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The electrochemical behavior of titanium dioxide film in Lewis basic AlCl3-1-butyl-3-methylimidizolium ionic liquid
Electrochimica Acta 63 (2012) 197–203
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
The electrochemical behavior of titanium dioxide film in Lewis basic AlCl3 -1-butyl-3-methylimidizolium ionic liquid Xiao-Ying Zhang a,b , Yi-Xin Hua a,b,∗ , Cun-Ying Xu a,b , Nan Xu a,b , Hui Xue c a b c
Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, PR China State Key Laboratory Breeding Base of Complex Nonferrous Metal Resources Cleaning Utilization in Yunnan Province, Kunming 650093, PR China City College, Kunming University of Science and Technology, Kunming 650093, PR China
a r t i c l e
i n f o
Article history: Received 28 September 2011 Received in revised form 12 December 2011 Accepted 20 December 2011 Available online 27 December 2011 Keywords: Titanium dioxide film Ionic liquids Potentiodynamic polarization Cyclic voltammetry EIS
a b s t r a c t The electrochemical behavior of titanium dioxide film in Lewis basic AlCl3 -1-butyl-3-methylimidizolium ionic liquid (AlCl3 -BMIC) at room temperature was investigated by means of potentiodynamic polarization; cyclic voltammetry; sampled current voltammetry and electrochemical impedance spectroscopy (EIS). It was found that as the TiO2 film thickness increased, a cathode inhibition effect of the oxide film on the reduction process was strengthened, which means that a greater overpotential was required to initiate the reduction process. In the EIS test, the preferable equivalent circuit was made to fit the experimental data and fitted parameters were obtained. Additionally, increasing reaction temperature can make the cathode depolarization. In the end, the model for this reduction process has been given on the basis of the pictures of reduced TiO2 film and the electronic energy band structure models. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Due to its excellent properties, a predictable demand for titanium and its alloy is huge in many parts of society [1]. However, the disadvantages of currently used titanium production method, Kroll process, have restricted its extensive usage. Therefore, researchers around the world have never stopped the attempts to find a low-cost, continuous and simplified way for high purity titanium production. Different from conventional Kroll method, some emerging titanium production technologies [2–5] have given us an enlightenment of cost reduction. In our opinion, electrochemical reduction of TiO2 in low temperature may be a preferable method, at least an idea, to overcome the shortcomings of old ones. Accordingly, finding a preferable electrolyte is a key point for this purpose. The discovery of the new class of substance with low melting points, good conductivity and wide electrochemical window, named as room temperature ionic liquids (RTILs), has come into our insight. There have been several decades from the nativity of ionic liquids to now. As well known, ionic liquids have been used in many fields, such as synthesis, catalysis [6] and lubricant [7]. Especially, it is a wonderful
∗ Corresponding author at: Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, PR China. Tel.: +86 871 5162008; fax: +86 871 5161278. E-mail address: [email protected] (Y.-X. Hua). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.12.085
electrolyte for us to obtain many refractory metals, such as Al [8,9], Mg [10,11] and Li [12,13], at low temperature, even at room temperature, which cannot be achieved in aqueous solution. The research on titanium in ionic liquids is what we most concern about. In the early time, the application of RTILs in electro-deposition of titanium has been rarely reported. Due to its several different variable valencies, electro-deposition of titanium is more complicated than other common metals. However, much more efforts have been made by a number of researchers on the study in this excellent electrolyte. Linga et al. [14] have found that Ti(IV) exited as two electrochemically reducible species and one oxide complex in basic Bu(py)Cl. When the E1/2 of the first reduction wave was −0.343 V vs. Al, the species was assumed to be TiCl6 −2 . In the case of the second reduction wave of −0.77 V vs. Al, the species was assumed to be TiOCl4 −2 . Only one oxidation wave, presumably that of a Ti(III) chloro complex, was found. From the research of Sun et al. [15], Ti(IV) appeared to be complexed as TiBr6 −2 in the Lewis basic AlBr3 -MeEtimBr ionic liquid, which was in good agreement with the result of AlCl3 -MeEtimCl. Carlin et al. [16] have examined the electrochemistry of Ti(III) in Lewis acidic AlCl3 -EMIC. Their study implied that titanium was reduced to Ti(III) and Ti(II) through two slow one-electron steps. Moreover Ti(III), as a state of -TiCl3 , was formed as a brown layer on electrode without obtaining titanium. Ding et al. [17] reported the investigation of usage of polymeric and nucleating agent assist the electro-deposition of titanium. Other researches [18,19] on electro-deposition of titanium from TiCl4 in [BMIm][BTA] at room temperature demonstrated that a ultrathin
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titanium film could be formed on highly oriented pyrolytic graphite and Au(1 1 1) substrate. The investigation concluded that reduction of TiCl4 , which was first reduced to TiCl2 and subsequently to Ti, included two steps. During this process, the researchers obtained a brown film, probably the TiCl3 , which possibly inhibited the reduction. Coincidently, the study by Endres et al. [20] indicated that non-stoichiometric halides have been formed from [EMIm]Tf2 N, [BMP]Tf2 N and [P14,6,6,6 ]Tf2 N containing TiCl4 as a source of titanium. Different from the research mentioned above, however, they believed that it was impossible to reduce TiCl4 to elemental Ti or very difficult. The studies [21,22] on the electrochemistry of TiCl4 and TiF4 in BMMImN3 and BMIMBF4 also denoted that formation of some poorly soluble low valence intermediates kinetically hindered the reduction process. Besides, it is interestingly found that the potential difference of some metals becomes close in ionic liquids, which can assist the formation of alloy, such as Al–Ti [23]. In our previous study [24], it was tried to electro-chemically reduceTiO2 , and its feasibility has been successfully confirmed. Additionally, the preliminary investigation on the reduction mechanism was carried out. In this paper, the major purpose is to investigate the electrochemical behavior of TiO2 film in Lewis basic AlCl3 -BMIC and analyze its reduction mechanism in greater depth. As well known, the electrochemistry of TiO2 film in aqueous solution has been widely investigated [25–28]. In spite of the nonstoichiometric composition of TiO2 film thermally formed on Ti foil, the low interfacial resistance and ease of charge transfer can make the better electrochemistry results. Therefore this kind of TiO2 film was chose to characterize electrochemical behavior of TiO2 in ionic liquid by following techniques: potentiodynamic polarization; cyclic voltammetry; sampled current voltammetry and electrochemical impedance spectroscopy (EIS). 2. Experimental
Fig. 1. Cyclic voltammograms of titanium foils with different oxide film thickness (0.28, 0.56 and 0.67 m) in Lewis basic AlCl3 -BMIC vs. Ag obtained at a sweep rate of 50 mV/s at 343 K.
