- Email: [email protected]
Influence of atmospheric water uptake on the hydrolysis of stannous chloride in the ionic liquid 1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate
Journal of Molecular Liquids 230 (2017) 209–213
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Influence of atmospheric water uptake on the hydrolysis of stannous chloride in the ionic liquid 1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate H.K. Farag a, S.R. El-Kiey b, S. Zein El Abedin b,⁎ a b
Inorganic Chemistry Department, National Research Centre, El Behoth St. 33, Dokki, Giza, Egypt Electrochemistry and Corrosion Laboratory, National Research Centre, El Behoth St. 33, Dokki, Giza, Egypt
a r t i c l e
i n f o
Article history: Received 29 October 2016 Received in revised form 6 January 2017 Accepted 9 January 2017 Available online 11 January 2017 Keywords: Ionic liquid Hydrolysis Electrodeposition Tin Water uptake
a b s t r a c t In this paper we report on the influence of water uptake on the hydrolysis of anhydrous stannous chloride in the ionic liquid 1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate ([Py1,4]TfO) under ambient atmosphere. The effect of water uptake on the electrodeposition of tin in the employed ionic liquid containing 0.1 M SnCl2 was also demonstrated. The hydrolysis products as well as the Sn electrodeposits were characterized by different techniques comprising; scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). The results show that the employed ionic liquid electrolyte strongly absorbs water under ambient conditions. Colloidal particles were formed in the electrolyte after an exposure time of about 4 weeks as a result of the hydrolysis of Sn(II) species. The electrochemical behaviour of the employed electrolyte and the morphology of tin electrodeposits were significantly influenced by the presence of water. Tin dendrites were obtained from IL-electrolyte exposed to air for one month, whereas dendrite free deposits were formed in the freshly prepared electrolyte. © 2017 Elsevier B.V. All rights reserved.
1. Introduction In recent years, ionic liquids have shown a huge potential as superior electrolytes for electrodeposition of metals and semiconductors. Ionic liquids offer some exceptional characteristics such as, e.g., wide electrochemical window, non-flammability, non-volatility, high thermal stability and low toxicity [1–4]. In view of these unique properties, ionic liquids can be regarded as a potential replacement for many aqueous electroplating baths, especially the toxic ones like cyanide baths for silver electroplating. Furthermore, non-volatility of ionic liquids makes them more environmentally desirable than volatile organic solvents which release harmful vapours into atmosphere. The wide electrochemical windows of ionic liquids enable the electrodeposition of reactive metals such as, e.g., lithium, magnesium or aluminium, which are not viable from aqueous electrolytes [5–8]. Also, the use of ionic liquids for the electrodeposition of less reactive metals that can be obtained from aqueous electrolytes, such as, e.g., tin [9,10], zinc [11–13], Fe [14–17], In [18,19] and other ones, is of advantages as the size and shape of the particles can be tuned without additives and the problems associated with hydrogen evolution can be avoided. Tin is one of the important metals and it is presently regarded as a promising anode candidate for ⁎ Corresponding author. E-mail address: [email protected] (S. Zein El Abedin).
http://dx.doi.org/10.1016/j.molliq.2017.01.025 0167-7322/© 2017 Elsevier B.V. All rights reserved.
the future generation Li ion batteries [20–22]. Electrodeposited Sn thin films directly obtained on a current collector showed a high capacity when investigated as anode for lithium ion batteries [23–26]. The electrodeposition of tin in different air and water stable ionic liquids such as, [EMIm]DCA [27], [Py1,4]DCA [28], [Py1,4]TfO [10], [Py1,4]TFSA [29], and ZnCl2-[EMIm]Cl [30], was reported. It was reported that dendrite free, Sn deposits can be obtained from the ionic liquid [Py1,4]TfO [10]. The ionic liquid [Py1,4]TfO has some interesting characteristics. The [Py1,4]+ cation can act as a grain refiner due to its strong adsorptive interaction with surfaces leading to nanocrystalline deposits [31]. Moreover, the TfO− anion exhibits an intense structure dissociation capability due to its bulky size facilitating the detachment of [Py1,4]+ cations from the transient bonding with anions [32]. This can enhance the adsorptive characteristics of the cation. To our knowledge, all reported studies on the electrodeposition of Sn from ionic liquids were performed under inert gas conditions which, from a practical point of view, limit the widespread application of ionic liquids as electrochemical baths for Sn deposition. In the present study we have performed the electrodeposition experiments under open air conditions to make the electrodeposition process more versatile and simple. The employed ionic liquid is air and water stable so it can be employed as an electrolyte in open galvanic baths. However, all ionic liquids, whether they are hydrophobic or hydrophilic, can quickly absorb considerable amounts of water when they are exposed to air [33,
H.K. Farag et al. / Journal of Molecular Liquids 230 (2017) 209–213
34]. As a consequence, the physical properties of the ionic liquids, such as viscosity, conductivity, solubility, ion diffusion and electrochemical stability, are strongly influenced. It should be noted that, the presence of water in ionic liquids might be a disadvantage for some applications, but not for others. Nevertheless, in all cases the water content should be controlled as the properties of ionic liquids are dependent not only on the presence of water but also on its concentration. As electrochemical liquids, the presence of a small amount of water is of advantage as the viscosity decreases, the conductivity and ion diffusion increase. On the other hand, the electrochemical window is reduced [34]. The extent of absorbed water is correlated to the strength of the hydrogen bonding among water molecules and also to the strength of the interaction between water and the anions of the ionic liquids [33]. The present paper aims at demonstrating the effect of atmospheric water uptake on the hydrolysis of stannous chloride in the ionic liquid 1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate with the sake of exploring the viability of employing the SnCl2/[Py1,4]TfO ionic liquid electrolyte as an electroplating bath for Sn. The extent of water absorption of the employed electrolyte on exposure to air, as a function of time, was determined in conditions of ambient humidity and room temperature. The influence of water uptake on the electrodeposition of Sn in the employed electrolyte was also investigated. 