Hydrogen-induced microstructural changes of Pd films

The development of the microstructure in nanocrystalline, polycrystalline and epitaxial Pd films loaded with hydrogen is investigated. Structural changes in Pd films loaded with hydrogen were characterized by positron annihilation spectroscopy combined with electron microscopy and X-ray diffraction. It was found that hydrogen charging causes plastic deformation which leads to an increase of the defect density in all Pd films studied. Moreover, the formation of buckles was observed in nanocrystalline and polycrystalline Pd films loaded above a certain critical hydrogen concentration. Buckling leads to detachment of the film from the substrate and this is accompanied with in-plane stress relaxation and plastic deformation of the film.     

Dielectric function of palladium capped zirconium thin films as a function of absorbed hydrogen

We report measurements of the optical transmission, between 240 and 1040 nm, and electrical resistivity of polycrystalline zirconium thin films as they absorb hydrogen. Both are measured as H₂ pressure is increased up to 880 mbar, at room temperature. Films, 20–22 nm thick, are deposited on fused quartz substrates by e-beam evaporation at 5.3 × 10^?7 mbar base pressure and covered with a 8.0 nm Pd over-coat. The morphology of the films is studied by means of AFM images. The complex refractive indices of Zr and Pd are extracted numerically from the transmission spectra by using a spectral projected gradient method for different hydrogen pressures. The corresponding dielectric functions for various Zr hydrogen concentrations are described with the parametric Drude-Lorentz and Brendel-Bormann (DL & BB) models. The Acceptance-Probability-Controlled Simulated Annealing approach is applied to calculate the parameters of the DL & BB model. This allows us to describe the effect of increasing hydrogen absorption on these parameters and in derived quantities, like the relaxation time and the effective mass of conduction electrons, the electrical resistance, the Fermi energy, and the electronic density of states at the Fermi level.     

Hydrogen microscopy – Distribution of hydrogen in buckled niobium hydrogen thin films

Hydrogen absorption in thin metal films clamped to rigid substrates results in mechanical stress that changes the hydrogen's chemical potential by Δμ_H(σ) = −1.124σ kJ/mol_H for σ measured in [GPa]. In this paper we show that local stress relaxation by the detachment of niobium hydrogen thin films from the substrate affects the chemical potential on the local scale: using coincident proton–proton scattering at a proton microprobe, the hydrogen concentration is determined with μm resolution, revealing that hydrogen is not homogenously distributed in the film. The local hydrogen solubility of the film changes with its local stress state, mapping the buckled film fraction. In niobium hydrogen thin films loaded up to nominal concentrations in the two-phase coexistence region, the clamped film fraction remains in the solid solution phase, while the buckles represent the hydride phase. These results are compared to a simple model taking the stress impact on the chemical potential into account.     

The dependence of the chemical potential of WO3 films on hydrogen insertion

Knowledge of the chemical potential of electrochromic films coloured by hydrogen is important for matching the elements of an electrochromic device, for understanding the colouring mechanism and for obtaining information about the microscopic structure of the film. The dependence of the chemical potential on the hydrogen concentration was measured electrochemically for tungsten oxide films of different crystallinity and water content. A new method for determining the chemical potential by catalytic coloration by hydrogen gas is introduced. It revealed that the increase in electromotive force with increasing crystallinity is due only to different binding energies of the protons. We expect the protons to be located in the centres of hexagons, which are created by WO₆ octahedra. According to our model, amorphous sputtered films show a hexagonal structure which is similar to that of evaporated films, but the hexagons are connected, leading to more hexagon centre sites, which increases the electromotive force.     

Hydrogen absorption and optical properties of Pd/Mg thin films prepared by DC magnetron sputtering

Hydrogen storage in metallic thin films in the form of metal hydride have a great potential to solve the hydrogen storage challenges and also thin films offer an opportunity to grow new samples fast with novel structures. In the present work the ex situ study on structural, optical and hydrogen storage properties of Pd-capped Mg thin films have been investigated. The nano structured Pd-capped Mg thin films have been prepared by DC magnetron sputtering on glass substrate. The as deposited and hydrogenated samples have been characterized by XRD and FESEM. The content of hydrogen in thin films has measured by using Elastic Recoil Detection Analysis (ERDA) technique with 120 MeV₁₀₇ Ag⁺⁹ ions. The temperature dependent hydrogen contents in thin film samples have been estimated and the saturation of hydrogen absorption has been observed at 250 °C among all studied samples. In the optical reflectance spectra, Mg hydride samples have been observed partially transparent in comparison to as deposited Mg film.     

