Langmuir 1991, 7, 556-558
556
Scanning Tunneling Microscope Study of Electropolymerized Polypyrrole with Polymeric Anion R. Yang, K. Naoi, D. F. Evans,* and W. H. Smyrl Department of Chemical Engineering a n d Materials Science, University of Minnesota, Minneapolis, Minnesota 55455
W. A. Hendrickson 3M Company, Corporate Research Laboratory, 3M Center, Building 208-1-01, S t . Paul, Minnesota 55144 Received August 28, 1990 The scanning tunneling microscope was used to study the structure of electropolymerized polypyrrole formed with the large polymeric anion poly(4-styrenesulfonate) (PSS-) on graphite and Au/Si surfaces. The samples were imaged in air and analyzed to address questions concerning the individual chain conformation, film morphology, structure differences between films doped with small and large anions, and the addition and removal of both monomeric and polymeric counterions. The images of the helical chain structure of polypyrrole coated with poly(4-styrenesulfonate)(PPY/PSS) are presented. The results are evidence for crystalline structure of PPY/PSS films at the nucleation stage of growth.
Introduction Conducting polymers have a wide range of useful applications in electronic devices such as batteries, capacitors, ion sensors, and electrochromic devices. Electropolymerization produces good quality electroactive polymers with doped anions. T h e structure of t h e conducting film has been shown t o depend upon both the dopant anion and t h e conditions of polymer formation.' In recent papers2J we described t h e structure of polypyrrole a n d polythiophene doped with small counterions, tetrafluoroborate and p-toluenesulfonate. In this paper, we discuss t h e microscopic structure of electropolymerized polypyrrole formed with t h e large polymeric anion, poly(Cstyrenesu1fonate) (PSS-)on graphite and Au-silicon surfaces a n d compare t h e structures formed with those of small dopant anions.
Experimental Section PPY /PSS films were prepared by electropolymerizationonto the surfaces of highly oriented pyrolytic graphite (HOPG) (from Union Carbide) and on Au evaporatedonto silicon wafers. HOPG sampleswere cleaved in air prior to the electrochemical deposition. All the chemicals used in the investigation were reagent grade (from Aldrich) without further treatment. Double distilled water was used as the solvent. The solution used for the polypyrrole film growth contained 0.1 M pyrrole monomer and 0.01 M electrolyte (NaPSS). This solution was completely purged with nitrogen gas before starting the electropolymerization. The potentials were referred to a saturated calomel electrode (SCE),and stainless steel was used as the counterelectrode. The PPY/PSS films were grown onto the HOPG or Au-silicon surface at a constant potential of 0.85 V by passing 10-20 mC of charge per square centimeter. The polymer films were electrochemically reduced by reversing the current at a constant potential of -1.0 V. During the sample preparation the anode (HOPG or Ausilicon) was partially immersed in the solution so that approximately half of the sample was available for polymer deposition.
* To whom correspondence should be addressed.
(1)Warren, L. F.; Walker, J. A.; Anderson, D. P.;Rhodes, C. G.; Buckley, L. J. J . Electrochem. SOC.1989, 136, 2286. (2) Yang, R.; Dalsin, K. M.; Evans, D. F.; Christensen, L.; Hendrickson, W. A. J . Phys. Chem. 1989, 93, 511. ( 3 ) Yang, R.; Evans, D. F.; Christensen, L.; Hendrickson, W. A. J . Phys. Chem. 1990, 94,6117.
0743-7463f 9112407-0556$02.50/0
Partially submerging the anode electrode in the electrolyte results in a gradation of film thickness appropriate for studying the nucleation and growth of the polymer. All samples were rinsed with distilled water and completely dried in a desiccator after film formation. The samples were analyzed in ambient condition at both constant height and constant current modes with a Nanoscope I1 scanning tunneling microscope (Digital Instruments). The best imaging parameters during scanning were found to be a bias voltage = 50 to 200 mV, a set point tunneling current = 0.5 to 2.0 nA, and scan rate = 3.9 to 8.7 Hz, depending on the conductivity of the film. The contrast was optimized with these settings. In all cases, multiple images were obtained on three to five independently prepared samples. Atomic resolution images of the graphite substrate were routinely obtained to serve as an internal check on the STM apparatus and on the STM tip quality. Similar images of the structures of the polypyrrole doped with poly(4-styrenesulfonate)were observed on both graphite and Ausilicon surfaces.
