Metal Transition Research Articles

Insulator–metal transition in VO2 film on sapphire studied by broadband dielectric spectroscopy

Vanadium dioxide () is a favorable material platform of modern optoelectronics, since it manifests the reversible temperature-induced insulator-metal transition (IMT) with an abrupt and rapid changes in the conductivity and optical properties. It makes possible applications of such a phase-change material in the ultra-fast optoelectronics and terahertz (THz) technology. Despite the considerable interest to this material, data on its broadband electrodynamic response in different states are still missing in the literature. This hampers the design and implementation of the -based devices. In this paper, we combine the Fourier-transform infrared (FTIR) spectroscopy, THz pulsed spectroscopy (TPS), and four-contact probe method to study the films prepared by magnetron sputtering on a c-cut sapphire substrate. Considering different temperatures of a substrate and pressures of atmosphere, we reconstruct complex dielectric permittivity of film in the frequency range of 0.2–150 THz, along with its static conductivity. The dielectric response is modeled using Lorentz and Drude kernels, which make possible splitting contributions from vibrational modes and free charge carriers to the total dynamic conductivity. By studying at different substrate temperatures and atmosphere pressures, we show that IMT appears to be pressure-dependent, which we attribute to the different thermostatic conditions of a sample. Finally, we estimate somewhat optimal thickness and temperature of the film in metallic phase for the THz optoelectronic applications. Our finding should be useful for further developments of the -based devices and technologies.

Structural Evolution and Metal–Insulator–Metal Transitions in Hafnium Oxides: Implication for Memristive Devices

Structural Evolution and Metal–Insulator–Metal Transitions in Hafnium Oxides: Implication for Memristive Devices

A HTO-Type Nonlinear Optical Fluorophosphate with Ultrawide Bandgap.

Compounds having hexagonal tungsten oxides (HTO) topology are of intense research interests owing to their potential functional properties, such as nonlinear optical (NLO) performances. However, most of the reported HTO-type compounds exhibit narrow optical bandgaps because of the d-d electronic transition of compositional d0 transition metals and lone pair electrons effect of Se4+/Te4+, which hinder their applications in the high-energy field, such as deep-ultraviolet (deep-UV) region. In this work, a new fluorophosphate, (NH4)3[Sc3F5(PO4)](PO3F)2 exhibiting HTO-topological structures is reported. Remarkably, the compound shows an ultrawide optical bandgap (Eg(exp.) >6.2eV, Eg(cal.) = 6.724eV), surpassing those of reported HTO-type compounds. The fact is mainly ascribed to the collaborative effect of Sc3+ without destructive d-d transition, the PO4 and PO3F units having large LUMO-HOMO gaps. Furthermore, it exhibits a phase-matchable second-harmonic generation (SHG) response of ca. 0.65 × KDP, underscoring its potential use for NLO applications. This work provides new opportunities for discovering HTO-type NLO crystals with ultrawide bandgaps.

Multistate Transition Metal Carbonyl Bonding Beyond Minimum Energy Pathway: Nonlocality of Spin-Orbit Interaction.

Transition metal carbonyl and transition metal dinitrogen are fundamental chemical complexes in many important biological and catalytic processes. Interestingly, binding between a transition metal (TM) atom and carbonyl or dinitrogen results in spin state change. However, no study has evaluated the spin-orbit (SO) effect along the association pathway of any TM-CO or TM-N2 bond. Using multireference calculations with SO interaction, we calculated the association potential energy curve for 11 electronic states for the spin crossover reactions: Ni + CO → NiCO and Ni + N2 → NiN2. Through this multistate calculation, we found that the commonly used minimum energy pathway (MEP) gives reasonable energies for the asymptotes but has an incorrect physical picture in the intermediate bond length. MEP assumes strong SO at the energy crossing point that allows for direct spin crossover from the triplet to singlet state, but multireference calculations showed that SO interactions strengthen at bond length regions, 2.3-2.5 Å before the crossing point. Furthermore, this results in a spin barrier of 0.15 eV along the Ni adsorbate association pathway. These calculations provide a new understanding of the overlooked yet important effect of the spin barrier on the association process, which can change the association rate by several orders of magnitude.

