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Comparative study on hydrogenation properties of Pd cappedMg and Mg/Al films

Pragya Jain a,*, Ankur Jain a, Devendra Vyas a, D. Kabiraj b, S.A. Khan b, I.P. Jain a

aCentre for Non-Conventional Energy Resources, University of Rajasthan, 14 Vigyan Bhawan, Jaipur 302055, Rajasthan, Indiab Inter University Accelerator Centre, New Delhi 110 067, India

a r t i c l e i n f o

Article history:

Received 15 February 2011

Received in revised form

23 February 2011

Accepted 25 February 2011

Available online 29 March 2011

Keywords:

Metal hydrogen systems

Mg thin films

Hydrogen content

ERDA

* Corresponding author. Tel./fax: þ91 141271E-mail addresses: pragya.2604@gmail.com

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.02.144

a b s t r a c t

Recent emergence of Mg as a promising hydrogen storage material with 7.6 wt% hydrogen

encourages study on its thin films to understand physics of storage mechanism. The

present study investigates the variations in hydrogen storage properties of Pd sandwiched

Mg films upon introduction of Al layer. Multilayered stack of Pd/Mg/Pd and Pd/Al/Mg/Pd

were grown on Si substrate using vapor deposition method and further hydrogenated at

150� C under 2 bar H2 pressure for 2 h. Elastic Recoil Detection Analysis (ERDA) technique

with 120 MeV Ag9þ ions was used to obtain hydrogen concentration versus incident ion

fluence. ERDA study reveals that Pd/Mg/Al/Pd films absorb 6.01 � 1018hydrogen atoms/cm2

in comparison to 4 � 1017 atoms/cm2 absorbed by Pd/Mg/Pd system.

Atomic force Microscopy (AFM) and X-ray Diffraction (XRD) techniques were utilized to

analyze the morphological and structural changes in the hydrogenated films. Results

indicate that addition of Al to the base system has led to the formation of Mg(AlH4)2 along

with MgH2 causing an increment in the hydrogen storage capacity and reduction in the

oxygen content.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction difficult to activate. Various efforts have been made to over-

Hydrogen storage for mobile applications such as fuel cell

driven cars has been a very active research area for decades.

Magnesium has been considered as a strong candidate for

these applications because of its high gravimetric (w 7.6 wt%

for MgH2) and volumetric (w 150 kg H2/m2 MgH2) efficiency,

light weight, low cost and abundance on earth crust. Unfor-

tunately, the practical application of Mg is limited by its slow

hydriding/dehydriding kinetics even at high temperatures.

The slow kinetics is attributed to (i) low dissociation rate of

hydrogen onMg surface due to high energy barrier (Eaw 72 kJ/

mol H2) [1] and (ii) slow diffusibility of hydrogen atoms in

MgH2 phase (DH w 10�16 cm2/sec) [2]. Additionally, the Mg

metal is very sensitive to contamination, which makes it

1049., ipjain46@gmail.com (P2011, Hydrogen Energy P

come the thermodynamic and kinetic barriers by alloying Mg

with various elements to alter the crystal structure of the

hydride [3], reducing the particle and grain size via mechan-

ical milling with e.g nanostructured carbon [4], or by addition

of catalytic additives such as Ni, LaNi5 or LaeMgeNi alloys

during milling [5e7].

A possible and better solution would come from the addi-

tion of light and cheap elements like Al. Guo and co-workers

[8,9], have shown that Al addition to MgH2 reduces the

stability of the hydride leading to an improvement in the

dehydrogenation conditions. The heat of formation predicted

for the MgH2þAl system is 28 kJ/mol H2 [10]. Additionally, it

has been found that the thermodynamics and kinetics of

MgeAl as compared to Mg are improved along with resistance

. Jain).ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 7 7 9e3 7 8 53780

toward oxygen contamination [11]. Moreover, there is

a possibility of formation of complex hydrides, such as alanate

compounds, in particular Mg(AlH4)2, which has a storage

capacity of 9.3 wt%.

The best way to understand the reaction mechanism of

MgeAl system is to prepare thin film by sputtering or evapo-

ration method. As in thin film structure, it is easy to control

the thickness, composition, interface and structural order.

