Valence Transition Research Articles

Effects of B- and A/B-sites Codoping on lattice distortion and defect concentration for high electromechanical response of lead-free piezoelectrics

Effects of different thermal conditions on lead-free (Bi0.65Ba0.35)(Fe0.65Ti0.35)O3 ceramics were studied to reduce secondary phase formation, regulate bismuth vacancies and resulting oxygen vacancies, and Fe ions valence transition during the heat treatment process. The optimal thermal conditions led to a higher d33 = 200 pC/N and a d33* = 420 pm/V. A/B-sites ions replacement in BF35BT, BF35BT-Zr, and BF35BTSZ ceramics were prepared to reveal the effects of different Zr or Sr-Zr-contents. A maximum tetragonality (cT/aT) was estimated for the single doping (Zr) and co-doping (Sr and Zr) cases. For Zr = 0.02, the electric field-induced strain reached its maximum value of d33* = 562 pm/V. Similarly, a strain of d33* = 701 pm/V was observed for Sr-Zr = 0.02. Additionally, the d33* improved to 870 pm/V (90 °C) in Zr = 0.02 and 1130 pm/V (120 °C) in Sr-Zr=0.02. The presence of local structural heterogeneity causes the domain size to decrease as it moves from the micro to the nanoscale. The results demonstrate that improved performance can be associated with reduced concentration of oxygen vacancies, nanodomains by local structural heterogeneity, high lattice anisotropy, higher relative density, and appropriate grain size.

Core Hole Effect to Valence Excitations: Tracking and Visualizing the Same Excitation in XPS Shake-Up Satellites and UV Absorption Spectra.

Introducing a core hole significantly alters the electronic structure of a molecule, and various X-ray spectroscopy techniques are available for probing the valence electronic structure in the presence of a core hole. In this study, we visually demonstrate the influence of a core hole on valence excitations by computing the ultraviolet absorption spectra and the shake-up satellites in X-ray photoelectron spectra for pyrrole, furan, and thiophene, as complemented by the natural transition orbital (NTO) analysis over transitions with and without a core hole. Employing equivalent core hole time-dependent density functional theory (ECH-TDDFT) and TDDFT methods, we achieved balanced accuracy in both spectra for reliable comparative analysis. We tracked the same involved valence transition in both spectra, offering a vivid illustration of the core hole effect via the change in corresponding particle NTOs introduced by a 1s core hole on a Cα, Cβ, or O atom. Our analysis deepens the understanding of the core hole effect on valence transitions, a phenomenon ubiquitously observed in general X-ray spectroscopic analyses.

The magnetic modulation by the valence transition of polyoxometalate

Three novel vanadoborates with crown-shaped V12B32 cluster cages were successfully synthesized, differing only in their countercation. X-ray photoelectron spectroscopy (XPS) and Bond valence sum (BVS) analyses revealed distinct V4+/V5+ ratios (11:1, 10:2, and 10:2) in the cluster cage of the three compounds. Structural analyses indicated that the valence state ratio was modulated by the countercation. The magnetic properties of these compounds were examined using variable-temperature magnetic susceptibilities and two-dimensional correlation infrared spectroscopy (2D-COS-IR) under magnetic perturbation. This approach elucidated the origin and variability of their magnetic properties, demonstrating that the vanadium valence ratio and countercation significantly influence the magnetic behaviour of vanadoborates. The present study highlights the effectiveness of the transition of the vanadium valence state in the cluster cage by altering the countercation, providing a new avenue for the study of magnetically modulated materials.

