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Electrodeposition of zinc–tin alloys from deep eutectic solvents based on choline chloride Andrew P. Abbott * , Glen Capper, Katy J. McKenzie, Karl S. Ryder Chemistry Department, University of Leicester, Leicester LE1 7RH, UK Received 18 November 2005; received in revised form 24 March 2006; accepted 3 April 2006 Available online 21 June 2006 Dedicated to David Schiffrin Abstract Here we describe the electrolytic deposition of Zn, Sn and Zn/Sn alloys from a solution of the metal chloride salts separately in urea and ethylene glycol/choline chloride based ionic liquids. We show that the deposition kinetics and thermodynamics differ from the aque- ous processes and that qualitatively different phases, compositions and morphologies are obtained for the metal coatings in the different ionic liquid systems. We have quantified the electrochemical stripping responses using cyclic voltammetry together with compositional analysis using SEM/EDAX and X-ray diffraction. The differences in electrochemical responses are rationalised in terms of the speciation of both Zn and Sn chlorides in the ionic liquids that have been identified using FAB mass spectrometry. Also we demonstrate that com- posite metal coatings, e.g. containing Al 2 O 3 particles, can be obtained from these liquid systems by virtue of the stable liquid suspensions. This novel feature of these liquids is a function of their relatively high viscosity. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Electrodeposition; Zinc; Alloy; Ionic liquid; Eutectic 1. Introduction Ionic liquids are classed as salts that are liquid at below 100 °C [1]. They have been extensively studied primarily for synthetic and electrochemical applications [2]. The major- ity of investigations have concentrated on imidazolium cations with discrete anions such as BF 4 ; PF 6 and (F 3 CSO 2 ) 2 N . These ionic liquids have large potential win- dows and exhibit relatively high conductivities and low vis- cosities. Two recent reviews by Endres [3,4] cover all aspects of electrodeposition from these liquids, Compton and coworkers [5] have reviewed fundamental aspects of electrochemistry in ionic liquids and a book by Ohno [6] describes other electrochemical aspects. While ionic liquids with discrete anions show significant potential for the elec- trodeposition of electronegative metals such as aluminium [3], issues such as toxicity and availability will limit their practical use for larger scale applications of other metals. An alternative approach to making ionic liquids is to start with a simple quaternary ammonium halide and decrease the freezing point by complexing the anion to effectively delocalise the charge. These eutectic-based ionic liquids can be described by the general formula: R 1 R 2 R 3 R 4 N þ X: zY and we have characterised these materials into three types depending on the complexing agent Y; Type 1 Y = MCl x , M = Zn, Sn, Fe, Al, Ge Type 2 Y = MCl x yH 2 O Type 3 Y = RZ, Z = CONH 2 , COOH, OH The electrochemistry, physical properties and speciation occurring in Types 1 and 2 ionic liquids has been reported for a variety of metals [7–10]. Considerably fewer studies 0022-0728/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2006.04.024 * Corresponding author. Fax: +44 116 252 3789. E-mail address: [email protected] (A.P. Abbott). www.elsevier.com/locate/jelechem Journal of Electroanalytical Chemistry 599 (2007) 288–294 Journal of Electroanalytical Chemistry

Electrodeposition of Zinc–Tin Alloys From Deep Eutectic Solvents Based on Choline Chloride

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    Abstract

    1. Introduction

    describes other electrochemical aspects. While ionic liquidswith discrete anions show signicant potential for the elec-trodeposition of electronegative metals such as aluminium

    [3], issues such as toxicity and availability will limit theirpractical use for larger scale applications of other metals.

    Type 2 Y = MClx yH2OType 3 Y = RZ, Z = CONH2, COOH, OH

    The electrochemistry, physical properties and speciationoccurring in Types 1 and 2 ionic liquids has been reportedfor a variety of metals [710]. Considerably fewer studies

    * Corresponding author. Fax: +44 116 252 3789.E-mail address: [email protected] (A.P. Abbott).

