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Thin Solid Films 491

Electrochemical study of Pd capped samarium hydride

thin film switchable mirror

Pushpendra Kumar *, L.K. Malhotra

Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India

Received 8 October 2004; received in revised form 14 April 2005; accepted 25 May 2005

Available online 7 July 2005

Abstract

A 55-nm samarium film capped with a 15-nm palladium overlayer switched from a metallic reflecting to a semiconducting, transparent in

visible state during ex-situ hydrogen loading via electrochemical means in 1 M KOH electrolytic aqueous solution at room temperature. The

optical transmittance exhibits a hysteresis effects on loading and unloading of hydrogen in SmHx thin film between x =2 and 3. The

hysteresis is discussed in terms of strain (and consequently stress) at the interface between the dihydride and trihydride. The diffusion

coefficients and switching kinetics are shown to depend on applied current density. The changes of anodic overpotential with time during

applied constant current discharge have been used to determine the electrochemical parameters the exchange current density i0 and the

symmetry factor b.D 2005 Elsevier B.V. All rights reserved.

Keywords: Hydrides; Electrochemistry; Adsorption; Galvanometric properties

1. Introduction

The reversible switching between the metallic reflecting

dihydride state and the insulating, transparent in visible

range, nearly trihydride state by controlling hydrogen

concentration is called FSwitchable Mirror_ effect [1].

Palladium capped yttrium and lanthanum thin films, in

which this effect was first reported by Huiberts et al. [2] and

other trivalent rare earth thin films, which have subse-

quently been shown to exhibit similar behavior are called

first generation switchable mirrors. Reversible electrical and

optical switching has, since then, also been observed in Gd–

Mg [3–5] and Mg–Ni [6–9] alloys upon hydrogenation

and these have been considered second and third generation

switchable mirrors. Besides gas phase loading of hydrogen

in rare earth metal films reported by Huiberts et al. [2],

electrochemical loading [10,11] has been used as means of

hydrogen loading. Electrochemical loading offers many

0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2005.05.042

* Corresponding author.

E-mail address: pkumar_iitd@yahoo.com (P. Kumar).

advantages: the concentration of hydrogen in the film can be

controlled accurately and extremely low pressures can be

achieved. Since hydrogen transport in metal hydride thin

films may be important in determining the dynamic

response of a device. Electrochemical techniques can be

used to study transport. Essential efforts have been

dedicated to understand the kinetics and mechanism of the

hydrogenation reaction, its optimization and reversibility

[12–17]. The kinetics of the electrochemical charge transfer

reaction at the metal hydride electrode have been inves-

tigated by Notten et al. [18] by studying the relationship

between exchange current density and partial pressure of

hydrogen. During electrochemical loading and unloading of

hydrogen between di- and trihydride states, hysteresis in the

physical properties of some of the rare earth metal hydride

thin film systems has also been reported in the literature.

Kooij et al. [14] found that YHx thin films exhibit

anomalously large hysteresis in the optical, electrical and

structural properties as a function of hydrogen concentration

during loading and unloading of hydrogen. The hysteresis

was attributed to different behavior of hydrogen during

absorption/desorption [19]. To further explore kinetics

(2005) 270 – 275

P. Kumar, L.K. Malhotra / Thin Solid Films 491 (2005) 270–275 271

taking place at the working electrode/electrolyte and

hysteresis observed during electrochemical hydrogen load-

ing and unloading, we have examined the effect in

samarium thin films through a study of the electrochemical

parameters such as exchange current density, symmetry

factor, diffusion coefficient and their dependence on the

applied current density and the optical behavior. Results are

presented in this paper.

0 25 50 75 100 125 1500,0

0,5

1,0

1,5

2,0

2,5

i = 2.04 mA/cm2

i = 1.02 mA/cm2

i = 0.6 mA/cm2

i = 0.2 mA/cm2

- E

(V

)

Time (s)

0

1

2i = - 2.04 mA/cm2

i = - 1.02 mA/cm2

i = - 0.6 mA/cm2

i = - 0.2 mA/cm2

(a)

2. Experimental details

Sm films of thickness 55 nm were deposited by vacuum

evaporation on 40 mm�40 mm�2 mm ultrasonically

cleaned glass substrates. The base pressure in the vacuum

system was 7�10�5 Pa prior to deposition. Pd overlayer of

thickness 15 nm was deposited on top of the Sm films

without breaking the vacuum. Pd overlayer acts as a catalyst

to dissociate hydrogen molecule into atomic hydrogen and

also protects the underlying films from oxidation. The

experimental details for in-situ measurements of film

thickness and rate of deposition have been described in an

earlier paper [20]. The electrochemical measurements were

performed at room temperature in an aqueous 1 M KOH

solution using a Pt strip as a counter electrode and an Hg/

HgO electrode as a reference electrode. The effective area of

palladium capped samarium films (working electrode: WE)

exposed to 1 M KOH electrolyte solution was 4.9 cm2 [21].

