Search

It has been observed that the properties of room temperature ionic liquids near solid substrates are different from those of bulk liquids, and these properties play an important role in the development of catalysts, lubricants, and electrochemical devices. In this paper, we report microscopic studies of ionic liquid/solid interfaces performed using molecular dynamics simulations. The structural and dynamic properties of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMIM-TFSI) on mica and graphite interfaces were thoroughly investigated to elucidate the microscopic origins of the formation of layered structures at the interfaces. Our investigation included the observation of structural and orientational changes of ions as a function of distance from the surfaces, and contour mappings of ions parallel and perpendicular to the surfaces. By virtue of such detailed analyses, we found that, during the 5 ns simulation, the closest layer of BMIM-TFSI behaves as a two-dimensional ionic crystal on mica and as a liquid or liquid crystal on graphite.

Electric double-layer transistors based on ionic liquid/organic semiconductor interfaces have been extensively studied during the past decade because of their high carrier densities at low operation voltages. Microscopic structures and the dynamics of ionic liquids likely determine the device performance; however, knowledge of these is limited by a lack of appropriate experimental tools. In this study, we investigated ionic liquid/organic semiconductor interfaces using molecular dynamics to reveal the microscopic properties of ionic liquids. The organic semiconductors include pentacene, rubrene, fullerene, and 7,7,8,8-tetracyanoquinodimethane (TCNQ). While ionic liquids close to the substrate always form the specific layered structures, the surface properties of organic semiconductors drastically alter the ionic dynamics. Ionic liquids at the fullerene interface behave as a two-dimensional ionic crystal because of the energy gain derived from the favorable electrostatic interaction on the corrugated periodic substrate.

The functionalization of graphene is important in practical applications of graphene, such as in catalysts. However, the experimental study of the interactions of adsorbed molecules with functionalized graphene is difficult under ambient conditions at which catalysts are operated. Here, the adsorption of CO2 on an oxygen-functionalized epitaxial graphene surface was studied under near-ambient conditions using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). The oxygen-functionalization of graphene is achieved in situ by the photo-induced dissociation of CO2 with X-rays on graphene in a CO2 gas atmosphere. The oxygen species on the graphene surface is identified as the epoxy group by XPS binding energies and thermal stability. Under near-ambient conditions of 1.6 mbar CO2 gas pressure and 175 K sample temperature, CO2 molecules are not adsorbed on the pristine graphene, but are adsorbed on the oxygen-functionalized graphene surface. The increase in the adsorption energy of CO2 on the oxygen-functionalized graphene surface is supported by first-principles calculations with the van der Waals density functional (vdW-DF) method. The adsorption of CO2 on the oxygen-functionalized graphene surface is enhanced by both the electrostatic interactions between the CO2 and the epoxy group and the vdW interactions between the CO2 and graphene. The detailed understanding of the interaction between CO2 and the oxygen-functionalized graphene surface obtained in this study may assist in developing guidelines for designing novel graphene-based catalysts.

An understanding of the characteristics of ionic liquid/graphite interfaces is highly important for electrochemical devices such as batteries and capacitors. In this paper, we report microscopic studies of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMIM-TFSI) on charged graphite electrodes using molecular dynamics simulations to reveal the two-dimensional arrangement of the ions and their dynamics at the interfaces. Analyses of surface distribution and mobility of ions revealed that the ion arrangement changes from a bilayer type to a checkerboard type with increasing applied potential. Whereas the bilayer type arrangement increases the ionic mobility parallel to the interfaces with the negative potential, the ions arranged in the checkerboard type tend to localize because of the increased lateral electrostatic interactions. Furthermore, we revealed that the inhomogeneity of ionic distribution at the positive potential propagates up to a few nanometers from the interface.

By means of the van der Waals density functional and quasi-harmonic approximation, we predict that the adsorption potential energies of CD4 on various fcc metal (1 1 1) surfaces are shallower than those of CH4, whereas the equilibrium distances from the surface are larger. This isotope effect originates from the softening of the C H bond pointing toward the surface, leading to the significant lowering of its vibrational frequency and hence the large zero-point energy difference between CH4 and CD4. The C H bond softening is attributed to the hybridization of the lowest unoccupied molecular orbital with the substrate state.

