Electrolytic Hydrogen Release from Hydrogen Boride Sheets.

Solid-state hydrogen storage materials are safe and lightweight hydrogen carriers. Among the various solid-state hydrogen carriers, hydrogen boride (HB) sheets possess a high gravimetric hydrogen capacity (8.5 wt%). However, heating at high temperatures and/or strong ultraviolet illumination is required to release hydrogen (H2 ) from HB sheets. In this study, the electrochemical H2 release from HB sheets using a dispersion system in an organic solvent without other proton sources is investigated. H2 molecules are released from the HB sheets under the application of a cathodic potential. The Faradaic efficiency for H2 release from HB sheets reached >90%, and the onset potential for H2 release is -0.445 V versus Ag/Ag+ , which is more positive than those from other proton sources, such as water or formic acid, under the same electrochemical conditions. The total electrochemically released H2 in a long-time experiment reached ≈100% of the hydrogen capacity of HB sheets. The H2 release from HB sheets is driven by a small bias; thus, they can be applied as safe and lightweight hydrogen carriers with economical hydrogen release properties.

Electronic and optical properties of the hydrogen boride sheet from the many-body perturbation theory.

We study the electronic and optical properties of the hydrogen boride sheet by using the many-body perturbation theory with the perturbativeGW(G0W0) approximation. It was found that the hydrogen boride sheet shows a semimetallic electronic structure, supporting the previous theoretical study based on the semilocal density functional theory calculations. It was also found that the optical spectrum calculated based on the quasiparticle energies agrees well with the experiments. This work suggests thatG0W0approximation may be useful for predicting precise electronic and optical properties of the hydrogen boride sheet and its derivatives.

Nanoscale coherent phonon spectroscopy.

Coherent phonon spectroscopy can provide microscopic insight into ultrafast lattice dynamics and its coupling to other degrees of freedom under nonequilibrium conditions. Ultrafast optical spectroscopy is a well-established method to study coherent phonons, but the diffraction limit has hampered observing their local dynamics directly. Here, we demonstrate nanoscale coherent phonon spectroscopy using ultrafast laser-induced scanning tunneling microscopy in a plasmonic junction. Coherent phonons are locally excited in ultrathin zinc oxide films by the tightly confined plasmonic field and are probed via the photoinduced tunneling current through an electronic resonance of the zinc oxide film. Concurrently performed tip-enhanced Raman spectroscopy allows us to identify the involved phonon modes. In contrast to the Raman spectra, the phonon dynamics observed in coherent phonon spectroscopy exhibit strong nanoscale spatial variations that are correlated with the distribution of the electronic local density of states resolved by scanning tunneling spectroscopy.

Hydrogenation of Formate Species Using Atomic Hydrogen on a Cu(111) Model Catalyst.

The reaction mechanism of the CH3OH synthesis by the hydrogenation of CO2 on Cu catalysts is unclear because of the challenge in experimentally detecting reaction intermediates formed by the hydrogenation of adsorbed formate (HCOOa). Thus, the objective of this study is to clarify the reaction mechanism of the CH3OH synthesis by establishing the kinetic natures of intermediates formed by the hydrogenation of adsorbed HCOOa on Cu(111). We exposed HCOOa on Cu(111) to atomic hydrogen at low temperatures of 200-250 K and observed the species using infrared reflection absorption (IRA) spectroscopy and temperature-programmed desorption (TPD) studies. In the IRA spectra, a new peak was observed upon the exposure of HCOOa on Cu(111) to atomic hydrogen at 200 K and was assigned to the adsorbed dioxymethylene (H2COOa) species. The intensity of the new peak gradually decreased with heating from 200 to 290 K, whereas the IR peaks representing HCOOa species increased correspondingly. In addition, small amounts of formaldehyde (HCHO), which were formed by the exposure of HCOOa species to atomic hydrogen, were detected in the TPD studies. Therefore, H2COOa is formed via hydrogenation by atomic hydrogen, which thermally decomposes at ∼250 K on Cu(111). We propose a potential diagram of the CH3OH synthesis via H2COOa from CO2 on Cu surfaces, with the aid of density functional theory calculations and literature data, in which the hydrogenation of bidentate HCOOa to H2COOa is potentially the rate-determining step and accounts for the apparent activation energy of the methanol synthesis from CO2 on Cu surfaces.

