Langmuir 1992,8, 1645-1653
1645
Electrochemical Deposition of a Conducting Polymer, Poly(thiophene-3-acetic acid): The First Observation of Individual Events of Polymer Nucleation and Two-Dimensional Layer-by-Layer Growth Feng-Bin Li*pt and W. John Alberyt Department of Chemistry, Imperial College, London SW7 2AY, U.K. Received October 1 , 1991. In Final Form: February 3, 1992 Cyclic voltammetric and chronoamperometric studies reveal that the electrochemical deposition of a conducting polymer, poly(thiophene-3-aceticacid) (poly(TPAA)), proceeds via a mechanism of twodimensional (2-D) layer-by-layer nucleation and growth, following the first monolayer deposited through oxidative adsorption of the TPAA monomer on a bare Pt surface. This finding is to be compared with previous investigations of conducting polymer deposition which have reported consistentlya mechanism of three-dimensionalnucleation and growth. The 2-D layer-by-layer mechanism is embodied in a variety of phenomena observed under various experimental conditions. The most striking evidence arises from the unique features of the cyclic voltammograms (CVs), in which multipeaks resulting from oxidative monomer adsorption, and single events of polymer nucleation and successive monolayer spreading, respectively, have been observed for the first time. The nucleation peaks in the CVs are characterized by a linearly rising edge as expected theoretically and the amount of charge involved, ca. 200 pC/cm2,in agreement with the deposition of a poly(TPAA) monomolecular layer. The initial propagation rate of a poly(TPAA) monolayer is between ca. 0.08 and 0.14 cm/s. The new mechanism indicates the possibility of preparing conducting polymer films with a more uniform and smooth morphology for use, e.g., as electronic materials,
Introduction Recently there has been a growing interest in the kinetics and mechanism of electrochemical deposition of conducting p01ymersl-l~alongside the in-depth research on the microstructure and conducting mechanismlE20 and applications21122of these materials. Apparently a detailed insight into the deposition process would promote a better + Present address: School of Chemistry, Universityof Bristol, Bris-
tol, BS8 l T S , U.K. 1 Present address: Physical Chemistry Laboratories, South Parks
Rd.,Oxford OX1 lQZ, U.K. (1) Genies, E. M.; Bidan, G.; Diaz, A. F. J.ElectroanaL Chem. 1983, 149, 101. Chandler, G. K.; Gunawardena, G. A,;Pletcher, (2) Asavapiriyanont,S.; D. J . Electroanal. Chem. 1984, 177, 229. (3) Hillman, A. R.; Mallen, E. F. J.Electroanal. Chem. 1987,220,351. (4) Miller, L. L.; Zinger, B.; Zhou, Q. X. J. Am. Chem. SOC.1987,109, 2267. (5) Hamnett, A,; Hillman, A. R. J. Electrochem. SOC.1988,135,2517. (6) Marcos, M. L.;Rodrigues, I.; Velasco, J. G. Electrochim. Acta 1987, 32, 1453. (7) Dian, G.; Merlet, N.; Barkey, G.;Outurquin, F.; Paulmier, C. J. Electroanul. Chem. 1987,238, 225. (8) Beck, F. Electrochim. Acta 1988, 33, 839. (9) Rubinstein, I.; Rishpon, J.; Sabatani, E.; Redondo, A.; Gottesfeld, S.J. Am. Chem. SOC.1990,112,6135. (10) Baker, C. K.;Reynolds, J. R. J. Electroanal. Chem. 1988, 251, 307. (11) Rishpon, J.; Redondo, A.; Derouin, C.; Gottesfeld, S.J. Electroanul. Chem. 1990,296, 73. (12) Scharifker, B. R.; Pastoriza, E. G.; Marino, W. J. Electroanal. Chem. 1991,300,85. (13) Gratzl, M.; Hus, D.-F.; Riley, A. M.; Janata, J. J. Phys. Chem. 1990,94, 5973. (14) Cade. G.: Wheller. B. L.: Swift., R.:. Porter. T. L.: Jeffers. S. J. Phys. k2&ta. ‘1990, 94,5639. (15) Yang, R.; Evans, D. F.; Christersen, L.; Hendrickson, W. A. J. Phys. Chem. 1990,94,6117. (16) Everson, M. P.; Helms, J. H. Synth. Met. 1991, 40, 97. (17) Soubiran, P.; Aeiyach, S.;Lacaze, P. C. J. Electroanal. Chem. 1991,303, 125. (18) Skotheim, T. A., Ed.Handbook of Conducting Polymers; Marcel Dekker: New York, 1986. (19) Charge Transfer in Polymeric Systems. Faraday Discuss. Chem. SOC.1989, 88. (20) Li, F.-B.; Albery, W. J. J. Chem. SOC.,Faraday Trans. 1991,87, 2949.
