3d Transition Metal Research Articles

Excitonic-Vibrational Interaction at 2D Material/Organic Molecule Interfaces Studied by Time-Resolved Sum Frequency Generation.

The hybrid heterostructures formed between two-dimensional (2D) materials and organic molecules have gained great interest for their potential applications in advanced photonic and optoelectronic devices, such as solar cells and biosensors. Characterizing the interfacial structure and dynamic properties at the molecular level is essential for realizing such applications. Here, we report a time-resolved sum-frequency generation (TR-SFG) approach to investigate the hybrid structure of polymethyl methacrylate (PMMA) molecules and 2D transition metal dichalcogenides (TMDCs). By utilizing both infrared and visible light, TR-SFG can provide surface-specific information about both molecular vibrations and electronic transitions simultaneously. Our setup employed a Bragg grating for generating both a narrowband probe and an ultrafast pump pulse, along with a synchronized beam chopper and Galvo mirror combination for real-time spectral normalization, which can be readily incorporated into standard SFG setups. Applying this technique to the TMDC/PMMA interfaces yielded structural information regarding PMMA side chains and dynamic responses of both PMMA vibrational modes and TMDC excitonic transitions. We further observed a prominent enhancement effect of the PMMA vibrational SF amplitude for about 10 times upon the resonance with TMDC excitonic transition. These findings lay a foundation for further investigation into interactions at the 2D material/organic molecule interfaces.

Geometrical and Electronic Structures of Red Phosphors Doped with 3d Transition Metals

Phosphors have become increasingly important in our lives with the spread of white LEDs in recent years. The most basic structure of a white LED is a combination of a blue LED and a yellow phosphor, but this combination results in a white color with a high color temperature that leans toward bluish-white. Therefore, red and green phosphors are demanding in order to adjust the color temperature and improve color rendering property. In addition to the demands of high efficiency and long-term stability for such phosphors, it is also important that the raw materials are low-priced. Many red phosphors achieve red fluorescence by adding a few percent or less concentration of rare earth elements. However, their supply may become unstable, since rare earth elements are unevenly distributed in certain regions of the earth. Therefore, it is requested that the phosphors without rare-earth elements. Recently we have reported stable rare-earth free res phosphors using 3d transition metals with 3d3 configuration, such as Cr3+ and Mn4+, to replace rare-earth elements [1-3]. In the current study, geometrical and electronic structures of phosphor materials doped with Cr3+ and Mn4+ are demonstrated both experimentally and theoretically. Especially, we focus upon the local environment of doped 3d transition element, which are studied in detail by X-ray absorption near edge structure (XANES) and electron spin resonance (ESR) analysis with the aid of the first-principles calculations.

Electrodeposition of Re-Mo Alloy

Rhenium is a 5d transition metal in the VIIB group well-known for its high density, high melting temperature, and wide variety of oxidation states1. It has been used in applications such as high temperature machineries, aeronautics, as well as catalysis. It is also a type I superconductor of interest for cryogenic electronic applications. On the other hand, molybdenum is a 4d group VIB transition metal also known for its exceptional mechanical properties, including strength, resilience, and high-temperature resistance, rendering Mo a fundamental alloying element in aerospace and heating equipment. In addition, Mo exhibits a high melting point and superior electrical conductivity in small dimensions due to a short mean free path, and self-limiting formation of oxide, making it potentially useful in the fabrication of electrical contacts and interconnects2, 3,4. The incorporation of Re into Mo significantly lowers the plastic-brittle transition temperature. Furthermore, it raises the recrystallization temperature of Mo and enhances the high-temperature performance5. More importantly an enhanced Tc, the superconducting transition temperature, has been reported for ReMo alloys and makes this alloy of more interest for cryogenic electronics6. Although ReMo alloys have been successfully prepared using vacuum deposition methods such as magnetron sputtering7, to the best of our knowledge there have not been any reports on the electrodeposition of such alloys. In our previous work, electrodeposition of metallic Mo using concentrated acetate solutions has been systematically studied8. The present work continues from previous study, reports a new electrodeposition route for ReMo alloys, and presents a systematic study on the chemistry and process.ReMo electrodeposition is carried out on rotating Cu disk electrodes. The electrolytes contain ammonia perrhenate (NH4ReO4), sodium molybdate (Na2MoO4), citric acid, and highly concentrated acetates such as lithium acetate or ammonia acetate. The effects of the concentrations of metal precursors (ReO4 -, MoO4 2-), citric acid, and acetates, as well as the deposition potential and time are systematically studied. Alloy film composition and thickness are measured using x-ray fluorescence spectroscopy. The faradic efficiency and the partial current densities of Re and Mo are calculated and compared. The surface morphology of films is characterized by SEM.Figure 1 shows the partial current densities of Re and Mo at various citric acidconcentrations. It is worth noting that Mo and Re salts cannot be co-dissolved into a clear solution at the concentrations used without the addition of citric acid. Thus, a minimum concentration of citric acid of 0.1 M is used in this study. As shown in Figure 1, the Re deposition rate increases with citric acid concentration when the concentration is low, reaching a maximum partial current density at 0.2 M before it decreases gradually. A similar trend is also observed for the Mo deposition rate. However, the partial current density of Mo drops quickly to zero when the concentration of citric acid reaches 0.7 M. This suggests that Mo deposition is completely inhibited in highly concentrated citric acid solutions. On the other hand, the acetic solutions consisting of ammonium acetate and lithium acetate is however found necessary for Mo deposition. Details will be further discussed in the poster presentation.1 Woolf, A. A., "An outline of rhenium chemistry," Quarterly Reviews, Chemical Society 15, 372 (1961).2 Gardner, D. S., Onuki, J., Kudoo, K., Misawa, Y. & Vu, Q. T., "Encapsulated copper interconnection devices using sidewall barriers," Thin Solid Films 262, 104-119 (1995).3 Gall, D., "Electron mean free path in elemental metals," Journal of Applied Physics 119, 085101 (2016).4 Founta, V., Soulié, J.-P., Sankaran, K., Vanstreels, K., Opsomer, K., Morin, P., Lagrain, P., Franquet, A., Vanhaeren, D., Conard, T., Meersschaut, J., Detavernier, C., Van de Vondel, J., De Wolf, I., Pourtois, G., Tőkei, Z., Swerts, J. & Adelmann, C., "Properties of ultrathin molybdenum films for interconnect applications," Materialia 24, 101511 (2022).5 Leonhardt, T., Carlén, J.-C., Buck, M., Brinkman, C. R., Ren, W. & Stevens, C. O., "Investigation of mechanical properties and microstructure of various molybdenum-rhenium alloys," AIP Conference Proceedings 458, 685-690 (1999).6 Singh, V., Schneider, B. H., Bosman, S. J., Merkx, E. P. J. & Steele, G. A., "Molybdenum-rhenium alloy based high-Q superconducting microwave resonators," Applied Physics Letters 105 (2014).7 Andreone, A., Barone, A., Dichiara, A., Mascolo, G., Palmieri, V., Peluso, G. & Diuccio, U. S., "MO-RE SUPERCONDUCTING THIN-FILMS BY SINGLE TARGET MAGNETRON SPUTTERING," Ieee Transactions on Magnetics 25, 1972-1975 (1989).8 Liu, Q. & Huang, Q.," Electrodeposition of molybdenum from water-in-acetate electrolytes", manuscript submitted for publication Figure 1