Application of Submerged arc Welding at Different Amperages in the Manufacture of Storage Tanks and Examination of the Weld Zone

In the study, ASTM A36 steel materials were combined with submerged arc welding using different amperages. Non-destructive magnetic particle (MT), liquid penetrant (SP), radiographic (RT), and ultrasonic (UT) examinations were performed on the joints. In addition, optical microscope, microhardness, bending, tensile, and notch impact tests were carried out on the welds. As a result of the RT and UT examinations, a lack of root penetration was found in the welds made at 450 A and 475 A. In the optical microscope examinations, the areas formed by the HAZ-weld metal transition were found to have a similar appearance for each amperage. In the microhardness studies, the hardness values are listed as weld metal, HAZ, and base material from high to low. From the notch impact tests, it was found that increasing the temperature increased the toughness value. From the bending tests performed on the joints where 450A and 475A were used, it was found that cracks and tears occurred. In addition, a rupture occurred in the weld metal during the tensile tests conducted on the joints made at 450 A and 475 A. For the joints made at other amperage values, it occurred in the base material.

Application investigations of force fields for molecular dynamic simulations of medical aluminum particles

The phase transition of metals is a research hotspot in molecular dynamic (MD) simulation. With the increase of the development of MD, different force fields with different requirements have emerged, which helps to select the appropriate force field and also increases the difficulty of selection. In addition, previous studies have been affected by particle size, which affects the melting point accuracy. MD simulation of the melting and condensing behavior of Aluminum (Al) particles was carried out by using EAM force fields from different sources. The thermal behavior of phase transition was studied by means of potential energy, specific heat, MSD, FCC lattice number, etc. The melting and condensation process of 2–10[Formula: see text]nm cubic without boundary under the NPT system was simulated, and the steady-state simulation was discussed by means of an isometric temperature point. It is proved that the borderless simulation can completely eliminate the boundary effect. The parameter setting of efficient use of EAM is widely discussed by using this method, and the effectiveness of the EAM force field is compared, so as to improve the efficiency of scholars engaged in related research and avoid the difficult choice of force field files. The results show that the borderless simulation has quite high advantages in terms of phase transition potential energy and melting point, while the current EAM force field is inherently deficient in this kind of discussion, and the development of new force field is urgent.

Layered NbS2 contacts formed via H2S reaction with Nb for high-performance WSe2-channel p-type transistors

Abstract Metallic transition metal dichalcogenides (TMDCs) are garnering attention as source/drain contact materials for TMDC-channel transistors. We demonstrate that NbS2, a metallic TMDC with a high effective work function of 4.69 eV, improves the performance of WSe2 p-type transistors when utilized as a contact material at the source/drain. NbS2 was synthesized through H2S annealing of Nb on WSe2. The annealing temperature was optimized to 600 °C, effectively balancing Nb sulfidation while minimizing damage to WSe2. The NbS2 contacts formed through this process yielded an improvement in on-state current for WSe2 p-type transistors, attributed to reduced contact resistance. Thus, NbS2 emerges as a promising contact material.

Heavy metal transitions from cooking utensils to different solutions

ABSTRACT The study aimed to elucidate metal transitions from cooking utensils to the solutions at different pH. Alkaline, acidic, drinking water solutions were boiled in themost preferred cooking utensils determined by a survey. The metal concentrations were measured using ICP-MS for Aluminium, Iron, Nickel, and Lead. Theresults showed that the most preferred utensils were stainless-steel, granite, teflon and cast-iron. There was a considerable difference between the transition amounts of the metals in acidic and alkaline solutions depending on the cookware.Cooking in stainless-steel, teflon, and cast-iron lead to metal concentrations exceeding WHO guidelines in acidic and alkaline media.Granite was the safest pot to cook in all media. Different brands of utensils made of the same material showed different amounts of metals released in different environmental conditions. Choosing the right cooking utensil and standardizing the metal release is important to minimize heavy metal exposure and the related health impacts.

Heterostructures with transition metal sulfides and transition metal phosphide: NiMnS/FeP achieve efficient seawater OER

Heterostructures with transition metal sulfides and transition metal phosphide: NiMnS/FeP achieve efficient seawater OER

Catalytic effect of the in-situ rhodium(III) complexes in surfactant medium on the auto-oxidation of 1,2-hydroxyphenylthiourea: A theoretical and experimental study for rhodium(III) sensing

1,2-hydroxyphenylthiourea(HPTU) undergoes auto-oxidation to form an yellow colored dimer. The process of auto-oxidation is enhanced by the presence of trace quantities of transition metal ions and transition metal complexes that act as catalysts. In the current study, catalytic potential of rhodium(III) complexes on the auto-oxidation of 1,2-hydroxyphenylthiourea(HPTU) was studied theoretically using quantum mechanical calculations and experiments were carried out using photometry and fluorometry. DFT studies inferred that [Rh(Py)6]Cl3 showed a better catalytic activity in the dimerization of HPTU. Theoretical studies were subsequently validated through experiments using photometric methods and the range of detection was determined to be 6 ng/mL–100 ng/mL. The detection limit was observed to be 2 ng/mL. Analytical parameters such as pH, reagent concentration, metal ion concentration, appropriate activators and surfactants were established.

