Formation Of H2 Molecules Research Articles

A Density Functional Theory Study of the Water−Gas Shift Reaction Promoted by Neutral, Anionic, and Cationic Gold Dimers

While nanoscale gold particles show exceptional catalytic activity toward the water−gas shift (WGS) reaction, not much is known about the detailed reaction mechanism and the influence of charge state of Au nanoparticles on the reactivity. We here report a systematic theoretical study by carrying out density functional theory calculations for the WGS reaction promoted by cationic, neutral, and anionic Au dimers, which represent three simplest prototypes of Au nanoparticles with different charge states. The reaction mechanism is explored along two possible entrances: one involves the complexes of the dimers with CO and the other is related to the complexes of the dimers with H2O. In all cases, it is found that the catalytic cycle proceeds via the formate mechanism and involves two sequential elementary steps: the rupture of the O−H bond in H2O and the formation of H2 molecule. The calculated results show that the reaction mediated by Au2+ is energetically most favorable compared to those promoted by Au2 and Au2−, indicating that the charge state of Au dimers plays an essential role for the catalyzed WGS. The present theoretical study rationalizes the early experimental findings well and enriches our understanding of the catalytic WGS by Au-based catalysts.

Dynamics of Hydrogen Spillover on Carbon-Based Materials

We investigate the dynamics of physisorbed atomic hydrogen on several carbon based materials (various fullerenes and a graphene sheet) using first principles molecular dynamics simulations. The physisorbed H atoms, generated upon H2 dissociative chemisorption on metal catalysts and interaction with carbonized “bridge” materials and substrates (Chen, L.; et al. J. Phys. Chem. C 2007, 111, 18995), can diffuse freely on carbon surfaces with high mobility. Our results indicate that physisorption of H atoms is a metastable state and the atoms will readily recombine to form H2 molecules, which can be recycled to generate more H atoms, or attack the substrates to form C−H bonds. The strength of the resulting C−H bonds exhibits a strong dependency on the carbon material surface curvature. The implication of C−H bond strength on the dehydrogenation of hydrogenated carbon materials to form molecular H2 is discussed.

Measurements of Energy Partitioning in H 2 Formation by Photolysis of Amorphous Water Ice

We demonstrate experimentally that photodissociation of amorphous solid water at 100 K results in formation of H2 molecules with an ortho/para ratio of gOPR = 3. Two distinct mechanisms can be identified: endothermic abstraction of a hydrogen atom from H2O by a photolytically produced H atom yields vibrationally cold H2 products, whereas exothermic recombination of two H-atom photoproducts yields translationally and internally hot H2. These results are in accord with predictions by molecular dynamics calculations and their astrophysical implications are discussed.

First‐principles analysis of He and H atom incidence onto hydrogen‐terminated Si(001) 2 × 1 surface

AbstractIn order to analyze the fundamental processes in atmospheric pressure plasma chemical vapor deposition, which has an ability to form silicon epitaxial films under low‐substrate‐temperature conditions, first‐principles microcanonical molecular‐dynamics simulations of collision process between plasma gas atoms (H and He) and Si(001) 2 × 1 hydrogen‐terminated surface are performed. In contrast to the He collision which results in simple bound, the formation of H2 molecule with exothermally generated kinetic energy of 1.1 eV, and the absorption of H atom, are observed in the case of H collision. The generated H2 molecule occasionally hits the adjacent H‐termination atom transferring a considerable amount of the kinetic energy of about 0.25 eV to the surface. It is suggested that this transferred energy might enhance the migration of reaction precursors, which is important for the improvement of the crystal quality of epitaxial films. In the comparison of induced energies to the surface, the collision process of H atom is more important than that of He. Copyright © 2008 John Wiley & Sons, Ltd.