A comparative study on the stability of the furfural molecule on the low index Ni, Pd and Pt surfaces.

We present a comparative density functional theory investigation of the furfural (Ff) molecule on the low index Ni, Pd and Pt surfaces to understand its geometrical and electronic properties to gain mechanistic insights into the experimentally measured catalytic reactivities of these metal catalysts. We show that the number of metal d-states, which hybridize with the nearest C and O p-orbitals of the Ff molecule, can be used to explain the stability of the Ff molecule on these surfaces. We find that the hybridization between atoms with higher electronegativity and the metal d-states plays a crucial role in determining the stability of these systems. Furthermore, we also find electron transfer from metal to the Ff molecule on the Ni and Pd surfaces, with a reverse process occurring on the Pt surface.

Carbon dioxide adsorption and conversion to methane and ethane on hydrogen boride sheets.

Hydrogen boride (HB) sheets are metal-free two-dimensional materials comprising boron and hydrogen in a 1:1 stoichiometric ratio. In spite of the several advancements, the fundamental interactions between HB sheets and discrete molecules remain unclear. Here, we report the adsorption of CO2 and its conversion to CH4 and C2H6 using hydrogen-deficient HB sheets. Although fresh HB sheets did not adsorb CO2, hydrogen-deficient HB sheets reproducibly physisorbed CO2 at 297 K. The adsorption followed the Langmuir model with a saturation coverage of 2.4 × 10-4 mol g-1 and a heat of adsorption of approximately 20 kJ mol-1, which was supported by density functional theory calculations. When heated in a CO2 atmosphere, hydrogen-deficient HB began reacting with CO2 at 423 K. The detection of CH4 and C2H6 as CO2 reaction products in a moist atmosphere indicated that hydrogen-deficient HB promotes C-C coupling and CO2 conversion reactions. Our findings highlight the application potential of HB sheets as catalysts for CO2 conversion.

Mechanistic insight into oxygen vacancy migration in SrFeO3-δ from DFT+U simulations.

SrFeO3-δ is known to be an effective oxygen ion conductor and oxygen vacancies are central to its performance. SrFeO3-δ displays four crystallographic structural transitions as it undergoes oxygen reduction over a broad range of operating temperatures. In this work, systematic density functional theory calculations using the Hubbard U correction were performed to understand oxygen vacancy interactions and migration as a function of vacancy concentrations in SrFeO3-δ (δ = 0-0.5). We found strong repulsion between oxygen vacancies at close distance while these oxygen vacancies are stabilized at further distance. We also found that the oxygen migration is highly anisotropic and the calculated effective migration energy for the oxygen migration tends to be high and increases from 0.91 eV to 1.30 eV as δ goes from 0.125 (tetragonal phase) to 0.25 (orthorhombic phase). In the ordered brownmillerite SrFeO2.625, the oxygen migration is restricted in the one-dimensional channel because of the highly anisotropic nature of the crystal structure, resulting in the relatively low effective migration energy of 0.49 eV. This explains the experimental activation energy of 0.55 ± 0.05 eV. These results suggest the importance of regulating the oxygen migration path via the crystal structure design toward development of a SrFeO3-δ based fast oxygen conductor.

A flat-lying dimer as a key intermediate in NO reduction on Cu(100).

The reaction of nitric oxide (NO) on Cu(100) is studied by scanning tunneling microscopy, electron energy loss spectroscopy and density functional theory calculations. The NO molecules adsorb mainly as monomers at 64 K, and react and dissociate to yield oxygen atoms on the surface at ∼70 K. The temperature required for the dissociation is significantly low for Cu(100), compared to those for Cu(111) and Cu(110). The minimum energy pathway of the reaction is via (NO)2 formation, which converts into a flat-lying ONNO and then dissociates into N2O and O with a considerably low activation energy. We propose that the formation of (NO)2 and flat-lying ONNO is the key to the exceptionally high reactivity of NO on Cu(100).

