11580
J. Phys. Chem. 1993,97, 11580-11582
Langmuir-Blodgett Films of Processable Polyaniline M. K. Ram,. N. S. Sundaresan, and B. D. Malhotra National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi 110012, India Received: September 7, 1993'
The deposition of quasi-ordered Langmuir-Blodgett films of preformed processable polyaniline is described. The development of surface irregularities during multilayer deposition resulting in poor redox kinetics of the deposited film is also discussed.
Processable conducting polymers have been extensively investigated for device applications.'.* The deposition of ordered monomolecular layers of conducting polymers by the LangmuirBlodgett (LB) technique is an important goal in this c~nnection.~ Conventionally, a lipophilic fatty acid tail is attached to the monomer to facilitate the deposition of an oriented layer; subsequently, the monomer is polymerized by suitable means,4 or the preformed polymer (e.g., polyalkylthiophene) mixed with stearic acid is dissolved in a suitable solvent and depo~ited.~ The former, however, usually requires a difficult chemical manipulation, and in addition, the polymeric films thus produced tend tocrack. Recently, in an attempt to obviate the abovedifficulties, Agbor and co-workershave reported the deposition of preformed polyemeraldine base (PEB) by the LB technique.6 In their method, polyaniline dissolved in N-methylpyrrolidinone(NMP)/ CHCl3 can be cast as a film with an aqueous subphase containing acetic acid. However, we have found that an LB film can be deposited without acetic acid and can be subsequently doped. In this Letter, we report the results of deposition of such LB films. We have not used stearic acid to orient the polymer. Multilayer films with varying number of layers have been deposited in an attempt to study the changes in surface upon going from monolayers to multilayers and to compare the results with a theoretical study7which predicted the developmentof an irregular surface after a certain number of layers. Additionally, the deposited films have been examined using cyclic voltammetry in an acid medium in order to assess the effect on the redox characteristics of changes in surface morphology in going from thin films to thick multilayers. The LB film of PEB was deposited using a Joyce-Loebl LB trough (model 4). The emeralidine base waschemically prepared as reported elsewheren and is dissolved in NMP/CHC& solvents. Though the material thus prepared may have some oligomeric impurities and thus a distribution of molecular weights (50 000100 000), this factor by itself is not expected to hinder the deposition of the LB film. This has been observed in other experimentsinvolving LB f i l m ~ . ~Indeed, J ~ visually smooth films have been deposited as expected from the nature of the pressurearea isotherm. Deionized water (Millipore, pH 7)has been used for the aqueous subphase. A dipping speed of 3 mm min-1 has been employed to effect Y-type deposition with a drying time of 4 min between successive dips. The pressure-area isotherm for the deposition is shown in Figure la. The area per molecule is 16 A*based on the molecular weight of the aniline repeating unit at an optimum pressure of 25 mN/m. Since a value of 23-26 A2 is estimated for substituted anilines," this suggests that monolayers are deposited, at least in the initial stages. The deposition of successive layers was monitored by the change in optical absorption at 360 nm as shown in Figure 2. The nearlinear variation in the initial stage is an indication of thedeposition ~
*Abstract published in Advance ACS Absrrucfs. October
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Number of Layers Figure 4. (top) Cyclic voltammogram of LB films of polyemeraldine on I T 0 in hydrochloric acid (1 M) medium: six-layer LB film (-); 38layer LB film (- - -). A voltage ramp of 100 mV/s has been employed. (bottom) Plot of Ep - E p p versus the number of layers. Figure 3. (a, top) Scanning electron micrograph of a six-layer polyemeraldineLB film on indium tin oxide (ITO) glass substrate. (b, bottom) Scanning electron micrograph of a 38-layer polyemeraldine base film on ITO.
