A study of the electroactivity decay of polythiophene film electrodes

Synthetic Metals, 32 (1989) 141 - 150

141

A STUDY OF THE ELECTROACTIVITY DECAY OF POLYTHIOPHENE FILM ELECTRODES SHENGLONG WANG, KAZUYOSHI TANAKA* and TOKIO YAMABE

Department of Hydrocarbon Chemistry and Division of Molecular Engineering, Faculty of Engineering, Kyoto University, Sakyo-ku, Kyoto 606 (Japan) (Received June 1, 1989; in revised form June 22, 1989, accepted June 28, 1989)

Abstract A study of the electroactivity decay of polythiophene (PT) film electrodes is reported. It has been found that PT film loses its electroactivity when the applied potential is scanned between 0 - 2.2 V (versus SCE) in the monomer-free electrolyte solution. Based on the analyses of the structures and the properties of such degraded PT films, a possible passivation process of PT film has been proposed.

1. Introduction Polythiophene (PT) is one of the most interesting conducting polymers because of its environmental stability against oxidation (compared with, for example, polyacetylene), good electroactivity and high electrical conductivity [1, 2]. PT can be prepared as a powder by chemical coupling polymerization or as a free-standing film through oxidative electrochemical polymerization [3, 4]. Recent development of soluble PT utilizing 3-hexyl- or 3-eicosylthiophene [5, 6] seems to widen further the applicability of this polymer. As to applications of PT, a display device [7], radiation detector [8], rechargeable battery [9], ion-gate membrane [10], etc. have been proposed. For these kinds of purposes, it is most important to examine the endurability of PT during the potential scanning as an electrode material. For instance, a preliminary check of the degradation of PT caused by electrical operation has been attempted by ESCA technology [11], in which it was found that the C(ls) peak (288.6 eV) in the degraded sample remarkably increases in comparison with the C(ls) (288.1 eV) in the doped sample and with the C(ls) (284.5 eV) in the undoped sample. Based on these observations, it has been proposed that the degradation of the PT film is triggered by the F- anion formed by the hydrolysis of BF4-. It has also been reported *Author to whom correspondence should be addressed. 0379-6779/89/$3.50

Q Elsevier Sequoia/Printed in The Netherlands

142 that PT is sensitive to water [12] ; i.e. on deliberate exposure of the polymer to water vapour, there occurs a significant degradation of PT, the electrical conductivity decreasing rapidly over several orders of magnitude in a few hours. Recently, changes in the electrical conductivity and in the capacity of PT films have been studied b y a.c. impedance technology [13], where it has been found that the polarization when larger positive potentials are applied promotes an irreversible decrease in the electrical conductivity of PT. On the other hand, however, there has never been a systematic study of the electroactivity decay including the change of morphology, molecular structure and the electronic properties of PT films. Hence, in this article, we present analyses of experimental results on the morphology, molecular structure and electronic properties of a PT film that has lost its electroactive properties. Based on these results, we propose a mechanism for the loss of electroactivity and conductivity for PT films.

2. Experimental The PT film sample used in Fig. 1 was prepared by electropolymerization on a platinum wire from an acetonitrile solution of 0.1 M thiophene and 0.1 M tetra-n-butylammonium perchlorate (TBAP) by potential scanning twice between 0 to 2.2 V (versus SCE), ending at 0 V. The samples used in Figs. 2, 3 and 4 were similarly prepared on an indium/tin oxide (ITO) plate from the same solution by a constant d.c. current (2 m A / c m 2) method. Then, as a conscious degradation operation, PT-coated electrodes were transferred into a 0.1 M TBAP-acetonitrile solution and potential-scanned repeatedly until the oxidation peak of the PT film at 1.1 V disappeared: the scan ended at 0 V (i.e., undoped) except for the samples used in Fig. 3(d) and Table 4. In order to clearly find the effect of the degradation, the upper potential limit of the scan was selected to be 2.2 V. Throughout this article, the PT sample thus degraded is called DPT. All the electrochemical measurements were performed with the combination of a potentiogalvanostat (Nikko NPGS-301) and a function generator (Nikko NFG-3) under N2 atmosphere, and a saturated Calomel electrode (SCE) was employed as the reference. The Fourier-transform infrared (FT i.r.) transmission measurements were carried out directly on the film samples by a Nicolet 20DXB FT i.r. spectrometer. The u.v. absorption spectra of the PT films on the ITO plate were measured by a Shimadzu MPS-2000 spectrophotometer. The morphology of the PT and DPT films was observed by a JEOL JSM-T220 scanning electron micrograph (SEM).

