J . Phys. Chem. 1990, 94, 5973-5981
5973
Electrochemically Deposited Polythiophene. 1. Ohmic Drop Compensation and the Polythiophene Paradox Miklos Gratzl, Duan-Fu Hsu, Andrew M. Riley, and Jiii Janata* Department of Materials Science and Engineering, University of Utah, Salt Lake City, Utah 841 12 (Received: November 27, 1989)
High-quality polythiophene cannot be directly electrodeposited on many metallic substrates such as titanium or even gold, because at the required high potential the oxidation of those substrates is preferred to polymerization. We found that a thin polypyrrole coating ensures the deposition of polythiophene on these substrates. Due to the low conductivity of 1-100 mM thiophene 0.1 M tetrabutylammonium tetrafluoroborate in acetonitrile or 1:l acetonitri1e:water (about 95 52 cm in acetonitrile as determined in this work), a significant ohmic (IR) voltage drop occurs between the working and reference electrode at any reasonable distance during deposition. To maintain a constant and known voltage drop across the solutionfpolymer interface, IR-drop control must be applied. By using dynamic IR compensation with current interrupts and 100%error compensation, the quality of the resulting film is better controlled. At these deposition conditions, the so-called "polythiophene paradox" has been proven to be the result of the degradation of polythiophene which starts solely at the solutionffilm interface and reaches the bulk of the film only at a further stage. This overoxidation reaction is competing with polymerization and oxidation of the monomer. In the presence of the thiophene monomer these latter 'useful" reactions prevail while in the absence of thiophene the degradation takes place. The cell geometry, solution composition, and deposition potential have been optimized.
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Introduction Both the electrochemical preparation and the following relaxation of polythiophene are extremely complex processes. Many research groups have investigated the conditions of preparation and focused on factors that seemed most influential on the final properties of this material. These factors, individually or in combination, include the choice of the solvent and particularly the presence or absence of water,'-3 the temperature? the type and level of doping during and after the d e p o ~ i t i o n ,and ~ ? ~the concentration of the m ~ n o m e r . ~ , ~ Part 1 of this series deals with the electrodeposition of polythiophene (PT) films on various substrates and also from mixed organicfaqueous media. The effect of the uncompensated ohmic (IR) voltage drop during the growth of the film is also investigated and related to the so-called "polythiophene para do^".^ Optimum conditions of deposition are discussed. In part 2, the relaxation processes following deposition which lead to a final stable state of polythiophene are investigated. In addition to growth at a constant potential, the pulsed mode of deposition has also been employed in this study, for the following reasons. Even though a (chemical) kinetical control of the electrodeposition of PT3 (and P P ) has been observed under usual circumstances, mass-transport limitation may become important in the deposition of PT. There are also special applications that require the deposition of the synthetic metal in confined spaces, such as gaps and holes9 in which the mass transport of the monomer is restricted. l t is also known that polymer films do not form if the solution is stirred,I0 which is consistent with the nucleation mechanism and the formation of soluble oligomers in the first stages of growth. These circumstances may favor in some cases the pulsed-mode deposition of polythiophene films. The possibility of electrochemical deposition of PT on different materials depends largely on the width of the "electrochemical window" that these materials exhibit in different electrolyte solutions. Thus, PT has been successfully grown on platinum from concentrated solutions of thiophene in acetonitrile. There are, however, problems associated with this type of deposition that originate mainly from the effects of the high resistivity of acetonitrile and from the resulting uncompensated IR voltage drop. The competing electrochemical reactions in which the electrode material itself andfor the electrolyte undergo electrochemical oxidation can be the major limitation in electrochemical deposition of thiophene on substrates other than platinum. This is apparently *To whom correspondence should be addressed.
0022-3654/90/2094-5913$02.50/0
the case with deposition on gold electrodes" which proceeds satisfactorily only from relatively concentrated (0.1 M) thiophene solutions. Under these conditions the thiophene polymerization is favored with respect to the formation of gold oxide. This type of competition is even more severe for less noble metals which are generally considered unsuitable for electrodeposition of PTe7 However, for large-scale applications platinum is too expensive and sometimes its material properties (e.g., mismatching thermal expansion coefficients, low ductility) may not be suitable from the point of view of mechanical requirements in construction of heterostructures in which PT would be used. It is known that polypyrrole (PP) can be deposited under conditions a t which the PT deposition fails. Thus, PP has been successfully grown on substrates such as AI or Fe.I2 On the other hand, competing metal dissolution has been found to prevent the formation of PT on the same metalsS7 The growth of PT on substrates such as n-GaAs is highly desirable from technological point of view.I3 However, it can be accomplished only if an intermediate layer of Pt is deposited on the substrate in order to prevent its degradation during electrooxidation. The possibility of deposition of PP under those conditions is mainly due to a considerably lower oxidation potential for pyrrole (0.6 V vs 0.01 M Ag'fAg) as opposed to 1.6 V for thiophene. It needs to be mentioned here that copolymerization of PP and PT from mixed thiophene:pyrrole solutions has been recently described.I4
( I ) Christensen, P. A.; Hamnett, A.; Hillman, A. R. J . Elecrroanal. Chem. 1988, 24, 47-62. (2) Downard, A. J.; Pletcher, D. J . Electroanal. Chem. 1986, 206, 147-1 52. (3) Hamnett, A.; Hillman, A. R. Ber. Bunsen.-Ges. Phys. Chem. 1987, 91, 329-326. (4) Kritsche, B.; Zagorska, M. Synth. Mer. 1989, 28, C263-C268. (5) Tourillon, G.; Garnier, F. J . Electroanal. Chem. 1984, 161, 407-414. (6) Glenis. S.; Tourillon, G.; Garnier, F. Thin Solid F i l m 1984, 122.9-17. (7) Tanaka, K.; Shichiri, T.; Wang, S.;Yamabe, T. Synth. Mer. 1988, 24, 203-215. (8) Genies, E. M.; Bidan, G. J . Electroanal. Chem. 1983, 149, 101-1 13. (9) Josowicz, M.; Janata, J. In Sensor Technology; Seyiama. T., Ed.; Elsevier: Amsterdam, 1988; Vol. 1. (10) Beck, F.; Oberst, M. Makromol. Chem., Macromol. Symp. 1987,8, 97-125. (11) Eales, R. M.; Hillman, A. R. J . Electroanal. Chem. 1988, 250,
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(12) Cheung, K. H.; Bloor, D.; Stevens, G. C. Polymer 1988, 29, 1709-1 7 17. (13) Garnier, F.; Horowitz, G. Synth. Met. 1987, 18, 693-698.