All potentials were measured with respect to the reference

electrode using an electrometer (Keithley, Model-6517A). A

constant current source (Keithley, Model-224) was used to

apply a current. For in-situ optical transmission measure-

ments, WE was illuminated with a diode laser (LA 12-10-

650 nm) and the transmission intensity measured with a

Photodyne radiometer/photometer (Model 88XLA), placed

on the opposite end. Before each measurement, high purity

argon gas was bubbled through the solution at least for 15

min to remove oxygen and a constant argon flow was

maintained over the electrolyte during the measurements.

Ex-situ measurements were made for the determination of

the structure. A Rigaku X-ray diffractometer (Giegerflex D/

MAX-RB-RV200B) in the glancing angle (Glancing Angle

X-ray Diffraction) mode was used for recording the X-ray

diffractograms. The glancing angle was kept at 3-.

0 20 40 60 80 100-3

-2

-1

E (

V)

Time (s)

(b)

Fig. 1. Change of the electrode potential with time for several current

densities: (a) loading, (b) unloading.

3. Results and discussion

During electrochemical loading, the electrolytic reduc-

tion of a proton donating species, water in our case, results

in the following reaction

H2Oþ e�YHad þ OH� ð1Þ

The mechanism of hydrogen entry into palladium involves

proton discharge H++e�YH followed by immediate hydro-

gen adsorption in the palladium layer. The adsorbed

hydrogen subsequently diffuses into the underlying Sm

film and is absorbed therein. The reaction of hydrogen with

Sm proceeds as follows:

Smþ ð3=2ÞH2YSmH2 þ ð1=2ÞH2SSmH3 ð2Þ

The second step is a reversible transition, which can easily

be induced by changing the polarity of the cell or in open

circuit condition, whereas the first step is unidirectional.

This is because of the relative small heat of formation for the

second step (�39.6 kJ/mol H) compared to the heat of

formation for the first step (�202.6 kJ/mol H) [15].

The working electrode was reversibly switched, in a 1 M

KOH electrolytic aqueous solution, between dihydride state

(dark brownish color) and trihydride state (golden greenish

color) galvanostatically at different constant current den-

sities (0.2, 0.6, 1.02, 2.04 mA/cm2). All the measurements

were made on the same sample. As mentioned in one of our

earlier paper [21], there was no noticeable degradation of

the sample even after 40 cycles of hydrogen loading/

unloading. Fig. 1a shows the change in potential with time

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.0 0.2 0.4 0.6 0.8 1.0 1.20.35

0.40

0.45

0.50

0.55

0.60

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0.15

0.20

0.25

0.30

0.35

0.40

Ove

rpot

enti

al η

(V)

Ove

rpot

enti

al η

(V)

Ove

rpot

enti

al η

(V)

-log[1-(t/τ)1/2]

-log[1-(t/τ)1/2]

-log[1-(t/τ)1/2]

i = - 0.2 mA/cm2

i = - 0.6 mA/cm2

i = - 1.02 mA/cm2

Fig. 2. The overpotential g vs. � log[1� (t /s)1/2] for several unloading

current densities.Table 1

Variation of response and recovery times, exchange current density and

symmetry factor with current density

Current density

(mA/cm2)

Response

times (s)

Recovery

times (s)

i0(mA/cm2)

b

0.2 70 50 0.013 0.7

0.612 25 20 0.063 0.66

1.02 15 12 0.23 0.7

2.04 8 6 – –

P. Kumar, L.K. Malhotra / Thin Solid Films 491 (2005) 270–275272

on loading of hydrogen. The fall in potential directly

depends on applied current density between working and

counter electrodes. If the current density is high, the rate of

evolution of hydrogen ions at the electrode surface will be

high, resulting in a high chemical potential generated in the

solution. It may be pointed out that during hydrogen

loading, Pd also forms a hydride–PdH0.6 in which hydrogen

is bound with Pd in such a way that it cannot be desorbed at

room temperature. Heating to high temperature can only

lead to desorption of hydrogen from Pd. The kinetics of

hydrogen loading/unloading at room temperature between

the dihydride and trihydride states therefore essentially

involves Sm and hydrogen.

The dependence of the anodic potential on applied

current density with reverse polarity is shown in Fig. 1b.