transfer ratio is nearly unity, leading to Y-type deposition. An interesting aspect here is that beyond the 10th layer the area displaced from the trough gradually decreases, indicating the onset of irregular deposition. This result is consistent with the theoretical calculations of Momose and Hirai7 using Monte Carlo methods which predict the development of inhomogeneous film surface after a certain number of layers. The electron micrograph of two such LB films (6 and 38 layers) show the smoothquasi-ordered nature of the six-layer film (Figure 3a). The development of large pinholes of the order of about 0.5 pm in diameter with irregularities in the surface is clearly visible in the case of the 38-layer film (Figure 3b). The electrochemistry of the LB films prepared as above has been investigated using cyclic voltammetry. The cyclic voltammogram (CV) of an LB film of PEB containing six monolayers in 1 M hydrochloricacid medium is shown in Figure 4. The peak currents scale linearly with sweep rate indicating a surfaceconfined species, though the peaks are broader than expected for such a situation. In the same figure is shown the CV of an LB film of PEB of 38 layers which exhibits only a broad hump from -0.5 to about 0.8 V. In an attempt to understand this behavior, the CV of different films containing varying number of layers has been recorded, and the half peak width E, - Eppis plotted as a functionof the number of layers as shown in Figure 4 (bottom). It is seen that the change is gradual as we go from six layers to 40 layers, indicating the slowing down of the electron transfer as we go to multilayer films. The use of half peak width for evaluating charge transfer is well established.12 Slow electron transfer has often been seen with electroactive species immobilized on
electrodes.13 We attempt to explain the results obtained with the LB films in the framework of these earlier studies. The redox behavior of polyaniline deposited electrochemically has been well studied previously,14 including such aspects as its dependance on the pH of the bathing solution15and its relation to the method of preparation whether by pulsed16 or by potentiodynamic and potentiostatic methods.17J8 The effect of the acid used in the synthesis has also been investigated.19 These studies suggest that better quality films exhibiting good CV response are obtained for pulsed and potentiodynamic methods of preparation probably due to a growth activation during the earlier stages of film formation leading to well-ordered films. It is important to note that though good CV response has been observed for fairly thick films (>1 pm), much broader CV peaks have also been observed for films of comparable thickness, underlining the notion that the redox kinetics is probablycontrolled by the degree of order in the film rather than ohmic effects due to thickness. Though it may seem anomalous that electrochemically prepared films thicker than themultilayer LB film described here should show good CV response, we emphasize that such a situation is seen only with those electrochemical films which are regular. The key factor that determines CV response is surface regularity. This effect of order and surface regularity on redox behavioris well illustrated by the CV of electrochemicallyprepared polyaniline films deposited by two different electrochemical techniques, i.e., controlled potential (CP) and potential pulse shown in Figure 5. In the CV of the CP deposited film, the first pair of redox peaks of polyaniline is seen only as a broad rise while the second couple exhibits broader peaks, though both films have similar thickness as estimated from their optical transmittance. It might be mentioned that the peak broadening observed with the multilayer LB film of polyaniline (e.g., 38 layers) is reminiscent of the behavior of redox polymer modified electrodes20
11582 The Journal of Physical Chemistry, Vol. 97, No. 45, 1993
Letters In view of the results presented above, it should be interesting to carry out detailed investigationsof the surface structure of LB films of polyaniline and poly(ani1ine-co-o-anisidine)(24) using X-ray diffraction and scanning tunneling microscopy (STM).
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Acknowledgment. We are grateful to Prof. E. S. R. Gopal, Director, NPL New Delhi, for his continued interest and constant encouragement in this work. We are grateful to Dr. Subhas Chandra for his interest in this work and Dr. S.Annapoorni for many helpful discussions. Thanks are due to Mr. S.S.Pandey for his help in the chemical synthesis of polyemeraldine. M.K.R. and N.S.S. thank the CSIR for financial support. Financial support for research from the European Economic Community under Contract CIl -CT 92-0102 is gratefully acknowledged.
