Physical Properties of Polyaniline Films: Assembled by the Layer-by

The uniformity of the PSS/PANI layer-by-layer (LBL) films could be well-maintained, undoping the films in NaOH for obtaining an emeraldine base form o...
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Langmuir 1999, 15, 1252-1259

Physical Properties of Polyaniline Films: Assembled by the Layer-by-Layer Technique Manoj K. Ram,*,† Marco Salerno,† Manuela Adami,† Paolo Faraci,† and Claudio Nicolini‡ Polo Nazionale Bioelettronica, Via Roma 28, 57030 Marciana (LI), Italy, and Institute of Biophysics, University of Genoa, Corso Europa 30, 16132 Genoa, Italy Received June 19, 1998. In Final Form: November 9, 1998 Sequential addition of a polyanion, poly(styrene sulfonate), and a polycation, polyaniline, lead to the formation of layer-by-layer films at different solid surfaces. The prime variables which determine the films formation of poly(styrene sulfonate) (PSS)/polyaniline (PANI) were the polymer charge and ionic strength. The films were deposited by selecting organic/inorganic acid media at pH 2.8. The building up of such multilayer films was characterized by the increment of the adsorbed amount through UV-visible spectroscopy. A linear increase in the absorption magnitude was measured from 1 to 25 bilayers. The uniformity of the PSS/PANI layer-by-layer (LBL) films could be well-maintained, undoping the films in NaOH for obtaining an emeraldine base form of polyaniline. The built-up multilayers were investigated by atomic force microscopy, scanning tunneling microscopy, and cyclic voltammetric and electrical conductivity measurements. The interesting feature of the nearly equal grain size was noticed between 4 and 15 bilayer films of PSS/PANI. The surface roughness was distinguished beyond 15 bilayers of LBL films. The cyclic voltammogram showed the change in the peaks potential value going from 1 to 20 bilayers. The inhomogeneity incorporated inside the films slowed down the electrochemical kinetics in the PSS/ PANI bilayers while going from 1 to 25 bilayer films. The diffusion coefficient (D0) of PSS/PANI 10 bilayers was estimated to be 2 × 10-8 cm2 s-1. Such multilayer films exhibit conductivity in the area of 0.1 S/cm.

Introduction It is most interesting and challenging to construct ultrathin films with a supramolecular architecture in which the individual organic molecules are macroscopically oriented and where the molecules with different functionality can be incorporated into individual layers.1,2 Recently, layer-by-layer (LBL) assembly processes based on electrostatic or other molecular forces are a unique technique that presents a new approach to the formation of supramolecular architectures by adsorption of consecutively alternating polyelectrolytes.3-5 The selfassembly of charged polyelectrolytes (i.e., proteins, conducting polymers, zirconium phosphate, optical dyes, metal nanoparticles, aluminosilicates, and clay) by LBL self-assembly can be considered as an alternative to the Langmuir-Blodgett, spin-coating, and chemical vapor deposition techniques.6-13 The most substantial advan* To whom correspondence should be addressed. † Polo Nazionale Bioelettronica. ‡ University of Genoa. (1) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 831-835. (2) Decher, G. Templating, Self-Assembly and Self-Organization. In Comprehensive Supramolecular Chemistry; Sauvage, J.-P., Hosseini, M. W., Eds.; Pergamon Press, Oxford, 1996; Vol. 9, pp 507-528. Decher, G. Fuzzy nanoassemblies: towards layered polymeric multicomposites. Science 1997, 277, 1232-1237. (3) Lvov, Y.; Decher, G.; Mo¨hwald, H. Langmuir 1993, 9, 481-486. Decher, G.; Eckle, M.; Schmitt, J.; Struth, B. Curr. Opin. Colloid Interface Sci. 1998, 3, 32-39. (4) Decher, G.; Lvov, Y.; Schmitt, J. Thin Solid Films 1994, 244, 772. (5) Ferreira, M.; Rubner, M. F. Macromolecules 1995, 28, 71077114. Fou, A. C.; Rubner, M. F. Macromolecules 1995, 28, 7115-7120. (6) Lvov, Y.; Haas, H.; Decher, G.; Mo¨hwald, H.; Kalachev, A. J. Phys. Chem. 1993, 97, 12835-12841. Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir 1997, 13 (13), 3427-3433. (7) Cheung, J. H.; Fou, A. C.; Rubner, M. F. Thin Solid Films 1994, 244, 985-989. Fou, A. C.; Onitsuka, O.; Ferreira, M.; Rubner, M. F.; Hsieh, B. R. J. Appl. Phys. 1996, 79, 7501. (8) Keller, S. W.; Kim, H. N.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 8817-8818.