Pd as a promoter to reduce Co cluster films at room temperature

The optical and resistivity properties of Pd/Co films are investigated and related to the interaction of palladium and the underlying cobalt (Co) clusters layer when exposed to hydrogen. Co is used because it is known to absorb hydrogen only on its surface. Co clusters with mean size of 1.8 nm in the gas phase were deposited on sapphire substrates at room temperature (RT), 300 °C, and 500 °C, respectively, and capped by a continuous Pd film. Light transmission and reflection in the visible range were measured during hydrogen exposure at different pressures in consecutive cycles. An unusual change in the transmittance and the resistance during the first hydrogenation cycle of a sample suggests that a reduction of the surface oxide on the Co clusters layer occurs at room temperature. The reduction of the native oxide on Co thin films or bulk does not happen without the Pd capping when exposed to hydrogen under similar conditions.     

Hydrogenation and annealing effect on electrical properties of nanostructured Mg/Mn bilayer thin films

Bilayer Mg/Mn thin films have prepared using thermal evaporation method at pressure 10⁻⁵ torr. Hydrogenation process has been done on pristine and annealed bilayer structure of films at different hydrogen pressure for half an hour. In case of annealed samples partially semiconductor nature is observed and conductivity of films found to decrease with hydrogen pressure and increased with annealing temperature. The XRD analysis shows microcrystalline nature of as-deposited films and after annealing it produce crystalline nature. After hydrogenation an additional peaks of magnesium hydride are also observed that suggesting the presence of hydrogen and hydrogen storage capacity of thin film bilayer structure. Optical band gap of annealed bilayer thin films found to increase with hydrogen pressure. It means hydrogenation process is capable to change bilayer structure from metallic to semi-conducting. The variation in relative resistivity is found nonlinear with time and increases with hydrogen pressure, due to the net effect of hydrogen absorption. Raman spectra show the decrease in intensity of peaks with hydrogen pressure that confirm the presence of hydrogen. Optical photographs are taken in reflection mode that shows a change of color from brown to dark black state with increasing hydrogen gas pressure. This dark black state may be used as solar thermal energy collector because black body is good absorber of heat.     

NiOxHy薄膜电致变色性能的研究

利用直流磁控溅射法，在Ｏ２＋Ｈ２的气氛下制备了ＮｉＯｘＨｙ薄膜，研究了不同氢气含量对薄膜的初始沉积态、漂白态和着色态透光性能的影响，含氢量为８０％时，薄膜的初始沉积态的平均可见光透射比最高。含氢量为６０％时，薄膜的电致变色能力最佳。用获得的ＮｉＯｘＨｙ薄膜制备的反射型全固态电致变色器件的控光范围可达７７％。对ＮｉＯｘＨｙ薄膜三种状态的红外吸收光谱分析表明，ＮｉＯｘＨｙ薄膜的变色机理可用：Ｎｉ（ＯＨ）２（漂白态）＝ＮｉＯＯＨ（着色态）＋Ｈ＋＋ｅ－表示。     

Hydrogen absorption by metallic thin films detected by optical transmittance measurements

The hydrogen absorption by bilayers of Pd/Nb and Pd/Ti, grown on glass substrates, was studied by measuring changes in optical transmittance and reflectance in the visible range (wavelengths between 400 nm and 1000 nm) of the films at hydrogen pressures between 3.99 × 10² and 4.65 × 10⁴ Pa. The electrical resistance of the films was also measured during absorption to correlate with the optical data. All the films were grown by a controlled sputtering technique in high vacuum. Pd films ranging in thickness between 4 nm and 45 nm were also characterized when the films were exposed to a hydrogen pressure. The resistance and transmittance of all the Pd samples increased with the uptake of hydrogen until saturation occurred. For Pd/Ti bilayers, fast uptake of hydrogen was deduced from a transmittance increase, indicating hydrogen absorption in the Ti layer. In the case of the Pd/Nb bilayer, a decrease in transmittance was observed, indicating that hydrogen was not absorbed in the Nb layer. The transmittance decrease could be explained by a reduction of Nb native oxide by the hydrogen at the surface.     

The nature of the photochromism arising in the nanosized MoO3 films

We carried out the experiments which show unambiguously that not one, as it was commonly accepted, but three optical absorption bands corresponding to different color centers arise in the nanosized MoO₃ films due to hydrogen atoms. Varying the conditions for the photoinjection of hydrogen into the MoO₃ films, it is possible to change drastically the correlation between the concentrations of different arising centers, which in turn yields the great difference in the optical absorption spectra. Our findings are crucial for understanding of the nature of electrochromism and photochromism not only in MoO₃ films but also in the series of the transition metal oxide films.     

