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Electrochromism in Mischmetal-Based AB2 Alloy Hydride Thin Film M. Krishna Kumar,† N. Rajalakshmi,‡ and S. Ramaprabhu*,† AlternatiVe Energy Technology Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai - 600 036, India, and Centre for Fuel Cell Technology, MedaVakam, Chennai - 601 302, India ReceiVed: February 11, 2007
Electrical resistivity studies on mischmetal-substituted, C15 type AB2 alloy (Mm0.2Tb0.8Co2) hydrides with varied hydrogen interstitial concentrations have been carried out in the temperature range 80-300 K. Hydrogen concentration-dependent metal-semiconductor-like transition has been observed and explained based on the charge transfer between hydrogen and the 3d band and the reduction in 6S component of the electron density. Polycrystalline thin film of the alloy capped with palladium has been obtained by electron beam evaporation technique on clean glass substrates. Switching properties from reflecting to a transparent state by electrochemical polarization in an alkaline electrolyte of pH 13 at room temperature have been investigated. In addition, the cyclic durability of the films has been studied and compared with that of rare-earth-based switchable mirror films. Preferential interstitial site occupancy for the hydrogen atoms has been analyzed based on the curent voltage characteristics of alloy thin film.
1. Introduction It is well known that many intermetallic compounds absorb large quantities of hydrogen. Apart from their technological applications like hydrogen storage, catalytic material, and permanent magnet applications, the metal hydrides are of special interest because of the profound influence of the absorbed hydrogen on electrical, thermal, and mechanical properties.1 Particularly, the hydrogen absorption in AB2 (A ) rare earth and B ) Fe, Co, Ni) type Laves phase compounds leads to charge transfer between hydrogen and the 3d band, thereby modifying the various exchange interactions,2-3 which will affect the magnetic scattering contribution to electrical resistivity significantly. Furthermore, the electrical resistivity studies on rare earth hydrides4-5 have revealed metal-semiconductor transitions on the increase of hydrogen concentration. This spectacular change in the electronic property is sometimes accompanied by optical switching effect also. In 1996, Huiberts et al. discovered the reversible optical switching in rare earth metal hydrides.6 They found that a mirrorlike reflective state of metal is transformed to a transparent state when it is converted into trihydride through hydrogen absorption.7-8 Color neutral switchable mirrors were obtained by hydrogenation of rare earthmagnesium solid solution alloys.9-11 Recently, metal-transparent hydride switching was also found in nickel-magnesium thin films.12 It is known that rare earth-based C15 type AB2 alloys have excellent hydrogen storage properties.13,14 We have systematically investigated the hydrogen storage properties of Mischmetal (Mm)-substituted rare earth-based AB2 alloys and have observed that some of these alloys have metal-tosemiconductor transition upon hydrogenation in bulk form indicating the potential of these thin film alloys for reversible optical switches. In this paper, we report the preparation, characterization, and temperature variation of electrical resistivity in mischmetal * Corresponding author. E-mail:
[email protected]. Tel: +91-4422574862. Fax: +91-44-22570509. † Indian Institute of Technology Madras. ‡ Centre for Fuel Cell Technology.
substituted C15 type AB2 alloy (Mm0.2Tb0.8Co2) and their hydrides. Palladium-capped thin films of this alloy have been obtained in situ by high-temperature electron beam evaporation and characterized by X-ray diffraction (XRD), scanning electron micrograph (SEM), and energy dispersive X-ray analysis (EDX). The switching action in these films from reflecting to transparent state upon electrolytic hydrogenation has been studied and the results have been presented. In addition, the cyclic stability of the thin films during repeated hydrogenation and dehydrogenation have been studied and discussed. 2. Experimental Details Mischmetal (Mm, a mixture of light rare earth elements (Ce, La, Pr, Nd and low concentration of other rare earths)) substituted rare earth-based AB2 alloy was prepared by arc melting the constituent elements under argon atmosphere and subsequent annealing at 850 °C for 7 days and was slow cooled to room temperature to obtain single phase. A 6 wt % of excess Tb was added to the total stoichiometric mixture of the alloy during arc melting to avoid the occurrence of additional phases. Alloys thus prepared were silver-white and brittle. Powder XRD showed the formation of materials in single phase with C15 type Laves phase structure. The direct current electrical resistivity measurements were performed using a four-probe technique in the temperature range 80-300 K. The samples required for the measurements were obtained from hydrided