Hydrogen-induced Reconstruction Research Articles

Hydrogen-Induced Reconstruction of Cu(100): Two-Dimensional and One-Dimensional Structures of Surface Hydride

Adsorption of atomic hydrogen has a remarkable influence on the structure and morphology of the Cu(100) surface. We have examined two specific situations; the well-known reconstructed p(2 × 2)-H surface and a one-dimensional (6 × 1)-H surface resembling the stripe reconstruction observed in an electrochemical environment. High resolution electron energy loss spectroscopy and density functional calculations show that the H atoms occupy only surface sites and that H bonding in induced 3-fold Cu surface hollow sites is a structural key element. The calculations support a transient scenario in which H absorption in subsurface sites is a critical intermediate step in the reconstruction of the Cu surface. We find clear evidence in terms of characteristic properties, like the H–Cu bond lengths and the dipole excited vibrational mode spectra, that these structures consist of two-dimensional and one-dimensional structures of surface hydride.

Magnetic States in Fe, Co, Ni hydrides under High Pressure Probed by X-ray Magnetic circular dichroism

Magnetic states of Fe, Co, and Ni hydrides under pressure have been investigated by X-ray magnetic circular dichroism (XMCD) at the K-edge. At room temperature, Fe, Co, and Ni metals are hydrogenated to ferromagnetic FeH, ferromagnetic CoH, and paramagnetic NiH, respectively. A sharp XMCD profile is observed after hydrogenation, which is attributed to the hydrogen-induced reconstruction of the electronic structure. The pressure dependence of XMCD demonstrates that the ferromagnetic state of FeH is less stable than that of CoH under pressure.

In situ video STM studies of the hydrogen-induced reconstruction of Cu(100): potential and pH dependence

The surface structure of Cu(100) electrodes in perchloric acid solutions of pH 1 to 3 was studied in the potential range of hydrogen evolution by video-rate scanning tunneling microscopy, focusing on the recently reported hydrogen-induced surface reconstruction [H. Matsushima et al., J. Am. Chem. Soc. 2009, 131, 10362]. Potential-dependent measurements reveal a two step formation process: at potentials close to the onset of hydrogen evolution a p(1×8) phase emerges, where Cu surface atoms in stripe-like structures are laterally and vertically displaced; at ≈30 mV more negative potentials a transition to a c(p×8) structure with an expanded Cu surface lattice occurs. Correlation of these observations with electrochemical data and studies on hydrogen interactions with Cu(100) surfaces under vacuum conditions support that these phases are induced by hydrogen in subsurface sites, pointing towards a high hydrogen coverage on this electrode surface under reaction conditions.

Simulation of Hydrogen-Induced Reconstruction of Oxygen on Pd(111)

The oxygen and hydrogen atoms co-adsorption experiment on Pd(111) is modelled using kinetic Monte Carlo method. We consider O–O, H–H, and O–H interactions and several kinetic processes (diffusion, surface and subsurface H-atoms exchange and water-formation reaction) leading to the phase transition (2×2)O → ( √ 3×3)O with increase in H-coverage. We also demonstrate how reverse phase transition ( √ 3×3)O → (2×2)O occurs due to hydrogen dissolution into subsurface in the absence of hydrogen gas. At higher temperature we observe the disappearance of (2× 2)O oxygen islands due to water-formation reaction.

The initial stages of the hydrogen-induced reconstruction of Pd(1 1 0) studied with STM

The hydrogen-induced reconstruction of the Pd(1 1 0) surface was investigated in situ with scanning tunneling microscopy (STM). Focusing on the initial stages of the restructuring, which ultimately leads to a stable (1 × 2) reconstructed surface, we find an exponential increase of the reconstructed surface area with hydrogen exposure, up to 8 Langmuir, which can be explained by an autocatalytic behavior. Moreover, the steps, especially those running along the [0 0 1] direction, play a distinctive role in the buildup of the (1 × 2) reconstruction.

Hydrogen-induced reconstruction of the GaP(001) surface studied by scanning tunneling microscopy

We report results of a scanning tunneling microscopy investigation on the reconstruction of GaP(001) surfaces. We have observed a 4×2 structure, accompanied locally with c(8×2) domains, at a surface prepared by using ion sputtering and annealing method. On the contrary, both 2×4 [with local c(2×8)] and 4×2 [with local c(8×2)] structures are obtained by hydrogenation followed by annealing. The former consists of a unit structure of three P dimers plus one dimer vacancy, whereas the latter is a parallel arrangement of zig–zag-chained structures along the [110] direction. It has been shown that the surface hydrogenation before annealing induces new types of reconstruction.