0 1990 American Chemical Society
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5974 The Journal of Physical Chemistry, Vol. 94, No. 15, I990
For both economical and technological reasons it would be advantageous to deposit polythiophene from aqueous solutions and/or at low monomer concentrations. It has been stated that water inhibits the polymerization of the thiophene radical cation2 but, on the other hand, small amounts of water have been found beneficial for the nucleation mechanism of the polymer growth.' Limited solubility of thiophene in water precludes the use of purely aqueous solutions. The possibility of electropolymerization of thiophene from mixed organic/aqueous solutions has not yet been fully investigated. In this work we have tested the idea of using electrochemically grown PP as the protective coating on which PT could be deposited without the risk of competing electrode and/or electrolyte degradation. The deposition was investigated both in pure acetonitrile and in 1:1 water:acetonitrile mixture. (The solubility of thiophene in water:acetonitrile system at 25 "C was determined to be 0.26 M thiophene, while the maximum concentration of the monomer used in this work was 0.1 M.) It has been shown repeatedly that the potential difference at the growing polymer/solution interface has a major effect on the final properties of the film. However, the resistivity, p, of acetonitrile (ACN) solutions is generally quite high. In the majority of the reported work, polythiophene is prepared from 0.1 M thiophene 0.1 M tetrabutylammonium tetrafluoroborate (TBATFB)/ACN at Pt electrodes. Even with this high concentration of the supporting electrolyte the resistivity remains significant: from the equivalent conductivity for infinite dilution (173 cm/(Q mol)) and from the resistivity of a 1 M solution (31 D cm),15p = 78 D cm can be calculated with Kohlrausch's rule for, e.g., 0.1 M TBATFB/ACN. In the absence of a convective mass transport the current density during the deposition done under these conditions can be typically 35-40 mA cm-2, but in some cases as high as 100 mA cm-2 for a stationary disk electrode. Consequently, for a Luggin capillary placed at some distance away from the working electrode, the resulting IR voltage drop in the solution can be significant. There is, in addition, another IR drop within the growing film. According to our observations, the total IR drop during deposition can be as high as 2-3 V, for reasonable distances between the Luggin tip and the working electrode. (Very small distances are disadvantageous because of the "screening" effect of the reference electrode.) Thus, without an IR drop compensation it is not possible to control the effective potential difference at the polymer/solution interface, i.e., the quality of the deposited film. It is necessary to take into account the IR compensating capability of the potentiostat and weigh it against other deposition conditions, namely, the concentration of the monomer, the solution resistivity, and the geometry of the cell. The maximum amount of the IR compensation available on the PAR 273 potentiostat used in this work is k2 V, which sets a limit on the distance of the Luggin capillary from the film/solution interface and, possibly, on the area-specific resistance (Le., the thickness) of the polymer layer. We believe that some of our observations obtained under IRdrop control may also shed some light on the so called "polythiophene paradox": which has been stated as follows: the formation of polythiophene by electrooxidation of thiophene requires potentials which are above +1.5 V vs 0.01 M Ag+/Ag. On the other hand, at those potentials the "ready" film of PT is irreversibly damaged, due to overoxidation. (In fact, this happens already above 1.3-1.4 V.) This is known to occur also in polypyrrole films. However, without an IR-drop control one cannot guarantee that the effective interfacial potential is as high during deposition as that which destroys the film after preparation. Besides using IR compensation, the composition of the monomer solution, the cell geometry, and the potential of deposition can also be optimized. These factors are discussed in this report as well.
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(14) Kuwabata, S.; Ito, S.;Yoneyama, H.J. Electrochem. Soc. 1988,135, 1691. (1 5) Sawyer, D. T.; Roberts, J. L. Experimental Electrochemistry for Chemists; Wiley: New York, 1974.
CE
Figure 1. Schematic diagram of the cell used for electrodeposition of polythiophene. Arrow: surface of the monomer solution. T: glass tube attached to the electrode with joint J (PVC). The inside diameter of the tube is equal to the diameter of the Pt disk. WE: Pt disk working electrode. CE: Pt disk counter electrode with a hole in its center. RE: reference electrode with a capillary (Luggin) tip.
