Studies on interdiffusion in Pd/Mg/Si films: Towards improved cyclic stability in hydrogen storage

The paper reports the diffusion coefficients of grain boundary diffusion and grain boundary assisted lattice diffusion of Pd in Mg in Pd/Mg/Si system, a useful material for hydrogen storage, at 473 K in vacuum. The grain boundary diffusivity is measured by Whipple model and grain boundary assisted lattice diffusivity by plateau rise method using Pd depth profiles constructed by Rutherford backscattering spectrometry. It is established that grain boundary diffusivities are about six orders of magnitude faster than lattice diffusivities. Fine grained microstructure of Pd film, high abundance of defects in Mg film and higher stability associated with Pd-Mg intermetallics are responsible for the diffusion of Pd into grain boundaries and subsequently in the interiors of Mg. Besides the indiffusion of Pd, annealing also brings about an outdiffusion of Mg into Pd film. Examination by nuclear reaction analysis involving ²⁴Mg(p,p′γ)²⁴Mg resonance reaction shows the occurrence of Mg outdiffusion. Minimization of surface energy is presumably the driving force of the process. In addition to Pd/Mg interface, diffusion occurs across Mg/Si (substrate) interface as well on increasing the annealing temperature above 473 K. These studies show that dehydrogenation of films accomplished by vacuum annealing should be limited to temperatures less than 473 K to minimize the loss of surface Pd, the catalyst of the hydrogen absorption-desorption process and Mg, the hydrogen storing element, by way of interfacial reactions.     

High-throughput concept for tailoring switchable mirrors

The optical properties, the switching kinetics and the lifetime of hydrogen switchable mirrors based on Mg-Ni alloys are determined with particular regard to the composition of the optically active metal-hydride layer in combination with the thickness of the catalytic capping layer. For this, a high-throughput experiment is introduced. The switching kinetics and the reversibility of switchable mirrors are strongly thickness dependent, though the details hinge on the fine structure of the clustered capping layer. Therefore, the kinetics is correlated with the surface structures of Pd on Mg_yNi_1−y as investigated by scanning tunneling microscopy. The results are explained by the so-called strong metal-support interaction (SMSI) state, characterized by a complete encapsulation of the capping layer clusters by oxidized species originating from the support. The SMSI-effect is less important with increasing Pd-layer thickness, and is suppressed by a good wetting of the Pd-clusters on the optically active film. This explains the critical thickness for the catalyzed hydrogen uptake observed in many switchable mirror systems. Moreover, the degradation of the kinetics during cycling is found to depend on the Pd-layer thickness and on the gas environment. Only films, covered with at least 15 nm Pd, show small degradation caused by the SMSI-effect. The SMSI-effect is partly reversible: after changing the gas environment from hydrogen to oxygen, the oxide on the Pd-clusters can be partly removed.     

Formation, stability and CO adsorption properties of PdAg/Pd(1 1 1) surface alloys

The formation, stability and CO adsorption properties of PdAg/Pd(1 1 1) surface alloys were investigated by X-ray photoelectron spectroscopy (XPS) and by adsorption of CO probe molecules, which was characterized by temperature-programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). The PdAg/Pd(1 1 1) surface alloys were prepared by annealing (partly) Ag film covered Pd(1 1 1) surfaces, where the Ag films were deposited at room temperature. Surface alloy formation leads to a modification of the electronic properties, evidenced by core-level shifts (CLSs) of both the Pd(3d) and Ag(3d) signal, with the extent of the CLSs depending on both initial Ag coverage and annealing temperature. The role of Ag pre-coverage and annealing temperature on surface alloy formation is elucidated. For a monolayer Ag covered Pd(1 1 1) surface, surface alloy formation starts at ∼450 K, and the resulting surface alloy is stable upon annealing at temperatures between 600 and 800 K. CO TPD and HREELS measurements demonstrate that at 120 K CO is exclusively adsorbed on Pd surface atoms/Pd sites of the bimetallic surfaces, and that the CO adsorption behavior is dominated by geometric ensemble effects, with adsorption on threefold hollow Pd₃ sites being more stable than on Pd₂ bridge sites and finally Pd₁ a-top sites.     

