Hydrogen Adatoms Research Articles

Chlorine Adsorption on Graphene: Chlorographene

We perform first-principles structure optimization, phonon frequency, and finite temperature molecular dynamics calculations based on density functional theory to study the interaction of chlorine atoms with graphene predicting the existence of possible chlorinated graphene derivatives. The bonding of a single chlorine atom is ionic through the transfer of charge from graphene to chlorine adatom and induces negligible local distortion in the underlying planar graphene. Different from hydrogen and fluorine adatoms, the migration of a single chlorine adatom on the surface of perfect graphene takes place almost without barrier. However, the decoration of one surface of graphene with Cl adatoms leading to various conformations cannot be sustained due to strong Cl–Cl interaction resulting in the desorption through the formation of Cl2 molecules. On the contrary, the fully chlorinated graphene, chlorographene CCl, where single chlorine atoms are bonded alternatingly to each carbon atom from different sides of graphene with sp3-type covalent bonds, is buckled. We found that this structure is stable and is a direct band gap semiconductor, whose band gap can be tuned by applied uniform strain. Calculated phonon dispersion relation and four Raman-active modes of chlorographene are discussed.

Magnetic Moment Formation in Graphene Detected by Scattering of Pure Spin Currents

Hydrogen adatoms are shown to generate magnetic moments inside single layer graphene. Spin transport measurements on graphene spin valves exhibit a dip in the nonlocal spin signal as a function of the applied magnetic field, which is due to scattering (relaxation) of pure spin currents by exchange coupling to the magnetic moments. Furthermore, Hanle spin precession measurements indicate the presence of an exchange field generated by the magnetic moments. The entire experiment including spin transport is performed in an ultrahigh vacuum chamber, and the characteristic signatures of magnetic moment formation appear only after hydrogen adatoms are introduced. Lattice vacancies also demonstrate similar behavior indicating that the magnetic moment formation originates from p(z)-orbital defects.

Ammonia synthesis using a stable electride as an electron donor and reversible hydrogen store

Industrially, the artificial fixation of atmospheric nitrogen to ammonia is carried out using the Haber-Bosch process, but this process requires high temperatures and pressures, and consumes more than 1% of the world's power production. Therefore the search is on for a more environmentally benign process that occurs under milder conditions. Here, we report that a Ru-loaded electride [Ca(24)Al(28)O(64)](4+)(e(-))(4) (Ru/C12A7:e(-)), which has high electron-donating power and chemical stability, works as an efficient catalyst for ammonia synthesis. Highly efficient ammonia synthesis is achieved with a catalytic activity that is an order of magnitude greater than those of other previously reported Ru-loaded catalysts and with almost half the reaction activation energy. Kinetic analysis with infrared spectroscopy reveals that C12A7:e(-) markedly enhances N(2) dissociation on Ru by the back donation of electrons and that the poisoning of ruthenium surfaces by hydrogen adatoms can be suppressed effectively because of the ability of C12A7:e(-) to store hydrogen reversibly.

Enhanced Screening in Chemically Functionalized Graphene

Resonant scatterers such as hydrogen adatoms can strongly enhance the low-energy density of states in graphene. Here, we study the impact of these impurities on electronic screening. We find a two-faced behavior: Kubo formula calculations reveal an increased dielectric function ε upon creation of midgap states but no metallic divergence of the static ε at small momentum transfer q→0. This bad metal behavior manifests also in the dynamic polarization function and can be directly measured by means of electron energy loss spectroscopy. A new length scale l(c) beyond which screening is suppressed emerges, which we identify with the Anderson localization length.

