J . Phys. Chem. 1990, 94, 5982-5989
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Electrochemically Deposited Polythiophene. 2. Dry and Wet Relaxation Duan-Fu Hsu, Miklos Cratzl, 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) Polythiophene (PT) freshly electrodeposited at 1.9 V vs 0.01 M Agt/Ag from 0.1 M thiophene + 0.1 M tetrabutylammonium tetrafluoroborate (TBATFB)/acetonitrile is not in equilibrium either as an isolated phase or when it is in contact with the solution it was deposited from. Consequently, spontaneous relaxation processes take place during and after growth until equilibrium conditions are established. These simultaneous processes happen at very different speeds. Relaxation of the PT film begins already during its growth, electroneutrality of its bulk being ensured by the fast uptake of anions. If the fresh PT is quickly washed and then kept dry and electrically isolated, its electron work function decreases. A further decrease in work function can be measured if dry relaxation is followed with wet relaxation in the supporting electrolyte. If the freshly prepared PT is kept in the dark in the solution where it was deposited from, the electrode potential of PT decays slowly down to about -450 mV. White light causes the potential to jump fast up to 140-160 mV, while when PT is put in the dark again the potential drops down to about -450 mV again. This light effect diminishes in the order of blue > green > red light. If the fresh PT film is washed and then relaxed in a supporting electrolyte without thiophene, the final potential is only 30-200 mV, and the observed light effect is smaller. Parallel changes in color (increase in the reflectance of red light) and an increase in real impedance can be recorded during relaxation. Similar changes in color and impedance can be measured when the electrode potential is cycled between 1.2 and 0 V. The spontaneous relaxation phenomena are caused by many factors, including the slow reduction of PT by certain impurities (e.g., water and, if present, thiophene). Its stable final state depends on the environment where it is kept (dry or wet, in monomer or in other solutions, in the presence or absence of water and oxygen). Poly(3-methylthiophene) (PMT) electrodeposited at 1.3 V from 0.1 M 3methylthiophene 0.1 M TBATFB/acetonitrile exhibited a behavior similar to that of PT with respect to all types of experiments, except for the very fast potential decay of PT from 1.9 V to about 0.8 V during potential relaxation, a part of which was necessarily missing in the case of PMT.
+
Introduction In part 1 we described results concerning the electrochemical growth of polythiophene (PT) films. For some substrates we suggested, as a method of PT deposition, to grow first a polypyrrole (PP) film and then to deposit PT on the top of PP. Thus, due to the protective effect of PP, PT has been successfully deposited on a variety of substrates that would normally be oxidized at the high potential necessary for PT deposition. We studied also the possibility of IR-drop compensation during growth and found that significant ohmic drops exist in the solution and, to a lesser extent, across the growing PT film. These experiments shed some light on the “polythiophene paradox”:* the controversial behavior of PT can only be resolved if degradation by overoxidation polymerization and doping are considered as competing reactions, the latter (“useful”) processes being faster. On the basis of proper IR-drop control the electrical parameters of the PT film during growth (in situ) and optimum potential and cell geometry have been determined. Thus, laterally homogeneous, metallic, compact, and hard PT could be grown in a reproducible way. In part 2 the behavior of the PT film following the deposition will be characterized. The film is apparently in an unstable state at the end of its deposition, and hence, relaxation processes will follow and continue until the PT reaches a stable low-energy state. These processes include several competing paths with very different mechanisms and rates (time constants) which depend also on the conditions of deposition and on the environment of the film (the surrounding solution or gas) where it is being kept. The environment also defines the final relaxed (equilibrium) state of PT, which in turn determines its potential applications. Though formation was the subject of part 1, the following relaxation processes cannot be understood separately, particularly because some of them (those with short time constants) are taking place during the growth of the film. Consequently, the mechanism of PT formation is also dealt with in this paper, though from a different aspect than in part 1 . The apparent “irreproducibility” of the quality of electrodeposited polythiophene can be avoided if correct IR compensation and cell geometry (tubular, uniform current pathway) are ensured, as was shown in part 1. However, the relaxation of PT still may To whom correspondence should be addressed.
0022-3654/90/2094-5982$02.50/0
lead to irreproducible final DroDerties due to the large number of competithe processes thai take place during and a t e r deposition. The most important processes during growth are as follows: the electrooxidation of thiophene to radical cation followed by its dimerization which occurs mainly in the a-a position but also, to some extent, in the a-0 p ~ s i t i o n . ~The next step is the restoration of the condition of electroneutrality in the film in which the diffusion of anions into the film is the obvious process. Another possibility is the elimination of protons observed in both polypyrrole4 and polythiophene.s Nucleophilic attack of water on the 0 position6 and reaction with molecular oxygen’ followed in both cases by the elimination of proton are other possibilities. These are likely processes also during the relaxation. There is enough evidence in the literature which shows that PTs prepared under different conditions have different electronic and ionic conductivities and different morphology. Thus, the chemical complexity is superimposed on the variation of the resistivity of the film during the growth process and on the change of the effective surface area. Because growth of PT is an oligomerization process, the relative contribution of all these reactions and parameters depends on the experimental conditions. In short, there are so many experimental variables involved in this seemingly simple process that PT layers prepared under slightly different experimental conditions may possess a significantly different “quality”. It is also known that bithiophene (BT) and terthiophene (TT), which can be regarded as the precursors and/or intermediates during the formation of PT, have significantly lower oxidation potentials than the monomer itself and that the PT layers prepared from these oligomers as starting materials have a very different “quality” than the films prepared by the direct electrooxidation of thiophene.2 In this work we present experimental observations of the relaxation and the final properties of polythiophene films formed (l)Gratzl, M.; Hsu, D. F.; Riley, A. M.; Janata, J. J . Phys. Chem., preceding paper in this issue. (2) Kritsche, 9.; Zagorska, M. Synth. Met. 1989, 28, C2634268. (3) Tanaka, K.; Shichiri, T.; Yamabe, T. Synth. Met. 1986, 14, 271-277. (4) Qian, R.; Li, Y.; Yan, B.; Zhang, H. Synth. Met. 1989 28, C51-C58. (5) Christensen, P. A.; Hamnett, A.; Hillman, A. R. J . Electroanal. Chem. 1988. 242.47-62. (6) Otero, T. F.; Tejada, R.; Elola, E. S. Polymer 1987, 28, 651-658. (7) Billingham, N. C.; Calvert, P. D.; Foot, P. J. S.; Mohammad, F. Polym. Degrad. Stab. 1987, 19, 323-341.
