Search

Entropy benchmarking of different sized molecules in aqueous phase is carried out for known solvation models, where we compare geometry and solvation cavity packing parameters, which allows us to improve the accuracy of the obtained entropy values using empirical corrections. A comparison of solvation entropy models is conducted for a benchmarking set of 56 molecules, showing how an accurate description of cavitation entropy and its hindrance on other entropy values is important for large-sized solute molecules. Finally, we compare reaction free energies with entropies calculated using the most accurate solvation model considered, where we demonstrate a significant improvement in the accuracy relative to experimental values.

Carbon-supported first-row transition metal complexes drive electroreduction of CO2 to CO in aqueous medium with remarkable activity and selectivity. However, their durability under negative potent...

We have performed density functional theory calculations to explore the possibility to overcome the linear scaling relations in the oxygen reduction reaction (ORR) using local inhomogeneities on Pt...

The use of solid-state electrolytes to provide safer, next-generation rechargeable batteries is becoming more feasible as materials with greater stability and higher ionic diffusion coefficients are designed. However, accurate determination of diffusion coefficients in solids is problematic and reliable calculations are highly sought-after to understand how their structure can be modified to improve their performance. In this paper we compare diffusion coefficients calculated using nonequilibrium and equilibrium ab initio molecular dynamics simulations for highly diffusive solid-state electrolytes, to demonstrate the accuracy that can be obtained. Moreover, we show that ab initio nonequilibrium molecular dynamics can be used to determine diffusion coefficients when the diffusion is too slow for it to be feasible to obtain them using ab initio equilibrium simulations. Thereby, using ab initio nonequilibrium molecular dynamics simulations we are able to obtain accurate estimates of the diffusion coefficients of Li ions in Li6PS5Cl and Li5PS4Cl2, two promising electrolytes for all-solid-state batteries. Furthermore, these calculations show that the diffusion coefficient of lithium ions in Li5PS4Cl2 is higher than many other potential all-solid-state electrolytes, making it promising for future technologies. The reasons for variation in conductivities determined using computational and experimental methods are discussed. It is demonstrated that small degrees of disorder and vacancies can result in orders of magnitude differences in diffusivities of Li ions in Li6PS5Cl, and these factors are likely to contribute to inconsistencies observed in experimentally reported values. Notably, the introduction of Li-vacancies and disorder can enhance the ionic conductivity of Li6PS5Cl.

Computational methods are used to show that graphene nanoribbons bind sodium (Na) and calcium (Ca) more strongly than graphene sheets. The binding strength is further enhanced by functionalizing the edge of the nanoribbon with oxygen-containing groups. Strengthening of the binding of these metal atoms to graphitic materials is important for applications including metal-ion batteries. Our results are obtained using density functional theory calculations of the binding of sodium and calcium to hydrogen, hydroxyl, carbonyl, and carboxyl groups at the edge of zigzag and armchair nanoribbons. Both hydrogen passivation and hydroxyl functionalization result in moderate binding of Na and Ca with binding energies varying from -1.0 to -1.9 eV for the nanoribbons considered. An increase in binding compared to graphene does not just occur at the edge, but extends across the nanoribbon. Furthermore, carbonyl and carboxyl groups bound both metal atoms more strongly, with binding energies between -1.6 and -3.1 eV. Increasing the number of these groups at the edge increases the binding strength of the metal adatoms. When there is a high number of oxygen-containing groups at the edge, the effect of the oxygen-containing groups is also evident away from the edge of the nanoribbon for sodium and calcium. It is demonstrated that this is at least partly due to the change in the electronic structure spanning the entire width of the nanoribbons considered.