understanding of the polymer internal structure and its importance on the physicochemical properties and possible applications. A number of groups have reported for the well-known polypyrrole, polythiophene, polyaniline, and some of their derivatives that the early stages of electrodeposition proceed through a nucleation and growth mechanism, similar to the case of metal d e p o s i t i ~ n . ~ ~ ~ ~ ~ Under this rather broad mechanism, however, there are more specific routes such as two-dimensional (2-D) nucleation and growth and three-dimensional (3-D) nucleation and growth, which actually distinguish the internal morphology of these polymers and their theoretical modeling and practical uses. For example, unlike the 3-D mechanism, by which a “packed grain” internal morphology with substantial amount of empty space may be dominant as a result of continuous growth of hemispherical or conical nuclei, a 2-D layer-by-layer mechanism may lead to polymer films which have a more uniform and compact arrangement of polymer chains and thus be more suitable for applications, for example, as ultrathin electronic films. The electrochemical observations so far are confined to a conclusion of 3-D nucleation and growth2-9*26 which is supported in some rare cases by SEM pictures.17 Recent STM images have revealed consistently the presence of relatively flat polymer islands joined by strands on electrode surfaces.14-16 Although these islands may be viewed as 2-D patches in view of the larger lateral sizes than their heights, no clear conclusion of 2-D nucleation and growth has been reached in these reports. This is
’
0743-7463/92/2408-l645$03.00/0
(21) Li, F.-B.; Albery, W. J. J. Electroanal. Chem. 1991,302, 279. (22) Garnier,F.; Horowitz, G.; Peng, X.; Fichou, D. Adu. Mater. 1990, 592. (23) Harrision, J. A.; Thirsk, H.R. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1971; Vol. 5, p 67. (24) Levie. de R. In Advances inElectrochemktrv andElectrochemical Engineerink Gerischer, H., Tobias, C. W., Eds.; JGhn Wiley: New York, 1984: Vol. 13. D 1. (25) Two-dimensional nucleation was mentioned briefly in ref 1, but no hard evidence was presented. One-dimensional nucleation was mentioned in ref 8.
0 1992 American Chemical Society
Li and Albery
1646 Langmuir, Vol. 8, No. 6, 1992 rather surprising since there are no kinetic or thermodynamic reasons for the 3-D mechanism to be universal and the 2-D mechanism to be impossible or unfavorable. In a previous communicationz6it is demonstrated, for the deposition of poly(thiophene-3-acetic acid) (poly(TPAA))27under potentiostatic conditions, that the nucleation and growth are unequivocally a 2-D layer-by-layer process rather than a 3-D process. Among the supporting evidence for the 2-D mechanism, a particularly interesting feature is the discrete current peaks in a potentiostatic transient, resulting from individual events of nucleation and 2-D monolayer spreadings. In this paper we demonstrate further that the 2-D layer-by-layer mechanism is not limited to potentiostatic conditions only. It is possible to observe single events of nucleation and 2-Dmonolayer spreading under potentiodynamic (cyclic voltammetric) conditions.
10
5
0
f/ P
Experimental Section A three-electrode glass cell was used. The working electrode was a Pt-Pt ring-disk electrode with geometric parameters of rl = 2.01, r2 = 2.13, and r3 = 2.26". The experiments were carried out on the disk electrode unless it is indicated specifically. The counter electrode was a Pt gauze. The reference electrode of Ag/Ag+ was made of a silver wire dipping into an acetonitrile internal solution containing0.01 M AgClOd (approximatevalue) and 0.1 M LiClOd. This internal solution was confined within a glass tube by a piece of porous ceramics and thus separated from the bulk electrolyte in which polymer deposition was carried out. The leakage of silver ion into the bulk electrolyte,and hence its possible effect on the electrode surface%or on the polymer deposition, is negligible. This reference electrode was 0.243 V relative to a saturated calomel electrode (SCE) and thus 0.491 V relative to the standard hydrogen electrode (SHE).All the potentials were recorded and are quoted with respect to this Ag/Ag+ reference electrode. The electrodeposition of poly(TPAA) was carried out in acetonitrile (refluxedand distilled over calcium hydride) solutions. The concentration of TPAA monomer (Aldrich,used as received) was typically 0.5 M. This relatively high concentration enabled the deposition to be carried out at low potentials, so that polymer degradation during the deposition2' is less important, and the deposition kinetics was not too fast to be observed conveniently. For the particular system of poly(TPAA),continuous deposition cannot be sustained in dilute monomer solutions (below0.1 M).n Pure nitrogen gas was bubbled through or over the solution throughout to purge oxygen. Before each run of deposition,the Pt working electrode was polished carefully with alumina (1hm size) slurry and washed with double distilled water and then acetonitrile. After ca. 5 to 10runs, thorough polishing was carried out on a specialized rotating polishing disk in order to maintain a macroscopically smooth surface. No appreciable difference was found between the hand-polished and disk-polishedsurfaces.
I
1
L
2
t/s
Figure 1. (a)Potentiostatic transients for the deposition of poly(TPAA),0.3 M TPAA and 0.1 M LiClO, in acetonitrile. Curves 1to 6 correspond to potential steps to 1.240,1.250,1.260,1.270, 1.280and 1.290V,respectively. (b)i l W plotsfor theearlyrieing part the transients shown in part a with corresponding l i e numbers.
electrode surface exceeds that allowed by the solubility. When reaching a certain size, these clusters become thermodynamically stable and begin to expand rapidly (the rising section of the transients) by further incorporation of the polymer into the nuclear periphery before a steady state of nucleation and overlapping is reached (the plateau). While not ruling out completely the possibility of nucleation via adsorption of poly(TPAA1 molecules on the surface of the polymer film already deposited, the precipitation pathway is supported by RRDE (rotating ring-disk electrode) o b s e r ~ a t i o nwhich s ~ ~ show clearly the presence of a large amount of oligomeric/polymeric TPAA in solution during the deposition. The adsorption pathway is important only for the first layer on a bare Pt surfacem as will be further discussed below. In the nucleation model, the possible rate-determining step includes the initial charge-transfer of the neutral monomer to form radical cations at low the nucleation process, diffusion of the monomer toward the electrode surface and/or diffusion of the depositing species Results and Discussion (which are polymer/large oligomers, and are not to be confused with the monomer) to the nuclear edges a t high Nucleation and Growth under Potentiostatic Conpotentials (see below), and the incorporation step of these ditions. A series of current-time transients for the species into the bulk polymer phase.23124The detailed deposition of poly(TPAA) under a potential-step regime mechanism of the polymerization, especially the process are shown in Figure la. In these transients there is an of chain elongation, is quite complicated and not generally initial spike, a minimum, a rising section, and a plateau. well understood yet. It is, however, commonly agreed that These are characteristic of electrochemical phase formation via a nucleation and growth m e c h a n i ~ m . ~ - ~In 1 ~ 3 * ~the ~ initial charge transfer of the neutral monomer produces a radical cation,I8which is added to the oligomericchains this mechanism, the oligomeric/polymeric TPAA species by coupling with other radicals18 or by aromatic elecin the solution precipitate onto the electrode surface to trophilic s u b s t i t ~ t i o nreaction ~~ with neutral species. In form clusters (nuclei) as their concentration near the (26) Li, F.-B.; Albery, W. J. Electrochim. Acta 1992,37, 393.
