Research Article www.acsami.org
Hydrogen Absorption of Palladium Thin Films Observed by in Situ Transmission Electron Microscopy with an Environmental Cell Tengfei Zhang,*,† Yuki Nakagawa,† Takenobu Wakasugi,† Shigehito Isobe,*,† Yongming Wang,‡ Naoyuki Hashimoto,† and Somei Ohnuki† †
Graduate School of Engineering, Hokkaido University, N-13, W-8, Sapporo 060-8628, Japan Creative Research Institution, Hokkaido University, N-21, W-10, Sapporo 001-0021, Japan
‡
S Supporting Information *
ABSTRACT: A window type of the environmental cell system for a high-voltage electron microscope was developed and applied to in situ observation of a palladium (Pd) thin film. For in situ hydrogenation of Pd thin films, the distances of the lattice fringes were 0.20 and 0.23 nm, which correspond to the lattice d spacings of β-phase (200) and (111) planes. Expansion of the Pd lattice happened as a result of phase transformation from the α phase to the β phase. In particular, the lattice fringes were clearly distinguished, and the dislocation behavior during Pd hydrogenation was easily recognized according to the corresponding inverse fast fourier transform images. Furthermore, significant growth in the number of dislocations was observed at the grain boundary during increasing hydrogen pressure in the cell. KEYWORDS: environmental cell, Pd catalysis, thin film, hydrogenation, dislocation
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INTRODUCTION Nowadays, energy structures are mainly dependent on fossil fuels, which are nonrenewable resources and cause negative environmental problems. Building a sustainable energy system is one of the most essential issues in current modern society.1,2 Hydrogen as an energy carrier is widely considered a sustainable alternative to the current energy predicament, involving hydrogen generation, storage, purification, and energy conversion.3−7 It is well-known that some kinds of metals or alloys are reactive to hydrogen gas and can store hydrogen through chemical bonds. Palladium (Pd) is a typical hydrogenstorage metal and presents extraordinary features for practical use, such as absorbing hydrogen at ambient temperature and pressure. Therefore, Pd has been studied as a model to clarify the hydrogenation properties, and hydrogen storage in Pd has been investigated for over the past half-century.8−12 Recently, nanosize effects have attracted much attention not only to modify the electronic, optical, catalytic, and magnetic properties of materials but also to improve the hydrogen-storage performance of metals.13,14 Nanostructured thin films of Pd contribute significantly to increasing the total hydrogen content. Some recent studies have further illustrated the catalytic behavior of nanostructured Pd for dissociation of molecular hydrogen, but direct observations in the nanoscale on how hydrogen atoms are trapped in Pd ultrathin films are absent.13−15 This work presents the effect of dislocation on the hydriding process of a Pd thin film via the in situ observation carried out by high-voltage transmission electron microscopy (TEM; details are shown in the Supporting Information). In situ study into the interaction of nanostructured Pd with © 2016 American Chemical Society
hydrogen is important for developing its potential applications in the future. A Pd thin film with a thickness of ∼10 nm was prepared on a silicon nitride (SiN) membrane TEM grid (ALLIANCE Biosystems) by electron-beam vacuum evaporation (EB1500R). The Pd target (Nilaco) purity was 99.95%. The film was annealed for grain coarsening in an argon-filled glove box (oxygen bellow 5 ppm) at 300 °C for 30 min and subsequently mounted in the self-developed environmental cell holder (further details are shown in the Supporting Information).
