Langmuir 1995,11, 5-7
5
Insertion of Polypyrrole into Arachidic Acid Langmuir-Blodgett Films C. W. Yuan,* C. R. Wu, J. J. Bai, W. Y. Yang,t and Y. Wei Department of Biomedical Engineering, Southeast University, Nanjing 210018, People's Republic of China Received August 31, 1993. In Final Form: November 7, 1994@ A new route to insert polypyrrole (PPy)into arachidicacid (Cz& multilayers is investigated. It consists of three steps of reactions: (i)CzoA films are first converted to silver arachidate (CpoAg.)films; (ii) exposing CzoAg films to perchloric acid (HC104)gas leads to recovery of Cz& films that contain AgC104; (iii)finally,
pyrrole molecules, introducedby vapor diffusion into the films, are polymerized under action ofintercalated AgC104 to form oxidized PPy. The insertion reactions and film structure throughout PPy manipulation are characterized by means of Fourier transform infrared spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy.
Introduction Polypyrrole (PPy) is one member of the poly(heter0cycles) family that presents both the polymeric and electroactive properties. Their structural characteristic is the conjugated n system extending over a large number of polymer units.'P2 There is a great deal of activity directed to these conducting polymers in view of their multiple potential applications. One of the challenges in this field is the fabrication of thin film of polymeric electroactive materials with desired structure and proper tie^.^ Insertion chemistry in Langmuir-Blodgett (LB)films attracts increasing interest in regard to preparation of lowdimensional materiah4z5 Insertion of inorganic compounds in LB films has been greatly developed.6-8 Recently, we reported results on synthesis of AgTCNQ complexes by the insertion m e t h ~ d .Rubner's ~ group extended this technique to fabrication of polypyrrole that is confined in a LB matrix.l0 The insertion process in their works consists of two steps: Ferric stearate LB films were exposed to hydrogen chloride gas to yield ferric chloride. The latter then acted to polymerize pyrrole molecules which were introduced into the films by vapor diffusion, leading to formation of polypyrrole. By careful manipulation, they overcome serious difficultiesencounted in the preparation of ferric stearate LB films: iron(II1) is partly reduced to iron(I1) or hydrolyzed; films have poor quality (transfer ratio inferior to 0.9).l1 These disadvantages may otherwise bring undesired influence in sequent t Department of Photoelectronics, East China Institute of Technology, Nanjing 210014, The People's Republic of China. Abstract published in Advance ACS Abstracts, December 15, @
1994. (1) Baeriswyl, D.; Harbeke, G.; Keiss, H.; Meyer, W. Electronic Properties ofPolymers; Mort, J.; Fisher, C. P., Eds.; Wiley: New York, 1982. (2) Street, G. B. In Handbook of Conducting Polymers; Skotheim, T. A., Ed.; Marcel Dekker: New York, 1986; Chapter 8. (3)For a recent review see Rubner, M. F.; Skotheim, T. A. In Conjugated Polymers; Bredas, J. L., Silbey, R., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1991; pp 363-493. (4) Barraud, A.; Rosilio, C.; Ruaudel-Teixier, A. Thin Solid Films 1980, 68, 7. (5) Ruaudel-Teixier, A. J. Chim.Phys. 1988,85, 1067. (6) Ruaudel-Teixier,A.; Leloup, J.;Barraud, A. Mol. Cryst.Liq. Cryst. 1986,134,347. (7)Zylberajch, C.; Ruaudel-Teixier, A.; Barraud, A. Thin Solid Films 1989, 179, 9. (8)Perez, H.; Ruaudel-Teixier, A.; Roulliay, M. Thin Solid Films 1992,210l211, 410. (9) Yuan, C. W.; Lu, W.; Chen, C. Y.; Wei, Y. Synth. Met. 1993,59, 235. (10) Rosner, R. B.; Rubner, M. F. J. Chem. SOC.,Chem. Commun. 1991, 1449. (11) Prakash, M.; Peng, J. B.; Ketterson, J. B.; Dutta, P. Thin Solid Films 1987, 146, L15.
reactions in respect of film structure and chemical reactivity. In this paper we present a new route to fabricate PPy layers within arachidic acid (CZ&)LB films. The polymerization of pyrrole under the action of silver perchloride is essentially concerned. The Czdmultilayers are used as a template that receives polypyrrole through sequential insertion reactions.