Hydrogen in semiconductors: From basic physics to technology

AbstractHydrogen is incorporated unintentionally in most semiconductors during crystal or epitaxial growth and during many processing steps. The knowledge of how hydrogen interacts with itself or other defects is of fundamental scientific interest but also of crucial importance for the understanding of processes in device technology and the performance of devices. A few examples will be presented from our work on the electrical and optical properties of hydrogen in semiconductors and connections will be made to device related issues. Subjects addressed are the passivation of shallow donors and acceptors, the interaction of hydrogen with transition metals and the formation of H2 molecules. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Feedback from First Radiation Sources: H - Photodissociation

During the epoch of reionization, the formation of radiation sources is accompanied by the growth of a H- photodissociating flux. We estimate the impact of this flux on the formation of molecular hydrogen and cooling in the first galaxies, assuming different types of radiation sources (e.g., Population II and Population III stars, miniquasars). We find that H- photodissociation reduces the formation of H2 molecules by a factor of Fs ~ 1 + 103ksxfδ-1, where x is the mean ionized fraction in the intergalactic medium, fesc is the fraction of ionizing photons that escape from their progenitor halos, δ is the local gas overdensity, and ks is an order unity constant that depends on the type of radiation source. By the time a significant fraction of the universe becomes ionized, H- photodissociation may significantly reduce the H2 abundance and, with it, the primordial star formation rate, delaying the progress of reionization.

Evolution of Hydrogen Related Defects in Plasma Hydrogenated Crystalline Silicon under Thermal and Laser Annealing

Boron doped [100]-oriented Cz Si wafers are hydrogenated with a plasma enhanced chemical vapor deposition setup at a substrate temperature of about 260 °C. In-situ Raman spectroscopy is applied on samples under thermal and laser annealing. It is found that different Si-H species have different stabilities. The most stable one is the Si-H bond at the inner surfaces of the platelets. The dissociated energy of Si-H bonds is deduced based on the first order kinetics. It is found that the hydrogen atoms which are released during annealing are trapped again by the platelets and passivate the silicon dangling bonds at the inner surfaces of the platelets or form H2 molecules in the open platelet volume, possibly relating to the basic mechanism of the hydrogen-induced exfoliation of the silicon wafer and the socalled “smart-cut” process.

Properties of hydrogen induced voids in silicon

After heat treatment, silicon samples implanted with high doses of hydrogenexhibit blistering and defoliation of thin silicon layers. The process is usedcommercially in the fabrication of thin silicon-on-insulator layers (SmartCut®). In the present study we investigate the behaviour of hydrogen after different processingsteps, which lead to thin Si layers bonded to glass substrates. A set of hydrogenimplanted samples is studied by means of low temperature photoluminescence, Ramanspectroscopy, x-ray diffraction and optical microscopy (visible and infrared). Theformation of Si–H bonds is detected after implantation together with a build-up ofinternal strain. After annealing, the relaxation of the implanted layers is found to beconnected with the formation of hydrogen saturated vacancies and the formation ofH2 molecules filling up larger voids. A comparison is made with hydrogen plasma treatedsamples, where well defined platelets on {111} planes are found to trap hydrogen molecules.No direct evidence of the role of {111} and {100} platelets in the blistering process is found inthe implanted layers from our study. We determine considerable compressive stressesin the bonded Si layers on glass substrates. The photoluminescence is stronglyenhanced in these bonded layers but red-shifted due to a strain reduced band gap.

Recombination and Exchange Reactions of Hydrogen and Dihydrogen Molecular Condensation in Single-Walled Carbon Nanotubes

The hydrogen recombination reaction, H + H → H2, and the hydrogen exchange reaction, H + H2→ H2 + H, taking place inside single-walled carbon nanotubes have been studied by using quantum mechanics molecular dynamics simulations. Two types of hydrogen exchange reactions have been found. The fast reaction lasts in less than 5 fs, and the slow one lasts for about 40 fs with a clear intermediate state manifested. The collisional reaction for two H atoms to form a H2 molecule usually takes place in a time interval of 25 fs. We also describe the detailed process of the reaction of individual H atoms to form H2 molecules and, with an increase of the density of H2 molecules confined in the tubes, H2 molecular condensation to form clusters and finally to form condensed H2 molecular lattices.

