Remarkable Hydrogen Storage, Structural and Optical Properties in Multi layered Pd/Mg Thin Films
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(2) 2722. H. Fujii et al. -6. 1.0x10. Table 1 Sputtering conditions for two-layered Pd/Mg films.. Mg (A) (B) (C) (D) Pd. Sputtering conditions Ar 7.0 × 10−2 Pa Ar 7.0 × 10−2 Pa Ar 7.0 × 10−1 Pa Ar 7.0 × 10−1 Pa Ar 7.0 × 10−2 Pa. RF RF RF RF RF. 0W 100 W 0W 200 W 50 W. DC 0.05 A DC 0.05 A DC 0.05 A DC 0.05 A DC 0.10 A. films were prepared under four different conditions as listed in Table 1, while all the Mg films in three-layered Pd/Mg/Pd films were prepared at an Ar pressure of 7.0 × 10−1 Pa with the target D.C. current of 0.10 A and the R.F. coil power of 200 W, the condition of which gave the best hydrogen storage properties in the two-layered Pd/Mg films. The thickness of the Pd layer was 10 and 25 nm for two-layered film and 50 nm for multi-layered films, respectively, while those of the Mg layers were 200 nm for two-layer one, and 25, 50, 200, 400 and 800 nm for three-layered one, respectively. After the film preparation, these films were examined by Auger electron spectroscope (AES) to clarify the characterization of the interface boundary between Pd and Mg films. The AES spectra indicated no oxidation in the interface boundary and the existence of sharp interface without Pd and Mg mixing phase. Then, the Pd/Mg films were hydrogenated under a hydrogen pressure of 0.10 MPa at 373 K for 24 hrs, and the dehydriding properties were examined by thermal desorption spectroscopy (TDS) method in a heating process at 4 K/min from room temperature to 773 K. The amount of desorbed hydrogen was determined by integration of mass spectroscopy profiles and the hydrogen concentration was calculated using the thickness and specific gravity of Mg and Pd. The calibration of the amount of absorbed hydrogen was done by the throught put (orifice) method. The accuracy of hydrogen capacity measured was within ±3%. The structural properties in the films were characterized by X-ray diffraction (XRD) using Cu Kα-radiation and transmission electron microscopy (TEM), just after the films were prepared. Prior to TEM observation, the films were cut into slices with thickness less than one hundred nanometer along the cross section of the films using a focused ion beam thinning technique with 30 kV Ga+ . TEM observation was operated at 300 kV using a JEOL-JEM-3000F. The optical properties were examined by ultra-violet and visible spectrophotometry at room temperature using a Shimazu UV-160A. 3. Hydrogen Storage, Structural and Optical Properties in Two-layered Pd/Mg Thin Films Figure 2 shows TDS-spectra for four kinds of the twolayered Pd(25 nm)/Mg(200) films (A), (B), (C) and (D) prepared in different sputtering conditions, the conditions of which are listed in Table 1. The desorption temperatures for all the films, which are corresponding to those showing the peak in the TDS-spectra, being much lower than that in pure Mg bulk metal (∼ 623 K), are close to those observed by Krözer et al.4, 5) in the Pd/Mg films with the α and β mixed. Ion current (m/e=2) (A). Films. -7. 8.0x10. (D) -7. 6.0x10. (A) (C). -7. 4.0x10. (B) -7. 2.0x10. 0.0 300. 400. 500. 600. 700. Temperature (K) Fig. 2 TDS-spectra of the hydriding two-layered Pd/Mg thin films prepared under different sputtering conditions.. phases. The interesting features are summarized as follows:3) (1) All the spectra display a common structure with a small peak at low temperature and a large peak at high temperature. (2) The small peak is due to hydrogen desorption from Pd films, while the large peak corresponds to hydrogen desorption from Mg films. (3) The total hydrogen contents deduced by the mass spectroscopy method are between 2.9 and 6.6 mass%. (4) The higher Ar pressure and higher R.F. power leads to lower hydrogen desorption temperature. (5) The optimum