Article pubs.acs.org/JPCC
UV-Assisted Electrodeposition of Germanium from an Air- and Water-Stable Ionic Liquid Abhishek Lahiri,† Sherif Zein El Abedin,†,‡ and Frank Endres*,† †
Institute of Particle Technology, Clausthal University of Technology, Arnold-Sommerfeld-Str. 6, 38678 Clausthal-Zellerfeld, Germany ‡ National Research Center, Dokki, Cairo, Egypt
ABSTRACT: The effect of UV light during electrodeposition of germanium on Au and on indium tin oxide (ITO) from the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py1,4]Tf2N) has been investigated. From cyclic voltammetric analysis, two reduction peaks were observed and were assigned to the reduction of Ge(IV) to Ge(II) species, and Ge(II) to Ge(0) species. On using 254 nm UV light during electrodeposition, a decrease of 140 mV occurred on the ITO substrate for the reduction of Ge(IV) to Ge(II) compared with that observed without UV, and a decrease of 204 mV was observed in the reduction of Ge(II) to Ge. The shift in the reduction peak seems to be related to the photoabsorption of UV by the electrolyte, possibly altering the ionic liquid/electrode interface. By characterizing the Ge deposit using scanning electron microscopy, it was observed that the presence of UV irradiation significantly decreased the Ge particle size. A green luminescence was also observed from the electrolyte during the cyclic voltammetry experiments in the presence of UV. The significant influence of UV on the reduction potential and on the electrodeposit might open up new avenues for electrodeposition processes in ionic liquids.
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INTRODUCTION Electrodeposition is, in general, a low-cost, viable technique for the synthesis of metals and alloys,1 and a lot of research has been done on the electrodeposition of metals from aqueous solutions.2 However, due to the limited electrochemical window of water, semiconductors cannot be easily electrodeposited.3 Only thin layers (few monolayers) of germanium were electrodeposited from aqueous solutions in the past.3−5 Recently, however, Stickney et al were successful in using electrochemical atomic layer deposition (E-ALD) for Ge deposition, and germanium nanofilms could be made in an aqueous solution.6 The technique is based on the underpotential deposition of germanium on tellurium, followed by stripping of the sacrificial tellurium layer. Germanium films were also deposited from organic electrolytes. Chandrasekhran and Sevov7 showed the possibility of electrodepositing Ge from an ethylenediamine containing K4Ge9. Macroporous germanium structures were, in principle, obtained from a solution of GeCl4 in ethylene glycol.8 Huang et al9 reported the deposition of germanium films on silicon © 2012 American Chemical Society
substrates from 1,3-propanediol electrolyte, wherein the effect of the substrate on the deposit and on crystallization was investigated. However, the presence of an organic electrolyte limits the electrochemical window and introduces impurities into the electrodeposit.5,9 In direct comparison, ionic liquids have a much wider electrochemical window. Furthermore, water can be removed easily from them, ensuring water free conditions.10−12 Our group has extensively studied the electrodeposition of germanium from ionic liquids.13−16 It has been previously shown that germanium can be electrodeposited from GeX4 (X = Cl, Br, I) in 1-butyl-3-ethylimidazolium hexafluorophosphate ([BMIm]PF6) ionic liquid.13,14 It should be mentioned here that ionic liquids containing PF−6 or BF−4 are very sensitve to decomposition in the presence of water; thus, highest care already in the synthesis of these liquids is needed. Nevertheless, Received: June 25, 2012 Revised: July 30, 2012 Published: August 3, 2012 17739
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Figure 1. Cyclic voltammetry of [Py1,4]Tf2N on the Au substrate in the presence of normal light and 254 and 365 nm UV light. The scan rate was 10 mV/s.
