Embedded Files
2025年
2025年
Yongkang Wang, Fujie Tang, Xiaoqing Yu, Kuo-Yang Chiang, Chun-Chieh Yu, Tatsuhiko Ohto, Yunfei Chen, Yuki Nagata, and Mischa Bonn, Nature Communications, 16, 7288 (2025).
Nanoconfinement of aqueous electrolytes is ubiquitous in geological, biological, and technological contexts, including sedimentary rocks, water channel proteins, and applications like desalination and water purification membranes. The structure and properties of water in nanoconfinement can differ significantly from bulk water, exhibiting, for instance, modified hydrogen bonds, altered dielectric constant, and distinct phase transitions. Despite the importance of nanoconfined water, experimentally elucidating the nanoconfinement effects on water, such as its orientation and hydrogen bond (H-bond) network, has remained challenging. Here, we study two-dimensionally nanoconfined aqueous electrolyte solutions with tunable confinement from nanoscale to angstrom-scale sandwiched between a graphene sheet and calcium fluoride (CaF2) achieved by capillary condensation. We employ heterodyne-detection sum-frequency generation (HD-SFG) spectroscopy, a surface-specific vibrational spectroscopy capable of directly and selectively probing water orientation and H-bond environment at interfaces and under confinement. The vibrational spectra of the nanoconfined water can be described quantitatively by the sum of the individual interfacial water signals from the CaF2/water and water/graphene interfaces until the confinement reduces to angstrom-scale (<~8 Å). Machine-learning-accelerated ab initio molecular dynamics simulations confirm our experimental observation. These results manifest that interfacial, rather than nanoconfinement effects, dominate the water structure until angstrom-level confinement for the two-dimensionally nanoconfined aqueous electrolytes.
Akihiro Mitsuhara, Kei Saito, Hiroshi Yukawa, Kenichi Takai, and Hajime Kimizuka, Journal of Membrane Science, 734, 124401 (2025).
Thorough assessment of hydrogen and lattice defect interactions in hydrogen-permeable alloys is essential because of their significant influence on hydrogen diffusion, the dominant elementary process in hydrogen permeation. However, the underlying mechanisms are not adequately understood. In this study, theoretical and experimental analyses revealed the controlling factors leading to a significant reduction in the hydrogen diffusivity of B2-structured PdCu alloys, which are promising materials for low-temperature hydrogen purification in practical applications. Using a density functional theory (DFT)-informed kinetic Monte Carlo (KMC) approach, we quantitatively evaluated point defect (i.e., antisite Cu atoms and Cu vacancies) effects on the hydrogen diffusivity (D) of PdCu alloys. It was found that antisite Cu atoms decreased D in proportion to their number because of the blocking effect, whereas Cu vacancies strongly decreased D at temperatures below 500 K due to the trapping effect. Moreover, the D values predicted by KMC simulations considering Cu vacancy effects were in good agreement with their experimental counterparts. Thermal desorption spectroscopy measurements were performed to detect the presence of lattice defects that trapped hydrogen in the PdCu alloys. The results indicate that lattice defects with a hydrogen binding energy of 0.4 eV exist in the sample, and this value is comparable to the DFT result for the Cu vacancy. This suggests that point defects, analogous to Cu vacancies, were responsible for the significant reduction in D for PdCu alloys observed in experiments. This study contributes to a detailed understanding of the interplay between hydrogen and lattice defects in Pd-based alloys.
[Invited] Hajime Kimizuka, Shigenobu Ogata, Bo Thomsen, and Motoyuki Shiga, Journal of Physics: Condensed Matter, 37, 193001 (2025).
Lighter isotopes typically diffuse faster than heavier isotopes; however, the case is not necessarily true for H. Predicting the kinetics of H isotope transport and reactions in substances remains a fundamental challenge in material and condensed matter physics. The peculiar experimentally observed isotope effect on H diffusivities in face-centred cubic (fcc) metals has long been an unresolved problem. Using an ab initio path-integral approach to explore the quantum mechanical nature of both electrons and nuclei, this study successfully predicts H isotope diffusivities in fcc Pd over a wide temperature range. The temperature dependence of the diffusivities follows an unusual 'reversed-S' shape on Arrhenius plots. This irregular behaviour, arising from the competition between different nuclear quantum effects (NQEs) with different temperature dependencies, reveals the mechanism of anomalous crossovers between normal and reversed isotope effects. The results illustrate that this phenomenon is common in other fcc metals (e.g., Cu and Ag), where H atoms prefer to occupy octahedral (O) sites. Conversely, in Al, where H atoms prefer to occupy tetrahedral (T) sites, the dependence of H diffusivities on temperature exhibits a familiar 'C' shape. A lattice expansion of approximately 1–2% causes the stable position of H atoms dissolved in Pd to shift from the O to T sites, and H diffusion in expanded Pd is no longer suppressed by NQEs, as observed in Al. This finding has important implications for interpreting kinetic processes involving the crossover from classical to quantum behaviour of H atoms moving between different interstitial sites. Path-integral simulation results describing the approximate quantum dynamics of the Pd–H system, using a machine-learning-based interatomic potential with accuracy similar to the density functional theory calculations, are presented. This computational approach paves the way for elucidating the quantum behaviour of H isotopes in various materials.
Akihiro Mitsuhara, Hiroshi Yukawa, and Hajime Kimizuka, Acta Materialia, 289, 120838 (2025).
To efficiently explore and design alloys with excellent hydrogen permeability, it is critical to clarify the bottleneck steps in the hydrogen permeation process. Hydrogen dissolution is one of the most important elementary processes in hydrogen permeation, regarding not only hydrogen permeability but also the suppression of hydrogen embrittlement. In this paper, we propose an effective and robust thermodynamic model for hydrogen dissolution in metals that can precisely describe the pressure–composition isotherm (i.e., the PCT curve) over a wide range of hydrogen concentrations. With this model, the PCT curves for the V and V–Fe systems were described nonempirically in a manner consistent with their experimental counterparts using only parameters obtained via density functional theory (DFT) calculations. The essential factors that determine the hydrogen concentration dependence of hydrogen dissolution in metals are the cooperative interactions between multiple hydrogen atoms in the metal, including the site-blocking effect. By quantifying these factors using DFT calculations and introducing them into the model, it was confirmed that the hydrogen dissolution behavior deviated from Sieverts' law, as observed for many metals and alloys. Furthermore, we found that the addition of Fe to V increased the interaction energy between the hydrogen atoms, reducing the solubility of hydrogen in V. Our DFT-informed model advances the understanding of the mechanism of hydrogen dissolution in metals and offers new insights into controlling hydrogen solubility at target temperatures to suppress hydrogen embrittlement in hydrogen-permeable metals. The findings of this study provide theoretical guidance for the efficient design of hydrogen-permeable and hydrogen-storage materials.