2. Experimental The ionic liquid 1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate with purity 99% (IoLiTec., Germany) was employed in this study. The ionic liquid was used as received without further purification or drying. The water content in the as-received was found to be 138 ppm. The water content in the employed ionic liquid electrolyte was determined by Karl Fischer titration, using a Metrohm 899 coulometer. Tin(II) chloride (Aldrich 99.99%) was used as the precursor for Sn electrodeposition. An electrolyte of 0.1 M SnCl2 in [Py1,4]TfO was employed in this study. Isopropanol (Alfa 99.5%) and bi-distilled water were used for washing the hydrolysis products of the employed ionic liquid electrolyte. The cyclic voltammetry and chronoamperometry measurements were performed using a Verstat 263A Potentiostat/Galvanostat (Princeton Applied Research) controlled by PowerCV and PowerStep software, respectively. The experiments were carried out under air with a relative humidity of 60 ± 5%. The electrochemical cell was made of polytetrafluoroethylene (Teflon) and clamped over a Teflon covered Viton O-ring onto the working electrode, thus yielding a geometric surface area of 0.3 cm2. A platinum ring (Alfa, 99.99%) and a platinum wire (Alfa, 99.99%) were used as counter and reference electrodes, respectively. Copper sheets (99.9%) were used as working electrodes and prior to use, the substrates were successively polished with silicon carbide emery paper of increasing fineness up to 1000, then degreased with acetone in an ultrasonic bath for 2 min. The hydrolysis products of the employed ionic liquid electrolyte were calcined in a muffle oven at the desired temperature for two hours and were cooled down to the ambient temperature in the oven. Various techniques were employed for the characterization of the obtained materials. A high resolution field emission scanning electron microscope (Carl Zeiss DSM 982 Gemini) was utilized to investigate the surface morphology of the obtained products, and energy dispersive X-ray analysis was used to determine the chemical composition. The phase composition of the obtained materials was investigated by Xray diffraction (XRD) using a PANalytical diffractometer with CuKα radiation. 3. Results and discussion
exposure time to ambient atmosphere. A glass bottle containing 50 ml of the employed electrolyte was exposed to air and left without stirring. The average humidity in the atmosphere was 60 ± 5% throughout the duration of the experiment and the room temperature was 20 ± 3 °C. Then the water content in the ionic liquid electrolyte was measured at different exposure time. The initial water content was found to be 138 ppm. As can be seen in Fig. 1, three different absorption intervals featuring the water absorption behaviour of the ionic liquid electrolyte, depending on the exposure time, are observed. In the early days of air exposure the IL-electrolyte strongly absorbs water from atmosphere reaching a concentration of 2500 ppm in the first three days, and in one week it reaches 16,000 ppm. The rate of water absorption in the first absorption interval, estimated from the slope of the extrapolated line, Fig. 1 is about 2000 ppm/day. In the absorption interval II, from 7 to 50 days, the water uptake continued to increase, but with a slower rate, with the increase in the exposure time reaching about 30,000 ppm after about 5 weeks. The rate of water absorption in this zone was found to be 500 ppm/day which is lower than that of the absorption interval I. After about 50 days of air exposure, interval III, the tendency of water absorption is considerably reduced as revealed from the very low rate of absorption in this region, 40 ppm/day. It can be stated that the IL-electrolyte slowly approaches its water saturation limit in this interval. It is worth mentioning that the freshly prepared 0.1 M SnCl2/ [Py1,4]TfO electrolyte was a clear solution, and after long exposure to air visible colloidal particles were formed. The particles slowly coagulated forming obvious transparent precipitates. The precipitates were obtained after an air exposure time of about four weeks, i.e. when the water content in the electrolyte was about 27,000 ppm. The obtained precipitates are the hydrolysis products of Sn(II) species in the ionic liquid electrolyte. It was reported that aqueous solution of SnCl2 undergoes hydrolysis by ongoing time forming a mixture of SnO2, Sn3O4 and Sn4(OH)6Cl2 [35]. The formed precipitates were retrieved from the electrolyte, washed with isopropanol, dried under vacuum and then characterized by SEM-EDX and XRD to investigate both morphology and composition. Fig. 2 shows the SEM-EDX analysis of the as-obtained hydrolysis product and of the calcined product at 500 °C for 2 h. As shown in the SEM micrograph of Fig. 2a the as obtained precipitates have a regular sheet-like structure. The accompanied EDX profile, Fig. 2b, shows the peaks of Sn, Cl and O indicating the formation tin hydroxychloride. The peaks of C, S, N and F are also recorded signifying the presence of ionic liquid residues in the precipitates. The sheet-like structure of the as-formed precipitates disintegrates on calcinations as shown in the SEM micrograph of the calcined product, Fig. 2c. The EDX profile of the calcined product reveal the transformation of tin hydroxychloride to tin oxide as the Cl peak was vanished (inset of Fig. 2c). The XRD patterns of the obtained precipitates without and with calcination at 200 °C and at 500 °C are depicted in Fig. 3. The XRD patterns of the as obtained
40000
[Py1,4]TfO + 0.1 M SnCl2
III
35000
Water Uptake / ppm
210
30000 25000
II
20000 15000 10000
I
5000 0
0
3.1. Hydrolysis of 0.1 M SnCl2/[Py1,4]TfO The water absorption tendency of the employed ionic liquid (IL) electrolyte ([Py1,4]TfO/0.1 M SnCl2) was investigated as a function of
2
4
6
8
10
12
14
16
18
Time / week Fig. 1. Water uptake as a function of the exposure time of [Py1,4]TfO/0.1 M SnCl2 to the ambient atmosphere.
H.K. Farag et al. / Journal of Molecular Liquids 230 (2017) 209–213
(110)
211
(101) (211)
Intensity
(200)
(220)
(301)
(302) (321) 500 oC
200 oC
x x * x
* x
20
30
x
* Tin oxide x: Tin hydroxychloride
x x x
40
50
2θ
as-prepared
60
70
80
90
Fig. 3. XRD patterns of the hydrolysis product of [Py1,4]TfO/0.1 M SnCl2, obtained after air exposure for one month, without and with calcination at 200 °C and 500 °C.