Platinum single-atom catalysts (PtSACs) on 3D support are emerging as the new frontier in catalysis due to their atom-economy, outstanding performance, and the advantage to bridge the gap between homogeneous and heterogeneous catalysis. Here we report on a simple, single-step electrochemical grafting attachment of a metal-selective ligand, 2,6:2’,2”-terpyridine, and the synthesis of platinum single-atom electrocatalyst via metal uptake from aqueous salt solution. At an ultra-low loading of 0.26 ±0.02 μgcm-2 of platinum, the single-atom catalysts supported on porous 3D carbon cloth electrode via chemical bonding revealed higher mass activity ratios, up to 83.0 at η = 50 mV/RHE, compared to the commercial catalyst 20 % Pt/C. Using hydrogen binding energy as the descriptor for efficiency, density functional theory calculations show that promoting hydrogen evolution using terpyridine-Pt proceeds when the catalyst is in a 2+ charge state and with a single hydroxy co-ligand attached to Pt catalytic center. Calculations predict an HER overpotential of about 0.28 V. The electro-grafted terpyridine ligand proved to act as effective scavenger for leached platinum from the counter electrode during extended operational hours. The method to make the PtSAC is facile, non-hazardous and versatile without involving any elaborate pre- and/or post-treatment steps and, the cost of the added platinum to the ligand is only 0.1 US$m-2.

The knowledge of nonequilibrium electron transfer rates is paramount for the design of modern hybrid electrocatalysts. Herein, we propose a general simulation-based approach to interpret variable-frequency square wave voltammetry (VF-SWV) for heterogeneous materials featuring reversible redox behavior. The resistive and capacitive corrections, inclusion of the frequency domain, and statistical treatment of the surface redox kinetics are used to account for the non-ideal nature of electrodes. This approach has been validated in our study of CoII/CoI redox transformation for Co tetraphenylporphyrin (CoTPP) immobilized on carbon cloth and multiwalled carbon nanotubes (CNTs) - one of the most active heterogeneous molecular catalysts in carbon dioxide (CO2) electroreduction. It is demonstrated that the modeling of experimental data furnishes the capacitance of the surface double layer C, uncompensated resistance Ru, symmetry coefficients α, kinetic constants k0, and equilibrium redox potentials E0 in one experiment. Moreover, the proposed method yields a stochastic map of the redox kinetics rather than a single value, thus exposing the inhomogeneous nature of the electrochemically active layer. The computed parameters are in excellent agreement with the results of the classic methods such as cyclic voltammetry and fall in line with the reported CoTPP catalytic activity. Thus, VF-SWV is suitable for the study of high-level composites such as covalent organic frameworks and organometallic-CNT mixtures. The resulting insights into the electron transfer mechanisms are especially useful for the rational development of the catalyst-support interfaces and immobilization methods.

The adsorbate structures on electrode surfaces in an electrochemical environment are controlled by thermodynamic parameters such as temperature, concentration, pH and electrode potential. Knowledge of these structures is important as specifically-adsorbed ions on an electrode impact catalytic reactions that take place at the electrode-electrolyte interface. From a theoretical point of view, the equilibrium structures of adsorbates can be conveniently estimated using the concept of the computational hydrogen electrode. Here we extend this concept to determine equilibrium co-adsorption structures of halides with hydrogen on Pt(111) as a function of the corresponding electrochemical potentials. We find that hydrogen-halide co-adsorption is of a competitive character, which means that mainly dense-packed structures of either halides or hydrogens are stable on the surface, in good agreement with experiment.

Advances and progress in computational research that aims to understand and improve solid-state electrolytes (SSEs) are outlined. One of the main challenges in the development of all-solid-state batteries is the design of new SSEs with high ion diffusivity that maintain chemical and phase stability and thereby provide a wide electrochemical stability window. Solving this problem requires a deep understanding of the diffusion mechanism and properties of the SSEs. A second important challenge is the development of an understanding of the interface between the SSE and the electrode. The role of molecular simulations and modeling in dealing with these challenges is discussed, with reference to examples in the literature. The methods used and issues considered in recent years are highlighted. Finally, a brief outlook about the future of modeling in studying solid-state battery technology is presented.

In spite of the strong relevance of electrochemical energy conversion and storage, the atomistic modeling of structures and processes in electrochemical systems from first principles is hampered by severe problems. Among others, these problems are associated with the theoretical description of the electrode potential, the characterization of interfaces, the proper treatment of liquid electrolytes, changes in the bulk structure of battery electrodes, and limitations of the functionals used in first-principles electronic structure calculations. We will illustrate these obstacles, but also indicate strategies to overcome them.