Experimental Section Cz& is the most classical film-forming material, the preparation of C&LB films was performed accordingto the conventional LB technique.lZ The slides (CaF2or glass)coated with 27 layers on both sides were immersed in AgNO3 aqueous solution M) and allowedto stand overnight. Then, the slides were dipped carefully into pure water so that the excess of &No3 was removed. After drying under reduced pressure, the slides were exposed to the HClOl gas that evolved from perchloric acid solution for about 10 h. The slides were suspended in a reduced pressure environment again then exposed to pyrrole vapor for 1 day. FTIR spectra were recorded on a Perkin-Elmer Model 983. Small-angleX-raydiffraction W t D ) patternswere obtained on D/max-RAX-ray diffractometerby use of Cu Ka at 40 kV and 100 mA. XPS profiles were recorded on PE Phi 5300 electron spectrometer using Mg Ka radiation. Results and Discussion Direct fabrication of good quality silver arachidate ((220Ag) LB films is unsuccessful. We have tried to prepare the Cz&g LB films by spreading C2& onto the subphase composed of AgN03 aqueous solution MI. The main problem appearing during film preparation referred to precipitation of silver micrograin even under a safety lamp. The monolayer deposition did not show satisfactory transfer ratio, probably due to rigidity of the film. Thus, C z d g LB films were derived from C2& LB films using the insertion technique: silver cations entered into the polar planes of Cz& multilayers by diffusion and reacted with -COOH groups to form -COOAg. FTIR spectra show clearly this conversion (Figure la,b). The absorption at 1702 cm-l attributed to -COOH groups of free acid13 is eliminated. Instead, the characteristic peak of carboxylate groups appears at 1518 cm-1.13J4 Meanwhile, the band progressions15in the region 1180-1350 cm-l are greatly diminished after reaction and CH2 scissoring vibrations, initially in the form of double peak around 1472 cm-l (12) Gaines, G. Jr. Insoluble Monolayers at the Liquid-Gas Interface, Interscience: New York, 1966. (13) Silverstein, R. M.; Basslert, G. C.; Morrill, T. C. Spectroscopic Identification of Organic Compounds; Wiley: New York, 1981. (14) Kagrise, R. F. J. Phys.Chem. 1955, 59, 271. (15) Kimura, F.; Umemura, J.;Takenaka, T. Langmuir 1986,2,96.
0743-7463/95/2411-0005$09.00/0 0 1995 American Chemical Society
Letters
6 Langmuir, Vol. 11, No. 1, 1995
1
3
5
7
9
D i f f r a c t i o n a n g l e 26
IO
1600
1400
1200
1000
800
Firmre 2. Small-anele XRD Datterns acauired from CZOA(a). Cz&g(b),AgC104coGainingdz&(c), andPPycontainingCz& (d) LB films.
becoming a single sharp peak. This is recognized as an indication of variation in hydrocarbon chain package.16 In other words, replacing of free acid by silver salt in arachidic acid multilayers is accompanied by rearrangement of aliphatic chains." The film structure of C2&g LB films was probed using the XRD technique. The profile shows well-ordered arrays of hydrocarbon chains. A set of at least ten (001) Bragg peaks are readily seen which are sharp and regular (Figure 2b). A bilayer distance of about 51.7 A is extracted from XRD data which is about 2 A larger than the long-spacingof C 2 4 multilayers (49.8 A) (Figure 2a). The value is comparable with that derived from XRD data of cadmium arachidata multilayers (53.8 Exposing the C2dg LB films to HC1O4gas leads to recovery of C2& LB films that contain now AgC104. As shown in the FTIR spectrum (Figure IC),strong absorption of -COOH groups at 1710 cm-' is re-established at the expense of -COOAg peaks a t 1518 cm-'. The latter is greatly reduced, signifying that the transformation from salt to free acid is almost completed under the experimental conditions used. On the other hand, the XRD pattern of the AgClO4-C2& films shows a decrease in diffraction peak number, and the peaks become broader and weaker in intensity (Figure 2c). The same phenomenon was observed in the transformation of ferric stearate films to ferric chloride-containing stearic acid films. The modification of XRD patterns may also reflect the solidstate reaction. It is well recognized that the reflection
intensity of multilayers is dependent on the electron density.lg For the films composed of metal soap of fatty acid, metal moieties are well localized in polar planes. The films can be used as pseudocrystals and the unit cell of the films as a unit cell of metal ions.20r21As the salt of fatty acid is converted to free acid, the liberated inorganic compounds are no longer anchored to head groups of fatty acid and may become scattered,1° causing separate domains of resultant free acid and AgC104. The weaker and broader diffraction peaks can be reasonably ascribed to the retained multilayer organization of a portion of nonconverted silver salt. Furthermore, the salt-acid conversion in this case does not restore hydrocarbon chain package that exists in the pristine arachidic acid films. This is evidenced in the FTIR spectrum: the single peak a t 1472 cm-' remains the same and no signals for band progressions can be gleaned. The net result of the reactions until now is the embedding of scattered AgC104, most likely in a form of cluster, in the C2& multilayers that undergo a structural decay due to the inserting treatment. The last step of the manipulation is the polymerization of pyrrole under the action of inserted AgC104. The role of AgC104 for polymerizing pyrrole in bulk has been recognized22though this compound is used not so widely as ferric chloride. HavingAgClO4 molecules localized in the films, we introduced pyrrole vapor into the places. This makes two species contact each other. As consequence, the polymerization of pyrrole triggered by AgC104 occurs in the LB matrix. Meanwhile, polypyrrole is doped byAgC104 to an oxidative state. Figure I d shows the FTIR data ofthe PPy-containing LB films. The region ranging from 800 to 1800 cm-l contains bands consistent with the polypyrrole.22 The XRD profile of the sample is
(16) Kawai, T.;Umemura, J.; Takenaka, T. Bull. Znst. Chem. Res. 1983, 61, 314. ( 1 7 ) Leloup, J.; Maire, P.; Ruaudel-Teixier, A.; Barraud, A. J. ChimPhys. 1986,82, 695. (18) Blodgett, K.B.; Langmuir, I. Phys. Rev. 1937, 51, 964.
