Desorption Of Hydrogen Atoms Research Articles

First-principles time-dependent simulation of laser assisted desorption of hydrogen atoms from H–Si(111) surface

The dynamics of hydrogen desorption from H-terminated silicon surface clusters has been simulated in the framework of real space time-dependent density functional theory complemented with molecular dynamics for ions. It has been demonstrated that by choosing an appropriate frequency and intensity of the laser it is possible to remove the hydrogen layer from the surface without destroying the structure of underlying silicon. At the laser field intensities used in the current study (0.5–2.0 V/Å) the desorption process is notably nonlinear.

Brightening of Triplet Dark Excitons by Atomic Hydrogen Adsorption in Single-Walled Carbon Nanotubes Observed by Photoluminescence Spectroscopy

Adsorption and desorption of atomic hydrogen on single-walled carbon nanotubes were observed by photoluminescence spectroscopy. A satellite peak appeared at the lower energy side of the E11 photoluminescence emission peak after exposure to atomic hydrogen and then disappeared after annealing at 300 °C in vacuum. The energy difference between the satellite peak and E11 peak was 40-80 meV, depending on the tube diameter. The satellite peak was attributed to the triplet dark exciton that became optically active because of the effectively enhanced spin-orbit interaction induced by adsorbed hydrogen atoms.

Electronic properties of then-doped hydrogenated silicon (100) surface and dehydrogenated structures at 5 K

We present a comparative study of the electronic properties of the clean Si(100) and the hydrogenated Si(100):H surfaces performed with a low-temperature (5 K) scanning tunneling microscope. Various surface structures such as single silicon dangling bonds and bare silicon dimers created by local desorption of hydrogen atoms from the Si(100):H surface are also investigated. The experimental scanning tunneling spectroscopy (STS) curves acquired locally on each of these structures are compared with STS measurements performed on the Si(100) and Si(100):H surfaces. First principle density-functional theory calculations of the projected local density of states, taking into account the influence of the dopant atoms (As), enable to assign the observed STS spectra.

Near coalescent submicron polycrystalline diamond films deposited on silicon: Hydrogen bonding and thermal enhanced carbide formation

The influence of high temperature annealing up to 1200 °C in vacuum on ∼100 nm nearly continuous thick diamond films consisting of 30–50 nm crystallites, deposited onto silicon substrates is reported. The hydrogen bonding and phase composition of the films were studied with Raman spectroscopy, while the surface microstructure and composition were studied with high resolution scanning electron microscopy (SEM), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS), respectively. Annealing to 800–900 °C of ∼100 nm thick films results in a decrease in the intensities of the peaks associated with hydrogen bonding (Raman), as well as changes to the morphological microstructure at the film surface. Heating the films to 1000 °C resulted in the complete disappearance of the Raman peaks associated with hydrogen bonding at grain boundaries, and an increase in the relative intensity of the diamond peak relative to the graphite-related D and G Raman peaks, concomitant with changes to the microstructure (SEM and TEM). Ex situ XP analysis of the films annealed to 800 and 1000 °C provides clear evidence for the formation of SiC on the films surface and near surface region. However a sharp SiC Raman peak at 796 cm−1 appears only after annealing to 1200 °C and it is concomitant with a decrease in the Raman peaks associated with sp2 bonded carbon. Our results suggest that formation of SiC phase preferentially consumes sp2/sp hybridized carbon matrix, produced by thermal desorption of hydrogen atoms at diamond grain boundary and at the diamond film—silicon substrate interface.

Hydrogen Absorption and Diffusion in Bulk α-MoO3

The absorption, diffusion, and desorption of atomic hydrogen in layered orthorhombic molybdenum trioxide (α-MoO3) was investigated using density functional theory. Hydrogen atoms are absorbed in bulk α-MoO3 to form the hydrogen molybdenum bronze HxMoO3 (x = 0.25, 0.5, 0.75, 1, 1.25, and 1.5). The semiconductor band gap of bulk α-MoO3 shifts to metallic upon hydrogen bronze formation at the H atom loadings selected in the present study. The hydrogen atoms become protonic when coordinated to oxygen, which gives rise to a charge reduction on the Mo atoms adjacent to the absorption sites. Hydrogen migration along a prescribed diffusion pathway in the lattice was found to be facile due to small energy barriers for H atom transfer between O atoms, facilitated by a hydrogen bonding network. The sequential hydrogen desorption from the bronze and the mechanisms of hydrogen spillover in α-MoO3 are also discussed.