condition gives that hydrogen of ∼ 5.6 mass% in Mg film is absorbed at 373 K under a H2 -gas pressure of 0.1 MPa and the hydrogen desorbs at T < 463 K in vacuum. (6) On the other hand, the Mg film without Pd coating never absorbs hydrogen at 373 K under 0.1 MPa, confirming that Pd really acts as a catalyst for dissociating hydrogen molecule into hydrogen atoms. For clarifying the origin of improvement in hydrogen storage properties, we examined the structural properties by XRD-profile analysis (θ –2θ scan). The results are shown in Figs. 3(a) and (b). Before hydrogenation, the profiles of the two-layered Pd/Mg films in Fig. 3(a) indicate the c-axis preferred orientation in the hcp Mg films, but no preferred orientation in Pd films. It is to be noted that the preferred orientation is irrespective of the R.F. power as well as Ar gas pressure. However, we can see that the Mg(002) peak intensity is strongly affected by the sputtering conditions. The weakest intensity is obtained at highest Ar pressure of 7.0 × 10−1 Pa and highest R.F. power of 200 W (film (D)). From the measurement of pole figures, we confirmed that the c-axis of Mg film widely distributes around the perpendicular direction to the film plane in the film (D), while the c-axis of Mg film in the film (B) is almost in perpendicular direction to film plane without wide distribution. Before hydrogenation, TEM observation of the cross section in Fig. 4 suggests that the Mg layer in film (D) is composed of slightly tilted columnarlike textures with smaller diameter than one hundred nanometer, while the Mg layer in film (B) consists of columnar ones along the c-axis with larger than one hundred nanometer. It seems therefore that the improvement of hydrogen storage.
(3) Remarkable Hydrogen Storage, Structural and Optical Properties in Multi-layered Pd/Mg Thin Films. Fig. 3 (a) X-ray diffraction profiles of the two-layered Pd/Mg films prepared under various sputtering conditions. (S) is the profile from substrate alone. (b) X-ray diffraction profiles of the hydriding two-layered Pd/Mg films prepared under various sputtering conditions listed in Table 1. The inset shows the MgH2 (110) peak intensities for comparison.. properties is originated in the development of smaller columnar Mg grains in nanometer scales. After hydrogenation, we can recognize that the (110) peak of MgH2 only grows in the XRD patterns in Fig. 3(b), suggesting that the peak intensity of the Mg (002) peak before hydrogen uptake transforms to the (110) peak intensities of MgH2 after hydrogenation for all the two-layered Pd/Mg films. This indicates that the Mg atoms in the thin films exhibit such a martensite-like transformation upon hydrogen uptake that the hcp Mg (00) plane corresponds to the rutiletype tetragonal MgH2 (hh0) plane; the directional relation,. 2723. Mg(001)[001]//MgH2 (110)[110]. As a result of the transformation, we can deduce that the distance between the Mg atoms is expanded by ∼ 22% along the perpendicular direction to the substrate plane as is seen in Fig. 5(a), while the Mg–Mg distance is expanded by only 6% in the substrate plane on average. Thus, the anisotropic deformation upon hydrogenation gives rise to a slight compression stress in the Mg films so as to minimize the increase in elastic energy upon hydrogenation. On the contrary, TEM observation of Mg/MgH2 boundary for hydriding bulk Mg ribbon11) indicated that the transformation due to hydrogenation satisfies the following directional relation, Mg(001)[−1, −1, 0]//MgH2 (100)[001]. This transformation leads to such strains that the basal c plane expands ∼ 50% and the c-axis shrinks ∼ 14% as shown in Fig. 5(b). This type of transformation could not be realized in the Pd/Mg thin films because of causing too large increase in elastic energy upon hydrogen introduction. In 1996, Huiberts et al.12, 13) found that the rare-earth hydride thin films