Function Of Hydrogen Coverage Research Articles

Effects of mechanical stress, chemical potential, and coverage on hydrogen solubility during hydrogen-enhanced decohesion of ferritic steel grain boundaries: A first-principles study

Hydrogen-enhanced decohesion (HEDE) is one of the many mechanisms of hydrogen embrittlement, a phenomenon that severely impacts structural materials such as iron and iron alloys. Grain boundaries (GBs) play a critical role in this mechanism, where they can provide trapping sites or act as hydrogen diffusion pathways. The interaction of H with GBs and other crystallographic defects, and thus the solubility and distribution of H in the microstructure, depends on the concentration, chemical potential, and local stress. Therefore, for a quantitative assessment of HEDE, a generalized solution energy in conjunction with the cohesive strength as a function of hydrogen coverage is needed. In this paper, we carry out density functional theory calculations to investigate the influence of H on the decohesion of the Σ5(310)[001] and Σ3(112)[11¯0] symmetrical tilt GBs in bcc Fe, as examples for open and close-packed GB structures. A method to identify the segregation sites at the GB plane is proposed. The results indicate that at higher local concentrations, H leads to a significant reduction of the cohesive strength of the GB planes, significantly more pronounced at the Σ5 than at the Σ3 GB. Interestingly, at finite stress, the Σ3 GB becomes more favorable for H solution, as opposed to the case of zero stress, where the Σ5 GB is more attractive. This suggests that, under certain conditions, stresses in the microstructure can lead to a redistribution of H to the stronger grain boundary, which opens a path to designing H-resistant microstructures. To round up our study, we investigate the effects of typical alloying elements in ferritic steel, C, V, Cr, and Mn, on the solubility of H and the strength of the GBs. Published by the American Physical Society 2024

Impact of Hydrogen Coverage Trend on Methyl Formate Adsorption on MoS2 Surface: A First Principles Study.

Adsorbates coverage plays a crucial role in a catalysis reaction. In hydrodeoxygenation (HDO), which involves high hydrogen pressure, hydrogen coverage on the surface may affect the adsorption of other adsorbates. The HDO is used in green diesel technology to produce clean and renewable energy from organic compounds. This motivates us to study the hydrogen coverage effect on methyl formate adsorption on MoS2 as a model case of the actual HDO. We calculate the methyl formate adsorption energy as a function of hydrogen coverage using density functional theory (DFT) and then comprehensively analyze the physical origin of the results. We find that methyl formate can have several adsorption modes on the surface. The increased hydrogen coverage can stabilize or destabilize these adsorption modes. However, finally, it leads to convergence at high hydrogen coverage. We extrapolated the trend further and concluded that some adsorption modes might not exist at high hydrogen coverage, while others remain.

A computational study of hydrogen detection by borophene

We present a quantum-classical molecular dynamics study of hydrogen irradiation of a single corrugated boron sheet in the incident energy range of 0.25–100 eV and report the resulting reflection, adsorption, and transmission probability, as well as quantum conductance in function of hydrogen coverage.

Uncovering the dominant scatterer in graphene sheets onSiO2

We have measured the impact of atomic hydrogen adsorption on the electronic transport properties of graphene sheets as a function of hydrogen coverage and initial, pre-hydrogenation field-effect mobility. Our results are compatible with hydrogen adsorbates inducing intervalley mixing by exerting a short-range scattering potential. The saturation coverages for different devices are found to be proportional to their initial mobility, indicating that the number of native scatterers is proportional to the saturation coverage of hydrogen. By extrapolating this proportionality, we show that the field-effect mobility can reach $1.5 \times 10^4$ cm$^2$/V sec in the absence of the hydrogen-adsorbing sites. This affinity to hydrogen is the signature of the most dominant type of native scatterers in graphene-based field-effect transistors on SiO$_2$.