The adsorption of halides on Cu(111) and Pt(111) has been studied using periodic density functional theory calculations. The equilibrium coverage of the halides as a function of the electrode potential was determined using a thermodynamic approach in which the electrochemical environment is not explicitly taken into account. For all considered systems, halide coverages between 1/3 and 3/8 should be stable over a wide potential range. Although some quantitative discrepancies with experiment are obtained, the qualitative trends derived from the calculations are consistent with experimental observations. The reasons for the remaining discrepancies with the experiment are discussed.

We study the transport properties of single-atom-thick Cu wires submerged in an electrochemical solvent containing HCl. As a first step, we investigate the stability of hydrogen coadsorbing with chlorine on the Cu(111) surface in an implicit electrochemical environment. We find that adding hydrogen to a Cl-covered Cu surface is energetically unfavorable. The result serves as an estimate for the number of Cl atoms that adsorb near the single-atom wire. We use it to construct model junctions (Cu wire plus adsorbates), the electron transport properties of which we investigate with density functional theory. We find that the Cl and H adsorbates tend to deplete the density of states of the Cu wire near the Fermi energy. As a consequence, the transmission is reduced. Interestingly, we observe that in the case of H-adsorption, the amount of depletion is quite sensitive to the wire geometry (relaxed vs unrelaxed), but this is not the case with Cl.

A model for the low-temperature catalysis of hydrogen molecules on graphene and graphite, relevant for interstellar chemistry, is proposed: hydrogen atoms are either chemisorbed at the edges, or physisorbed on graphene and transported to a chemisorbed state at the edges. A second atom can then produce a molecule via a hot atom or an Eley–Rideal process. Since much of the energy is needed to desorb the molecule from the tightly-bound chemisorbed state, the desorbing molecules have only low internal excitation energy, in agreement with astronomical observations.

Two mechanisms have been cited as the reason for unexpected work function decrease upon adsorption of electronegative adatoms: electron spillout depletion [Michaelides et al., Phys. Rev. Lett., 2003, 90, 246103] and polarization of the adatom [Roman et al., Phys. Rev. Lett., 2013, 110, 156804]. We attempt to bridge the two pictures in this work. Work function changes due to the adsorption of halides on (111) surfaces of fcc metals (Ca, Sr, Ni, Pd, Pt, Cu, Ag, Au, Al and Pb) were studied using periodic density functional theory. The two mechanisms were found to be clearly independent of each other because of the opposite factors that lead to the work function decrease, and are therefore easy to distinguish. A more general picture of interpreting bond ionicity based on observed work function changes is discussed.

The structure of water on metal electrodes is addressed based on first-principles calculations. Special emphasis is placed on the competition between water-metal and water-water interaction as the structure determining factors. Thus the question will be discussed whether water at metal surfaces is ice- or rather liquid-like. The proper description of liquid phases requires to perform thermal averages. This has been done by combining first-principles electronic structure calculations with molecular dynamics simulations. After reviewing recent studies about water on flat, stepped and pre-covered metal electrodes, some new results will be presented.

The structure of water layers above hydrogen-covered Pt(1 1 1) surfaces at room temperature has been studied by ab initio molecular dynamics simulations based on periodic density functional theory calculations. Fully hydrogen-covered Pt(1 1 1) with additionally either a hydrogen vacancy or another hydrogen adatom have been considered. The resulting structures have been analyzed in detail as a function of the hydrogen coverage. In particular, the thermal disorder in the water layer is examined in terms of deviations from the ice lattice, orientational inhomogeneity within a water bilayer, as well as the onset of proton transfer. On hydrogen-covered Pt(1 1 1), the water layer is located at a much larger distance from the Pt atoms than on the pure metal surfaces. Surprisingly, the more weakly bound water layer on the hydrogen-covered Pt(1 1 1) electrode exhibits a greater order than the water layer on clean Pt(1 1 1) which is attributed to the stronger water–water interaction above hydrogen-covered Pt(1 1 1). The relevance of our findings for the understanding of electrochemical electrode/water interfaces is discussed.

