Electrochimica Acta 49 (2004) 3355–3360

Electrochemical loading of hydrogen in palladium capped samariumthin film: structural, electrical, and optical properties

Pushpendra Kumar, L.K. Malhotra∗

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

Received 25 August 2003; received in revised form 6 February 2004; accepted 13 February 2004

Available online 10 May 2004

Abstract

A 50 nm samarium film capped with a 7 nm palladium overlayer switched from a metallic to semiconducting state during ex-situ hydrogenloading via electrochemical means at room temperature. The transition is accompanied by a change in transmittance measured duringhydrogen loading and the associated optical appearance. The monitoring of working electrode (WE) potential, the transmittance and chipotential difference (�χ) has been used to identify the phases present during hydrogen loading. Deloading of hydrogen has been studiedin open circuit potential condition. Glancing angle X-ray diffraction (GAXRD) studies show that the rhombohedral structure of metallicsamarium film (a0 = 8.989 Å) changes to hexagonal structure of the SmH3−δ film with average lattice parameters ofa = 3.775 Å andc = 6.743 Å. A direct optical band gap of 2.9 eV has been obtained for SmH3−δ film and 2.0 eV for SmH2±ε film from reflectance andtransmittance data. Removal of hydrogen from SmH3−δ leads to the formation of localized states within the band whose signature is clearlyseen in transmittance and Tauc’s plot curves of SmH2±ε film. The Hall coefficientRH measured as a function of hydrogen concentration,changes from a metal-like value−14.23× 10-10 m3/C to−1001.1 × 10−10 m3/C for SmH3−δ films. On unloading hydrogen, the value ofRH

changes to−3.56× 10−10 m3/C at the dihydride composition.© 2004 Elsevier Ltd. All rights reserved.

Keywords: Electrochemical switching; Lanthanides; Hydrides; Metal to semiconductor transition; Optical switching

1. Introduction

In the continuing research effort on rare earth hydrides(REHx), the discovery of switchable mirror effect by Huib-erts et al.[1] occupies a special place. By varying the hydro-gen concentration in palladium capped yttrium/lanthanumthin films, these materials were shown to switch reversiblyfrom metallic, reflecting (x = 2) to insulating transparent(x ∼ 3) state. Although metal to insulator (M–I) transitionsin rare earth hydrides were known[2], the main benefitof this reversible M–I transition is that it is accompaniedby drastic changes in the optical properties in the visibleregion and occurs at room temperature and normal pressure(1.0 × 105 Pa) of hydrogen gas. This type of transition hassince been shown to occur in all hydrides of the trivalentrare earths in thin film form even though the crystal struc-tures of the nearly trihydride state for different rare earthsmay be different[3,4]. The observation of visual changes

∗ Corresponding author. Tel.:+91-11-6591325; fax:+91-11-6581114.E-mail address: lalit@physics.iitd.ac.in (L.K. Malhotra).

accompanying the M–I transition has led to many theo-retical investigations of the phenomena and currently twodifferent approaches have been used to explain it—strongelectron correlation models[5] and the band structure mod-els [6]. The effect has also received considerable attentionfor its use in various technological applications such assmart windows, hydrogen sensors, solid state displays andelectrochemical devices[7–9].

A convenient and widely used procedure by various re-searchers for hydrogen loading of thin rare earth metal filmsis by gas-phase loading[10]. However, the gas-phase load-ing has the disadvantage that the hydrogen concentrationgetting incorporated into the films cannot be continuouslymonitored easily. Moreover it is not very convenient to usehydrogen gas for commercial purposes because of its explo-sive nature in air. An alternative way of hydrogen loadingis via electrochemical means[11–13].

One of the rare earths, which has received comparativelyless attention, is samarium. In an earlier investigation on Smthin films, the films were sputter deposited on transparentconducting tin oxide substrates and then capped with sput-

0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.electacta.2004.02.044

3356 P. Kumar, L.K. Malhotra / Electrochimica Acta 49 (2004) 3355–3360

ter deposited Pd films. The galvanostatic loading was car-ried out in 5 M NaOH solution[14]. In the present work wehave deposited Sm film of thickness 50 nm by vacuum evap-oration and capped them with a Pd overlayer of optimizedthickness 7 nm. Systematic studies of structural, electricaland optical properties have been carried out and the resultsare presented in this paper.