samples suitably pressed to pellet form of 3 mm thickness. Hydrides of different concentrations were prepared using the pressure-composition hydrogen absorption isotherm data of the alloy. Alloy thin films of thickness of 100 nm were deposited in deep vacuum conditions on clean glass substrates of size 50 mm × 20 mm at 300 °C with precise thickness control using a precalibrated quartz crystal thickness monitor. The substrate heating was achieved by irradiation with high power IR radiation. This enabled a region of high temperature over the path of evaporated species before reaching down onto to the substrate. A chemically cleaned glass substrate surface was subjected to Ar+ ion etching in the deposition chamber to achieve enhanced adhesion of the
10.1021/jp071163d CCC: $37.00 © 2007 American Chemical Society Published on Web 05/27/2007
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alloy film to the glass. Prior to deposition, the chamber is purged repeatedly with argon to remove the presence of any oxygen species. A 10 nm Pd capping was achieved in situ without breaking vacuum lest the films get oxidized. Fine pieces of Pd foil (purity 99.99%, Sigma Aldrich) in a graphite crucible were used as the target. Pd capping was carried out at a substrate temperature of 100 °C. A lower substrate temperature was chosen to avoid any alloying at the interface. Electrochemical behavior of the samples was studied using a three-electrode assembly with Pt wire counter-electrode and Ag/AgCl reference electrode (standard potential is 0.198 V). Experiments were performed with Pd/alloy as the working electrode with Cu wire contact made of conductive silver paste. Working area of the Pd/alloy electrodes was 5 cm2 and was patterned using nonconductive epoxy resin. 1 M KOH solution of pH 13 was used as the electrolyte. Base metals like the rare earths dissolve in acidic solution but are stable in alkaline solution.15 All the experiments were carried out using AUTOLAB 30 electrochemistry workbench. For in situ optical transmission, the Pd-capped alloy film was illuminated with white light source (DH 2000, Mikropack make). The light transmitted was collected by a fiber optic cable and was transmitted to a spectrometer (USB2000, ocean optics make) for data analysis. The operational band of the spectrophotometer is in the range 250-800 nm. Light intensity variation (at 632 nm) with time was measured during repeated cycles of electrochemical charging and discharging. All experiments were performed at 25 °C with dry N2 gas, bubbled continuously through the electrolyte to remove the dissolved O2. 3. Results and Discussion Figure 1a shows the room-temperature Cu KR powder X-ray diffractograms confirming the single phase nature of Mm0.2Tb0.8Co2, which crystallizes in cubic (MgCu2 type) C15 structure with space group Fd3hm. The lattice parameters determined by least-square refinement technique of a is 7.212 ( 0.004 Å. The temperature variation of resistivity (F) curves, normalized to F at 300 K, obtained for the hydrides is presented in Figure 1d. A typical value of resistivity at 80 K for the hydride samples is found to be about in the range 10-2 Ω cm which is 4 orders of magnitude higher than the resistivity for the alloy ingots (10-6 Ω cm). This is due to the increase in the number of scattering centers and the decrease in the conduction electron density with an increase in hydrogen content. The F versus T curves for the hydrogen content of 3.4 and 2.6 hydrogen atoms per formula unit are characterized by a change in sign of the temperature coefficient of resistance when compared with the hydride of lower hydrogen concentration. The decrease in resistivity with an increase in temperature is characteristic of a semiconducting material. A change in the electronic structure involving a strong decrease in the conduction electron density may lead to metalsemiconductor transition as the hydrogen concentration is increased. Hydrogen absorption in AB2 alloy systems leads to a decrease in the conduction electron density due to chargetransfer taking place between hydrogen and the 3d band and the reduction in 6S component of the electron density. Further increase in the hydrogen concentration above a critical concentration may decrease N(EF) to zero that could lead to the opening up of a small band gap at the Fermi level and therefore the material behaves like a semiconductor.16,17 Figure 1b shows the room-temperature Cu KR glancing angle X-ray diffractogram of the alloy thin film confirming the polycrystalline C15 structure for the thin film. However, there may be the presence of localized regions of amorphous phase
Figure 1. (a) Cu KR powder X-ray diffractogram of Mm0.2Tb0.8Co2; (b) glancing angle X-ray diffractogram of Mm substituted Mm0.2Tb0.8Co2 alloy thin film; (c) glancing angle X-ray diffractogram of Mm0.2Tb0.8Co2-H after three cycles of hydrogenation and dehydrogenation; and (d) temperature variation of electrical resistivity of Mm0.2Tb0.8Co2-H normalized to ′F at 300 K.