Surface stress of partially H-covered Si(001) surfaces

Surface stress and chemisorption energy have been calculated for partially hydrogen-covered Si(001) surfaces by means of density-functional total-energy calculations. A stronger variation of the stress component ${\ensuremath{\sigma}}_{\ensuremath{\perp}}$ perpendicular to the Si-dimer bond with hydrogen coverage and adsorption geometry is observed as for the stress component ${\ensuremath{\sigma}}_{\ensuremath{\Vert}}$ parallel to the dimer bond. An approximation is suggested to generalize the ab initio results computed for periodically repeated $(2\ifmmode\times\else\texttimes\fi{}2)$ surface unit cells to arbitrary H-adsorption patterns on macroscopic surfaces. Surface stress as function of hydrogen coverage is presented for surfaces randomly covered with hydrogen atoms and for surfaces in thermodynamic equilibrium.

Quenching of optical second harmonic generation at the Si(001) surface by hydrogen adsorption

A detailed study of the surface second harmonic generation (SHG) efficiency at $\mathrm{Si}(001)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}1)$ is presented. We have measured the SHG efficiency as a function of hydrogen coverage, relative to that of the clean surface, at a fundamental wavelength of 1064 nm, for substrate temperatures between 200 and 600 K, and for different polarization directions of both the fundamental and second harmonic light beams. The dependence of hydrogen-induced SHG quenching on H coverage is explained with a statistical mechanical model that has previously been invoked to describe the kinetics and dynamics of ${\mathrm{H}}_{2}$ adsorption/desorption. The model predicts an increase in the SHG intensity upon rearranging randomly adsorbed H atoms by thermal annealing, which was confirmed experimentally. An empirical parametrization of the SHG efficiency curves is presented which may be used as a practical calibration to determine hydrogen coverage on Si(001) as a function of SHG signal and temperature.

Cluster expansion method for adsorption: Application to hydrogen chemisorption on graphene

A systematic method is presented for determining the ground state of absorbed species on a substrate based on a cluster expansion of the configurational energy. It is shown that the method can determine the ground state of a strongly relaxing system using a few first-principles total energy calculations of small cells only. The method is applied to a particularly challenging case, the two-sided hydrogen chemisorption on a free standing graphene sheet, where, as a function of hydrogen coverage, the carbon hybridization goes from ${\mathrm{sp}}^{2}$ to ${\mathrm{sp}}^{3}.$ The method should require still fewer calculations and yield still more accurate results in the case of physisorption where longer-ranged strain effects are less important.

Electron affinity and band bending of single crystal diamond (111) surface

Abstract The electron affinity and band bending of a single crystal diamond (111) surface have been investigated as a function of hydrogen coverage via work function measurements, photoelectron yield spectroscopy, X-ray-excited photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). The hydrogen desorption and the 1×1–2×1 surface phase transition were achieved by electron-beam irradiation at room temperature as well as annealing at high temperature. The electron affinity ranges from −1.27 eV for the fully hydrogen-covered 1×1 surface to 0.38 eV for the hydrogen-free 2×1 reconstructed surface. Electron beam irradiation is demonstrated to be an effective method to induce hydrogen desorption and surface reconstruction. However, this irradiation process does not change the surface band bending on type IIb diamond, which is generally increased by annealing. It is concluded that the electron-affinity change is caused by the C–H dipole layer, whereas the band bending is not directly related to hydrogen coverage or reconstruction but rather due to charged surface defects that are created simultaneously with the thermal treatment.

Microscopic Theory of Second Harmonic Generation at Si(100) Surfaces

We apply a microscopic formulation to calculate the second harmonic spectra of clean and hydrogenated Si(100) surfaces. We find a trend of the surface-allowed ${E}_{1}$ resonance as a function of the hydrogen coverage, which is consistent with experiment. Another resonance at lower energy, recently observed at the clean surface, is also reproduced and explained in terms of transitions across surface states. Its quenching is clearly demonstrated as a function of hydrogen coverage. Our results for the energetically more favorable $c(4\ifmmode\times\else\texttimes\fi{}2)$ structure are in better agreement with low temperature experimental data than those calculated for the $2\ifmmode\times\else\texttimes\fi{}1$ reconstruction. We find that the spectra are dominated by the ${\ensuremath{\chi}}_{\ensuremath{\parallel}\ensuremath{\parallel}\ensuremath{\perp}}^{s}$ component of the second-order nonlinear surface susceptibility.

