Dissociation Of Hydrogen Molecules Research Articles

On the suppression of hydrogen degradation in PbZr0.4Ti0.6O3 ferroelectric capacitors with PtOx top electrode

The effect of oxygen content in the sputtered Pt oxide (PtOx, x=0.4∼1), top electrode on suppressing the hydrogen degradation of ferroelectric capacitors of PbZr0.4Ti0.6O3 (PZT) was studied. It was found that the ferroelectric property becomes significantly degraded along with a positive voltage offset in the polarization-field hysteresis loop after forming gas (N2+H2) annealing at 200°C of the PZT capacitors made with Pt top electrode. The leakage current in the degraded capacitors also greatly increases due to the reduction of Schottky barrier. However, the degradation of electrical property can be suppressed by using PtOx as top electrode, and the ferroelectric characteristics are well retained ever for capacitors with size shrunk down to submicron scale. The secondary-ion-mass spectroscopy and elastic recoil detection analysis reveal that the catalytic reaction of Pt in dissociation of hydrogen molecules from forming gas can be effectively interrupted by increasing the oxygen content in the PtOx electrode, and the property degradation induced from hydrogen segregation at the interface and incorporation with the PZT layer is thus suppressed. In addition to the superior hydrogen blocking capability, which prevents the detrimental effect of hydrogen doping, a high Schottky barrier is also obtained from the high work function of PtOx electrode.

Dehydrogenation and hydrogenation characteristics of mechanically alloyed Mg-11 at.% In-1 at.% Fe alloy

The ionic bonding of MgH 2 must be made weaker in order to improve its dehydrogenation performance. Alloying of Mg with In, which has a larger electronegativity than Mg and is soluble to Mg, has a possibility to improve the dehydrogenation performance by weakening the ionic bonding of the hydride. A Mg–In–Fe alloy was, therefore, made by mechanical alloying and its behavior of dehydrogenation and hydrogenation was examined. Fe was added to expect its catalytic effect on hydrogen molecule dissociation. As a result, the mechanically alloyed Mg–In–Fe alloy powder showed better dehydrogenation and hydrogenation performance than a pure Mg powder made by thermal decomposition of a commercial MgH 2 powder. Activation energy of both dehydrogenation and hydrogenation reactions of Mg–In–Fe alloy powder decreased as compared with the pure Mg powder.

Adsorption and dissociation of hydrogen molecules on bare and functionalized carbon nanotubes

We investigated interaction between hydrogen molecules and bare as well as functionalized single-wall carbon nanotubes (SWNT) using first-principles plane wave method. We found that the binding energy of the H$_{2}$ physisorbed on the bare SWNT is very weak, and can be enhanced neither by increasing the curvature of the surface through radial deformation, nor by the coadsorption of Li atom that makes the semiconducting tube metallic. Though the bonding is strengthened upon adsorption directly to Li atom, yet its nature continues to be physisorption. However, the character of the bonding changes dramatically when SWNT is functionalized by the adsorption of Pt atom. Single H$_{2}$ is chemisorbed to Pt atom on the SWNT either dissociatively or molecularly. If Pt-SWNT bond is weakened either by displacing Pt from bridge site to a specific position or by increasing number of the adsorbed H$_{2}$, the dissociative adsorption of H$_{2}$ is favored. For example, out of two adsorbed H$_{2}$, first one can be adsorbed dissociatively, second one is chemisorbed molecularly. The nature of bonding is weak physisorption for the third adsorbed H$_{2}$. Palladium also promotes the chemisorption of H$_{2}$ with relatively smaller binding energy. Present results reveal the important effect of transition metal atom adsorbed on SWNT and advance our understanding of the molecular and dissociative adsorption of hydrogen for efficient hydrogen storage.

Molecular hydrogen adsorption and dissociation on the plutonium (111) surface

Hydrogen molecule adsorption and the reaction barrier for dissociation on the plutonium (111) surface have been studied in detail using the generalized gradient approximation to density functional theory. All calculations have been performed at both non-spin-polarized and spin-polarized levels of the theory. Weak molecular adsorptions with a layer by layer alternate spin arrangement of the plutonium atoms were observed. Horizontal approaches on the top site both without and with spin polarization were found to be the most favorable molecular adsorption sites. For dissociative adsorption it was found that the most favorable dissociation channel needs activation energies of 0.216 and $0.305\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ at the non-spin-polarized and spin-polarized levels, respectively, with considerably higher adsorption energies than those of the molecular cases. Compared to dissociation on the (100) surface, hydrogen molecule dissociation on the (111) surface is easier because the activation energies are much lower in the later case. In general the adsorption of hydrogen molecule pushes the Pu $5f$ band away from the Fermi level, except for spin-polarized dissociative adsorption, where $5f$ orbitals come closer to the Fermi level. Charge transfer to the hydrogen atoms for dissociative adsorption is larger than that of molecular adsorption due to the reduced distances of hydrogen atoms to the plutonium surface. In that case, the ionic part of $\mathrm{H}\ensuremath{-}\mathrm{Pu}$ bonding contributes along with the covalent part due to Pu $5f--\mathrm{H}$ $1s$ hybridization.