Synthesis of Multicomponent Pt Alloy Catalysts for Oxygen Reduction Reaction and Durability Evaluation

Introduction In order to popularize PEFCs, it is important to increase the oxygen reduction reaction (ORR) activity and reduce the amount of expensive Pt catalyst used. It is well known that the Pt-based alloy catalysts show high ORR activity, but the problem is that the 3d transition metals are oxidatively dissolved in the PEFC cathode environment, deteriorating the ORR activity. Studies using single crystal model electrodes have reported that a high-entropy alloy (HEA) catalyst (PtFeCoNiCu) exhibits higher ORR activity and durability compared to a PtCo binary alloy catalyst [1]. Chida et al. demonstrated that the coexistence of Mn and Cr in a model PtCoNiMnCrFe catalyst suppresses the thermal diffusion of the constituent elements [2]. Furthermore, it has been reported that the addition of melamine suppresses the dissolution of 3d transition metals and improves the durability of PtCo alloy catalyst [3]. In this study, we synthesized PtCo, PtCoMn and PtCoMnCr alloy catalysts and investigated the effects of Mn and Cr additions on ORR activity and durability, along with the effect of melamine addition. Experimental 500 mg mesoporous carbon (MPC) support (CNovel MH-18, primary particle size: 800 nm, central mesopore diameter: 4 nm, SBET: 1345 m2/g, TOYO TANSO) and Pt(NO2)2(NH3)2 equivalent to 500 mg of metallic Pt were added to a EtOH/H2O mixed solvent (37.5 mL/250 mL), and refluxed at 90℃ under N2 gas atmosphere for 4 h to synthesize Pt/MPC catalyst (Pt particle size: 2.5 nm, Pt loading: 50 wt.%). The obtained Pt/MPC catalyst (600 mg) was dispersed in 200 mL DI water and metal nitrates of Co, Mn and Cr were added in the following molar ratios; (i) Pt50Co50, (ii) Pt50Co37.5Mn12.5 and (iii) Pt50Co25Mn12.5Cr12.5. An aqueous NaBH4 solution (30 mL) corresponding to 100 times the equivalent of the metal nitrates was added dropwise to this solution over 2 h while stirring at 300 rpm at 30° C. The solution was filtered, dried at 60℃ in air, and the dried powder was heat treated at 800℃ for 2 h under 15% H2-Ar atmosphere to obtain PtCo/MPC, PtCoMn/MPC and PtCoMnCr/MPC alloy catalysts. Characterizations of the alloy catalysts were performed using XRD, XRF, TG-DTA and TEM. The initial CV and LSV of the catalysts were recorded in Ar/O2 saturated 0.1 M HClO4 at 25°C. The accelerated durability test (ADT) of the catalysts was conducted by using a square wave potential cycling of 0.6 V (3 s)-0.95 V (3 s) vs. RHE performed in Ar-saturated 0.1 M HClO4 at 80°C for 10 k cycles with and without addition of 10 μM of melamine to the electrolyte. Results and Discussion Figure 1 shows cross-sectional TEM images of the three Pt alloy/MPC catalysts thermally reduced at 800℃. The Pt alloy catalyst NPs were uniformly deposited inside the MPC support with a particle size of approximately 4.5 nm. In the outer part of the MPC support, large particles were observed due to a lack of cage effects of mesopores. The XRD analysis in Fig. 2 indicated that the Pt alloy catalysts partially transformed to the L10 ordered phase by the heat treatment at 800℃. LSV measurements showed that ORR mass activity (MA) at 0.9 V vs. RHE of the PtCo/MPC binary alloy catalyst increased from 1,430 to 1,700 A/g by the addition of Mn. Figure 3 summarizes MA of the three types of Pt alloy catalysts before and after ADT performed at 80°C for 10 k cycles. MA of all the Pt alloy catalysts decreased to ca 500 A/g after ADT. Figure 4 shows LSVs before and after ADT of PtCo/MPC and PtCoMnCr/MPC alloy catalysts with and without the addition of melamine. PtCo/MPC and PtCoMnCr/MPC alloy catalysts retained high MA of 1050 and 800 A/g, respectively, even after ADT due to the addition of melamine, and there was little difference in MA between the two alloy catalysts. From these results, it is concluded that the addition of Mn to the PtCo/MPC alloy catalyst improved the ORR activity, but the addition of Cr did not improve the durability of the PtCoMn/MPC alloy catalyst and that the addition of melamine significantly improved the durability of the Pt alloy catalysts. Further studies are required to investigate the effect of Mn and Cr addition on the durability of PtCo/MPC catalysts when the particle size increases (>5 nm). Reference [1] T. Chen et al., Science, 26, 105890 (2023).[2] Y. Chida et al., Nat. Commun., 14, 4492 (2023).[3] H. Daimon et al., ACS Catal., 12, 8976 (2022). Figure 1