Heting Liao, Jun-Ping Du, Hajime Kimizuka, and Shigenobu Ogata, Computational Materials Science, 251, 113771 (2025).
The kinetic Monte Carlo (kMC) method is employed to simulate time-dependent precipitation nucleation via vacancy jumps during alloy aging. Unlike pure metals, the activation energy for vacancy jumps in alloy systems depends on the local chemical structure, and needs to be recalculated at each kMC step. Traditionally, approximated activation energies derived from that of pure metal and the energy difference before and after the vacancy jump are used, however, they lack quantitative reliability. This study developed a neural network (NN) for face-centered cubic Al–Cu alloys to predict activation barriers based on local chemical structures, significantly accelerating barrier estimation compared to on-the-fly nudged elastic band analyses. NN-based kMC simulations revealed single-layer and double-layer Guinier–Preston (GP) zone formation in Al–2.0 at%Cu alloys. The incubation times of GP zones at 300 and 350 K were quantitatively determined, showing good agreement with experimental observations.
Ryo Asakawa, Soichi Yokoyama, Ryo Yamada, Tatsuhiko Ohto, Hirokazu Tada, and Yutaka Ie, Bull. Chem. Soc. Japan, 98, uoae146 (2025).
To realize efficient intramolecular hopping transport, molecular wires must possess both a small reorganization energy (λ) at the hopping site and a minimal energy difference (ΔEhs) between them. While we showed that periodically twisted molecular wires using an annelated unit (TBID) as a hopping site improves hopping transport, the influence of the chemical structure of the hopping site remains unclear. Here, to investigate the impact of the annelated structure on hopping transport, we designed a non-annelated unit (BPB) and developed molecular wires based on periodically twisted BPB. Physical properties of BPB were compared with those of TBID. Molecular wires based on BPB showed lower single-molecule conductance than those based on TBID. Theoretical calculations indicate that BPB has a larger λ, ΔEhs, as well as a higher injection barrier compared to TBID. These results demonstrate the importance of the annelated structure at the hopping site for achieving efficient hopping transport.
Xiushang Xu, Liliia Moshniaha, Serhii Vasylevskyi, Ryota Kabe, Tatsuhiko Ohto, and Akimitsu Narita, Angew. Chem. Int. Ed., 64, e202418334 (2025).
Dibenzoperioctacene (DBPO) with extended zigzag edges was synthesized and unambiguously characterized by a combination of nuclear magnetic resonance (NMR), mass spectrometry, and single-crystal X-ray diffraction analysis. Variable-temperature (VT) NMR analysis indicated the closed-shell character of DBPO, yet an electron paramagnetic resonance (EPR) signal was observed at room temperature suggesting its potential open-shell diradicaloid nature. The bond lengths in the single-crystal structure of DBPO aligned more closely with those of open-shell teranthene than closed-shell bisanthene. Spin-unrestricted density functional theory (DFT) calculations using various methods supported that ground state of DBPO might be on the borderline between closed- and open-shell singlet states, with a large singlet-triplet energy gap (ΔEST), consistent with the VT-NMR and EPR results. UV-vis-near infrared (NIR) absorption spectrum showed the longest-wavelength peak at 905 nm, marking the NIR optical properties of DBPO. DBPO exhibited a tendency to react with atmospheric oxygen, which additionally hinted at its possible open-shell character. Furthermore, the oxidized species of non-fluorescent DBPO exhibited strong emission at 820 and 915 nm, which opens its potential for oxygen detection via a "turn-on" NIR fluorescence response.
Takuya Ishimoto, Naotaka Morita, Ryosuke Ozasa, Aira Matsugakia, Ozkan Gokcekaya, Shota Higashino, Masakazu Tane, Tsuyoshi Mayama, Ken Cho, Hiroyuki Y. Yasuda, Masayuki Okugawa, Yuichiro Koizumi, Masato Yoshiya, Daisuke Egusa, Taisuke Sasaki, Eiji Abe, Hajime Kimizuka, Naoko Ikeo, and Takayoshi Nakano, Acta Materialia, 286, 120709 (2025).
This study demonstrates the control of the crystalline orientation through metal additive manufacturing, enabling the development of component design guidelines that incorporate the inherent anisotropy of the mechanical properties (e.g., Young's modulus) in crystalline materials. We, for the first time, successfully fabricated a <111>//build direction (BD)-oriented single-crystalline-like texture in an alloy with a cubic crystal structure via laser powder bed fusion (LPBF) and completed a series of three single-crystalline-like microstructures with <001>, <011>, and <111>//BD orientations in a single material. The <001> and <111> directions exhibited the lowest and highest Young's moduli, respectively, demonstrating a wide range of control over the anisotropy of the mechanical properties of the product. To achieve a <111>//BD-oriented single-crystalline-like texture, a novel three-layer cyclic strategy of "uni" directional laser scanning at 120° angular intervals was developed by considering the easy growth direction and crystal symmetry. To the best of our knowledge, no previous study has reported this unique strategy. By superimposing the realized <111> orientation and shape-based anisotropy, products exhibiting high Young's modulus anisotropy, which cannot be expressed by shape and texture alone, were obtained via the LPBF single process. This achievement holds promise for the realization of a new component design guideline that integrates texture (mechanical properties) design for each internal location—modifiable through scanning strategies—with traditional shape optimization techniques typically used in computer-aided design. This approach enables tailored mechanical performance through optimized design strategies.
Aimi A. H. Tajuddin, Tatsuhiko Ohto, Hisanori Tanimoto, Takeshi Fujita, Samuel Jeong, Atsushi Fukazawa, Yuto Shimoyama, Yoshitatsu Misu, Kaori Takano, Koji Matsuoka, and Yoshikazu Ito, ChemSusChem, 18, e202401071 (2025).