The adsorption of water on bimetallic PtRu/Pt(111) surface alloys has been studied based on periodic density functional theory calculations including dispersion corrections. The Ru atoms of the PtRu surface alloy interact more strongly with water than Pt atoms, as far as both single water molecules and ice-like hexagonal structures are concerned. Within the surface alloy layer, the lateral ligand effect reducing the local reactivity of the surface atoms with increasing Ru content is more dominant than the opposing geometric effect due to the tensile strain. The structural preference for the Ru atoms also prevails at room temperature, as ab initio molecular dynamics simulations show.

We evaluate the reactivity between the surface of the film and gas molecules and determine the optimum gas combinations by calculating the total energy based on the density functional theory. We propose a model for the etching process of titanium oxide thin films. In this model, we consider three specific states, namely, the initial, intermediate and final states. On etching TiO (anatase), we observed that it is more efficient to have fluorine-based reactive gases (e.g. CHF) than other considered reactive gases (e.g. CH, HO) in order to enhance the etching.

We investigate the quantum mechanical behavior of adsorbed hydrogen (H, D, T) on Cu(1 0 0) and (1 1 0) surfaces. We construct potential energy surfaces (PESs) for the motion of the hydrogen H atom on Cu(1 0 0) and (1 1 0) surfaces within the framework of density functional theory. The potential energy takes a minimum value on the hollow site of Cu(1 0 0) and on the short bridge site of Cu(1 1 0). Moreover, we calculate the quantum states of hydrogen atom motion on these calculated PESs. The ground state wave function of the hydrogen atom motion is strongly localized around the hollow site on the Cu(1 0 0) surface. On the other hand, the ground state wave function of the hydrogen atom motion on Cu(1 1 0) is distributed from the short bridge site to two neighboring pseudo-threefold sites. We finally show isotope effects on the quantum states of the motion of hydrogen on both surfaces.

The adsorption of the amino acid glycine (NH2CH2COOH) on (3,3) armchair carbon nanotubes has been investigated through calculations based on density functional theory (DFT). A comparison of the adsorption energies for four adsorption models is presented, showing glycine in its zwitterionic form is generally bound stronger to the nanotube sidewall as compared to its nonionic counterparts. Adsorption through the carboxylate group of the glycine zwitterion exhibited the strongest interaction, favourably adsorbing with an adsorption energy of 0.53 eV. Further investigations on the adsorption of glycine on graphite sheets have showed weaker interactions as compared to adsorption onto the nanotube surface, suggesting a decrease in glycine adsorption strength with increasing nanotube diameter.

We investigate and discuss the adsorption of a few amino acids on (3,3) carbon nanotubes and on graphite sheets through calculations within density functional theory. Results show weak binding of the biomolecules on both substrates, but through generally favourable adsorption pathways. Zwitterion adsorption through the charged amine and carboxylate groups are bound stronger to the nanotube surface in comparison to their nonionic counterparts, as well as on histidine, phenylalanine, and cysteine side chain groups fixed in specific orientations. Binding strengths on graphite suggest dissimilar trends for amino acid interactions with increasing nanotube diameter.

The strength of adhesion of polybutylene terephthalate (PBT) on aluminium is investigated using density functional theory-based energy calculations. The aluminium atom is connected to a PBT monomer at different orientations, and total energies are calculated and compared to determine the most stable orientation. The binding is strongest when the Al is oriented at 180° to the ester group of the monomer. Using this orientation as a basis, PBT adhesion on Ti, Ag, and Au is also investigated.

We investigate the adhesion strength of poly(butylene terephthalate) or PBT on aluminum using density functional theory based energy calculations on periodic models. Instead of dealing with a bigger polymer system, we considered, as a first step, a small scale configuration to examine the key point of adhesion between PBT and Al surface. We placed the PBT monomer horizontally and vertically on aluminum surface. We also considered placing aluminum on top of the bulk PBT. By calculating the total energy of the system when PBT monomer approaches aluminum surface and comparing them with their energies when they are isolated, we obtain the binding energy of PBT on aluminum surface. The adhesion is stronger when the PBT monomer is oriented vertically than horizontally. Strong binding is also observed when the aluminum atom is placed on bulk PBT.