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EXPERIMENTAL SECTION 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py1,4]Tf2N) was purchased in the highest available quality from Io-Li-Tec (Germany) and was used after drying under vacuum at 100 °C to remove the water content to below 2 ppm. GeCl4 (99.9999%) was purchased from Alfa Aesar. The working electrode in the experiment was either a sputtered film of gold on glass or indium tin oxide (ITO) on glass. The ITO and Au were cleaned by refluxing in isopropanol at 90 °C for 2 h. Prior to the experiments, the gold was carefully heated in a hydrogen flame to remove any surface contamination. Platinum wires were used as a counter and a quasi-reference electrode, which gave sufficient stability in the ionic liquid throughout the experiments. To ensure that the potential of the quasi-reference Pt electrode was stable in the presence of UV light, a twocompartment cell setup was made, wherein two platinum wires were placed in two cells connected with a salt bridge containing the same electrolyte. A multimeter was connected to the Pt electrodes, and the open-circuit potential (OCP) was monitored. To evaluate the effect of UV light, one compartment was covered with Al foil and the other one was exposed to 254 nm UV light. The OCP was continuously monitored during the experiment for 30 min, and no change in OCP was found, confirming that the Pt quasi-reference electrode was stable in the presence of UV light. For electrochemistry experiments, the electrochemical cell was made of Teflon and clamped over a Teflon-covered Viton O-ring onto the substrate, yielding a geometric surface area of 0.3 cm2. Prior to experiments, the Teflon cell and the O-ring were cleaned in a mixture of 50:50 vol % of concentrated H2SO4 and H2O2 (35%), followed by refluxing in distilled water.
with in situ scanning tunnelling microscopy and in situ current/ voltage tunnelling spectroscopy, a band gap of 0.7 ± 0.1 eV was observed for the germanium deposit, which is in good agreement with the band gap of intrinsic microcrystalline bulk Ge (0.67 eV at 300 K).14 X-ray photoelectron spectroscopy (XPS) was also used to characterize the deposit, which proved the formation of the elemental semiconductor.14 Recently, Al-Salman et al showed the formation of germanium nanowires,17 SixGe1−x alloys,18 and also the formation of SixGe1−x nanowires19 from ionic liquids. During the SixGe1−x alloy deposition, an interesting optical behavior was observed during cyclic voltammetry that was related to the quantum confinement effect.18 Meng et al20 also showed the possibility of electrodepositing 3D Ge macroporous structures on ITO, which showed various optical colors by changing the angle of incident light. In comparison to organic solvents, ionic liquids do not attack artificial polystryene opal structures; furthermore, their low surface tension ensures a complete wetting of the opal interstices and the electrode surface underneath. In this paper, we show in a fundamental approach the influence of UV light on the deposition of germanium on ITO and Au at room temperature from 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py1,4]Tf2N) ionic liquid. On performing cyclic voltammetric experiments on ITO, it was observed that a considerable shift to lower reduction potentials was achieved on using UV light. Furthermore, a green luminescence was observed on exposing the electrolyte to UV. Constant potential deposition of germanium was also performed in the presence of UV light on both gold and ITO substrates, and the deposits were characterized using scanning electron microscopy (SEM). 17740
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Figure 2. (a) CV of 0.1 M GeCl4 in [Py1,4]Tf2N at room temperature on the ITO substrate. (b) In the presence of 254 nm UV light. Inset shows the green luminescence at OCP after one CV cycle. (c) CV in the presence of 365 nm UV light. The scan rate was 10 mV sec−1.
