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Spectroscopic studies of the ionic liquid during the electrodeposition of Al–Ti alloy in 1-ethyl-3-methylimidazolium chloride melt
Materials Chemistry and Physics 132 (2012) 34–38
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Spectroscopic studies of the ionic liquid during the electrodeposition of Al–Ti alloy in 1-ethyl-3-methylimidazolium chloride melt Abhishek Lahiri ∗ , Rupak Das Department of Metallurgy and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487 United States
a r t i c l e
i n f o
Article history: Received 2 December 2010 Received in revised form 31 May 2011 Accepted 31 October 2011 Keywords: Electrodeposition Ionic liquid Al-Ti alloy Spectroscopy
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 at various potentials to understand the effect of voltage on the reaction rate. It was observed that at low potentials, the presence of titanium hinders the reaction and diminishes the formation of aluminum and aluminum–titanium alloy at the cathode. However, at higher potentials there was substantial formation of Al3 Ti alloy. To understand the reaction mechanism during the electrolysis, the electrolyte was characterized using Fourier transform infrared spectroscopy (FTIR), UV–visible spectroscopy (UV–vis) and nuclear magnetic resonance (NMR) techniques. The material on the cathode was characterized using scanning electron microscope (SEM) and Energy dispersive X-ray (EDX) techniques. From the above analysis it was found that titanium forms a complex with aluminum and also assists in formation of AlCl4 − phase in the acidic ionic liquid. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Aluminum–titanium alloys have some remarkable properties one of which is the resistance to corrosion at high temperature [1]. Much research has been undertaken on making Al–Ti alloys using high temperature melting process [2–4]. However relatively less amount of study has been performed on the formation of these alloys in room temperature ionic liquids (RTIL) [1,5]. As ionic liquids have low melting point, high conductivity, negligible vapor pressure, high thermal stability, it would be ideal to produce different alloys using this medium. The use of ionic liquid will also reduce the 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 Magneli phases [6] which make it a difficult metal to electrodeposit. The high temperature molten salt electrolysis for the production of titanium has been promising [6]. However, the low temperature production of titanium has not yet been possible. Mukhopadhyay et al. [7] showed the electrodeposition of titanium from TiCl4 on Au cathode which was in nanometer scale. Recent papers demonstrated that titanium deposit is perhaps not feasible in large scale using ionic liquids [8,9]. There are fewer studies performed on the electrodeposition of Al–Ti alloy due to its complexity [1,5,10]. Tsuda
∗ Corresponding author. Present address: World Premier International Research Center, Advanced Institute for Materials Research, Tohoku University, Sendai, Japan. E-mail address: [email protected] (A. Lahiri). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.10.048
et al. [5] added TiCl2 to EmimCl:AlCl3 solution to produce Al3 Ti alloy. They said that titanium forms a complex with aluminum from which Al–Ti alloy is deposited. However no proof for the formation of aluminum–titanium complex was established. In our investigation we have attempted to understand the reaction mechanism by performing electrolysis at various potentials. It was observed that as the potential across the electrodes was increased from 1.5 to 3.0 V in steps of 0.5 V, the deposit of Al3 Ti increased. However, the presence of titanium in the electrolyte significantly reduced the deposition of Al3 Ti. We did not find any deposition of aluminum. At low potentials there is a formation of TiCl3 product layer over the electrodes which could be one of the reasons for decreasing the deposition rate. Furthermore, titanium might have led to the formation of AlCl4 − phase due to which the deposition of aluminum also diminished. Although direct proof for the formation of Al–Ti complex in the electrolyte could not be established, NMR studies indicated the formation of a complex phase which 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 without further purification. Anhydrous AlCl3 (99.985%) was obtained from Alfa Aesar. The chloroaluminate molten salt was prepared by mixing weighed quantities of AlCl3 and EmimCl in a Pyrex beaker. The experiment was conducted in a Labconco glove box filled with argon. As the reaction between AlCl3 and EmimCl is exothermic,
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aluminum chloride was added step-wise. The mixture was stirred continuously for complete dissolution. The electrolysis experiments were conducted in a 40 ml Pyrex beaker fitted with a Teflon cap. The Teflon cap had holes for introducing the electrodes and thermometer. A schematic diagram of the electrolysis setup is shown elsewhere [11]. The titanium anode and cathode had a dimension of 40 × 15 × 0.5 mm. The two electrodes were immersed into the electrolyte and a potential was applied across it. Experiments were carried out for 4 h at 100 ± 2 ◦ C. After the experiment, the electrolyte was stored in a glass bottle. The material deposited at the cathode was thoroughly washed with acetone and water and was analyzed under SEM in JEOL 7000. Corresponding EDX measurements was also performed to determine the compositions of Al and Ti. The electrolyte was analyzed using FTIR, NMR and UV–vis spectroscopy. FTIR analysis was carried out using PerkinElmer FTIR-ATR instrument. For FTIR analysis one drop of the sample was placed on the diamond base and the absorption data was collected on a computer-based software. UV–vis was measured by introducing the sample into a silica sample holder with the reference being air. For NMR studies samples were introduced into the NMR tube and were performed on Bruker AM360 or Bruker AM500 spectrometer. As EmimCl was solid at room temperature, it was dissolve in dimethyl sulfoxide (DMSO) solution.