0 1990 American Chemical Society
Electrochemically Deposited Polythiophene. 2
under our experimental conditions (discussed in part 1) and offer our interpretation of the formation/relaxation mechanism. The “quality” of the film in this context refers to many different parameters. Unusually it means the conductivity of the film, but it may also mean optical or mechanical properties or any combination of the above. Another fundamental physical property that can be added to the “quality” list is the electron work function. It refers to the affinity of electrons for the phase and determines their distribution in solid-state junctions formed by these materials. It also influences the optical properties and the charge-transfer kinetics across the interfaces, etc. As such, it has a dominating influence on the operating characteristics of the devices in which these materials are or would be used. Among other techniques, electron work function can be conveniently measured with a Kelvin probe. This, and conventional electrochemical techniques (open cell potential, cyclic voltammetry, complex impedance analysis) have been the main tools used in this study. Color changes were followed using reflectance measurements. Poly(3-methylthiophene), which can be deposited at a much lower potential, has also been investigated with all these techniques.
Experimental Section Measurements of the rest potentials were done with the Radiometer PHM 84 pH meter with respect to the 0.01 M Ag+/Ag reference electrode. Chemicals and the potentiostats used in this work were the same as those described in part 1. Poly(3methylthiophene) was prepared from 3-methylthiophene,Aldrich (analytical grade). The color changes were monitored by measuring the reflectance of red light the source of which was an SLA-591LT3 highbrightness light-emitting diode (LED) (Radio Shack; luminous intensity 500 mcd, peak wavelength 660 nm). The LED was driven at a 1-kHz frequency with a Wavetek Model 185 Signal Generator. A photodiode (Silicon Detector Corp., SDl 00-1 I11-021) sensed the light reflected by the PT surface. The angles of both the incoming and reflected light were 45’ (with 90’ between source and sensor). A Teflon chamber with windows for both light beams contained 0.1 M tetrabutylammonium (TBATFB)/acetonitrile(ACN) solution and the PT film deposited on a Pt plate with a small reference electrode. The potential of PT could be either measured or cycled by the potentiostat (PAR 273). A Keithley 840 Autoloc lock-in amplifier was used to determine the in-phase and out-of-phase components of the reflected intensity, using the input of the LED as a reference signal. Thus, a relative measurement of red-light reflectance and its changes was possible during cycling or during spontaneous relaxation. Impedance was measured under identical circumstances with fl5-mV excitation voltage at 85 Hz and 8.5 kHz with the same apparatus (the signal generator was connected to the external input of the potentiostat). Both the in-phase and out-of-phase components were determined. A Tungsten lamp (Cole Parmer, Model 9741-50) was used to produce white light, and an universal monochromator illuminator (Orzel, Model 7340) was used with a Hg-Xe lamp as a monochromatic light source in the study of photopotentials. The relative changes in work function were measured by use of a vibrating capacitor (Kelvin probe). The schematic diagram of the purpose-built instrument is shown in Figure 1. The reference electrode is a stainless steel (304) disk inserted into a plastic rod and glued onto the cone of a 20-W loudspeaker. A flow cell was constructed by milling a channel in a block of Plexiglas and fitting the reference electrode through a hole drilled in the base of the channel. This bottom half of the cell was attached to the loudspeaker. The top of the flow cell was formed by bolting on a second Plexiglas block which had an entry port for the introduction of the test gas and a hole opposite the reference disk for the sample electrode. A grounded steel plate was placed between the loudspeaker and the flow-cell body to screen out electromagnetic emission from the loudspeaker. The rough adjustment of distance between the sample and reference electrodes could be made by moving the sample electrode with a micrometer linear positioner. The fine adjustment was done by the application of
The Journal of Physical Chemistry, Vol. 94, No. 15, 1990 5983 ...............................................................
:
I-v
PREAMPLIFIER
i
I
AMPLIFIER AND FILTER
GENERATOR
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OSCl LLOSCOPE
LOCK-IN AMPLIFIER
I I NT EGRATOR
DC BIAS
1 ’ 1
PLEXIGLASS
Figure 1. Circuit diagram of the Kelvin probe measurement. Dotted lines symbolize Faraday cages. The magnified insert shows the design of the vibrating capacitor. 1 , 1/8-in. pipe thread/gas inlet; 2, linear positioner; 3, loudspeaker (40 W); 4, test electrode; 5, reference electrode; 6, spring in electrical contact with the reference electrode.