Growth of epitaxial thin Pd(1 1 1) films on Pt(1 1 1) and oxygen-terminated FeO(1 1 1) surfaces

Thin Pd films (1-10 monolayers, ML) were deposited at 35 K on a Pt(1 1 1) single crystal and on an oxygen-terminated FeO(1 1 1) monolayer supported on Pt(1 1 1). Low energy electron diffraction, Auger electron spectroscopy, and Kr and CO temperature programmed desorption techniques were used to investigate the annealing induced changes in the film surface morphology. For growth on Pt(1 1 1), the films order upon annealing to 500 K and form epitaxial Pd(1 1 1). Further annealing above 900 K results in Pd diffusion into the Pt(1 1 1) bulk and Pt-Pd alloy formation. Chemisorption of CO shows that even the first ordered monolayer of Pd on Pt(1 1 1) has adsorption properties identical to bulk Pd(1 1 1). Similar experiments conducted on FeO(1 1 1) indicate that 500 K annealing of a 10 ML thick Pd deposit also yields ordered Pd(1 1 1). In contrast, annealing of 1 and 3 ML thick Pd films did not result in formation of continuous Pd(1 1 1). We speculate that for these thinner films Pd diffuses underneath the FeO(1 1 1).     

Oxidation and annealing of thin FeTi layers covered with Pd

The hydrogen storage material FeTi has the disadvantage to lose its sorption capacity in contact with impurities such as O₂ and H₂O. A possibility to overcome this problem is to coat it with an anti-corrosive layer which is permeable for hydrogen. In this study we prepared FeTi layers covered with a (4 or 20 nm) thin Pd layer. We used ion beam and sputter profiling techniques, X-ray photoelectron spectrometry and scanning probe techniques to investigate the response of these bi-layers upon annealing up to 300°C in vacuum, air and 10⁻⁵ mbar O₂. The layered structure remains intact up to 150°C. At 200°C in air and O₂, Fe and (some) Ti move towards the Pd surface where they form oxide regions. At higher temperatures thicker oxide regions, presumably along the Pd grains, are formed. These processes are more pronounced for the case of 4 nm Pd. A model is presented to explain the observed phenomena. We conclude that up to 150°C 4 nm of Pd is sufficient to act as a protective layer. For a temperature of 200°C, 20 nm Pd may still provide sufficient protection against oxidation.     

Hydrogenation of Pd-capped Mg thin films prepared by DC magnetron sputtering

Structural and optical properties of pure Mg thin film coated with Pd have been investigated. Pd-capped Mg thin films had been prepared by DC magnetron sputtering. This work presents an ex situ study on hydrogenation and dehydrogenation kinetics of Pd/Mg films at different conditions using XRD, AFM and optical spectrophotometer. We have succeeded to load thin films of Mg to MgH₂ at normal temperature and normal pressure of hydrogen gas. In hydrogenation, α-MgH₂ phase of magnesium hydride was observed in hydrogenated films at 200 °C and γ-MgH₂ at 250 °C respectively. The desorption kinetics in vacuum also revealed the phase transformation α-MgH₂ to γ-MgH₂. A reflectance change was observed in hydrogenated films in comparison of as deposited thin film. Hydrogenated (H loaded) samples were observed partially transparent in comparison of as deposited.     

Adsorption, reaction and desorption of hydrogen on modified Pd(1 1 1) surfaces

The interaction of hydrogen (deuterium) with different modified Pd(1 1 1) surfaces has been investigated. The focus was put on the energy and angel dependence of the desorbing molecules from oxygen covered, potassium covered and vanadium oxide covered surfaces. Conventional adsorption/desorption as well as permeation/desorption experiments were performed. For the oxygen covered surface optimum reaction rates for water production and the energy distribution of the reaction products were determined, both for the reaction of oxygen with molecular hydrogen as well as with atomic hydrogen. Potassium on the surface enhances the activation barrier for hydrogen adsorption resulting in a hyper-thermal desorption flux and a forward focused angular distribution of desorption. Permeation/desorption of deuterium from ultra-thin vanadium oxide films yield mainly thermalized desorbing molecules or slightly hyper-thermal, depending on the oxidation state of the surface oxide.     

Temperature- and coverage-dependent evolution of the Au/Pd(1 1 0) surface structure

The morphology and the atomic scale structure of thin gold films (up to 2.5 ML) on Pd(1 1 0) were studied by means of scanning tunneling microscopy and surface X-ray diffraction. At room temperature the films exhibit a multilayer growth mode accompanied by the formation of highly anisotropic islands. Annealing above 500 K significantly increases the smoothness of the gold films, which are in registry with the substrate. Above a critical threshold of two monolayers a (1 × 2) missing-row reconstructed film is found. This reconstructed surface is well ordered after annealing at temperatures above 580 K. The specific gold film morphology is envisaged as a way to relax the strain caused by the mismatch between gold and palladium.     

