Zhang-Rice physics and anomalous copper states in A-site ordered perovskites

Stabilizing lattice oxygen redox in layered sodium transition metal oxide through spin singlet state

Reversible lattice oxygen redox reactions offer the potential to enhance energy density and lower battery cathode costs. However, their widespread adoption faces obstacles like substantial voltage hysteresis and poor stability. The current research addresses these challenges by achieving a non-hysteresis, long-term stable oxygen redox reaction in the P3-type Na2/3Cu1/3Mn2/3O2. Here we show this is accomplished by forming spin singlet states during charge and discharge. Detailed analysis, including in-situ X-ray diffraction, shows highly reversible structural changes during cycling. In addition, local CuO6 Jahn-Teller distortions persist throughout, with dynamic Cu-O bond length variations. In-situ hard X-ray absorption and ex-situ soft X-ray absorption study, along with density function theory calculations, reveal two distinct charge compensation mechanisms at approximately 3.66 V and 3.99 V plateaus. Notably, we observe a Zhang-Rice-like singlet state during 3.99 V charging, offering an alternative charge compensation mechanism to stabilize the active oxygen redox reaction.

Magnetic Interplay between π-Electrons of Open-Shell Porphyrins and d-Electrons of Their Central Transition Metal Ions

Magnetism is typically associated with d- or f-block elements, but can also appear in organic molecules with unpaired π-electrons. This has considerably boosted the interest in such organic materials with large potential for spintronics and quantum applications. While several materials showing either d/f or π-electron magnetism have been synthesized, the combination of both features within the same structure has only scarcely been reported. Open-shell porphyrins (Pors) incorporating d-block transition metal ions represent an ideal platform for the realization of such architectures. Herein, the preparation of a series of open-shell, π-extended Pors that contain magnetically active metal ions (i.e., Cu^II , Co^II , and Fe^II ) through a combination of in-solution and on-surface synthesis is reported. A detailed study of the magnetic interplay between π- and d-electrons in these metalloPors has been performed by scanning probe methods and density functional theory calculations. For the Cu and FePors, ferromagnetically coupled π-electrons are determined to be delocalized over the Por edges. For the CoPor, the authors find a Kondo resonance resulting from the singly occupied Co^II d_z ² orbital to dominate the magnetic fingerprint. The Fe derivative exhibits the highest magnetization of 3.67 μ_B (S≈2) and an exchange coupling of 16 meV between the π-electrons and the Fe d-states.

An Open-Ended Ni3 S2 -Co9 S8 Heterostructures Nanocage Anode with Enhanced Reaction Kinetics for Superior Potassium-Ion Batteries

Sulfides are perceived as promising anode materials for potassium-ion batteries (PIBs) due to their high theoretical specific capacity and structural diversity. Nonetheless, the poor structural stability and sluggish kinetics of sulfides lead to unsatisfactory electrochemical performance. Herein, Ni₃ S₂ -Co₉ S₈ heterostructures with an open-ended nanocage structure wrapped by reduced graphene oxide (Ni-Co-S@rGO cages) are well designed as the anode for PIBs via a selective etching and one-step sulfuration approach. The hollow Ni-Co-S@rGO nanocages, with large surface area, abundant heterointerfaces, and unique open-ended nanocage structure, can reduce the K⁺ diffusion length and promote reaction kinetics. When used as the anode for PIBs, the Ni-Co-S@rGO exhibits high reversible capacity and low capacity degradation (0.0089% per cycle over 2000 cycles at 10 A g^-1 ). A potassium-ion full battery with a Ni-Co-S@rGO anode and Prussian blue cathode can display a superior reversible capacity of 400 mAh g^-1 after 300 cycles at 2 A g^-1 . The unique structural advantages and electrochemical reaction mechanisms of the Ni-Co-S@rGO are revealed by finite-element-simulation in situ characterizations. The universal synthesis technology of bimetallic sulfide anodes for advanced PIBs may provide vital guidance to design high-performance energy-storage materials.