tages of the LBL self-assembly is the quite accurately controlled average thickness of the polyelectrolyte layers, where the macroscopic properties of the molecular film can be controlled by the microscopic structure.14 Besides the technological interest for the use of LBL bilayers in biosensors and microelectronics (i.e., LED and displays), a number of issues more exciting for fundamental science are the investigation of surface morphology, the kinetics of deposition, and the ionic strength on the deposition of polyelectrolyte LBL layers.15-18 The stability of the film, the stoichiometry of multilayers, and the better understanding of the film formation of the various polyelectrolytes are also under investigation.19 This technique was originally developed by Decher for polyelectrolytes and later extended to doped conjugated polymers by Rubner.2,20,21 (9) Feldheim, D. L.; Crabar, K. C.; Natan, M. J.; Mallouk, T. E. J. Am. Chem. Soc. 1996, 118, 7640-7641. (10) Schmitt, J.; Decher, G.; Dressik, W. J.; Brandow, S. L.; Geer, R. E.; Sashidhar, R.; Calvert, J. M. Adv. Mater. 1997, 9, 61-65. (11) Kleinfeld, E. R.; Ferguson, G. S. Chem. Mater. 1996, 8 (8), 15751578. (12) Kotov, N. A.; De`ka`ny, I.; Fendler, J. H. Adv. Mater. 1996, 8, 637-641. Onda, M.; Lvov, Y.; Ariga, K.; Kunitake, T. Biotechnol. Bioeng. 1996, 51, 163-167. Kotov, N. A.; Magonov, S.; Tropsha, E. Chem. Mater. 1988, 10 (3), 886-895. (13) Hotta, Y.; Inukai, K.; Taniguchi, M.; Nakata, M.; Yamagishi, A. Langmuir 1997, 13 (25), 6697-6703. (14) Decher, G.; Schmitt, J. Prog. Colloid Polym. Sci. 1992, 89, 160. (15) Lehr, B.; Seufert, M.; Wenz, G.; Decher, G. Supramol. Sci. 1995, 2, 199-207. Onoda, M.; Yoshino, K. Jpn. J. Appl. Phys. 1995, 34, L260L263. (16) Lowack, K.; Helm, C. A. Macromolecules 1998, 31 (3), 823-833. (17) Klitzing, R. V.; Mo¨hwald, H. Macromolecules 1996, 29 (21), 69016906. (18) Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir 1997, 13 (13), 3427-3433. (19) Laurent, D.; Schlenoff, J. B. Langmuir 1997, 13 (6), 1552-1557. (20) Fou, A. C.; Rubner, M. F. Macromolecules 1995, 28, 7115-7120. (21) Cheung, J. H.; Fou, A. C.; Rubner, M. F. Thin Solid Films 1994, 244, 895.

10.1021/la9807255 CCC: $18.00 © 1999 American Chemical Society Published on Web 01/26/1999