Experimental Section Platinum and old disk electrodes, pure titanium foil, and also layers of 1000 of sputtered Pt,Au and Ti,W (10/90%) on Pt/glass have been used as working electrodes. The metal disks (diameter 4.5 and 7 mm) were mounted in Teflon while the Ti foil and the sputtered specimens (1 cm2) were directly immersed into the solution. Acetonitrile (HPLC grade, 99.99+%) (ACN), tetrabutylammonium tetrafluoroborate (TBATFB), pyrrole (P), thiophene (T), ferrocene (all from Aldrich), and LiC104 (G. Frederick & Smith, Inc.) were used as received. Neither the acetonitrile nor the supporting electrolytes were dried, which means that the concentration of water was at least 10 mM in the solutions. In pure ACN 0.1 M TBATFB and in the 1:l water:ACN mixture 0.1 M LiClO, were used as supporting electrolytes, except for the determination of the background currents of different substrates when in all cases LiC104 was employed. The 1:l mixture was made by mixing equal volumes. An IBM EC 225 potentiostat was used for testing the PP coating on different substrates. In the cell a large stainless steel counter electrode was used, and the tip of the Luggin capillary was placed 2-3 mm away from the working electrode, outside from the main current path. An EG&G PAR 273 potentiostat was applied under computer control (IBM PS/2) for the IR-drop experiments. During a current interrupt, 32 open cell potential samples were taken 5 ps apart. Sample points 2 and 4 were used to extrapolate back to the IR drop. The degree of compensation was 100%. The cell itself (Figure 1) was a glass cylinder with 35 mm i.d. The 12-mm tip of the Luggin capillary ( R E diameter, 1.5 mm) was introduced through a hole in the center of the Pt disk counter electrode (CE; diameter, 13 mm), and the center of the Pt working electrode (WE; diameter, 7 mm) coincided with the axis of the Luggin capillary. Thus, a symmetrical geometry and a minimum "screening" effect were ensured for the IR drop experiments. In some experiments a glass tube (T in Figure 1) with 7-mm i.d. was mounted on the working electrode with a PVC joint (J), and the tip of the Luggin capillary was kept outside, but close (1 mm) to the lower end of this tube. The length of tube T was varied according to the distance of the Luggin from the working electrode. Theoretically, the best place of the tip of the Luggin probe would have been beside the working electrode body and somewhat
1
Electrochemically Deposited Polythiophene. 1 behind the plane of the Pt disk, because between such a point and the W E there is no current flowing. Consequently, most of the IR drop could have been avoided with such an asymmetrical geometry. The use of tube T for preparation of homogeneous films was the reason for placing the Luggin capillary into the center axis of the cell throughout this work. The conductivity (or resistivity) measurements of different solutions have been done in a glass tube of 5.05-mm i.d., the two ends of which were closed with two polished brass rod electrodes (diameter, 5 mm). The distance of these electrodes was varied from 2 to 80 mm, and the 100-kHz high-frequency limit of the measured real impedance component was determined at 10 different distances. The resistivity was then calculated with linear regression of these resistances with respect to distance. The regression coefficient, 9,was always better than 0.999. An H P 4274A Multi-Frequency LCR meter controlled with an H P 9000-300 computer was used to measure the impedance with f10 mV ac excitation voltage. The same apparatus was used to measure the ohmic resistance of the cell used to deposit PT (without and with film on the Pt working electrode, and without and with the 7-mm-i.d. glass tube on). All potentials were referred to the Ag10.01 M AgN03, 0.1 M XY,ACN((O.1 M XY,ACNlldouble-junction reference electrode filled with a 1: 1 water:acetonitrile mixture or with pure acetonitrile, as appropriate. The electrolyte XY was TBATFB in pure ACN, and LiC104 in the aqueous mixture and in all PP coating experiments. The offset of this reference scale with respect to NHE is about 0.4 V.
Results Deposition of Polythiophene on Polypyrrole Substrates. The potential range of stability of metals (within which they do not dissolve in solutions) can generally be characterized by their Pourbaix diagrams which show the stable potential window as a function of pH. For Pt, Au, Ti, and W such diagrams could not be found in the literature except for pure aqueous solutions.I6 So, these electrochemical "windows" for ACN and aqueous ACN had to be determined experimentally. For this purpose normal pulse voltammetry has been used, some results of which are shown in Figure 2. As expected, the background current on a Pt substrate rises at the most positive potentials, followed by gold both in nonaqueous (Figure 2A1) and in aqueous (Figure 2B) acetonitrile. Ti,W exhibits a typical passivation/breakdown behavior, due to the formation of a protective oxide film. Some applications of titanium, however, particularly in the field of chemical electronics, require growth of Pt on Ti,W substrates or on noble metal materials contaminated with Ti,W." Attempts to deposit PT on these materials even at high concentrations and from pure acetonitrile ended up in inconsistent results. The deposition of PP film onto such substrates shift the oxidation background curve in the positive direction (Figure 2A2,B), closer to the background obtained on Pt. It is this extension of the electrochemical window that makes the further deposition of PT on low overvoltage substrates possible. Ti,W (and Ti) in pure ACN are exceptions, however (Figure 2A1): the current on the bare metal is much lower at higher potentials than on the PP coated substrate, due to the passivation of the metal. (The curves for the first and second scans for bare Ti,W display the progress of this passivation. A similar but less pronounced behavior can be observed also with Au: its passivation is shown in its first stage in Figure 2A1 and in a developed stage in Figure 2A2.) Nevertheless, a compact PT layer cannot be grown directly on Ti,W but can be grown on Ti,W/PP. The quality of PT grown on Au is also poor (nonuniform, loose) while it becomes compact on Au/PP. Thus, a polypyrrole underlayer seems to be a generally applicable protective coating for many substrates on which ( I 6) Pourbaix, M.; et al. Ailas of Electrochemical Equilibria in Aqueous Solutions; Pergamon: Oxford, 1966. (17) Josowicz, M.; Janata, J.; Levy, M. J . Electrochem. Soc. 1988, 135, 1 1 2- I 1 5.
The Journal of Physical Chemistry, Vol. 94, No. 15, 1990 5975 1.2s-
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Figure 2. Pulse voltammograms of different substrates without and with polypyrrole coatings, in pure and aqueous acetonitrile. (A) Pulse voltammogram of 0.1 M LiCIOJACN on bare Pt, Au, and Ti.W and on Au/PP, Au/PP/FT, Ti,W/PP and Ti,W/PP/PT. A l , metal foils: A2, sputtered metal as substrates. (B) Pulse voltammogram of 0.1 M LiC104/l:l ACN:water on bare Pt, Au, and Ti,W and on Ti,W/PP and Ti,W/PP/PT. The deposition of PT on PP was done from 0.1 M thiophene in both solvents.