(27)Albery,W.J.; Li, F.-B.; Mount, A. R. J . Electroanal. Chem. 1991, 310, 239. (28) Holland-Mortz, E.; Gordan, J., 11; Kanazawa, K.; Sonnenfeld, R. Langmuir 1991, 7, 1981.
(29)Batina, N.;Gui, J. Y.; Kahn, B. E.; Lin, C.-H.; Lu,F.; McCargar, J. W.; Saiaita, G. N.; Stern, D. A.; Hubbard, A. T.; Mark, H. B., Jr.; Zimmer, H. Langmuir 1989,5,588. (30)Satoh, M.; Imanishi, K.; Yoshino, K. J. ElectroanaL Chem. 1991, 31 7, 139.
Electrochemical Deposition of a Conducting Polymer
Langmuir, Vol. 8, No. 6, 1992 1647
either case, the polymerization occurs mainly in the solution near the electrode surface,8~~~ and the rate of chain elongation is considered to be fast,l-I7 compared with that of the incorporation step. Therefore, the effect of polymerization kinetics on the overall rate of polymer deposition is considered negligible. Cross-linking of polymer chains inside the film during the electrochemical deposition or during the subsequent redox switching in a pure background electrolyte is possible, but linear chain elongation during the depositionby adding monomer units to the polymer chain ends on the surface in contact with the electrolyte is unimportant statistically. The early rising part of the i-t transients for poly(TPAA) deposition follows a relationship of i 0: t2,as can be derived from Figure lb, where plots of i1/2vs t present straight lines before overlap of the growing nuclei becomes significant. The use of the i1/2vs t plots instead of i vs t2 plots has the advantage of better accuracy.26 This dependence of the overall current on time is consistent with the theoretical predictions for a progressive 2-D nucleation (2-DP) process23
i = ?rzFMhANh2t2/p
(1)
where M is the molar mass, NOis the number of nucleation sites available,k is the growth rate constant (mol.cm-2.s-1), A is the nucleation rate constant (s-l), p is the density of the material deposited, z is the charge transferred from each monomer, h is the height of the 2-D disk-shaped nucleus, and F is the Faraday constant. This feature, however, is not unique to the 2-D mechanism. A 3-D growth of instantaneously formed nuclei (3-DI), for example, predicts the same23
i = 2.rrzFMNok3t2/p
(2)
Therefore, establishment of the 2-D mechanism needs other supporting evidence. Polymer Growth on Preformed Nuclei under a Double-Potential Step Regime. One of the characteristics which distinguishes the 2-DP mechanism from the 3-DI one is the variation of nucleationlaw with the strength of the driving force, mainly the electrode potential. An instantaneous nucleation observed at relatively low potentials should remain the same when the process is examined at high potentials. A progressive nucleation process, governed by23
N = ANot (3) however, is expected to switch to an instantaneous nucleation, in which a certain number of nuclei (although may not be the same as NO)are created virtually at the instant the potential is applied, under such conditions. Consequently, the dependence of the overall deposition current on time is expected to be the same as predicted byeq 2, if the nucleation of poly(TPAA)is an instantaneous process, and to change from a t2-dependence to a linear time-dependen~e~~ i = 2?rzFMhNok2t/p (4) if the nucleation in Figure 1 is progressive. Practically, polymer deposition on instantaneously formed nuclei can be achieved in a double-potential step experiment, in which the nucleation is completed at a high potential (the nucleation potential), and the nuclei created are then allowed to grow at a second, lower potential (the growth potential). Ideally, a certain number of nuclei will have been produced during the nucleation process, but nuclear overlap will not have become significant. The
31
. -2 I 1
E
OO
10
t/
s
Figure 2. Current-time transients for the deposition of poly(TPAA) under a double-potential step scheme, 0.5M TPAA and 0.1 M LiClOl in acetonitrile. The first potential step for nucleationisfrom0.0to 1.600Vfor0.5s,andthesecondpotential step for polymer growth is from 1.600 V to (from the bottom to top) 1.280, 1.290, 1.310, and 1.320 V, respectively.