Fig. 2. a) SEM micrograph of the hydrolysis product of [Py1,4]TfO/0.1 M SnCl2 obtained after exposure to air for one month. b) EDX profile of the as-obtained product. c) SEM micrograph of the calcined product at 500 °C for 2 h. Inset: EDX profile of the calcined product.
precipitate reveal the formation of a multicomponent product as mixed diffraction peaks of tin oxides and tin hydroxychloride are recorded. If the obtained precipitate was calcined at 200 °C for 2 h a pure SnO2 is formed. The recorded XRD peaks are indexed and compared with the JCPDS-file number 41-1445 of tetragonal SnO2. The peaks are broad indicating the formation of nanosized particles. Calcination of the obtained SnO2 at 500 °C did not show a significant improvement in the crystallinity. By treatment of the as-obtained hydrolysis product with hot water SnO2 is obtained, as revealed from XRD patterns of Fig. 4c. Consequently, the morphology of the obtained hydrolysis product after hot water treatment is altered, Fig. 4a. As shown in the SEM micrograph of Fig. 4a, spherical particles with sizes in the nanometer regime are obtained. No Cl is detected as revealed from the corresponding EDX profile, Fig. 4b, indicating the formation of tin oxide. Traces of N and C are detected signifying the incomplete removal of the ionic liquid residues even after hot water treatment. 3.2. Electrodeposition of Sn from 0.1 M SnCl2/[Py1,4]TfO The cyclic voltammetry behaviour of 0.1 M SnCl2/[Py1,4]TfO, freshly prepared and after being exposed to ambient atmosphere for one
Fig. 4. a) SEM micrograph of the obtained hydrolysis product after treatment with hot water. b) EDX profile of obtained product. c) XRD patterns of the obtained product.
212
H.K. Farag et al. / Journal of Molecular Liquids 230 (2017) 209–213
week and for one month, was investigated. We aim at exploring to which extent the electrochemical/deposition behaviour can be influenced by the water uptake and the subsequent hydrolysis of the electrolyte. Fig. 5 shows the cyclic voltammograms of the ionic liquid [Py1,4]TfO containing 0.1 M SnCl2, recorded on copper substrates, under open air conditions. The measurements were performed using freshly prepared solution and using solutions exposed to air for 1 week and for one month, respectively. The potential was first scanned from the open circuit potential towards the negative direction up to −1.5 V (vs. Pt), then scanned back in the positive direction up to a potential of +0.5 V, and finally terminated at the starting potential. The cyclic voltammogram of the freshly prepared electrolyte, Fig. 5a, exhibits a similar behaviour as that obtained for the same electrolyte under inert gas conditions on a gold substrate [10]. In the forward scan four cathodic processes, C1, C2, C3 and C⁎3 are recorded. The cathodic processes C1 and C2 can be ascribed to the formation of Cu-Sn phases. The bulk deposition of Sn takes place at C3, and C⁎3 can be correlated with further deposition on the electrodeposited Sn. The anodic counterparts of the cathodic processes C1, C2 and C3 are obtained on the backward scan. The cyclic voltammograms recorded for the IL-electrolytes exposed to air for one week, Fig. 5b, and for one month, Fig. 5c, show the same general features as those of the freshly prepared electrolyte. However, the deposition potential shifts to more negative values and the currents of both cathodic anodic peaks decrease as the water content increases. There are several factors can account for the observed behaviour. The presence of nanosized colloidal particles in the electrolyte, as a result of the hydrolysis of Sn(II) species, might contribute to the observed behaviour. Adsorption of fine colloidal particles onto the electrode surface can exert some sort of passivity to the electrode surface that can account for the
a2
a3 -2 400 μΑ.cm
c2
c1
a1
(a)
c*3
a2
c3 a3
-2 400 μΑ.cm
c2
c1
-2 400 μΑ.cm
a2
a3 c2 c1
(b)
Cu/Cu+
c3
c3
observed decrease in current values and the negative shift in the deposition potential. Furthermore, the decrease in the concentration of Sn(II) species as a result of hydrolysis causes a decrease in the current values. The alteration in the kind and the concentration of the electroactive species present in the electrolyte can also influence electrochemical behaviour and the morphology of the electrodeposits. It was reported [10] that changing the anion of the ionic liquid significantly affects the morphologies of tin electrodeposits. Tin dendrites were obtained in the ionic liquid 1-butyl-1-methylpyrrolidinium dicyanamide ([Py1,4]DCA), while dendrite free deposits were formed in [Py1,4]TfO [10]. The significant increase in the anodic current starting at 0.0 V (vs. Pt) is associated with the anodic dissolution of copper and the reduction peak recorded at −0.1 V is attributable to the redeposition of dissolved Cu. The anodic dissolution of Cu moves to less positive values as the water content in the electrolyte increases. Fig. 6 shows the surface morphology of tin electrodeposits obtained potentiostatically on copper in the employed IL-electrolyte at − 0.5 V (vs. Pt) for the freshly prepared electrolyte (Fig. 6a) and at − 1.2 V (vs. Pt) for the electrolyte exposed to air for one month (Fig. 6b). As seen in Fig. 6a, a dense, dendrite free Sn deposit is obtained. The deposit comprises coarse particles with sizes ranging from 0.2 to 1 μm (inset of Fig. 6a). The Sn deposits obtained from the air exposed electrolyte exhibits a different morphology as Sn dendrites are formed above a compact Sn layer. The aforementioned results reveal that the employed SnCl2/ [Py1,4]TfO electrolyte undergoes hydrolysis on long exposure to air which obviously influences the electrodeposition behaviour of Sn. Consequently, for the application of the employed ionic liquid electrolyte in Sn electroplating the long exposure to ambient atmosphere should be avoided. Alternatively, the increase in the temperature of the electrolyte
a1
(c) +
Cu /Cu
-1.5 -1.2 -0.9 -0.6 -0.3
0.0
0.3
0.6
Potential vs. Pt / V Fig. 5. Cyclic voltammograms of the ionic liquid [Py1,4]TfO containing 0.1 M SnCl2, recorded on copper substrates, after air exposure for different intervals: a) freshly prepared electrolyte, b) after air exposure for one week, c) after air exposure for one month. Scan rate: 10 mV s−1.
Fig. 6. SEM micrographs of the Sn deposits obtained potentiostatically in a) freshly prepared 0.1 M SnCl2/[Py1,4]TfO (Inset: SEM micrograph of higher magnification) and in b) 0.1 M SnCl2/[Py1,4]TfO after air exposure for one month.