sweep voltammetry with a scan rate of 5 mV/s. The Tafel test was conducted at the same scan rate. In the sampled current voltammetry test, the voltage range was from −0.5 to −2.4 V with the sample period of 10 s and step voltage of 0.1 V. All the Ac impedance data of one oxidized Ti foil in AlCl3 -BMIC was obtained from −1 to −1.6 V, with Ac excitation amplitude of 5 mV. The scanning frequency ranged from 10 kHz down to 50 mHz. During all experiments the cell was purged by an argon flow to avoid the ingress of air. The surface area of working electrode was estimated from the depth of the electrode immersed in the electrolyte. In each test, a new oxidized Ti foil was used. All the electrochemical experiments were performed on a Gamry PCI4/300 electrochemical work station for the experimental control and data acquisition.
2.1. Chemicals 3. Results and discussion Anhydrous AlCl3 (99 wt.%) was used without further treatment. 1-butyl-3-methylimidizolium chloride (BMIC) was synthesized a procedure described in details elsewhere [29]. The electrolyte of Lewis basic ionic liquid AlCl3 -BMIC used in this paper was prepared by the slow addition of a certain weight of AlCl3 into BMIC in a mole ratio of 0.8:1.0 respectively, giving a mole fraction of AlCl3 of 0.44. 2.2. Preparation of Ti electrodes A titanium foil (commercial purity, 30 mm × 10 mm × 0.1 mm) was mechanically polished with a sequence of emery papers of different grades (400, 600 and 800), degreased with acetone in ultrasonic bath for 5 min, washed three times with distilled water, and dried at room temperature. Subsequently, the titanium foil was oxidized in a furnace at 823 K in air for different time (48, 96 and 144 h). Obviously, the more oxidation time, the thicker oxide film (0.28, 0.56 and 0.67 m). 2.3. Electrochemical experiments Electrochemical tests were performed in a standard threeelectrode electrochemical cell using the oxidized Ti foil mentioned above as the working electrode, a platinum wire as a counter electrode, and a silver wire as a reference electrode. Cyclic voltammetry, sampled current voltammetry, cathode polarization, electrochemical impedance spectroscopy and chronoamperometry were carried out to study on the electrochemistry of oxidized Ti foil in AlCl3 -BMIC. The scan rate in cyclic voltammetry was 50 mV/s and the first scan direction was negative. The cathode polarization curves were measured by linear
3.1. Influence of TiO2 film thickness on its electrochemical behavior In Fig. 1, three typical cyclic voltammograms of titanium electrode with different thickness oxide film (0.28, 0.56 and 0.67 m) in AlCl3 -BMIC melt at 343 K are illustrated. In the scan voltage range of 0.5 to −0.9 V, two reduction waves A, B were observed at negative direction and their possible anodic corresponding waves C at positive direction in each cyclic voltammogram, the peak currents of which was at nearly same positions. By the comparison of three cyclic voltammograms, however, reduction peak current decreased along with the thickness of TiO2 film increased. Increasing thickness of TiO2 film might enhanced the cathode polarization of this reduction process. Additionally, a relatively negative shift of the initial cathode potentials of reduction current can be found, due to the voltage loss across the oxide film. Increase of TiO2 film thickness, which made the charge transfer difficult, inhibited the reduction of TiO2 . In the precious paper [24], potentiodynamic polarization test can further confirm the results obtained above. The thicker is oxide film, the more negative is reduction potential. Alternatively, a thicker oxide film required a greater cathode overpotential to be reduced, which was resulted from the cathode inhibition effect of the oxide film on the reduction process. In Fig. 2, a negative shift of equilibrium potential can be observed from the Tafel curves. From Table 1, when TiO2 film became thicker, the exchange current density and cathode Tafel slope decreased along with an increasing linear polarization resistance. Obviously, direct electrochemical reduction of bulk TiO2 should need a greater driving force to be
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Table 1 Parameters for oxidized Ti foils (0.28, 0.56 and 0.67 m) obtained from Fig. 3. Film thickness (m)
Eeq (mV)
j0 (A cm−2 )
0.28 0.56 0.67
162 126 117
0.352 0.279 0.170
LPR (ohm cm2 )
104,071.2 140,513.5 441,382.5
Tafel slope ˇa (mV)
ˇc (mV)
288.6 463.6 221.5
120.4 112.1 105.2
completed can be found. This conclusion was in agreement with the cyclic voltammetry. In EIS test of oxidized Ti foil (0.67 m), it is clearly found that the charge resistance of the reduction decreased as the cathode overpotential increased (Fig. 3). In the high frequency region, the Nyquist plots at different cathode potentials all present one semicircle, when the charge transfer at the interface between the TiO2
film and the ionic liquid was the rate-limiting step. In contrast, the nearly linear relationship between Z and −Z in the low frequency region may be attributed to the mass transfer in the ionic liquid. Many equivalent circuits have been used by ZsimpWin software to simulate the Ti/TiO2 film/IL system, only the model shown in Fig. 5 fitted the experimental data best, and the fitting line in Fig. 4. Fitted results can be seen in Table 2. In this system (Fig. 5), Rfilm and
Fig. 2. Tafel curves in Lewis basic AlCl3 -BMIC at 343 K for oxidized Ti foils with different oxide film thicknesses (0.28, 0.56 and 0.67 m) at a sweep rate of 5 mV/s.