2. Experimental

Sm films of thickness 50 nm were deposited by vacuumevaporation on 40 mm× 40 mm × 2 mm ultrasonicallycleaned glass substrates. The adhesion of the film to thesubstrate is very good provided the glass substrate is prop-erly cleaned and degassed prior to deposition. The basepressure in the vacuum system was 7× 10−5 Pa prior todeposition. Pd overlayers of thickness 7 nm were subse-quently deposited on top of the Sm films without breakingthe vacuum. The geometry of working electrode (WE),shown inFig. 1awas obtained by using a mask during theevaporation process. Pre-calibrated quartz crystal monitorswere used to monitor the rate of deposition and film thick-ness, the rate being kept constant at 1.0 nm/s in each run.Four contacts (A, B, C, and D) were then made for elec-trical measurements using copper wires and silver paste.The experimental setup used for electrochemical loading isshown inFig. 1b. The electrochemical measurements were

Fig. 1. (a) Sample geometry and (b) electrochemical setup.

performed in an aqueous 1 M KOH solution (the rare earthsbeing base metals, they dissolve in acidic solution but arestable in alkaline solution[11]) using a Pt strip as a counterelectrode and an Hg/HgCl2 electrode as a reference elec-trode. The circular portion of palladium capped samariumfilms (hereafter referred to as sample) was immersed in theelectrolyte whereas the contacts were not in contact withthe solution. The effective area of the sample exposed to1 M KOH electrolyte solution was 4.9 cm2. All potentialswere measured with respect to the reference electrode us-ing a constant current source (Keithley, Model-224) andelectrometer (Keithley, Model-6517A). For in situ opticaltransmission measurements, the sample (WE in the elec-trochemical setup) was illuminated with a diode laser (LA12-10 650 nm) and the transmission intensity measuredwith a Photodyne radiometer/photometer (Model 88XLA),placed on the opposite end. A digital multimeter was usedto monitor the surface potential difference across the WE.Before each measurement, high purity argon gas was bub-bled through the solution at least for 15 min to remove theoxygen and a constant argon flow was maintained over theelectrolyte during the measurements.

Ex situ measurements were made for determination ofthe structure, optical behavior and Hall effect parametersof loaded and unloaded films. A Rigaku X-ray diffractome-ter (Giegerflex D/MAX-RB-RV200B) in the glancing angle(GAXRD) mode was used for recording the X-ray diffrac-tograms. The glancing angle was kept at 3◦. The opticalbehavior of the films was investigated by measurement ofthe reflectance (R) and transmittance (T) in the 350–850 nmrange using a UV/VIS/NIR Perkin Elmer Lambda 900 spec-trophotometer with barium sulphate (BaSO4) coated inte-grating sphere.

Hall measurements were made on 5 mm× 5 mm× 2 mmsize samples. Van der pauw configuration[15,16] was usedfor these measurements, applying a direct current 1.0 mAand a magnetic field 0.2 T. A constant current source (Keith-ley, Model-224) and electrometer (Keithley, Model-6517A)were used for these measurements.

3. Results and discussion

During electrochemical loading, the electrolytic reductionof a proton donating species, water in our case, results inthe following reaction

H2O + e− → H+ + OH−

The mechanism of hydrogen entry into palladium involvesproton discharge H+ + e− → H followed by immediatehydrogen adsorption in the palladium layer. The adsorbedhydrogen subsequently diffuses into the underlying Sm filmand is absorbed therein.

The reaction of hydrogen with Sm proceeds as follows:

Sm+ 3

2H2 → SmH2 + 1

2H2 ⇔ SmH3

P. Kumar, L.K. Malhotra / Electrochimica Acta 49 (2004) 3355–3360 3357

Fig. 2. (a) Electrode potential (b) transmission, and (c) surface potentialdifference of a 50 nm Sm/7 nm Pd film as a function of hydrogen con-centrationx = H/Sm determined from galvanostatic loading experiments.

The second step is a reversible transition, which can eas-ily be induced by changing the polarity of the cell or inopen circuit condition, whereas the first step is unidirec-tional. This is because of the relative small heat of for-mation for the second step (−39.6 kJ/mol H) compared tothe heat of formation for the first step (−202.6 KJ/mol H)[17].