3. Results

In Fig. 1, the cyclic voltammograms (CV) indicating the degradation in acetonitrile solution of 0.1 M TBAP are shown. The PT film in the first scan

143

0.4

~" 02 E

-02 0-0

1.0 E(V) vs S C E

20

Fig. 1. Cyclic voltammogram of the process to prepare a DPT film by potential scanning in the range 0 to 2 . 2 V (vs. S C E ) . TABLE 1

Effect of change of the positive limit of the scanning potential on electroactivity Positive potential limit (in V v s . S C E )

ipa (after scana)/ipa (before scan)

1.2 1.4 1.6 1.8 2.0 2.2

0.864 0.850 0.842 0.170 ~ 0 ~ 0b

aData after ten scans. b Data after five scans.

shows definite current peaks for the oxidation and reduction reactions at 1.1 V (Epa) and 0.6 V (Epc), respectively, the values of which are well known [14, 15]. But if the upper potential limit exceeds 1.2 V, the electroactivity of the PT film is lost, that is, the current values corresponding to Epa and Epc (ipa and i~c, respectively) gradually decrease. This tendency was found to become remarkable as the upper potential limit increases, as listed in Table 1. The main characteristics of DPT are that the electroactivity of the original PT film is completely lost (ipa and ipe become zero) and that the well-known reversible colour change from pale red (undoped state) to blackblue (doped state) disappears, rendering the film black coloured, poorly adhesive and brittle. These facts imply that the original PT film undergoes a rapid and irreversible electroactivity decay. In the following, the structure and properties of the DPT film thus prepared are described and from which we try to find out the essential aspects of the phenomenon of electroactivity decay of PT films.

3.1. Morphology It is well known that the oxidation and the reduction reactions of a PT film can be controlled by diffusion of the anions of the electrolyte into and

144

out of the film [16]. It is possible that the redox reactions of the PT film are affected by a change of morphology. Hence, we examined the morphologies of both PT and DPT films by the SEM pictures (Fig. 2). The morphology of the PT film is similar to that reported previously [17], but that of DPT shows quite a different morphology, looking flat and less ramentaceous. This macroscopic change of morphology of DPT seems to result basically from the rearrangement of polymer chains. The diffusion of anions through a DPT film seriously influencing the electroactivity ought to be affected by this rearrangement of polymer chains. 3.2. U.v. spectra The u.v. absorption spectra of PT and DPT films are shown in Fig. 3. The undoped PT film has an absorption peak at 488 nm (2.54 eV) corresponding to the 7r-Tr* interband transition, as usual [18], and does not show other remarkable absorptions in the visible region. The doped PT shows in addition to this ~-ir* transition a broad absorption with a peak at 871 nm (1.42 eV), originating from what is called the bipolaron absorption [19 - 21].

7

(a)

(b)

(e)

(d)

Fig. 2. SEM pictures of (a) PT (solution side); (b) PT (electrode side); (c) DPT (solution side); and (d) DPT (electrode side) samples.

145 30

h w(eV) 2.0

o ~.c o OE

O0

I

J

400

I 600

800

Wavelength(rim) 3.0

hw(eV) 2.0

cu

~

0.~

O.C

I 400

h

I 600 Wave length(nm)

,

I 800

I

Fig. 3. U.v. absorption spectra of (a) undoped PT; (b) undoped DPT; (c) doped PT; and (d) 'doped' DPT samples.