The direct one-step hydrogenation of toluene to methylcyclohexane facilitated by a proton-exchange membrane water electrolyzer driven by renewable energy has garnered considerable attention for stable hydrogen storage and safe hydrogen transportation. However, a persistent challenge lies in the crossover of toluene from the cathode to the anode chamber, which deteriorates the anode and decreases its energy efficiency and lifetime. To address this challenge, the catalyst-poisoning mechanism is systematically investigated using IrO2 and high-entropic non-noble-metal alloys as anodes in acidic electrolytes saturated with toluene and toluene-oxidized derivatives, such as benzaldehyde, benzyl alcohol, and benzoic acid. Benzoic acid plays an important role in polymer-like carbon-film formation by blocking the catalytically active sites on the anode surface. Moreover, Nb and the highly entropic state on the surface of the multi-element alloy lower the adsorbing ability of toluene and prevent polymer-like carbon film formation. This study contributes to the design of catalyst-poisoning-resistant anodes for organic hydride technology, advanced fuel cells, and batteries.
2024年
2024年
Alessandro Greco, Tatsuhiko Ohto, Yuki Nagata, Mischa Bonn, and Ellen H. G. Backus, J. Chem. Phys., 161, 174706 (2024).
Water interfaces play a crucial role in regulating interactions and energy flow. Vibrational sum-frequency generation (vSFG) spectroscopy provides structural and dynamic information on water molecules at interfaces. It has revealed, for instance, the presence of the hydrogen-bonded and free OH groups at the air–water interface. Here, using temperature-dependent, time-resolved vSFG, we focus on the vibrational energy relaxation dynamics of interfacial heavy water (D2O). We reveal that while the relaxation timescale for hydrogen-bonded OD stretch modes is temperature-independent, the lifetime of the free OD stretch mode decreases with increasing temperature. Our data, supported by simulations, suggest that both intramolecular energy transfer and rotational reorientation mechanisms jointly contribute to the energy relaxation process of the free OD, with temperature influencing these mechanisms differently.
Ryo Asakawa, Soichi Yokoyama, Ryo Yamada, Seiya Maeda, Tatsuhiko Ohto, Hirokazu Tada, and Yutaka Ie, J. Am. Chem. Soc., 146, 23529–23536 (2024).
Realizing efficient long-distance intramolecular charge transport based on a hopping mechanism is a key challenge in molecular electronics. In hopping transport, smaller reorganization energy (l) and energy difference between hopping sites (ΔEhs) should lead to smaller activation energy and faster charge transfer. However, the development of π-extended molecules that meet these requirements is challenging. In this study, we successfully synthesized several nanometer-scale π-extended molecules composed of a fused π-conjugated unit as a hopping site for reducing l. Conformational twists between fused units effectively localize π-conjugation in each unit, contributing to reducing ΔEhs. The expected electronic structures of the oligomers were confirmed using spectroscopic and electrochemical measurements. Single-molecule conductance measurements exhibited higher conductance and lower activation energy than those of non-fused oligothiophenes. First-principles calculations indicated that smaller l and ΔEhs explain the high conductance. These results highlight the efficiency of the proposed molecular design for effective intramolecular hopping transport.
Heting Liao, Hajime Kimizuka, Hiroshi Miyoshi, and Shigenobu Ogata, Computational Materials Science, 242, 113069 (2024).
Metastable precipitates of solute atoms, known as Guinier–Preston (GP) zones, play a significant role in the age hardening of Al–Cu alloys. In this study, we utilized the classical nucleation theory (CNT) to obtain time-temperature-transformation (TTT) diagrams for steady-state nucleation over a wide temperature range. The CNT model, which is based on atomistic simulations using artificial neural network potentials, was constructed to predict the nucleation kinetics of GP zones in the Al–Cu system with solute concentrations ranging from 1.3 to 2.5 at% (3.0 to 5.7 wt%). The nose temperature at which the steady-state nucleation time for GP zone formation is the shortest was calculated. The nose temperature increased and the minimum nucleation time decreased as the solute concentration increased. These predictions are comparable to previous theoretical results and are in good qualitative agreement with experimental observations. Furthermore, two formation scenarios of double-layer GP (GP2) zones, considering synchronous and asynchronous attachments of solute atoms to clusters, were compared in terms of nucleation efficiency. This provides new insights into the nucleation pathways of the GP2 zone in Al–Cu alloys.
Yongkang Wang, Fujie Tang, Xiaoqing Yu, Tatsuhiko Ohto, Yuki Nagata, and Mischa Bonn, Angew. Chem. Int. Ed., 63, e202319503 (2024).
Graphene, a transparent two-dimensional conductive material, has brought extensive new perspectives and prospects to various aqueous technological systems, such as desalination membranes, chemical sensors, energy storage, and energy conversion devices. Yet, the molecular-level details of graphene in contact with aqueous electrolytes, such as water orientation and hydrogen bond structure, remain elusive or controversial. Here, we employ surface-specific heterodyne-detection sum-frequency generation (HD-SFG) vibrational spectroscopy to re-examine the water molecular structure at a freely suspended graphene/water interface. We compare the response from the air/graphene/water system to that from the air/water interface. Our results indicate that the χ(2)yyz spectrum recorded from the air/graphene/water system arises from the topmost 1-2 water layers in contact with the graphene, with the graphene itself not generating a significant SFG response. Compared to the air/water interface response, the presence of monolayer graphene weakly affects the interfacial water. Graphene weakly affects the dangling O-H group, lowering its frequency through its interaction with the graphene sheet, and has a small effect on the hydrogen-bonded O-H group. Molecular dynamics simulations confirm our experimental observation. Our work provides molecular insight into the interfacial structure at a suspended graphene/water interface, relevant to various technological applications of graphene.
Motoyuki Shiga, Bo Thomsen, and Hajime Kimizuka, Physical Review B, 109, 054303 (2024).
Inelastic neutron scattering (INS) spectra of hydrogen in face-centered cubic palladium have been calculated considering nuclear quantum effects (NQEs) at finite temperatures. The calculations were performed using semiclassical Brownian chain molecular dynamics (MD) [Shiga, J. Comput. Chem. 43, 1864 (2022)] and artificial neural network potentials with an accuracy of generalized gradient approximation of density functional theory. The calculated spectra are in good agreement with experimental spectra with respect to the peak positions and intensities corresponding to the fundamental tone and the first overtone of the vibrational excitation of hydrogen atoms. These results differ significantly from those of classical MD, indicating that NQE plays an essential role in the correct estimation of the INS spectrum. Importantly, the NQE acts as a blueshift of the INS spectrum for hydrogen in the octahedral site due to strong anharmonic vibrations of hydrogen on the potential surface with even symmetry. The calculated peak shifts associated with Pd lattice distortion were also in agreement with experimental results.