Temperature and pressure dependencies of Sm valence in SmB[formula omitted] Kondo insulator probed by x-ray absorption spectroscopy

Pressure and temperature induced valence changes in the Kondo insulator SmB6 have been investigated using x-ray absorption spectroscopy (XAS) along several isobaric cooling runs and one isothermal compression. The average valence of Sm at various pressure and temperature conditions is extracted from high quality XAS data by peak fitting procedures. The temperature dependence of the Sm valence at ambient pressure shows changes of slope near the known characteristic temperatures of gap formation (120 K) and the in-gap state (50 K). At higher pressures the average valence also shows similar temperature dependent trends upon cooling but the slope changes are less pronounced. This behavior is discussed in relation to the possible existence of an underlying first order valence transition and its critical end point. At constant temperature (T=20 K), the Sm valence increases rapidly with pressure; however, it remains far below the expected integer value (3+) near the onset of the magnetic ordering and gap closure. In addition, the local structure of SmB6 at ambient conditions has been analyzed using extended x-ray absorption fine structure.

Mo4+-Doped CuS Nanosheet-Assembled Hollow Spheres for CO2 Electroreduction to Ethanol in a Flow Cell.

Electrocatalytic CO2 reduction reaction (CO2RR) to ethanol has been widely researched for potential commercial application. However, it still faces limited selectivity at a large current density. Herein, Mo4+-doped CuS nanosheet-assembled hollow spheres are constructed to address this issue. Mo4+ ion doping modifies the local electronic environments and diversifies the binding sites of CuS, which increases the coverage of linear *COL and produces bridge *COB for subsequent *COL-*COH coupling toward ethanol production. The optimal Mo9.0%-CuS can electrocatalyze CO2 to ethanol with a faradaic efficiency of 67.5% and a partial current density of 186.5 mA cm-2 at -0.6 V in a flow cell. This work clarifies that doping high valence transition metal ions into Cu-based sulfides can regulate the coverage and configuration of related intermediates for ethanol production during the CO2RR in a flow cell.

Endogenous iron-enriched biochar derived from steel mill wastewater sludge for tetracycline removal: Heavy metals stabilization, adsorption performance and mechanism

Steel mill wastewater sludge, as an iron-enriched solid waste, was expected to be converted into iron-enriched biochar with acceptable environmental risk by pyrolysis. The purpose of our study was to evaluate the chemical speciation transformation of heavy metals in biochar under various pyrolysis temperatures and its reutilization for tetracycline (TC) removal. The experimental data indicated that pyrolysis temperature was a key factor affecting the heavy metals speciation and bioavailability in biochar, and biochar with pyrolysis temperature at 450 °C was the most feasible for reutilization without potential risk. The endogenous iron-enriched biochar (FSB450) showed highly efficient adsorption towards TC, and its maximum adsorption capacity could reach 240.38 mg g−1, which should be attributed to its excellent mesoporous structure, abundant functional groups and endogenous iron cycling. The endogenous iron was converted to a stable iron oxide crystalline phase (Fe3O4 and MgFe2O4) by pyrolysis, which underwent a valence transition to form a coordination complex with TC by electron shuttling in the FSB450 matrix. The study provides a win-win approach for resource utilization of steel wastewater sludge and treatment of antibiotic contamination in wastewater.

Oxidation state regulation of iron-based bimetallic nanoparticles for efficient and simultaneous electrochemical detection of Pb2+ and Cu2+

In this work, bimetallic nanocomposites were synthesized by an emulsion hydrothermal method, and the electronic structure of the nanocomposite’s surface was adjusted to make it optimal by introducing different transition metal salts, providing more active adsorption sites for detecting heavy metal ions (HMIs). The results showed that different transition metal cations underwent redox reactions with Fe3+ at high temperature, resulting in generation of more Fe2+ as well as high valence transition metal ions. And this process formed more oxygen vacancies (OVs) on the material surface, enhancing the adsorption properties of HMIs. Among these nanocomposites, CoFe2O4 produced the most Fe2+ (Fe2+/Fe3+ = 1.29) and Co3+ (Co3+ = 44.0%) at a high temperature, suggesting the generation of the most active sites on the material surface. The corresponding modified electrode CoFe2O4 (CoFe2O4/GCE) also exhibited good electrochemical performance for simultaneously detecting Pb2+ and Cu2+, with the limit of detections of 3.94 and 8.27 nM, respectively. Compared with other nanocomposites, adsorption experiments showed that the CoFe2O4/GCE had the largest adsorption capacity of Pb2+ and Cu2+ (QPb2+=49.82 μC, QCu2+=48.88 μC). Therefore, this work showed that the interaction between metal ions in bimetallic nanomaterials can regulate the electronic structures of the material surface at high-temperature treatment, improve the adsorption capacity of nanomaterials, and lead to good electrochemical detection performance of HMIs.

