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Materials Chemistry and Physics 132 (2012) 34– 38

Contents lists available at SciVerse ScienceDirect

Materials Chemistry and Physics

jo u rn al hom epage : www.elsev ier .com/ locate /matchemphys

pectroscopic studies of the ionic liquid during the electrodeposition of Al–Tilloy in 1-ethyl-3-methylimidazolium chloride melt

bhishek Lahiri ∗, Rupak Dasepartment of Metallurgy and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487 United States

r t i c l e i n f o

rticle history:eceived 2 December 2010eceived in revised form 31 May 2011ccepted 31 October 2011

eywords:

a b s t r a c t

A new approach for the formation of Al–Ti alloy in 1-ethyl-3-methylimidazolium chloride (EmimCl)was investigated. The dissolution of titanium electrodes in presence of EmimCl:AlCl3 was performed atvarious potentials to understand the effect of voltage on the reaction rate. It was observed that at lowpotentials, the presence of titanium hinders the reaction and diminishes the formation of aluminum andaluminum–titanium alloy at the cathode. However, at higher potentials there was substantial forma-

lectrodepositiononic liquidl-Ti alloypectroscopy

tion of Al3Ti alloy. To understand the reaction mechanism during the electrolysis, the electrolyte wascharacterized using Fourier transform infrared spectroscopy (FTIR), UV–visible spectroscopy (UV–vis)and nuclear magnetic resonance (NMR) techniques. The material on the cathode was characterized usingscanning electron microscope (SEM) and Energy dispersive X-ray (EDX) techniques. From the above anal-ysis it was found that titanium forms a complex with aluminum and also assists in formation of AlCl4−

phase in the acidic ionic liquid.

. Introduction

Aluminum–titanium alloys have some remarkable propertiesne of which is the resistance to corrosion at high temperature1]. Much research has been undertaken on making Al–Ti alloyssing high temperature melting process [2–4]. However relatively

ess amount of study has been performed on the formation of theselloys in room temperature ionic liquids (RTIL) [1,5]. As ionic liquidsave low melting point, high conductivity, negligible vapor pres-ure, high thermal stability, it would be ideal to produce differentlloys using this medium. The use of ionic liquid will also reducehe overall energy consumption.

Titanium has various oxidation states such as Ti4+, Ti3+ and Ti2+

6]. Besides, it also has sub-oxide phases generally termed as Mag-eli phases [6] which make it a difficult metal to electrodeposit.he high temperature molten salt electrolysis for the productionf titanium has been promising [6]. However, the low temperatureroduction of titanium has not yet been possible. Mukhopadhyayt al. [7] showed the electrodeposition of titanium from TiCl4 onu cathode which was in nanometer scale. Recent papers demon-

trated that titanium deposit is perhaps not feasible in large scalesing ionic liquids [8,9]. There are fewer studies performed on thelectrodeposition of Al–Ti alloy due to its complexity [1,5,10]. Tsuda

∗ Corresponding author. Present address: World Premier International Researchenter, Advanced Institute for Materials Research, Tohoku University, Sendai, Japan.

E-mail address: lahiri.abhishek@gmail.com (A. Lahiri).

254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2011.10.048

© 2011 Elsevier B.V. All rights reserved.

et al. [5] added TiCl2 to EmimCl:AlCl3 solution to produce Al3Tialloy. They said that titanium forms a complex with aluminum fromwhich Al–Ti alloy is deposited. However no proof for the formationof aluminum–titanium complex was established.

In our investigation we have attempted to understand the reac-tion mechanism by performing electrolysis at various potentials.It was observed that as the potential across the electrodes wasincreased from 1.5 to 3.0 V in steps of 0.5 V, the deposit of Al3Tiincreased. However, the presence of titanium in the electrolytesignificantly reduced the deposition of Al3Ti. We did not find anydeposition of aluminum. At low potentials there is a formation ofTiCl3 product layer over the electrodes which could be one of thereasons for decreasing the deposition rate. Furthermore, titaniummight have led to the formation of AlCl4− phase due to which thedeposition of aluminum also diminished. Although direct proof forthe formation of Al–Ti complex in the electrolyte could not be estab-lished, NMR studies indicated the formation of a complex phasewhich has been discussed.