Epitaxial Growth of Highly Crystallized InSb films on Si Substrate by MBE and Their Devices Properties

AbstractHigh crystallized thin InSb epitaxial growth directly on Si substrate was investigated by molecular-beam epitaxy (MBE). Experimental results indicated that suppressing the desorption of hydrogen atoms which terminated the dangling bonds of Si wafer surface and incorporation of As around the interface between film and Si substrate were the most important to obtained high crystallized InSb film. It could be achieved by the irradiation of As4 cluster beam onto the Si wafer just before film growth. Obtained thin InSb film showed mirror like surface, and its thickness was 0.7 μm. Its electron mobility was 47,600 cm2/V-s, and FWHM of HR-XRD rocking curve was about 300 arcsec. This InSb film on Si wafer was applied to Hall element, and it passed ordinary reliability tests.

Desorption force on hydrogen atoms from resonant excitations of the H:Si(0 0 1)-(2 × 1) surface

Laser excitation at 7.9 eV photon energy leads to the desorption of hydrogen atoms from the H:Si(0 0 1)-(2 × 1) monohydride surface [T. Vondrak, X.-Y. Zhu, Phys. Rev. Lett. 82 (1999) 1967]. In the present paper we address the electronic excitations relevant for this desorption process and investigate the corresponding force on the hydrogen atom. The excited state is described by ab-initio many-body perturbation theory. A particular problem is posed by the resonant character of the excited state in question, leading to delocalization and fast decay. Here we suggest to employ an additional short-range confinement potential to localize the excited state on the Si–H bond which is broken in the desorption process.

Model for thermal desorption of hydrogen atoms from a graphite surface based on kinetic Monte Carlo simulations

Kinetic Monte Carlo simulations are carried out to describe the mechanism which is responsible for the occurrence of two peaks in the thermal recombinative desorption of molecular ${\text{H}}_{2}$ (or ${\text{D}}_{2}$) from clean graphite surfaces, after chemisorption of H (or D) atoms. At sufficiently low hydrogen exposure to prevent formation of clusters on the surface, the main peak is shown to be due to paradimers, while the second, less intense peak is due to the loss of orthodimers and results of a competition between diffusion and evaporation of monomers and paradimers.

Highly Ordered Pd Nanowire Arrays as Effective Electrocatalysts for Ethanol Oxidation in Direct Alcohol Fuel Cells

