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Electrodeposition of Al from a 1-butylpyrrolidine-AlCl3 ionic liquid
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Progress in Natural Science Materials International Progress in Natural Science: Materials International 25 (2015) 603–611 www.elsevier.com/locate/pnsmi www.sciencedirect.com
Original Research
Electrodeposition of Al from a 1-butylpyrrolidine-AlCl3 ionic liquid Giridhar Pulletikurthi1, Björn Bödecker1, Andriy Borodin1, Bernd Weidenfeller1, Frank Endresn,1 Institute of Electrochemistry, Clausthal University of Technology, Arnold-Sommerfeld-Str. 6, D-38678 Clausthal-Zellerfeld, Germany Received 10 September 2015; accepted 30 October 2015 Available online 31 December 2015
Abstract The addition of 1-butylpyrrolidine to AlCl3 results in the formation of an electrolyte that is suited to Al deposition. The feasibility of electrodepositing Al from the synthesized electrolyte was investigated. Several compositions of AlCl3 and 1-butylpyrrolidine were prepared for this purpose. These mixtures show a different phase behavior at various compositions of AlCl3 and 1-butylpyrrolidine. IR, Raman and NMR spectroscopy were employed to characterize the synthesized liquids. Among the prepared compositions, 1:1.2 mol ratio of 1-butylpyrrolidine: AlCl3 and the upper phase of 1:1.3 mol ratio of 1-butylpyrrolidine:AlCl3 were found to be suitable for Al electrodeposition at room temperature (RT). Uniform and thick ( mm thick) layers of Al were obtained on copper at RT. Al deposition occured from the cationic species of AlCl3 x Lyþ (where x r2, y ¼ 1–2, and L¼1-butylpyrrolidine) in this electrolyte. This behavior is contrary to the well investigated classic AlCl3 based ionic liquids, where the deposition of Al occurs mainly from anionic Al2 Cl7 ions. & 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of Chinese Materials Research Society. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Aluminium; Al cations; 1-butylpyrrolidine; Ionic liquid; Electrodeposition
1. Introduction The electrodeposition of aluminium has been extensively studied in ionic liquids composed of organic halides and aluminium halides. An attempt to electrodeposit aluminium from a mixture of tetraethyl ammonium bromide and aluminium bromide at 100 1C was reported as early as in 1928 [1]. Hurley and Wier electrodeposited aluminium and a few metals such as Cu and Zn from solutions composed of ethylpyridinium bromide and the respective metal chlorides [2,3]. These liquids are often called ‘‘first generation ionic liquids’’ [4–19] and in most cases microcrystalline aluminium was obtained from acidic AlCl3/1,3-dialkylimidazolium chlorides. The use of liquids with pyrrolidinium cations, however, leads to the deposition of nanocrystalline aluminium [20,21]. As a possible interpretation the interaction of the pyrrolidinium cations with the substrate leads to the deposition of nanocrystalline aluminium [22]. n
Corresponding author. Fax: þ49 5323 722460. E-mail address: [email protected] (F. Endres). 1 Tel.: þ49 5323 72 3141; fax: þ49 5323 72 99 3141. Peer review under responsibility of Chinese Materials Research Society.
These organic halides/aluminium halides mixtures are highly sensitive to moisture. From liquids with trifluoromethylsulfonate and bis(trifluoromethylsulfonyl)amide ions the electrodeposition also occurs from the anionic species of e.g. Al with Cl and TFSA , as revealed by NMR and Raman spectroscopic studies [23–25]. The major problem associated with anionic species of metal ions for electrodeposition is that during the reduction reaction, the anionic species move against the electrostatic field. To circumvent such problems, cationic complexes of Al containing neutral ligands are interesting for the electrodeposition, but they have been rarely reported in the literature [26,27]. The neutral ligands are e.g. composed of amines or ligands derived from tertiary amines. Abbott et al. investigated the Al deposition from cationic aluminium species [28]. Recently, it was reported that cationic complexes of AlCl3 form with 4-propylpyridine [29] and with dipropyl sulfide [30]. This kind of liquids allows Al deposition, in principle, under ambient conditions. In an effort to make an electrolyte that is suitable for electrodeposition of Al from cationic species, we have chosen 1-butylpyrrolidine instead of alkyl substituted pyridine or amides. In this paper, we report on the synthesis of an ionic liquid
http://dx.doi.org/10.1016/j.pnsc.2015.11.003 1002-0071/& 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of Chinese Materials Research Society. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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synthesized from AlCl3 and a neutral ligand 1-butylpyrrolidine for Al deposition. The characterization is done using thermal gravimetry, NMR and vibrational spectroscopy. 2. Experimental 1-butylpyrrolidine was purchased from SIGMA ALDRICH, Germany, in a purity of 98% and used without further purification. Aluminium chloride grains (99.99%) were procured from FLUKA, Germany. The ionic liquids were prepared by careful mixing 1-butylpyrrolidine and AlCl3 in the glove box. The reaction is exothermic and the weighed amount of AlCl3 grains was added in small quantities to avoid thermal decomposition. Various compositions were prepared ranging from 1.3:1 to 1:1.3 mol ratios of AlCl3 and 1butylpyrrolidine. A viscous rather gel-like solid was obtained at mol ratios of 1.3:1 and 1.2:1 of 1-butylpyrrolidine:AlCl3 with a dark brown/red color. Clear yellowish orange liquids were obtained from 1.1:1 to 1:1.1 mol ratios of 1-butylpyrrolidine:AlCl3. Clear and yellow liquids were obtained at 1:1.2 and 1:1.25 mol ratios of 1-butylpyrrolidine:AlCl3. However, a biphasic mixture was obtained at a mol ratio of 1:1.3 of 1-butylpyrrolidine:AlCl3 with a thin solid lower phase at the bottom and an upper clear yellow phase. Thermal gravimetric analysis was carried out with a TA Instruments TGA Q5000 V3.17 build 265 under nitrogen at a heating rate of 20 K/min. The FT-IR measurements were performed on a Bruker VERTEX 70FT-IR spectrometer. The instrument was equipped with an extension for measurements in the Far-IR region, which consists of a multilayer mylar beam splitter, a room temperature DLATGS detector with preamplifier and polyethylene (PE) windows for the internal optical path. The available spectral range for this configuration was between 30 cm 1 and 680 cm 1. Raman spectra were recorded with a Bruker Senterra Raman microscope using 50 objective with a laser excitation of 532 nm. Raman measurements were carried out with a Raman module FRA 106 (Nd:YAG laser, 1064 nm) attached to a Bruker IFS 66v interferometer. The samples were sealed under an argon atmosphere in glass tubes and data were recorded at RT. The 27 Al NMR spectra were recorded using a Bruker Avance III spectrometer equipped with 14.1 T and 600 MHz proton frequency. The spectra were externally referenced to AlCl3 in D2O at 0.0 ppm (chemical shift). The samples were placed in Wilmads NMR tubes with 5 mm diameter. The electrochemical measurements were carried out using a PARSTAT 2263 potentiostat/galvanostat controlled by PowerCV and PowerStep software. Copper substrates were used as working electrodes (WE). Al sheets and Al wires were used as counter and reference electrodes, respectively. Prior to use the WEs were cleaned in acetone and dried. High resolution SEM (Carl Zeiss DSM 982 Gemini) was employed to investigate the morphology of the deposited films. X-ray diffraction (XRD) patterns were recorded at RT using a PANalytical Empyrean Diffractometer (Cabinet no. 9430 060 03002) with CuKα radiation.