Observation of perfect diamagnetism and interfacial effect on the electronic structures in infinite layer Nd0.8Sr0.2NiO2 superconductors

Nickel-based complex oxides have served as a playground for decades in the quest for a copper-oxide analog of the high-temperature superconductivity. They may provide clues towards understanding the mechanism and an alternative route for high-temperature superconductors. The recent discovery of superconductivity in the infinite-layer nickelate thin films has fulfilled this pursuit. However, material synthesis remains challenging, direct demonstration of perfect diamagnetism is still missing, and understanding of the role of the interface and bulk to the superconducting properties is still lacking. Here, we show high-quality Nd_0.8Sr_0.2NiO₂ thin films with different thicknesses and demonstrate the interface and strain effects on the electrical, magnetic and optical properties. Perfect diamagnetism is achieved, confirming the occurrence of superconductivity in the films. Unlike the thick films in which the normal-state Hall-coefficient changes signs as the temperature decreases, the Hall-coefficient of films thinner than 5.5 nm remains negative, suggesting a thickness-driven band structure modification. Moreover, X-ray absorption spectroscopy reveals the Ni-O hybridization nature in doped infinite-layer nickelates, and the hybridization is enhanced as the thickness decreases. Consistent with band structure calculations on the nickelate/SrTiO₃ heterostructure, the interface and strain effect induce a dominating electron-like band in the ultrathin film, thus causing the sign-change of the Hall-coefficient.

Nickelate superconductors attract enormous attention in the field of high-temperature superconductivity. Here the authors report observation of perfect diamagnetism and interfacial effect on the electronic structures in infinite layer Nd_0.8Sr_0.2NiO₂ superconductors.

In Situ Formation of Nano Ni-Co Oxyhydroxide Enables Water Oxidation Electrocatalysts Durable at High Current Densities

The oxygen evolution reaction (OER) limits the energy efficiency of electrocatalytic systems due to the high overpotential symptomatic of poor reaction kinetics; this problem worsens over time if the performance of the OER electrocatalyst diminishes during operation. Here, a novel synthesis of nanocrystalline Ni-Co-Se using ball milling at cryogenic temperature is reported. It is discovered that, by anodizing the Ni-Co-Se structure during OER, Se ions leach out of the original structure, allowing water molecules to hydrate Ni and Co defective sites, and the nanoparticles to evolve into an active Ni-Co oxyhydroxide. This transformation is observed using operando X-ray absorption spectroscopy, with the findings confirmed using density functional theory calculations. The resulting electrocatalyst exhibits an overpotential of 279 mV at 0.5 A cm^-2 and 329 mV at 1 A cm^-2 and sustained performance for 500 h. This is achieved using low mass loadings (0.36 mg cm^-2 ) of cobalt. Incorporating the electrocatalyst in an anion exchange membrane water electrolyzer yields a current density of 1 A cm^-2 at 1.75 V for 95 h without decay in performance. When the electrocatalyst is integrated into a CO₂ -to-ethylene electrolyzer, a record-setting full cell voltage of 3 V at current density 1 A cm^-2 is achieved.

Doping evolution of the Mott-Hubbard landscape in infinite-layer nickelates

Copper-based (cuprate) oxides are not only the original but also one of the best-studied families of “high-temperature” superconductors. With nominally identical crystal structure and electron count, nickel-based (nickelate) compounds have been widely pursued for decades as a possible analog to the cuprates. The recent demonstration of superconductivity in nickelate thin films has provided an experimental platform to explore the possible connections between the copper- and nickel-based superconductors. Here, we perform highly localized spectroscopic measurements to reveal a number of key differences between the two systems, particularly with regard to the hybridization between the O and metal (Cu or Ni) orbitals.

