Hydrogen absorption and desorption properties of Pd/Mg/Pd tri-layersprepared by magnetron sputtering

Yogendra K. Gautam, Mukesh Kumar, Ramesh Chandra ⁎

Nano Science Laboratory, Institute Instrumentation Centre, Indian Institute of Technology Roorkee, Roorkee 247667, India

a b s t r a c ta r t i c l e i n f o

Available online 8 July 2013

Keywords:

Nanostructured filmsXRDMetal hydridesMg thin filmERDA

In this present work, a study on hydrogen absorption and desorption properties of Pd/Mg/Pd tri-layers, pre-pared by DC/RF magnetron sputtering has been conducted using XRD, FE-SEM and AFM. Hydrogenation ofas-deposited Pd/Mg/Pd tri-layers was carried out at (50–150 °C) temperatures in a fixed amount of hydrogengas (100,000 Pa). Hydrogen content in as-deposited and hydrogenated tri-layers have been estimated byElastic Recoil Detection Analysis (ERDA) technique with 120 MeV 107Ag

+9 ions. The XRD study supportedby ERDA data reveals the absorption of hydrogen in hydrogenated Pd/Mg/Pd tri-layers. The hydrogen desorp-tion from Pd/Mg/Pd tri-layers is enhanced by “cooperative effect” caused by elastic interaction within inter-facial region of Pd and Mg films. The maximum hydrogen absorption (8.4 × 1017 H atoms/cm2) has beenobserved at 125 °C among all samples studied. Low desorption temperature (80 °C) has been observed forPd/Mg/Pd tri-layers.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, hydrogen-driven economy has been proposed thathydrogen be an important candidate for environmentally compatibleenergy source in the future. Most of the available hydrogen storagematerials reveal low hydrogen storage capacity and too high desorp-tion temperatures. To attain high performance with high hydrogenabsorbing capacity and low absorption/desorption temperatures, itis necessary to develop technologies and storing materials that canstore large amounts of hydrogen effectively and safely [1]. Magne-sium is a very interesting material because of its high hydrogen stor-age capacity (7.6 wt.% for MgH2), low density and low cost. The bestway to make suitable materials for hydrogen storage is to form thinfilms/multilayers. In thin film process, it is easy to control the thick-ness or grain size of film layer in nanometer scale and there are favor-able changes in properties at that scale [2], like large surface area [3],coupling interactions between layers of a multilayer system etc. [4].Mg film in nanoscales leads to better kinetics for hydrogen absorp-tion/desorption [5]. Higuchi et al. investigated hydrogen storageproperties of bi-layered Pd/Mg films prepared by a RF sputteringmethod [1]. The results show that hydrogen storage propertiescould be remarkably improved by controlling the Mg sputtering con-ditions and preparing fine structured Mg and Pd films. The experi-mental investigations of Pd and Mg based thin films for hydrogen

storage materials have been performed by Fischer et al. [6–9]. Highhydrogen dissociation rate and high hydrogen diffusivity of the Pdsurface layer have been observed by Ryden et al. [10]. The enhance-ment in H2 absorption and desorption kinetics, offered by multilayersof Pd and Mg have been reported by Fujii et al. [11–13]. Some hydro-gen storage properties of two or more layers of Pd and Mg havealso been studied and found strongly dependent on the Mg deposi-tion method and parameters [14–20]. For instance, Pd/Mg films pre-pared by sputtering under higher Ar pressure and higher RF powerexhibited better dehydrogenation properties due to fine grained Mgfilms [14]. Mg films prepared by thermal evaporation has shownthe faster kinetics of H-sorption as the evaporation rate increases,which may be related to the c-axis preferred orientation of the Mgstructure [15]. In addition, depending on the deposition conditions,an extension of the Pd/Mg interface region may enhance the hydroge-nation kinetics. It is associated with alloying between Pd andMg layeror with the penetration of Pd atoms in the pores of the Mg sub layer[16–20]. The expected amount of absorbed hydrogen in thin film istoo small for gravimetric analysis and so it is very important to finda convenient method to evaluate the hydrogen absorption in metallicfilms. ERDA (Elastic Recoil Detection Analysis) is a useful tool to ob-serve hydrogen in thin film as demonstrated by Avasthi et al. [21].Pranevicius et al. studied on Mg–Ni films after different stages of hy-drogenation by ERDA. The observed experimental results wereexplained assuming that properties of the surface barrier layerchanges during hydrogenation and modify hydrogen transport mech-anism [22]. ERDA study on the hydrogen concentration of Pd/Mg/Ni/Pd system revealed the addition of Ni or Mg2Ni layers enhance its hy-drogenation contents [23]. But major drawbacks in Pd/Mg/Ni/Pd

Surface & Coatings Technology 237 (2013) 450–455

⁎ Corresponding author at: Institute Instrumentation Centre, IIT Roorkee, Roorkee247667, India. Tel.: +91 1332 285743; fax: +91 1332 286303.