growth potential should be high enough to sustain the polymer growth on the preformed nuclei but not high enough to create an appreciable amount of new nuclei. A series of i-t transients for the deposition of poly(TPAA) by this double-potentialstep technique are shown in Figure 2. The most important feature of these transients is that the current at the growth potentials is linear with time, as expected from eq 4. Such an excellentlinearity indicates that polymer growth at the second potential is predominantly on the nuclei preformed by the first potential step, and fresh nucleation is negligible. More importantly, these results prove that the electrochemicaldeposition of poly(TPAA), as shown in Figure 1,proceeds via a mechanism of 2-D nucleation and layer-by-layer growth, rather than 3-D nucleation and growth, as proposed previously for the deposition of other conducting polymers. The mechanism of 2-D nucleation and growth of poly(TPAA) is consistent with the surface morphology of the polymer observed in the SEMs. For a relatively thin poly(TPAA) film on the order of a few dozens nanometers thick, the polymer surface in contact with the solution is smooth and uniform seen on a scale of 4000 times magnification. Some flat bumps with lateral sizes of ca. 0.5to 1pm are visible, but no 3-D structures, such as cones or (hemi-)spheres,are observed. The edges of these bumps are smoothly fused with the surrounding background as seen on this scale. Therefore, these bumps are likely the growing 2-D nuclei. These observations also ruled out the possibility of one-dimensional nucleation and growth of a limited number of macroscopic rods, which also follows a linear i-t relationship, since such a mechanism would result in a highly uneven morphology. Macroscopic Two-DimensionalGrowth under Diffusion Control. One of the interesting features of the poly(TPAA)deposition is that the polymer film can grow, starting from the disk electrode on which the potential is applied, laterally along the electrode surface to cover the outside ring electrode and the Teflon mantle, under suitable conditions. Figure 3a shows an i-t transient for such a macroscopic 2-D lateral growth. Due to the difference in polymerization potential and the time scale, the shape of this i-t curve is quite different from those shown above. The horizontal arrow indicates the current at the early stages of deposition, corresponding to the steady-state growth (the plateau region seen on a short time scale, as those shown in Figure la) confined to the original geometric area of the disk. After this point, the polymer film already deposited on the disk electrodeserves
Li and Albery
1648 Langmuir, Vol. 8, No. 6,1992
V
c
0 ‘
b
4-
6
-
E
.-
\
2-
I
0
0
0
i
V
Figure 4. ‘Collection” experiments of the ring-disk electrode connected with a poly(TPA.4) film, resulting from macroscopic 2-D growth as shown in Figure 3a. The electrolyte is 0.1 M LiClO, in acetonitrile. Potential sweep rate is 5 mV/s. The crosses indicate the origin of the current axes and the upper potential of the scan. These current-potential curves indicate good electrical conductivity, and thus physical uniformity of the poly(TPAA) film on top of the insulating Teflon gap between the , ring current, ring and the disk electrodes. (a),Disk current, i ~ and i ~as, functions of the disk electrode potential, ED,with the ring potential, ER,kept at -0.4 V. (b) Similar to part a but the ring and the disk are used interchangeably.
6
ti/2/
s’/2
12
Figure 3. (a) A current-time curve for the deposition of poly(TPAA), showing the macroscopic 2-D growth beyond the disk electrode, 0.5 M TPAA and 0.1 M LiClOr in acetonitrile. The potential for the deposition is 1.600 V and is applied to the disk electrode. The whole ring-disk electrode is rotated at 4Hz during the deposition. See the text for details. (b) i-N2 plot for the steadily rising section of part a. The origin of the current axis is the point marked by the horizontal arrow in part a.
as a macroscopic 2-D “nucleuss, whose growth results in a lateral size expansion of the polymer film beyond the disk electrode. Because the whole polymer film is active for monomer oxidation and polymer deposition, owing to its conductive nature in the oxidized state, the lateral size expansion leads to a steady increase in current with polymerization time. The appearance of a plateau current at the late stages of deposition after ca. 3.5 min corresponds to a situation, where all the surface of the Teflon mantle in the electrode surface plane has been used up and further lateral expansion has stopped, due to a lack of supporting substrate. The ratio between the plateau current and the current marked by the horizontal arrow is the same as that between the geometric area of the poly(TPAA) film finally obtained (ca. 0.8 cm in diameter) and
of the Pt disk electrode. At the end of the polymerization, the polymer film has grown ca. 2 mm beyond the disk electrode, while the film thickness is on the order of 1pm. Therefore, the growth rate of the polymer film in the lateral direction is faster than that in the direction perpendicular to the electrode surface by 3 orders of magnitude. During the size-expansion process, part of the polymer generated on the central area of the film will certainly deposit locally, which results in an increase in film thickness. But most of these polymer molecules are transported downstream to the edges of the film and deposited there to contribute to the lateralgrowth. (Some of them are transported to the bulk of the solution.) In this case, one would expect a mass-transport control of the whole process. Figure 3b displays the polymerization current during the steadily rising section of Figure 3a m a function of t1I2. The linearity of the plot indicates that the process is indeed controlled by mass transport, as expected from eq 531
i=
(5)
where c is the concentration of the species being deposited. The small intercept on the abscissa represents the time lapse from the beginning of the polymerization to the moment when lateral size expansion of the film beyond the disk electrode becomes significant. This time obtained from the intercept, 1.3 s, is in good agreement with that directly read from the i-t curve in Figure 3a. The uniformity of the poly(TPAA) film deposited through such a continuous lateral growth can be seen in a ‘collection” experiment using the ring and the disk electrodes jointly, as shown in Figure 4. Figure 4a shows the variation of both the disk current, iD, and the ring current, iR,as functions of the disk potential, ED,with the ring potential, ER,kept at a constant potential -0.400 V. In this scheme, the polymer oxidized by the disk can be reduced by the ring, when the section of the film covering the insulating gap between the disk and the ring is switched to a conducting state. This way of using the RRDE is similar to the “normal” collection experimenV2 for redox (31)Sluyters-Rehbach, M.; Wijenberg, J. H. 0. J.; BOBCO, E.; Sluyters,
J. H.J.Electroanal. Chem. 1987, 236, 1.
(32) Albery, W. J.; Hitchmann, M. L. Ring-Disc Electrodes; Clarendon Press: Oxford, 1971.