H.K. Farag et al. / Journal of Molecular Liquids 230 (2017) 209–213
would also be a possible solution to minimize the water concentration in the electrolyte which, in turn, lessens the hydrolysis of the Sn(II) species. 4. Conclusion We have demonstrated the water absorption tendency of the ionic liquid electrolyte [Py1,4]TfO/0.1 M SnCl2 under ambient atmosphere and its influence on the hydrolysis of Sn(II) species and on the electrodeposition of tin. It was shown that the employed ionic liquid electrolyte strongly absorbs water under ambient conditions reaching a concentration of 42,000 ppm after 10 weeks. Dispersed colloidal particles were formed in the electrolyte after an exposure time of about 4 weeks as a result of the hydrolysis of Sn(II) species. The XRD and EDX results of the hydrolysis products of the ionic liquid electrolyte [Py1,4]TfO/0.1 M SnCl2 revealed the formation of tin hydroxychloride. The presence of water as well as the hydrolysis product affects the electrochemical behaviour of the employed electrolyte and the morphology of tin electrodeposits. In the IL-electrolyte exposed to air for one month dendritic deposits were obtained while dendrite free deposits were formed in the freshly prepared electrolyte. Acknowledgement The authors would like to thank Prof. Dr. Frank Endres, Institute of Electrochemistry, Clausthal University of Technology, Germany, for the support with ionic liquid samples. The authors also thank Barbara Holly for carrying out the Karl-Fischer measurements. References [1] F. Endres, A.P. Abbott, D.R. MacFarlane, Electrodeposition From Ionic Liquids, WileyVCH, 2008. [2] M. Armand, F. Endres, D.R. MacFarlane, H. Ohno, B. Scrosati, Nat. Mater. 9 (2009) 621. [3] F. Endres, S. Zein El Abedin, Phys. Chem. Chem. Phys. 8 (2006) 2101. [4] A.P. Abbott, K.J. McKenzie, Phys. Chem. Chem. Phys. 8 (2006) 4265.
213
[5] Y. Katayama, T. Morita, M. Yamagata, T. Miura, Electrochemistry 71 (2003) 1033. [6] P.C. Howlett, D.R. MacFarlane, A.F. Hollenkamp, Electrochem. Solid-State Lett. 7 (2004) A97. [7] G.T. Cheek, W.E. O'Grady, S. Zein El Abedin, E.M. Moustafa, F. Endres, Electrochem. Soc. 155 (2008) D91. [8] S. Zein El Abedin, E.M. Moustafa, R. Hempelmann, H. Natter, F. Endres, ChemPhysChem 7 (2006) 1535. [9] W. Yang, H. Cang, Y. Tang, J. Wang, Y. Shi, J. Appl. Electrochem. 38 (2008) 537. [10] P. Giridhar, A.M. Elbassiouny, S. Zein El Abedin, F. Endres, ChemElectroChem 1 (2014) 1549. [11] S.I. Hsiu, J.F. Huang, I.W. Sun, C.H. Yuan, J. Shiea, Electrochim. Acta 74 (2002) 4367. [12] J.F. Huang, I.W. Sun, Electrochim. Acta 49 (2004) 3251. [13] Z. Liu, S. Zein El Abedin, F. Endres, Electrochim. Acta 89 (2013) 635. [14] S. Pye, J. Winnick, P.A. Kohl, J. Electrochem. Soc. 144 (1997) 1933. [15] R.T. Carlin, H.C. De Long, J. Fuller, P.C. Trulove, J. Electrochem. Soc. 145 (1998) 1598. [16] H.-Y. Huang, C.-J. Su, C.-L. Kao, P.-Y. Chen, J. Electroanal. Chem. 650 (2010) 1. [17] P. Giridhar, B. Weidenfeller, S. Zein El Abedin, F. Endres, ChemPhysChem 15 (2014) 3515. [18] Y. Hsieh, Y. Chen, I.-W. Sun, ChemElectroChem 3 (2016) 638. [19] S. Zein El Abedin, A.Y. Saad, H.K. Farag, N. Borisenko, Q.X. Liu, F. Endres, Electrochim. Acta 52 (2007) 2746. [20] H. Ma, K. Teng, Y. Fu, Y. Song, Y. Wanga, X. Dong, Energy Environ. Sci. 4 (2011) 3067. [21] W. Zhang, J. Hu, Y. Guo, S. Zheng, L. Zhong, W. Song, L. Wan, Adv. Mater. 20 (2008) 1160. [22] K. Kravchyk, L. Protesescu, M.I. Bodnarchuk, F. Krumeich, M. Yarema, M. Walter, C. Guntlin, M.V. Kovalenko, J. Am. Chem. Soc. 135 (2013) 4199. [23] C.-H. Hsu, C.-H. Yang, Y.-C. Wang, J.-K. Chang, J. Mater. Chem. A 2 (2014) 16547. [24] H. Morimoto, S. Tobishima, H. Negishi, J. Power Sources 146 (2005) 469. [25] J. Hassoun, M. Wachtler, M. Wohlfahrt-Mehrens, B. Scrosati, J. Power Sources 196 (2011) 349. [26] K. Ui, S. Kikuchi, Y. Kadoma, N. Kumagai, S. Ito, J. Power Sources 189 (2009) 224. [27] M. Deng, J. Chang, T. Leong, S. Fang, P.-Y. Chen, I.-W. Sun, Electrochemistry 77 (2009) 588. [28] A.M. Elbasiony, S. Zein El Abedin, F. Endres, J. Solid State Electrochem. 18 (2014) 951. [29] N. Tachikawa, N. Serizawa, Y. Katayama, T. Miura, Electrochim. Acta 53 (2008) 6530. [30] J.-F. Huang, I.-W. Sun, J. Electrochem. Soc. 150 (2003) 299. [31] R. Atkin, S. Zein El Abedin, L.H.S. Gasparotta, R. Hayes, N. Borisenko, F. Endres, J. Phys. Chem. C 113 (2009) 13266. [32] C.W. Liew, S. Ramesh, Materials 7 (2014) 4019. [33] L. Cammarata, S.G. Kazarian, P.A. Salter, T. Welton, Phys. Chem. Chem. Phys. 3 (2001) 5192. [34] A.M. O'Mahony, D.S. Silvester, L. Aldous, C. Hardacer, R.G. Compton, J. Chem. Eng. Data 53 (2008) 2884. [35] O.V. Reva, T.N. Vorob'eva, Russ. J. Appl. Chem. 75 (2002) 700.
