Physical properties of polypyrrole films containing trisoxalatometallate anions and prepared from aqueous solution

IL,u,JFfiiTlUI TIIE mlIT ILS ELSEVIER

Synthetic Metals 87 (t997) 237-247

Physical properties of polypyrrole films containing trisoxalatometallate anions and prepared from aqueous solution Natalie S. Allen a, Keith S. Murray a,,, Robert J. Fleming b, Brian R. Saunders a,1 a Department of Chemistry, Monash University, Clayton, Vic. 3168, Australia b Department of Physics, Monash University, Clayton, Vic. 3168, Australia

Received 14 July 1996~revised 1 October 1996; accepted6 December1996

Abstract

Polypyrrole (PPy) thin films containing trisoxalatometallate anions (M(ox)3 3- , M =Cr and A1; ox =C204 z- ) were prepared electrochemically from aqueous solution in a single-compartment celt. Thicknesses were in the range 30-70 Ixm. They were characterized by elemental analysis and diffuse specular reflectance FT-IR spectroscopy. Their structural and morphological features were probed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Room-temperature electrical conductivities ( 11-50 S cm- t) were comparable to those of the well-studied PPy-p-toluenesulfonate system. Some limited studies of the effect of oxygen and nitrogen on the variation of conductivity with time were made. Some of the films displayed excellent conductivity stability over a period of 50 days, comparable to the stability found in other commonly reported polymers. Keywords: Polypyrrole;Transitionmetal complexes;Conductivity;Environmentalstability

1. Introduction

In the last 15 years or so, there has been considerable interest in the development of conducting organic polymers such as polypyrrole, polyacetylene, polythiophene and polyaniline, which have found application as lightweight, electrically conductive and mechanically robust materials in a variety of contexts [ 1,2]. The semiconducting properties of potypyrrole (PPy) were first recognized, in Australia, by McNeill et al. [3] in 1969. Diaz et al. [4] and Kanazawa et al. [5] were the first to prepare a thin film of PPy electrochemically. The physical and spectroscopic properties of PPy have been the subject of a number of reviews [6-11]. A number of factors have been found to influence the conductivity of PPy. These include the nature of the solvent, monomer and counteranion, preparation temperature, applied potential, current density and electrolyte concentration [ 12lS]. Inorganic anions such as BF4-, C104- and .pCH3C6H¢SO3- (p-toluenesulfonate-, P T S - ) are well known as counteranions for PPy films. Conductivities in the range of 10-100 S cm -1 are common, and occasionally * Correspondingauthor. E-mail: [email protected] 1Present address: School of Chemistry,Universityof Bristol, Cantock's Close, Bristol BS8 ITS, UK, 0379-6779/97/$17.00 © 1997 ElsevierScienceS.A. All rights reserved PI1S0379-6779(97)03820-4

higher values have been reported [ 18 ]. The size of the incorporated counteranion has been shown by Yarnaura et al. [ 18 ] to influence the conductivity of PPy. Larger counteranions obstruct interchain charge transport and reduce the overall conductivity. Earlier work in our laboratories [ 6,17,19-21 ] has shown that the incorporated counteranion influences the stability of the conductivity and the effect appears related to the shape and size of the counteranion. Transition-metalcomplexes provide an opportunity to vary systematically the molecular architecture of the anionic species incorporated into PPy and, in this work, we use such complexes in an attempt to improve the conductivity of PPy and the stability of the conductivity in air. These metal complexes often have useful spectral and magnetic features which can be exploited [ 19,20]. In particular, Takakubo [ 13,14] studied PPy films incorporating Cr(ox)33- grown at various temperatures and found that, under certain conditions, films with conductivities of the order 100 S cm - 1 could be produced. Amongst other anionic metal complex systems, Cervini et al. [ 17,21] studied PPy containing planar Ni(CN)42- and linear A u ( C N ) z - counteranions, and reported conductivities as high as 303 S c m - 1 for the latter case. One of the films containing Ni(CN)42showed a decrease in conductivity of about 7% when exposed to the atmosphere over 50 days [ 17 ], whilst a film containing

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N.S. Allen etal./Synthetic Metals 87 (1997) 237-247