    Journal of Electroanalytical Chemis

    Journal ofIonic liquids are classed as salts that are liquid at below100 C [1]. They have been extensively studied primarily forsynthetic and electrochemical applications [2]. The major-ity of investigations have concentrated on imidazoliumcations with discrete anions such as BF4 ; PF

    6 and

    (F3CSO2)2N. These ionic liquids have large potential win-

    dows and exhibit relatively high conductivities and low vis-cosities. Two recent reviews by Endres [3,4] cover allaspects of electrodeposition from these liquids, Comptonand coworkers [5] have reviewed fundamental aspects ofelectrochemistry in ionic liquids and a book by Ohno [6]

    An alternative approach to making ionic liquids is to startwith a simple quaternary ammonium halide and decreasethe freezing point by complexing the anion to eectivelydelocalise the charge. These eutectic-based ionic liquidscan be described by the general formula:

    R1R2R3R4NX: z Y

    and we have characterised these materials into three typesdepending on the complexing agent Y;

    Type 1 Y = MClx, M = Zn, Sn, Fe, Al, GeHere we describe the electrolytic deposition of Zn, Sn and Zn/Sn alloys from a solution of the metal chloride salts separately in ureaand ethylene glycol/choline chloride based ionic liquids. We show that the deposition kinetics and thermodynamics dier from the aque-ous processes and that qualitatively dierent phases, compositions and morphologies are obtained for the metal coatings in the dierentionic liquid systems. We have quantied the electrochemical stripping responses using cyclic voltammetry together with compositionalanalysis using SEM/EDAX and X-ray diraction. The dierences in electrochemical responses are rationalised in terms of the speciationof both Zn and Sn chlorides in the ionic liquids that have been identied using FAB mass spectrometry. Also we demonstrate that com-posite metal coatings, e.g. containing Al2O3 particles, can be obtained from these liquid systems by virtue of the stable liquid suspensions.This novel feature of these liquids is a function of their relatively high viscosity. 2006 Elsevier B.V. All rights reserved.

    Keywords: Electrodeposition; Zinc; Alloy; Ionic liquid; EutecticElectrodeposition of zinctin albased on cho

    Andrew P. Abbott *, Glen Capper

    Chemistry Department, University

    Received 18 November 2005; received in reviAvailable onl

    Dedicated to0022-0728/$ - see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.jelechem.2006.04.024ys from deep eutectic solventsine chloride

    aty J. McKenzie, Karl S. Ryder

    eicester, Leicester LE1 7RH, UK

    form 24 March 2006; accepted 3 April 200621 June 2006

    vid Schirin

    www.elsevier.com/locate/jelechem

    try 599 (2007) 288294

    ElectroanalyticalChemistry

  • analhave been carried out using Type 3 based liquids, which usehydrogen bond donors as the complexing agents [11,12].The use of simple amides, acids and alcohols as complexingagents makes the liquids very versatile and to distinguishthem from other ionic liquids the term deep eutectic sol-vents (DES) has been coined. DESs have been used forelectropolishing [13], polymer synthesis [14] and metaloxide processing [15]. Most of our previous studies haveconcentrated on choline chloride as the quaternary ammo-nium salt as it is non-toxic, biodegradable and is alreadyused as a common component to numerous householdand industrial products. Hence it can be applied economi-cally to large-scale processes. In the current work we showhow DESs formed with choline chloride and either urea orethylene glycol can be used for the electrodeposition ofzinc, tin, and zinctin alloys. We also show that the choiceof hydrogen bond donor aects the type of alloy and theelectrochemistry of the components in solution.

    Zinctin alloys have been deposited from a variety ofaqueous baths including sulphate [16], gluconate [17] andpyrophosphate baths [18]. The alloys are found to havebetter corrosion resistance than pure zinc, particularly inhigh humidity conditions and are also reported to be supe-rior to cadmium deposits in marine environments.

    2. Experimental

    Choline chloride HOC2H4NCH33 Cl (ChCl)(Aldrich 99%) was recrystallised from absolute ethanol, l-tered and dried under vacuum. Urea (Aldrich > 99%) wasdried under vacuum prior to use. Ethylene glycol (EG)(Aldrich 99+%), tin chloride (Aldrich) and zinc chloride(BDH) were used as received. The eutectic mixtures wereformed by stirring the two components together, in the sta-ted proportions, at 100 C until a homogeneous, colourlessliquid formed. Voltammetry was carried out using anAutolab PGSTAT12 potentiostat controlled with GPESsoftware. A three-electrode system consisting of a platinummicroelectrode (0.5 mm diameter) (made in-house) a plati-num counter electrode and a silver wire reference electrodewere used. The working electrode was polished with 0.3 lmalumina paste, rinsed and dried prior to all measurements.All voltammograms were performed at 40 C and a scanrate of 20 mV s1. Stainless steel Hull cell panels were pre-pared by washing with deionised water and dried, thendegreased by placing in dichloromethane (DCM) for2 min, then removed and allowed to dry. The panel wasplaced in the Hull Cell with a nickel cathode (also rinsedwith water and DCM prior to use). The cell was lled with1ChCl:2urea ionic liquid containing 0.5 M Zn:0.05 M Snand 3 wt% Al2O3. The panel was plated at 10 V for30 min, rinsed with deionised water and allowed to dry.The current density at dierent positions across the Hullcell was calibrated using a Ni strip 5 mm wide. Surfaceanalysis was carried out using scanning electron micros-