Electrochemical Deposition of a Conducting Polymer reactions in solution phase, in that the disk is used as the generator and the ring as the detector of electroactive species. The difference is that, in the case of solution reactions, the speciesbeing detected have to be transported to the ring mainly by radial convection of the solution. In the case of solid polymer film, the electroactive sites on the polymer chains are essentially fixed, the redox processes being achieved through the flow of electrons and counterions through the polymer film. Therefore, the continuity of the polymer film can be decerned from its redox behavior during the process. Figure 4b shows a similar experiment but with the disk and ring used interchangeably. The cross-over of the current on the generating electrodes, and the anodic delay and cathodic tailing of the current on the detecting electrodes are due to the effect of the negative potential applied to the detecting electrodes, and the kinetic factors arising from the remote distance to the electrodes of part of the film (the edge of the film is ca. 2 mm away from the ring). These points have been explained in detail elsewhere.33 What we are interested in here is the fact that the magnitude of the current on the detecting electrode is very significant, and the current patterns are closely similar, in whatever way the two electrodes are used. These indicate that the whole polymer film is electrochemically active in the process and the electrical conduction through the film between the ring and the disk is good. Therefore, the conducting properties of the poly(TPAA) film deposited under such conditions of macroscopic 2-D growth is the same as those of relatively thin film confined to the disk. The same conclusion is reached from the CVs, which are similar to each other and to those of thin films, when the potential is applied to either the ring or the disk or both. The macroscopic lateral growth observed in Figure 3 is different, in the thickness of the growing layer (or the height of the growing nucleus) and the time scale, from the 2-D nucleation and growth discussed above (and further below). In the latter case, the emphasis is on the early stages of lateral spreading of microscopic polymer clusters (nuclei) with a basic thickness of a monomolecular layer. The phenomena and their underlying principles,however, are the same. The microscopic 2-D growth of nuclei, for example, requires a suitable dispersibility (lyophilicity)of the depositing species,which enables them to travel a certain distance to reach the growing nuclear edges before disposition, and predicts a much faster lateral growth rate than the perpendicular growth rate. These have been clearly demonstrated in the macroscopic 2-D growth. Two-Dimensional Layer-by-Layer Growth in the Presence of a Structurally Similar Inhibitor. One of the typical exemplificationsof the 2-D layer-by-layermechanism is the multiple peaks during the early part of a potential step t r a n ~ i e n t . ~ In~a. relatively ~~ concentrated coating solution and at relatively high potentials, however, the deposition of poly(TPAA) tends to proceed continuously, which makes it difficult for the multipeaks to be observed (transients in relatively dilute coating solutions and at low potentials have been analyzed previously26). One strategy is to slow down the process through the use of an inhibitor, which is ideally analogous in chemical structure to the TPAA monomer and thus does not divert the polymerization and deposition mechanism significantly. One such an inhibitor is the homologous compound, 3-thiophenecarboxylic acid (TPCA). This compound exhibits adsorption features on a p t surfacemsimilar ~~
~
(33) Li, F.-B. Ph.D. Thesis, Imperial College, London, 1990.
Langmuir, Vol. 8, No. 6,1992 1649 41
,lst
I
I
t/s
t/s
Figure 5. Potentiostatic i-t transient for the depositionof poly(TPAA) in the presence of inhibiting TPCA, 0.3 M TPAA and 0.3 M TPCA in acetonitrile. E = 2.10 V. The inset shows the early stages of the same transient recorded simultaneously with
a digital oscilloscope.
to TPAA but cannot be deposited electrochemically to form a bulk film under various conditions, probably due to the electron-drawing effect34 of the -COOH group. Therefore the presence of TPCA during the polymerization of TPAA serves as an inhibitor. Figure 5 shows an i-t transient for the deposition of TPAA in the presence of TPCA. Here a higher potential (2.1 V) is used, because no polymer film can be obtained a t lower potentials under these conditions. The observation of several well-resolved current peaks indicates clearly the nature of a 2-D layer-by-layer deposition. A subsequent visual check confirmed that a uniform polymer film has been deposited over the whole electrode surface. The smaller magnitude of the current (compared with the polymerization of TPAA alone under similar conditions) and the decay of the peak heights confirm the inhibitory effect of TPCA. The charge under the first two peaks is equivalent to ca. 80 TPAA monolayers as measured previously26(and below). This suggests that the process is different from the current oscillation at the early stages of the 2-D layer-by-layer deposition on a homogeneous s~rface,3~ which may not be observed due to the factor of a limited number of nucleation sites36on a heterogeneous surface. The exact role played by the inhibiting TPCA during the polymerization and deposition of TPAA is not pursued in detail. The possibilities include (a) copolymerization of TPCA with TPAA, and thus formation of a polymer which is significantlydifferent from poly(TPAA), (b) chain termination of poly(TPAA) by the monomeric TPCA intermediate, and (c) formation of a TPCA blanket, likely through adsorption, on the bare electrode surface prior to poly(TPAA) deposition and on the poly(TPAA) film surface during the deposition. Given the fact that TPAA itself is relatively difficult to polymerize37(relativelyhigh potentials and monomer concentrations are required to obtain a conducting film27)and that TPCA alone cannot (34) Waltmann, R.J.; Bargon, J. Can. J. Chem. 1986, 64,76. (35) Armstrong, R.D.; Metcalfe, A. A. J.Electroanal. Chem. 1975,63, 19. (36) Obretenov, W.Electrochim. Acta 1988,33,487. (37) Waltman, R.J.; Bargon, J.; Dim, A. F. J.Phys. Chem. 1983,87, 1459.
