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Contents lists available at ScienceDirect

Electrochimica Acta

jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta

lectrodeposition of Fe–Ga thin films from eutectic-based ionic liquid

. Zhao, S. Franz ∗, A. Vicenzo1, M. Bestetti, F. Venturini, P.L. Cavallotti1

ipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”, Politecnico di Milano, Via L. Mancinelli, 7, 20131 Milano, Italy

r t i c l e i n f o

rticle history:eceived 30 March 2013eceived in revised form 24 July 2013ccepted 24 July 2013vailable online xxx

eywords:agnetostrictive material

e–Ga thin filmlectrodepositionutectic based ionic liquidholine chloride

a b s t r a c t

In the present work, a novel process for Fe–Ga thin films electrodeposition is addressed and the mag-netic properties of the films are studied. The electrodeposition was carried out under ambient conditionsusing an ionic liquid electrolyte consisting of a mixture of choline chloride and ethylene glycol in themolar ratio 1:2, containing 0.3 M FeCl2 and 0.1 M GaCl3, either in the absence or in the presence of oxalicacid at 4 or 17 mM concentration. The effect of oxalic acid on the discharge reaction of the single ionicspecies Fe2+ and Ga3+ and on their codeposition was investigated by linear sweep voltammetry, sug-gesting a specific action of oxalic acid on the cathodic reduction of Ga3+ ionic species. Depending ondeposition potential and oxalic acid concentration, alloy films with Ga content variable in the rangeup to about 20 at.% could be obtained. The Fe–Ga thin films showed a disordered body-cantered cubicphase (A2) with (1 1 0) preferred orientation and columnar microstructure, with a drastic change frompyramidal to granular surface morphology revealed raising the deposition potential. No superlattice

reflections, indicating the formation of the D03 structure, were observed. A vibrating sample magnetome-ter was employed to measure hysteresis loops by applying longitudinal and transversal magnetic fieldon the Fe–Ga film plane. The saturation magnetization of as-deposited film reached 1.75 T for Fe83Ga17

thin films, confirming that good quality films were obtained. For the same alloy composition, the coer-civity values were 67 Oe and 200 Oe, with applied field parallel and perpendicular to the film plane,

respectively.

. Introduction

Owing to their good magnetostrictive and mechanical proper-ies, Fe–Ga alloys are considered a valid alternative to piezoelectricsr Terfenol-D for compact actuators and sensors to be operated inarsh and mechanical shock environments. Single-crystal Fe–Galloy with 17 at.% Ga shows a moderate magnetostriction (about00 ppm) compared to Terfenol-D (about 2000 ppm). However,he bias field required is just about 100 Oe (10 times lower thanerfenol-D), and it can be achieved with a permanent magnet,aking them suitable for compact devices. Furthermore, Fe–Ga

lloys show tensile strength of about 500 MPa (20 times higher thanerfenol-D) [1] and a limited dependence of magnetostriction onhe operating temperature in the range from −20 to 80 ◦C [2]. Fe–Galloys with Ga content lower than 20 at.% are machinable, duc-

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ile, weldable and show good corrosion resistance [3]. Fe–Ga alloyshow high crystalline anisotropy and the magnetoelastic behaviourhanges with the crystallographic direction [4]. Textured polycrys-alline Fe–Ga alloys usually show lower magnetostriction (up to

∗ Corresponding author. Tel.: +39 0223993102.E-mail address: silvia.franz@polimi.it (S. Franz).

1 ISE member.

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.07.172

© 2013 Elsevier Ltd. All rights reserved.

170 ppm) than the single crystals of similar composition, thoughare likely to be more commercially viable. Polycrystalline Fe–Gafilms have been produced in the form of sheets, rods, ribbons andthin films. Fe–Ga thin films offer great advantages to the MEMStechnology due to the possibility of downscaling without giving uprobustness and sensitivity of the device.

Compared to pure Fe, the addition of Ga greatly increasesthe magnetostriction of over 10-fold up to 400 ppm in single-crystalline alloy containing 17 at.% Ga [5]. The increase ofmagnetostriction over that of pure iron is attributed to the highermagnetoelastic coupling constant of the alloy due to short rangeordering between Ga–Ga atoms in bcc �-Fe with randomly substi-tuted Ga atoms (A2 structure) [6,7]. According to the phase diagramof the Fe–Ga binary system [8], the equilibrium solubility of Gain �-Fe is about 11 at.% at room temperature, and the solubilitylimit is 36 at.% at 1037 ◦C. As much as 20 at.% Ga may be retained inmetastable solid solution at room temperature [9]. At Ga contentgreater than 20 at.%, the bcc terminal solid solution phase evolvesinto ordered phases based on the B2, D03, D019 and finally into theL12 structure at low temperature [10]. Magnetostriction in bulkFe–Ga alloys was first discussed by Clark et al. [11] and has been

thin films from eutectic-based ionic liquid, Electrochim. Acta (2013),

studied by several groups [12–15]. Studies on bulk Fe–Ga alloysshowed that in �-phase Fe–Ga single crystal the maximum mag-netostriction occurs around ∼17 to 20 at.% Ga, depending on thecooling conditions, while a further increase in Ga content induces

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ong range ordering (D03 and B2 structure) that mitigates the mag-etostriction.