The atomic scale characterization of dopant atoms in semiconductor devices to establish correlations with the electrical activation of these atoms is essential to the advancement of contemporary semiconductor process technology. Spectro-photoelectron holography combined with first-principles simulations can determine the local three-dimensional atomic structures of dopant elements, which in turn affect their electronic states. In the work reported herein, this technique was used to examine arsenic (As) atoms doped into a silicon (Si) crystal. As 3d core level photoelectron spectroscopy demonstrated the presence of three types of As atoms at a total concentration of approximately 1020 cm–3, denoted as BEH, BEM, and BEL. On the basis of Hall effect measurements, the BEH atoms corresponded to electrically active As occupying substitutional sites and exhibiting larger thermal fluctuations than the Si atoms, while the BEM atoms corresponded to electrically inactive As embedded in the AsnV (n = 2–4) type cluste...

We investigated the transmetalation step of the Suzuki–Miyaura cross coupling reaction (SMR) catalyzed by ligand-free Pd atom or Pd-X– (X = Cl or Br) using first-principles molecular dynamics simulations with an explicit solvent model. When starting from the single Pd atom, the halogen anion bound to the Pd was not replaced by organoboronate species and instead remained bound to the Pd throughout the transmetalation step. However, when starting from the Pd-X– catalyst, one of the two halogen anions was released from the first coordination sphere of the Pd during transmetalation. Therefore, the products after the transmetalation starting with either the single Pd atom or the Pd-X– were the same. We concluded that Pd-X– is the active species of the ligand-free Pd catalyst for the SMR. The overall activation free energies for transmetalation and reductive elimination were relatively low, estimated to be at most, 8.1 kcal/mol for X = Br and 8.4 kcal/mol for X = Cl, respectively, leading to the efficient turno...

Electronic states and growth modes of the Zn-deposited Cu(111) surface at 300 K were quantitatively studied using core-level and valence photoelectron spectroscopies. Both Cu 2p and Zn 2p core-levels shifted to higher binding energy with increasing the amount of deposited Zn up to multilayer. The origin of the core-level shift of Cu 2p was further investigated by density functional theory calculations; the shift of the Cu 2p peak results from the change in the effective electrostatic potential (initial state effect) caused by the formation of Zn-Cu surface alloy, and the increase of coordination numbers of surface Cu atoms by Zn overlayer. The observed valence photoelectron spectra show the formation of the two atomic-layer Zn-Cu alloy up to the Zn coverage of 1 ML, followed by the formation of three-dimensional Zn islands on the alloyed surface at 300 K.

We present a density functional theory study of atomic and molecular adsorption on a single Pt atom deposited at the edges of graphene. We investigate geometric and electronic structures of atoms (H, C, N, and O) and molecules (O2, CO, OH, NO, H2O, and OOH) on a variety of Pt deposited graphene edges and compare the adsorption states with those on a Pt(111) surface and on a Pt single atom. Furthermore, using the calculated adsorption energy and simple kinetic models, the catalytic activities of a Pt single-atom catalyst for the oxygen reduction reaction and CO oxidation are discussed.

A catalytically assisted etching system was developed for the ultra-precision fabrication of optical components, such as X-ray mirrors and extreme-ultraviolet mask blanks. This study demonstrates that an atomically smooth surface with a sub-Angstrom root-mean-square roughness could be achieved on a SiO

We present a density functional theory study on the adsorption and decomposition mechanisms of monomeric formic acid (HCOOH) on a Cu(111) surface. We used Perdew-Burke-Ernzerhof (PBE) functional, PBE with dispersion correction (PBE-D2), and van der Waals density functionals (vdW-DFs). We found that the adsorption energy of HCOOH by using the PBE functional is smaller than the experimental value, while the PBE-D2 and vdW-DFs give better agreement with experimental results. The activation energies of decomposition calculated by using PBE-D2 and vdW-DFs are lower compared with desorption energies, seemingly in contradiction with experimental findings at room temperature, in which no decomposition of HCOOH on Cu(111) is observed when the surface is exposed to the gas phase HCOOH. We performed the reaction rate analysis based on the first-principles calculations for desorption and decomposition processes to clarify this contradiction. We found that the desorption of monomeric HCOOH is faster than that of its decomposition rate at room temperature because of a much larger pre-exponential factor. Thus, no decomposition of monomeric HCOOH should take place at room temperature. Our analysis revealed the competition between desorption and decomposition processes of HCOOH.