2. Experimental

1-ethyl-3-methylimidazolium chloride and titanium metal(99.999%) was obtained from Sigma Aldrich and was used as is with-out further purification. Anhydrous AlCl3 (99.985%) was obtained

from Alfa Aesar. The chloroaluminate molten salt was prepared bymixing weighed quantities of AlCl3 and EmimCl in a Pyrex beaker.The experiment was conducted in a Labconco glove box filled withargon. As the reaction between AlCl3 and EmimCl is exothermic,

mistry and Physics 132 (2012) 34– 38 35

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Fig. 1. FTIR of EmimCl, EmimCl:AlCl3(1:2) and EmimCl:AlCl3 on application of var-ious potentials across the titanium electrodes.

A. Lahiri, R. Das / Materials Che

luminum chloride was added step-wise. The mixture was stirredontinuously for complete dissolution.

The electrolysis experiments were conducted in a 40 ml Pyrexeaker fitted with a Teflon cap. The Teflon cap had holes for intro-ucing the electrodes and thermometer. A schematic diagram of thelectrolysis setup is shown elsewhere [11]. The titanium anode andathode had a dimension of 40 × 15 × 0.5 mm. The two electrodesere immersed into the electrolyte and a potential was applied

cross it. Experiments were carried out for 4 h at 100 ± 2 ◦C. Afterhe experiment, the electrolyte was stored in a glass bottle. The

aterial deposited at the cathode was thoroughly washed withcetone and water and was analyzed under SEM in JEOL 7000. Cor-esponding EDX measurements was also performed to determinehe compositions of Al and Ti.

The electrolyte was analyzed using FTIR, NMR and UV–vis spec-roscopy. FTIR analysis was carried out using PerkinElmer FTIR-ATRnstrument. For FTIR analysis one drop of the sample was placedn the diamond base and the absorption data was collected on aomputer-based software. UV–vis was measured by introducinghe sample into a silica sample holder with the reference being air.or NMR studies samples were introduced into the NMR tube andere performed on Bruker AM360 or Bruker AM500 spectrome-

er. As EmimCl was solid at room temperature, it was dissolve inimethyl sulfoxide (DMSO) solution.

. Results

Fig. 1 compares the FTIR of EmimCl, EmimCl:AlCl3 (1:2) andmimCl:AlCl3 on application of various potentials across the tita-ium electrodes for a fixed time of 60 min.

A number of peaks are observed in Fig. 1, most of which corre-pond to the aromatic, aliphatic and ring structure groups [12,13].n pure EmimCl the aromatic C–H bonds occurred at 3043 andliphatic C–H bond at 2973 cm−1. There is a peak at 2864 cm−1

hich could arise due to the N–H bond [14]. There are ring stretch-ng symmetries which occur at 1566, 1453, 1334 and 1173 cm−1.he out of plane bending of C–H occurs at 1383 and 1334 cm−1 [13].he C–H ring structure in-plane bending is observed at 1092 cm−1

nd an N–H bond occurs at 957 cm−1. A broad outer plane asym-etric ring bending is observed at 758 cm−1. On addition of AlCl3

o EmimCl, a number of changes take place. The biggest change ishe shift and reduction in peak intensities of aromatic compounds.n addition of 1:2 ratio of EmimCl:AlCl3 the C–H stretching occurst 3161 cm−1 and 3118 cm−1 which is in good agreement with theiterature data [14]. The intensity of the C–H stretching of aliphaticroup at 2990 cm−1 also reduces. The ring stretching symmetry at566 cm−1 splits to show the C N combination bond at 1600 cm−1

s observed from Fig. 1 [12]. There is a reduction of intensities ofhe ring stretching symmetry at 1453, 1334 and 1173 cm−1. Theroad band at 758 cm−1 splits to give predominant peaks of C–H

n-plane bending at 826 cm−1 and an outer plane asymmetric ringending at 740 cm−1 [15]. The small peak at 790 cm−1 could behe C–H bending in the methyl group. The peak at 700 cm−1 cor-esponded to a combination bond [16]. On introducing titaniumnto the molten salt by passing constant voltage across the tita-ium electrodes, further decrease of aromatic and aliphatic bondsccurred. The peak intensity at 790 cm−1 increased and grew withncrease in the voltage across the titanium electrodes.

To identify the oxidation states of titanium present in themimCl:AlCl3 melt, UV–vis spectroscopy was performed on theonic liquids. Fig. 2 compares the UV–vis spectra of EmimCl:AlCl3

elt when potential was applied in the system through the tita-ium electrodes.