a dc voltage to the cone of the loudspeaker. The two plates of the Kelvin probe were connected together through a current-to-voltage converter (1-MQ feedback resistor and additional X 100 amplification) whose output was conditioned with a band-pass filter (100 Hz-1 kHz) and input into a lock-in amplifier (EG&G PAR Model 124A). In order to minimize the error due to misalignment of the two plates after the insertion of the new sample, the phase angle was always tuned to a maximum value of the output. A nulling technique was used to measure the voltage between the plates due to the work-function difference. The in-phase signal at the operating frequency of 450 Hz was integrated (time constant, t = 4.7 ms). The output of the integrator which is equivalent to the work-function difference of the two plates was fed back to the reference electrode. The diameter of the reference electrode (8 mm) was slightly larger than that of the sample used in part 1 and in this work. The aim was to minimize stray capacitances and average out specimen heterogeneity. Unless stated otherwise, all experiments were done at room temperature. Although conceptually simple, Kelvin probe measurements can be subject to a large error due to parasitic impedances and due to uncontrolled adsorption of contaminants during sample manipulation in open atmosphere. Because the results of these measurements are important in further analysis of the behavior of synthetic metals, we have assessed the confidence limits of our measurements. This was done by using two “identical”stainless steel plates, one being permanently attached to the moving core of the loudspeaker (reference plate) and the other representing the “sample” plate. The repeatability test was done simply by removing the sample plate from the test chamber and putting it back again. This sequence corresponds to the steps done with the real sample plate on which the material of interest is electrodeposited. It is interesting to note that even “identical” stainless steel plates exhibited a substantial difference of the work function (WF). When the sample plate is removed from the chamber, it is exposed to normal uncontrolled laboratory atmosphere for 10-30 s (i.e., it is subject to uncontrolled adsorption). Otherwise both plates are kept in a stream of helium. The second part of the test consisted of adding 100 mV from a floating power supply placed in series with the sample. This test simulates the situation in which the W F difference changes
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The Journal of Physical Chemistry, Vol. 94, No. 1.5, I990
TABLE I: Estimate of the Accuracy and Confidence Limits of Kelvin Probe Measurements 0.2 v p-p phase max
DWF
DWF
+ 100 mV 158 178 206 195 208
% error
0.8 V p-p
phase max 40.5 47.0 43.5 53.0 53.5
47.5 39.5 48.5 45.4 54.0
154 174 204 194 206
9, mV 90% confidence limit 95% confidence limit
186.4 f 1 1
173.6 f 8
f10
f7 f10
f 13.6
due to chemical modulation (Table I). In each measurement the optimum value of the phase angle was recorded. These experiments confirm that measurements of the absolute difference of W F (second column) with Kelvin probe are indeed subject to a large error (relative standard deviation is approximately 5%). This is due to parasitic impedances which are very sensitive to the relative position of the two plates. These effects relate to the distance between the plates, the overlap, and the relative tilt of the plates. Upon dismantling/reassembling the probe, all these geometrical factors may change, as is seen from the changing values of the phase angle yielding the maximum output of the lock-in amplifier. Such a cause of inaccuracy is well-known8 and would occur when different samples were introduced to the probe. The evaluation of precision was done for two spacings of the plates. As expected, for the smaller mean distance of the plates (0.8 V peak to peak (p-p), Table I) the measurement is more precise. On the other hand, the relative measurements of the W F difference (column 3), obtained by the addition of 100 mV from the floating source, are much more precise. This precision would be obtained in the situation where the Kelvin probe would be calibrated and used in a continuous mode as a monitor. For thermodynamical reasons, it is impossible to measure the absolute value of work function of a single phase. By performing the Kelvin probe measurement on samples prepared under different conditions, we can obtain only the change of the relative value of W F while assuming that the W F of the reference electrode remains unchanged. When this measurement is performed in an inert atmosphere, such an assumption is justified.
Results Dry Relaxation (No Solution Contact): Work-Function Measurements. The assessment of the precision of work function measurements on identical samples has been discussed in the Experimental Section. The large error in the determination of the absolute difference of W F (DWF) caused by the deviations in the alignment allows us to comment only on the trends but not on the absolute values of the DWF. We measured changes of DWF for samples prepared with and without IR compensation for three different concentrations of monomer (0.1, 0.01, 0.001 M) after three postdeposition steps. The DWF was measured immediately after the sample had been removed from electrolytic cell and rinsed with acetonitrile (step I ) . There is a small and rapid decrease of the DWF due to the drying off of the acetonitrile from the sample placed in a stream of He. The value of the DWF is recorded as soon as it becomes stable. The sample is then removed and placed in a saturated vapor of acetonitrile at 60 O C for minimum of 8 h. The DWF is again measured (step 2) after it has reached a stable value. The sample is then placed in 0.1 M TBATFB/acetonitrile solution and allowed to reach a stable open cell (equilibrium) potential. This takes typically 5-10 h. It is then rinsed and D W F is measured again (step 3). First we need to review again the sign convention used in our experiment. When the W F of the sample decreases, its electron affinity decreases causing the sample to become more an electron donor, Le., more positive (because as a consequence, the sample plate looses electrons). in order to restore the balanced condition, a negative voltage must be applied by the feedback loop to the (8) Bonnet, J.; Soonckindt. 1.: Lassabatere, L. Vacuum 1984, 34,693-698.
"""
600
DWF 150 166 180 180 192
DWF+100mV 151 168 181 182 194
%error 1 2 1 1 2
I
I
0
1
2
3
step no.
Figure 2. Changes of the work-function difference with postdeposition treatment. Step I , freshly deposited PT film; step 2, relaxation of the PT film at 6 0 "C in saturated acetonitrile vapor for 8 h; step 3, relaxation of the PT film in 0.1 M TBATFB under open circuit conditions for 5-10 h. The vertical bars indicate standard deviations characterizing the different PTs at the step in question. The symbolized vertical "width" of the changes indicates the relative standard deviation of the same change for all samples (Le., the standard deviation of the changes divided by the average change). As most changes were parallel, these deviations are much smaller than those belonging to a certain step (vertical bars) because the PT's themselves differed from each other quite significantly, due to their different growth conditions.
