Electroless Deposition of Poly(2-alkoxyaniline)s - Langmuir (ACS

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Langmuir 2004, 20, 3471-3476

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Electroless Deposition of Poly(2-alkoxyaniline)s Maciej Mazur† and G. J. Blanchard*,‡ Department of Chemistry, Laboratory of Electrochemistry, University of Warsaw, 02-093 Warsaw, Pasteura 1, Poland, and Department of Chemistry, Michigan State University, East Lansing, Michigan 48823-1322 Received December 30, 2003. In Final Form: February 3, 2004 W This paper contains enhanced objects available on the Internet at http://pubs.acs.org/ journals/langd5. The in situ deposition of poly(2-alkoxyaniline)s onto oxide surfaces is reported. It is demonstrated that the identity of the substrate can have a pronounced effect on the polymerization rate of these substituted polyanilines. Poly(2-alkoxyaniline)s deposit efficiently onto indium-doped tin oxide (ITO), but deposition onto quartz proceeds slowly. The critical stage in the deposition process is shown to be polymerization of the adsorbed oligomeric species. When this polymerization process is catalyzed by the surface, polymer growth is enhanced, and we find that conducting substrates mediate this apparent catalytic process. We demonstrate selective deposition by growing poly(2-alkoxyaniline) adlayers onto patterned ITO/quartz substrates.

Introduction The field of conducting polymers has attracted a great deal of interest in the past 25 years, because of the utility of these materials in numerous technological applications, such as secondary batteries, sensors, catalysts, electrochromic materials, and organic semiconductors.1-3 Conducting polymers can be prepared by two general methods, electrochemical or chemical polymerization. Electrochemical deposition can be performed on conducting substrates, resulting in the deposition of a thin polymer layer on the substrate. Chemical polymerization can lead to a more bulklike material, which is precipitated from the solution, depending on reaction conditions. It has been shown that solid substrates introduced into a chemical polymerization reaction can become coated by a thin layer of the polymer.4 This observation led to development of in situ or electroless deposition of conducting polymers on solid substrates. Electroless polymer deposition allows coating of a variety of materials ranging from fibers and textiles to noble metals.5-25 * To whom correspondence should be addressed. E-mail: [email protected]. † University of Warsaw. ‡ Michigan State University. (1) MacDiarmid, A. G. Synth. Met. 1997, 84, 27. (2) Skotheim, T. A., Ed., Handbook of Conducting Polymers; Marcel Dekker: New York, 1998. (3) Gerard, M.; Chaubey, A.; Dmalhorta, B. Biosens. Bioelectron. 2002, 17, 345. (4) Gregory, R. V.; Kimbrell, W. C.; Kuhn, H. H. Synth. Met. 1989, 28, C823. (5) Tzou, K.; Gregory, R. V. Synth. Met. 1992, 47, 267. (6) Dhawan, S. K.; Singh, N.; Venkatachalam, S. Synth. Met. 2002, 129, 261. (7) Stejskal, J.; Sapurina, I.; Prokes, J.; Zemek, J. Synth. Met. 1999, 105, 195. (8) Chen, Y.; Kang, E. T.; Neoh, K. G.; Huang, W. Langmuir 2001, 17, 7425. (9) Goren, M.; Qi, Z.; Lennox, R. B. Chem. Mater. 2000, 12, 1222. (10) Rajapakse, R. M. G.; Chandani, A. D. L.; Lankenshwara, L. P. P.; Kumarasiri, N. L. W. L. Synth. Met. 1996, 83, 73. (11) Li, Z. F.; Ruckenstein, E. J. Colloid Interface Sci. 2002, 251, 343. (12) Armes, S. P.; Gottesfeld, S.; Beery, J. G.; Garzon, F.; Agnew, S. F. Polymer 1991, 32, 2325. (13) Mazur, M.; Tagowska, M.; Pałys, B.; Jackowska, K. Electrochem. Commun. 2003, 5, 403. (14) Mazur, M.; Krysinski, P. Thin Solid Films 2001, 396, 131.