    A.P. Abbott et al. / Journal of Electrocopy (Philips XL30 ESEM) and energy dispersive analysisby X-rays (EDAX). Powder X-ray diraction was carriedout using a Philips model PW 1730 X-ray generator, witha PW 1716 diractometer and PW 1050/25 detector. Thetube was a long ne focus Cu anode, Ni ltered Ka radia-tion. The normal current operating conditions are 40 kV30 mA. Scans are run from 15 to 110 2h with a step sizeof 0.02 2h, at a speed of 1/min. Angle calibration is by asynthetic Si sintered standard.

    3. Results and discussion

    As with all non-aqueous electrochemistry, the denitionof a reference potential is dicult due to the unknownliquid junction potential. Most studies in ionic liquids haveused either ferrocene as an internal standard or a silver wirequasi-reference electrode. With the DESs based on urea andethylene glycol the latter approach is used for two reasons.Firstly: ferrocene along with many of its functionalisedderivatives is largely insoluble in the ionic liquids. Second,the reference potential of the silver wire in both IL environ-ments is likely to be dominated by the activity of chlorideion. In all the experiments described here the concentrationof chloride ion is very much larger than that of any otherspecies in solution. The potential window of the urea: cho-line chloride eutectic mixture is relatively small on a plati-num electrode (1.2 V to +1.25 V vs. Ag) [15]. However,metals can be deposited with high current eciencies (videinfra) because the reduction kinetics of the eutectic are con-siderably slower on other metal surfaces e.g. on Zn no sig-nicant decomposition of the eutectic occurs above 2 V.

    Fig. 1A and B show the voltammetry of ZnCl2 and SnCl2in ChClmixtures with urea and ethylene glycol, respectively.The peak reduction potentials for tin are very similar in boththe urea and glycol based liquids (0.38 V and 0.35 V,respectively) suggesting rstly that the silver wire does actas a stable reference electrode and secondly that the hydro-gen bond donor does not signicantly aect the mechanismof tin ion reduction (where this is a combination of thermo-dynamic and kinetic eects). The reduction potentials forzinc are dierent in the urea and glycol based liquids(1.00 V and0.85 V, respectively) suggesting that the zinccomplexes present in solution may dier in the two liquids.In aqueous solutions the dierence between the standardreduction potentials for the two metals is 0.62 V. The dier-ence in the metal reduction potentials in the urea-basedliquid is exactly the same as that in water, 0.62 V, while thatin the ethylene glycol based liquid is only 0.50 V. Fig. 1Balso illustrates the dierence in deposition kinetics for Znand Sn, as the charges associated with deposition and strip-ping of the metal are approximately equal despite a 10-folddierence in the concentration of the two metal salts.

    It is also evident from Fig. 1 that the currents for reduc-tion and oxidation of zinc are signicantly dierentbetween the two liquids despite the concentrations beingsimilar. For example the ratio of the cathodic peak cur-rents, ip(EG)/ip(urea), observed during Zn deposition,

    ytical Chemistry 599 (2007) 288294 289Fig. 1, shows a ratio of approximately 15. The magnitudeof this ratio can be explained, in the most part, by the

  • naldierence in viscosity, g, of the two liquids (gurea = 1072 [email protected] 20 C; gEG = 50 cP @ 20 C) [11,13]. These viscositiesare related to the diusion coecients, D, of reacting spe-cies by Waldens Rule (below) [19], where the subscripts 1and 2 denote the viscosities and diusion coecients of areacting species in two diering media.

    D g D g

    Fig. 1. (A) Voltammograms (scan rate 20 mV s1) for a Pt microdiscelectrode (0.5 mm diameter) immersed in 1ChCl:2urea containing 0.05 MSnCl2 (solid line) and in 1ChCl:2urea containing 0.5 M ZnCl2 (dottedline). (B) Voltammograms (scan rate 20 mV s1) for a Pt microdiscelectrode (0.5 mm diameter) immersed in 1ChCl:2EG containing 0.05 MSnCl2 (solid line) and in 1ChCl:2EG containing 0.5 M ZnCl2 (dotted line).

