- Email: [email protected]
Study on electroreduction of Eu(III) and electrodeposition of Eu–Co in europium toluenesulfonate+DMF
Journal of Electroanalytical Chemistry 456 (1998) 223 – 227
Study on electroreduction of Eu(III) and electrodeposition of Eu– Co in europium toluenesulfonate+ DMF Qiqin Yang *, Peng Liu, Yansheng Yang, Yexiang Tong Facalty of Chemistry and Chemical Engineering, Zhongshan Uni6ersity, Guangzhou 510275, People’s Republic of China Received 15 April 1998; received in revised form 18 June 1998
Abstract Europium toluenesulfonate highly soluble in organic solvent was prepared for the first time and was used as the electrolyte in DMF. The composition of its crystalline hydrate was determined as (p-CH3C6H4SO3)3Eu · 7H2O. Cyclic voltammetry and the E –t curve resulting from a current step were used to study the electroreduction of Eu(III) in (p-CH3C6H4SO3)3Eu+ DMF. The electrode process of Eu(III) reduced on a Pt electrode occurs in two steps: Eu(III) +e − = Eu(II), Eu(II) +2e − = Eu; the first step is quasi-reversible. The diffusion coefficient of Eu(III) was calculated as 5.8 ×l0 − 7 cm2 s − 1 at 25°C. A europium film with a bluish violet lustre and a Eu–Co film with black metallic lustre were obtained by potentiostatic electrolysis. SEM, EDAX and XRD were used to analyze the film. The composition of Eu – Co film is 69.4 at.%Co and 30.6 at.%Eu. It adheres firmly to the copper substrate. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Europium toluenesulfonate; Dimethylformamide, Electrodeposition of europium; Electroreduction of Eu(III); Diffusion coefficient; Electrodeposition of Eu–Co
1. Introduction The films of rare earth metals and their alloys have many special properties and can be used to prepare functional materials. So far, these films have usually been produced by sputtering or vacuum plating. If such thin films were prepared by electrodeposition, the production process would be simplified and the manufacturing cost would be reduced. Since the rare earth elements are very active, non-aqueous electrolytes are often used. The electrodeposition of Y, Nd, La, Dy, Sm – Co, Gd–Co, Nd – Fe, Dy – Fe from organic solvents has been reported [1 – 7], but in these papers the low solubility in organic solvent of the electrolytes (RE(CH3COO)3, RE(NO3)3, RECl3) resulted in low electric conductivity and severe concentration polarization. The coulometric efficiency is low and the state of * Corresponding author. [email protected]
Fax:
+86
20
84189173;
e-mail:
the deposit is poor. Therefore, an increase in the solubility is the key for the utilization of electrodeposition of rare earth alloys. In this paper we have prepared europium toluenesulfonate. It was used as the electrolyte in DMF from which the electroreduction of Eu(III) and the electrodeposition were studied.
2. Experimental DMF(AR) was distilled under vacuum after MgSO4 addition to remove water and then molecular sieves were added to the distilled DMF. The supporting electrolyte, (n-Bu)4NBF4 was prepared following the literature [8]. Europium toluenesulfonate was prepared by (99.95%) and pthe reaction of Eu2O3 CH3C6H4SO3H · H2O(AR). After recrystallization the salt was analyzed by thermogravimetry (TG), differential scanning calorimetry (DSC) and X-ray energy dis-
0022-0728/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S0022-0728(98)00283-6
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Q. Yang et al. / Journal of Electroanalytical Chemistry 456 (1998) 223–227
persive analysis (EDAX). The hydrated salt was dehydrated at 120°C under 0.5 – 1 kPa and preserved in a desiccator containing P2O5 for use in the electrochemical experiments. Pt (99.9%) and Cu (99.95%) were used as the working electrodes, spectroscopically pure graphite or Pt were used as the counter electrode and the reference electrode was an SCE which was connected to the cell through a salt bridge. The salt bridge contains the gel agaropectin and tetrabutylammonium picrate. The tip of the salt bridge was filled with asbestos. All the potential values in this paper are versus SCE. Argon was bubbled through the electrolytic solution to remove oxygen. The electrochemical measurements were carried out in an argon atmosphere. The electrodeposit obtained was analyzed by EDAX, SEM and XRD.