(19) Pomerantz, M.; Segmuller, A. Thin Solid Films 1980, 68, 33. (20) Henke, B. L. Adv. X-ray Anal. 1964,7 , 460. (21) Hans, R. P.; Davidson, F. D. Rev. Sci. Instrum. 1965,36,230. (22) Street, G.B.;Clarke, T. C.;Krounbi,M.; Kanazawn, K.; Lee, V.; Pfluger, P.; Scott,J. C.; Weiser, G .MdCryst. Lip. C y s t . 1982,83,253.
Wavenumber (cm-')
Figure 1. FTIR spectra acquired from CZ& (a), CZ& (b), kc104 containing CZ& (c), and PPy containing Cz& (d) LB films.
Langmuir, Vol. 11, No. 1, 1995 7
Letters displayed in Figure 2d. The (001)Bragg reflections are similar to those of AgC104-C~& films in terms of peak number, location, breadth, and intensity, being indicative of persistent multilayer organization of nonconverted CzoAg molecules as they do not interfere in the polymerization of pyrrole. It is the previously formed AgC104aggregates that make the polymerization occur so as to the polypyrrole chains are believed to be dispersed in the separated phase of arachidic acid films. Lateral conductivity of the films was measured using the two-probe method, which is of the order of S cm-’. The loss of high conductivity may be associated with the domain structure of polypyrrole. XPS investigation provides more information on PPY formation. Figure 3b presents a C 1s XPS spectrum of PPy-containing CZ& multilayers. For comparison, the spectrum of Cp& multilayers is also presented (Figure 3a). The principal peaks for both films are centered at binding energy (BE) of 284.6 eV. The peak for PPycontaining films is somewhat broader and asymmetric compared to that for reference. Two or more minor peaks are required, in addition to the dominant peak associated with aliphatic chain carbons, for better fitting of the envelope, suggesting the presence of multiple forms of carbon. It was reported that in the PPy unit, &carbons have a peak position very close to that of saturated carbon,23and the peak for a-carbons is centered at the binding energy 0.98eV higher than that for /?-carbon.24 We believe that the asymmetrictail located in high energy wings of main peak is connected not only with photoemission from the a-carbon but also with n-n* transition and transition of polaronic states in the band gap.25 The latter is a characteristic feature for oxidized PPy which will be further proved with N 1s XPS data. For P-carbons, the peak is overlapped by that of long chain carbons of fatty acid. PPy-containing CZ& films show a N 1s signal centered at 399.7eV (Figure 3c) which is not present in the N 1s region for CZ& films. The peak is asymmetric with a shoulder located at the binding energy about 2 eV higher than the main peak, being indicative of the oxidative state of P P Y . The ~ ~ N 1s shape agrees well with that reported in literature for electrochemically synthesized PPy perchl~rate.~’ In conclusion, pyrrole polymerization is achieved in Cz& LB films under the action of AgC104 that was embedded in the films. The insertion reactions and film structure throughout PPy manipulation are characterized utilizing (23) Clark, D. T.; Thomas, H. R. J . Polym. Sci., Polym. Chem. Ed. 1978,16, 791. (24) Gelius, U.;Allan, C. J.; Johansson, G.;Siegbahn, H.; Allison, D. A.;Siegbahn, K.Physica Scripta 1971,3,237. (25) Bredas, J. L. In Handbook of Conducting Polymers; Skotheim, T.A.,Ed.; Marcel Dekker: New York, 1986; Chapter 25. (26) Eavas, J. G.; Munro, H. S.;Parker, D. Polym. Commun. 1987, 28,38. (27) F’fluger, P.;Street, G. B. J . Chem.Phys. 1984,80,544.
404
292
400
288
396
284
B i n d i n g enerqy
280
(ev)
Figure 3. C 1s XPS spectra acquired from C d (a) and PPy containingC2& (b)LB films and N 1s XPS spectrum acquired from PPy containing C2& (c) LB films.
a variety of techniques. The dispersed PPy chains within CZ& multilayers are proposed. We are currently investigating the aggregate behavior of silver perchloride molecules with the help of atomic force microscopy.
Acknowledgment. The Authors thank the reviewers for very informative suggestions. LA9305334