Physical Properties of Polyaniline Films

A potential application of the LBL technique could be the fabrication of a homogenous ultrathin film of conjugated polymers. Molecular-level processing of conjugated polymers (i.e., polypyrrole, polyaniline, poly(phenylene vinylene), poly(o-anisidine)) by the LBL technique was shown in the literature.22-24 Amongst conducting polymers, the polyaniline class has received greater attention because of its environmental stability, ease in preparation, and exciting electrochemical, optical, and electrical properties.24-26 The polyaniline class has also been postulated as a potential candidate for numerous electronic applications such as electrochromic displays, rechargeable batteries, microelectronics devices, biosensors, protective coating against corrosion, and chemical sensors.27-32 For many device applications, it is desirable to have polyaniline in a thin films structure, preferably with known thickness and molecular packing.23,29 Polyaniline is known as an intractable material in most of the organic solvents, but Cheung et al. found the route to create dilute aqueousbased solutions of partially doped polyaniline, which could have sufficient stability to allow multilayer deposition with a polyanion, poly(styrene sulfonate).23 It was shown that the polyaniline and poly(styrene sulfonate) bilayers can be formed using pH ranging from 2 to 4, in HCl or methyl sulfonic acid.23 The role of such acids for the deposition of PSS/PANI LBL films and various physical properties of multilayer films could be useful for molecular devices. Keeping this in mind, we determined the object of this manuscript was the fabrication of PSS/PANI LBL films and their characterizations by using UV-visible spectroscopy, electrochemical properties, and electrical conductivity. The electrochemical response time and diffusion coefficient of such film was also investigated. The surface morphology of such bilayers films was investigated by using atomic force microscopy and scanning tunneling microscopy, respectively. Experimental Details Substrate Preparation. The chemicals were procured from Sigma. Microscopic glass slides, quartz plates, indium tin oxide (ITO)-coated glass plates, and platinum foils were used as the substrates for the fabrication of PSS/PANI LBL films. The substrates (except ITO-coated glass plates) were treated with 7:3 concentrated sulfuric acid/hydrogen peroxide solution for 1 h. Care must be taken before using sulfuric acid/hydrogen peroxide solution, which reacts violently with organic solvents. Further, the substrates were treated in 1:1:5 ammonium hydroxide/hydrogen peroxide/water for another hour. These substrates were cleaned using sufficient deionized water and (22) Raposo, M.; Pontes, R. S.; Mattoso, L. H. C.; Oliveira, O. N., Jr. Macromolecules 1997, 30 (20), 6095-6101. (23) Cheung, J. H.; Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30 (9), 2712-2716. Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30 (9), 2717-2725. (24) Onitsuka, O.; Fou, A. C.; Ferreira, M.; Hsieh, B. R.; Rubner, M. F. J. Appl. Phys. 1996, 80, 4067. (25) Epstein, A. J.; MacDiarmid, A. G. In Electronic Properties of Conjugated Polymers; Kuzmany, H., Mehring, M., Roth, S., Eds.; Springer Verlag: Berlin, 1989. (26) Rubner, M. F.; Skotheim, T. A. In Conjugated Polymers; Bredas, J. L., Silbey, R., Eds.; Kluwer: Amsterdam, 1991; pp 363-403. (27) Paul, E. M.; Ricco, A. J.; Wrighton, M. S. J. Phys. Chem. 1985, 89, 1441. (28) Langer, J. J. Synth. Met. 1990, 36, 35. (29) Ram, M. K.; Sundaresan, N. S.; Malhotra, B. D. J. Phys. Chem. 1993, 97, 11580. (30) Ram, M. K.; Carrara, S.; Paddeu, S.; Nicolini, C. Thin Solid Films 1997, 302, 89-97. (31) Ram, M. K.; Paddeu, S.; Carrara, S.; Maccioni, E.; Nicolini, C. Langmuir 1997, 13 (10), 2760-2765. Paddeu, S.; Ram, M. K.; Nicolini, C. J. Phys. Chem. B 1997, 101 (24), 4759-4766. (32) Ando, M.; Watanabe, Y.; Iyoda, T.; Honda, K.; Shimidzu, T. Thin Solid Films 1989, 178, 373.