Reactivity of gold thin films grown on iridium: Hydrogen dissociation

Dissociative adsorption of H 2 (D 2) on Au thin films grown on an Ir{1 1 1} surface has been studied with temperature-programmed desorption using a quadrupole mass spectrometer and nuclear reaction analysis. Thin Au{1 1 1} films are epitaxially grown on Ir{1 1 1}, as confirmed by low-energy electron diffraction and scanning tunneling microscopy. H 2 (D 2) was dissociatively adsorbed on these Au{1 1 1} films, although it is well known that Au surfaces are noble enough not to dissociate hydrogen molecules. We propose a model to explain the unexpected high reactivity of the thin Au{1 1 1} surface, where narrowing of the s-band is responsible. Moreover, we found that H (D) atoms can be confined into the interface between the Ir surface and the Au thin film. We demonstrate how reactive Au thin films grown on an Ir surface are, and also compare the Au films with Ag films grown on Ir in order to test the validity/generality of our proposed model.

Hydrogen Storage in Ni Nanoparticle-Dispersed Multiwalled Carbon Nanotubes

Hydrogen storage properties of mutiwalled carbon nanotubes (MWCNTs) with Ni nanoparticles were investigated. The metal nanoparticles were dispersed on MWCNTs surfaces using an incipient wetness impregnation procedure. Ni catalysts have been known to effectively dissociate hydrogen molecules in gas phase, providing atomic hydrogen possible to form chemical bonding with the surfaces of MWCNTs. Hydrogen desorption spectra of MWCNTs with 6 wt % of Ni nanoparticles showed that approximately 2.8 wt % hydrogen was released in the range of 340-520 K. In Kissinger's plot to evaluate the nature of interaction between hydrogen and MWCNTs with Ni nanoparticles, the hydrogen desorption activation energy was measured to be as high as approximately 31 kJ/mol.H(2), which is much higher than the estimates of pristine SWNTs. C-H(n)() stretching vibrations after hydrogenation in FTIR further supported that hydrogen molecules were dissociated when bound to the surfaces of MWCNTs. During cyclic hydrogen absorption/desorption, there was observed no significant decay in hydrogen desorption amount. The hydrogen chemisorption process facilitated by Ni nanopaticles could be suggested as an effective reversible hydrogen storage method.

A Study on Hydrogen Desorption Properties of Multiwall Carbon Nanotubes

Hydrogen desorption properties in multiwall carbon nanotubes(MWNTs) were investigated from the view points of not only physical hydrogen molecules adsorption in nano-hole but also chemical hydrogen adsorption on graphite surface. MWNTs with intrinsic closed structure having the blocked hole and MWNTs with open structure were studied through hydrogen thermal desorption technique equipped with gas chromatograph. In order to grow MWNTs with open structure, novel method using oxygen added plasma(CH4/H2/O2) has been introduced. The structure of MWNTs was gradually changed with increasing oxygen addition amount, showing aligned MWNTs bundles and open tube cap at 10% oxygen addition. The precise analysis on thermal desorption spectra on MWNTs with closed structure showed that hydrogen gas was released at three major temperature ranges such as 100-230K, 290-350K, and 600-625K, where the evolved hydrogen amounts were about 1.65wt%, 0.64wt%, and 0.03wt%, respectively. However, in case of aligned and open MWNTs, the evolution peak around 290-350K was highly developed (about 1.9wt%) and the desorption peak at 600-625K was disappeared. The hydrogen desorption activation energy was also calculated. The obtained hydrogen desorption activation energy of MWNTs with closed and open structure at ambient temperature was -18.5kJ/mol H2 and 16.5kJ/molH2, respectively. The hydrogen desorbed between 290 and 330K in MWNTs with open structure was the hydrogen physi-sorbed in nano-hole. And Ni-doping (dispersion) effect on hydrogen storage properties was investigated comparatively. Ni nano-catalysts were expected to effectively dissociate hydrogen molecules in gas phase, providing atomic hydrogen possible to form chemical bonding on carbon surface. The metal nanoparticles were homogeneously dispersed using incipient wetness impregnation method. Hydrogen desorption spectra of MWNTs with Ni nanoparticles showed that about 2.8wt% of hydrogen was released in the range of 340-520K.