The diffusion properties of an excess H atom adsorbed on a fully H-terminated Si(100)(2 × 1)-H surface have been analyzed by means of density functional theory-generalized gradient approximation calculations. Our diffusion pathway models consist of sequences of hops between metastable Si surface atomic structures with an excess H atom. We analyzed the reaction path and corresponding barrier height in each hop using the climbing image nudged elastic band method. The activation energies for diffusion along intra-dimer, intra-row, and inter-row pathways are found to be 0.11, 0.54, and 0.74 eV, respectively, which are quite small compared to the common H diffusion running with a vacant site of hydrogen termination. The weak adsorption energy of the excess H atom is responsible for the small activation energy of diffusion. The present diffusion mechanism suggests that the physical substance of the “hot atom mechanism” proposed in the previous investigations on H exposure onto the Si surface is related to the metastable structures of excessively adsorbed hydrogen atoms.

Bonding energies of N–N, C–C, and C–H in Na–Ga melts and C–H in Ga melts were investigated using first-principles calculations. Activation free energies for formation and dissociation of the above bonds were estimated via the blue-moon ensemble method using constrained molecular dynamics simulations. The dissociation activation energies of the N–N, C–C, and C–H bonds in the Na–Ga melt were about 1.13, 2.71, and 1.58 eV, respectively, and that of the C–H bond in the Ga melt was about 1.40 eV.

Boron-based two-dimensional materials are of interest for use in electronic devices and catalytic applications, for which it is important that they are chemically stable. Here, we explore the chemical stability of hydrogen boride nanosheets in water. Experiments reveal that mixing hydrogen boride and water produces negligible amounts of hydrogen, suggesting that hydrolysis does not occur and that hydrogen boride is stable in water, which is in contrast to most boron hydride materials. First-principles calculations reveal that the sheets interact weakly with water even in the presence of defects and that negatively charged boron prevents the onset of hydrolysis. We conclude that the charge state of boron and the covalent boron-boron bond network are responsible for the chemical and structural stability. On the other hand, we found that proton exchange with hydrogen boride nanosheets does occur in water, indicating that they become acidic in the presence of water. Nanoscale boron-based materials are promising for electronic devices, and it is important to understand their chemical stability. Here, hydrogen boride nanosheets are stable against hydrolysis in water, which ab initio calculations attribute to the charge state of boron and the boron-boron bond network.

The adsorption of CO2 molecules on monolayer epitaxial graphene on a SiC(0001) surface at 30 K was investigated by temperature-programmed desorption and X-ray photoelectron spectroscopy. The desorption energy of CO2 on graphene was determined to be (30.1–25.1) ± 1.5 kJ/mol at low coverages and approached the sublimation energy of dry ice (27–25 kJ/mol) with increasing the coverage. The adsorption of CO2 on graphene was thus categorized into physisorption, which was further supported by the binding energies of CO2 in core-level spectra. The adsorption states of CO2 on graphene were theoretically examined by means of the van der Waals density functional (vdW-DF) method that includes nonlocal correlation. The experimental desorption energy was successfully reproduced with high accuracy using vdW-DF calculations; the optB86b-vdW functional was found to be most appropriate to reproduce the desorption energy in the present system.

We investigated the oxidative addition of PhX (X = Cl, Br) to a single Pd(0) atom or a PdX– complex in water using first-principles molecular dynamics simulations, with solvent H2O molecules explicitly included in the calculation models, to clarify the origin of the extremely high reactivity of a ligand-free Pd catalyst in an aqueous solution for the Suzuki–Miyaura reaction. The free-energy profiles are estimated using blue moon ensemble sampling to include the entropy effect in chemical reactions in a water solvent. The free-energy barrier of the oxidative addition step is quite low for PhBr, whereas the barrier for PhCl is sizable, indicating that the reaction can proceed at room temperature with a high rate for PhBr but a rather low rate for PhCl. We also investigated the effect of the additional halogen anion on the Pd catalyst as a “supporting ligand”. The activation barrier of the oxidative addition step is not affected by the supporting halogen ligand, but the final state is significantly destabili...