We have reexamined hydrogen-platinum systems, giving emphasis on the atomic migration on the metal surface. The ideal (111) surface was firstly revisited, with the model potential energy surface for the motion of hydrogen created from calculations using a slab of platinum atoms fixed at the bulk separation obtained from experimental measurements. Calculated hydrogen hopping states in an adiabatic approximation are presented, with the lowest of these found at only 30 meV above the ground state. Effects of the presence of vacancies on the atomic migration are discussed from a potential energy surface constructed using a similar model. [DOI: 10.1380/ejssnt.2006.619]

We investigate the electric and magnetic properties of a (3,3) single-walled carbon nanotube filled with a linear Co nanowire. We carry out first-principle calculations based on the spin-polarized density functional theory, and find that in the stable structure, it shows half metallic ferromagnetic behavior, i.e., the majority-spin electrons show metallic behavior while the minority-spin electrons have a semiconducting bandgap.

Local magnetic moments in benzenes of a multiple-decked sandwich cluster, namely, Mnn(C6H6)n+1 (n = 1,2) are investigated using density functional theory. A significant increase in the magnetic moments of benzenes was found in the 2pz orbitals. From the values of the local magnetic moments of Mn(C6H6)2 and Mn2(C6H6)3, we find that the local magnetic moment of benzene in the centre of the chain is larger than that at the edge position. These results indicate a promising possibility of changing the magnetic properties of benzenes using a magnetic material.

We investigate the adhesion of poly(phenylene sulfide) (PPS) on aluminum, copper, silver and gold through simulations involving PPS monomers and single metal atoms. Density functional theory based-total energy calculation results show that PPS binds best to aluminum through the sulfur end of the monomer. Although adhesion to copper is strongest in the other two schemes examined, the energy curves suggest that these may not proceed directly. The models involving silver in general showed the least affinity for PPS adhesion.

A model for the RIE process design of metal oxide surfaces based on density functional theory-based total energy calculations has been developed. Cluster and periodic systems were employed in order to develop the model and hence gain a deeper understanding of the process mechanisms on an atomic scale. In the present study, Fe 3 O 4 was used for the cluster system, and NiO for the periodic system as metal oxide surfaces. Possible gas combinations and by-products were studied with the aim of producing a more efficient and effective RIE process for metal oxide thin films. This study can be considered as fundamental groundwork required for the understanding of the reactivity in the etching of metal oxide surfaces and a means of decreasing the processing period of RIE for metal oxide thin films by optimum selection of gas combinations with a knowledge of their probable by-products.

The reactive ion etching (RIE) of the binary transition-metal oxides (TMOs) NiO, CuO and CoO, which are expected to be key materials of resistance random access memory (RRAMTM), was investigated. We found that inductively coupled plasma using CHF3-based discharge, which is highly compatible with conventional semiconductor RIE, is effective for the TMOs studied here. Furthermore, device fabrication using Pt/CoO/Pt trilayers is carried out, and a large change in resistance, which is an essential functionality of RRAM, was successfully observed. This should be definite evidence of a successful RIE realized in the present device fabrication.

We propose an intricate method of Reactive Ion Etching (RIE) process design for transition-metal (TM) materials using ab-initio calculations. The TM materials are inert in dry etching processes since volatile etching by-products cannot be formed easily. However, to achieve new high-performance memories based on the TM materials, a selective dry etching technique such as RIE is eagerly required instead of the conventional ion milling method. In this work, we would like to introduce our scenario of the RIE design for TM materials using the results of CoFe as an example.

We investigate the adhesion of PBT on aluminum using density functional theory-based calculations. The geometric structure of the PBT monomer is first relaxed then an aluminum atom is connected to the monomer in different orientations. We calculate their total energies and determine the orientation that gives the strongest binding between the monomer and the aluminum atom. Binding is strongest when the Al connects linearly with the carbonyl oxygen in the ester group. We present binding mechanisms and total energy relationships for the different orientations.

