Hydrogen absorption by metallic thin films detected by optical transmittance measurements
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(2) 10614. i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 0 6 1 3 e1 0 6 1 9. [5e7], including its electrical resistivity [8] and optical permittivity [9]. Both of these effects have been considered as possible ways of developing hydrogen sensing technology. The absorption of hydrogen in light-weight materials for hydrogen storage applications has been recently studied using optical methods [10,11]. Optical measurements rely on changes in the complex index of refraction when the sample absorbs hydrogen [11,12] or when a surface oxide is reduced by hydrogen [13]. Unlike electrical resistance measurements, optical measurements allow the absorption of hydrogen process to be easily distinguished from surface oxide reduction. Optical measurements are also simpler and quicker to perform than X-ray or neutron scattering measurements. In the case of Pd, it is known that the generation of subsurface hydrogen atoms provides an important link between the chemisorbed surface hydrogen and the dissolved bulk species [14,15]. The interaction of molecular hydrogen with other metallic surfaces such as Ni, Fe, Nb, Ta and V has been extensively studied for a number of years [5]. The absorption of hydrogen by these metals is negligible under ambient conditions, firstly because the dissociation of the hydrogen molecule on the surface is blocked by a native oxide, which is usually difficult to remove, and secondly, in cases such as Fe and Ni the dissociation of hydrogen is endothermal and the diffusion of atomic hydrogen into the lattice is thermodynamically unfavourable. The study of hydrogen adsorption on the sputter-cleaned metal surfaces, conducted under high vacuum conditions, has been conducted over the last 40 years. The results of these studies have established that hydrogen remains chemisorbed mainly on the surface without diffusing into the bulk in many transition metals [16]. According to the studies of Alefeld and Volkl [5], hydrogen diffusion in Ni is three orders of magnitude slower than in pure Pd. On the other hand, in the case of Nb and Ta, hydrogen diffusion is comparable to the value obtained in Pd at room temperature. At 700 K, the diffusion of hydrogen in Nb is one order of magnitude faster than in Pd [17]. Moreover, in the case of Ti, it is known that this metal has great affinity for gaseous elements such as hydrogen [18,19]. Thus, Nb, Ta and Ti are potential candidates to be part of hydrogen storage materials once the problem of molecular hydrogen dissociation at their surfaces is solved. This can be done by capping their surfaces with a thin film of Pd [20,21]. It is well known that this prevents the oxidation of the underlying metal and thus facilitates the hydrogen atom diffusion into the metal. Despite the extensive aforementioned research, it is still unclear whether significant hydrogen absorption can be induced in metallic layers capped with a Pd films at relatively low hydrogen pressures and room temperature conditions, which is a requirement for energy-efficient (i.e., room temperature operation) hydrogen storage materials. Here we explore this issue by using optical measurements to study hydrogen absorption in Nb and Ti films capped with Pd. The absorption of hydrogen by a sample is determined, in most cases, by two factors: (1) an increase in the transmittance of light in the visible range and (2) an increase in the resistance of the films when hydrogen absorption has occurred. When hydrogen absorption does not take place in the film, but instead the oxide of the film is reduced, a decrease in the. optical transmittance is observed. These results are validated by simultaneous electrical resistance measurements.. 2.. Experimental details. 2.1.. Sample preparation. One set of samples consisted of pure Pd and Nb films and Nb films capped with a thin Pd film. Another set of samples consisted of Pd and Ti films and Ti films of different thicknesses capped with a thin film of Pd. All samples were grown on glass substrates at room temperature via magnetron sputter deposition [22]. The glass substrates were cleaned with a standard degreasing technique using acetone, methanol, a rinse with distilled water, and finally a drying step with nitrogen gas. The base pressure of the sputter system was below 1.33 106 Pa. Pd, Ti and Nb targets (>99.9% purity) were used. The films were grown in an Ar gas atmosphere at 332.5 Pa. The thicknesses of the films were monitored with independent quartz crystal balances during growth but their true value was determined by X-ray reflectivity (XRR). All the samples were made in triplicate.. 2.2. films. Thickness and morphology determination of the. XRR of the films was measured with a Bruker D8 Advance diffractometer using CuKa radiation (wavelength of 0.154 nm). The diffraction plots were fitted using the LEPTOS software package [23] to determine thickness and roughness parameters using bulk densities for the elements. In all cases a good fit was found with a roughness of less than 0.6 nm for almost all the films. These values agreed with the 0.5e0.8 nm roughness obtained using atomic force microscopy (AFM). Samples were inspected with scanning electron microscopy (SEM) using a LEO VP1400 system equipped with energy dispersive spectroscopy (EDS). The samples before and after hydrogen absorption were continuous without pin holes and did not present any damage at 5000 magnification. EDS showed the presence of the typical elements in microscope glass (Si, O, Na and Ca) and weak signals of Nb and Pd, for the Nb/Pd bilayers, due to the small thickness of the films.. 