Anomalous surface core-level shifts in the hydrogen-induced reconstruction of W {100}

Shifts are reported in the W 4 f 7 2 core-level binding energy of surface atoms involved in the hydrogen-induced restructuring of W {100}. A detailed analysis of the data reveals anomalously large shifts peaking at hydrogen fractional surface coverages θ H ≈ 0.33 and θ H ≈ 0.53, amounting to a shift to higher binding energy in the clean-surface core level of as much as 430 ± 50 meV. By comparison with data from W {100} at saturation ( θ H = 2), where the shift is only ∼ 100 meV to higher energy compared to the clean surface core level, and data for H adsorption on other crystal planes, the large shift is mostly attributed to the pinching of pairs of surface W atoms induced by H adsorption at low coverages. The shift of ∼ 370 ± 50 meV associated with this reconstruction alone, to the exclusion of the influence of the adsorbate, is considerably larger than expected by comparison with extensive data for both the clean surface reconstruction on W {100} itself and on a range of other single-crystal planes of W. It is concluded that the anomalous shift is consistent with a hydrogen-induced inward displacement of the surface potential barrier, as deduced by Herlt and Bauer from electron reflection coefficient measurements. The coverage dependence of the core-level shifts (in particular their two maxima at θ H ≈ 0.33 and 0.53) provides evidences for the nature of both commensurate and incommensurate phases formed at the surface.

First-principles study of the H-induced reconstruction of W(110).

We studied the hydrogen-induced reconstruction of the W(110) surface using the pseudopotential plane-wave approach. The calculations for a full monolayer of hydrogen coverage showed that the quasithreefold hollow site (distorted bridge) has the lowest energy, and that for this geometry a surface reconstruction, consisting of a small uniform shift of the W top layer in the [11\ifmmode\bar\else\textasciimacron\fi{}0] direction, is energetically favorable. We also studied the surface states for clean and H-covered W(110) and investigated the effect of the reconstruction on electronic structure. \textcopyright{} 1996 The American Physical Society.

Hydrogen-induced reconstruction of W(001): Predictions of the four-position model.

Predictions of the four-position model of the surface reconstruction of W(001) induced by ${\mathrm{H}}_{2}$ chemisorption are obtained by Monte Carlo simulation. Results are compared with those of the simplified two-position model, finding qualitative differences, and with experiments.

Numerical studies of the two-dimensional XY model with symmetry-breaking fields.

We present results of numerical studies of the two-dimensional XY model with four- and eightfold symmetry-breaking fields. This model has recently been shown to describe hydrogen-induced reconstruction on the W(100) surface. Based on mean-field and renormalization-group arguments, we first show how the interplay between the anisotropy fields can give rise to different phase transitions in the model. When the fields are compatible with each other there is a continuous phase transition when the fourth-order field is varied from negative to positive values. This transition becomes discontinuous at low temperatures. These two regimes are separated by a multicritical point. In the case of competing four- and eightfold fields, the first-order transition at low temperatures opens up into two Ising transitions. We then use numerical methods to accurately locate the position of the multicritical point, and to verify the nature of the transitions. The different techniques used include Monte Carlo histogram methods combined with finite-size scaling analysis, the real-space Monte Carlo renormalization-group method, and the Monte Carlo transfer-matrix method. Our numerical results are in good agreement with the theoretical arguments.

Effects of the hydrogen-induced (1×2) surface reconstruction on the kinetics of ethylene hydrogenation on Ni(110)

Effects of the hydrogen-induced (1×2) surface reconstruction on the kinetics of ethylene hydrogenation on Ni(110)

Hydrogen-induced reconstruction of a W(110) surface studied by angle-resolved photoemission spectroscopy

Abstract We have observed changes in the electronic structure of a W(110) surface caused by the hydrogen-induced reconstruction using angle-resolved photoemission spectroscopy. For the unreconstructed H/W(110) surface, a number of hydrogen-induced states are observed, and some of them are found to be in the projected bulk band gap. As W(110) undergoes the surface reconstruction induced by further hydrogen adsorption, one of those states is rapidly quenched, while others simply shift downward in energy by a fraction of an electron volt. To understand the surface reconstruction mechanism for this system, the surface electronic states before and after the surface reconstruction are calculated using the “simple” tight-binding method. The driving mechanism for the hydrogen-induced surface reconstruction is briefly discussed.

Hydrogen-induced reconstruction of a W(110) surface studied by angle-resolved photoemission spectroscopy

We have observed changes in the electronic structure of a W(110) surface caused by the hydrogen-induced reconstruction using angle-resolved photoemission spectroscopy. For the unreconstructed H/W(110) surface, a number of hydrogen-induced states are observed, and some of them are found to be in the projected bulk band gap. As W(110) undergoes the surface reconstruction induced by further hydrogen adsorption, one of those states is rapidly quenched, while others simply shift downward in energy by a fraction of an electron volt. To understand the surface reconstruction mechanism for this system, the surface electronic states before and after the surface reconstruction are calculated using the “simple” tight-binding method. The driving mechanism for the hydrogen-induced surface reconstruction is briefly discussed.

