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Surface study of liquid water treated and water vapor treated Mg2.35Ni alloy
International Journal of Hydrogen Energy 27 (2002) 79–83
www.elsevier.com/locate/ijhydene
Surface study of liquid water treated and water vapor treated Mg2:35Ni alloy M.D. Hamptona;∗ , J.K. Lomnessa;b , L.A. Giannuzzib a Department
of Chemistry, University of Central Florida, 4000 Central Florida Boulevard, Orlando, FL 32816-2366, USA b Mechanical Materials and Aerospace Engineering, University of Central Florida, 4000 Central Florida Boulevard, Orlando, FL 32816-2450, USA
Abstract Magnesium nickel alloy (Mg2:35 Ni) has been considered an excellent hydrogen storage medium because it has a high hydrogen capacity, forms a very stable hydride, is inexpensive, and it presents no environmental hazards. One of the major problems associated with the use of Mg2:35 Ni alloy for hydrogen storage is its initial activation for hydrogen uptake. Earlier work in this laboratory showed that treatment of Mg2:35 Ni with either liquid water or water vapor, activates the alloy for hydrogen up-take. In the present study, the surface modi6cation of Mg2:35 Ni by liquid water and water vapor is characterized. x-ray photoelectron spectroscopy, and transmission electron microscopy suggest the presence of Mg(OH)2 on the surface. It is believed that this is the 6rst report showing the presence of hydroxides on the surface of an active hydrogen storage alloy. ? 2001 Published by Elsevier Science Ltd on behalf of the International Association for Hydrogen Energy. Keywords: Activation; FIB; Magnesium–nickel alloy; Water activation; Surface analysis; Surface hydroxide; TEM
1. Introduction Hydrogen has been considered an ideal alternative fuel because of its exceedingly high energy density, its well-known reaction characteristics, and its environmental friendliness when utilized properly. While it is di>cult to improve on hydrogen as a fuel source, it is necessary to improve methods of storing hydrogen. The three major forms for storing hydrogen are the liquid form, as a compressed gas, or in the solid form as metallic hydrides. Storage of liquid hydrogen is not e>cient because of the energy required to liquefy H2 , the storage tanks are exceedingly heavy, and the loss of hydrogen from boil-o? may be extensive if stored for long periods of time [1]. Compressed gas also requires the use of extremely large heavy containers for storage [2]. These methods not only
∗ Corresponding author. Tel.: +1-407-823-2136; fax: +1-407823-2252. E-mail address: [email protected] (M.D. Hampton).
involve pressure, volume, and weight concerns, but also major safety issues. If a tank containing large amounts of liquid or gaseous hydrogen ruptured, allowing the release of this very reactive element into the environment, catastrophic consequences could result. Storage of hydrogen in the form of a metal hydride o?ers distinct advantages in terms of both volumetric e>ciency and safety. As a result, an enormous amount of research is currently being performed on metal-hydride systems. Practical application of these systems for the storage of hydrogen can only occur when these systems are better understood. Most metals and alloys react directly and reversibly with hydrogen to form metal hydrides. One major obstacle encountered with the application of these materials for hydrogen storage is their inability to absorb hydrogen at reasonable temperatures and pressures. This problem has been attributed to the ambient formation of oxide and=or hydroxide surface layers that act as a barrier for hydrogen di?usion. Since this surface contamination may act as a barrier to hydrogen, a surface treatment is necessary for initial activation for hydrogen uptake and to achieve stable hydriding
0360-3199/02/$ 20.00 ? 2001 Published by Elsevier Science Ltd on behalf of the International Association for Hydrogen Energy. PII: S 0 3 6 0 - 3 1 9 9 ( 0 1 ) 0 0 0 9 2 - 1
80
M.D. Hampton et al. / International Journal of Hydrogen Energy 27 (2002) 79–83
properties. Several approaches have been utilized to overcome the di>culty of initial activation and improve hydriding characteristics of metals and alloys [3– 6]. One method reported, involved the use of a dilute solution of HCl to etch the surface of Mg2 Ni [3]. It was observed that the HCl etch resulted in the removal of MgO and dissolved away NiO from the surface. The new surface consisted mainly of metallic porous Ni microspheres that possessed a high catalytic activity towards hydrogen chemisorption. Another study reported the use of aqueous Iuoride to remove oxides and=or hydroxides from the surface of La–Ni–Al and Mg–Ni alloys [4]. Suda et al. reported that Iuoride treatment of LaNi4:7 Al0:3 produced a Ni-rich sub-surface under a surface of LaF3 [4]. They proposed that LaF3 protected the sub-layer from contamination by substances such as water and oxides, while