3. Results Fig. 1 compares the FTIR of EmimCl, EmimCl:AlCl3 (1:2) and EmimCl:AlCl3 on application of various potentials across the titanium electrodes for a fixed time of 60 min. A number of peaks are observed in Fig. 1, most of which correspond to the aromatic, aliphatic and ring structure groups [12,13]. In pure EmimCl the aromatic C–H bonds occurred at 3043 and aliphatic C–H bond at 2973 cm−1 . There is a peak at 2864 cm−1 which could arise due to the N–H bond [14]. There are ring stretching symmetries which occur at 1566, 1453, 1334 and 1173 cm−1 . The out of plane bending of C–H occurs at 1383 and 1334 cm−1 [13]. The C–H ring structure in-plane bending is observed at 1092 cm−1 and an N–H bond occurs at 957 cm−1 . A broad outer plane asymmetric ring bending is observed at 758 cm−1 . On addition of AlCl3 to EmimCl, a number of changes take place. The biggest change is the shift and reduction in peak intensities of aromatic compounds. On addition of 1:2 ratio of EmimCl:AlCl3 the C–H stretching occurs at 3161 cm−1 and 3118 cm−1 which is in good agreement with the literature data [14]. The intensity of the C–H stretching of aliphatic group at 2990 cm−1 also reduces. The ring stretching symmetry at 1566 cm−1 splits to show the C N combination bond at 1600 cm−1 as observed from Fig. 1 [12]. There is a reduction of intensities of the ring stretching symmetry at 1453, 1334 and 1173 cm−1 . The broad band at 758 cm−1 splits to give predominant peaks of C–H in-plane bending at 826 cm−1 and an outer plane asymmetric ring bending at 740 cm−1 [15]. The small peak at 790 cm−1 could be the C–H bending in the methyl group. The peak at 700 cm−1 corresponded to a combination bond [16]. On introducing titanium into the molten salt by passing constant voltage across the titanium electrodes, further decrease of aromatic and aliphatic bonds occurred. The peak intensity at 790 cm−1 increased and grew with increase in the voltage across the titanium electrodes. To identify the oxidation states of titanium present in the EmimCl:AlCl3 melt, UV–vis spectroscopy was performed on the ionic liquids. Fig. 2 compares the UV–vis spectra of EmimCl:AlCl3 melt when potential was applied in the system through the titanium electrodes. From the spectroscopy it is observed that EmimCl:AlCl3 contains one peak at 330 nm which corresponds to Al3+ absorption [17].
Fig. 1. FTIR of EmimCl, EmimCl:AlCl3 (1:2) and EmimCl:AlCl3 on application of various potentials across the titanium electrodes.
Fig. 2. Comparison of UV spectra of EmimCl:AlCl3 with EmimCl:AlCl3 on application of various constant potentials across titanium electrodes.
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A. Lahiri, R. Das / Materials Chemistry and Physics 132 (2012) 34–38
Table 1 Chemical shifts (ppm) in NMR spectra of 1 H for different solutions.
EmimCl EmimCl:AlCl3 (1:2) 1.5 V 2.0 V 2.5 V 3.0 V
H2
H4
H5
9.46 7.38 7.43 7.41 7.42 7.43
7.87 6.47 6.51 6.47 6.49 6.51
7.76 6.43 6.46 6.43 6.45 6.46
Table 2 Chemical shifts (ppm) in NMR spectra of 13 C for different solutions.
C2 C5 C4 N–CH2 N–CH3 CH3
EmimCl
EmimCl:AlCl3 (1:2)
1.5 V
2.0 V
2.5 V
3.0 V
136.91 123.99 122.44 44.52 36.13 15.63
133.39 123.28 121.43 44.56 36.05 14.6
133.49 123.29 121.62 44.85 36.19 14.47
133.52 123.22 121.56 44.79 36.16 14.46
133.51 123.26 121.60 44.83 36.17 14.46
133.51 123.28 121.61 44.84 36.18 14.47
Table 3 Chemical shifts (ppm) in NMR spectra of 27 Al for different solutions.
Al
EmimCl:AlCl3 (1:2)
1.5 V
2.0 V
2.5 V
3.0 V
99.8
97.37
97.22
97.34
97.52
When a potential was applied between the titanium electrodes, Ti dissolved into the ionic liquid forming various oxidation states. At 1.5 V, the UV spectra show two prominent peaks at 336 and 285 nm. The 336 nm could be attributed to Al3+ whereas the 285 nm is Ti4+ absorption [18]. On increasing the potential to 2.0 V we find that the Al3+ peak at around 335 is not present. However there are two peaks at 504 nm and 285 nm which corresponds to Ti2+ and Ti4+ absorption respectively [18,19]. The UV spectra of 2.5 and 3.0 V showed lot of noise and therefore were not analyzed. The NMR of 1 H, 12 C and 27 Al of EmimCl, EmimCl:AlCl3 and EmimCl:AlCl3 when potential was passed across the titanium electrodes are compared in Tables 1, 2 and 3. The structure of EmimCl is shown in Fig. 3 in which 1 and 3 position are occupied by ethyl and methyl groups, respectively. The NMR of 1 H shows a significant chemical shift at H2 when AlCl3 is added to EmimCl. There is also a smaller shift in H4 and H5 positions. When a potential is applied and titanium dissolves into the solution, there is a small positive shift in H2, H4 and H5. Similar observation is noted in the 13 C NMR in Table 2. The maximum shift occurs in C2 position compared to the shifts in C4 and C5 positions. There is also a small negative shift in the ethyl CH3 group on the addition of AlCl3 and titanium in the EmimCl. In the 27 Al NMR we find a significant negative shift when titanium dissolves into the EmimCl:AlCl3 melt. However on increasing the potential across the electrodes, i.e. when the concentration of titanium in the electrolyte increases, there is a slight positive shift. A discrepancy is observed
Fig. 3. Structure of EmimCl.