Half-century of Efros–Shklovskii Coulomb gap: Romance with Coulomb interaction and disorder

The Efros–Shklovskii (ES) Coulomb gap in the one-electron density of localized states and the ES law of the variable range hopping conductivity were coined 50 years ago. The theory and its first confirmations were reviewed in the Shklovskii–Efros (SE) monograph published 40-years ago. This paper reviews the subsequent experimental evidence, theoretical advancements, and novel applications of the ES law. Out of hundreds of experimental validations of the ES law in a diverse range of materials, I focus on those where the dynamic range of conductivity exceeds four orders of magnitude. These include three- and two-dimensional semiconductors under both zero and high magnetic fields, localized phases in the quantum Hall effect, granular metals, nanocrystal arrays, and conducting polymers. Additionally, I discuss the non-ohmic ES law and the Coulomb gap near insulator–metal transition. Recent developments of other concepts of the SE book are also discussed.

Machine learning-based prediction of CoCrFeNiMo0.2 high-entropy alloy weld bead dimensions in wire arc additive manufacturing

Wire arc additive manufacturing (WAAM) is a promising method to fabricate large-sized components. By adjusting WAAM process parameters, dimensions of WAAM-produced weld beads can be optimized. To this end, the current study adopted the cold metal transition (CMT) process to produce different-sized beads of CoCrFeNiMo0.2 high-entropy alloy (HEA), varying WAAM parameters and linking them with the formed bead dimensions and bead-substrate contact angles. Three machine learning algorithms, namely back propagation neural network (BPNN), support vector regression (SVR), and random forest regression (RFR), were used to predict bead dimensions and contact angles under various WAAM parameters. The BPNN model has great prediction performance in the height of beads. The SVR model has the highest accuracy in predicting the width and cross-sectional area of beads. The RFR outperforms the other two models in contact angles prediction. This work not only provides a reference for the WAAM of high-entropy alloys, but also provides new ideas for predicting bead size in WAAM.

The transition-metal-dichalcogenide family as a superconductor tuned by charge density wave strength

In metallic transition metal dichalcogenides (TMDs), which remain superconducting down to single-layer thickness, the critical temperature Tc decreases for Nb-based, and increases for Ta-based materials. This contradicting trend is puzzling, impeding the development of a unified theory. Here we study the thickness-evolution of superconducting tunneling spectra in TaS2 heterostructures. The upper critical field Hc2 is strongly enhanced towards the single-layer limit – following Hc2∝Tc2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${H}_{c2}\\propto {T}_{c}^{2}$$\\end{document}. The same ratio holds for the entire family of intrinsically metallic 2H-TMDs, covering 4 orders of magnitude in Hc2. Using Gor’kov’s theory, we calculate the suppression of Tc by the competing charge density wave (CDW) order, which affects the quasiparticle density of states and the resulting Tc and Hc2. The latter is found to be universally enhanced by two orders of magnitude. Our results substantiate CDW as the key determinant factor limiting Tc across the TMD family.

Documenting weld pool behavior differences in variable-gap laser self-melting and wire-filling welding of titanium alloys

In large-scale welding processes involving complex geometries, variations in gap dimensions and the transition of filler wire form distinctive molten pool flow behaviors, significantly affecting weld formation and quality. This paper investigates the differences in the laser self-melting and wire-filling welding pool behavior of titanium alloy with variable gaps through both simulation and experimentation. An innovative three-dimensional transient heat-flow coupling model is developed to simulate laser welding with variable gap structures, incorporating the dynamics of the welding wire-filling process. The different laser welding modes and the filling process under the variable gap structure were numerically simulated. The results indicate that the maximum flow rate of the laser self-melting pool remains stable in proximity to the keyhole, reaching a maximum of approximately 2.491 m/s, with molten metal entering through the area surrounding the keyhole. As the gap size gradually increases to 0.2 mm during the laser self-fusion welding stage, keyhole instability becomes more pronounced. In transitioning from laser self-fusion to wire-filling welding, the welding wire behavior can be divided into three stages. The molten metal at the end of the welding wire is transferred to the liquid melt pool through a liquid metal bridge. The flow rate of this bridge initially rises before gradually decreasing, reaching a maximum flow rate of 1.606 m/s. Notably, the welding wire lags approximately 0.6 mm behind the laser's focal point, with the narrowest width of the liquid bridge measuring about 0.89 mm. Based on the simulation results and considering the melting transition time of the welding wire, the initiation time for the welding wire has been further adjusted to the first 10 ms when the gap threshold reaches 0.2 mm. Experimental verification confirms the successful laser welding of a 2 mm variable gap structure thin plate. This study elucidates the melt pool flow, keyhole fluctuations, and metal transition behaviors during the laser welding process, contributing to a deeper understanding of the complex heat and mass transfer dynamics inherent in variable gap structure laser self-melting and wire-filling welding. Ultimately, these insights aim to enhance the application of laser welding in adaptive welding for large structural components.