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References and Notes
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Figure 5. Cyclic voltammograms of plyaniline film: film deposited using constant potential (curve a); film deposited using a voltage pulse (curve b). Medium = 1 M hydrochloric acid, scan rate = 50 mV/s, Pt = wire quasi-reference electrode (QRE).
where slowness of electron transfer between the metal electrode and one of the layers leads to a large Upvalue. Chronopotentiometric studies of polyaniline2' indicating that the rate of oxidation of the leucoemeraldine is kinetically limited support theconclusion of slowness of electron transfer in multilayer films. We rule out anisotropy in conductivityas being a probable cause, since it has been shown in earlier work with polypyrrole LB films22 that solution doping yields much less anisotropy in conductivity. A recent theoretical treatment by Feldberg23has discussed the effect of domain size in altering the kinetics and thus determining the shape of cyclic voltammograms. Changes in domain size can be brought by a number of factors including molecular disarray during LB film deposition. Thus, we may expect the inhomogeneous 38-layer film to display poor kinetics leading to the broad hump in the CV. This result has important implications for the performance of electroactive devices fabricated using polyaniline LB films. In summary,we have shown that it is possible to directly deposit films of emeraldine base by the Langmuir-Blodgett technique without incorporating fatty acid tails in the molecule. However, on deposition of multilayers, irregularities begin to form in the film and the ordered nature of the LB films is lost, leading to poor electrochemistry.
(1) Malhotra, B. D.; Kumar, N.; Chandra, S . Prog. Polym. Sci. 1986, 12. 179. (2) Annapoorni, S.;Sundarcsan, N. S.; Pandey, S.S.; Malhotra, B. D. J. Appl. Phys, in press. (3) Watanabe, I.; Hong, K.; Rubner, M. F. Lungmuir 1990,6, 1164. (4) Hodge, P.; D a w F.; Tredgold, R. H. Philos. Trans. R. Soc.London, A 1990. 330. 153. (5)'Skoiheim, T. A.; Yang, X.G.; Chen, J.; Inagaki, T.; Dcnboer, M.; Tripthy& Samuelson, L.; Rubncr, M. F.; Hong, K.;Watanabe, I.; Okamoto, Y. Thin Solid F i l m 1993,178,233. (6) Agbor, N. E.; Monkman, A. P.; Petty, M. C.; Hams, M. Extended
Absrracrs of Inrernational Conference on Science and Technologvof Syntheric Merals, Goteborg, Sweden, 1992;p 442. (7) Momoae, A,; Hirai, Y . A. Thin Solid F i l m 1991,204, 175. (8) Mishra,S.C. K.;Ram, M. K.;Pandcy,S.S.;Malhotra,B. D.;Chandra, S . Appl. Phys. Lett. 1992,61,1. (9) Taumura, M.; Ishida, H.;Sekia, A. ThinSolidFilms 1989,178,373. (10) Nerner. D.: Ost.. H.:- SchoDDer. H. C.: Wehrmann. R. Thin Solid F i l k i989.378. 253. (1 1) Ando, hi.; Watanabe, Y.; Iyoda, T.; Honda, K.; Shimidzu, T. Thin Solid Films 1989,179,225. (12) Bard, A. J.; Faullcner, L. Electrochemical Methods; Wiley: New York, 1980. (13) Murray, R. W. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York. 1989: Vol. 13. (14) Huang, W.; Humphrey, B. D.; McDiarmid, A. G. Trans. Faraday SOC.1986,82, 2385. (15) Rudzinski, W. E.; Lozano, L.;Walker, M. J . ElectrochemSoc. 1990, 137, 3132. (16) Tsakova, V.; Milchev, A.;Schulze, J. W. J. Electroanal. Chem. 1993,
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346. 85. (17) Qui, C.; Long, L.H.; Tan, T. C.; Lee,J. Y. J. Electroanal. Chem. 1993,346,477. (18) Diaz, A. F.; Logan, J. A. J. Elecrroanal. Chem. 1980,Ill, 1 1 1 . (19) Lapkowski, M. Synrh. Mer. 1990,35, 169. (20) Fukui, M.; Degrand, C.; Miller, L. L.J. Am. Chem. Soc. 1982,104, 28. (21) Kalaji, M.; Nyholm, L.; Peter, L. M. J. Electroanal. Chem. 1992, 269. -325. - - , -. .. (22) Cheung, J. H.; Rosner, R. B.; Watanabe, I.; Rubner, M. F. Mol. Cryst. Liq. Cryst. 1990,190, 133. (23) Feldberg, S. W.; Rubinstein, I. J . Electroanal. Chem. 1988,240, 1. (24) Pandey, S. S.; Annapoorni, S.; Malhotra, B. D. Macromolecules 1993,26,3190.