3. Results and discussion
3.1. Analysis of europium toluenesulfonate EDAX was used to analyze the content of the elements Eu and S in the prepared salt; the result shows that the molar ratio of Eu:S equals 1:3, thus it can be inferred that the composition of the salt is as follows: p-(CH3C6H4SO3)3Eu · xH2O. Fig. 1 shows the DSC and TG curves of the prepared hydrous salt. From the DSC curve, it can be seen that three phase changes occurred in the temperature range 30 – 730°C and two phase changes with weak heat absorption from 47.5 to 119.2°C and 119.2 to 177.4°C, one phase change with strong heat evolution from 387.5 to 520°C. On the TG curve in the temperature range mentioned above the weight loss occurred in three regions. The losses of weight from 47.5 to 119.2°C and 119.2 to 177.4°C are owing to water loss; according to the values of weight loss on the TG curve, the numbers of water molecules lost are calculated as 5 and 2, respectively. Hence, the number of waters of crystalliza-
Fig. 1. TG and DSC curves of europium teluenesulfonate crystalline hydrate.
Fig. 2. Cyclic voltammogram of a Pt electrode (0.54 cm2) in pCH3C6H4SO3Na (0.03 mol l − 1) +(n-Bu)4NBF4 (0.03 mol l − 1) + DMF at 25°C 0.100 V s − 1.
tion in europium toluenesulfonate crystalline hydrate is 7. The 5H2O lost at lower temperature may be water in crystal lattice, the 2H2O lost at the higher temperature may be water coordinated to Eu(III). The change with strong heat evolution and large loss of weight happened from 387.5 to 520°C; this must be decomposition or oxygenolysis reaction of the salt before it melts. The DSC and TG experiments show that europium toluenesulfonate hydrate can lose all the water of crystallisation by 177.4°C at 1 atm and that europium toluenesulfonate is stable to heat and not easy to oxidize or hydrolyze below 387.5°C. Therefore, the anhydrous salt is easy to prepare and further experiment shows that it does not deliquescence in the air. A solubility test shows that europium toluenesulfonate is easy to dissolve in organic solvents, such as DMF, DMSO, ethylenediamine, formamide. For example, at 25°C, 100 ml DMF can dissolve 100 g of europium toluenesulfonate compared with RECl3, RE(NO3)3, the solubility is high.
3.2. Electrochemical beha6ior of Eu(III) 3.2.1. Electrode process of the reduction of Eu(III) Fig. 2 shows the cyclic voltammogram of a Pt electrode in p-CH3C6H4SO3Na+ (n-Bu)4NBF4 +DMF. The lower limit is − 1.9 V, the upper limit is 1.5 V, so the electrochemical window is 3.4 V. Thus, in a large potential range, no electrochemical reaction of p-CH3C6H4SO3Na or (n-Bu)4NBF4 occurs. The cyclic voltammogram of a Pt electrode in (pCH3C6H4SO3)3Eu+(n-Bu)4NBF4 + DMF is shown in Fig. 3. Two cathodic peaks starting at −0.53 and − 0.87 V and on the reverse sweep, two anodic peaks appeared correspondingly. Because of the known valence states of europium, it is considered preliminarily that the first cathodic peak is due to the reduction of Eu(III) to Eu(II) and the second cathodic peak is due to the reduction of Eu(II) to Eu. The two anodic peaks then correspond to the anodic stripping of Eu and the oxidation of Eu(II) to Eu(III).
Q. Yang et al. / Journal of Electroanalytical Chemistry 456 (1998) 223–227
225
Fig. 3. Cyclic voltammogram of a Pt electrode (0.54 cm2) in (pCH3C6H4SO3)3Eu (0.04 mol l − 1)+ (n-Bu)4NBF4 (0.10 mol l − 1) + DMF at 25°C 0.040 V s − 1.
The galvanostatic E – t curve on a Pt electrode in (p-CH3C6H4SO3)3Eu +(n-Bu)4NBF4 +DMF is shown in Fig. 4. Two potential plateaus at − 0.60 and −0.93 V appeared on the E – t curve. These potentials are in accord with the starting potentials of the cathodic peaks in the CV curve. This suggests that the reduction of Eu(III) may occur in two steps. For a multi-step electrode process, the relation between the ratio of transition times and the number of electron transfer is as follows [9]. t2/t1 =2n2/n1 +(n2/n1)2
(1)
From Fig. 4 the value of t2/t1 is 8, hence from Eq. (1), n2 = 2n1 is obtained. According to the results deduced from the CV curve and the galvanostic E – t curve, it can be determined that the electroduction of Eu(III) to Eu is through the two steps: Eu(III) +e − = Eu(II) Eu(II)+2e − =Eu
Fig. 4. E – t curve resulting from a current step (3.01 mA) of a Pt electrode (0.54 cm2) in (p-CH3C6H4SO3)3Eu (0.04 mol l − 1) +(nBu)4NBF4 (0.03 mol l − 1)+ DMF at 25°C.