Anisotropic magnetic interactions in a candidate Kitaev spin liquid close to a metal-insulator transition

In the Kitaev honeycomb model, spins coupled by strongly-frustrated anisotropic interactions do not order at low temperature but instead form a quantum spin liquid with spin fractionalisation into Majorana fermions and static fluxes. The realization of such a model in crystalline materials could lead to major breakthroughs in understanding entangled quantum states, however achieving this in practice is a very challenging task. The recently synthesized honeycomb material RuI3 shows no long-range magnetic order down to the lowest probed temperatures and has been theoretically proposed as a quantum spin liquid candidate material on the verge of an insulator to metal transition. Here we report a comprehensive study of the magnetic anisotropy in un-twinned single crystals via torque magnetometry and detect clear signatures of strongly anisotropic and frustrated magnetic interactions. We attribute the development of sawtooth and six-fold torque signal to strongly anisotropic, bond-dependent magnetic interactions by comparing to theoretical calculations. As a function of magnetic field strength at low temperatures, torque shows an unusual non-parabolic dependence suggestive of a proximity to a field-induced transition. Thus, RuI3, without signatures of long-range magnetic order, displays key hallmarks of an exciting candidate for extended Kitaev magnetism with enhanced quantum fluctuations.

Influence of Strong Metal-Support Interaction to the Noble Metal Catalyst for the Activity and Durability Oxygen Reduction Reaction

The stability and the activity of electrocatalysts in fuel cells and other applications can be improved by the strong interaction between noble metal nanoparticles and transition metal oxide, a phenomenon known as “strong metal-support interaction (SMSI).” Understanding and harnessing this interaction could lead to the development of more efficient and durable electrocatalysts. When SMSI occurs, the d-band center of noble metal as the catalyst changes to the favored state for the catalytic reaction or stability. Naturally, the variation of the d-band center depends on the combination of noble metal elements and transition metal oxide species, and the optimum state of the d-band is determined by the reaction. We focused on the oxygen reduction reaction (ORR), which is the cathode reaction of polymer electrolyte fuel cells (PEFCs). Because PEFCs require highly active and durable electrocatalysts.Generally, in catalysts where SMSI occurs, the electron-poor metal oxide covers the electron-rich noble metal particles in the reduction environment. On the other hand, the covered noble metal particles recover by conducting the oxidation treatment. An essential point for this phenomenon is that noble metal particles are smaller enough than metal oxide as the support or substrate. Therefore, the metal oxide layer can cover the noble metal particles. Theoretically, if the oxide layer is a few nanometers thick, it is difficult to cover the noble metal particles because the oxide layer is insufficient around them. In other words, it is expected that the electron structure of catalyst particles may be dramatically changed.The aim of this study was to find the optimum combination between noble metal and oxide for ORR in cathode catalysts in PEFCs. As the first step, Ir nanoparticles were deposited on the TiO2 layer, which was a few nanometers thick, and its ORR activity was evaluated. Electrooxidation by potential scanning was employed as the oxidation method for a model electrode. Moreover, the prepared Ir/TiO2 model electrode was conducted in the thermal reduction treatments, and the XPS analysis was performed to investigate the change in its electron structure. Other combinations, for example, Pt/TiO2, Ir/CeO2, etc., will be also done. Ir/TiO2 model electrode was composed of a glassy carbon substrate, a 1 nm TiO2 middle layer made from titanium oxide nanosheet, and Ir nanoparticles on the top deposited using the arc plasma deposition method. The electrochemical measurements were conducted using a three-electrode cell with a working electrode, a carbon rod counter electrode, an RHE reference electrode, and 0.1 M HClO4 at room temp. The reduction method was calcination at 500ºC under the reduction environment. The XPS results indicated that the TiO2 middle layer suppressed the valence variation of Ir nanoparticles. In this study, it was confirmed that the effects on ORR by introducing the TiO2 middle layer were stabilization of ORR activity in the Ir/TiO2 model electrode and inhabitation of the dissolution of Ir nanoparticles. In the day's presentations, we will compare with other combination model electrodes and discuss the relationship between valence variation and ORR activities of several model electrodes.

