Langmuir 1991, 7, 1447-1452
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Morphology and Structure of Conducting Polymers S. P. Armes* School of Chemistry, University of Sussex, Brighton B N l 9QJ, U.K.
M. Aldissi and M. Hawley Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
J. G. Beery and S. Gottesfeld Electronics Research Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Received September 7,1990. I n Final Form: December 28, 1990 We have examined polyaniline and polypyrrole colloids, polyaniline- and polypyrrole-coated textile substrates, and electrochemically synthesized polyaniline films by a combination of transmission and scanning electron microscopy, scanning tunneling microscopy, and energy-dispersive analytical X-rays. Our results shed new light on various aspects of these systems, including conduction mechanisms, coating homogeneity, and nanomorphology.
Introduction In the last five years the conducting polymer community has focused on relatively air-stable materials, such as polypyrrole and polyaniliie. These polymers have usually been chemically synthesized as bulk powders*-' and composites* and have been electrochemically grown as thin films on various electrodes.*lS In the last few years several novel forms of these conducting polymers have been reported. For example, we have described the preparation and characterization of colloidal dispersions of both p ~ l y p y r r o l e ~and " ~ ~polyani1ine.l"a These systems significantly improve the processability of the electroactive component while retaining reasonably high conductivities. Recently, Gregory et al. described a technique for the surface polymerization of pyrrole or aniline on various
* Author to whom correspondence should be addrewed.
(1) Myers, R. E. J. Electron. Mater. 1986.2, 61. (2)Chaq T. H.; March, J. J. Polym. Sci. Polym. Chem. 1988,26,743. (3)Armes, 5. P.; Miller, J. F. Synth. Met. 1988,22,385. (4)Pron, A.; Genoud, F.; Menardo, C.; Nechtechein, M. Synth. Met. 1988,24,193. (5)Ojio, T.; Mijata, 5.Polym. J. 1986, 18 (l), 95. (6)Yosomiva. R.: Hirata. M.: Hana. Y.:. An,. H.:. Seki., M. Makromol. Chem. Rapid-Commun. 1986,7,69f. (7)Bjorklund, R. B.; Lundstrom, I. J. Electron. Mater. 1984,13,211. (8) Mohammadi, A.; Lundstrom, 1.; Inganas, 0.;Salaneck, W. R. Polymer 1990,31 (3),395. (9)Diaz, A. F.;Logan,J. J. Electroanal. Chem. Interfacial Electrochem. 1990,111,111. (10)Kanazawa, K. K.; Diaz, A. F.; Gill, W.D.; Grant, P. B.; Street, G. B.; Gardini, G. P.; Kwak, J. F. Synth. Met. 1980,1,329. (11)Pfluger, P.; Street, G.B. J. Chem. Phys. 1984,80,544. (12)Genies, E. M.; Syed, A. A. Synth. Met. 1984,10,21. (13)International Conferenceon Science and Technology of Synthetic Metals (ICSM88). Aldisei, M., Ed. Synth. Met. 1989,27-29. (14)Armes, S.P.; Miller, J. F.; Vincent, B. J. Colloid Interface Sci. 1987,118 (2),410. (15)Armes, 5.P.; Aldiasi, M. Polymer 1990,31, 569. (16)Armes, S.P.; Vincent, B. J. Chem. SOC.,Chem. Commun. 1987, 288. (17)Armes, S.P.; Aldissi, M.; Agnew, S. F . Synth. Met. 1989,28,C837. (18)Armes, S.P.; Aldisei, M. J.Chem. Soc., Chem. Commun. 1989,88. (19)Armes, S.P.; Aldisei, M.; Agnew, S. F.; Gottesfeld, S. Langmuir 1990,6,1745. (20)Armes, S.P.; Aldissi, M.; Agnew, S. F.; Gottesfeld, S. Mol. Cryst. Liquid Cryst. (21)Armes, 5.P.; Aldiwi, M. R o c . ACS Division Polym. Mater.: Sci. Eng. 1989,60,751. (22)Armes, S . P.; Aldisei, M. Mater. Res. SOC.Symp. R o c . 1990,173, or 1
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(23)Bay,C.;Armes,S. P.;Pickett,C.;Ryder,C. SubmittedtoPolymer.
substrates, including textiles, glass, and q ~ a r t z Fi. ~ ~ ~ ~ ~ nally, Gottesfeld and his co-workers have described the electrochemical growth of polyaniline f i i s on a chemically modified gold substrate.26 Such films have been shown to be significantly more compact than those prepared previously. In the present work we examined the three new systems, described above, by various techniques, including transmission and scanning electron microscopy (TEM, SEM), scanning tunneling microscopy (STM), and energy dispersive analytical X-ray spectroscopy (EDAX). Our results shed new light on various aspects of these systems including conduction mechanisms, coating homogeneity and nanomorphology.