cracks in the Iuoride layer provided a path for hydrogen molecules to di?use to the Ni-rich sub-surface, where dissociation into atomic hydrogen occurs. Previous work in this laboratory has shown that both liquid water and water vapor treatment of Mg2:35 Ni provided very rapid activation for hydrogen uptake and greatly improved hydriding and dehydriding properties [5,6]. Mg2:35 Ni, activated using liquid water, absorbed hydrogen at relatively low temperatures (413 K, at 827 kPa H2 ) resulting in a hydrogen content approaching the theoretical maximum (Mg2:35 NiH4:7 ). Water vapor treatment also essentially caused complete activation of the alloy. Many studies have commented on the deleterious e?ects of oxide and hydroxide layers on the reactivity of metals and alloys with hydrogen. Water is commonly listed as a poison to hydrogen systems [7,8]. In the light of the studies above, more work is obviously required on the role of oxides and hydroxides in the interaction of metals and alloys with hydrogen. It has been shown that the e?ect of a pressure of 10−8 Torr at room temperature on chemical compounds corresponds ◦ to a temperature of 350 – 400 C at atmospheric pressure [9]. Since surface analysis techniques utilize vacuum chambers in this range, it is likely that surface hydroxides are commonly destroyed in the process of analysis. Thus, the role of hydroxides in hydrogen-storage systems is not well understood. The purpose of this paper is to present the results of a study of the surfaces of magnesium–nickel alloys. Alloys that have been rendered totally unreactive with hydrogen, as well as those that have been activated with liquid water and with water vapor, have been studied. The results obtained, suggest the presence of a metal-hydroxide on the surface. To the best of our knowledge, this is the 6rst time that hydroxides have been shown to be present on the surface of an active hydrogen storage alloy. Supporting data obtained from x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) will be presented.
2. Experimental The Hy–Stor 301 (Ergenics) alloy was used in this study. This alloy has the formula Mg2:35 Ni and is slightly enriched in Mg to prevent disproportionation of Mg2 Ni to Mg and MgNi2 . Deionization of water was achieved using a Culligan ion-exchange system and then further puri6ed with a Barnstead B-pure polishing system. High-purity grade argon was used without further puri6cation. As previously reported [5,6], the as-received Hy–Stor 301 readily absorbed hydrogen before treatment. As a result, the following procedure was used to deactivate the as-received Hy–Stor 301. This process involved stirring the alloy in water for 2:5 h and then drying the alloy in air for 20 h. The alloy was further dried in reduced pressure at approximately 370 K in a vacuum oven for 30 – 40 min. The dried alloy was ground using a mortar and pestle and placed in an atmosphere containing 37.1% relative humidity (using sulfuric acid with a speci6c gravity of 1.40 in a sealed chamber) for 21:5 h. In the 6nal step of the deactivation process, the alloy was placed in the furnace of a SETARAM DSC 111 di?erential scanning calorimeter (DSC) under a slow Iow of argon and heated from 323 to 723 K at 3 K=min before cooling to room temperature. A portion of the sample was checked for deactivation by attempting to hydride it in the DSC, according to the procedures described in earlier investigations [5,6]. Liquid water treatment of the samples involved placing the ground alloy in deionized water with constant stirring for 2:5 h and then drying in air for 12 h. A portion of the ground alloy was also placed in an atmosphere containing 100% humidity for 1 h and is de6ned as the water vapor treated alloy. XPS analyses of the surface of the liquid water and water vapor treated samples were carried out using a Kratos DS800. Microstructural analyses were performed using TEM with a Philips EM430 operating at 300 keV. TEM specimens were prepared with the lift-out technique using an FEI 200TEM focused ion beam (FIB) workstation [10,11]. The particles were sputter-coated with gold–palladium (Au–Pd), prior to insertion in the FIB, to protect the outer surface from spurious sputtering due to FIB imaging and to protect the surface from the high vacuum. A suitable particle of the alloy was located and platinum was deposited over the region of interest to protect it from subsequent FIB milling operations. Preliminary trench FIB cuts were then milled on the front, back, bottom, and sides of the region of interest, resulting in a specimen geometry of approximately 10 m long ×5 m wide×1 m thick. The specimen was milled to electron transparency (∼ 100 nm thick) before 6nishing cuts were made at the sides to free the specimen from the bulk. The free-standing specimen was then lifted out and placed on a carbon-coated Cu grid using a hydraulic micromanipulator with the aid of a light optical microscope.