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5916 The Journal of Physical Chemistry, Vol. 94, No. 15, 199(3
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Figure 3. Optical microphotograph of PT ( 1 C cm-2) grown on bare Ti,W and Ti,W/PP (0.67 C cm-2). The light band at the top of the picture is the bare substrate, the middle region is PP, and the bottom one is PP covered with PT.
highquality polythiophene cannot be directly grown. Accordingly, PP has been grown with pulsing between 0.6 and 1 V while for the deposition of PT 1.6 and 2 V have been used in this study in both ACN and ACN/water mixture, with a 100-ms pulse width and 100-ms pauses between pulses. The substrate seems to influence also the kinetics of growth of both PP and PT: the deposition on Pt or Au is much faster (=30 s for both PP and PT on PP) than on Ti,W (4min for PP and -15 min for PT on PP) up to identical charge density (200 mC cm'2 for both PP and PT). This is probably due to the presence of a thin semiinsulating passivated oxide layer on Ti,W, which is growing further during the deposition of the second (PT) layer, thus diminishing the current efficiency of the PT deposition. To decrease the IR drop originating from the solution, we have investigated the possibility of growing PT films on such substrates from more conducting, mixed aqueous/acetonitrile solutions. The PT film has been grown on the working electrode in a sequential manner: first two-thirds of the electrode was immersed in the pyrrole solution and the PP film was grown up to 666 mC cm-* charge density (Figure 3). This photograph shows the boundary between the pristine Ti,W substrate and TiW/PP. After this step the PT was deposited at the bottom third of the electrode with the charge density of 1 C cm-2. The boundary between the PP and PP/PT is clearly visible. We note that in these experiments no IR compensation was used, because the Luggin probe has been placed near the surface of the working electrode but in the same time out of the path of current, and so, the IR drop was negligible. The apparent current densities (see Figure 3) were larger than usual, because of the contribution of capacitive (nonfaradaic) currents due to the pulsed mode of deposition (see later). Growth of Polythiophene with IR-Drop Compensation. The uncompensated resistance of both the solution (R,) and the polymer film ( R f )can lead to irreproducibility during the prep aration of the film, because the effective potential across the solution/polymer interface may differ significantly from its nominal value. Thus, the lack of IR compensation in most studies of electrodeposition of PT may have been an important factor of the large deviations in properties of the resulting polymers. The IR drop within the film, Rf,if there is any, is independent of the position of the Luggin capillary, but does depend on the
Gratzl et al.
film thickness, i.e., on time. Consequently, it is not possible to use a preselected value of the residual resistance in the positive feedback mode. Instead, it is necessary to use a current interrupt IR compensation in dynamic mode (DIRC).'**I9 This feature, being available on the PAR 273 potentiostat, uses brief (190 ps) openings of the cell circuit. During such an opening it measures the potential decay and extrapolates back to the potential of the working electrode established immediately after current interruption. For this extrapolation the potentials at 10 and 20 ps after opening (points 2 and 4) were used. (The point 1 at 5 ps was not yet reliable enough, while the use of the range 50-160 ps would have caused a false "increase in IR drop", due to the mismatch between the exponential discharge of the double layer and the linear character of the extrapolation.) The highest repetition rate of the interruption available on PAR 273 is once every 4 ms. This means that the use of current interrupts at this fastest rate superimposes a 250-Hz (asymmetrical) square-wave signal onto the charge-transfer process, which is characterized itself by a typical cutoff frequency of a similar order (around 30-35 Hz; see later). In order to avoid the obvious artifact, a much lower repetition rate of current interrupt, e.g., 100-200 ms, must be chosen (=IO or 5 Hz). This is, however, meaningful only at a dc deposition. At pulsed mode a higher rate (e.g., 8 ms, or 125 Hz) must be preserved, to have the interfacial potential controlled for most of the duration of the 1OO-ms pulse. Thus, a large part of the current will be purely capacitive (i.e., "useless"). Examples of the dynamic IR compensation as applied both to constant potential and pulsed-mode depositions of polythiophene from a 0.1 M solution of thiophene are shown in Figure 4. The thick traces contain the interfacial potential difference plus the "added voltage" due to the IR compensation. The thin traces display the current. Because both the film resistance and the current change during the deposition (and during each pulse), the IR compensating voltage is often a complex function of time. These films were grown at 1.9-V interfacial potential for 10 s up to a total charge density of about 350 mC cm-2. This value was obtained by integrating the current, which contained only a minor capacitive ("useless") component as the repetition rate of the current interrupt was low (100 ms or larger). However, in some cases (e.g., at pulsed mode) 8- or 4-ms repetition rates were employed, which resulted in a 2-3 times higher current or apparent charge density for the same 10-s deposition and identical geometry. Then, only a fraction of the total current has been used for the faradaic process. DlRC is particularly suitable for experiments in which the resistance changes in time, such as electrodeposition of polymer films. It is also possible to record the value of the applied IR compensation (in volts) and current during each deposition pulse. From these data the actual value of the purely ohmic resistance, including the film resistance, can be obtained. Parts A-C of Figure 4 show DlRC during the polymer growth with a high time resolution, while Figure 4D demonstrates the iterative character of DlRC during a 100-ms pulse. Each interrupt is followed by a change in the total applied voltage (seethe arrows in Figure 4A,B), which has been calculated on the basis of the previous step. To show the dynamics of compensation, a 4-ms repetition rate for current interrupt has been applied in the examples shown. The ohmic voltage drop in the solution depends on several factors. One of them is the resistivity of the solution. For 0.1 M TBATFB/ACN, p = 78 Q cm is calculated from literature data.Is As strong deviations from Kohlrausch's rule may exist in organic electrolytes, this parameter was also experimentally determined in this study: p = 94 Q cm was obtained at the 100-kHz high-frequency limit. For 0.1 M thiophene + 0.1 M TBATFB/ACN (the solution most used in this work for electrodeposition), p = 95 Q cm was found. (The presence of thiophene probably decreased the mean activity coefficient, which resulted in a slight decrease of conductivity.) For 1 M TBATFB we obtained p = 30 Q cm, which was in good agreement with the (18) Ncwman, J. J . Electrochem. Soc. 1970, 117, 507-509. (19) Britz, D.J . Elecrroanul. Chem. 1978. 88, 309-352.
The Journal of Physical Chemistry, Vol. 94, No. 15. 1990 5977
Electrochemically Deposited Polythiophene. 1
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Figure 4. Examples of DIRC. Depositions were done at 1.9 V, from a 0.1 M thiophene t 0.1 M TBATFB/ACN solution, at different time resolutions. At pulsed mode an applied 1.9 V and cell off were alternated. The thin trace in all figures corresponds to the current (I,scale on the right axes) and the thick trace to the total output voltage (E,, scale on the left axes) including the added IR compensation. Arrows indicate the changes in IR compensation voltage (A, B). (A) dc, 0.2 ms/division. (B) dc, 1 ms/division. (C) dc, 5 ms/division. (D) One 1s" pulse with 4-ms current interrupts, 20 ms/division. (E) A IO-s deposition with dc voltage (dashed lines: deposition on the top of a previously formed PT layer). (F) A 10-s deposition with pulses on top of a previously formed PT layer (100 ms cell on, 100 ms cell off). The connecting lines between zero and nonzero currents are omitted.