1650 Langmuir, Vol. 8, No. 6, 1992
Li and Albery
1 be polymerized as described above,process a is statistically unimportant, because such a copolymer can be highly unstable chemically or electrochemically. The ability of copolymerization of TPCA with the unsubstituted thiophene3s*39 may be related to the polymerization reactivity Q of the parent thiophene which is greater than that of the .-3 thiophenes with an electron-drawing or sterically unfavorable substituent g r o ~ p Process . ~ ~ b ~may ~ be ~ ~ ~ ~ ~ ~ ~ important, but it does not provide a rationalization of the separated current peaks observed. A preferential initial I deposition of long, less soluble polymer chains followed by a subsequent deposition of short, more soluble chains EIV E/V is unlikely. Characterization with gel permeation chromatography of molar mass and molar mass distribution of poly(TPAA1 revealed2’ a wide range of chain lengths. If such a preferential deposition of long chains is important, similar multipeaks should be observed for the deposition of poly(TPAA) as well, in the absence of TPCA under analogous conditions, which in fact has never been seen (previousobservation of discrete current peaks26is under very different conditions). Therefore, process c is considered the most likely case. Since TPCA cannot be polymerized to form a conducting film, its adsorption and subsequent oxidation on the electrode surface must produce a deactivated thin “blanket”. The generation of a monomeric TPAA radical intermediate, which is the basic building unit of poly(TPAA), is obviously more f difficult on such a blanket, and thus higher potential is required. Once the deposition of poly(TPAA)is initiated, it tends to proceed continuously at this high polymerization potential, until a drop in the concentration of poly(TPAA)in the solution near the electrode surface becomes significant, and thus poly(TPAA) deposition becomes vulnerable to the inhibitory effect of TPCA. After a period of time, the intermediates of TPAA accumulate again and new nuclei are created, which lead to another series of continuous monolayer growths of poly(TPAA). In this way, separated current peaks are produced. Figure 6. Cyclic voltammograms with multiple anodic peaks, Oxidative Adsorption of Monomer and Nucleation 0.5 M TPAA and 0.1 M LiClO, in acetonitrile. Sweep rata is 20, and Growth of Polymer under Potentiodynamic 30,40,50, 60, and 70 mV/s for curves a to f, respectively. See (Cyclic Voltammetric) Conditions. Figure 6 shows a the text for explanations. set of CV curves in a TPAA monomer solution at various sweep rates, Y. The upper potential of these CVs is limited ization. Then on top of this adsorbed layer, 2-Dnucleation to a relatively low value of 1.0 V, so that the initial stages and monolayer growth of poly(TPAA) occur layer-by-layer of monomer adsorption and polymer nucleation and (peaks nl and n2). The details of this process are analyzed growth can be examined in detail. It is very interesting in the following sections. that multiple peaks with different features are observed Characteristics of the Adsorption Peak. The adclearly. In curve a, for example, initially, the current sorption nature of peak a is supported by the following increases smoothly and then flattens out forming a relative evidence. Firstly, the peak appears at potentials below flat peak (peak a). On top of peak a, the current suddenly 1.0 V with a peak potential between 0.91 and 0.95 V rises steeply, resulting in a very sharp peak nl which is (observed from seven CVs at various sweep rates), which followed by another relatively sharp anodic (not cathodic) is significantly below the onset potential (ca. 1.2 V) and peak n2 during the reverse (cathodic) scan. This pattern the peak potential (ca. 1.4-1.6 V, observed in dilute of a flat peak followed by sharp peaks is seen repeatedly. monomer solutions containing 0.01-0.1 M TPAA, in a As the sweep rate is increased, the initiation of peak nl is concentrated solution the peak is shifted to more positive generally shifted to increasingly late stagesof the CV course potentials due to continuous growth and overoxidation of with peak n2 gradually disappearing. The position of peak poly(TPAA))for bulk oxidation of the mon0mer.~6*27 This a, on the other hand, remains essentially the same, which is possible only when the intermediate resulting from enables itself to be better separated from the subsequent peaks (nl and n2) and thus its whole feature to be seen TPAA oxidation is adsorbed on the electrode surface where clearly. the attractive interactions enable the monomer to be oxidized and polymerized at much less positive potentials On the basis of the characteristics of these peaks, it is as in the case of underpotential deposition (UPD) of considered that the mechanism of the process is as metaha A solution reaction not involving an adsorbed follows: a monolayer of TPAA is first adsorbed oxidaspecies should not produce a separate prepeak, because tively on the bare Pt surface (peak a) prior to polymer-
A! /7
(38) Mount, A. R. Ph.D. Thesis, Imperial College, London, 1987. (39) Wheeler, B.L.; Caple, G.; Henderson, A.; Francis, J.; Cantrell, K.; Vogel, S.; Grey, S.J . Electrochem. SOC.1989,136, 2769.
(40)Adzic, R. R. In Advances in Electrochemistry and Electrochemical Engineering; Gerisher, H., Tobias, C. W., Eds.; John Wiley: New York, 1984;Vol. 13,p 159.