revealed optical transparency. Those are expected as one of the functional materials for optical switching devices. Recently, Richardson et al.15) have reported an electrochromic mirror electrode based on reversible uptake of hydrogen in nickel-magnesium alloy films, resulting from reversible formation of Mg2 NiH4 and MgH2 . However, for pure Mg hydride thin film without any rare earth and nickel elements, the optical properties have not been sufficiently studied because it was quite difficult to load enough hydrogen into the Pd/Mg thin films previously reported.4–8, 14) Since we have found the optimizing sputtering condition to fully hydrogenate the Pd/Mg films at relatively low temperature, we carried out the experiments of the optical transmission in this work.10) Figure 6 shows the photographs in transmitted light as a function of charging time under a H2 gas pressure of 0.1 MPa at 393 K for the two-layered Pd(10 nm)/Mg(200 nm) film. With increasing the charging time, hydrogenation proceeds from the edge of the films, leading to optical transparency. We can see that the whole film shows complete transparency after 24 hrs charging. As shown in Fig. 7, the transmission in the Mg film increases with increasing a light wavelength and reaches 82% at nearly 900 nm after the transmission corrections for the quartz-glass substrate and Pd thin film. It is noteworthy that the optical transmission of Mg film at short wavelength side is better than those in the rare earth hydride films. 4. Hydrogen Storage and Structural Properties in Three-layered Pd/Mg/Pd Thin Films Figure 8 shows the TDS spectra for the hydrogenated threelayered Pd(50 nm)/Mg(x nm)/Pd(50 nm) films with x = 25, 50, 200, 400 and 800.9) Here, all the Mg films were prepared under the optimizing sputtering condition for obtaining best hydrogen storage properties in two-layered films; that is at an Ar gas atmosphere of 7.0 × 10−1 Pa with the target DC current of 0.10 A and the RF power of 200 W. We notice that each spectrum displays two-peak structure in low and high temperature sides for all the films. These two peaks correspond to hydrogen desorption from the Pd and Mg films, respectively, as described above. The most interesting feature.
(4) 2724. H. Fujii et al.. Fig. 4 TEM micrographs for the cross sections of sample (B) and (D) before hydrogenation.. Mg[00.1]. Mg[11.0]. (a). (b). Fig. 5 (a) Distortion of Mg atoms in Mg thin film by hydrogen uptake, and (b) distortion in bulk Mg sample by hydrogenation given by Schober et al. : Mg and ●: MgH2 (showing only the Mg atoms).. is that the peak position in the TDS spectra remarkably shifts to low temperature side with increasing the thickness of Mg films. We can deduce that the peak temperature due to hydrogen desorption from the Mg film decreases from 463 to 363 K with increasing the thickness of the Mg films, while that from the Pd film decreases from 373 to 320 K. It is to be noted that the hydrogen desorption temperature from the Mg film in the three-layered Pd/Mg/Pd films are significantly lower than that of the two layered Pd/Mg films. The hydrogen content absorbed in the Mg films in three-. Fig. 6 Photographs in transmitted light in the hydrogenation process for two-layered Pd(10 nm)/Mg(200 nm) thin films under hydrogen gas pressure of 0.1 MPa at 393 K.. layered Pd/Mg/Pd films were deduced to be ∼ 5.0 mass%, that was almost independent of the thickness of Mg film. This is in contrast with the result that hydrogen content in twolayered Pd/Mg films rapidly decreased with increasing the Mg film thickness. It should be noted that hydrogen content in Mg film for three-layered film was almost two times larger than that for two-layered one for the Mg film thickness of 800 nm at the same hydrogenation process. On the other hand, the hydrogen content in Pd layer was deduced to be 0.15–0.30 mass% irrespective of the Mg film thickness..