Fig. 3. Ac impedance spectra of oxidized Ti foil (0.67 m) in Lewis basic AlCl3 -BMIC at 343 K with a frequency range of 10 kHz to 0.05 Hz and a signal amplitude of 5 mV.
Fig. 4. Ac impedance experimental and approximation spectra diagrams for oxidized Ti foils (0.28 m (a), 0.56 m (b) and 0.67 m (c)) in Lewis basic AlCl3 -BMIC at 343 K with a cathode potential of −1.3 V.
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Table 2 EIS parameters from the equivalent circuit for the oxidized Ti foils (0.28, 0.56 and 0.67 m) at 343 K. Thickness (m)
Rfilm (ohm cm2 )
Rt (ohm cm2 )
CPEdl (F cm−2 )
Rs (ohm cm2 )
W/Y0 (Ss0.5 cm−2 )
2
0.28 0.56 0.67
16.09 19.67 24.53
3.12 3.07 3.19
19.40 14.24 21.48
1.43 2.88 1.49
0.146 0.377 0.07
2.61E−3 1.93E−3 1.97E−3
CPEfilm represent the resistance and capacity of oxide film respectively; Rt refers to the charge transfer resistance process occurring on the oxide film surface; CPEdl delegates the double capacity at the interface between the oxide film and tested IL; Rs can be explained to be the resistance of the tested IL; W can describe the diffusion control process at low frequency. As shown in Table 2, the increase of Rfilm is the direct evidence of the bodiness of oxide film, corresponding well to the result of potentiodynamic polarization measurement. 3.2. Influence of temperature on electrochemical behavior of TiO2 film For increasing the reduction rate of this process, the dependence of electrochemical behavior of TiO2 film on the reaction temperature was investigated. Cyclic voltammograms of titanium foil with oxide film (0.67 m) at different temperatures in AlCl3 BMIC are illustrated in Fig. 6. Similar with the former findings, the two reductions A, B and one oxidation waves C are still present. But it is easy to find that increasing temperatures makes the increased both reduction and oxidation currents. Thereafter from the cathode polarization curves of different temperatures (Fig. 7), it is found that a higher reaction temperature resulted in a less cathode overpotential. Alternatively, this implies that increasing reaction temperature can make the depolarization of the reduction of TiO2 film. Tafel tests were carried out to confirm the results above (Fig. 8). With temperature increased, we observed a displacement of equal potential toward more positive value, which is characteristic of the cathode depolarization. Furthermore, on analysis of the parameters obtained from Tafel curves (Table 3), the exchange current density
Fig. 5. Equivalent circuit of the oxidized Ti foils (0.28, 0.56 and 0.67 m) tested in Lewis basic AlCl3 -BMIC at 343 K.
Fig. 6. Cyclic voltammograms of titanium foil with oxide film (0.67 m) at different temperatures in Lewis basic AlCl3 -BMIC vs. Ag obtained at a sweep rate of 50 mV/s.
and cathode Tafel slope increased with decreased linear polarization resistance. Accordingly, it is concluded that increasing reaction temperature can weaken the cathode inhibition of this reduction process. As shown in Fig. 9, EIS experiments of TiO2 films at different temperatures were conducted. Intuitively, there is a decrease of resistance of the electrochemical reduction when the reaction temperature increased. At 313 K, the Warburg resistance is seriously weakened at low frequency, which appears again at higher ones (333, 353 and 373 K). It is mostly because that the reaction rate is slow at low temperature and thus the diffusion process is very weak or nearly absent. Given the equivalent circuit mentioned above, the parameters fitted from experimental data are exhibited in Table 4. Clearly, increasing temperature makes the charge transfer resistance Rt and tested IL resistance Rs decline. The conclusion of depolarization effect of increasing reaction temperature on this reduction process is identical with cathode polarization experiments.
Fig. 7. Cathode polarization curves in Lewis basic AlCl3 -BMIC at different temperatures for oxidized Ti foil (0.67 m) at a sweep rate of 5 mV/s.
Fig. 8. Tafel curves in Lewis basic AlCl3 -BMIC at different temperatures for oxidized Ti foil (0.67 m) at a sweep rate of 5 mV/s.