The measured electrode potential (E) between WE andreference electrode, transmittance (T) of the WE, and sur-face or “chi” potential difference (�χ) across the WE, as afunction of hydrogen concentration getting incorporated inthe WE on applying a constant currentI = 1 mA, are plottedin Fig. 2. To avoid problems related to the polarizing cur-rent in resistivity measurements, we measured�χ by takingtwo contacts on the WE rather than measuring resistance.Since�χ is related to resistance, one can get an idea aboutthe variation in resistance from the measured surface poten-tial difference. In the case of a metal-solution interface, both

phases contribute to�χ [18]

�χ = χ(metal) − χ(solution)

Fig. 2ashows that the potential of WE drops from−0.190to −1.0 V immediately after applying the constant current.This immediate drop in potential is due to excess chargeaccumulated on the interface of electrolyte and WE. Threeregions can thereafter be clearly distinguished (1) a nearplateau from−1.0 to−1.05 V (2) a gradual drop in poten-tial from −1.05 to−1.29 V, and again (3) a constancy at∼−1.29 V.T and�χ curves are shown inFig. 2b and c, re-spectively. Five regions can be distinguished in the trans-mittance curve.T is initially almost constant at 6% and thenrises gradually to attain a value of 20%. It again remainsconstant for some time followed by a sharper rise in its valueto 42% and then remaining constant at that value thereafter.Similarly, �χ curve shows an immediate rise in�χ from 0to 87 mV, a small constant region followed by a decrease to70 mV, rising again to 200 mV and saturating at that value.Since hydrogen is getting incorporated in the Sm film dueto the passage of current, these changes in the three curvesrepresent the changes taking place in Sm film due to theincorporation of hydrogen. Using Faraday’s law and takingthe film thickness, the electrode surface area, and the inte-grated charge into account, we have calculated the hydrogenconcentration in Sm film. Based on an earlier report[19],we have taken an initial hydrogen concentrationx = 0.08,which gets incorporated into the film during deposition, intoaccount while making our calculations. These calculationsshow that the observed transitions inT and∆χ curves cor-respond to hydrogen content of about 0.25, 1.7, 2.1, and 2.6,respectively in Sm films. It is very important that the thick-ness taken into account while calculating hydrogen concen-tration is measured very accurately. Based on our experi-mental results and calculations, the reported phase diagramfor bulk samarium–hydrogen system[20] and the publisheddata on similar studies on rare earth metal films[12], we caninfer the following: For hydrogen concentrationsx ≤ 0.25,T remains constant and�χ, after an initial rise from 0 to87 mV also remains constant. ConstantT and�χ are signa-tures of a single-phase solid solution of hydrogen in samar-ium. For 0.25 < x < 1.7, �χ decreases gradually whichindicates moving towards the dihydride state. The rise in Tin this region is because of the small thickness (50 nm) ofour film, a feature also observed in an earlier work[14]. Tremains constant for 1.7 < x < 2.1 and the minima in∆χ

occurs at 1.85 which is an indication of the formation ofthe dihydride state at that composition. Thereafter all threecurves show a sharp rise and attain constant values beyondx = 2.6 which is an indication of the formation of trihydridestate. Based on these results, we have constructed the phasediagram drawn at the bottom ofFig. 2.

By increasing the hydrogen concentrationx in SmHx atroom temperature, we thus observe three stable hydridephases:� (a single-phase solid solution of hydrogen insamarium),� (the dihydride phase), and� (the trihydride

3358 P. Kumar, L.K. Malhotra / Electrochimica Acta 49 (2004) 3355–3360

Fig. 3. (a) Electrode potential (b) transmission, and (c) surface potentialdifference of a 50 nm Sm/7 nm Pd film as a function of time on sponta-neous discharging.

phase). In addition, there are two regions of coexistingphases (� + � and� + �). It may be noted fromFig. 2athatthere is a flat plateau during the conversion of�-phase to�-phase. However, potential gradually falls during the�- to�-phase conversion and no plateau is observed. More hy-drogen getting incorporated into the films with increasingxleads to higher stresses in films and this may be a possiblecause for the observed behavior.