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 Al3 Ti alloy and the corresponding EDX in Fig. 4b shows the presence of Al and Ti peaks. The microstructure of Al3 Ti in Fig. 4a shows a nodular structure which is consistent with the results in the literature [5,11,20]. The semi quantitative analysis from the EDX showed that the ratio of Al to Ti was 75:25 at% which corresponds well with the formation of Al3 Ti alloy. Further proof of the Al3 Ti alloy was confirmed using X-ray diffraction analysis which showed a disordered aluminum structure. On calculating the lattice parameter, a 0.52% shift was observed as Ti atoms substitute for Al and agrees with the data presented in the literature [5].
4. Discussion Comparing the FTIR of EmimCl with EmimCl:AlCl3 (1:2), we find significant changes. There is a decrease in the peak intensities for aromatic and aliphatic bonds in the region between 3135 and 2865 cm−1 . Dieter et al. [21] identified that C2 hydrogen bond as shown in Fig. 3 lies at 3118 cm−1 whereas C4 and C5 will lie between 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 could arise due to the presence of AlCl3 which forms bonds with H and Cl− forming a C–H–Al2 Cl7 − type structure. We considered the presence of Al2 Cl7 − as we were working in the acidic medium (EmimCl:AlCl3 (1:2)). The Al–Cl bond is found to be more electronegative compared to that of the C–H bond [22] and therefore leads to the decrease in the intensity of aromatic and aliphatic bonds. There is also a small shift observed in the C–H bond at 3140 cm−1 which relates to the C4 and C5 positions in EmimCl to 3161 cm−1 on addition of AlCl3 in acidic region. It is interesting to observe that there is no C N bonds observed in EmimCl which could be due to the broad peak of ring stretching symmetry at 1564 cm−1 . As AlCl3 is added to EmimCl, the ring stretching symmetry splits to show the C N peak. The addition of AlCl3 also decreases the ring stretching symmetry at 1173 cm−1 . As electrolysis is performed at different voltages using titanium electrodes, there is a further decrease in the intensity of the aromatic, aliphatic and ring structure symmetry peaks. The decrease could be attributed to the formation of a complex between titanium and Al2 Cl7 − . However, there was no peak shift observed in C4 and C5 on the addition of titanium. A peak growth at 790 cm−1 is observed when a potential is applied through the electrolysis setup which varies with increase in the current and was identified to be C–H bending in methyl group. However, the NMR of 13 C of N–CH3 does not show a significant chemical shift on introducing titanium into the electrolyte from the anode. Thus, the peak growth could only be due to the presence of titanium and corresponded well with Ti3+ and Ti4+ oxidation states forming complexes with imidazole [15]. The random variation of the peak at 790 cm−1 indicates that the concentration of Ti3+ and Ti4+ ions does not depend on the potential applied. There could also be a possibility of sampling error as only one drop of sample is used during 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 intensity increases on the application of 1.5 and 2.0 V. However a decrease in the intensity is observed on the application of greater than 2.0 V. This suggests that there is a complex formed within the ionic liquid whose threshold lies around 2.0 V and is confirmed by the fact that greater amount of Al3 Ti deposits on application of 2.5 and 3.0 V. Thus, from the FTIR characterization it looks like Al2 Cl7 − bonds with
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Fig. 4. (a) Microstructure of Al3 Ti alloy deposited on the cathode when a potential of 2 V was applied across the titanium electrodes. (b) EDX showing the presence of both aluminum and titanium.
C2–H2 and alters the imidazole structure which then contributes to the shift in C4 and C5 bonds. The addition of titanium in the system further affects the C2–H2 structure. However, the effect is not that significant to show a shift in C4 and C5 which could be explained based on the oxidation state of Ti in the melt. From the UV–visible spectroscopy it is observed that EmimCl:AlCl3 contains one peak at 330 nm which could only be the Al3+ absorption[17]. When a potential was applied between the titanium electrodes, Ti dissolved into the ionic liquid forming various oxidation states. At 1.5 V, the UV spectra showed Al3+ and Ti4+ peaks, whereas on increasing the potential to 2.0 V we find peaks of Ti2+ and Ti4+ . Thus from the FTIR and UV–visible spectra analysis we can conclude that Ti anode dissolves into the ionic liquid in various oxidation states. As the thermodynamic stability of TiCl4 and TiCl3 is high, they exist in the ionic liquid as a separate phase. However Ti2+ ions which are unstable form an aluminum–titanium chloride complex as shown in Eq. (1). Although direct proof for the formation of the aluminum–titanium complex cannot be established, however charge balance technique show the possibility for the formation of [Ti (Al2 Cl7 )4 ]2− . Further proof of aluminum–titanium complex is established by the fact that when a potential is passed through the electrodes in presence of titanium, there is no deposition of aluminum or titanium on the cathode. It must also be noted that the solubility of TiCl4 in EmimCl:AlCl3 is insignificant [5,18] and therefore the production of Al3 Ti at the cathode must occur from the Al–Ti complex.
decreased. The decrease in the line broadening can take place by the formation of titanium aluminum complex as described in Eq. (1) or by forming AlCl4 − phase as described via Eq. (3).