3. Results and discussion A comparison of TG curves is shown in Fig. 1. 1butylpyrrolidine evaporates slowly in the beginning with a major evaporation onset temperature of 50 1C. The onset of AlCl3 evaporation occurred at about 160 1C. When AlCl3 was added to 1-butylpyrrolidine the behavior changed. At 1:1 mol ratio the first weight loss was observed at about 80–100 1C. A plateau followed with an increased weight loss beginning at about 220 1C. At 1:1.1 mol ratio the first weight loss was observed at 150 1C followed by a steep increase at 250 1C. The beginning weight loss of 1:1.2 mol ratio occurred at about 200 1C, which was followed by a plateau and accelerated at the temperature of 250 1C. In order to investigate further the coordination of Al species in the liquids, 27Al NMR spectra were recorded. The NMR spectra of three mixtures with different concentrations of AlCl3 are shown in Fig. 2. The upper phase is analyzed in the case of 1:1.3 mol ratio of 1-butylpyrrolidine: AlCl3. At a composition of 1:1 two signals were observed, one is at 103.4 and the other
Fig. 1. Thermal gravimetry of 1-butylpyrrolidine, AlCl3, and their mixtures at various mol ratios under N2 gas at a heating rate of 20 K/min.
Fig. 2. RT.
27
Al NMR spectra of 1-butylpyrrolidine:AlCl3 at various mol ratios at
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one is at 108.3 ppm. The peak at 103.4 ppm is attributed to be AlCl4 , according to the literature, where it is found 103 ppm [31]. The peak at 108.3 ppm is due to cationic species like AlCl3 x Lyþ (where x¼ 1 2), possibly to [AlCl2(1butylpyrrolidine)2] þ . Only a single bell shaped peak was observed when the mol ratio of AlCl3 increased to 1.2 (or to 1.3). Furthermore, the peak became broader with an increase in the mol fraction of AlCl3. This line broadening is due to an increase in viscosity or due to a difference in the chemical environment around the ions in the mixtures. The peaks at about 108 ppm are assigned to complexes of Al with the neutral ligand. Coleman et al. characterized liquid coordination complexes formed from AlCl3 and several donor molecules such as urea, acetamide, and thiourea [31]. Their peaks at δ¼ 119 ppm and δ¼ 107.8 ppm were assigned to [AlCl2(L)2] þ , where L is thiourea and trioctylphosphine, respectively. The complexes formed from acetamide and AlCl3 showed the peaks at 101.1 and 73.6 ppm, corresponding to [AlCl2(Acet)2] þ and [AlCl2(Acet)] þ , respectively [28]. For details on 27Al NMR spectra of chloroaluminate ionic liquids, we refer to references [32,33]. Nara et al. allocated the chemical shifts at 79.3 and 73.9 ppm for 1-butyl-3-methylimidazolium chloride: AlCl3 at mol ratios of 1:1 and 1:2 to AlCl4 and Al2Cl7 , respectively [34]. Abbott et al. reported the NMR spectra of cationic complexes derived from AlCl3 and urea mixtures. They also outlined the effect of diluent on the resolution of the NMR spectra of these species [35]. Atwood reported the formation of cationic complexes of Al, Ga, and In etc. [26]. The complex formation involved the asymmetric cleavage of AlCl3 in the presence of Lewis bases like NH3 and pyridine, for example [AlCl2(NH3)4] þ [AlCl4] and [AlCl2(Py)4] þ [AlCl4] [36,37]. In the case of 1-butylpyrrolidine (1-bupy), the complexes formed should be of the form ([AlCl2(1bupy)x] þ [AlCl4] , where x¼ 1–2). Fang et al. also observed such AlCl3 complexes formed with 4-propylpyridine [29]. Cation–anion interaction of ionic liquids has been studied by Far-IR spectroscopy. Ludwig and co-workers used this technique to study the interactions between the ions of ILs [38]. Metal– chloride and metal–ligand bonds are observed in the regions between 100 and 500 cm 1. Clark and Williams investigated the Far-IR spectra of metal–halide complexes with pyridine and related ligands [39]. The weak bands in the region of 100– 150 cm 1 were assigned to metal–nitrogen vibrations. The metal– nitrogen–halide vibrational modes appeared in the regions between 200 and 300 cm 1 and halide–metal–halide bending vibrations below 200 cm 1. The Far-IR spectra of the neat liquid and its mixtures with AlCl3 are compared in Fig. 3. As seen, there were no bands in the regions between 100 and 450 cm 1 for 1butylpyrrolidine. Upon adding AlCl3 to the ligand, a few distinct peaks were noticed in the same region. These bands are attributed to the complexation of AlCl3 with 1-butylpyrrolidine. A sharp peak was observed in all the mixtures at 180 cm 1 along with a broad shoulder at 155 cm 1. These peaks are correlated with the bending modes of Cl–Al–Cl, the band at 180 cm 1 is associated with the Cl–Al–Cl bending mode as such a bending mode which is reported for the bending mode of AlCl4 . Furthermore, the peak
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Fig. 3. Far-IR spectra of 1-butylpyrrolidine and mixtures of 1-butylpyrrolidine: AlCl3.
intensity at 180 cm 1 decreases upon increasing the AlCl3 concentration. For example, the metal–nitrogen bond vibration for pyridine is between 200 and 290 cm 1, and these bands are strongly dependent on the ligand to which the nitrogen atom is bound [39]. Thus, a shift is expected for metal–nitrogen bond vibration for the 1-butylpyrrolidine ligand. The vibrational bands for AlCl4 were reported in the literature and these were observed at 170, 300, 355, and 475 cm 1 [40, 41]. Two clear peaks were observed here at 297 and 335 cm 1 for the spectra of 1.2:1 mol ratio of (1-butylpyrrolidine: AlCl3) and 1:1 mol ratio of (1butylpyrrolidine:AlCl3). The peak at 297 cm 1 is allocated to the antisymmetric stretching of Al–Cl and this peak was relatively unaffected by a change in the AlCl3 concentration. The intensity of the peak at 335 cm 1 was shifted to 331 cm 1 and also the peak intensity increased with an increasing amount of AlCl3 in the mixtures. Similarly, quite a strong peak at 418 cm 1 with a shoulder at 427 cm 1 is observed in all spectra. These peaks are related with cationic Al species. Furthermore, a clear and distinct new peak is observed at 385 cm 1 for 1:1.2 mol ratio of 1-butylpyrrolidine:AlCl3. In the case of a 2:1 liquid composed of AlCl3 and 1-butylpyridinium chloride two strong peaks at 334 and 386 cm 1 are observed in the IR spectra due to Al2Cl7 ion along with very strong peaks at 500 and 545 cm 1, however, the peaks at 500 and 545 cm 1 are not observed instead of that those at 490 and 525 cm 1. This clearly indicates that the species form at 1:1.2 mol ratio of 1-butylpyrrolidine:AlCl3 are different from mixtures at mol ratios of 1.1 and 1.2:1 of 1-butylpyrrolidine:AlCl3. Fig. 4a illustrates the IR absorption spectra of the mixtures with varying compositions of 1-butylpyrrolidine and AlCl3 in the spectral region of 450 to 1700 cm 1. Several peaks are observed between 1150 and 1400 cm 1 for 1-butylpyrrolidine, which are associated with the C–H bending modes of the pyrrolidine ring.The peaks originating from 1-butylpyrrolidine are assigned based on the values of alkyl substituted pyrrolidine vs. pyrrolidine from literature [42–44]. The intensities of these modes of vibrations of the neat ligand decrease upon the addition of AlCl3. The ring bending modes are observed at 580 and 620 cm 1. The ring stretching modes are observed
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Fig. 4. IR spectra of 1-butylpyrrolidine and 1-butylpyrrolidine:AlCl3 at various mol ratios (a) in the regions between 450 and 1700 cm 1 þ : ring bending modes, *: ring stretching modes, o: peaks due complexation of AlCl3 with ligand, X: peaks due to AlCl4 and (b) in the regions from 1700 to 3400 cm 1.
