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On the initiation and growth of polymer films onto electrode surfaces
PERGAMON
Electrochimica Acta 44 (1999) 1901±1910
On the initiation and growth of polymer ®lms onto electrode surfaces L.M. Abrantes a, *, J.P. Correia b a
CECUL, Departamento QuõÂmica, Faculdade de CieÃncias, Universidade de Lisboa, R. Ernesto Vasconcelos, 1700 Lisboa, Portugal b INETI, Departamento de Energias RenovaÂveis, Est. do Pac° o do Lumiar, 1699 Lisboa Codex, Portugal Received 27 July 1998
Abstract The probe beam de¯ection technique (PBD) has been successfully used to detect in situ the in¯uence of electrochemical mode and conditions on the initial stages of poly-3-methylthiophene formation. The interpretation of the results is coherent with the previously reported oligomer formation in solution prior to deposition. The eect of the initial polymerization steps in the subsequent stages of the polymer growth has also been investigated, analysing the redox behaviours along with the correspondent de¯ectograms obtained for thin ®lms synthesized under dierent deposition charges. The data indicate that up to a given thickness the polymer responses re¯ect the nature of the ®rst deposited layer. The electropolymerization behaviour of methylthiophene has also been contrasted to that of pyrrole under the same experimental conditions (solvent, monomer concentration, anionic and cationic components of the electrolyte). The results clearly reveal an earlier deposition process and that oligomer formation, if any, is not determinant for polypyrrole deposition. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Conducting polymers; Electropolymerization; Probe beam de¯ection; Poly-3-methylthiophene; Initiation and growth
1. Introduction During the last two decades the considerable attention devoted to electronically conducting polymers has been mainly focused on their redox behaviour and inherent potential useful properties [1, 2]. Although the polymer ®lms properties, and therefore the success of the envisaged applications, strongly depends upon the preparation route, it is relatively more recent that investigation towards the understanding of basic mechanism of the ®lm growth and its interrelation with the electrochemical conditions has been reported [3, 4]. Two models have been put forward to describe the initiation and growth of polymer ®lms prepared electrochemically. The anodic electropolymerization process has been described either as adsorption of monomers on the electrode surface followed by the gradual addition of
* Corresponding author. Tel.: +35-7573624; fax: +3517573625; e-mail: [email protected]
monomers to the surface-bound species [5, 6] or as oligomer formation in solution which at a given stage `precipitate' on the electrode surface [7, 8]. In spite of the dierences among the electrodeposition of metals and the electropolymerization process, electrochemical data has been used to test nucleation models [9]; however, purely electrochemical information is unable to identify produced species or to provide information on the ®rst stages of the polymer formation. The coupling of electrochemical and UVVIS spectroscopic techniques can provide the evolution of spectral characteristics at dierent stages of the electropolymerization process [10], but information whether the species are produced only in solution or also at the electrode surface cannot be obtained. Evidence for deposition of solution-formed oligomers at early stages of the electropolymerization has been mainly given by ellipsometric measurements [11±13] and more recently by photocurrent spectroscopy [14, 15], which are well-known reliable techniques to characterize surface modi®cations.
0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 8 ) 0 0 2 9 9 - 0
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L.M. Abrantes, J. Correia / Electrochimica Acta 44 (1999) 1901±1910
Not withstanding the anodic electropolymerization the electrode process also involves ¯uxes of neutral and/or ionic species from and to the solution which in turn are also dependent on the ®lm growth mechanism and on the electrochemical experimental conditions employed to produce the polymer. Those ¯uxes give rise to concentration gradients of electrolyte species in front of the electrode, which call for the utilization of in situ optical methods like refractive index gradients measurements. Thus, the probe beam de¯ection (PBD) technique, which is based on the de¯ection of a laser beam aligned parallel to the electrode surface [16, 17], appears as a very convenient tool for studying the mass transport and reaction mechanism at the electrochemical interface. Furthermore, the PBD technique can be used simultaneously with electrochemical methods such as voltammetry or chronoamperometry, allowing a direct correlation between the charge transfer and the ion ¯ow process. Although there is a propagation delay due to the location of the beam (e.g. 100 mm away from the electrode) [18], the low detection limit (1 nrad) [19] and the reported successful use of PBD to monitor the ion exchange during conducting polymers redox processes [20, 21] have prompted our interest in employing it as an alternative tool to detect the in¯uence of the electrochemical mode and conditions on the electropolymerization initial stages and on the subsequent redox behaviour of conductive polymer layers. The PBD results for the polymerization of 3methylthiophene are consistent with oligomers formation in solution prior to deposition in contrast to the case of polypyrrole formation which, under the same experimental conditions, appears to follow an instantaneous nucleation model. Interpretation of PBD data from potential cycling experiments with dierent ®lms also enables a better appraisal of the eects of change in polymer growth potential/current and charge. 2. Experimental With the exception of the acetonitrile (Merck, Uvasol spectroscopy grade) which was distilled under argon atmosphere prior to use, all the electrolyte solutions were prepared from `Analar' grade chemicals as received. They were thoroughly deoxygenated directly in the cell with argon (purity >99.9997%) prior to measurements. All the experiments were performed at room temperature. Films of P(3-MeTh) and PPy were electrosynthesized under potentiostatic and galvanostatic control in solutions containing 0.6 mol dm ÿ 3 monomer and 0.2 mol dm ÿ 3 LiClO4 in acetonitrile. After holding the potential at 0 V for 12 min for polymer discharge, the
Table 1 Refractive indices of monomers and electrolyte and their derivatives at 258C
3 MeTh Py LiClO4
n
@n/@c (mol ÿ 1 cm3)
1.3434 + 0.0153C 1.3435 + 0.0130C 1.3434 + 0.0111C
15.3 13.0 11.1
characterization of P(3-MeTh) ®lms was carried out in a monomer free solution by scanning the electrode potential between 0 and 1 V at v = 50 mV s ÿ 1. The PBD experiments were performed by analysis of the de¯ection of a 2 mW He±Ne laser beam (Oriel model 79200) caused by concentration gradients near the electrode surface. The laser with a 1/e2 beam diameter of 0.63 mm is focused to the centre of the cell by a lens, converging to a waist diameter of about 1/ e2 = 50 mm. In this work the estimated distance electrode-probe beam is kept at ca 100 mm. The deviation of the beam is measured by a position-sensitive detector consisting of a SPOT-2 DMI Optilas double photodiode (10 mm gap between the active surfaces) placed behind the electrochemical cell being the setup rigidly mounted on an Oriel optical bench. A more detailed description of the PBD setup is reported elsewhere [22]. In this work a positive de¯ection (y > 0) indicates an ion-¯ow towards the electrode while the movement away from the electrode is taken as negative (y < 0). A three-electrode quartz cuvette cell ®xed in the optical bench was employed being the Pt disk (5 mm é) working electrode potential controlled with respect to the saturated calomel electrode (SCE). A platinum grid counter electrode was placed far enough away to ensure that the reactions occurring on it did not aect the deviation of the laser caused by the process taking place at the working electrode. The refractive indices of the electrolyte and the monomers in acetonitrile (0.1±1.0 M) were measured at 258C with a Abbe `60' refractometer using the 589.6 nm line from a sodium D1 lamp, and the results are summarized in Table 1.