One disadvantage of electroless polymerization, to date, has been the relatively low selectivity of polymer deposition onto various substrates. Although it was shown that polyaniline or polypyrrole can be polymerized selectively on patterned glass,26 the difference in polymerization rates on various substrates is modest.14,15 Electroless deposition involves immersion of the substrate into the solution containing a monomer and an oxidant. Oxidation of the monomers in solution results in the formation of oligomeric species, some of which adsorb onto the substrate surface. These oligomers can subsequently react on the substrate to form a polymer. Growth of the substrate-bound polymer can proceed by attachment of monomeric and oligomeric species from solution to the already-formed polymeric layer.27 The polymer is also formed in solution and when precipitated can accumulate on the substrate surface.28 These two processes, “growth from” and “deposition onto” the surface, proceed in parallel, and their relative efficiency depends on the experimental conditions as well as the substrates used. Catalytic effects which serve to enhance the “growth from” mechanism are tempered by the leveling effect of the “deposition onto” process. To achieve substrate selectivity in the deposition of conducting polymers, we need to identify ways to enhance (15) Mazur, M., Krysinski, P. Electrochim. Acta 2001, 46, 3963. (16) Mazur, M.; Krysinski, P. Langmuir 2001, 17, 7093. (17) Fedorova, S.; Stejskal, J. Langmuir 2002, 18, 5630. (18) Sapurina, I.; Osadchev, A. Y.; Volchek, B. Z.; Trchova, M.; Riede, A.; Stejskal, J. Synth. Met. 2002, 129, 29. (19) Fu, Y, Weiss, R. A. Synth. Met. 1997, 84, 129. (20) Sapurina, I.; Riede, A.; Stejskal, J. Synth. Met. 2001, 123, 503. (21) Job, A. E.; Herrmann, P. S. P., Jr.; Vaz, D. O.; Mattoso, L. H. C. J. Appl. Polym. Sci. 2001, 79, 1220. (22) Laska, J.; Widlarz, J.; Wozny, E. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 3562. (23) Oh, S. Y.; Koh, H. C.; Choi, J. W.; Rhee, H. W.; Kim, H. S. Polym. J. 1997, 29, 404. (24) Ayad, M. M.; Gemaey, A. H.; Salahuddin, N.; Shenashin, M. A. J. Colloid Interface Sci. 2003, 263, 196. (25) Riede, A.; Helmstedt, M.; Sapurina, I.; Stejskal, J. J. Colloid Interface Sci. 2002, 248, 413. (26) Huang, Z.; Wang, P.-C.; MacDiarmid, A. G.; Xia, Y.; Whitesides, G. Langmuir 1997, 13, 6480. (27) Stejskal, J.; Trchova, M.; Fedorova, S.; Sapurina, I.; Zemek, J. Langmuir 2003, 19, 3013. (28) Sapurina, I.; Fedorova, S.; Stejskal, J. Langmuir 2003, 19, 7413.

10.1021/la036475w CCC: $27.50 © 2004 American Chemical Society Published on Web 03/09/2004