between 870 and 1110 cm 1. The rocking mode of C–H and N– H bending mode are noticed at 854 and 740 cm 1, respectively. The intensity of the C–H bending modes relative to the ligand drastically decreases on adding AlCl3. Furthermore, a new peak is observed at 844 cm 1 in the mixtures, compared to 855 cm 1 for the ligand. A similar observation is noticed at 902 cm 1 peak (ring breathing mode) for the ligand, which is shifted to 916 cm 1 after complexation with AlCl3. These new peaks (denoted by ‘O’ in Fig. 4a) are observed in the mid-IR spectra from 440 to 1050 cm 1 and at 1650 cm 1. The new peaks observed at 490, 522, 688, and 980 cm 1 are related to the tetrahedral symmetrical AlCl4 ion, as such modes of vibrations were reported in literature for the MAlCl4 (where M¼ alkali metal) at temperatures above 200 1C [40]. The discrepancies observed in the frequencies are attributed to the differences in electrostatic interactions where the alkali metal cation induces the perturbation of tetrahedral symmetry of the AlCl4 ion. For example, the IR frequencies of AlCl4 in 1- butylpyridinium chloride:AlCl3 (1:1 mol ratio) were observed at 490 and at 525 cm 1 by Gale et al. [45]. The ligand vibrations in 1-butylpyrrolidine/AlCl3 mixtures were observed between 630 and 1080 cm 1. In the literature quite a strong
and broad peak was observed for the Al2Cl7 ion at 545 cm 1 along with a strong peak at 500 cm 1 [45]. However, such strong peaks at these wavenumbers are not observed in the present work. Thus, the mid-IR spectra of the mixtures indicate the formation of AlCl4 species in the mixtures, but there is no hint for Al2Cl7 ions. Fig. 4b compares the IR spectra of 1-butylpyrrolidine and its mixtures with AlCl3 in the region between 1700 and 3400 cm 1. The aliphatic stretching bands of 1butylpyridinium chloride-AlCl3 were reported by Tait and Osteryoung [46]. The aliphatic C–H stretching frequencies can be observed between 2870 and 2970 cm 1 for the free ligand. Three clear peaks were observed for the free ligand and for the mixtures with AlCl3. The intensity of the peaks at 2873 and at 2929 cm 1 decreases with the addition of AlCl3 to 1butylpyrrolidine. The aliphatic stretching band at 2929 cm 1 is shifted to 2937 and the intensity of the peak at 2873 cm 1 also decreases with an increase in the AlCl3 concentration in the mixtures. The band at 2780 cm 1 originates from C–H bonds of N–C–H of the free ligand [47]. Furthermore, this peak starts to vanish upon increasing the AlCl3 amount in the mixtures. A significant change is also observed above 3150 cm 1. Quite a low intensity band is observed at 3153 cm 1 which is due to the N–H stretching of the free Lewis base. This stretching band is shifted to 3170 cm 1 with an increase in AlCl3 concentration in the mixtures, which also indicates an interaction of AlCl3 with the neutral ligand. A comparison of the Raman spectra of neat 1-butylpyrrolidine and its mixtures with AlCl3 below 1600 cm 1 is shown in Fig. 5a. In order to characterize the Raman spectra, comparisons are made with the peak positions of AlCl4 from literature [48]. The peaks originating from 1-butylpyrrolidine (black curve) are marked in Fig. 5a. The Raman vibrational frequencies for the AlCl4 and Al2Cl7 ions are reported in literature to occur at 126, 184, 351, and 484 cm 1 for AlCl4 and 102, 163, 315, and 434 cm 1 for Al2Cl7 ion. In our spectra we did not observe peaks at 102 and 315 cm 1. The peaks due to AlCl4 were observed approximately at 120, 178, and at 348 cm 1 for the mixture 1.2:1 mol ratio of 1-butylpyrrolidine:AlCl3. However, a shift to lower wavenumbers, which are denoted by ‘þ ’ at 114 and 158 cm 1 in the Figure, is observed for the mol ratios of 1:1 for 1-butylpyrrolidine:AlCl3 and of 1:1.2 mol ratio for 1-butylpyrrolidine:AlCl3. Furthermore, the peak due to AlCl4 at 348 cm 1 vanishes for 1:1.2 mol ratio of 1-butylpyrrolidine: AlCl3. A few new peaks are also observed for the mixtures which are denoted by asterisks in the Figure, probably due to cationic Al species. Fig. 5b shows a comparison of the Raman spectra of the neat ligand and its mixtures with AlCl3. Here also a few changes are observed in the C–H stretching bands of 1-butylpyrrolidine. The peaks at 2743 and 2783 cm 1 disappear upon complexation with AlCl3. In addition, the peak intensities are found to decrease for the mixtures when compared to the neat liquid. Thus, the Raman spectra of the mixtures further prove that the ILs are formed from the reaction/complexation of AlCl3 with 1-butylpyrrolidine. Taking together the results of the vibrational spectra we conclude at a minimum that these liquids contain a cationic Al species and tetrachloroaluminate anions, but there is no clear hint for [Al2Cl7] .
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Fig. 6. Comparison of Raman spectra of the neat ligand and its mixtures with AlCl3 at two different mol ratios between 230 and 330 cm 1.
mixtures at two different mol ratios of 1-butylpyrrolidine to AlCl3. From the peak fit analysis, the average coordination number (N) is obtained. The average coordination/solvation number of the ligands surrounding Al3 þ is calculated according to the following equation [49,50].
Fig. 5. (a) Raman spectra of 1-butylpyrrolidine and 1-butylpyrrolidine:AlCl3 at various mol ratios (a) below 1600 cm 1. þ : shift in the peaks due to AlCl4 , *: new peaks due complexation of AlCl3 with 1-butylpyrrolidine (b) between 1700 and 3400 cm 1.
Since the most significant changes in the peaks are observed in the regions between 230 and 330 cm 1 in neat 1butylpyrrolidine and its mixtures with AlCl3, we focused on this region to analyze the Raman spectra. The average coordination number for the species is determined from signals in this region. A clear peak is observed at 313 cm 1 in the neat ligand (black line), which is attributed to the out-of-plane ring vibration mode of the ligand. This peak is shifted to 300 cm 1 and also the peak intensity is found to decrease at this position upon complexation with AlCl3. The lowering of the intensity and a shift of the peak to 300 cm 1 results from the formation of a complex between the ligand and AlCl3. The Raman peaks of the neat ligand and its mixtures at 1.2:1 and 1:1.2 mol ratios in this region are shown in Fig. 6. In comparison, the Raman peak of the neat ligand occurs at 313 cm 1. Adding 1 mol of AlCl3 to the ligand, the intensity of the peak is decreased and a new peak appears at 300 cm 1. Increasing the AlCl3 concentration further, the intensity of the new peak is also changed. To obtain more information from the vibrational spectra, the Raman peaks are deconvoluted with Voigt functions. Fig. 7a and b shows the peak fits of the
where, aCO is the Raman integral intensity of the coordinated Al3 þ with ligand, aL is the Raman integral intensity of the neat ligand, nAlCl3 is the number moles of AlCl3 and nL is the number moles of the neat ligand. The number of the ligands surrounding Al3 þ is found to be 1.9 ( 2) for 1.2:1 mol ratio and 1.5 for 1:1.2 mol ratio of 1-butylpyrrolidine:AlCl3, respectively. The average coordination number of ligands around Al3 þ was found to decrease with an increase in AlCl3 concentration in the mixtures. From the vibrational spectral analysis the possible complexes formed may be of the form [AlCl2L2] þ , where L ¼ 1-butylpyrrolidine. Cyclic voltammetry is employed to evaluate the electrochemical behavior of the prepared mixtures. The cyclic voltammograms at various compositions of the mixtures are shown in Fig. 8. The CVs were carried out on Cu electrodes at a scan rate of 10 mV s 1 at RT. CVs were recorded for the mixtures that are liquids at RT. For the mixture 1:1.3 mol ratio of 1-butylpyrrolidine:AlCl3, the upper phase is taken to acquire the CV. The potential of the electrode is scanned initially in the negative direction from the open circuit potential to the chosen switching potential (vs. Al). The CVs of 1-butylpyrrolidine: AlCl3 for 1.3:1, 1.2:1, and 1:1 mol ratios are quite similar with a single reduction process with an onset of reduction at 0.2 V in the forward scan. No oxidation process is observed in the reverse scan. However, two reduction processes and two oxidation processes are observed in the CV for 1:1.2 mol ratio of 1-butylpyrrolidine:AlCl3. The reduction process, C1, is due to the alloying of Al with Cu and its corresponding oxidation process is observed at A1I. The reduction process at C2 is
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due to the formation of different Al(III) species which are not reducible in the investigated potential regime (ca. 1 V) at RT. The reduction and oxidation processes may be described as follows, where L¼ 1-butylpyrrolidine. Abbott et al. reported that the oxidation process of Al in AlCl3/urea favors the formation of a cationic complex [AlCl2L] þ [35]. [AlCl2L2] þ þ 3e -Al þ 2Cl þ 2L and Alþ 2 Cl þ 2L-2 [AlCl2L] þ þ 3e
Fig. 7. Voigt fit to the Raman spectrum of 1-butylpyrrolidine:AlCl3 (a) (1.2:1) mol ratio and (b) (1:1.2) mol ratio.