ZnO thin films deposited by RF magnetron sputtering: Effects of the annealing and atmosphere conditions on the photocatalytic hydrogen production

This work studied the effect of different annealing conditions of ZnO thin films grown by RF magnetron sputtering and their application as photocatalysts for hydrogen production without any sacrificial agent or co-catalyst. ZnO films were annealed in air, nitrogen, and argon atmospheres to study the effect of their physical properties in the photocatalytic activity. ZnO films showed high crystallinity and optical transparence of around 75–90% after annealing. Changes in composition and optical properties of the ZnO films were studied by x-ray photoelectron spectroscopy (XPS) and ellipsometry spectroscopy (SE), and results were correlated with the photocatalytic performance in hydrogen production. The highest photocatalytic hydrogen production was obtained with the ZnO thin film annealed in an air atmosphere with a result of 76 μmol.     

Sprayed CuInS2 thin films for solar cells: The effect of solution composition and post-deposition treatments

CuInS₂ thin films were prepared by spray pyrolysis from solutions with different compositions. Etching in KCN solution and thermal treatments in vacuum and hydrogen were applied to as-deposited films. KCN etching removes conductive copper sulfide from the surface of Cu-rich films but has no effect on matrix composition. Vacuum annealing at 500°C and hydrogen treatment at 400–500°C purifies the films, prepared from the solutions with the Cu/In=1, from secondary phases, reduces chlorine content and improves crystallinity. Vacuum annealing results in n-type films due to the formation of In₂O₃ phase. Treatment in hydrogen reduces oxygen-containing residues and results in p-type CuInS₂ films with resistivity close to 10 Ω cm.     

Characterization of gas sensing behavior of multi walled carbon nanotube polyaniline composite films

In this paper, we report multiwall carbon nanotube (MWNT) doped polyaniline (PANI) composite thin films for hydrogen gas sensing applications. PANI was synthesized by in- situ chemical oxidative polymerization of aniline using ammonium persulfate in acidic medium.This emeraldine salt form of PANI was converted into emeraldine PANI base and doped with MWNT (4 wt%) in presence of champhor sulfonic acid (CSA) by solution mixing method. The MWNT/PANI composite films were deposited onto ITO coated glass substrate using spin cast technique. The gas sensitivity of these composite films was evaluated by measuring the change in electrical resistance of composite films in presence of hydrogen gas for different pressures at room tempeature. It is observed that the MWNT/PANI composite film shows a higher sensitivity in comparison to pure PANI and it decreases on increasing hydrogen gas pressure. The composite films have also been characterized by X-ray diffraction (XRD) analysis and Atomic force microscopy (AFM).     

Studies on microstructure of silicon thin films and its effect on solar cells

Intrinsic microcrystalline silicon films have been deposited at high-power–high-pressure regime. Effects of pressure and hydrogen dilution on the microstructures of the films have been investigated. Crystalline size decreases at high pressure although the deposition rate increases up to 10 Å/s. Microstructure depends sensitively on pressure as well as on hydrogen dilution of silane. Single junction solar cells have been fabricated with Si films having different degrees of crystallinity and grain size and the performances have been studied.     

Thermodynamics, kinetics and microstructural evolution during hydrogenation of iron-doped magnesium thin films

New results are reported suggesting that with appropriate levels of Fe doping Mg can rapidly and reversibly absorb up to 7 mass fraction (%) hydrogen at moderate temperatures and pressures useful for hydrogen storage applications. Hydrogenation kinetics and thermodynamics of Mg-4Fe at.% (+/− 1 at.%) thin films capped with Pd at temperatures ranging from 363 K to 423 K were studied by a number of different methods: in situ infrared imaging, volumetric pressure-composition isotherm (PCI) measurements, and ex situ X-ray diffraction and transmission electron microscopy. The hydride growth rate was determined by utilizing wedge-shaped films and infrared imaging; assuming formation of a continuous hydride layer, the growth rate was found to range from ≈3.8 nm/s at lower temperature to ≈36.7 nm/s at higher temperature. The apparent activation energy of the thermally activated hydrogenation kinetics was measured to be 56 kJ/mol; this value suggests that at low temperatures hydrogen diffusion along grain boundaries of MgH₂ is the mechanism controlling the hydride layer growth.Reproducible PCI measurements of 600 nm-thick uniform films showed a pressure plateau and large hysteresis; from these measurements enthalpy and entropy were estimated as 66.9 kJ/mol and 0.102 kJ/(mol∗K), respectively, which are both slightly less than values for pure magnesium (as either films or bulk). The extremely rapid and cyclable kinetics of Mg-4 at.% Fe films suggest that properly grown Mg-Fe powders of 1-2 μm size can be fully charged with hydrogen within 1 min at temperature near 150 °C (423 K), with possible practical hydrogen storage applications.     

