properties

Manipulating the interlayer twist angle is a powerful tool to tailor the properties of layered two-dimensional crystals. The twist angle has a determinant impact on these systems’ atomistic structure and electronic properties. This includes the corrugation of individual layers, formation of stacking domains and other structural elements, and electronic structure changes due to the atomic reconstruction and superlattice effects. However, how these properties change with the twist angle, θ, is not yet well understood. Here, we monitor the change of twisted bilayer (tBL) MoS2 characteristics as a function of θ. We identify distinct structural regimes, each with particular structural and electronic properties. We employ a hierarchical approach ranging from a reactive force field through the density-functional-based tight-binding approach and density-functional theory. To obtain a comprehensive overview, we analyzed a large number of tBLs with twist angles in the range of . Some systems include up to half a million atoms, making structure optimization and electronic property calculation challenging. For , the structure is well-described by a moiré regime composed of two rigidly twisted monolayers. At small twist angles ( and ), a domain-soliton regime evolves, where the structure contains large triangular stacking domains, separated by a network of strain solitons and short-ranged high-energy nodes. The corrugation of the layers and the emerging superlattice of solitons and stacking domains affects the electronic structure. Emerging predominant characteristic features are Dirac cones at K and kagome bands. These features flatten for θ approaching 0∘ and 60∘. Our results show at which range of θ the characteristic features of the reconstruction, namely extended stacking domains, the soliton network, and superlattice, emerge and give rise to exciting electronics. We expect our findings also to be relevant for other tBL systems.

Cs2 SnCl6 : To Emit or to Catalyze? Te4+ Ion Calls the Shots.

A low concentration of Te4+ doping is found to be capable of endowing the lead-free Cs2 SnCl6 perovskites with excellent photoluminescence quantum yield (PLQY), while further increasing Te4+ concentration leads to PLQY deterioration. The mechanism behind the improved PLQY is intensively studied and reported elsewhere. However, little work is conducted to understand the decreased PLQY at high doping levels and to explore its implications for non-PL-related applications. Here, it is demonstrated that the Te4+ -incorporated Cs2 SnCl6 can be promising candidate for efficient CO2 photocatalysis. An optimum photocatalytic performance is achieved when Te4+ concentration reaches as high as 50%, at which point significant PL quenching has occurred. Through a detailed spectral characterization, such concentration-dependent functionality is attributed to systematic changes in both electronic and local crystal structure, which allow a robust regulation of excitation energy relaxation channels. These findings expand the scope of available photocatalysts for CO2 reduction and also inform synthetic planning for the preparation of multifunctional Pb-free metal halide perovskites.

Modified flake Al powder by ZrO2 coating as potential low infrared radiation material for high temperature over 500 °C

Electronic structure adjustment on materials' surfaces often leads to materials obtaining novel performances. Flake Al powder is a classical low infrared emissivity material at room temperature. However, this powder shows intense infrared radiation over 500 °C due to its high-temperature oxidation, which restricts flake Al powder's application as low infrared radiation material in higher temperatures. For this reason, the Al@ZrO2 composite was prepared by a solvothermal method to improve its thermostability. This composite can be used as low infrared radiation material over 500 °C for the long term, even at 700 °C for a short time. According to the experimental and theoretical investigation, a delocalization bond of Al–O–Zr is formed in the interface between the Al surface and ZrO2 so that the Al surface's electronic structure is adjusted. This change of electronic structure weakens the electroaffinity of Al atoms to bond with the electron-rich groups, and the oxidation reaction of Al needs to proceed in a higher-temperature range. In this case, flake Al powder coating with ZrO2 holds its metal properties with low infrared emissivity over 500 °C. The Al@ZrO2 composite could be used in a higher temperature range as a low infrared radiation material.

The Morphology-Controllable Synthesis of Ni–Co–O Nanosheets on a 3D Porous Ni Template as a Binder-Free Electrode for a Solid-State Symmetric Supercapacitor