because complete phase purity is not assured by the X-ray diffractogram. The lattice parameter a determined by leastsquare refinement technique is 7.018 ( 0.012 Å. A strained lattice is observed due to lattice mismatch between the substrate (amorphous) and film. The broadened peaks indicate smaller grain size of the thin film when compared with the bulk alloy (Figure 1a). SEM shows a smooth surface morphology of alloy thin film showing the absence of pin holes or agglomerations (Figure 2a). This is the result of stringent chemical treatment followed by a thorough Ar+ ion etch-cleaning procedure adapted on the glass substrates prior to coating. High-quality Pd-capping was then possible to be coated on the alloy film. Pd-capping plays a dual role, namely, a good electrocatalytic material for the dissociation of hydrogen and thus enhancing reaction kinetics, and it is a protective layer minimizing any oxidation of the highly reactive rare earth-based alloy thin film. The compositional analysis gives details regarding the presence and concentration of the constituent elements. Comparing the EDX of the alloy thin film (Figure 2b) and Pd-capped thin film (Figure 2c) reveals a good capping because peaks only due to Pd are seen in the Pd-capped alloy thin film. Contrary to the observation of islandlike growth of Pd thin film over rare earth thin film base, in the present study it has been observed as a closed top
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Figure 3. (a) The transmittance variation in Mm0.2Tb0.8Co2 thin film (100 nm) capped with 10 nm Pd measured during the initial hydrogen loading at a current density 1 mA/cm2 in 1 M KOH. (b) Development of electrode potential and in situ measured optical transmission as a function of time for working electrode covered with Pd overlayer during galvanostatic hydrogen loading and unloading at current density 1 mA/ cm2 in 1 M KOH.
can be written as15,18
H2O + e- T Had + OHThe adsorbed Had subsequently diffuses into the underlying alloy thin film and is then absorbed according to the reaction
Alloy + 2H f (Alloy)H2(δ + H T (Alloy)H3(δ
Figure 2. (a) SEM image showing smooth surface morphology for the alloy thin film. (b) EDX of Mm0.2Tb0.8Co2 alloy thin film. (c) EDX of Pd-capped Mm0.2Tb0.8Co2 alloy thin film.
layer of Pd.6,18 This could be due to the cubic crystal structure of Pd matching with the cubic structure of the alloy. Pure rare earth film crystallizes with hexagonal crystal structure and, hence, favors islandlike growth of Pd over them.6 Electrochemical hydrogen loading in thin film has the advantage of precisely being able to control the hydrogen concentration by careful selection of the electrical current. The charge-transfer reaction in an alkaline electrolyte leading to accumulation/adsorption of Had atoms at the thin film/electrolyte
The first step is unidirectional; however, the second step leading to the transition from dihydride to trihydride is a reversible reaction, and the reversibility can be achieved by changing the cell polarity. The absorbed hydrogen concentration is calculated based on Faraday’s law. The optical transmittance variation (at 632 nm) during initial hydrogen loading of the thin film at constant current density of 1mA/cm2 is shown in Figure 3a. The optical transmittance is close to zero during the initial stages of loading until a hydrogen concentration of ∼2.4 hydrogen atoms per formula unit. Transparency of ∼60-70% is achieved on higher hydrogen concentration (g3 hydrogen atoms per formula unit). Figure 3b shows the development of potential on discharging and charging of the working electrode with a current density of 1 mA/cm2 and the corresponding in situ measured variation of light transmittance at 632 nm. Charging of the working electrode results in accumulation of charges on the electrode/electrolyte interface and hence a sharp fall in the electrode potential by ∼0.5 V. Thereafter hydrogen atoms
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Figure 4. (a) Dependence of switching time with current density during hydrogen charging and discharging. (b) Change in percentage transmission and switching time versus number of cycles of hydriding/ dehydriding in 1 M KOH solution at a current density of 1 mA/cm2.