Langmuir, Vol. 15, No. 4, 1999 1253 further dried by nitrogen flux. This procedure created the hydrophilic substrates. Later, such substrates were immersed into toluene solution for 2 h and further treatment was performed to make the glass plate positively charged by immersing it in the solution of toluene with 5% of (N-2-aminoethyl-3-aminopropyl) trimethoxysilane (TMS) for 24 h. The films were then successively washed with toluene, methanol/toluene, and then toluene before being washed in deionized water several times. The ITO glass was first washed with methanol/chloroform and later treated with aqueous ammonia for 5 min to create the hydrophilic surface. The positively charged surface on the ITO glass plate was created similar to the glass treatment. The substrates were kept in the deionized water prior to their use in LBL deposition. Solution Preparation and Deposition. Sulfonated polystyrene (molecular weight, MW ) 70 000) obtained from Sigma was used without further purification. An emeraldine base form of polyaniline was synthesized by oxidative coupling of aniline as reported previously. The MW of polyaniline was found to lie between 25 000 and 30 000 with a polydispersity of 2. We have also checked the previous studies by Cheung et al. for the pH dependence of the depositing solution.23 It was shown that when the pH of the solution is lying between 5 and 6, it was not possible to control the film deposition because polyaniline is not sufficiently charged to be deposited on the polyanion pSS surface because of the pH value of the solution. When the solution pH was decreased from 3.5 to 2.7, polyaniline was sufficiently charged and the uniform deposition took place. The increase in UV absorption was also noted for the films deposited from pH to pH 2.7. There was not much difference in the UV-absorption magnitude for the films deposited in the pH range 2.85-2.7. The polyaniline solution was adjusted to a 2.8 pH for the deposition of the films, and this solution was quite stable for 2 weeks. When the pH of the solution was decreased from 2.5, it showed the precipitation of the depositing solution. The amount of polyaniline in solution was taken from literature.23 The emeraldine base of polyaniline was dissolved in dimethyl acetamide (DMAc) at a concentration of 20 mg/mL by stirring the solution overnight and then was sonicated for 12 h. The solution was then filtered with 7 µm filter paper to remove the trace of undissolved polyaniline particles. This solution was left for another 24 h in an ultrasonic bath, so that the polyaniline dissolved in the DMAc solvent. Later, such polyaniline solution was added to the ninth part of water and the pH was adjusted to be 3.5. Then the pH of the solution was lowered to 2.8 by adding slowly 1 M HCl or methyl sulfonic acid (MeSA) solution. It could be noted that the solution, lowered to 2.5, showed the aggregation and precipitation of polyaniline. The prepared polyaniline dipping solution can be used for a period of 1 week. Poly(styrene sulfonate) solution was prepared by using 2 mg/mL PSS in water with a pH adjusted to 2.8. The solution was filtered by 4 µm filter paper. Alternating layers of PSS and polyaniline were deposited on to the positively charged glass plates, ITO-coated glass plates, mica, and platinum foil, by alternating submersions of the film samples in an electrolytes solution to build multilayer structures, up to 25 bilayers. After 5 min of deposition, the films were rinsed with water containing either MeSA or HCl at pH 2.8. Optical Measurements. The UV-visible spectra of POAS LS films deposited on glass substrates were recorded by using the UV-visible spectrophotometer (Jasco model 7800). Electrical and Electrochemical Measurements. The electrical characterization was performed using an electrometer (Keithley model 6517). Current-voltage (I-V) characteristics were obtained by a potential step of 0.05 V. The interdigitated electrodes were fabricated on a quartz plate by the photolithography technique. Any pair of electrodes was spaced 50 µm and each tract was spaced 50 µm in width and 40 nm in height. The electrochemical measurements were made by Potentiostat/ Galvanostat (EG & G PARC, model 263A) with a supplied software (M270). A standard three-electrodes configuration was used, where LS films on a glass ITO plate acted as a working electrode, platinum as a counter, and Ag/AgCl as a reference electrode. Atomic Force Microscopy. The used AFM was a homebuilt instrument (Polo Nazionale Bioelettronica) working in contact mode. It operated in air, at constant deflection (i.e., vertical contact force) with triangular-shaped gold-coated Si3N4