Ti(Anode) + 4(Al2 Cl7 )− = [Ti(Al2 Cl7 )4 ]2− + 2e−
[Ti(Al2 Cl7 )4 ]2− + 2e− = Ti + 4(Al2 Cl7 )−
(4)
4(Al2 Cl7 )− + 3e− = Al + 7AlCl4 −
(5)
(1)
The formation of titanium aluminum complex was further clarified by performing NMR on the ionic liquid samples. Tables 1, 2 and 3 show the NMR of 1 H, 13 C and 27 Al, respectively. As C2 is located between two electronegative nitrogen atoms, it will have a low electron density. Therefore the AlCl3 will localize in the N1-C2-N3 region [23]. The negative shift in H2 corresponds well with the shift in C2 thus confirming the AlCl3 complex bonding in the C2 position. In acidic region, AlCl3 will be in Al2 Cl7 − phase from which aluminum can be deposited [24]. However we cannot neglect the presence of other aluminum species as there will be equilibrium between Al2 Cl7 − , AlCl3 and AlCl4 − via the reaction shown in Eq. (2). AlCl3 + AlCl4 − = Al2 Cl7 −
(2)
From 27 Al NMR studies it was observed that there was line broadening when the ratio of AlCl3 :EmimCl was greater than one. This is because of the presence of Al2 Cl7 − which does not have a tetrahedral symmetry [25]. However when potential was applied and titanium dissolved into the ionic liquid, the line broadening
Al2 Cl7 − + TiCl4 + e− = 2AlCl4 − + TiCl3
(3)
However, if there is formation of AlCl4 − , the chemical shift will be positive [25]. Therefore the negative shift in the 27 Al NMR can only be due to the complex formation of titanium aluminum chloride as shown in Eq. (1). On contrary, as the potential is increased across the titanium electrodes there is a slight positive increase in the chemical shift. There is some discrepancy in 1.5 V shift which might be due to the presence of only Ti4+ ion as evident from the UV–vis spectra in Fig. 2. Although UV–vis spectra did not show any Ti3+ oxidation state, FTIR showed a peak at 790 cm−1 which corresponded well with the formation of titanium complex [15]. As TiCl3 is a stable structure, it forms a coating over the electrodes and reduces the overall reaction rate. The formation of TiCl3 coating has been confirmed previously [11]. On application of higher potential this layer can be decomposed which has also been shown previously [5]. This could be the reason that no deposition of Al3 Ti was observed when 1.5 V was applied across the electrodes. As the potential across the electrodes was increased, the TiCl3 layer decomposed and facilitated the formation of aluminum–titanium complex via Eq. (2) which then results in the formation of Al3 Ti maybe according to Eqs. (4) and (5).
As Al3 Ti deposits at the cathode, the formation of AlCl4 − increases in the ionic liquid which could be the reason for observing a positive shift on the application of higher potential. 5. Conclusions The electrolysis of titanium in the electrolyte of EmimCl:AlCl3 led to the formation of Al3 Ti alloy which was confirmed using SEMEDX techniques. It was observed that titanium considerably reduces the reaction rate, which was due to the formation of TiCl3 coating over the electrodes and TiCl4 in the electrolyte which was clarified using FTIR spectroscopy. The NMR and UV–visible spectroscopy techniques proved that titanium forms a complex with aluminum which results in the formation of Al3 Ti alloy.