sample, or a positive voltage to the reference plate. In other words, the change of the feedback voltage of the reference plate in the positive direction corresponds to a decrease in the W F of the sample. Thus, there is a general trend toward decrease of the work function (Figure 2) both during dry heat treatment (from step 1 to step 2) and during all wet posttreatments (from step 2 to step 3). Although the application of the IR compensation affects the morphology of the film, its effect on the WF, if any, is hidden within the experimental errors of the measurement. Hence, Figure 2 shows the changes in the average with the standard deviations, instead of showing all data individually. There is a noticeable decrease in the W F observed for all concentrations and all cases when the film is kept in acetonitrile vapor a t 60 OC. Because the sample is electrically neutral at the end of growth, no net charge flows between the PT film and the adjacent phases during this heat treatment. Indeed, when after the acetonitrile vapor conditioning the sample is placed in 0.1 M TBATFB solution, the E , (open cell electrode potential) decreases in the usual way (see later). Therefore, the decrease of the electron affinity of the polymers during the dry treatment (step 1 to step 2) cannot be explained by a supposed "restoration of electroneutrality" or by any related process. Electroneutrality is, in fact, maintained always, even during growth: it cannot be violated but locally and for a very short time only, at the growting end of the PT chains. No new element is attached to the chain before the electroneutrality of the previously grown chain section is ensured by the uptake of an appropriate anion. As the deposition is presumably kinetically controlled (see part I ) , there is sufficient
Electrochemically Deposited Polythiophene. 2
The Journal of Physical Chemistry, Vol. 94, No. 15, 1990 5985 TABLE 11: Relative Exchange Current Densities of PT Film" exchange current electrolyte density, wA/cm2 0.01 M LiCIOl 36.2 11.6 0.01 M LiN03 4.3 0.01 M sodium tosylate 3.3 0.01 M TBATPB 0.01 M TBATFB 2.9 28.4 0.1 M TBATFB
I
cE
1
w
'All measurements were done in ACN vs a double-junction 0.01 M AgN03/Ag reference electrode. The current densities were calculated with respect to the geometrical electrode area. The solution was moderately stirred during the Tafel plot measurements. time (h)
Figure 3. Relaxation of the open cell potential (electrode potential of PT): (A) in the monomer solution (0.1 M thiophene + 0.1 M TBATFB/ACN); (9) in the (virtual) absence of monomer (0.1 M TBATFB/ACN); (C) in the presence of 1 M water (0.1 M TBATFB 1 M water/ACN). At r = 5 h white light was shed on the surface of the film. This light has been turned off at r = 11 h. The effect is only shown for curve A but is reproducible in all other cases (the potential when the light is on is about 140-160 mV in each case). Films grown up to about 300 mC/cm2 were used.
+
time for the anions to move into the film and to preserve macroscopic electroneutrality from the beginning on (apart from the growing edge, of course, the net charge of which can be nonzero). We note that if mass-transport limitation were the case, the condition of macroscopic electroneutrality within the film would still hold, because then either the transport of the monomer would be slow and then the ions still would move with a sufficient speed to ensure electroneutrality all time or the anions would move more slowly. In this case their transport would limit the rate of growth, as otherwise electroneutrality would be macroscopically violated (which is impossible). Thus, charge neutralization processes cannot account for the observed "dry" changes in WF, partly because they are impossible without electrical contacts and partly because they have already happened during deposition. A possible explanation of the decrease in W F during the heat treatment may be the relaxation of the polymer backbone, resulting in a decrease in electron affinity of the PT matrix, which has been suggested by Heinze et aL9 A reduction by residual water in the atmosphere of the oven may also contribute to the observed changes (see later). A possible reaction with residual oxygen' would cause an increase in WF. According to our results, this process is less likely to happen. It has not been possible to detect, by XPS, the presence of other anions in the PT but an increased amount of oxygen.' This oxygen, however, may have come from a reducing agent, e.g., water. There is also a thermodynamical reason for the observed W F changes at dry relaxation: the PT as an isolated single phase can spontaneously change only in the direction of equilibrium. Such a change plausibly coincides with a decrease, and not with an increase, in WF: a smaller W F means that the phase needs to uptake electrons less than when the changes began. In other words, the phase became more stable. That there is a change at all is not surprising either, as the freshly prepared PT is, e.g., nonhomogeneous for many parameters (due to obvious reasons), and steric stresses may also be there which all tend to relax. The following wet relaxation also involves a (further) decrease in WF. An analogous thermodynamic explanation is possible for this observation, too. Wet Relaxation (with Solution Contact): Potential, Color, Impedance, and Light Effect. The change in the work function during wet treatment (from step 2 to step 3; see Figure 2) parallels the usual change in the potential of PT as an electrode (or open cell potential) that can be observed in solutions: during relaxation, the potential always changes in the negative direction (Figure 3). (9) Heinze, J.; Bilger, R.; Meerholz, Klaus Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 1266-1271.
If PT is kept in the dark and in the same solution where it has been prepared (curve A), first a fast (and almost instantaneous) potential drop occurs from the deposition potential to 600-700 mV. Then, the change becomes much slower, and a potential plateau appears at about 200-300 mV. After a longer time (hours), the decrease becomes again faster (though happening in the hour range) and, through an inflection point at about -100 mV, finally slows down again and the potential stabilizes around -450 mV. After the film is formed, it can be rinsed and placed in the supporting electrolyte solution only. If the monomer is absent from the solution where relaxation is taking place, the potential stabilizes at much higher values, around 30-200 mV over the same period of time (curve B in Figure 3). The two curves are similar though in their common potential range. We have monitored relaxation behavior for PT films deposited on all substrates including those covered with PP layer and obtained similar results. These experiments show that PT films grown under various deposition conditions and on any substrate are not in an equilibrium state. Though we are not aware of any relaxation measurements done in the presence of the monomer (Figure 3A), some observations similar to Figure 3B (relaxation in the absence of the monomer) have been published for polypyrrole and C10, anion.I0 A detailed study of the open cell potential for PP has been carried our by Beck et al." Polypyrrole has been also investigated as an ion selective electrode material for Clod- ions.12 The relaxation of the polymer can be done in any electrolyte (without the monomer). In such cases an ion-exchange process is superimposed on the other relaxation processes, because the anions and/or cations in which the polymer was formed (here TFB- and TBA+) are exchanged for the bathing anions and/or cations. Not surprisingly, the kinetics and the final potential are different for each electrolyte. We have compared the relative values of the exchange current density (referred to the geometrical area of the electrode) for five electrolytes. The most facile ion transfer (Le., lowest chargetransfer resistance) was found for LiC104. The TBATFB routinely used as the supporting electrolyte was 10 times slower (Table 11). We have also verified the high rate of electron transfer at the same PT electrode using ferrocene (ACN) and ferro/ferricyanide(aq) redox couples. While the difference between exchange rates for previous electrolytes was low and within 1 order of magnitude, the electron-transfer rate was several orders of magnitude faster (not measurable on a stationary electrode). This observation is in agreement with Elfenthal et aI.I3 It should be noted that the comparison of exchange current densities measured on different materials was difficult, presumably due to the different effective area of different preparations. We note also that the shape of most of the recorded I-E curves was far from one corresponding (10) Panero, S.; Prosperi, P.; Scrosati, B. Synth. Met. 1989, 28, C133C137. ( 1 1 ) Beck, F.; Jiang, J.; Kolberg, M.; Krohn, H.; Schloten, F. Z . Phys. Chem. 1988, 160, 83-97. (12) Lu, Z.; Sun, Zh.; Dong. Sh. Electroanalysis 1989, 7, 271-277. (13) Elfenthal, H.;Schultze,J. W.;Thyssen, A. Z . Phys. Chem. 1988, 160, 83-97.