    290 A.P. Abbott et al. / Journal of Electroa1 1 2 2

    It can easily be shown for a diusion-controlled process ina linear sweep voltammogram that the corresponding ratioof peak currents for Zn ion reduction in the two media isgiven by the expression:

    ipEG=ipurea DEG=Durea

    p

    Inserting the ratio of diusion coecients derived from theviscosity values (Waldens Rule) into this expression gives avalue for the ratio ip(EG)/ip(urea) of 5. This indicates thatviscosity eects only account for approximately one-thirdof the observed dierences, and the remaining discrepancycould be accounted for by variations in surface area of theelectrode together with dierences in the deposition kinet-ics (since the deposition peaks have qualitatively dierentshapes).

    SnCl2 is considerably less soluble in the ChCl:urea sys-tems and at the concentration shown in Fig. 1 produces aslightly turbid yellow solution suggesting that the solutionis actually beyond its saturation limit for tin chloride. Thismay account, in part, for the smaller voltammetricresponse of SnCl2 (in relation to ZnCl2) observed for theurea system (see Fig. 1).

    To elucidate the cause for the dierent voltammetricresponses FAB mass spectra were run of both ChCl:2ureaand ChCl:2EG each containing ZnCl2, then SnCl2 andnally a mixture of the two metals in a 10:1 ratio. In thepositive ion analysis mode, peaks of m/z = 104 and 243were the only signals observed and these are known to cor-respond to the species Choline+ and [2Choline Cl]+ andshows that no signicant cationic metal-containing speciesare formed. The negative ion spectra are considerably morecomplex due to the isotope splitting of the metal chloridespecies. A signal at 174 is common to all spectra [Choline2Cl]. The signals at m/z = 95 and 97 were also observedcorresponding to [Cl. urea] and [Cl. EG]. Where justtin is present the only species identied in both liquids iscentred at m/z = 225 [SnCl3]

    . We have previously studiedeutectic mixtures between just SnCl2 and ChCl and foundboth SnCl3 and Sn2Cl

    5 [9], but none of the di-tin species

    were detected in either the urea or EG case.For the zinc containing liquids a signicant dierence is

    observed between the urea and EG systems. In urea theonly zinc containing species is ZnCl3 whereas in EGZnCl3 ; Zn2Cl

    5 and Zn3Cl

    7 were detected. The most

    probable explanation for the observed dierences betweenzinc chloride in these two liquids is the ligand propertiesof the two complexing agents. Urea will act as a far stron-ger ligand for ZnCl3 than ethylene glycol. Its absence fromthe FAB-MS spectrum is not unexpected as it is rarely seenas a metal complex with this technique. No mixed metalcomplexes (e.g. ZnSnCl5 were observed in either the ureaor EG liquids, which is in contrast to what was found in theZnCl2/SnCl2/ChCl eutectic where it was the dominant spe-cies [7]. Eutectic mixtures between ChCl and ZnCl2 arereported to contain ZnCl3 ; Zn2Cl

    5 and Zn3Cl

    7 . The rela-

    tive proportions of each species have been quantied usingpotentiometry [8]. It was found that Zn2Cl

    5 was the pre-

    dominant species. The observation that no Zn2Cl5 was

    observed in the urea based liquid suggests that urea actsas a better complexing agent than ZnCl2 for ZnCl

    3 .

    The dierence in the species present will certainly resultin a change in reduction potential of the metal. Fig. 1 sug-gests that the mixed zinc species present in 1ChCl:2EG areeasier to reduce than ZnCl3 in urea lending further weightto the idea that the urea is involved in the coordinationsphere. There is a signicant dierence between the voltam-metric behaviour of the two liquids containing both tin andzinc chlorides. Fig. 2 shows the response for a solution con-taining 0.5 M ZnCl2 and 0.05 M SnCl2. In the ethylene gly-col based ionic liquid separate deposition and strippingsignals are observed for tin and zinc. Stopping the reduc-tive scan at 0.5 V the deposition of only tin is observedwith QC = QA. Extending the scan potential down to1.5 V results in a separate reduction signal for zinc. Thezinc deposition response is not as sharp as that observedin Fig. 1 for pure zinc, but this would be expected as thedeposition of zinc is now occurring on a fresh tin surfacerather than platinum. The two stripping potentials occur-ring on the anodic sweep occur at approximately the samepotentials as the individual metals shown in Fig. 1B sug-

    ytical Chemistry 599 (2007) 288294gesting that this is stripping of the pure metals and that thisis a two-phase alloy i.e. discrete zinc and tin phases. It is