μ-Raman investigations of plasma hydrogenated silicon

Standard [001]-oriented p- (12–20 Ω cm) and n-type (1.8–2.6 Ω cm) Czochralski (Cz) silicon wafers were treated by a RF (13.56 MHz) hydrogen plasma at a substrate temperature of 250 °C. After the plasma hydrogenation subsequent annealing was applied up to 600 °C in air. The formation of H2 molecules in voids/platelets was investigated by Raman spectroscopy. The Raman intensities of the H2 vibration modes at ~4150 cm−1 exhibited significant intensity modulations in dependence on the annealing temperature. The intensities of the H2 Raman lines indirectly monitor the evolution of the voids/platelets upon annealing. This assumption was verified by cross-sectional transmission electron microscopy (XTEM), which was applied for comparison. The intensity modulations of the H2 Raman signal can be explained by the evolution of and platelets. At lower annealing temperatures (<500 °C) platelets laying in planes are dominant, while at elevated temperatures (≥500 °C) [001]-oriented platelets become more and more important. platelets were also observed using XTEM measurements in p-type material. In case of p-type substrates the Raman intensities were significant higher than for n-type material. The higher H2 Raman intensities in p-type Cz Si can be explained by the amphoteric character of hydrogen in silicon.

Growth mechanism of amorphous hydrogenated carbon

Amorphous hydrogenated carbon (a-C:H) films offer a wide range of applications due to their extraordinary material properties like high hardness, chemical inertness and infrared transparency. The films are usually deposited in low temperature plasmas from a hydrocarbon precursor gas, which is dissociated and ionized in the plasma and radicals and ions impinging onto the surface leading to film growth. Final stoichiometry and material properties depend strongly on composition, fluxes and energy of the film forming species. The underlying growth mechanisms are investigated by means of quantified particle beam experiments employing radical sources for atomic hydrogen (H) and methyl (CH3) radicals as well as an argon ion beam. The interaction of these species with amorphous hydrogenated carbon films is investigated in real time by ellipsometry and infrared spectroscopy. The formation of hydrocarbon films from beams of CH3, H and Ar+ is considered a model system for growth of amorphous hydrogenated carbon films in low temperature plasmas from a hydrocarbon precursor gas. The growing film surface can be separated in a chemistry-dominated growth zone with a thickness of 2 nm on top of an ion-dominated growth zone. In the chemistry-dominated growth zone incident atomic hydrogen governs the surface activation as well as film stoichiometry. In the ion-dominated growth zone, especially hydrogen ions, displace bonded hydrogen in the film. Displaced hydrogen recombine and form H2 molecules, which desorb. This leads to a low hydrogen content of the films.

Nitrogen-Stabilized H2* Defects in GaP:N

AbstractThe presence of N in III-V compounds such as GaAs and GaP drastically changes the behavior of H. We studied atomic structures of N-related hydrogen complexes in GaP:N using first-principles method. We show that the formation of the strong N-H bond is responsible for the stabilization of H2* complex that is otherwise unstable against the formation of H2 molecule. This provides the first theoretical proof that H2* can be stable in III-V semiconductor. The previously proposed H-N-H dihydride model is found to be unstable against spontaneously transforming into H2*, which involves only monohydrides, H-N and H-Ga. The calculated local vibration frequencies and isotope shifts are in good agreement with experiment.

The evolution of hydrogen molecule formation in hydrogen-plasma-treated Czochralski silicon

The formation of H2 molecules in voids/platelets and their evolution upon annealing were studied. Standard p- and n-type Czochralski (Cz) silicon wafers have been treated by a hydrogen plasma at 250°C. After the plasma hydrogenation the samples were annealed at temperatures up to 600°C in air. Investigations were done by Raman spectroscopy. H2 did appear as nearly free molecules in the platelets/voids (Raman shift ∼4150cm−1), but not on the tetrahedral interstitial position in the Si lattice. The normalized Raman intensities of the H2 vibration modes exhibited significant sensitivity to the annealing temperature for both p- and n-type Cz Si. These peculiarities can be explained by the evolution of the platelets during annealing.