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Influence of atmospheric water uptake on the hydrolysis of stannous chloride in the ionic liquid 1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate H.K. Farag a, S.R. El-Kiey b, S. Zein El Abedin b,⁎ a b
Inorganic Chemistry Department, National Research Centre, El Behoth St. 33, Dokki, Giza, Egypt Electrochemistry and Corrosion Laboratory, National Research Centre, El Behoth St. 33, Dokki, Giza, Egypt
a r t i c l e
i n f o
Article history: Received 29 October 2016 Received in revised form 6 January 2017 Accepted 9 January 2017 Available online 11 January 2017 Keywords: Ionic liquid Hydrolysis Electrodeposition Tin Water uptake
a b s t r a c t In this paper we report on the influence of water uptake on the hydrolysis of anhydrous stannous chloride in the ionic liquid 1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate ([Py1,4]TfO) under ambient atmosphere. The effect of water uptake on the electrodeposition of tin in the employed ionic liquid containing 0.1 M SnCl2 was also demonstrated. The hydrolysis products as well as the Sn electrodeposits were characterized by different techniques comprising; scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). The results show that the employed ionic liquid electrolyte strongly absorbs water under ambient conditions. Colloidal particles were formed in the electrolyte after an exposure time of about 4 weeks as a result of the hydrolysis of Sn(II) species. The electrochemical behaviour of the employed electrolyte and the morphology of tin electrodeposits were significantly influenced by the presence of water. Tin dendrites were obtained from IL-electrolyte exposed to air for one month, whereas dendrite free deposits were formed in the freshly prepared electrolyte. © 2017 Elsevier B.V. All rights reserved.
1. Introduction In recent years, ionic liquids have shown a huge potential as superior electrolytes for electrodeposition of metals and semiconductors. Ionic liquids offer some exceptional characteristics such as, e.g., wide electrochemical window, non-flammability, non-volatility, high thermal stability and low toxicity [1–4]. In view of these unique properties, ionic liquids can be regarded as a potential replacement for many aqueous electroplating baths, especially the toxic ones like cyanide baths for silver electroplating. Furthermore, non-volatility of ionic liquids makes them more environmentally desirable than volatile organic solvents which release harmful vapours into atmosphere. The wide electrochemical windows of ionic liquids enable the electrodeposition of reactive metals such as, e.g., lithium, magnesium or aluminium, which are not viable from aqueous electrolytes [5–8]. Also, the use of ionic liquids for the electrodeposition of less reactive metals that can be obtained from aqueous electrolytes, such as, e.g., tin [9,10], zinc [11–13], Fe [14–17], In [18,19] and other ones, is of advantages as the size and shape of the particles can be tuned without additives and the problems associated with hydrogen evolution can be avoided. Tin is one of the important metals and it is presently regarded as a promising anode candidate for ⁎ Corresponding author. E-mail address: [email protected] (S. Zein El Abedin).
http://dx.doi.org/10.1016/j.molliq.2017.01.025 0167-7322/© 2017 Elsevier B.V. All rights reserved.
the future generation Li ion batteries [20–22]. Electrodeposited Sn thin films directly obtained on a current collector showed a high capacity when investigated as anode for lithium ion batteries [23–26]. The electrodeposition of tin in different air and water stable ionic liquids such as, [EMIm]DCA [27], [Py1,4]DCA [28], [Py1,4]TfO [10], [Py1,4]TFSA [29], and ZnCl2-[EMIm]Cl [30], was reported. It was reported that dendrite free, Sn deposits can be obtained from the ionic liquid [Py1,4]TfO [10]. The ionic liquid [Py1,4]TfO has some interesting characteristics. The [Py1,4]+ cation can act as a grain refiner due to its strong adsorptive interaction with surfaces leading to nanocrystalline deposits [31]. Moreover, the TfO− anion exhibits an intense structure dissociation capability due to its bulky size facilitating the detachment of [Py1,4]+ cations from the transient bonding with anions [32]. This can enhance the adsorptive characteristics of the cation. To our knowledge, all reported studies on the electrodeposition of Sn from ionic liquids were performed under inert gas conditions which, from a practical point of view, limit the widespread application of ionic liquids as electrochemical baths for Sn deposition. In the present study we have performed the electrodeposition experiments under open air conditions to make the electrodeposition process more versatile and simple. The employed ionic liquid is air and water stable so it can be employed as an electrolyte in open galvanic baths. However, all ionic liquids, whether they are hydrophobic or hydrophilic, can quickly absorb considerable amounts of water when they are exposed to air [33,
H.K. Farag et al. / Journal of Molecular Liquids 230 (2017) 209–213
34]. As a consequence, the physical properties of the ionic liquids, such as viscosity, conductivity, solubility, ion diffusion and electrochemical stability, are strongly influenced. It should be noted that, the presence of water in ionic liquids might be a disadvantage for some applications, but not for others. Nevertheless, in all cases the water content should be controlled as the properties of ionic liquids are dependent not only on the presence of water but also on its concentration. As electrochemical liquids, the presence of a small amount of water is of advantage as the viscosity decreases, the conductivity and ion diffusion increase. On the other hand, the electrochemical window is reduced [34]. The extent of absorbed water is correlated to the strength of the hydrogen bonding among water molecules and also to the strength of the interaction between water and the anions of the ionic liquids [33]. The present paper aims at demonstrating the effect of atmospheric water uptake on the hydrolysis of stannous chloride in the ionic liquid 1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate with the sake of exploring the viability of employing the SnCl2/[Py1,4]TfO ionic liquid electrolyte as an electroplating bath for Sn. The extent of water absorption of the employed electrolyte on exposure to air, as a function of time, was determined in conditions of ambient humidity and room temperature. The influence of water uptake on the electrodeposition of Sn in the employed electrolyte was also investigated. 