Direct alcohol fuel cells (DAFCs) based on liquid fuels have attracted enormous attention as power sources for portable electronic devices and fuel-cell vehicles owing to the much higher energy density of liquid fuels than gaseous fuels such as hydrogen (e.g., the energy densities of ethanol and methanol are 6.34 kWh L and 4.82 kWh L, respectively, as compared to 0.53 kWh L for gaseous hydrogen at 20 MPa. Among various liquid fuels, ethanol is particularly attractive because it is less toxic than methanol and can be produced in large quantities from agricultural products. Ethanol is also the major renewable biofuel from the fermentation of biomass. Pt and Pt-based catalysts such as PtRu/C have been extensively investigated as electrocatalysts for the electrooxidation of liquid fuels such as methanol and ethanol. However, the high cost and limited supply of Pt constitute a major barrier to the development of DAFCs. We studied recently Ptfree electrocatalysts for the electrooxidation reactions of ethanol and methanol, and the results revealed that Pd is a good electrocatalyst for ethanol oxidation in alkaline media. Metal nanowire arrays (NWAs) have attracted much attention and interest owing to their excellent physical and chemical properties and have been extensively investigated for applications such as high-density magnetic recording devices and sensors. Ordered nanowire or nanotube arrays have also been applied for methanol oxidation and H2O2 electrocatalytic reduction because of their high active surface area. Among various techniques to synthesize NWAs, the anodized aluminum oxide (AAO) template method is probably the simplest and most versatile approach to creating highly ordered metal NWAs with uniform and tunable porous structure and good mechanical and thermal stability. Electrodeposition has been shown to be an efficient method for the growth of uniform and continuous metallic nanowire arrays. Despite the exceptional physical, chemical, and electrical properties of metal NWAs, there is little information on the electrocatalytic properties of Pd NWAs. Here, we report the fabrication of highly ordered Pd NWA electrodes by the AAO templateelectrodeposition method, and the results show that Pd NWAs are highly active for ethanol oxidation in alkaline media, demonstrating the potential of applying Pd NWAs as effective electrocatalysts for DAFCs. Figure 1 shows typical scanning electron microscopy (SEM) images of Pd NWAs after the AAO template has been fully dissolved. The Pd nanowires (NWs) are highly ordered with uniform diameter and length. The average length and diameter of the Pd NWs are ca. 800 and ca. 80 nm, respectively. The NWs are uniform, well isolated, parallel to one another, and standing vertically to the electrode substrate surface. The hexagonal shape of the Pd NWs is due to the AAO porous structure during anodization. The X-ray diffraction (XRD) pattern (inset in Fig. 1a) indicates that the Pd NWAs exhibit a typical face-centered cubic (fcc) lattice structure. The strong diffraction peaks at 40.10°, 46.49°, and 68.08° correspond to the (111), (200), and (220) facets of Pd. This indicates that Pd NWAs have been successfully fabricated. Figure 2a shows cyclic voltammograms (CVs) of ethanol oxidation in a 1.0 M KOH + 1.0 M C2H5OH solution on a Pd film electrode (curve a, Pd loading: 1.10 mg cm), a Pd NWA electrode (curve c, Pd loading: 0.24 mg cm), and an E-TEK PtRu/C electrode (curve b, Pt loading: 0.24 mg cm). The cyclic voltammograms of the Pd film, E-TEK PtRu/C, and Pd NWA electrodes in 1.0 M KOH solution without ethanol are shown in Figure 2b. In the CVs obtained in 1.0 M KOH electrolyte solution, the anodic peaks appearing between –0.73 and –0.53 V versus Hg/HgO on Pd and –0.8 and –0.5 V versus Hg/HgO on Pt originate from the desorption of atomic hydrogen on the electrocatalysts (Fig. 2b). Thus the area of H desorption after the deduction of the double layer region on the CV curves represents the charge passed for the H desorption, QH, and is proportional to the electrochemically active area (EAA) of the electrocatalysts. The value QH = 10.6 mC cm –2 for the Pd NWA electrode is much higher than 3.4 mC cm for the Pd film electrode and 4.6 mC cm for the E-TEK PtRu/C electrode. This shows that the Pd NWA electrode has high EAA, most likely due to the well-defined and uniform porous structure of the nanowires in the arrays (Fig. 1). Such well-defined nanowire structure enhances the active sites for the electrooxidation reaction of ethanol. The high electrocatalytic activity of the Pd NWA electrode is also indicated by its superior performance for the electrooxC O M M U N IC A TI O N

Hydrogen desorption from nanostructured graphite: ab initio molecular-dynamicsstudies

We have carried out hybrid ab initio/classical molecular-dynamics simulations for themodel system of hydrogen-adsorbed nanostructured graphite. We have investigated theeffect of the recrystallization of the nanostructured graphite to the bonding states ofhydrogen atoms at 1000 K and the desorption mechanism of the hydrogen dimer from thegraphite at 2000 K. We have shown that the recrystallization weakens the bond between thehydrogen and the carbon atoms and the desorption of hydrogen atoms, as the hydrogendimer occurs at 2000 K.

On the Sequential Hydrogen Dissociative Chemisorption on Small Platinum Clusters: A Density Functional Theory Study

We present a simple cluster model to understand the dissociative chemisorption of molecular hydrogen and the desorption of atomic hydrogen on platinum small clusters using the gradient-corrected density functional theory. Successive H2 decomposition and sequential H desorption on the selected Ptn (n = 2−5, 7−9) clusters were systematically studied, and the H2 dissociative chemisorption energies and the H desorption energies at the full H saturation were identified. The reaction processes are driven by charge transfer from Pt atoms to H atoms assisted by strong orbital overlaps between Pt 5d orbitals and H 1s orbital, which leads to electron delocalization in large clusters of metal hydrides. It was found that the number of H atoms chemisorbed on the small Pt clusters increases almost linearly with the size of the selected Pt cluster.