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Table 3 Parameters for oxidized Ti foil (0.67 m) at different temperatures obtained from Fig. 10. Temperature (K)
Eeq (mV)
j0 (A cm−2 )
333 353 373
135 277 289
0.389 0.443 1.823
LPR (ohm cm2 )
173,207.3 118,057.1 25,226.8
Tafel slope ˇa (mV)
ˇc (mV)
407.5 441.7 326.1
480.2 593.7 751.3
Table 4 EIS parameters from the equivalent circuit for the oxidized Ti foil (0.67 m) at different temperatures. Temperature (K)
Rfilm (ohm cm2 )
Rt (ohm cm2 )
CPEdl (F cm−2 )
Rs (ohm cm2 )
W/Y0 (Ss0.5 cm−2 )
2
313 333 353 373
18.46 21.14 23.26 20.87
7.54 5.67 3.59 3.06
33. 65 33.03 44.35 49.35
5.21 3.89 1.78 0.86
0.308 0.300 0.230 0.248
3.491E−3 2.854E−3 2.152E−3 1.161E−3
3.3. Further discussion During the electrochemical reduction process of TiO2 film, we found that a lot of bubbles were generated at the bottom of cathode and less at anode. In our opinion, the reduced TiO2 film on the cathode was temporarily employed as anode this moment, when the former anode just acted as a conductor. Simultaneously, SEM graphs show the different microstructures of the reduced TiO2 film
Fig. 9. Ac impedance experimental and approximation spectra diagrams for oxidized Ti foil (0.67 m) at 313 (), 333 (), 353 (♦) and 373 () K in Lewis basic AlCl3 -BMIC with a cathode potential of −1.3 V.
from the original one (Fig. 10). Compared to the un-reduced oxide film, the reduced one obviously has metallic luster (Fig. 11). It can be predicted that the reduction of oxidized Ti foil in basic AlCl3 -BMIC was possibly occurred at the edges of the foil, which is mostly due to the different potential distribution at the edges of the electrode. Many studies have been carried out on the mechanism of electrochemical reduction of TiO2 in molten CaCl2 bath. As well acknowledged by most people, oxygen ionization at cathode is a more credible explanation [2]. We believe that the reduction process of TiO2 film in ionic liquid is an interfacial electrochemical reaction. From the viewpoint of solid physics, TiO2 film thermally formed on titanium, well known as an n-type semiconductor, has a wide bandgap (Eg ≈ 3.2 eV) [29–31], which makes its charge transfer more difficult than other narrow ones. For the aim of reduction of TiO2 film, increasing the cathodic polarization made the electronic bands bend downward at the TiO2 film/ionic liquid interface, leading to degeneration of the space-charge region of the TiO2 film with an electron-accumulation layer formed at the film surface as shown in Fig. 12, where the oxygen in the TiO2 cathode was ionized, and as a result, the reduction of TiO2 film occurred [32]. Accordingly, we can suppose that the reduction of TiO2 film in AlCl3 -BMIC melt might occur in the following steps (Fig. 13):
(1) As cathode potential increased, the electronic bands bent downward the TiO2 /ionic liquid interface, where charge transfer happened. Alternatively, the oxygen was ionized at the interface of TiO2 film and ionic liquid (TiO2 + 4e → Ti + 2O2− ). (2) Space between reduced Ti atoms was formed, which is immediately filled with ionic liquid.
Fig. 10. SEM micrographs of oxidized Ti foil after electrolysis in Lewis basic AlCl3 -BMIC at 373 K.
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Fig. 11. Metallurgical microscope photographs of oxidized Ti foil before ((b) and (d)) and after electrolysis ((a) and (c)) in Lewis basic AlCl3 -BMIC at 373 K.
(3) Diffusion of O2− dissolved in ionic liquid through the partly reduced porous film. (4) Mass transfer of O2− from the cathode surface through the melt to the anode and charged with O2 formed (2O2− → O2 ↑ + 4e).
(5) When reduced TiO2 film formed, which then served as anode, O2 was generated at the interface of reduced film, oxide film and ionic liquid. At this moment, the anode just acted as a conductor. 4. Conclusions (1) Thickness increase of oxide film can strengthen the cathode polarization effect of reduction. (2) More negative the set cathode potential can make less the resistance of the reduction process. (3) Increasing reaction temperature can weaken the cathode polarization. (4) The model for this reduction process has been given on the basis of the pictures of reduced TiO2 film and the electronic energy band structure models.
Fig. 12. Schematic presentation of the electronic energy band structure model [32] for the reduction of the n-type semiconducting oxide film on titanium in Lewis basic AlCl3 -BMIC.
Acknowledgement The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Project NO. 50864009) and National Natural Science Foundation of Yunnan province (Project NO. 2011FA009) for this work. The technical support of Sion-Metrohm Technology Ltd. should be appreciated. References [1] [2] [3] [4]
Fig. 13. Model for the reduction process of oxides film on titanium foil in Lewis basic AlCl3 -BMIC.
[5] [6] [7] [8] [9] [10] [11] [12]
H.M. Flower, Nature 407 (2000) 305. G.Z. Chen, D.J. Fray, T.W. Farthing, Nature 407 (2000) 361. K. Ono, R.O. Suzuki, J. Met. 54 (2002) 59. T. Abiko, I. Park, T.H. Okabe, Proceedings 10th World Conference on Titanium, vol. 7, Hamburg, Germany, 14 July, 2003, p. 13. T.H. Okabe, T. Oda, Y. Mitsuda, J. Alloys Compd. 364 (2004) 156. T. Welton, Chem. Rev. 99 (1999) 2071. H. Kamimura, T. Kubo, I. Minami, S. Mori, Tribol. Int. 40 (2007) 620. J. Robinson, R.A. Osteryoung, J. Electrochem. Soc. 127 (1980) 122. B.J. Welch, R.A. Osteryoung, J. Electroanal. Chem. 118 (1981) 456. Y. Nuli, J. Yang, R. Wu, Electrochem. Commun. 7 (2005) 1105. Z. Feng, Y. Nuli, J. Wang, J. Yang, J. Eelectrochem. Soc. 135 (2006) C689. Y. Katayama, T. Morita, M. Yamagata, T. Miura, Electrochemistry 71 (2003) 1033.