After charging the WE, the current was switched off andthe spontaneous (ocp) discharging of WE done. TheE, T,and�χ curves during spontaneous discharging are shown inFig. 3. E immediately increases from−1.3 to−1.0 V. Fourregions, subsequently encountered inFig. 3a are regionsof gradual increase, a near constant region followed by asharper increase and then a slow increase to attain a constantvalue. The occurrence of these regions can be explainedwith the help ofFig. 3b and c. In the first region, till 120 s,the excess charge (H+ ion) accumulated on the interfaceof WE and electrolyte starts to diffuse slowly through theelectrolyte. It may be noted that a slow increase ofE meansthat the capacitive layer diffuses towards the anode. How-ever, there is no deloading of hydrogen from the WE. Thiscan be seen fromFig. 3b and cwhich show almost constantvalues ofT (42%) and∆χ (26 mV). T and�χ are mostlyconcerned with the properties of WE whileE takes into ac-count both the reactions taking place in the electrolyte andthe WE. In the second region, between 120 and 260 s,Ebecomes approximately constant but a drastic change inT

from 42 to 19% and in∆χ from 26 to 6.5 mV occurs. Thisregion could be understood by noting that in this region theWE starts to discharge its hydrogen and that the dischargedhydrogen stays at the interface of WE which is reflected inthe near constantE. In the range between 260 and 400 s,Eincreases sharply from−0.790 to−0.270 V, indicating thatthe accumulated hydrogen on the WE again starts to diffusethrough the electrolyte and a very small change inT and�χ occurs. In the fourth region beyond 400 s, all the threecurves stay almost constant which indicate that the dihy-dride state has been attained and no hydrogen can then bedeloaded. Though the potential on deloading falls to about0.2 V a value comparable to that obtained (correspondingto �-phase) during loading, it does not mean reaching the�-phase during deloading. The nearly equal potential valuesin the two phases are due to their comparable electrical con-ductivity; however, the transmittance (a distinguishing fea-ture between the two phases) on deloading—16% is muchhigher than that obtained an loading till�-phase∼6%. Thetransition between the di and trihydride state is reversible.We have tried ten cycles of reversible switching and no dete-riorating effect on the films was observed. The dark brown-ish color of dihydride state changes to the golden greenishcolor of the trihydride state which again reverts back todark brownish color. It may be pointed out that the entirearea of working electrode was uniformly on/off as revealedby the uniform observed color throughout the WE as wellas uniform observed transmittance during laser scanningof WE.

Since the surface energy of Pd cap layer is higher thanthat of any rare earth metal[21], the nucleation and growththeories suggest an activation barrier to the nucleation of thecondensed phase[22]. Pd will therefore deposit in the formof electrically disconnected small clusters. It has been the-oretically predicted and experimentally verified that the ge-ometry and the electronic band structure of the Pd clustersvary with cluster size and they show corresponding change intheir properties; for example, these clusters cease to show re-versible hydrogen deloading capacity[23–25]but they con-tinue to show catalytic hydrogen dissociation behavior. Thisexplains slow discharge of hydrogen in Sm films capped witha thin 7 nm Pd overlayer. We were thus able to store hydro-gen in the hydrogen saturated film (nearly trihydride state)approximately for an hour. This was confirmed by measure-ment ofR andT as well visual appearance. This enabled usto carry out all our measurements during this period.

Hall effect measurements were performed for all thethree states metallic, hydrogen saturated, and the nearlydihydride state. From an electron dominated conductionwith a metal-like Hall coefficient (RH) value of −14.2 ×10−10 m3/C, the magnitude ofRH increases as the free carrierdensity decreases.RH = −1001.06×10−10 m3/C for hydro-gen saturated (nearly trihydride) and−3.85× 10−10 m3/Cfor the dihydride state. This decrease in the value ofRHtowards zero is an indication of the existence of holes inthe dihydride state.

P. Kumar, L.K. Malhotra / Electrochimica Acta 49 (2004) 3355–3360 3359

Fig. 4. X-ray diffractograms for a Sm film of thickness 50 nm coveredwith a 7 nm Pd cap layer (a) as deposited (b) in hydrogen saturatedSmH3−δ-phase (c) in SmH2±ε dihydride phase.