The recent observation of superconductivity in Nd0.8Sr0.2NiO2 has raised fundamental questions about the hierarchy of the underlying electronic structure. Calculations suggest that this system falls in the Mott–Hubbard regime, rather than the charge-transfer configuration of other nickel oxides and the superconducting cuprates. Here, we use state-of-the-art, locally resolved electron energy-loss spectroscopy to directly probe the Mott–Hubbard character of Nd1−xSrxNiO2. Upon doping, we observe emergent hybridization reminiscent of the Zhang–Rice singlet via the oxygen-projected states, modification of the Nd 5d states, and the systematic evolution of Ni 3d hybridization and filling. These experimental data provide direct evidence for the multiband electronic structure of the superconducting infinite-layer nickelates, particularly via the effects of hole doping on not only the oxygen but also nickel and rare-earth bands.

Revealing the Improved Catalytic Properties of Modified Graphene-like Structures

The surface morphology and electronic structure of hexagonal graphene onion rings (HGORs), a modified graphene structure, were investigated to confirm the possibility as an efficient catalyst when compared to graphene. After confirming the formation of HGORs with a smaller width (~4.2 μm) from scanning electron microscopy (SEM) and optical microscopy images, we compared the catalytic activities of HGORs and graphene by measuring the rate of oxidation of thiophenol using high-resolution photoemission spectroscopy (HRPES). In addition, we also assessed in 4-chlorophenol degradation and the OH radical formation with a benzoic acid to confirm the possibility for photocatalytic activities of HGORs. As a result, we confirmed that HGORs, which has an increased active site due to its three-dimensional structure formed by the reaction of graphene with hydrogen, can act as an effective catalyst. In addition, we could also realize the possibility of optical applicability by observing the 0.13 eV of band gap opening of HGORs.

A Sequential Electron Doping for Quadruple Perovskite Oxides ACu3Co4O12 (A = Ca, Y, Ce)

A novel quadruple perovskite oxide CeCu₃Co₄O₁₂ has been synthesized in high-pressure and high-temperature conditions of 12 GPa and 1273 K. Rietveld refinement of the synchrotron X-ray powder diffraction pattern reveals that this oxide crystallizes in a cubic quadruple perovskite structure with the 1:3-type ordering of Ce and Cu ions at the A-site. X-ray absorption spectroscopy analysis demonstrates the valence-state transitions in the ACu₃Co₄O₁₂ series (A = Ca, Y, Ce) from Ca²⁺Cu³⁺₃Co^3.25+₄O₁₂ to Y³⁺Cu³⁺₃Co³⁺₄O₁₂ to Ce⁴⁺Cu^2.67+₃Co³⁺₄O₁₂, where the electrons are doped in the order from B-site (Co^3.25+ → Co³⁺) to A'-site (Cu³⁺ → Cu^2.67+). This electron-doping sequence is in stark contrast to the typical B-site electron doping for simple ABO₃-type perovskite and quadruple perovskites CaCu₃B₄O₁₂ (B = V, Cr, Mn), further differing from the monotonical A'-site electron doping for Na_1-xLa_xMn₃Ti₄O₁₂ and A'- and B-site electron doping for AMn₃V₄O₁₂ (A = Na, Ca, La). The differences in the electron-doping sequences are interpreted by rigid-band models, proposing a wide variety of electronic states for the complex transition-metal oxides containing the multiple valence-variable ions.

Comparative study on the catalytic activity of Fe-doped ZrO2 nanoparticles without significant toxicity through chemical treatment under various pH conditions

Despite advances in the construction of catalysts based on metal oxide nanoparticles (MO NPs) for various industrial, biomedical, and daily-life applications, the biosafety concerns about these NPs still remain. Recently, the need to analyze and improve the safety of MO NPs along with attempts to enhance their catalytic performance has been strongly perceived. Here, we prepared multiple variants of Fe-doped zirconium oxide (Fe@ZrO₂) NPs under different pH conditions; then, we assessed their toxicity and finally screened the variant that exhibited the best catalytic performance. To assess the NP toxicity, the prepared NPs were introduced into three types of human cells originally obtained from different body parts likely to be most affected by NPs (skin, lung, and kidney). Experimental results from conventional cellular toxicity assays including recently available live-cell imaging indicated that none of the variants exerted severe negative effects on the viability of the human cells and most NPs were intracellular localized outside of nucleus, by which severe genotoxicity is unexpected. In contrast, Fe@ZrO₂ NPs synthesized under a basic condition (pH = 13.0), exhibited the highest catalytic activities for three different reactions; each was biochemical (L-cysteine oxidation) or photochemical one (4-chlorophenol degradation and OH radical formation with benzoic acid). This study demonstrates that catalytic Fe@ZrO₂ NPs with enhanced activities and modest or insignificant toxicity can be effectively developed and further suggests a potential for the use of these particles in conventional chemical reactions as well as in recently emerging biomedical and daily-life nanotechnology applications.