Jiung Jang, Jeong Woo Jo, Tatsuhiko Ohto, and Hyo Jae Yoon, J. Am. Chem. Soc, 146, 4922-4929 (2024).
The study of molecular wires facilitating long-range charge transport is of fundamental interest for development of various technologies in (bio)organic and molecular electronics. Defining the nature of long-range charge transport is challenging as electrical characterization does not offer the ability to distinguish a tunneling mechanism from the other. Here we show that investigation of Seebeck effect provides the ability. We examine the length-dependence of Seebeck coefficient in electrografted bis-terpyridine Ru(II) complex films. The Seebeck coefficient ranges from 307 to 1027 µV/K, with an increasing rate of 95.7 µV/(K·nm), as the film thickness increases to ten nanometers. Quantum-chemical calculations unveil that nearly overlapped molecular-orbital energy level of the Ru-complex with Fermi level accounts for the giant thermopower. Landauer-Büttiker probe simulations indicate the significant length-dependence evinces the Seebeck effect dominated by coherent near-resonant tunneling rather than thermal hopping. This study enhances our comprehension of long-range charge transport, paving the route to efficient electronic and thermoelectric materials.
Fumiya Shiokawa, Aimi Asilah Haji Tajuddin, Tatsuhiko Ohto, Yue Yu, Takeshi Fujita, Hisanori Tanimoto, Zeyu Xi, Samuel Jeong, and Yoshikazu Ito, Chem. Eng. J, 479, 147862 (2024).
Enhancing hydrogen production is key to achieving a carbon–neutral world. Among the various technologies available, using renewable energy for water electrolysis stands out as a leading method for creating green hydrogen. However, standard water electrolysis requires substantial quantities of ultra-clean fresh water for optimal energy efficiency and operations. This poses a problem as fresh water only represents a minuscule 0.01 % of our planet’s water reserves. Seawater electrolysis is a potential solution to resolve this limitation and attain sustainable hydrogen production. Nevertheless, electrolysis of seawater in its neutral state demands stringent reaction conditions, making it imperative to thoughtfully design the electrodes, particularly the anodes. This study presents the development of highly durable, noble-metal-free, high-entropy alloy anodes suitable for neutral seawater under highly fluctuating power operation. The anode demonstrates impressive stability (up to 6,000 cycles), lasting over 100 h at 100 mA cm−2 in a 0.5 M aqueous NaCl electrolyte. Its Faradaic efficiency for O2 production is 42 %. Density functional theory calculations reveal that the oxidized surface (passivation layer) of the anode prevents Cl− absorption, helping to protect the catalytically active sites and prevent catalyst degradation. This finding suggests that noble-metal-free anodes can serve as practical alternatives to those made from noble metals for neutral seawater electrolysis. Moreover, the utilization of seawater without acid or alkali treatment paves the way for safer and more sustainable hydrogen production. This is a significant stride toward realizing our goal of carbon neutrality.
Gyu Don Kong, Jiung Jang, Suin Choi, Gayoung Lim, In Soo Kim, Tatsuhiko Ohto, Seiya Maeda, Hirokazu Tada, and Hyo Jae Yoon, Small, 20, 2305997 (2024).
Functionality in molecular electronics relies on inclusion of molecular orbital energy level within a transmission window. This can be achieved by designing the active molecule with accessible energy levels or by widening the window. While many studies have adopted the first approach, the latter is challenging because defects in the active molecular component cause low breakdown voltages. Here we show that control over the packing structure of monolayer via supramolecular mixing transforms an inert molecule into a highly tunable rectifier. Binary mixed monolayer composed of alkanethiolates with and without carboxylic acid head group as a proof-of-concept is formed via a surface-exchange reaction. The monolayer withstands high voltages up to |4.5 V| and shows a dynamic rectification-external bias relationship in magnitude and polarity. Sub-HOMO levels activated by the widened transmission window account for our observations. Our work demonstrates that simple supramolecular mixing can imbue new electrical properties in electro-inactive organic molecules.
眞山剛, 君塚肇, まてりあ, 63, 49-53 (2024).
2023年
2023年
Samuel Jeong, Tatsuhiko Ohto, Tomohiko Nishiuchi, Yuki Nagata, Jun-ichi Fujita, and Yoshikazu Ito, Advanced Science, 10, 2304082 (2023).
Proton exchange membranes with high proton conductivity and low crossover of fuel molecules are required to realize advanced fuel-cell technology. The selective transportation of protons (by blocking the transportation of fuel molecules across a proton exchange membrane) is crucial to suppress crossover while maintaining a high proton conductivity. In this study, we propose a simple yet powerful method for optimizing the crossover-conductivity relationship by pasting sulfanilic-functionalized holey graphene onto a Nafion membrane. Our results show that the sulfanilic-functionalized holey graphene (supported by the membrane) suppresses the crossover by 89% in methanol and 80% in formate compared with that in the self-assembled Nafion membrane; an approximately 60% reduction is observed in the proton conductivity. This method exhibits the potential for application in advanced fuel cells that use methanol and formic acid as chemical fuels to achieve high energy efficiency.
Hungu Kang, Jiung Jang, Gyu Don Kong, Sangmin Jung, Tatsuhiko Ohto, and Hyo Jae Yoon, Journal of Materials Chemistry A, 11, 16233-16242 (2023).
N-Heterocyclic carbene (NHC) is an emerging anchor moiety for surface engineering and various applications. While various deposition conditions have been reported, how these affect the charge transport properties of NHC adsorbates remains uncertain. A NHC salt precursor based on benzo[d]imidazole with a PF6- counterion was deposited onto an ultrasmooth gold substrate using three different conditions for creating NHC monolayers: ambient incubation (AI), base (tert-butoxide)-induced deprotonation (BD), and electrochemical deprotonation (ED). Junction measurements using the EGaIn technique revealed that current density increased by ~10^1.7 in the order of AI < BD < ED, while the Seebeck coefficient showed the opposite trend, ranging from 9.3 to 13.5 μV/K. First-principles calculation reproduced the experimentally observed positive sign of the Seebeck coefficient. Further surface characterizations and computational calculations indicated that the different deposition conditions cause variation in surface coverage in the order of AI < BD < ED. This variation has a significant influence on the broadening of HOMO level and marginal impact on the energy offset between HOMO and Fermi levels, accounting for the opposite trends of electrical conductance and thermopower as a function of the deposition conditions. Finally, we found that the power factor of the NHC monolayer can increase by ~10^2 depending on the deposition condition.