M.D. Hampton et al. / International Journal of Hydrogen Energy 27 (2002) 79–83
permeable to hydrogen. While these data do not con6rm the formation of Mg(OH)2 , they do support the possibility. This is not surprising, since the surfaces of the samples were not protected from the high-vacuum conditions during XPS analysis, resulting in the probable partial decomposition of the surface layer. The cross section bright 6eld (BF) TEM images of the surface regions of the deactivated, liquid water, and water vapor treated Mg2 Ni alloy are shown in Figs. 1–3, respectively. Fig. 1 shows a BF image of the alloy, the Au–Pd sputter coating and the Pt FIB layer. There is no identi6able surface layer within the resolution of this image. The lack of a surface layer is consistent with the apparent removal of organic contaminants during the deactivation process. The TEM images in Figs. 2 and 3 show a substantial change in the characteristics of the surface of the alloy, upon treatment with liquid water and water vapor. The BF TEM images of the liquid water treated (Fig. 2) and water vapor treated (Fig. 3) alloy shows the formation of surface layers, approximately, 470 and 130 nm thick, respectively. These results support the XPS analysis of the alloy and con6rm that the alloy surface was more a?ected by the liquid water treatment than by the water vapor treatment, since the liquid water treated sample shows a ¿ 3-fold increase in the thickness of the surface layer. The surface of the liquid water treated specimen (Fig. 2) shows a greater degree of di?raction contrast than that of the water vapor treated specimen (Fig. 3). However, the di?raction contrast from both images indicates that these surface layers are 6ne grained and polycrystalline in nature. The indexed selected area di?raction patterns obtained from the surface layers of the liquid water and water vapor treated samples are shown in Figs. 4 and 5, respectively, and support the contrast in the images. The ring patterns from the surface layers of the liquid water treated specimen (Fig. 4) and the water vapor treated specimen, Fig. 5, indicate that the surface layers are polycrystalline in nature. The d-spacings, intensities, and the (h k l) values for the Mg(OH)2 structure are presented in Table 2 [12]. The ring pattern for the liquid water treated specimen (Fig. 4) shows the (0 0 1), (1 0 1), and (1 1 0) planes. Upon closer inspection, the (1 0 0), and (1 0 2) planes may also be observed. The ring pattern for the water vapor treated specimen (Fig. 5) shows the (1 0 1); (1 0 2), and (1 1 0) planes. The (1 0 0) plane may also be observed in this di?raction
3. Results and discussion The unexpected activity of the as-received alloy was attributed to the presence of a surface contaminant. In order to remove the possible surface contaminant and restore the alloy to its inactive state, a deactivation procedure was developed, in which the surface was actively reacted and eroded away. Then conditions were adjusted to allow a thick, insulating oxide coating to form on the alloy surface. Following completion of this procedure, deactivation was con6rmed with the technique described earlier [3,4]. The XPS data obtained from the as-received, deactivated, liquid water treated, and water vapor treated alloy are presented in Table 1. The as-received material had a relatively large amount of carbon (34 at%) and oxygen (54 at%), a small amount of magnesium (12 at%), and only a trace of nickel (¡ 1 at%) on the surface. After the deactivation procedure, the new surface contained less than half the carbon that the original surface contained, indicating the possibility that an organic contaminant was removed during deactivation. The deactivation procedure also resulted in a doubling of the amount of surface magnesium and an oxygen increase of about 3 at%. There was no signi6cant change in the amount of nickel present after the deactivation procedure. These changes are consistent with the removal of an organic surface contaminant and the establishment of a magnesium-oxide coating. The lack of reactivity with hydrogen con6rmed that the coating present was thick and continuous enough to be protective (i.e., prevent hydrogen di?usion). Treatment of the deactivated alloy with water, either in the liquid or the vapor form, resulted in a decrease in the amount of magnesium and an increase in the amount of oxygen present in the surface layer. These treatments produce an increase in the oxygen–magnesium ratio, as would be expected if magnesium hydroxide was formed from magnesium oxide as shown by MgO + H2 O → Mg(OH)2 :
81
(1)
Also, as expected, the decrease in magnesium and increase in oxygen was greatest for the liquid water treated sample due to the more vigorous reaction conditions. Both treatments, liquid water and water vapor, rendered the alloy active towards hydrogen, indicating that the new coating was
Table 1 Data obtained from X-ray photoelectron spectroscopy for deactivated, water treated and water vapor treated Mg2:35 Ni Sample
Mg 2p (at%)
C 1s (at%)
O 1s (at%)
Ni 2p (at%)
Increase in at% O
Increase in O : Mg ratio
As-received Deactivated Liquid water treated Water vapor treated
11.8 28.4 23.8 25.2
33.6 14.4 8.7 15.6
53.8 56.7 67.1 58.9
0.7 0.5 0.4 0.3
— — 10.4 0.2
— — 2.8 2.3
82
M.D. Hampton et al. / International Journal of Hydrogen Energy 27 (2002) 79–83
Fig. 1. Cross section BF TEM image of the surface of the deactivated Mg2:35 Ni.