Fig. 5. Cyclic voltammogram of the first cathodic peak on a Pt electrode (0.54 cm2) in (p-CH3C6H4SO3)3Eu (0.04 mol l − 1) +(nBu)4NBF4 (0.10 mol l − 1) +DMF at 25°C.
3.2.2. Diffusion coefficient of Eu(III) Fig. 5 shows the cyclic voltammograms of the first cathodic peak at different sweep rates. With the increase of the sweep rate, the cathodic peak potential shifts negatively, and the anodic peak potential shifts positively. The shape of the cyclic voltammogram has the character of a quasi-reversible charge transfer [9]. Although the peak separation is large the reaction is by no means irreversible; it may be caused by IR drop. The relation between cathodic peak current and the square root of the sweep rate is shown in Fig. 6. At lower sweep rates (less than 0.070 V s − 1) the relation between Ip and 6 1/2 is linear. With the increase of sweep rate Ip deviates below the line. All the above characteristics indicate that the electro-reduction of Eu(III) to Eu(II) is quasireversible. At lower sweep rates the relation between Ip
Fig. 6. Plot of Ip – 6 1/2 for the first cathodic peak.
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Q. Yang et al. / Journal of Electroanalytical Chemistry 456 (1998) 223–227
Fig. 7. Cyclic voltammogram of a Cu electrode (1.1 cm2) in CoCl2 (0.03 mol l − 1)+ (n-Bu)4NBF4 (0.06 mol l − 1)+ DMF (A) and CoCl2 (0.03 mol l − 1)+ (p-CH3C6H4SO3)3Eu (0.08 mol l − 1)+ (n-Bu)4NBF4 (0.06 mol l − 1)+ DMF (B) at 25°C 0.040 V s − 1.
and 6 1/2 can be treated approximately as that for a reversible charge transfer [10], with the soluble electrode product. The Randles – Sevc' ik equation for the peak is as follows [9]: Ip = 0.4463(nF)3/2(Dn/RT)1/2Ac
+ DMF. Three cathodic peaks appeared, the starting potential and peak potential of the first peak being in accord with the CV of Co(II) reduction (Fig. 7B), so it is caused by the reduction of Co(II) to Co. The second and third peaks are due to the reduction of Eu(III) to Eu in two steps. The first anodic peak is very sharp, due to the stripping of Eu and then Eu(II) is oxidized to Eu(III). The last anodic peak is due to the stripping of Co. Electrodeposition was carried out on a Cu sheet by potentiostatic electrolysis at −1.1 V for 30 min and a deposit with a black metallic lustre was obtained. The current density was 4.7 mA cm − 2, its thickness was 1.4 mm. EDAX was used to analyzed the deposit as shown in Fig. 8. The Ka and Kb, peaks of Eu and Co are strong, other peaks belong to the L series; the composition was determined as 69.4 at.%Co and 30.6 at.%Eu. The deposit can be stripped in 0.1 mol l − 1 HCl with a large amount of gas released at the beginning and it stripped completely after 5 min. SEM (4 keV) shows the film is dense and uniform. No diffraction peaks were found in the XRD pattern, so the Eu–Co film also exist as an amorphous state.
Acknowledgements
(2)
According to the slope of the linear part in Fig. 6 and Eq. (2), the diffusion coefficient of Eu(III) at 25°C is calculated as 5.8×10 − 7 cm2 s − 1. The low value in a solvent with low viscosity suggests the reacting species must be large. In fact the solvate exists between Eu(III) and DMF [11], the order of affinity is as: DMF \ TBP \NO3− \ H2O [12].
One of the authors Liu Peng, a Ph.D. candidate, thanks Mr Ma Chanan for a Ma Chanan scholarship.