Akihiro Mitsuhara, Hiroshi Yukawa, and Hajime Kimizuka, International Journal of Hydrogen Energy, 48, 35997 (2023).
To improve the performance of hydrogen-permeable alloys, quantitatively elucidating the metal–hydrogen interactions and identifying the controlling factors that govern the bottleneck step of the hydrogen permeation process are essential. In this study, based on the density functional theory, we characterized the interactions of hydrogen with point defects (such as antisite atoms and vacancies) in B2-type non-stoichiometric PdCu alloys, which are promising candidate materials for low-temperature working membranes for hydrogen purification. We proposed a "double defect" mechanism that produces a new type of defect complex with an atomic configuration similar to the Cu vacancy in B2-type intermetallics. We found that both Cu vacancies and double defects can act as strong hydrogen-trapping sites and are thus responsible for the marked decrease in hydrogen diffusivity. In contrast, antisite Cu atoms only repel H atoms and do not significantly hinder hydrogen diffusion. Whereas the binding energy of hydrogen to Cu vacancies and double defects exceeded 0.3 eV when relatively few (one or two) H atoms were trapped, the binding energy of hydrogen to Pd vacancies was substantially smaller (0.02 eV at most). Therefore, in Cu-rich non-stoichiometric PdCu alloys, the double defects may be the major hydrogen trapping sites. Furthermore, the effects of hydrogen on the formation of Pd vacancies, Cu vacancies, and double defects in B2-type PdCu alloys were clarified. Notably, double-defect formation from Pd vacancies in B2-type PdCu was significantly enhanced by the presence of nearby H atoms. This phenomenon, in which vacancies in B2-type intermetallic compounds transform into defect complexes in the presence of hydrogen, is postulated for the first time in this study. The findings of this study contribute to further understanding the driving forces for the formation of vacancy-hydrogen complexes in PdCu alloys at the atomic level and provide theoretical support for identifying the lattice defects that inhibit hydrogen permeability in Pd-based membranes.
Rana Hossain, Hajime Kimizuka, and Shigenobu Ogata, Physical Review Materials, 7, 053608 (2023).
MAX phases are a unique class of atomically layered ternary ceramics that deform plastically at room temperature owing to highly mobile basal dislocations (BDs). To understand and control the ductility of these materials, it is crucial to study the core structures and mobilities of BDs. In this study we developed a machine-learning-based spectral neighbor analysis potential (SNAP) to perform atomistic simulations of edge, screw, and mixed BDs in a Ti3SiC2 MAX phase. The SNAP was trained on density functional theory (DFT)-calculated data. The SNAP calculations demonstrate that the BD core structure exhibits significant asymmetry that is dependent on the position of the weakly bonded Si layer relative to the Ti(4f)–Si slip plane. The dislocation core either splits into Shockley partials or remains compact, depending on whether compressive or tensile stresses act on the Si layer. This asymmetry in the BD core structure agrees well with DFT results. Differential displacement and Nye tensor distribution analyses reveal that undissociated BD cores are centered on Si layers and spread over adjacent parallel basal planes. Additionally, they are three orders of magnitude less mobile than partial BDs. The Peierls stresses of partial edge BDs (∼71 MPa) are closer to those of metals than those of ceramics, suggesting that edge BDs play a key role in the incipient plasticity of Ti3SiC2 MAX phases at low temperatures. The findings of this study contribute to a deeper understanding of the complex behavior of BDs at the atomic level and provide theoretical support for elucidating the unique deformation modes of crystals with atomically layered structures owing to the motion of BDs.
Hiroshi Okuda, Kohei Kinstsu, Shu Kurokawa, Masao Tabuchi, Hiroaki Nitani, Hajime Kimizuka, Shin-ichi Inoue, Michiaki Yamasaki, Yoshihito Kawamura, Acta Materialia, 253, 118963 (2023).
A coarse-grained picture of segregation layers in Mg–Y–Zn alloys as a two-dimensional cluster systems bound in stacking faults has been justified by a combined use of extended X-ray absorption fine structure (EXAFS), small-angle X-ray scattering, and scanning tunneling microscopy. EXAFS analysis showed an identical relaxed atomic configuration around Y and Zn atoms as a finger print of molecular-like L12 clusters, characteristic of long period stacking ordered (LPSO) structures. The fingerprint was confirmed for wide range of compositions of cast ternary alloys containing bulk LPSO structures. The clusters in dilute Mg–Y–Zn alloys, whose segregation layers were sparsely dispersed in the α-Mg matrix, showed the same fingerprint. The results suggested that the Y–Zn clusters in the segregation layers of Mg–Y–Zn alloys are identical, regardless of the cluster density in the layers or the number of segregation layers.
Hyukjoon Kwon, Motoyuki Shiga, Hajime Kimizuka, Takuji Oda, Acta Materialia, 247, 118739 (2023).
Hydrogen diffusivity in metals has been extensively investigated owing to its rich physical characteristics and importance in materials engineering. However, there are large deviations in the reported experimental data of diffusion coefficients due to surface and trapping effects, indicating that accurate measurements are inherently difficult, especially at low temperatures. For computational studies, several atomistic simulation methods have been proposed and used to determine the true hydrogen diffusivity in the lattice; however, their accuracy remains questionable as most studies have not accurately simulated the force field, dynamic effects, or nuclear quantum effects. In this study, using three bcc metals (Nb, Fe, and W) as test cases, we estimated the diffusivity of dilute hydrogen from long-time path integral simulations using machine-learning moment tensor potentials with the accuracy of density functional theory, which accurately handles the three factors simultaneously. Calculations based on such accurate modeling revealed that existing measurements are reliable only if the experimental methods and conditions are appropriate for representing bulk hydrogen diffusion. In the temperature range where the experiments seem reliable (<800 K for Nb, >500 K for Fe, and >1500 K for W), our calculations show excellent agreement for all three bcc metals. Isotope effects were also consistent with the experimental data. These results demonstrate that precise measurements over a wide temperature range remain a challenge in experimental studies and that predicted data from accurate computer simulations can compensate for missing experimental data.