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Figure 1. (a) The optical spectra of PSS/PANI as a function of the number of bilayers made in 2.8 MeSA vs (1) 1 bilayer, (2) 2 bilayers, (3) 4 bilayers, (4) 5 bilayers, (5) 8 bilayers, (6) 11 bilayers, (7) 15 bilayers, (8) 18 bilayers, (9) 20 bilayers, and (10) 25 bilayers. (b) The optical spectra of the emeraldine base form of polyaniline in PSS/PANI bilayers as a function of bilayers vs (1) 1 bilayer, (2) 2 bilayers, (3) 4 bilayers, (4) 5 bilayers, (5) 8 bilayers, (6) 11 bilayers, (7) 15 bilayers, (8) 18 bilayers, (9) 20 bilayers, and (10) 25 bilayers. microlevers (commercially available Park Scientific Instrument chips).33 The tips of the microlevers had a standard aspect ratio (about 1:1) and the levers had a nominal force constant of 0.03 N/m. The constant force set point was about 0.1 nN, while the images acquired were 256 × 256 pixel maps. During acquisition the row scanning frequency was 4 Hz (i.e., a physical tip sample motion speed of 8-4-2 µm/s in the 2-1-0.5 µm scan size images, respectively. All images are standard top-view topographic maps, where the brightness is proportional to the quote of the features over the sample surface (i.e., light means mountain, dark means valley). Some images present high features that were saturated in the postprocessing redistribution of the available gray levels, because they are much higher than most of the data points. Henceforth, it was possible to observe the finest structure of the samples. The images shown are representative of the samples, since the same-looking images appeared in four different regions of the analyzed samples, positioned at the vertices of a 4 mm side square, centered at the specimen. Scanning Tunneling Microscopy. The STM analysis of the PSS/PANI film was performed by a commercial instrument (Asse-Z, Italy). The STM is equipped with a small scanning range piezo tube to achieve high-resolution images.

Results and Discussion Figure 1a shows the optical spectra of PSS/PANI as a function of a number of bilayers from 1 to 25. It reveals the two sharp absorption bands at 340 and 800-850 nm for the film made at a pH of 2.8 using MeSA. The observed peak at 340 nm can be attributed to a π-π* transition (33) Ram, M. K.; Adami, M.; Sartore, M.; Paddeu, S.; Nicolini, C. Presented at the fifth Foresight Conference of Molecular Nanotechnology, Palo Alto, CA, Nov 5-8, 1997.

centered on the benzoid ring (interband transition) and the band seen at 850 nm is due to the dopant incorporated, when the films were formed at pH 2.8 (partial doped state of polyaniline). A small band at 450 nm can also be seen in Figure 1a, which could be due to the polarons when the films are formed at pH 2.8 using MeSA. The UV-visible absorbance increases gradually with the increase in the number of bilayers of PANI/PSS LBL films from 1 to 25 bilayers, which reveals the uniformity in deposition. The films were undoped using NaOH solution for 10 min each and the respective UV-visible spectra of the films were recorded. The optical spectrum of the PANI base as a function of bilayers is shown in Figure 1b. There is an induced absorption peak at 330 nm and a broad band is located near 620 nm which characterizes an emeraldine base (giving the familiar blue color) form of polyaniline.34 The uniformity of the film could be well-maintained, while undoping the film for the emeraldine base form of polyaniline. It demonstrates a linear increase in the measured absorption magnitude until 25 bilayers is reached. By the fact that polyaniline absorbs on to the PSS, polyanion surfaces, we have recorded the time period of absorption at 340 nm as a function of time period (minutes) as shown in Figure 2a. It could be seen that the absorption of polyaniline on PSS starts getting deposited as soon as it is inserted into the polyaniline solution. The absorption gets saturated after 5 min of the deposition. Later, there (34) Kim, Y. H.; Foster, C. M.; Chiang, J. C.; Heeger, A. J. Synth. Met. 1989, 298, E285-E290.

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Figure 3. AFM pictures for the PSS/PANI LBL films deposited on glass substrates as a function of bilayers: (a) 2 bilayers, (b) 4 bilayers, and (c) 20 bilayers. The image dimension is 1 × 1 µm2.

Figure 2. Plot of absorbance at 340 nm vs time of deposition when polyaniline was deposited on the PSS surface at pH 2.8 using a MeSA medium. (b) Plot of absorbance at 340 nm for the emeraldine salt form of polyaniline in PSS/PANI LBL films as a function of layers deposited using MeSA (2) and HCl (b) media. (c) Plot of absorbance at 330 nm for the emeraldine base form of polyaniline in PSS/PANI LBL films as a function of bilayers deposited in MeSA (2) and HCl (b) media.

is a very small increase in the UV absorption after 40-80 minutes. We tried to deposit the film after 5 min of the insertion in the polyaniline solution. The multilayered films in this time period are homogenous and uniform as we have seen in Figure 1a. The deposition of successive bilayers was monitored by the change in the optical absorption at 340 nm using either a MeSA or HCl medium as shown in Figure 2b. The nearlinear variation in the absorbency indicates the ordered deposition of PSS/PANI LBL prepared films using either