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RESULTS AND DISCUSSION
The Pd thin film (10 nm thick) deposited on a SiN window (15 nm thick) was observed in the environmental cell at room temperature. To confirm whether high resolution can be obtained with extradouble 15-nm-thick SiN window films of the environmental cell, the observation was first performed in vacuum conditions, with the pressure inside the cell about 5.0 × 10−2 Pa. Figure 1 shows a high-resolution TEM (HRTEM) image of Pd grains under these conditions. Lattice fringes of the Pd(111) and Pd(200) planes, corresponding to distances of 0.22 and 0.19 nm, respectively, were clearly achieved. This result indicates that the image resolution was prior to 0.19 nm; thus, in situ high-resolution observation using the environmental cell is possible for the Pd thin film. Received: March 10, 2016 Accepted: May 23, 2016 Published: May 23, 2016 14548
DOI: 10.1021/acsami.6b02971 ACS Appl. Mater. Interfaces 2016, 8, 14548−14551
Research Article
ACS Applied Materials & Interfaces
at hydrogen pressures of 20, 40, and 100 kPa. At the lowest pressure of 20 kPa, the distances of the lattice fringes were 0.23 and 0.20 nm, corresponding to lattice d spacings of PdHx (0.6 ≤ x ≤ 0.75) (111) and (200), respectively. These changes in the lattice d spacing from Pd (α phase) to palladium hydride (β phase) are in good agreement with the published PCT data.8,16,17 The image quality was influenced progressively with increasing pressure. At a pressure of 100 kPa, the definition of the fringes and FFT spots was less clear than that of the lower pressure results. It should be noted that the difference in the lattice−fringe spacing between the β and α phases is distinguishable by HRTEM, but it is impossible to precisely determine the hydrogen content of the β phase by the distance of the lattice fringes because the differences in the lattice parameter due to hydrogen content changes are very small, within the errors of the lattice fringes observed under HRTEM. During increasing hydrogen pressure in the environmental cell, expansion of the Pd lattice happened as a result of phase transformation from the α phase to the β phase (Figure 3). The Figure 1. HRTEM image of the Pd grains of the Pd thin film and (inset) its FFT image. The observation was under 5.0 × 10−2 Pa of inside pressure of the environmental cell. The planes of Pd(111) and Pd(200), with distances of 0.22 and 0.19 nm, respectively, can be clearly identified by their lattice-fringe images and corresponding FFT spots.
The effect of increasing hydrogen pressure inside the cell on Pd thin film hydrogenation at room temperature was examined by in situ observation. The hydrogen pressure was built up from 5.0 × 10−2 Pa to 100 kPa during the observation. From the pressure−composition−temperature (PCT) diagram,8,16,17 when the hydrogen pressure is higher than the equilibrium pressure (about 2.4 kPa) at room temperature, the Pd metal should be transformed entirely into the palladium hydride (βphase) PdHx, the hydrogen content x = H/Pd of which slightly changes from 0.6 to 0.75 with increasing hydrogen pressure up to 100 kPa. Figure 2 shows the HRTEM images and corresponding fast Fourier transform (FFT) images obtained
Figure 3. HRTEM images with FFT insets of the β phase (a, b, and c) and corresponding IFFT images (d, e, and f) obtained at hydrogen pressures of 10 kPa (a and d), 20 kPa (b and e), and 40 kPa (c and f). Both FFT and IFFT images are from the white square-marked regions in the HRTEM images. The dislocation cores (yellow circles) formed near the grain boundary (yellow dashed lines) during increasing hydrogen pressure.
inverse FFT (IFFT) images show grain expansion, and the yellow dashed line illustrates the grain boundary. The dislocation cores (yellow circles) formed near the grain boundary during increasing hydrogen pressure. In order to know the details about the interaction between hydrogen and Pd, an in situ observation of the β phase was carried out to understand the dislocation behavior with increasing hydrogen pressure. The HRTEM images (and the insets of FFT images) of the grain and corresponding IFFT images (white squares) as a function of changing hydrogen pressure are shown in Figure 4. Under all pressure conditions, the distance of the lattice fringes was 0.23 nm, corresponding to the lattice d spacing of the β-phase (111) planes. Some dislocation cores (yellow circles) are obviously seen. Additionally, the increment in the number of dislocation cores and its distribution change were confirmed with increasing pressure. Figure 5 presents a distribution change throughout the grain. Yellow dots indicate the positions of the dislocation cores. In particular, significant growth in the number of dislocations was recognized at the
Figure 2. HRTEM images and corresponding FFT images of palladium hydride (β phase) obtained at hydrogen pressures of 20, 40, and 100 kPa. The (111) and (200) planes of the β phase are clearly identified by d-spacing measurements in each image, being 0.23 and 0.20 nm, respectively. Additionally, a high image resolution of 0.12 nm was confirmed even at a pressure of 100 kPa. 14549
DOI: 10.1021/acsami.6b02971 ACS Appl. Mater. Interfaces 2016, 8, 14548−14551
Research Article
ACS Applied Materials & Interfaces
atoms could diffuse through the boundary and be picked back up by dislocations in the neighboring grain.22 According to the above results, dislocations in the grains were produced by hydrogen absorption of Pd. However, dislocations in thin films may sometimes also be generated by the deposition process or the application of pressure regardless of the gas. To draw a definite conclusion, further in situ observations using inert argon gas have been performed to rule out these possibilities. The results were shown in Figure 7.