We performed ab initio molecular dynamics analysis of formate decomposition to CO2 and H on a Cu(111) surface using van der Waals density functionals. Our analysis shows that the desorbed CO2 has approximately twice larger bending vibrational energy than the translational, rotational, and stretching vibrational energies. Since formate synthesis, the reverse reaction of formate decomposition, has been suggested experimentally to occur via the Eley–Rideal mechanism, our results indicate that the formate synthesis can be enhanced if the bending vibrational mode of CO2 is excited rather than the translational and/or stretching vibrational modes. Detailed information on the energy distribution of desorbed CO2 as a formate decomposition product may provide new insights for improving the catalytic activity of formate synthesis.

Ferrocene-terminated self-assembled monolayers (SAMs) have been widely studied in the past quarter century to reveal the electrochemical properties of chemically modified electrodes. It has been well-known that the formal potential of the system strongly depends on the local environment of the ferrocene moiety. Although electronic states of ferrocene-terminated SAMs should directly affect the electrochemical properties, knowledge concerning electronic structures with respect to different local environment is very limited. In this study, we performed density functional theory calculations of ferrocene-terminated SAMs with different coadsorbed species to reveal the relationship between the electronic structures and the local environment of ferrocene moieties. Depending on the local electrostatic potential, density of states derived from the highest-occupied molecular orbital (HOMO) and its vicinities (HOMO–1 and HOMO–2) of the ferrocene moiety were found to largely shift with respect to the electrode Fermi ...

Formic acid (HCOOH) can be catalytically decomposed into H2 and CO2 and is a promising hydrogen storage material. As H2 production catalysts, Cu surfaces allow selective HCOOH decarboxylation; however, the on-surface HCOOH decomposition reaction pathway remains controversial. In this study, the temperature dependence of the HCOOH/Cu(111) adsorption structures is elucidated by scanning tunneling microscopy and non-contact atomic force microscopy, establishing the adsorbate chemical species using density functional theory. 2D HCOOH islands at 80 K, linear chains of HCOOH and monodentate formate at 150 K, chain-like assemblies of monodentate and bidentate formate at 200 K, and bidentate formate clusters at 300 K are observed. At each temperature, the adsorbates experience attractive interactions among themselves. Such aggregation stabilizes them against desorption and decomposition. Thus, accurate evaluation of intermolecular interactions is essential to understand catalytic reactivity.

Absolute surface energies of reconstructed polar, nonpolar, and semi-polar GaN surfaces formed under oxide vapor phase epitaxy (OVPE) growth conditions were investigated via first-principles calculations. The relationships of stable surface orientations and surface structures with the growth conditions of Ga pressure and temperature were examined in terms of absolute surface energies. The stability of the surface orientation was in the order (0 0 0 1 ¯ ), (1 1 ¯ 0 1), (1 1 ¯ 0 0), (1 1 2 ¯ 0), (0 0 0 1), and (1 1 ¯ 0 1 ¯ ). High O concentrations on OVPE-grown GaN could be explained in relation to the stability of the (1 1 ¯ 0 1) surface. The stable (1 1 ¯ 0 1) surface consisted of N vacancies and H and O adatoms. Therefore, it was deduced that its appearance results in increased O impurity incorporation.

Single-atom catalysis, which utilizes single atoms as active sites, is one of the most promising ways to enhance the catalytic activity and to reduce the amount of precious metals used. Platinum atoms deposited on graphene are reported to show enhanced catalytic activity for some chemical reactions, e.g. methanol oxidation in direct methanol fuel cells. However, the precise atomic structure, the key to understand the origin of the improved catalytic activity, is yet to be clarified. Here, we present a computational study to investigate the structure of platinum adsorbed on graphene with special emphasis on the edges of graphene nanoribbons. By means of density functional theory based thermodynamics, we find that single platinum atoms preferentially adsorb on the substitutional carbon sites at the hydrogen terminated graphene edge. The structures are further corroborated by the core level shift calculations. Large positive core level shifts indicate the strong interaction between single Pt atoms and graphene. The atomistic insight obtained in this study will be a basis for further investigation of the activity of single-atom catalysts based on platinum and graphene related materials.