Some Considerations Relating to Growth Chemistry of Amorphous SI and (SI,GE) Films and Devices

ABSTRACTRecent experiments suggest that the chemistry responsible for the growth of amorphous and microcrystalline Si and (Si,Ge) alloys is more complicated than suggested by the standard model of growth. In particular, unexpected phenomena related to the density of ion flux, magnitude of ion energy and the presence of inert gas ions in the discharge suggest that both inert and reactive ion flux profoundly influence the growth of amorphous and microcrystalline films and devices. The influence of the ion flux is particularly important in understanding the growth of high quality a-(Si,Ge):H films. By carefully controlling this flux, we can change significantly the H bonding environment in a-(Si,Ge):H and produce high quality materials and devices. These observations suggest that the standard model of growth, which postulates that the growth is primarily limited by surface diffusion of radicals, and that H is eliminated by spontaneous bond breaking between neighboring Si-H bonds and formation of H2 molecule, does not describe the actual growth chemistry very well. Rather, the growth is limited by both surface diffusion of radicals and more fundamentally, by how rapidly H desorbs from surface and subsurface bonds. Ion induced desorption of excess H is shown to be very important in controlling the growth of both a-Si and a-(Si,Ge):H.

Sensitivity studies of methane photolysis and its impact on hydrocarbon chemistry in the atmosphere of Titan

The photodissociation of methane at Lyman α (1216 Å) has been the focus of much scrutiny over the past few years. Methane photolysis leads to the formation of H2 molecules as well as H, CH, 1CH2, 3CH2, and CH3 radicals, which promote the propagation of hydrocarbon chemistry. However, recent studies [Mordaunt et al., 1993; Romani, 1996; Smith and Raulin, 1999] have not fully resolved the issue of methane photolytic product yields at this wavelength. We use a one‐dimensional photochemical model with updated chemistry to investigate the significance of these quantum yield schemes on the hydrocarbon chemistry of Titan's atmosphere, where Lyman α radiation accounts for 75% of methane photolysis longward of 1000 Å. Sensitivity studies show that while simple hydrocarbons like C2H2 (acetylene) and C2H4 (ethylene), which serve as important intermediates to the formation of more complex hydrocarbons, show virtually no variation in abundance, minor C3 molecules do show substantial sensitivity to choice of quantum yield scheme. We find that the C3H4 isomers (methylacetylene, allene) and C3H6 (propylene) display major variation in atmospheric mixing ratios under the implementation of these schemes, with a maximum variation of approximately a factor of 5 in C3H4 abundance and approximately a factor of 4 for C3H6. In these cases our nominal scheme, recommended by Romani [1996], offers an intermediate result in comparison with the other schemes. We also find that choice of pathway for non‐Lyman α methane absorption does affect hydrocarbon chemistry in the atmosphere of Titan, but this effect is minimal. A 65% variation in C2H6 (ethane) abundance, a value within observational uncertainty, is the largest divergence found for a wide range of possible non‐Lyman α photofragment quantum yields. These results will have significance in future modeling and interpretation of observations of the atmosphere of Titan.

Hydrogen Effect on Damage Structure of Si(100) Surface Studied by in Situ Raman Spectroscopy

In situ Raman measurements have been performed on a Si(100) surface under irradiation by low-energy H+, D+ and He+. The intensity of the 520 cm-1 Raman peak of crystalline Si decreased almost linearly with the square root of displacement per atom (dpa), suggesting that the peak reduction originates from defect clusters but not single vacancies or interstitials. At a high dpa, the peak intensity became very low and broadened due to amorphization for all incident ions. In addition, the chemical effect of hydrogen was clearly observed, i.e., the reduction rate at low dpa was slightly enhanced by H+ and D+ irradiation as compared to that with He+, whereas H+ and D+ decelerated the amorphization as compared to He+. The initial damage enhancement is attributed to Si–H bond formation, whereas the later deceleration of the amorphization is attributed to the formation of H2 molecules recovering Si–Si bonds.

Plasma-insulator transition of spin-polarized hydrogen.