(5) Remarkable Hydrogen Storage, Structural and Optical Properties in Multi-layered Pd/Mg Thin Films 100. Transmission (%). 80. 60. 40. 20. 0. 300. 500. 900. 700. Wavelength (nm) Fig. 7 Transmission spectrum of the Mg film as a function of wavelength for hydrogenated two-layered Pd(10 nm)/Mg(200 nm) film after transmission corrections for quartz-glass and Pd film.. 1.0x10- 6. Ion current (m/e=2) (A). (e) x=800nm. (d) x=400nm. 8.0x10- 7. 6.0x10- 7 (c) x=200nm. 4.0x10- 7. 2.0x10- 7. (b) x=50nm (a) x=25nm. 0.0. 250. 300. 350. 400. 450. 500. 550. Temperature (K) Fig. 8 TDS-spectra for hydrogen absorbing three-layered Pd(50 nm)/ Mg(x nm)/Pd(50 nm) films with x = 25, 50, 200, 400 and 800.. For clarifying the origin of remarkable hydrogen storage properties in the three-layered Pd/Mg/Pd film, we performed outward watching with eyes and TEM image-figure observation for the three-layered film. To outward seeing, we noticed the peeling off for the films with Mg film thickness of (d) 400 nm and (e) 800 nm from the grass substrate at the interface between the Pd film and substrate, where the adhesive force is weakest in the film. Degradation due to peeling by appearances increases with increasing the Mg film thickness. This may be due to the fact that the stress induced between the substrate and Pd film upon hydrogen uptake increases with increasing the Mg film thickness. TEM image for the cross section of the three-layered Pd(50 nm)/Mg(200 nm)/Pd(50 nm) film before hydrogenation is shown in Fig. 9. The Mg layer is composed of fine columnar structures along the c-axis with the diameter of 10–30 nm, which is much smaller than those observed in two-layered Pd/Mg films. This indicates that the structure of the Mg film is significantly influenced by its foundation (Pd or glass) and the. 2725. Mg columnar grain with smaller diameter leads to lower dehydriding temperature. As mentioned above, significant improvement in hydrogen storage properties was induced in the three-layered Pd/Mg/Pd films. Next, we will discuss the origin of such improvement on the basis of cooperative phenomena that hydrogen reveals in nano-composite regions. In Fig. 10, a schematic diagram of nano-structure in three-layered Pd/Mg/Pd film is shown for understanding the cooperative phenomena. The scenario of dehydrogenation is drawn as follows: As an initial condition, we assume that the Pd/Mg/Pd film with thicker Mg layer is fully hydrogenated and peels off the substrate, while the film with thinner Mg layer is partly hydrogenated and sticks on the substrate. When the temperature is raised for the thicker Mg film, hydrogen in Pd films becomes at first unstable and easily desorbs from the up and down Pd films. As a result of dehydrogenation from both sides of the Pd films, the compression stress is induced on the top and down planes of the Mg film. This stress strongly influences for hydrogen existing at the grain boundaries in the Mg films, by which hydrogen at the grain boundaries in the Mg films becomes unstable and brings low temperature desorption. At the next step, hydrogen in the grains of the Mg films becomes unstable and all the hydrogen desorb at low temperature. Thus, hydrogen in Mg film can be dehydrogenated as soon as hydrogen in Pd is desorbed at lower temperature than 373 K. This phenomenon is called “cooperative phenomena”1, 2) that hydrogen exhibits in nano-composite system by elastic interactions. Otherwise, the Pd/Mg/Pd film with thinner Mg layer is not peeled off the substrate by hydrogen introduction. In this case, the Mg atoms in the films might exhibit such a transformation upon hydrogenation that the hcp Mg (00) plane corresponds to the tetragonal MgH2 (hh0) plane. As a result of the transformation, the film are expanded ∼ 22% along the direction perpendicular to the film plane as described above, while the film is only expanded ∼ 6% in the substrate plane. Therefore, only a small elastic stress acts in the Mg film upon hydrogenation, giving rise to a weak cooperative interaction. This leads to no good hydrogen storage properties for the Pd/Mg/Pd films with thinner Mg layers without peeling off. Furthermore, since the diameter of Mg columnar grains in the three-layered films is smaller than that in the two-layered films, there are many hydrogen path ways (the number of grain boundaries) in three-layered films, which also leads to lower dehydriding temperature due to faster hydrogen storage reaction as well. 5. Hydrogen Storage Properties in Multi-layered Pd/Mg/ - - -Thin Films Furthermore, we can expect better hydrogen storage properties in multi-layered Pd/Mg films with more than three layers if the cooperative interaction is profound in this system. To clarify this point, we examined the hydrogen storage and structural properties in five- and sevenlayered films. In Fig. 11, the TDS spectra of multi-layered Pd(50 nm)/Mg(200 nm)/- - -thin films are shown as a function of the number of layers. We can see the two-peak structure in them as well. The dehydriding temperature from the Mg film, which is defined as the temperature showing a.