In response to the current need for an efficient, safe, and compact system for storing hydrogen in mobile applications, a scheme for maximizing and controlling hydrogen storage in graphite is proposed by modifying substrate reactivity through the exploitation of intrinsic vibrational modes in pristine and fully-hydrogenated graphite systems. Calculations within density functional theory suggest that infrared radiation of distinct frequencies can be used to independently induce graphite lattice restructuring and recrystallization for promoting hydrogen uptake and discharge, respectively. Effects of the initial attachment of hydrogen on graphite sheets are discussed, with computational results showing that additional hydrogen adsorption can proceed through easier reaction routes.

We discuss effects of the adsorption of glycine, histidine, phenylalanine, and cysteine on the electronic structure of the metallic (3,3) carbon nanotube through calculations within density functional theory. Results show good nanotube electronic structure stability in the presence of the biomolecules. Molecule-nanotube band mixing and band degeneracy lifting are observed, though the limited extents to which these effects are manifested on the system electronics affirm the weak, noncovalent connection at their interface.

H2 adsorption on Fe(OH)3 is investigated by using the density functional theory (DFT) for the H2 molecular orientation θ = 10° with respect to the surface normal and that for the H2 molecular orientation θ = 70°. From the PES results of H2-Fe(OH)3 system, we find that for the H2 molecular orientation θ = 10°, a potential well is at Z = 4.2 A with a depth of around 4.4 meV while for the H2 molecular orientation θ = 70°, a potential well is at Z = 3.0 A with a depth of around 17.7 meV. The small binding energy and the positions of potential well with respect to the surface of these adsorption states are typical for physisorption.

Density functional theory-based calculations on representative cluster models have been performed to investigate the adhesion strength of poly(phenylene sulfide) (PPS) onto magnesium, silicon and titanium, in addition to previously studied aluminum, copper, silver, and gold. Energy relations of substrate atom interactions with the sulfur end of a PPS monomer showed strongest binding with titanium and aluminum, while the larger transition metals showed the weakest results. A further investigation on the interaction of aluminum with the sulfur atom of a diphenyl sulfide model suggests a strong orientation dependence of the adhesion process, as shown in the greater preference for the in-plane case.

We have investigated several ways of introducing a solvated proton into a DFT calculation in order to mimic an electrochemical interface: an extra hydrogen introduced into a metal bilayer, a Zundel and an Eigen ion. In all these cases the charge on the supposed proton is substantially less than a unit positive charge. In contrast, when the electrode is represented as a cluster, the charge on Zundel ion is indeed plus one. However, the distribution of the compensating charge on the cluster is quite different from that on a plane metal surface. Fil: Quaino, Paola Monica. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Programa de Electroquímica Aplicada e Ingeniería Electroquímica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Luque, Noelia Beatriz. Universitat Ulm; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Soldano, Germán. Universitat Ulm; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Nazmutdinov, R.. Kazan National Research Technological University; Rusia Fil: Santos, Elizabeth del Carmen. Universitat Ulm; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina Fil: Roman, T.. Universitat Ulm; Alemania Fil: Lundin, E.. Chalmers University Of Technology; Suecia Fil: Groß, A.. Universitat Ulm; Alemania Fil: Schmickler, Wolfgang. Universitat Ulm; Alemania

Using periodic density-functional theory calculations, we address the work-function change induced by the adsorption of chlorine and iodine on Cu(111) which are shown to change the work function in opposite ways, contrary to what one may expect for these two electron acceptors. In contrast to previous studies, we demonstrate that substrate effects play only a minor role in work-function changes brought about by halogen adsorption on metals. Instead, polarization on the adsorbate not only explains the sign of the work-function change as a contributor to a positive surface dipole moment, but it is also the decisive factor in the dependence of adsorption-induced work-function changes on the coverage of halogens on metal surfaces.

We review the physical mechanisms that lead toward the conversion of graphene into its fully hydrogenated counterpart, which is a material that possesses properties closer to those of diamond than graphene. These are discussed from a theoretical perspective, i.e., from calculations based on density functional theory. We first discuss stability trends in small clusters of adsorbed hydrogen, as well as surface structure and concurrent reactivity changes for graphene one-face and two-face hydrogenation. Effects of adsorbed hydrogen on graphene electronic states, which are essential to adsorbed hydrogen structure discrimination, are also discussed.