3.3. Electrodeposition of Eu and Eu– Co The cyclic voltammogram of Eu(III) on a Cu electrode is similar to that on a Pt electrode; the electroreduction of Eu(III) is in two steps. A deposit was obtained by potentiostatic electrolysis at − 1.1 V on a Cu electrode for 30 min. The deposit film was 1.1 mm thick and adheres firmly to the copper substrate, and had a bluish violet lustre. Scanning electron microscope observation showed that the deposit was uniform and smooth. It can be stripped in 0.1 mol l − 1 HCl with a large amount of gas released. The deposit was analyzed by EDAX and Eu was the only element found in the deposited film. Thus, the deposit should be metallic europium. No diffraction peaks of Eu or Eu2O3 were found in the XRD pattern, but the base line of the pattern is not smooth, so it may be exist as amorphous state. Fig. 7A shows the cyclic voltammogram of a Cu sheet in CoCl2 +(p-CH3C6H4SO3)3Eu + (n-Bu)4NBF4
Fig. 8. EDAX pattern of Eu – Co film.
Q. Yang et al. / Journal of Electroanalytical Chemistry 456 (1998) 223–227
References [1] [2] [3] [4]
T. Moeller, P. Zimmerman, Science 120 (1954) 539. P.P. Kumbhar, C.D. Lokhande, Metal Finish. 4 (1995) 28. P.P. Kumbhar, C.D. Lokhande, Metal Finish 11 (1994) 70. Y. Sato, T. Takazawa, M. Akahashi, H. Ishida, K Kobayakawa, Plating Surf. Finish. 90 (1993) 72. [5] N. Usuzaka, H. Yamaguchi, T. Watanabe, Mater. Sci. Eng. 99 (1988) 105. [6] N. Yoshimoto, O. Shinoura, H. Miyauchi, Denki Kagaku 62 (1994) 982.
.
227
[7] Y. Matsuda, T. Fujii, N. Yoshimoto, M. Morita, J. Alloy Compd. 193 (1993) 23. [8] H.O. House, E. Feng, L. Meites, J. Org. Chem. 36 (1971) 2371. [9] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Wiley, New York, 1980, pp. 268, 230, 218. [10] Tian Zhaowu, Electrochemical Research Methods (in Chinese), Science Publishing House, Beijing, 1984, p. 230. [11] Handbook on the Physics and Chemistry of Rare Earth, vol. 3, North-Holland, Amsterdam, 1979, P233. [12] Coordination Chemistry Review, vol. 27, No. 3, 1979, P223.
Study on electroreduction of Eu(III) and electrodeposition of Eu– Co in europium toluenesulfonate+ DMF Qiqin Yang *, Peng Liu, Yansheng Yang, Yexiang Tong Facalty of Chemistry and Chemical Engineering, Zhongshan Uni6ersity, Guangzhou 510275, People’s Republic of China Received 15 April 1998; received in revised form 18 June 1998
Abstract Europium toluenesulfonate highly soluble in organic solvent was prepared for the first time and was used as the electrolyte in DMF. The composition of its crystalline hydrate was determined as (p-CH3C6H4SO3)3Eu · 7H2O. Cyclic voltammetry and the E –t curve resulting from a current step were used to study the electroreduction of Eu(III) in (p-CH3C6H4SO3)3Eu+ DMF. The electrode process of Eu(III) reduced on a Pt electrode occurs in two steps: Eu(III) +e − = Eu(II), Eu(II) +2e − = Eu; the first step is quasi-reversible. The diffusion coefficient of Eu(III) was calculated as 5.8 ×l0 − 7 cm2 s − 1 at 25°C. A europium film with a bluish violet lustre and a Eu–Co film with black metallic lustre were obtained by potentiostatic electrolysis. SEM, EDAX and XRD were used to analyze the film. The composition of Eu – Co film is 69.4 at.%Co and 30.6 at.%Eu. It adheres firmly to the copper substrate. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Europium toluenesulfonate; Dimethylformamide, Electrodeposition of europium; Electroreduction of Eu(III); Diffusion coefficient; Electrodeposition of Eu–Co
1. Introduction The films of rare earth metals and their alloys have many special properties and can be used to prepare functional materials. So far, these films have usually been produced by sputtering or vacuum plating. If such thin films were prepared by electrodeposition, the production process would be simplified and the manufacturing cost would be reduced. Since the rare earth elements are very active, non-aqueous electrolytes are often used. The electrodeposition of Y, Nd, La, Dy, Sm – Co, Gd–Co, Nd – Fe, Dy – Fe from organic solvents has been reported [1 – 7], but in these papers the low solubility in organic solvent of the electrolytes (RE(CH3COO)3, RE(NO3)3, RECl3) resulted in low electric conductivity and severe concentration polarization. The coulometric efficiency is low and the state of * Corresponding author. [email protected]
Fax:
+86
20
84189173;
e-mail:
the deposit is poor. Therefore, an increase in the solubility is the key for the utilization of electrodeposition of rare earth alloys. In this paper we have prepared europium toluenesulfonate. It was used as the electrolyte in DMF from which the electroreduction of Eu(III) and the electrodeposition were studied.