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polymer growth from the substrate while not affecting deposition from solution. We report here on the electroless polymerization of selected ortho-substituted aniline derivatives. It is shown that the chemical oxidation of these compounds results in the formation of stable, soluble oligomeric species in the solution. Introduction of a catalytic surface into the reaction vessel induces changes in the reactivity of the oligomers, enabling efficient growth of the polymer from the surface. For our experimental conditions, since the polymer is not formed efficiently in solution, and substrate-mediated growth prevails, differences in the catalytic behavior of different substrate materials can be resolved more clearly. We observe large enhancements in growth rates of poly(2-alkoxyaniline)s on ITO substrates compared to growth on quartz. These findings point the way toward a novel and simple means of generating patterned conducting polymer-based device structures using simple solution phase chemistry. Experimental Section Chemicals. All chemicals were of the highest quality commercially available: 2-methoxyaniline (OMA) (Aldrich, 99+%), 2-ethoxyaniline (OEA) (Aldrich, 98%), 2-hydroxyaniline (OHA) (Aldrich, 99%), aniline (Aldrich, 99.5+%), ammonium peroxodisulfate (NH4)2S2O8 (Fluka, >98%), hydrochloric acid (Aldrich), p-phenylenediamine (Aldrich, 99.5+%), adipoyl chloride (Aldrich, 98%), 4-methylmorpholine (Aldrich, 99%), acetonitrile (Aldrich, anhydrous, 99.8%), ethyl acetate (Spectrum Chemical, ACS grade), sulfuric acid (Aldrich, 99.999%). Aqueous solutions were prepared from water distilled in-house. Instrumentation. Absorbance spectra were recorded using a Cary model 300 UV-visible spectrophotometer. All spectra were recorded with 1 nm resolution. Electrochemical measurements were performed using a PC-controlled Electrochemical Workstation (CH Instruments model 650A) with a conventional small volume three-electrode cell with Pt wire as the counter electrode. All potentials reported here are versus a Ag/AgCl/1 M KClaq reference electrode. A Digital Instruments Nanoscope IIIa was used to collect the contact mode AFM (atomic force microscopy) images in air. Substrate Preparation. Quartz slides (NSG Precision Cells, Inc., P/N 10040 UV fused silica windows, nonfluorescent) were cleaned by immersion in piranha solution (Caution! Strong oxidizer) for ca. 20 min. Indium-doped tin oxide (ITO) planar (Delta Technologies, Minneapolis, MN) and patterned films (Bayview Optics, Dover-Foxcroft, ME) on fused silica were cleaned by rinsing with ethanol and dried. Conducting Polymer Deposition. Quartz and ITO-coated quartz slides were immersed into the polymerization solution of 2 mL of 0.24 M monomer in aqueous 1 M HCl mixed with 2 mL of aqueous 22 mM (NH4)2S2O8 in 1 M HCl. After a specified period of time (typically 20 min for POMA and 70 min for POEA), the substrates were removed, rinsed with 1 M HCl and distilled water, and dried. The deposition method is substantially the same as the widely used procedure for electroless deposition of polyaniline.29 Bonding p-Phenylenediamine to Quartz and ITO. Quartz and ITO substrates were reacted with adipoyl chloride (0.3 mL) in dry acetonitrile (10 mL), using 4-methylmorpholine (0.3 mL) as a Lewis base, under reduced pressure for 1 h. The reacted substrates were removed from the reaction vessel, rinsed with dry acetonitrile and ethyl acetate, and dried under a stream of nitrogen. Then the acid chloride terminal functionalities were reacted with p-phenylenediamine by exposing substrates with adipoyl chloride adlayer to a 10 mM solution of the amine in dry acetonitrile for 1 h. The substrates were then removed from the reaction vessel, washed with dry acetonitrile and ethyl acetate, and dried under a stream of nitrogen.

Results and Discussion We investigated the oxidation of three aniline derivatives, 2-hydroxyaniline (OHA), 2-methoxyaniline (OMA), (29) Malinauskas, A. Polymer 2001, 42, 3957.

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Figure 1. UV-visible absorbance spectra of aqueous solutions: 0.12 M monomer, 11 mM NH4S2O8 in 1 M HCl (OHA, 360 s; OMA, 1200 s; OEA, 4200 s after mixing the solutions of the corresponding monomer and oxidant).