2. Experimental The ionic liquid 1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate with purity 99% (IoLiTec., Germany) was employed in this study. The ionic liquid was used as received without further purification or drying. The water content in the as-received was found to be 138 ppm. The water content in the employed ionic liquid electrolyte was determined by Karl Fischer titration, using a Metrohm 899 coulometer. Tin(II) chloride (Aldrich 99.99%) was used as the precursor for Sn electrodeposition. An electrolyte of 0.1 M SnCl2 in [Py1,4]TfO was employed in this study. Isopropanol (Alfa 99.5%) and bi-distilled water were used for washing the hydrolysis products of the employed ionic liquid electrolyte. The cyclic voltammetry and chronoamperometry measurements were performed using a Verstat 263A Potentiostat/Galvanostat (Princeton Applied Research) controlled by PowerCV and PowerStep software, respectively. The experiments were carried out under air with a relative humidity of 60 ± 5%. The electrochemical cell was made of polytetrafluoroethylene (Teflon) and clamped over a Teflon covered Viton O-ring onto the working electrode, thus yielding a geometric surface area of 0.3 cm2. A platinum ring (Alfa, 99.99%) and a platinum wire (Alfa, 99.99%) were used as counter and reference electrodes, respectively. Copper sheets (99.9%) were used as working electrodes and prior to use, the substrates were successively polished with silicon carbide emery paper of increasing fineness up to 1000, then degreased with acetone in an ultrasonic bath for 2 min. The hydrolysis products of the employed ionic liquid electrolyte were calcined in a muffle oven at the desired temperature for two hours and were cooled down to the ambient temperature in the oven. Various techniques were employed for the characterization of the obtained materials. A high resolution field emission scanning electron microscope (Carl Zeiss DSM 982 Gemini) was utilized to investigate the surface morphology of the obtained products, and energy dispersive X-ray analysis was used to determine the chemical composition. The phase composition of the obtained materials was investigated by Xray diffraction (XRD) using a PANalytical diffractometer with CuKα radiation. 3. Results and discussion
exposure time to ambient atmosphere. A glass bottle containing 50 ml of the employed electrolyte was exposed to air and left without stirring. The average humidity in the atmosphere was 60 ± 5% throughout the duration of the experiment and the room temperature was 20 ± 3 °C. Then the water content in the ionic liquid electrolyte was measured at different exposure time. The initial water content was found to be 138 ppm. As can be seen in Fig. 1, three different absorption intervals featuring the water absorption behaviour of the ionic liquid electrolyte, depending on the exposure time, are observed. In the early days of air exposure the IL-electrolyte strongly absorbs water from atmosphere reaching a concentration of 2500 ppm in the first three days, and in one week it reaches 16,000 ppm. The rate of water absorption in the first absorption interval, estimated from the slope of the extrapolated line, Fig. 1 is about 2000 ppm/day. In the absorption interval II, from 7 to 50 days, the water uptake continued to increase, but with a slower rate, with the increase in the exposure time reaching about 30,000 ppm after about 5 weeks. The rate of water absorption in this zone was found to be 500 ppm/day which is lower than that of the absorption interval I. After about 50 days of air exposure, interval III, the tendency of water absorption is considerably reduced as revealed from the very low rate of absorption in this region, 40 ppm/day. It can be stated that the IL-electrolyte slowly approaches its water saturation limit in this interval. It is worth mentioning that the freshly prepared 0.1 M SnCl2/ [Py1,4]TfO electrolyte was a clear solution, and after long exposure to air visible colloidal particles were formed. The particles slowly coagulated forming obvious transparent precipitates. The precipitates were obtained after an air exposure time of about four weeks, i.e. when the water content in the electrolyte was about 27,000 ppm. The obtained precipitates are the hydrolysis products of Sn(II) species in the ionic liquid electrolyte. It was reported that aqueous solution of SnCl2 undergoes hydrolysis by ongoing time forming a mixture of SnO2, Sn3O4 and Sn4(OH)6Cl2 [35]. The formed precipitates were retrieved from the electrolyte, washed with isopropanol, dried under vacuum and then characterized by SEM-EDX and XRD to investigate both morphology and composition. Fig. 2 shows the SEM-EDX analysis of the as-obtained hydrolysis product and of the calcined product at 500 °C for 2 h. As shown in the SEM micrograph of Fig. 2a the as obtained precipitates have a regular sheet-like structure. The accompanied EDX profile, Fig. 2b, shows the peaks of Sn, Cl and O indicating the formation tin hydroxychloride. The peaks of C, S, N and F are also recorded signifying the presence of ionic liquid residues in the precipitates. The sheet-like structure of the as-formed precipitates disintegrates on calcinations as shown in the SEM micrograph of the calcined product, Fig. 2c. The EDX profile of the calcined product reveal the transformation of tin hydroxychloride to tin oxide as the Cl peak was vanished (inset of Fig. 2c). The XRD patterns of the obtained precipitates without and with calcination at 200 °C and at 500 °C are depicted in Fig. 3. The XRD patterns of the as obtained
40000
[Py1,4]TfO + 0.1 M SnCl2
III
35000
Water Uptake / ppm
210
30000 25000
II
20000 15000 10000
I
5000 0
0
3.1. Hydrolysis of 0.1 M SnCl2/[Py1,4]TfO The water absorption tendency of the employed ionic liquid (IL) electrolyte ([Py1,4]TfO/0.1 M SnCl2) was investigated as a function of
2
4
6
8
10
12
14
16
18
Time / week Fig. 1. Water uptake as a function of the exposure time of [Py1,4]TfO/0.1 M SnCl2 to the ambient atmosphere.
H.K. Farag et al. / Journal of Molecular Liquids 230 (2017) 209–213
(110)
211
(101) (211)
Intensity
(200)
(220)
(301)
(302) (321) 500 oC
200 oC
x x * x
* x
20
30
x
* Tin oxide x: Tin hydroxychloride
x x x
40
50
2θ
as-prepared
60
70
80
90
Fig. 3. XRD patterns of the hydrolysis product of [Py1,4]TfO/0.1 M SnCl2, obtained after air exposure for one month, without and with calcination at 200 °C and 500 °C.