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[23] T. Tsuda, C.L. Hussey, G.R. Stafford, J.E. Bonevich, J. Electrochem. Soc. 150 (2003) C234. [24] X.Y. Zhang, Y.X. Hua, C.Y. Xu, Q.B. Zhang, X.B. Cong, N. Xu, Electrochim. Acta 56 (2011) 8530. [25] K. Azumi, M. Seo, Corros. Sci. 43 (2001) 533. [26] D.S. Kong, Langmuir 24 (2008) 5324. [27] D.S. Kong, Y.Y. Feng, J. Electrochem. Soc. 156 (2009) C283. [28] D.S. Kong, Langmuir 26 (2010) 4880. [29] X.H. Xu, C.L. Hussey, J. Electrochem. Soc. 140 (1993) 619. [30] J.W. Schultze, M.M. Lohrengel, Electrochim. Acta 45 (2000) 2499. [31] S. Piazza, M. Santamaria, C. Sunseri, F. Di Quarto, Electrochim. Acta 48 (2003) 1105. [32] D.S. Kong, J.X. Wu, J. Electrochem. Soc. 155 (2008) C32.
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
The electrochemical behavior of titanium dioxide film in Lewis basic AlCl3 -1-butyl-3-methylimidizolium ionic liquid Xiao-Ying Zhang a,b , Yi-Xin Hua a,b,∗ , Cun-Ying Xu a,b , Nan Xu a,b , Hui Xue c a b c
Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, PR China State Key Laboratory Breeding Base of Complex Nonferrous Metal Resources Cleaning Utilization in Yunnan Province, Kunming 650093, PR China City College, Kunming University of Science and Technology, Kunming 650093, PR China
a r t i c l e
i n f o
Article history: Received 28 September 2011 Received in revised form 12 December 2011 Accepted 20 December 2011 Available online 27 December 2011 Keywords: Titanium dioxide film Ionic liquids Potentiodynamic polarization Cyclic voltammetry EIS
a b s t r a c t The electrochemical behavior of titanium dioxide film in Lewis basic AlCl3 -1-butyl-3-methylimidizolium ionic liquid (AlCl3 -BMIC) at room temperature was investigated by means of potentiodynamic polarization; cyclic voltammetry; sampled current voltammetry and electrochemical impedance spectroscopy (EIS). It was found that as the TiO2 film thickness increased, a cathode inhibition effect of the oxide film on the reduction process was strengthened, which means that a greater overpotential was required to initiate the reduction process. In the EIS test, the preferable equivalent circuit was made to fit the experimental data and fitted parameters were obtained. Additionally, increasing reaction temperature can make the cathode depolarization. In the end, the model for this reduction process has been given on the basis of the pictures of reduced TiO2 film and the electronic energy band structure models. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Due to its excellent properties, a predictable demand for titanium and its alloy is huge in many parts of society [1]. However, the disadvantages of currently used titanium production method, Kroll process, have restricted its extensive usage. Therefore, researchers around the world have never stopped the attempts to find a low-cost, continuous and simplified way for high purity titanium production. Different from conventional Kroll method, some emerging titanium production technologies [2–5] have given us an enlightenment of cost reduction. In our opinion, electrochemical reduction of TiO2 in low temperature may be a preferable method, at least an idea, to overcome the shortcomings of old ones. Accordingly, finding a preferable electrolyte is a key point for this purpose. The discovery of the new class of substance with low melting points, good conductivity and wide electrochemical window, named as room temperature ionic liquids (RTILs), has come into our insight. There have been several decades from the nativity of ionic liquids to now. As well known, ionic liquids have been used in many fields, such as synthesis, catalysis [6] and lubricant [7]. Especially, it is a wonderful
∗ Corresponding author at: Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, PR China. Tel.: +86 871 5162008; fax: +86 871 5161278. E-mail address: [email protected] (Y.-X. Hua). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.12.085
electrolyte for us to obtain many refractory metals, such as Al [8,9], Mg [10,11] and Li [12,13], at low temperature, even at room temperature, which cannot be achieved in aqueous solution. The research on titanium in ionic liquids is what we most concern about. In the early time, the application of RTILs in electro-deposition of titanium has been rarely reported. Due to its several different variable valencies, electro-deposition of titanium is more complicated than other common metals. However, much more efforts have been made by a number of researchers on the study in this excellent electrolyte. Linga et al. [14] have found that Ti(IV) exited as two electrochemically reducible species and one oxide complex in basic Bu(py)Cl. When the E1/2 of the first reduction wave was −0.343 V vs. Al, the species was assumed to be TiCl6 −2 . In the case of the second reduction wave of −0.77 V vs. Al, the species was assumed to be TiOCl4 −2 . Only one oxidation wave, presumably that of a Ti(III) chloro complex, was found. From the research of Sun et al. [15], Ti(IV) appeared to be complexed as TiBr6 −2 in the Lewis basic AlBr3 -MeEtimBr ionic liquid, which was in good agreement with the result of AlCl3 -MeEtimCl. Carlin et al. [16] have examined the electrochemistry of Ti(III) in Lewis acidic AlCl3 -EMIC. Their study implied that titanium was reduced to Ti(III) and Ti(II) through two slow one-electron steps. Moreover Ti(III), as a state of -TiCl3 , was formed as a brown layer on electrode without obtaining titanium. Ding et al. [17] reported the investigation of usage of polymeric and nucleating agent assist the electro-deposition of titanium. Other researches [18,19] on electro-deposition of titanium from TiCl4 in [BMIm][BTA] at room temperature demonstrated that a ultrathin