Influence of Dopant Atoms in TiO2 for Its Electron Structure and Activity of Oxygen Reduction Reaction in Acidic Environmental

Polymer electrolyte fuel cells (PEFCs) are attracting attention, but their widespread commercialization has various challenges. In particle, the Pt-based cathode catalyst has two significant problems: durability and cost. These problems will persist as long as the Pt-based catalyst is used in PEFCs. Therefore, we focused on TiO2 as the alternative Pt-based catalyst, which is highly durable and low-cost. However, TiO2 has low electron conductivity and must obtain the electron conduction path to progress the oxygen reduction reaction (ORR). In general, doping transition metal and/or introducing the oxygen vacancy into the crystal structure is conducted to enhance the electron conductivity of TiO2. It was reported that Nb-doped TiOx having oxygen vacancies showed relatively higher ORR activity. Experimentally, it is known that other metal doping and oxygen vacancies bring in high ORR activity. However, the detailed enhancement mechanism for ORR activity is still unclear.Some research groups were using theoretical calculations to investigate the mechanism of ORR, such as the reaction path and active site on the TiO2 surface [1, 2]. These reports suggest one of the potentials for the ORR mechanism but don’t completely describe the ORR on the TiO2-based catalyst because it is hard to compute the state of a realistic, accurate ORR system. Therefore, we planned the combination of experiment data and theoretical calculations, and the balance of experimental data was heavier.In this study, the 3d transition metal atom was doped to TiO2 and conducted for the reduction treatment for doped TiO2 to introduce oxygen vacancies. Their ORR activities and electron structure were evaluated experimentally. In addition, the electron structures of all samples were computed using DFT calculation and compared with experimental data.5 at% of transition metal-doped TiO2 and TiO2 were synthesized by sol-gel method. The reduction treatment in this study consisted of calcining at 800°C for 10 min under the H2+Ar mixed gas flow conditions. The electrochemical measurements used a three-electrode cell with a working electrode, a carbon plate counter electrode, an Ag/AgCl reference electrode, and 0.1 M HClO4(60°C). The potential in this study was converted from versus Ag/AgCl to versus RHE. The ORR current was obtained by subtracting currents under inert gas flow conditions from currents under O2 flow conditions. The ORR activity was judged by the potential at 5 mA g-1 of ORR current. DFT calculation was performed with Quantum ESPRESSO.All the synthesized samples had the anatase phase structure, but crystallinity was low since they formed nanoparticles or might not react with precursors completely. XPS results suggested that the dopant atom was probably introduced into the TiO2 crystal structure. The ORR activity of TiO2 was improved by doping the 3d transition metal atoms.The density of states (DOS) of modeled our doped TiO2 samples showed a new intermediate band in the forbidden band of TiO2. Now, we expect that the location, amount, density, and other circumstances of this new intermediate band may be related to ORR activity.[1] F. Zhang, et al. Applied Surface Science, 598 (2022) 153873.[2] Y. Yamamoto, et al. J.phys.Chem.C, 123 (2019) 19486-19492.

Fluoride-Free Sonochemical Exfoliation of Titanium Aluminium Carbide to Few-Layered Titanium Carbide Nanosheets

Two-dimensional (2D) transition metal carbides and carbonitrides, also known as MXenes, possess a unique blend of hydrophilic surfaces and excellent electrical conductivity,[1] characteristics seldom found in other 2D nanostructures like graphene and transition metal dichalcogenides. Particularly, titanium carbide (Ti3C2Tx) and its composites have garnered increasing attention across diversified fields such as catalysis,[2] energy conversion and storage,[3] membrane separation,[4] photothermal conversion,[5] and field-effect transistors.[6] Thus far, chemical etching of aluminium (Al) layers from titanium aluminium carbide (Ti3AlC2) remains the predominant method to synthesize Ti3C2Tx.[7] However, fluoride-based synthetic procedures remain a hindrance to the practical applications of these promising materials. Despite the identification of various etching conditions, current state-of-the-art strategies involve handling hazardous hydrofluoric acid or fluoride-based compounds such as LiF/HCl, NaHF2, KHF2 and NH4HF2, resulting in highly toxic liquid waste and surface functionalization of Ti3C2Tx with fluorine and oxygen-containing terminations.[8] Achieving selective removal of Al layers while preserving the 2D structure of Ti3C2Tx necessitates careful design of electrolytes.Hence, in this work, we present an efficient, fluoride-free sonochemical approach for the etching and delamination of Ti3AlC2 using aqueous tetramethylammonium hydroxide (TMAOH) solution. During the sonochemical exfoliation, ultrasound waves facilitate penetration of solvent ions through Ti3AlC2 to selectively etch out Al, forming few-layered Ti3C2Tx nanosheets. In this particular study, the effect of sonication time and effect of TMAOH concentration is explored.