and 2-ethoxyaniline (OEA), and have studied the efficiency of polymer deposition on the identity of the substrates on which they are deposited. The solutions containing monomer and oxidant were prepared and monitored for formation of the oxidation products with time. We show in Figure 1 the UV-visible absorbance spectra of 2-hydroxyaniline (after 360 s of reaction time), 2-methoxyaniline (after 1200 s), and 2-ethoxyaniline (after 4200 s), respectively. For all three compounds, an absorption band with a maximum near 460 nm is seen. The presence of the 460 nm band indicates that short oligomeric species are formed by the oxidation reaction.30,31,13,15,16 Stassen and Hambitzer reported that only those aniline oligomers which consist of not more than three aniline monomer units are soluble in an acid solution, whereas the products of higher polymerization cannot be found in solution.32 It is believed that such soluble short oligomers (n e 3) are obtained in our case, and we designate all the longer, nonsoluble products reported here as polymeric species. The important point for this work is that the oxidation products are soluble (no precipitation is seen), and apparently longer polymeric species are not present in significant number, based on the absence of absorption bands at wavelengths longer than 500 nm, which are characteristic of oxidized forms of polyaniline-like polymers. The oligomeric species are stable on the time scale of the experiment, and their concentration increases with time as shown in Figure 2. It should be noted that the growth of the 460 nm band, corresponding to the rate of soluble oligomer formation, decreases with increasing substituent size; 2-hydroxyaniline oligomers form most rapidly and 2-ethoxyaniline oligomers form most slowly. This can be understood in the context of the differences in the oxidation potentials of the monomers. 2-Ethoxyaniline, which is characterized by the highest oxidation potential, reacts to form the oligomeric species with the slowest rate. However, this phenomenon can be explained equally well by differences in the monomer reactivity due to the presence of different substituents of the benzene ring. The absence of polymeric (polyaniline-like) species in solution after the chemical oxidation of the monomers (30) Goncalves, D.; Faria, R. C.; Yonashiro, M.; Bulhoes, L. O. S. J. Electroanal. Chem. 2000, 487, 90. (31) Viva, F. A, Andrade, E. M.; Florit, M. I.; Molina, F. V. Phys. Chem. Chem. Phys. 2002, 4, 2293. (32) Stassen, I.; Hambitzer, G., J. Electroanal. Chem. 1997, 440, 219.

Electroless Deposition of Poly(2-alkoxyaniline)s

Figure 2. Dependence of absorbance at 460 nm (OHA, OMA, OEA) with time for the solutions: 0.12 M monomer, 11 mM NH4S2O8 in 1 M HCl.

Figure 3. UV-visible absorbance spectra of ITO immersed in aqueous solutions: 0.12 M monomer, 11 mM NH4S2O8 in 1 M HCl (OHA, 360 s; OMA, 1200 s; OEA, 4200 s after mixing the solutions of the corresponding monomer and oxidant).

raises the question of whether we can cause polymer growth using a heterogeneous (solid) catalyst in the reaction solution. Two different substrates were used to evaluate this possibility, ITO and quartz, both in the form of planar substrates (slides). We consider growth on the ITO surface first. The substrates were immersed into solutions containing the monomer and oxidant, and we recorded the UV-visible absorption spectra of the films formed on the substrates (Figure 3). For OMA and OEA, the band at 460 nm appears in addition to a feature centered at 700 nm. The 700 nm band indicates the formation of polymeric species, and these polymers are present on the ITO surface. The formation of a blue-green film can be seen on the ITO substrate in the absence of any precipitate forming in the bulk solution. No absorption band is seen at 700 nm for OHA, and this finding correlates with the absence of a detectable polymer film on the ITO substrate surface. Similar observations have been reported by Goncalves et al.30 who suggested that chemical oxidation of OHA results in the formation of oligomeric, not polymeric, species. For OMA and OEA, the deposited polymer thin film adheres strongly to the ITO surface. The optical and electrochemical properties of these polymer films are comparable to those of the same

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Figure 4. Cyclic voltammogram of ITO with covalently attached amido-4-aminophenyl adipate in aqueous 0.5 M H2SO4. Sweep rate: 0.05 V/s.