3. Results and discussion A typical current transient for the electropolymerization of 3-MeTh under potentiostatic control is shown in Fig. 1. Previous in situ ellipsometric studies on this polymer growth [12, 13] enables us to de®ne three dierent ranges: from A to B the diusion controlled monomer oxidation in solution takes place; from B to C there is a diusion controlled nucleation of oligomers produced in solution; from C to D the overlap of growing centres. The movement of solution species
L.M. Abrantes, J. Correia / Electrochimica Acta 44 (1999) 1901±1910
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Table 2 Data obtained during the potentiostatic growth of P(3-MeTh) at dierent potentials (Fig. 2) Eg/V vs SCE
Charge/mC cm ÿ 2 and (time/s) consumed until B
Charge/mC cm ÿ 2 and (time/s) consumed until C
Time/s until y
1.36 1.38 1.40
0.7 (1.2) 0.4 (0.5) 0.1 (0.1)
2.5 (3.4) 2.0 (1.6) 1.3 (0.6)
4.5 3.5 3.0
which accompanies these process are (due to diusion and migration) the displacement of monomer species towards the electrode surface (A±B) whereas after B the movement of the protons, formed during the 3MeTh oxidation, away from the electrode must also be accounted for and as soon as a polymer is formed and oxidized at the surface (C), other ¯uxes are expected such as anion diusion towards the electrode for polymer charge compensation or non-consumed oligomers away from its surface. Although the dierences in the diusion coecients (Di) and the refractive index change with concentration (@n/@c) when many species are involved, analysis of the beam de¯ection signal during the electropolymerization process can be dicult, and changes in the contribution of the partial ¯uxes involved are expected which eventually can cause the inversion of the predominant species transport. Depending on the applied potential, dierences in the rate of formation of soluble oligomers/polymer onto the electrode should be detected in the corresponding de¯ectograms. Current pro®les and respective beam de¯ections simultaneously recorded during the potentiostatic synthesis of P(3-MeTh) under dierent growth potentials are presented in Fig. 2 where the contribution of migration due to the electric ®eld in a monomer free solution is also illustrated. These experiments display some important characteristics. The onset of beam deviation increase strongly depends on the applied potential being positive since the very early stages for high values (1.40 V); at a given time there is a change in the de¯ection slope, which may even change sign; eventually y can display a constant value.
max
These results clearly reveal the presence of more than the single monomer ¯ux towards the electrode. Considering the polymer growth at Eg = 1.38 V (Fig. 2b), the current density at point C of the chronoamperogram (i = 2 mA cm ÿ 2) gives rise to a mass ¯ux density (J), of approximately 20 nmol cm ÿ 2 s ÿ 1 (z = 1) which, according to Fick's law, creates a concentration gradient (@c/@x) of 2 mmol cm ÿ 4 (assuming a diusion coecient of D = 1.10 ÿ 5 cm2 s ÿ 1). The beam de¯ection for a single ¯ux is given by y x; t L
@n @c @c @x
1
where L and @n/@c are, respectively, the electrode length and the refractive index gradient with the concentration, one obtains considering only the ¯ux of the monomer, a de¯ection of 16 mrad, which is two orders of magnitude higher than the maximum value of the recorded beam deviation (0.16 mrad). This simple computation allows us to infer from the simultaneous occurrence of dierent and opposite ¯uxes which generate a de¯ectogram with a maximum. Table 2 summarizes a semi-quantitative analysis of the results shown in Fig. 2. It is known that the polymerization of P(3-MeTh) requires a previous generation and oxidation of oligomers in solution, since the radical cations of oligomeric species act as a major propagation intermediate for the polymer growth, its concentration being maintained at a steady state level during the synthesis [3]. At short times (e.g. B) the consumed charges are coherent with those observed for the formation of intermediates in solution [12, 14, 23] whereas at slightly longer times (C) the consistency with ellipsometric data [12, 13] allows
Fig. 1. Current transient for the electropolymerization of P(3-MeTh) at Eg = 1.38 V vs SCE.
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Fig. 2. Current and probe beam de¯ection transients for the synthesis of P(3-MeTh) under potentiostatic control at Eg = 1.36 (a), 1.38 (b) and 1.40 V vs SCE (c).
us to state that from that point (C) on there will be an expansion of nuclei which eventually overlaps and then, at a given time, a polymer monolayer is produced, oxidized, and the growth proceeds. In agreement with this interpretation are the probe beam
de¯ection features, and apart from the inherent time shift also the lapses for y max observation follow the expected behaviour occurring earlier as the electrode potential increases. Being the net beam deviation the result of opposite ¯uxes of species with dierent Di
L.M. Abrantes, J. Correia / Electrochimica Acta 44 (1999) 1901±1910
and @n/@c [16], an inversion of y slope corresponds to a change of the dominant contribution for the laser de¯ection onset. This feature must be related to a modi®cation of the electrode surface, the likely formation of the polymer ®rst monolayer being the time lapse required for electrode coverage being smaller as the growth potential increases [24]. At low growth charges the polymerization proceeds mainly by regular incorporation of radical cations on
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the grafted chain [25, 26] and thus this process will be favoured for synthesis carried out at low electrode potentials. Indeed, the low growth rate allows the radical cations in solution to position at the edges of the polymeric chain, creating long and regular ®brils. On the other hand, for higher deposition potentials the oligomeric radical cations will react more randomly, producing branched chains with short segments covering the electrode surface faster. The values of the charge con-
Fig. 3. Electrode potential and probe beam de¯ection pro®les for the galvanostatic growth of P(3-MeTh) at ig = 1 (a), 2 (b) and 5 mA cm ÿ 2 (c).