A comparison of cathodoluminescence and photoluminescence of porous silicon and the influence of aging and electron irradiation of these properties

In this paper, Cathodoluminescence and Photoluminescence in porous silicon prepared by different etching times were reported. The Cathodoluminescence emission may originate from the presence of hydrogen atoms in oxide phase, which is reduced by electron beam irradiation, but the Photoluminescence emission may be associated with the defects in dioxide in porous silicon surface, which can be proved by scanning electron microscopy images, Fourier transform infrared spectra and Raman spectra. Under electron beam irradiation, the degradation of Cathodoluminescence intensity from as-prepared sample is attributed to hydrogen-atom desorption from porous silicon surface or destruction of irradiative center on porous silicon surface. During electron-beam radiation, the stabilization of Cathodoluminescence emission of the samples, which are post treated by electron beam exposure for 60 s and then aged for 2 min, is due to the protection of surface oxidized layer generated from electron beam heating.

Multi-scale modeling of hydrogen isotope transport in porous graphite

We describe a general multi-scale method to model hydrogen isotope transport, over length scales from angstroms to centimeters and time scales from picoseconds to several seconds, in a complex three-dimensional porous geometry using molecular dynamics, kinetic Monte Carlo and Monte Carlo diffusion simulations. we present comparisons with experimental results for hydrogen diffusion and hydrogen atom desorption from graphite. Finally, we demonstrate the flexibility of the computational tools used to tackle problems at different scales.

Evidence for enhanced desorption of hydrogen atoms from a Si(1 0 0) surface induced by slow highly-charged ions

We report evidence for an enhanced desorption of hydrogen atoms from a Si(100) surface bombarded by 30keV Xeq+ (q=6–22) ions. The measured desorption yield amounts to 0.76 and 2.2 hydrogen atoms per incident Xe10+ and Xe18+ ion, respectively. For understanding the behaviour of hydrogen desorption from Si, another experiment was carried out to see the hydrogen signals as a function of time for about 140min after deliberately introducing hydrogen into the target chamber and then shut off the valve. The results are discussed in the light of potential sputtering which essentially dominates for ions at higher charge states and the interpretation is supported by theoretical estimates.

Accumulation of hydrogen near the interface between ultrathin SiO2 and Si(0 0 1) under ion irradiation in high-resolution elastic recoil detection

Depth profiles of hydrogen in a ultrathin (2.1nm) SiO2 film grown on Si(001) have been measured using high-resolution elastic recoil detection (ERD) to investigate redistribution of hydrogen atoms by ion-beam irradiation in ERD itself. During irradiation of 400keV C+, steep decrease of hydrogen density at the surface and gradual increase of hydrogen density near the SiO2/Si interface was observed. A model that involves irradiation-induced desorption of hydrogen atoms from the surface and re-adsorption of residual gas molecules containing hydrogen atoms on the surface successfully explains the decrease of hydrogen density at the surface. The increasing hydrogen atoms near the SiO2/Si interface originate in the adsorbed hydrogen on the surface, suggesting that hydrogen atoms recoiled inward from the surface accumulate near the SiO2/Si interface. According to this “recoil” model, the recoil cross section for hydrogen by 400keV C+ is estimated to be 7×10−17cm2.

Quantum study of Eley-Rideal reaction and collision induced desorption of hydrogen atoms on a graphite surface. I. H-chemisorbed case

Collision induced (CI) processes involving hydrogen atoms on a graphite surface are studied quantum mechanically within the rigid, flat surface approximation, using a time-dependent wave packet method. The Eley-Rideal (ER) reaction and collision induced desorption (CID) cross sections are obtained with the help of two propagations which use different sets of coordinates, a "product" and a "reagent" set. Several adsorbate-substrate initial states of the target H atom in the chemisorption well are considered, and CI processes are studied over a wide range of projectile energy. Results show that (i) the Eley-Rideal reaction is the major reactive outcome and (ii) CID cross sections do not exceed 4 A2 and present dynamic thresholds for low values of the target vibrational quantum number. ER cross sections show oscillations at high energies which cannot be reproduced by classical and quasiclassical trajectory calculations. They are related to the vibrational excitation of the reaction products, which is a rather steep decreasing function of the collision energy. This behavior causes a selective population of the low-lying vibrational states and allows the quantization of the product molecular states to manifest itself in a collisional observable. A peak structure in the CID cross section is also observed and is assigned to the selective population of metastable states of the transient molecular hydrogen.