Ultrathin Gallium Oxide As a Gate Dielectric and Passivation Layer for Two-Dimensional Devices with Conductive Filament Contacts

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are generally regarded as the channel material for post-silicon nanoelectronics owing to their distinctive crystal structure consisting of atomically thin dangling-bond-free layers, tunable bandgap and high carrier mobility. However, 2D devices based on TMDs typically suffer from performance degradation due to unwanted atmospheric reactions or adsorbates and require surface passivation to ensure that device performance is maintained. Recently, ultrathin metal oxides obtained through liquid metal printing or squeezing technique have emerged as a new class of 2D material with nanometer-scale thickness and large-area uniformity. Among them, ultrathin amorphous gallium oxide (GaOX) has garnered significant attention for its ease of fabrication owing to the low melting point of gallium (29.76 °C), large bandgap, and unique insulating and passivating properties. The ultrathin GaOX has been utilized for large-area passivation of 2D materials and as a dielectric for wide bandgap semiconductor devices. However, the integration of the ultrathin GaOX with 2D devices based on TMDs has rarely been investigated. In this study, we fabricated 2D devices based on multilayer n-type tungsten disulfide (WS2), wholly enveloped by the ultrathin GaOX. The ultrathin GaOX served as a dielectric passivation layer in the gate and channel region, whereas conductive filaments (CFs) were created in the ultrathin GaOX in the contact region through electroforming to form electrical contacts to the passivated WS2.The WS2 multilayers for the channel were mechanically exfoliated from its bulk crystal and were aligned to and dry-transferred onto pre-patterned Ti/Pt bottom electrodes on Si/SiO2 substrates. Liquid gallium droplets were placed on prepared polydimethylsiloxane (PDMS)/polypropylene carbonate (PPC) films and were firmly compressed using a PDMS film supported on a glass slide to produce ultrathin GaOX monolayers. The fabricated ultrathin GaOX monolayers were dry-transferred onto the prepared WS2 channels by utilizing the PPC film as a sacrificial layer. The ultrathin GaOX monolayers were slowly lowered to fully contact the prepared substrates, after which the substrates were heated to 90 °C to soften and release the PPC films from the PDMS films. The PPC films held down the ultrathin GaOX monolayers on the substrates during the detachment of the PDMS films and were removed later by immersion in acetone. This process was repeated twice to form the ultrathin GaOX bilayers on the WS2 channels. Ti/Au drain and source top electrodes were formed above the Ti/Pt bottom electrodes before Ni/Au top-gate electrodes were formed to finalize the WS2/GaOX field-effect transistors (FETs). CF contacts to the drain and source region of the passivated WS2 channels were created via electroforming, where positive and negative bias voltages were applied to the top electrodes with the corresponding bottom electrodes grounded.The ultrathin GaOX monolayer (~4.7 nm) and bilayer (~10.1 nm) fabricated in this work were determined to be highly uniform with root-mean-square roughness of 0.45–0.57 nm using atomic force microscopy. The ultrathin GaOX bilayer was evaluated via X-ray photoelectron spectroscopy to have an oxygen-deficient chemical composition with a sub-stoichiometric Ga:O ratio of 42:58 (Ga2O2.8). From the capacitance–voltage and current–voltage analyses of Pt/GaOX/graphene metal–insulator–metal devices, dielectric constant of 3.1 and dielectric breakdown voltage of +8 V were extracted for the ultrathin GaOX bilayer, respectively. The electroforming process of the ultrathin GaOX bilayer in the contact region of the WS2/GaOX FETs was irreversible, only showing decrease in the resistance during consecutive sweeps regardless of the polarity of the applied bias voltages. After the CF formation in ultrathin GaOX bilayers in the contact region, the electrical properties of WS2/GaOX FETs were investigated. The devices exhibited excellent top-gated performance with low subthreshold swing of 68.0–84.6 mV/dec and minimal hysteresis of 0.10–0.12 V. Furthermore, the CF contacts and the devices exhibited great electrical stability even after 50 days. Also, dual-gate logic operations of AND gate and OR gate were demonstrated using both the top and back gates of the WS2/GaOX FETs. This study demonstrates the implementation of the ultrathin GaOX as a gate dielectric, a passivation layer, and a CF contact layer in a single 2D device structure. The WS2/GaOX FETs exhibited outstanding electrical characteristics and stability, indicating the successful implementation and the versatility of the ultrathin GaOX. These results indicate the potential of the ultrathin GaOX as a key component in future 2D electronics.

(Invited) Atomic Layer Deposition of “Group 16 Compounds” for Emerging Applications

The exclusive benefits of atomic layer deposition (ALD), including excellent conformality over nanoscale complex structures, high quality of deposited films even at low temperature down to room temperature, atomic scale thickness controllability, and high uniformity over nanoscale structure, make it viable tool for many emerging applications. Among varous materials which can be prepared by ALD, Group 16 compounds, including oxides and chalcogenides, are most widely studied materials group, which are very important for nanoscale device fabrications and other emrging applciations. In this presentation, I will summarize the process and applciaitons of emerging oxides and chalgogenides. Especially, focus will be given to two dimensional (2D) transition metal dischalcogenides (TMDCs) nanosheets, since they have exotic electronic and optical properties: Indirect-to-direct bandgap transition depending on the number of layers, high carrier mobility and strong spin-orbit coupling due to their broken inversion symmetry. In contrast to conventional chemical vapor deposition (CVD), ALD makes it a promising synthesis method for 2D TMDCs. I will present the synthesis of various 2D TMDCs nanosheets including MoS2, WS2, WSe2 and their alloys such as Mo1-xWxS2 based on ALD process. Also, various appliations of these materials will be presented such as gas sensors and catalysis for hydrogen evolution rate. In addition, ALD for other related materials including other 3D chalcogenides such as GeS, RuS2 and carbon. Basic growth charateristics and applilcations for ovonic threshold switwching (OTS), Lidar and next generation patterning tehnology will be presented.

Exclusive Generation of Single-Atom Sulfur for Ultrahigh Quality Monolayer MoS2 Growth.

Preparation of high-quality two-dimensional (2D) transition metal dichalcogenides (TMDCs) is the precondition for realizing their applications. However, the synthesized 2D TMDCs (e.g., MoS2) crystals suffer from low quality due to the massive defects formed during the growth. Here, we report single-atom sulfur (S1) as a highly reactive sulfur species to grow ultrahigh-quality monolayer MoS2. Derived from battery waste, sulfurized polyacrylonitrile (SPAN) is found to be exclusive and efficient in releasing S1. The monolayer MoS2 prepared by SPAN exhibits an ultralow defect density of ∼7 × 1012 cm-2 and the narrowest photoluminescence (PL) emission peak with full-width at half-maximum of ∼47.11 meV at room temperature. Moreover, the statistical resonance Raman and low-temperature PL results further verify the significantly lower defect density and higher optical quality of SPAN-grown MoS2 than those of the conventional S-powder-grown samples. This work provides an effective approach for preparing ultrahigh-quality 2D single crystals, facilitating their industrial applications.