From the point of view of the application, Fe–Ga thin films aref current interest for use in micro- and nano-electromechanicalystems (MEMS and NEMS) and in particular for integrated mag-etostrictive devices (MagMEMS) which require a thin film ofagnetoelastic material with high magnetostriction constant and

ow saturation field [16–18]. Thin film fabrication techniquesor Fe–Ga alloys with optimized parameters are not yet wellstablished. Fe–Ga thin films deposition is mostly carried out byputtering, either DC [19,20] or RF sputtering [21], due to thereat, inherent flexibility of the technique but more in particu-ar because it was believed that sputtering could duplicate the

etastable effects produced by quenching that were proved toause an increase in the magnetostriction.

More recently, a few studies on the electrodepostion of Fe–Galloy thin films have been published by several research groups22–24]. Compared to sputtering, electrochemical deposition pro-ides a cost-effective and non-equipment-intensive method for thereparation of thin films over substrates of large area and arbi-rary shape. However, the electrodeposition of Fe–Ga has beenardly studied due to the difficulty of depositing Ga and its alloy

rom aqueous electrolytes [25]. The difficulty comes from theighly negative standard potential of Ga3+/Ga (−0.53 V), mean-

ng that hydrogen evolution reaction (HER) interferes with thelectrodeposition process. The side-reaction reduces the platingfficiency, induces the precipitation of oxides and hydroxides anddversely affects the quality of thin films, due to pinhole forma-ion and morphological defects. To overcome the limits imposedy aqueous electrolytes, a promising strategy is to use elec-rolytes based on ionic liquids. So far, a few papers have reportedn the electrodeposition of Ga and its alloys from electrolytesased on ionic liquids. In the early 1990s Carpenter and Ver-rugge [26] investigated the electrodeposition of Ga from Lewisasic GaCl3/1-ethyl-3-methylimidazolium chloride ionic liquid onlatinum microelectrodes. By adding AsCl3 to the Lewis basicaCl3/1-ethyl-3-methylimidazolium chloride ionic liquid the elec-

rodeposition of GaAs was realized at room temperature. Chen et al.27] studied the electrodeposition of Ga from a Lewis acid AlCl3/1-thyl-3-methylimidazolium chloride ionic liquid (60/40 mol%) onlassy carbon and tungsten electrodes. However, chlorogallateonic liquids did not obtained wide attention, because of theirggressive chemical nature. Recently, Abbott et al. [28,29] haveeveloped several types of deep eutectic ionic liquids based onholine chloride. Many research groups [30–32] have applied themn the electrodeposition of metallic coatings. Compared to otherypes of ionic liquids, choline chloride-based eutectic ionic liq-ids are cheap, easy to manipulate, non-toxic and biodegradable33]. Moreover, these liquids are able to dissolve a large range of

etal oxides and chlorides [34]. These properties qualify cholinehloride-based eutectic ionic liquids as good candidates for thelectrodeposition of metallic films for large scale production.

In the present paper, the potentiostatic electrodeposition ofe–Ga films from a eutectic based ionic liquid, consisting of aixture of choline chloride and ethylene glycol (Ethaline), under

mbient conditions has been studied. The influence of operatingariables, namely the addition of oxalic acid (OA) and the deposi-ion potential, on composition, morphology and crystal structure oflectrodeposited Fe–Ga films was investigated in order to optimizehe electrodeposition process of Fe–Ga thin films.

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. Experimental

All the experiments were carried out using a deep eutecticolvent, Ethaline, which consists of a 1:2 molar ratio mixture of

PRESScta xxx (2013) xxx– xxx

choline chloride (2-hydroxy-ethyl-trimethyl ammonium chloride)and ethylene glycol (both of purity ≥99%, Sigma–Aldrich). Beforemixing with ethylene glycol, choline chloride was dried in a vacuumdryer at 120 ◦C for 24 h. Anhydrous ferrous chloride and anhydrousgallium chloride (both of purity ≥99.999%, Sigma–Aldrich) wereadded to Ethaline to a final concentration of 0.3 and 0.1 M, respec-tively, under an inert atmosphere in a glove box and subsequentlydissolved by ultrasonication at 70 ◦C. Dihydrated oxalic acid (purity≥99%, Riedel-de Haën) was used as an additive, with concentrationof 4 and 17 mM. Karl-Fischer analysis was performed to determinethe water content of the as-prepared electrolyte using a MettlerToledo titrator (Model DL31). The water content was found to beabout 1.8 vol.% for the as-prepared electrolyte and slightly higherin aged electrolyte, up to 2.2 vol.%.

All experiments were carried out by using a three-electrode cellconfiguration with a porous glass diaphragm separating the anodicand cathodic compartments. Cu sheets (purity 99.95%, thickness700 �m) were used as substrate, with an exposed area of 1 cm2.The counter electrode was a dimensionally stable anode (DSA). AnAg/AgCl electrode was employed as reference. The electrolyte wasdeaerated with N2 before deposition.