Fig. 4. Indexed selected area di?raction pattern for the surface layer of the liquid water treated Mg2:35 Ni.
Fig. 2. Cross section BF TEM image of the surface of the liquid water treated Mg2:35 Ni.
Fig. 3. Cross section BF TEM image of the surface of the water vapor treated Mg2:35 Ni.
pattern, however, the (0 0 1) plane is obscured by the center bright spot. The actual positions of the rings as well as the ratios of their d-spacings, obtained from the rings, are consistent with the hexagonal Mg(OH)2 structure. Thus, TEM analyses of these water and vapor treated materials con6rm the presence of Mg(OH)2 on the surface. The formation of this surface layer most likely results from the disruption of the insulating oxide layer by the formation of magnesium hydroxide upon treatment of the alloy with liquid water and water vapor, according to Eq. (1). Thus, the activation of Mg2:35 Ni for hydrogen uptake under reasonable temperature and pressure conditions appears to be directly related to the disruption of the oxide layer by the formation of Mg(OH)2 .
Fig. 5. Indexed selected area di?raction pattern for the surface layer of the water vapor treated Mg2:35 Ni.
4. Conclusions Mg2:35 Ni alloys that have been rendered totally unreactive with hydrogen as well as those that have been activated with liquid water and with water vapor have been studied. It has been shown that upon treatment with liquid water and water vapor, a prominent surface layer forms on the surface of the magnesium–nickel alloy. XPS analysis of these surface layers supports the conclusion that the surface layer of the liquid water and water vapor treated Mg2:35 Ni is Mg(OH)2 .
M.D. Hampton et al. / International Journal of Hydrogen Energy 27 (2002) 79–83 Table 2 The 6rst 5 d-spacings, intensities, and (h k l) values for magnesium hydroxide d(nm)
Intensity
h
k
l
0.477 0.2725 0.2365 0.1794 0.1573
90 6 100 55 35
0 1 1 1 1
0 0 0 0 1
1 0 1 2 0
The selected area di?raction patterns obtained for both the liquid water treated and water vapor treated specimen indicate that the surface layer consists of the polycrystalline Mg(OH)2 phase, and thus, corroborate the XPS results. It is believed that this is the 6rst report showing the presence of hydroxides on the surface of an active hydrogen storage alloy. Acknowledgements This work was made possible through the generous support of the Florida Space Grant Consortium, the Advanced Materials Processing Center and the Department of
83
Chemistry at the University of Central Florida, and NSF DMR 9703281. Special thanks are extended to Cirent Semiconductor and NASA, Kennedy Space Center for their gracious instrumentation support. References [1] Ivey DG, Northwood DO. J Mater Sci 1983;18:321. [2] Sandrock GD, Huston EL. Chemtech 1981;754. [3] Zhu HY, Chen CP, Lei YQ, Wu J, Wang QD. J Less-Common Met 1991;174:873. [4] Wang X, Inoue AI, Watanabe H, Ebihara T, Suda S. Res Rep Kogakuin Univ 1992;73:57. [5] Hampton MD, Lomness JK. Int J Hydrogen Energy 1999;24:175. [6] Hampton MD, Juturu R, Lomness JK. Int J Hydrogen Energy 1999;24:981. [7] Liu FJ, Sandrock G, Suda S. J Alloys Compounds 1992;190:57. [8] Sandrock G, Goodell P. J Less-Common Met 1980;73:161. [9] Hirokawa K, Danzaki Y. Surf Interface Anal 1982;4:63. [10] Giannuzzi LA, Drown JL, Brown SR, Irwin RB, Stevie FA. Mat Res Soc Symp Proc 1997;480:19. [11] Giannuzzi LA, Drown JL, Brown SR, Irwin RB, Stevie FA. Microsc Res Tech 1998;41:285. [12] JCPDS File 07-0239. International Centre for Di?raction Data, 1996.