Multi-scale Simulation of Equilibrium Step Fluctuations on Cu(111) Surfaces.

Understanding the nature of active sites is a non-trivial task, especially when the catalyst is sensitively affected by chemical reactions and environmental conditions. The challenge lies on capturing explicitly the dynamics of catalyst evolution during reactions. Despite the complexity of catalyst reconstruction, we can untangle them into several elementary processes, of which surface diffusion is of prime importance. By applying density functional theory-kinetic Monte Carlo (DFT-KMC) simulation employed with cluster expansion (CE), we investigated the microscopic mechanism of surface diffusion of Cu with defects such as steps and kinks. Based on the result, the energetics obtained from CE have shown good agreement with DFT calculations. Various diffusion events during the step fluctuations are discussed as well. Aside from the adatom attachment, the diffusion along the step edge is found to be the dominant mass transport mechanism, indicated by the lowest activation energy. We also calculated time correlation functions at 300, 400, and 500 K. However, the time exponent in the correlation function does not strictly follow the power law behavior due to the limited step length, which inhibits variation in the kink density.

Role of Intermolecular Interactions in the Catalytic Reaction of Formic Acid on Cu(111).

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.

Adsorption of toxic gases on borophene: surface deformation links to chemisorptions.

ß12 borophene has received great attention because of its intriguing mechanical and electronic properties. One of the possible applications of borophene is gas sensing. However, the interaction between common gases and ß12 borophene remains to be clarified. In this work, we study the interactions of ß12 borophene towards five hazardous gases, namely, CO, NO, NH3, NO2, and CO2 using various non-empirical van der Waals density functionals and provide an insight into the adsorption behavior of borophene. The adsorption mechanism and molecular vibrations are discussed in great detail. Among the gases considered, CO2 is physisorbed while other gases are chemically bonded to ß12 borophene. We also demonstrate that the deformation at the ridge of borophene enables its active p z orbital to strongly hybridize with frontier orbitals of the studied polar gases. Consequently, borophene is predicted to interact strongly with CO, NO, NH3, and especially NO2, making it a sensitive sensing material for toxic gases.

Manipulable Metal Catalyst for Nanographene Synthesis.

Performing bottom-up synthesis by using molecules adsorbed on a surface is an effective method to yield functional polycyclic aromatic hydrocarbons (PAHs) and nanocarbon materials. The intramolecular cyclodehydrogenation of hydrocarbons is a critical process in this synthesis; however, thus far, its elementary steps have not been elucidated thoroughly. In this study, we utilize the metal tip of a low-temperature noncontact atomic force microscope as a manipulable metal surface to locally activate dehydrogenation for PAH-forming cyclodehydrogenation. This method leads to the dissociation of a H atom of an intermediate to yield the cyclodehydrogenated product in a target-selective and reproducible manner. We demonstrate the metal-tip-catalyzed dehydrogenation for both benzenoid and nonbenzonoid PAHs, suggesting its universal applicability as a catalyst for nanographene synthesis.

Dramatic Enhancement of Tip-Enhanced Raman Scattering Mediated by Atomic Point Contact Formation.

Tip-enhanced Raman scattering (TERS) in ångström-scale plasmonic cavities has drawn increasing attention. However, Raman scattering at vanishing cavity distances remains unexplored. Here, we show the evolution of TERS in transition from the tunneling regime to atomic point contact (APC). A stable APC is reversibly formed in the junction between an Ag tip and ultrathin ZnO or NaCl films on the Ag(111) surface at 10 K. An abrupt increase of the TERS intensity occurs upon APC formation for ZnO, but not for NaCl. This remarkable observation is rationalized by a difference in hybridization between the Ag tip and these films, which determines the contribution of charge transfer enhancement in the fused plasmonic junction. The strong hybridization between the Ag tip and ZnO is corroborated by the appearance of a new vibrational mode upon APC formation, whereas it is not observed for the chemically inert NaCl.