2. Experimental DMF(AR) was distilled under vacuum after MgSO4 addition to remove water and then molecular sieves were added to the distilled DMF. The supporting electrolyte, (n-Bu)4NBF4 was prepared following the literature [8]. Europium toluenesulfonate was prepared by (99.95%) and pthe reaction of Eu2O3 CH3C6H4SO3H · H2O(AR). After recrystallization the salt was analyzed by thermogravimetry (TG), differential scanning calorimetry (DSC) and X-ray energy dis-
0022-0728/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S0022-0728(98)00283-6
224
Q. Yang et al. / Journal of Electroanalytical Chemistry 456 (1998) 223–227
persive analysis (EDAX). The hydrated salt was dehydrated at 120°C under 0.5 – 1 kPa and preserved in a desiccator containing P2O5 for use in the electrochemical experiments. Pt (99.9%) and Cu (99.95%) were used as the working electrodes, spectroscopically pure graphite or Pt were used as the counter electrode and the reference electrode was an SCE which was connected to the cell through a salt bridge. The salt bridge contains the gel agaropectin and tetrabutylammonium picrate. The tip of the salt bridge was filled with asbestos. All the potential values in this paper are versus SCE. Argon was bubbled through the electrolytic solution to remove oxygen. The electrochemical measurements were carried out in an argon atmosphere. The electrodeposit obtained was analyzed by EDAX, SEM and XRD.
3. Results and discussion
3.1. Analysis of europium toluenesulfonate EDAX was used to analyze the content of the elements Eu and S in the prepared salt; the result shows that the molar ratio of Eu:S equals 1:3, thus it can be inferred that the composition of the salt is as follows: p-(CH3C6H4SO3)3Eu · xH2O. Fig. 1 shows the DSC and TG curves of the prepared hydrous salt. From the DSC curve, it can be seen that three phase changes occurred in the temperature range 30 – 730°C and two phase changes with weak heat absorption from 47.5 to 119.2°C and 119.2 to 177.4°C, one phase change with strong heat evolution from 387.5 to 520°C. On the TG curve in the temperature range mentioned above the weight loss occurred in three regions. The losses of weight from 47.5 to 119.2°C and 119.2 to 177.4°C are owing to water loss; according to the values of weight loss on the TG curve, the numbers of water molecules lost are calculated as 5 and 2, respectively. Hence, the number of waters of crystalliza-
Fig. 1. TG and DSC curves of europium teluenesulfonate crystalline hydrate.
Fig. 2. Cyclic voltammogram of a Pt electrode (0.54 cm2) in pCH3C6H4SO3Na (0.03 mol l − 1) +(n-Bu)4NBF4 (0.03 mol l − 1) + DMF at 25°C 0.100 V s − 1.
tion in europium toluenesulfonate crystalline hydrate is 7. The 5H2O lost at lower temperature may be water in crystal lattice, the 2H2O lost at the higher temperature may be water coordinated to Eu(III). The change with strong heat evolution and large loss of weight happened from 387.5 to 520°C; this must be decomposition or oxygenolysis reaction of the salt before it melts. The DSC and TG experiments show that europium toluenesulfonate hydrate can lose all the water of crystallisation by 177.4°C at 1 atm and that europium toluenesulfonate is stable to heat and not easy to oxidize or hydrolyze below 387.5°C. Therefore, the anhydrous salt is easy to prepare and further experiment shows that it does not deliquescence in the air. A solubility test shows that europium toluenesulfonate is easy to dissolve in organic solvents, such as DMF, DMSO, ethylenediamine, formamide. For example, at 25°C, 100 ml DMF can dissolve 100 g of europium toluenesulfonate compared with RECl3, RE(NO3)3, the solubility is high.