Reactive gold thin films grown on iridium

We report results of our studies on the dissociative adsorption of hydrogen on Au thin films grown epitaxially on Ir{1 1 1} surface, using nuclear reaction analysis. We found that H 2 dissociatively adsorbs on these Au{1 1 1} films. This feature can be contrasted to the well-known noble bulk Au surfaces, which do not dissociate hydrogen molecules. We attribute this to the local surface properties, e.g., electron localization (the narrowing of the s-band, the s-band center model), which can explain the unexpected high reactivity of a thin Au{1 1 1} film.

The surface temperature dependence of the inelastic scattering and dissociation of hydrogen molecules from metal surfaces.

High-dimensional, wave packet calculations have been carried out to model the surface temperature dependence of rovibrationally inelastic scattering and dissociation of hydrogen molecules from the Cu(111) surface. Both the molecule and the vibrating surface are treated fully quantum-mechanically. It is found, in agreement with experimental data, that the surface temperature dependence of a variety of dynamical processes has an Arrhenius form with an activation energy dependent on molecular translational energy and on the initial and final molecular states. The activation energy increases linearly with decreasing translational energy below the threshold energy. Above threshold the behavior is more complex. A quasianalytical model is proposed that faithfully reproduces the Arrhenius law and the translational energy dependence of the activation energy. In this model, it is essential to include quantized energy transfer between the surface and the molecule. It further predicts that for any process characterized by a large energy barrier and multiphonon excitation, the linear change in activation energy up to threshold has slope-1. This explains successfully the universal nature of the unit slope found experimentally for H2 and D2 dissociation on Cu.

Kinetic study of hydrogen adsorption on Pt/WO 3-ZrO 2 and WO 3-ZrO 2

Rates of hydrogen adsorption on Pt/WO 3-ZrO 2 and WO 3-ZrO 2 were measured in the adsorption temperature range 323–573 K and the hydrogen pressure range 20–70 Torr (1 Torr = 133 Pa) to elucidate the rate-controlling step involved in the hydrogen adsorption, the corresponding energy barrier, and the heat of adsorption. The hydrogen adsorption was faster on Pt/WO 3-ZrO 2 than on WO 3-ZrO 2. On both Pt/WO 3-ZrO 2 and WO 3-ZrO 2, the adsorption continued for a long time. The adsorption continued for more than 10 h for Pt/WO 3-ZrO 2 below 373 K and for WO 3-ZrO 2 below 473 K. The adsorption of hydrogen involved the surface diffusion of hydrogen atoms for both adsorbents. For WO 3-ZrO 2, each hydrogen molecule was dissociated on a specific site of WO 3-ZrO 2 to form hydrogen atoms, which undergo surface diffusion over the surface of WO 3-ZrO 2. The rate-controlling step was the surface diffusion of hydrogen atoms. The activation energy was 25.9 kJ/mol for the surface diffusion. For Pt/WO 3-ZrO 2, two routes were operating. One route was same as that for WO 3-ZrO 2, and the other route involved dissociation of hydrogen molecules on the Pt sites to form hydrogen atoms that undergo spillover onto the WO 3-ZrO 2 surface, followed by surface diffusion. For the latter route, the rate-controlling step was the spillover step; its activation energy was 35.5 kJ/mol. The isosteric heats of adsorption of hydrogen on Pt/WO 3-ZrO 2 were obtained as 11.6–8.0 kJ/mol for the hydrogen uptake range 2–3×10 19 atom H/g cat. Similarities and differences in hydrogen adsorption between SO 4 2−-ZrO 2 and WO 3-ZrO 2 types are discussed.

H density in a hydrogen plasma post-glow reactor

A nickel catalytic probe was used to determine the density of neutral hydrogen atoms in a hydrogen post-glow reactor. The plasma was created in a side quartz tube by a surfatron microwave generator. The charged particles were recombined on the quartz surface before reaching the post-glow reactor, so that the gas entering the reactor was just a mixture of molecular and atomic hydrogen. The density of hydrogen atoms in the post-glow reactor was measured at different output power of the microwave generator and different flow of hydrogen through the vacuum system. The H density increased linearly with increasing microwave power, and it did not depend much on the hydrogen flow. At the lowest power tested, i.e. 40 W, the H density was 5×10 20 m −3, while at the highest power of 145 W it was about 2×10 21 m −3. The degree of dissociation of hydrogen molecules, on the other hand, depended on the hydrogen flow. At the flow of 80 cm 3 min −1 the degree of dissociation was between 4% (at the power of 40 W) and 14% (at 145 W), while at 300 cm 3 min −1 it dropped to between 1.5% (at 40 W) and 5% (at 145 W). The results were explained taking into account the collision phenomena in ionized gases.