ATR-SEIRAS--an approach to probe the reactivity of Pd-modified quasi-single crystal gold film electrodes

Quasi-single crystalline gold films of 20 nm thickness and preferential (1 1 1) orientation on Si hemispheres were modified by controlled potentiostatic deposition of Pd (sub-ML, ML, multi-L) from sulphate and/or chloride-containing electrolyte. The electrochemical properties of these model electrodes were characterised for hydrogen and (hydrogen-) sulphate adsorption as well as for surface oxide formation by cyclic voltammetry. Conditions were developed to fabricate defined and stable Pd monolayers. In situ ATR-SEIRAS (Attenuated Total Reflection Surface Enhanced Infrared Reflection Absorption Spectroscopy) experiments were carried out to describe the electrochemical double layer of Pd modified gold film electrodes in contact with aqueous 0.1 M H₂SO₄ with focus on interfacial water and anion adsorption. Based on an analysis of the non-resonant IR background signal the potential of zero charge is estimated to 0.10-0.20 V (vs. RHE). CO was found to be weakly physisorbed in atop sites on Au(1 1 1-20 nm)/0.1 M H₂SO₄ only in CO saturated electrolyte. CO, deposited on a quasi-single crystal gold film modified with 1 ML Pd, is chemisorbed with preferential occupation of bridge sites and atop positions at step edges. Saturated CO adlayers, as obtained by deposition at 0.10 V, contain isolated water species and are covered by a second layer of hydrogen bonded water. Potentiodynamic SEIRAS experiments of CO electro-oxidation on Pd-modified gold film electrodes demonstrate clearly the existence of a “pre-oxidation” region. They also provide spectroscopic evidence that isolated water and weakly hydrogen bonded water are consumed during the reaction and that atop CO on defect sites is a preferential reactant. The simultaneous in situ monitoring of the potential- and time-dependent evolution of characteristic vibrational modes in the OH- and CO-stretching regions are in agreement with the Gilman (“reactant pair”) mechanism of CO oxidation.     

Hydrogen Permeability of a Foil of Pd–Ag Alloy Modified with a Nanoporous Palladium Coating

Thin films with the composition Pd–23% Ag are obtained via magnetron sputtering. The magnetron sputtering of Pd–50% Zn films with subsequent diffusion annealing and etching of the active component is used to modify the surfaces of palladium–silver films to improve their hydrogen permeability. Modifying the surfaces of the resulting Pd–Ag films using a nanoporous palladium coating with a predominant distribution of particles ranging from 0 to 50 nm allows a hydrogen flux density of up to 0.4 mmol s^?1 m^?2 to be achieved for sufficiently thin palladium membranes (<10 μm) under conditions of low temperature (<90°C) and pressure (<0.6 MPa). Experimental evidence is gathered that under these conditions, the velocity of hydrogen transport is limited by dissociative–associative processes at membrane boundaries and can be greatly (by an order of magnitude) increased, due to acceleration of the limiting stage of the process via the formation of a palladium nanoporous coating on the film’s surface.     

H uptake kinetics of FeTi films coated with Ni

The H₂ storage material FeTi can be fully charged through a Ni coating in the course of hours. A mild activation, either by long H₂ exposure at RT, or annealing in 35 mbar H₂ at 150 °C, leads to a full uptake in minutes. The enhancement of the uptake rate after activation is caused by a reduction of the Ni-oxide surface. Ni does not limit the hydrogen uptake rate once the surface is clean. However, the Ni surface is sensitive to contamination, as for instance by CO adsorbed from air.     

Structural changes of diamond-like carbon films due to atomic hydrogen exposure during annealing

We have deposited diamond-like carbon (DLC) films by radio-frequency magnetron sputtering, and have annealed the films under various conditions to investigate the effects of annealing on the structural properties by visible Raman spectroscopy, X-ray photoelectron spectroscopy and atomic force microscopy. The structural ordering of hydrogenated DLC films occurs during annealing below 400 °C in a vacuum and a hydrogen gas atmosphere, while unhydrogenated DLC films are not ordered during annealing even at 700 °C. On the other hand, the ordering and the decrease of the sp³ content are observed for both the films after annealing under an atomic hydrogen exposure. The ordering progresses as the annealing temperature and time are increased. The reduction of the film thickness after annealing is suppressed with increasing annealing temperature. The results suggest that both the preferential etching by atomic hydrogen and the hydrogen evolution encourage the structural changes under an atomic hydrogen exposure.     