Using the density functional method, we investigated the crystal and electronic structures and the electrochemical properties of NaxVOPO4 (x = 0, 1) and explored the diffusion mechanism of Na ions in these materials. The van der Waals interaction was also taken into account to include the non-local electron correlation in the calculation of structural parameters and voltage. The diffusion of Na ions is treated as a process of the Na vacancy-positive small polaron complex in NaVOPO4 and the Na ion-negative small polaron complex in VOPO4, respectively. During the charging (discharging) process, the removal (insertion) of a Na ion would result in the formation of a positive (negative) small polaron at one of the two first nearest vanadium sites to the Na vacancy. Three elementary diffusion processes, including the single, crossing and parallel diffusion processes, are explored. It is found that the [010] direction is preferable for Na ion diffusion in both the charging and discharging processes. The influence of small polaron migration on Na ion diffusion in the charging state is negligible, whereas such effect is considerably strong in the discharging process. Moreover, while three elementary diffusion processes in NaVOPO4 require the same energy, the parallel diffusion process in VOPO4 is not preferred. The diffusion of Na vacancy accompanied by a positive polaron in the full charging process requires an activation energy of 395 meV, while the diffusion of Na ion accompanied by a negative polaron in the discharging state, VOPO4, has a higher activation energy of 627 meV. With a voltage and activation barrier similar to that of the olivine phosphate LiFePO4, these sodium-based materials are expected to be promising cathode materials for sodium ion batteries.

Understanding gas-surface reaction dynamics, such as the rupture and formation of bonds in vibrationally and translationally excited ('hot') molecules, is important to provide mechanistic insight into heterogeneous catalytic processes. Although it has been established that such excitation can affect the reactions occurring via dissociative mechanisms, for associative mechanisms-in which the gas-phase reactant collides directly with a surface-adsorbed species-only translational excitation has been observed to affect reactivity. Here we report a bond-formation reaction that is driven by the vibrational energy of reactant molecules and occurs via an (associative) Eley-Rideal-type mechanism, in which the reaction takes place in a single collision. Hot CO

The functionalization of graphene is important in practical applications of graphene, such as in catalysts. However, the experimental study of the interactions of adsorbed molecules with functionalized graphene is difficult under ambient conditions at which catalysts are operated. Here, the adsorption of CO2 on an oxygen-functionalized epitaxial graphene surface was studied under near-ambient conditions using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). The oxygen-functionalization of graphene is achieved in situ by the photo-induced dissociation of CO2 with X-rays on graphene in a CO2 gas atmosphere. The oxygen species on the graphene surface is identified as the epoxy group by XPS binding energies and thermal stability. Under near-ambient conditions of 1.6 mbar CO2 gas pressure and 175 K sample temperature, CO2 molecules are not adsorbed on the pristine graphene, but are adsorbed on the oxygen-functionalized graphene surface. The increase in the adsorption energy of CO2 on the oxygen-functionalized graphene surface is supported by first-principles calculations with the van der Waals density functional (vdW-DF) method. The adsorption of CO2 on the oxygen-functionalized graphene surface is enhanced by both the electrostatic interactions between the CO2 and the epoxy group and the vdW interactions between the CO2 and graphene. The detailed understanding of the interaction between CO2 and the oxygen-functionalized graphene surface obtained in this study may assist in developing guidelines for designing novel graphene-based catalysts.