E-mail address: ramesfic@gmail.com (R. Chandra).

0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.surfcoat.2013.06.125

Contents lists available at ScienceDirect

Surface & Coatings Technology

j ourna l homepage: www.e lsev ie r .com/ locate /sur fcoat

systemwere higher absorption temperature (150 °C) and unavailabilityof data for hydrogen desorption.

In the present investigation we have enhanced the hydrogen ab-sorption and desorption properties of Mg by depositing two Pd layers,one on top and another one at bottom of Mg film. A systematic studyof hydrogen content in as-deposited and hydrogenated Pd/Mg/Pdtri-layers has also been carried out using ERDA.

2. Experimental

2.1. Synthesis of Pd/Mg/Pd tri-layers

Pd/Mg/Pd tri-layers were deposited on Si (100) substrate by DC/RF magnetron sputtering. Prior to deposition of Pd/Mg/Pd tri-layersa few nanometer thick Cr buffer layer was sputter deposited ontothe cleaned substrates in order to enhance the adhesion of the sub-sequent layers. Targets disk (2 in. diameter and 5-mm thick) Pd(99.99%), Mg (99.95%) and Cr (99.9%) were used in the deposition.Deposition of thin films was carried out in a custom built 12 in. diam-eter sputtering chamber (Excel Instruments, India). Before depositionthe Si (100) substrates were etched in HF solution then ultrasonicallycleaned in propanol. The sputtering chamber was initially evacuatedto a high vacuum by turbo molecular pump backed by a rotarypump. High purity (99.95%) inert gas (Ar) was used as sputteringgas. The targets were pre-sputtered for 5 min for removing anyunwanted layer on the target surface. Substrate to target distancewas kept constant at 45 mm during all depositions.

The sputtering parameters for deposition are shown in Table 1. Forhydrogenation/dehydrogenation process the as-deposited sampleswere transferred into a custom built stainless steel thin film hydroge-nation set-up (Excel Instruments, India). The Pd/Mg/Pd tri-layerswere exposed to hydrogen gas (purity: 99.99%) at 100,000 Pa pres-sure and various temperatures (50 °C to 150 °C) for 6 h. The samplehydrogenated at 50 °C is marked as HD50; similarly at 80 °C,100 °C, 125 °C, 150 °C are marked as, HD80, HD100, HD125, HD150,respectively. After annealing, all samples were cooled down to roomtemperature to conduct structural and ERDA studies. For dehydroge-nation, all hydride samples were annealed at 80 °C for 3 h in vacuumusing rotary pump. The dehydrogenated samples have marked asDHD80, DHD100, DHD 125 and DHD150.

2.2. Characterization

XRD analyses were carried out using CuKα radiation in glazingangle X-ray diffractometer (Bruker AXS, D8 Advance) in (θ − 2θ) ge-ometry. The crystallite size (dXRD) was calculated using well knownScherrer's formula. Field emission scanning electron microscopy(FE-SEM) (FEI, Quanta 200 F) was used to measure film thickness.Surface morphology of the samples was analyzed using atomic forcemicroscopy (AFM) (NT-MDT, Ntegra), operated in semi contact (tapping)mode.