  • -1.250 -0.750 -0.250 0.250 0.750 1.250-4

    -0.100x10

    -4-0.075x10

    -4-0.050x10

    -4-0.025x10

    0

    -40.025x10

    -40.050x10

    -40.075x10

    E / V

    i / A

    Fig. 3. Voltammograms (scan rate 20 mV s1) for a Pt microdisc electrode(0.5 mm diameter) immersed in a 1:2ChCl:urea DES containing 0.05 MSnCl2 and 0.5 M ZnCl2.

    analytical Chemistry 599 (2007) 288294 291only when the reductive limit is extended to more negativepotentials that a third stripping peak is observed at+50 mV and this could be due to a third phase consistingof predominantly tin with some zinc. The relative areasunder the oxidation peaks vary with the lower reductionlimit. Comparing these data to the 2ZnCl2/ChCl eutecticwith 3 wt% SnCl2 described previously [10] it is interestingto note that the voltammetry is very similar to that shownin Fig. 1B. However, the dierence between the reductionpotentials of the two metals in the 2ZnCl2/ChCl eutecticis only 0.341 V which is less than that observed in eitherof the two solvents seen here and less than the standardaqueous reduction potentials (0.62 V). Interestingly, thiscould be due to the presence of mixed metal complexesi.e. ZnSnCl5 which are known to exist in the Type 1 sys-tems [7].

    Guaus and Torrent-Burgues [17] studied the depositionof tinzinc alloys from aqueous sulphategluconate bathsand found a considerably more complex voltammetricresponse than that shown in Fig. 2. It was shown thatthe four reduction peaks observed were due to dierentzinc and tin containing species. The response observed onthe anodic sweep is comparable with that observed with

    -1.500 -1.000 -0.500 0 0.500 1.000 1.500-4

    -0.150x10

    -4-0.050x10

    -40.050x10

    -40.150x10

    -40.250x10

    -40.350x10

    -40.450x10

    E / V

    i / A

    Fig. 2. Voltammograms (scan rate 20 mV s1) for a Pt microdisc electrode(0.5 mm diameter) immersed in a 1:2 ChCl:EG DES containing 0.05 MSnCl2 and 0.5 M ZnCl2.

    A.P. Abbott et al. / Journal of ElectroChCl:2EG. The stripping of pure zinc and pure tin areclearly discernable and the third oxidative peak increasesits relative size depending on the lower reductive limit, akinto that observed above.

    Fig. 3 shows an analogous voltammogram to thatshown in Fig. 2 but using urea as the hydrogen bond donorin place of EG. The electrochemical response in the twoionic liquids is clearly dierent and signicantly theresponse is also dierent to the response of the two individ-ual component voltammograms for SnCl2 and ZnCl2. Thereductive potentials for tin and zinc are shifted to morepositive potentials compared to those in Fig. 2. A shift inreference potential is not likely to be a large contributoryfactor because the reference electrode appears to remainstable (vide supra) as can be seen by comparing Figs. 1and 2. The single anodic process occurs at a voltage thatis in between those for the zinc and tin processes shownin Fig. 1. Even taking the shift in reference potential intoaccount the dierence between the main onset of reduction(0.88 V) and the oxidation peak potential (0.55 V) isnevertheless greater than that shown in Fig. 1. Interestinglyno stripping of a tin rich phase is observed. It is probabletherefore, that the use of urea in the ionic liquid tends tolead to less of the separate zinc and tin phases and insteadyields a zinc rich phase.

    Bulk deposition of zinc from both EG and urea basedliquids leads to zinc deposits with small crystallites thathave negligible residual chloride. The deposits are dulland silver coloured in all cases. Fig. 4 shows an SEM imageof a zinc lm grown in 1ChCl:2EG containing 0.5 MZnCl2:0.05 M SnCl2 at a current density of 10 mA cm

    2

    for 1 h. This was typical of the morphology in both theEG and urea based liquids and was relatively unaectedby the current density. We have previously shown that zinccan be deposited as a crack free lm with high current e-ciency from a 1ChCl:2ZnCl2 liquid [7]. These lms arewhite in appearance and are made up of crystallites thatare slightly larger than those shown in Fig. 4 (c.a. 1025 lm) [20]. The morphology is also similar to thatobtained from deposition from the 2ZnCl2/ChCl eutecticFig. 4. Scanning electron micrograph obtained by the electrolysis of 0.5 MZnCl2/0.05 M SnCl2 in 1ChCl:2EG at a current density of 10 mA cm

    2 for1 h.