Linear sweep voltammetry (LSV) measurements were per-formed in the range from OCP (Open Circuit Potential, about −0.8 V)and −1.6 V with a sweep rate of 5 mV/s in electrolytes with a ten-fold dilution of the metal components, maintaining unchanged theconcentration of oxalic acid, at 50 ◦C.

The electrodeposition was performed by imposing a constantpotential in the range from −1.1 to −1.5 V vs. Ag/AgCl, and sup-plying a total charge of 3 C, i.e. to a final nominal thickness ofabout 1 �m. During the deposition, moderate stirring was pro-vided by using a magnetic stirrer. The deposition temperature wasmaintained at 50 ◦C. All the experiments were repeated at leastfive times. The partial c.d. jM for the deposition of Fe and Ga wascalculated by the Faraday’s law, using the total mass depositeddetermined from the sample weight gain and the measured com-position of the film. The current efficiency � was calculated as theratio of the charge used for alloy deposition, as already noted above,and the total charge supplied to the cell (3 C).

The thickness and Ga content of deposited Fe–Ga films weremeasured by X-ray fluorescence using a Fischerscope-XAN®-FD BC instrument. The mean value and the standard deviationwere obtained by performing measurements on different samplesdeposited under the same conditions. Phase structure and textureof the deposits were determined by acquiring X-ray diffraction(XRD) patterns with Cu K� radiation (� = 1.5405 A) and a powdergoniometer (Philips PW-1830). The surface morphology of Fe–Gadeposits was investigated by Scanning Electron Microscopy (SEM)(Zeiss EVO-50 microscope) and by Atomic Force Microscopy (AFM)in contact mode by sampling an area of the desired size with anNT-MDT Solver SPM instrument (Solver PRO-M). Glow dischargeoptical emission spectrometry (GD-OES) depth profiling analysiswas performed with a Spectruma GDA750 analyser using argonions for sputtering with a beam spot size of 2.5 mm. A vibratingsample magnetometer (VSM) was employed to measure hysteresisloops by applying longitudinal and transversal magnetic field onthe Fe–Ga film plane.

3. Results and discussion

3.1. Linear sweep voltammetry

thin films from eutectic-based ionic liquid, Electrochim. Acta (2013),

The cathodic behaviour of the ionic liquid baths was investi-gated by LSV with the objective to define the potential range foralloy deposition and to shed some light on the effect of oxalic acidaddition. The latter were investigated at concentration levels as in

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FEG

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ig. 1. LSV curves at Cu electrode for different electrolytes at 50 ◦C: (a) Ethaline; (b)thaline, 10 mM Ga3+; (c) Ethaline, 30 mM Fe2+; (d) Ethaline, 30 mM Fe2+ and 10 mMa3+.

he deposition electrolyte, i.e. 4 and 17 mM, in the assumption that

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ts action was of a surface character, and at reduced concentrationf the metal ionic species, in the aim to get a relative enhancementf the effect of the additive on the polarization behaviour. Fig. 1hows LSV curves over the potential range from −0.8 to −1.6 V

ig. 2. LSV curves at Cu electrode for various electrolytes. (I-a) Ethaline; (I-b) Ethaline, 4

thaline, 30 mM Fe2+ and 4 mM OA; (II-d) Ethaline, 30 mM Fe2+ and 17 mM OA. (III-a) Ethathaline, 10 mM Ga3+ and 17 mM OA. (IV-a) Ethaline; (IV-b) Ethaline, 30 mM Fe2+ and 100 mM Fe2+, 10 mM Ga3+ and 17 mM OA.

PRESScta xxx (2013) xxx– xxx 3

corresponding to the base electrolyte Ethaline (a), Ethaline withaddition of 10 mM GaCl3 (b), Ethaline with addition of 30 mM FeCl2(c), and in the presence of both 10 mM GaCl3 and 30 mM FeCl2(d). In the case of the base electrolyte, a low current density (c.d.)is observed over the potential range from OCP to −1.4 V, slightlyincreasing towards the lower limit of the scanning range, probablydue to the electrochemical decomposition of the water containedin the electrolyte – which is approximately 1 M – or the reductionof ionic liquid components. By adding 10 mM GaCl3 to pure Etha-line, a broad cathodic peak associated to Ga3+ reduction appearsat about −1.32 V, with a peak c.d. of about 0.36 mA/cm2. At highercathodic potential, the c.d. approaches the value observed for thebase electrolyte. By adding 30 mM FeCl2 to pure Ethaline, curve (c)in Fig. 1, the Fe2+ reduction occurs at potential negative to −1.10 V,as revealed by a slowly rising c.d., peaking at about 2.8 mA/cm2 at−1.23 V. Further increasing the cathodic potential, a steep increaseof c.d. is revealed with visible gas formation, suggesting enhanceddecomposition of water at the freshly deposited Fe, overlappingwith metal ion discharge. By adding 10 mM GaCl3 to this electrolyte,curve (d) in Fig. 1, a cathodic peak is seen at potential of about−1.36 V, and c.d. of 3.7 mA/cm2, indicating that the potential foralloy deposition is shifted towards a more negative value compared

thin films from eutectic-based ionic liquid, Electrochim. Acta (2013),

to the reduction potential of either cations from the respective elec-trolyte. Notably, the peak is preceded by a slowly rising c.d. in thepotential range where – according to curve (c) – Fe2+ ions can bereduced.