The shapes of unloading curves for all the current densities

are very similar even though their time scales are different.

The unloading of hydrogen from hydrogen saturated film

(SmH3�d) takes place in the plateau region, which is

confirmed from the decreases in transmission in that reason.

The potential at which the plateau occurs shifts to positive

values as the current density increases with reverse polarity.

For the higher current density, the curve loses the sigmoid

shape.

According to Yayama et al., the general dependence of

overpotential g (g =E�E0, where E is the electrode

potential and E0 is standard electrode potential) on current

density for hydrogen absorbing electrodes is [22]

g ¼ 2:3RT

bFlog i=i0ð Þ � log 1F t=sð Þ1=2

h in oð3Þ

where b is the symmetry factor and i0 the exchange current

density: the sign in the parenthesis is + for cathodic and �for anodic processes.

In agreement with this equation, our experimental data

shows that for activated samples (WE), the overpotential

shifts to negative values during loading at constant current

density and to positive value during unloading. The above

equation has been used in chronopotentiometric experi-

ments to determine the electrochemical parameters b and

i0 from the slope and the intercept of the straight lines

obtained from a plot of g as a function of � log[1� (t /

s)1/2]. The results are shown in Fig. 2. The exchange

current density i0 is calculated from g at t =0. The

exchange current density, symmetry factor, response and

recovery times with different current densities are listed in

Table 1. The exchange current density does uniquely

define the rate of the heterogeneous charge transfer

reaction, which takes place at working electrode/electro-

0.0 0.5 1.0 1.5 2.00

2

4

6

8

10

12

Current density (mA/cm2)

D (

10-1

0 cm

2 s-1)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

1/t s

(s)

Fig. 3. The applied current dependence of the diffusion coefficient and the

reciprocal response time of Pd (15 nm) capped Sm (55 nm) film.

P. Kumar, L.K. Malhotra / Thin Solid Films 491 (2005) 270–275 273

lyte increases with increase in applied current density (i).

The symmetry factor (slop of curves) is independent of

applied current density. The response time (defined as the

time required to reach 90% of maximum transmittance in

2.0 2.2 2.4 2.6 2.8 3.0

15

20

25

30

35

40

45

x = H/Sm

T %

15

20

25

30

35

40

45

T %

1.8 2.0 2.2 2.4 2.6 2.8 3.0

15

20

25

30

35

40

45

x = H/Sm

T %

15

20

25

30

35

40

45

T %

i = 1.02 mA/cm2

i = 0.2 mA/cm2+–

+–

Fig. 4. Hysteresis in Sm–Hx thin film on loading and unloading between dihyd

overlayer at different current densities.

hydrogen saturated state) and recovery time (defined as

the time required to attain 30% of maximum trans-

mittance) decrease with an increase in current density. The

recovery time is less than the response time (Table 1). It

is due to the higher surface energy of WE in the

electrolyte. The transition time s (defined as the time

required in going from one state to other state i.e.

response and recovery time) decreases with increasing

current density. This dependence is in agreement with

Sand’s equation [23]

s1=2 ¼ nFAp1=2D1=2cH

2ið4Þ

where D is the diffusion coefficient for the H atoms in

the Sm film, A is the electrode surface area and cH is the

hydrogen concentration. The latter was evaluated using

the Faraday’s law from the amount of charge supplied

during different loading periods. It was found that the

diffusion coefficient of hydrogen in Sm film varied with

applied current density. The diffusion coefficient is plotted

as a function of the applied current density in Fig. 3. The

1.8 2.0 2.2 2.4 2.6 2.8 3.0

1.8 2.0 2.2 2.4 2.6 2.8 3.0

15

20

25

30

35

40

45

x = H/Sm

T %

15

20

25

30

35

40

45

T %

15

20

25

30

35

40

45

x = H/Sm

T %

15

20

25

30

35

40

45

T %

i = 2.04 mA/cm2

i = 0.6 mA/cm2

+–

+–

ride and trihydride state in a 55-nm-thick Sm film capped with 15 nm Pd

Table 2

Experimentally determined structures and lattice constants of Pd:H/SmHx

(i.e. hydrogen saturated), Pd:H/SmHx V(i.e. hydrogen desorbed) films

Sample Structure a (A) c (A) d– d�

SmHx (x�3�d) Hexagonal 3.775 6.743 3.372 3.775

Pd:H (Hydrogen saturated) Fcc 3.881 – 2.241 2.744

SmHx V(x¨2T e) Fcc 5.372 – 3.10 3.798

Pd:H (Hydrogen desorbed) Fcc 3.899 – 2.251 2.757

The separation between consecutive planes, i.e. d– and nearest neighbor

distance, i.e. d� within the planes are also shown.