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Fig. 4. Cyclic voltammograms and de¯ectograms for P(3-MeTh) ®lms (Qg = 30 mC cm ÿ 2) potentiostatically grown at Eg = 1.36 (a) and 1.40 V vs SCE (b).
sumed until B and the corresponding rate of growth give support to the assumption that at low potentials the polymer is formed mainly by long segments. Results of the electropolymerization of 3-MeTh under galvanostatic control, displayed in Fig. 3, present similar qualitative featuresÐthe higher the applied current the sooner y inversion occurs. Since the data obtained for current values above 2 mA cm ÿ 2 present the same trends as that observed for polymer growth at E = 1.40 V, the polymer formation from short segments will be considered in these cases. The assumptions made so far being correct, the cyclic voltammograms and simultaneously recorded de¯ectograms of thin P(3-MeTh) ®lms will display characteristics re¯ecting the polymer formation from dierent sized chains, the in¯uence of polymerization conditions under potentiostatic mode as well as the eect of preparation at a constant rate. The redox behaviour of thin P(3-MeTh) ®lms prepared under potentiostatic control at Eg = 1.36 and 1.40 V is contrasted in Fig. 4 and Table 3. It can be seen that ®lms prepared at higher potentials are easily oxidized (lower E p0, higher ip0) the conversion being less reversible (lower Q0/Qr), but the polymerization eciency, evaluated through the Q0/Qg ratio, slightly
higher. On the other hand, the beam de¯ection denotes the earlier oxidation (y increases at a lower potential) and the easier switching (when the potential is reversed the decrease in y is larger) of the ®lm grown at the higher potential. It must also be pointed out that, for the ®lm prepared with Eg = 1.36 V, before the oxidation current peak there is an initial decrease in y which can be attributed to cation expulsion, a feature that is likely related to the presence of somewhat longer chains. Fig. 5 displays the cyclic voltammogram and de¯ectogram recorded for a polymer galvanostatically synthesized at ig = 2 mA cm ÿ 2 which, contrasted with the potentiostatically grown ®lms, support the usual choice of potentiostatic mode for this polymer preparation. Actually, the voltammetric features of the galvanostatically produced ®lm, compared to those of polymers prepared at constant potentials, shows a more dicult oxidation (E p0 = 0.665 V, ip0 = 0.36 mA cm ÿ 2, Q0 = 2.75 mC cm ÿ 2), lower reversibility for the redox conversion (Q0/Qr = 1.38) and polymerization eciency (Q0/Qg = 8.0%). The de¯ection recorded during the anodic potential sweep denotes the involvement of cations in the ®rst stages of oxidation; reversing the potential leaves y almost insensitive which re¯ects (in
Table 3 Voltammetric data for the redox behaviour of P(3-MeTh) thin ®lms (Qg = 30 mC cm ÿ 2) grown under potentiostatic control at dierent potentials (Fig. 4) Eg V ÿ 1
Ep0 mV ÿ 1
ip0 mA ÿ 1 cm ÿ 2
Q0 mC ÿ 1 cm ÿ 2
Q0/Qr
Q0/Qg(%)
1.36 1.40
653 620
0.53 0.60
3.36 3.29
1.07 1.10
10.4 11.4
L.M. Abrantes, J. Correia / Electrochimica Acta 44 (1999) 1901±1910
Fig. 5. Cyclic voltammogram and de¯ectogram for a P(3MeTh) ®lm (Qg = 30 mC cm ÿ 2) grown under galvanostatic control at ig = 2 mA cm ÿ 2.
agreement to the high value observed for Q0/Qr) a dif®cult anion expulsion. The in¯uence of the polymerization conditions are still noticeable on the displayed characteristics of thicker P(3-MeTh) potentiostatically prepared ®lms (Fig. 6). For the case of Eg = 1.40 V growth, after the potential scan is reversed, a ¯ow towards the electrode
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persists as suggested by the sign y. Indeed, y is still positive (and even larger than during the anodic scan) for a much larger potential range than observed for ®lms prepared under Eg = 1.38 V. This eect was seen to become more pronounced as the growth potential increases. The continuous ¯ux of non-charged species (very likely ion pair (A ÿ , C + )) is de®nitively associated to ®lms where the oxidation takes place at relatively lower potentials and higher rates, as appears to be the case of polymers prepared at high potentials and therefore where short chains are more probable. Voltammetric information taken in systematic experiments has shown that the polymer oxidation potential increases with the ®lm thickness, whereas the Q0/Qr ratio and the polymerization eciency decrease until a given value. It is worthwhile to compare the cyclic voltammograms and de¯ectograms obtained for P(3MeTh) ®lms prepared under charges of 50 mC cm ÿ 2 (Fig. 6b) and 100 mC cm ÿ 2 (Fig. 7), where the values of the ratio Q0/Qr and growth eciency are similar, but there is a clear increase in the oxidation peak potential for the latter. The change in the redox behaviour can easily be observed in the de¯ectogram. As
Fig. 6. Cyclic voltammograms and de¯ectograms for P(3-MeTh) ®lms (Qg = 50 mC cm ÿ 2) potentiostatically grown at Eg = 1.38 (a) and 1.40 V vs SCE (b).