The behavior of hydrogen near a platinum-surface-adsorbed carbon monoxide molecule is described using a potential energy term constructed from density functional theory. A clear nonattractive interaction of hydrogen with CO is confirmed, most notably with oxygen, which retains its strong H-repulsive traits in the Pt-bound CO case. Inhibiting effects of CO greater than what is expected from simple adsorption site exclusion are discussed with regard to adsorption/desorption and mobility on platinum, as well as possibilities of COH and HCO formation.

Quantum states of a hydrogen atom adsorbed on $\mathrm{Cu}(100)$ and $\mathrm{Cu}(110)$ are studied theoretically. In calculating eigenenergies and wave functions of hydrogen atom motion, three-dimensional adiabatic potential energy surfaces (PESs) are constructed within density functional theory and the Schr\"odinger equation for hydrogen atom motion on the PESs is solved by the variation method. The wave function on $\mathrm{Cu}(100)$ indicates a localized mode on the hollow (HL) site at the ground state. Wave functions of the first few excited states indicate vibrational modes on the HL site and suggest migration from an HL site to a neighboring HL site over the bridge (BR) site. In the case of $\mathrm{Cu}(110)$, the ground state wave function is spread from the short bridge (SB) site and to the pseudothreefold (PT) site. The first few excited states are vibrational modes centered at the SB and long bridge (LB) sites. The excited state wave function of the hydrogen atom motion on $\mathrm{Cu}(110)$ show isotope effects as follows. The fourth excited state wave function for the H atom motion shows a localized character on the LB site, and those for D and T atom motion show vibrational modes parallel to the surface. On the other hand, the fifth excited state wave functions for D and T atom motion show localized characters on the LB site and that for H atom motion shows a vibrational mode parallel to the surface. Our calculated eigenenergies of the hydrogen atom motion in excited states on $\mathrm{Cu}(100)$ and $\mathrm{Cu}(110)$ are fairly in agreement with their corresponding experimental findings.

ion reactions of copper from a (111) surface using halogen (F, Cl, and Br) atoms are studied as benchmark processes involving halogen-based etching agents. Reaction energetics are discussed, as well as potential energy surfaces that were created from ab initio energy calculations as functions of the center of mass and interatomic separation of the desorbing copper monohalide molecule. We support these results with a simple quantum dynamics model for the reaction leading to the desorption of CuF, which shows very high abstraction reaction probabilities. We also comment on analogous reactions on a Cu-terminated CuO surface.