Effects of rapid thermal annealing on structural, magnetic and optical properties of Ni-doped ZnO thin films

XPS depth profiles were used to investigate the effects of rapid thermal annealing under varying conditions on the structural, magnetic and optical properties of Ni-doped ZnO thin films. Oxidization of metallic Ni from its metallic state to two-valence oxidation state occurred in the film annealed in air at 600 °C, while reduction of Ni²⁺ from its two-valence oxidation state to metallic state occurred in the film annealed in Ar at 600 and 800 °C. In addition, there appeared to be significant diffusion of Ni from the bottom to the top surface of the film during annealing in Ar at 800 °C. Both as-deposited and annealed thin films displayed obvious room temperature ferromagnetism (RTFM) which was from metallic Ni, Ni²⁺ or both with two distinct mechanisms. Furthermore, a significant improvement in saturation magnetization (M_s) in the films was observed after annealing in air (M_s = 0.036 μ_B/Ni) or Ar (M_s = 0.033 μ_B/Ni) at 600 °C compared to that in as-deposited film (M_s = 0.017 μ_B/Ni). An even higher M_s value was observed in the film annealed in Ar at 800 °C (M_s = 0.055 μ_B/Ni) compared to that at 600 °C mainly due to the diffusion of Ni. The ultraviolet emission of the Ni-doped ZnO thin film was restored during annealing in Ar at 800 °C, which was also attributed to the diffusion of Ni.     

Pd, Co and Co-Pd clusters on the ordered alumina film on NiAl(1 1 0): Contact angle, surface structure and composition

We have investigated the structure and morphology of Co and Pd clusters grown at room temperature on an alumina film on NiAl(1 1 0) by scanning tunneling microscopy, low energy ion scattering and Auger electron spectroscopy. We have also studied the clusters after annealing to 300 °C and Pd clusters deposited at 300 °C. Mixed Co-Pd clusters obtained by sequential deposition at room temperature were also studied. Pure Co deposited at room temperature forms a single type of clusters, most or all of them with close-packed planes parallel to the oxide surface. Their shape can be approximated by truncated spheres with a high contact angle of 115-125°. These clusters are stable upon annealing up to 300 °C.Pd clusters deposited at room temperature grow in two different modes. At the reflection domain boundaries the clusters grow in their thermodynamically favorable shape. The clusters do not have a single crystallographic orientation and their shape can be approximated by a truncated sphere with a high contact angle of about 110°, especially at very low coverages (below 0.05 ML). At the antiphase domain boundaries, the Pd clusters grow in (1 1 1) orientation and on some of them small (1 1 1) facets appear at their tops already at low coverages. For higher coverages of Pd, the majority of Pd clusters are rather flat with a large Pd(1 1 1) facet on top. The clusters’ shape at the antiphase domain boundaries differs from the thermodynamically favorable one, due to kinetic limitations, especially at higher coverages. Annealing the Pd clusters to 300 °C leads to re-structuring of these Pd clusters. They transform into higher and more rounded clusters and a thin disordered alumina film is formed on top of the clusters. When Pd is deposited at 300 °C, about 16% of the Pd clusters have a steep slope and rounded tops. The rest of the Pd forms lower clusters, goes subsurface and is covered by a disordered alumina film. When Co and Pd are deposited sequentially, Pd covers the Co clusters forming a shell. The resulting mixed clusters are still truncated spheres with a lowered contact angle. For deposition in the reverse order (first Pd and then Co) we found that Co forms an alloy with Pd already at room temperature.     

Effect of annealing on structural, optical and electrical properties of nanostructured Ge thin films

Ge thin films with a thickness of about 110 nm have been deposited by electron beam evaporation of 99.999% pure Ge powder and annealed in air at 100-500 °C for 2 h. Their optical, electrical and structural properties were studied as a function of annealing temperature. The films are amorphous below an annealing temperature of 400 °C as confirmed by XRD, FESEM and AFM. The films annealed at 400 and 450 °C exhibit X-ray diffraction pattern of Ge with cubic-F structure. The Raman spectrum of the as-deposited film exhibits peak at 298 cm⁻¹, which is left-shifted as compared to that for bulk Ge (i.e. 302 cm⁻¹), indicating nanostructure and quantum confinement in the as-deposited film. The Raman peak shifts further towards lower wavenumbers with annealing temperature. Optical band gap energy of amorphous Ge films changes from 1.1 eV with a substantial increase to ∼1.35 eV on crystallization at 400 and 450 °C and with an abrupt rise to 4.14 eV due to oxidation. The oxidation of Ge has been confirmed by FTIR analysis. The quantum confinement effects cause tailoring of optical band gap energy of Ge thin films making them better absorber of photons for their applications in photo-detectors and solar cells. XRD, FESEM and AFM suggest that the deposited Ge films are composed of nanoparticles in the range of 8-20 nm. The initial surface RMS roughness measured with AFM is 9.56 nm which rises to 12.25 nm with the increase of annealing temperature in the amorphous phase, but reduces to 6.57 nm due to orderedness of the atoms at the surface when crystallization takes place. Electrical resistivity measured as a function of annealing temperature is found to reduce from 460 to 240 Ω-cm in the amorphous phase but drops suddenly to 250 Ω-cm with crystallization at 450 °C. The film shows a steep rise in resistivity to about 22.7 KΩ-cm at 500 °C due to oxidation. RMS roughness and resistivity show almost opposite trends with annealing in the amorphous phase.     

