Reticular Imine-linked Coordination Polymers Based on Paddlewheel Diruthenium/Dirhodium Nodes: Synthesis and Metal-site Dependent Photocatalytic Reduction of CO2.

The paddlewheel-type dimetal core ([M2]) is a ubiquitous motif in the nodes in coordination polymers (CPs) and metal-organic frameworks (MOFs). However, their preparation has relied on ligand-substitution-labile metal ions owing to challenges associated with crystallization. Consequently, examples featuring ligand-substitution-inert metal ions, such as Ru or Rh, are scarce. This study presents the synthesis of novel reticular imine-linked CPs incorporating the paddlewheel-type diruthenium(II, II) ([Ru2II,II]; 1-Ru) or dirhodium(II, II) ([Rh2II,II]; 1-Rh) subunits. The synthetic approach involved a Schiff base dehydration condensation reaction between p-formylbenzoate-bridged [Ru2II,II] or [Rh2II,II] precursors (i.e., CHO-Ru and CHO-Rh, respectively) and 2,5-dimethyl-1,4-phenylenediamine in a 1:2 ratio. The catalytic activities of 1-Ru and 1-Rh for the photochemical reduction of CO2 in a heterogeneous system depended on the metal site. The 1-Rusystem exhibited exceptional selectivity, generating 3.0 ´ 104 mmol g-1 of CO after 24 h of irradiation, whereas the 1-Rhsystem generated a lower amount of CO (3.2 ´ 103 mmol g-1). The catalytic activity of 1-Ru ranked with that of all relevant catalytic systems. This study paves the way for the exploration of [Ru2II,II]- or [Rh2II,II]-based polymers with open metal site-dependent functional properties.

Direct Oxidation of Methane to Methanol over Transition-Metal-Free Ferrierite Zeolite Catalysts.

Direct oxidation of methane to methanol was reported to be highly dependent on the transition- or noble-metal-loading catalysts in the past decades. Here, we show that the transition-metal-free aluminosilicate ferrierite (FER) zeolite effectively catalyzed methane and N2O to methanol for the first time. The distorted tetracoordinated Al in the framework and pentacoordinated Al on the extra framework formed during calcination, activation, and reaction processes were confirmed as the potential active centers. The possible reaction pathway similar to the Fe-containing zeolites was advocated based on the reaction results using different oxidants, N2O adsorption FTIR spectra, and 27Al MAS NMR spectra. The stable and efficient methanol production capacity of FER zeolite was ascribed to the two-dimensional straight channels and its distinctive Al distribution of FER zeolite (CP914C) from Zeolyst. The transition-metal-free FER zeolite performed better than the record in the literature and our recent results using transition-metal-containing catalysts in terms of selectivity and formation rate of methanol and stability. This work has great significance and prospects for utilizing CH4 and N2O as resources and will open new avenues for methane oxidation.

Molecular dynamics simulations for interfacial structure and affinity between carboxylic acid-modified Al2O3 and polymer melts.

Controlling the dispersion state of nanoparticles in a polymer matrix is necessary to produce polymer nanocomposites. The surface modification of nanoparticles is used to enable their dispersion in polymers. Moreover, molecular dynamics (MD) simulations are useful for revealing the interfacial properties between nanoparticles and polymers to aid in the design of materials. In this study, the effect of surface coverage, modifier length, and polymer species on the interfacial structure and affinity between surface-modified Al2O3 and polymer melts were investigated using all-atom MD simulations. Hexanoic, decanoic, and tetradecanoic acids were used as surface modifiers, and polypropylene (PP), polystyrene (PS), and poly (methyl methacrylate) (PMMA) were used as polymers. The work of adhesion Wadh and the work of immersion Wimm were selected as quantitative measures of affinity. Wadh was calculated using the phantom-wall approach, and Wimm was calculated by simply subtracting the surface tension of polymers γL from Wadh. The results showed that Wadh and Wimm were improved by surface modification with low coverage, owing to a good penetration of the polymer. The effect of modifier length on Wadh and Wimm was small. Whereas Wadh increased in the following order: PP < PS < PMMA, Wimm increased as follows: PMMA < PS < PP. Finally, the trend of Wadh and Wimm was organized using the Flory-Huggins interaction parameter χ between the modifier and the polymer. This study demonstrates that the interfacial affinity can be improved by tuning the surface coverage and modifier species depending on the polymer matrix.