3.2. Electrochemical beha6ior of Eu(III) 3.2.1. Electrode process of the reduction of Eu(III) Fig. 2 shows the cyclic voltammogram of a Pt electrode in p-CH3C6H4SO3Na+ (n-Bu)4NBF4 +DMF. The lower limit is − 1.9 V, the upper limit is 1.5 V, so the electrochemical window is 3.4 V. Thus, in a large potential range, no electrochemical reaction of p-CH3C6H4SO3Na or (n-Bu)4NBF4 occurs. The cyclic voltammogram of a Pt electrode in (pCH3C6H4SO3)3Eu+(n-Bu)4NBF4 + DMF is shown in Fig. 3. Two cathodic peaks starting at −0.53 and − 0.87 V and on the reverse sweep, two anodic peaks appeared correspondingly. Because of the known valence states of europium, it is considered preliminarily that the first cathodic peak is due to the reduction of Eu(III) to Eu(II) and the second cathodic peak is due to the reduction of Eu(II) to Eu. The two anodic peaks then correspond to the anodic stripping of Eu and the oxidation of Eu(II) to Eu(III).
Q. Yang et al. / Journal of Electroanalytical Chemistry 456 (1998) 223–227
225
Fig. 3. Cyclic voltammogram of a Pt electrode (0.54 cm2) in (pCH3C6H4SO3)3Eu (0.04 mol l − 1)+ (n-Bu)4NBF4 (0.10 mol l − 1) + DMF at 25°C 0.040 V s − 1.
The galvanostatic E – t curve on a Pt electrode in (p-CH3C6H4SO3)3Eu +(n-Bu)4NBF4 +DMF is shown in Fig. 4. Two potential plateaus at − 0.60 and −0.93 V appeared on the E – t curve. These potentials are in accord with the starting potentials of the cathodic peaks in the CV curve. This suggests that the reduction of Eu(III) may occur in two steps. For a multi-step electrode process, the relation between the ratio of transition times and the number of electron transfer is as follows [9]. t2/t1 =2n2/n1 +(n2/n1)2
(1)
From Fig. 4 the value of t2/t1 is 8, hence from Eq. (1), n2 = 2n1 is obtained. According to the results deduced from the CV curve and the galvanostic E – t curve, it can be determined that the electroduction of Eu(III) to Eu is through the two steps: Eu(III) +e − = Eu(II) Eu(II)+2e − =Eu
Fig. 4. E – t curve resulting from a current step (3.01 mA) of a Pt electrode (0.54 cm2) in (p-CH3C6H4SO3)3Eu (0.04 mol l − 1) +(nBu)4NBF4 (0.03 mol l − 1)+ DMF at 25°C.
Fig. 5. Cyclic voltammogram of the first cathodic peak on a Pt electrode (0.54 cm2) in (p-CH3C6H4SO3)3Eu (0.04 mol l − 1) +(nBu)4NBF4 (0.10 mol l − 1) +DMF at 25°C.
3.2.2. Diffusion coefficient of Eu(III) Fig. 5 shows the cyclic voltammograms of the first cathodic peak at different sweep rates. With the increase of the sweep rate, the cathodic peak potential shifts negatively, and the anodic peak potential shifts positively. The shape of the cyclic voltammogram has the character of a quasi-reversible charge transfer [9]. Although the peak separation is large the reaction is by no means irreversible; it may be caused by IR drop. The relation between cathodic peak current and the square root of the sweep rate is shown in Fig. 6. At lower sweep rates (less than 0.070 V s − 1) the relation between Ip and 6 1/2 is linear. With the increase of sweep rate Ip deviates below the line. All the above characteristics indicate that the electro-reduction of Eu(III) to Eu(II) is quasireversible. At lower sweep rates the relation between Ip
Fig. 6. Plot of Ip – 6 1/2 for the first cathodic peak.