diffuse into the thin film matrix to form hydride with accompanied change in electronic and optical properties. The corresponding changes in optical transmittance are shown in Figure 3c. The metallic dark color of the film (corresponding to point A in the Figure) changes to a bright yellowish color (point B). When a potential of -1.2 V versus Ag/AgCl is applied, the film reaches the high-hydrogen state and becomes transparent. Note that the maximum transmission of a sample is limited to 60% by the reflection and absorption in the Pd topcoat, and the discharged state shows a minimum of 40% transmittance. Hence, we see a switching action from the dihydride to trihydride state with a transmittance variation of 20-30%. The switching time (defined as the time required to reach 90% of the maximum transmittance) is about 50 s for a current density of 1 mA/cm2. Figure 4a is the plot of current density versus switching time. Current density plays a major role in determining switching time. The switching time decreases on applying higher-current density due to increased reaction rate resulting because of the increased driving force. However, we observe bubble formation above a current density of around 2.5 mA/cm2, which is due to excess availability of electrons during the cathodic polarization leading to evolution of hydrogen gas. To study the cyclic durability of the films, the hydrogenation/ dehydrogenation was performed until the optical data show a significant change. Figure 4b shows the analysis of cyclic stability during repeated electrochemical switching of the film. The plot with square symbols represents the percentage transmittance for every fifth cycle, and the plot with star symbols
Figure 5. The visual observation of electrochromic effect in hydrogenation and dehydrogenation states in Mm0.2Tb0.8Co2 alloy thin film.
represents the switching time required during every consecutive fifth cycle. The cyclic durability of the films are comparable with that of rare earth-based switchable mirror films.19 Visible damage to the films were not observed even after 50 cycles of charging and discharging but there is a considerbale decrease in the maximum transmitted intensity and increase in switching time. The cyclic stability of thin films of Mm0.2Tb0.8Co2 is due to the La content in Mm, which contracts while going from
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Kumar et al. repulsive and energetic concerns result in a maximum of about 3-4 hydrogen atoms per formula unit. Site occupancy predictions in C15 compounds based on geometrical considerations have usually been made using the Westlake criteria, which states that the radius of the interstitial “hole” is calculated assuming touching hard spheres should be larger than 0.4 Å and that the distance between hydrogens should be greater than 2.1 Å.23 This predicts the g site is preferentially occupied at lower hydrogen concentrations and as the H concentration increases, the e site is partially occupied. The above estimation has been observed experimentally by neutron diffraction studies in these systems.24 We also observe the preferential site occupancy from the CV characteristics (Figure 6a). The kink around -0.5 V in the cathodic polarization curve could be due to the g site occupation of the interstitials. However, the e site occupation is not shown by the CV graph due considerable oxygen evolution side reaction at higher voltages. The dehydriding cycle, however, clearly shows two distinct peaks around -0.7 and +0.3 V, respectively, corresponding to both g and e sites. 4. Conclusion
Figure 6. (a) The CV curve for 100 nm Mm-substituted C15 type AB2 thin film capped with 10 nm Pd measured at 10 mV s-1. (b) Unit cell of the cubic C15 structure showing three types of tetrahedral sites, g, e, and b.
dihydride to trihydride, whereas other rare earths mostly expand.19 Structural characterization of hydrogenated thin films (three cycles) carried out by ex situ glancing angle XRD show the amorphous nature of the thin film due to hydrogenation (Figure 1c). Rare earth-based AB2 alloy hydrides at higher hydrogen (>3) concentrations usually are amorphous.20 Amorphous compounds have quite different manifestation of the macroscopic properties like the magnetization behavior that involves the long-range exchange interactions and the hydrogen storage properties.21 However, these compounds do still have the short range order, and the electronic charge-transfer continues to behave similarly. This is evidenced from the isomer shifts at higher concentrations (amorphous systems) not showing any drastic changes; however, they do continue