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Acknowledgments The authors gratefully acknowledge the financial support from National Science Foundation and The University of Alabama. The authors would also like to thank Prof. R. Reddy, Dr. Scott Spear and Dr. Ken Belmore for their lab facilities and useful suggestions. References [1] G.R. Stafford, The electrodeposition of Al3 Ti from chloroaluminate electrolytes, Journal of the Electrochemical Society 141 (4) (1994) 945–953. [2] B.G. Guo, J.S. Zhou, S.T. Zhang, H.D. Zhou, Y.P. Pu, J.M. Chen, Tribological properties of titanium aluminides coatings produced on pure Ti by laser surface alloying, Surface & Coatings Technology 202 (17) (2008) 4121–4129, doi:10.1016/j.surfcoat.2008.02.026. [3] H. Bahmanpour, S. Heshmati-Manesh, Role of process control agent on mechanical alloying of nano structured TiAl-based alloy, International Journal of Modern Physics B 22 (18–19) (2008) 2933–2938. [4] V. Auradi, S.A. Kori, Influence of reaction temperature for the manufacturing of Al-3Ti and Al-3B master alloys, Journal of Alloys and Compounds 53 (1–2) (2008) 147–156, doi:10.1016/j.jallcom.2006.11.119. [5] T. Tsuda, C.L. Hussey, G.R. Stafford, J.E. Bonevich, Electrochemistry of titanium and the electrodeposition of Al–Ti alloys in the Lewis acidic aluminum chloride1-ethyl-3-methylimidazolium chloride melt, Journal of the Electrochemical Society 50 (4) (2003) C234–C243, doi:10.1149/1.1554915. [6] G.Z. Chen, D.J. Fray, T.W. Farthing, Direct electrochemical reduction of titanium dioxide to titanium in molten calcium chloride, Nature 407 (6802) (2000) 361–364. [7] I. Mukhopadhyay, C.L. Aravinda, D. Borissov, W. Freyland, Electrodeposition of Ti from TiCl4 in the ionic liquid 1-methyl-3-butyl-imidazolium bis (trifluoro methyl sulfone) imide at room temperature: study on phase formation by in situ electrochemical scanning tunneling microscopy, Electrochimica Acta 50 (6) (2005) 1275–1281, doi:10.1016/j.electacta.2004.07.052. [8] F. Endres, S. Zein El Abedin, A.Y. Saad, E.M. Moustafa, N. Borissenko, W.E. Price, G.G. Wallace, D.R. MacFarlane, P.J. Newmanc, A. Bundd, On the electrodeposition of titanium in ionic liquids, Physical Chemistry Chemical Physics 10 (2008) 2189–2199. [9] J. Ding, J. Wu, D. MacFarlane, W.E. Price, G. Wallace, Induction of titanium reduction using pyrrole and polypyrrole in the ionic liquid ethylmethyl-imidazolium bis(trifluoromethanesulphonyl)amide, Electrochemistry Communications 10 (2) (2008) 217–221, doi:10.1016/j.elecom.2007.11.021. [10] C.L. Aravinda, I. Mukhopadhyay, W. Freyland, Electrochemical in situ STM study of Al and Ti-Al alloy electrodeposition on Au(111) from a room temperature molten salt electrolyte, Physical Chemistry Chemical Physics 6 (22) (2004) 5225–5231, doi:10.1039/b407846m.
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Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Spectroscopic studies of the ionic liquid during the electrodeposition of Al–Ti alloy in 1-ethyl-3-methylimidazolium chloride melt Abhishek Lahiri ∗ , Rupak Das Department of Metallurgy and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487 United States
a r t i c l e
i n f o
Article history: Received 2 December 2010 Received in revised form 31 May 2011 Accepted 31 October 2011 Keywords: Electrodeposition Ionic liquid Al-Ti alloy Spectroscopy
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 at various potentials to understand the effect of voltage on the reaction rate. It was observed that at low potentials, the presence of titanium hinders the reaction and diminishes the formation of aluminum and aluminum–titanium alloy at the cathode. However, at higher potentials there was substantial formation of Al3 Ti alloy. To understand the reaction mechanism during the electrolysis, the electrolyte was characterized using Fourier transform infrared spectroscopy (FTIR), UV–visible spectroscopy (UV–vis) and nuclear magnetic resonance (NMR) techniques. The material on the cathode was characterized using scanning electron microscope (SEM) and Energy dispersive X-ray (EDX) techniques. From the above analysis it was found that titanium forms a complex with aluminum and also assists in formation of AlCl4 − phase in the acidic ionic liquid. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Aluminum–titanium alloys have some remarkable properties one of which is the resistance to corrosion at high temperature [1]. Much research has been undertaken on making Al–Ti alloys using high temperature melting process [2–4]. However relatively less amount of study has been performed on the formation of these alloys in room temperature ionic liquids (RTIL) [1,5]. As ionic liquids have low melting point, high conductivity, negligible vapor pressure, high thermal stability, it would be ideal to produce different alloys using this medium. The use of ionic liquid will also reduce the 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 Magneli phases [6] which make it a difficult metal to electrodeposit. The high temperature molten salt electrolysis for the production of titanium has been promising [6]. However, the low temperature production of titanium has not yet been possible. Mukhopadhyay et al. [7] showed the electrodeposition of titanium from TiCl4 on Au cathode which was in nanometer scale. Recent papers demonstrated that titanium deposit is perhaps not feasible in large scale using ionic liquids [8,9]. There are fewer studies performed on the electrodeposition of Al–Ti alloy due to its complexity [1,5,10]. Tsuda
∗ Corresponding author. Present address: World Premier International Research Center, Advanced Institute for Materials Research, Tohoku University, Sendai, Japan. E-mail address: [email protected] (A. Lahiri). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.10.048