2.3. ERDA measurements

ERDA measurements were performed at material science beamline in IUAC, New Delhi, India using 120 MeV Ag9+ beam to deter-mine the areal concentration of hydrogen. The H-recoils from thefilms were detected in a silicon surface barrier detector (SSBD), keptat 30° recoil angle preceded by a 1.5 μm polypropylene stopper foilin front of it to stop other recoils. The areal concentration of hydrogen(NH = H atoms/cm2) was calculated using the following Eq. (1):

NH ¼YH sin α

ΩNP

dσ

dΩ

� �

R

ð1Þ

where YH is number of recoils detected in the detector subtending asolid angle of Ω, α is the target tilt angle, Np is the number of incidentions, dσ

dΩ

� �

Ris the Rutherford recoil cross section for H in laboratory

frame, given by Eq. (2)

dσ

dΩ

� �

R

¼ Zp Zte2

mp þmt

� �h i24E2

Pm

2t cos

3φ

� �

−1ð2Þ

wheremp, mt, Zp and Zt are the atomic masses and atomic numbers ofthe projectile and the target materials respectively, Ep is the incidention energy, ϕ is the recoil angle, and e is the electronic charge. The de-tector signals were arranged with conventional electronics and ener-gy spectra was stored for different ions dose [22,23]. Schematic ofexperimental setup of ERDA, indicating path of ion beam and the de-tectors position is shown in Fig. 1.

3. Results and discussion

3.1. Hydrogenation of the samples

From Fig. 2, it has been observed that the as-deposited polycrys-talline Pd/Mg/Pd tri-layers show major XRD peaks corresponding tohcp-Mg along (002) and fcc-Pd along (111) with dominant orienta-tion at 34.29° (JCPDS, 040770) and 40.02° (JCPDS, 461043) respec-tively. The other reflections at 32.72°, 38.11° and 44.17° have alsobeen observed for hcp-Mg along (100), Mg5Pd2 (212) and cubic Cralong (110), respectively [24]. Scherrer's formula (Eq. (3)) was usedto calculate the crystallite size of the samples [25].

d ¼ 0:9λ=β cos θB ð3Þ

where λ, θB and β are the X-ray wavelength of Cu (1.54056 Å), Braggdiffraction angle and full width at half maximum (FWHM), respec-tively. The average crystallite size of Pd comes out to be ~40 nmcorresponding to Pd (111) while ~15 nm for Mg (002). After hydro-genation at 50 °C (HD50), XRD pattern does not show any hydrideformation and it is similar to as-deposited one, as shown in Fig. 2.XRD patterns clearly reveals that hydrogen absorption in Mg filmstarts at 80 °C temperature as no detectable change is observedbelow this temperature at 50 °C. It is because of the fact that temper-ature (b80 °C) is not sufficient to overcome the activation barrier forthe reaction (hydride formation) [26]. The thicknesses were mea-sured by the cross-sectional views of tri-layers by FE-SEM. Fig. 3(a)and (b) show the FE-SEM cross section views of as-deposited and hy-drogenated Pd/Mg/Pd tri-layers (HD125), respectively. The observedthickness values of as-deposited Pd (cap and bottom layers) and Mgfilm are ~30 and ~780 nm, respectively. After hydrogenation, the ob-served thickness of MgH2 magnesium hydride is ~950 nm for hydro-genated sample at 125 °C (HD125). It is found that the film thicknessincreases after hydrogen absorption process and it is increased by21.7% as compared to the as-deposited one. The reason for this maybe the volume expansion following hydride formation [17]. In allthe hydrogenated samples, α-MgH2 magnesium hydride phase is

Table 1

Sputtering parameters.

Sputtering parameters

Target Mg, Pd, CrBase pressure 2.6 × 10−4 PaGas used ArDeposition power [50 W (Mg)] DC [40 W (Pd), 50 W (Cr)] RFDeposition time 3 min (Mg), 1 min (Pd), 1 min (Cr)Sputtering pressure .6 PaSubstrate Si (100)Substrate temperature RT

451Y.K. Gautam et al. / Surface & Coatings Technology 237 (2013) 450–455

found to be present; however a small amount of Mg still remains inthe samples [27]. The XRD peaks corresponding to tetragonalα-MgH2 (110) and α-MgH2 (101) phases are observed at 27.52°and 35.39°, respectively in all hydrogenated samples, while theα-MgH2 (200) is marked as fcc-Pd (111) (Fig. 2) (JCPDS, 740934).Hydride peak positions are found at lower angles as compared tothe reference data, indicating the presence of strain caused volumeexpansion on hydride formation [28,29]. The fcc-Pd (111) orientationin the hydrogenated samples does not suffer any change in angle anddoes not present any peak corresponding to palladium hydride in hy-drogenated samples. During hydride formation the hydrogen absorp-tion process involves two distinct stages: (i) The Pd (cap and bottomlayers) acts as catalyst to dissociate the hydrogen molecules into hy-drogen atoms, (ii) hydrogen atoms diffuse into Mg and form MgH2

[30]. From Fig. 2, we observed the presence of α-MgH2 withrutile-type structure for hydrogenated samples. The experiments ondynamics of nucleation of hydride in Pd-capped Mg thin filmsshowed that: (i) the hydride phase starts nucleating at the Pd/Mg in-terface, (ii) the diffusion of hydrogen through the hydride phase isthe rate-limiting step in the hydride formation [17]. Upon absorption

Fig. 1. A schematic of experimental setup of ERDA indicating the path of ion beam and position of detector.