A u ( C N ) 2 - showed a tess than 2% decrease in conductivity over a similar period. Saunders et al. [ 19], on the other hand, investigated the physical and spectroscopic properties of PPy containing large, planar tetrasulfonated metallophthalocyanine counteranions, MPcTs 4-. The films of PPyMPcTs exhibited lower conductivities (0.1-10 S cm - I ) than PPyAu(CN)2- and PPyNi(CN) 4a - , and aiso tended to be less stable to the environment with conductivities dropping to as low as 2.5% (for PPyCoPcTs) of their initial value, after 21 days. These observations have led to the postulate that the smaller the counteranion, the higher the conductivity. Saunders et al. [20] recently found that films containing large spherical [metal(III)-(edta) ] - counteranions (edta= ethylenediaminetetraacetato anion) had low conductivities (e.g., 6-11 S c m - ~for Co (edta) - ), as expected for a large counteranion, but with excellent stabilities (conductivities were a little more than the original value, after 100 days). Work performed within our laboratories on a number of PPy films containing a variety of counteranions [6,17,19-21] suggests that the architecture of the counteranion (size and shape) influences the long-term stability of the conductivity when the film is exposed to the environment. Of course the specific preparation conditions employed during each synthesis will influence the structure, morphology and stoichiometry of PPy films and these factors, in turn, affect the rate of diffusion of oxygen (and its chemical attack on the PPy backbone) and hence the conductivity stability. With the aim of testing out some of these structure-property relations it was decided to extend the work of Takakubo [13,14] on trisoxalatometallate anion systems. These medium-sized, almost spherical series of anionic complexes were chosen as counteranions in the present work. To our knowledge, the only published data [ 13,14] consist of some cyclic vottammetry and room-temperature conductivity measurements on PPyCr(ox)3 prepared under various conditions. We have therefore undertaken an investigation of the morphology, bulk structure and environmental stability of the conductivity of PPyCr(ox) 3. In addition, PPyAI (ox)3 has been prepared and characterized for the first time.

2. Experimental 2.1. Reagents

Pyrrole monomer (Aldrich) was vacuum distilled prior to use and stored at - 18 °C under a nitrogen atmosphere, in the dark. The water used in film growth was of 'milli-Q' quality. The sodium salt of toluene-4-sulfonic acid was used as received. The potassium salts of chromium, aluminium, cobalt and iron oxalate (I(3 [ M (ox) 3]" 3H20) were prepared using procedures described by Bushra and Johnson [22] for the chromium complex, McNeese and Wierda [23] for the aluminium complex, and Bailar and Jones [24] for both the cobalt and iron complexes. The complexes were purified by

recrystallization, and compositions and purities were confirmed using magnetic moment measurements, electron microprobe analysis, oxalate analysis, as well as UV-Vis and infrared spectroscopy. The latter spectra were similar to those published elsewhere [25]. All other reagents and solvents were of standard laboratory or analytical grade quality, and used without further purification. 2.2. Polymer synthesis

Polymer films were grown in a large-scale (volume, 100 mt), one-compartment electrochemical cell equipped with gold-plated stainless-steel electrodes, each having an area of (7.5 ?<2.5) cm 2 and spaced 5 mm apart by Teflon spacers. Unless stated to the contrary, aqueous solutions were used which contained pyrrole and the appropriate metal oxalate complex at concentrations of 0.1 and 0.02 M, respectively. Films were usually grown at solution temperatures maintained in the range 0.6--4 °C. Prior to polymerization, the electrochemical cell and electrolyte were purged with nitrogen, and a nitrogen atmosphere was maintained over the solution throughout synthesis. A constant potentialdifference of 0.80 V was maintained between the anode and cathode during film growth, by using either a Bioanalytical Systems SP-2 or Princeton Applied Research model 174A potentiostat. The reference electrode (Ag/AgCI in 3 M NaCl(aq) ) was separated from the bulk electrolyte by a sintered glassfrit arrangement. The half-wave potential of the ferrociniumferrocene couple (measured in 0.1 M tetrabutylammonium perchlorate/dimethyl sulfoxide) was in the range 0.5260.546 V relative to the above reference electrode, and 0.4560.471 V relative to a saturated calomel electrode. The films obtained ranged from 30 to 70 ~m in thickness. The current densities employed were 0.22--0.35 mA cm -2 for PPyM(ox)3 ( M = C r , A1) films and 0 . t l mA cm -2 for PPyPTS. Given the measured thicknesses of several films, and the charge passed during their preparation, the average charge density required to produce a film of 0.5 txm thickness was calculated as 80 mC cm -z. Using this value, the thicknesses of the films studied by infrared spectroscopy were estimated to be 0.4-0.6 ~m. The films grown on the anode were then washed with 200 ml of milli-Q grade water, peeled off and washed thoroughly with a further 2 × 200 ml of water. Samples were then dried either under dynamic vacuum at 120 °C for 24 h (method A) or under dynamic vacuum over P205 for 24 h at room temperature (method B). Because of the experimental arrangement used, the dried films were exposed to the atmosphere for approximately 24 h prior to initial conductivity measurements being taken. In the cases of Co(ox)33-, Fe(ox)33- and VO(ox)a a - , free-standing thick films could be obtained, but a number of side reactions occurred which resulted in poor mechanical and morphological properties being observed; conductivities of 1, 8 and 0.4 S cm - i were noted for each of the systems,

N.S. Allen et al. / Synthetic Metals 87 (1997) 237-247

239

Table 1 Atom mole ratios, growth conditions, initial conductivitiesand film thickness of various polypyrrole metal-oxalate films No.