From the spectroscopy it is observed that EmimCl:AlCl3 con-ains one peak at 330 nm which corresponds to Al3+ absorption [17].

Fig. 2. Comparison of UV spectra of EmimCl:AlCl3 with EmimCl:AlCl3 on applicationof various constant potentials across titanium electrodes.

36 A. Lahiri, R. Das / Materials Chemistry

Table 1Chemical shifts (ppm) in NMR spectra of 1H for different solutions.

H2 H4 H5

EmimCl 9.46 7.87 7.76EmimCl:AlCl3(1:2) 7.38 6.47 6.431.5 V 7.43 6.51 6.462.0 V 7.41 6.47 6.432.5 V 7.42 6.49 6.453.0 V 7.43 6.51 6.46

Table 2Chemical shifts (ppm) in NMR spectra of 13C for different solutions.

EmimCl EmimCl:AlCl3(1:2)

1.5 V 2.0 V 2.5 V 3.0 V

C2 136.91 133.39 133.49 133.52 133.51 133.51C5 123.99 123.28 123.29 123.22 123.26 123.28C4 122.44 121.43 121.62 121.56 121.60 121.61N–CH2 44.52 44.56 44.85 44.79 44.83 44.84N–CH3 36.13 36.05 36.19 36.16 36.17 36.18CH3 15.63 14.6 14.47 14.46 14.46 14.47

Table 3Chemical shifts (ppm) in NMR spectra of 27Al for different solutions.

EmimCl:AlCl3 (1:2) 1.5 V 2.0 V 2.5 V 3.0 V

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Al 99.8 97.37 97.22 97.34 97.52

hen a potential was applied between the titanium electrodes, Tiissolved into the ionic liquid forming various oxidation states. At.5 V, the UV spectra show two prominent peaks at 336 and 285 nm.he 336 nm could be attributed to Al3+ whereas the 285 nm is Ti4+

bsorption [18]. On increasing the potential to 2.0 V we find that thel3+ peak at around 335 is not present. However there are two peakst 504 nm and 285 nm which corresponds to Ti2+ and Ti4+ absorp-ion respectively [18,19]. The UV spectra of 2.5 and 3.0 V showedot of noise and therefore were not analyzed.

The NMR of 1H, 12C and 27Al of EmimCl, EmimCl:AlCl3 andmimCl:AlCl3 when potential was passed across the titanium elec-rodes are compared in Tables 1, 2 and 3. The structure of EmimCls shown in Fig. 3 in which 1 and 3 position are occupied by ethylnd methyl groups, respectively.

The NMR of 1H shows a significant chemical shift at H2 whenlCl3 is added to EmimCl. There is also a smaller shift in H4 and H5ositions. When a potential is applied and titanium dissolves intohe solution, there is a small positive shift in H2, H4 and H5. Similarbservation is noted in the 13C NMR in Table 2. The maximum shiftccurs in C2 position compared to the shifts in C4 and C5 positions.here is also a small negative shift in the ethyl CH3 group on theddition of AlCl3 and titanium in the EmimCl. In the 27Al NMR wend a significant negative shift when titanium dissolves into the

mimCl:AlCl3 melt. However on increasing the potential across thelectrodes, i.e. when the concentration of titanium in the electrolytencreases, there is a slight positive shift. A discrepancy is observed

Fig. 3. Structure of EmimCl.

and Physics 132 (2012) 34– 38

when 1.5 V was applied which could be due to the presence of Ti4+

as seen from the UV–vis spectra.The deposit at the cathode was characterized using SEM-EDX

technique. Fig. 4a illustrates the microstructure of the Al3Ti alloyand the corresponding EDX in Fig. 4b shows the presence of Al andTi peaks.

The microstructure of Al3Ti in Fig. 4a shows a nodular structurewhich is consistent with the results in the literature [5,11,20]. Thesemi quantitative analysis from the EDX showed that the ratio ofAl to Ti was 75:25 at% which corresponds well with the formationof Al3Ti alloy. Further proof of the Al3Ti alloy was confirmed usingX-ray diffraction analysis which showed a disordered aluminumstructure. On calculating the lattice parameter, a 0.52% shift wasobserved as Ti atoms substitute for Al and agrees with the datapresented in the literature [5].