20 25 30 35 40 45

Mg

5P

d2

(212)

-Mg

H2

(10

1)

Cr(110)

Pd

(111)

Mg

(00

2)

Mg

(10

0)

-Mg

H2

(11

0)

as-deposited

HD50

HD80

HD100

HD125

HD150

Inste

nsit

y (

arb

.)

2 (deg.)

Fig. 2. XRD patterns of as-deposited and hydrogenated Pd/Mg/Pd tri-layers.

a

b

Fig. 3. FE-SEM cross-sectional views of: (a) as-deposited and (b) hydrogenated Pd/Mg/Pd tri-layers at 125 °C for 6 h.

452 Y.K. Gautam et al. / Surface & Coatings Technology 237 (2013) 450–455

of hydrogen into a metal, the metal undergoes a structural change.When the absorbed concentration of hydrogen is low then it pene-trates the bulk phase to occupy mainly interstitial positions. Thisphase in which the H atoms are in a diluted solid solution is so calledα-phase and in this case H atoms can move easily through the metalmatrix [31]. The intensity of peak corresponding to the dominant hy-dride phases depend on the concentration of hydrogen in Pd/Mg/Pdtri-layers system [31]. The peaks in XRD pattern correspondingto MgH2 phase were observed to be highly intense up to 125 °C but in-crease in annealing temperature (N125 °C) favors hydrogen desorptionalso, because of its endothermic nature, however peak corresponding toMgH2 at 150 °C was found to be of lower intense compare to the peakintensities of hydride phases in temperatures range of (80–125 °C).The 2D AFM images (scan area 2 × 2 μm) of the top surface ofas-deposited and hydrogenated samples are shown in Fig. 4(a–b).Fig. 4(a) shows very smooth surface morphology of as-deposited Pd/Mg/Pd tri-layers. The average surface roughness of as-deposited ismea-sured to be 3 nm, while it is 12 nm for hydrogenated tri-layers at

125 °C. Increase in surface roughness can be attributed to the surface de-formation caused by hydrogen absorption [17]. The average grain sizehas also been measured using AFM and it is 22 nm and 38 nm foras-deposited and hydrogenated Pd/Mg/Pd tri-layers, respectively. The in-crement in grains size of hydrogenated sample compare to as-depositedis the cause of volume expansion in hydride formation [17].

3.2. Hydrogen in as-deposited and hydrogenated Pd/Mg/Pd tri-layers

The hydrogen recoil spectra taken during ERDA experiment ofas-deposited and hydrogenated Pd/Mg/Pd tri-layers are shown inFigs. 5 and 6, respectively. It is observed that the recoil counts aremore in case of hydrogenated tri-layers (Fig. 6) as compared to that ofas-deposited (Fig. 5). This is the result of hydrogen absorption after hy-drogenation of Pd/Mg/Pd tri-layers. H content (areal density) wasmea-sured using Eq. (1) and it is found to be 6.8 × 1017 atoms/cm2, 7.9 ×1017 atoms/cm2, 8.4 × 1017 atoms/cm2 and 3.3 × 1017 atoms/cm2 forhydrogenated samples HD80, HD100, HD125, and HD150 respectively.The error in ERDA measurement was counted ~ ±2.5%. A noticeableamount ~6.21 × 1015 atoms/cm2 of hydrogen was measured inas-deposited tri-layers, and it has been also revealed from H recoilcounts in Fig. 5. However, XRD does not show any hydride formationin as-deposited film, implying that the H signals in ERDAmay be causedby two reasons: firstly, the films were deposited in ~10−4 Pa vacuum,resulting in some hydrogen absorption during sample formation, sec-ondly atmospheric water vapor absorbed in the micro and nano chan-nels on surface of the films, which may be responsible for presence ofhydrogen content in the as-deposited Pd/Mg/Pd tri-layers [31]. Thetemperature dependent hydrogen contents in the Pd/Mg/Pd tri-layershave been estimated and shown in Fig. 7. We can observe that at80 °C a significant amount of H is detected and it is because of thatMg starts to absorb hydrogen after hydrogenation process. It impliesthat the activation of H absorption occurs at 80 °C. The hydrogenationprocess is mainly governed by the H diffusion in Mg matrix and nucle-ation and growth of the hydride phase. For the H diffusion in a thinfilm, it is determined not only by the diffusion in Mg but also in theMgH2 (formed at interface) layer because the further H diffusion indepth of Mg matrix is significantly affected by MgH2 layer [32]. As thetemperature increases, the vibrational amplitude of hydrogen atomsalso increases and there is statistical distribution of vibrational ampli-tude and the probability that atomwill have sufficient vibrational ener-gy to overcome activation barrier, however hydrogen contents in Pd/Mg/Pd system increase with increase in temperature up to 125 °C. Fur-ther increase in temperature is not always favorable for the exothermichydrogen absorption, there is an endothermic nature in which H atom