HCl or MeSA acid media. The absorbance curves, shown in Figure 2b represent almost the same magnitude as a function of bilayers, either made in MeSA or HCl media. When prepared films using MeSA or HCl medium were undoped in NaOH solution and subsequent UV spectra were recorded at 330 nm, the linear characteristics were also obtained. The change in the absorption magnitude for an emeraldine base (Figure 2c) for the same bilayer films obtained after undoping could have different realignment of the molecules. The UV-visible studies reveal that films could be formed by selection of an organic and inorganic acid. AFM Observations. The morphology and uniformity of multilayer films of PSS/PANI were investigated by the AFM technique. The AFM images in Figure 3 present the surface topography of a 1 × 1 µm2 dimensions of 2, 4, and 20 bilayers of PSS/PANI LBL films on a glass or mica surface. The AFM image of two bilayer films is clearly visible in Figure 3a. It attributes that the whole surface is covered by particles of 16 ( 2 nm in diameter. The

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Table 1. Physical Parameters of PSS/PANI LBL Films Measured by Atomic Force Microscopy layers no.

RMS (nm)

∆Z (nm)

density of grains (µ-2)

2 3 4 10 15 20 25

14 ( 2 15 ( 1 19 ( 5 25 ( 3 26 ( 9 29 ( 4 37 ( 4

84 ( 8 92 ( 7 127 ( 15 122 ( 13 175 ( 67 127 ( 28 54 ( 20

33 ( 1 30 ( 3 31 ( 2 19 ( 3 32 ( 3 29 ( 2 30 ( 3

average grain size a (nm) b (nm) 127 ( 52 140 ( 99 137 ( 51 163 ( 89 102 ( 41 139 ( 55 137 ( 68

88 ( 42 71 ( 21 85 ( 26 117 ( 71 79 ( 23 100 ( 36 94 ( 50

aspect ratio a/b 1.44 1.97 1.61 1.39 1.29 1.39 1.46

Figure 4. STM picture for 20 bilayers of PSS/PANI LBL films. The image dimension is 0.5 × 0.5 µm2.

regular spacing of the bright alley shows the presence of polyaniline films. Interesting features can be noticed in the four bilayer films of PANI in Figure 3b, where nearly equal grain size and compact film can be seen. The obtained thin thickness and size are greater than those attributed to true polyaniline monolayers and most likely due to the documented propensity of polyaniline to aggregate into units larger in dimension than isolated molecules. The grain size PSS/PANI increases with the number of layers similar to that of polyaniline Langmuir-Blodgett films.35 The bundle size of LBL film is found to be less than polyaniline LB films.35 The mean surface roughness is area-dependent when calculated over a 1 × 1 µm2 size of PSS/PANI films. The AFM images of such films reveal an increasing fine-grained structure up to 15 layers. The size of the grains simply depends upon the nature of polyaniline molecules. A smooth, completed, and continuous surface with fine globular grains could be seen from 1 to 15 bilayers of PSS/PANI films, whereas inhomogeneity is incorporated in the film beyond 15 layers. Figure 3c shows a somewhat different surface topography of 20 bilayers of PSS/PANI films. Some typical parameters of the grains are shown in Table 1, such as the density of grains, averaged grain size in both directions such as a and b, aspect ratio, root mean square (RMS) of the surface roughness, and height (∆Z). The uniform distribution of polyaniline granules is lost and different granule size distributions in the films can be viewed. The limitations in uniform structures in 20 bilayer films are somewhat (35) Porter, T. L.; Thompson, D.; Bradley, M. Thin Solid Films 1996, 288, 268-271.