A mixed classical-quantum density functional theory is used to calculate pair correlations and the free energy of a spin-polarized hydrogen plasma. A transition to an atomic insulator phase is estimated to occur around r(s)=2.5 at T=10(4) K, and a pressure P is approximately equal to 0.5 Mbar. Spin polarization is imposed to prevent the formation of H2 molecules.

First Structure Formation. I. Primordial Star‐forming Regions in Hierarchical Models

We investigate the possibility of very early formation of primordial star clusters from high-\sigma perturbations in cold dark matter dominated structure formation scenarios. For this we have developed a powerful 2-level hierarchical cosmological code with a realistic and robust treatment of multi-species primordial gas chemistry, paying special attention to the formation and destruction of hydrogen molecules, non-equilibrium ionization, and cooling processes. We performed 3-D simulations at small scales and at high redshifts and find that, analogous to simulations of large scale structure, a complex system of filaments, sheets, and spherical knots at the intersections of filaments form. On the mass scales covered by our simulations (5x10^5 - 1x10^9\Ms) that collapse at redshifts z>25, we find that only at the spherical knots can enough H2 be formed (n_{H_2}/n_H > 5x10^-4) to cool the gas appreciably. Quantities such as the time dependence of the formation of H2 molecules, the final H2 fraction, and central densities from the simulations are compared to the theoretical predictions of Abel (1995) and Tegmark et al. (1997) and found to agree remarkably well. Comparing the 3-D results to an isobaric collapse model we further discuss the possible implications of the extensive merging of small structure that is inherent in hierarchical models. Typically only 5-8% percent of the total baryonic mass in the collapsing structures is found to cool significanlty. Assuming the Padoan (1995) model for star formation our results would predict the first stellar systems to be as small as ~30\Ms. Some implications for primordial globular cluster formation scenarios are also discussed.

Topology of density difference and force analysis

The density difference Δρ(r) of a molecule A-B is defined as the difference of the density ρ(r) of the molecule A-B and the density ρA(r) + ρB(r) put at the position of the atoms A and B. We investigate here the topological features of the density difference and define electron density flow (EDF) as representing the direction and the amount of the electron density flow in the course of the nuclear displacement processes. As such examples, we study H2 molecule formation reaction and the interaction of two He atoms. By the topological analysis of Δρ(r), and by using the Hellmann-Feynman force and its partition into the AD, EC, and EGC forces, the characteristic behaviors of the Δρ(r) map are clarified. In particular, the electron cloud preceding and incomplete following are represented by using the concept of the EDF. The natures of the covalent bond are clarified based on the topological properties of the difference density Δρ(r) rather than that of the total density ρ(r).

Hydrogen release kinetics during reactive magnetron sputter deposition of a-Si:H: An isotope labeling study

The release of molecular hydrogen from the growing surface of hydrogenated amorphous silicon films is determined using an isotope labelling technique. The results demonstrate that surface-bonded H atoms are readily abstracted by atomic hydrogen arriving from the gas phase. The films are deposited by dc reactive magnetron sputtering of a silicon target in an argon-hydrogen atmosphere. To achieve isotope labeling, we first deposit a deuterated amorphous silicon film, then commence growth of hydrogenated amorphous silicon and measure the transient release of HD and D2 from the growing surface using mass spectrometry. Release occurs when the supply of reactive hydrogen in the growth flux exceeds the incorporation rate into the film, and is observed under all experimental conditions. The net rate of H incorporation is known from ex situ measurements of film growth rate and hydrogen content. We combine the H release and incorporation data in a mass balance argument to determine the H-surface kinetics. Under conditions which produce electronically useful films, (i) 0.5–1.0 hydrogen atoms react with the growing surface per incorporated silicon atom, (ii) the near surface of the growing film contains 1–3×1015/cm2 of excess hydrogen, (iii) the dominant hydrogen release mechanism is by direct abstraction to form H2 molecules, and (iv) the kinetics of H release and incorporation can be described by constant rate coefficients. These data are supported by studies of H interactions with single-crystal silicon and amorphous carbon surfaces.