Computational Discovery of Optimal Dopants for Electrode to Enhance Sea Water Splitting

A strategic approach has been proposed for designing robust, high-performing oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) catalysts tailored for saline/sea water splitting. By employing a density functional theory (DFT)-based computational screening process, a set of promising dopants were identified from a range of twenty-six 3d to 5d transition metals, with the aim of enhancing the activity and saline water resilience of the catalysts. The screening methodology was threefold, encompassing evaluations of OER / HER energetics, chlorine evolution reaction (ClER) energetics, and chloride-corrosion energetics. The screening led to the promising dopants, which substantially elevated the performance of each of the host catalysts. This study contributes to the understanding of alkaline ClER and chloride-corrosion mechanisms, providing insights into catalyst behavior in saline/sea conditions.

In Situ Synchrotron Based Soft XAS and Resonant Inelastic X-Ray Scattering (RIXS) Characterizations for ε-Cu0.9V2O5 Oscillator

As traditional semiconductor technology nears its limits in terms of energy efficiency and capacity, the imperative to explore new avenues in semiconductor design grows ever more pressing. One promising direction is neuromorphic computing, which holds the potential to provide real-time learning and embedded intelligence capabilities. Among the materials showing promise in this field, vanadium dioxide (VO2) received extensive research interest due to its unique properties, which undergo a reversible transition from a metallic to a semiconductor state at 340 K. The low-entropy resistive-switching behavior, which involves multiple internal states of vanadium, makes VO2 an ideal candidate for neuromorphic applications, poised to drive advancements in next-generation real-time AI. Studies have explored the effects of composition (doping), electric field, temperature, and pressure on the material properties of VO2, including its electronic device performance. However, despite these investigations, the underlying mechanism governing resistance switching for a neuromorphic response and the associated material properties remain ambiguous.Synchrotron X-ray absorption spectroscopy (XAS) and resonance inelastic X-ray scattering (RIXS) techniques are powerful tools for investigating the element-specific electronic structure of the materials. The techniques within the soft X-ray regime could provide atomic-level insights into transition metal oxides based on core-level excitations of oxygen K-edge and 3d transition metals L-edges. RIXS reveals electron configuration, especially d-orbital configuration and band gap that majorly contribute to the electric conductivity of neuromorphic transition metal oxides.Here, temperature/electric field dependent in situ sXAS and RIXS, with a homemade sample holder and devices designed to apply bias (electric field), are employed and demonstrated on the single crystal ε-Cu0.9V2O5 oscillator to provide fundamental insight on metal-insulator transition behaviors. This integrated approach to characterization holds considerable promise in advancing our comprehension of neuromorphic materials and establishing connections between target neuronal behavior and material properties.

Active Sites of 3d Transition Metal Oxyhydroxides for OER

3d transitional metal oxyhydroxides, such as Ni and Co oxyhydroxides with Fe doping, are among the most active and most studied OER catalysts in alkaline electrolytes. They are also the catalysts of choice for industrial alkaline water electrolysis. Despite decades of research, the atomic-scale structures of these materials, particularly key Ni (oxy)hydroxide hosts, remain elusive. This presentation aims to fill this critical knowledge gap by combining rigorous Density Functional Theory calculations with in situ Wide Angle X-ray Scattering techniques under quasi-steady state electrochemical cycling conditions. This unique combination has enabled us to unveil the in-situ structures and (ir)reversible transformations of pure undoped Ni (oxy)hydroxides. Such an advance consequently allows us to transcend the limitations of previous modeling based on approximated structures. Specifically, based on the newly identified actual in-situ structures, we discover the existence of a previously unknown synergy at the reaction centers, coupled with the polaron effects, in promoting the oxygen evolution reaction. Such a finding sheds light on the catalytic active sites and oxygen evolution reaction pathways, particularly the exceptional activity of multiple Fe reaction centers on Ni oxyhydroxides. In addition, we found that such a phenomenon is general and also exists on NiM and CoM layered double hydroxides,1, 2 A2B2O7 pyrochlores,3 layered Co oxyhydroxides intercalated with phenanthroline through non-covalent ligand-oxide interaction.4