Fig. 4. AFM images of: (a) as-deposited and (b) Hydrogenated Pd/Mg/Pd tri-layers at125 °C for 6 h.

190 209 228 247 266 285 304 323

0

5

10

15

20

25

30

35

Co

un

ts

Channel No.

Fig. 5. Elastic recoil spectrum of as-deposited Pd/Mg/Pd tri-layers.

453Y.K. Gautam et al. / Surface & Coatings Technology 237 (2013) 450–455

release from its hydride phase as a result hydrogen desorption alsooccurred at higher temperature (150 °C). Thus we have been observedsufficient hydrogen absorption up to 125 °C and further increase inthe temperature (N125 °C) favored hydrogen desorption as confirmedfrom XRD pattern also (Fig. 2). Thus, there is an optimal temper-ature for hydrogen absorption as described in the literature [33]. Inour experimental conditions, the maximum hydrogen absorption(8.4 × 1017 H atoms/cm2) for HD125 is observed at 125 °C (Fig. 7).The areal density of hydrogen (8.4 × 1017 H atoms/cm2) for Pd/Mg/Pdtri-layer system is found to be more as compared to the areal density(~4.3 × 1017 H atoms/cm2) for Pd/Mg bi-layer system [31]. The reasonof this result is that the latticemismatch at the Pd/Mg interface certainlycreates defectswith low atomic density [34]. The hydrogen is trapped atdefects on the surface or interface. The interface hydride forms after thenucleation of hydride at the interface, followed by growth and hydrogendiffusion is predominantly along the interface. So, the hydrogen istrapped at interface as well as in Mg matrix. Hence, Pd/Mg/Pd tri-layerstake up more hydrogen than the Pd/Mg bi-layer, since the Pd/Mg/Pdstructure has two Pd/Mg interfaces and the Pd/Mg only one [34].

3.3. Dehydrogenation of hydrogenated Pd/Mg/Pd tri-layers

Dehydrogenation of all hydrogenated samples was performed at80 °C temperature for 3 h in vacuum using rotary pump. XRD pat-terns of dehydrogenated samples are shown in Fig. 8. Fig. 8 shows

the highly intense XRD peaks corresponding to hcp-Mg along (002)and fcc-Pd along (111) at angle of 34.41 and 40.14°, respectively. Theother peaks are observed at 32.84°, 38.12° and 44.19°corresponding tohcpMgalong (100),Mg5Pd2 (212) and cubic Cr along (110), respective-ly [24].

The XRD patterns of dehydrogenated Pd/Mg/Pd tri-layers (Fig. 8)are found to be similar as XRD pattern of as-deposited Pd/Mg/Pdtri-layers (Fig. 2) i.e. peaks corresponding to MgH2 are lost. It meanscomplete hydrogen desorption occurred after the dehydrogenationof the samples at 80 °C temperature. During desorption the hydrogenis first released from the Pd layers. The compressive stress induced onthe top and down surface of Mg film, forces desorption of hydrogenfrom Mg at low temperature. This desorption process is attributedto the “cooperative effect” caused by the elastic interaction in thePd/Mg interfacial region [35]. The XRD peak positions are found tobe at higher angles as compared to the XRD pattern of as-depositedPd/Mg/Pd tri-layers in Fig. 2, indicating volume contraction on hydro-gen desorption [28]. Also, the number of grain boundaries (hydrogenpath way) in the Pd/Mg/Pd tri-layers is larger than that in two-layered Pd/Mg system [14,17]. Therefore, these grains lead to thelower temperature for hydrogen desorption. The improvement in hy-drogen absorption/desorption properties is induced in the Pd/Mg/Pdtry-layers with thick Mg layer and thin Pd layers. It has been observedthat the Pd/Mg/Pd tri-layers are attending to volume expansion and

190 209 228 247 266 285 304 323

0

32

64

96

128

160

192

224

Co

un

ts

Channel No.