et al. [5] added TiCl2 to EmimCl:AlCl3 solution to produce Al3 Ti alloy. They said that titanium forms a complex with aluminum from which Al–Ti alloy is deposited. However no proof for the formation of aluminum–titanium complex was established. In our investigation we have attempted to understand the reaction mechanism by performing electrolysis at various potentials. It was observed that as the potential across the electrodes was increased from 1.5 to 3.0 V in steps of 0.5 V, the deposit of Al3 Ti increased. However, the presence of titanium in the electrolyte significantly reduced the deposition of Al3 Ti. We did not find any deposition of aluminum. At low potentials there is a formation of TiCl3 product layer over the electrodes which could be one of the reasons for decreasing the deposition rate. Furthermore, titanium might have led to the formation of AlCl4 − phase due to which the deposition of aluminum also diminished. Although direct proof for the formation of Al–Ti complex in the electrolyte could not be established, NMR studies indicated the formation of a complex phase which 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 without further purification. Anhydrous AlCl3 (99.985%) was obtained from Alfa Aesar. The chloroaluminate molten salt was prepared by mixing weighed quantities of AlCl3 and EmimCl in a Pyrex beaker. The experiment was conducted in a Labconco glove box filled with argon. As the reaction between AlCl3 and EmimCl is exothermic,
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aluminum chloride was added step-wise. The mixture was stirred continuously for complete dissolution. The electrolysis experiments were conducted in a 40 ml Pyrex beaker fitted with a Teflon cap. The Teflon cap had holes for introducing the electrodes and thermometer. A schematic diagram of the electrolysis setup is shown elsewhere [11]. The titanium anode and cathode had a dimension of 40 × 15 × 0.5 mm. The two electrodes were immersed into the electrolyte and a potential was applied across it. Experiments were carried out for 4 h at 100 ± 2 ◦ C. After the experiment, the electrolyte was stored in a glass bottle. The material deposited at the cathode was thoroughly washed with acetone and water and was analyzed under SEM in JEOL 7000. Corresponding EDX measurements was also performed to determine the compositions of Al and Ti. The electrolyte was analyzed using FTIR, NMR and UV–vis spectroscopy. FTIR analysis was carried out using PerkinElmer FTIR-ATR instrument. For FTIR analysis one drop of the sample was placed on the diamond base and the absorption data was collected on a computer-based software. UV–vis was measured by introducing the sample into a silica sample holder with the reference being air. For NMR studies samples were introduced into the NMR tube and were performed on Bruker AM360 or Bruker AM500 spectrometer. As EmimCl was solid at room temperature, it was dissolve in dimethyl sulfoxide (DMSO) solution.
3. Results Fig. 1 compares the FTIR of EmimCl, EmimCl:AlCl3 (1:2) and EmimCl:AlCl3 on application of various potentials across the titanium electrodes for a fixed time of 60 min. A number of peaks are observed in Fig. 1, most of which correspond to the aromatic, aliphatic and ring structure groups [12,13]. In pure EmimCl the aromatic C–H bonds occurred at 3043 and aliphatic C–H bond at 2973 cm−1 . There is a peak at 2864 cm−1 which could arise due to the N–H bond [14]. There are ring stretching symmetries which occur at 1566, 1453, 1334 and 1173 cm−1 . The out of plane bending of C–H occurs at 1383 and 1334 cm−1 [13]. The C–H ring structure in-plane bending is observed at 1092 cm−1 and an N–H bond occurs at 957 cm−1 . A broad outer plane asymmetric ring bending is observed at 758 cm−1 . On addition of AlCl3 to EmimCl, a number of changes take place. The biggest change is the shift and reduction in peak intensities of aromatic compounds. On addition of 1:2 ratio of EmimCl:AlCl3 the C–H stretching occurs at 3161 cm−1 and 3118 cm−1 which is in good agreement with the literature data [14]. The intensity of the C–H stretching of aliphatic group at 2990 cm−1 also reduces. The ring stretching symmetry at 1566 cm−1 splits to show the C N combination bond at 1600 cm−1 as observed from Fig. 1 [12]. There is a reduction of intensities of the ring stretching symmetry at 1453, 1334 and 1173 cm−1 . The broad band at 758 cm−1 splits to give predominant peaks of C–H in-plane bending at 826 cm−1 and an outer plane asymmetric ring bending at 740 cm−1 [15]. The small peak at 790 cm−1 could be the C–H bending in the methyl group. The peak at 700 cm−1 corresponded to a combination bond [16]. On introducing titanium into the molten salt by passing constant voltage across the titanium electrodes, further decrease of aromatic and aliphatic bonds occurred. The peak intensity at 790 cm−1 increased and grew with increase in the voltage across the titanium electrodes. To identify the oxidation states of titanium present in the EmimCl:AlCl3 melt, UV–vis spectroscopy was performed on the ionic liquids. Fig. 2 compares the UV–vis spectra of EmimCl:AlCl3 melt when potential was applied in the system through the titanium electrodes. From the spectroscopy it is observed that EmimCl:AlCl3 contains one peak at 330 nm which corresponds to Al3+ absorption [17].
Fig. 1. FTIR of EmimCl, EmimCl:AlCl3 (1:2) and EmimCl:AlCl3 on application of various potentials across the titanium electrodes.
Fig. 2. Comparison of UV spectra of EmimCl:AlCl3 with EmimCl:AlCl3 on application of various constant potentials across titanium electrodes.
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Table 1 Chemical shifts (ppm) in NMR spectra of 1 H for different solutions.
EmimCl EmimCl:AlCl3 (1:2) 1.5 V 2.0 V 2.5 V 3.0 V
H2
H4
H5
9.46 7.38 7.43 7.41 7.42 7.43
7.87 6.47 6.51 6.47 6.49 6.51
7.76 6.43 6.46 6.43 6.45 6.46
Table 2 Chemical shifts (ppm) in NMR spectra of 13 C for different solutions.
C2 C5 C4 N–CH2 N–CH3 CH3
EmimCl
EmimCl:AlCl3 (1:2)
1.5 V
2.0 V
2.5 V
3.0 V
136.91 123.99 122.44 44.52 36.13 15.63
133.39 123.28 121.43 44.56 36.05 14.6
133.49 123.29 121.62 44.85 36.19 14.47
133.52 123.22 121.56 44.79 36.16 14.46
133.51 123.26 121.60 44.83 36.17 14.46
133.51 123.28 121.61 44.84 36.18 14.47
Table 3 Chemical shifts (ppm) in NMR spectra of 27 Al for different solutions.