Moreover, the co-operative phenomena and the spill over

effects can be induced by synthesis of sandwich structured

films, leading to an improved kinetics [12e14]. Ferrer et al. [15]

investigated the Mg/Al layer sandwiched between Pd/Fe(Ti)

and observed improved storage capacity by the formation of

MgxAly intermetallic. Some authors have reported the

formation of Mg(AlH4)2 from MgeAl thin films under different

conditions [16e18].

This papermainly focuses on the hydrogenation properties

of Mg and Mg/Al thin films sandwiched between Pd layers.

The increase in the hydrogen content has been studied by

Elastic Recoil Detection Analysis (ERDA). X-ray diffraction and

Atomic force microscope have been used to investigate the

structural and morphological changes.

20 30 40 50

0

200

400

600

800

1000

1200

*

*

Δ

a

Inte

nsity

2θ

Δ

Pd*Mg

2. Experiment

2.1. Thin film preparation technique

The thin film sample of Pd/Mg/Pd was prepared by vapor

deposition method at a base pressure of 10�7 mbar. The

evaporation unit is equipped with 3 KW electron gun, for Pd

deposition and two thermal evaporation units used forMg and

Al deposition. 150 nm Mg layer is sandwiched between 20 nm

layers of Pd to protect it against oxidation and to promote

hydrogen dissociation. The deposition rates ofMg and Pdwere

kept constant at 0.15 nm/s and 0.1 nm/s respectively. In the

second sample 50 nm Mg layer is replaced by 50 nm Al layer

deposited by resistive heating at a deposition rate of 0.15 nm/

s, to form Pd/Al/Mg/Pd system. Thus, in the present study two

systems are being investigated (i) as-deposited (AD1) and

hydrogenated (HD1) Pd/Mg/Pd system and (ii) as-deposited

(AD2) and hydrogenated (HD2) Pd/Al/Mg/Pd system.

1500

2000

2500

3000

3500

4000

*

ΔPd

b

Inte

nsity

Δ Mg* Al

2.2. Thin film hydrogenation technique

Hydrogenation of thin film samples was carried out at 150� C

and 2 bar H2 pressure for 2 h in the system described by

Agarwal et al. [19]. Three cycles of hydrogen absorption/

desorption were performed to ensure complete hydrogena-

tion of the films.

20 30 40 50

0

500

1000

*

2θ

Fig. 1 e XRD spectra of as-deposited (a) Pd/Mg/Pd and (b)

Pd/Al/Mg/Pd samples.

2.3. Structural characterization using GI-XRD

The structures of the as-deposited and hydrogenated samples

were studied by GI-XRD technique using monochromated

CuKa radiation of wavelength 1.54060 A with model Brucker

DX 8-Advance.

The spectra were recorded in the 2q range of 20e50� with

scan speed of 0.5�/min and step width of 0.02�. The average

crystallite dimension DP (nm) was calculated using the

formula:

Dp ¼ 0:9lb1=2cosq

(1)

where l is the X-ray wavelength, q is the Bragg Diffraction

angle and b1/2 is the FWHM of the peak after correction for the

instrument broadening.

2.4. Morphological characterization using AFM

The surface morphology of all the samples have been inves-

tigated by AFM (Nanoscope IIIE model from Digital Instru-

ments, USA), in contact mode at room temperature. The scan

area and rate were kept as 5 � 5 mm and 1.526 Hz respectively.

2.5. Hydrogen content measurement using ERDA

Elastic recoil detection analysis (ERDA) measurements for

areal concentration of hydrogen (NH in atoms cm�2) in as-

deposited and hydrogenated films of both systems were

carried out at Material Science beam line in IUAC, New Delhi.