Oxygen diffusion through thin Pt films on Si(100)

We have investigated the diffusion of oxygen through evaporated platinum films on Si(100) upon exposure to air using substrates covered with Pt films of spatially and continuously varying thickness (0–500 Å). Film compositions and morphologies before and after silicidation were characterized by modified crater edge profiling using scanning Auger microscopy, energy-dispersive X-ray microanalysis, scanning tunneling microscopy, and transmission electron microscopy. We find that oxygen diffuses through a Pt layer of up to 170 Å forming an oxide at the interface. In this thickness range, silicide formation during annealing is inhibited and is eventually stopped by the development of a continuous oxide layer. Since the platinum film consists of a continuous layer of nanometer-size crystallites, grain boundary diffusion of oxygen is the most probable way for oxygen incorporation. The diffusion constant is of the order of 10^–19 cm²/s with the precise value depending on the film morphology.     

Solid phase epitaxy of Ge films on CaF2/Si(1 1 1)

At room temperature deposited Ge films (thickness < 3 nm) homogeneously wet CaF₂/Si(1 1 1). The films are crystalline but exhibit granular structure. The grain size decreases with increasing film thickness. The quality of the homogeneous films is improved by annealing up to 200 °C. Ge films break up into islands if higher annealing temperatures are used as demonstrated combining spot profile analysis low energy electron diffraction (SPA-LEED) with auger electron spectroscopy (AES). Annealing up to 600 °C reduces the lateral size of the Ge islands while the surface fraction covered by Ge islands is constant. The CaF₂ film is decomposed if higher annealing temperatures are used. This effect is probably due to the formation of GeF_x complexes which desorb at these temperatures.     

The effect of heating rate on the structural and electrical properties of sol-gel derived Al-doped ZnO films

Al-doped ZnO (AZO) films are prepared by sol-gel method with a proper annealing procedure. For the first time, we find that the heating rate which is normally neglected during the post annealing process plays a significant role in improving AZO properties. The AZO film with nanorod structure is obtained by using a rapid heating rate. The AZO nanorods can provide a faster conduction pathway for charge transport due to the high crystal quality and thus enhance the conductivity of the film significantly. After hydrogen treatment, the AZO nanorod film exhibits a minimum resistivity of 1.4 × 10⁻³ Ω cm. This approach to the preparation of AZO nanorods by a simple rapid annealing process may be helpful for the development of sol-gel-derived TCO films.     

Influence of films thickness and structure on the photo-response of ZnO films

ZnO thin films were grown using Successive Ionic Layer Adsorption and Reaction (SILAR) method on glass substrates at room temperature. Annealing temperatures and film thickness effect on the structural, morphological, optical and electrical properties of the films were studied. For this as-deposited films were annealed at 200, 300, 400 and 500 °C for 30 min in oxygen atmosphere. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies showed that the films are covered well with glass substrates and have good polycrystalline structure and crystalline levels. The film thickness effect on band gap values was investigated and band gap values were found to be within the range of 3.49-3.19 eV. The annealing temperature and light effect on electrical properties of the films were investigated and it was found that the current increased with increasing light intensity. The resistivity values were found as 10⁵ Ω-cm for as-deposited films from electrical measurements. The resistivity decreased decuple with annealing temperature and decreased centuple with light emission for annealed films.     

Effect of substrate temperature on the surface and interface oxidation of NiTi thin films

NiTi shape memory alloy thin films are deposited on pure Cu substrate at substrate ambient temperatures of 300 °C and 450 °C. The surface and interface oxidation of NiTi thin films are characterized by X-ray photoelectron spectroscopy (XPS). After a subsequent annealing treatment the crystallization behavior of the films deposited on substrate at different temperatures is studied by X-ray diffraction (XRD). The effects of substrate temperature on the surface and interface oxidation of NiTi thin films are investigated. In the film surface this is an oxide layer composed of TiO₂. The Ni atom has not been detected on surface. In the film/substrate interface there is an oxide layer with a mixture Ti₂O₃ and NiO in the films deposited at substrate temperatures 300 °C and 450 °C. In the films deposited at ambient temperature, the interface layer contains Ti suboxides (TiO) and metallic Ni.