Experimental Section Systems. 1. Poly(viny1 alcohol)-Polyaniline Colloids. Their preparation has been described in detail in refs 20-22. 2. Poly(viny1alcohol)-Polypyrrole Colloids. Their preparation has been described in detail in ref 14. 3. Polypyrrole-Polyester and Polyaniline-Polyester Textiles. These samples were prepared by using FeCla and NaVOs oxidants, respectively, under previously described condition~.~~" 4. Electrochemically Grown Polyaniline Films. The syntheticprocedure was as previously reported.% The electrode surface was pretreated as follows: The gold substrate was immersed in a 7 m M anilinefbicyclohexyl ether ( 1 5 1 (vfv)) solution for -24 h at 25 O C . The polyaniline films were grown galvanostatically at 5 pA cm-* for 17 and 125 min, respectively. Techniques. 1. Transmissionmicroscopy studieswere made on diluted,dried-downpolyanilinedispersions on carbon-coated copper grids using an EM410 300 keV Philips instrument. 2. Scanning electron microscopy studies were made on goldcoated samples (unless specificallystated otherwise in the text) by using three different instruments: a Camscan series 4, a Philips SEM 505, or a JEOL 100-CTEM with an ultrahigh resolution scanning attachment. The former instrument was used for the EDAX studies. (24)Gregory,R.V.;Kimbrell, W. C.; Kuhn, H.H.;MacDiarmid, A. G.; Epstein, A. J. 3rd Int. SAMPE Electron. Conf. R o c . 1989,570. (25! Gregory,R.V.;Kimbrell, W.C.; Kuhn, H. H.; MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1989,28,C823. (26)Rubenstein, I.; Epetein, A. J.; Riehpon, J.; Sabatani, E.; Gottesfeld, S. J. Am. Chem. Soc., in press.
0743-7463/91/2407-1447$02.50/0 0 1991 American Chemical Society
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Figure 2. Scanning electron micrograph of a solution-cast, freestanding film of poly(viny1 alcohol)-stabilized polyaniline colloid.
Figure 1. Transmission electron micrograph of a dilute, drieddown poly(viny1alcohol)-stabilizedpolyaniline colloid. The scale of the micrograph is indicated by the rice-grain particles which are typically 80 X 150 nm in size.
3. Scanning-tunneling microscopy: All STM images were obtained in air by using a Nanoscope I1 instrument with coldworked PtIr tips a t constant current (high gain). Colloidal silver paint was used to make electrical contact to the samples. With two exceptions, data acquisition was performed in the height mode. For those two cases, tunneling currents were measured relative to graphite (5&log I) to give a pseudorelative conductivity profile. Because of the relatively low conductivity of the sample (compared to graphite or metals), scan speeds were kept to less than 2.5 Hz per line for a 400 X 400 data array to improve the image resolution, minimize sample damage, and reduce image smearing. In addition, tunneling resistances were kept in excess of 100 MQ, with applied bias sample voltages in the range *400 to *2000 mV and set point current generally below 0.3 nA. STM data presented in this work generally received only lowpass filtering, and profile views were generated by using a computer graphics package. No obvious differences were noted in the nanomorphology on reversal of the sample bias voltage polarity. Atomic resolution was not achieved, and only poor image quality was obtained, even after offline software filtering. All samples appeared to be unchanged over several months. We believe that adsorbed contaminants, solvent residues, uncertain tip quality, and the presence of loose material (particularly for the colloid samples) probably account for most of the resolution limitations.
Results and Discussion (i) Polyaniline Colloids. A typical transmission electron micrograph of the dried-down dispersion is shown in Figure 1. The individual particles have a "rice-grain" morphology (average length 140-180 nm, average width -60-90 nm) as previously A scanning electron micrograph of a solution-cast free-standing film is shown in Figure 2. This image was taken near the instrument's resolution limit but, nevertheless, suggests
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Figure 3. Constant current-height STM image of a solutioncast, free-standing film of poly(viny1 alcohol)-stabilized polyaniline colloid.