Oxygen Electrocatalysis of Perovskite Oxyfluorides in Alkaline Media

Introduction Fossil fuels are indispensable as the main energy source in modern society, but there are concerns about the environmental problems and depletion caused by increased use of them. To realize a sustainable society, high-performance energy conversion devices are required, and various secondary batteries are being developed. Among these, metal-air secondary batteries, which use atmospheric oxygen as the active material, are attracting a lot of attention. Their theoretical energy density are approximately 3 to 30 times higher than that of lithium-ion batteries.[1] At the air electrode of this battery, oxygen evolution reaction (OER) occurs during charging and oxygen reduction reaction (ORR) occurs during discharging, but the overpotentials of these reactions are high, which is an issue for practical application that needs high current density. Therefore, cost effective bifunctional catalysts that can promote both reactions are being extensively researched.Perovskites have high catalytic activity in alkaline solutions and are inexpensive compared to noble metal catalysts such as Pt and IrO2.[2] It has been reported that Ruddlesden-Popper-type layered perovskite oxychlorides showed high OER and ORR activities,[3] and this study focused on fluorine, which has high electronegativity. The purpose is to clarify the effects of partial fluorination of SrFeO3 -δ and SrCo0.8Fe0.2O3 -δ containing Fe, which is one of the 3d transition metal elements reported to have OER and ORR activity.[4] Experimental SrFeO3 -δ and SrCo0.8Fe0.2O3 -δ were synthesized by the sol-gel method.[5] Fluorination was performed by filling a fluororesin tube with the synthesized catalyst powder and fluorine gas at various pressures, and allowing it to stand overnight at room temperature or 100 °C. As for the characterization of the obtained catalyst, X-ray diffraction measurements were used to analyze the crystal structure, and a fluorine ion meter was used for the solution in which the catalyst powder was dissolved to quantify the amount of F- introduced.Catalytic activity was evaluated by using cyclic voltammetry (CV) measurements. The catalyst ink was prepared by dispersing the catalyst, acetylene black, and a cation-conducting ionomer (Nafion) in tetrahydrofuran. This catalyst ink coated on a glassy carbon rotating disk electrode was used as a working electrode. A platinum wire was used as the counter electrode, a reversible hydrogen electrode (RHE) was used as the reference electrode, and an oxygen-saturated 1 mol dm- 3 potassium hydroxide aqueous solution was used as the electrolyte. For OER activity measurement, the rotational speed of the working electrode was 1600 rpm, the scanning speed was 10 mV s- 1, and the scanning range was set from 1.1 V to 1.6 V vs. RHE, and measurements were performed for 50 cycles. For ORR activity measurement, the rotational speed of the working electrode was 1600, 1200, 800, and 400 rpm, the scanning speed was 10 mV s- 1, and the scanning range was set from 0.4 V to 1.0 V vs. RHE, and measurements were performed for 1 cycle at each rotational speed. Results As a result of X-ray diffraction measurements, it was confirmed that SrFeO3 -d and SrCo0.8Fe0.2O3 -d were synthesized before the reaction with F2. After the reaction, the peak positions were shifted, which confirmed that F- was doped, and it became clear that SrF2 was produced as an impurity.Figure 1 shows the results of CV measurements normalized by the electrochemically active surface area (ECSA) obtained in the potential range of 1.1 – 1.2 V vs. RHE (non-Faradaic reaction region) or electrode surface area. Regarding OER, the activity and cycling stability of SrFeO3 -δ decreased due to fluorination. Although the surface area of SrCo0.8Fe0.2O3 -δ increased due to fluorination, the activity normalized by ECSA decreased. Regarding ORR, it was confirmed that fluorination increased the electron transfer number and improved the ORR onset potential for all catalysts. Although the details are not clear so far, it is assumed that fluorination changes the valence of transition metal, local structure, and amount of oxygen vacancies, which may change the catalytic activity. Conclusion As a result of evaluating the catalytic activity of SrFeO3 -δ and SrCo0.8Fe0.2O3 -δ in which F- was doped, it was revealed that the OER activity decreased, but the ORR activity was improved.

(Keynote) Electrocatalysts Design for Carbon Dioxide Reduction and Valorization

There are several materials design strategies that have been utilized with various levels of success for CO2 reduction reaction (CO2RR) to C1, C2 and occasionally, C3 products (Ci denotes the number of carbon atoms in the product of CO2RR). Among those Cu based materials take leading role as this class of catalysts is relatively simple to synthesize and implement.1 Adding co-catalyst in the composite materials allow for regulating of selectivity towards a specific product while sustaining some simplicity of the synthesis and deployment strategies.2 Alternatively, CO2RR can be carried on carbon-based electrocatalysts with atomically dispersed transition metals, stabilized by nitrogen (known as M-N-C). We synthesized a library of nitrogen-doped carbonaceous materials with atomically dispersed 3d transition metals and corresponding metal-free electrocatalysts. The sacrificial support method (SSM) was used yielding catalyst materials of high dispersity and high graphitic content. The resulting electrocatalysts were impurity free, hence allowing a better understanding of the mechanism of CO2 reduction. By combining the electrochemical results with density functional theory, we were able to separate the electrocatalysts into several categories, based on their CO2 → COOHads free energy and their COads binding strength.3 The ‘strong-CO binder’ electrocatalysts (e.g. Cr, Mn and Fe – N – C) achieved a Faradaic efficiency up to 50% at – 0.35 V vs. RHE (at pH = 7.5, in 0.1 M phosphate buffer). Such Faradaic efficiency was also achieved for a metal-free electrocatalyst, therefore showing the high activity of the metal-free, N-containing, moieties toward the CO2 reduction reaction. A separate study showed materials hydrophobicity as one of the major factors of metal-free catalysts selectivity.4 Among the many practical products of CO2RR syngas (an H2/CO mixture) attracts special attention. Appropriate electrocatalysts, such as the metal–nitrogen–carbon (M-N-C) materials, allow for the production of CO alongside H2(from the hydrogen evolution reaction), and thus leads to syngas generation.5 Selectivity of mono- and bi-metallic (M-N-C, M = Fe, Mo or FeMo) electrocatalysts towards syngas production have been extensively studied.6 The ratio of the CO:H2 in the syngas was tuned by modifying the ratio of metallic precursors in the bi-metallic FeMo-N-C catalysts, tailoring the catalysts’ selectivity towards the CO2RR or the hydrogen evolution reaction (HER).Further development of CO2RR towards valorization of its products may lay through its integration with bioprocesses.7 Success of that strategy will rely on the ability of such systems to result in highly selective synthesis of formate, acetate, or propionate as major products.8,9 Those examples show the path to designer catalyst materials for a desired scalable CO2RR-based electrosynthesis technology. References S. Ozden et al., Journal of Catalysis 404 (2021) 512-517M. Ferry et al., ACS Energy Letters, 7 (2022) 2304-2310T. Asset et al., ACS Catalysis, 9 (2019) 7668-7678D. Hursán et al., Joule, 3 (2019) 1719-1733L. Delafontaine et al., ChemSusChem, 13 (2020) 1688-1698L. Delafontaine et. al, ChemElectroChem, 9 (2022) e202200647S. Guo et. al, ACS Catalysis, 11 (2021) 5172-5188S. Guo et. al, Applied Catalysis B – Environmental, 316 (2022) 121659S. Guo et. al, ACS Energy Lett., 8 (2023) 935-942