NbTe4 Phase-Change Material: Breaking the Phase-Change Temperature Balance in 2D Van der Waals Transition-Metal Binary Chalcogenide.

2D van der Waals (vdW) transition metal di-chalcogenides (TMDs) have garnered significant attention in the nonvolatile memory field for their tunable electrical properties, scalability, and potential for phase engineering. However, their complex switching mechanism and complicated fabrication methods pose challenges for mass production. Sputtering is a promising technique for large-area 2D vdW TMD fabrication, but the high melting point (typically Tm > 1000 °C) of TMDs requires elevated temperatures for good crystallinity. This study focuses on the low-Tm 2D vdW TM tetra-chalcogenides and identifies NbTe4 as a promising candidate with an ultra-low Tm of around 447 °C (onset temperature). As-grown NbTe4 forms an amorphous phase upon deposition that can be crystallized by annealing at temperatures above 272 °C. The simultaneous presence of a low Tm and a high crystallization temperature Tc can resolve important issues facing current phase-change memory compounds, such as high Reset energies and poor thermal stability of the amorphous phase. Therefore, NbTe4 holds great promise as a potential solution to these issues.

Mechanisms of chemical-reaction-induced tensile deformation of an Fe/Ni/Cr alloy revealed by reactive atomistic simulations.

High entropy alloys (HEAs) have demonstrated excellent potential in various applications owing to the unique properties. One of the most critical issues of HEAs is the stress corrosion cracking (SCC) which limits its reliability in practical applications. However, the SCC mechanisms have not been fully understood yet because of the difficulty of experimental measuring of atomic-scale deformation mechanisms and surface reactions. In this work, we conduct atomistic uniaxial tensile simulations using an FCC-type Fe40Ni40Cr20 alloy as a typical simplification of normal HEAs, in order to reveal how a corrosive environment such as high-temperature/pressure water affects the tensile behaviors and deformation mechanisms. In a vacuum, we observe the generation of layered HCP phases in an FCC matrix during tensile simulation induced by the formation of Shockley partial dislocations from surface and grain boundaries. While, in the corrosive environment of high-temperature/pressure water, the alloy surface is oxidized by chemical reactions with water and this oxide surface layer can suppress the formation of Shockley partial dislocation as well as the resulting FCC-to-HCP phase transition; instead, a BCC phase is preferred to generate in the FCC matrix for releasing the tensile stress and stored elastic energy, leading to a reduced ductility as the BCC phase is typically more brittle than the FCC and HCP. Overall, the deformation mechanism of the FeNiCr alloy is changed by the presence of a high-temperature/pressure water environment-from FCC-to-HCP phase transition in vacuum to FCC-to-BCC phase transition in water. This theoretical fundamental study may contribute to the further improvement of HEAs with high resistance to SCC in experiments.

Definition of Atomic-Scale Contact: What Dominates the Atomic-Scale Friction Behaviors?

The definition of atomic-scale contact is a very ambiguous issue owing to the discrete atomic arrangement, which hinders the development of contact theory and nano-tribological techniques. In this work, we studied the atomic-scale contact area and their correlations with friction force based on three distinct contact definitions (interatomic distance, force, and interfacial chemical bonds) by performing large-scale atomistic simulations on a typical ball-on-disk contact model. In the simulations, the measured contact areas defined by interatomic distance, force, and interfacial chemical bonds (referred as to Adist, Aforce, and Abond, respectively) are not equivalent at all, while we interestingly clarify that only Adist is consistent with the one calculated by continuum Hertz contact mechanics, and moreover, only Abond is proportional to the friction force indicating that Abond is the dominant one for determining materials' frictional behaviors. The above fundamental insights into the atomic-scale contact problems are useful to deeply understand the origins of tribological phenomena and contribute to the further prediction of atomic-scale friction.

Atom-by-Atom and Sheet-by-Sheet Chemical Mechanical Polishing of Diamond Assisted by OH Radicals: A Tight-Binding Quantum Chemical Molecular Dynamics Simulation Study.