4f electron temperature driven ultrafast electron localization

Valence transitions in strongly correlated electron systems are caused by orbital hybridization and Coulomb interactions between localized and delocalized electrons. The transition can be triggered by changes in the electronic structure and is sensitive to temperature variations, applications of magnetic fields, and physical or chemical pressure. Launching the transition by photoelectric fields can directly excite the electronic states and thus provides an ideal platform to study the correlation among electrons on ultrafast timescales. The EuNi2(Si0.21Ge0.79)2 mixed-valence metal is an ideal material to investigate the valence transition of the Eu ions via the amplified orbital hybridization by the photoelectric field on subpicosecond timescales. A direct view on the 4f electron occupancy of the Eu ions is required to understand the microscopic origin of the transition. Here we probe the 4f electron states of EuNi2(Si0.21Ge0.79)2 at the sub-ps timescale after photoexcitation by x-ray absorption spectroscopy across the Eu M5-absorption edge. The observed spectral changes due to the excitation indicate a population change of total angular momentum multiplet states J = 0, 1, 2, and 3 of Eu3+ and the Eu2+ J = 7/2 multiplet state caused by an increase in 4f electron temperature that results in a 4f localization process. This electronic temperature increases combined with fluence-dependent screening accounts for the strongly nonlinear effective valence change. The data allow us to extract a time-dependent determination of an effective temperature of the 4f shell, which is also of great relevance in the understanding of metallic systems' properties, such as the ultrafast demagnetization of ferromagnetic rare-earth intermetallic and their all-optical magnetization switching. In addition, our results elucidate the energetics of charge fluctuations in valence-mixed electronic systems, which provide enriched knowledge regarding the role of valence transitions and orbital hybridization for quantum critical phenomena. Published by the American Physical Society 2024

Pressure-induced large valence transitions in Yb-Cu binary intermetallic systems

Pressure-induced large valence transitions in Yb-Cu binary intermetallic systems

Oxygen-Independent Radiodynamic Therapy: Radiation-Boosted Chemodynamics for Reprogramming the Tumor Immune Environment and Enhancing Antitumor Immune Response.

Radiodynamic therapy (RDT) has emerged as a promising modality for cancer treatment, offering notable advantages such as deep tissue penetration and radiocatalytic generation of oxygen free radicals. However, the oxygen-dependent nature of RDT imposes limitations on its efficacy in hypoxic conditions, particularly in modulating and eliminating radioresistant immune suppression cells. A novel approach involving the creation of a "super" tetrahedron polyoxometalate (POM) cluster, Fe12-POM, has been developed for radiation boosted chemodynamic catalysis to enable oxygen-independent RDT in hypoxic conditions. This nanoscale cluster comprises four P2W15 units functioning as energy antennas, while the Fe3 core serves as an electron receptor and catalytic center. Under X-ray radiation, a metal-to-metal charge transfer phenomenon occurs between P2W15 and the Fe3 core, resulting in the valence transition of Fe3+ to Fe2+ and a remarkable 139-fold increase in hydroxyl radical generation compared to Fe12-POM alone. The rapid generation of hydroxyl radicals, in combination with PD-1 therapy, induces a reprogramming of the immune environment within tumors. This reprogramming is characterized by upregulation of CD80/86, downregulation of CD163 and FAP, as well as the release of interferon-γ and tumor necrosis factor-α. Consequently, the occurrence of abscopal effects is facilitated, leading to significant regression of both local and distant tumors in mice. The development of oxygen-independent RDT represents a promising approach to address cancer recurrence and improve treatment outcomes.

