Physical properties of polypyrrole films containing sulfonated metallophthalocyanine anions

Synthetic Metals, 47 (1992) 167-178

167

Physical properties of polypyrrole films containing sulfonated metallophthalocyanine anions Brian R. S a u n d e r s a, Keith S. Murray ~ a n d R o b e r t J. F l e m i n g b aDepartment of Chemistry, bDepartment of Physics, Monash University, Clayton, Vic. 3168 (Australia) (Received October 7, 1991; accepted in revised form November 11, 1991)

Abstract Thin films of polypyrrole (PPy) containing tetrasulfonated metallophthalocyanine anions (MTsPc*-, M = Cu, Ni and Co) were prepared electrochemically from aqueous solution in a single-compartment cell. The films were deposited on a platinum anode and had thicknesses in the range 20-100 ~m. Their room temperature electrical conductivities were of the order 1-10 S cm -1 and were compatible with a variable range hopping model of interchain charge transport between 77 and 300 K. The conductivities fell by approximately 50% when the films were exposed to laboratory air for one week; this reduction appears to be due to irreversible oxygen attack. ESR measurements suggested intercalation of the MTsPc anions between the PPy chains, while X-ray diffraction data indicated a level of structural order comparable with that of PPy-para-toluenesulfonate (PPyPTS).

Introduction E l e c t r o p o l y m e r i z a t i o n o f p y r r o l e to f o r m p o l y p y r r o l e ( P P y ) in thin film f o r m w a s first r e p o r t e d in 1979 [1]. Since then, a g r e a t deal o f r e s e a r c h on e l e c t r o p o l y m e r i z a t i o n o f pyrrole a n d o t h e r o r g a n i c m o l e c u l e s , e.g., t h i o p h e n e , p h e n y l e n e a n d aniline, h a s b e e n carried out, driven in p a r t b y the large c o m m e r c i a l m a r k e t for light-weight, electrically c o n d u c t i v e and m e c h a n i c a l l y r o b u s t materials w h i c h c o u l d be s u b s t i t u t e d for m e t a l s in a v a r i e t y o f c o n t e x t s [2]. P o l y m e r i z a t i o n o c c u r s at an e l e c t r o d e s u r f a c e (usually Pt o r i n d i u m - t i n oxide) i m m e r s e d in a solvent, s u c h as w a t e r o r acetonitrile c o n t a i n i n g salts o f a n i o n s s u c h as t e t r a f l u o r o b o r a t e , p e r c h l o r a t e , etc. The p o l y m e r i z a t i o n m e c h a n i s m is t h o u g h t to c o m m e n c e with o x i d a t i o n o f t h e m o n o m e r to yield a radical c a t i o n a n d h a s b e e n d e s c r i b e d in c o n s i d e r a b l e detail b y several a u t h o r s [3, 4]. The electrical conductivities o f films g r o w n in this w a y d e p e n d m a r k e d l y o n a n u m b e r of f a c t o r s including the identity o f t h e c o u n t e r - i o n s (anions) i n c o r p o r a t e d in their structure, the t e m p e r a t u r e of e l e c t r o o x i d a t i o n a n d o n the potential applied d u r i n g the p o l y m e r i z a t i o n p r o c e s s , i.e., the d e g r e e o f oxidation; values in t h e r a n g e 1 0 - 1 0 0 S c m -1 are c o m m o n , while v a l u e s m u c h h i g h e r t h a n t h e s e have b e e n r e p o r t e d [5]. P P y films are usually h o m o g e n e o u s a n d a m o r p h o u s , with densities of a r o u n d 1.5 g c m -3. Coul o m e t r i c m e a s u r e m e n t s a n d m i c r o a n a l y s i s d a t a usually s u g g e s t the p r e s e n c e of o n e a n i o n f o r e v e r y t h r e e to f o u r p y r r o l e units in the p o l y m e r .

0379-6779/92/$5.00

© 1992- Elsevier Sequoia. All rights reserved

168 Inorganic anions such as BF4- and CIO4- and p-toluenesulfonate (PTS) have usually been favoured in PPy films. Such films are reasonably stable mechanically and chemically when exposed to the atmosphere, but their electrical conductivities (10-100 S cm -1) are much too small for most commercial applications. In searching for higher conductivities and higher environment stabilities we have investigated a family of anions based on tetrasulfonated metallophthalocyanines (MTsPc). These planar macrocycles are inherently robust and so might be expected to enhance the mechanical stability of the films in which they are incorporated. More importantly, they are electrochromic in their own right and thus their influence on the structure and electrical characteristics of the films may be examined in considerable detail via UV-Vis, ESR and magnetic susceptibility measurements. Some of this work is described below. There have been a few other recent studies of PPy films containing sulfonated metal phthalocyanines as anions, including Cu(II), Fe(III) and Co(II) derivatives, and reference is made to these data where appropriate [6-10].