Ultraflat and damage-free single-crystal diamond is a promising material for use in electronic devices such as field-effect transistors. Diamond surfaces are conventionally prepared by the chemical mechanical polishing (CMP) method, although the CMP efficiency remains a critical issue owing to the extremely high hardness of diamond. Recently, OH radicals have been demonstrated to be potentially useful for improving the CMP efficiency for diamond; however, the underlying mechanisms are still elusive. In this work, we applied our previously developed CMP-specialized tight-binding quantum chemical molecular dynamics simulator to comprehensively elucidate the CMP mechanisms of diamond assisted by OH radicals. Our simulation results indicate that the diamond surface is oxidized by reactions with OH radicals and then a concomitant surface reconstruction takes place due to the distorted and unstable nature of the oxidized diamond surface structure. Furthermore, we interestingly reveal that the reconstruction of the diamond surface ultimately leads to two distinct removal mechanisms: (i) gradual atom-by-atom removal through the desorption of gaseous molecules (e.g., CO2 and H2CO3) and (ii) drastic sheet-by-sheet removal through the exfoliation of graphitic ring structures. Hence, we propose that promoting the oxidation-induced graphitization of the diamond surface may provide a route to further improving the CMP efficiency.

Role of OH Termination in Mitigating Friction of Diamond-like Carbon under High Load: A Joint Simulation and Experimental Study.

Diamond-like carbon (DLC) has recently attracted much attention as a promising solid-state lubricant because it exhibits low friction, low abrasion, and high wear resistance. Although we previously reported the reason why H-terminated DLC exhibits low friction based on a tight-binding quantum chemical molecular dynamics (TB-QCMD) simulation, experimentally, the low-friction state of H-terminated DLC is not stable, limiting its application. In the present work, our TB-QCMD simulations suggest that H/OH-terminated DLC could give low friction even under high loads, whereas H-terminated DLC could not. By using gas-phase friction experiments, we confirm that OH termination can indeed provide much more stable lubricity than H termination, validating the predictions from simulations. We conclude that H/OH-terminated DLC is a new low-friction material with high load capacity and high stable lubricity that may be suitable for practical use in industrial applications.

Cooperative roles of chemical reactions and mechanical friction in chemical mechanical polishing of gallium nitride assisted by OH radicals: tight-binding quantum chemical molecular dynamics simulations.

Chemical mechanical polishing (CMP) is a key manufacturing process for applying gallium nitride (GaN), especially the Ga-face GaN, to semiconductor devices such as laser diodes. However, the CMP efficiency for GaN is very low due to its high hardness and chemical stability. Experimentally, OH radicals appear able to improve the CMP efficiency of GaN polished by a SiO2 abrasive grain, whereas the mechanisms of the OH-radical-assisted CMP process remain unclear because experimental elucidation of the complex chemical reactions occurring among GaN substrate, abrasive grain, and OH radicals is difficult. In this work, we used our previously developed tight-binding quantum chemical molecular dynamics simulator to study the OH-radical-assisted CMP process of the widely employed Ga-face GaN substrate polished by an amorphous SiO2 abrasive grain in an effort to understand how OH radicals assist the CMP process and then aid the development of next-generation CMP techniques. Our simulations revealed that the OH-radical-assisted CMP process of GaN occurs via the following three basic reaction steps: (i) first, all hydrogen terminations on the GaN surface are replaced by OH terminations through continuous reactions with OH radicals; (ii) after the substrate is fully terminated by OH, the hydrogen atoms of these OH terminations are removed by reacting with newly added OH radicals, which forms H2O molecules and leaves energetic oxygen atoms with dangling bonds on the surface; and (iii) finally, these energetic oxygen atoms intrude inside the substrate with concomitant dissociation of Ga-N bonds and the generation of N2 and gallium hydroxide molecules, which accumulatively lead to the removal of N and Ga atoms from the substrate.

Non-Empirical Law for Nanoscale Atom-by-Atom Wear.

Wear of contact materials results in energy loss and device failure. Conventionally, wear is described by empirical laws such as the Archard's law; however, the fundamental physical and chemical origins of the empirical law have long been elusive, and moreover empirical wear laws do not always hold for nanoscale contact, collaboratively hindering the development of high-durable tribosystems. Here, a non-empirical and robustly applicable wear law for nanoscale contact situations is proposed. The proposed wear law successfully unveils why the nanoscale wear behaviors do not obey the description by Archard's law in all cases although still obey it in certain experiments. The robustness and applicability of the proposed wear law is validated by atomistic simulations. This work affords a way to calculate wear at nanoscale contact robustly and theoretically, and will contribute to developing design principles for wear reduction.