Fig. 6. Elastic recoil spectrum of hydrogenated Pd/Mg/Pd tri-layers at 125 °C for 6 h.

25 50 75 100 125 150

0.0

3.0x1017

6.0x1017

9.0x1017

H C

on

ten

t (H

ato

ms

/cm

2)

Temperature(oC)

Fig. 7. Hydrogen content, areal density (atoms/cm2) vs. hydrogenation temperature.

20 25 30 35 40 45

Cr(110)DHD150

DHD125

DHD100

DHD80

Mg

(100)

Mg

(002)

Mg

5P

d2(2

12

)

Pd

(11

1)

Inte

nsit

y (

arb

.)

2 (deg.)

Fig. 8. XRD patterns after dehydrogenation of hydride Pd/Mg/Pd tri-layers.

454 Y.K. Gautam et al. / Surface & Coatings Technology 237 (2013) 450–455

contraction due to the hydrogen absorption and desorption reactions,respectively in addition to the poor adhesion between layers and sub-strate. Therefore, Pd/Mg/Pd tri-layer is peeled off, leading to the deg-radation of the hydrogen absorption/desorption properties. Becauseof this reason Pd/Mg/Pd tri-layers posses poor recyclability of hydro-gen absorption/desorption.

4. Conclusion

Hydrogen absorption and desorption study of sputtered Pd/Mg/Pdtri-layers at low temperature and pressure has been conducted. Ab-sorption of hydrogen has been observed at 80 °C temperature and1 bar hydrogen pressure. The temperature dependent hydrogen con-tent in the Pd/Mg/Pd tri-layers system has been estimated by ERDAand the maximum hydrogen contents (8.4 × 1017 H atoms/cm2) hasbeen observed among all samples measured. Low desorption temper-ature (80 °C) have also been observed for Pd/Mg/Pd tri-layers. Theimprovement in the hydrogen absorption/desorption properties ofMg film has been achieved due to the cooperative effect caused bythe elastic interaction in the interfaces of nanostructured Mg and Pdlayers. Above study indicates that the multilayer structure with thinlayers of Pd and Mg can be effective in improving the hydrogen stor-age properties of Mg.

Acknowledgments

One of the authors, Yogendra K. Gautam is thankful to the Councilof Scientific & Industrial Research (CSIR), India for the awarding ofResearch Associateship (RA) (Grant No. 9/143 (786)/10-EMR-I) forthis work. The authors are also thankful to Inter University Accelera-tor Centre (IUAC), New Delhi, India for providing ERDA facility.Special thanks to Mr. Saif. A. Khan, Material Science Lab for his sup-port in ERDA measurement and data analysis.

References

[1] K. Higuchi, K. Yamamoto, H. Kajioka, K. Toiyama, M. Honda, S. Orimo, H. Fujii,J. Alloys Compd. 330–332 (2002) 526–530.

[2] H. Fujii, S. Orimo, Physica B 328 (2003) 77–80.[3] L.Z. Ouyang, H.Wang, C.Y. Chung, J.H. Ahn,M. Zhu, J. Alloys Compd. 422 (2006) 58–61.

[4] K. Klose, Ch. Rehm, D. Nagengast, H. Maletta, A. Weidinger, Phys. Rev. Lett. 78(1997) 1150–1153.

[5] Wipf, Hydrogen inMetals III: Properties and Applications, Topics in Applied Physics,vol. 73, Springer, Berlin, 1997.

[6] A. Fischer, H. Kostler, L. Schlapbach, J. Less-Common Met. 172–174 (1991)808–815.

[7] A. Fischer, A. Krozer, L. Schlapbach, Surf. Sci. 269–270 (1992) 737–742.[8] Y. Liu, L. Chen, T.M. Lu, G.C. Wang, Int. J. Hydrogen Energy 36 (2011)

11752–11759.[9] A. Krozer, B. Kasemo, J. Less-Common Met. 160 (1990) 323–342.