polymers synthesized electrochemically (cyclic voltammograms and potential-dependent absorption spectra not shown). We compare our results for polymer deposition on ITO to deposition on quartz. The formation of polymer films was not observed, for any of the monomers, on quartz, and the absorption spectra of quartz substrates exposed to the polymer-forming solution did not exhibit absorption features characteristic of polymer formation. The next issue to consider is why polymer formation is facile on ITO but not on quartz. For these substituted polyanilines, polymer growth on a solid substrate involves at least three steps. These are adsorption of monomers or oligomers on the surface, polymerization of the adsorbed species, and subsequent growth of a thicker polymeric layer from these surface-bound polymers. Each growth regime can give rise to differences in the formation of polymer on the substrate surface. A series of experiments were performed to elucidate which of these steps influences the overall polymer deposition process most strongly. First we tried to elucidate the effect of adsorption of oligomeric species onto the substrates. Both ITO and quartz were modified with covalently attached aniline to enhance the accumulation of oligomers on the surface and find out how this modification affects polymer formation. A similar approach has been reported recently by Li and Ruckenstein,11 who coated monomer-primed glass fibers by in situ polymerization of aniline. The modification of the substrate resulted in the formation of an ultrathin, smooth, and well-bonded polyaniline adlayer.11 To form an aniline-terminated surface, we used a well-established procedure,33,34 in which a monomolecular adlayer of adipoyl chloride is first bound to the substrate through surface -OH groups and then p-phenylenediamine is bound to the acid chloride terminated adlayer through the formation of an amide linkage. Thus, we obtained both ITO and quartz surfaces with aniline moieties attached. We detected the presence of aniline groups on ITO by cyclic voltammetry (Figure 4). In the first scan, an irreversible anodic peak is seen at 0.80 V, due to oxidation of the surface-bound aniline, followed by formation of a new redox pair at ca. 0.45 V in subsequent scans. This peak can be attributed to newly formed aniline dimers. These results are in quantitative agreement with those for p-phen(33) Mazur, M.; Blanchard, G. J. J. Phys. Chem. B 2004, 108, 1038. (34) Kelepouris, L.; Krysin´ski, P.; Blanchard, G. J. J. Phys. Chem. B 2003, 107, 4100.

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Figure 5. (a) Dependence of absorbance at 700 nm with time: i, ITO modified with amido-4-aminophenyl adipate; ii, bare ITO; iii, quartz modified with amido-4-aminophenyl adipate; iv, bare quartz. Immersed in the solution: 0.12 M OEA, 11 mM NH4S2O8 in 1 M HCl. Inset: UV-vis spectrum of quartz (modified with amido-4-aminophenyl adipate) removed from the polymerization solution and placed in 11 mM NH4S2O8 in 1 M HCl. (b) Dependence of absorbance at 700 nm with time: i, ITO modified with amido-4-aminophenyl adipate; ii, bare ITO; iii, quartz modified with amido-4-aminophenyl adipate; iv, bare quartz. Immersed in the solution: 0.12 M OMA, 11 mM NH4S2O8 in 1 M HCl.

ylenediamine attached to a 11-mercaptoundecanoic acid monolayer on gold.16 The redox reaction of the terminal aniline groups allows the estimate of surface adlayer concentration. On the basis of the charge released during the oxidation of the aniline moiety, we calculate the surface aniline concentration to be ca. 7.8 × 10-11 mol cm-2. This is a relatively low surface concentration, but likely high enough to enhance surface deposition of OMA and OEA oligomers. We show in Figure 5a the time dependence of the 700 nm absorbance band for bare and aniline-modified ITO and bare and modified quartz substrates immersed in a solution containing OEA and the oxidant. It can be seen that for bare and modified ITO after ca. 2500 s the absorbance increases due to the growth of a poly(2ethoxyaniline) layer on the substrate, and the deposition on the aniline-modified ITO is slightly greater than that