mM OA; (I-c) Ethaline, 17 mM OA. (II-a) Ethaline; (II-b) Ethaline, 30 mM Fe2+; (II-c)line; (III-b) Ethaline, 10 mM Ga3+; (III-c) Ethaline, 10 mM Ga3+ and 4 mM OA; (III-d)

mM Ga3+; (IV-c) Ethaline, 30 mM Fe2+, 10 mM Ga3+ and 4 mM OA; (IV-d) Ethaline,

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Fig. 3. Ga content in Fe–Ga films deposited from electrolytes without OA (�), with4g

arOa−wtamaaebrftUawlotupcp

oaa(affiftwtfat

efficiency for Fe–Ga electrodeposition are reported in Fig. 4a–c,

mM OA (©) and 17 mM OA (�), as a function of the deposition potential (lightrey-masked points indicate powdery deposits).

In Fig. 2 the cathodic LSV curves for different electrolytes in thebsence and in the presence of OA are shown. LSV curves in Fig. 2Iefer to the base electrolyte Ethaline (a), and to Ethaline containingA 4 mM (b) and 17 mM (c). The trend of the LSV curve changes byppearance of a seeming peak or a small wave at potential about1.0 V, at the higher and lower OA concentration respectively,hich may be ascribed to the reduction of protons coming from

he dissociation of OA. Moreover, a distinct c.d. wave is revealedt potential of −1.40 V, increasing with the concentration, whichust be attributed to the reduction of OA, possibly to glyoxylic

cid [35]. In particular, the latter feature of the LSV curves can belso taken as an indication of the adsorption of oxalic acid at thelectrode. LSV cathodic curves (a) and (b) in Fig. 2II refer to thease electrolyte Ethaline and to the same containing 30 mM FeCl2,espectively, both already shown in Fig. 1 and reported here againor comparison purposes; curves (c) and (d) refer to the Fe2+ con-aining electrolyte in the presence of OA 4 and 17 mM, respectively.pon addition of OA, a c.d. wave appears at potential slightly neg-tive to −1.0 V, with c.d. increasing with the concentration. In lineith the above interpretation, this wave is probably due to over-

apping of the reduction of protons released by the dissociationf OA and the slowly rising c.d. for Fe2+ discharge. Interestingly,he peak potential for Fe2+ discharge remains virtually unchangedpon addition of OA 4 mM, showing instead a slight shift to higherotential at OA 17 mM. Also notably, the peak c.d. change closelyorresponds to that of the preceding c.d. wave, and therefore itrobably only reflects the superposition of the relevant processes.

LSV curves presented in Fig. 2III show the effect of OA additionn the reduction of Ga3+. Following the same presentation schemes in Fig. 2II, curves (a) and (b) refer to the base electrolyte Ethalinend to the same in the presence of 10 mM GaCl3; curves (c) andd) refer to the Ga3+ containing electrolyte in the presence of OA 4nd 17 mM, respectively. With addition of OA 4 mM, the peak c.d.or the Ga3+ reduction, still observed at about −1.32 V, increasesrom 0.4 to 0.55 mA/cm2, revealing a promotion of Ga3+ discharge,n agreement with the result of increased Ga content in alloy filmsrom the electrolyte containing OA 4 mM (see further in Fig. 3). Fur-her significant changes are seen in the LSV curves, in agreementith the OA effects already noted in commenting on Fig. 2I: namely,

he c.d. wave at about −1.0 V for the reduction of protons coming

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rom OA dissociation and the c.d. wave for the reduction of oxaliccid. The latter appears to strongly overlap with the Ga3+ reduc-ion, in particular at OA concentration of 17 mM, resulting in fact in

PRESScta xxx (2013) xxx– xxx

a slight shift of the relevant peak to higher cathodic potential and,more notably, in a broad shoulder on the higher potential side of thepeak. The corresponding peak, and certainly the c.d. increase, can-not therefore be attributed solely to Ga3+ discharge, but also to theoverlapping processes, namely proton and oxalic acid reduction.

Finally, the LSV curves presented in Fig. 2IV refer to Ethaline con-taining both 30 mM FeCl2 and 10 mM GaCl3, in the absence (curve(b)) and in the presence of OA 4 (curve (c)) and 17 mM (curve (d)).Apparently, a similar effect as noted in the previous case for theelectrolyte containing only Fe2+ or Ga3+ is revealed by LSV curves(c) and (d), i.e. the increase of the peak c.d. upon the addition of OA;moreover, a slight cathodic shift of the peak, from −1.35 to −1.37 V,is seen for the highest OA concentration. A further effect worth not-ing is the steeper c.d. rise preceding the c.d. peak in the presence ofOA, possibly associated to more defined and intense nucleation ofthe alloy phase.

According to the above results, the discharge of the Fe2+ speciesis only slightly affected by OA addition, while a stronger activatingeffect is observed on that of Ga3+ species, namely at OA concentra-tion of 4 mM. The nature of this effect is yet to be clarified, thoughwe can reasonably suggest that it is related to surface activation,possibly due to the adsorption of both hydrogen and oxalate ions atthe electrode, which translates into a relative lowering of chloridesactivity at the surface and in the implication of oxalic acid in thedischarge mechanism of Ga3+ species, as discussed further on.