Silver (Ag9þ) beam of energy 120 MeV and current 7e9 nA was

20 30 40 50

50

100

150

200

250

300

350

O

O

Δ

Δ

*

a

2θ

Inte

nsity

MgH2

Δ

Mg5Pd2O

Pd*

PdH0.7

20 30 40 50

0

100

200

300

400

500

600

700

800

Inte

nsity

2θ

ΔΔ

Δ

*

*

Δ

Δ

MgH2

Δ

Pd*Mg(AlH4)2

Al

b

Fig. 2 e XRD spectra of hydrogenated (a)Pd/Mg/Pd and (b)

Pd/Al/Mg/Pd samples.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 7 7 9e3 7 8 5 3781

obtained from 15UD Accelerator. ERDA experiment was per-

formedwith a beamof spot size 1� 1mm2 incident at an angle

of 20�with respect to sample under a base pressure of

4.5 � 10�6 mbar. The H-recoils from the film were detected in

a silicon surface barrier detector (SSBD) kept at 30� recoil

angle, assembled with a 1.5 mm polypropylene stopper foil in

front of it to stop other recoils. The recoils of oxygen present

Table 1 e DebyeeScherer analysis for crystallite sizecalculation of as-deposited and hydrogenated samples.

Specifications Element 2q b1/2 DP(nm)

As-deposited Mg 34.4� 0.228 47.91

Al 38.47� 0.203 43.38

Pd(1) 40.14� 0.332 26.62

Pd(2) 46.84� 0.947 9.55

Hydrogenated MgH2 27.98� 0.229 37.26

Al 38.47� 0.606 24.40

Pd(1) 40.14� 0.495 17.87

PdH0.7 46.7� 1.606 5.63

as impurity in the films were detected in isobutane gas filled

detector placed at 45� recoil angle. The areal concentration of

hydrogen (H) (NH atoms cm�2 � 5%) atoms were calculated

using the following equation:

NH ¼ Ysina

NpdsdU

U

(2)

where, Y is the integral counts obtained by the recoil energy

spectra, a is the target tilt angle,U is the solid angle subtended

by the detector and ds/dU is the Rutherford recoil cross

section. The fluence-dependent NH was estimated from the

on-line data, taken in event-by-event mode [20].

3. Result and discussion

3.1. Effect of hydrogenation on structural properties

In Fig. 1(a) and (b), the patterns of as-deposited samples, AD1

and AD2 respectively are presented. In both the samples,

peaks at around 34.4�, 40.19� and 46.84� characteristic of pure

Mg [002], Pd [111] and [200] respectively were observed. Thus

the profile reveals that the Mg grains prefer the c-axis orien-

tation while no preferred orientation occurs for Pd. No peak

corresponding to MgO was observable, suggesting that the

Fig. 3 e AFM images of as-deposited and hydrogenated Pd/

Mg/Pd samples.

Table 2 e Particle size and roughness calculation fromAFM images of as-deposited and hydrogenated samples.

Sample Particle Size (nm) Roughness (nm)

Pd/Mg/Pd 410.16 31.37

Pd/Mg/PdeH 341.80 38.05

Pd/Mg/Al/Pd 537.11 48.53

Pd/Mg/Al/PdeH 419.92 67.34

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 7 7 9e3 7 8 53782

sandwiched structure effectively protects Mg from oxidation.

In AD2, an additional peak at around 38.4�, characteristic of Al

[111] is also an observant. The XRD pattern of Pd/Mg/Pd (HD1)

sample, exposed to 2 bar H2 at 150� C for 2 h is shown in

Fig. 2(a). It was observed that hydrogenation leads to the

disappearance of pure magnesium [002] peak and appearance

ofmagnesium hydride (MgH2) [110] and [101] peaks, indicating

the transformation from hexagonal close packed Mg to

tetragonal MgH2 phase. The X-ray peak related to Pd hydride

(PdH0.7) was also detected, however most of the Pd remains

unhydrogenated as evident from the presence of an intense

peak corresponding to pure Pd element. This is in agreement

with several reports [21]. However, a broadening in the Pd

peak is noticed, which is caused by particle/grain refining and

lattice strains introduced by several cycles of hydrogenation.

In addition, the formation of MgePd intermetallic (Mg5Pd2)

due to intermixing of Mg and Pd occurring at the interfacial

region was also observed. XRD findings of hydrogenated Pd/

Al/Mg/Pd (HD2) shows the appearance of complex hydride

Mg(AlH4)2 peak along with MgH2. Garcia et al. [22] have made

similar studies on Mg/Al systems of varying composition and

have suggested the formation of complex hydride phase with

composition above 35% Mg but they were unable to observe

any peak corresponding to this hydride in the XRD pattern.