A DFT calculation study on the temperature-dependent hydrogen electrocatalysis on Pt(1 1 1) surface

DFT calculations are combined with thermodynamic and kinetic modeling to study the effect of temperature on the electrochemical adsorption of hydrogen and the exchange current density (j0) of the hydrogen electrode reactions (HERs) on Pt(111). The nature of the underpotential and overpotential deposited hydrogen (UPD H and OPD H), the potential dependence of their coverage and the corresponding voltammetric curves for hydrogen adsorption are evaluated for temperatures from 273K to 373K. By introducing a coverage-dependent correction term in the free energy expression to account for the effect of the interaction between hydrogen ad-atoms on the configurational entropy, the DFT calculations give isotherms and voltammetric curves for hydrogen adsorption agreeing reasonably with those observed in experiments. It is shown that both the UPD and OPD H on Pt(111) surface could be the H atoms adsorbed at the 3-fold fcc hollow sites. A linear dependence of lnj0 on T−1 is found under temperatures from 273K to 373K, which, however, deviates from that predicted by the Arrhenius relation. The values of j0 and activation enthalpy for HERs on Pt(111) surface are estimated.

Structure of water layers on hydrogen-covered Pt electrodes

The structure of water layers above hydrogen-covered Pt(111) surfaces at room temperature has been studied by ab initio molecular dynamics simulations based on periodic density functional theory calculations. Fully hydrogen-covered Pt(111) 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(111), 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(111) electrode exhibits a greater order than the water layer on clean Pt(111) which is attributed to the stronger water–water interaction above hydrogen-covered Pt(111). The relevance of our findings for the understanding of electrochemical electrode/water interfaces is discussed.

Water dissociation on well-defined platinum surfaces: The electrochemical perspective

This paper discusses three important discrepancies in the current interpretation of the role of water dissociation on the blank cyclic voltammetry of well-defined single-crystalline stepped platinum surfaces. First, for H adsorption both H-terrace and H-step contributions have been identified, whereas for OH adsorption only OH-terrace has been identified. Second, different shapes (broad vs. sharp) of the H-terrace and H-step voltammetric peaks imply different lateral interactions between hydrogen adatoms at terraces and steps, i.e. repulsive vs. attractive interactions. Third, the H-step peak exhibits an unusual pH-dependent shift of 50mVNHE/pH unit. We propose here a model that can explain all these observations. In the model, the H-step peak is not due to only ad- and desorption of hydrogen, but to the replacement of H with O and/or OH. The O:OH ratio in the step varies with step geometry, step density and medium. In alkaline media relatively more OH is adsorbed in (or on) the step than in acidic media, under which conditions more O is adsorbed in (or on) the step. This would explain the anomalous pH dependence and would provide a possible explanation for the higher catalytic activity of alkaline media for electro-oxidation reactions. Although the model certainly still contains speculative elements, we believe it provides the most consistent interpretation of platinum single-crystal electrochemistry currently available, and presents an important and significant improvement over previous interpretations. In situ spectroscopic data are ultimately needed to confirm or disprove some of the assumptions of the model.

On the alleviation of Fermi-level pinning by ultrathin insulator layers in Schottky contacts

With a few exceptions, metal-semiconductor or Schottky contacts are rectifying. Intimate n-Ge Schottky contacts are the most extreme example in that their barrier heights are almost independent of the metal used. Such behavior is characterized as pinning of the Fermi level. Quite recently, ultrathin insulator layers placed between the metal and the semiconductor were found to lower the barrier heights of Schottky contacts and to increase their dependence on the metals used. In this way ohmic behavior was achieved without alloying. The barrier heights of intimate Schottky contacts and the valence-band offsets of heterostructures are well described by the intrinsic interface-induced gap states (IFIGS). Insulators fit in this concept because they are large-gap semiconductors. This article demonstrates that the IFIGS concept also explains the experimentally observed alleviation of the Fermi-level pinning or, as it is also addressed, the Fermi-level depinning in metal-ultrathin insulator-semiconductor or MUTIS structures. Their barrier heights are determined by the IFIGS branch-point energy of the semiconductor and the dependence of the barrier heights of the insulator Schottky contacts on the metals used. Furthermore, saturation of the semiconductor dangling bonds by, for example, sulfur or hydrogen adatoms prior to the deposition of the metals also reduces or increases the barrier heights of Schottky contacts irrespective of the metals applied. In other words, no alleviation of the Fermi-level pinning or depinning occurs. These modifications of the barrier heights are explained by the partial ionic character of the covalent bonds between the adatoms and the semiconductor atoms at the interface, i.e., by an extrinsic electric-dipole layer.