Hydrogen-induced reconstruction of transition metal surfaces

This study compares the results of a number of recent papers on hydrogen adsorption on Rh(110), Rh(311) and Fe(211) as well as on Ni(111) and Fe(110) surfaces. It particularly deals with the structural aspect of these low energy electron diffraction (LEED) investigations and correlates them, if available, with respective thermodesorption data. Upon dissociative adsorption by a non activated process hydrogen induces local displacements of the atoms about the adsorption sites. With increasing coverage these displacements order to form a sequence of weakly reconstructed phases and gradually lift the surface layer relaxation of the formerly clean surface. Along close packed rows of metal surface atoms hydrogen atoms tend to occupy threefold coordinated adsorption sites which, in turn, arrange in single or double chains. The coverage dependent periodicity of these adlayer structure elements together with the respective shift buckling of the substrate surface generates the observed superstructures. Since not only open but also close packed surfaces show this weak (and sometimes strong) reconstruction upon hydrogen adsorption it should be generally considered in all adsorption systems.

X-ray photoelectron diffraction study of the reconstructed Ni(110)-1 × 2-H surface

Hydrogen-induced reconstruction of Ni(110) was studied by X-ray photoelectron diffraction (XPD) in the polar angle (θ) scanning mode. The angle-resolved emission of Ni 2p and Ni 3p photoelectrons along the [001] and [11̄0] azimuths for the 1 × 1 and the reconstructed 1 × 2-H surface of Ni(110) was measured at 120 K. The XPD patterns showed intensity differences, Δ I rec(θ) = I(θ, 1 × 2)- I(θ, 1 × 1), which are characteristic of the reconstruction process. These differences, amounting to about 8% of the minimum intensity in a single distribution, exhibit positive and negative excursions for the [001] azimuth. The maxima in Δ I rec are frequently observed at the same angles where minima exist in I(θ), and minima in Δ I rec correlate with maxima in I(θ). These changes are attributed to altered forward scattering and diffraction directions, elastic diffuse and inelastic scattering (attenuation), all due to the shifting of near-surface atoms. The differences Δ I rec for the [11̄0] azimuth are negative over the whole range of polar angles indicative of elastic diffuse scattering and inelastic damping of the bulk forward scattering peaks. Single scattering cluster calculations for the 1 × 1- and 1 × 2-H-Ni(110) surface are compared with the experimental results; the intensity differences are approximately consistent with a row-pairing reconstruction.

Hydrogen-induced reconstruction of Si(111)-√3-Ag surface studied by TOF-ICISS

The structural change of Si(111)-√3 × √3-Ag surfaces caused by atomic hydrogen adsorption has been studied using low-energy ion scattering spectroscopy. Computer simulations of ion scattering spectra have been carried out for various kinds of the morphology of the Ag deposit. Comparison of the simulation results with the observed change of the spectra reveals that the Ag monolayer with the √3 × √3-Ag structure is transformed into small Ag(111) clusters epitaxially grown on Si(111) by hydrogen adsorption at room temperature.

Experimental Fermi surface for W(011)

The Fermi surfaces for the surface-localized states on W(011) are experimentally determined using high-resolution angle-resolved photoemission. The significance of our results in terms of phonon anomalies, hydrogen-induced reconstruction, and work function changes is examined.

The face dependence of the effective electron mean free path derived from spherical-wave corrections in photoelectron diffraction of W(110) and W(100) surfaces

The authors show that spherical-wave corrections can be of importance in a single-scattering calculation of the angular anisotropy of the photoemission from W(110) and W(100) 4f core levels. More precisely, for photon energies of about 65 eV, these corrections are sufficiently large to show the necessity, when using an isotropic electron mean free path lambda ee, to vary it according to the face under study. In our case, this leads to an effective electron mean free path lambda ee of 8 AA for W(110) and 5 AA for W(100). When this variation is allowed, the plane-wave approximation leads to results for W(110) which are in as good agreement with experiments as those for W(100). In particular, the influence of the hydrogen-induced reconstruction on the azimuthal patterns of W(110) is now very well reproduced.

Electronic contributions to the hydrogen-induced reconstruction of W(110).

A band of localized states of the W(110) surface has been observed along $\overline{\ensuremath{\Sigma}}$ which undergoes a considerable shift to higher binding energy with hydrogen adsorption. We interpret this state and the associated shift in energy to be related to the hydrogen-induced reconstruction of this surface. The change in the nature of the electronic states intrinsic to this surface which result from hydrogen adsorption is fundamental to an understanding of the mechanism which drives this reconstruction.

Hydrogen-induced reconstruction on W(100) and Mo(100) by surface infrared spectroscopy

Infrared absorption and low-energy electron-diffraction measurements of H adsorbed on W(100) and Mo(100) show that on each surface, distinct wavenumbers characterize the H-substrate stretching modes associated with the different long-range structures of the complicated T-θ phase diagram. Hydrogen is bonded at a two-fold bridge site at all temperatures and coverages investigated and the wavenumber of the symmetric stretch mode, v 1, is determined by the local geometry, i.e. the substrate dimer length. Analysis of the coverage dependence of the v 1 wavenumber shows that, at low coverages (θ ≲0.3), the effective H-H interactions are very different for the two substrates, leading to a uniform H layer on W(100) and to island formation on Mo(100). In general, the phase transitions are continuous on W(100), with regions of intermediate structures, and first order on Mo(100), with regions of coexisting phases.