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RESULTS AND DISCUSSION The electrochemical window of [Py1,4]Tf2N was evaluated on a Au substrate in the presence and absence of UV irradiation. Figure 1 compares the electrochemical window from which it is observed that the electrochemical window in the presence and absence of UV is around 5.4 V at room temperature. The decomposition of [Py1,4]+ starts around −2.9 V vs the Pt quasireference electrode, and the bulk oxidation of gold starts at about +2.5 V. However, comparing the cyclic voltammetry curves in Figure 1, it is evident that the presence of 254 and 365 nm UV light leads to a rather sluggish decomposition of the [Py1,4]+ ion. Similar electrochemical windows are observed on the ITO substrate. Figure 2 compares the cyclic voltammetry (CV) curves of 0.1 M GeCl4 in [Py1,4]Tf2N on ITO in the absence and presence of UV light. From Figure 2a, we find two cathodic peaks at −1.74 and at −2.17 V, without UV illumination. The first reduction peak corresponds to the reduction of Ge(IV) to Ge(II) ions without visual deposits, and the second one from Ge(II) reduction to Ge(0), giving a black deposit. These observations are consistent with previously published results.18 If the cathodic limit is set to values < −3 V, an irreversible reduction of the organic cation is observed. During the anodic scan, some incomplete oxidation of Ge occurs at > −0.3 V. On
The electrochemical measurements were performed in an argon-filled glovebox with water and oxygen contents below 2 ppm (OMNI-LAB from Vacuum Atmospheres) by using a VersaStatt II (Princeton Applied Research) potentiostat/ galvanostat controlled by PowerCV and PowerSTEP software. The scan rate during cyclic voltammetry was 10 mV sec−1. A UV lamp having both 254 and 365 nm wavelengths was suspended over the electrochemical cell. Care was taken to direct the UV light into the cell. The distance between the cell and the UV lamp was about 5 cm. The deposits obtained by constant potential deposition were characterized using scanning electron microscopy (SEM, Carl Zeiss DSM 982 Gemini) and energy-dispersive X-ray spectroscopy (EDX). To understand the absorption properties and to evaluate the photoluminescence effect, the ionic liquid was dissolved in spectroscopic grade isopropanol (Merck) to obtain a solution concentration of 10 mM. The solution was then transferred into a quartz cell and analyzed using a PerkinElmer LAMBDA 950 UV−vis-IR spectrometer. Initially, the absorption was measured with pure isopropanol (0.5 cm optical path length) to establish the zero calibration curve, after which the absorption curves of the ionic liquids were measured. 17741
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deposited in all the cases. F and C are due to some remaining ionic liquid. The presence of Cl (1.5 atomic %) could be due to the halide termination of Ge surface atoms. The oxygen peak is a result of surface oxidation of Ge nanoparticles due to an unavoidable handling outside the glovebox. To evaluate whether the ITO was affecting the electrochemical reaction, studies were performed on the Au working electrode for comparison purposes. Figure 4 shows the cyclic voltammetry curves of GeCl4 in [Py1,4]Tf2N obtained on Au in the presence and absence of UV light. The CV scan in Figure 4a shows two reduction peaks at −1.71 and −2.11 V, which correspond to the reduction of Ge(IV) to Ge(II) and Ge(II) to Ge(0), respectively. The electrode potentials are the same as those for ITO. During the anodic scan, an increase in current above +0.5 V is observed, which can be related to the oxidation of Ge and of the gold surface. In the presence of 254 nm UV (Figure 4b), again two reduction peaks at −1.53 and −1.88 V are observed. A shift in the reduction potential (in the presence of the UV light) to lower values is evident. Also, the appearance of green luminescence is seen (inset in Figure 4b), as previously noticed on using the ITO substrate. The observed electrochemical behavior and the positions of the reduction peaks are pretty much the same ones as for ITO. Different gold substrates can give slightly different peak positions; however, there is a dependence on crystal orientation. A detailed study on the dependence of CVs on crystal orientation is beyond the scope of the present paper. On running the CV in the presence of 365 nm UV light (Figure 4c), negative shifts in the potential to −1.62 and −2.07 V are observed compared to the CV without UV. Similar to the ITO substrate, the shift is less prominent compared to 254 nm UV light. Figure 5 compares the microstructure of