On the other hand, a reduction of the current efficiency can beanticipated with OA at the concentration of 17 mM, as a result ofboth the reduction of OA (Fig. 2I) and the enhancement of cathodicparasitic reactions (Fig. 2II). The modification of the LSV curvescaused by OA addition to the electrolyte containing both metal ionicspecies should be considered in the light of the comments above,thus suggesting that oxalic acid behaves as a surface active species,with effects depending however on its concentration. The concen-tration dependence of the OA influence may be an indirect effectrelated to its dissociation, i.e. to the effective increase of acidityleading to the activation of parasitic reactions and the consequentdepression of the rate of discharge of metal ionic species.

3.2. Potentiostatic electrodeposition of Fe–Ga on Cu

The electrodeposition of Fe–Ga thin films was realized underpotentiostatic conditions from an Ethaline 0.1 M GaCl3 and 0.3 MFeCl2 electrolyte, on Cu substrate, at potential in the range from−1.1 to −1.5 V, and at 50 ◦C. In Fig. 3 the Ga content in the alloythin films is reported as a function of the deposition potential. Inthe absence of OA, Ga content increases with increasing cathodicpotential. However, the deposits become dull and powdery at depo-sition potential of −1.35 V (grey solid point in Fig. 3) or higher, dueto precipitation of hydroxides, apparently triggered by change ofthe chemical environment at the interface caused by hydrogen evo-lution and possibly other side reactions. Upon addition of OA 4 mM,alloy films of higher Ga content can be obtained, notably at cathodicpotential in excess of −1.25 V, and, more significantly, the depositsbecome dull only for deposition potential of −1.45 V (light grey hol-low point in Fig. 3) or higher. On the contrary, with 17 mM oxalicacid, the Ga content is limited to less than 5%, regardless of thedeposition potential. Interestingly, Fe–Ga thin films deposited withaddition of 17 mM OA show a bright appearance over the wholerange of deposition potential, suggesting that the precipitation ofhydroxides is effectively prevented by the increased acidity or thehigher local buffering capability of the electrolyte.

The partial c.d. for Fe2+ and Ga3+ discharge and the current

thin films from eutectic-based ionic liquid, Electrochim. Acta (2013),

respectively, in the absence and in the presence of OA 4 and 17 mM.As shown in Fig. 4a, in the absence of OA (solid squares in Fig. 4a), thepartial c.d. for Fe2+ discharge increases with the increasing cathodic

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F (b) Ga(

pehtc−ldats

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ig. 4. The effect of oxalic acid concentration on the (a) Fe partial current density,�), with OA 4 mM (©) and OA 17 mM (�)).

otential in the range −1.1 to −1.3 V. At deposition potential inxcess of the latter value, due to precipitation of metal oxides orydroxides, the partial c.d. could not be determined. Upon addi-ion of 4 mM OA (hollow circles in Fig. 4a), jFe shows only minorhanges and the same trend as in the absence of OA, in the range1.1 to −1.3 V; on the contrary, in the presence of OA 17 mM (hol-

ow circles in Fig. 4a), a definite decrease of jFe is seen, probably

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ue to the reduction of protons released by the dissociation of OA,s already noted above. For potential more negative than −1.3 V, inhe presence of OA, the rate of change of jFe is apparently reduced,uggesting that the Fe2+ discharge rate is approaching the mass

able 1ormality inertia parameter of metals relevant to the Fe–Ga deposition.

Reaction �H◦ion

zF (V)�H◦

hydrzF (V)

�H◦ion+�H◦

hydrzF (V) �

�H◦io

H2 = 2H+ + 2e− 13.662 −11.307 2.355 5.802

Inert metalsCr = Cr3+ + 3e− 18.137 −15.754 2.384 7.609Ga = Ga3+ + 3e− 19.132 −16.237 2.895 6.609Cr = Cr2+ + 2e− 11.696 −9.888 1.809 6.467Fe = Fe2+ + 2e− 12.105 −10.126 1.979 6.117

Intermediate metalsCu = Cu+ + e− 7.794 −5.936 1.858 4.194Cu = Cu2+ + 2e− 14.075 −10.877 3.197 4.402

Normal metalsGa = Ga2+ + 2e− 13.271 −9.628 3.643 3.643

a Electrochemical electronegativity RNI, defined as RMNI = [�H◦

ion/(�H◦ion + �H◦

hydr )]M

b Normality inertia parameter PNI, defined as PNI = RMNI/RH

NI.

partial current density (c) current efficiency for Fe–Ga codeposition (without OA

transport limiting value. In this potential range, an enhancementof the Fe partial current is revealed at OA concentration of 17 mM,in connection with the attendant reduction of Ga3+ discharge, asshown by the results for the partial c.d. jGa in Fig. 4b. In fact, whilein the presence of OA 4 mM (hollow circles in Fig. 4b), a strongenhancement of the Ga3+ discharge rate is seen (notably, in thepotential range −1.3 to −1.4 V), increasing the concentration of OA

thin films from eutectic-based ionic liquid, Electrochim. Acta (2013),

to 17 mM (solid circles in Fig. 4b), jGa drastically decreases overthe whole deposition potential range. Current efficiency data forFe–Ga deposition (Fig. 4c) show a slight increase in the presence ofOA 4 mM; on the other hand, in the presence of oxalic acid 17 mM,