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titanium film could be formed on highly oriented pyrolytic graphite and Au(1 1 1) substrate. The investigation concluded that reduction of TiCl4 , which was first reduced to TiCl2 and subsequently to Ti, included two steps. During this process, the researchers obtained a brown film, probably the TiCl3 , which possibly inhibited the reduction. Coincidently, the study by Endres et al. [20] indicated that non-stoichiometric halides have been formed from [EMIm]Tf2 N, [BMP]Tf2 N and [P14,6,6,6 ]Tf2 N containing TiCl4 as a source of titanium. Different from the research mentioned above, however, they believed that it was impossible to reduce TiCl4 to elemental Ti or very difficult. The studies [21,22] on the electrochemistry of TiCl4 and TiF4 in BMMImN3 and BMIMBF4 also denoted that formation of some poorly soluble low valence intermediates kinetically hindered the reduction process. Besides, it is interestingly found that the potential difference of some metals becomes close in ionic liquids, which can assist the formation of alloy, such as Al–Ti [23]. In our previous study [24], it was tried to electro-chemically reduceTiO2 , and its feasibility has been successfully confirmed. Additionally, the preliminary investigation on the reduction mechanism was carried out. In this paper, the major purpose is to investigate the electrochemical behavior of TiO2 film in Lewis basic AlCl3 -BMIC and analyze its reduction mechanism in greater depth. As well known, the electrochemistry of TiO2 film in aqueous solution has been widely investigated [25–28]. In spite of the nonstoichiometric composition of TiO2 film thermally formed on Ti foil, the low interfacial resistance and ease of charge transfer can make the better electrochemistry results. Therefore this kind of TiO2 film was chose to characterize electrochemical behavior of TiO2 in ionic liquid by following techniques: potentiodynamic polarization; cyclic voltammetry; sampled current voltammetry and electrochemical impedance spectroscopy (EIS). 2. Experimental
Fig. 1. Cyclic voltammograms of titanium foils with different oxide film thickness (0.28, 0.56 and 0.67 m) in Lewis basic AlCl3 -BMIC vs. Ag obtained at a sweep rate of 50 mV/s at 343 K.
sweep voltammetry with a scan rate of 5 mV/s. The Tafel test was conducted at the same scan rate. In the sampled current voltammetry test, the voltage range was from −0.5 to −2.4 V with the sample period of 10 s and step voltage of 0.1 V. All the Ac impedance data of one oxidized Ti foil in AlCl3 -BMIC was obtained from −1 to −1.6 V, with Ac excitation amplitude of 5 mV. The scanning frequency ranged from 10 kHz down to 50 mHz. During all experiments the cell was purged by an argon flow to avoid the ingress of air. The surface area of working electrode was estimated from the depth of the electrode immersed in the electrolyte. In each test, a new oxidized Ti foil was used. All the electrochemical experiments were performed on a Gamry PCI4/300 electrochemical work station for the experimental control and data acquisition.
2.1. Chemicals 3. Results and discussion Anhydrous AlCl3 (99 wt.%) was used without further treatment. 1-butyl-3-methylimidizolium chloride (BMIC) was synthesized a procedure described in details elsewhere [29]. The electrolyte of Lewis basic ionic liquid AlCl3 -BMIC used in this paper was prepared by the slow addition of a certain weight of AlCl3 into BMIC in a mole ratio of 0.8:1.0 respectively, giving a mole fraction of AlCl3 of 0.44. 2.2. Preparation of Ti electrodes A titanium foil (commercial purity, 30 mm × 10 mm × 0.1 mm) was mechanically polished with a sequence of emery papers of different grades (400, 600 and 800), degreased with acetone in ultrasonic bath for 5 min, washed three times with distilled water, and dried at room temperature. Subsequently, the titanium foil was oxidized in a furnace at 823 K in air for different time (48, 96 and 144 h). Obviously, the more oxidation time, the thicker oxide film (0.28, 0.56 and 0.67 m). 2.3. Electrochemical experiments Electrochemical tests were performed in a standard threeelectrode electrochemical cell using the oxidized Ti foil mentioned above as the working electrode, a platinum wire as a counter electrode, and a silver wire as a reference electrode. Cyclic voltammetry, sampled current voltammetry, cathode polarization, electrochemical impedance spectroscopy and chronoamperometry were carried out to study on the electrochemistry of oxidized Ti foil in AlCl3 -BMIC. The scan rate in cyclic voltammetry was 50 mV/s and the first scan direction was negative. The cathode polarization curves were measured by linear
3.1. Influence of TiO2 film thickness on its electrochemical behavior In Fig. 1, three typical cyclic voltammograms of titanium electrode with different thickness oxide film (0.28, 0.56 and 0.67 m) in AlCl3 -BMIC melt at 343 K are illustrated. In the scan voltage range of 0.5 to −0.9 V, two reduction waves A, B were observed at negative direction and their possible anodic corresponding waves C at positive direction in each cyclic voltammogram, the peak currents of which was at nearly same positions. By the comparison of three cyclic voltammograms, however, reduction peak current decreased along with the thickness of TiO2 film increased. Increasing thickness of TiO2 film might enhanced the cathode polarization of this reduction process. Additionally, a relatively negative shift of the initial cathode potentials of reduction current can be found, due to the voltage loss across the oxide film. Increase of TiO2 film thickness, which made the charge transfer difficult, inhibited the reduction of TiO2 . In the precious paper [24], potentiodynamic polarization test can further confirm the results obtained above. The thicker is oxide film, the more negative is reduction potential. Alternatively, a thicker oxide film required a greater cathode overpotential to be reduced, which was resulted from the cathode inhibition effect of the oxide film on the reduction process. In Fig. 2, a negative shift of equilibrium potential can be observed from the Tafel curves. From Table 1, when TiO2 film became thicker, the exchange current density and cathode Tafel slope decreased along with an increasing linear polarization resistance. Obviously, direct electrochemical reduction of bulk TiO2 should need a greater driving force to be