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for the ITO substrate. On the basis of this finding, we expect an analogous enhancement in oligomer adsorption on modified quartz, relative to bare quartz, and there is indeed an increase in the signal at 700 nm (Figure 5a, profile iii). It is important to note that the increase in the 700 nm signal for the quartz substrates is not resolvable as a discrete band, but appears to be a red-edge band tail. We recorded the absorption spectrum of the modified quartz substrate (Figure 5a inset) and observed the band centered at 460 nm only but no well-resolved 700 nm feature. These data suggest that polyaniline-like polymer is not formed on the aniline-modified quartz surface; predominantly oligomers accumulate at this interface. Similar results are obtained for poly(2-methoxyaniline) (POMA) (Figure 5b). The only difference between OMA and OEA deposition is that initial OMA deposition occurs prior to 500 s, due its lower oxidation potential. The results obtained are different from those reported for aniline polymerization on a monomer-primed glass surface. For aniline polymerization, a conducting film is obtained, with FTIR and conductivity data indicating the film to be polyaniline (PANI).11 We surmise from these experiments that adsorption of the oligomeric species onto the surface is not the critical step in determining the different polymer growth rates on ITO and quartz. It is the subsequent reaction of adsorbed oligomers to form polymers that is mediated by the substrate. If this assertion is correct, we should be able to deposit POEA or POMA on quartz by electroless polymerization if the quartz surface is precoated with a thin polymer film. First, the surface of a quartz substrate was covered with a thin layer of polyaniline and then we studied how this polymer film influenced the deposition of POMA and POEA. We deposited the polyaniline film on quartz by electroless polymerization; its deposition mechanism is substantially different from that of POMA and POEA.13,14,28 The reason for a different deposition mechanism of PANI originates from relatively high reactivity of aniline oligomers in the bulk polymerization solution. Although the catalytic effect of the surface on the deposition rate is noticeable, as for POMA or POEA, it is significantly less pronounced for PANI, due to the reactivity of solution phase aniline oligomers and the consequent efficient formation of the polymer in solution. Thus, even if formation of the polymer is not initiated through the surface catalytic effect, it can be initiated by physisorption of the polymer from solution. Once initiated, the polymer growth occurs rapidly. The electroless deposition of PANI is only weakly selective for the substrate. The immersion of the polyaniline-coated quartz substrate into an OMA or OEA solution containing oxidant results in the rapid deposition of the corresponding polymer on the surface. We show in Figure 6 the absorption spectra of POMA deposited on a thin polyaniline film-coated quartz substrate as a function of deposition time. The absorption band at 700 nm, indicative of polymer formation, increases almost immediately after immersion of the substrate into the solution. The POMA grows even faster on the modified quartz substrate than it does on the ITO substrate, and we obtain the same result for POEA deposition. Similar behavior was reported by Malinauskas et al., who observed increased polyaniline deposition on polymer-coated quartz, due to an autocatalytic effect.35,36 These findings show that the deposition of POEA and POMA on ITO and quartz is not influenced significantly by the adsorption of oligomeric species on the surface. The critical step that (35) Mazeikiene, R.; Malinauskas A. J. Chem. Res. (S) 1999, 622. (36) Mazeikiene, R.; Malinauskas A. Synth. Met. 2000, 108, 9.

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Figure 6. Dependence of UV-vis spectra of polyaniline-coated quartz immersed in the solution: 0.12 M OMA, 11 mM NH4S2O8 in 1 M HCl.

Figure 8. AFM image of (a) patterned ITO layer on quartz and (b) POMA selectively deposited on patterned ITO on quartz. W A video showing selective deposition of POMA on macroscopically patterned ITO on quartz is available.

Figure 7. Absorbance spectra of POMA formed on an ITO substrate, grown in room light (top spectrum) and under dark conditions (bottom spectrum). Both polymer growth reactions were performed for ca. 15 min using the same solution phase monomer and oxidant concentrations.

mediates the polymer deposition process is the reaction of the adsorbed monomers and oligomers to form polymeric species. It can be assumed that the monomer and oligomer species accumulate at similar rates on both quartz and ITO, but only on ITO can these species react efficiently to form an ultrathin polymeric layer. Once a polymeric layer is initiated by reaction of the adsorbed species, further polymer growth can occur. Our data point to the ITO surface functioning catalytically toward the reaction of monomeric and oligomeric 2-alkoxyanilines. The differences between ITO and quartz, which does not manifest this catalytic behavior, probably stem from differences in the electron density of the adsorbed monomers and oligomers, a property which influences reactivity. It is believed it is the availability of electron density from the conduction band of the substrate that facilitates polymerization of the oligomers. The deposition rate of poly(2alkoxyanilines) on ITO is substantially slower when the deposition reaction is performed in the dark (Figure 7). We observe a significant difference in polymer deposition rate, with the layer formed in the presence of room light producing an absorbance of 1.6 at 700 nm (ca. 15 min