Polymer

Qo (C cm -2) f

Average film thickness (lxm)

Atom mole ratio g

Oxygen equivalent h

Extent of oxidation

Initial conductivity (S cm -~)

1~ 2 ,,c 3~ 4b 5 a,d 6 b.~ 7~ 8b

PPyPTS PPyCr(ox)3 PPyCr(ox)3 PPyCr(ox)3 PPyCr(ox)3 PPyCr(ox)3 PPyAI(ox)3 PPyAi(ox)3

10.9 10.4 10.2-t5.7 10.0-11.1 11,0 9.3-11.5 11.2 9.6-12.4

66 38 69 69 72 6t 33 33

(C,.ooH3.14Nt.o2)(PTS)o.28 -J -J (C4.ooH3.51Nl.o2)[Cr(ox)3]o.io -J (C4,ooH3.65N1.02) [Cr(ox)3]o.ti -J (C4.00H3.3oN0.95)[Al(ox)3] o.o5

0.12 -J -J 0.48 -J 0.38 -J 0.86

0.28 _J _J 0.30 _J 0.33 _J 0.15

26.4 (12.9-35.8) 12.1 (6.8-16.5) 21.4 (13.5-26.7) 20.8 (11.5-27.8) 11.3 (7.4-13.1) 50.2 (34.2-67.9) 19.0 (9.4-24.0) 18.4 (5.%34.4)

Samples were dried by method 'A', under dynamic vacuum at 120 °C for 24 h. b Samples were dried by method 'B', under dynamic vacuum over P205 for 24 h. ° Grown at room temperature (23.0-23.1 °C), all others grown at solution temperatures maintained in the range 0.6-4 °C. d 0.06 M K3[Cr(ox)3] .3HaO used in synthesis. e Grown in ethanol-water (1:3). All others grown in water. f Electropolymerization charge density. g Values normalized to carbon. h Oxygen equivalent of missing weight. From C/S or C/metal ratios. J Not measured. respectively. A film could not be produced incorporating the free oxalate anion. 2.3. P h y s i c a l methods

Infrared spectra of complexes and detached PPy films incorporated in KBr disks were recorded on a Perkin-Elmer 1600 F T - I R spectrophotometer in the range of 4000--400 cm -~. Diffuse specular reflectance FT-IR spectra were obtained using a Bio-Rad Digi-Lab FTS-60 FT-IR spectrophotometer for very thin films (about 0.4-0.6 ~zm) still attached to the gold-plated stainless-steel anode. The spectra were corrected for the gold substrate background. Microanalyses on selected PPy films were performed by the University o f O t a g o Microanalytical Service ( N e w Zealand) and Galbraith Laboratories ( U S A ) . Prior to analysis, all films were washed continuously for 24 h in 2 × 500 ml of milli-Q grade water and subsequently dried at room temperature over P205 under dynamic vacuum for the same period. The drying procedure was performed again immediately prior to analysis. X-ray diffractograms were obtained using Cu Kc~ radiation on a S C I N T A G P A D ( V ) diffractometer. Scattered intensity data were collected over the 20 range, 2 - 4 0 °. Diffractograms were recorded using both transmission and reflection geometries. Scanning electron micrographs of the growing faces of selected films were obtained on a JEOL JSM-840A scanning electron microscope operating at' an accelerating voltage of 20 kV, and also on a Hitachi S-2300 scanning electron microscope at accelerating voltages of 10 and 20 kV. A n o d e faces o f PPy films were also studied, but were usually featureless and are not discussed here. Electron microprobe analyses were obtained using a JEOL JSM-840A scanning electron microscope. Samples were mounted on

aluminium planchettes and coated with carbon using 'Aquadag' colloidal carbon dispersion. Conductivity measurements were made at 2 0 - 2 4 °C using linear four-probe geometry [26]. As indicated above, the first measurement was always made approximately 24 h after exposure of the film to air. In agreement with the observations of Saunders et al. [ 20] and Takakubo [ 14], the thickness of the films in relation to their position on the electrode was found to increase with increasing depth in the electrolyte solution, an effect which has been attributed to current density variations over the electrode surface. It was found that the conductivity also generally increased with increasing depth in the electrolyte solution. Hence, initial conductivities were measured on four sections of films taken from regions corresponding to the maximum and minimum depths in the solution, and from two intermediate positions. Table 1 shows the averages of these, along with the conductivity ranges representing the minimum and maximum values obtained on four samples of each film, for all the polymers except nos. 3, 4 and 8. In the case of nos. 3 and 4, four preparations were undertaken; for no. 8, three preparations were undertaken, using identical conditions, and the range reflects the averages obtained for each batch.