Results and Discussion I n previous papers we showed that electropolymerized polypyrrole and polythiophene with small counterions exist in a variety of structural forms depending on t h e thickness of the conducting polymer film. Isolated polymer helical chains with pitches of 0.5-0.9 nm and diameters of 1.5-1.8 n m were observed on graphite. Small microislands with diameters u p t o 100 nm and t h e edges of polymer films u p t o thicknesses of approximately 20 nm were found t o consist of crystalline ordered arrays of supercoils. These supercoils have cross-sectional diameters of 5-6 n m with pitches of 2.6 nm formed by coiling of the simple helix described above. At intermediate film thicknesses (2090 nm) a fibular structure was observed which transformed with increasing film thickness t o the nodular amorphous polymer structure associated with bulk conducting polymer materials. T h e sizes of the nodules increased with film thickness and were found to be similar t o the structures observed by scanning electron microscopy (SEM) a n d transmission electron microscopy (TEM). PPY/PSS also displays a variety of structures t h a t depend upon the thickness of the deposited material. T h e morphology of thick films (Figure 1)has t h e characteristic nodular structure seen p r e v i ~ u s l y . On ~ 1000 n m thick
0 1991 American Chemical Society
Electropolymerized Polypyrrole with Polymeric Anion
Langmuir, Vol. 7, No. 3, 1991 557
Figure I. Morphology of the thick film showing a nodular growth where the size of the spherical grains depends on the polymer film thickness. The structure and behavior are similar to those observed previously with 'the small diameter anions, BF4- and TOS-, doped into polypyrrole films. The nodules were part of islands of deposited polymer where size and thickness increase with distance from the gas/liquid junction.
polymer films, the nodules were 100-250 nm in diameter and increased in diameter with increasing film thickness. We were unable to image individual polymer chains in the thick film samples. Before considering the structure of PPY/PSS, we present STM images of NaPSS. Samples prepared by depositing a drop of an aqueous solution of NaPSS (Figure 2a) on a graphite surface followed by drying gave a random distribution of polymer chains without discernible orientation. In contrast, when NaPSS is deposited under the influence of a constant electrical field, 1.0 V and 15 mC/cm2, an ordered array of the polymer chains was obtained (Figure 2b). The average measured width of these polymer chains is 1.20 f 0.09 nm. The random polymer chains were easily removed by rinsing with water; however some of the electrodeposited film remained attached even after several minutes of rinsing with water. Ordered arrays of PPY/PPS polymer chains were commonly observed on microislands and on thin polymer films in all of the preparations. Figure 3a shows an image of several individual chains on top of a thin polymer film. These parallel chains extend for several tens to several hundreds of nanometers. The chain diameter is not uniform, but ranges in width from 2 to 4 nm. However, closer estimation of some of the chains reveals a regular axial periodicity, especially a t the end of the chains or along their sides (Figure 4a). Fourier transform spectra of polymer chains like those shown in Figure 4b yield a characteristic distance of 0.43 f 0.02 nm. This value is close to the dedoped spiral helical polypyrrole chain periodicity of 0.5 nm observed in previous studies.3 These observations suggest that PPY/PSS forms a helical polypyrrole chain with a 0.43-nm axial periodicity which is overcoated by PSS- instead of the PSS- being incorporated inside the helical structure as is observed with small counterions. With small counterions such as BFg, reduction of electrodeposited polypyrrole leads to a decrease in the helical pitch from 0.9 to 0.5 nm, consistent with labile small anions moving into and out of the polymer matrix during oxidation/ reduction processes. In order to clarify counterion behavior of the large polymeric anions during oxidation/reduction cycles, PPY/PSS samples were re-
Figure 2. (a) Image showing the many adsorbed poly(4-styrenesulfonate) polymer chains randomly spreading on the graphite surface. No preferred shape and orientation were observed. (b) When the PSS- is deposited under the condition, which is similar to the PPY/PSS electropolymerization conditions, some of the adsorbed polymer chains started to line up due to the influence of the applied potential on charged PSS-. The average chain width is about 1.2 nm.