4. Discussion

Comparing the FTIR of EmimCl with EmimCl:AlCl3 (1:2), wefind significant changes. There is a decrease in the peak intensi-ties for aromatic and aliphatic bonds in the region between 3135and 2865 cm−1. Dieter et al. [21] identified that C2 hydrogen bondas shown in Fig. 3 lies at 3118 cm−1 whereas C4 and C5 will liebetween 3150 and 3200 cm−1. The Cl− band lies around 3049 cm−1

in EmimCl [21] which is consistent with the result obtained in Fig. 1.On addition of AlCl3, the intensity of this band decreases. This couldarise due to the presence of AlCl3 which forms bonds with H and Cl−

forming a C–H–Al2Cl7− type structure. We considered the presenceof Al2Cl7− as we were working in the acidic medium (EmimCl:AlCl3(1:2)). The Al–Cl bond is found to be more electronegative com-pared to that of the C–H bond [22] and therefore leads to thedecrease in the intensity of aromatic and aliphatic bonds. Thereis also a small shift observed in the C–H bond at 3140 cm−1 whichrelates to the C4 and C5 positions in EmimCl to 3161 cm−1 on addi-tion of AlCl3 in acidic region. It is interesting to observe that thereis no C N bonds observed in EmimCl which could be due to thebroad peak of ring stretching symmetry at 1564 cm−1. As AlCl3 isadded to EmimCl, the ring stretching symmetry splits to show theC N peak. The addition of AlCl3 also decreases the ring stretchingsymmetry at 1173 cm−1. As electrolysis is performed at differentvoltages using titanium electrodes, there is a further decrease inthe intensity of the aromatic, aliphatic and ring structure symme-try peaks. The decrease could be attributed to the formation of acomplex between titanium and Al2Cl7−. However, there was nopeak shift observed in C4 and C5 on the addition of titanium. A peakgrowth at 790 cm−1 is observed when a potential is applied throughthe electrolysis setup which varies with increase in the current andwas identified to be C–H bending in methyl group. However, theNMR of 13C of N–CH3 does not show a significant chemical shifton introducing titanium into the electrolyte from the anode. Thus,the peak growth could only be due to the presence of titaniumand corresponded well with Ti3+ and Ti4+ oxidation states form-ing complexes with imidazole [15]. The random variation of thepeak at 790 cm−1 indicates that the concentration of Ti3+ and Ti4+

ions does not depend on the potential applied. There could also bea possibility of sampling error as only one drop of sample is usedduring FTIR analysis which could have led to the random growth.Furthermore, there is an initial increase in the peak at 700 cm−1

which corresponds to a combination bond [16]. The peak intensityincreases on the application of 1.5 and 2.0 V. However a decreasein the intensity is observed on the application of greater than 2.0 V.

This suggests that there is a complex formed within the ionic liq-uid whose threshold lies around 2.0 V and is confirmed by the factthat greater amount of Al3Ti deposits on application of 2.5 and 3.0 V.Thus, from the FTIR characterization it looks like Al2Cl7− bonds with

A. Lahiri, R. Das / Materials Chemistry and Physics 132 (2012) 34– 38 37

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ig. 4. (a) Microstructure of Al3Ti alloy deposited on the cathode when a potentialluminum and titanium.

2–H2 and alters the imidazole structure which then contributes tohe shift in C4 and C5 bonds. The addition of titanium in the systemurther affects the C2–H2 structure. However, the effect is not thatignificant to show a shift in C4 and C5 which could be explainedased on the oxidation state of Ti in the melt.

From the UV–visible spectroscopy it is observed thatmimCl:AlCl3 contains one peak at 330 nm which could onlye the Al3+ absorption[17]. When a potential was applied betweenhe titanium electrodes, Ti dissolved into the ionic liquid formingarious oxidation states. At 1.5 V, the UV spectra showed Al3+

nd Ti4+ peaks, whereas on increasing the potential to 2.0 V wend peaks of Ti2+ and Ti4+. Thus from the FTIR and UV–visiblepectra analysis we can conclude that Ti anode dissolves into theonic liquid in various oxidation states. As the thermodynamictability of TiCl4 and TiCl3 is high, they exist in the ionic liquids a separate phase. However Ti2+ ions which are unstable formn aluminum–titanium chloride complex as shown in Eq. (1).lthough direct proof for the formation of the aluminum–titaniumomplex cannot be established, however charge balance techniquehow the possibility for the formation of [Ti (Al2Cl7)4]2−. Furtherroof of aluminum–titanium complex is established by the facthat when a potential is passed through the electrodes in presencef titanium, there is no deposition of aluminum or titanium onhe cathode. It must also be noted that the solubility of TiCl4 inmimCl:AlCl3 is insignificant [5,18] and therefore the productionf Al3Ti at the cathode must occur from the Al–Ti complex.