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Table 1 Parameters for oxidized Ti foils (0.28, 0.56 and 0.67 m) obtained from Fig. 3. Film thickness (m)
Eeq (mV)
j0 (A cm−2 )
0.28 0.56 0.67
162 126 117
0.352 0.279 0.170
LPR (ohm cm2 )
104,071.2 140,513.5 441,382.5
Tafel slope ˇa (mV)
ˇc (mV)
288.6 463.6 221.5
120.4 112.1 105.2
completed can be found. This conclusion was in agreement with the cyclic voltammetry. In EIS test of oxidized Ti foil (0.67 m), it is clearly found that the charge resistance of the reduction decreased as the cathode overpotential increased (Fig. 3). In the high frequency region, the Nyquist plots at different cathode potentials all present one semicircle, when the charge transfer at the interface between the TiO2
film and the ionic liquid was the rate-limiting step. In contrast, the nearly linear relationship between Z and −Z in the low frequency region may be attributed to the mass transfer in the ionic liquid. Many equivalent circuits have been used by ZsimpWin software to simulate the Ti/TiO2 film/IL system, only the model shown in Fig. 5 fitted the experimental data best, and the fitting line in Fig. 4. Fitted results can be seen in Table 2. In this system (Fig. 5), Rfilm and
Fig. 2. Tafel curves in Lewis basic AlCl3 -BMIC at 343 K for oxidized Ti foils with different oxide film thicknesses (0.28, 0.56 and 0.67 m) at a sweep rate of 5 mV/s.
Fig. 3. Ac impedance spectra of oxidized Ti foil (0.67 m) in Lewis basic AlCl3 -BMIC at 343 K with a frequency range of 10 kHz to 0.05 Hz and a signal amplitude of 5 mV.
Fig. 4. Ac impedance experimental and approximation spectra diagrams for oxidized Ti foils (0.28 m (a), 0.56 m (b) and 0.67 m (c)) in Lewis basic AlCl3 -BMIC at 343 K with a cathode potential of −1.3 V.
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Table 2 EIS parameters from the equivalent circuit for the oxidized Ti foils (0.28, 0.56 and 0.67 m) at 343 K. Thickness (m)
Rfilm (ohm cm2 )
Rt (ohm cm2 )
CPEdl (F cm−2 )
Rs (ohm cm2 )
W/Y0 (Ss0.5 cm−2 )
2
0.28 0.56 0.67
16.09 19.67 24.53
3.12 3.07 3.19
19.40 14.24 21.48
1.43 2.88 1.49
0.146 0.377 0.07
2.61E−3 1.93E−3 1.97E−3
CPEfilm represent the resistance and capacity of oxide film respectively; Rt refers to the charge transfer resistance process occurring on the oxide film surface; CPEdl delegates the double capacity at the interface between the oxide film and tested IL; Rs can be explained to be the resistance of the tested IL; W can describe the diffusion control process at low frequency. As shown in Table 2, the increase of Rfilm is the direct evidence of the bodiness of oxide film, corresponding well to the result of potentiodynamic polarization measurement. 3.2. Influence of temperature on electrochemical behavior of TiO2 film For increasing the reduction rate of this process, the dependence of electrochemical behavior of TiO2 film on the reaction temperature was investigated. Cyclic voltammograms of titanium foil with oxide film (0.67 m) at different temperatures in AlCl3 BMIC are illustrated in Fig. 6. Similar with the former findings, the two reductions A, B and one oxidation waves C are still present. But it is easy to find that increasing temperatures makes the increased both reduction and oxidation currents. Thereafter from the cathode polarization curves of different temperatures (Fig. 7), it is found that a higher reaction temperature resulted in a less cathode overpotential. Alternatively, this implies that increasing reaction temperature can make the depolarization of the reduction of TiO2 film. Tafel tests were carried out to confirm the results above (Fig. 8). With temperature increased, we observed a displacement of equal potential toward more positive value, which is characteristic of the cathode depolarization. Furthermore, on analysis of the parameters obtained from Tafel curves (Table 3), the exchange current density
Fig. 5. Equivalent circuit of the oxidized Ti foils (0.28, 0.56 and 0.67 m) tested in Lewis basic AlCl3 -BMIC at 343 K.
Fig. 6. Cyclic voltammograms of titanium foil with oxide film (0.67 m) at different temperatures in Lewis basic AlCl3 -BMIC vs. Ag obtained at a sweep rate of 50 mV/s.
and cathode Tafel slope increased with decreased linear polarization resistance. Accordingly, it is concluded that increasing reaction temperature can weaken the cathode inhibition of this reduction process. As shown in Fig. 9, EIS experiments of TiO2 films at different temperatures were conducted. Intuitively, there is a decrease of resistance of the electrochemical reduction when the reaction temperature increased. At 313 K, the Warburg resistance is seriously weakened at low frequency, which appears again at higher ones (333, 353 and 373 K). It is mostly because that the reaction rate is slow at low temperature and thus the diffusion process is very weak or nearly absent. Given the equivalent circuit mentioned above, the parameters fitted from experimental data are exhibited in Table 4. Clearly, increasing temperature makes the charge transfer resistance Rt and tested IL resistance Rs decline. The conclusion of depolarization effect of increasing reaction temperature on this reduction process is identical with cathode polarization experiments.