N-doped CuFe-MOF-919 derivatives in activation of peroxymonosulfate for enhanced degradation of organic pollutants: 1O2 dominated non-radical pathway

Metal organic framework (MOF)-derived carbon materials have received significant attention as catalysts for peroxymonosulfate (PMS)-mediated degradation of organic pollutants. Despite the growing interest, the challenge persists in designing MOF derivatives where non-radicals serve as the primary active species. This study introduces an N-doped carbon-based material denoted as CuFe@NC-10, featuring highly dispersed Cu and Fe bimetallic active sites. The design involves co-doping MOF-919(Fe) with melamine. Under identical experimental conditions, CuFe@NC-10 exhibited remarkable performance, degrading 99.5 % of Rhodamine B (RhB) within 30 min, with a mineralization rate of 60 %. Notably, the pseudo-primary kinetic constant was 5.8 times higher than that observed for the pure MOF derivative (CuFe@NC). CuFe@NC-10 displayed consistent activation capacity across a broad pH range, and its catalytic performance remained largely unaffected by the coexistence of inorganic anions and humic acid. Quenching experiments and electron paramagnetic resonance (EPR) analysis indicated that singlet oxygen (1O2) primarily drove RhB degradation. X-ray photoelectron spectroscopy (XPS) findings suggested that the valence transition between Cu and Fe facilitated 1O2 generation, confirming PMS as the primary source of 1O2. The RhB degradation pathway was deduced through Q Exactive mass spectrometer and density-functional theory (DFT) calculations. Additionally, the study evaluated the ecotoxicity of RhB and its intermediates, providing essential insights into ecological risks associated with its degradation.

Tristable TaOx-based memristor by controlling oxygen vacancy transportion based on valence transition mechanism

It is always a hot and difficult problem of how to realize multi-bit data storage by preparing multi-resistance memristors by a simple process compatible with standard CMOS processes. In this work, a tristable TaOx-based memristor with tunable oxygen vacancies is fabricated by physical vapor deposition, consisting of a high-resistance state and two low-resistance states. Its set and reset voltages are low −1.1V, −2.1V, and -1V, 3.3V, respectively. The XPS characterization demonstrates the presence of three valence states within the TaOx film. It has been suggested that different barriers lead to different switching voltages during the transition of the trivalent state, resulting in the formation of stable states. Interestingly, when increasing the concentration of oxygen during preparation to decrease the concentration of oxygen vacancies in TaOx film, the volt-ampere properties of memristor change from tristable to bistable, which further proves the above point. This work provides a viable strategy for switching between bistable and tristable states in memristors prepared from transition metal oxides.

Effect of Yb doping on the optical and photoelectric properties of CsPbCl3 single crystals

In this paper, the effect of Yb dopant on the optical and photoelectric properties of CsPbCl3:Yb single crystals is studied. The position of the main energy level of Yb3+ ions relative to the energy band was determined. Two channels of electron photoionization into the conduction band were identified, which are photo-stimulated transition of valence electrons and transition of electrons from the ground level of the Yb3+ dopant ion. The crucial role of Yb2+ ions in the manifestation of the quantum cutting effect was shown. A non-uniform distribution of ytterbium dopant in the CsPbCl3:Yb single crystal volume was revealed. It was established that the absolute quantum yield of ytterbium luminescence in CsPbCl3:Yb single crystals in the near-infrared region at Yb concentration of 2 mol.% reaches 86%.

