Yasuharu Okamoto

Electrode Dynamics from First Principles

The investigation of electrode dynamics has been a major topic in the field of electrochemistry for a century. Electrode dynamics consist of electron transfer reactions that give rise to, or are caused by, a bias voltage, and are influenced by surface catalysis, electrolyte solution, transport of electrons and ions. The first-principles molecular dynamics simulation of the electrochemical system has been hampered by the difficulty to describe the bias voltage and the complex solution-electrode interface structure. Here we utilize a new algorithm called the effective screening medium to characterize the biased interface between platinum and liquid water, revealing the microscopic details of the first, Volmer, step of the platinum-catalyzed hydrogen evolution reaction. By clarifying the important roles played by both the water and the bias, we show why this reaction occurs so efficiently at the interface. Our simulations make a significant step towards a deeper understanding of electrochemical reactions.

Ab initio study on the structures of Th(IV) hydrate and its hydrolysis products in aqueous solution

Ab initio calculations have been performed to study the structures of thorium(IV) hydrate and its hydrolysis products in aqueous solution. The conductor-like polarizable continuum model (CPCM) has been used to perform geometry optimization calculations in aqueous solution. The calculated results demonstrate that the molecule geometries obtained in solvent are generally consistent with the experiments. The coordination number of thorium(IV) hydrolysis products has been investigated. The effect of the relativistic effective core potential (RECP) on the structures is also discussed.

First-principles calculations on Mg impurity and Mg-H complex in GaN

First-principles calculations are performed in order to investigate the atomic geometry and stability of the Mg impurity and Mg–H complex in GaN. We find that a doped Mg atom is stable at a Ga substitutional site with only slight lattice relaxation, suggesting that the Mg impurity induces a shallow acceptor level. It is also found that this Mg impurity forms a very stable complex with an incorporated hydrogen; the hydrogen atom intervenes between the N and Mg atoms to form the stable Mg–H complex. The formation of this stable Mg–H complex is expected to be the reason for the experimental result that the acceptor is passivated by the hydrogen. Local vibrational frequencies of the Mg–H complexes are also discussed.

A new dioxin decomposition process based on a hybrid density-functional calculation

Abstract Using hybrid density-functional theory, a new decomposition process for 2,3,7,8-tetrachlorinated dibenzo- p -dioxins (TCDD) is proposed. This hypothetical process is composed of two steps, combining the use of hydrogen radicals with proton irradiation of dioxins. The first step is to remove chlorine atoms bound to the dioxin ring by selective abstraction using hydrogen radicals. This leads to non-chlorinated dibenzo- p -dioxin (NCDD) which is much less toxic than 2,3,7,8-TCDD. In the second step, a dative bond is formed between an oxygen atom of an NCDD and an introduced proton; then further irradiation with hydrogen radicals results in the breaking of the C–O bond of the dioxin ring. The rate-determining step of the whole process is the chlorine abstractions, and their computed activation energies are less than 0.4 eV.

Multivacancy and Its Hydrogen Decoration in Crystalline Si

We present first-principles total-energy calculations that reveal microscopic structures of multivacancies in Si and their feasibility of hydrogen incorporation. We find that the hexavacancy V6 and the decavacancy V10 are stable, and that the stable multivacancies are either free from or fully decorated with hydrogen depending on its chemical potential. We also find that the H-decorated multivacancy is capable of containing an additional H2 molecule and hereby exhibits peculiar vibration spectra related to the hydrogen.

The charged interface between Pt and water: First principles molecular dynamics simulations

The charged interface between a platinum electrode and an aqueous solution is investigated by first-principles molecular dynamics simulations in which charges in the system are controlled by the effective screening medium method under periodic boundary conditions. H3O+ and OH are located above or on the Pt surface. Water molecules rotate to screen the electric field induced by the charge accumulated on the Pt surface. The time-averaged electrostatic potential near the Pt surface is structured with a flattened “bulk” region. The potential difference between the Pt Fermi level and the bulk potential is proportional to the charge and is used to estimate the Pt electrode potential via the PZC (potential of the zero charge). The surface charge significantly polarizes the water molecules near the Pt surface. The OH stretching frequency of molecules on the negatively charged (7 ∼ 14 μC/cm2) Pt electrode shift to lower values (red shift) by 100 ∼ 200 cm−1. For the positively charged Pt lattice, a complex feature ...

Elastic scattering of electrons by ozone molecules

Abstract Electron scattering from an ozone molecule is studied theoretically. Differential, integral, and momentum-transfer cross sections are calculated for the (vibrationally) elastic scattering at 5–20 eV. An ab initio static potential is used with electron exchange and target polarization taken into account approximately. The resulting cross section is about 2–3 times larger than those for molecular oxygen at the energies considered.

Theoretical study of hydrolysis reactions of tetravalent thorium ion

Abstract The hydrolysis reaction of the tetravalent thorium (Th4+) ion was investigated by ab initio theoretical calculations on hydration complex models. The transition state structure was optimized, and the intrinsic reaction coordinate was traced. The results showed that the hydrolysis is highly exothermic and the transition state is close to the reactant. Reaction dependency upon the number of non-reactive water molecules was examined.

Theoretical Study of the Surface Reaction Mechanism of GaN with HCl

Ab initio molecular orbital calculations are performed to investigate the three successive surface reactions of GaN with HCl. These reactions correspond to the simplest model which accounts for GaN chemical etching by HCl. We observed two characteristic points about these reactions. First, all of these reactions are exothermic and the reaction energies are about 20?29 kcal/mol. Second, the activation energy of the reaction decreases as the number of Ga?Cl bonds increases. The calculation shows that the first reaction of the GaN surface with HCl is the rate-determining one and the calculated reaction barrier is less than 10 kcal/mol.

Graphene-Like-Graphite as Fast-Chargeable and High-Capacity Anode Materials for Lithium Ion Batteries

Here we propose the use of a carbon material called graphene-like-graphite (GLG) as anode material of lithium ion batteries that delivers a high capacity of 608 mAh/g and provides superior rate capability. The morphology and crystal structure of GLG are quite similar to those of graphite, which is currently used as the anode material of lithium ion batteries. Therefore, it is expected to be used in the same manner of conventional graphite materials to fabricate the cells. Based on the data obtained from various spectroscopic techniques, we propose a structural GLG model in which nanopores and pairs of C-O-C units are introduced within the carbon layers stacked with three-dimensional regularity. Three types of highly ionic lithium ions are found in fully charged GLG and stored between its layers. The oxygen atoms introduced within the carbon layers seem to play an important role in accommodating a large amount of lithium ions in GLG. Moreover, the large increase in the interlayer spacing observed for fully charged GLG is ascribed to the migration of oxygen atoms within the carbon layer introduced in the state of C-O-C to the interlayer space maintaining one of the C-O bonds.