Experimental Pyrrole (Aldrich) was purified by distillation under reduced pressure prior to use. The tetrasodium salts of copper, nickel and cobalt tetrasulfophthalocyanine (MTsPcNa4) were prepared using a modification of the procedure described by Weber and Busch [11 ]. Tetrabutylammonium perchlorate and the sodium salt of toluene-4-sulfonic acid were used as received. PPy films were deposited on the platinum anode in a single-compartment electrochemical cell operated in the constant potential mode using a BAS Model SP-2 potentiostat. Anode potentials were in the range 0.45-1.20 V relative to a saturated calomel electrode, and the current densities were 0.06-2.20 mA c m - 2. Aqueous solutions containing pyrrole and the appropriate MTsPcNa4 salt at concentrations of 0.2 M and 10 -a M, respectively, were used; these solutions were deoxygenated by nitrogen bubbling before electropolymerization and a nitrogen atmosphere was maintained in the cell during film deposition. The film thicknesses were in the range 2 0 - 1 0 0 ~m. The films were thoroughly washed in distilled water and dried under a nitrogen stream. Microanalytical data on the parent MTsPcNa4 complexes and on the films were obtained by the Australian National University Research School of Chemistry, Canberra, and by the H. Malissa and G. Reuter Analytische Laboratorien, Elbach, Germany. Large macrocyclic complexes such as phthalocyanines and porphyrins are, in our experience, notoriously difficult to microanalyse accurately and reproducibly so considerable effort was put in to obtain reliable data using a number of similar film preparations. It should be noted that the C/N ratio of pyrrole is the same as that for MTsPc, i.e. 4:1. Thus, while this is a useful ratio, it is often more pertinent to obtain

169

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170 C/S or N/S ratios, The analytical values given in Table 1 are representations of the many observed data. The temperature dependence of d.c. conductivity of the films was measured over the range 3 0 0 - 7 7 K using a cell based on the design of Cahen et al. [ 12] by means of a linear four-probe technique. Temperatures were measured using a copper-constantan thermocouple. The films had four parallel 1 m m wide gold strips vacuum evaporated across the surface to which were attached fine copper wires by use of conducting epoxy glue. The normal checks for ohmic behaviour were carried out prior to the variable temperature studies. A JEOL JSM-840A instrument was used at 20 kV for SEM measurements on the growing face of the films. X-ray diffractograms were obtained using Cu Ka radiation on a SCINTAG PAD(V) diffractometer. Electron spin resonance (ESR) measurements on unoriented strips of PPyMTsPc films (M--Cu, Co) were made on a Varian E12 X-band instaxunent at 77 K with microwave frequency of 9.11 GHz. The spectra of the parent MTsPcNa4 complexes were found to correspond to those published [13]. Results and discussion Chemical composition

Following Wynne and Street [14], elemental analysis of the films was carried out before and after drying in vacuum at elevated temperature. Some typical data are shown in Table 1, along with results for the precursor materials CuTsPcNa4 and NiTsPcNa4. As-synthesized PPy films contained 10-11 wt.% H20. The presence of small amounts of excess oxygen was also inferred, consistent with the findings of several workers [14-16] who synthesized PPy films containing various other anions. Presumably this excess oxygen is bonded to the PPy chain. The amounts of hydrogen found were consistently less than those calculated from the assumed empirical formulae, even after drying; Buckley et al. [17] found both excess and deficit of hydrogen in a series of PPy films containing various sulfonate anions. The number of pyrrole units per CuTsPc molecule may be calculated from the C/S ratios as 16.6 and 18.8 for the two PPy films detailed in Table 1. These correspond to average oxidation states for each pyrrole unit of + 0 . 2 4 and + 0 . 2 1 , respectively, a little lower than the range 0.24-0.26 reported by Kuwabata et al. [8]. However, these authors used aqueous CuTsPcNa4 solutions ten times more concentrated than in the present work. In a very recent communication, Walton e t al. [9] have reported mixed CuTsPc 4-/CIO4- doped films prepared in methanol/water (1:3) solution, and obtained analytical data for their galvanostatically prepared 100% PPyCuTsPc film very similar to that given in Table 1 for the film dried under a N2 stream. Conditions of washing and drying were not specified by Walton et al. T h e i r H figure, while higher than that obtained here, was still lower than the calculated value. This group assumed that the O content was all due to water which does not seem likely in view of the present data on predried films. The pyrrole/CuTsPc ratio was the same as that found here.