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Figure 4. Relaxation of the red-light reflectance in the virtual absence of monomer (FTfilm rinsed off 5 times). The film was grown up to about 50 mC/cm2. The reflectance was considered to be 0% at the beginning of relaxation (blue film) and 100%at the end (red film). Zero amplitude as measured by the lock-in amplifier practically coincided with the previously defined level of 0%. The thick line is reflectance and the thin line is open cell potential.
0;
0.2
0.4
0.8
0.8
B1
to a Buttler-Volmer kinetics. Some of them were, in addition, asymmetrical (e.g., convex at one side and concave at the other). For these reasons the values of exchange current density given in Table 11 have a limited reliability and can only be used for a qualitative comparison. As for the final rest potentials, no pattern or correlation could be found with the nature of the electrolyte or with its concentration (no ion selective electrode (ISE) type "calibration" was possible). No calibration was possible in the presence of water ( 1 M) in any solution either. We tried to potentiostatically reduce the PT to -200 mV and then to calibrate it, with a similar lack of success. However, all final potential values lay within the range of 30-200 mV in all cases. We note that in the presence of AgCIO,, and especially A g N 0 3 (not included in Table 11) the potential relaxation was found to be much faster than in all other cases: the rest potential was established within a couple of minutes in a 0.01 M AgNOJACN electrolyte. We monitored, in addition to the potential, red-light reflectance (Figure 4), and cell impedance during some of the relaxation experiments. The changes in all these parameters correlated with the change in potential: both the red reflectance and the real part of impedance increased significantly while the potential decreased. If the relaxed PT film immersed in the monomer containing solution is illuminated with white light, the potential jumps almost instantaneously up to about 140-160 mV (curve A, Figure 3). Turning the light off causes an instantaneous change back to the original value. This effect remains reproducible after any number of repetition. Even a very small light intensity can induce a small but instantaneous rise in potential (60-80 mV). To reach 140-160 mV, however, a relatively strong light source is needed. When relaxation took place in the absence of monomer, a similar light effect could be observed, except that the potential jump was slower and much smaller (the final values being still 140-160 mV). When monochromatic light was used, the effect became slower and smaller in the order of blue (420 nm), green (560 nm), and red (630 nm) light. In the case of red light, virtually no effect could be observed. The effects were found to be proportional to the intensity of light in all cases. Current, Color, and Impedance as Functions of Enforced Potentials (Cycling). It is possible to obtain similar changes in color and impedance if, instead of a slow, spontaneous decay in potential, a potential decrease is enforced potentiostatically upon PT. Repetitive potential cycling between 1.2 and 0 V also produces the same effects. In addition to the response belonging to the decreasing potential sweep, the reflectance and impedance changes in the opposite direction (which do not occur spontaneously) can also be determined (Figure 5). The corresponding cyclic voltammograms are presented in Figure 6, at three different electrolyte concentrations. The phenomena causing the changes during the enforced sweeps are analogous to those found during open-circuit decay (Figures 3 and 4). Thus, the understanding of the results
1.2
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1.4
00
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04 0.2
0.4
0.6
0.8
1.0
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1.4
04 0.2
0.4
0.6
0.8
1.0
1.2
1.4
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Figure 5. Changes of the red-light reflectance of PT (A) and the complex impedance of the cell (B) in the absence of monomer at enforced potential sweeps, in 0.1 M TBATFB/ACN. A film grown up to about 300 mC/cm2 was used. Oo and 90° correspond to the in-phase and out-ofphase components, respectively. (A) Red-light reflectance as a function of cycling potential (the reflectance axis is defined in accordance with that in Figure 4). (B) Complex impedance as a function of cycling potential ( B l , at 85 Hz;B2, at 8.5 kHz).
obtained at potential sweeps may aid in the interpretation of the changes at spontaneous relaxation, too. The cyclic voltammograms show the oxidation/reduction of the PT polymer. The process is apparently nonideal (see the broad waves); however, it is reproducible. It is mass transport limited only at low electrolyte concentrations (CO.01 M; see Figure 6).
The Journal of Physical Chemistry, Vol. 94, No. 15, 1990 5987
Electrochemically Deposited Polythiophene. 2 0.1 M 0.01 M
15.0-
12’5-
lo,ol 6
3000
0.M)l M
h
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2000
7.5
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. . . e * *
Figure 6. Cyclic voltammograms of PT in TBATFB/ACN. Films grown up to about 300 mC/cm2 were used. (A) 0.1 M TBATFB (0);(B) 0.01 M TBATFB (+); (C) 0.001 M TBATFB (e).