Al
EmimCl:AlCl3 (1:2)
1.5 V
2.0 V
2.5 V
3.0 V
99.8
97.37
97.22
97.34
97.52
When a potential was applied between the titanium electrodes, Ti dissolved into the ionic liquid forming various oxidation states. At 1.5 V, the UV spectra show two prominent peaks at 336 and 285 nm. The 336 nm could be attributed to Al3+ whereas the 285 nm is Ti4+ absorption [18]. On increasing the potential to 2.0 V we find that the Al3+ peak at around 335 is not present. However there are two peaks at 504 nm and 285 nm which corresponds to Ti2+ and Ti4+ absorption respectively [18,19]. The UV spectra of 2.5 and 3.0 V showed lot of noise and therefore were not analyzed. The NMR of 1 H, 12 C and 27 Al of EmimCl, EmimCl:AlCl3 and EmimCl:AlCl3 when potential was passed across the titanium electrodes are compared in Tables 1, 2 and 3. The structure of EmimCl is shown in Fig. 3 in which 1 and 3 position are occupied by ethyl and methyl groups, respectively. The NMR of 1 H shows a significant chemical shift at H2 when AlCl3 is added to EmimCl. There is also a smaller shift in H4 and H5 positions. When a potential is applied and titanium dissolves into the solution, there is a small positive shift in H2, H4 and H5. Similar observation is noted in the 13 C NMR in Table 2. The maximum shift occurs in C2 position compared to the shifts in C4 and C5 positions. There is also a small negative shift in the ethyl CH3 group on the addition of AlCl3 and titanium in the EmimCl. In the 27 Al NMR we find a significant negative shift when titanium dissolves into the EmimCl:AlCl3 melt. However on increasing the potential across the electrodes, i.e. when the concentration of titanium in the electrolyte increases, there is a slight positive shift. A discrepancy is observed
Fig. 3. Structure of EmimCl.
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 Al3 Ti alloy and the corresponding EDX in Fig. 4b shows the presence of Al and Ti peaks. The microstructure of Al3 Ti in Fig. 4a shows a nodular structure which is consistent with the results in the literature [5,11,20]. The semi quantitative analysis from the EDX showed that the ratio of Al to Ti was 75:25 at% which corresponds well with the formation of Al3 Ti alloy. Further proof of the Al3 Ti alloy was confirmed using X-ray diffraction analysis which showed a disordered aluminum structure. On calculating the lattice parameter, a 0.52% shift was observed as Ti atoms substitute for Al and agrees with the data presented in the literature [5].
4. Discussion Comparing the FTIR of EmimCl with EmimCl:AlCl3 (1:2), we find significant changes. There is a decrease in the peak intensities for aromatic and aliphatic bonds in the region between 3135 and 2865 cm−1 . Dieter et al. [21] identified that C2 hydrogen bond as shown in Fig. 3 lies at 3118 cm−1 whereas C4 and C5 will lie between 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 could arise due to the presence of AlCl3 which forms bonds with H and Cl− forming a C–H–Al2 Cl7 − type structure. We considered the presence of Al2 Cl7 − as we were working in the acidic medium (EmimCl:AlCl3 (1:2)). The Al–Cl bond is found to be more electronegative compared to that of the C–H bond [22] and therefore leads to the decrease in the intensity of aromatic and aliphatic bonds. There is also a small shift observed in the C–H bond at 3140 cm−1 which relates to the C4 and C5 positions in EmimCl to 3161 cm−1 on addition of AlCl3 in acidic region. It is interesting to observe that there is no C N bonds observed in EmimCl which could be due to the broad peak of ring stretching symmetry at 1564 cm−1 . As AlCl3 is added to EmimCl, the ring stretching symmetry splits to show the C N peak. The addition of AlCl3 also decreases the ring stretching symmetry at 1173 cm−1 . As electrolysis is performed at different voltages using titanium electrodes, there is a further decrease in the intensity of the aromatic, aliphatic and ring structure symmetry peaks. The decrease could be attributed to the formation of a complex between titanium and Al2 Cl7 − . However, there was no peak shift observed in C4 and C5 on the addition of titanium. A peak growth at 790 cm−1 is observed when a potential is applied through the electrolysis setup which varies with increase in the current and was identified to be C–H bending in methyl group. However, the NMR of 13 C of N–CH3 does not show a significant chemical shift on introducing titanium into the electrolyte from the anode. Thus, the peak growth could only be due to the presence of titanium and corresponded well with Ti3+ and Ti4+ oxidation states forming complexes with imidazole [15]. The random variation of the peak at 790 cm−1 indicates that the concentration of Ti3+ and Ti4+ ions does not depend on the potential applied. There could also be a possibility of sampling error as only one drop of sample is used during 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 intensity increases on the application of 1.5 and 2.0 V. However a decrease in the intensity is observed on the application of greater than 2.0 V. This suggests that there is a complex formed within the ionic liquid whose threshold lies around 2.0 V and is confirmed by the fact that greater amount of Al3 Ti deposits on application of 2.5 and 3.0 V. Thus, from the FTIR characterization it looks like Al2 Cl7 − bonds with
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Fig. 4. (a) Microstructure of Al3 Ti alloy deposited on the cathode when a potential of 2 V was applied across the titanium electrodes. (b) EDX showing the presence of both aluminum and titanium.