Bathocuproine (BCP) is a promising organic material of a hole blocking layer in organic light-emitting diodes or an electron buffer layer in organic photovoltaic cells. The nature of the unoccupied electronic states is a key characteristic of the material, which play vital roles in the electron transport. To elucidate the electronic properties of the molecular or crystalline BCP, we use the GW approximation for calculation of the crystalline band gap, and the long-range corrected (LC) density functional theory (DFT) for the molecular optical absorption. It is found that the band gap of the crystalline BCP is 4.39 eV, in agreement with the recent low-energy inverse photoemission spectroscopy measurement. The polarization energy is estimated to be 1.05 eV, demonstrating the large polarization effects induced by the electronic clouds sur- rounding the injected charge. The theoretical optical absorption energy is 3.68 eV, and the exciton binding energy is estimated to be 0.71 eV, implying the large binding in the eletron-hole pair distributed around the small part of the molecular region.

The electrochemical properties of chemically modified electrodes have long been a significant focus of research. Although the electronic states are directly related to the electrochemical properties, there have been only limited systematic efforts to reveal the electronic structures of adsorbed redox molecules with respect to the local environment of the redox center. In this study, density functional theory (DFT) calculations were performed for ferrocene-terminated self-assembled monolayers with different electron-donating abilities, which can be regarded as the simplest class of chemically modified electrodes. We revealed that the local electrostatic potentials, which are changed by the electron donating/withdrawing functional groups at the ferrocene moiety and the dipole field of coadsorbed inert molecules, practically determine the density of states derived from the highest occupied molecular orbital (HOMO) and its vicinities (HOMO−1 and HOMO−2) with respect to the electrode Fermi level. Therefore, to design new, sophisticated electrodes with chemical modification, one should consider not only the electronic properties of the constituent molecules, but also the local electrostatic potentials formed by these molecules and coadsorbed inert molecules.

The structural and electronic properties of naphthalene adsorbed on graphene are studied from first principles using the van der Waals density functional method. It is shown that naphthalene molecules are stabilized by forming a superstructure with the periodicity of $(2\sqrt{3}\ifmmode\times\else\texttimes\fi{}2\sqrt{3})$ and a tilted molecular adsorption geometry on graphene, in good agreement with the scanning tunneling microscopy (STM) experiments on highly oriented pyrolytic graphite. Our results predict that image potential states (IPSs) are induced by intermolecular interaction on the naphthalene overlayer, hybridizing with the IPSs derived from graphene. The resultant hybrid IPSs are characterized by anisotropic effective mass reflecting the molecular structure of naphthalene. By means of STM simulations, we reveal that one of the hybrid IPSs manifests itself as an oval protrusion distinguishable from naphthalene molecular orbitals, which identifies the origin of an experimental STM image previously attributed to the lowest unoccupied molecular orbital of naphthalene.

We investigated the adsorption of CO

We performed ab initio molecular dynamics analysis of formate decomposition to CO

We hypothesized that a nitrogen transportation process mediated by cyanide (CN) ions causes the observed increase in growth rate by carbon addition in the Na-flux growth of GaN. Using first-principles calculations, we investigated the probability of the nitrogen transportation process from the viewpoint of change of free energy. In addition, to verify whether the CN can separate into N and C atoms, we estimated the C–N bond energy on the GaN crystal surface by the blue-moon method using constrained molecular dynamics simulations.

H2 dissociation by Ag clusters supported on the θ-Al2O3(110) surface has been investigated using density functional theory calculations. The crucial role of the dual perimeter site of Ag cluster and the surface oxygen (O) site of the alumina support is demonstrated with three theoretical models: anchored cluster, isolated cluster, and anchored cluster on hydroxylated alumina. The heterolytic cleavage of H2 at the silver–alumina interface, yielding Ag–Hδ− and O–Hδ+, is thermodynamically and kinetically preferred compared with H2 cleavage at two Ag atomic sites on top of the Al2O3-supported Ag cluster and the homolytic cleavage of H2 on the isolated Ag cluster. The hydroxylation at the O site of the alumina reduces the H2 dissociation activity, which indicates that the interfacial bare O site is indispensible. It is concluded that the interfacial cooperative mechanism between the Ag cluster and Lewis acid–base pair site (bare Al–O site) is essentially relevant for the H2 activation over Ag-loaded Al2O3 cata...