Fig. 8. Comparison of CVs at various mol ratios of 1-butylpyrrolidine:AlCl3 at RT and at a scan rate of 10 mV s 1.
attributed to the bulk deposition of Al on Cu and the oxidation process at A1 is associated to the dissolution of deposited Al on Cu. The bulk deposition of Al commences around 0.15 V (vs. Al). The absence of Al deposition from other mixtures is
To obtain thick Al deposits, bulk electrolysis was carried out at 1 V from 1:1.2 mol ratio of 1-butylpyrrolidine:AlCl3 for one hour at RT. The deposits were washed thoroughly in isopropanol to remove traces of the IL. A homogeneous, thick, and greyish Al layer is obtained. The XRD of the deposited Al on Cu is shown in Fig. 9a. The XRD pattern shows peaks related to Al (JCPDS no: 04-0787) and Cu (JCPDS no: 040836) only, as shown in Fig. 9a. The crystal size is found to be 50 nm using the Scherrer equation [51]. The SEM micrograph of an Al deposit on Cu is shown in Fig. 9b, the particles are agglomerated. The optical photograph of the deposit shows that the Cu substrate is covered by a greyish white Al layer after electrolysis (Fig. 9c). The CV of 1:1.3 mol ratio of 1-butylpyrrolidine:AlCl3 is depicted in Fig. 10. Two reduction processes, C1 and C2, are observed in the forward scan. The process at C1 is either related to under potential deposition (UPD) of Al or to an alloying of Al with Cu. The reduction process at C2 is clearly due to the deposition of Al on Cu. An oxidation wave is observed in the reverse scan. The process at A1 is correlated with the dissolution of Al on Cu. To obtain thick Al deposits, bulk electrolysis was carried out at –1.0 V from 1:1.3 mol ratio of 1-butylpyrrolidine:AlCl3 for one hour at RT. The deposits were washed in isopropanol to remove the remainders of the IL. A homogeneous and greyish white Al layer is obtained. The XRD of the deposited Al on Cu is shown in Fig. 11a. The XRD pattern shows peaks related to Al (JCPDS no: 04-0787) along with peaks of the Cu substrate (JCPDS no: 04-0836). The crystal size is found to be 20– 25 nm using the Scherrer equation. The SEM micrograph of the Al deposit on Cu is shown in Fig. 11b. Tiny particles are seen in the image at higher magnification. Fig. 11c shows that the Cu substrate is coated by a greyish deposit of Al after electrolysis. A mixture with 1:1.5 mol ratio of 1-butylpyrrolidine:AlCl3 was prepared. The mixture slowly gets solid after a few days and 10 vol% of toluene was added to this mixture to make it liquid. The corresponding CV of such a mixture at 10 mV s 1 at RT is depicted in Fig. 12. In this case, a clear reduction peak is observed due to bulk deposition of Al in the forward scan and its corresponding stripping peak is observed in the reverse scan. Furthermore, we did not observe any surface reduction process before the bulk deposition of Al from this mixture. To make Al deposits, bulk electrolysis was carried out at 1.0 V from 1:1.5 mol ratio of 1-butylpyrrolidine:AlCl3 þ toluene
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Fig. 9. (a) X-ray diffraction pattern of Al on Cu from 1-butylpyrrolidine:AlCl3 (1:1.2) mol ratio at 1.0 V for 1 h at RT, (b) SEM image of Al deposited on Cu from 1-butylpyrrolidine:AlCl3 (1:1.2) mol ratio at 1.0 V for 1 h at RT and (c) optical photograph Al deposit on Cu.
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Fig. 11. (a) X-ray diffraction pattern of Al deposited on Cu from the upper phase of 1-butylpyrrolidine:AlCl3 (1:1.3) mol ratio at 1.0 V for 1 h at RT, (b) SEM image of Al deposited on Cu from the upper phase of 1butylpyrrolidine:AlCl3 (1:1.3) mol ratio at 1.0 V for 1 h at RT and (c) optical photograph of Al deposit on Cu.
spherical particles are seen in the SEM image ( Fig. 13b). The photograph of the Al deposit on Cu is shown in Fig. 13c, a greyish Al is seen on Cu. 4. Conclusions
Fig. 10. Cyclic voltammogram of the upper phase of 1-butylpyrrolidine:AlCl3 (1:1.3) mol ratio at RT and ν¼ 10 mV s 1.
mixture for 1 h at RT. A homogeneous and greyish white Al layer is obtained. The XRD of as-deposited Al on Cu is shown in Fig. 13a. The XRD pattern shows that the peaks are related to Al (JCPDS no: 04- 0787) and Cu (JCPDS no: 04-0836). The crystal size is found to be 40 nm using the Scherrer equation. Small
Ionic liquids were made with 1-butylpyrrolidine and AlCl3. It was found that 1:1.2 mol ratio of 1-butylpyrrolidine:AlCl3 is suitable to deposit Al at RT. Agglomerated nanocrystalline Al deposits were obtained from 1:1.2 mol ratio of 1-butylpyrrolidine:AlCl3. A biphasic mixture is obtained at 1:1.3 mol ratio of 1-butylpyrrolidine:AlCl3, and Al is electrodeposited from the upper phase of this mixture. Nanocrystalline Al is obtained from the upper phase of 1:1.3 mol ratio of 1-butylpyrrolidine: AlCl3. Al deposits were obtained from 1:1.5 mol ratio of 1-butylpyrrolidine:AlCl3 and toluene mixtures at RT. Al electrodeposition occurs from cationic species of AlCl3. This is in contrary to conventional ILs where the deposition occurs mainly from anionic heptachloroaluminate species. It is expected that analogous mixtures can be made using various metal salts to synthesize ILs for electrodepositing metals from them.
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Bode, Institut für Anorganische Chemie, for IR and Raman spectroscopy measurements, and Dr. Jan C Namyslo, Institut für Organische Chemie, for NMR measurements. This project is funded by DFG proposal EN 370/27-1.
References
Fig. 12. Cyclic voltammogram of 1:1.5 mol ratio of 1-butylpyrrolidine: AlCl3 þ 10 vol% toluene on Cu at RT and ν ¼10 mV s 1.
Fig. 13. (a) XRD pattern of Al deposited on Cu from 1:1.5 mol ratio of -butylpyrrolidine:AlCl3 þ 10 vol% toluene at 1.0 V for 1 h at RT, (b) SEM image of Al deposited on Cu from 1:1.5 mol ratio of 1-butylpyrrolidine: AlCl3 þ10 vol% toluene at 1.0 V for 1 h at RT, and (c) optical photograph of Al deposit on Cu.