Fig. 4ashows the X-ray diffractogram of the as depositedsample. The peaks corresponding to Sm and Pd can beclearly identified. Analysis of the diffraction peaks revealsrhombohedral structure of samarium with an average latticeparametersa0 = 8.989 Å. This value is slightly differentfrom the reported value[18], a0 = 8.982 Å for bulk samar-ium. This may be due to the incorporation of small amountof hydrogen into Sm film during deposition as it is difficultto completely remove hydrogen from the vacuum chamber.Fig. 4bshows the diffractogram for the hydrogen saturatedsample of SmH3−δ. Analysis of this pattern revealed a hexag-onal structure. The average lattice parameters determinedarea = 3.775 Å andc = 6.743 Å; these values are in prox-imity to values reported for bulk SmH3−δ (a = 3.782 Å andc = 6.779 Å). The difference may be because of the differ-ence inδ values between our films and the bulk. The diffrac-tion pattern of hydrogen desorbed (dihydride state) film isshown inFig. 4c. The structure of the film is fcc, CaF2 typestructure with an average lattice parameter 5.374 Å which isclose to the value 5.374 Å reported for bulk SmH2.

In principle, R and T measurements can be inverted toyield the complex refractive index, but this is numericallycomplicated for a double layer film (PdHx/SmHx) due tomultiple solutions. In this work, we calculated the absorp-tion coefficient,α(�), which is proportional to the imaginarypart of the refractory index. The absorption coefficient foran absorbing film is related toT andR by the following ex-pression[26,27]:

α(λ) = 1

dln

(1 − R(λ)

T(λ)

)

whered denotes the thickness of the film.

Fig. 5. (a) Reflection and transmission spectra in the wavelength range350–850 nm for a 50 nm SmH3−δ film covered with 7.0 nm Pd film (b)Plot of (αhν)2 vs hν for the same film.

The functional form of absorption coefficient (α) for thedirect band-to-band transition is well described by the rela-tion

(αhν)2 = constant(hν − Eg)

whereas(αhν)1/2 = constant(hν − Eg) gives the relationfor indirect band gap materials. The best linear relationshipwas obtained by plotting (αhν)2 against photon energy (hν),indicating that the absorption edge in these films is due to adirect allowed transition. The line fit to the (αhν)2 versushν

plot was obtained by fitting a straight line to linear portion ofthe curve using linear regression software.Fig. 5bshows thedirect band gap to be 2.93 eV calculated fromR andT givenin Fig. 5afor hydrogen saturated sample.Fig. 6 shows thechange in optical properties of the same film after deloading.Fig. 6ashows the reflectance and transmittance spectra forthe sample in the state SmH2±ε. The absorption edge in thetransmittance curve was found at a wavelength of 630 nm(edge is defined as 90% of total transmittance) while in thehydrogen saturated film (SmH3−δ), it was at a wavelength of530 nm. This is attributed to a transition involving localizedstates, which are formed between valence and conductionbands, after deloading of hydrogen from hydrogen saturatedfilm. The observed transition energy due to vacancy states isfound to be 1.96 eV (Fig. 6b), quite close to the theoretically

3360 P. Kumar, L.K. Malhotra / Electrochimica Acta 49 (2004) 3355–3360

Fig. 6. (a) Reflection and transmission spectra in the wavelength range350–850 nm for a 50 nm SmH2±ε film covered with 7.0 nm Pd film (b)Plot of (αhν)2 vs hν for the same film.

calculated value, i.e. 1.9 eV reported for LaHx films [5].Observation of this localized band is in agreement with thetheoretical prediction of Ng et al.[5], which suggested theformation of an impurity band of strongly localized electronsnear the conduction bands due to the removal of hydrogenfrom octahedral sites of insulating LaH3. Kooij et al. [28]interpreted the concentration dependence of optical resultsin the �-YHx-phase using Ng et al.’s theory for LaHx, andthey also reported the formation of strongly localized statesnear the conduction band during removal of hydrogen fromYHx films.

In conclusion, we have shown that Pd capped Sm films canbe reversibly switched between the metallic reflecting dihy-dride state to semiconducting transparent (in the visible partof electromagnetic spectrum) nearly trihydride state by load-ing/deloading hydrogen by Galvanostatic means. Changesin hydrogen concentration lead to changes in crystal struc-ture, the color and the carrier concentration.

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