2.3.. Optical and resistance measurements. Near-normal incidence reflectance and transmittance measurements were performed in a small cylindrical aluminum vacuum chamber fitted with two transparent quartz windows, a gas/vacuum line and electrical feedthrough used for connections. A schematic diagram of the experimental set-up is shown in Fig. 1. Light from a tungsten halogen lamp was focused on a Triax 180 Jobin Yvon-Horiba monochromator fitted with a 1200 grooves/mm diffraction grating. The beam of light was diverted 2.8 from the center and the incidence angle on the sample was 7 . Light detection was done with Si diodes (PD1, PD2 and PD3 in Fig. 1) connected to a preamplifier and to a lock-in amplifier (Stanford Research Systems). A reference signal for the lock-in was provided by an optical chopper.
(3) i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 0 6 1 3 e1 0 6 1 9. 10615. Fig. 1 e Experimental set-up to measure transmittance and reflectance of visible light through a metallic thin film in a controlled atmosphere chamber.. placed between the light source and the monochromator. The transmittance and reflectance data were not corrected for the Si diode response (PD1 and PD2) depending on changes of the ambient temperature because another Si diode (PD3) of the same characteristics was used to measure the incident intensity to normalize the optical signals. The optical experiments were performed by recording the transmittance and reflectance of the samples initially in vacuum and subsequently in hydrogen atmosphere pressures of up to 4.66 104 Pa. The response of the sample was monitored as a function of time and wavelength until it stabilized for each pressure increment. The wavelength scans were taken in steps of 10 nm from 400 nm to 900 nm, and repeated every 40 or 50 s. The transmittance was measured to within a 5% uncertainty below 400 nm and approximately 1% uncertainty above 500 nm. The resistance of the films was measured with a Keithley resistivity bridge model 580 which allows measurements with 4-point connections. A similar system is described elsewhere [12,13]. As soon as hydrogen was allowed to enter the chamber, optical and electrical data were simultaneously recorded until the signals stopped changing. Data were recorded in some cases for up to 2500 s with the samples at room temperature.. 3.. Results. 3.1.. Pure Pd films. The transmittance of light as a function of wavelength and time for a 4.1 nm Pd film, exposed to 4.39 103 Pa of hydrogen, is shown in Fig. 2. The transmittance is defined as T¼. IT I0. (1). where IT and I0 are the transmitted and incident light intensities, respectively. A dramatic increase in transmittance at all wavelengths was observed when the film absorbed hydrogen. In order to compare the optical data as a function of the final hydrogen pressure, the optical data were analyzed at 600 nm. The transmittance changed from 0.51 to 0.54 at 600 nm when the sample was exposed to hydrogen. The whole saturation process took place within approximately 200 s (each scan was. Fig. 2 e Transmittance of a sample of 4.1 nm Pd on glass as a function of wavelength in hydrogen at 4.4 3 103 Pa. Time increases from the bottom line- each scan takes about 50 s. Lines overlapping occurred at hydrogen saturation.. completed in 50 s). The blue line corresponds to the first scan (elapsed time 50 s), the green line to the second scan (elapsed time 100 s) and the red line corresponds to the third scan (elapsed time 150 s). The thick black line corresponds to an elapsed time of 200 s, and thereafter the change was negligible. The hydrogen absorption by Pd was reversible, that is, all of the hydrogen was desorbed from Pd film when the hydrogen gas was removed. It was possible to perform several cycles of absorption and desorption on the same Pd sample. Pd films with different thicknesses were studied in order to obtain an analytical expression that could describe the transmittance as a function of thickness with and without hydrogen absorption in order to determine if Nb or Ti under layer films absorb hydrogen in Pd/Nb and Pd/Ti bilayers. Since the relative transmittance change was the same for all wavelengths between 450 and 1000 nm (see Fig. 2), the data taken at 600 nm as a function of hydrogen pressure was used to analyze the dependence on hydrogen absorption. A plot of relative transmittance, resistance and reflectance changes for a Pd sample 9.1 nm thick as a function of hydrogen pressure is displayed in Fig. 3(a).The relative change in transmittance, defined as DT TðPH Þ Tð0Þ h T Tð0Þ. (2). where T(PH) is the transmittance of the Pd film loaded with hydrogen and T(0) is the transmittance of the Pd film in vacuum. The relative resistance (DR) and the relative reflectance (Dr) are defined in the same way as Eq. (2). The transmittance and resistance increased with hydrogen pressure while the reflectance decreased. Clearly all three quantities are correlated. The data were acquired by filling the aluminum chamber with a fixed hydrogen pressure and then recording the transmittance and reflectance after waiting 5 min for the signals to stabilize. The hydrogen pressure was then increased to the next value and the process was repeated. The change in.