H◦ion

n+�H◦hydr

�G◦M

zF (V) RNIa PNI

b Log (j◦H/A cm−2)

4.6142 26.771 1.000

3.874 29.477 1.101 4.085 26.997 1.008

3.714 24.020 0.897 −6.4 4.174 25.531 0.954 −5.6

5.134 21.533 0.804 −7.8 4.954 21.808 0.815

4.164 15.170 0.567 −10.4

· �G◦M/zF .

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F 35 V vo

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ig. 5. Surface morphology and EDS spectrum of Fe–Ga thin films deposited at −1.f oxalic acid 4 mM (b) and 17 mM (c).

strong reduction of the current efficiency is seen, particularly forathodic potential in excess of −1.3 V, in correspondence with theotential region of limiting rate of deposition.

These results are in line with the above analysis on the effectsf oxalic acid addition, highlighting on the one hand the activa-ion effect of OA on the reduction of Ga3+ at the concentration of

mM, and on the other hand the strong enhancement of competi-ive cathodic processes at the concentration of 17 mM, resulting inhe apparent inhibition of metal deposition.

In Fig. 5a–c the surface morphology and the corresponding EDSpectrums of the Fe–Ga thin films deposited at −1.35 V from thelectrolytes without OA and containing OA 4 and 17 mM, respec-ively, are reported. The cracked mud morphology and the EDS

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pectrum of Fe–Ga alloy deposited from the electrolyte without OAlearly indicate a high oxygen content in the film. On the other hand,he Fe–Ga film deposited from the electrolyte with OA 4 mM has aelatively smooth and homogenous surface morphology, and much

s. Ag/AgCl from the electrolyte in the absence of oxalic acid (a) and in the presence

lower oxygen content, as shown by the EDS spectrum. Fe–Ga thinfilms deposited from the electrolyte containing OA 17 mM showlarge spherical particles and sub-micro sized pinholes at the sur-face, the latter probably associated to hydrogen evolution, and nooxygen content according to EDS analysis.

3.3. Electrokinetics of Fe–Ga electrodeposition

Overall, it can be noticed that, at the concentration of 4 mM,oxalic acid has a definite activating effect on the reduction of gal-lium ionic species. The role of oxalic acid on Ga3+ discharge wasnot studied in detail in the present work, however we believe thatthe results herein presented provide a reasonable framework for a

thin films from eutectic-based ionic liquid, Electrochim. Acta (2013),

preliminary scrutiny of the discharge mechanism.The very first remark, in this respect, is that a multi-steps reac-

tion mechanism can be assumed for the cathodic reduction of Ga3+,by comparison with the reduction of Cr3+ complex species in the

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Table 2Crystallite size of Fe–Ga thin films of variable composition with the A2 phase vs.deposition potential.

Depositionpotential (V)

Thickness (�m) Ga content(at.%)

Crystallite size� (nm)

−1.2 0.8 5.7 ± 0.5 40

puGodotAiotitdbeits

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Fig. 6. (a) XRD patterns of samples deposited from the electrolyte containing OA4 mM with different applied potential: (1) −1.2 V, 6 at.% Ga; (2) −1.3 V, 10 at.% Ga;

−1.3 1.0 9.6 ± 0.5 23−1.4 1.2 17 ± 0.5 20

resence of OA in aqueous environment [36]. Moreover, it is spec-lated that the role of oxalic acid in the discharge mechanisms ofa3+ may be like that assumed for Cr3+ in [36], i.e. the formation ofxalate complexes of Ga2+ as relatively stable adsorbed interme-iates. A rational basis in support of this interpretation is brieflyutlined and discussed in the following, by making reference tohe normality-inertia parameter PNI [37–40] of the ions involved.lthough the PNI approach obviously underlines the role of water

n electrode kinetics, we believe that it can be also significant forur current study, in consideration of the relatively high water con-ent in the ionic liquid electrolyte. In fact, the implication of watern the discharge reactions cannot be overlooked, being experimen-ally proved by the observation of high oxygen content in Fe–Gaeposits obtained at high cathodic potential and further suggestedy the current efficiency results. Accordingly, it is also a reasonablexpectation that the discharge behaviour of the metal ionic species,.e. their reaction pathway at the interface, could be affected byhe presence of water at the surface, especially in the case of Ga3+

pecies, in view of the high hydration enthalpy of the ions.Piontelli [41,42] introduced a classification of metals according

o their electrokinetic behaviour as normal, intermediate and inert.e showed that inert metals have very low metal ion exchange cur-

ent densities, whilst normal metals have an electrodic reversibleehaviour. He also underlined the general anti-correlation betweenetal ion and hydrogen exchange at metal electrodes. Our group

ntroduced a parameter, the normality-inertia parameter PNI,hich can be used as a classification criterion according to Pio-telli’s view.