Lithium Battery Cathode Materials, Li-Rich Li-Fe-M-O, Based on Tetrahedrally-Coordinated Structure

The development of high-energy-density batteries with lithium (de-)intercalation reaction is one of the most promising solutions to realize electric vehicles and power storage system applications with long-term stability. The cathode materials exhibit smaller capacity than the anodes, which limits the energy density of the battery cells [1]. Consequently, the development of new cathode materials with higher capacity than previously reported is required. In the lithium 3d transition metal system, Li-rich materials based on tetrahedrally-coordinated structure, ex. Li5FeO4 and Li6CoO4, exhibited a reversible two lithium (de-)intercalation reaction over 350 mAh/g with mainly anionic redox [2,3]. The capacity was much higher than the practical cathode materials. Therefore, these Li-rich transition metal oxides with tetrahedrally coordinated structure have potential as high capacity cathode materials. In this study, Li-rich cathode materials based on tetrahedrally-coordinated structure were synthesized, and electrochemical properties in the lithium battery cell were characterized by electrochemical and structural investigations.One of the Li-rich transition metal oxides; Li5+x Fe1-x Mn x O4 was synthesized by solid state reaction [4]. Phase identification by X-ray diffraction (XRD) measurement was conducted using a diffractometer with Cu Kα radiation. Observation of particle morphology and elemental distribution was performed using scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDX). The prepared active material samples were mixed with Ketjen black as conductor and polytetrafluoroethylene as a binding agent using an agate mortar to prepare a composite electrode. A 2032-type coin cell was prepared using Li metal as the anode and 1 mol/dm3 LiPF6/ethylene carbonate:diethylene carbonate = 50:50 vol% as the electrolyte. Charge-discharge measurements were conducted in the voltage range of 1.7−3.9 V at 50 °C. The ball-milling process with acetylene black (AB) was applied before electrode fabrication to decrease the particle size of the prepared samples and achieve homogeneous electrical conduction in the composite electrode. The active material and AB were mixed with ZrO2 balls in a ZrO2 pot. A similar electrode fabrication process was conducted using a mixture of active material and AB.An antifluorite structured Li6MnO4-type phase with the small starting materials was obtained in the range between 0.6 and 1.0 composition. The amount of residual reagents decreased with increasing excess lithium content in the starting mixture. A single phase with an Li6MnO4-type structure was obtained for an excess lithium composition of 10 mol%. The particle size of the synthesized samples was about 50 µm, which is quite large compared with conventional cathode materials for lithium batteries. The as-synthesized Li5.6Fe0.4Mn0.6O4 exhibited first charge and discharge capacities of 680 and 300 mAh/g, respectively, with a large irreversible capacity of 380 mAh/g (coulombic efficiency = 44%). This indicates that the new iron-manganese composition with ordered antifluorite-type structure is lithium insertion/extraction-active although a large irreversible electrode reaction was observed.The ball milling process with AB provided that the particle size of the sample was decreased from ca. 50 µm to 1 μm, and a uniform distribution of Mn, Fe and C signals was realized for the mixture without any local signals. This suggests that a fine and homogeneous composite material composed of the active material and AB was obtained by the ball-milling process. The first discharge capacity of the Li5.6Fe0.4Mn0.6O4 electrode treated by ball-milling was 450 mAh/g. The coulombic efficiency was 60%. A reversible reaction continued to proceed with ca. 200 mAh/g after the following cycles. The differential capacity plots for the ball-milled Li5.6Fe0.4Mn0.6O4 (x = 0.6) at the second, third, and tenth cycles exhibited two pair of redox peaks at around 2.5 V and 3.2 V, which reveals that the lithium (de-)intercalation reaction successfully occurred after the 2nd cycle. A new iron-manganese based ordered antifluorite-type compound has potential as a high-capacity oxide cathode material and thus further investigation is required. Phase formation and electrochemical properties of the Li-rich materials based on tetrahedrally-coordinated structure including the other cation will also be discussed in the presentation. Acknowledgement: This work was financially supported by Tokuyama Science Foundation, Nippon Sheet Glass Foundation for Materials Science and Engineering, and JSPS KAKENHI Grant Number 24K01582.

Proton Solubilities in Candidate Protonic Ceramic Fuel Cell Air Electrode Materials from First Principles

Using a mixed protonic-electronic conducting oxide as the air electrode of protonic ceramic fuel cells is one strategy for increasing their electrical efficiency, as this should enable a larger particle surface area to participate in the oxygen reduction reaction. However, because of their complex defect chemistry, few studies have focused on proton solubility in electrode materials, and hence little is known about which materials are able to absorb sufficient protons to support high conductivity. To help identify materials likely to have both high protonic and electronic conductivity, we performed first-principles calculations of 3d transition metal oxide perovskites LnMO3 (Ln = La, Pr, Nd, Sm, Gd; M = Sc, V–Ni), focusing on their hydration enthalpies calculated according toH2O+v O ⋅⋅+OO ×→2OHO ⋅, where v O ⋅⋅ is an oxide-ion vacancy and the proton bonds to an O atom to form a hydroxide ion, OHO ⋅.We found that hydration becomes less favorable as the number of 3d electrons increases, i.e., as the M-O bonds become less basic. In contrast, electronic conductivity, as assessed from band structure plots and effective masses, generally increases. By comparing the hydration enthalpy to that of prototypical proton electrolyte BaZrO3, good mixed proton-hole conducting oxides are thus likely to be found using transition metals from the middle of the period (M = Cr–Co) when Ln = La. Smaller lanthanides favor easier hydration, however, so that good mixed conducting perovskites containing Fe, Co, and/or Ni may be achievable using lanthanides Pr, Nd or Sm.AcknowledgmentThis work was performed as part of the Development of Ultra-High Efficiency Protonic Ceramic Fuel Cell Devices project (JPNP20003), commissioned by the New Energy and Industrial Technology Development Organization.