171

CrystaUinity X-ray diffractograms for PPyPTS and PPyMTsPc (M =Ni, Cu and Co) films are presented in Fig. 1. The broad maximum in the 2~ range 20--26 ° for PPyPTS is indicative of an amorphous material and is believed to be associated with scattering from the PPy chains [14, 18, 19]. Presumably the peak around 25 ° in each of the PPyMTsPc diffractograms has the same origin. (Very little of the CoTsPc material was available and this may explain the more diffuse nature of its diffractogram.) The widths of the peaks observed for the PPyMTsPc films suggest a comparable degree of ordering of the PPy chains to those in PPyPTS, and this is consistent with the higher flotation densities of the present films (1.50, 1.47 and 1.37 g cm -3 for PPyCoTsPc, PPyCuTsPc and PPyPTS, respectively). Rosenthal et al. [7] studied the crystallinity of a PPyCoTsPc film and concluded that it was much higher than that of PPyBF4; this is consistent with the present data. However, these structural conclusions must remain tentative in view of the broad diffractograms obtained. Electron spin resonance spectra The precise molecular geometry of the metallophthalocyanine anions and the PPy chains within PPyMTsPc is still unknown. Rosenthal et al. [7] suggested that a strong interaction occurs between the pyrrole nitrogen and the Co ion in PPyCoTsPc, but it has also been postulated that dimerization of the anions occurs in PPyFeTsPc [20]. Since CuTsPc and CoTsPc, with

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14

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I

I

18 22 26 28(deg)

I

30

34

2200

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2600

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3000 FIELD (gauss)

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Fig. i. X-ray diffractograms for (a) PPyPTS, CO) PPyNiTsPc, (c) PPyCuTsPc and (d) PPyCoTsPc. Note that these are smoothed curves taken from the experimental diffractograms. Fig. 2. E S R spectra for (a) PPyCuTsPc and Co) PPyCoTsPc films at 77 K and microwave frequency 9.11 GHz.

172 d 9 and d T (low-spin) configurations, contain unpaired spins, preliminary ESR measurements were made on PPyCuTsPc and PPyCoTsPc films placed in random orientation in a conventional ESR tube. From such measurements it was hoped that some electronic and structural information, or degree of association, of the anion could be obtained. The spectra measured at 77 K, are shown in Fig. 2. The narrow line at g = 2.00 in the spectrum of PPyCoTsPc is probably due to polarons within the PPy chains [21] rather than to coordinated dioxygen species of type CoTsPc. 02, since the latter normally yield signals containing superimposed 57Co hyperfine splitting [22, 23], albeit of a reduced size. The overall lineshape of the present spectrum is typical of Co n (low-spin) macrocyclic species with a dz2 ground state [13, 23-25]. There is no indication of N-hyperfine splitting on the signals which might be expected [23, 25] if the postulated coordination of the polypyrrole N atom [7] had occurred. The PPyCuTsPc spectrum has a lineshape generally indicative of isolated planar Cu(II) molecules [13], although it is not the same as the spectrum of the CuTsPc precursor which displays a rich hyperfine split lineshape. Any radical signal at g ~ 2 . 0 will be masked by the CuII g± line. No 'half-field' (AMs = 2) signals were obtained as would be expected for dimeric [CuTsPc]2 species [26]. These data are consistent with intercalation of the MTsPc anions between the PPy chains without any significant covalent interactions. Further ESR measurements are being made to elucidate this point.

Morphology Scanning electron microscopy was used to examine the surfaces of PPyCuTsPc and PPyCoTsPc films, and some of the resulting data are presented in Fig. 3. These micrographs are typical of PPy films, revealing small nodules clustered together to form larger structure [27-29].

(a) Co) Fig. 3. Scanning electron micrographs for (a) PPyCoTsPc and (b) PPyCuTsPc surfaces. The scales are shown on the photograph.