3. Results and discussion 3.1. C h e m i c a l constitution o f the f i l m s

Table t shows the compositions, determined from elemental analysis, for selected examples of the polymers studied. The formulae shown have been derived directly from the microanalytical data. Analyses of Cr, A1 and S confirmed the

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constitution of the anionic components of the films. The anions are also known to stay intact from the spectroscopic and physical data presented here. Cyclic voltammetric studies on M(ox)33- solutions also show that the anions are stable under the potential applied in film growth. PPy is hygroscopic, so films were dried immediately prior to analysis. The washing procedure of the films also ensured that no residual potassium was incorporated, as confirmed by electron microprobe analysis. The electron microprobe data also showed that the samples were free of chloride, so contamination from the reference electrode could be excluded. Elemental analysis of PPy is frequently complicated by the presence of surplus oxygen and hydrogen [ 19]. The unidentified mass in the present analyses is assigned to excess oxygen. Although each film was dried before analysis, some residual water may have been present, accounting for the excess oxygen and hydrogen. The excess oxygen may have been generated electrochemically [27] and/or atmospheric oxygen may have been incorporated as either carbonyl or hydroxyl groups upon exposure of the films to air. Some of the excess hydrogen may also be due to the presence of saturated carbon atoms and chain ends within the polymer [28]. As seen in Table 1, the extent of oxidation for PPyPTS is close to that observed for films grown elsewhere under similar conditions [ 13]. The amount of counteranion incorporated in PPyCr(ox)3 (film no. 4) is identical to that reported by Takakubo [ 13] despite the fact that much higher electrolyte concentrations were used than in the present study. This result implies that counteranion incorporation is affected by other factors, one of which could be counteranion size. Also, the synthesis conditions of the PPyCr(ox)3 films appear to have little effect on the average charge per pyrrole unit, yet the conductivity of the films grown from an ethanol-water solution (1:3) was twice that grown from water. This result also suggests that counteranion volume affects the extent of incorporation. From this point of view, it is surprising that the amount of Al(ox)33- incorporated is much lower than that of Cr(ox)33-. As each of the M(ox)33. complexes carries three negative charges, the extent of oxidation of the PPy in the three analysed films is in the range normally observed [28]. 3.2. Influence of synthesis conditions on electrical conductivity

Table 1 lists the growth conditions and electrical conductivities obtained for the polymers in the present study. The metal oxalate-doped PPy films (M--Cr, A1) were found to have conductivity values of about 20 S cm - 1, comparable to those obtained for the frequently studied PPyPTS films when grown and dried under similar conditions (see nos. 1, 4 and 8). Perusal of the data for the PPyCr(ox)3 and PPyAl(ox)3 films shows that, under the same synthesis conditions (for example, samples 3 and 7), the conductivity of the films is largely independent of the particular metal present. Saunders

et al. [ 19 ] found that this was also true for PPyMPcTs films (M = Cu, Ni and Co). The Cr(ox)3-doped films were synthesized under several different conditions. When the concentration of the Cr(ox)33- anion in the electrolyte was increased from 0.02 to 0.06 M (see nos. 3 and 5 in Table 1), the conductivity decreased by about one-half. As mentioned previously, a number of researchers [ 13,15,17] have found that the concentration of the anion in the electrolyte affected the film conductivities. For instance, Cervini et al. [ 17] found that the conductivity initially increased with increasing anion concentration, and then decreased. It was proposed that the degree of chain ordering would be lower at higher anion concentrations as a result of increased growth rates. The conductivities of the PPyCr(ox)3 films grown at low temperatures (no. 3) are almost twice those of the same films grown at room temperature (no. 2). The temperature of polymerization appears therefore to affect the bulk structure, i.e., how the PPy chains orientate with respect to the anode surface and the degree to which defects such as branching and crosslinking occur. At lower temperatures, coupling via the c~-carbons is favoured over ~-{3 coupling and, as a result, charge carrier mobility and conductivity are increased. The same effect was observed by Takakubo [ 13,14] in his earlier study on PPyCr(ox) 3 films. It was also found in this work that addition of ethanol to the growth solution had a marked effect on the conductivity. For instance, a PPyCr(ox)3 film deposited from a 1:3 ethanol-water solution (no. 6) had a conductivity two-and-a-half times higher than that of a film deposited from water. A similar solvent-dependent effect has recently been attributed to the marked influence of the solvent on the morphology, molecular ordering and packing of PPy [29]. 3.3. Variation of the room-temperature electrical conductivity of the films as a function of time

A major obstacle to the commercial application of PPy films is the variation of their conductivity with time. It was therefore of interest to determine what influence the incorporated anion had on this behaviour. The variation of the conductivity of the films during the first 30 to 80 days of exposure to air is shown in Figs. 1-3. The conductivities of PPyM(ox) 3 (M = Cr and A1) were respectively 93 and 89% of the initial value after 50 days of air exposure. Fig. 2 shows the time dependence of the conductivities of films prepared using the same anion, but under different growth conditions. It can be seen that the preparation conditions significantly affect the time dependence of the conductivity. The film prepared at room temperature has a greater stability over time. The effect of the film-drying procedure on the time dependence of the conductivity of PPyCr(ox) 3 and PPyAl(ox) 3 is shown in Fig. 3. Drying at 120 °C (method A) postpones the initial fall in conductivity and this may be due to a change in the morphology of the polymer.