The electrochemical electronegativity (RMNI) of the metals is

efined as [39,40]:

MNI =

[�H◦

ion

�H◦ion + �H◦

hydr

]M

· �G◦M (1)

here �H◦ion and �H◦

hydr are the ionization and hydrationnthalpies for the metal ions in solution, �G◦

M is the metaltandard free energy for the metal ion reaction in solution, cal-ulated with reference to the hydrogen value assumed to beG◦

H/zF = 4.614 V [39].PNI is obtained from PNI = RM

NI/RHNI and is directly related to the

tandard exchange current density for the metals in aqueous solu-ion, reversely proportional to their hydrogen discharge currentensity.

Table 1 reports the values of PNI for the iron and gallium ions,ogether with those for Cu (as the substrate used in the presentork) and Cr, for comparison. Gallium deposition from Ga3+ solu-

ion in the presence of water is known to be very difficult, becausef the strong interaction of Ga3+ ions with water molecules. In thisespect, as shown by the PNI value, the Ga3+ ions behaves as annert metal, implying a high overvoltage for the ionic exchangeeaction and consequently strong overlapping with the hydrogenvolution reaction, resulting in the concurrent hydrolysis of theons and hydroxides precipitation. Similar conditions, in our view,

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ay be established at the electrode, in the present case, either inhe absence of oxalic acid or in the presence of oxalic acid 17 mM;n the latter case, as a result of the increased acidity and therefore

ith suppression of hydroxide formation.

(3) −1.4 V vs. Ag/AgCl, 17 at.% Ga and (b) lattice parameters vs. Ga content in Fe–Gathin films in this work (4 mM OA) (�); Dunlap et al. [43] 800-nm-thick films (�);Luo et al. [44] bulk Fe–Ga alloy (©).

At the variance with the behaviour of Ga3+, the Ga2+ oxidationstate exhibits a significantly lower kinetic inertness, and an PNIvalue lower than that of an intermediate metal, as copper. There-fore, in order to deposit Ga in the presence of water, conditions mustbe obtained where the intermediate Ga2+ is stabilized by adsorptionat the metal surface. This behaviour is typical of some three-valentions discharge, such as Cr3+, as noted above, where also the stabi-lization of the two-valent ion Cr2+ determines its discharge [36].The determinant role of the low valence intermediate is partic-ularly highlighted when considering the kinetic character of thehydrogen discharge reaction. In fact, hydrogen evolution occurson gallium with strong irreversibility, as shown by the hydrogenexchange current density reported in the last column in Table 1.In view of the anti-correlation between the degree of kinetic inert-ness for the ionic exchange reaction – in other word the PNI value –and j◦H [39,40], it appears that the ionization of the metal must beconsidered with respect to its lowest possible valence: the very lowhydrogen exchange current density on gallium, even lower than oncopper, is in fact typical of a normal metal. Moreover, and at vari-ance with gallium, iron – which is an inert metal according to thePNI value – has a high hydrogen exchange current density.

thin films from eutectic-based ionic liquid, Electrochim. Acta (2013),

Accordingly, the observed increase of current efficiency (Fig. 4c)and Ga content in the alloy films (Fig. 3) in the presence of OA 4 mMcan be taken as indicative of its role in the stabilization of Ga2+ sur-face species and the resulting enhancement of gallium deposition.

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ig. 7. AFM and SEM images of the surface of Fe–Ga films deposited from the electro0 at.% Ga; (c) −1.4 V vs. Ag/AgCl, 17 at.% Ga.

n the other hand, at higher concentration of OA, the increasedcidity of the environment is believed to be responsible for thebrupt changes observed, in particular the strong decrease in the Gaontent of the alloy film and, as a more general effect, of the currentfficiency. This interpretation, it is worth repeating, is based on thessumption of an active role of water at the interface, which is sug-ested by its relatively high content in the ionic liquid electrolytend supported by the experimental evidence.

.4. Characterization of deposited thin films

Fig. 6a shows the XRD patterns of Fe–Ga films deposited at1.2 V, −1.3 V and −1.4 V from the electrolyte containing 4 mM OA.he Ga content in the deposits is about 6, 10 and 17 at.%, respec-ively. As revealed from XRD patterns, in the composition range

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rom 3 to 17 at.% Ga, Fe–Ga thin films show a disordered bcc �-FeGatructure (A2 phase), with a strong (1 1 0) preferred orientation. Nouperlattice reflections identified by the splitting of the character-stic peaks was observed, indicating that the ordering to give the

ontaining OA 4 mM with different applied potential: (a) −1.2 V, 6 at.% Ga; (b) −1.3 V,

D03 phase in the A2 matrix has not occurred. Theoretically, as theatomic radius of the Ga atom (0.181 nm) is slightly greater than thatof Fe (0.172 nm), when Ga is randomly distributed into the Fe latticea linear increase in the lattice parameter is expected, according tothe Vegard’s law. As shown in Fig. 6a, with an increase of Ga content,the (1 1 0) peak slightly shifted to lower 2� value. The lattice param-eter of the alloy films was calculated from the diffraction angle ofthe (1 1 0) peak and is reported in Fig. 6b together with values takenfrom the literature [43,44]. A comparison of the data shows that themeasured values follow the same trend of those reported in the lit-erature for 800 nm thick films and for polycrystalline bulk Fe–Gaalloy, in agreement with the Vegard’s law.