173

An optical micrograph of a cleaved cross section of a PPyCuTsPc film, presented in Fig. 4, shows continuous oblique striations similar to those recently observed in PPyPTS [30]. Vork and Janssen [31] also observed similar tube-like structures in PPyPTS films and postulated that they reflected a spiralled macroscopic structure which itself grew out of spiralled PPy chains and was stabilized by the toluenesulfonate anions. The MTsPc anions may likewise be interspaced between such spiralled PPy chains. Thus, the nodules shown in Fig. 3 may be located at the top of tube-like structures formed by such spiralling. The regularity of the striations seen in Fig. 4 is consistent with the higher degree of order discussed above. E l e c t r i c a l c o n d u c t i v i t y versus t e m p e r a t u r e The d.c. electrical conductivity ~ of several films was measured over the temperature range 77-300 K. The data are presented in Fig. 5 in the

Fig. 4. Optical micrograph of a cleaved cross section o f a PPyCuTsPc film. Note that the fine horizontal parallel lines are due to microtome cuts.

1000

100~

'-N

~

.1

Ol

0.)5

0.27

0.)9

T-1/4 (K-1/4)

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0.33

Fig. 5. Electrical conductivity vs. T -°25 for various films: (A) PPyCIO4; (m) PPyCuTsPc; and (Q) PPyNiTsPc.

174

form of plots of In q versus T-°2s, linearity in such a plot usually being interpreted as evidence of the variable range hopping regime [32]. PPyC104 and PPyCuTsPc show such linearity throughout but there is s o m e curvature in the PPyNiTsPc data at lower temperatures. Maddison et al. [33] observed similar deviations from linearity in PPyPTS films and attributed them to changes in the effective 'dimensionality' of the conduction process, larger anion concentrations causing deviations at lower temperatures. For variable range hopping [32] a(T) = (ro exp[ - (To/T) °'2s ]

(1)

where (to is a constant and To is the characteristic temperature. Furthermore, To = 7.64a ~ [kN(Zf) ] - ' and

R(T) = 0.623[kN(Ef)aT]-°2s where N(Ef) is the density of states at the Fermi level, R(T) is the average hopping distance, a : ~ is the electron localization length and k is the Boltzmann constant. After extracting q0 and To from least-squares fits of the Fig. 5 data to eqn. (1), and assuming a -1 = 0 . 3 nm [34], the parameters shown in Table 2 were calculated. Although the N(Ef) values expressed in units eV- z cm-3 are reasonable, the corresponding values in units of eV"1 m o n o m e r - ' exceed the limiting value of 0.03 eV -x monomer -z deduced for PPy from ESR measurements [35]. It may be that the conduction mechanism proposed for PPyBF4 by Watanabe et al. is applicable to the present films [36]. According to these authors the conductivity has inter- and intrachain components, the former involving transfer of charge carriers between adjacent chains and the latter involving transport of the same carriers along the conjugated pyrrole ring system. One would usually expect the former to be the rate-determining process in any system with a low degree of crystallinity, but the intrachain TABLE 2 Best fit p a r a m e t e r s for data of Fig. 5 to the Mort variable range hopping model (see text for parazneter definitions) Parameter

To (104 tO a0 (103 S cm -1) R, 3 0 0 K (/~) N(Ec) (eV -1 cm -3) N(Ef.) (eV -1 m o n o m e r -1)

Sample PPyC].O 4

PPyCuTsPc

PPyNiTsPc

1.02 1.08 2.7 3.2)< 1023 35"

113 13.9 8.8 2.9)< 1021 0.45 b

340 74.2 11.6 9.7)< 1020 0.15 b

abased o n (C4HaN)0.33(CIO4); density 1.51 g c m -a. bBased o n (C4HaN)0.057(MTsPc)0.35(O)0.85(H20); density 1.47 g cm -z.

175

contribution to the conductivity should not be neglected. Only one transport mechanism is envisaged in the variable range hopping model and so it is not surprising that N(Ef) values deduced from this model exceed the theoretical upper limit deduced from ESR data. Since the N(Ef) values for PPyMTsPc are closer to the theoretical limit than those for PPyC104, we deduce that interchain transfer is more dominant in the former.