Acknowledgements The Authors would like to thank Mrs. Sylvia Löffelholz, Institut für Elektrochemie, for SEM measurements, Mrs. Karin
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Progress in Natural Science Materials International Progress in Natural Science: Materials International 25 (2015) 603–611 www.elsevier.com/locate/pnsmi www.sciencedirect.com
Original Research
Electrodeposition of Al from a 1-butylpyrrolidine-AlCl3 ionic liquid Giridhar Pulletikurthi1, Björn Bödecker1, Andriy Borodin1, Bernd Weidenfeller1, Frank Endresn,1 Institute of Electrochemistry, Clausthal University of Technology, Arnold-Sommerfeld-Str. 6, D-38678 Clausthal-Zellerfeld, Germany Received 10 September 2015; accepted 30 October 2015 Available online 31 December 2015
Abstract The addition of 1-butylpyrrolidine to AlCl3 results in the formation of an electrolyte that is suited to Al deposition. The feasibility of electrodepositing Al from the synthesized electrolyte was investigated. Several compositions of AlCl3 and 1-butylpyrrolidine were prepared for this purpose. These mixtures show a different phase behavior at various compositions of AlCl3 and 1-butylpyrrolidine. IR, Raman and NMR spectroscopy were employed to characterize the synthesized liquids. Among the prepared compositions, 1:1.2 mol ratio of 1-butylpyrrolidine: AlCl3 and the upper phase of 1:1.3 mol ratio of 1-butylpyrrolidine:AlCl3 were found to be suitable for Al electrodeposition at room temperature (RT). Uniform and thick ( mm thick) layers of Al were obtained on copper at RT. Al deposition occured from the cationic species of AlCl3 x Lyþ (where x r2, y ¼ 1–2, and L¼1-butylpyrrolidine) in this electrolyte. This behavior is contrary to the well investigated classic AlCl3 based ionic liquids, where the deposition of Al occurs mainly from anionic Al2 Cl7 ions. & 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of Chinese Materials Research Society. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Aluminium; Al cations; 1-butylpyrrolidine; Ionic liquid; Electrodeposition
1. Introduction The electrodeposition of aluminium has been extensively studied in ionic liquids composed of organic halides and aluminium halides. An attempt to electrodeposit aluminium from a mixture of tetraethyl ammonium bromide and aluminium bromide at 100 1C was reported as early as in 1928 [1]. Hurley and Wier electrodeposited aluminium and a few metals such as Cu and Zn from solutions composed of ethylpyridinium bromide and the respective metal chlorides [2,3]. These liquids are often called ‘‘first generation ionic liquids’’ [4–19] and in most cases microcrystalline aluminium was obtained from acidic AlCl3/1,3-dialkylimidazolium chlorides. The use of liquids with pyrrolidinium cations, however, leads to the deposition of nanocrystalline aluminium [20,21]. As a possible interpretation the interaction of the pyrrolidinium cations with the substrate leads to the deposition of nanocrystalline aluminium [22]. n
Corresponding author. Fax: þ49 5323 722460. E-mail address: [email protected] (F. Endres). 1 Tel.: þ49 5323 72 3141; fax: þ49 5323 72 99 3141. Peer review under responsibility of Chinese Materials Research Society.
These organic halides/aluminium halides mixtures are highly sensitive to moisture. From liquids with trifluoromethylsulfonate and bis(trifluoromethylsulfonyl)amide ions the electrodeposition also occurs from the anionic species of e.g. Al with Cl and TFSA , as revealed by NMR and Raman spectroscopic studies [23–25]. The major problem associated with anionic species of metal ions for electrodeposition is that during the reduction reaction, the anionic species move against the electrostatic field. To circumvent such problems, cationic complexes of Al containing neutral ligands are interesting for the electrodeposition, but they have been rarely reported in the literature [26,27]. The neutral ligands are e.g. composed of amines or ligands derived from tertiary amines. Abbott et al. investigated the Al deposition from cationic aluminium species [28]. Recently, it was reported that cationic complexes of AlCl3 form with 4-propylpyridine [29] and with dipropyl sulfide [30]. This kind of liquids allows Al deposition, in principle, under ambient conditions. In an effort to make an electrolyte that is suitable for electrodeposition of Al from cationic species, we have chosen 1-butylpyrrolidine instead of alkyl substituted pyridine or amides. In this paper, we report on the synthesis of an ionic liquid
http://dx.doi.org/10.1016/j.pnsc.2015.11.003 1002-0071/& 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of Chinese Materials Research Society. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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synthesized from AlCl3 and a neutral ligand 1-butylpyrrolidine for Al deposition. The characterization is done using thermal gravimetry, NMR and vibrational spectroscopy. 2. Experimental 1-butylpyrrolidine was purchased from SIGMA ALDRICH, Germany, in a purity of 98% and used without further purification. Aluminium chloride grains (99.99%) were procured from FLUKA, Germany. The ionic liquids were prepared by careful mixing 1-butylpyrrolidine and AlCl3 in the glove box. The reaction is exothermic and the weighed amount of AlCl3 grains was added in small quantities to avoid thermal decomposition. Various compositions were prepared ranging from 1.3:1 to 1:1.3 mol ratios of AlCl3 and 1butylpyrrolidine. A viscous rather gel-like solid was obtained at mol ratios of 1.3:1 and 1.2:1 of 1-butylpyrrolidine:AlCl3 with a dark brown/red color. Clear yellowish orange liquids were obtained from 1.1:1 to 1:1.1 mol ratios of 1-butylpyrrolidine:AlCl3. Clear and yellow liquids were obtained at 1:1.2 and 1:1.25 mol ratios of 1-butylpyrrolidine:AlCl3. However, a biphasic mixture was obtained at a mol ratio of 1:1.3 of 1-butylpyrrolidine:AlCl3 with a thin solid lower phase at the bottom and an upper clear yellow phase. Thermal gravimetric analysis was carried out with a TA Instruments TGA Q5000 V3.17 build 265 under nitrogen at a heating rate of 20 K/min. The FT-IR measurements were performed on a Bruker VERTEX 70FT-IR spectrometer. The instrument was equipped with an extension for measurements in the Far-IR region, which consists of a multilayer mylar beam splitter, a room temperature DLATGS detector with preamplifier and polyethylene (PE) windows for the internal optical path. The available spectral range for this configuration was between 30 cm 1 and 680 cm 1. Raman spectra were recorded with a Bruker Senterra Raman microscope using 50 objective with a laser excitation of 532 nm. Raman measurements were carried out with a Raman module FRA 106 (Nd:YAG laser, 1064 nm) attached to a Bruker IFS 66v interferometer. The samples were sealed under an argon atmosphere in glass tubes and data were recorded at RT. The 27 Al NMR spectra were recorded using a Bruker Avance III spectrometer equipped with 14.1 T and 600 MHz proton frequency. The spectra were externally referenced to AlCl3 in D2O at 0.0 ppm (chemical shift). The samples were placed in Wilmads NMR tubes with 5 mm diameter. The electrochemical measurements were carried out using a PARSTAT 2263 potentiostat/galvanostat controlled by PowerCV and PowerStep software. Copper substrates were used as working electrodes (WE). Al sheets and Al wires were used as counter and reference electrodes, respectively. Prior to use the WEs were cleaned in acetone and dried. High resolution SEM (Carl Zeiss DSM 982 Gemini) was employed to investigate the morphology of the deposited films. X-ray diffraction (XRD) patterns were recorded at RT using a PANalytical Empyrean Diffractometer (Cabinet no. 9430 060 03002) with CuKα radiation.