www.elsevier.com/locate/ijhydene
Surface study of liquid water treated and water vapor treated Mg2:35Ni alloy M.D. Hamptona;∗ , J.K. Lomnessa;b , L.A. Giannuzzib a Department
of Chemistry, University of Central Florida, 4000 Central Florida Boulevard, Orlando, FL 32816-2366, USA b Mechanical Materials and Aerospace Engineering, University of Central Florida, 4000 Central Florida Boulevard, Orlando, FL 32816-2450, USA
Abstract Magnesium nickel alloy (Mg2:35 Ni) has been considered an excellent hydrogen storage medium because it has a high hydrogen capacity, forms a very stable hydride, is inexpensive, and it presents no environmental hazards. One of the major problems associated with the use of Mg2:35 Ni alloy for hydrogen storage is its initial activation for hydrogen uptake. Earlier work in this laboratory showed that treatment of Mg2:35 Ni with either liquid water or water vapor, activates the alloy for hydrogen up-take. In the present study, the surface modi6cation of Mg2:35 Ni by liquid water and water vapor is characterized. x-ray photoelectron spectroscopy, and transmission electron microscopy suggest the presence of Mg(OH)2 on the surface. It is believed that this is the 6rst report showing the presence of hydroxides on the surface of an active hydrogen storage alloy. ? 2001 Published by Elsevier Science Ltd on behalf of the International Association for Hydrogen Energy. Keywords: Activation; FIB; Magnesium–nickel alloy; Water activation; Surface analysis; Surface hydroxide; TEM
1. Introduction Hydrogen has been considered an ideal alternative fuel because of its exceedingly high energy density, its well-known reaction characteristics, and its environmental friendliness when utilized properly. While it is di>cult to improve on hydrogen as a fuel source, it is necessary to improve methods of storing hydrogen. The three major forms for storing hydrogen are the liquid form, as a compressed gas, or in the solid form as metallic hydrides. Storage of liquid hydrogen is not e>cient because of the energy required to liquefy H2 , the storage tanks are exceedingly heavy, and the loss of hydrogen from boil-o? may be extensive if stored for long periods of time [1]. Compressed gas also requires the use of extremely large heavy containers for storage [2]. These methods not only
∗ Corresponding author. Tel.: +1-407-823-2136; fax: +1-407823-2252. E-mail address: [email protected] (M.D. Hampton).
involve pressure, volume, and weight concerns, but also major safety issues. If a tank containing large amounts of liquid or gaseous hydrogen ruptured, allowing the release of this very reactive element into the environment, catastrophic consequences could result. Storage of hydrogen in the form of a metal hydride o?ers distinct advantages in terms of both volumetric e>ciency and safety. As a result, an enormous amount of research is currently being performed on metal-hydride systems. Practical application of these systems for the storage of hydrogen can only occur when these systems are better understood. Most metals and alloys react directly and reversibly with hydrogen to form metal hydrides. One major obstacle encountered with the application of these materials for hydrogen storage is their inability to absorb hydrogen at reasonable temperatures and pressures. This problem has been attributed to the ambient formation of oxide and=or hydroxide surface layers that act as a barrier for hydrogen di?usion. Since this surface contamination may act as a barrier to hydrogen, a surface treatment is necessary for initial activation for hydrogen uptake and to achieve stable hydriding
0360-3199/02/$ 20.00 ? 2001 Published by Elsevier Science Ltd on behalf of the International Association for Hydrogen Energy. PII: S 0 3 6 0 - 3 1 9 9 ( 0 1 ) 0 0 0 9 2 - 1
80
M.D. Hampton et al. / International Journal of Hydrogen Energy 27 (2002) 79–83
properties. Several approaches have been utilized to overcome the di>culty of initial activation and improve hydriding characteristics of metals and alloys [3– 6]. One method reported, involved the use of a dilute solution of HCl to etch the surface of Mg2 Ni [3]. It was observed that the HCl etch resulted in the removal of MgO and dissolved away NiO from the surface. The new surface consisted mainly of metallic porous Ni microspheres that possessed a high catalytic activity towards hydrogen chemisorption. Another study reported the use of aqueous Iuoride to remove oxides and=or hydroxides from the surface of La–Ni–Al and Mg–Ni alloys [4]. Suda et al. reported that Iuoride treatment of LaNi4:7 Al0:3 produced a Ni-rich sub-surface under a surface of LaF3 [4]. They proposed that LaF3 protected the sub-layer from contamination by substances such as water and oxides, while cracks in the Iuoride layer provided a path for hydrogen molecules to di?use to the Ni-rich