Fig. 2. a) SEM micrograph of the hydrolysis product of [Py1,4]TfO/0.1 M SnCl2 obtained after exposure to air for one month. b) EDX profile of the as-obtained product. c) SEM micrograph of the calcined product at 500 °C for 2 h. Inset: EDX profile of the calcined product.
precipitate reveal the formation of a multicomponent product as mixed diffraction peaks of tin oxides and tin hydroxychloride are recorded. If the obtained precipitate was calcined at 200 °C for 2 h a pure SnO2 is formed. The recorded XRD peaks are indexed and compared with the JCPDS-file number 41-1445 of tetragonal SnO2. The peaks are broad indicating the formation of nanosized particles. Calcination of the obtained SnO2 at 500 °C did not show a significant improvement in the crystallinity. By treatment of the as-obtained hydrolysis product with hot water SnO2 is obtained, as revealed from XRD patterns of Fig. 4c. Consequently, the morphology of the obtained hydrolysis product after hot water treatment is altered, Fig. 4a. As shown in the SEM micrograph of Fig. 4a, spherical particles with sizes in the nanometer regime are obtained. No Cl is detected as revealed from the corresponding EDX profile, Fig. 4b, indicating the formation of tin oxide. Traces of N and C are detected signifying the incomplete removal of the ionic liquid residues even after hot water treatment. 3.2. Electrodeposition of Sn from 0.1 M SnCl2/[Py1,4]TfO The cyclic voltammetry behaviour of 0.1 M SnCl2/[Py1,4]TfO, freshly prepared and after being exposed to ambient atmosphere for one
Fig. 4. a) SEM micrograph of the obtained hydrolysis product after treatment with hot water. b) EDX profile of obtained product. c) XRD patterns of the obtained product.
212
H.K. Farag et al. / Journal of Molecular Liquids 230 (2017) 209–213
week and for one month, was investigated. We aim at exploring to which extent the electrochemical/deposition behaviour can be influenced by the water uptake and the subsequent hydrolysis of the electrolyte. Fig. 5 shows the cyclic voltammograms of the ionic liquid [Py1,4]TfO containing 0.1 M SnCl2, recorded on copper substrates, under open air conditions. The measurements were performed using freshly prepared solution and using solutions exposed to air for 1 week and for one month, respectively. The potential was first scanned from the open circuit potential towards the negative direction up to −1.5 V (vs. Pt), then scanned back in the positive direction up to a potential of +0.5 V, and finally terminated at the starting potential. The cyclic voltammogram of the freshly prepared electrolyte, Fig. 5a, exhibits a similar behaviour as that obtained for the same electrolyte under inert gas conditions on a gold substrate [10]. In the forward scan four cathodic processes, C1, C2, C3 and C⁎3 are recorded. The cathodic processes C1 and C2 can be ascribed to the formation of Cu-Sn phases. The bulk deposition of Sn takes place at C3, and C⁎3 can be correlated with further deposition on the electrodeposited Sn. The anodic counterparts of the cathodic processes C1, C2 and C3 are obtained on the backward scan. The cyclic voltammograms recorded for the IL-electrolytes exposed to air for one week, Fig. 5b, and for one month, Fig. 5c, show the same general features as those of the freshly prepared electrolyte. However, the deposition potential shifts to more negative values and the currents of both cathodic anodic peaks decrease as the water content increases. There are several factors can account for the observed behaviour. The presence of nanosized colloidal particles in the electrolyte, as a result of the hydrolysis of Sn(II) species, might contribute to the observed behaviour. Adsorption of fine colloidal particles onto the electrode surface can exert some sort of passivity to the electrode surface that can account for the
a2
a3 -2 400 μΑ.cm
c2
c1
a1
(a)
c*3
a2
c3 a3
-2 400 μΑ.cm
c2
c1
-2 400 μΑ.cm
a2
a3 c2 c1
(b)
Cu/Cu+
c3
c3
observed decrease in current values and the negative shift in the deposition potential. Furthermore, the decrease in the concentration of Sn(II) species as a result of hydrolysis causes a decrease in the current values. The alteration in the kind and the concentration of the electroactive species present in the electrolyte can also influence electrochemical behaviour and the morphology of the electrodeposits. It was reported [10] that changing the anion of the ionic liquid significantly affects the morphologies of tin electrodeposits. Tin dendrites were obtained in the ionic liquid 1-butyl-1-methylpyrrolidinium dicyanamide ([Py1,4]DCA), while dendrite free deposits were formed in [Py1,4]TfO [10]. The significant increase in the anodic current starting at 0.0 V (vs. Pt) is associated with the anodic dissolution of copper and the reduction peak recorded at −0.1 V is attributable to the redeposition of dissolved Cu. The anodic dissolution of Cu moves to less positive values as the water content in the electrolyte increases. Fig. 6 shows the surface morphology of tin electrodeposits obtained potentiostatically on copper in the employed IL-electrolyte at − 0.5 V (vs. Pt) for the freshly prepared electrolyte (Fig. 6a) and at − 1.2 V (vs. Pt) for the electrolyte exposed to air for one month (Fig. 6b). As seen in Fig. 6a, a dense, dendrite free Sn deposit is obtained. The deposit comprises coarse particles with sizes ranging from 0.2 to 1 μm (inset of Fig. 6a). The Sn deposits obtained from the air exposed electrolyte exhibits a different morphology as Sn dendrites are formed above a compact Sn layer. The aforementioned results reveal that the employed SnCl2/ [Py1,4]TfO electrolyte undergoes hydrolysis on long exposure to air which obviously influences the electrodeposition behaviour of Sn. Consequently, for the application of the employed ionic liquid electrolyte in Sn electroplating the long exposure to ambient atmosphere should be avoided. Alternatively, the increase in the temperature of the electrolyte
a1
(c) +
Cu /Cu
-1.5 -1.2 -0.9 -0.6 -0.3
0.0
0.3
0.6
Potential vs. Pt / V Fig. 5. Cyclic voltammograms of the ionic liquid [Py1,4]TfO containing 0.1 M SnCl2, recorded on copper substrates, after air exposure for different intervals: a) freshly prepared electrolyte, b) after air exposure for one week, c) after air exposure for one month. Scan rate: 10 mV s−1.
Fig. 6. SEM micrographs of the Sn deposits obtained potentiostatically in a) freshly prepared 0.1 M SnCl2/[Py1,4]TfO (Inset: SEM micrograph of higher magnification) and in b) 0.1 M SnCl2/[Py1,4]TfO after air exposure for one month.