Figure 4. (Top) HRTEM images with FFT insets of the β-phase grains obtained, in turn, at hydrogen pressures of 10, 20, and 40 kPa. (Bottom) Corresponding IFFT images. Both FFT and IFFT images are from the white square-marked areas in the top images.
Figure 7. HRTEM images of Pd grains observed under pressures of 20, 40, and 100 kPa, respectively. The distances of the lattice fringes are 0.22 and 0.19 nm, corresponding to the FFT spots of α-phase (111) and (200), respectively. Figure 5. HRTEM images for the distribution changes of dislocation cores (yellow dots). A significant increment in the number of the dislocations is recognized at the grain boundary.
From the HRTEM images and the corresponding FFT images, the dislocation cores were not generated with an increase in the argon gas pressure from 5.0 × 10−2 Pa to 100 kPa. Consequently, these results verified that the formation of dislocation cores came from hydrogen absorption of Pd, where they accommodated the lattice mismatch associated with βphase nucleation and growth in the solid solution phase.
grain boundary. These edge dislocations could interact strongly with hydrogen atoms,18 indicating that the diffusion of hydrogen at the grain boundary becomes faster during increasing hydrogen concentration. This corresponds with previous reports.19−21 The relationship between the dislocation count and hydrogen pressure is shown in Figure 6. These results suggested that dislocations act as prior pathways for hydrogen transportation and cause the local stress concentration at the boundary and hence expansion or rotation of the grain. Because dislocations cannot travel from grain to grain, it was also implied that the dislocations that concentrated near grain boundary would offload hydrogen at or near the boundary. Then the hydrogen
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CONCLUSION In this study, a high-gas-pressure environmental cell system for high-voltage electron microscopy was designed and fabricated. An in situ observation of Pd thin film hydrogenation was carried out by this technique at room temperature. Under all pressure conditions, the distances of the lattice fringes were 0.23 and 0.20 nm, corresponding to the lattice d spacings of βphase (111) and (200), respectively. Expansion of the Pd lattice happened as a result of phase transformation from the α phase to the β phase. In particular, using the corresponding IFFT images allowed us to distinguish the fringes more clearly and to recognize the dislocation behavior more easily. Furthermore, a significant increment in the number of dislocations that interacted intensely with hydrogen atoms was observed at the grain boundary during increasing hydrogen pressure in the cell. These results suggested that hydrogen could diffuse through dislocations as prior pathways, which produced a local stress concentration at the boundary and led to expansion or rotation of the grain to generate more hydrogen-carrying dislocation cores. It is also indicated that these dislocations would drop hydrogen at or near the boundary, where the hydrogen atoms could diffuse through the boundary and then be picked back up by dislocations in the neighboring grain. Herein, the dislocation behavior of Pd thin film hydrogenation was clarified. This direct
Figure 6. Dislocation count per unit area growth with increasing hydrogen pressure. 14550
DOI: 10.1021/acsami.6b02971 ACS Appl. Mater. Interfaces 2016, 8, 14548−14551