Figure 5. (a) Cyclic voltammogram of one bilayer of PSS/ PANI films on a glass ITO-coated plate in 1 M HCl acid medium. (b) Cyclic voltammogram of one bilayer of PSS/PANI films on platinum foil in a 1 M HCl acid medium.

similar to Langmuir-Blodgett films of polyaniline after a certain number of deposited bilayers.29 STM Characterization. The distribution of the grain (bundle) size in 20 bilayers of PSS/PANI films was investigated by scanning tunneling microscopy as shown in Figure 4. The size of the images shown is 0.5 × 0.5 µm2. Such images reveal a similar morphology as shown in Figure 3c by AFM study. The surface is organized in spherical objects (grains) of about 20-200 nm in diameter. The STM image shows that the increment in the grain size is related to the number of bilayers. Electrochemical Investigation. The electrochemistry of the bilayer films deposited on an ITO-coated glass plate and platinum was investigated using cyclic voltammetry. The cyclic voltammograms (CVs) of one bilayer PSS/PANI film in a 1 M HCl acid medium at a scan rate of 50 mV/s are shown in Figure 5a,b. The CV shows the

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Figure 6. (a) Cyclic voltammogram of PSS/PANI LBL films on a glass ITO-coated plate in a 1 M HCl acid medium as a function of bilayers deposited. (b) Cyclic voltammogram of 10 bilayer PSS/PANI films on ITO glass plates at different scan rates: (1) 10, (2) 20, (3) 25, (4) 50, (5) 100, and (6) 200 mV/s.

oxidation peaks at around 232, 558, and 795 mV, while the redox peaks at 84, 532, and 706 mV. Figure 5b shows the oxidation peaks at 239, 574, and 791 and redox peaks at 87, 593, and 765 mV for the same bilayer deposited on a platinum surface. It reveals that the polyaniline can be deposited on metallic surfaces prior to the deposition of PSS. Figure 6a shows the CVs as a function of bilayers in a 1 M HCl solution at a scan rate of 50 mV/s. The peaks are associated with the oxidation and reduction processes of PANI films. The peak current scale increases linearly as a function of bilayers as shown in Figure 6. It shows the little change in the peaks potential (757-764 mV) value going from 8 to 20 bilayer deposition. In fact, such peaks’ potential shift can be related to the increase in polaron/ bipolaron states for the increment in bilayers for PSS/ PANI films. In general, there is little shift in the redox potential at 86 mV, whereas there is a gradual increase in the oxidized potential at around 760 mV as a function of bilayers.36 The oxidation electrochemical response of PANI was practically independent of the number of (36) Pasquali, M.; Pistoia, G.; Rosati, R. J. Adv. Mater. Opt. Elec. 1992, 1, 263-270. Pasquali, M.; Pistoia, G.; Fiordiponti, P. J. Adv. Mater. Opt. Elec. 1992, 1, 271-279.

bilayers, whereas the redox couple at around 558 mV is affected by changing the number of bilayers. In addition to that, the higher value of the electrochemical oxidation potential (814 mV) for bilayers in comparison to the electrochemical polyaniline (at 800 mV) can also be noticed. The change in the oxidation potential value is linked to the higher electronic density states because of the surface modification. Curve 7 in Figure 6a shows the electrochemical response of 25 bilayers. It shows the quenching of the peak at 81 mV as well as the broadening and intensity decreasing of the peak potential at 757 mV (it may be perhaps linked to the inhomogeneity incorporated inside the films for a higher number of bilayer-deposited films). A widening in the peak potential (764-814 mV) from 15 to 25 bilayers was observed. It is linked to the homogeneity incorporated inside the film in the process of deposition. It can also be related to the fact that while going from 1 to 25 bilayers, the electrochemical kinetics in the PSS/PANI bilayer films are slower. The resulted cyclic voltammograms (CVs) for a lower number of LBL films are linked to higher oxidation and reduction peak potentials and a faster electrochemical response, whereas the CVs of the films containing a higher number of bilayers

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Figure 7. Oxidation and reduction current response of 10 bilayer PSS/PANI films on a 1 M HCl acid medium.

behave little differently, showing the bulk effect of the polyaniline films.29 The CVs of 10 bilayers of PSS/PANI film on an ITO glass plate in 1 M HCl acid medium at a different scan rate are shown in Figure 6b. The shape conveys the surface confined (i.e., the redox peak current scale linearly increases with the sweep rate indicating the surfaceconfined species and diffusion-controlled system).29 In order to verify the oxidation and reduction processes of PANI conducting polymer films and diffusion coefficients, the redox potential for ten bilayers was recorded at a different scanned rate. There is an enhancement of the reversibility of the electrode process since the oxidation and reduction processes are similar in nature. The observed oxidation and reduction responses in Figure 7 underlay the notion that the redox kinetics is probably controlled by the Ohmic effect. The current response time was shown to be 75 ms. The value of the diffusion coefficient (D0) in different media for PANI films was determined using the Randles-Sevics equation. The