Low-Temperature Opto-Galvanic Spectroscopy without Discharges

A new versatile method for opto-galvanic spectroscopy is described. The method is based on sensitive detection of impedance changes in dilute plasma without the necessity for a self-sustaining discharge. A one-dimensional model for the effect in a high-pressure electron beam pumped gas is developed and tested. Test spectra for the Balmer-α transition in hydrogen, produced by resonant dissociation of excited hydrogen molecules in high pressure Ne, demonstrate the high resolution and sensitivity achievable for potential spectroscopic and chemical kinetics applications.

Dissociative adsorption of hydrogen on thin Au films grown on Ir{ [formula omitted

Adsorption and reaction of hydrogen (deuterium) on thin gold films grown on Ir{1 1 1} surface at ∼100 K have been studied with temperature-programmed desorption using a quadrupole mass spectrometer. Thin Au{1 1 1} films are epitaxially grown on the Ir{1 1 1}. Surprisingly, H 2 (D 2) can dissociatively adsorb on these Au{1 1 1} films, although it is well known that Au surfaces are noble enough not to dissociate hydrogen molecules [Nature 376 (1995) 238]. Moreover, we found that H (D) atoms can be confined into a Au thin film and/or the interface between an Ir surface and the thin Au film. We demonstrate how reactive thin Au films are.

Stability of hydrogenated in pure hydrogen plasma p-channel polycrystalline silicon thin-film transistors

The stability of p-channel polycrystalline silicon thin-film transistors, hydrogenated in pure hydrogen plasma, is investigated. The hot-carrier induced degradation mechanisms are studied for operation in the saturation region and different gate bias voltages V g. During on-state stress at high | V g|, first an effective shortening of the channel length is observed due to trapping of hot electrons. As the stress proceeds further, donor-type interface states are generated, resulting in an increase of the electric field near the drain due to built-up of positive charge in these states by trapping of hot holes. During on-state stress at low | V g|, the transistor parameters are improved due to further passivation of grain boundary deep and tail states, caused by dissociation of hydrogen molecules by hot electrons near the drain region. During off-state stress, hot electrons are injected in the gate oxide near the drain causing an effective shortening of the channel length and a reduction of the minimum leakage current.

Effect of LaNiO3 Top Electrode on the Resistance of Pb(Zr,Ti)O3 Ferroelectric Capacitor to Hydrogen Damage and Fatigue

The polarization hysteresis characteristics of Pb(Zr,Ti)O3 (PZT) capacitors with a Pt top electrode are degraded by annealing in a hydrogen-containing environment due to the catalytic effect of Pt. This degradation can be avoided by adopting an LaNiO3 (LNO) conductive film as a top electrode. The LNO film does not act as a catalyst for the dissociation of hydrogen molecules and it is also a good diffusion barrier to the hydrogen molecules. The resistance of the LNO film to hydrogen degradation is further enhanced by inserting a thin Pt layer between the LNO and PZT films due to the blocking ability of Pt against the out-diffusion of the Pb component from the PZT film. The polarization characteristics of LNO/Pt/PZT/Pt capacitors are well preserved after hydrogen annealing even at 400°C for 30 min. The use of the LNO film as a top electrode also has a significant effect on improving the fatigue property of PZT capacitors.

Collisional Radiative Kinetics of Molecular Assisted Recombination in Edge Plasmas

Recombination of hydrogen plasma in the divertor volume is a simple explanation for divertor detachment phenomena experimentally observed on many tokamaks and divertor plasma simulators. The presence of vibrationally excited molecules in detached plasma provides an additional mechanism for plasma recombination, the so-called molecular assisted recombination (MAR). The cross-section data on atomic and molecular processes relevant for divertor plasma conditions are reviewed. Collisional radiative kinetics for cascade excitation of vibrational and electronic states as well as for the dissociation, recombination, and ionization of hydrogen molecule and molecular ions is studied. The results of calculations on the effective rate coefficient for MAR in detached divertor plasmas are presented. A comparison of the MAR rate coefficient with the rate coefficients for molecular dissociation, ionization, and electron-ion recombination is given. The sensitivity of MAR rate to the accuracy and completeness of atomic physics data is discussed

Dissociation dynamics from a de Broglie–Bohm perspective

Within the framework of Bohm’s reformulation of quantum physics we revisit the activated dissociation of hydrogen molecules at metal surfaces. The quantum-mechanical force, which accounts for most of quantum effects in the method, and is caused by nonlocal, topographical properties of the wave function, is computed using time-dependent wave packets obtained using conventional, spectral methods. Driven by a combination of the classical force together with the quantum force, trajectories carrying probability density either succeed in overcoming the barrier for dissociation or are scattered back into the gas phase. The Bohmian picture for the dissociation process has enabled us to develop a novel mechanism to account for vibrationally enhanced molecular dissociation. This is relevant to the recently observed promotion of dissociation of very highly vibrationally excited NO molecules at Cu surfaces.