We investigate the dependence of the Curie temperature (T C), electronic structure and magnetic properties on hole doping by Zn vacancy (VZn) generations in Zn(Sn,Mn)As2 and (Zn,Mn)SnAs2 using Korringa-Kohn-Rostoker method incorporated with the coherent potential approximation and a local spin density approximation (KKR-CPA-LSDA) within density functional theory (DFT). We find that T C of (Zn,VZn)(Sn,Mn)As2 is strongly reduced by hole doping due to the reduced double-exchange interaction. In contrast to this, (Zn,VZn,Mn)SnAs2 shows ferromagnetism because of the introduction of holes into paramagnetic (Zn,Mn)SnAs2. In this case, T C is significantly enhanced by the hole doping because of the increased double exchange interaction.

Daisetsu Toh, Pho Van Bui, Ai Isohashi, Naotaka Kidani, Satoshi Matsuyama, Yasuhisa Sano, Yoshitada Morikawa, and Kazuto Yamauchi, "Catalyzed chemical polishing of SiO2 glasses in pure water", Review of Scientific Instruments 90, 045115 (2019), https://doi.org/10.1063/1.5090320., A catalytically assisted etching system was developed for the ultra-precision fabrication of optical components, such as X-ray mirrors and extreme-ultraviolet mask blanks. This study demonstrates that an atomically smooth surface with a sub-Angstrom root-mean-square roughness could be achieved on a SiO2 glass substrate using pure water and Pt as the etching solution and catalyst, respectively. Density functional theory calculations confirmed that the mechanistic pathway was involved in catalyzed hydrolysis. The significant roles of the catalyst were clarified to be the dissociation of water molecules and the stabilization of a meta-stable state, in which a hypervalent silicate state is induced, and the Si-O backbond is elongated and loosened. To confirm the role of the catalyst, the Pt metal was replaced by Au, and the observed drastic difference in the removal rate was attributed to the degree of stabilization of the metastable state.

These years, an “intelligent” automotive catalyst has been proposed and used in practical applications, which self-regenerates its precious metal nanoparticles during the redox cycles in automotive engines (Nishihata et al. Nature 418:164, 2002). In eliminating the use of expensive precious metals, a self-regenerating catalyst using non-precious metals such as Cu and Fe is necessary. We investigated the self-regeneration properties of Cu dissolved in layered perovskites with K2NiF4 structure under redox conditions of exhaust gas. Using ab initio thermodynamics calculations, we show that the self-regeneration of Cu on LaCuO3 may be hampered by the formation of La2CuO4, a well-known stable layered perovskite. Next, using the same calculation method, we examined the self-regeneration of Cu on a variety of layered perovskites, and proposed some layered perovskites as potential materials of the self-regenerating catalyst.

We have developed a novel abrasive-free planarization method, which we term catalyst-referred etching (CARE). In silicon carbide (SiC) CARE, Pt is used as a catalyst and HF solution is used as an etchant. CARE produces a crystallographically undamaged and smooth SiC surface. To understand the removal mechanism at the topmost surface of SiC in the CARE process, we performed first-principles reaction path simulations using the simulation tool for atom technology (STATE) program package. These calculations are based on the density functional theory within the generalized gradient approximation of Perdew et al. The barrier height of the dissociative adsorption of HF on a SiC surface was evaluated by the climbing image nudged elastic band method. We present simulation results for the initial stages of the etching process. The reaction barrier height for adsorption of the first HF is 1.2 eV.

An “intelligent” automotive catalyst that self-regenerates its precious-metal nanoparticles during reduction–oxidization cycles was proposed and has been used in realistic applications (Nishihata e...

In this study, we developed aA novel abrasive-free polishing method called the catalyst-referred etching (CARE) has been developed. CARE can chemically remove SiC chemically with using an etching agent activated by a catalyst. Platinum and hydrofluoric (HF) acid are used for the planarization of SiC substrates as a catalyst and etchant, respectively. CARE can produce an atomically flat surface of 4H–SiC (0001) with a root-mean-square roughness of less than