(4) 10616. i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 0 6 1 3 e1 0 6 1 9. sample in vacuum we obtained T0 ¼ 0.70 0.02 and b ¼ 0.085 0.002 nm1, while for the sample in hydrogen T0 ¼ 0.64 0.05 and b ¼ 0.067 0.004 nm1. The fittings obtained (with correlation coefficients of 0.999 for measurements in vaccum and 0.997 for measurements in hydrogen) are not valid for film thicknesses smaller than 4 nm. For Pd films thinner than 4 nm there was no measurable change in transmittance when the Pd film was exposed to hydrogen.. a. 3.2.. A bare Nb film 8.2 nm thick, exposed to 4.4 103 Pa of hydrogen, showed a reduction in transmittance of approximately 0.4% at 600 nm. A larger change of 4.8% was observed when the hydrogen pressure was increased to 9.8 103 Pa. As shown in Fig. 4, the transmittance decreased across the entire spectrum during 2500 s, after which the transmittance remained constant. In contrast to the case of pure Pd, the transmittance decreased for the Nb sample. A possible explanation for this behavior is that the native oxide of Nb was reduced with hydrogen and no hydrogen absorption occurred in the film. This is the reverse of removal of hydrogen from Nb using oxygen gas [18] which confirms the findings that this reaction takes place at room temperature.. b. 3.3.. Fig. 3 e (a) Relative changes in transmittance (triangles), resistance (squares) and reflectance (circles) for the 9.4 nm Pd/glass sample as a function of hydrogen pressure. The transmittance and reflectance were measured at a wavelength of 600 nm. (b) Transmittance of pure Pd films deposited on glass substrates in vacuum (full circles) and exposed to 8.0 3 103 Pa of hydrogen pressure (open circles). The solid and dashed lines are fits to Eqn. (3).. reflectance depends strongly on the roughness of the films and substrates [24] and thus might not be a good indication that hydrogen absorption is occurring. Moreover, the transmittance depended exponentially on the thickness due to absorbance in the metallic film (Eq. (3)), and therefore changes in the transmittance are directly proportional to changes in the absorbance. Therefore, in this work we focus on the transmittance measurements. A summary of transmittance values of the Pd films as a function of thickness and after being exposed to hydrogen is shown in Fig. 3(b). The transmittance of the Pd films in vacuum decreased exponentially with the thickness of the films as expected. The transmittance of the Pd films loaded with hydrogen presented the same type of exponential decay but shifted to the right side of the plot. Both curves can be fitted to an exponential decay of the form TPd ¼ T0 ebd. Nb films. (3). where T0 is the transmittance for zero thickness, b is an absorption coefficient, and d is the film thickness. For the. Nb films capped with Pd. A 14 nm Nb film capped with 6.5 nm Pd deposited on glass was studied with the same technique. The transmittance of light as a function of wavelength and time was obtained for a hydrogen pressure of 4.8 103 Pa. A decrease in transmittance in all wavelengths was observed. The transmittance changed from 0.094 to 0.086, or a 9% decrease, for the 600 nm wavelength in hydrogen. Again, our interpretation is that the Nb oxide formed at the interface between Pd and Nb is reduced by hydrogen. This oxide reduction was also observed in the case of Pd coated Co cluster films [13]. Since water (the. Fig. 4 e Transmittance for a sample of 8.2 nm Nb on glass when exposed to 9.8 3 103 Pa of hydrogen. The transmittance is taken as a function of time during hydrogen exposure. Time increases from the top line to bottom lines- each scan takes about 50 s. Lines overlapping occurred when no more changes are detected..