Synergistic Co/S co-doped CeO2 sulfur-oxide catalyst for efficient catalytic reduction of toxic organics and heavy metal pollutants under dark conditions

The rising accumulation of pollutants, including heavy metals and toxic organics, necessitates effective environmental remediation to protect health and biological systems. In this study, a cubic cerianite CeO2-like Co/S co-doped CeO2 sulfur-oxide catalyst (labeled as CeCoOS) with super catalytic reduction performance of pollutants with NaBH4 as a reducing agent under dark was prepared via a green and facile preparation method. Transition metal Co2+-cation and S2−-anion co-doped CeO2 adjusted the energy bandgap and introduced oxygen vacancy (Vo), which converted Ce4+ around Vo into Ce3+ to maintain electric neutrality. The heterovalent Ce4+/Ce3+ states in Co/S-CeO2 enhance the electron lifetime and facilitate fast electron transfer. The valence transition between Ce4+/Ce3+ and Vo make the Co/S co-doped in CeO2 catalyst increase its electrochemically active surface sites and accelerate the pollutants reduction reaction. The CeCoOS-3 with Co/S dopants has an excellent catalytic reduction activity to completely reduce 20 ppm toxic 4-NP in 10 min, 50 ppm organic dyes of MB, RhB, and MO in 6 min, and 50 ppm heavy metal Cr6+ in 10 min. The CeCoOS-3 sulfur-oxide catalyst also exhibited excellent stability in the reusability test. Therefore, CeCoOS sulfur-oxide catalysts are expected to be used for dye decolorization, detoxification of organic toxins, and reduction of electroplating wastewater.

Nonvolatile Isomorphic Valence Transition in SmTe Films.

The burgeoning field of optoelectronic devices necessitates a mechanism that gives rise to a large contrast in the electrical and optical properties. A SmTe film with a NaCl-type structure demonstrates significant differences in resistivity (over 105) and band gap (approximately 1.45 eV) between as-deposited and annealed films, even in the absence of a structural transition. The change in the electronic structure and accompanying physical properties is attributed to a rigid-band shift triggered by a valence transition (VT) between Sm2+ and Sm3+. The stress field within the SmTe film appears closely tied to the mixed valence state of Sm, suggesting that stress is a driving force in this VT. By mixing the valence states, the formation energy of the low-resistive state decreases, providing nonvolatility. Moreover, the valence state of Sm can be regulated through annealing and device-operation processes, such as applying voltage and current pulses. This investigation introduces an approach to developing semiconductor materials for optoelectrical applications.

Current-Induced Metallization and Valence Transition in Black SmS

Current-Induced Metallization and Valence Transition in Black SmS

High‐Performance Reversible Oxygen Reduction/Evolution Gas Diffusion Electrodes with Multivalent Cation Doped Core‐Shell Mn/Mn3O4 Catalysts

AbstractThe development of precious‐metal‐free catalysts with bifunctional activities for both oxygen reduction and evolution reactions (ORR/OER) is crucial for the advancement of regenerative fuel cells and rechargeable metal−air batteries. Manganese oxides (MnOx) have emerged as promising bifunctional catalysts. However, MnOx electrodes typically exhibit poor ORR/OER cycling stability owing to polarization‐induced MnOx redox reactions and phase transition. To address this issue, we developed metallic cation (i. e., Co2+, Ni2+, Cu2+, or Bi3+) doped MnOx/carbon electrodes using potentiodynamic, potentiostatic and galvanostatic methods. Among the explored dopant cations Ni2+ intercalated into MnOx under acidic conditions using a slow‐scan cyclic voltammetry method, significantly enhanced the ORR/OER activity and stability of MnOx. Alongside electrochemical doping, MnOx also underwent redox reactions leading to changes in Mn valence and phase transitions. The Ni‐incorporated MnOx gas diffusion electrode (GDE) demonstrated exceptional stability for over 120 accelerated OER and ORR cycles at ±10 mA cm−2 in 5 M KOH, surpassing the performance of the Pt/C−IrO2 benchmark. Furthermore, it achieved OER current densities of approximately 22 mA cm−2 at 1.65 VRHE, which was twice as high as that of Pt/C−IrO2.