Furthermore, the crystallite size � was estimated by the Scher-rer’s equation [45]:

K�

thin films from eutectic-based ionic liquid, Electrochim. Acta (2013),

� =ˇ2 cos �

(2)

where K is the shape factor (taken as 0.94 for cubic crystals), �is the X-ray wavelength (1.54 for Cu K� radiation), is the line

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t (a) −

baep

ststtbs

F−

Fig. 8. SEM micrographs of the cross-section of Fe–Ga films deposited a

roadening (full width at half maximum, FWHM), and � is the Braggngle. As reported in Table 2, the average crystallite size of thelectrodeposited Fe–Ga alloys decreases with increasing depositionotential, i.e. with increasing Ga content.

In Fig. 7 the AFM images and SEM micrographs taken from theurface of the same Fe–Ga thin films are reported. The AFM image ofhe Fe–Ga thin film deposited at −1.2 V shows that the average grainize is in the range of 40–60 nm, comparable with the average crys-allite size estimated from the Scherrer’s equation. The grain size of

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he Fe–Ga thin films deposited at more negative potential could note estimated from their corresponding AFM images because of thetrong grain aggregation. Besides, as shown by SEM micrographs of

ig. 9. GD-OES depth profile analysis of Fe83Ga17 film (∼1.2 �m thick) deposited at1.4 V vs. Ag/AgCl.

1.2 V, 6 at.% Ga; (b) −1.3 V, 10 at.% Ga; (c) −1.4 V vs. Ag/AgCl, 17 at.% Ga.

films, a drastic change of the surface morphology from pyramidalto granular with increasing deposition potential can be observed.

Fig. 8 shows SEM micrographs of the cross-section of Fe–Gathin films deposited at −1.2, −1.3 and −1.4 V. The SEM micrographof the cross-section reveals that the Fe–Ga films have a columnarstructure.

GD-OES depth profiling analysis of the Fe83Ga17 film (about1.2 �m thick) was carried out in order to investigate the oxygencontent in Fe–Ga thin films. As seen in the GD-OES profile in Fig. 9,

thin films from eutectic-based ionic liquid, Electrochim. Acta (2013),

a very low oxygen content was measured, confirming the goodquality of the film.

Fig. 10. Hysteresis loops of as-deposited Fe83Ga17 films measured with applied filedparallel and perpendicular to the film plane.

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ARTICLEA-20965; No. of Pages 11

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Fig. 10 shows the hysteresis loops of the as-deposited Fe83Ga17lm measured with applying the external magnetic field parallelnd perpendicular to the film plane. Several features of the mea-ured loop indicate the good quality of the film: (a) the saturationagnetization of the as-deposited Fe83Ga17 film reaches 1.75 T,hich can be also taken as a further evidence of the low oxygen

ontent; (b) a very small coercivity is observed corresponding to good soft magnetic behaviour; (c) the parallel loop shows highemanence, while the perpendicular loop is tilted due to the demag-etizing field, as expected considering the strong anisotropy of thelm.

. Conclusions

In the present paper, the electrodeposition of Fe–Ga thin filmsrom a eutectic based ionic liquid consisting in a mixture of cholinehloride and ethylene–glycol (1:2) with addition of oxalic acid (OA)as addressed.

Linear sweep voltammetry (LSV) demonstrated that the ioniciquid had a sufficiently wide potential window to allow thelectrodeposition of Fe–Ga alloys under ambient conditions,otwithstanding a relative high water content in the electrolyte.

n particular, the addition of a small concentration of oxalic acid4 mM) promoted the discharge of Ga3+ species, and improved theuality of deposits. On the other hand, the addition of a higher con-entration of oxalic acid (17 mM) caused a strong reduction bothn the Ga content in the deposits and in the current efficiency, as aesult of the increased acidity.

In the assumption of an active role of water at the interface, theinetics of Fe–Ga codeposition was discussed in the frame of theormality inertia parameters (PNI) approach, concluding that theeposition of Ga may occur through the formation of Ga2+ inter-ediate species, stabilized at the surface by oxalic acid.The electrodeposited Fe–Ga thin films with Ga content up to

7 at.% showed a disordered bcc � structure (A2 phase) with a strong1 1 0) preferred orientation. No D03 phase was observed. Magneticysteresis cycles of Fe–Ga films containing 17 at.% Ga showed aaturation magnetization of about 1.7 T, confirming a good qualityf thin films.

Overall, the electrodeposition of Fe–Ga from the cholinehloride–ethylene glycol based ionic liquid, even in the presencef a relatively high content of water, and with addition of a smalloncentration of oxalic acid, was shown to be a very promisingrocess for the deposition of magnetostrictive Fe–Ga thin films.

cknowledgement

The authors would like to express their gratitude to Prof. Andreaele (CMIC, Politecnico di Milano) for the assistance and valuable

dvice provided during the reviewing process.

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