Ram et al.

diffusion coefficient (D0) has been calculated to be 2 × 10-8 cm2 s-1. It shows a faster electrochromic response than the electrochemical and Langmuir-Blodgett films of polyaniline.37 Electrical Conductivity. We performed the electrical measurements of layer-by-layer PSS/PANI films on interdigitated electrodes with the perspectives as (1) the interdigitated chromium electrodes do not react with the polyanilines films, (2) there is no pinning effect of conducting polymer films, which generally occurs in the sandwiched (metal-LB-metal)-type of configuration, (3) the interdigitated electrodes have a fixed size and equal separation from one tract to another, and (4) the electrodes are fabricated before the deposition of the films and postprocessing of electrode formation are not able to change the films’ property.31 The electrical conductivity of five and ten bilayer PSS/PANI films was studied by deposition on interdigitated electrodes. The polyaniline films produced by the LBL technique were in partially doped states. The doping levels of polyaniline LBL films were increased by treatment in 1 M HCl acid for 1 h. Figure 8 shows the I-V characteristics of LBL films measured at a scan rate of 20 mV/s. The current-voltage characteristics for PANI do not show the Ohmic behavior, which can be related to the possible redox reaction of the interdigitated electrodes with HCl during the preparation of LBL films, and also to some potential barrier with the degenerately doped conducting polyaniline. Both curves showed similar behavior to the I-V characteristics. Typical multilayers films exhibited conductivity in the range of 0.1 S/cm. Such a value of conductivity has been achieved in the literature.23 Conclusions Layer-by-layer films of PSS/PANI were fabricated using either HCl or MeSA acid. The LBL multilayers of the conducting polymer have been monitored by UV-visible spectroscopy. The stage of adsorption was clearly identified by using either MeSA or HCl, which shows the same amount of adsorption on different substrates. It revealed that there was little surface modification for the films

Figure 8. Current-voltage (I-V) characteristic of PSS/PANI LBL films doped in a 1 M HCl acid medium: curve 1 corresponds to 5 bilayers and curve 2 to 10 bilayers.

Physical Properties of Polyaniline Films

made in HCl or MeSA acid after the undoping was made in NaOH solution. The nucleation and growth of the deposited films were performed by scanning microscopies, showing a compact structure up to 15 bilayers and a decrease in uniformity from 15 to 20 bilayers of PSS/PANI films. The various distributions of grain size in 20 bilayers of PSS/PANI films were also confirmed by STM studies. Three well-defined redox peaks could be seen from 1 to 8 bilayer films, but further deposited films showed oxidation and reduction potential similar to that of electrochemical polyaniline films. The diffusion coefficient (D0) was calculated to be 2 × 10-8 cm2 s-1, which revealed a faster electrochromic response than the electrochemical and Langmuir-Blodgett films of polyaniline.37 Typical multilayer films have conductivity in the area of 0.1 S/cm. In view of these results, it should be interesting to investigate the detailed electrochromic and electrical properties of

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LBL PSS/PANI films for display applications. We are investigating the structural properties by low-angle X-ray diffraction and synchrotron radiation source for our future work. Acknowledgment. We are thankful to Drs. M. Sartore and G. Martinazzo for their interesting discussions during the preparation of the manuscript. Thanks are due to Mrs. M. Panza, D. Nardelli, and A. Sardi for their help in carrying out the experiments. Financial supports from the EL.B.A. Foundation (CAP 2102) and Polo Nazionale Bioelettronica are gratefully acknowledged. LA9807255 (37) Ram, M. K.; Sundaresan, N. S.; Malhotra, B. D. J. Mater. Sci. Lett. 1994, 13, 1490. Ram, M. K.; Maccioni, E.; Nicolini, C. Thin Solid Films 1997, 303, 27-33.