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Fig. 7. Cyclic voltammogram and de¯ectogram for a P(3MeTh) ®lm (Qg = 100 mC cm ÿ 2) grown under potentiostatic control at Eg = 1.40 V vs SCE.
revealed by ellipsometric experiments [27, 28], above a given charge a dierent structured material is produced. Thus the de¯ectogram in Fig. 7 corresponds to a two-layered ®lm and denotes the cation expulsion
since the beginning of the oxidation process (y < 0), the anion entrance in correspondence to the current oxidation peak (y > 0) with a reduction in y magnitude when the potential scan is reversed, which will change sign at the end of the potential scan. According to the previous discussion, these trends of the angular de¯ection can be linked to the formation of long segments, which means that on the top of a ®rst layer of polymer mainly formed by short segments, the second layer may present longer chains. The above mentioned assumptions are also consistent with the results obtained for the redox behaviour of galvanostatically prepared P(3-MeTh) ®lms (Fig. 8). In this case (longer chains), the second layer appears to be produced earlier since the de¯ectogram for a ®lm prepared with Qg = 50 mC cm ÿ 2 re¯ects in a very de®ned way the expected features of cations expulsion at the beginning of the polymer oxidation, followed by anions entrance for the charge compensation and expulsion when the potential scan is reversed, being the cations entrance well marked as a second stage of ®lm reduction. As shown in Fig. 9, preliminary results have also been obtained for pyrrole electropolymerization. Under potentiostatic or galvanostatic control a continuous decrease in the angular deviation, which reach saturation very shortly for Eg = 0.85 V, is observed. If the polymerization is an instantaneous process, this re¯ects the movement of the produced proton away from the electrode (much higher diusion coecient than the monomer or anion). The absence of a positive
Fig. 8. Cyclic voltammogram and de¯ectogram for a P(3-MeTh) ®lm (Qg = 50 mC cm ÿ 2) grown under galvanostatic control at ig = 2 mA cm ÿ 2.
L.M. Abrantes, J. Correia / Electrochimica Acta 44 (1999) 1901±1910
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Fig. 9. Transients for the potentiostatic and galvanostatic growth of PPy and respective probe beam de¯ection pro®les. Eg = 0.80 (a) and 0.85 V vs SCE (b); ig = 1 mA cm ÿ 2 (c).
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de¯ection at the ®rst instance of the polymerization (monomer diusion towards the electrode surface) is probably due to the high monomer concentration used in this work for polypyrrole synthesis, in order to maintain the same conditions as used for P(3-MeTh) preparation. Further evidence of the eventual participation of reaction products on the initial stages of PPy formation requires other experimental conditions which are currently under investigation. Due to the complexity of the involved mechanism, the mathematical technique of temporal convolution [29] was extended in order to analyse more than a single ¯ux. The quantitative treatment of the data on the 3-MeTh and Py electropolymerization, using that approach, will be reported in a separate paper. References [1] S. Glenis, G. Tourillon, F. Garnier, Thin Solid Films 122 (1984) 9. [2] R.J. Cushman, P.M. McManus, S.C. Yang, J. Electroanal. Chem. 291 (1986) 335. [3] S.N. Hoier, S.M. Park, J. Electrochem. Soc. 140 (1990) 2454. [4] T.F. Otero, J. RodrõÂ guez, Electrochim. Acta 39 (1994) 245. [5] S. Asavapiriyanont, G.K. Chandler, G. Gunawardea, G.A. Pletcher, J. Electroanal. Chem. 177 (1984) 229. [6] J. Roncali, Chem. Rev. 92 (1992) 711. [7] K. Tanaka, T. Schichiri, S. Wang, T. Yarmabe, Synth. Met. 24 (1988) 203. [8] C. Visy, J. Lukkari, J. Kankare, J. Electroanal. Chem. 319 (1991) 85. [9] B.R. Scharifker, E. GarcõÂ a-Pastoriza, W. Marino, J. Electroanal. Chem. 300 (1991) 85.
[10] A.R. Hillman, E.F. Mallen, J. Electroanal. Chem. 243 (1988) 403. [11] P.A. Christensen, A. Hammett, Electrochim. Acta 36 (1991) 1263. [12] F. Chao, M. Costa, E. Museux, E. Levart, L.M. Abrantes, J. Chim. Phys. 89 (1992) 1009. [13] C. Tian, G. Jin, F. Chao, M. Costa, J.P. Roger, Thin Solid Films 233 (1993) 91. [14] J. Lukkari, M. Alanko, V. PitkaÈnen, K. Kleemola, J. Kankare, J. Phys. Chem. 98 (1994) 8525. [15] J. Lukkari, Mat. Sci. Forum 191 (1995) 219. [16] J. Rudnicki, G. Brisard, H. Gasteiger, R. Russo, F. McLarnon, E. Cairns, J. Electroanal. Chem. 362 (1993) 55. [17] C. Barbero, M.C. Miras, R. Kotz, Electrochim. Acta 37 (1992) 429. [18] E. Vieil, K. Meerholz, T. Matencio, J. Heinze, J. Electroanal. Chem. 368 (1994) 183. [19] W.B. Jackson, N.M. Amer, A.C. Boccara, D. Fournier, Appl. Opt. 20 (1981) 1333. [20] O. Haas, J. Rudnicki, F.R. McLarnon, E.J. Cairns, J. Chem. Soc. Faraday Trans. 87 (1991) 939. [21] C. Barbero, M.C. Miras, O. Haas, R. Kotz, J. Electrochem. Soc. 138 (1991) 669. [22] L.M. Abrantes, M.C. Oliveira, E. Vieil, Electrochim. Acta 41 (1996) 1515. [23] J. Lukkari, J. Kankare, Synth. Met. 69 (1995) 353. [24] P. Lang, F. Chao, M. Costa, E. Lheritier, F. Garnier, Ber. Bunsenges. Phys. Chem. 92 (1988) 1528. [25] F. Chao, M. Costa, C. Tian, Synth. Met. 53 (1993) 127. [26] P. Lang, F. Chao, M. Costa, F. Garnier, Polymer 28 (1987) 668. [27] L.M. Abrantes, F. Chao, M. Costa, E. Museux, Portugaliñ Electrochim. Acta 9 (1991) 163. [28] F. Chao, M. Costa, G. Jin, C. Tian, Electrochim. Acta 39 (1994) 197. [29] E. Vieil, J. Electroanal. Chem. 264 (1994) 9.