As seen in Figs. 3(b) and (d), absorption peaks at 412 nm (3.01 eV) and 409 nm (3.03 eV) are observed in the undoped DPT and the doped DPT, respectively. These absorptions are considerably blue-shifted and rather similar to that of quinquethienyl (416 nm) [22]. The d o p e d DPT has lost the 871 nm peak, that is, the DPT film no longer has the bipolaronic absorption and it is 'not doped' any more. This result is consistent with that of the CV of DPT in the loss of the redox peaks. It is known that quinquethienyl is soluble in organic solvents such as chloroform [22]. However, DPT has been confirmed to be still insoluble in organic solvents.

3.3. I.r. spectra Figure 4 shows the FT i.r. spectra of PT and DPT samples. Main absorption bands and their assignments are listed in Table 2. These assignments are based on those of the 2,5-disubstituted thiophene m o n o m e r [23] and were adopted in the previous report on normal PT [24]. It is seen that DPT further exhibits a strong band at 1700 cm -1 with slight shoulders, that can be attributed to the C = O stretching vibration on referring to the similarity of the PT sample prepared in an oxygen atmosphere [25]. The unidentified bands at 1343, 1212, 1121 and 1031 cm -1 of the doped PT sample have been proved to be independent of the difference in dopant species and contents [24]. In this sense, these bands seem to be similar to the soliton bands in trans-polyacetylene, the origin of which is still highly controversial [26 - 30]. Although these bands are also seen in the undoped PT and DPT, it is highly possible that in these samples the undoping process is still not complete (see Table 3), even after the electrical conduc-

146

q

•

i,~

/

"

\

J,

\

(c) ., ,,j .,"

bl

IJ II , Z.600

I 3000

,

, 1800

I 1000

J,

I 600

Wave number(cm-I) Fig. 4. FT i.r. spectra of (a) doped PT; (b) undoped DPT; and (e) undoped PT samples.

TABLE 2 I.r. band positions (in cm -1) and their assignments Undoped PT

Doped PT

Undoped DPT

Assignments

1523(m) 1491(s)

1523(w) 1482(m)

1523(w) 1482(w)

Ring deformation

1441(m) 1230(w)

1441(s)

1441(m)

1065(w) 785(vs) 697(m) 1343(s) 1040(w)

6 (C--H) in-plane 785(s) 1343(vs) 1212(m) 1121(vs) 1031(vs)

785(m) 1343(vs) 1212(s) 1121(s) 1031(s) 1700(vs)

}

(5(C--H) out-of-plane

~(c=o)

aNot identified; see text.

tivities decrease down to 10 -7 to 10 -8 S/cm (see Table 4). Incidentally, these bands have been assigned to the generation of sulfone and sulfoxide groups in the PT chain [31]. Our theoretical calculations, however, on poly(thiophene dioxide) and poly(thiophene oxide) claim the occurrence of the red shift of the 7r-Tr* transition energy [32] and, hence, we do not accept this assignment here. The appearance of the C= O group in DPT will be discussed later.

147 TABLE 3 Elemental analysis of DPT Elemental composition (in wt.%)

Empirical formula of DPT

C

H

S

C1

Oa

Total

50.43

2.00

31.82

2.46

11.14

97.85

C4.0H1.9S0.94(C104)0. 0600.40

aEstimated value (see text).

TABLE 4 Electrical conductivities at room temperature (in S/cm) Doped PT

Undoped PT

'Undoped' DPT

2.3

1.1 x 10 - s

3.0 × 10 -7

3.4. Elemental analysis and electrical conductivity The results of the elemental analysis of the DPT sample are shown in Table 3. It is obvious that DPT still contains a small a m o u n t of dopant although the redox peaks in the CV cannot be observed. From the above u.v. and i.r. analyses, it is suggested that the C= O group exists in the DPT chain and that the 7r-conjugation length in DPT is similar to that in quarterthienyl. Based on this information, it can be estimated that there is one thiophene ring with a C= O group for every five m o n o m e r units, on average. Assuming that these degraded thiophene rings have two C = O groups, as discussed in the next section, the total elemental composition of DPT would become 97.85%. The electrical conductivities (o) of the samples at r o o m temperature are listed in Table 4. The o value of DPT is larger than that of the undoped PT by one order of magnitude. This seems to have some relationship with the small a m o u n t of C104- anions in the DPT.