Research Article
ACS Applied Materials & Interfaces
(11) Delmelle, R.; Amin-Ahmadi, B.; Sinnaeve, M.; Idrissi, H.; Pardoen, T.; Schryvers, D.; Proost, J. Effect of Structural Defects on the Hydriding Kinetics of Nanocrystalline Pd Thin Films. Int. J. Hydrogen Energy 2015, 40, 7335−7347. (12) Amin-Ahmadi, B.; Idrissi, H.; Delmelle, R.; Pardoen, T.; Proost, J.; Schryvers, D. High Resolution Transmission Electron Microscopy Characterization of Fcc 9R Transformation in Nanocrystalline Palladium Films due to Hydriding. Appl. Phys. Lett. 2013, 102, 071911-1−071911-4. (13) Kobayashi, H.; Morita, H.; Yamauchi, M.; Ikeda, R.; Kitagawa, H.; Kubota, Y.; Kato, K.; Takata, M.; Toh, S.; Matsumura, S. NanosizeInduced Drastic Drop in Equilibrium Hydrogen Pressure for Hydride Formation and Structural Stabilization in Pd−Rh Solid-Solution Alloys. J. Am. Chem. Soc. 2012, 134, 12390−12393. (14) Kobayashi, H.; Yamauchi, M.; Kitagawa, H. Finding HydrogenStorage Capability in Iridium Induced by the Nanosize Effect. J. Am. Chem. Soc. 2012, 134, 6893−6895. (15) Kobayashi, H.; Yamauchi, M.; Ikeda, R.; Kitagawa, H. AtomicLevel Pd-Au Alloying and Controllable Hydrogen-Absorption Properties in Size-Controlled Nanoparticles Synthesized by Hydrogen Reduction Method. Chem. Commun. 2009, 4806−4808. (16) Lewis, F. A. The Hydrides of Palladium and Palladium Alloys. Platinum Metals Rev. 1960, 4, 132−137. (17) Knapton, A. G. Palladium Alloys for Hydrogen Diffusion Membranes. Platinum Metals Rev. 1977, 21, 44−50. (18) Myers, S. M.; Baskes, M. I.; Birnbaum, H. K.; Corbett, J. W.; Deleo, G. G.; Estreicher, S. K.; Haller, E. E.; Jena, P.; Johnson, N. M.; Kirchheim, R.; Pearton, S. J.; Stavola, M. J. Hydrogen Interactions with Defects in Crystalline Solid. Rev. Mod. Phys. 1992, 64, 559−617. (19) Janssen, S.; Natter, H.; Hempelmann, R.; Striffler, T.; Stuhr, U.; Wipf, H.; Hahn, H.; Cook, J. C. Hydrogen Diffusion in Nanocrystalline Pd by Means of Quasielastic Neutron Scattering. Nanostruct. Mater. 1997, 9, 579−582. (20) Kirchheim, R. Hydrogen Solubility and Diffusivity in Defective and Amorphous Metals. Prog. Mater. Sci. 1988, 32, 261−325. (21) Oudriss, A.; Creus, J.; Bouhattate, J.; Savall, C.; Peraudeau, B.; Feaugas, X. The Diffusion and Trapping of Hydrogen along the Grain Boundaries in Polycrystalline Nickel. Scr. Mater. 2012, 66, 37−40. (22) Tien, J. A.; Thompson, A. W.; Bernstein, I. M.; Richards, R. J. Hydrogen Transport by Dislocations. Metall. Trans. A 1976, 7, 821− 829.
observation of palladium hydride formation at room temperature could be significant not only in the mechanism of Pd hydrogenation but also in the development of novel catalysts in practical applications.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b02971. Equipment and development details of an environmental cell holder (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (T.Z.). *E-mail:
[email protected] (S.I.). Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by the X-ray Free Electron Laser Priority Strategy Program and Nanotechnology Platform Program from the Ministry of Education, Sports, Culture, Science and Technology (MEXT), Japan. We thank Prof. Y. Nishino and Prof. T. Kimura for their valuable discussion on the environmental cell.
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REFERENCES
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DOI: 10.1021/acsami.6b02971 ACS Appl. Mater. Interfaces 2016, 8, 14548−14551