Enhancement of Electrocatalytic Oxygen Evolution Activity in Perovskite Oxides Via Fluorination Approach

The electrochemical oxygen evolution reaction (OER) is an important anode reaction for water electrolysis to generate green hydrogen using renewable energy sources. Numerous studies have been conducted on developing OER catalysts composed of transition metal oxides. Among them, perovskite oxides (PVs) are a group of metal oxides composed of alkali-earth metal ions and transition metal ions. It is reported that the OER activity of PVs can be improved by choosing the appropriate metal cations and tuning their composition. Recently, in addition to controlling the composition of metal cations, doping hetero-anions such as fluoride ions (F−) into the PV structure or substituting oxide ions (O2−) with F− in PVs have been employed to enhance their OER activity. Certain amounts of F− substitution (F-substitution) are expected to be a promising approach to tune the valence states of metal cations in PVs due to the difference in valence of F− and O2−. Since the valence state of the metal active center is an important factor determining the OER activity of PVs, F-substitution in PVs have been attempted in previous researches. However, since the synthesis of F-containing PVs was often conducted under high-temperature heat treatment conditions, the content of fluoride ions and enhancement of OER activity were limited. Based on these backgrounds, here we synthesized barium (Ba), iron (Fe), and cobalt (Co) containing PVs in the form of BaFe1–xCoxO3–d (x = 0, 0.1, 0.2) and attempted to enhance their OER activity through broad-range fluorine substitution via low-temperature fluorination.BaFe1–xCoxO3–d (x = 0, 0.1, 0.2) was synthesized by a sol-gel method using metal nitrates as precursors referring to a previous report.[1] F-substitution was performed by heat-treating a mixture of the as-synthesized BaFe1–xCoxO3–d with polyvinylidene fluoride (PVDF) by modifying previous reports.[2,3] The OER activity was evaluated using rotating disk electrodes in O2-saturated 1M KOH.This abstract reports the results for BaFe0.8Co0.2O3-d (x = 0.2, BFCO-20), BaFe0. 9Co0. 1O3-d (x = 0.1, BFCO-10) and their F-substituted samples. The F/Ba ratios of F-substituted samples in this abstract are 1 and 0.8 for BFCO-20 (F-BFCO-20) and BFCO-10 (F-BFCO-10), respectively. Powder X-ray diffraction (XRD) patterns confirmed that single-phase PVs were successfully synthesized. No formation of any impurity due to the F-substitution process was observed. The peaks of BFCO-20 and BFCO-10 were slightly shifted to the lower angle region by the F-substitution process. The Co K-edge and Fe K-edge X-ray absorption near edge structure (XANES) spectra shifted to the lower energy region by F-substitution as shown in Fig. 1a, indicating that the valence was reduced by F-substitution. The cyclic voltammograms for pristine BFCO-20, BFCO-10, and F-substituted samples revealed that F-substitution enhanced the OER activity (Fig. 1b). The F-substitution increased the OER current at 1.65 V vs. RHE about 3.9 times and 2.6 times for BFCO-20 and BFCO-10 (Fig. 1b), respectively.[4] In the presentation, we will also discuss details of the structural analysis and the electrochemical analysis of the synthesized samples.[1] A. el Hadri et al., ACS Catal. 7 (2017) 8653-8663. [2] R. Heap et al., Solid State Commun. 141 (2007) 467–470. [3] O. Clemens et al., J Solid State Chem., 198 (2013) 262–269. [4] K. Iwase et al., Chem. Mater., 35, (2023) 2773–2781. Figure 1