Triboemission of hydrocarbon molecules from diamond-like carbon friction interface induces atomic-scale wear.

Understanding atomic-scale wear is crucial to avoid device failure. Atomic-scale wear differs from macroscale wear because chemical reactions and interactions at the friction interface are dominant in atomic-scale tribological behaviors, instead of macroscale properties, such as material strength and hardness. It is particularly challenging to reveal interfacial reactions and atomic-scale wear mechanisms. Here, our operando friction experiments with hydrogenated diamond-like carbon (DLC) in vacuum demonstrate the triboemission of various hydrocarbon molecules from the DLC friction interface, indicating its atomic-scale chemical wear. Furthermore, our reactive molecular dynamics simulations reveal that this triboemission of hydrocarbon molecules induces the atomic-scale mechanical wear of DLC. As the hydrogen concentration in hydrogenated DLC increases, the chemical wear increases while mechanical wear decreases, indicating an opposite effect of hydrogen concentration on chemical and mechanical wear. Consequently, the total wear shows a concave hydrogen concentration dependence, with an optimal hydrogen concentration for wear reduction of around 20%.

Ionic Conductivity in Ionic Liquid Nano Thin Films.

Thin film approaches are powerful methods for gaining a nanoscale understanding of interfacial ionic liquids (ILs) in the vicinity of solids. These approaches are used to directly elucidate the interfacial contributions to the physical properties of ILs as nanoscale thin films have significant proportions of the surface or interface region with respect to their total volume. Here, we report the growth of a uniform [emim][TFSA] thin film ionic liquid on a chemically modified, well-wettable sapphire, thereby allowing the in situ measurement of its ionic conductivity on the nanoscale. We observed the thickness-dependent behavior of the ionic conductivity, which gradually decreased especially when the thickness was less than 10 nm, and found it to be quantitatively analyzed well by using an empirical two-layer model. The molecular dynamics (MD) simulations show that the thickness-dependent ionic conductivity originates from the solid-like structuring of the IL near the substrate, reproducing a thickness-dependent ionic conductivity. The MD simulation results suggest that the thickness of the low conductivity region determined in the two-layer model should roughly correspond to the thickness of the solid-like structuring of the IL near the substrate.

Molecular dynamics study of mesophase transitions upon annealing of imidazolium-based ionic liquids with long-alkyl chains.

Molecular dynamics simulations are performed on a 1-dodecyl-3-methylimidazolium hexafluorophosphate ([C12mim][PF6]) ionic liquid using a united-atom model. The ionic liquid exhibits second step relaxation at temperatures below a crossover point, where the diffusion coefficient shows an Arrhenius to non-Arrhenius transition. Annealing below this crossover temperature makes an isotropic to mesophase transition, where the smectic A (SmA) phase or crystal-like smectic B (SmB) phase forms. Hundreds of nanoseconds are required for completing these transitions. A normal diffusion process is found for anions along the layer-normal and -lateral directions in the SmA phase, but only in the lateral directions in the SmB phase. We find a preserved orientational order for the imidazolium-ring rotational and the alkyl-chain reorientational dynamics in both of the smectic phases.

Tight-Binding Quantum Chemical Molecular Dynamics Study on the Friction and Wear Processes of Diamond-Like Carbon Coatings: Effect of Tensile Stress.

Diamond-like carbon (DLC) coatings have attracted much attention as an excellent solid lubricant due to their low-friction properties. However, wear is still a problem for the durability of DLC coatings. Tensile stress on the surface of DLC coatings has an important effect on the wear behavior during friction. To improve the tribological properties of DLC coatings, we investigate the friction process and wear mechanism under various tensile stresses by using our tight-binding quantum chemical molecular dynamics method. We observe the formation of C-C bonds between two DLC substrates under high tensile stress during friction, leading to a high friction coefficient. Furthermore, under high tensile stress, C-C bond dissociation in the DLC substrates is observed during friction, indicating the atomic-level wear. These dissociations of C-C bonds are caused by the transfer of surface hydrogen atoms during friction. This work provides atomic-scale insights into the friction process and the wear mechanism of DLC coatings during friction under tensile stress.