the obtained germanium in the presence and absence of UV light deposited on Au for 30 min, by applying a constant potential electrolysis at −2.11, −1.88 (254 nm UV light), and −2.07 V (365 nm UV light). The microstructure in Figure 5a shows Ge agglomerates having sizes of about 200 nm, which themselves are composed of smaller grains. In comparison, Ge deposition in the presence of 254 nm UV light (Figure 5b) leads to 100−150 nm agglomerates, also composed of smaller particles. Upon illumination with 365 nm UV light, the germanium deposit shows a different mushroom-type structure. These mushroom structures are also built up of smaller grains with a size range between 20 and 50 nm. The EDX spectra of these particles are presented in Figure 5d, wherein a prominent Ge peak is observed. The F, C, and S impurities are due to some residual ionic liquid. Oxygen is due to a particle oxidation under air. In the following section, we suggest an interpretation for the above-described result. It is evident from cyclic voltammetric and from microstructural analysis (Figures 1−5) that UV light somehow affects both the reduction potential and the morphology of the deposits. There is no increase of the electrolyte temperature due to UV illumination. Also, no decomposition of pure ionic liquid due to UV illumination was observed. Therefore, the possible reasons for switching the reduction potential to more positive values could be the absorption of energy by GeCl4-[Py1,4]Tf2N, thereby altering the ionic liquid−substrate interface, generation of photovoltage by the substrates, or a change in the potential of the Pt quasireference electrode. However, using a two-compartment cell setup, no change in the potential of the Pt quasi-reference electrode in the presence and absence of UV light was noticed,
continuing the scan, a rise in current occurs above +0.4 V, which can be correlated to the oxidation and dissolution of germanium in the ionic liquid. Above +0.4 V, the disappearance of a black Ge deposit could be visually observed. In comparison, when the CV was performed in the presence of 254 nm UV, a shift in the reduction potential occurred, as observed in Figure 2b. The first reduction peak occurred at −1.6 V, and the second peak, related to the deposition of germanium, occurred at −1.93 V. An interesting observation was the evolution of a green luminescence on the GeCl4containing ionic liquid at open-circuit potential (OCP) after one CV cycle. During the cathodic scan, a faint green luminescence starts to appear during the reduction of Ge(IV) to Ge(II) species. The green luminescence becomes prominent on the anodic scan at a potential above +0.6 V, which could be due to the oxidation of electrodeposited Ge. On performing the experiments with a 365 nm UV light, the shift in the reduction peak was less compared with that of 254 nm UV light. Again, two reduction peaks occur at −1.65 and −2.0 V, corresponding to the reduction of Ge(IV) to Ge(II) and Ge(II) to Ge(0), respectively. Figure 3a−c shows the microstructure of the
Figure 3. (a) SEM of Ge nanoparticles obtained after constant potential electrodeposition at −2.17 V. (b) Microstructure of Ge nanoparticles obtained after 30 min at −1.93 V in the presence of 254 nm UV light. (c) Ge nanoparticles obtained at −2.0 V in the presence of 365 nm UV light. (d) EDX spectrum of germanium deposit in (b).
germanium deposit obtained after 30 min at −2.17, −1.93 (254 nm UV light), and −2.0 V (365 nm UV light), in the absence and presence of UV light. It is evident from the microstructure that Ge forms a spherical morphology of agglomerates built up from smaller clusters. In the presence of 254 nm UV light, fewer clusters are formed. The cluster size of Ge in the absence of UV was found to be around 400−500 nm. The presence of 254 and 365 nm UV light decreased the Ge cluster size to about 100 and 150−200 nm, respectively. Primarily, these results mean that, with higher energy of the UV lamp, the cluster size decreases. It must be mentioned here that, during high-resolution SEM, there was sample damage due to the interaction of the electron beam with the Ge nanoparticles, and therefore, clear information about the Ge particles size could not be obtained. Figure 3d shows a representative EDX of Figure 3b. EDX shows that Ge was 17742
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Figure 4. (a) CV of 0.1 M GeCl4 in [Py1,4]Tf2N at room temperature on the Au substrate. The scan rate was 10 mV sec−1. (b) In the presence of 254 nm UV light. Inset shows the green luminescence after first CV scan. (c) CV in the presence of 365 nm UV light.