to obey the trend as is seen for the lower hydrogen concentrations.17 Note that this amorphous film still continues to have the switching properties, suggesting the structural independence of switching properties and the robust nature of the switching mechanism in the rare earth-based system. The visual observation of electrochromic effect in hydrogenation and dehydrogenation states is shown in Figure 5. In Figure 6a, the current-voltage (CV) characteristics of the electrode are shown. The electrode had previously been charged and discharged with hydrogen and had been brought to equilibrium at +0.6 V versus Ag/AgCl. The shape of the dehydriding is more complex than the hydriding curve. Apart from the dehydriding, release of hydrogen gas can be expected to attribute to the curve. Theoretically, in these systems the available interstitials for hydrogen inclusion are 12 A2B2 (g site), 4 AB3 (e site), and 1 B4 (b site). An example of each of these sites can be seen in Figure 6b.22 However, electronic
Electrochromism in Mm0.2Tb0.8Co2 alloy hydride is demonstrated. Bulk alloy hydrides show a hydrogen concentration dependent metal-to-semiconductor transition due to chargetransfer taking place between hydrogen and the 3d band, which leads to a strong decrease in the conduction electron density. Thin film of the alloy capped with Pd switches from the reflecting to a transparent state during electrochemical polarization in an alkaline electrolyte at room temperature with a switching time of 50 s. The film is stable with no visible damage even after 50 cycles of charging and discharging with a considerable reduction in switching kinetics. Cyclic voltametry results show preferencial hydrogen interstitial occupation. Acknowledgment. The authors are grateful to DST, IITM, and ARCI for supporting this work. This work is already covered by an application for a patent, and the authors thank Indian Institute of Technology, Madras (IITM) and International Advanced Research Centre for powder Metallurgy and New materials, Hyderabad (ARCI) for filing the patent. References and Notes (1) (a) Hydrogen in Intermetallic Compounds - I: Topics in Applied Physics; Schlapbach, L., Ed.; Springer-Verlag: Berlin, 1988; Vol. 63. (b) Hydrogen in Intermetallic Compounds - II: Topics in Applied Physics; Schlapbach, L., Ed.; Springer-Verlag: Berlin, 1992; Vol. 67. (2) Pourarian, F.; Wallace, W. E.; Malik, S. K. J. Magn. Magn. Mater. 1982, 25, 299. (3) Buschow, K. H. J.; Van der Kraan, A. M. J. Less-Common Met. 1983, 91, 203. (4) Buschow, K. H. J. J. Less-Common Met. 1977, 51, 173. (5) Libowitz, G. G.; Pack, J. G.; Binnie, W. P. Phys. ReV. B 1972, 6, 4540. (6) Huiberts, J. N.; Griessen, R.; Rector, J. H.; Wijngaarden, R. J.; Dekker, J. P.; de Groot, D. G.; Koeman, N. J. Nature 1996, 380, 231. (7) Kremers, M.; Koeman, N. J.; Griessen, R.; Notten, P. H. L.; Tolboom, R.; Kelly, P. J.; Duine, P. A. Phys. ReV. B 1998, 57 (8), 4943. (8) Van Gogh, A. T. M.; Nagengast, D. G.; Kooij, E. S.; Koeman, N. J.; Rector, J. H.; Griessen, R.; Flipse, C. J.; Smeets, R. J. J. G. A. M. Phys. ReV. B 2001, 63, 195105. (9) Van der Sluis, P.; Ouwerkerk, M.; Duine, P. A. Appl. Phys. Lett. 1997, 70 (25), 3356. (10) Von Rottkay, K.; Rubin, M.; Michalak, F.; Armitage, R.; Richardson, T.; Slack, J.; Duine, P. A. Electrochim. Acta 1999, 44, 3093. (11) Nagengast, D. G.; Van Gogh, A. T. M.; Kooij, E. S.; Dam, B.; Griessen, R. Appl. Phys. Lett. 1999, 75 (14), 2050. (12) Richardson, T. J.; Slack, J. L.; Armitage, R. D.; Kostecki, R.; Farangis, B.; Rubin, M. D. Appl. Phys. Lett. 2001, 78, 3047.
Mischmetal-Based AB2 Alloy Hydride Thin Film (13) Krishna Kumar, M.; Ramaprabhu, S. Int. J. Hydrogen Energy 2006 doi:10.1016/j.ijhydene.2006.09.016. (14) Aoki, K.; Li, H. W.; Ishikawa, K. J. Alloys Compd. 2005, 404406, 559. (15) Notten, P. H. L.; Kremers, M.; Griesen, R. J. Electrochem. Soc. 1996 143, 3348. (16) Buschow, K. H. J.; Van Dlepen, A. M. Solid State Commun. 1976 19, 79. (17) Cohen, I. R. L.; West, K. W.; Oliver, F.; Buschow, K. H. J. Phys. ReV. B 1980, 21, 941. (18) Pushendra, K.; Malhothra, L. K. Electrochim. Acta 2004, 49, 3355.
J. Phys. Chem. C, Vol. 111, No. 24, 2007 8537 (19) Janner, A.-M.; van der Sluis, P.; Mercier, V. Electrochim. Acta 2001, 46 (13-14), 2173. (20) Aoki, K. Mater. Sci. Eng., A 2001, 304-306, 45. (21) Li, X. G.; Chiba, A.; Takahashi, S.; Aoki, K.; Masumoto, T. Intermetallics 1999, 7 (2), 207. (22) Fernandez, J. F.; Kemali, M.; Ross, D. K.; Sanchez, C. J. Phys.: Condens. Matter 1999, 11, 10353. (23) Westlake, D. G. J. Alloys Compd. 1983, 90, 251. (24) Somenkov, V. A.; Irodova, A. V. J. Less-Common Met. 1984, 101, 481.