Realizing high hydrogen uptakes on surfaces is one of the essential aspects of practical hydrogen storage in solid-state materials. To achieve this, it is always beneficial to know how the road to adsorption saturation on the surface looks like in terms of the physical mechanisms involved, and how we can control required reactions given this knowledge. On this topic, work has been carried out on how the simplest groups—pairs—of hydrogen behave on graphite/graphene. In that study it was shown that hydrogen pair interaction cannot be described by a simple function of interadsorbate separation, and that only certain pairing geometries on the surface are energetically favored (we note here that full relaxation of the substrate atoms was not found to change these conclusions). As detecting and discriminating singly adsorbed and small groups of adsorbates is an essential step to knowing how saturation can be reached, we have subsequently shown how probing surface electronic states can be used to identify an atomic hydrogen adsorbate, and distinguish it from the closely-spaced hydrogen pairs on the surface, affirming previously published experimental work on this subject, particularly that in ref. 3. In this paper we comment on the next step towards saturation: the formation of hydrogen clusters of three, i.e. hydrogen trios, on graphene. Results are discussed with respect to results obtained from hydrogen pairs adsorbed on graphene. Stable hydrogen adsorption configurations were determined through geometry optimization calculations using the VASP code, which implements the projector augmentedwave method for density functional theory-based electronic structure calculations. All calculations were spin-polarized, and utilized the exchange–correlation functional based on the PBE version of the generalized gradient approximation. We applied a 400 eV cutoff to limit the plane-wave basis set without compromising computational accuracy, and a 4 4 1 Monkhorst–Pack special k point grid for Brillouin zone sampling. Three H atoms on a 48-C atom single sheet comprise the unit cell, with C–C nearest-neighbor distances of 1.42 A before relaxation. All atoms were completely unrestricted in the geometry optimization. A 15.0 A vacuum separating adjacent sheets was used. Figure 1 shows the different clusters of three hydrogen atoms systems included in the computations of this study. We specifically choose the fourteen most closely-packed combinations of three atoms adsorbed on C atom ‘‘top’’ sites. Upon reconstruction hydrogen atom lateral positions generally don’t deviate much from the positions on receiving C atoms shown in Fig. 1. A trio is named based on its smallest H pairing component (o = ortho, m = meta, p = para) and distance of the third member of the trio from the pair center. This means, for example, that the trio labeled to1 is the three-hydrogen cluster comprised of a pair of adjacently adsorbed (ortho) hydrogen, and a third H atom adsorbed in the closest possible distance from the aforementioned pair. Table I shows the trios arranged by adsorption energy, starting with the most stable geometry. Table values were computed using the following expressions: Eads 1⁄4 Egr+3H(ads) ðEgr þ 3EH(g)Þ; Es 1⁄4 Eads=3; Eo 1⁄4 Egr+3H(ads) ðEgr+2H(ortho) þ EH(g)Þ; Em 1⁄4 Egr+3H(ads) ðEgr+2H(meta) þ EH(g)Þ; and Ep 1⁄4 Egr+3H(ads) ðEgr+2H(para) þ EH(g)Þ, where the terms Egr, EH(g), Egr+3H(ads), and Egr+2H are the total energies for the graphene sheet, a gas phase H atom, the system comprised of an adsorbed H trio and graphene, and the system comprised of an adsorbed H pair and graphene, respectively. The adsorption energy of an isolated H atom on graphene Eiso 1⁄4 Egr+H(ads) ðEgr þ EH(g)Þ is 0:77 eV, a value which differs slightly from previous calculations due to the different adsorbate coverage used in these studies. For the same reason pair and trio adsorption energies reported here also differ slightly from corresponding values reported in refs. 2 and 8. Adsorbed trios are generally stable: the Eads (and Es) values are all negative, meaning all adsorbed three-hydrogen groups are stable with respect to hydrogen atoms located far from the graphene surface. If Es for a given trio is less than Eiso, the trio is more stable compared with a system comprised of three isolated adsorbed H on graphene. In other words it would be more energetically favorable for the hydrogen atoms to clump together than to separate from each other on the surface, i.e., the net interaction shows an ‘attractive’ character. Only one trio— tm1— is found to1 to4

Using first principles calculations with local density approximation (LDA) and LDA+U methods, we present a detailed theoretical study of reactive gas combinations (O2, N2, CH4, and CHF3) for the etching processes of CoO(001) surfaces. The calculation results show that the best possible gas combinations for the etching process contain CH4. Despite differences in the intermediate state total energies predicted by the two methods, the favorable results in the final state lead to the same reaction products.

We comment on the identification of a hydrogen atom adsorbed on graphite and distinguishing it from closely-spaced pairs under the scanning tunneling microscope (STM) through electronic state calculations based on density functional theory. The presence of the H atom should be very well observable through a distinct feature most directly associated with the adsorbate itself, a threefold symmetry most apparent through the third-nearest neighbor C atoms, and through sublattice visibility differences. A comparison with effects on the electronic states brought about by closely-spaced pairs shows visible differences in the aforementioned three factors, which should enable us to discriminate among adsorbed structures. Results compare well with STM measurements of adsorbed deuterium on graphite.

For hydrogen fuel cells to become commercially viable, an alternative catalyst to platinum surfaces that is both efficient and affordable must be discovered. We consider haem and haem derivatives as potentials substitutes. In this paper, we discuss the oxygen reduction reaction on both the platinum surface and on haem. We then introduce our suggestions based on density-functional studies on how to improve haem's oxygen-reduction capabilities, which can be summarized as follows: inducing the singlet state, inducing side-on interaction, mimicking cytochrome c oxidase by adding a copper-imidazole complex, using platinum deposited on tin porphyrin instead of haem, and using oxomolybdenum porphyrin instead of haem. We shall focus on the last three methods because of their experimental practicability.