Quantum study of Eley-Rideal reaction and collision induced desorption of hydrogen atoms on a graphite surface. II. H-physisorbed case

Following previous investigation of collision induced (CI) processes involving hydrogen atoms chemisorbed on graphite [R. Martinazzo and G. F. Tantardini, J. Chem. Phys. 124, 124702 (2006)], the case in which the target hydrogen atom is initially physisorbed on the surface is considered here. Several adsorbate-substrate initial states of the target H atom in the physisorption well are considered, and CI processes are studied for projectile energies up to 1 eV. Results show that (i) Eley-Rideal cross sections at low collision energies may be larger than those found in the H-chemisorbed case but they rapidly decrease as the collision energy increases; (ii) product hydrogen molecules are vibrationally very excited; (iii) collision induced desorption cross sections rapidly increase, reaching saturation values greater than 10 A2; (iv) trapping of the incident atoms is found to be as efficient as the Eley-Rideal reaction at low energies and remains sizable (3-4 A2) at high energies. The latter adsorbate-induced trapping results mainly in formation of metastable hot hydrogen atoms, i.e., atoms with an excess energy channeled in the motion parallel to the surface. These atoms might contribute in explaining hydrogen formation on graphite.

Structural transitions and stabilization of palladium nanoparticles upon hydrogenation

Using an explicit empirical potential and a combination of Monte Carlo and moleculardynamics simulations, we investigate the energetic, thermal and dynamical stability ofhydrogenated palladium nanoclusters containing a few hundred atoms. At zerotemperature, icosahedral clusters favour tetrahedral absorption sites, while cubic clusterspreferentially absorb at octahedral sites. As a consequence, icosahedra can absorb a largerquantity of hydrogen than cubic clusters. However, even a moderate amount of absorbedhydrogen is found to favour cubic structures. There are two distinct effects of temperature.First, thermal desorption of hydrogen atoms can take place spontaneously even at lowtemperatures. Second, hydrogen rich cubic particles undergo a structural transitiontoward icosahedra before melting occurs. A simplified diagram is proposed toaccount for the observed behaviour. Finally, the coalescence between two clusters isstudied dynamically for several clusters at various temperatures, both in the gasphase and for particles embedded in a simple model solvent. The heat producedfrom coalescence results in the desorption of hydrogen molecules. This leads to asignificant cooling of the nanoparticle, thereby contributing to its stabilization.

A Desorption Mechanism of Hydrogen Atoms Adsorbed on A Graphite Layer: Ab initio Molecular-Dynamics Simulations

We have carried out ab initio molecular-dynamics simulations for the model system of hydrogen-adsorbed graphite to investigate the temperature-induced desorption of hydrogen atoms from the graphite and found for the first time by the ab initio electronic structure calculations the desorption of hydrogen atoms as a hydrogen dimer, which moves almost freely between graphite layers. We have also investigated the microscopic mechanism of the desorption and the dimerization of the hydrogen atoms based on the electron density distribution as well as the overlap population analysis, and clarified that the dimerization occurs through the intermediate `bridge' state such as –C–H–H–C–. Considering the increase in the temperature due to the recrystallization occurred in the nanostructured graphite, our results are consistent with the experimental results, in the sense that the recrystallization of the nanostructured graphite is necessary for the desorption of hydrogen atoms observed experimentally at higher temperature.

Molecular-dynamics Studies on Hydrogen Atoms in Nanostructured Graphite

We have investigated the effect of crystallization on the desorption of hydrogen atoms from nanostructured graphite by carrying out the hybrid molecular-dynamics simulation on model systems. It is shown by our simulation that the bond between hydrogen and carbon atoms becomes weaker due to the recrystallization of the nanostructured graphite and that the hydrogen dimer is formed with increasing temperature. We have thus found one possible mechanism of the hydrogen desorption from the nanostructured graphite.