Transition metal doped Sb@SnO2 nanoparticles for photochemical and electrochemical oxidation of cysteine

Transition metal-doped SnO₂ nanoparticles (TM-SnO₂) were synthesized by applying a thermos-synthesis method, which first involved doping SnO₂ with Sb and then with transition metals (TM = Cr, Mn, Fe, or Co) of various concentrations to enhance a catalytic effect of SnO₂. The doped particles were then analyzed by using various surface analysis techniques such as transmission electron microscopy (TEM), X-ray diffraction (XRD), scanning transmission X-ray microscopy (STXM), and high-resolution photoemission spectroscopy (HRPES). We evaluated the catalytic effects of these doped particles on the oxidation of L-cysteine (Cys) in aqueous solution by taking electrochemical measurements and on the photocatalytic oxidation of Cys by using HRPES under UV illumination. Through the spectral analysis, we found that the Cr- and Mn-doped SnO₂ nanoparticles exhibit enhanced catalytic activities, which according to the various surface analyses were due to the effects of the sizes of the particles and electronegativity differences between the dopant metal and SnO₂.

Enhancement of Catalytic Activity of Reduced Graphene Oxide Via Transition Metal Doping Strategy

To compare the catalytic oxidation activities of reduced graphene oxide (rGO) and rGO samples doped with five different transition metals (TM-rGO), we determine their effects on the oxidation of L-cysteine (Cys) in aqueous solution by performing electrochemistry (EC) measurements and on the photocatalytic oxidation of Cys by using high-resolution photoemission spectroscopy (HRPES) under UV illumination. Our results show that Cr-, Fe-, and Co-doped rGO with 3+ charge states (stable oxide forms: Cr³⁺, Fe³⁺, and Co³⁺) exhibit enhanced catalytic activities that are due to the charge states of the doped metal ions as we compare them with Cr-, Fe-, and Co-doped rGO with 2+ charge states. The SEM images and the corresponding EC measurements for (a) Cr3+, (b) Fe3+, and

Determining the Catalytic Activity of Transition Metal-Doped TiO2 Nanoparticles Using Surface Spectroscopic Analysis

The modified TiO₂ nanoparticles (NPs) to enhance their catalytic activities by doping them with the five transition metals (Cr, Mn, Fe, Co, and Ni) have been investigated using various surface analysis techniques such as scanning electron microscopy (SEM), Raman spectroscopy, scanning transmission X-ray microscopy (STXM), and high-resolution photoemission spectroscopy (HRPES). To compare catalytic activities of these transition metal-doped TiO₂ nanoparticles (TM-TiO₂) with those of TiO₂ NPs, we monitored their performances in the catalytic oxidation of 2-aminothiophenol (2-ATP) by using HRPES and on the oxidation of 2-ATP in aqueous solution by taking electrochemistry (EC) measurements. As a result, we clearly investigate that the increased defect structures induced by the doped transition metal are closely correlated with the enhancement of catalytic activities of TiO₂ NPs and confirm that Fe- and Co-doped TiO₂ NPs can act as efficient catalysts.