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Fig. 7. Cyclic voltammogram of a Cu electrode (1.1 cm2) in CoCl2 (0.03 mol l − 1)+ (n-Bu)4NBF4 (0.06 mol l − 1)+ DMF (A) and CoCl2 (0.03 mol l − 1)+ (p-CH3C6H4SO3)3Eu (0.08 mol l − 1)+ (n-Bu)4NBF4 (0.06 mol l − 1)+ DMF (B) at 25°C 0.040 V s − 1.
and 6 1/2 can be treated approximately as that for a reversible charge transfer [10], with the soluble electrode product. The Randles – Sevc' ik equation for the peak is as follows [9]: Ip = 0.4463(nF)3/2(Dn/RT)1/2Ac
+ DMF. Three cathodic peaks appeared, the starting potential and peak potential of the first peak being in accord with the CV of Co(II) reduction (Fig. 7B), so it is caused by the reduction of Co(II) to Co. The second and third peaks are due to the reduction of Eu(III) to Eu in two steps. The first anodic peak is very sharp, due to the stripping of Eu and then Eu(II) is oxidized to Eu(III). The last anodic peak is due to the stripping of Co. Electrodeposition was carried out on a Cu sheet by potentiostatic electrolysis at −1.1 V for 30 min and a deposit with a black metallic lustre was obtained. The current density was 4.7 mA cm − 2, its thickness was 1.4 mm. EDAX was used to analyzed the deposit as shown in Fig. 8. The Ka and Kb, peaks of Eu and Co are strong, other peaks belong to the L series; the composition was determined as 69.4 at.%Co and 30.6 at.%Eu. The deposit can be stripped in 0.1 mol l − 1 HCl with a large amount of gas released at the beginning and it stripped completely after 5 min. SEM (4 keV) shows the film is dense and uniform. No diffraction peaks were found in the XRD pattern, so the Eu–Co film also exist as an amorphous state.
Acknowledgements
(2)
According to the slope of the linear part in Fig. 6 and Eq. (2), the diffusion coefficient of Eu(III) at 25°C is calculated as 5.8×10 − 7 cm2 s − 1. The low value in a solvent with low viscosity suggests the reacting species must be large. In fact the solvate exists between Eu(III) and DMF [11], the order of affinity is as: DMF \ TBP \NO3− \ H2O [12].
One of the authors Liu Peng, a Ph.D. candidate, thanks Mr Ma Chanan for a Ma Chanan scholarship.
3.3. Electrodeposition of Eu and Eu– Co The cyclic voltammogram of Eu(III) on a Cu electrode is similar to that on a Pt electrode; the electroreduction of Eu(III) is in two steps. A deposit was obtained by potentiostatic electrolysis at − 1.1 V on a Cu electrode for 30 min. The deposit film was 1.1 mm thick and adheres firmly to the copper substrate, and had a bluish violet lustre. Scanning electron microscope observation showed that the deposit was uniform and smooth. It can be stripped in 0.1 mol l − 1 HCl with a large amount of gas released. The deposit was analyzed by EDAX and Eu was the only element found in the deposited film. Thus, the deposit should be metallic europium. No diffraction peaks of Eu or Eu2O3 were found in the XRD pattern, but the base line of the pattern is not smooth, so it may be exist as amorphous state. Fig. 7A shows the cyclic voltammogram of a Cu sheet in CoCl2 +(p-CH3C6H4SO3)3Eu + (n-Bu)4NBF4
Fig. 8. EDAX pattern of Eu – Co film.
Q. Yang et al. / Journal of Electroanalytical Chemistry 456 (1998) 223–227
References [1] [2] [3] [4]
T. Moeller, P. Zimmerman, Science 120 (1954) 539. P.P. Kumbhar, C.D. Lokhande, Metal Finish. 4 (1995) 28. P.P. Kumbhar, C.D. Lokhande, Metal Finish 11 (1994) 70. Y. Sato, T. Takazawa, M. Akahashi, H. Ishida, K Kobayakawa, Plating Surf. Finish. 90 (1993) 72. [5] N. Usuzaka, H. Yamaguchi, T. Watanabe, Mater. Sci. Eng. 99 (1988) 105. [6] N. Yoshimoto, O. Shinoura, H. Miyauchi, Denki Kagaku 62 (1994) 982.
.
227
[7] Y. Matsuda, T. Fujii, N. Yoshimoto, M. Morita, J. Alloy Compd. 193 (1993) 23. [8] H.O. House, E. Feng, L. Meites, J. Org. Chem. 36 (1971) 2371. [9] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Wiley, New York, 1980, pp. 268, 230, 218. [10] Tian Zhaowu, Electrochemical Research Methods (in Chinese), Science Publishing House, Beijing, 1984, p. 230. [11] Handbook on the Physics and Chemistry of Rare Earth, vol. 3, North-Holland, Amsterdam, 1979, P233. [12] Coordination Chemistry Review, vol. 27, No. 3, 1979, P223.