Hydrogenation of defects in edge-defined film-fed grown aluminum-enhanced plasma enhanced chemical vapor deposited silicon nitride multicrystalline silicon

Gettering of impurities and hydrogen passivation of defects in edge-defined film-fed grown (EFG) multicrystalline silicon were studied by low-cost manufacturable technologies such as emitter diffusion by a spin-on phosphorus dopant source, back surface field formation by screen-printed aluminum, and a post-deposition anneal of plasma enhanced chemical vapor deposited (PECVD) silicon nitride antireflection coating. These processes were carried out in a high-throughput lamp-heated conveyor belt furnace. PECVD silicon nitride-induced hydrogenation of defects in EFG silicon was studied in conjunction with screen-printed aluminum back surface field formation to investigate the synergistic effect of aluminum gettering and silicon nitride hydrogenation of bulk defects. It was found that post-deposition anneal of PECVD silicon nitride at temperatures ranging from 450 to 850 °C, without the coformation of aluminum back surface field on the back, does not provide appreciable passivation or hydrogenation of bulk defects in EFG material. However, simultaneous anneal of PECVD silicon nitride and formation of aluminum back surface field at 850 °C significantly enhanced the hydrogenation ability of the PECVD silicon nitride film. PECVD silicon nitride deposition and a subsequent anneal, after the aluminum back surface field formation, was found to be less effective in passivating bulk defects. It is proposed that aluminum-enhanced hydrogenation from a PECVD silicon nitride film is the result of vacancy generation at the aluminum-silicon interface due to the alloying process. The affinity of hydrogen to react with vacancies provides a chemical potential gradient that increases the flux of atomic hydrogen from the silicon nitride film into the bulk silicon. In addition, vacancies can dissociate hydrogen molecules, increasing the atomic hydrogen content of the bulk silicon. This enhances defect passivation and improves the minority carrier lifetime.

Novel method for absolute quantification of the flux and angular distribution of a radical source for atomic hydrogen

In this article we present a novel method for absolute quantification of the flux and angular distribution of a beam source for atomic hydrogen or atomic deuterium. It is based on quantitative determination of the erosion of an amorphous, hydrogenated carbon (a-C:H) film. A heated tungsten capillary serves as the radical source. Atomic hydrogen is produced in this capillary by thermal dissociation of hydrogen molecules. A large-area a-C:H film is exposed to the source at a substrate temperature of 650 K. Interaction of atomic hydrogen with the a-C:H film causes erosion. From the spatial variation of the erosion rate one can deduce the angular distribution of the impinging hydrogen atoms. This angular distribution was also measured by mass spectrometry and showed excellent agreement with the erosion profile. The absolute flux of atomic hydrogen was also determined by mass spectrometry. With the absolute flux of atomic hydrogen known from mass spectrometry, measurement of the lateral variation of the erosion rate can be directly used as a probe for absolute quantification of the angular distribution of the impinging H (D) flux. The erosion yield is (2±0.7)×10−2, which is consistent with the microscopic erosion mechanisms of a-C:H by atomic hydrogen known from the literature.

Effect of macrostructure and composition of the top metal electrode on properties of MIS gas sensors

Pd and Pd/Cu gate MIS gas sensitive structures are studied by the high-frequency CV method. The loss of the gas sensitivity due to interaction of the top electrode with the environment are characterised by Auger spectroscopy and scanning electron microscopy. Pd/Cu structures are more sensitive to the adsorption of the gas molecules compared to Pd gate. The base-line inversion after air recovery is observed and this effect is discussed. The response time of MIS structure with a thin fresh Pd/Cu electrode (∼500 Å) is slower compared to the structures with a thick Pd gate (∼1000 Å). Similar results have also been obtained for thick aged Pd electrodes. Slow responses of these structures may be associated with a Pd oxide growth on a Pd surface and/or an enhanced adsorption of oxygen and water species at the boundaries of Pd/Cu grains. The higher active hydrogen sensitivity catalyst Pd/Cu has been modelled on a more intensive dissociation of hydrogen molecules on Pd surface doped with Cu.