Langmuir, Vol. 8, No. 6, 1992 1651
Electrochemical Deposition of a Conducting Polymer
91
/
L’ 0
I
I
I
2
4
6
Y
/’
1’2/(,vs-‘)”~
Figure 7. Relationship between peak current and sweep rate. Line a is for the adsorption peak a in Figure 6 and line b is for the nucleation peak nl in Figure 8.
there are evidently no other reactants with different oxidation potentials present in these fresh coating solutions. Secondly, after deduction of the background doublelayer charging current, the peak current of the adsorption peak, ipa,is a linear function of Y (Figure 7, line a), as expected the0retically.4~~~~ The charge under peaka during the anodic scan, Qa, is 18.0 (i0.7) p C (after the same deduction of double-layer charging current as for the measurement of ipa,and deduction of nucleation current where it is appropriate), independent of potential, again as expected.42In terms of apparent coverage, this Qavalue (143.2pC/cmzbased on the apparent area of the electrode) corresponds to ca. 6.6 X 10-lO mol/cm2 TPAA monomer units, assuming two-electron oxidation per monomer for polymerization and 25% polymer doping,18which is on the order of a monomolecular layer, and is comparable with the packing density of typically (3-4) X 10-lo mol/ cmz on an atomically smooth Pt(ll1) surface reported by Hubbard et aLZ9Given the diversity of the methods used and the adsorption conditions, especially the roughness of electrode surfaces, this is a fairly good agreement. Apart from the dispersibility of the poly(TPAA) molecules discussed above, the adsorption of TPAA on the bare Pt surface is also an important factor of the 2-D deposition mechanism. The adsorbed TPAA may form 2-D nucleation centers which are more favorablethan other parts of the surfacez3and thus promote the subsequent deposition proceeding in a layer-by-layer manner. Characteristics of the Nucleation Peaks. Figure 8 displays another series of CVs for the deposition of poly(TPAA) in which the sharp nucleation peak nl is seen more clearly. These curves (together with those shown in Figure 6) show that the nucleation peak is generally shifted to progressively later stages of the CV course (individual exceptions are discussed below), and the charge covered by the adsorption peak up to the onset point of the first nucleation peak, though fairly close to the value of Qa in general, increases systematically, as the sweep rate is (41) Laviron, E. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1982; Vol. 12, p 53. (42) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley: New York, 1980.
Ldq
00
E/ V
-0
E/V
1
Figure 8. Cyclic voltammogramswith better definednucleation peaks, 0.5 M TPAA and 0.1M LiClOd in acetonitrile. Sweeprate is 20, 30,40,and 50 mV/s for curves a to d, respectively.
increased. This suggests that the kinetics of the first nucleation event is relatively slow under these conditions, and perhaps an induction timez3124~31 is involved. The nucleation starts on top of the adsorbed first monolayer when the latter is nearly or already completed.z6 Therefore, the nucleation process is not competing for favorable sites with the adsorption pr0cess.4~ The sharply rising edge of the nucleation peak is linear with time. The charge under the first nucleation peak, Q,, typically around 20 pC (the value becomes smaller when the nucleation peak emerges at relatively late times during the reverse scan, due to a decrease in potential), is comparable with the monolayer charge, Qml = 27 pC obtained previously under potentiostatic conditions.26A second nucleation peak is repeatedly observed even in the case where Qn, is appreciably smaller than Qml. These features are unique to the 2-D layer-by-layer mechanism and ruled out any possibility of 3-D nucleation and growth. A 3-D nucleation cannot, for example, account for the linear change of the deposition current with time under any circumstance. The amount of charge involved in a such a 3-D process up to the stage where the first batch of the 3-D conical or hemispherical nuclei overlap and the next batch of nuclei are formed would be much greater than that of a monomolecular layer. (Progressive 3-D nucleation, which predicts a t3 dependence of the deposition current, has been ruled out by the results of Figure 1.) The characteristicsof these CVs indicate that eachsharp peak corresponds to the spreading of a single poly(TPAA) monomolecular layer starting from a single nucleus. The shape of the 2-D nucleus, which must be circular because other geometries predict more than one maximumud6 of the current resulting from each monolayer, is preserved during the monolayer Under potentiostatic (43) Noel, M.; Chandrasekaran, S.;Basha, C. A. J. Electroanal. Chem. 1987,225,93. (44) Barradas, R. G.; Porter, J. J. Electroanal. Chem. 1982,132,26. (45) Obretenov,W.; Bostanov, V.; Budevski, E. J.Electroanal. Chem. 1984, 170, 51. (46) Obretenov, W.; Kossev, T.; Bostanov, V.; Budevski, E. J. Electroanal. Chem. 1983, 159, 257.
Li and Albery
1652 Langmuir, Vol. 8, No. 6, 1992 conditions,the spreadingrate, V, during the linearly arising section can be described by45 ailat = 2rQmlV2 In the present CV scheme, this potentiostatic condition is valid approximately, since the potential change is slight during the short time span of the rising edge at these relatively slow sweep rates. From the slope of the rising edge, ailat, Vis calculated to be between ca. 0.08 and 0.14 cm/s (depending mainly upon the position of the peak), which is close to those of ca. 0.08 cm/s for the first few layers of a series of monolayer spreadings under potentiostatic conditions.26 At the later stages of these monolayer spreadings, which quite often straddle the anodic switch potential, however, the spreading rate is no longer the same because the potential has been swept steadily to significantly different values. The peak current of the first nucleation peak, ipn,is proportional to u1I2, as can be seen from line b in Figure 7. This feature has been predicted theoreti~ally4~1~~ for a 2-DIprocess. Sincethere is only one nucleus formed during each monolayer spreading, the nucleation is apparently instantaneous. In the CVs shown, some of the nucleation peaks, for example, curve a in Figure 8, start on a fairly flat top of peak a and thus have a well-defined height. In other cases the first nucleation peak emerges early in the reverse scan, during which the background current resulting from oxidative monomer adsorption is decaying quickly (see,e.g., curves d and e in Figure 61, which renders the apparent height of peak nl smaller that its real value. This effect has been corrected in plotting the ipn-~1/2 figure by assuming that the background current falls at the same speed as the case where no nucleation has occurred at a similar sweep rate (for example, the section of early reverse scan before peak nl of curve d in Figure 6). Then the value of the background current by which it has decreased during the rising portion of peak nl is added to the apparent ipn. This assumption is appropriate in this case of noncompetitive nucleation and adsorption, in which the total current is simply the sum of the two processes.43 Monolayer Structure and the Stochastic Nature of Nucleation. Apparently, the packing density of a monolayer depends upon factors, such as the geometric size and the orientation of the component molecules, and the surface morphology. In the case of metals with relatively small atomic radii, such as Ag and Cu, close packing with a coverage on the order of 10-9 mol/cm2are observed.4O For larger organic molecules, smaller coverage on the order of 10-lomol/cm2is more When the coverage data are derived from the amount of charge involved, it also depends on the current efficiency. For the oxidative adsorption of TPAA, the current efficiency may be higher than that for the subsequent monolayer deposition through nucleation. In the latter case, some of the polymer/oligomers may transport to the bulk solution, owing to the significant solubility. As a result, the actual monolayer coverage is smaller than that calculated by assuming a 100% deposition. This effect, however, is significant only when the solution is stirred or the electrode is rotated during the polymerization and deposition at relatively high potentials. In the case of a stationary electrode and quiescent solution during the deposition at fairly low potentials, on which the monolayer-coverage data are based, dissipation of the soluble poly(TPAA)into (47) Rangarajan, S. K. Faraday Symp. Chem. SOC.1977,12, 103. (48) Murray, R. W . In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, p 191. (49) Hubbard, A. T. Langmuir 1990, 6, 97.