Atomic and molecular adsorption on single platinum atom at the graphene edge: A density functional theory study.

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.

Resolving the Correlation between Tip-Enhanced Resonance Raman Scattering and Local Electronic States with 1 nm Resolution.

Low-temperature tip-enhanced Raman spectroscopy (TERS) enables chemical identification with single-molecule sensitivity and extremely high spatial resolution even down to the atomic scale. The large enhancement of Raman scattering obtained in TERS can originate from physical and/or chemical enhancement mechanisms. Whereas physical enhancement requires a strong near-field through excitation of localized surface plasmons, chemical enhancement is governed by resonance in the electronic structure of the sample, which is also known as resonance Raman spectroscopy. Here we report on tip-enhanced resonance Raman spectroscopy (TERRS) of ultrathin ZnO layers epitaxially grown on a Ag(111) surface, where both enhancement mechanisms are operative. In combination with scanning tunneling spectroscopy (STS), it is demonstrated that the TERRS intensity strongly depends on the local electronic resonance of the ZnO/Ag(111) interface. We also reveal that the spatial resolution of TERRS is dependent on the tip-surface distance and reaches nearly 1 nm in the tunneling regime, which can be rationalized by strong-field confinement resulting from an atomic-scale protrusion on the tip apex. Comparison of STS and TERRS mapping clearly shows a correlation between resonantly enhanced Raman scattering and the local electronic states at near-atomic resolution. Our results suggest that TERRS is a new approach for the atomic-scale optical characterization of local electronic states.

Vibration-driven reaction of CO2 on Cu surfaces via Eley-Rideal-type mechanism.

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 CO2 in a molecular beam is found to react with pre-adsorbed hydrogen atoms directly on cold Cu(111) and Cu(100) surfaces to form formate adspecies. The vibrational energy of CO2 is more effective at promoting the reaction than translational energy, the reaction rate is independent of the surface temperature and the experimental results are consistent with density functional theory calculations.

Van der Waals density functional study of formic acid adsorption and decomposition on Cu(111).

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.

Platinum single-atom adsorption on graphene: a density functional theory study.

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.

Hydrogen adsorption on Pt(111) revisited from random phase approximation.

Hydrogen adsorption on Pt(111) has been actively studied using semilocal approximations within the density functional theory featuring simultaneous adsorption of hydrogen on multiple sites, i.e., fcc, atop, and hcp. Considering the accuracy needed to detail the feature, we revisit this problem with the help of higher level of theory, the adiabatic connection fluctuation dissipation theorem within the random phase approximation. Our simulation emphasizes important roles played by the equilibrium lattice parameter of the surface, mass of the hydrogen isotope, and hydrogen coverage. The insight acquired in this study provides a way to consistently interpret electrochemical and spectroscopic data.

Insight into the Solvation Structure of Tetraglyme-Based Electrolytes via First-Principles Molecular Dynamics Simulation.

Glyme-lithium salt equimolar mixtures, as solvate ionic liquid electrolytes for rechargeable lithium secondary batteries, are of great interest, due to the desirable properties such as high oxidative stability, low vapor pressure, and nonflammability. However, the fundamental understanding of the solvation shell structure in glyme electrolytes has not been clearly established. Herein, we employ first-principles molecular dynamics (FPMD) simulation to study the lithium bis(trifluoromethylsulfonyl)-amide (LiTFSA) and tetraglyme (G4) electrolyte system. For the case of equimolar ratio, a positive correlation between the total coordination number of Li+ ions and the phase stability is clearly established. At the ground state of equimolar LiTFSA-G4 electrolyte, most of the Li+ ions are coordinated to four O atoms of a curled G4 molecule and one O atom of a TFSA- anion, equivalent to the second most stable contact ion pair in gas-phase cluster calculations. By contrast, Li+ ions prefer to be coordinated by two G4 molecules and not in direct contact with TFSA- anions at a low concentration of Li salt. The significantly increased probability of pairing between the Li-G4 complexes and TFSA- anions at the equimolar ratio could be highly relevant to its ionic-liquid-like properties.