Electrochimica Acta 44 (1999) 1901±1910
On the initiation and growth of polymer ®lms onto electrode surfaces L.M. Abrantes a, *, J.P. Correia b a
CECUL, Departamento QuõÂmica, Faculdade de CieÃncias, Universidade de Lisboa, R. Ernesto Vasconcelos, 1700 Lisboa, Portugal b INETI, Departamento de Energias RenovaÂveis, Est. do Pac° o do Lumiar, 1699 Lisboa Codex, Portugal Received 27 July 1998
Abstract The probe beam de¯ection technique (PBD) has been successfully used to detect in situ the in¯uence of electrochemical mode and conditions on the initial stages of poly-3-methylthiophene formation. The interpretation of the results is coherent with the previously reported oligomer formation in solution prior to deposition. The eect of the initial polymerization steps in the subsequent stages of the polymer growth has also been investigated, analysing the redox behaviours along with the correspondent de¯ectograms obtained for thin ®lms synthesized under dierent deposition charges. The data indicate that up to a given thickness the polymer responses re¯ect the nature of the ®rst deposited layer. The electropolymerization behaviour of methylthiophene has also been contrasted to that of pyrrole under the same experimental conditions (solvent, monomer concentration, anionic and cationic components of the electrolyte). The results clearly reveal an earlier deposition process and that oligomer formation, if any, is not determinant for polypyrrole deposition. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Conducting polymers; Electropolymerization; Probe beam de¯ection; Poly-3-methylthiophene; Initiation and growth
1. Introduction During the last two decades the considerable attention devoted to electronically conducting polymers has been mainly focused on their redox behaviour and inherent potential useful properties [1, 2]. Although the polymer ®lms properties, and therefore the success of the envisaged applications, strongly depends upon the preparation route, it is relatively more recent that investigation towards the understanding of basic mechanism of the ®lm growth and its interrelation with the electrochemical conditions has been reported [3, 4]. Two models have been put forward to describe the initiation and growth of polymer ®lms prepared electrochemically. The anodic electropolymerization process has been described either as adsorption of monomers on the electrode surface followed by the gradual addition of
* Corresponding author. Tel.: +35-7573624; fax: +3517573625; e-mail: [email protected]
monomers to the surface-bound species [5, 6] or as oligomer formation in solution which at a given stage `precipitate' on the electrode surface [7, 8]. In spite of the dierences among the electrodeposition of metals and the electropolymerization process, electrochemical data has been used to test nucleation models [9]; however, purely electrochemical information is unable to identify produced species or to provide information on the ®rst stages of the polymer formation. The coupling of electrochemical and UVVIS spectroscopic techniques can provide the evolution of spectral characteristics at dierent stages of the electropolymerization process [10], but information whether the species are produced only in solution or also at the electrode surface cannot be obtained. Evidence for deposition of solution-formed oligomers at early stages of the electropolymerization has been mainly given by ellipsometric measurements [11±13] and more recently by photocurrent spectroscopy [14, 15], which are well-known reliable techniques to characterize surface modi®cations.
0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 8 ) 0 0 2 9 9 - 0
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L.M. Abrantes, J. Correia / Electrochimica Acta 44 (1999) 1901±1910
Not withstanding the anodic electropolymerization the electrode process also involves ¯uxes of neutral and/or ionic species from and to the solution which in turn are also dependent on the ®lm growth mechanism and on the electrochemical experimental conditions employed to produce the polymer. Those ¯uxes give rise to concentration gradients of electrolyte species in front of the electrode, which call for the utilization of in situ optical methods like refractive index gradients measurements. Thus, the probe beam de¯ection (PBD) technique, which is based on the de¯ection of a laser beam aligned parallel to the electrode surface [16, 17], appears as a very convenient tool for studying the mass transport and reaction mechanism at the electrochemical interface. Furthermore, the PBD technique can be used simultaneously with electrochemical methods such as voltammetry or chronoamperometry, allowing a direct correlation between the charge transfer and the ion ¯ow process. Although there is a propagation delay due to the location of the beam (e.g. 100 mm away from the electrode) [18], the low detection limit (1 nrad) [19] and the reported successful use of PBD to monitor the ion exchange during conducting polymers redox processes [20, 21] have prompted our interest in employing it as an alternative tool to detect the in¯uence of the electrochemical mode and conditions on the electropolymerization initial stages and on the subsequent redox behaviour of conductive polymer layers. The PBD results for the polymerization of 3methylthiophene are consistent with oligomers formation in solution prior to deposition in contrast to the case of polypyrrole formation which, under the same experimental conditions, appears to follow an instantaneous nucleation model. Interpretation of PBD data from potential cycling experiments with dierent ®lms also enables a better appraisal of the eects of change in polymer growth potential/current and charge. 2. Experimental With the exception of the acetonitrile (Merck, Uvasol spectroscopy grade) which was distilled under argon atmosphere prior to use, all the electrolyte solutions were prepared from `Analar' grade chemicals as received. They were thoroughly deoxygenated directly in the cell with argon (purity >99.9997%) prior to measurements. All the experiments were performed at room temperature. Films of P(3-MeTh) and PPy were electrosynthesized under potentiostatic and galvanostatic control in solutions containing 0.6 mol dm ÿ 3 monomer and 0.2 mol dm ÿ 3 LiClO4 in acetonitrile. After holding the potential at 0 V for 12 min for polymer discharge, the
Table 1 Refractive indices of monomers and electrolyte and their derivatives at 258C
3 MeTh Py LiClO4
n
@n/@c (mol ÿ 1 cm3)
1.3434 + 0.0153C 1.3435 + 0.0130C 1.3434 + 0.0111C
15.3 13.0 11.1
characterization of P(3-MeTh) ®lms was carried out in a monomer free solution by scanning the electrode potential between 0 and 1 V at v = 50 mV s ÿ 1. The PBD experiments were performed by analysis of the de¯ection of a 2 mW He±Ne laser beam (Oriel model 79200) caused by concentration gradients near the electrode surface. The laser with a 1/e2 beam diameter of 0.63 mm is focused to the centre of the cell by a lens, converging to a waist diameter of about 1/ e2 = 50 mm. In this work the estimated distance electrode-probe beam is kept at ca 100 mm. The deviation of the beam is measured by a position-sensitive detector consisting of a SPOT-2 DMI Optilas double photodiode (10 mm gap between the active surfaces) placed behind the electrochemical cell being the setup rigidly mounted on an Oriel optical bench. A more detailed description of the PBD setup is reported elsewhere [22]. In this work a positive de¯ection (y > 0) indicates an ion-¯ow towards the electrode while the movement away from the electrode is taken as negative (y < 0). A three-electrode quartz cuvette cell ®xed in the optical bench was employed being the Pt disk (5 mm é) working electrode potential controlled with respect to the saturated calomel electrode (SCE). A platinum grid counter electrode was placed far enough away to ensure that the reactions occurring on it did not aect the deviation of the laser caused by the process taking place at the working electrode. The refractive indices of the electrolyte and the monomers in acetonitrile (0.1±1.0 M) were measured at 258C with a Abbe `60' refractometer using the 589.6 nm line from a sodium D1 lamp, and the results are summarized in Table 1.