Fig. 7. Cathode polarization curves in Lewis basic AlCl3 -BMIC at different temperatures for oxidized Ti foil (0.67 m) at a sweep rate of 5 mV/s.
Fig. 8. Tafel curves in Lewis basic AlCl3 -BMIC at different temperatures for oxidized Ti foil (0.67 m) at a sweep rate of 5 mV/s.
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Table 3 Parameters for oxidized Ti foil (0.67 m) at different temperatures obtained from Fig. 10. Temperature (K)
Eeq (mV)
j0 (A cm−2 )
333 353 373
135 277 289
0.389 0.443 1.823
LPR (ohm cm2 )
173,207.3 118,057.1 25,226.8
Tafel slope ˇa (mV)
ˇc (mV)
407.5 441.7 326.1
480.2 593.7 751.3
Table 4 EIS parameters from the equivalent circuit for the oxidized Ti foil (0.67 m) at different temperatures. Temperature (K)
Rfilm (ohm cm2 )
Rt (ohm cm2 )
CPEdl (F cm−2 )
Rs (ohm cm2 )
W/Y0 (Ss0.5 cm−2 )
2
313 333 353 373
18.46 21.14 23.26 20.87
7.54 5.67 3.59 3.06
33. 65 33.03 44.35 49.35
5.21 3.89 1.78 0.86
0.308 0.300 0.230 0.248
3.491E−3 2.854E−3 2.152E−3 1.161E−3
3.3. Further discussion During the electrochemical reduction process of TiO2 film, we found that a lot of bubbles were generated at the bottom of cathode and less at anode. In our opinion, the reduced TiO2 film on the cathode was temporarily employed as anode this moment, when the former anode just acted as a conductor. Simultaneously, SEM graphs show the different microstructures of the reduced TiO2 film
Fig. 9. Ac impedance experimental and approximation spectra diagrams for oxidized Ti foil (0.67 m) at 313 (), 333 (), 353 (♦) and 373 () K in Lewis basic AlCl3 -BMIC with a cathode potential of −1.3 V.
from the original one (Fig. 10). Compared to the un-reduced oxide film, the reduced one obviously has metallic luster (Fig. 11). It can be predicted that the reduction of oxidized Ti foil in basic AlCl3 -BMIC was possibly occurred at the edges of the foil, which is mostly due to the different potential distribution at the edges of the electrode. Many studies have been carried out on the mechanism of electrochemical reduction of TiO2 in molten CaCl2 bath. As well acknowledged by most people, oxygen ionization at cathode is a more credible explanation [2]. We believe that the reduction process of TiO2 film in ionic liquid is an interfacial electrochemical reaction. From the viewpoint of solid physics, TiO2 film thermally formed on titanium, well known as an n-type semiconductor, has a wide bandgap (Eg ≈ 3.2 eV) [29–31], which makes its charge transfer more difficult than other narrow ones. For the aim of reduction of TiO2 film, increasing the cathodic polarization made the electronic bands bend downward at the TiO2 film/ionic liquid interface, leading to degeneration of the space-charge region of the TiO2 film with an electron-accumulation layer formed at the film surface as shown in Fig. 12, where the oxygen in the TiO2 cathode was ionized, and as a result, the reduction of TiO2 film occurred [32]. Accordingly, we can suppose that the reduction of TiO2 film in AlCl3 -BMIC melt might occur in the following steps (Fig. 13):
(1) As cathode potential increased, the electronic bands bent downward the TiO2 /ionic liquid interface, where charge transfer happened. Alternatively, the oxygen was ionized at the interface of TiO2 film and ionic liquid (TiO2 + 4e → Ti + 2O2− ). (2) Space between reduced Ti atoms was formed, which is immediately filled with ionic liquid.
Fig. 10. SEM micrographs of oxidized Ti foil after electrolysis in Lewis basic AlCl3 -BMIC at 373 K.
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Fig. 11. Metallurgical microscope photographs of oxidized Ti foil before ((b) and (d)) and after electrolysis ((a) and (c)) in Lewis basic AlCl3 -BMIC at 373 K.
(3) Diffusion of O2− dissolved in ionic liquid through the partly reduced porous film. (4) Mass transfer of O2− from the cathode surface through the melt to the anode and charged with O2 formed (2O2− → O2 ↑ + 4e).
(5) When reduced TiO2 film formed, which then served as anode, O2 was generated at the interface of reduced film, oxide film and ionic liquid. At this moment, the anode just acted as a conductor. 4. Conclusions (1) Thickness increase of oxide film can strengthen the cathode polarization effect of reduction. (2) More negative the set cathode potential can make less the resistance of the reduction process. (3) Increasing reaction temperature can weaken the cathode polarization. (4) The model for this reduction process has been given on the basis of the pictures of reduced TiO2 film and the electronic energy band structure models.
Fig. 12. Schematic presentation of the electronic energy band structure model [32] for the reduction of the n-type semiconducting oxide film on titanium in Lewis basic AlCl3 -BMIC.
Acknowledgement The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Project NO. 50864009) and National Natural Science Foundation of Yunnan province (Project NO. 2011FA009) for this work. The technical support of Sion-Metrohm Technology Ltd. should be appreciated. References [1] [2] [3] [4]
Fig. 13. Model for the reduction process of oxides film on titanium foil in Lewis basic AlCl3 -BMIC.
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