Stability o f room temperature electrical conductivity of the f i l m s Typical room temperature electrical conductivities of PPyCuTsPc, PPyNiTsPc, PPyCoTsPc and PPyPTS films were 2-6, 2, 1 - 7 and 35 S cm -I, respectively. Clearly, the choice of metal has little influence on the conductivities of the Pc films. Our result for PPyCuTsPc is in good agreement with that of Kuwabata et al. [8] and is significantly higher than those reported by Walton et al. [9] using films grown from aqueous methanol ((r c. 0.05 S c m - I). It has been reported that the conductivity of PPyCoTsPc films increases markedly on ageing [7]. We investigated the conductivity of several of our films that had been allowed to stand in laboratory air for several days and the results are presented in Fig. 6. PPyCuTsPc showed an initial rapid decrease of 50% within one week, the rate being independent of the voltage applied during polymerization ( 0 . 4 5 - 1 . 5 0 V). PPyCoTsPc and PPyNiTsPc also showed a decrease with time. On the other hand, PPyPTS increased very slowly over the entire measurement period, in agreement with the findings of other workers [14, 37]. Several authors have ascribed the decreasing conductivity of PPy films on standing in air to the absorption of oxygen [16, 38]. We investigated this proposal by storing one PPyCuTsPc sample (A) under nitrogen, and a

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~bo

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3bo

4bo

sbo

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Time (hrs) Fig. 6. Effects of ageing at r o o m t e m p e r a t u r e o n the electrical conductivity of (I-1) PPyPTS and ( A ) PPyCuTsPc films.

176

second sample (B) cut from the same film in a dust-free container exposed to laboratory atmosphere. After eight days the conductivity of sample A had increased by 5%, while that of sample B had decreased by 47%, as shown in Fig. 7. Sample A was then exposed to a stream of dry oxygen, as a result of which the conductivity first increased by a further 5% over a period of about 30 min and then decreased rapidly. The original conductivity was not recovered on storing the sample in vacuum, suggesting an irreversible reaction. The initial increase in conductivity could be due to a non-destructive oxidation of the PPy chains similar to that which occurs upon exposure of neutral PPy to oxygen [39]. The subsequent decrease in ¢r is most likely the result of an irreversible reaction which generates a covalently bonded oxygen species [40]. The difference in behaviour between the PPyPTS and PPyMTsPc films could be due to the difference in their anion concentrations. Each pyrrole monomer in PPyCuTsPc has an average oxidation state in the range 0.21-0.24, whereas the corresponding figure in PPyPTS is expected to exceed 0.3 [14, 41]. As indicated above, it has also been shown that neutral PPy reacts readily with oxygen [39], and so the greater sensitivity of the PPyMTsPc films could be due to their lower initial state of oxidation. The environmental stability of PPyCoTsPc films reported by Rosenthal e t al. [7] could also be explained along the same lines; the anion concentration in those films decreased significantly as the films grew and, since their thickness was typically only 0.65 /~m, they would have been more highly oxidized and therefore much less susceptible to oxygen attack. We are presently extending these environmental degradation studies to include the effect of temperature, gaseous environment, etc. [42].

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80" 0 70r- 60"~- 50" 0 400 o~ 30-

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Time (hrs) Fig. 7. Comparison of the effects of ageing under nitrogen and in laboratory air on the electrical conductivity of two PPyCuTsPc films: (O) sample A s t o r e d under nitrogen for 8 days and then exposed to oxygen; (A) sample B stored in dust-free laboratory air.

177

Conclusions The following conclusions may be drawn from this work: (1) Incorporation of metallophthalocyanine anions in PPy films leads to room temperature conductivities between one and two orders of magnitude smaller than those found in films containing smaller related inorganic anions, such as PTS. However, the levels of structural order are comparable. (2) The electrical conductivity of PPyMTsPc films is dominated by interchain transport, which can be satisfactorily modelled as a variable range hopping process. (3) PPyCuTsPc synthesized as described above is unstable towards chemical attack by oxygen on ageing in air. PPyPTS is more stable in this regard. While these physical data on PPyMTsPc films are somewhat disappointing from the solid-state device point of view, it is feasible, despite the low room temperature conductivity values, that such films may have useful application in sensor or modified electrode arenas where their electrochromic and redox features come to the fore. Work is continuing to explore such features.

Acknowledgements The authors wish to acknowledge the receipt of a Commonwealth Postgraduate Scholarship (to B.R.S.) and a grant from the Australian Research Council (to K.S.M. and R.J.F.).

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