Heting Liao, Hajime Kimizuka, Hiroshi Miyoshi, Shigenobu Ogata, Journal of Alloys and Compounds, 938, 168559 (2023).
The age-hardening response during the heat-treatment process of Al–Cu alloys is significantly and nonlinearly influenced by the type and size of metastable precipitates formed. In Al–Cu alloys, a semicoherent theta' phase, usually observed after the formation of coherent Guinier–Preston (GP) zones during aging, is the key strengthening precipitate. Thus, identifying the energetics of preferential nucleation of these precipitates is essential for clarifying the optimal conditions for the formation of precipitates that effectively contribute to hardening. In this study, using classical nucleation theory (CNT) along with a recently developed machine-learning-based interatomic potential with near first-principles accuracy, we characterized the nucleation preference of coherent GP zones and semicoherent theta' nanoprecipitates in Al–Cu alloys at various temperatures and solute concentrations. Our atomistically informed CNT model revealed the overall temperature and solute-concentration dependencies of the nucleation barriers of the nanoprecipitates, which determine the crossover temperatures at which the ease of formation of each precipitate alternates at the solute concentration of interest. The predicted results were in good agreement with the previous experimental observations. The findings of this study contribute to furthering the understanding of the driving forces for nucleation of precipitates in Al–Cu alloys at an atomic level and provide theoretical guidance for identifying the optimal age-hardening response.
2022年
2022年
[Invited] Hajime Kimizuka, Bo Thomsen, Motoyuki Shiga, Journal of Physics: Energy, 4, 034004 (2022).
The contribution of nuclear quantum effects (NQEs) to the kinetics and dynamics of interstitial H isotopes in face-centered cubic Pd was intensively investigated using several path-integral techniques, along with a newly developed machine-learning interatomic potential based on artificial neural networks for Pd–H alloys. The diffusion coefficients (D) of protium, deuterium, and tritium in Pd were predicted over a wide temperature range (50–1500 K) based on quantum transition-state theory (QTST) combined with path-integral molecular-dynamics simulations. The importance of NQEs even at high temperatures was illustrated in terms of the characteristic temperature dependence of the activation free energies for H-isotope migration in Pd. This illuminates the overall picture of anomalous D crossovers among the three H isotopes in Pd. In addition, the D of protium in Pd was directly computed using two approximate quantum-dynamics methods based on Feynman's path-integral theory, i.e. centroid molecular dynamics (CMD) and ring-polymer molecular dynamics (RPMD), in the temperature range 370–1500 K. The D values obtained from the CMD and RPMD simulations were very similar and agreed better with the reported experimental values than the QTST results in this temperature range. Our machine learning-based path-integral calculations elucidate the underlying quantum nature of the 'reversed S'-type nonlinear behavior of D for the three H isotopes in Pd on the Arrhenius plots.
三津原晟弘, 湯川宏, 君塚肇, 日本金属学会誌, 86, 140-148 (2022).
Pd-Cu alloys are of particular interest for use as hydrogen purification membranes, and identifying the effects of stoichiometry on hydrogen permeability is the key to exploring the possibility of improving this material. In this study, we carried out hydrogen permeation tests for Pd-51.5 mol％Cu alloy in order to understand the effects of excess Cu atoms on the hydrogen permeability of Pd-Cu alloys with B2-type crystal structure. Even though the Pd-51.5 mol％Cu alloy contained a small amount of a face-centered cubic phase, its hydrogen permeability was higher than that of practical Pd-53 mol％Cu alloy with a B2 single phase in the temperature range of 298 K to 623 K. These results were analyzed according to the consistent description of hydrogen permeation based on hydrogen chemical potential. It was found that the hydrogen solubility was improved by reducing the excess Cu atoms. On the other hand, the hydrogen diffusivity of Pd-51.5 mol％Cu alloy was still comparable to that of Pd-53 mol％Cu alloy and decreased significantly at low temperatures, just like that of the practical alloy. First-principles calculations suggested that the excess Cu atoms act as repellers and reduce the hydrogen solubility in the alloy, in a manner consistent with experimental results. It was also suggested that the excess Cu atoms reduce the apparent hydrogen diffusivity as the hydrogen atoms rarely occupy the interstitial sites around the excess Cu atoms. However, no significant differences in the hydrogen diffusivity due to the excess Cu atoms were observed in the experiment. Therefore, other factors apart from the excess Cu atoms may have inhibited the hydrogen diffusivity at low temperatures. Our findings will serve as a basis for enhancing the hydrogen permeability of Pd-Cu alloys by tuning their composition and processing.
Rana Hossain, Hajime Kimizuka, Yoshinori Shiihara, and Shigenobu Ogata, Computational Materials Science, 209, 111366 (2022).
In the MAX phase, the nucleation and motion of basal dislocations dominate the plastic deformation at room temperature. Therefore, information on the dislocation core properties and energy barrier for dislocation glide is essential to understand their plasticity. Dislocations in MAX phases are mainly <a>-type basal edge dislocations (b = a/3[11-20]) and can glide in the basal plane (typical slip) or form kink bands depending on the load direction and crystal orientation owing to the strong plastic anisotropy. In this study, we determined the core structure, Peierls barrier, and corresponding stress of a/3(0001)[11-20] edge dislocations in Ti3AlC2 MAX phase by multiple computational approaches using the semidiscrete variational Peierls–Nabarro method, density functional theory, and classical bond-order-potential-based molecular statics calculations. The a/3[11-20] basal edge dislocation was dissociated into two Shockley partial dislocations with a 2–2.5b-wide stacking fault in between. These partial dislocations glide between the Ti(4f) and Al atomic layer. For straight dislocation motion, the Peierls barrier was single-humped and the corresponding Peierls stress was estimated to be approximately 182 MPa.
Heting Liao, Hajime Kimizuka, Akio Ishii, Jun-Ping Du, and Shigenobu Ogata, Scripta Materialia, 210, 114480 (2022).