sub-surface, where dissociation into atomic hydrogen occurs. Previous work in this laboratory has shown that both liquid water and water vapor treatment of Mg2:35 Ni provided very rapid activation for hydrogen uptake and greatly improved hydriding and dehydriding properties [5,6]. Mg2:35 Ni, activated using liquid water, absorbed hydrogen at relatively low temperatures (413 K, at 827 kPa H2 ) resulting in a hydrogen content approaching the theoretical maximum (Mg2:35 NiH4:7 ). Water vapor treatment also essentially caused complete activation of the alloy. Many studies have commented on the deleterious e?ects of oxide and hydroxide layers on the reactivity of metals and alloys with hydrogen. Water is commonly listed as a poison to hydrogen systems [7,8]. In the light of the studies above, more work is obviously required on the role of oxides and hydroxides in the interaction of metals and alloys with hydrogen. It has been shown that the e?ect of a pressure of 10−8 Torr at room temperature on chemical compounds corresponds ◦ to a temperature of 350 – 400 C at atmospheric pressure [9]. Since surface analysis techniques utilize vacuum chambers in this range, it is likely that surface hydroxides are commonly destroyed in the process of analysis. Thus, the role of hydroxides in hydrogen-storage systems is not well understood. The purpose of this paper is to present the results of a study of the surfaces of magnesium–nickel alloys. Alloys that have been rendered totally unreactive with hydrogen, as well as those that have been activated with liquid water and with water vapor, have been studied. The results obtained, suggest the presence of a metal-hydroxide on the surface. To the best of our knowledge, this is the 6rst time that hydroxides have been shown to be present on the surface of an active hydrogen storage alloy. Supporting data obtained from x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) will be presented.
2. Experimental The Hy–Stor 301 (Ergenics) alloy was used in this study. This alloy has the formula Mg2:35 Ni and is slightly enriched in Mg to prevent disproportionation of Mg2 Ni to Mg and MgNi2 . Deionization of water was achieved using a Culligan ion-exchange system and then further puri6ed with a Barnstead B-pure polishing system. High-purity grade argon was used without further puri6cation. As previously reported [5,6], the as-received Hy–Stor 301 readily absorbed hydrogen before treatment. As a result, the following procedure was used to deactivate the as-received Hy–Stor 301. This process involved stirring the alloy in water for 2:5 h and then drying the alloy in air for 20 h. The alloy was further dried in reduced pressure at approximately 370 K in a vacuum oven for 30 – 40 min. The dried alloy was ground using a mortar and pestle and placed in an atmosphere containing 37.1% relative humidity (using sulfuric acid with a speci6c gravity of 1.40 in a sealed chamber) for 21:5 h. In the 6nal step of the deactivation process, the alloy was placed in the furnace of a SETARAM DSC 111 di?erential scanning calorimeter (DSC) under a slow Iow of argon and heated from 323 to 723 K at 3 K=min before cooling to room temperature. A portion of the sample was checked for deactivation by attempting to hydride it in the DSC, according to the procedures described in earlier investigations [5,6]. Liquid water treatment of the samples involved placing the ground alloy in deionized water with constant stirring for 2:5 h and then drying in air for 12 h. A portion of the ground alloy was also placed in an atmosphere containing 100% humidity for 1 h and is de6ned as the water vapor treated alloy. XPS analyses of the surface of the liquid water and water vapor treated samples were carried out using a Kratos DS800. Microstructural analyses were performed using TEM with a Philips EM430 operating at 300 keV. TEM specimens were prepared with the lift-out technique using an FEI 200TEM focused ion beam (FIB) workstation [10,11]. The particles were sputter-coated with gold–palladium (Au–Pd), prior to insertion in the FIB, to protect the outer surface from spurious sputtering due to FIB imaging and to protect the surface from the high vacuum. A suitable particle of the alloy was located and platinum was deposited over the region of interest to protect it from subsequent FIB milling operations. Preliminary trench FIB cuts were then milled on the front, back, bottom, and sides of the region of interest, resulting in a specimen geometry of approximately 10 m long ×5 m wide×1 m thick. The specimen was milled to electron transparency (∼ 100 nm thick) before 6nishing cuts were made at the sides to free the specimen from the bulk. The free-standing specimen was then lifted out and placed on a carbon-coated Cu grid using a hydraulic micromanipulator with the aid of a light optical microscope.