The work function of ITO is around 4.6 eV21 and the energy provided by 254 nm UV light is around 4.8 eV, thus enough to excite the substrate and generate electrons. It has been shown that UV−ozone treatments can change the work function of the ITO substrate.22 In our experiments, a small photovoltage was observed on switching the UV light. Figure 6a,b shows the photovoltage observed on ITO on illumination with 254 and 365 nm UV light. A maximum change of about 17 and 22 mV is seen on exciting the ITO substrate with 254 and 365 nm UV light, respectively. By directly using the UV light, no reduction of GeCl4 took place. It has been previously reported that the energy needed to form GeCl from GeCl4 by photolysis is >10 eV,23 and therefore, no reduction to Ge is possible using only UV light. However, the lowering of reduction potential by 200 mV cannot be explained by the generation of photovoltage alone and may be a combination of effects. The absorption of the UV light by the electrolyte is also a possibility, which then changes the interface at the substrate. Also, the absorption of energy by GeCl4-[Py1,4]Tf2N would change the thermodynamics and, therefore, lead to a lowering of the reduction potentials, as observed in Figure 2b. As the energy of 365 nm UV light is only 3.4 eV, the shift in the reduction potential in Figure 2c should be lower compared with that using 254 nm UV light. In comparison to ITO, the work function of polycrystalline Au is 5.2 eV, which corresponds to 238 nm.
Figure 5. (a) SEM of Ge obtained after constant potential electrodeposition at −2.11 V for 30 min. (b) Microstructure of Ge obtained after 30 min at −1.88 V in the presence of 254 nm UV light. (c) Ge obtained at −2.07 V in the presence of 365 nm UV light. (d) EDX spectrum of germanium deposit in (c).
which indicates that the changes observed in the reduction potential might have been due to the absorption of UV light. 17743
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Figure 6. (a) Change in OCP on exciting the ITO substrates with 254 nm UV light. (b) On excitation with 365 nm UV light. Pt wires were used as reference and counter electrodes.
sulfonyl group25 and a sharp peak at 212 nm. On addition of GeCl4, there is no additional peak observed in the UV scan until 225 nm, where a sharp increase in absorption starts, resulting in a peak at 208 nm. As the UV light used in our experiments (254 nm) falls within this region, the electrolyte would have absorbed the UV light, leading to a shift of the reduction potential. After one CV cycle, there is an increased absorption in the overall UV scan, and a prominent rise in the absorption occurs around 250 nm, giving rise to a peak at 216 nm. This rise in the absorption could be related to the formation of Ge(II) species during the CV cycle, which gives rise to the green luminescence in the presence of 254 nm UV light. It was previously reported by Nikol et al26 that GeCl3− shows a photoluminescence effect on exciting the sample with 250 nm light, giving rise to a green luminescence. Also, an absorption peak at 218 nm was observed by Nikol et al,26 which further supports that Ge(II) species formed in the ionic liquid in our experiments during the CV cycle gives rise to the photoluminescence effect.
However, the adsorption of ions can reduce the work function. Mukhopadhyay et al24 had shown that the work function of Au(111) was reduced to 2 eV in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide. However, in our experiments, no photovoltage was observed on illuminating the Au substrate with 254 nm UV light, which suggests that an insignificantly small amount of change of work function might have taken place. Alternatively, underpotential deposition of germanium was reported by Endres and El-Abedin.14 The underpotential deposition of Ge on Au and ITO might have generated a photovoltage that contributed to the lowering of the reduction potential. At present, we do not have a conclusive proof for the above-mentioned interpretation, and it is an open question that cannot be answered completely. To further evaluate the influence of UV light on electrodeposition, UV−visible spectroscopy measurement of the ionic liquid was performed. Figure 7 compares the UV spectra of the pure ionic liquid and the ionic liquid containing GeCl4 before and after one CV cycle. The [Py1,4]Tf2N ionic liquid shows a broad absorption peak between 225 and 325 nm that could be due to the presence of a
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CONCLUSIONS In this paper, we have shown that the UV light affects the reduction potential of the germanium species on both Au and ITO substrates. The results showed that, with decreasing the wavelength of the UV light, a shift to less negative values in the reduction potential takes place. The negative shift may be due to a combination of effects, such as the absorption of UV light by the electrolyte and the generation of photovoltage by the substrate/underpotential deposition of Ge. A green luminescence was also noticed after one CV cycle, which was attributed to the presence of GeCl2 in the electrolyte. On performing constant potential electrodeposition on Au and ITO in the presence and absence of UV light, the clustered growth of germanium was observed. However, the cluster formation was less in the presence of UV light.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
Figure 7. Comparison of pure [Py1,4]Tf2N, and [Py1,4]Tf2N containing 0.1 M GeCl4 and after one CV cycle. These samples were diluted in spectroscopic grade isopropanol, giving a final concentration of 10 mM.