The beta'' precipitate is a primary strengthening precipitate in Mg–Y alloys. It nucleates as localized zigzag- and hexagonal-shaped clusters. Studies on the nucleation kinetics of beta'' precipitate are scarce. In this study, we applied the kinetic Monte Carlo (KMC) approach to explore the nucleation kinetics of the beta'' precipitates in the Mg–3.0 at.%Y system using a density functional theory-based interatomic potential. The time evolution of nucleation of the beta'' precipitates was characterized based on the KMC results. Using these results, we predicted the existence of an optimum temperature for the formation of the beta'' precipitates to be 550 K, at which the time necessary for nucleation is the shortest. Moreover, an upper temperature limit, above which the beta'' precipitates cannot nucleate, was computed as 700 K. This study explains precipitate nucleation in Mg–Y alloys at an atomic level and provides the theory for obtaining an optimal age-hardening response.
Kai Guan, Mariko Egami, Daisuke Egusa, Hajime Kimizuka, Michiaki Yamasaki, Yoshihito Kawamura, and Eiji Abe, Scripta Materialia, 207, 114282 (2022).
We have investigated short-range order (SRO) solute clusters in the long-period stacking/order (LPSO) phases with an intrinsic-I (I1) type stacking faults (SFs), which have been uniquely formed in Mg-Co-Y alloys, based on atomic-resolution scanning transmission electron microscopy (STEM) combined with first-principles calculations. The Co3Y5 SRO cluster model embedded across the I1-type SFs has been successfully constructed to satisfy the observed electron diffraction, STEM images and computed energetic stabilities. The optimized Co3Y5 cluster configurations appear to be significantly deviated from the host Mg atom positions and consequently provide a significant gain for the energetic stability of the I1-type LPSO phases. This is a similar phenomenon observed for the major LPSO phases with an intrinsic-II (I2) type SFs, and therefore the solute SRO clusters play an important key role for the formation and stability of the LPSO phases in ternary Mg alloys.
2021年
2021年
君塚肇, 三好宙, 石井明男, 尾方成信, まてりあ, 60, 467-474 (2021).
筆者らは，第一原理電子状態計算を活用して，合金中のナノ析出物の形成を支配する溶質原子間の相互作用を識別・同定するとともに，溶質原子の規則化・クラスター化の原子描像を明らかにしてきた．最近では，原子レベルの速度論と熱力学の詳細情報を採り入れて，置換型合金中のナノ析出物の核生成過程を記述することができるアプローチを構築している．本稿では，代表的な時効硬化型アルミニウム合金である Al-Cu 合金を対象に，時効硬化過程において重要な役割を果たすGPゾーンの核生成過程を原子論的情報に基づいてモデル化するとともに，温度・組成に依存して変化するGPゾーンの核生成・成長のクライテリアを予測的に獲得した結果について紹介する．
Hajime Kimizuka and Motoyuki Shiga, Physical Review Materials, 5, 065406 (2021).
Nuclear quantum effects (NQEs) are highly important for understanding a host of kinetic processes that occur with the participation of H (e.g., H adsorption, diffusion, permeation, and trapping in materials). In this paper, ab initio path integral molecular dynamics simulations were used to investigate NQEs on the lattice diffusion of H in common face-centered cubic (fcc) metals such as Al, Ag, and Cu over a wide temperature range of 75–1200 K (75–900 K for Al). We determined that the dependence of H diffusivities on temperature in Ag and Cu has a “reversed-S” shape on Arrhenius plots, as confirmed for fcc Pd in our recent study [H. Kimizuka et al., Phys. Rev. B 100, 024104 (2019)]. This result illustrates that the phenomenon is common in many fcc metals in which H atoms prefer to occupy octahedral sites. On the other hand, in the case of Al, in which H atoms prefer to occupy tetrahedral sites, the dependence of H diffusivities on temperature exhibits a familiar “C” shape. Such counterintuitive behavior is ascribed to differences in the dependence on temperature of the activation barriers for H migration between both types of fcc metals; this is due to the NQEs involving a competition between deceleration of H migration, which becomes effective at high temperatures because of zero-point vibrations, and acceleration of H migration, which becomes effective at low temperatures because of quantum tunneling. The dominance of the two mechanisms is determined by the coupling of the NQEs and the site preference of H depending on the metal. This finding has important implications for the interpretation of kinetic processes involving the crossover from classical to quantum behavior of H atoms jumping between different types of interstitial sites.
Ruxia Liu, Masakazu Tane, Hajime Kimizuka, Yuji Shirakami, Ken-ichi Ikeda, Seiji Miura, Koji Morita, Tohru S. Suzuki, Yoshio Sakka, Lianmeng Zhang, and Tohru Sekino, Journal of the European Ceramic Society, 41, 2278-2289 (2021).
The elastic properties of a single-crystalline Ti3SiC2 MAX phase with a nanolayered crystal structure, comprising Ti-Si and two distinct Ti-C bonding layers, that had remained unclear because of the difficulty in growing large single crystals, were studied. Rather than unavailable large single crystals, polycrystalline samples with a crystallographic texture were prepared. By analyzing the polycrystalline elastic constants on the basis of an inverse Voigt–Reuss–Hill approximation, the elastic properties of a single crystal Ti3SiC2 with a hexagonal symmetry were determined. This revealed that the single-crystalline Young's modulus was almost isotropic despite its highly anisotropic layered structure. The shear modulus for (0001)<11-20> was higher than that for {11-20}[0001] in contrast to the basal slip-dominated plastic deformation reflecting the layered structure. Furthermore, first-principles calculations revealed that heterogeneous interlayer elastic deformation caused by the stabilization of Ti-Si bonding is the origin of the elastic isotropy in a Ti3SiC2 MAX phase.
Hiroshi Miyoshi, Hajime Kimizuka, Akio Ishii, and Shigenobu Ogata, Scientific Reports, 11, 4503 (2021).
Solid-state precipitation is a key heat-treatment strategy for strengthening engineering alloys. Therefore, predicting the precipitation process of localized solute-rich clusters, such as Guinier–Preston (GP) zones, is necessary. We quantitatively evaluated the critical nucleus size and nucleation barrier of GP zones in Al–Cu alloys, illustrating the precipitation preferences of single-layer (GP1) and double-layer (GP2) GP zones. Based on classical nucleation theory using an effective multi-body potential for dilute Al–Cu systems, our model predicted GP1 and GP2 precipitation sequences at various temperatures and Cu concentrations in a manner consistent with experimental observations. The crossover between formation enthalpy curves of GP1 and GP2 with increasing cluster size determines the critical conditions under which GP2 zones can nucleate without prior formation of GP1 zones. This relationship reflects competing interactions within and between clusters. The results illustrate the underlying mechanisms of competing nucleation between zones, and provide guidance for tailoring aging conditions to achieve desired mechanical properties for specific applications.