Piezo-Photocatalytic Water Splitting of MoS2@Mo2CTx Heterostructure Catalyst

A heterostructure comprised of two-dimensional transition metal dichalcogenides (2D-TMDs) and transition metal carbide/nitride (MXenes) was fabricated. The substrate for the growth of 2D MoS2 nanoflowers (NFs) was established using the highly conductive metal phase Mo2CTx MXene. The resulting MoS2@Mo2CTx catalysts exhibited a remarkably enhanced piezocatalytic hydrogen (H2) generation rate, reaching 1164.8 μmol･g-1 h-1, being 4.01 times and 3.06 times higher than the individual pristine Mo2CTx and MoS2 counterparts, respectively. Moreover, with the introduction of light irradiation, the piezo-photocatalytic H2 yield rate of MoS2@Mo2CTx reached 2191.1 μmol･g-1 h-1, showing a 1.88-fold increase compared to the piezocatalytic yield rate. The hetero-catalyst demonstrated commendable reusability and long-term stability, yielding an impressive 9039.1 μmol･g-1 of hydrogen gas under simultaneous ultrasonic vibration and light irradiation within an 8-hour timeframe. Finite element method (FEM) simulations were also employed to validate the piezo-voltage output of the ultrasonic-force-triggered hydrogen evolution reaction (HER) systems. Figure 1

Vanadium Oxide Device with Tunable Oxygen Vacancy Concentration for Neuromorphic Computing

Analog, non-volatile memories that can enable in-memory data processing are attractive for exploring non-von-Neumann architectures to realize substantially more efficient computing for AI applications. Among the various device concepts, NVMs based on tuning of point defects via insertion of ions such as alkali metals, protons, or oxygen vacancies are attractive due to the ability to finely tune the electronic conductivity by controlling the defect concentration. Previously, oxygen vacancy concentration in vanadium oxide (VO2-x) has been linked to electronic conductivity and the insulator-to-metal transition temperature. However, while previous studies have mainly concentrated on single crystal epitaxial films of VO2, polycrystalline films deposited by technique compatible with Si back end of the line (BEOL) fabrication scheme are considerably easier to integrate with CMOS circuitry. We employ two distinct approaches for device integration of VO2-x thin films: (1) an electron beam evaporation deposition of bulk vanadium (V) precursor followed by annealing in an oxygen (O2) environment, and (2) reactive sputtering of vanadium oxide followed by annealing in air. Raman spectroscopy reveals the presence of VO2 and V2O5 phases, with differing ratios indicative of device conductance states [Figure 1b].1 Previous research has demonstrated that the characteristics of VO2 insulator to metal transition (IMT) can be adjusted by manipulating oxygen vacancy concentration using an electrolyte.2 However, the presence of the electrolyte interface can introduce noise due to uncontrolled parasitic reactions. To mitigate this issue, our study utilized electron beam irradiation to deliberately induce vacancies in the VO2 without relying on an electrolyte. This controlled alteration of the device state was found to lead to a significant change in noise levels, particularly when the device was in proximity to the IMT transition temperature.The analysis of our films provides detailed structural and compositional dynamics of VO2-x and V2O5 phases which exhibit analog synaptic and threshold tunability with remarkable state retention2 [Figure 1cd]. This abstract provides a comparative analysis of these approaches, focusing on their respective advantages and disadvantages in the context of vanadium oxide based analog device fabrication.

A FeCo-Se@NiCo-PO4 Electrode Designed by Hierarchical Strategy for Supercapacitors and NiCo//Bi Batteries.

In this work, FeCo-Se and NiCo-PO4 were electrodeposited on nickel foam (NF) successively to prepare a cathode material for asymmetric supercapacitors (ASCs) and NiCo//Bi batteries. FeCo-Se@NiCo-PO4 combines the advantages of transition metal selenides (TMSs) and transition metal phosphates (TMPs). FeCo-Se electrodeposited in the underlying layer can facilitate electron transfer for higher conductivity. NiCo-PO4 in the outer layer can facilitate OH- ions diffusion because TMPs can be intercalated into ions readily and the outer robust P-O bond of TMPs can stabilize the structure. Precisely because the hierarchical structure maximizes the synergy between FeCo-Se and NiCo-PO4, FeCo-Se@NiCo-PO4 delivers a rapid electron/ion transfer capability and superior electrochemical performance. The FeCo-Se@NiCo-PO4 exhibits a high specific capacitance of 2221.5 F g-1 (888.6 C g-1) at 1 A g-1. Its aqueous ASC shows specific capacitance of 115.8 F g-1 at 1 A g-1 and all-solid-state ASC presents high reversibility. Its aqueous NiCo//Bi battery has superior durability of about 60% capacity retention and 98% Coulombic efficiency after 2300 cycles. And its all-solid-state NiCo//Bi battery possesses a higher energy density and power density.