Notes
The authors declare no competing financial interest. 17744
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REFERENCES
(1) Gamburg, Y. D.; Zangari, G. Theory and Practice of Metal Electrodeposition; Springer: Berlin, 2011. (2) Dini, J. W. Electrodeposition: The Materials Science of Coatings and Substrates; Noyas Publication: New York, 1993. (3) Colin, G. F.; Vasant, M. D. J. Electrochem. Soc. 1949, 95, 80−97. (4) Liang, X.; Kim, Y.-G.; Gebergziabiher, D. K.; Stickney, J. L. Langmuir 2009, 26, 2877−2884. (5) Jayakrishnan, S.; Pushpavanam, M.; Shenoi, B. A. Surf. Technol. 1981, 13, 225−240. (6) Liang, X.; Zhang, Q.; Lay, M. D.; Stickney, J. L. J. Am. Chem. Soc. 2011, 133, 8199−8204. (7) Chandrasekhran, N.; Sevov, S. C. J. Electrochem. Soc. 2010, 157, C140−C145. (8) van Vugt, L. K.; van Driel, A. F.; Tjerkstra, R. W.; Bechger, L.; Vos, W. L.; Vanmaekelbergh, D.; Kelly, J. J. Chem Commun. 2002, 2054−2055. (9) Huang, Q.; Bedell, S. W.; Saenger, K. L.; Copel, M.; Deligianni, H.; Romankiw, L. T. Electrochem. Solid-State Lett. 2007, 10, D124− D126. (10) Endres, F.; Zein El Abedin, S. Phys. Chem. Chem. Phys. 2006, 8, 2101−2116. (11) Endres, F. ChemPhysChem 2002, 3, 144−154. (12) Plechkova, N. V.; Seddon, K. R. Chem. Soc. Rev. 2008, 37, 123− 150. (13) Endres, F. Phys. Chem. Chem. Phys. 2001, 3, 3165−3174. (14) Endres, F.; Zein El Abedin, S. Phys. Chem. Chem. Phys. 2002, 4, 1649. (15) Endres, F.; Zein El Abedin, S. Chem Commun. 2002, 892−893. (16) Freyland, W.; Zell, C. A.; Zein El Abedin, S.; Endres, F. Electrochim. Acta 2003, 48, 3053−3061. (17) Al-Salman, R.; Mallet, J.; Molinari, M.; Fricoteaux, P.; Martineau, F.; Troyon, M.; Zein El Abedin, S.; Endres, F. Phys. Chem. Chem. Phys. 2008, 10, 6233−6237. (18) Al-Salman, R.; Zein El Abedin, S.; Endres, F. Phys. Chem. Chem. Phys. 2008, 10, 4650−4657. (19) Al-Salman, R.; Endres, F. J. Mater. Chem. 2009, 19, 7228−7231. (20) Meng, X.; Al-Salman, R.; Zhao, J.; Borissenko, N.; Li, Y.; Endres, F. Angew. Chem., Int. Ed. 2009, 48, 2703−2707. (21) Kiyoshi, S.; Hisao, I.; Yukio, O.; Kazuhiko, S. J. Appl. Phys. 2000, 87, 295−298. (22) Kim, S. Y.; Lee, J.; Kim, K.; Tak, Y. H. J. Appl. Phys. 2004, 95, 2560−2563. (23) Ibuki, T. Chem. Phys. Lett. 1990, 169, 64−68. (24) Mukhopadhyay, I.; Aravinda, C. L.; Borissov, D.; Freyland, W. Electrochim. Acta 2005, 50, 1275−1281. (25) Ranieri, G.; Hallett, J. P.; Welton, T. Ind. Eng. Chem. Res. 2008, 47, 638−644. (26) Nikol, H.; Becht, A.; Vogler, A. Inorg. Chem. 1992, 31, 3277− 3279.
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