(6) 2726. H. Fujii et al.. Fig. 9 TEM microphotograph for the cross section of the three-layered Pd(50 nm)/Mg(200 nm)/Pd(50 nm) thin film before hydrogenation. -6. Desorbing Hydrogen Contraction. 1.0x10. Contraction. Mg. Pd Contraction. Contraction Desorbing Hydrogen. Fig. 10 Schematic diagram of hydrogrnated Pd/Mg/Pd thin film with thicker Mg layer.. peak at high temperature, decreases with increasing the number of the layers and reaches 360 K for the seven-layered Pd/Mg/Pd/Mg/Pd/Mg/Pd film. On the other hand, the amount of absorbed hydrogen in Mg film is ∼ 5 mass%, which is almost independent of the number of film layers. TEMobservation also confirmed that the Mg layer in seven-layered film was composed of fine columnar structures along the caxis with the diameter of 10–30 nm as in three-layered films. From the above experimental results, we can conclude that the cooperative phenomena are also induced in multi-layered thin films with more than three layers and leads to excellent hydrogen storage properties. In addition, we examined the reversibility test of hydrogen absorption-desorption cycles up to ten times at 373 K. After ten cycles, the film showed almost the same hydrogen desorption temperature and absorbing hydrogen concentration as well. After changing the thickness of Pd film to 10 nm from 50 nm, the hydrogen storage properties were examined for three-layered Pd/Mg(x)/Pd film with x = 800 nm, the results of which showed no change in the hydrogen storage. Ion current (m/e=2) (A). -7. Pd. 8.0 x10. -7. 5layers. 7layers. 2layers. 6.0 x10. -7. 4.0 x10. 3layers. -7. 2.0 x10. 0.0 250. 300. 350. 400. 450. 500. 550. Temperature (K) Fig. 11 TDS-spectra of multi-layered Pd(50 nm)/Mg(200 nm)/- - -thin films as a function of number of layers.. properties in Mg film. These results indicates that it is possible to design the hydrogen storage materials that hydrogen of ∼ 5 mass% absorbs at 373 K under a hydrogen pressure of 0.1 MPa and all the hydrogen desorbs at 360 K in vacuum. 6. Summary In this work, we examined the hydrogen storage, structural and optical properties in multi-layered Pd/Mg thin films prepared using a RF assisted magnetron sputtering method. The results obtained are summarized as follows: (1) Two-layered Pd(25 nm)/Mg(200 nm) film prepared under the highest RF power of 200 W and highest Ar pressure of 7.0 × 10−1 Pa indicated the best hydrogen storage prop-.