3. Results and discussion A comparison of TG curves is shown in Fig. 1. 1butylpyrrolidine evaporates slowly in the beginning with a major evaporation onset temperature of 50 1C. The onset of AlCl3 evaporation occurred at about 160 1C. When AlCl3 was added to 1-butylpyrrolidine the behavior changed. At 1:1 mol ratio the first weight loss was observed at about 80–100 1C. A plateau followed with an increased weight loss beginning at about 220 1C. At 1:1.1 mol ratio the first weight loss was observed at 150 1C followed by a steep increase at 250 1C. The beginning weight loss of 1:1.2 mol ratio occurred at about 200 1C, which was followed by a plateau and accelerated at the temperature of 250 1C. In order to investigate further the coordination of Al species in the liquids, 27Al NMR spectra were recorded. The NMR spectra of three mixtures with different concentrations of AlCl3 are shown in Fig. 2. The upper phase is analyzed in the case of 1:1.3 mol ratio of 1-butylpyrrolidine: AlCl3. At a composition of 1:1 two signals were observed, one is at 103.4 and the other
Fig. 1. Thermal gravimetry of 1-butylpyrrolidine, AlCl3, and their mixtures at various mol ratios under N2 gas at a heating rate of 20 K/min.
Fig. 2. RT.
27
Al NMR spectra of 1-butylpyrrolidine:AlCl3 at various mol ratios at
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one is at 108.3 ppm. The peak at 103.4 ppm is attributed to be AlCl4 , according to the literature, where it is found 103 ppm [31]. The peak at 108.3 ppm is due to cationic species like AlCl3 x Lyþ (where x¼ 1 2), possibly to [AlCl2(1butylpyrrolidine)2] þ . Only a single bell shaped peak was observed when the mol ratio of AlCl3 increased to 1.2 (or to 1.3). Furthermore, the peak became broader with an increase in the mol fraction of AlCl3. This line broadening is due to an increase in viscosity or due to a difference in the chemical environment around the ions in the mixtures. The peaks at about 108 ppm are assigned to complexes of Al with the neutral ligand. Coleman et al. characterized liquid coordination complexes formed from AlCl3 and several donor molecules such as urea, acetamide, and thiourea [31]. Their peaks at δ¼ 119 ppm and δ¼ 107.8 ppm were assigned to [AlCl2(L)2] þ , where L is thiourea and trioctylphosphine, respectively. The complexes formed from acetamide and AlCl3 showed the peaks at 101.1 and 73.6 ppm, corresponding to [AlCl2(Acet)2] þ and [AlCl2(Acet)] þ , respectively [28]. For details on 27Al NMR spectra of chloroaluminate ionic liquids, we refer to references [32,33]. Nara et al. allocated the chemical shifts at 79.3 and 73.9 ppm for 1-butyl-3-methylimidazolium chloride: AlCl3 at mol ratios of 1:1 and 1:2 to AlCl4 and Al2Cl7 , respectively [34]. Abbott et al. reported the NMR spectra of cationic complexes derived from AlCl3 and urea mixtures. They also outlined the effect of diluent on the resolution of the NMR spectra of these species [35]. Atwood reported the formation of cationic complexes of Al, Ga, and In etc. [26]. The complex formation involved the asymmetric cleavage of AlCl3 in the presence of Lewis bases like NH3 and pyridine, for example [AlCl2(NH3)4] þ [AlCl4] and [AlCl2(Py)4] þ [AlCl4] [36,37]. In the case of 1-butylpyrrolidine (1-bupy), the complexes formed should be of the form ([AlCl2(1bupy)x] þ [AlCl4] , where x¼ 1–2). Fang et al. also observed such AlCl3 complexes formed with 4-propylpyridine [29]. Cation–anion interaction of ionic liquids has been studied by Far-IR spectroscopy. Ludwig and co-workers used this technique to study the interactions between the ions of ILs [38]. Metal– chloride and metal–ligand bonds are observed in the regions between 100 and 500 cm 1. Clark and Williams investigated the Far-IR spectra of metal–halide complexes with pyridine and related ligands [39]. The weak bands in the region of 100– 150 cm 1 were assigned to metal–nitrogen vibrations. The metal– nitrogen–halide vibrational modes appeared in the regions between 200 and 300 cm 1 and halide–metal–halide bending vibrations below 200 cm 1. The Far-IR spectra of the neat liquid and its mixtures with AlCl3 are compared in Fig. 3. As seen, there were no bands in the regions between 100 and 450 cm 1 for 1butylpyrrolidine. Upon adding AlCl3 to the ligand, a few distinct peaks were noticed in the same region. These bands are attributed to the complexation of AlCl3 with 1-butylpyrrolidine. A sharp peak was observed in all the mixtures at 180 cm 1 along with a broad shoulder at 155 cm 1. These peaks are correlated with the bending modes of Cl–Al–Cl, the band at 180 cm 1 is associated with the Cl–Al–Cl bending mode as such a bending mode which is reported for the bending mode of AlCl4 . Furthermore, the peak
605
Fig. 3. Far-IR spectra of 1-butylpyrrolidine and mixtures of 1-butylpyrrolidine: AlCl3.
intensity at 180 cm 1 decreases upon increasing the AlCl3 concentration. For example, the metal–nitrogen bond vibration for pyridine is between 200 and 290 cm 1, and these bands are strongly dependent on the ligand to which the nitrogen atom is bound [39]. Thus, a shift is expected for metal–nitrogen bond vibration for the 1-butylpyrrolidine ligand. The vibrational bands for AlCl4 were reported in the literature and these were observed at 170, 300, 355, and 475 cm 1 [40, 41]. Two clear peaks were observed here at 297 and 335 cm 1 for the spectra of 1.2:1 mol ratio of (1-butylpyrrolidine: AlCl3) and 1:1 mol ratio of (1butylpyrrolidine:AlCl3). The peak at 297 cm 1 is allocated to the antisymmetric stretching of Al–Cl and this peak was relatively unaffected by a change in the AlCl3 concentration. The intensity of the peak at 335 cm 1 was shifted to 331 cm 1 and also the peak intensity increased with an increasing amount of AlCl3 in the mixtures. Similarly, quite a strong peak at 418 cm 1 with a shoulder at 427 cm 1 is observed in all spectra. These peaks are related with cationic Al species. Furthermore, a clear and distinct new peak is observed at 385 cm 1 for 1:1.2 mol ratio of 1-butylpyrrolidine:AlCl3. In the case of a 2:1 liquid composed of AlCl3 and 1-butylpyridinium chloride two strong peaks at 334 and 386 cm 1 are observed in the IR spectra due to Al2Cl7 ion along with very strong peaks at 500 and 545 cm 1, however, the peaks at 500 and 545 cm 1 are not observed instead of that those at 490 and 525 cm 1. This clearly indicates that the species form at 1:1.2 mol ratio of 1-butylpyrrolidine:AlCl3 are different from mixtures at mol ratios of 1.1 and 1.2:1 of 1-butylpyrrolidine:AlCl3. Fig. 4a illustrates the IR absorption spectra of the mixtures with varying compositions of 1-butylpyrrolidine and AlCl3 in the spectral region of 450 to 1700 cm 1. Several peaks are observed between 1150 and 1400 cm 1 for 1-butylpyrrolidine, which are associated with the C–H bending modes of the pyrrolidine ring.The peaks originating from 1-butylpyrrolidine are assigned based on the values of alkyl substituted pyrrolidine vs. pyrrolidine from literature [42–44]. The intensities of these modes of vibrations of the neat ligand decrease upon the addition of AlCl3. The ring bending modes are observed at 580 and 620 cm 1. The ring stretching modes are observed
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Fig. 4. IR spectra of 1-butylpyrrolidine and 1-butylpyrrolidine:AlCl3 at various mol ratios (a) in the regions between 450 and 1700 cm 1 þ : ring bending modes, *: ring stretching modes, o: peaks due complexation of AlCl3 with ligand, X: peaks due to AlCl4 and (b) in the regions from 1700 to 3400 cm 1.