The XRD pattern (Fig. 2(b)) in this work gives a clear evidence

of the formation of complex hydride due to the reaction

Fig. 4 e AFM images of as-deposited and hydrogenated

Pd/Al/Mg/Pd samples.

between Al and Mg at Mg/Al interface under 150� C and 2 bar

H2 pressure, while the Mg atoms located far away from the

interface contributes to the formation of magnesium hydride.

Reduction in Al peak area further supports these observations.

Further, Al addition benefited in the form suggests that it

eliminates the formation of MgxPdy intermetallic phase which

is crucial for hydrogenation, as shown in Fig. 2(b). The

DebyeeScherer analysis for both Pd/Mg/Pd and Pd/Al/Mg/Pd

systems shows a decrease in the crystalline size upon

hydrogenation, which is given in Table 1.

3.2. Effect of hydrogenation on morphological properties

AFM surface analysis provides additional evidence that

hydrogenation process drastically changes surface topog-

raphy. Figs. 3 and 4 shows the AFM images of (a) as-deposited

and (b) hydrogenated Pd/Mg/Pd and Pd/Al/Mg/Pd films

respectively. Table 2 shows that hydrogen loading in thin film

samples causes a reduction in particle size leading to an

enhanced roughness.

3.3. Hydrogen content measurement

The ERDA spectrum of hydrogenated Pd/Mg/Pd and Pd/Mg/Al/

Pd films taken during the first minute of the ERDA measure-

ments is shown in Fig. 5. The area under the hydrogen recoil

600 800 1000 1200

0

10

20

30

40

50

60

70

80

90

100

Energy/Channel (MeV)

H-C

ount

s (a

.u)

(Pd/Mg/Pd) (Pd/Mg/Al/Pd)

Fig. 5 e ERDA spectra of hydrogenated Pd/Mg/Pd (black

line) and Pd/Al/Mg/Pd (red line) samples taken during 1st

minute of the ERDAmeasurement.(For interpretation of the

references to color in this figure legend, the reader is

referred to the web version of this article.)

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 7 7 9e3 7 8 5 3783

spectra can be used to obtain the hydrogen concentration NH

atoms/cm2 in the samples at a particular time during experi-

ment [23]. Using Eq. (2), amount of hydrogen absorbed by the

films (NH) under 150� C and 2 bar H2 pressure is calculated for

different ion doses. Fig. 6(a) and (b) represents the plot of NH

(atoms/cm2) v/s incident ion fluence (ions/cm2) after hydro-

genation for both the films. The decrease in the hydrogen

content is due to H loss on ion irradiation during ERDA anal-

ysis [24]. The data is fitted using equation:

NH ¼ NOexpð�sfÞ (3)

where NO is the initial concentration of hydrogen,atoms/cm2,

s is the hydrogen release cross section (cm2) and v is the ion

fluence (ions/cm2) [24]. This type of variation has been studied

by Gupta et al. [24]. According to them the interaction of the

ions with the sample releases hydrogen from a cylindrical

zone. After certain duration of ion beam exposure, the

damaged zone is modified due to hydrogen loss and ion beam

0.00E+000 2.00E+013 4.00E+013 6.00E+013

2.35385E17

6.39843E17

Hyd

roge

n co

nc. (

at/c

m2 )

Fluence(ions/cm2)

1

2

a

0.00E+000 2.00E+013 4.00E+013

6.39843E17

1.73927E18

4.72784E18

b

Hyd

roge

n co

nc. (

at/c

m2 )

Fluence (ions/cm2)

1

2

Fig. 6 e Variation of hydrogen concentration with fluence

of Ag9D 120 MeV ion beam irradiation on hydrogenated (a)

Pd/Mg/Pd and (b) Pd/Al/Mg/Pd films. Solid lines (red) are

linear fits of region 1 and 2 (see text). Error bars show

statistical error in the calculated regions.(For interpretation

of the references to color in this figure legend, the reader is

referred to the web version of this article.)

induced modifications. This leads to the existence of two

curves of different slopes explaining the nature of hydrogen

loss from the sample.