(7) Remarkable Hydrogen Storage, Structural and Optical Properties in Multi-layered Pd/Mg Thin Films. erties with the absorbed hydrogen content of ∼ 5.6 mass% in Mg and the desorption temperature of 463 K, in which characteristic columnar-like textures with smaller diameter than a hundred nanometer develop in the Mg film. (2) In the three-layered Pd(50 nm)/Mg(800 nm)/ Pd(50 nm) film, hydrogen of ∼ 5 mass% absorbed in Mg film and the hydroden desorbed at 363 K, in which the columnarlike textures with the diameter of a few ten nanometer develop in Mg film and the peeling off the substrate occurs as well. The smaller size of columnar textures leads to better hydrogen storage properties. (3) Seven-layered Pd(50 nm)/Mg(200 nm)/Pd(50 nm)/ Mg(200 nm)/Pd(50 nm)/Mg(200 nm)/Pd(50 nm) showed similar excellent hydrogen storage properties to that in the threelayered Pd(50 nm)/Mg(800 nm)/Pd(50 nm) film. (4) Such significant improvement of hydrogen storage properties in multi-layered film could be explained by the cooperative phenomena that hydrogen reveals in nanocomposite regions through some elastic interactions. (5) By reducing Pd film thickness, we confirmed that it was possible to design new hydrogen storage material such that hydrogen of ∼ 5 mass% absorbed at 373 K under a hydrogen pressure of 0.1 MPa, and all the hydrogen desorbed at 360 K. (6) In addition, we found that the rare earth and nickel free two-layered Pd(10 nm)/Mg(200 nm) film revealed colorneutral transparency upon hydrogenation. Acknowledgements This work was financially supported in part by the Proposal-Based Industry Creative Type Technology R&D. 2727. Promotion Program from the New Energy and Industrial Technology Development Organization (NEDO), and by the Grant-Aid for Scientific Research on Priority Areas (A) and by the Grant-in-Aid for COE Research (No, 13CE2002) of the Ministry of Education, Science, Sports and Culture of Japan. REFERENCES 1) H. Fujii, S. Orimo and K. Ikeda: J. Alloys Compd. 253–254 (1997) 80. 2) S. Orimo, H. Fujii and K. Ikeda: Acta. Meter. 45 (1997) 331. 3) K. Higuchi, H. Kajioka, K. Toiyama, H. Fujii, S. Orimo and Y. Kikuchi: J. Alloys Compd. 293–295 (1999) 484. 4) A. Krözer and B. Kasemo: J. Vac. Sci. Technol. A5 (1987) 1003. 5) A. Krözer and B. Kasemo: J. Less-Common Met. 160 (1990) 323. 6) A. Fischer, H. Köstler and L. Schlapbach: J. Less-Common Met. 172– 174 (1991) 808. 7) A. Fischer, H. Köstler and L. Schlapbach: Sur. Sci. 269/270 (1992) 737. 8) P. Spatz, H. A. Aebischer, A. Krözer and L. Schlapbach: Zeitschrift für Physikalische Chemie, Bd. 181, S. 393–397 (1993) 955. 9) K. Higuchi, K. Yamamoto, H. Kajioka, K. Toiyama, M. Honda, S. Orimo and H. Fujii: J. Alloys Compd. 330–332 (2002) 526. 10) K. Yamamoto, K. Higuchi, H. Kajioka, H. Sumida, S. Orimo and H. Fujii: J. Alloys Compd. 330–332 (2002) 352. 11) T. Schober: Metall. Trans. A 12 (1981) 951. 12) J. N. Huiberts, R. Griessen, J. H. Rector, R. J. Wijngaarden, J. P. Dekker, D. G. de Groot and N. J. Koeman: Nature 380 (1996) 231. 13) J. N. Huiberts, J. H. Rector, R. J. Wijngaarden, S. Jetten, D. G. de Groot, B. Dam, N. J. Koeman, R. Griessen, B. Hjörvarsson and S. Olafsson: J. Alloys Compd. 239 (1996) 158. 14) L. Schlapbach, J. Osterwalder and T. Riesterer: J. Less-Common Met. 103 (1984) 295. 15) T. J. Richhadson, J. L. Slack, R. D. Armitage, R. Kostecki, B. Farangis and M. D. Rubin: Appl. Phys. Lett. 78 (2001) 3047..
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