The nonideality is caused by the fact that the PT film consists of chains of different length and conformation, which is characterized by a spectrum of formal potentials and other parameters. The parallel red reflectance curves show that at cycling, indeed, a “switching” between the oxidized and reduced state of PT happens (PT is red in reflectance in its reduced stateI4). The out-of-phase component is practically constant and zero, which means that the response of the circuit measuring reflectance is virtually instantaneous at 1 kHz. The interpretation of the impedance curves at low frequency (85 Hz;Figure 5B1) are as follows: At high potentials PT is in its oxidized (conducting) state and therefore the real part of impedance of the cell must be much smaller than at low potentials where PT is nonconducting. Even though the change in conductivity is of several orders of magnitude,ls the recorded curves show only a 10-100-fold change. This is caused by the fact that the overall impedance of the cell is determined by its most resistive element, which is not the PT film at high potentials (it is probably the charge-transfer process in the range of 0-1.2 V, in either the absence or presence of thiophene). At 85 Hz the curves belong to the region of Warburg impedances; hence the real and imaginary parts are equal. According to the analysis discussed in part 1, at low voltages the 8.5-kHz point lies on the (depressed) semicircle characterizing the impedance of the film bulk as shown in Figure 7. This semicircle shrinks and practically vanishes when the applied voltage is increased. Accordingly, in Figure 5B2 a trend of a decrease in the real part of impedance can be observed. Simultaneously, the measured point moves onto the semicircle of charge transfer, causing an increase in the imaginary part. (Generally, the coordinates of the 8.5-kHz point change in a complex way as a function of the applied voltage.) Finally, the hysteresis in both color and impedance curves is a natural consequence of the hysteresis in the cyclic voltammograms. Interpretation of the Wet Relaxation Results. The relaxation of potential (and reflectance) as shown in Figure 4 were much faster than those in Figure 3, which correlates well with the film being much thinner in the case of Figure 4. This observation is a first indication that the entire bulk of the film rather than only the film/solution interface is affected by the spontaneous relaxation processes. On the basis of the close similarity of the results obtained with cycling and the corresponding relaxation curves, it is plausible to assume that PT is being reduced also during its spontaneous, slow relaxation. This explains both the color changes (Figure 4) and the increase in cell impedance. We note here that in part 1 the resistance of a relaxed PT film has already been determined: (14) Garnier, F.; Tourillon, G.; Gazardand, M.; Dubois, J. C . J . Elecrroanal. Chem. 1983, 148, 299-303. ( I 5 ) Tourillon, G.:Garnier, F. J . Electroanal. Chem. 1982, 135, 173-178.
Figure 7. Complex impedance plot of the cell with a relaxed PT film on a 7-mm-diameter Pt electrode. (For details see part I , Experimental Section.)
the corresponding impedance plot is shown in Figure 7. The surface area specific resistance of that film (350 mC/cm*) is about 500 Q cm2. The film bulk semicircle is very much depressed, which means that the parameters of PT are widely distributed. This explains also why the cyclic voltammetric waves are so wide (Figure 6). During open-circuit relaxation (Figures 3 and 4) only a chemical (not electrochemical) reduction of PT may happen. As our solutions were not water free (all of them contained about 10 mM water as impurity), one of the possible reducing agents was certainly water. By increasing the water concentration to 0.1 and 1 M, the relaxation as shown in Figure 3B became significantly faster in proportion to the concentration increase. At 1 M, the relaxation speed was at least 10 times larger than with the original solution, and also the rest potential became more negative, as shown in Figure 3C. Thus, it is possible that water is able to reduce PT to a potential close to 0 mV. If the monomer is absent, this reduction can never reach negative potentials. Even when PT was potentiostatically reduced to -200 mV, the final potential slowly drifted back to within 30-200 mV. This means that PT is sensitive to oxidizing agents as well: probably oxygen in the solution does not allow the rest potential to drop below 0 V. However, if other, more powerful reducing agents are present, a further decrease in potential can occur. Such an agent can be a substrate made of a more reactive (less inert) element than Pt or Au, such as, e.g., Ti: the oxidation of this substrate can liberate electrons which then chemically reduce the polymer deposited onto its surface.I6 According to our results thiophene itself seems to be able to reduce PT down to about -450 mV (Figure 3A). In the presence of thiophene, even an inflection point could be observed in the relaxation curve at low potentials ( ~ 2 0 mV), 0 being very similar to an end point in titration curves (Figure 3A). We also recorded potential-time curves (not shown in this paper) during galvanostatic reduction of PT films from 1.2 to 0 V. Similar plateaus and an inflection point could be observed in the same potential range in these curves, too. This provides further evidence that the entire bulk of the film is reduced (“titrated”) during the spontaneous relaxation process. The chemical mechanism of the spontaneous reduction of PT in the presence of thiophene is still t.iclear. It is possible that different oligomers act as reducing agents. They are always present in trace amounts in a thiophene solution, and even more so in a solution previously used for growing PT films. It is known (16) Deng, Zh.; Smyrl, W. H.; White, H. S. J . Electrochem. SOC.1989, 136. 2152-2157.
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The Journal of Physical Chemistry, Vol. 94, No. 15, 1990
that the oxidation potential of any oligomer is much lower than that of thiophene itself.” Generally a whole spectrum of oligomers are present characterized with different oxidation potentials, which can account for the observed very low final rest potential. If thiophene itself acted as the reducing agent, a faster relaxation should be expected because of its high (0.1 M) concentration, similar to the case of I M water. The actual low rate of the process is in agreement with the low concentration of oligomers. This additional role of thiophene and its oligomers can explain also w h y no consistent stable rest potentials and no “calibration pattern” could be found in different electrolytes at the (virtual) absence of the monomer. After deposition, PT could never be rinsed so perfectly that some thiophene (and oligomer) residue would remain in the wet film. The uncertainty in their residual amount can be responsible for the final potentials scattering randomly within the range 30-200 mV. The explanation of the observed light effects is another still unclear issue. There has to be a species in the solution that can exchange electrons/holes with polythiophene during exposure to light. At the very low potential range in question (between -450 and 160 mV), this may again be one or more oligomers. This would also explain why a much weaker light effect could be found when the monomer was (virtually) absent. Apparently, a light with either larger energy or a larger intensity causes a more pronounced effect. It is interesting to note that the applied red light (630 nm) corresponded exactly to the 2.2-eV redox band gap of polythiophenei8 but did cause only a barely detectable effect. This correlates well with the plausible fact that any light effect can only be related to interfacial processes rather than to the bulk of PT (such as, e.g., its oxidation or reduction). Thus, all discussed relaxation processes are bulk processes while the light effect is certainly an interfacial one. We note that when the potential of the already relaxed film is potentiostatically forced back to its original (unrelaxed) high value for a couple of seconds, the relaxation curves can be reproduced in all electrolytes: a slowly relaxing system will relax again slowly, while one that relaxed quickly will again show the same fast stabilization (in, e.g., AgNO,, or in 1 M water/ACN). This means that the relaxation processes are fully reversible and that there is only one factor-the potential-that completely defines the state of the polymer. The same reversibility could be observed also with respect to the light effect; i.e., the changes of both the solutionlfilm interface and the film bulk are reversible. With poly(3-methylthiophene) (PMT) prepared at 1.3 V all wet experiments gave results similar to the analogous experiments with FT,except that the first part of the relaxation (down to about 600 mV) happened much faster than in the case of PT. The interpretation of these results is similar to those of PT.