M.D. Hampton et al. / International Journal of Hydrogen Energy 27 (2002) 79–83
permeable to hydrogen. While these data do not con6rm the formation of Mg(OH)2 , they do support the possibility. This is not surprising, since the surfaces of the samples were not protected from the high-vacuum conditions during XPS analysis, resulting in the probable partial decomposition of the surface layer. The cross section bright 6eld (BF) TEM images of the surface regions of the deactivated, liquid water, and water vapor treated Mg2 Ni alloy are shown in Figs. 1–3, respectively. Fig. 1 shows a BF image of the alloy, the Au–Pd sputter coating and the Pt FIB layer. There is no identi6able surface layer within the resolution of this image. The lack of a surface layer is consistent with the apparent removal of organic contaminants during the deactivation process. The TEM images in Figs. 2 and 3 show a substantial change in the characteristics of the surface of the alloy, upon treatment with liquid water and water vapor. The BF TEM images of the liquid water treated (Fig. 2) and water vapor treated (Fig. 3) alloy shows the formation of surface layers, approximately, 470 and 130 nm thick, respectively. These results support the XPS analysis of the alloy and con6rm that the alloy surface was more a?ected by the liquid water treatment than by the water vapor treatment, since the liquid water treated sample shows a ¿ 3-fold increase in the thickness of the surface layer. The surface of the liquid water treated specimen (Fig. 2) shows a greater degree of di?raction contrast than that of the water vapor treated specimen (Fig. 3). However, the di?raction contrast from both images indicates that these surface layers are 6ne grained and polycrystalline in nature. The indexed selected area di?raction patterns obtained from the surface layers of the liquid water and water vapor treated samples are shown in Figs. 4 and 5, respectively, and support the contrast in the images. The ring patterns from the surface layers of the liquid water treated specimen (Fig. 4) and the water vapor treated specimen, Fig. 5, indicate that the surface layers are polycrystalline in nature. The d-spacings, intensities, and the (h k l) values for the Mg(OH)2 structure are presented in Table 2 [12]. The ring pattern for the liquid water treated specimen (Fig. 4) shows the (0 0 1), (1 0 1), and (1 1 0) planes. Upon closer inspection, the (1 0 0), and (1 0 2) planes may also be observed. The ring pattern for the water vapor treated specimen (Fig. 5) shows the (1 0 1); (1 0 2), and (1 1 0) planes. The (1 0 0) plane may also be observed in this di?raction
3. Results and discussion The unexpected activity of the as-received alloy was attributed to the presence of a surface contaminant. In order to remove the possible surface contaminant and restore the alloy to its inactive state, a deactivation procedure was developed, in which the surface was actively reacted and eroded away. Then conditions were adjusted to allow a thick, insulating oxide coating to form on the alloy surface. Following completion of this procedure, deactivation was con6rmed with the technique described earlier [3,4]. The XPS data obtained from the as-received, deactivated, liquid water treated, and water vapor treated alloy are presented in Table 1. The as-received material had a relatively large amount of carbon (34 at%) and oxygen (54 at%), a small amount of magnesium (12 at%), and only a trace of nickel (¡ 1 at%) on the surface. After the deactivation procedure, the new surface contained less than half the carbon that the original surface contained, indicating the possibility that an organic contaminant was removed during deactivation. The deactivation procedure also resulted in a doubling of the amount of surface magnesium and an oxygen increase of about 3 at%. There was no signi6cant change in the amount of nickel present after the deactivation procedure. These changes are consistent with the removal of an organic surface contaminant and the establishment of a magnesium-oxide coating. The lack of reactivity with hydrogen con6rmed that the coating present was thick and continuous enough to be protective (i.e., prevent hydrogen di?usion). Treatment of the deactivated alloy with water, either in the liquid or the vapor form, resulted in a decrease in the amount of magnesium and an increase in the amount of oxygen present in the surface layer. These treatments produce an increase in the oxygen–magnesium ratio, as would be expected if magnesium hydroxide was formed from magnesium oxide as shown by MgO + H2 O → Mg(OH)2 :
81
(1)
Also, as expected, the decrease in magnesium and increase in oxygen was greatest for the liquid water treated sample due to the more vigorous reaction conditions. Both treatments, liquid water and water vapor, rendered the alloy active towards hydrogen, indicating that the new coating was
Table 1 Data obtained from X-ray photoelectron spectroscopy for deactivated, water treated and water vapor treated Mg2:35 Ni Sample
Mg 2p (at%)
C 1s (at%)
O 1s (at%)
Ni 2p (at%)
Increase in at% O
Increase in O : Mg ratio
As-received Deactivated Liquid water treated Water vapor treated
11.8 28.4 23.8 25.2
33.6 14.4 8.7 15.6
53.8 56.7 67.1 58.9
0.7 0.5 0.4 0.3
— — 10.4 0.2
— — 2.8 2.3
82
M.D. Hampton et al. / International Journal of Hydrogen Energy 27 (2002) 79–83
Fig. 1. Cross section BF TEM image of the surface of the deactivated Mg2:35 Ni.