Voltammetry of Basal Plane Platinum Electrodes in Acetonitrile Electrolytes: Effect of the Presence of Water

The first part of this report studies the electrochemical properties of single-crystal platinum electrodes in acetonitrile electrolytes by means of cyclic voltammetry. Potential difference infrared spectroscopy in conjunction with linear voltammetry was used to obtain a molecular-level picture of this interface. The second part of this report studies the hydrogen evolution and the hydrogen oxidation reactions on the three low-index faces of Pt electrodes in acetonitrile electrolytes. The data (CVs and IR spectra) strongly suggest that acetonitrile and CN(-) molecules are adsorbed on single-crystal platinum electrodes in the range of -1.5 to 0.3 V versus Ag/AgCl. Those species block part of the adsorption sites for hydrogen adatoms, and they decompose on the surface in the presence of water. The nature of the cation and the presence of water strongly affect the onset of acetonitrile electrolysis and the kinetics and stability of the adsorbed species on the electrode. Finally, the hydrogen evolution and the hydrogen oxidation reactions on platinum single-crystal surfaces in acetonitrile electrolytes are strongly affected by the surface-energy state of Pt electrodes.

Mechanism of nitrate electroreduction on Pt(100)

Kinetics and mechanism of nitrate anion reduction on the Pt(100) electrode in perchloric and sulfuric acid solutions are studied. Analysis of the results of electrochemical measurements (combination of potentiostatic treatment and cyclic voltammetry) and the data of in situ IR spectroscopy allow suggesting the following scheme of the nitrate reduction process on Pt(100) differing from that in the literature. If the potential of 0.85 V is chosen as the starting potential for a clean flame-annealed electrode surface and negativegoing (cathodic) potential sweep is applied, then an NO adlayer with the coverage of about 0.5 monolayer is formed on Pt(100) in the nitrate solution already at 0.6 V. The further decrease in the potential results in NO reduction to hydroxylamine or/and ammonia, desorbing products vacate the adsorption sites for nitrate and hydrogen adatoms. At E < 0.1 V, adsorbed hydrogen is mostly present on the surface. During positive-going (anodic) potential sweep, the process of nitrate reduction starts after partial hydrogen desorption, the cathodic peak of nitrate reduction to hydroxylamine or ammonia is observed at 0.32 V on cyclic voltammograms. The process of nitrate anion reduction continues up to 0.7 V; at higher potentials, the surface redox process with participation of hydroxylamine or ammonia (the anodic peak at 0.78 V) and nitrate (the cathodic peak at 0.74 V is due to nitrate reduction to NO on the vacant adsorption sites) occurs.

A problem on a dimer adsorbed at single-sheet graphene

In the context of the previously proposed M model for the density of states in graphene (Davydov and Sabirova [3]), two adatoms coupled via the amplitude of electron transitions from one adatom to the other (direct exchange) are considered. An analytical expression for the energy density of states in/for the dimer is derived, and the dependence of the charge of the dimer’s constituent atoms on the parameters of the problem is analyzed. The effect of image forces is investigated. In the context of the surface molecule model, a dimer composed of hydrogen adatoms is considered. The dissociation and desorption energies of such a dimer are estimated.

Effect of Shuttling Catalyst on the Migration of Hydrogen Adatoms: A Strategy for the Facile Hydrogenation of Graphene

Using density functional theory calculations, we have investigated a strategy for the facile hydrogenation of graphene. We show that the presence of polar hydride molecules, such as H2O, HF, and NH3 (physisorbed molecules that mediate/assist the migration of atomic H adatoms on graphene, named “shuttle gases” in the present work), can provide a favorable catalytic effect (lowering the H migration barrier) and a favorable thermodynamic effect (activating the direct transition to the second-nearest-neighbor site). In comparison with the widely known fact that the migration of chemisorbed H is kinetically unfavorable on graphene, this mechanism for shuttling catalysis provides an easier migration channel. We propose that randomly distributed hydrogen adatoms on graphene can transform into a compactly aggregated hydrogenation domain (similar to graphane, as suggested in the literature) by heat treatment in the presence of a shuttling catalyst.