Introducing Ionic-Current Detection for X-ray Absorption Spectroscopy in Liquid Cells

Photons and electrons are two common relaxation products upon X-ray absorption, enabling fluorescence yield and electron yield detections for X-ray absorption spectroscopy (XAS). The ions that are created during the electron yield process are relaxation products too, which are exploited in this study to produce ion yield for XA detection. The ionic currents measured in a liquid cell filled with water or iron(III) nitrate aqueous solutions exhibit characteristic O K-edge and Fe L-edge absorption profiles as a function of excitation energy. Application of two electrodes installed in the cell is crucial for obtaining the XA spectra of the liquids behind membranes. Using a single electrode can only probe the species adsorbed on the membrane surface. The ionic-current detection, termed as total ion yield (TIY) in this study, also produces an undistorted Fe L-edge XA spectrum, indicating its promising role as a novel detection method for XAS studies in liquid cells.

Superconductor to Mott insulator transition in YBa2Cu3O7/LaCaMnO3 heterostructures

The superconductor-to-insulator transition (SIT) induced by means such as external magnetic fields, disorder or spatial confinement is a vivid illustration of a quantum phase transition dramatically affecting the superconducting order parameter. In pursuit of a new realization of the SIT by interfacial charge transfer, we developed extremely thin superlattices composed of high Tc superconductor YBa2Cu3O7 (YBCO) and colossal magnetoresistance ferromagnet La0.67Ca0.33MnO3 (LCMO). By using linearly polarized resonant X-ray absorption spectroscopy and magnetic circular dichroism, combined with hard X-ray photoelectron spectroscopy, we derived a complete picture of the interfacial carrier doping in cuprate and manganite atomic layers, leading to the transition from superconducting to an unusual Mott insulating state emerging with the increase of LCMO layer thickness. In addition, contrary to the common perception that only transition metal ions may respond to the charge transfer process, we found that charge is also actively compensated by rare-earth and alkaline-earth metal ions of the interface. Such deterministic control of Tc by pure electronic doping without any hindering effects of chemical substitution is another promising route to disentangle the role of disorder on the pseudo-gap and charge density wave phases of underdoped cuprates.

Investigation of continuous changes in the electric-field-induced electronic state in Bi1−xCaxFeO3−δ

Systematic changes in the electronic structure of Bi_1−xCa_xFeO_3−δ, which are induced by electrically controlled hole carrier doping, are observed by photoemission electron microscopy (PEEM).

Possible Kondo physics near a metal-insulator crossover in the a-site ordered perovskite CaCu3Ir4O12

The A-site ordered perovskite (AA(3)')B(4)O(12) can accommodate transition metals on both A' and B sites in the crystal structure. Because of this structural feature, it is possible to have narrow-band electrons interacting with broadband electrons from different sublattices. Here we report a new A-site ordered perovskite (CaCu(3))Ir(4)O(12) synthesized under high pressure. The coupling between localized spins on Cu(2+) and itinerant electrons from the Ir-O sublattice makes Kondo-like physics take place at a temperature as high as 80 K. Results from the local density approximation calculation have confirmed the relevant band structure. The magnetization anomaly found at 80 K can be well rationalized by the two-fluid model.

Hollow fibers networked with perovskite nanoparticles for H2 production from heavy oil

Design of catalytic materials has been highlighted to build ultraclean use of heavy oil including liquid-to-gas technology to directly convert heavy hydrocarbons into H₂–rich gas fuels. If the H₂ is produced from such heavy oil through high-active and durable catalysts in reforming process that is being constructed in hydrogen infrastructure, it will be addressed into renewable energy systems. Herein, the three different hollow fiber catalysts networked with perovskite nanoparticles, LaCr_0.8Ru_0.2O₃, LaCr_0.8Ru_0.1Ni_0.1O₃, and LaCr_0.8Ni_0.2O₃ were prepared by using activated carbon fiber as a sacrificial template for H₂ production from heavy gas oil reforming. The most important findings were arrived at: (i) catalysts had hollow fibrous architectures with well-crystallized structures, (ii) hollow fibers had a high specific surface area with a particle size of ≈50 nm, and (iii) the Ru substituted ones showed high efficiency for H₂ production with substantial durability under high concentrations of S, N, and aromatic compounds.