(5) i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 0 6 1 3 e1 0 6 1 9. 10617. product of the reaction) cannot diffuse through the Pd overlayer, the most plausible explanation is that water gas is released from the edges of the sample given that there is no coating acting as a barrier at the edges. When the hydrogen pressure was increased to 9.7 103 Pa the transmittance at all wavelengths and at 600 nm changed from 0.085 to 0.090 for a 6% increase. The saturation time was w200 s, the same as in the case of pure Pd film. This can be explained by calculating the transmittance of the bilayer with TPdNb ¼ TPd TNb. (4). This is justified because the wavelength of light used is much greater than the thickness of the films. According to Eq. (3), TPd ¼ 0:40 0:02 for d ¼ 6.5 nm, and since TPdNb ¼ 0:085 0:004, TNb ¼ 0:22 0:02 before hydrogen absorption. Similarly, TPdHx ¼ 0:42 0:02 for d ¼ 6.5 nm and since TPdHx Nb ¼ 0:090 0:004, TNb ¼ 0:21 0:02 after hydrogen absorption. Therefore, the 6% increase in the Pd/Nb bilayer is a result of hydrogen absorption in the Pd layer only. Assuming that the Nb-H system is within the regime where Sievert’s Law applies [25], the change in transmittance should have a linear relationship with the square root of the hydrogen pressure. In the best case scenario, Nb would absorb 2% of atomic hydrogen. From Fig. 3, a 25% change in transmittance in the Pd layer corresponds to roughly 50% of atomic H absorption. Thus, a change in transmittance for Nb would correspond to only a 2% change in transmittance, which we were unable to detect. Our results are consistent with a hydrogen depth profile performed at room temperature in a 50 nm thick Nb film exposed to hydrogen at 500 K using the 1 H(15N, ag)12C nuclear resonance reaction, which showed that the absorbed hydrogen is concentrated at a depth smaller than 10 nm [25].. 3.4.. in the Ti film. The final increase scales with the Ti thicknesses since the Pd thickness was kept near constant for the three samples. A plot of relative resistivity change for all the samples is displayed in Fig. 6(b). In this case, the resistance. a. Pure Ti films. Ti films adhered very well to the glass and similar experiments were done with Pd/Ti bilayers. The relative transmittance, resistance, and reflectance changes for a 26.4 nm thick Ti film are displayed in Fig. 5. The transmittance of the sample decreased less than 0.3% up to w4 103 Pa of hydrogen and did not change further in pressures of up to 4.7 104 Pa. The decrease in transmittance can be related to the reduction of the Ti native oxide. There is a marginal increase in the resistance of 1.5%, but the resistance of a metallic film can also increase on the order of 1% when hydrogen is chemisorbed on the surface [26]. To determine that a film absorbs hydrogen, its transmittance and resistance should increase together, as is the case for Pd. We can conclude from these results that the Ti film does not absorb hydrogen under our experimental conditions.. 3.5.. Fig. 5 e Relative changes in transmittance (squares), resistance (triangles) and reflectance (circles) for a sample of 26.4 nm Ti on glass as a function of hydrogen pressure. The lines are guides to the eye.. b. c. Ti films capped with Pd. The relative transmittance changes for several Pd/Ti bilayer samples are shown in Fig. 6(a) and the corresponding plot of relative reflectance change is displayed in Fig. 6(c). The transmittance increased in all the samples while the reflectance decreased. This is an indication of hydrogen absorption. Fig. 6 e Relative change in (a) transmittance, (b) electrical resistance and (c) reflectance of 7.6 nm (squares), 6.5 nm (triangles), and 4.2 nm (circles) thick Ti films with a w2 nm Pd capping layer as a function of hydrogen pressures. The solid curves are guides to the eye..