+
Conclusions The results presented in these papers reflect the complex nature of the parallel reactions and processes of polythiophene (and poly(3-methylthiophene)) films during and after the electrodeposition. The common feature of the electropolymerization done under any conditions is the fact that the resulting material is not in the state of equilibrium. The final state and the “quality” of the film then depends on the relative contribution of all processes both during the growth and during the equilibration. I t has been pointed out beforeI9 that during the growth of the film the faradaic charge is invested in two principal electrode reactions: in the electron transfer which results in the growth of the polymer and in the oxidationlion transfer which tends to restore the charge neutrality in the polymer. A similar conclusion has been reached by Inzelt and HoranyLZDwho treated the polymer growth as a charge-coupled process involving ionic and electronic (17) Eales, R. M.; Hillman, A. R . J . E/ecrroana/. Chem. 1988, 250, 219-223. (18) Bredas. J. L.; Themans, B.: Andre, J. M. Synth. Met. 1984, 9, 265-274. ( I 9) Beck, F.; Oberst, M. Makromol. Chem., Macromol. Symp. 1987,8, 97-1 25, (20) Inzelt. G.;Horanyi. G. J. Electrochem. SOC.1989, 136, 1747-1752.
Hsu et al. charge transfer and supported their claims by radiotracer study. Our view of the mechanism of the formation of these films is described in part 1 and in the section dealing with work function relaxation data. Accordingly, the film is in its oxidized (conducting) state at the end of deposition, with anions taken up in a sufficient amount to ensure macroscopic charge neutrality. However, the fresh PT film is not an equilibrium phase even when viewed as isolated from the environment: this has been proven by the decrease in the electron work function when PT was kept dry and electrically isolated. A slow sterical relaxation (and, possibly, some other processes) may cause this decrease in affinity for electrons. In chemical terms, the PT tends to develop a more reducing character even without any solution contact. When the film is placed in a solution of the supporting electrolyte (with or without the monomer), it approaches equilibrium in the same direction: the PT is slowly reduced further. The same changes happen also when wet relaxation immediately follows deposition. The final rest potential, however, does not depend solely on the degree of reduction of PT: ion-exchange equilibria also contribute to it. A solution/PT interface can be treated also as an ion-selective electrode (ISE), generally with very “poor” selectivity coefficients. Accordingly, the final equilibrium potential will be
where aoxand ad are activities of the oxidized and reduced forms of PT within the PT film, a’is the activity of the “primary ion” of the “ISE” (a’is here unknown and equals 0), and uif is the activity of any “interfering“ ion present (Le., the ions of the supporting electrolyte or protons). The Ki;s are the corresponding “selectivity coefficients”. E’is the sum of the formal potential of the redox process (Eordox)and that of the ion-exchange process + EO,,,. with respect to the “primary” ion (Eoise):E’ = Eoredox The two slopes are Sldox = RT/nrdoxF, and Six = RT/niseF. If a calibration were made in terms of a,, and urd with all activities in the solution kept constant, the first slope could be determined, from which the number of charges per mole involved in the oxidation/reduction of the polymer could be concluded. The ratio aOx/ared can be determined by optical means2’ Similarly, the character (positive or negative) and the charge of the “primary ion” could be determined with a calibration at changing activity of any “interfering ion”. This has been tried in this work, but no clear pattern in the rest potentials was obtained. This is caused by the undefined redox state of the PT (second term in eq 1). It was not possible to fix this state even by adding water or reducing the PT potentiostatically. We believe that the cause of this problem is the presence of residual monomer and/or oligomers in the film during relaxation. When a freshly formed film is rinsed and placed in the supporting electrolyte solution, its open potential decreases generally over a period of hours until it reaches the equilibrium potential. The relaxation times for different ions correlated qualitatively with the exchange current densities obtained for those ions (Table 11). However, we have not been able to establish a unique characteristic relaxation time for each curve. It was not possible to interpret the potential decays in terms of a simple exponential relationship or as a function of the inverse square root of time either. The relaxation process is quite complex and involves many different changes taking place in the layer. At higher water concentrations faster relaxation was found, and in the presence of the monomer a section similar to a titration curve appeared on the relaxation curves. These findings indicate that a chemical reaction, probably reduction of PT by reducing agents, was taking place (Figure 3). It is well-known that the quality of conducting polymers depends very much on the composition of the monomer solution. This is underlined by the findings described above or by the results shown in Figure 6: even the basic mechanism of electrodeposition depends on, e.g., the concentration of the supporting electrolyte and on that of the monomer as well. At higher concentrations a kinetic (21) Cunningham, D. D.;Galal, A.; Pham, C. V.; Lewis, E. T.; Burkhardt, A,; Laguren-Davidson, L.; Nkansah, A,; Ataman, 0. Y.; Zimmer, H.; Mark, H . B. J . Electrochem. SOC.1988, 135, 2750-2754.