the bulk solution is not important. The difference between the charge involved in the adsorbed first layer and the subsequent layers deposited via nucleation and lateral spreading is considered mainly due to a difference in molecular orientation. For example, one can speculate that the thiophene ring of the adsorbed TPAA may lay in parallel to the electrode surfaceM and thus occupy a relatively large area. While in the subsequent layers, the polymer chains may lay parallel to each other and to the electrode surface, but the individual thiophene rings may not be parallel to the electrode surface. Thus each monomer unit may occupy a smaller area on average. In either case, the monolayer coverage data of TPAA are on the correct order of magnitude. Another interesting point, which is well-known for the case of metal deposition, is that nucleation is a stochastic process in time and space (on a homogeneous surface) best described by statistical methods.24At relatively high potentials, however, the nucleation is so frequent that its stochastic nature is obscured. The related macroscopic properties, such as the current, behave as continuous functions of time (Figure la). In cases where single events of nucleation are observed separately at low potentials (Figures6and 8),the stochasticnaturemayplayadecisive role in the detailed behavior of the i-t transient. Therefore, it is not surprising that a specific result is not reproduced in exactly the same way, or deviates from a general trend observed in a series of experiments. This is considered to be the main reason for some of the apparent ‘irregular” phenomena. For example, in Figure 6b and Figure 8d, the nucleation peaks appear slightly earlier than expected from the tendency of peak-shift with sweep rate described above. Finally, at a slow deposition rate, the poly(TPAA) f i b may be deactivated to a certain degree, as revealed from previous studiest7resultingin CVs such as those in Figures 6 and 8 with the cathodic charge consumed for polymer reduction being less than expected (ca. 10% of the anodic Faradaic c h a r g e 9 The possible structural change of the Pt surface as previously observed51over a period of 3-15 h, on the other hand, is unlikely, partly because of the much shorter time scale of the CVs presented here and mainly because of the monomer adsorption and polymer deposition, which form a ‘protection” layer.
Conclusions The electrochemical nucleation and growth of poly(TPAA) are shown to proceed through a 2-D layer-by-layer mechanism under potentiodynamic (cyclicvoltammetric) as well as potentiostatic conditions. This process takes place on top of a first monolayer deposited via oxidative adsorption of TPAA monomer on a bare Pt surface prior to polymerization. The new mechanism may result in conducting polymer films of ordered layer structure and polymer chain arrangement and is fundamentallydifferent from previous suggestions of 3-D nucleation and growth, which may lead to a ‘packed grain” internal morphology with a substantial amount of intergrain space. The 2-D layer-by-layer deposition mechanism is supported by evidence such as the linear dependence on time of the deposition current for poly(TPAA) growth on preformed nuclei in a double-potential step experiment, the macroscopic lateral growth of the polymer film far beyond the electrode surface, and the multipeaks in a po(50) Christensen, P. A.; Hamnett, A.; Hillman, A. R. J.Electroanal. Chem. 1988,242,47. (51) Canullo, J. C.; Triaca, W. E.; Arvia, A. J. J. Electroanal. Chem. 1986,200, 397.
Langmuir, Vol. 8, No. 6, 1992 1653
Electrochemical Deposition of a Conducting Polymer tentiostatic transient in the presence of an inhibitor which is structurally similar to the TPAA monomer. The most convincing evidence, however, is the observation of individual events of nucleation and 2-Dmonolayer spreadings under a cyclic voltammetric scheme. No similar result has been reported previously. The monolayers are characterized by an apparent coverage of ca. mol/cm2 monomer units, an initial spreading rate of 0.0&0.14 cm/ s, and a stochastic initiation. Such a highly ordered deposition of one molecular layer after another, however, is observed for thin films and may not carry over to thick
films, especially when the film is deposited at relatively high potentials at which there are normally a number of nuclei growing concurrently within a monolayer or the spreading layer is thicker than a monomolecular layer.
Acknowledgment. F.B.L. sincerely thanks the Royal Society/ICI for their fellowship, and the School of Chemistry, University of Bristol, for the use of office facilities during the preparation of the paper. Registry No. TPAA,114815-74-6; AgClO4,7783-93-9; LiClO,, 7791-03-9.