Todeterminethe initialhydrogenconcentration inPd/Mg/Pd

and Pd/Al/Mg/Pd hydrogenated samples, the graphs were

extrapolated to zero ion doses as explained by Singh et al.

[25]. ERDA result shows that the HD1 film absorbs 4.34

� 1017hydrogen atoms/cm2 whereas HD2 absorbs 6.01

� 1018hydrogen atoms/cm2. The present study shows that Al

layer enhances hydrogen content in Pd sandwiched Mg film to

a greater extend than that observed for Pd sandwiched Mg/Ni

andMg/Mg2Ni systems inourpreviouswork [26]. Followingmay

be the reasons responsible for this enhancement:

i) ERDA plots for O-content in hydrogenated Pd/Mg/Pd and

Pd/Mg/Al/Pd films are shown in Fig. 7(a) and (b) respec-

tively. Table 3 summarizes the atomic concentration of

hydrogen and oxygen atoms in both the samples. ERDA

0.00E+000 2.00E+013 4.00E+013 6.00E+013

1.00E+017

1.20E+017

1.40E+017

1.60E+017

1.80E+017

2

1

Oxy

gen

Con

c. (a

tom

s/cm

2 )

Fluence (ions/cm2)

a

0.00E+000 2.00E+013 4.00E+013

3.00E+016

6.00E+016

9.00E+016

1.20E+017

1.50E+017

1.80E+017

2.10E+017

2.40E+017b

Oxy

gen

Con

c. (a

tom

s/cm

2 )

Fluence (ions/cm2)

2

1

Fig. 7 e Variation of oxygen concentration with fluence of

Ag9D 120 MeV ion beam irradiation on hydrogenated (a)

Pd/Mg/Pd (reprint from ref. 27 with permission from

Elsevier) and (b) Pd/Al/Mg/Pd films. Solid lines (red) are

linear fits of region 1 and 2 (see text). Error bars show

statistical error in the calculated regions.(For interpretation

of the references to color in this figure legend, the reader is

referred to the web version of this article.)

Table 3 e ERDA measurements of hydrogen and oxygencontent in hydrogenated Pd/Mg/Pd & Pd/Al/Mg/Pd films.

Sample Areal H-Content(atoms/cm2)

Areal O-Content(atoms/cm2)

Pd/Mg/PdeH 4.00 � 1017 2.83 � 1017

Pd/Mg/Al/PdeH 6.01 � 1018 1.80 � 1017

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 7 7 9e3 7 8 53784

findingsshowlessoxygencontent inPd/Al/Mg/Pdsystem

in comparison to thebase system, suggesting that adding

Al creates a compound with less oxygen concentration

probablydueto the formationof lessdenseoxide layere.g

amorphous alumina with improved hydrogen diffusion

properties compared to close packed MgO [27].

ii) Further, ERDA results well supported by XRD pattern

suggest that at 50%Mg thereoccursa reactionbetweenAl

and Mg at Mg/Al interface under 150� C and 2 bar H2

pressure which leads to the formation of complex

hydride while the Mg atoms located far away from the

interface contributes to the formation of magnesium

hydride.

iii) Al layer also prevents the formation of any MgePd inter-

metallic alloy which was formed in Pd/Mg/Pd system,

resulting in complete conversion of Mg to MgH2 phase.

4. Conclusion

The present study reports that hydrogenation properties of

Pd/Mg/Pd system can be enhanced by the introduction of Al

layer. The XRD patternwell supported by ERDA finding reveals

that the formation of complex hydride phase along with

magnesium hydride is responsible for increase in hydrogen

content from 4.34 � 1017 atoms/cm2 to 6.01 � 1018 atoms/cm2.

Acknowledgments

The authors are thankful to Inter University Accelerator

Centre (IUAC), NewDelhi, India for providing financial support

and permission for availing research facility under Project No.

4133. Special thanks to Mr. S.A.Khan, Scientist IUAC for

support during ERDA experiment.

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