Using ab initio molecular dynamics as implemented in periodic, self-consistent (generalized gradient approximation Perdew-Burke-Ernzerhof) density functional theory, we investigated the mechanism of methanol electrooxidation on Pt(111). We investigated the role of water solvation and electrode potential on the energetics of the first proton transfer step, methanol electrooxidation to methoxy (CH3O) or hydroxymethyl (CH2OH). The results show that solvation weakens the adsorption of methoxy to uncharged Pt(111), whereas the binding energies of methanol and hydroxymethyl are not significantly affected. The free energies of activation for breaking the C-H and O-H bonds in methanol were calculated through a Blue Moon Ensemble using constrained ab initio molecular dynamics. Calculated barriers for these elementary steps on unsolvated, uncharged Pt(111) are similar to results for climbing-image nudged elastic band calculations from the literature. Water solvation reduces the barriers for both C-H and O-H bond activation steps with respect to their vapor-phase values, although the effect is more pronounced for C-H bond activation, due to less disruption of the hydrogen bond network. The calculated activation energy barriers show that breaking the C-H bond of methanol is more facile than the O-H bond on solvated negatively biased or uncharged Pt(111). However, with positive bias, O-H bond activation is enhanced, becoming slightly more facile than C-H bond activation.

The adsorption of benzene on the Si(100) surface is studied theoretically using the self-consistent van der Waals density functional (vdW-DF) method. The adsorption energies of two competing adsorption structures, butterfly (BF) and tight-bridge (TB) structures, are calculated with several vdW-DFs at saturation coverage. Our results show that recently proposed vdW-DFs with high accuracy all prefer TB to BF, in accord with more accurate calculations based on exact exchange and correlation within the random phase approximation. Detailed analyses reveal the important roles played by the molecule-surface interaction and molecular deformation upon adsorption, and we suggest that their precise description is prerequisite for accurate prediction of the most stable adsorption structure of organic molecules on semiconductor surfaces., 9 pages, 4 figures, 2 tables

In this article, we discuss recent progress in theoretical studies on the electronic properties of organic/metal interfaces, especially on the origin of the interface dipoles. We first discuss the effect of the interface dipole on the charge injection barriers at organic/metal interfaces. Then, we observe the importance of the interface structure, especially of the organic–metal distances in physisorption systems. The experimentally observed substrate dependence of the interface dipole can be attributed mainly to the difference in the organic–metal distance. In the case of chemisorption systems, the induced density of interface states tends to pin the Fermi level relative to the HOMO and LUMO levels of molecules. We also point out an important role of van der Waals (vdW) interaction between organic molecules and metal substrates. Density functional theory (DFT) within a generalised gradient approximation (GGA) plus a recently proposed semi-empirical vdW correction can describe atomic geometries of organic/metal interfaces reasonably well.

First-principles calculations were carried out to study the difference in the segregation behavior of Pd on the LaO and FeO 2 terminated surfaces of LaFe 1− x Pd x O 3− y perovskite oxide. Without oxygen vacancy, the tendency of Pd to segregate near to the LaO- and FeO 2 -terminated surfaces is rather weak. Oxygen vacancies enhance the surface segregation of Pd significantly at FeO 2 terminated surfaces, while they do not at LaO terminated surfaces. Combined with the previous thermodynamic calculations, we propose that Pd-containing perovskite catalyst should be prepared at high temperature to enhance the surface segregation of Pd and to form suitable surface geometries for rapid switching between oxidized and reduced states of Pd catalyst on perovskite support. The origin for the large difference in the segregation tendency of Pd between LaO and FeO 2 terminated surfaces is also discussed.

We have carried out first-principles reaction path simulations of hydrogen fluoride (HF) adsorption at steps and terraces of 3C-silicon carbide (SiC) (111) surfaces to investigate initial processes in catalyst-referred etching (CARE). CARE is a novel abrasive-free planarization method and has been invented in our group. In SiC CARE, we use platinum as a catalyst and hydrofluoric acid (HF) solution as an etchant. A crystallographically undamaged and smooth SiC surface is obtained after CARE. In this study, we performed first-principles reaction path simulations using the Simulation Tool for Atom Technology (STATE) program package. Our results showed that the adsorption of HF molecules at step edges has lower activation barrier than that at terraces, and that HF adsorption at F-terminated surfaces is easier than that at OH-terminated surfaces.