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105

I

100

c) 90

75 0

I 10

1

I

20

30

I

I

I

l

I

40 50 60 70 80 90 TIME (DAYS) Fig. 1. Conductivity vs. time of exposure to laboratory air for PPyPTS (no. 1 ( * )) and PPyM(ox)3 (nos. 3 and 7; M = Cr ( • ) and A1 (O), respectively), prepared under similar conditions. Refer to Table 1. 115

i

i10

~

95

90

~ 85 80 30 40 50 60 70 TIME (DAYS) Fig. 2. Conductivity vs. time of air exposure for various PPyCr(ox)3 films grown under different conditions (nos. 2-6 in Table 1): no. 2, grown at 23 °C ( • ) ; no. 3, dried using conditions 'A' (,ik); no. 4, dried using conditions 'B' (O); no. 5, grown using 0.06 M anion ( • ) ; no. 6, grown in a 25% ethanol-water solution ( * ). 0

20

10

105 ~100 95

8o

© ~ 75 ~ 70 10

20

30

40 50 60 70 80 90 Tn~m (DAYS) Fig. 3. Conductivity vs. time of air exposure for PPyAl(ox)3 and PPyCr(ox)3 films dried under different conditions: 'A', heating at 120 °C for 24 h under dynamic vacuum ((©) and ([]), respectively); 'B', drying over P205 under dynamic vacuum at room temperature for 24 h ((O) and ( • ) , respectively). It can be seen in Figs. 1 a n d 2 that some of the trisoxalatometallate-doped PPy films s h o w e d an increasing conductivity w h e n first exposed to air, and then a decrease after about 10 to 15 days. By a n a l o g y to polyacetylene, the initial conductivity increase has been attributed to the attack of m o l e c u l a r o x y g e n on the p o l y m e r b a c k b o n e to form a chargetransfer c o m p l e x i n v o l v i n g the superoxide a n i o n [30]. The

conductivity decrease was suggested to be the product of a destructive oxidation m e c h a n i s m , where the superoxide a n i o n reacts further with the p o l y m e r b a c k b o n e to form carb o n y l groups, thus disrupting the c o n j u g a t i o n [ 31 ]. The C = O groups act as barriers to intrachain charge m o v e m e n t , effectively lowering the m o b i l i t y o f the charge and hence the conductivity.

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N.S. Allen et aL / SyntheticMetals87 (1997)237-247 105 ~100 95 90

80

75 2

3 4 5 TIME (DAYS) Fig. 4. Effect of oxygenand nitrogen on the conductivityof PPyCr(ox)3. To investigate this phenomenon further, a study of the effects of oxygen and nitrogen was carried out on a PPyCr(ox)3 film (Fig. 4). A sample was initially stored under an oxygen atmosphere. After 5 days, the conductivity had decreased by about 17%. The film was then exposed to dry nitrogen, and the conductivity stabilized. The initial conductivity loss is attributed to irreversible oxidation by molecular oxygen as discussed above. 3.4. Scanning electron microscopy (SEM) and film morphology SEM micrographs of the growth surfaces (facing the bulk solution) o f P P y C r ( o x ) 3 (nos. 2, 3, 4 and 6) andPPyAl(ox)3 (no. 8) are shown in Fig. 5. As predicted by Shapiro et al. [32], the surfaces facing the smooth anode were generally found to be featureless and are not shown here, whereas micrographs of the groWth surfaces showed much more detail. These micrographs are typical of many PPy films grown from aqueous solutions, and reveal small nodules clustered together to form larger structures [ 19]. The growth mechanism is believed to involve nucleation and growth. In Fig. 5(a) and (b), the nodule size depends on the metal in the counteranion. Although the PPyAl(ox)3 film had a lower extent of oxidation than the PPyCr(ox)3 film grown

under the same conditions, it exhibited superior morphology (i.e., a more uniform/smoother surface) as exemplified by SEM. X-ray diffraction (XRD) studies also indicated a superior molecular ordering (see below). A higher degree of order within and between the PPy chains should then yield higher charge carrier mobilities which might then compensate for the lower number of carriers compared to the PPyCr(ox)3 film. When the micrographs of PPyCr(ox)3 films grown using different conditions are compared (see Fig. 5(a), (c), (d) and (e)), it can be seen that the surface morphologies also differ slightly. During the syntheses of the films a gradual increase in the number of gaseous bubbles attached to cell components was observed. The gas bubbles probably result from the oxidation of water, to form oxygen, and perhaps from partial oxidation of the oxalate ligand to form carbon dioxide. Such bubbles on the surface of the anode could act as nucleation or polymerization growth centres, resulting in the films having varying degrees of cup-like or bell-like dendrites on their surfaces (see Fig. 5 ( f ) ) . The shape of the dendrites appeared to be related to the counteranion and growth conditions. Notably, the SEM micrograph shown in Fig, 5(g) clearly shows two types of growth. The SEM micrograph in Fig. 5(f) is taken from the edge of a dendrite that had not grown completely

Table 2 XRD data" No.