Electron Affinity of the Bare and Hydrogen Covered Single Crystal Diamond (111) Surface

The electron affinity $\ensuremath{\chi}$ and the band diagram of single crystal diamond (111) surfaces was determined as a function of hydrogen coverage by combining work function measurements with photoelectron yield and core level photoemission spectroscopy. $\ensuremath{\chi}$ ranges from $\ensuremath{-}1.27\mathrm{eV}$ for the fully hydrogen covered $(1\ifmmode\times\else\texttimes\fi{}1)$ surface to $+0.38\mathrm{eV}$ for the hydrogen free $(2\ifmmode\times\else\texttimes\fi{}1)$ reconstructed surface. This change is quantitatively explained by a surface dipole model provided a coverage dependent depolarization is included. The dipole moment of the C-H bond on diamond (111) is found to be $0.09e\AA{}$.

Hydrogen adsorption states on silica-supported Ru–Ag and Ru–Cu bimetallic catalysts investigated via microcalorimetry

The nature of hydrogen adsorption states on silica supported Ru, Ru–Ag and Ru–Cu was investigated using microcalorimetry and selective hydrogen chemisorption. The differential heats of hydrogen adsorption as a function of hydrogen coverage were determined for Ru and bimetallics with varying Ag and Cu contents. The initial heats of hydrogen adsorption (differential heat at coverages approaching zero) were found to be similar on all three catalyst systems suggesting the absence of effects that directly alter the chemisorptive bond (e.g. electron transfer between Ru and the added metal). The addition Ag to Ru significantly reduced the total amount of hydrogen adsorbed per surface Ru atom. That is, the stoichiometry of adsorption was altered by the addition of Ag even though Ag does not adsorb hydrogen. Similarly, the addition of Cu reduced the amount of hydrogen adsorbed per surface metal atom. In both cases, the reduced amount of hydrogen adsorbed per surface atom (adsorptive stoichiometry) was associated with the loss of hydrogen adsorbed from weak and intermediate energy adsorption states (<80 kJ/mol). The population of sites with adsorption energies ≥80 kJ/mol were unaffected. From atomistic simulations it is known that, at low concentrations, Ag and Cu preferentially occupy the low coordination metal sites. The loss of the intermediate and low adsorption energy sites is correlated with replacement of Ru atoms at edge, corner and other defectlike sites by Ag and Cu. Possible reasons for this structure sensitivity are discussed.

Structure of the hydrogen covered Cu(110) surface studied with thermal energy helium scattering

The adsorption of atomic hydrogen on the Cu(110) surface has been studied with thermal energy helium scattering. Upon hydrogen adsorption the structure exhibits a variety of structures: At temperatures above 140 K the helium diffraction spectra show a clear (1 × 2) structure even at very low H coverages. This phase is the well known (1 × 2) Cu(110)-H “missing row” reconstructed phase. Based on an analysis of the specularly reflected He-intensity as a function of hydrogen coverage we have characterized the nucleation and growth of this phase. After the formation of small reconstructed areas the development of the reconstructed phase proceeds via the growth of these nuclei into large (1 × 2) Cu(110)-H islands indicating an attractive interaction between adsorbed H atoms within the reconstructed unit cells. Adsorption at temperatures below 80 K yields a sequence of adsorbate phases as a function of hydrogen exposure: at low exposures a (1× 4) phase is observed. With increasing hydrogen exposure the (1 × 4) structure changes continuously into a well ordered (1 × 3) phase, which then evolves into a (1 × 2) phase at higher exposures. This (1 × 2) structure is only poorly ordered and transforms upon further hydrogen exposure into a (1 × 1) structure. This sequence of adsorbate phases, as well as the continuous transition between them, suggests a repulsive interaction between the adsorbed H atoms. All adsorbate phases formed at surface temperatures T s below 120 K undergo an irreversible transition into a disordered phase upon heating to 120 K < T s < 140 K. Heating above T s = 140 K irreversibly yields the (1 × 2) “missing row” phase.