2020年
2020年
経路積分法で探る金属中の水素の拡散メカニズム（最近の研究から）
君塚肇, 尾方成信, 志賀基之, 日本物理学会誌, 75, 484-490 (2020).
水素は元素のうちで最も軽く量子性が強いため，金属中に導入された場合，他の不純物原子と比べて種々の特異な性質・挙動を示す．その中の一つに水素の拡散がある．通常，軽い同位体は重いものよりも速く物質内を拡散する．しかし，これは水素の場合には必ずしも当てはまらない．実際，面心立方構造を有するパラジウム，銅，ニッケル中の水素同位体（軽水素，重水素，三重水素）の拡散係数の測定値は特異な同位体効果，すなわち，高温および低温では軽い同位体ほど速く拡散が進行するものの，ある中間の温度域では重い同位体ほど速く拡散が進むことが報告されている．この現象は，金属結晶中の格子間サイトに位置する水素同位体の局所振動モードの違いのみからでは説明できず，水素同位体の物質中の輸送や反応の速度論の予測が今なお，材料物理における挑戦的な課題の一つであることを示すものである．
多くの遷移金属は水素と広い非化学量論的組成にわたって金属的性質をもつ水素化物相を形成する．中でもパラジウムは，室温において容易に水素と反応し，結晶格子間に多量の水素（パラジウムの体積の約1,000倍の気体水素）を吸収する．このような性質からパラジウムは水素吸蔵金属や水素透過膜材料のプロトタイプとして注目され，研究が盛んに行われている．パラジウムを始めとする種々の材料の水素透過・吸蔵特性を評価する上で，材料中の水素の拡散速度はその性能を決める重要な因子となる．温度等に依存して多様に変化する水素同位体の振る舞いを根本から理解し，その拡散メカニズムの真の姿を明らかにするためには，水素の原子核に対する量子効果を適切に考慮することができる理論的モデルを構築することが欠かせない．
最近，我々はこの課題を解決するために，経路積分法と電子状態計算を連成させた第一原理手法を用いて電子と原子核の双方の量子力学的性質を考慮し，面心立方構造を有するパラジウム結晶中の水素同位体の拡散挙動を幅広い温度で予測することに成功した．その結果，パラジウム中の水素同位体の拡散係数のアレニウスプロットは，温度の低下とともに量子ゆらぎの影響が顕在化することによって“上に凸”の折れ曲がり（拡散の抑制）を示すこと，更に低温域では量子トンネル効果の発現によって拡散障壁は減少に転じ，“下に凸”の折れ曲がり（拡散の促進）が生じることを見出した．このようなアレニウスプロット上の“逆S字型”の挙動は，異なる温度・質量依存性をもつ量子効果の競合によって生じるものであり，実験により示唆されていながらその機構の詳細が不明であった水素拡散の特異な同位体効果を明瞭に説明する．
パラジウム中の格子間サイトに固溶する水素はエネルギー的に四面体サイトよりも八面体サイトの方を好むが，1%程度の格子膨張を与えると水素原子の安定位置が四面体サイトに遷移する．我々の解析では，これによって量子効果の影響が潜在化し，水素拡散が抑制されなくなることを見出した．このような量子ゆらぎと格子ひずみの協調によって発現する高速な格子拡散モードは，水素透過特性に対して弾性ひずみを工学的に利用した材料設計の可能性を開くものである．
我々のアプローチは様々な材料における水素同位体の量子的振る舞いを理解する際に幅広く適用できる．今後，本研究の成果を発展させることで，水素エネルギー材料の性能向上に対する有用な設計指針が得られるようになることを期待する．
多くの遷移金属は水素と広い非化学量論的組成にわたって金属的性質をもつ水素化物相を形成する．中でもパラジウムは，室温において容易に水素と反応し，結晶格子間に多量の水素（パラジウムの体積の約1,000倍の気体水素）を吸収する．このような性質からパラジウムは水素吸蔵金属や水素透過膜材料のプロトタイプとして注目され，研究が盛んに行われている．パラジウムを始めとする種々の材料の水素透過・吸蔵特性を評価する上で，材料中の水素の拡散速度はその性能を決める重要な因子となる．温度等に依存して多様に変化する水素同位体の振る舞いを根本から理解し，その拡散メカニズムの真の姿を明らかにするためには，水素の原子核に対する量子効果を適切に考慮することができる理論的モデルを構築することが欠かせない．
最近，我々はこの課題を解決するために，経路積分法と電子状態計算を連成させた第一原理手法を用いて電子と原子核の双方の量子力学的性質を考慮し，面心立方構造を有するパラジウム結晶中の水素同位体の拡散挙動を幅広い温度で予測することに成功した．その結果，パラジウム中の水素同位体の拡散係数のアレニウスプロットは，温度の低下とともに量子ゆらぎの影響が顕在化することによって“上に凸”の折れ曲がり（拡散の抑制）を示すこと，更に低温域では量子トンネル効果の発現によって拡散障壁は減少に転じ，“下に凸”の折れ曲がり（拡散の促進）が生じることを見出した．このようなアレニウスプロット上の“逆S字型”の挙動は，異なる温度・質量依存性をもつ量子効果の競合によって生じるものであり，実験により示唆されていながらその機構の詳細が不明であった水素拡散の特異な同位体効果を明瞭に説明する．
パラジウム中の格子間サイトに固溶する水素はエネルギー的に四面体サイトよりも八面体サイトの方を好むが，1%程度の格子膨張を与えると水素原子の安定位置が四面体サイトに遷移する．我々の解析では，これによって量子効果の影響が潜在化し，水素拡散が抑制されなくなることを見出した．このような量子ゆらぎと格子ひずみの協調によって発現する高速な格子拡散モードは，水素透過特性に対して弾性ひずみを工学的に利用した材料設計の可能性を開くものである．
我々のアプローチは様々な材料における水素同位体の量子的振る舞いを理解する際に幅広く適用できる．今後，本研究の成果を発展させることで，水素エネルギー材料の性能向上に対する有用な設計指針が得られるようになることを期待する．
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