Fig. 4. Indexed selected area di?raction pattern for the surface layer of the liquid water treated Mg2:35 Ni.
Fig. 2. Cross section BF TEM image of the surface of the liquid water treated Mg2:35 Ni.
Fig. 3. Cross section BF TEM image of the surface of the water vapor treated Mg2:35 Ni.
pattern, however, the (0 0 1) plane is obscured by the center bright spot. The actual positions of the rings as well as the ratios of their d-spacings, obtained from the rings, are consistent with the hexagonal Mg(OH)2 structure. Thus, TEM analyses of these water and vapor treated materials con6rm the presence of Mg(OH)2 on the surface. The formation of this surface layer most likely results from the disruption of the insulating oxide layer by the formation of magnesium hydroxide upon treatment of the alloy with liquid water and water vapor, according to Eq. (1). Thus, the activation of Mg2:35 Ni for hydrogen uptake under reasonable temperature and pressure conditions appears to be directly related to the disruption of the oxide layer by the formation of Mg(OH)2 .
Fig. 5. Indexed selected area di?raction pattern for the surface layer of the water vapor treated Mg2:35 Ni.
4. Conclusions Mg2:35 Ni alloys that have been rendered totally unreactive with hydrogen as well as those that have been activated with liquid water and with water vapor have been studied. It has been shown that upon treatment with liquid water and water vapor, a prominent surface layer forms on the surface of the magnesium–nickel alloy. XPS analysis of these surface layers supports the conclusion that the surface layer of the liquid water and water vapor treated Mg2:35 Ni is Mg(OH)2 .
M.D. Hampton et al. / International Journal of Hydrogen Energy 27 (2002) 79–83 Table 2 The 6rst 5 d-spacings, intensities, and (h k l) values for magnesium hydroxide d(nm)
Intensity
h
k
l
0.477 0.2725 0.2365 0.1794 0.1573
90 6 100 55 35
0 1 1 1 1
0 0 0 0 1
1 0 1 2 0
The selected area di?raction patterns obtained for both the liquid water treated and water vapor treated specimen indicate that the surface layer consists of the polycrystalline Mg(OH)2 phase, and thus, corroborate the XPS results. It is believed that this is the 6rst report showing the presence of hydroxides on the surface of an active hydrogen storage alloy. Acknowledgements This work was made possible through the generous support of the Florida Space Grant Consortium, the Advanced Materials Processing Center and the Department of
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Chemistry at the University of Central Florida, and NSF DMR 9703281. Special thanks are extended to Cirent Semiconductor and NASA, Kennedy Space Center for their gracious instrumentation support. References [1] Ivey DG, Northwood DO. J Mater Sci 1983;18:321. [2] Sandrock GD, Huston EL. Chemtech 1981;754. [3] Zhu HY, Chen CP, Lei YQ, Wu J, Wang QD. J Less-Common Met 1991;174:873. [4] Wang X, Inoue AI, Watanabe H, Ebihara T, Suda S. Res Rep Kogakuin Univ 1992;73:57. [5] Hampton MD, Lomness JK. Int J Hydrogen Energy 1999;24:175. [6] Hampton MD, Juturu R, Lomness JK. Int J Hydrogen Energy 1999;24:981. [7] Liu FJ, Sandrock G, Suda S. J Alloys Compounds 1992;190:57. [8] Sandrock G, Goodell P. J Less-Common Met 1980;73:161. [9] Hirokawa K, Danzaki Y. Surf Interface Anal 1982;4:63. [10] Giannuzzi LA, Drown JL, Brown SR, Irwin RB, Stevie FA. Mat Res Soc Symp Proc 1997;480:19. [11] Giannuzzi LA, Drown JL, Brown SR, Irwin RB, Stevie FA. Microsc Res Tech 1998;41:285. [12] JCPDS File 07-0239. International Centre for Di?raction Data, 1996.