that the polyaniline particles retain their morphological integrity within the film during the drying process. This situation is verified by our scanning tunneling microscopy studies. Figure 3 shows a constant current-height image. The much higher resolution of this technique clearly shows both the "rice-grain" particulates and their internal structure. They appear to be composed of small globular or spherical nuclei of approximately 5-10 nm diameter. The macroscopicsolid-statedc conductivity of the polyaniline colloids (either as solution-cast films or as compressed pellets) typically lie in the range 0.1-1.0 S cm-l. These conductivities are approximately 1 order of magnitude lower than those of bulk polyaniline powders or films (1-10 S cm-l). However, in view of the long-term stability of the polyaniline colloids toward particle aggregation (precipitation), it is clear that, in the dispersed phase, each particle must be completely covered with a thick adsorbed layer of solvated, electrically insulating poly(viny1 alcohol) s t a b i l i ~ e r . ~Thus, ~ , ~ ~it is perhaps surprising that such composites exhibit reasonably high
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Morphology and Structure of Conducting Polymers
E Figure 4. High-resolution STM current image of a solutioncast, free-standing film of poly(viny1 alcohol)-stabilized polyaniline colloid.
conductivities. Our STM studies offer a tentative explanation for this observed macroscopicconductivity. Figure 4 shows a high magnification STM current image. The polyaniline particles appear as valleys in this mode because the software analysis is calibrated to the conductivity of graphite and therefore yields a profile relative to the graphite conductivity (see Experimental Section). The periphery of the particles are the mountain tops and are distinctly lighter in tone than the other topograhical features. Since the tunneling current has an exponential dependence on distance, this suggests that interparticle contacts are more conducting than the top-side surface of the particles and that the observed macroscopic conductivity is therefore due to efficient interparticle chargetunneling processes. Because the PVA acts as an insulating layer between the STM tip and the conductive polymer, changes from spot to spot in the image give a pseudomap of the relative thickness of the insulating coating. High points indicate greater tunneling current and therefore a thinner layer of the insulator. For comparison Figure 5 is a computer-generated 3-D topographical rendering of the same material but in the height mode with no reference to the electrical property of another metal. This observation implies that the adsorbed layer of insulating poly(vinyl alcohol) stabilizer is nonuniformly distributed over the surface of each particle in the solid state. The redistribution of the poly(viny1alcohol) coating must occur during the drying process, and it is rather surprising in view of the likely chemical grafting of the stabilizer to the polyaniline particle We note that the above explanation is consistent with the observed weak pressure dependence of the dc conductivity of poly(viny1alcohol)polyaniline colloid films29(if the particles were uniformly coated with an insulating poly(viny1alcohol) layer, a greater pressure dependence of the conductivity might be expected since electron tunneling between these layers would become much more favorable as the interparticle distance decreased). (27) Hunter, R. J. Foundations of Colloid Science; Clarendon Press: Oxford, 1933;Vol. 1. (28)Napper, D. H. Polymer Stabilization of Colloidal Dispersions; Academic Press: London, 1983. (29)Armes,S . P.; Aldissi, M.; Aronson,M. Manuscript in preparation.
Figure 5. High-resolution STM height image of a sample similar to that in Figure 4. 4 J*”’.#&y$qr-** -? I
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f Figure 6. Scanning electron micrograph of a solution-cast, freestanding film of poly(viny1 alcohol)-stabilized polypyrrole colloid. The particle diameters are -100 nm.
(ii) Polypyrrole Colloids. A typical scanning electron micrograph of a solution-cast, free-standing poly(viny1 alcohol)-polypyrrole colloid film is shown in Figure 6. Again, it is evident that the morphologicalintegrity of the polypyrrole microspheres is retained during the drying process. Our STM studies were made on similar films, for which the mean polypyrrole particle diameter was -100 nm. A typical high-resolution constant current-height image is shown in Figure 7. The curvature of the polypyrrole particles has been artificially flattened by the software analysis. Clearly, each particle is composed of much smaller nanosized particulates of 5-10 nm diameter.
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Figure 9. High-resolution STM height image of the same area of the sample in Figure 8. Figure 7. High-resolution constant current-height STM image of a solution-cast, free-standing film of poly(viny1 alcohol)stabilized polypyrrole colloid.
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Figure 10. Scanning electron micrograph of a polypyrrolepolyester textile composite (example from Milliken Research Corp.).
Figure 8. High-resolution STM current image of a solutioncast, free-standing film of poly(viny1 alcohol)-stabilized polypyrrole colloid.