Adsorption kinetics for ethylsilane, diethylsilane, and diethylgermane on Si(111) 7×7

The adsorption kinetics for ethylsilane (ES), diethylsilane (DES), and diethylgermane (DEG) on Si(111) 7×7 were studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. The initial reactive sticking coefficients were determined as a function of surface temperature using LITD measurements. In these experiments, the ethyl coverage vs adsorption time was monitored using CH2=CH2 (ethylene) LITD signals that were produced by the β-hydride elimination of the surface ethyl groups, e.g. Si–CH2CH3(ad)→Si–H(ad)+CH2=CH2(g). The initial reactive sticking coefficients were S0≊2×10−3, 4×10−3, and 5×10−2 for DES, ES, and DEG, respectively, at 200 K. As expected from a precursor-mediated adsorption model, the initial reactive sticking coefficients were observed to decrease with increasing surface temperature. Experiments with preadsorbed hydrogen also demonstrated that the initial reactive sticking coefficients of DES and DEG were reduced as a function of hydrogen coverage. This behavior indicated that alkylsilane and alkylgermane adsorption on Si(111) 7×7 requires free dangling bond sites. LITD experiments revealed that the ethyl surface coverage saturated after large exposures. The saturation coverage corresponded to a deposited Si or Ge coverage of Θ=0.13 monolayer for DES and DEG and was independent of surface temperature between 200–400 K. DEG adsorption cycles were used to deposit increasing amounts of Ge on Si(111) 7×7. The deposited coverage was examined using H2 TPD studies which indicated that the germanium may be forming islands. LITD experiments were also used to monitor ethyl surface diffusion on Si(111) 7×7 after DES adsorption. No evidence of significant ethyl surface mobility (D≤1.0×10−10 cm2/s) was found for surface temperatures as high as 600 K.

Surface reactions in Si chemical vapor deposition from silane

The kinetics and mechanisms of silane adsorption and decomposition on clean single crystal Si surfaces have been investigated under ultrahigh vacuum conditions. Individual reaction steps are studied experimentally in the surface temperature range −163–500 °C, and modeling results are extrapolated to Si film growth temperatures (400–1000 °C) for comparison with the literature. Static secondary ion mass spectrometry (SSIMS) has been used to identify the silicon hydride species formed by silane adsorption on both the Si(100)-(2×1) and Si(111)-(7×7) surfaces. Temperature programmed desorption of H2 has been used to measure the SiH4 reactive sticking probability (S) as a function of hydrogen coverage (ΘH). Kinetics of SiH3, SiH2, and SiH decomposition are compared here, and direct evidence for SiH decomposition as the rate-determining surface step leading to Si film growth is presented. The experimental data provide a firm basis for kinetic modeling of the growth of Si films from SiH4 by chemical vapor deposition (CVD). Preliminary modeling results are outlined here, including predictions of Si growth rate versus surface temperature (compared with data from the literature).

High-resolution photoemission study of the interaction of hydrogen with GaAs(110) surfaces.

We present a synchrotron-radiation photoemission investigation of hydrogen chemisorption on GaAs(110) surfaces cleaved in situ. Analysis of the valence-band emission and high-resolution core photoemission studies as a function of hydrogen coverage and photoelectron escape depth show that at least in the high-coverage regime hydrogen chemisorption occurs via the formation of surface bonds between atomic hydrogen and both As and Ga atoms. Decomposition of the As 3d and Ga 3d core lines in terms of bulk and surface components demonstrates that hydrogen adsorption is accompanied by preferential etching of As, roughening of the surface, and a consequent relatively large variation of surface stoichiometry toward a Ga-rich composition.