  • with 3 wt% SnCl2 [10]. The issue associated with the use ofthese Type 1 zinc eutectics is the low conductivity(36 lS cm1 at 40 C) whereas the two Type 3 eutectic mix-tures used in this work have much higher conductivities(ChCl:2urea = 1.8 mS cm1 and ChCl:2EG = 11 mS cm1

    both at 40 C) [11,13].Alternatively the deposition of tin, reported here, pro-

    duced more dendritic clusters. The dendrites are built upof simple cubic crystals as can be seen from Fig. 5. Thedeposition of whisker-like deposits, which is common forthe deposition of tin from aqueous solutions, was notobserved in either ionic liquid when the current densityapplied was 10 mA cm2.

    Electrolysis of the mixed SnCl2/ZnCl2 solutions pro-duced deposits with dierent morphologies depending onthe hydrogen bond donor used. With urea based liquidsthe deposit was made up of cubic crystallites, which looksimilar to those found in Fig. 4 (not shown). Energy Dis-persive Analysis by X-rays (EDAX) showed that they arepredominantly zinc (ca. 89%) with the remainder being

    Fig. 7. Scanning electron micrograph of a deposit grown from thesolution shown in Fig. 3 at a current density of 10 mA cm2 for 60 min(A + 1.0 V in situ anodic etch was performed in the solution for 1 minbefore deposition commenced).

    20 40 60 80 1000

    400

    800

    1200

    1600In

    ten

    sity

    / c

    ps

    2 / degrees

    EG Urea

    Fig. 8. X-ray diraction (XRD) analysis of the samples shown in Figs. 6and 7.

    Fig. 9. Scanning electron micrograph of an alloy deposit grown from thesolution of 0.5 M Zn:0.05 M Sn in 1ChCl:2urea containing 3 wt% Al2O3

    292 A.P. Abbott et al. / Journal of Electroanalytical Chemistry 599 (2007) 288294Fig. 5. Scanning electron micrograph obtained by the electrolysis of0.05 M SnCl2 in 1ChCl:2urea, plated onto copper substrate at a currentdensity of 10 mA cm2 for 30 min.

    Fig. 6. Scanning electron micrograph of a ca. 12 mm deposit grown from

    a solution of 0.5 M ZnCl2/0.05 M SnCl2 in 1ChCl:2urea + 3 wt% aluminaat a current density of 10 mA cm2 for 120 min.

    (shown in Fig. 3) at a current density of 10 mA cm2 for 30 min. Brightareas show high Al2O3 content.

  • largely tin with only traces of chloride. The two metalsseemed evenly distributed throughout the deposit. Thickdeposits (>1 mm) showed unusual morphologies. Fig. 6shows a ca. 12 mm deposit grown from a solution of0.5 M ZnCl2:0.05 M SnCl2 in 1ChCl:2urea, at a currentdensity of 10 mA cm2 for 120 min. Regular macroscopicpores of approximately 150 lm diameter are observed witha regular crystalline architecture surrounding them.

    Electrolysis of the ethylene glycol based liquid used inFig. 2 at 10 mA cm2 gave a deposit with similar underly-ing morphology to that shown in Fig. 7 but with dendriticgrowths on the top. These dendritic areas showed higher Sncontent than the bulk and we assume that the dendrites arepure tin. The overall Sn composition of the underlyingmetal is higher in tin than that found using urea based liq-uids (ca. 4045% Sn and 5055% Zn). Hence it can be seenthat changes in ionic liquid composition can aect the ther-modynamics of metal deposition by changing metalspeciation.

    X-ray diraction (XRD) analysis was carried out on thesamples shown in Figs. 6 and 7 and the results are shown inFig. 8. The XRD spectra are notably dierent with the dis-tinct tin signals at 2h = 30, 64 and 74 being absent fromthe sample deposited from the urea based liquid. Signals at2h = 34 and 36 are largely absent from the sample pre-pared in the EG based liquid and from those depositedfrom aqueous solutions [17] and could be due to a homoge-neous Zn based alloy.