J . Phys. Chem. 1990, 94, 5989-5994 limitation prevails while in more dilute solutions mass-transport limitation becomes more important, which evidently results in very different polymers. Even the cell geometry can influence the quality of the films significantly, as was shown in part I . Relaxation processes may also differ accordingly: if, e.g., the supporting electrolyte were more diluted than the monomer, the uptake of anions would certainly become much slower than in our experiments where this process happened to be practically instantaneous. Thus, the physical cause of the slow negative shift in open cell potential during relaxation in this work could not be the slow uptake of anions. Our relaxation results can rather be explained in terms of cations leaving the film, like H+, which is liberated in the reduction of PT by water. This process is interrelated with the decrease in the redox ratio in the second term of eq 1. In order to obtain a film with stable properties it is necessary to equilibrate it in the desired electrolyte solution until its rest potential has been reached. The redox environment (water, oxygen, monomer, oligomers, and substrate) seems to be the principal facter determining the final rest potential, though ion-exchange
5989
equilibria influence this value as well. However, these latter effects remain obscured by the redox ones as long as all redox parameters are not strictly controlled. For any practical application in the oxidized (conducting) state, PT and its surrounding electrolyte must be protected from any external redox effect. In other words, PT and the bathing solution must be sealed tightly, and then its metallic state is as stable as that of, e.g., polypyrrole under normal circumstances (for which the redox capacity of the environment favors the oxidized state). A significant and instantaneous light effect (==600-mVchange upon application of white light) has been observed with PT in the presence of thiophene monomer in the relaxation solution. Further investigation of this phenomenon and that of the unusually fast relaxation found in the presence of Ag+ ions are in progress.
Acknowledgment. This work was supported in part by a contract from the Office of Naval Research. Registry No. PT, 25233-34-5; TBATFB, 429-42-5; ACN, 75-05-8; 7732-1 8-5; Pt, 7440-06-4; thiophene, 1 10-02-1; PMT, 84928-92-7; H20, 3-methylthiophene,61 6-44-4.
Characterization of Acidity in ZSM-5 Zeolites: An X-ray Photoelectron and I R Spectroscopy Study R. Borade, A. Sayari, A. Adnot, and S. Kaliaguine* Dgpartement de gCnie chimique and CRAPS, Universite Laval, Ste-Foy, QuCbec. C I K 7P4. Canada (Received: December 4, 1989)
An X-ray photoelectron spectroscopic (XPS) method is proposed for the identification and quantitation of Br~nstedand Lewis acid sites in ZSM-5 zeolites. The method consists of deconvoluting the N,, XPS level of chemisorbed pyridine and measuring the relative intensities of the peak components. It was found that pyridine is chemisorbed in three different states on ZSM-5 zeolites corresponding to N,, binding energy of 398.7,400.0, and 401.8 eV, respectively. The first peak at 398.7 eV was assigned to N,, level of pyridine adsorbed on Lewis sites, while the second and third were assigned to N1, levels of pyridine adsorbed on relatively weak and strong Brplnsted acid sites, respectively. Comparison of the concentrations of the various acid sites as determined from the relative intensities of the N,, components with IR spectroscopic data showed that XPS has potential applications in the identification and the quantitative determination of Br~nstedand Lewis acid sites in zeolites.
Introduction The activity of zeolites in catalyzing a great number of hydrocarbon transformation reactions is attributed to their acidic character wherein Brensted and/or Lewis sites are involved.' Even though numerous investigations have dealt with zeolite acidity and its relationship to catalytic activity,* recent s t u d i e ~ ~ - ~ showed that our current knowledge of the nature and distribution of acid sites in zeolites as well as their involvement in catalytic processes is far from being thorough. In order to improve our understanding of this area of highly practical importance, one needs to develop quantitative methods for studying the acidity of zeolites. Ideal techniques or combination of techniques should provide a comprehensive picture of the catalyst and acid sites in terms of their nature (Brensted or Lewis sites), numbers and ultimately their strengths. ZSM-5 zeolite attracts much attention because of its unique activity and shape selectivity. The hydrogen form of ZSM-5 zeolite also contains both Brensted and Lewis acid sites in proportions that depend on the activation conditions. Dwyer and (1) Rabo, J. A. Zeolite Chemistry and Catalysis; ACS Monograph 171; American Chemical Society: Washington, DC, 1979 Chapter 8. (2) Jacobs, P. A. Carboniogenic Activity of Zeolites; Elsevier: Amsterdam, 1977. ( 3 ) Fritz, P. 0.;Lunsford, J. H. J. Caral. 1989, 118, 85. (4) Hall, W. K.; Engelhardt, J.; Sill, G. A. Srud. Surf. Sci. Caral. 1989, 42, 1253. ( 5 ) Lombardo, E. A.; Sill, G. A.; Hall, W. K. J . Caral. 1989, 119, 426.
0022-3654/90/2094-5989$02.50/0
OMalley6 reviewed various methods that have been employed to determine the acidity of zeolites, the most important of which are microcalorimetry,' temperature-programmed desorption of amm ~ n i a ? and - ~ infrared spectroscopy.lwl2 IR spectroscopy seems to be one of the most widely used techniques for determining the Brensted and Lewis sites concentrations in zeolites. The only drawback of this technique is that it only leads to relative concentrations of Bransted and Lewis acid sites. In order to determine the absolute concentration of each type of acidic centers, assumptions regarding the mechanism by which Lewis acid sites are generated during dehydroxylation have to be made. Such a mechanism would provide an additional independent relationship between the numbers of Brmsted (B) and Lewis (L) acid sites per unit cell. A dehydroxylation mechanism involving the disappearance of two Br~nstedsites for each Lewis site generated has been adopted by several authors. On the basis of this mechanism, Datka and Tuznik'O used IR spectroscopy to obtain quantitative information (6) Dwyer, J.; O'Malley, P.J. Srud. Surf. Sci. Catal. 1988, 35, 5 . (7) Vedrine, J. C.; Auroux, A.; Dejaifve, P.; Ducarme, V.; Hoser, H.; Zhou, S . J . Catal. 1982, 73, 147. (8) Tops%, N. Y.;Pedersen, K.; Derouane, E. G.J . Caral. 1981, 70,41. (9) Anderson, J. R.; Foger, K.; Mole, T.; Rajadhyaksha, R. A.; Sanders, J . V. J . Caral. 1979, 58, 114. (IO) Datka, J.; Tuznik, E. J. Caral. 1986, 102, 43. ( 1 I ) Jentys, A.; Wareca, G.; Lercher, J. A. J . Mol. Caral. 1989,51,309. (12) Rhee, K. H.; Rao, U. S.;Stencel, J. M.; Melson, G. A.; Crawford, J. E. Zeolites 1983, 3, 337.
0 1990 American Chemical Society