In this work, a porous Ni template (Ni–Co–O@3D Ni) with Ni–Co oxide nanosheets (Ni–Co–O)@3D was synthesized by incorporating Ni–Co oxide nanosheets within a 3D porous Ni template as a binder-free electrode for a supercapacitor. The 3D Ni template was synthesized with hydrogen bubble templates that possessed different applied voltages that marked differences in terms of physicochemical properties, as well as factors that affect the subsequent growth of Ni–Co–O nanosheets. Then, Ni and Co metal ion sources were introduced to produce the morphology adjustment of Ni–Co–O@3D Ni with a multiple hierarchical architecture with a hydrothermal process. Field emission scanning electron microscopy (FESEM), X-ray absorption spectroscopy (XAS), and an electrochemical analysis were employed to investigate the morphological, structural, and electrochemical characteristics. FESEM and XAS results evidenced that Ni–Co–O@3D Ni consists of a 3D, well-designed hierarchical interconnected network, and the local electronic structure change has a great influence on the capacitive performance. The electrochemical results of Ni–Co–O@3D Ni displayed an excellent electrochemical performance due to the synergistic effect of Ni and Co on Ni–Co–O@3D Ni, which possessed multiple oxidation states to enable various reversible Faradaic redox reactions. Remarkably, the solid-state symmetric supercapacitor fabricated with Ni–Co–O@3D Ni exhibited excellent capacitive behaviour at a wide operating voltage window and cycling performance. Also, the as-assembled solid-state symmetric supercapacitor (two devices in series) can successfully illuminate a desired parallel pattern consisting of 36 red LED lights, demonstrating its practical application as a supercapacitor.

Interface electronic coupling in FeOOH-Co9S8 heterostructure for efficient oxygen evolution reaction

The oxygen evolution reaction (OER) is a four-electron–proton coupling process with a high kinetic energy barrier, which affects the overall efficiency of water splitting. Rational construction of specific interfaces and optimization of electron interactions can effectively promote the kinetic behavior of OER. Herein, ultrathin FeOOH nanosheets were loaded on Co9S8 nanorods by hydrothermal method to construct heterogeneous structures (FeOOH-Co9S8/NF), and the modulation of catalytic activity by structural changes in the phase interface region was investigated. FeOOH-Co9S8/NF showed excellent performance as an OER catalyst, requiring an overpotential of only 215 mV to reach 100 mA cm−2. The rich heterogeneous interface can optimize the coordination environment of the Co site by redistributing the interfacial charge to obtain the appropriate chemisorption strength of oxygen-containing intermediates. This paper explores the relationship between interfacial electronic structure changes and catalyst performance enhancement, and provides an experimental basis for further practicalization of OER catalysts.

Effect of Coulomb impurities on the electronic structure of magic angle twisted bilayer graphene

In graphene, charged defects break the electron-hole symmetry and can even give rise to exotic collapse states when the defect charge exceeds a critical value which is proportional to the Fermi velocity. In this work, we investigate the electronic properties of twisted bilayer graphene (tBLG) with charged defects using tight-binding calculations. Like monolayer graphene, tBLG exhibits linear bands near the Fermi level but with a dramatically reduced Fermi velocity near the magic angle (approximately 1.1°). This suggests that the critical value of the defect charge in magic-angle tBLG should also be very small. We find that charged defects give rise to significant changes in the low-energy electronic structure of tBLG. Depending on the defect position in the moiré unit cell, it is possible to open a band gap or to induce an additional flattening of the low-energy valence and conduction bands. Our calculations suggest that the collapse states of the two monolayers hybridize in the twisted bilayer. However, their in-plane localization remains largely unaffected by the presence of the additional twisted layer because of the different length scales of the moiré lattice and the monolayer collapse state wavefunctions. These predictions can be tested in scanning tunneling spectroscopy experiments.

Oscillation of Electronic-Band-Gap Size Induced by Crystalline Symmetry Change in Ultrathin PbTe Films.

For the semiconductors of atomic length scales, even one atom layer difference could modify crystal symmetry and lead to a significant change in electronic structure, which is essential for modern electronics. However, the experimental exploration of such effect has not been achieved due to challenges in sample fabrication and characterization with atomic-scale precision. Here, we report the discovery of crystal symmetry alternation induced band-gap oscillation in atomically thin PbTe films by scanning tunneling microscopy. As the thickness of PbTe films is reduced from an 18- to 2-atomic layer, the band-gap size not only expands from 0.19eV to 1.06eV by 5.6 fold, but also exhibits an even-odd-layer oscillation, which is attributed to the alternating crystal symmetries between P4/mmm and P4/nmm. Our work sheds new light on electronic structure engineering of semiconductors at atomic scale for next-generation nanoelectronics.