31 Q cm found in the literature.I5 So, by using the equivalent conductivity at infinite dilution, ho= 173 cm2/(Qmol),I5 and the resistivities at 0.1 and 1 M, the coefficients of the linearly extended Kohlrausch's rule can be determined for TBATFBIACN: A = 173 ( I - I . ~ C I / ~ 0.59~) cmz/(Q mol) ( c is concentration in M). The other important parameter influencing the solution IR drop is the distance of the Luggin tip from the solutionlfilm interface. A cell of an ideal geometry would consist of parallel working and counter electrodes in a tube with an inside diameter equal to the diameter of the working electrode. In such a cell, with a current and p = 95 Q cm, the voltage drop on a density of 40 mA 1-cm distance would be 3.8 V. By using the cell described in the Experimental Section (Figure 1, without tube T), an IR drop of only about 250-300 mV can be measured under identical circumstances (same distance, current density, and solution resistivity). In addition, this drop includes the IR drop in the film, too. This latter term will, however, be temporarily neglected in the following considerations. The experimentally obtained values of IR drop were found to be always much smaller than those expected on the basis of the above three parameters. This is predominantly due to the nonideal
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cell geometry: the current flows across the entire cross section of the cell, which is generally much larger than the surface area of the working electrode. In fact, in our cell the effective current density at some distance away from the working electrode was necessarily about 72/352 = 0.04 times smaller than the "theoretically" expected 40 mA c d . (We used a 7-mm-diameter working electrode in a cell of 35-mm i.d. and, as a first approximation, assumed a uniform current density in the cell). This effect must reduce the IR voltage drop to a similar extent. Indeed, 3.8 V X 0.04 = 152 mV. This consistency with the measured values also means that no other major effect is responsible for the discrepancies. Accordingly, the possible "screening" effect of the Luggin probe was sufficiently minimized with the geometry described in the Experimental Section. The use of only the first couple of points (2 and 4) in DIRC during current interrupts to extrapolate back to the IR drop is apparently also correct, because when, e.g., points 15-30 were used, a much larger "ohmic drop" was obtained under identical circumstances (600-800 mV). These values would be inconsistent with the above geometrical considerations and also theoretically wrong, because the purely ohmic voltage drop must
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The Journal of Physical Chemistry, Vol. 94, No. 15, 1990 2018
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Figure 6. Ohmic (IR compensated)resistance as a function of deposition time for three positions of the Luggin probe: (A) 17 mm; (B) IO mm; (C) 4 mm (distances between the Luggin tip and the working electrode).
Identical circumstances as in Figure 5 , curve A. distance (mm)
the current should not depend on this parameter at all. (In other words, the rate of growth cannot depend on the distance.) With less frequent current interrupts (e.g., 100 ms or IO Hz) the current, + indeed, does not change with the distance to any major extent (though some minor change remains). We will discuss this question later. The ohmic voltage drop across the polymerfilm, which was neglected in the previous considerations, depends on its resistance, Rf,and (again) on the current. At Luggin tipworking electrode distances smaller than 1 mm, strong current oscillations occurred during IR compensation, which rendered the electrodeposition impossible at x 0. Thus, to obtain Rf,the curve A in Figure 5 had to be extrapolated to x = 0, which resulted in a small value, being certainly not larger than 1 52. This procedure is, however, decay by definition instantaneously when the current becomes zero. quite unreliable, because the distance dependence is strongly In this calculation a homogeneous current distribution was nonlinear. To provide a more reliable value, the ohmic resistance assumed within any cross section of the cell. The current density of the cell as a function of distance has been measured also in is, however, larger along the center of the cell than near its edges. the absence of the PT film, using a superimposed ac voltage a t This is why the calculated =I50 mV is smaller than the experthe 100-kHz high-frequency limit. After subtracting the real imentally determined IR drop. The assumption of a uniform impedance obtained at zero distance (Le., the solution resistance current density is even less acceptable at cross sections closer to between the reference and counter electrode), the ohmic resistance the solution/polymer interface. This is the principal reason why measured without the film can be plotted. The differences between the ohmic resistance at the end of deposition as a function of the two curves furnish an average of 2-4 52 for film resistance, distance deviates very much from the expected linear one, as shown in Figure 5, curve A. Its “theoretical” form would be R, = x ~ / A , ~ being valid at the end of deposition. These small values are quite plausible as PT is grown in its oxidized (conducting) state. For where x is the distance of the Luggin tip and the working (macomparison, the same ac measurements have been performed also cro)electrode, the surface area of which is A. This formula may, with a (relaxed) PT film on Pt. After a similar evaluation (Le., however, be only relevant a t very small distances: x = 2 mm is after substracting the resistance at zero distance), the high-frestill apparently too large in our cell because instead of the exquency limit of these results yielded a curve similar to that furperimentally found 4 52, 50 52 can be calculated. nished by the blank measurements in the absence of the film. In fact, the above formula provides with a good approximation Curve B in Figure 5 shows the average of the results of the blank at x 2 1 cm and with A as the cross-sectional area of the entire experiment (no film on the Pt electrode) and of the impedance cell, as we have seen. With A as the surface area of the electrode, measurements with the relaxed film. We note that the resistance which is the way it is usually used,m the formula is certainly wrong of the film bulk was found much larger (-1.3 k52) in the relaxed down to the submillimeter range. At distances on the order of state than during deposition. The measured complex impedance microns it might become more relevant, especially far from the plot of the relaxed film will be shown and explained in part 2. edges of the electrode, though we did not test this possibility. Even though the film resistance during growth can generally These simple calculations emphasize the probability of a strong be neglected with respect to that of the solution, the slight changes lateral inhomogeneity of large surface area films which