Understanding the Band Gap, Magnetism, and Kinetics of Graphene Nanostripes in Graphane

The electronic structure and kinetic stability of various graphene nanostripes (GNSs) in graphane are systematically studied by ab initio simulations. The band gap of armchair GNS (nonmagnetic) is determined by the quantum confinement of π electrons and modified by the contraction of the edge C–C bonds. The band gap of zigzag GNS is induced by the exchange splitting of the edge states and its magnetism closely correlates with the long-range nature of π electrons, quantum confinement, intraedge exchange interaction, and interedge superexchange interaction. The kinetic stability of these GNSs in graphane is probed by the potential barriers and reaction rates for the diffusion and desorption of the hydrogen adatoms at various graphene/graphane interfaces. These interfaces are very stable under conventional thermalization conditions. The conformation of graphane has a small effect on the electronic structure of GNS but has a significant effect on the kinetic stability of the interfacial adatoms. The hydrogen ...

Enantioselective Hydrogenation of α-Ketoesters: An in Situ Surface-Enhanced Raman Spectroscopy (SERS) Study

Adsorption of ethyl pyruvate (EP) and methyl pyruvate (MP) has been studied on a SERS-active platinum electrode. Gas-phase experiments under a hydrogen atmosphere show that decarbonylation to form adsorbed CO together with formation of a half-hydrogenated state of MP/EP are the dominant surface reaction pathways occurring under these conditions. Similar findings were obtained using a hydrogen-evolving platinum electrode under potentiostatic control in 0.1 M aqueous sulfuric acid containing EP or MP. DFT calculations have been used to support the assignment of vibrational bands in the SERS obtained from the half-hydrogenated state intermediate. At hydrogen evolution potentials and using concentrations of MP and EP < 0.02M, adsorbed hydrogen adatoms were detected. At higher EP concentrations, hydrogen-atom formation was inhibited by the greater surface coverages of the half-hydrogenated state of EP/MP and to a lesser extent adsorbed CO derived from decarbonylation side reactions. Addition of the chiral modifier cinchonidine (CD) to the acidic electrolyte under conditions of hydrogen evolution resulted in significantly less CO production and a reduction in the intensity of all SERS peaks associated with the half-hydrogenated state. Most importantly, however, the presence of surface hydrogen, previously absent on the unmodified surface, was always observed irrespective of the EP concentration. The significance of this result in relation to the question of the origin of rate enhancement during heterogeneously catalyzed enantioselective hydrogenation of α-ketoesters is discussed.

Magnetoresistance and Magnetic Ordering Fingerprints in Hydrogenated Graphene

Spin-dependent features in the conductivity of graphene, chemically modified by a random distribution of hydrogen adatoms, are explored theoretically. The spin effects are taken into account using a mean-field self-consistent Hubbard model derived from first-principles calculations. A Kubo transport methodology is used to compute the spin-dependent transport fingerprints of weakly hydrogenated graphene-based systems with realistic sizes. Conductivity responses are obtained for paramagnetic, antiferromagnetic, or ferromagnetic macroscopic states, constructed from the mean-field solutions obtained for small graphene supercells. Magnetoresistance signals up to ∼7% are calculated for hydrogen densities around 0.25%. These theoretical results could serve as guidance for experimental observation of induced magnetism in graphene.

Low-Temperature Hydrogenation of Acetaldehyde to Ethanol on H-Precovered Au(111)

Gold-based classical high surface area catalysts have been widely investigated for hydrogenation reactions, but fundamental studies on model catalysts are lacking. We present experimental measurements of the reaction of hydrogen adatoms and adsorbed acetaldehyde on the Au(111) surface employing temperature-programmed desorption. Here, we show that chemisorbed hydrogen adatoms bind weakly with desorption peaks at ∼110 K, indicating an activation energy for recombinative desorption of ∼28 kJ/mol. We further demonstrate that acetaldehyde (CH3CHO) can be hydrogenated to ethanol (CH3CH2OH) on the H-atom-precovered Au(111) surface at cryogenic temperatures. Isotopic experiments employing D atoms indicate a lower hydrogenation reactivity.