duced to different degrees and then examined with STM. No detectable differences in the PPY/PSS structures were observed, demonstrating that PSS- remains associated with the helical backbone. In a second series of oxidation/ reduction experiments, we reduced a PPY/BF4 film and then oxidized it in the presence of NaPSS. No PSS- was incorporated into the film. . We conclude that PSS- can only be incorporated into PPY films during electropolymerization. Two other studies, which will be described in more detail elsewhere,support the conclusionshere. In the first study: PPY films prepared with small anions such as C104- and BF4- were cycled between oxidized and reduced states and in situ mass changes were observed on the quartz crystal microbalance (QCM). Upon oxidation, the small anions were inserted in the films and the mass increased. Upon reduction, the anions were released and the mass decreased. Also, the mass changes were consistent with the amount of charge passed. This is contrasted with the (4) Naoi, K.;Lien, M.;Smyrl, W .H.J. Electroanal. Chem. Interfacial Electrochem. 1989,272, 273; J . Electrochem. SOC.1990, 138,440.
558 Langmuir, Vol. 7, No. 3, 1991
Figure 3. (a) Several chains lying parallel on top of the layer. The PPY/PSS chains showed strong parallel growth in the early nucleation and growth stages, and some of the chains extend to several tens to several hundreds of nanometers long. The chain width, as seen from the image, is not uniform. The measured chain width is between 2 and 4 nm. The exact internal structure and periodicity were not clear from the image. (b) The crosssection profile (across the polymer chain axis as indicated in line) shows two distinguished layers structure, which provides the polymer growth packing information.
behavior of PPY films prepared with PSS- anions. Upon oxidation in NaPSS or NaC104, the mass decreased which indicated that Na+ cations were released from the films and no anions were inserted. Reduction caused the mass to increase as Na+ ions were reinserted into the films. Therefore, the large polymeric anion is trapped in the structure and is not removed by redox cycling, which confirms the STM results above. In the second study,5X-ray photoelectron spectroscopy (XPS) measurements were carried out on PPY films with C104- and PSS- anions. With C 4 - anions (doped to the level of 1c104-/3 pyrrole units), each C104- is associated with an individual pyrrole ring. It interacts with the entire ring, however, and causes the development of inequivalent nitrogens on different rings. Again in constrast, the PSS--doped films contain no such evidence of close interaction. Instead the interaction appears to be of a "disordered" type where there are no inequivalent nitrogens, and the chemical shift of both C and N core level spectra indicate interaction of the large anions with the entire backbone of the PPY structure. The PPY helix cannot incorporate the PSS- anions, so the anion instead (5) Atanasoska, Lj.;Naoi, K.; Smyrl, W. H., submitted for publication in J. Vac. Sci. Technol.
Yang et al.
Figure 4. (a) Extensive examination of the polymer chains in the discontinuous region revealed a number of local areas with regular axial periodicity. This is especially evident at the sides or edges of the chains. Analyses of a number of these polymer chains give an average value of 0.43 nm, which is comparable to the periodicity seen for undoped polypyrrole. (b) The axial crosssection profile (along the polymer chain axis as indicate in line) of the polymer chain provides a better view of the chain regular periodicity. The average pitch value is about 0.43 nm.
wraps around the outside of the helix. This structural arrangement does not permit the close association of the large polymeric anion with the pyrrole units of the helix.
Conclusions By examining electropolymerizedpolypyrrolewith large polymeric anion using STM, we conclude that (1)thick PPY/PSS films have a nodular growth morphology, (2) thin films and microislands of PPY/PSS formed in the early growth stage possess crystalline structure, (3) polypyrrole with poly(4-styrenesulfoante)has a helix chain conformation, in which the polypyrrole spiral helix with about 0.43-nm axial periodicity is coated with poly(4-styrenesulfonate) on the outside of the helical chains instead of being incorporated fully within polypyrrole, and (4) reduced polymer films retain the polymeric anions and cations are incorporated into the polymer matrix to maintain electrical neutrality. Acknowledgment. U.S. Army Grant DA-DAAL-0389-K-058, The Center for Interfacial Engineering (CIE), a National Science Foundation Engineering Research Center, and The 3M Company, a CIE sponsor company, are gratefully acknowledged. ONR/DARPA provided partial support for K.N. and W.H.S. Registry No. NaPSS, 28038-50-8; PPY, 30604-81-0.