between 870 and 1110 cm 1. The rocking mode of C–H and N– H bending mode are noticed at 854 and 740 cm 1, respectively. The intensity of the C–H bending modes relative to the ligand drastically decreases on adding AlCl3. Furthermore, a new peak is observed at 844 cm 1 in the mixtures, compared to 855 cm 1 for the ligand. A similar observation is noticed at 902 cm 1 peak (ring breathing mode) for the ligand, which is shifted to 916 cm 1 after complexation with AlCl3. These new peaks (denoted by ‘O’ in Fig. 4a) are observed in the mid-IR spectra from 440 to 1050 cm 1 and at 1650 cm 1. The new peaks observed at 490, 522, 688, and 980 cm 1 are related to the tetrahedral symmetrical AlCl4 ion, as such modes of vibrations were reported in literature for the MAlCl4 (where M¼ alkali metal) at temperatures above 200 1C [40]. The discrepancies observed in the frequencies are attributed to the differences in electrostatic interactions where the alkali metal cation induces the perturbation of tetrahedral symmetry of the AlCl4 ion. For example, the IR frequencies of AlCl4 in 1- butylpyridinium chloride:AlCl3 (1:1 mol ratio) were observed at 490 and at 525 cm 1 by Gale et al. [45]. The ligand vibrations in 1-butylpyrrolidine/AlCl3 mixtures were observed between 630 and 1080 cm 1. In the literature quite a strong
and broad peak was observed for the Al2Cl7 ion at 545 cm 1 along with a strong peak at 500 cm 1 [45]. However, such strong peaks at these wavenumbers are not observed in the present work. Thus, the mid-IR spectra of the mixtures indicate the formation of AlCl4 species in the mixtures, but there is no hint for Al2Cl7 ions. Fig. 4b compares the IR spectra of 1-butylpyrrolidine and its mixtures with AlCl3 in the region between 1700 and 3400 cm 1. The aliphatic stretching bands of 1butylpyridinium chloride-AlCl3 were reported by Tait and Osteryoung [46]. The aliphatic C–H stretching frequencies can be observed between 2870 and 2970 cm 1 for the free ligand. Three clear peaks were observed for the free ligand and for the mixtures with AlCl3. The intensity of the peaks at 2873 and at 2929 cm 1 decreases with the addition of AlCl3 to 1butylpyrrolidine. The aliphatic stretching band at 2929 cm 1 is shifted to 2937 and the intensity of the peak at 2873 cm 1 also decreases with an increase in the AlCl3 concentration in the mixtures. The band at 2780 cm 1 originates from C–H bonds of N–C–H of the free ligand [47]. Furthermore, this peak starts to vanish upon increasing the AlCl3 amount in the mixtures. A significant change is also observed above 3150 cm 1. Quite a low intensity band is observed at 3153 cm 1 which is due to the N–H stretching of the free Lewis base. This stretching band is shifted to 3170 cm 1 with an increase in AlCl3 concentration in the mixtures, which also indicates an interaction of AlCl3 with the neutral ligand. A comparison of the Raman spectra of neat 1-butylpyrrolidine and its mixtures with AlCl3 below 1600 cm 1 is shown in Fig. 5a. In order to characterize the Raman spectra, comparisons are made with the peak positions of AlCl4 from literature [48]. The peaks originating from 1-butylpyrrolidine (black curve) are marked in Fig. 5a. The Raman vibrational frequencies for the AlCl4 and Al2Cl7 ions are reported in literature to occur at 126, 184, 351, and 484 cm 1 for AlCl4 and 102, 163, 315, and 434 cm 1 for Al2Cl7 ion. In our spectra we did not observe peaks at 102 and 315 cm 1. The peaks due to AlCl4 were observed approximately at 120, 178, and at 348 cm 1 for the mixture 1.2:1 mol ratio of 1-butylpyrrolidine:AlCl3. However, a shift to lower wavenumbers, which are denoted by ‘þ ’ at 114 and 158 cm 1 in the Figure, is observed for the mol ratios of 1:1 for 1-butylpyrrolidine:AlCl3 and of 1:1.2 mol ratio for 1-butylpyrrolidine:AlCl3. Furthermore, the peak due to AlCl4 at 348 cm 1 vanishes for 1:1.2 mol ratio of 1-butylpyrrolidine: AlCl3. A few new peaks are also observed for the mixtures which are denoted by asterisks in the Figure, probably due to cationic Al species. Fig. 5b shows a comparison of the Raman spectra of the neat ligand and its mixtures with AlCl3. Here also a few changes are observed in the C–H stretching bands of 1-butylpyrrolidine. The peaks at 2743 and 2783 cm 1 disappear upon complexation with AlCl3. In addition, the peak intensities are found to decrease for the mixtures when compared to the neat liquid. Thus, the Raman spectra of the mixtures further prove that the ILs are formed from the reaction/complexation of AlCl3 with 1-butylpyrrolidine. Taking together the results of the vibrational spectra we conclude at a minimum that these liquids contain a cationic Al species and tetrachloroaluminate anions, but there is no clear hint for [Al2Cl7] .
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Fig. 6. Comparison of Raman spectra of the neat ligand and its mixtures with AlCl3 at two different mol ratios between 230 and 330 cm 1.
mixtures at two different mol ratios of 1-butylpyrrolidine to AlCl3. From the peak fit analysis, the average coordination number (N) is obtained. The average coordination/solvation number of the ligands surrounding Al3 þ is calculated according to the following equation [49,50].
Fig. 5. (a) Raman spectra of 1-butylpyrrolidine and 1-butylpyrrolidine:AlCl3 at various mol ratios (a) below 1600 cm 1. þ : shift in the peaks due to AlCl4 , *: new peaks due complexation of AlCl3 with 1-butylpyrrolidine (b) between 1700 and 3400 cm 1.
Since the most significant changes in the peaks are observed in the regions between 230 and 330 cm 1 in neat 1butylpyrrolidine and its mixtures with AlCl3, we focused on this region to analyze the Raman spectra. The average coordination number for the species is determined from signals in this region. A clear peak is observed at 313 cm 1 in the neat ligand (black line), which is attributed to the out-of-plane ring vibration mode of the ligand. This peak is shifted to 300 cm 1 and also the peak intensity is found to decrease at this position upon complexation with AlCl3. The lowering of the intensity and a shift of the peak to 300 cm 1 results from the formation of a complex between the ligand and AlCl3. The Raman peaks of the neat ligand and its mixtures at 1.2:1 and 1:1.2 mol ratios in this region are shown in Fig. 6. In comparison, the Raman peak of the neat ligand occurs at 313 cm 1. Adding 1 mol of AlCl3 to the ligand, the intensity of the peak is decreased and a new peak appears at 300 cm 1. Increasing the AlCl3 concentration further, the intensity of the new peak is also changed. To obtain more information from the vibrational spectra, the Raman peaks are deconvoluted with Voigt functions. Fig. 7a and b shows the peak fits of the
where, aCO is the Raman integral intensity of the coordinated Al3 þ with ligand, aL is the Raman integral intensity of the neat ligand, nAlCl3 is the number moles of AlCl3 and nL is the number moles of the neat ligand. The number of the ligands surrounding Al3 þ is found to be 1.9 ( 2) for 1.2:1 mol ratio and 1.5 for 1:1.2 mol ratio of 1-butylpyrrolidine:AlCl3, respectively. The average coordination number of ligands around Al3 þ was found to decrease with an increase in AlCl3 concentration in the mixtures. From the vibrational spectral analysis the possible complexes formed may be of the form [AlCl2L2] þ , where L ¼ 1-butylpyrrolidine. Cyclic voltammetry is employed to evaluate the electrochemical behavior of the prepared mixtures. The cyclic voltammograms at various compositions of the mixtures are shown in Fig. 8. The CVs were carried out on Cu electrodes at a scan rate of 10 mV s 1 at RT. CVs were recorded for the mixtures that are liquids at RT. For the mixture 1:1.3 mol ratio of 1-butylpyrrolidine:AlCl3, the upper phase is taken to acquire the CV. The potential of the electrode is scanned initially in the negative direction from the open circuit potential to the chosen switching potential (vs. Al). The CVs of 1-butylpyrrolidine: AlCl3 for 1.3:1, 1.2:1, and 1:1 mol ratios are quite similar with a single reduction process with an onset of reduction at 0.2 V in the forward scan. No oxidation process is observed in the reverse scan. However, two reduction processes and two oxidation processes are observed in the CV for 1:1.2 mol ratio of 1-butylpyrrolidine:AlCl3. The reduction process, C1, is due to the alloying of Al with Cu and its corresponding oxidation process is observed at A1I. The reduction process at C2 is
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due to the formation of different Al(III) species which are not reducible in the investigated potential regime (ca. 1 V) at RT. The reduction and oxidation processes may be described as follows, where L¼ 1-butylpyrrolidine. Abbott et al. reported that the oxidation process of Al in AlCl3/urea favors the formation of a cationic complex [AlCl2L] þ [35]. [AlCl2L2] þ þ 3e -Al þ 2Cl þ 2L and Alþ 2 Cl þ 2L-2 [AlCl2L] þ þ 3e
Fig. 7. Voigt fit to the Raman spectrum of 1-butylpyrrolidine:AlCl3 (a) (1.2:1) mol ratio and (b) (1:1.2) mol ratio.