3. Results and discussion A typical current transient for the electropolymerization of 3-MeTh under potentiostatic control is shown in Fig. 1. Previous in situ ellipsometric studies on this polymer growth [12, 13] enables us to de®ne three dierent ranges: from A to B the diusion controlled monomer oxidation in solution takes place; from B to C there is a diusion controlled nucleation of oligomers produced in solution; from C to D the overlap of growing centres. The movement of solution species
L.M. Abrantes, J. Correia / Electrochimica Acta 44 (1999) 1901±1910
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Table 2 Data obtained during the potentiostatic growth of P(3-MeTh) at dierent potentials (Fig. 2) Eg/V vs SCE
Charge/mC cm ÿ 2 and (time/s) consumed until B
Charge/mC cm ÿ 2 and (time/s) consumed until C
Time/s until y
1.36 1.38 1.40
0.7 (1.2) 0.4 (0.5) 0.1 (0.1)
2.5 (3.4) 2.0 (1.6) 1.3 (0.6)
4.5 3.5 3.0
which accompanies these process are (due to diusion and migration) the displacement of monomer species towards the electrode surface (A±B) whereas after B the movement of the protons, formed during the 3MeTh oxidation, away from the electrode must also be accounted for and as soon as a polymer is formed and oxidized at the surface (C), other ¯uxes are expected such as anion diusion towards the electrode for polymer charge compensation or non-consumed oligomers away from its surface. Although the dierences in the diusion coecients (Di) and the refractive index change with concentration (@n/@c) when many species are involved, analysis of the beam de¯ection signal during the electropolymerization process can be dicult, and changes in the contribution of the partial ¯uxes involved are expected which eventually can cause the inversion of the predominant species transport. Depending on the applied potential, dierences in the rate of formation of soluble oligomers/polymer onto the electrode should be detected in the corresponding de¯ectograms. Current pro®les and respective beam de¯ections simultaneously recorded during the potentiostatic synthesis of P(3-MeTh) under dierent growth potentials are presented in Fig. 2 where the contribution of migration due to the electric ®eld in a monomer free solution is also illustrated. These experiments display some important characteristics. The onset of beam deviation increase strongly depends on the applied potential being positive since the very early stages for high values (1.40 V); at a given time there is a change in the de¯ection slope, which may even change sign; eventually y can display a constant value.
max
These results clearly reveal the presence of more than the single monomer ¯ux towards the electrode. Considering the polymer growth at Eg = 1.38 V (Fig. 2b), the current density at point C of the chronoamperogram (i = 2 mA cm ÿ 2) gives rise to a mass ¯ux density (J), of approximately 20 nmol cm ÿ 2 s ÿ 1 (z = 1) which, according to Fick's law, creates a concentration gradient (@c/@x) of 2 mmol cm ÿ 4 (assuming a diusion coecient of D = 1.10 ÿ 5 cm2 s ÿ 1). The beam de¯ection for a single ¯ux is given by y x; t L
@n @c @c @x
1
where L and @n/@c are, respectively, the electrode length and the refractive index gradient with the concentration, one obtains considering only the ¯ux of the monomer, a de¯ection of 16 mrad, which is two orders of magnitude higher than the maximum value of the recorded beam deviation (0.16 mrad). This simple computation allows us to infer from the simultaneous occurrence of dierent and opposite ¯uxes which generate a de¯ectogram with a maximum. Table 2 summarizes a semi-quantitative analysis of the results shown in Fig. 2. It is known that the polymerization of P(3-MeTh) requires a previous generation and oxidation of oligomers in solution, since the radical cations of oligomeric species act as a major propagation intermediate for the polymer growth, its concentration being maintained at a steady state level during the synthesis [3]. At short times (e.g. B) the consumed charges are coherent with those observed for the formation of intermediates in solution [12, 14, 23] whereas at slightly longer times (C) the consistency with ellipsometric data [12, 13] allows
Fig. 1. Current transient for the electropolymerization of P(3-MeTh) at Eg = 1.38 V vs SCE.
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Fig. 2. Current and probe beam de¯ection transients for the synthesis of P(3-MeTh) under potentiostatic control at Eg = 1.36 (a), 1.38 (b) and 1.40 V vs SCE (c).
us to state that from that point (C) on there will be an expansion of nuclei which eventually overlaps and then, at a given time, a polymer monolayer is produced, oxidized, and the growth proceeds. In agreement with this interpretation are the probe beam
de¯ection features, and apart from the inherent time shift also the lapses for y max observation follow the expected behaviour occurring earlier as the electrode potential increases. Being the net beam deviation the result of opposite ¯uxes of species with dierent Di
L.M. Abrantes, J. Correia / Electrochimica Acta 44 (1999) 1901±1910
and @n/@c [16], an inversion of y slope corresponds to a change of the dominant contribution for the laser de¯ection onset. This feature must be related to a modi®cation of the electrode surface, the likely formation of the polymer ®rst monolayer being the time lapse required for electrode coverage being smaller as the growth potential increases [24]. At low growth charges the polymerization proceeds mainly by regular incorporation of radical cations on
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the grafted chain [25, 26] and thus this process will be favoured for synthesis carried out at low electrode potentials. Indeed, the low growth rate allows the radical cations in solution to position at the edges of the polymeric chain, creating long and regular ®brils. On the other hand, for higher deposition potentials the oligomeric radical cations will react more randomly, producing branched chains with short segments covering the electrode surface faster. The values of the charge con-
Fig. 3. Electrode potential and probe beam de¯ection pro®les for the galvanostatic growth of P(3-MeTh) at ig = 1 (a), 2 (b) and 5 mA cm ÿ 2 (c).