Epitaxial thin films of binary Eu-compounds close to a valence transition

Intermetallic binary compounds of europium reveal a variety of interesting phenomena due to the interconnection between two different magnetic and 4f electronic (valence) states, which are particularly close in energy. The valence states, or magnetic properties are thus particularly sensitive to strain tuning in these materials. Consequently, we grew epitaxial EuPd2 (Eu2+, high magnetic moment) and EuPd3 (Eu3+, vanishing magnetic moment) thin films on MgO(001) substrates using molecular beam epitaxy. Ambient X-ray diffraction confirms an epitaxial relationship of cubic Laves-type (C15) EuPd2 with an [111]-out-of-plane orientation, whereby eight distinct in-plane crystallographic domains develop. For simple cubic EuPd3 two different out-of-plane orientations can be obtained by changing the substrate annealing temperature under ultra-high vacuum conditions from 873K to 1273K for one hour. A small resistance minimum evolves for EuPd3 thin films grown with low temperature substrate annealing, which was previously found even in single crystals of EuPd3 and might be attributed to a Kondo or weak localization effect. Absence of influence of applied magnetic fields and magnetotransport measurements suggest a purely trivalent Eu valence for EuPd3 thin films, as found in EuPd3 single crystals. For EuPd2 magnetic ordering below ∼72K is observed, quite similar to single crystal behavior. Field dependent measurements of the magnetoresistance and the Hall effect show hysteresis effects below ∼0.4T and an anomalous Hall effect below ∼70K, which saturates around 1.4T, thus proving a ferromagnetic ground state of the divalent Eu.

Ab initio study of the structural, electronic, and optical properties of MgTiO3 perovskite materials doped with N and P

This study investigates the electronic, optical, and structural properties of MgTiO3 perovskite materials, whether pure or doped with elements such as nitrogen (N) and phosphorus (P). The investigation utilizes density functional theory (DFT) with the GGA-mBJ approximation as implemented in the Wien2k code. The results show that the band gap energy of doped MgTiO3 is significantly lower than that of pure MgTiO3, which has a band gap of 2.933 eV, at oxygen sites with Y (N, and P). In particular, with N and P, the band gaps drop to 1.74 and 0.65 eV moreover, the Fermi energy (Ef) level shifts towards the valence band (VB) in a p-type semiconductor (SC). Further, we have analyzed the optical characteristics of these systems, including their dielectric function (ε1 and ε2), optical conductivity (σ), absorption coefficient (α), and refractive index (n). Furthermore, doping with N and P increases absorption in the visible spectrum, which raises the photocatalytic activity in the presence of light because the doped materials’ valence and conduction bands transition more readily, producing hydrogen. The discoveries above suggest that these materials possess a broad spectrum of applications, encompassing the creation of optoelectronic apparatus.

Enhanced photocatalytic CO2 conversion of a CdS/Co-BDC nanocomposite via Co(ii)/Co(iii) redox cycling.

Photocatalytic CO2 reduction into value-added chemical fuels using sunlight as the energy input has been a thorny, challenging and long-term project in the environment/energy fields because of to its low efficiency. Herein, a series of CdS/Co-BDC composite photocatalysts were constructed by incorporating CdS nanoparticles into Co-BDC using a dual-solvent in situ growth strategy for improving photocatalytic CO2 reduction efficiency. The composites were characterized through XRD, SEM, TEM, XPS, DRS and EPR techniques in detail. 18% CdS/Co-BDC composites showed superior performance for the photocatalytic CO2 reduction to CO, which was 8.9 and 19.6 times higher than that showed by the pure CdS and Co-BDC, respectively. The mechanism of enhanced photocatalytic CO2 reduction performance was analyzed. The CdS/Co-BDC composites showed better adsorption for CO2. Detailed analysis of XPS, transient photocurrent responses, and electrochemical impedance spectroscopy (EIS) shows the existence of strong charge interaction between CdS and Co-BDC and the photo-electrons of CdS can be transferred to Co-BDC. Additionally, Co-oxo of Co-BDC plays the role of a redox-active site and promotes the reduction performance via the method of valence transition of Co(ii)/Co(iii) redox.