is caused in the ohmic resistance as a function of deposition time (Figure by nonuniform current distribution (“edge effect”). This inho6) can only be attributed to changes in the film resistance. This mogeneity can be even visually observed during and after the is representative of the increase of the film thickness and of the growth of polymer films at these electrodes (a qualitatively difconcurrent changes in its conductivity. Figure SA displays the ferent and thicker layer close to the edges). values of the ohmic resistance during deposition obtained after Another parameter influencing the solution (and film bulk) IR 10 s of film growth. Similar values, or values even larger by 1-2 voltage drop is the current, which is mainly determined by the 52, were obtained at the beginning of deposition, which means that concentration of the monomer. The current was found to be a Rr must decrease from 4-6 il to 2-4 Q during growth (Figure 6). function of the Luggin tip-working electrode distance, too, when This implies an extremely high film resistivity at the early stages a 4-ms repetition rate of the current interrupt was employed: in of deposition, because then the film thickness is still very small, the case of 0.1 M thiophene concentration, at x > IO mm the virtually negligible. current density was about 40 mA cm-* while at x = 2 mm it was Electrochemical Processes and Electrical Properties of the Film almost 100 mA cm-*. With correct IR compensation, however, during Deposition. On the basis of observations obtained with IR drop control (in situ) and with independent impedance measurements, both qualitative and quantitative conclusions about (20) Bard, A. J.; Faulkner, L. R. EIectroehemicol Merhods; Wiley: New the PT film during deposition can be drawn. York, 1980. Figure 5. Ohmic (IR compensated) resistance as a function of distance between the tip of the Luggin capillary and the working electrode in a 0.1 M thiophene 0.1 M TBATFB/ACN solution. The cell shown in Figure 1 was used without the glass tube (T). (A) In situ data for a PT film (350 mC cm-2) electrodeposited at 1.9 V with DIRC at dc mode. The points were obtained at the end of the depositions calculated on the basis of IR compensation data. (B) Average of data measured with a bare Pt electrode (blank experiment) and of data with a relaxed F T film on the Pt electrode at the 100-kHzhigh-frequency limit at 0 V dc, with a &IO-mV excitation voltage. The value obtained at zero distance has been subtracted from all other values in both cases (Le., without film and with a relaxed film on Pt).
-
The Journal of Physical Chemistry, Vol. 94, No. 15, 1990 5979
Electrochemically Deposited Polythiophene. 1 RE
WE 'd
C f
I
A
4 0
I ("
--cr3
2-
- 20
-
- 0
-
I
Figure 7. Equivalent circuit of the cell as seen by dynamic IR drop compensation. IR, is the IR drop in the solution between the interface and the Luggin tip, Miis the interfacial potential difference (the "driving force" of charge transfer) with respect to NHE, and IRf is the IR drop within the growing polymer film. WE and RE are working and reference electrode. RE' is a reference electrode other than NHE, and AE( is the interfacial potential difference with respect to this latter reference scale.
The fact that a nearly constant applied potential results in a constant current during a period as long as 10 s indicates that hardly any mass-transport limitation is effective during growth (Figure 4E,F). This observation corroborates the result that a (chemical) kinetic limitation of growth is in effect when Pt is deposited,' similar to the case of PP. In a mass-transport-limited process a decay of current proportional to the square root of time would be expected. The transient current with dynamic IR compensation, as shown in Figure 4E,F, exhibited no peak in the early stages of the deposition if the deposition was started on well-polished, shiny bare Pt. When growth was restarted on an already existing PT layer, a peak appeared shortly after the application of the voltage to the working electrode (dashed lines in Figure 4E; Figure 4F). So did, of course, the total voltage, if IR compensation was on. This behavior is probably caused by the higher initial surface concentration of thiophene monomer, a significant amount of which can be adsorbed onto the large surface area of the existing PT film during a longer pause in growth. Its less adsorptive "affinity" to Pt and the smaller surface area of bare Pt can explain the lack of such peaks on finely polished bare metal. We note that a similar peak as shown by the dashed lines in Figure 4D,E was observed even when a bare Pt with rough surface has been used. This peak gradually diminished and eventually disappeared soon as the surface of the Pt electrode was covered with the polythiophene film. Similar current and voltage peaks are observed at a much smaller time scale, during each pulse at pulsed mode (Figure 4D,F). The adsorptive accumulation of thiophene during pulse pauses is the plausible explanation for this phenomenon, too. A current decay can be observed even on the smallest time scale, after each current interrupt (Figure 4A-C). This is, however, of a different origin: it is a consequence of the capacitive behavior of the double layer (see later). The 190-ps interrupts are apparently too short for any substantial adsorption to take place. A minor sign of it can, however, be observed even here: see the initial tiny peaks in Figure 4A. The short period of low current values observed sometimes on the largest time scale (Figure 4E) before these peaks is consistent
Ill
5 ms
Figure 8. Results of model IR compensation experiments made with the dummy cell under circumstances identical with those used in Figure 4 (A, division = 0.2 ms; C, division = 5 ms). The correspondingelectrical parameters are R, = 15 R, RCt= 125 R, C, = 40 pF, and Rf= 3 R. The film capacitance and the Warburg elements were omitted from the ex-
periments because they are insignificant during deposition. The arrow indicates the change in IR compensating voltage in (A). with the nucleation-growth mechanism that has been proposed for the formation of the early layers of the PT film." To characterize the entire deposition process, however, adsorption and kinetic limitation must also be taken into account, as we have just seen. From the data collected during deposition, in addition to R, (2-20 Q; Figure 5) and Rf(4-6 fl at the beginning and 2-4 fl at the end of the deposition, =3 kR when relaxed), it is possible to also estimate the charge-transfer resistance, R,. From the ohmic voltage drops and from the associated resistances, R,, i= 120-140 Q assuming that 1.9 V is the true interfacial potential drop. For the relaxed film, the cutoff frequency was found from the impedance measurements to be = l o kHz and Rf i= 1.3 kR (see Figure 7 in part 2). Thus, the film bulk capacitance, C , is about 12 nF. During deposition, Rfi= 2-6 9; Le., the cutoff frequency of the film bulk must be in the megahertz range if a similar capacitance is taken into