Polymer

20 (°)

d (/~) b

1 1 4 4 5 6 8

PPyPTS (reflection) PPyPTS (transmission) PPyCr(ox)3 (reflection) PPyCr(ox)3 (transmission) PPyCr(ox)3 ° PPyCr(ox)3 d PPyAI(ox)3

20,4 sh, w; 25.3 br, st 6.0 sh, m; 20.4 br, m 13.0 br, st; 23.0 sh, w; 25.3 br, st 13.0 br, st; 25.3 br, w 13.0 br, m; 23.0 sh, m; 25.4 br, st 9.7 br, m; 22.0 sh, w; 25.3 br, st 11.0 br, m; 22.0 sh, w; 25.3 br, st

4.3, 3.5 14.7, 4.3 6.8, 3,9, 3.5 6.8, 3,5 6.8, 3.9, 3.5 9.1, 4.0, 3.5 8,0, 4.0, 3.5

a st = strong, m = medium, w = weak, s = sharp, br= broad and sh = shoulder. b Evaluated from the Bragg condition. c Grown with 0.06 M anion. a Grown in ethanol-water (1:3).

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243

Fig. 5. SEM micrographs of growth surfaces: (a) PPyCr(ox) 3 (no. 3), showing 'cauliflower' structure typical of films grown from aqueous solutions; (b) PPyAI(ox)3 (no. 8), showing similar, regular nodule structure; (c) PPyCr(ox)3 (no. 6), film grown from a mixed ethanol-water solution (1:3); (d) PPyCr(ox)~ (no. 5), film grown using 0.06 M anion; (e) PPyCr(ox)3 (no. 2), film grown at room temperature; (f) PPyCr(ox)3 (no. 3), large dendrite has grown around a bubble on the anode surface; (g) magnified view of the edge of the dendrite in (f), showing two-dimensional and three-dimensional growth. Micrographs (b), (f) and (g) reprinted with permission from Ref. [41].

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<

2

12

22

20

(o)

32

40

Fig. 6. X-ray diffractograrns for (a) PPyPTS (no. 1 ) using reflection geometry, (b) PPyPTS using transmission geometry, (c) PPyCr(ox)3 (no, 4) using reflection geometry, and (d) PPyCr(ox)3 using transmission geometry.

PPyPTS) [36]. The lower angle maximum in the 20 range 20.4-23.8 ° (4.3-3.9 ,X,) may be due to scattering from sideby-side pyrrole rings [ 36]. It has been suggested that the low angle peaks at 20= 8-13 ° are related to the counteranion [35]. These peaks are pronounced in all PPyM(ox)3 diffractograms. In agreement with Warren et al. [35], two mutually dependent forms of solid-state order appear to be present in the PPy films. The first is associated with order in the polymer backbone itself, and generates the more sharply defined higher angle ( 15-30 °) diffraction peaks. The species responsible for these sharper peaks are located in a more ordered environment. The second type of order is associated with the incorporated counteranions, and generates the lower angle (3-13 ° ) peaks. There is now evidence that film conductivity is related to the degree of molecular order, assuming sharper high angle lines at 20 = 25.3 ° reflect a more ordered structure. For example, PPyCr(ox)3 grown in mixed solvent (Fig. 7 ( d ) ) , and PPyPTS (Fig. 7(a)) possessed higher conductivities and sharper high angle peaks than the film grown at 0.06 M Cr(ox)33- (Fig. 7 ( e ) ) . In this context, it should be noted that Sutton and Vaughan [29] also found that the solvent composition has a marked effect on the molecular ordering and electrical properties of PPyPTS.

over a bubble on the anode surface. It is an example of terrace formation as a result of two-dimensional growth also observed by Fujii and Yoshino [33 ]. The nodule-type clusters formed above the terraces are three-dimensional in character.

3.5. XRD studies offilm structure The physical properties of PPy films are inextricably linked to their molecular organization. For example, charge transport within PPy is comprised of interchain and intrachain components. Thus, the morphology and chain arrangement are expected to play a significant role in determining the electrical transport properties of the bulk material. X-ray diffractograms of PPyPTS (no. 1), several PPyer(ox)3 films and a PPyAl(ox)3 film are shown in Figs. 6 and 7. Associated data appear in Table 2. It should be noted that Mitchell [34] has pointed out that d-spacings for amorphous polymers calculated from Bragg's law are unlikely to be correct, even though it is a common practice

.<

[35]. Broad scattering maxima appear in the diffractograms, indicating that the films are largely amorphous. Nevertheless, peak positions and shape can yield useful information. In all the reflection diffractograms, a broad maximum at 20 = 25.325.4 ° (3.5/~) has been associated with the closest distance of approach of the planar aromatic rings of pyrrole, i.e., faceto-face pyrrole rings, or with pyrrole-toluene rings (in

2

12

22

20

(o)

32

40

Fig. 7. X-ray diffractograms for (a) PPyPTS (no. 1), (b) PPyCr(ox)3 (no. 4), (c) PPyAl(ox)3 (no. 8), (d) PPyCr(ox) 3 (no. 6) grown in ethanolwater (1:3), and (e) PPyCr(ox) 3 (no. 5) grown using 0.06 M anion. All were obtained using reflection geometry. Note that the sharp peaks at about 4 and 6 ° in (e) and (e) are due to reflection from the support, The small sharp peaks around 23 ° probably have the same origin.