C2–H2 and alters the imidazole structure which then contributes to the shift in C4 and C5 bonds. The addition of titanium in the system further affects the C2–H2 structure. However, the effect is not that significant to show a shift in C4 and C5 which could be explained based on the oxidation state of Ti in the melt. From the UV–visible spectroscopy it is observed that EmimCl:AlCl3 contains one peak at 330 nm which could only be the Al3+ absorption[17]. When a potential was applied between the titanium electrodes, Ti dissolved into the ionic liquid forming various oxidation states. At 1.5 V, the UV spectra showed Al3+ and Ti4+ peaks, whereas on increasing the potential to 2.0 V we find peaks of Ti2+ and Ti4+ . Thus from the FTIR and UV–visible spectra analysis we can conclude that Ti anode dissolves into the ionic liquid in various oxidation states. As the thermodynamic stability of TiCl4 and TiCl3 is high, they exist in the ionic liquid as a separate phase. However Ti2+ ions which are unstable form an aluminum–titanium chloride complex as shown in Eq. (1). Although direct proof for the formation of the aluminum–titanium complex cannot be established, however charge balance technique show the possibility for the formation of [Ti (Al2 Cl7 )4 ]2− . Further proof of aluminum–titanium complex is established by the fact that when a potential is passed through the electrodes in presence of titanium, there is no deposition of aluminum or titanium on the cathode. It must also be noted that the solubility of TiCl4 in EmimCl:AlCl3 is insignificant [5,18] and therefore the production of Al3 Ti at the cathode must occur from the Al–Ti complex.
decreased. The decrease in the line broadening can take place by the formation of titanium aluminum complex as described in Eq. (1) or by forming AlCl4 − phase as described via Eq. (3).
Ti(Anode) + 4(Al2 Cl7 )− = [Ti(Al2 Cl7 )4 ]2− + 2e−
[Ti(Al2 Cl7 )4 ]2− + 2e− = Ti + 4(Al2 Cl7 )−
(4)
4(Al2 Cl7 )− + 3e− = Al + 7AlCl4 −
(5)
(1)
The formation of titanium aluminum complex was further clarified by performing NMR on the ionic liquid samples. Tables 1, 2 and 3 show the NMR of 1 H, 13 C and 27 Al, respectively. As C2 is located between two electronegative nitrogen atoms, it will have a low electron density. Therefore the AlCl3 will localize in the N1-C2-N3 region [23]. The negative shift in H2 corresponds well with the shift in C2 thus confirming the AlCl3 complex bonding in the C2 position. In acidic region, AlCl3 will be in Al2 Cl7 − phase from which aluminum can be deposited [24]. However we cannot neglect the presence of other aluminum species as there will be equilibrium between Al2 Cl7 − , AlCl3 and AlCl4 − via the reaction shown in Eq. (2). AlCl3 + AlCl4 − = Al2 Cl7 −
(2)
From 27 Al NMR studies it was observed that there was line broadening when the ratio of AlCl3 :EmimCl was greater than one. This is because of the presence of Al2 Cl7 − which does not have a tetrahedral symmetry [25]. However when potential was applied and titanium dissolved into the ionic liquid, the line broadening
Al2 Cl7 − + TiCl4 + e− = 2AlCl4 − + TiCl3
(3)
However, if there is formation of AlCl4 − , the chemical shift will be positive [25]. Therefore the negative shift in the 27 Al NMR can only be due to the complex formation of titanium aluminum chloride as shown in Eq. (1). On contrary, as the potential is increased across the titanium electrodes there is a slight positive increase in the chemical shift. There is some discrepancy in 1.5 V shift which might be due to the presence of only Ti4+ ion as evident from the UV–vis spectra in Fig. 2. Although UV–vis spectra did not show any Ti3+ oxidation state, FTIR showed a peak at 790 cm−1 which corresponded well with the formation of titanium complex [15]. As TiCl3 is a stable structure, it forms a coating over the electrodes and reduces the overall reaction rate. The formation of TiCl3 coating has been confirmed previously [11]. On application of higher potential this layer can be decomposed which has also been shown previously [5]. This could be the reason that no deposition of Al3 Ti was observed when 1.5 V was applied across the electrodes. As the potential across the electrodes was increased, the TiCl3 layer decomposed and facilitated the formation of aluminum–titanium complex via Eq. (2) which then results in the formation of Al3 Ti maybe according to Eqs. (4) and (5).
As Al3 Ti deposits at the cathode, the formation of AlCl4 − increases in the ionic liquid which could be the reason for observing a positive shift on the application of higher potential. 5. Conclusions The electrolysis of titanium in the electrolyte of EmimCl:AlCl3 led to the formation of Al3 Ti alloy which was confirmed using SEMEDX techniques. It was observed that titanium considerably reduces the reaction rate, which was due to the formation of TiCl3 coating over the electrodes and TiCl4 in the electrolyte which was clarified using FTIR spectroscopy. The NMR and UV–visible spectroscopy techniques proved that titanium forms a complex with aluminum which results in the formation of Al3 Ti alloy.
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