Parallel Large-Scale Molecular Dynamics Simulation Opens New Perspective to Clarify the Effect of a Porous Structure on the Sintering Process of Ni/YSZ Multiparticles.

Ni sintering in the Ni/YSZ porous anode of a solid oxide fuel cell changes the porous structure, leading to degradation. Preventing sintering and degradation during operation is a great challenge. Usually, a sintering molecular dynamics (MD) simulation model consisting of two particles on a substrate is used; however, the model cannot reflect the porous structure effect on sintering. In our previous study, a multi-nanoparticle sintering modeling method with tens of thousands of atoms revealed the effect of the particle framework and porosity on sintering. However, the method cannot reveal the effect of the particle size on sintering and the effect of sintering on the change in the porous structure. In the present study, we report a strategy to reveal them in the porous structure by using our multi-nanoparticle modeling method and a parallel large-scale multimillion-atom MD simulator. We used this method to investigate the effect of YSZ particle size and tortuosity on sintering and degradation in the Ni/YSZ anodes. Our parallel large-scale MD simulation showed that the sintering degree decreased as the YSZ particle size decreased. The gas fuel diffusion path, which reflects the overpotential, was blocked by pore coalescence during sintering. The degradation of gas diffusion performance increased as the YSZ particle size increased. Furthermore, the gas diffusion performance was quantified by a tortuosity parameter and an optimal YSZ particle size, which is equal to that of Ni, was found for good diffusion after sintering. These findings cannot be obtained by previous MD sintering studies with tens of thousands of atoms. The present parallel large-scale multimillion-atom MD simulation makes it possible to clarify the effects of the particle size and tortuosity on sintering and degradation.

Diamond-like carbon coating under oleic acid lubrication: Evidence for graphene oxide formation in superlow friction.

The achievement of the superlubricity regime, with a friction coefficient below 0.01, is the Holy Grail of many tribological applications, with the potential to have a remarkable impact on economic and environmental issues. Based on a combined high-resolution photoemission and soft X-ray absorption study, we report that superlubricity can be realized for engineering applications in bearing steel coated with ultra-smooth tetrahedral amorphous carbon (ta-C) under oleic acid lubrication. The results show that tribochemical reactions promoted by the oil lubrication generate strong structural changes in the carbon hybridization of the ta-C hydrogen-free carbon, with initially high sp3 content. Interestingly, the macroscopic superlow friction regime of moving mechanical assemblies coated with ta-C can be attributed to a few partially oxidized graphene-like sheets, with a thickness of not more than 1 nm, formed at the surface inside the wear scar. The sp2 planar carbon and oxygen-derived species are the hallmark of these mesoscopic surface structures created on top of colliding asperities as a result of the tribochemical reactions induced by the oleic acid lubrication. Atomistic simulations elucidate the tribo-formation of such graphene-like structures, providing the link between the overall atomistic mechanism and the macroscopic experimental observations of green superlubricity in the investigated ta-C/oleic acid tribological systems.

Experimental and Quantum Chemical Approaches to Develop Highly Selective Nanocatalysts for CO2 -free Power Circulation.

Renewable electricity must be utilized to usefully suppress the atmospheric CO2 concentration and slow the progression of global warming. We have thus proposed a new concept involving CO2 -free electric power circulation systems via highly selective electrochemical reactions of alcohol/carboxylic acid redox couples. Design concepts for nanocatalysts able to catalyze highly selective electrochemical reactions are provided from both experimental and quantum mechanical perspectives.

Superionic Conduction in Co-Vacant P2-Nax CoO2 Created by Hydrogen Reductive Elimination.

The layered P2-Nax MO2 (M: transition metal) system has been widely recognized as electronic or mixed conductor. Here, we demonstrate that Co vacancies in P2-Nax CoO2 created by hydrogen reductive elimination lead to an ionic conductivity of 0.045 S cm(-1) at 25 °C. Using in situ synchrotron X-ray powder diffraction and Raman spectroscopy, the composition of the superionic conduction phase is evaluated to be Na0.61 (H3 O)0.18 Co0.93 O2 . Electromotive force measurements as well as molecular dynamics simulations indicate that the ion conducting species is proton rather than hydroxide ion. The fact that the Co-stoichiometric compound Nax (H3 O)y CoO2 does not exhibit any significant ionic conductivity proves that Co vacancies are essential for the occurrence of superionic conductivity.