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Table 3 FT-IR spectral peaks for PPyCr (ox) 3, PPyAl(ox) 3 and PPyPTS" Assignment [ 16,39,40]

M(ox)33- ~,~(C=O) u7 M(ox)33- v~(C=O) ~'i

(C=C), backbone stretching of pyrrole tings (C=C), backbone stretching of pyrrole rings (C=C), backbone stretching of pyrrole rings Ring stretching Ring stretching

PPyCr(ox)3 (cm- 1) r,t.

PPyCr(ox)3 (cm-i) 1.t.

PPyAt(ox)3 (cm- ~) 1.t.

PPyPTS (cm- i) l.t.

KBr

DSR

KBr

DSR

KBr

DSR

KBr

1720w 1703 w, sh; 1677 m, sh

1714 s 1697 s; 1667 s

1702 w, sh; 1677m, sh

1720 s 1703 s; 1671 s 1690 s 1659 s 1635 s

1727 s 1710 s; t674s 1692 s 1664s 1642 s

1565 s

1729m 1702m; 1669m 1686m, sh 1655 s 1638 s 1623 s 1560 s

1567 s

1652m 1636m 1620m 1560m

1549 m, sh

1544s

t544s

1554 s

1544s

1539 m

1526 s

1526m

1534 s

1528m

1506s

1518 s 1503 s 1487 s 1465 s 1453 s 1415m

1512m

1397 w, sh 1386 m 1304 m, br; l170s

1480m 1459m 1443m 1424m 1403 m 1377 m 1311 m; 1183m

1510 m 1498m 1476m 1460m 1439m 1423m 1402 m 1373 w 1294 m, br; 1165 s, br

1388 m 1305 m; l191m

1090 m

1096 m

1086 m

1091 m

1385 m 1293 m, br; 1186 s, br 1126 m, sh 1094 m, sh

1037 s 966 m; 906 s; 867 s 784 s 699 m 671 m

1047 m 976 w; 909 w

1034 s 964 w; 885 s

1045 m 974 m; 92l m; 867 m

1655m 1637m 1620m, sh 1560m

1641 s 1556 m, sh

1544 m 1532 s 1506 s 1460 m

1471 s 1453 s 1432 s

(C=C) stretch (=C-H) in-plane deformation

1663m, sh 1641 s 1620m, sh

1297 m, br; 1167 s, br

1301 m; 1t60m 1121 m

(C-OH; C-O) stretching (N-H) in-plane deformation (C-H) out-of-plane deformation

1034 s 964 m; 899 s, br

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

779 s, br 670 s

Crystal water?

1038 m 970 w; 907 m; 860 m 794 m 683 w 665 w 652 w

1467 w 1454 w

775 s 690 w 678 w 647 w 624 w 553 w 523 w

614 m 553 m 535 m

M(ox)33-; ring deformation + 8(O--C=O) Ulo M(ox)33-; u(MO) + ring deformation uH

499 m 474 m 445 w 410 w

1477m 1460m 14-41m

1034 s 962 m; 915 s, br; 867 m, sh 779 s, br

670 s

661 w

671 m

619 s

592 w 564 w 542 w

620 m, br 564 m

438 m 476 w 430 w

42t m 482 m

a DSR indicates diffuse specular reflectance FT-IR. KBr pellets of samples were studied by transmission FT-IR. Films grown at room temperature and at low temperature (0.6 to 4 °C) are denoted by r.t. and l.t., respectively. Band intensities are expressed as w (weak), m (medium), s (strong), sh (shoulder) and br (broad). All bands are quoted in cm- 1. It has b e e n s u g g e s t e d that P P y P T S has an a n i s o t r o p i c

side ring spacings. H o w e v e r , this p e a k also m a n i f e s t s i t s e l f

m o l e c u l a r structure [ 3 6 ] , s i n c e d i f f e r e n t p e a k m a x i m a are

in reflection d i f f r a c t o g r a m s ( F i g . 6 ( a ) ) ,

o b t a i n e d w h e n the i n c i d e n t X - r a y b e a m is p e r p e n d i c u l a r

degree

( t r a n s m i s s i o n ) r a t h e r than parallel ( r e f l e c t i o n ) to the film plane. S i n c e the m a x i m a r e p r e s e n t s c a t t e r i n g f r o m structural

this indicates that the global o r i e n t a t i o n o f t h e P P y c h a i n s is

c o r r e l a t i o n s w i t h i n the film p l a n e ( t r a n s m i s s i o n ) and t h r o u g h the t h i c k n e s s o f the film ( r e f l e c t i o n ) , any significant differ-

p r e d o m i n a n t l y a n i s o t r o p i c a n d the c h a i n s h a v e g r o w n parallel to the a n o d e [ 3 6 ] .

e n c e s i n d i c a t e d i r e c t l y an a n i s o t r o p i c m o l e c u l a r o r g a n i z a t i o n . The

peak

(Fig. 6 ( b ) )

in

of

disorder.