The thermodynamic and kinetic properties of hydrogen dimers on graphene

The thermodynamic and kinetic properties of hydrogen adatoms on graphene are important to the materials and devices based on hydrogenated graphene. Hydrogen dimers on graphene with coverages varying from 0.040 to 0.111 ML (1.0 ML = 3.8 × 10 15 cm − 2 ) were considered in this report. The thermodynamic and kinetic properties of H, D and T dimers were studied by ab initio simulations. The vibrational zero-point energy corrections were found to be not negligible in kinetics, varying from 0.038 (0.028, 0.017) to 0.257 (0.187, 0.157) eV for H (D, T) dimers. The isotope effect exhibits as that the kinetic mobility of a hydrogen dimer decreases with increasing the hydrogen mass. The simulated thermal desorption spectra with the heating rate α = 1.0 K/s were quite close to experimental measurements. The effect of the interaction between hydrogen dimers on their thermodynamic and kinetic properties was analyzed in detail.

Ab Initio Study of Hydrogen Interaction with Defective BC2N Nanotubes

The spin polarized density functional theory is used to investigate the incorporation of hydrogen adatoms and the interaction between molecular H2 with antisites and vacancies in both zigzag (4,0) and armchair (3,3) BC2N nanotubes. We find that the presence of antisites and vacancies increases the binding energy of hydrogen adatoms on the tube surface. In the most stable antisites (CB, CN, NCI and BCII), the hydrogen adatoms bind preferentially on carbon atoms of the defective site (CB, and CN) or closer to it (NCI and BCII). For a single adsorbed H, the calculated binding energies show that the H adsorption on a carbon vacancy (VCII) is the most stable site with a binding energy of −4.23 eV. The adsorption of a second H atom near the previous one is an exothermic process compared to of a single H2 molecule physisorbed on the nanotube surface.

Local semiconducting transition in armchair carbon nanotubes: The effect of periodic bi-site perturbation on electronic and transport properties of carbon nanotubes

In carbon nanotubes, the most abundant defects, caused for example by irradiation or chemisorption treatments, are small perturbing clusters, i.e. bi-site defects, extending over both A and B sites. The relative positions of these perturbing clusters play a crucial role in determining the electronic properties of carbon nanotubes. Using bandstructure and electronic transport calculations, we find out that in the case of armchair metallic nanotubes a band gap opens up when the clusters fulfill a certain periodicity condition. This phenomenon might be used in future nanoelectronic devices in which certain regions of single metallic nanotubes could be turned to semiconducting ones. Although in this work we study specifically the effect of hydrogen adatom clusters, the phenomenon is general for different types of defects. Moreover, we study the influence of the length and randomness of the defected region on the electron transport through it.

Direct imaging of hydrogen-atom columns in a crystal by annular bright-field electron microscopy

Enhancing the imaging power of microscopy to identify all chemical types of atom, from low- to high-atomic-number elements,would significantly contribute for a direct determination of material structures. Electron microscopes have successfully provided images of heavy-atom positions, particularly by the annular dark-field method, but detection of light atoms was difficult owing to their weak scattering power. Recent developments of aberration-correction electron optics have significantly advanced the microscope performance, enabling identification of individual light atoms such as oxygen, nitrogen, carbon, boron and lithium. However, the lightest hydrogen atom has not yet been observed directly, except in the specific condition of hydrogen adatoms on a graphene membrane. Here we show the first direct imaging of the hydrogen atom in a crystalline solid YH(2), based on a classic 'hollow-cone' illumination theory combined with state-of-the-art scanning transmission electronmicroscopy. The optimized hollow-cone condition derived from the aberration-corrected microscope parameters confirms that the information transfer can be extended to 22.5 nm(-1), which corresponds to a spatial resolution of about 44.4 pm. These experimental conditions can be readily realized with the annular bright-field imaging in scanning transmission electron microscopy according to reciprocity, revealing successfully the hydrogen-atom columns as dark dots, as anticipated from phase contrast of a weak-phase object.