P. Kumar, L.K. Malhotra / Thin Solid Films 491 (2005) 270–275274

diffusion coefficient increases from 9.9�10�11 to

11.0�10�10 cm2/s in the current density range from 0.2

to 2.04 mA/cm2. This is of the same order as the

diffusion coefficient of hydrogen in other rare earth metal

films 10�10 to 10�12 cm2/s [16]. The response time

decreases from 70 to 8 s with increasing applied current

density. The reciprocal response time, as shown in Fig. 3,

is inversely proportional to the applied current density.

Using Faraday’s law and taking the film thickness, the

electrode surface area, and the integrated charge into

account, we have calculated the hydrogen concentration in

the Sm films. Based on an earlier report [24], we have

taken an initial hydrogen concentration x =0.08 (which

gets incorporated into the film during deposition) into

account while making our calculations. A different current

density was applied for different time periods to attain an

appropriate amount of hydrogen in the film. Transmittance

of the WE measured during hydrogen loading and

unloading is shown in Fig. 4. There is a hysteresis in

transmittance in each of the four cases. The area under the

loop is almost the same for each current density indicating

that there is no degradation of the films (which may cause

hysteresis) nor is hysteresis a kinetic effect [14]. As more

and more hydrogen gets absorbed in samarium film during

loading, stresses get built up in the film due to repulsive

interaction between H–H atoms becoming much larger

than the interactive interaction. The clamping of film to the

substrate prevents powder formation as observed in bulk

[2]. On unloading, desorption of hydrogen takes place and

the stresses get released. The hysteresis in Fig. 4 appears

to be a consequence of stresses built up and released as a

function of hydrogen concentration meaning that the

20 40 60 80

Inte

nsit

y (a

rb. u

nits

)

2θ (deg)

SmH2 ± ε

SmH3 – δ

Pd

(111

)

(200

)

(111

)

(200

)(2

20)

(311

)(2

22)

(220

)(4

00)

(331

)(3

11)

(200

)

(003

)(1

11)

(101

)

(002

)(1

00)

(220

)(1

13)

(202

)(1

04)

(201

)(2

00)

(004

)(110

)

(204

)(3

11)

(211

)(2

10)

(114

)

(005

)

(a)

(b)

Fig. 5. X-ray diffractograms for a Sm film of thickness 55 nm covered with

a 15-nm Pd overlayer (a) in hydrogen saturated SmH3�d phase, (b) in

SmH2T( dihydride phase.

behavior of Sm:H system during hydrogen absorption

and desorption is different [19]. The behavior of the

system during loading (absorption) and unloading (desorp-

tion) is different which can be seen from cathodic

(loading) and anodic (unloading) curves shown in Fig.

1a and b, respectively. The different behavior of the system

on loading and unloading is mainly due to the fact that the

electrical resistance of SmH3 is higher than the SmH2

state. The mechanical stress at the interface between Pd:H

and SmHx may also be a contributing factor. It is generally

accepted that a phase transformation occurs in the plateau

regions [14,18], while the steeper part corresponds to a

solid solution region, which is a characteristic for a single

phase process. Fig. 5 shows the X-ray diffractogram for

samarium thin films in the dihydride (fcc) and trihydride

(hexagonal) states. The experimentally determined lattice

constants, the separation d– where {d–=c / 2 for hex;

d–=a / (3)1/2 for fcc} between consecutive planes and the

nearest-neighbor distance d� {where d�=a for hex;

d�=a / (2)1/2 for fcc} with in these planes are given in

Table 2. The out-of-plane expansion and in-plane contrac-

tion between dihydride and trihydride state calculated from

X-ray diffraction (XRD) data come out to be 8.8% and

0.6%, respectively. From these data, we infer that the

hysteresis is closely related to the combination of large

uniaxial change along the c-axis and lateral clamping of

film to substrate plays more important role during hydro-

gen desorption [25].

4. Conclusions

Pd capped samarium film was reversibly switched by

applying different current density between dihydride and

trihydride state. The electrochemical parameters have been

estimated. Switching time and diffusion coefficient have

been shown to vary with applied current density. Hysteresis

was observed in transmittance during loading and unloading

between dihydride and trihydride states and attributed to the

stresses induced/released on loading/unloading of hydrogen.

From XRD results, we infer that the hysteresis is closely

related to the combination of large uniaxial change along the

c-axis and lateral clamping between dihydride and trihy-

dride states.

P. Kumar, L.K. Malhotra / Thin Solid Films 491 (2005) 270–275 275

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