Since

the

indicating some

reflection diffractogram

( F i g . 6 ( a ) ) s h o w s o n l y a m a x i m u m at 2 0 ~ 25 ° ( d = 3.5 I t ) ,

A s the a n i o n s in P P y films take up a b o u t 5 0 % o f the v o l u m e

the t r a n s m i s s i o n s p e c t r u m o f P P y P T S

o f the p o l y m e r , their s h a p e is e x p e c t e d to h a v e a s i g n i f i c a n t

at 2 0 = 2 0 . 4 ° (4.3 ,~) c o r r e s p o n d s to s i d e - b y -

e f f e c t on the P P y c h a i n p a c k i n g . H e n c e , for m o l e c u l a r ani-

N.S. Allen et al. /Synthetic Metals 87 (1997) 237-247

246

sotropy to occur, with the pyrrole rings lying preferentially parallel to the electrode surface, a planar counteranion and a planar chain configuration would be required (Mitchell et al. [37] ), and if either component is replaced, then the molecular anisotropy would be disrupted. Thus, small spherical counteranions such as BF4- and SO42- result in isotropic molecular structures [37], whereas planar counteranions such as naphthalene disulfonate promote anisotropic structures [38]. Comparison of the transmission and reflection datafor PPyCr(ox)3 (Fig. 6(c) and (d)) shows that thepeak positions remained essentially unchanged between the two geometries. Thus, the Cr(ox)33- anions, and by analogy the Al(ox)33- anions, promote an isotropic distribution of the PPy chains as a result of their near-spherical shape.

3.6. Infrared spectra of PPyM(ox)3films Well-resolved FT-IR specular reflectance spectra were obtained for very thin (0.4-0.6 txm) films attached to the gold-plated anode. Powdered dendrites (outgrowths perpendicular to the surface of the films) from pre-dried film samples were incorporated into KBr pellets in order to measure their transmission FT-IR spectra. The band positions and relative intensities are recorded in Table 3. A typical diffuse reflectance spectrum of PPyAI (ox)3 is shown in Fig. 8. Peak positions for reflection data were obtained from the inflection point rather than from the peak maxima. In such spectra the inflection points correspond to the peak positions which occur in transmission data. A very broad absorption was observed between 4000 and 2000 cm-1, and this is indicative of the presence of free charge carriers. The v (NH) and u (CH) bands are not visible in the reflection spectra of conducting PPys, as they are masked by the tail of the 4000-2000 c m - 1 peak [9]. When diffuse reflectance spectra are compared with transmission spectra obtained from KBr pellets, the observed peak positions agree well. The region 1720-1635 cm-1 contains a number of metal oxalate carbonyl vibrations. Unlike the work of Cervini et al. [ 17] on PPyNi(CN)4 films, there is

l

no evidence to suggest a strong interaction between the trisoxalatometallate (M = Cr, A1) ions and the PPy chains. The C=O stretching frequencies of the free anions closely match those incorporated into the PPy matrix, indicating little interaction between polymer and anion. Observation of these peaks confirms the incorporation of the counteranions, which were not altered or destroyed during the electropolymerization process. Pyrrole ring vibrations, characteristic of PPy, are dominant in the region 1600-600 cm-1, and these mask the metal oxalate absorptions. However, several anion bands appear below 600 cm- 1 ( see Table 3, KBr disks of PPyCr(ox) 3 and PPyAl(ox)3). In comparing the reflectance spectrum of the PPyCr(ox)3 film grown at 1-4 °C with that grown at room temperature, it was found that the absorption peaks were sharper in the former, probably because more ordered chains were formed.

,

Conclusions

1. Conducting PPy films of the form PPyM(ox)3, where M = Cr and AI, have been successfully prepared and their conductivities measured. The conductivity values were largely independent of the choice of metal and, thus, the presence of unpaired d-electrons on the anion plays no part in the conductivity. When the electropolymerization conditions were varied, the conductivities of the PPyCr(ox)3 films varied considerably. PPy films with poor mechanical and morphological properties containing Co(ox)33-, Fe(ox)33- and VO(ox)2 a- were also prepared. . The stabilities of the conductivities in air of these metal oxalate-doped polymers are good, probably due to their spherical shape, in accordance with related work by Saunders et al. [20]. . As exemplified by the PPyCr(ox)3 film grown from an ethanol-water solution, there appears to be a relation between observed film conductivity, surface morphology (as probed by SEM) and bulk structure (as probed by XRD). The more ordered the film morphology and structure, the higher is the conductivity. . A comparison of the transmission and reflection XRD data for PPyCr (ox) 3 showed that incorp oration of a non-planar anion affected the packing of the PPy chains, resulting in a material with isotropic (bulk) structure.

8

Acknowledgements <

40 0

3 00

3000 2500 2000 Wavenumber (era"1)

1500

1000

500

Fig. 8. FT-IR diffuse reflectance spectrum of PPyAl(ox)3 grown at low temperature,

The authors wish to thank Mr Rod Mackie (Physics Department, Monash University) for the XRD measurements and Mr Adrian van den Bergen (Chemistry Department, Monash University) for the electron microprobe analyses.

N.& Allen et al. / Synthetic Metals 87 (1997) 237-247

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