H.K. Farag et al. / Journal of Molecular Liquids 230 (2017) 209–213
would also be a possible solution to minimize the water concentration in the electrolyte which, in turn, lessens the hydrolysis of the Sn(II) species. 4. Conclusion We have demonstrated the water absorption tendency of the ionic liquid electrolyte [Py1,4]TfO/0.1 M SnCl2 under ambient atmosphere and its influence on the hydrolysis of Sn(II) species and on the electrodeposition of tin. It was shown that the employed ionic liquid electrolyte strongly absorbs water under ambient conditions reaching a concentration of 42,000 ppm after 10 weeks. Dispersed colloidal particles were formed in the electrolyte after an exposure time of about 4 weeks as a result of the hydrolysis of Sn(II) species. The XRD and EDX results of the hydrolysis products of the ionic liquid electrolyte [Py1,4]TfO/0.1 M SnCl2 revealed the formation of tin hydroxychloride. The presence of water as well as the hydrolysis product affects the electrochemical behaviour of the employed electrolyte and the morphology of tin electrodeposits. In the IL-electrolyte exposed to air for one month dendritic deposits were obtained while dendrite free deposits were formed in the freshly prepared electrolyte. Acknowledgement The authors would like to thank Prof. Dr. Frank Endres, Institute of Electrochemistry, Clausthal University of Technology, Germany, for the support with ionic liquid samples. The authors also thank Barbara Holly for carrying out the Karl-Fischer measurements. References [1] F. Endres, A.P. Abbott, D.R. MacFarlane, Electrodeposition From Ionic Liquids, WileyVCH, 2008. [2] M. Armand, F. Endres, D.R. MacFarlane, H. Ohno, B. Scrosati, Nat. Mater. 9 (2009) 621. [3] F. Endres, S. Zein El Abedin, Phys. Chem. Chem. Phys. 8 (2006) 2101. [4] A.P. Abbott, K.J. McKenzie, Phys. Chem. Chem. Phys. 8 (2006) 4265.
213
[5] Y. Katayama, T. Morita, M. Yamagata, T. Miura, Electrochemistry 71 (2003) 1033. [6] P.C. Howlett, D.R. MacFarlane, A.F. Hollenkamp, Electrochem. Solid-State Lett. 7 (2004) A97. [7] G.T. Cheek, W.E. O'Grady, S. Zein El Abedin, E.M. Moustafa, F. Endres, Electrochem. Soc. 155 (2008) D91. [8] S. Zein El Abedin, E.M. Moustafa, R. Hempelmann, H. Natter, F. Endres, ChemPhysChem 7 (2006) 1535. [9] W. Yang, H. Cang, Y. Tang, J. Wang, Y. Shi, J. Appl. Electrochem. 38 (2008) 537. [10] P. Giridhar, A.M. Elbassiouny, S. Zein El Abedin, F. Endres, ChemElectroChem 1 (2014) 1549. [11] S.I. Hsiu, J.F. Huang, I.W. Sun, C.H. Yuan, J. Shiea, Electrochim. Acta 74 (2002) 4367. [12] J.F. Huang, I.W. Sun, Electrochim. Acta 49 (2004) 3251. [13] Z. Liu, S. Zein El Abedin, F. Endres, Electrochim. Acta 89 (2013) 635. [14] S. Pye, J. Winnick, P.A. Kohl, J. Electrochem. Soc. 144 (1997) 1933. [15] R.T. Carlin, H.C. De Long, J. Fuller, P.C. Trulove, J. Electrochem. Soc. 145 (1998) 1598. [16] H.-Y. Huang, C.-J. Su, C.-L. Kao, P.-Y. Chen, J. Electroanal. Chem. 650 (2010) 1. [17] P. Giridhar, B. Weidenfeller, S. Zein El Abedin, F. Endres, ChemPhysChem 15 (2014) 3515. [18] Y. Hsieh, Y. Chen, I.-W. Sun, ChemElectroChem 3 (2016) 638. [19] S. Zein El Abedin, A.Y. Saad, H.K. Farag, N. Borisenko, Q.X. Liu, F. Endres, Electrochim. Acta 52 (2007) 2746. [20] H. Ma, K. Teng, Y. Fu, Y. Song, Y. Wanga, X. Dong, Energy Environ. Sci. 4 (2011) 3067. [21] W. Zhang, J. Hu, Y. Guo, S. Zheng, L. Zhong, W. Song, L. Wan, Adv. Mater. 20 (2008) 1160. [22] K. Kravchyk, L. Protesescu, M.I. Bodnarchuk, F. Krumeich, M. Yarema, M. Walter, C. Guntlin, M.V. Kovalenko, J. Am. Chem. Soc. 135 (2013) 4199. [23] C.-H. Hsu, C.-H. Yang, Y.-C. Wang, J.-K. Chang, J. Mater. Chem. A 2 (2014) 16547. [24] H. Morimoto, S. Tobishima, H. Negishi, J. Power Sources 146 (2005) 469. [25] J. Hassoun, M. Wachtler, M. Wohlfahrt-Mehrens, B. Scrosati, J. Power Sources 196 (2011) 349. [26] K. Ui, S. Kikuchi, Y. Kadoma, N. Kumagai, S. Ito, J. Power Sources 189 (2009) 224. [27] M. Deng, J. Chang, T. Leong, S. Fang, P.-Y. Chen, I.-W. Sun, Electrochemistry 77 (2009) 588. [28] A.M. Elbasiony, S. Zein El Abedin, F. Endres, J. Solid State Electrochem. 18 (2014) 951. [29] N. Tachikawa, N. Serizawa, Y. Katayama, T. Miura, Electrochim. Acta 53 (2008) 6530. [30] J.-F. Huang, I.-W. Sun, J. Electrochem. Soc. 150 (2003) 299. [31] R. Atkin, S. Zein El Abedin, L.H.S. Gasparotta, R. Hayes, N. Borisenko, F. Endres, J. Phys. Chem. C 113 (2009) 13266. [32] C.W. Liew, S. Ramesh, Materials 7 (2014) 4019. [33] L. Cammarata, S.G. Kazarian, P.A. Salter, T. Welton, Phys. Chem. Chem. Phys. 3 (2001) 5192. [34] A.M. O'Mahony, D.S. Silvester, L. Aldous, C. Hardacer, R.G. Compton, J. Chem. Eng. Data 53 (2008) 2884. [35] O.V. Reva, T.N. Vorob'eva, Russ. J. Appl. Chem. 75 (2002) 700.