i(Anode) + 4(Al2Cl7)− = [Ti(Al2Cl7)4]2− + 2e− (1)

he formation of titanium aluminum complex was further clarifiedy performing NMR on the ionic liquid samples. Tables 1, 2 and 3how the NMR of 1H, 13C and 27Al, respectively. As C2 is locatedetween two electronegative nitrogen atoms, it will have a lowlectron density. Therefore the AlCl3 will localize in the N1-C2-N3egion [23]. The negative shift in H2 corresponds well with the shiftn C2 thus confirming the AlCl3 complex bonding in the C2 posi-ion. In acidic region, AlCl3 will be in Al2Cl7− phase from whichluminum can be deposited [24]. However we cannot neglect theresence of other aluminum species as there will be equilibriumetween Al2Cl7−, AlCl3 and AlCl4− via the reaction shown in Eq.2).

lCl3 + AlCl4− = Al2Cl7

− (2)

rom 27Al NMR studies it was observed that there was line broad-

ning when the ratio of AlCl3:EmimCl was greater than one. Thiss because of the presence of Al2Cl7− which does not have aetrahedral symmetry [25]. However when potential was appliednd titanium dissolved into the ionic liquid, the line broadening

was applied across the titanium electrodes. (b) EDX showing the presence of both

decreased. The decrease in the line broadening can take place bythe formation of titanium aluminum complex as described in Eq.(1) or by forming AlCl4− phase as described via Eq. (3).

Al2Cl7− + TiCl4 + e− = 2AlCl4

− + TiCl3 (3)

However, if there is formation of AlCl4−, the chemical shift will bepositive [25]. Therefore the negative shift in the 27Al NMR can onlybe due to the complex formation of titanium aluminum chloride asshown in Eq. (1). On contrary, as the potential is increased across thetitanium electrodes there is a slight positive increase in the chemi-cal shift. There is some discrepancy in 1.5 V shift which might be dueto the presence of only Ti4+ ion as evident from the UV–vis spectrain Fig. 2. Although UV–vis spectra did not show any Ti3+ oxidationstate, FTIR showed a peak at 790 cm−1 which corresponded wellwith the formation of titanium complex [15]. As TiCl3 is a stablestructure, it forms a coating over the electrodes and reduces theoverall reaction rate. The formation of TiCl3 coating has been con-firmed previously [11]. On application of higher potential this layercan be decomposed which has also been shown previously [5]. Thiscould be the reason that no deposition of Al3Ti was observed when1.5 V was applied across the electrodes. As the potential across theelectrodes was increased, the TiCl3 layer decomposed and facil-itated the formation of aluminum–titanium complex via Eq. (2)which then results in the formation of Al3Ti maybe according toEqs. (4) and (5).

[Ti(Al2Cl7)4]2− + 2e− = Ti + 4(Al2Cl7)− (4)

4(Al2Cl7)− + 3e− = Al + 7AlCl4− (5)

As Al3Ti deposits at the cathode, the formation of AlCl4− increasesin the ionic liquid which could be the reason for observing a positiveshift on the application of higher potential.

5. Conclusions

The electrolysis of titanium in the electrolyte of EmimCl:AlCl3led to the formation of Al3Ti alloy which was confirmed using SEM-EDX techniques.

It was observed that titanium considerably reduces the reactionrate, which was due to the formation of TiCl3 coating over the elec-trodes and TiCl4 in the electrolyte which was clarified using FTIR

spectroscopy.

The NMR and UV–visible spectroscopy techniques proved thattitanium forms a complex with aluminum which results in the for-mation of Al3Ti alloy.

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and deposit characteristics, Electrochimica Acta 50 (16–17) (2005) 3286–3295,

8 A. Lahiri, R. Das / Materials Chem

cknowledgments

The authors gratefully acknowledge the financial support fromational Science Foundation and The University of Alabama. Theuthors would also like to thank Prof. R. Reddy, Dr. Scott Spear andr. Ken Belmore for their lab facilities and useful suggestions.

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