account as that obtained for the relaxed PT. Because of the apparent lack of a mass-transport limitation, the role of Warburg impedances is negligible during deposition. Instead, a resistance corresponding to the kinetic limitation must be involved. This appears as a constant contribution to the charge-transfer resistance. Thus, an adequate equivalent circuit of the cell involves neither a Warburg element nor any capacitor representing C, during growth (though in Figure 7 all these elements are shown, because in the case of a relaxed film they too become significant). With a typical double-layer capacity of 10 pF/cm2,20the time constant of the faradaic process is about 0.5 ms, or its cutoff frequency =320 Hz. However, the equivalent circuit of Figure 7 with a c d of about 3.8 p F (for a 7-mm-diameter electrode) did not furnish any current profile similar to that obtained during a real deposition (with identical potentiostat settings). Very similar curves could be obtained with the above parameters if about 10 times larger c d was used (compare Figure 8A,B with Figure 4A,C). Thus, the specific double-layer capacity of the PT/monomer solution interface during growth is rather about 100 pF/cm2. This is
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The Journal of Physical Chemistry, Vol. 94, No. 15, 1990
TABLE I: Electrical Parameters of Polythiophene during Deposition and after Relaxationpc during deposition after relaxation R,, 46-54 0 cm2 large (not measured in this work) C, = IO0 rF =IO0 rF R, 0.7-2.3 il cm2 4 0 0 z2 cm2 C, -30 nFcm-* 230 nF cm-2 R, XPlA A 173(1 - 1.4~~1’ + 0.59~) cm2/il
“The data are relevant for a PT film grown up to about 300 mC/ cm2at 1.9 V. b A is the effective cross-sectional area of the entire cell, and x is the distance between the working electrode and the tip of the Luggin capillary. The formula is valid for the cylindrically symmetrical arrangement as shown in Figure I . The resistivity of the solution, p , can be calculated on the basis of the equivalent conductivity, A. c c is the concentration of TBATFB in M in a 0.1 M thiophene/ACN solution whose equivalent conductivity is A. probably due to its large specific surface area caused by its fractal structure. So, the cutoff frequency of the faradaic process is, in fact, about 30-35 Hz. Even though the measurements described in the previous section furnished strong evidence for the film resistance being extremely small during deposition, all three discussed ways of determining Rf relied on the values of total IR drop measured with the IR compensating circuitry itself. Therefore, an independent measurement has also been designed in the following way: a 50-kHz, f100-mV excitation signal was superimposed onto the 1.9-V dc voltage during growth at x = 10 mm, and the amplitude of the resulting current oscillation was measured on the oscilloscope screen. It was found to be 5-6 mA, and thus, the total ohmic resistance was about 17-20 $2,which agrees well within the values obtained from IR compensation data (see curve A in Figure 5 ) . We obtained identical current oscillations under identical circumstances with a dummy cell which consisted of the equivalent elements discussed above, which proved that our in situ measurement was not distorted by any unexpected phase shift or other effect either. As the dynamics of current and voltage decays in Figure 4A,C is extremely similar to that in Figure 8A,B, the equivalent-circuit experiments can be considered as a further independent proof of the parameters determined by the analysis of the IR compensation results. We note the unequivocal capacitive character of the current decay between interrupts (theoretically, also depletion and/or adsorptive effects could have resulted in such curves). The huge current spikes at the beginning of deposition (Figure 4B-D or Figure 8A) are also capacitive currents. It is also understandable on the basis of the simulation results (Figure 8) that too-frequent current interrupts (4 or 8 ms) can cause a 2-3-fold increase in the average current by the large capacitive contribution. As the cutoff frequency of the faradaic process is about 30-35 Hz, for current interrupts 10 Hz or lower repetition rates are appropriate. We note that the directions of current decay in Figures 4A and 8A after current interrupt are opposite to each other. This is explained by the opposite changes in the total applied voltage and by the capacitive character of the changes in current in the ms range. The electrical parameters governing the electrodeposition of PT are summarized in Table I. IR-Drop Control and the Polythiophene Paradox. The question of the ‘polythiophene paradoxn4 must be considered in the light of the IR-drop-control experiments. PT was grown by numerous research groups in many cases without IR compensation, which means that it was by no way sure that the effective potential at the solutionlpolymer interface was as high as the nominal 1.9-2 V. However, if the correctly applied DIRC is on, it can be guaranteed that the potential drop across the solution/polymer interface is very close to the nominal value. This is, of course, true only if, in addition to the time constant of the solution, that of the film bulk is also very small. DIRC works on the assumption that the “response” of the charge-transfer process to a current interrupt exhibits finite speed while the dy-
Gratzl et al. namics of the ”parasitic” elements (solution, film bulk) is instantaneous. In the case of PT deposition this is certainly true because the polymer is growing in its conducting pseudometallic state (see also the negligible resistance found in the previous section). The macroscopic film thickness ensures also a low bulk capacitance (in the nanofarad range, as obtained above). Thus, the film bulk exhibits a dynamics in the megahertz range. Figure 7 shows the equivalent circuit of the cell as it is seen by DIRC, and parts A and B of Figure 8 exhibit the responses of a corresponding model cell. With the DIRC on, the interfacial potential difference is indeed constant and exhibits the prescribed nominal value. (We note that this can be seen at later stages of the deposition, after the IR compensation already has reached a steady state. Figure 4A-C and Figure 8 show earlier stages of the growth.) In this work PT films were successfully grown even under such circumstances, Le., with 1.9-2.2 V across the solution/polymer interface, even though some products of overoxidation (soluble dark oligomers) were evidently present at 2 V and above. This means that the film is indeed growing at high positive potentials if thiophene is present while it is being destroyed (overoxidized) under otherwise identical circumstances in the absence of thiophene. Thus, there is only one possibility to resolve the paradox: we have to assume that polymerization and oxidation are always competing with overoxidation and that the first two reactions are kinetically preferred to overoxidation, which is more inhibited. Therefore, if the monomer is present, the film grows while in its absence it is being destroyed above 1.6 V. Moreover, the monomer can protect the film but it is in contact only with the surface of the polymer. Thus, it can also be stated that degradation can only start at the solution/film interface. This degradation later propagates into the polymer bulk, too, causing irreversible damage. The bulk cannot be, however, directly destroyed by any applied positive potential, because the damagingly high field exists only outside, at the polymer/solution interface. A part of this degradation mechanism has been recently suggested also on the basis of XPS studies.