    The relatively high viscosity of these solvents allowsimproved stability of suspensions. Colloidal dispersionsof 1 lm Al2O3 require over 3 h to settle and this is possiblyaided by the high ionic strength of the liquids. A dispersionof 3 wt% Al2O3 was made in the mixture whose voltamme-try is shown in Fig. 2. Mild agitation was sucient toretain the alumina as a homogeneous dispersion. Cyclic

    80 100 120 140

    0

    10

    20

    30

    40

    50

    60

    Wt %

    Current density / mA cm-2

    Zn Sn O Al Cl

    Fig. 10. Plot of elemental composition as a function of current density forthe deposits shown in Fig. 11 (data obtained from the hull cell panelshown in Fig. 11).

    A.P. Abbott et al. / Journal of Electroanalytical Chemistry 599 (2007) 288294 293Fig. 11. Scanning electron micrograph showing dierent regions of a hull ceZn:0.05 M Sn in 1ChCl:2urea containing 3 wt% Al2O3 (shown in Fig. 3) atD = 148.ll panel coated with an alloy deposit grown from the solution of 0.5 M

    various current densities for 30 min. A = 85 mA cm2; B = 97; C = 110;

  • voltammetry of the dispersion showed negligible dierencefrom that shown in Fig. 2.

    Fig. 9 shows an SEM image of the Zn/Sn alloy depos-ited from the EG based liquid using a current density of10 mA cm2 for 30 min. EDAX analysis conrms theinclusion of Al2O3 in the lms and these can be seen asbright areas in Fig. 9. The size of the Al2O3 inclusionsappears to suggest that the particles do not aggregate insolution or upon deposition in the lm but rather remainas discrete entities. The Al concentration in the lm isapproximately 1 wt% which is similar to that in the liquidsuggesting that the material is just dragged onto the surface

    for the deposition of coatings with improved wearresistance.

    Acknowledgement

    The authors acknowledge the EU under the FP6 pro-gramme for funding this work through the IONMETProject.

    References

    294 A.P. Abbott et al. / Journal of Electroanalytical Chemistry 599 (2007) 288294as the metal deposits.A Hull cell test was also carried out to determine the

    eect of current density on the Zn, Sn and Al2O3 composi-tion of the lm (determined by EDAX). Fig. 10 shows thepercentage of each component as a function of current den-sity and it can be seen that this is largely unaected over thecurrent density range studied. It was however found thatthe morphology of the surface layer was dependent onthe current density. At I = 85 mA cm2 a homogenousdeposit is formed as shown in Fig. 11A. Increasing the cur-rent density yields a similar underlying deposit with anamorphous surface coating (Fig. 11B) that is tin rich(42% Sn and 38% Zn). At higher current densities(Fig. 11C: I = 110 mA cm2) oral shaped tin depositsare found on the surface and these aggregate at even highercurrent densities (Fig. 11D: I = 148 mA cm2) these coa-lesce to give nodular deposits.

    4. Conclusions

    This work shows that ionic liquids based on eutecticmixtures of choline chloride and hydrogen bond donorssuch as ethylene glycol or urea can be used as electrochem-ical solvents. It is shown that zinc and tin can be electrode-posited from these liquids both individually and as alloys.It is shown for the rst time that the alloy morphologyand composition can be changed by judicious choice ofthe ionic liquid. It is proposed that metal speciation is acause of metal reduction thermodynamics. It is also dem-onstrated that composite materials can be deposited andAl2O3 is used as an example. This could open a new avenue[1] T. Welton, Chem. Rev. 99 (1999) 2071.[2] P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis, Wiley-VCH

    Verlag, Weinheim, Germany, 2003.[3] F. Endres, ChemPhysChem 3 (2002) 144.[4] F. Endres, Z. Phys. Chem. 218 (2004) 255.[5] M.C. Buzzeo, R.G. Evans, R.G. Compton, ChemPhysChem 5 (2004)

    1106.[6] H. Ohno (Ed.), Electrochemical Aspects of Ionic Liquids, John Wiley

    & Sons, New York, 2005.[7] A.P. Abbott, G. Capper, D.L. Davies, H.L. Munro, R.K. Rasheed,

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    Electrodeposition of zinc-tin alloys from deep eutectic solvents based on choline chlorideIntroductionExperimentalResults and discussionConclusionsAcknowledgementReferences