(Invited) Slit Pores Engineering in 2D Layered MXenes for Electrochemical Energy Storage and Conversion

MXenes constitute a diverse family of 2D transition metal carbides/nitrides, characterized by the formula Mn+1XnT x (where M represents an early transition metal, X denotes carbon or nitrogen, n ranges from 1 to 4, and T x indicates surface terminations). Their exceptional electrical conductivity and ion-hosting capacity make them outstanding candidates for electrode materials in electrochemical energy storage systems. The interlayer spacing in MXenes can be considered as tunable slit pores. While intercalation plays a pivotal role in the applications of MXenes as a mechanism for ion storage, the impact of pre-intercalation on their electrochemical performance is not well understood. In this context, we will discuss several examples where pre-intercalation and slit pores engineering have significantly altered the electrochemical behavior of MXenes. Manipulating Ti3C2T x MXene multilayers through pre-intercalation has yielded an ultra-high areal capacitance of up to 5.7 F/cm2—a record for MXenes. Notably, engineering the slit pores through pre-intercalation, with an interlayer spacing of approximately 2.2 nm, has achieved a capacitance of 257 F/g—an order of magnitude higher than that of pristine MXene—across a voltage window exceeding 3V in pure EMIMTFSI electrolyte. This outcome results in a high-energy density comparable to batteries without compromising their impressive power density. The slit pores in MXenes can also support the growth of other nanomaterials such as nitrogen-doped carbon sheets. The hybrid of Ti3C2T x with confined nitrogen doped carbon in the slit pores have demonstrated a reversible sodium-ion specific capacity of 305 mAh/g at a specific current of 20 mA/g, 2.3 times that of pristine multilayer MXene. For Li-ion batteries, a reversible capacity of 400 mAh/g at a specific current of 20 mA/g has been achieved, which is 1.5 times that of Ti3C2T x MXene.

Universal Platform for Robust Dual-Atom Doped 2D Catalysts with Superior Hydrogen Evolution in Wide pH Media

Layered 2D transition metal dichalcogenides (TMDs) have been suggested as efficient substitutes for Pt-group metal electrocatalysts in the hydrogen evolution reaction (HER). However, poor catalytic activities in neutral and alkaline electrolytes considerably hinder their practical applications. Furthermore, the weak adhesion between TMDs and electrodes often impedes long-term durability and thus requires a binder. Here, I present a universal platform for robust dual-atom doped 2D electrocatalysts with superior HER performance over a wide pH range media. V:Co-ReS2 on a wafer scale is directly grown on oxidized Ti foil by a liquid-phase precursor-assisted approach and subsequently used as highly efficient electrocatalysts. The catalytic performance surpasses that of Pt group metals in a high current regime (≥ 100 mA cm−2) at pH ≥ 7, with a high durability of more than 70 h in all media at 200 mA cm−2. First-principles calculations reveal that V:Co dual doping in ReS2 significantly reduces the water dissociation barrier and simultaneously enables the material to achieve the thermoneutral Gibbs free energy for hydrogen adsorption.

First-Principles Study of Electron-Defect Interaction and Carrier Mobility in Transition Metal Dichalcogenides Semiconductors

Electron-defect interaction is an important effect limiting the electronic properties of 2D transition metal dichalcogenides (TMD), especially to the low charge carrier mobility. We employ first-principles methods and develop scientific computation software "EDI" (Electron-Defect Interaction), which is used to accurately calculate the scatterings by some common defect types and study the mobility of various TMD systems. Our results indicate that traditional metric of effective mass is not enough to characterize the mobility. But the electron-defect coupling is the most important factor in the systems we research. In addition, we also identify good candidates with high mobility, and provide a potential annealing method to improve the electron transport in TMDs with common vacancy defects.

(Invited) Integration and Optimization of 2D Transition Metal Dichalcogenides as Switching Layers in Memristors

Memristive devices possess the ability to dynamically adjust resistance based on previous voltage and current inputs. They represent a frontier in next-generation computational hardware, offering unparalleled potential for brain-inspired computing and neuromorphic engineering. Among the various configurations, electrochemical metallization (ECM) memristors feature a solid electrolyte layer sandwiched between two electrodes and perform resistive switching via filament formation (SET) and dissolution (RESET) [1]. Recent studies have focused on the integration of two-dimensional (2D) materials into the design of ECM memristors, aiming at improving performance and unlocking new functionalities [2-4]. 2D transition metal dichalcogenides and hexagonal boron nitride, in particular, are promising candidates for realizing memristors with ultra-low switching voltages, suitable for neuromorphic computing and artificial synapses [5, 6].In this talk, I will focus on vertical ECM memristor in crossbar configuration using chemical vapor deposited (CVD), atomic-thick MoSe2 as switching material, and Au/Cu as bottom/top electrodes. The devices exhibit nonvolatility and bipolar resistive switching behavior, with well-defined SET/RESET voltages (below 1V in absolute value) [7]. We performed atomic-scale investigations by transmission electron microscopy to unveil the filament formation mechanism within the MoSe2 layer. Our results shed light on the Cu/MoSe2 interface, particularly critical for memristor performance, providing key information on the intermixing and diffusion mechanisms of Cu atoms within the device structure.Furthermore, we studied the effect of XeF2 fluorination of the MoSe2 films to tune their surface properties and improve the ion transport kinetics across the interface with Cu. By carefully controlling the XeF2 process, we were able to induce structural and chemical modifications in the MoSe2, such as layer thinning and doping/implantation. This could provide a way of controlling the interface morphology and ultimately the performance and reliability of the memristive devices. Transport properties and X-ray photoelectron spectroscopy analyses provided insights into the fluorination-induced modifications on the MoSe2 surface, offering a framework for performance optimization. Overall, these findings rationalize the 2D materials-based ECM memristor operation and highlight the potential for advanced applications.