*’ The same is probably true for PP, and in addition, PP can even be “protected” by the presence of the monomer of PT, as has been demonstrated in this work. This is why the conducting polymer films can be formed at an applied oxidation potential at which they are destroyed when the monomer is absent. Optimized Parameters of Deposition. A careful design of the cell geometry is necessary to grow more homogeneous and reproducible films. Well-centered counter, reference, and working electrodes (Figure 1) can contribute to laterally homogeneous film growth. Our cell with a 7-mm-i.d. narrow glass body attached to the working electrode (T in Figure 1) ensures a more favorable current distribution and the growth of laterally perfectly homogeneous polymer layers. The theoretical formula for solution resistance, if the Luggin tip is kept close to the narrow cell section, also becomes much more realistic. The other possibility is the use of a small-area working electrode (diameter C1 mm). Then, a more or less radial current pattern and spherical isopotential surfaces will form close to the electrode. In spite of this unfavorable geometry, the effective potential at the surface of the film will not vary laterally to any major extent because of the small size of the electrode. Thus, a better lateral uniformity of the prepared film can be expected in this case than when the substrate area is comparable to that of the counter electrode. In the case shown in Figure 5 , the Luggin tip is best placed at a distance of about 1 cm (in the absence of tube T). At shorter distances the IR drop would be too sensitive to any uncertainty in the position, while a larger distance would unnecessarily increase the voltage to be compensated. In general, as a feasible compromise, the distance of the Luggin probe from the working electrode can be somewhat larger than the electrode diameter, unless it is so large that the IR drop becomes difficult to compensate accurately. With the shielding tube on (T in Figure I ) , (21) Takenaka, Y . ;Koike, T.; Oka, T.; Tanahashi, M. Synth. Mer. 1987, 18, 207-121.
Electrochemically Deposited Polythiophene. 1 TABLE II: Oualitv of PT Films Deposited at Different Potentialsa potential vs 0.01 M film amearance charge, mC Ag+/Ag, V no film (yellowish Pt surface) 6 1.4 loose, thin film 30 1.5 47 homogeneous, compact film 1.6 71 perfect lateral homogeneity, 1.7 very compact, metallic, shiny same 100 1.8 1I 7 same I .9 rougher film surface, oligomers 121 2 forming and dissolving looser, less metallic film, 151 2.1 more oligomers less compact film, more oligomers 155 2.2 ~
“ A 4-mm glass tube 7-mm i.d., was put on the working electrode (see T in Figure I).
however, shorter distances are also good. The best quality films
can be obtained with, e.g., a 7-mmdiameter, 4-mm-long tube when the tip of the Luggin probe is kept 1 mm outside the tube. (At fixed working and counter electrode positions, the IR drop greatly increases when the Luggin tip is moved into the tube, because it causes the establishment of a narrow cross section between the tube walls and the tip. Hence, it is better to keep it outside the tube.) The solution composition is another parameter of deposition. The current can be diminished by lowering the concentration of the monomer. At 10 or 1 mM, e.g., the current densities would decrease, while the solution resistivity would still be determined and kept constant by the supporting electrolyte. Thus, the IR voltage drop can be significantly mitigated by lowering the concentration (current), which becomes another parameter in selecting the proper electrodeposition conditions. Smooth and compact films can be produced at lower (approximately 10 mM) concentration of the monomer. A 0.1 M supporting electrolyte (generally TBATFB or LiCI0.J is good enough to ensure a relatively low resistivity. Finally, with optimal IR compensation (100-ms current interrupts, the above geometry) we determined the potential window of deposition (Table 11). The best is a 1.7-1.9-V interfacial potential.
The Journal of Physical Chemistry, Vol. 94, No. 15, 1990 5981 Except for some special applicationsg the constant-potential mode is sufficiently effective for deposition.
Conclusions It has been shown in this study that the formation of PT layers is feasible on a variety of substrates if an underlayer of PP is first deposited. This enables the growth of PT films under otherwise difficult conditions: on electrode materials that are less noble than gold or platinum and from mixed acetonitri1e:water solutions. This technique is both simpler and cheaper than the use of a platinum coating. It is somewhat surprising that PT films can form on PP at potentials under which PP should be “overoxidized” and therefore nonconductive (Le., above 1.2 V). We believe that this may be due to reasons similar to those for which the PT film itself grows at otherwise prohibitively positive applied potentials (viz. the “PT paradox”). A close examination of the IR compensation data reveals that the film resistance changes in a complex way with the time of deposition (Figures 4 and 6) and so does the deposition current. Even though the effective potential at the polymer/solution interface may be much lower than the applied + 1.9 V without IR compensation, with this technique it is certainly close to its nominal value. This provides a better control of the quality of the resulting polymers. Our initial intention was to utilize the continuous IR-drop control to produce films under defined conditions. This technique provided, however, some insight into the growth process and its electrical parameters, too. Instead of using unnecessarily high potentials, with DIRC it is possible to determine the exact deposition potential, or its lowest possible value. We discussed also the cell geometry. The tube shielding can be effectively used to electrodeposit laterally homogeneous PT, PP, and other polymers. The ‘polythiophene paradox” is apparently caused by the competition between polymerization and film destruction by overoxidation, the latter being kinetically more inhibited. Acknowledgment. This work was supported in part by a contract from the Office of Naval Research. D.F.H. acknowledges support from Drager GmbH. Registry No. TBATFB, 429-42-5; ACN, 75-05-8; PT, 25233-34-5; PP, 30604-81-0; Pt, 7440-06-4; Au, 7440-57-5;Ti, 7440-32-6; LiCIO,, 7791-03-9; thiophene, 110-02-1; pyrrole, 109-97-7.