A STM current image is depicted in Figure 8. Like the polyaniline colloids previously discussed, the peripheries on the surface of the individual polypyrrole particles seem to be more conducting than the top-side surfaces. Again, we interpret this as being caused by some redistribution of the insulating poly(viny1 alcohol) outer layer (which is believed to be physically adsorbed oia a hydrogen-bonding type interaction) during the drying process. This results in a nonuniform stabilizer coating in the solid state and hence in macroscopic conductivities of approximately 0.5 S cm-l. Figure 9 is an image of the same area of the film as in Figure 8 but in the height or topographical mode. (iii)Polypyrrole-CoatedTextiles. Both polypyrroletextile and polyaniline-textile composites have been prepared and characterized by Gregory et al.24$25This group claimed that, because of specific adsorption processes not fully understood, the conducting polymer coating was remarkably smooth, featureless, and homogeneous, even at the submicrometer level. Their low magnification (X2500) SEM images supported this view
and, indeed, for certain polypyrrole samples, it was shown that selective dissolution of the textile substrate yielded a free-standing, thin, continuous film of the conducting polymer. In addition, XPS studies provided good evidence that the polypyrrole overlayer was considerably more ordered (contained fewer a@ cross-links) than bulk polypyrrole films or powder. Our SEM studies on polypyrrole-polyester textiles (kindly provided by Milliken Research Corp.) were undertaken at rather higher magnification than those of Gregory et al. They confirm that the conducting polymer coating is generally smooth and uniform (see Figure 10). However, there are also not insignificant areas of distinctly globular deposits which are randomly distributed along the textile fibers (Figure 11shows a close-up image of one of these deposits). Their morphology strongly resembles that of conventional chemically synthesized bulk polypyrrole powder films.2$30-32Because of the intrinsic conductivity of the polypyrrole-textile composite,we were able to obtain our SEM images without recourse to sputtercoating the sample. Thus, we were able to compare the elemental compositionof both the smooth and the globular (30) Armes, S. P. Unpublished results. (31) Armes, S. P.; Aldissi, M. Synth. Met. 1990, 31, 137. (32) Rueda, D. R.; Arribas, C.; Balta Calleja, F. J.; Fierro, J. L. G.; Palacios, J. M. Synth. Met. 1989, 28, C77.
Morphology and Structure of Conducting Polymers
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Figure 11. High-magnification scanning electron micrograph of a polypyrrole-polyester textile composite showing globular clusters of bulk polypyrrole.
regions of the textile surface using energy-dispersive analytical X-ray (EDAX) techniques. We observed only a sulfur peak and a very small chlorine peak (just above background) for each region. We draw two important conclusions from these results. First, the globular features are, indeed, bulk polypyrrole deposits. This suggests that the pyrrole polymerization proceeds predominately, but not exclusively, on the surface of the textile substrate, with a relatively small remaining fraction of monomer being polymerized in solution. It is possible that a minor modification in preparation procedure (change in monomer, oxidant concentration, etc.) might well result in a more optimized surface polymerization process. Alternatively, the textile surface may be influencing the order and morphology of the conducting polymer overlayer. If this were the case, this "local" effect should be reduced for increased coating thicknesses, and eventually the overlayer morphology would be expected to revert to that of conventional chemically synthesized polypyrrole. Interestingly, we have observed a similar surface effect on the morphology of electrochemically synthesized polyaniline films (see following section). Our second conclusion is that the aromatic sulfonic acid additive(s) used to moderate the pyrrole polymerization kinetics in the preparation of these textile composites is clearly incorporated as the major dopant counterion, in preference to chloride anion. Similar observations were made by Gregory et al. in their XPS studies.33 This would explain the reported superior environmentalstability (slower decay of conductivity) exhibited by the polypyrrole-textile samples relative to bulk chloride-doped polypyrrole powder,25because it is well-known that aromatic sulfonic acid dopants significantly enhance the conductivity stability of polypyrrole.l5JO We examined polypyrrole overlayers deposited on various materials (polyester, nylon, glass, and quartz-all samplesprovided by Milliken Research Corp.) by scanning tunneling microscopy. A typical constant-current height image for a polypyrrole-polyester composite is shown in Figure 12. In all four samples, the polypyrrole overlayer appeared to be composed of nanosized spherical particulates (5-10 nm diameter) similar to those observed in the polypyrrole colloid system. Qualitatively, the nature of the substrate surface appears to promote ordering of these (33) Kuhn H. H. Milliken Research Corporation. Private communication.
Figure 12. Constant current-height STM image of a polypyrrole-polyester textile composite (example from Milliken Research Corp.).