Fig. 8. Comparison of CVs at various mol ratios of 1-butylpyrrolidine:AlCl3 at RT and at a scan rate of 10 mV s 1.
attributed to the bulk deposition of Al on Cu and the oxidation process at A1 is associated to the dissolution of deposited Al on Cu. The bulk deposition of Al commences around 0.15 V (vs. Al). The absence of Al deposition from other mixtures is
To obtain thick Al deposits, bulk electrolysis was carried out at 1 V from 1:1.2 mol ratio of 1-butylpyrrolidine:AlCl3 for one hour at RT. The deposits were washed thoroughly in isopropanol to remove traces of the IL. A homogeneous, thick, and greyish Al layer is obtained. The XRD of the deposited Al on Cu is shown in Fig. 9a. The XRD pattern shows peaks related to Al (JCPDS no: 04-0787) and Cu (JCPDS no: 040836) only, as shown in Fig. 9a. The crystal size is found to be 50 nm using the Scherrer equation [51]. The SEM micrograph of an Al deposit on Cu is shown in Fig. 9b, the particles are agglomerated. The optical photograph of the deposit shows that the Cu substrate is covered by a greyish white Al layer after electrolysis (Fig. 9c). The CV of 1:1.3 mol ratio of 1-butylpyrrolidine:AlCl3 is depicted in Fig. 10. Two reduction processes, C1 and C2, are observed in the forward scan. The process at C1 is either related to under potential deposition (UPD) of Al or to an alloying of Al with Cu. The reduction process at C2 is clearly due to the deposition of Al on Cu. An oxidation wave is observed in the reverse scan. The process at A1 is correlated with the dissolution of Al on Cu. To obtain thick Al deposits, bulk electrolysis was carried out at –1.0 V from 1:1.3 mol ratio of 1-butylpyrrolidine:AlCl3 for one hour at RT. The deposits were washed in isopropanol to remove the remainders of the IL. A homogeneous and greyish white Al layer is obtained. The XRD of the deposited Al on Cu is shown in Fig. 11a. The XRD pattern shows peaks related to Al (JCPDS no: 04-0787) along with peaks of the Cu substrate (JCPDS no: 04-0836). The crystal size is found to be 20– 25 nm using the Scherrer equation. The SEM micrograph of the Al deposit on Cu is shown in Fig. 11b. Tiny particles are seen in the image at higher magnification. Fig. 11c shows that the Cu substrate is coated by a greyish deposit of Al after electrolysis. A mixture with 1:1.5 mol ratio of 1-butylpyrrolidine:AlCl3 was prepared. The mixture slowly gets solid after a few days and 10 vol% of toluene was added to this mixture to make it liquid. The corresponding CV of such a mixture at 10 mV s 1 at RT is depicted in Fig. 12. In this case, a clear reduction peak is observed due to bulk deposition of Al in the forward scan and its corresponding stripping peak is observed in the reverse scan. Furthermore, we did not observe any surface reduction process before the bulk deposition of Al from this mixture. To make Al deposits, bulk electrolysis was carried out at 1.0 V from 1:1.5 mol ratio of 1-butylpyrrolidine:AlCl3 þ toluene
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Fig. 9. (a) X-ray diffraction pattern of Al on Cu from 1-butylpyrrolidine:AlCl3 (1:1.2) mol ratio at 1.0 V for 1 h at RT, (b) SEM image of Al deposited on Cu from 1-butylpyrrolidine:AlCl3 (1:1.2) mol ratio at 1.0 V for 1 h at RT and (c) optical photograph Al deposit on Cu.
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Fig. 11. (a) X-ray diffraction pattern of Al deposited on Cu from the upper phase of 1-butylpyrrolidine:AlCl3 (1:1.3) mol ratio at 1.0 V for 1 h at RT, (b) SEM image of Al deposited on Cu from the upper phase of 1butylpyrrolidine:AlCl3 (1:1.3) mol ratio at 1.0 V for 1 h at RT and (c) optical photograph of Al deposit on Cu.
spherical particles are seen in the SEM image ( Fig. 13b). The photograph of the Al deposit on Cu is shown in Fig. 13c, a greyish Al is seen on Cu. 4. Conclusions
Fig. 10. Cyclic voltammogram of the upper phase of 1-butylpyrrolidine:AlCl3 (1:1.3) mol ratio at RT and ν¼ 10 mV s 1.
mixture for 1 h at RT. A homogeneous and greyish white Al layer is obtained. The XRD of as-deposited Al on Cu is shown in Fig. 13a. The XRD pattern shows that the peaks are related to Al (JCPDS no: 04- 0787) and Cu (JCPDS no: 04-0836). The crystal size is found to be 40 nm using the Scherrer equation. Small
Ionic liquids were made with 1-butylpyrrolidine and AlCl3. It was found that 1:1.2 mol ratio of 1-butylpyrrolidine:AlCl3 is suitable to deposit Al at RT. Agglomerated nanocrystalline Al deposits were obtained from 1:1.2 mol ratio of 1-butylpyrrolidine:AlCl3. A biphasic mixture is obtained at 1:1.3 mol ratio of 1-butylpyrrolidine:AlCl3, and Al is electrodeposited from the upper phase of this mixture. Nanocrystalline Al is obtained from the upper phase of 1:1.3 mol ratio of 1-butylpyrrolidine: AlCl3. Al deposits were obtained from 1:1.5 mol ratio of 1-butylpyrrolidine:AlCl3 and toluene mixtures at RT. Al electrodeposition occurs from cationic species of AlCl3. This is in contrary to conventional ILs where the deposition occurs mainly from anionic heptachloroaluminate species. It is expected that analogous mixtures can be made using various metal salts to synthesize ILs for electrodepositing metals from them.
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Bode, Institut für Anorganische Chemie, for IR and Raman spectroscopy measurements, and Dr. Jan C Namyslo, Institut für Organische Chemie, for NMR measurements. This project is funded by DFG proposal EN 370/27-1.
References
Fig. 12. Cyclic voltammogram of 1:1.5 mol ratio of 1-butylpyrrolidine: AlCl3 þ 10 vol% toluene on Cu at RT and ν ¼10 mV s 1.
Fig. 13. (a) XRD pattern of Al deposited on Cu from 1:1.5 mol ratio of -butylpyrrolidine:AlCl3 þ 10 vol% toluene at 1.0 V for 1 h at RT, (b) SEM image of Al deposited on Cu from 1:1.5 mol ratio of 1-butylpyrrolidine: AlCl3 þ10 vol% toluene at 1.0 V for 1 h at RT, and (c) optical photograph of Al deposit on Cu.
Acknowledgements The Authors would like to thank Mrs. Sylvia Löffelholz, Institut für Elektrochemie, for SEM measurements, Mrs. Karin
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