Reactive sticking coefficient of silane on the Si(111)-(7×7) Surface

The reactive sticking coefficient, S R, of SiH 4 on the Si (111)-(7×7) surface has been studied as a function of hydrogen coverage (Θ H) in the temperature range from 100 to 500 °C. Evidence is seen for two adsorption regimes which are proposed to correspond to minority and majority surface sites. On the minority sites (Θ H=0 to 0.08), S R is approximately 10 −5 and essentially no dependence of S R on surface temperature, T S, is found. Reactive sticking becomes a complicated function of T S and Θ H, with S R decreasing, on the majority sites (Θ H⪢008).

Laser-induced thermal desorption of Silicon-containing surface reaction intermediates from Si(111)7 × 7

Laser-induced thermal desorption (LITD) was used to remove surface reaction intermediates from Si(111)7 × 7. Silicon-containing species were detected in the LITD yield from Si(111)7 × 7 surfaces covered with hydrogen, water or methanol. These silicon-containing LITD products include SiH n , Si(OH) n and Si(OCH 3) n where n = 1 − 3. None of these species were observed in the conventional temperature programmed desorption (TPD) spectra. TPD and temperature-pro-grammed LITD studies were also performed and correlated for Si(111)7 × 7 surfaces as a function of hydrogen coverage. The SiH 2 LITD products were related directly to the low-temperature β 2-peak in the H 2 TPD spectra. Previous infrared studies have demonstrated that this β 2-peak corresponds to H 2 desorption from silicon dihydride species on the silicon surface. The ability of LITD to desorb silicon-containing surface reaction intermediates should be extremely useful in investigations of silicon surface reaction kinetics.

Kinetics and thermodynamics of hydrogen interaction with thin cobalt films

The kinetics and thermodynamics of hydrogen adsorption on thin cobalt films at 195, 273 and 298 K have been studied by means of sticking probability measurements, thermal desorption mass spectrometry (TDMS) and examination of adsorption isotherms. The dissociative adsorption occurs with a high initial sticking coefficient ( S 0 close to unity) at all temperatures. The behaviour of the sticking probability S as a function of hydrogen coverage θ has been analyzed. Two TDMS peaks were detected when hydrogen adsorption was carried out at 195 K. The isosteric heat of adsorption remains constant at 286 K up to θ ≈ 0.5 and then decreases indicating the arising of a new adsorbed state of hydrogen at θ > 0.5. A detailed analysis of the adsorption entropy for T ≈ 286 K has been performed.

Laser-induced thermal desorption of silicon-containing surface reaction intermediates from Si(111)7×7

Laser-induced thermal desorption (LITD) was used to remove surface reaction intermediates from Si(111)7×7. Silicon-containing species were detected in the LITD yield from Si(111)7×7 surfaces covered with hydrogen, water or methanol. These silicon-containing LITD products include SiH n , Si(OH) n and Si(OCH 3) n where n = 1–3. None of these species were observed in the conventional temperature programmed desorption (TPD) spectra. TPD and temperature-programmed LITD studies were also performed and correlated for Si(111)7×7 surfaces as a function of hydrogen coverage. The SiH 2 LITD products were related directly to the low-temperature β 2-peak in the H 2 TPD spectra. Previous infrared studies have demonstrated that this β 2-peak corresponds to H 2 desorption from silicon dihydride species on the silicon surface. The ability of LITD to desorb silicon-containing surface reaction intermediates should be extremely useful in investigations of silicon surface reaction kinetics.

MINDO/SR calculations of nickel surface properties as a function of hydrogen coverage

The MINDO/SR method is used to study surface properties such as the work function, heat of adsorption and magnetic moment as a function of hydrogen coverage. A good correlation between theory and experiment is found. Furthermore, a qualitative analysis of surface geometry changes due to hydrogen chemisorption is presented.