Bidirectional electron transfer in Triboelectrification caused by friction-induced change in surface electronic structure

Triboelectrification (TE) plays fundamental roles in a wide range of industrial and scientific fields but its charge transfer mechanisms still remains elusive. It has been widely believed that the electron transfer in TE between two homogeneous surfaces should be always unidirectional. But we found here that the electron transfer during friction process will change its direction as the position of the contact point on the friction surface changes, indicating that bidirectional electron transfer occurs between the contacting surfaces. The experiments further show that the electron transfer direction at any given contact position on the friction surface keeps unchanged over time during the whole friction process if the relative sliding direction keeps unchanged, and when the sliding direction is reversed, the electron transfer direction at a given contact position will also be reversed if the friction surface is crystalline. To explain the observed non-unidirectional electron transfer, a theoretical model for the electron transfer in TE has been proposed, assuming that the changes in the electron transfer direction can be attributed to the changes in the surface work function caused by the friction-induced tangential deformation on the friction surface. The theory has further been validated by the first principles calculations about the influence of the tangential deformation on the surface electronic structure. The present work may offer a new perspective for understanding the charge transfer mechanism in TE and may provide potential applications in numerous fields involving TE such as triboelectric generator, petrochemical industry, power electronics and textile industry.

First-principles investigation of divalent ion sensing with cesium lead trihalides

The first-principles calculations of the cesium lead trihalides, CsPbX3 (X = Cl, Br and I), doped with divalent ions such as Cd2+, Cu2+, Mg2+, Ni2+, Sn2+ and Zn2+ are carried out within the density functional theory to explore ion-sensing ability of CsPbX3 for divalent ions. Here the formation energy of the divalent ions doped CsPbX3 are calculated to evaluate the ability of absorption of the divalent ions. To investigate change in photoluminescence intensity of CsPbX3 due to the divalent ions incorporation, change in electronic structures of CsPbX3 due to the divalent ions dopings are also discussed by comparing the calculated electronic density of states.

Reshaping the coordination and electronic structure of single atom sites on the right branch of ORR volcano plot

The coordination environment of atomic metal sites in single atom catalysts is of vital significance in tailoring the d-orbital states and thus the catalytic behavior towards oxygen reduction reaction (ORR), thereby offering great promise to boost activity via regulating the local chelation structure. Herein we designed a carbon coordination environment for the atomic M sites reside on the right leg of the volcano plot, to effectively counter balance the low adsorption energy of the metal center to oxygen species. Combining time-of-flight secondary ion mass spectrometry and density functional theory calculations, we unveiled the M-C coordination structure and the thus induced changes in electronic structure and ORR catalytic behavior. The reshape in coordination structure, i.e., the replacement of typical nitrogen coordination by low electronegativity carbon coordination, gives rise to exceptional intrinsic activity towards ORR. This work brings a new perspective to boost the activity of single atom catalysts on the weak bonding leg, with exceptional ORR activity being further expected through advancing the chelation structure.

Ru supported on MnBCD derived from Mn MOF for thermo- and electro-catalytic synthesis of ethylene-d4 glycol.

Deuterium-labeled polyols were one of the most extensive applications in biochemistry and biophysics. However, the deuteriation reaction still exhibit low deuteriation rate and indistinct reaction mechanism. In this paper, Ru supported on MnBDC (derived from Mn-MOF) nano-catalyst, with agglomerated sheet-type structure, which ensure that possibility of achieving both thermo- and electro-catalytic hydrogen isotope exchange (HIE) reaction. Furthermore, XPS characterization affirmed that the specific structural changes in the electron density of Ru outer layers were modulated through the impregnation and reduction processes. According to the change of outer electronic structure, hydrogen spillover and electron-rich flow promote the reaction of catalyst in thermo- and electro-catalytic systems respectively. And, the results indicate that high deuterium rates of 97% can be obtained, which the catalytic technology has enormous potential for the synthesis of a broad variety of deuterium-labeled compounds.

Regulating the kinetics of zinc-ion migration in spinel ZnMn2O4 through iron doping boosted aqueous zinc-ion storage performance

Spinel ZnMn2O4 with a three-dimensional channel structure is one of the important cathode materials for aqueous zinc ions batteries (AZIBs). However, like other manganese-based materials, spinel ZnMn2O4 also has problems such as poor conductivity, slow reaction kinetics and structural instability under long cycles. Herein, ZnMn2O4 mesoporous hollow microspheres with metal ion doping were prepared by a simple spray pyrolysis method and applied to the cathode of aqueous zinc ion battery. Cation doping not only introduces defects, changes the electronic structure of the material, improves its conductivity, structural stability, and reaction kinetics, but also weakens the dissolution of Mn2+. The optimized 0.1 % Fe-doped ZnMn2O4 (0.1% Fe-ZnMn2O4) has a capacity of 186.8 mAh g−1 after 250 charge-discharge cycles at 0.5 A g−1 and the discharge specific capacity reaches 121.5 mAh g−1 after 1200 long cycles at 1.0 A g−1. The theoretical calculation results show that doping causes the change of electronic state structure, accelerates the electron transfer rate, and improves the electrochemical performance and stability of the material.

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.