4. Discussion The electrochemical oxidation and reduction reactions of PT films are controlled by diffusion processes of the anions of the electrolyte into and out of the film. If the diffusion of the anions through the polymer film cannot be carried out, the film does not show an electroactivity allowing the redox reactions, as a matter of course. The morphology of the polymer film is thus quite important for obtaining good electronic properties. For instance, a conducting polymer such as polyacetylene consists of a fibrillar mat which is about 30% dense. The individual fibrils have a diameter of about 20 nm so that the mat has a very high surface area, typically of the

148

order of 60 m2/g [33]. As shown in Fig. 2, the morphology of DPT is quite different from that of normal PT, which suggests a factor influencing the electroactivity in PT and DPT. The normal PT film exhibits a a value of more than 1 S/cm, induced from its electroactivity since it has a long r-conjugation chain. According to the u.v. analysis of the DPT film, it can be estimated that it has nearly the same r-conjugation length as quinquethienyl. The insolubility of DPT in organic solvents indicates that there still exists a macromolecular chain. I.r. analysis strongly suggests that there are C=O groups in the DPT chain. Based on these results, the molecular structure of DPT would have the following features: the carbon atoms in the thiophene ring react with oxygen producing the C=O groups in the polymer chain and leading to irreversible shortening of the r~conjugation. It is estimated that such a 'degraded' thiophene ring exists in every six m o n o m e r units. According to the above discussion, the passivation process of PT is proposed as illustrated in Fig. 5. There can be two pathways to make the ~conjugation in PT short, that is, the direct oxidation process and the hydrogen migration process. Both pathways eventually lead to the C=O group in the polythiophene chain. The elemental analysis results suggest that pathway (a) in Fig. 5 would not be dominant since the hydrogen composition in the empirical formula of DPT in Table 3 is near to 2.0. The existence of two C=O groups in a 'degraded' thiophene ring is suggested by the stoichiometry of the elemental analysis results given in Table 3 and from the suggested structure based on the mass spectrometry of polypyrrole film oxidized by exposure to air [34]. However, in Fig. 4(b), two branches of the C=O vibration peak characteristic of the planar s-cis diketone do not clearly appear, probably because of a smearing effect from the polymer chain where the degraded thiophene rings with one C=O group would also be present. In this sense, the formation of two C=O groups in Fig. 5 throughout all of the polymer chains should be taken as an 'ideal' or

Fig. 5. Possible passivation processes of PT by (a) direct oxidation and (b) hydrogen migration.

149

extreme case. The oxidation process proposed in Fig. 5 is considered to occur at the carbon radical sites generated at first by the degradation process with even a trace of oxygen remaining in the electrochemical system used for the degradation operation. Thus the u-conjugation length in DPT is estimated to be about that of four thiophene rings, on average, and the bipolaron expected in the doped state cannot be generated because the conjugation length is too short. Consequently, the electrical conductivity does not increase upon doping. It is interesting to note that even in such a situation the i.r. absorption bands characteristic of the 'doped' state are issued. In any case, DPT film produced by the application of excessive positive potential during the electrochemical doping/undoping process becomes 'undopable' in the usual sense of the conductive polymers.

5. Concluding remarks The electroactivity decay in a DPT sample has been experimentally studied. There are five major features in the present study. (i) The redox peaks quickly disappear in the DPT electrode prepared by potential scanning up to a higher limit (2.2 V v e r s u s SCE). (ii) The SEM picture of the DPT sample shows plain and less ramentaceous features compared with that of the normal PT electrode. (iii) The DPT sample shows a larger energy value of the 7r-~r* interband transition maximum (3.01 eV) compared with that of normal PT {2.54 eV), which suggests that the 7r-conjugation length in DPT extends to only about five thiophene rings. (iv) The DPT sample does not show bipolaronic absorption nor high electrical conductivity upon doping. Hence DPT is no longer dopable in the sense of the conductive polymers. (v) A passivation process to degrade thiophene ring to form C=O groups has been proposed. Acknowledgements The authors are grateful to Dr Tokushige Shichiri for his technical help in the electrochemical operations. W e are indebted to Mr Dejung Wang and Dr Koji Segawa for their help in performing the S E M measurements. This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.

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