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Fig. 4. Cyclic voltammograms and de¯ectograms for P(3-MeTh) ®lms (Qg = 30 mC cm ÿ 2) potentiostatically grown at Eg = 1.36 (a) and 1.40 V vs SCE (b).
sumed until B and the corresponding rate of growth give support to the assumption that at low potentials the polymer is formed mainly by long segments. Results of the electropolymerization of 3-MeTh under galvanostatic control, displayed in Fig. 3, present similar qualitative featuresÐthe higher the applied current the sooner y inversion occurs. Since the data obtained for current values above 2 mA cm ÿ 2 present the same trends as that observed for polymer growth at E = 1.40 V, the polymer formation from short segments will be considered in these cases. The assumptions made so far being correct, the cyclic voltammograms and simultaneously recorded de¯ectograms of thin P(3-MeTh) ®lms will display characteristics re¯ecting the polymer formation from dierent sized chains, the in¯uence of polymerization conditions under potentiostatic mode as well as the eect of preparation at a constant rate. The redox behaviour of thin P(3-MeTh) ®lms prepared under potentiostatic control at Eg = 1.36 and 1.40 V is contrasted in Fig. 4 and Table 3. It can be seen that ®lms prepared at higher potentials are easily oxidized (lower E p0, higher ip0) the conversion being less reversible (lower Q0/Qr), but the polymerization eciency, evaluated through the Q0/Qg ratio, slightly
higher. On the other hand, the beam de¯ection denotes the earlier oxidation (y increases at a lower potential) and the easier switching (when the potential is reversed the decrease in y is larger) of the ®lm grown at the higher potential. It must also be pointed out that, for the ®lm prepared with Eg = 1.36 V, before the oxidation current peak there is an initial decrease in y which can be attributed to cation expulsion, a feature that is likely related to the presence of somewhat longer chains. Fig. 5 displays the cyclic voltammogram and de¯ectogram recorded for a polymer galvanostatically synthesized at ig = 2 mA cm ÿ 2 which, contrasted with the potentiostatically grown ®lms, support the usual choice of potentiostatic mode for this polymer preparation. Actually, the voltammetric features of the galvanostatically produced ®lm, compared to those of polymers prepared at constant potentials, shows a more dicult oxidation (E p0 = 0.665 V, ip0 = 0.36 mA cm ÿ 2, Q0 = 2.75 mC cm ÿ 2), lower reversibility for the redox conversion (Q0/Qr = 1.38) and polymerization eciency (Q0/Qg = 8.0%). The de¯ection recorded during the anodic potential sweep denotes the involvement of cations in the ®rst stages of oxidation; reversing the potential leaves y almost insensitive which re¯ects (in
Table 3 Voltammetric data for the redox behaviour of P(3-MeTh) thin ®lms (Qg = 30 mC cm ÿ 2) grown under potentiostatic control at dierent potentials (Fig. 4) Eg V ÿ 1
Ep0 mV ÿ 1
ip0 mA ÿ 1 cm ÿ 2
Q0 mC ÿ 1 cm ÿ 2
Q0/Qr
Q0/Qg(%)
1.36 1.40
653 620
0.53 0.60
3.36 3.29
1.07 1.10
10.4 11.4
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Fig. 5. Cyclic voltammogram and de¯ectogram for a P(3MeTh) ®lm (Qg = 30 mC cm ÿ 2) grown under galvanostatic control at ig = 2 mA cm ÿ 2.
agreement to the high value observed for Q0/Qr) a dif®cult anion expulsion. The in¯uence of the polymerization conditions are still noticeable on the displayed characteristics of thicker P(3-MeTh) potentiostatically prepared ®lms (Fig. 6). For the case of Eg = 1.40 V growth, after the potential scan is reversed, a ¯ow towards the electrode
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persists as suggested by the sign y. Indeed, y is still positive (and even larger than during the anodic scan) for a much larger potential range than observed for ®lms prepared under Eg = 1.38 V. This eect was seen to become more pronounced as the growth potential increases. The continuous ¯ux of non-charged species (very likely ion pair (A ÿ , C + )) is de®nitively associated to ®lms where the oxidation takes place at relatively lower potentials and higher rates, as appears to be the case of polymers prepared at high potentials and therefore where short chains are more probable. Voltammetric information taken in systematic experiments has shown that the polymer oxidation potential increases with the ®lm thickness, whereas the Q0/Qr ratio and the polymerization eciency decrease until a given value. It is worthwhile to compare the cyclic voltammograms and de¯ectograms obtained for P(3MeTh) ®lms prepared under charges of 50 mC cm ÿ 2 (Fig. 6b) and 100 mC cm ÿ 2 (Fig. 7), where the values of the ratio Q0/Qr and growth eciency are similar, but there is a clear increase in the oxidation peak potential for the latter. The change in the redox behaviour can easily be observed in the de¯ectogram. As
Fig. 6. Cyclic voltammograms and de¯ectograms for P(3-MeTh) ®lms (Qg = 50 mC cm ÿ 2) potentiostatically grown at Eg = 1.38 (a) and 1.40 V vs SCE (b).