[10] P. Spatz, H.A. Aebischer, A. Krozer, L. Schlapbach, Z. Phys. Chem. 181 (1993)393–397.

[11] J. Ryden, B. Hjorvarsson, T. Ericsson, E. Karlsson, A. Krozer, B. Kasemo, J. Less-CommonMet. 152 (1989) 295–309.

[12] V.P. Zhdanov, A. Krozer, B. Kasemo, Phys. Rev. B 47 (1993) 11044–11048.[13] H. Fujii, K. Higuchi, K. Yamamoto, H. Kajioka, S. Orimo, K. Toiyama, Mater. Trans.

43 (2002) 2721–2727.[14] A. Leon, E. Knystautas, J. Huot, R. Schulz, Thin Solid Films 496 (2006) 683–687.[15] K. Higuchi, H. Kajioka, K. Toiyama, H. Fujii, S. Orimo, Y. Kikuchi, J. Alloys Compd.

293–295 (1999) 484–489.[16] S. Singh, S.W.H. Eijt, M.W. Zandbergen, W.J. Legerstee, V.L. Svetchnikov, J. Alloys

Compd. 441 (2007) 344–351.[17] Y.K. Gautam, A.K. Chawla, R. Walia, R.D. Agrawal, R. Chandra, Appl. Surf. Sci. 257

(2011) 6291–6295.[18] K. Yoshimura, Y. Yamada, M. Okada, Surf. Sci. 566–568 (2004) 751–754.[19] J. Paillier, L. Roue, J. Alloys Compd. 404–406 (2005) 473–476.[20] J. Paillier, S. Bouhtiyya, G. Ross, L. Roue, Thin Solid Films 500 (2006) 117–123.[21] D.K. Avasthi, Bull. Mater. Sci. 19 (1996) 3–14.[22] L. Pranevicius, E. Wirth, D. Milcius, M. Lelis, L. Pranevicius, A. Bacianskas, Appl.

Surf. Sci. 255 (2009) 5971–5974.[23] J. Pragya, J. Ankur, V. Devendra, V. Reena, S.A. Khan, I.P. Jain, J. Alloys Compd. 509

(2011) 2105–2110.[24] M.I. Stormer, C. Blawert, H. Hagen, V. Heitmann, W. Dietzel, Plasma Processes

Polym. 4S (2007) 557–561.[25] B.D. Cullity, Elements of X-ray Diffraction, 2nd edn Addison-Wesley, London,

1978. 102–103.[26] J. Ramón, A. Fernández, K. Francois, A. Zinsou, Catalysts 2 (2012) 330–343.[27] Q.U. Jianglan, W. Yuntao, X. Lei, J. Zheng, L. Yang, L. Xingguo, Int. J. Hydrogen

Energy 34 (2009) 1910–1915.[28] P. Vajeeston, P. Ravindran, B.C. Hauback, H. Fjellvag, A. Kjekshus, S. Furuseth,

Phys. B 73 (2006) 224102–2241088.[29] G. Siviero, V. Belloa, G. Mattei, P. Mazzoldia, G. Battaglin, N. Bazzanella, R.

Checchetto, A. Miotello, Int. J. Hydrogen Energy 34 (2009) 1417–4726.[30] I.P. Jain, L.K. Malhotra, K.S. Uppadhyay, Int. J. Hydrogen Energy 13 (1998) 15–23.[31] Y.K. Gautama, A.K. Chawla, S.A. Khan, R.D. Agrawal, R. Chandra, Int. J. Hydrogen

Energy 37 (2012) 3772–3778.[32] P. Selvam, B. Viswanathan, C.S. Swamy, V. Srinivansan, Int. J. Hydrogen Energy 11

(1986) 169–192.[33] H. Yuping, Z. Yiping, H. Liwei, H. Wang, J. Russell Composto, J. Appl. Phys. Lett. 93

(2008) 163114–1631113.[34] A. Krozer, B. Kasemo, J. Vac. Sci. Technol. A 5 (1987) 1003–1005.[35] H. Wang, L.Z. Ouyang, M.Q. Zeng, M. Zhu, J. Alloys Compd. 375 (2004) 317–318.

455Y.K. Gautam et al. / Surface & Coatings Technology 237 (2013) 450–455