Structure of PECVD Si:H films for solar cell applications

The structure of undoped SiH films and solar cells deposited under different hydrogen concentration and substrate temperatures were studied. The characterization techniques used were XRD, Raman spectroscopy, TEM, optical absorption, and hydrogen effusion. The high concentration films were amorphous in the as-deposited state but crystallized upon annealing at 700°C. Middle and low concentration films were nanocrystalline (nc) and remained nc up to 800°C annealing. A theoretical explanation is given for the stability of these films. Such films, on glass substrates, had optical absorption spectra close to those of amorphous material. The solar cell samples, showed some nc morphology in all-concentration states.     

MgH2 thin films deposited by one-step reactive plasma sputtering

The morphology, crystal structure, hydrogen content, and sorption properties of magnesium hydride thin films prepared by reactive plasma-assisted sputter deposition were investigated. Few micrometers-thick films were deposited on Si and SiO₂/Si substrates, at low pressure (0.4 Pa) and close to room temperature using (Ar + H₂) plasma with H₂ fraction in the range 15–70%. The microstructure and hydrogen content of the films are closely related to the surface temperature and hydrogen partial pressure during the deposition process. Operating in pulsed-plasma mode allows the hydrogenation rate of the MgH₂ thin film to top up to 98%, thereby producing a nearly fully hydrogenated film in a single-step process. The positive effect of the pulsed process is explained by the significant decrease in the whole energy flux incident on the surface and the favourable impact of the transient process for the rearrangement/relaxation of the materials. As for the hydrogen storage properties, desorption experiments and cycling of the films show the destabilizing effect of Mg₂Si formation at the interface between the film and the Si substrate resulting in a drastically increased desorption kinetics compared to less reactive SiO₂ substrate. However, the reaction is regrettably not reversible upon hydrogenation and the hydrogen storage capacity is consequently reduced upon cycling. Nevertheless, the deposition process carried out on inert substrates would offer true potential for reversible storage. Finally, our experimental results, which show the possibility to preferentially grow the metastable medium pressure γ-MgH₂ phase, open possibilities for the synthesis of more complex metastable phases such as magnesium-based ternary compounds.     

The dependence of anti-corrosion behaviors of iron sulfide films on different reactants

As the product of hydrogen sulfide corrosion, iron sulfide compound can be used as hydrogen permeation barrier. In this study, chemical vapor deposition was used to synthesize iron sulfide films on the surface of X80 steel with different sulfur sources, and the effects of different sulfur sources on the products were studied. The film was characterized by its morphology, composition and performance. The results show that the hydrogen resistance of our product and the bonding degree between the substrate and the film are strongly relevant with the compactness of the films. When we used TBDS as sulfur source, the highest impendance and the smallest hydrogen permeation current (0.53 μA/cm²) was obtained, which is due to the better adhesion and dense structure compare to the other two sulfur sourses.     

Correlation among hydrogenation,magnetoelastic coupling,magnetic anisotropy,and magnetoresistance in magnetostrictive,hydrogen-absorbing palladium-cobalt alloy films for hydrogen sensing

Hydrogen-absorbing magnetic alloy films, such as palladium-cobalt (PdCo) alloy films, are expected to play a significant role in the next generation of hydrogen sensors. However, effects of hydrogenation on such films are very complex, since these alloys show strong spin-orbit interaction, i.e., strong magnetoelastic coupling. Accordingly, we conducted integrated research on the hydrogenation, magnetoelastic coupling, magnetism, and galvanomagnetic effect of PdCo alloy films having different magnetic anisotropies of longitudinal and perpendicular magnetic anisotropies. As a result, it was revealed that the stress in the film determines its magnetic anisotropy. The magnetoresistance curves of films, consisting of ordinal and anisotropic magnetoresistance effects, correspond well to the magnetization-magnetic field curves. Hydrogenation results in the compressive stress and decreased magnetostriction, which both have a negative influence on the perpendicular magnetic anisotropy energy of the films. Moreover, the influence is observed also in ordinal and anisotropic magnetoresistances. In addition, the increases in coercivity and electronic resistivity due to the incorporated hydrogen atoms (and related defects) are detected. The results are summarized in a correlation diagram, which shows that hydrogen-absorbing magnetic alloy films are very suitable for use in hydrogen sensors—the films can detect hydrogen via various methods such as magnetic anisotropy, galvanomagnetic effect, coercivity, and resistivity.     

Cyclic charging and discharging of obliquely deposited FeTi thin films

The charging of FeTi thin films with hydrogen of 1000 Å thickness deposited at different angles (θ = 0°, 30°, 45°, 60° and 75°) was carried out at 1 atm hydrogen pressure and at room temperature. The discharging was carried out at 10⁻⁵ Torr pressure by heating. The resistance of FeTi films on charging with hydrogen increases and decreases on discharging. The change in resistance in subsequent charging cycles was found to decrease, indicating saturation stage.