Atomistic Mechanisms of Chemical Mechanical Polishing of a Cu Surface in Aqueous H2O2: Tight-Binding Quantum Chemical Molecular Dynamics Simulations.

We applied our original chemical mechanical polishing (CMP) simulator based on the tight-binding quantum chemical molecular dynamics (TB-QCMD) method to clarify the atomistic mechanism of CMP processes on a Cu(111) surface polished with a SiO2 abrasive grain in aqueous H2O2. We reveal that the oxidation of the Cu(111) surface mechanically induced at the friction interface is a key process in CMP. In aqueous H2O2, in the first step, OH groups and O atoms adsorbed on a nascent Cu surface are generated by the chemical reactions of H2O2 molecules. In the second step, at the friction interface between the Cu surface and the abrasive grain, the surface-adsorbed O atom intrudes into the Cu bulk and dissociates the Cu-Cu bonds. The dissociation of the Cu-Cu back-bonds raises a Cu atom from the surface that is mechanically sheared by the abrasive grain. In the third step, the raised Cu atom bound to the surface-adsorbed OH groups is removed from the surface by the generation and desorption of a Cu(OH)2 molecule. In contrast, in pure water, there are no geometrical changes in the Cu surface because the H2O molecules do not react with the Cu surface, and the abrasive grain slides smoothly on the planar Cu surface. The comparison between the CMP simulations in aqueous H2O2 and pure water indicates that the intrusion of a surface-adsorbed O atom into the Cu bulk is the most important process for the efficient polishing of the Cu surface because it induces the dissociation of the Cu-Cu bonds and generates raised Cu atoms that are sheared off by the abrasive grain. Furthermore, density functional theory calculations show that the intrusion of the surface-adsorbed O atoms into the Cu bulk has a high activation energy of 28.2 kcal/mol, which is difficult to overcome at 300 K. Thus, we suggest that the intrusion of surface-adsorbed O atoms into the Cu bulk induced by abrasive grains at the friction interface is a rate-determining step in the Cu CMP process.

Tight-binding quantum chemical molecular dynamics simulations for the elucidation of chemical reaction dynamics in SiC etching with SF6/O2 plasma.

We used our etching simulator [H. Ito et al., J. Phys. Chem. C, 2014, 118, 21580-21588] based on tight-binding quantum chemical molecular dynamics (TB-QCMD) to elucidate SiC etching mechanisms. First, the SiC surface is irradiated with SF5 radicals, which are the dominant etchant species in experiments, with the irradiation energy of 300 eV. After SF5 radicals bombard the SiC surface, Si-C bonds dissociate, generating Si-F, C-F, Si-S, and C-S bonds. Then, etching products, such as SiS, CS, SiFx, and CFx (x = 1-4) molecules, are generated and evaporated. In particular, SiFx is the main generated species, and Si atoms are more likely to vaporize than C atoms. The remaining C atoms on SiC generate C-C bonds that may decrease the etching rate. Interestingly, far fewer Si-Si bonds than C-C bonds are generated. We also simulated SiC etching with SF3 radicals. Although the chemical reaction dynamics are similar to etching with SF5 radicals, the etching rate is lower. Next, to clarify the effect of O atom addition on the etching mechanism, we also simulated SiC etching with SF5 and O radicals/atoms. After bombardment with SF5 radicals, Si-C bonds dissociate in a similar way to the etching without O atoms. In addition, O atoms generate many C-O bonds and COy (y = 1-2) molecules, inhibiting the generation of C-C bonds. This indicates that O atom addition improves the removal of C atoms from SiC. However, for a high O concentration, many C-C and Si-Si bonds are generated. When the O atoms dissociate the Si-C bonds and generate dangling bonds, the O atoms terminate only one or two dangling bonds. Moreover, at high O concentrations there are fewer S and F atoms to terminate the dangling bonds than at low O concentration. Therefore, few dangling bonds of dissociated Si and C atoms are terminated, and they form many Si-Si and C-C bonds. Furthermore, we propose that the optimal O concentration is 50-60% because both Si and C atoms generate many etching products producing fewer C-C and Si-Si bonds are generated. Finally, we conclude that our TB-QCMD etching simulator is effective for designing the optimal conditions for etching processes in which chemical reactions play a significant role.