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Fig. 7. Cyclic voltammogram and de¯ectogram for a P(3MeTh) ®lm (Qg = 100 mC cm ÿ 2) grown under potentiostatic control at Eg = 1.40 V vs SCE.
revealed by ellipsometric experiments [27, 28], above a given charge a dierent structured material is produced. Thus the de¯ectogram in Fig. 7 corresponds to a two-layered ®lm and denotes the cation expulsion
since the beginning of the oxidation process (y < 0), the anion entrance in correspondence to the current oxidation peak (y > 0) with a reduction in y magnitude when the potential scan is reversed, which will change sign at the end of the potential scan. According to the previous discussion, these trends of the angular de¯ection can be linked to the formation of long segments, which means that on the top of a ®rst layer of polymer mainly formed by short segments, the second layer may present longer chains. The above mentioned assumptions are also consistent with the results obtained for the redox behaviour of galvanostatically prepared P(3-MeTh) ®lms (Fig. 8). In this case (longer chains), the second layer appears to be produced earlier since the de¯ectogram for a ®lm prepared with Qg = 50 mC cm ÿ 2 re¯ects in a very de®ned way the expected features of cations expulsion at the beginning of the polymer oxidation, followed by anions entrance for the charge compensation and expulsion when the potential scan is reversed, being the cations entrance well marked as a second stage of ®lm reduction. As shown in Fig. 9, preliminary results have also been obtained for pyrrole electropolymerization. Under potentiostatic or galvanostatic control a continuous decrease in the angular deviation, which reach saturation very shortly for Eg = 0.85 V, is observed. If the polymerization is an instantaneous process, this re¯ects the movement of the produced proton away from the electrode (much higher diusion coecient than the monomer or anion). The absence of a positive
Fig. 8. Cyclic voltammogram and de¯ectogram for a P(3-MeTh) ®lm (Qg = 50 mC cm ÿ 2) grown under galvanostatic control at ig = 2 mA cm ÿ 2.
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Fig. 9. Transients for the potentiostatic and galvanostatic growth of PPy and respective probe beam de¯ection pro®les. Eg = 0.80 (a) and 0.85 V vs SCE (b); ig = 1 mA cm ÿ 2 (c).
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de¯ection at the ®rst instance of the polymerization (monomer diusion towards the electrode surface) is probably due to the high monomer concentration used in this work for polypyrrole synthesis, in order to maintain the same conditions as used for P(3-MeTh) preparation. Further evidence of the eventual participation of reaction products on the initial stages of PPy formation requires other experimental conditions which are currently under investigation. Due to the complexity of the involved mechanism, the mathematical technique of temporal convolution [29] was extended in order to analyse more than a single ¯ux. The quantitative treatment of the data on the 3-MeTh and Py electropolymerization, using that approach, will be reported in a separate paper. References [1] S. Glenis, G. Tourillon, F. Garnier, Thin Solid Films 122 (1984) 9. [2] R.J. Cushman, P.M. McManus, S.C. Yang, J. Electroanal. Chem. 291 (1986) 335. [3] S.N. Hoier, S.M. Park, J. Electrochem. Soc. 140 (1990) 2454. [4] T.F. Otero, J. RodrõÂ guez, Electrochim. Acta 39 (1994) 245. [5] S. Asavapiriyanont, G.K. Chandler, G. Gunawardea, G.A. Pletcher, J. Electroanal. Chem. 177 (1984) 229. [6] J. Roncali, Chem. Rev. 92 (1992) 711. [7] K. Tanaka, T. Schichiri, S. Wang, T. Yarmabe, Synth. Met. 24 (1988) 203. [8] C. Visy, J. Lukkari, J. Kankare, J. Electroanal. Chem. 319 (1991) 85. [9] B.R. Scharifker, E. GarcõÂ a-Pastoriza, W. Marino, J. Electroanal. Chem. 300 (1991) 85.
[10] A.R. Hillman, E.F. Mallen, J. Electroanal. Chem. 243 (1988) 403. [11] P.A. Christensen, A. Hammett, Electrochim. Acta 36 (1991) 1263. [12] F. Chao, M. Costa, E. Museux, E. Levart, L.M. Abrantes, J. Chim. Phys. 89 (1992) 1009. [13] C. Tian, G. Jin, F. Chao, M. Costa, J.P. Roger, Thin Solid Films 233 (1993) 91. [14] J. Lukkari, M. Alanko, V. PitkaÈnen, K. Kleemola, J. Kankare, J. Phys. Chem. 98 (1994) 8525. [15] J. Lukkari, Mat. Sci. Forum 191 (1995) 219. [16] J. Rudnicki, G. Brisard, H. Gasteiger, R. Russo, F. McLarnon, E. Cairns, J. Electroanal. Chem. 362 (1993) 55. [17] C. Barbero, M.C. Miras, R. Kotz, Electrochim. Acta 37 (1992) 429. [18] E. Vieil, K. Meerholz, T. Matencio, J. Heinze, J. Electroanal. Chem. 368 (1994) 183. [19] W.B. Jackson, N.M. Amer, A.C. Boccara, D. Fournier, Appl. Opt. 20 (1981) 1333. [20] O. Haas, J. Rudnicki, F.R. McLarnon, E.J. Cairns, J. Chem. Soc. Faraday Trans. 87 (1991) 939. [21] C. Barbero, M.C. Miras, O. Haas, R. Kotz, J. Electrochem. Soc. 138 (1991) 669. [22] L.M. Abrantes, M.C. Oliveira, E. Vieil, Electrochim. Acta 41 (1996) 1515. [23] J. Lukkari, J. Kankare, Synth. Met. 69 (1995) 353. [24] P. Lang, F. Chao, M. Costa, E. Lheritier, F. Garnier, Ber. Bunsenges. Phys. Chem. 92 (1988) 1528. [25] F. Chao, M. Costa, C. Tian, Synth. Met. 53 (1993) 127. [26] P. Lang, F. Chao, M. Costa, F. Garnier, Polymer 28 (1987) 668. [27] L.M. Abrantes, F. Chao, M. Costa, E. Museux, Portugaliñ Electrochim. Acta 9 (1991) 163. [28] F. Chao, M. Costa, G. Jin, C. Tian, Electrochim. Acta 39 (1994) 197. [29] E. Vieil, J. Electroanal. Chem. 264 (1994) 9.