(6) 10618. i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 0 6 1 3 e1 0 6 1 9. decreased upon hydrogen absorption. At first glance this behavior seems puzzling, but this can be explained by keeping in mind two properties of Ti: 1) Ti easily forms hydrides under the right conditions [27] and 2) the resistivity of Ti hydride is lower than that of pure Ti [28]. Normally, pure Ti must be heated above room temperature to absorb hydrogen and form Ti hydrides and the Ti native oxide must be removed before the hydrogen molecule is dissociated on the Ti surface, thus allowing atomic hydrogen to diffuse into the Ti lattice. In this case, the Pd overlayer dissociates hydrogen at room temperature, and once the hydrogen atoms go through the Pd layer they can easily diffuse into the Ti film and form Ti hydride once the film is saturated with hydrogen.. 4.. Conclusions. We have demonstrated that optical measurements, in particular transmittance measurements, are an effective way of determining the hydrogen absorbance in metallic films. In particular, we have shown that Nb films capped with Pd do not absorb significant amounts of hydrogen; rather, changes in optical transmittance are due to reduction of an interface Nb oxide during the first run of the experiment. Subsequent runs showed that the absorption occurs exclusively in the Pd layer. On the other hand, in Ti films capped with Pd hydrogen absorption and diffusion is enhanced by the Pd overlayer. Although a pure Ti film did not absorb hydrogen, some reduction of native oxide on Ti was observed when the film was exposed to hydrogen. With respect to possible technological applications of these results, hydrogen absorption in thin metallic films results in a hydrogen saturation concentration which occurs at low hydrogen pressures due to the small thickness of the films and light transmission studies are limited to a film thickness not larger than approximately 50 nm. Earlier studies on metallic membranes at the end of the 1980s were done on Pd foils with 100 mm thickness [29]. In these studies, high reactant pressures were used to speed up the permeation of hydrogen through the thick Pd foil. Nowadays, composite membranes are used in order to reduce the Pd film thickness or the thickness of other permselective metals such as Pt. Pd films 4.5 mm thick supported on porous materials are used for membrane applications [3,29] These films are still 90 times thicker than the films used in our studies and higher pressures are needed to increase hydrogen permeability. In order to have a good membrane material, increased permselectivity and good mechanical strength of the material are needed. Some researches claim that this can be obtained by a “surface diffusion mechanism” rather than through “solutionediffusion mechanism” [29]. Our studies might help to improve membrane technology by finding new permselective metals or alloys. For the case of hydrogen storage, the extrapolation of our results to thicker films is possible by taking into account slower absorption kinetics and decreased permeability for thicker films. Nevertheless, practical applications for hydrogen storage should look for a composite system made of a thin metallic film supported on a high surface area material.. These findings open a venue to further research on other metal films capped with thin Pd.. Acknowledgements Funds were provided by FONDECYT 1060634, MECESUP PUC00006 and UCh0205, Chile, and by the US National Science Foundation grant #DMR-0502825. We thank Guerau B. Cabrera and Eric Schiers for helping with the growth of the Pd/Nb and Pd/Ti films. We also thank Manuel Pino for taking the X-ray reflectometry data of the films, Marcela Soto for SEM characterization and Marcos Flores for AFM measurements.. references. [1] Dogan B. Hydrogen storage tank system and materials selection for transport applications. ASME Conference PVP2006-ICPVT-11, Vancouver, Canada, July 23e27, 2006 Conference Proceeding CD, Track: materials and fabrication. Session: materials for hydrogen service, Paper No 93868, p. 1e8. [2] Chen SC, CaryatHung CY, Tu CG, Rei MH. Perturbed hydrogen permeation of a hydrogen mixture e new phenomena in hydrogen permeation in Pd membranes. Int J Hydrogen Energy 2008;33:1880e9. [3] Tosti S. Supported and laminated Pd-based metallic membranes. Int J Hydrogen Energy 2003;28:1445e54. [4] Sakintuna B, Lamari-Darkrim