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Figure 13. Constant current-height STM image of a polyanilinepolyester textile composite (example from Milliken Research Corp.).
particulates in the following sequence: quartz 2 glass 2 polyester 2 nylon. We also examined a polyaniline-polyester textile sample (Milliken Research Corporation) by STM (see Figure 13). Again, we observed partially ordered, nanosized particulates (2-20 nm diameter). In this case it is tempting to suggest that these particulates are actually "metallic islands" (predicted to have dimensionsof 200 A)of highly conducting polyaniline separated by thin "insulating beaches". This granular metal description of polyaniline has been recently proposed by MacDiarmidand co-workers If this on the basis of various physical
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(34) Epstein, A. J.; Ginder, J. M.; Zuo, F.; Bigelow, R. W.; Woo,H. S.; Tanner, D. B.; Richter, A. F.; Huang, W. S.; MacDiarmid, A. G. Synth. Met. 1987, 18, 303. (35) Zuo, F.; Angelopoulos,M.; MacDiarmid,A. G.;Epstein, A. J. Phys. Rev. B: Condens. Matter 1987,36,3475. (36) Zuo, F.; Angelopoulos,M.; MacDiarmid,A. G.;Epstein, A. J. Phys. Rev. B: Condens. Matter 1989,39, 3570. (37) Kuzmany, H.; Sariciftci, N. S.; Neugebauer, H.; Neckel, A. Phys. Rev. Lett. 1988. 60. 212.
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Figure 14. Scanning electron micrograph of a thin electrochemically synthesized polyaniline film (growth time 17 min).
Figure 16. Scanning electron micrograph of a thick electrochemically synthesized polyaniline film (growth time 125 min).
20 nm diameter) similar to those observed in the polyaniline colloid and polyaniline textile samples. Interestingly, the thicker polyaniline film has a much rougher surface morphology,with regular features of submicrometer dimensions clearly visible by SEM (see Figure 16). STM images of this latter samplewere essentially identical with those of the thinner film.
Figure 15. Constant current-height STM image of a thin electrochemically synthesized polyaniline film.
is correct, the work of Travers et al.40would suggest that each particulate is actually a single chain of polyaniline. (iv) Electrochemically Synthesized Polyaniline Films. The thickness of the electrochemicallysynthesized polyaniline films were estimated to be 30 nm (17 min growth) and 58 nm (125 min growth), respectively, using Rutherford backscattering techniques. The SEM image of the thinner film was completely featureless, even at a magnification of 40 000 (see Figure 14). Fortunately, the greater resolving power of the STM allows us to examine the surface topography in some detail (see Figure 15).The polyaniline film is made up of clusters of nuclei (50-250 nm diameter) each of which contain nanoparticulates (2~~
(38) McCall, R. P.; Roe,M. G.; Ginder, J. M.; Kusumoto, T.;Epstein, A. J.; Asturias, G. E.; Scherr, E. M.; MacDiamid, A. G. Synth. Met. 1989, 29, E433. (39) Javadi, H. H. S.;Cromack, K. R.;MacDiarmid,A. G.;Epstein, A. J. Phys. Rev. B: Condens. Matter 1989,39,3579. (40) Travers, J. P.; Genoud, F.; Menardo, C.; Nechtschein, M. Synth. Met. 1990, 35, 159.
Conclusions 1. Our scanning tunneling microscopy (STM) studies clearly show that all of our polyaniline and polypyrrole samples are composed of nanosized particulates of 2-50 nm diameter. This nanomorphology seems to be a truly intrinsic property of these conducting polymers because it is apparently independent of the preparation conditions (chemical or electrochemical synthesis, surface or bulk polymerization, etc.). 2. STM current images of both polyaniline and polypyrrole colloids indicate that the distribution of poly(viny1 alcohol) coating is not uniform over the top surfaces of the particles, which suggests that some redistribution of the adsorbed poly(viny1alcohol) stabilizer layer occurs during film formation. 3. The polypyrrole textile composites consist of a relatively smooth overlayer, together with some globular deposites of the conducting polymer. We interpret the presence of the latter features as evidence that the pyrrole polymerization does not necessarily proceed exclusively at the surface of the textile substrate. Our EDAX studies confirm that the incorporated dopant anion is almost exclusively the aromatic sulfonic acid additives used to moderate the pyrrole polymerization rather than the chloride anion derived from the FeCl3oxidant. The nature of the preferred dopant anion accounts for the reported environmental stability of these composites. Acknowledgment. We gratefully acknowledge the following people for their contributions to this work: C. Mombourquette (SEM), Bob Sebring and J. Thorpe (TEM), and J. Smith (EDAX). Milliken Research Corporation is thanked for their generousgift of the conducting polymer textile samples. S. Armes (University of Sussex) wishes to thank the SERC for a travel grant which made this collaboration possible. This work was funded, in part, by the U.S. Department of Energy, Advanced Industrial Concepts Division.