F, Hirscher M. Metal hydride materials for solid hydrogen storage: a review. Int J Hydrogen Energy 2007;32:1121e40. [5] Alefeld G, Volk J, editors. Topics in applied physics: hydrogen in metals I. Berlin: Springer-Verlag; 1978. [6] Auer W, Grabke HJ. The kinetics of hydrogen absorption in palladium (a- and b-phase) and palladiumesilver alloys. Ber Bunsenges 1974;78:58. [7] Cabrera AL, Morales-Leal E, Hasen J, Schuller IK. Structural changes induced by hydrogen absorption in palladium and palladiumeruthenium alloys. Appl Phys Lett 1995;66:1216. [8] Cabrera AL, Aguayo-Soto R. Hydrogen absorption in palladium films sensed by changes in their resistivity. Catal Lett 1997;45:79. [9] Kalli K, Othonos A, Christofides C. Characterization of reflectivity inversion, alpha- and beta-phase transitions and nanostructure formation in hydrogen activated thin Pd films on silicon based substrates. J Appl Phys 2002;91:3829. [10] Gremaud R, Broedersz CP, Borsa DM, Borgschulte A, Mauron P, Schreuders H, et al. Hydrogenography: an optical combinatorial method to find new light-weight hydrogenstorage materials. Adv Mater 2007;19:2813. [11] Borsa DM, Gremaud R, Baldi A, Schreuders H, Rector JH, Kooi B, et al. Structural, optical and electrical properties of MgyTi1-yHx thin films. Phys Rev B 2007;75:205408. [12] Avila JI, Matelon RJ, Trabol R, Favre M, Lederman D, Volkmann UG, et al. Optical properties of Pd thin films exposed to hydrogen studied by transmittance and reflectance spectroscopy. J Appl Phys 2010;107:1e5. [13] Romero CP, Avila JI, Trabol RA, Wang Huan, Vantomme A, Van Bael MJ, et al. Pd as a promoter to reduce Co cluster films at room temperature. Int J Hydrogen Energy 2010;35:2262e7. [14] Behm RJ, Penka V, Cattania MG, Christmann K, Ertl G. Evidence for “subsurface” hydrogen on Pd(110): an intermediate between chemisorbed and dissolved species. J Chem Phys 1983;78:7586..
(7) i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 0 6 1 3 e1 0 6 1 9. [15] Ward TL, Dao T. Model of hydrogen permeation behavior in palladium membranes. J Membr Sci 1999;153:211. [16] Menzel D. Desorption phenomena in interaction on metal surfaces. In: Gomer R, editor. Berlin: Springer-Verlag; 1975. p. 118. [17] Zhang GX, Yukawa H, Watanabe N, Saito Y, Fukaya H, Morinaga M, et al. Analysis of hydrogen diffusion coefficient during hydrogen permeation through pure niobium. Int J Hydrogen Energy 2008;33:4419e23. [18] Setoyama Daigo, Matsunaga Junji, Muta Hiroaki, Uno Masayoshi, Yamanaka Shinsuke. Characteristic of titaniumehydrogen solid solution. J Alloys Compd 2004;385: 156e9. [19] Briant CL, Wang ZF, Chollocoop N. Hydrogen embrittlement of commercial purity titanium. Corros Sci 2002;44:1875e88. [20] Pick MA, Davenport JW, Strongin M, Dienes GJ. Enhancement of hydrogen uptake rates for Nb and Ta by thin surface overlayers. Phys Rev Lett 1979;43:286. [21] Zuchner H, Schluter HJ, Rauf T, Hergemoller R. Properties of thin palladium films and their hydrogen permeability. Ber Bunsen-Ges Phys Chem 1991;95:1401. [22] Shi H, Lederman D. Annealed Co thin films: pit formation and magnetic anisotropy. J Appl Phys 2000;87:6095.. 10619. [23] Feranchuk ID, Feranchuk SI, Minkevich AA, Ulyanenkov A. Description of X-ray reflection and diffraction from periodical multilayers and superlattices by the Eigenwave method. Phys Rev B 2003;68:235307. [24] Matelon RJ, Avila JI, Volkmann UG, Cabrera AL, Morales Erie H, Lederman D. Substrate effect on the optical response of thin palladium films exposed to hydrogen gas. Thin Solid Films 2008;516:7797. [25] Johans son E, Olsson S, Chacon C, Hjorvarsson B. Solubility of hydrogen at low concentrations in thin epitaxial Nb(110) films. J Phys Condens Matter 2004;16:1165. [26] Cabrera AL, Garrido-Molina W, Morales-Leal E, EspinosaGangas J, Schuller IK, Ledermann D. Surface roughness of metallic films probed by resistivity measurements. Langmuir 1997;14:3249. [27] Kandasamy K, Surplice NA. The effects of hydrogen sorption on the resistance and work-function of titanium films at 290 K. J Phys D Appl Phys 1984;17:387. [28] Tsuchida B, Nagata S, Shikama T, Konashi K, Yamawaki M. Heat conductions due to electrons and phonons for titanium hydride and deuteride. J Alloys Compd 2003;356e357:223. [29] Kikuchi E. Membrane reactor application to hydrogen production. Catal Today 2000;56:97e101..
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