reaction time), and an absorbance of 0.2 at 700 nm for the same reaction performed in the dark. Polymer formation in the dark reaction can be understood in terms of thermal population of the ITO conduction band. As light pumps electrons into the conduction band of ITO, its catalytic effect becomes more pronounced. It is likely that electrons from the conducting substrate decrease positive charge of the oxidized oligomers and lower Coulombic repulsion between them. Thus, the surface-bound species can react more easily and an ultrathin polymeric film is produced on the surface. Once the initial polymer film is formed, the further rapid growth of the polymer can occur autocatalytically, similar to what is observed for polyaniline growth.37 The mechanism for electroless deposition of POMA and POEA is significantly different than that of PANI. Although the issue of PANI growth by electroless deposition has not been addressed extensively in the literature, some generalizations can be made based on published data. The solution phase reaction of aniline with an oxidant, e.g., persulfate, results in formation of the polymer, which precipitates from solution.27 The introduction of a solid substrate into the polymerization solution may provide some acceleration to the polymerization rate,28 but surface coverage of the substrate occurs primarily as the result of simple precipitation. Thus, the surface catalytic effects for PANI are less pronounced than those for POMA and POEA, where no polymer is formed in bulk solution, but selective, efficient surface deposition is seen. The precipitation of PANI in solution can be (37) Chakraborty, M.; Mukherjee, D. C.; Mandal B. M. Langmuir 2000, 16, 2482.

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retarded or even completely inhibited, however, using 1,3phenylenediamine38 or sodium tungstate27 to delay precipitation of the polymer in the solution while allowing growth to occur on the substrate surface. Kang et al.39 and Gu et al.40 achieved formation of polyaniline on platinum and palladium simply by immersing the substrates into an oxygen-rich monomer solution for several days. Under these conditions the authors were able to deposit PANI through the “growth from” mechanism, while the “deposition onto” was not observed. Our synthetic procedures seem to be of advantage because of reaction time, control over film thickness, and spatial selectivity. The electroless polymerization of poly(2-alkoxyanilines) can be exploited for technological applications such as selective deposition on both the macro- and microscales. We show in Figure 8 microscopic images of a patterned ITO layer (ca. 200 nm thick) on quartz (Figure 8a). The chemical polymerization of POMA on such a substrate results in the selective deposition of the polymer on ITO, with the quartz regions remaining substantially uncoated (Figure 8b). In these images, the thickness of the polymer layer is ca. 500 nm, demonstrating our ability to spatially control the deposition of a conducting polymer by simply taking advantage of differences in polymer deposition rates on different substrate materials. Such patterns may subsequently be implemented as electrodes in the preparation of certain types of devices such as, e.g., liquid crystal displays, sensors, molecular recognition devices, or connecting wires in nanoscale electronic circuits. Regardless of the end use, the ability to kinetically mediate polymer growth on selected substrate materials is likely to prove useful in the future, and gaining a deeper understanding of the details of this polymerization process will lead to even larger differences in reactivity for selected substrates. (38) Stejskal, J.; Kratochvil, P.; Spirkova, M. Polymer 1995, 36, 4135. (39) Chen, Y.; Kang, E. T.; Neoh, K. G. Appl. Surf, Sci. 2002, 185, 267. (40) Liao, C.; Gu, M. Thin Solid Films 2002, 408, 37.

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Conclusions It has been shown that the kinetics of electroless deposition of poly(2-alkoxyaniline)s depends sensitively on the identity of the substrate. The critical stage in the deposition mechanism is the efficiency with which adsorbed oligomeric species can react to form longer polymers. If the surface catalyzes this reaction, polymer growth is efficient, leading to the deposition of relatively thick polymer layers in usefully short periods of time. ITO and quartz precoated with polyaniline both function as catalytic substrates for poly(2-alkoxyaniline) deposition while bare quartz does not. It is believed that this difference in reactivity is due to the availability of electron density from the conduction band of the substrate material. We anticipate that this finding will be of use in the spatially selective deposition of conducting polymers and that further work will allow the optimization of this process. Acknowledgment. We are grateful to the National Science Foundation for their support of this work through Grant 0090864. M.M. is grateful to the National Science Foundation and NATO for their support of a postdoctoral fellowship (DGE-0209459). Supporting Information Available: 1: The video presents selective deposition of POMA on macroscopically patterned ITO on quartz. The upper half of the substrate is coated with ITO, the lower part is bare quartz. The sample is immersed in the solution containing OEA and the oxidant in aqueous HCl solution (in a quartz cuvette). Originally the film lasted ca. 60 min and it was accelerated to 37 s. First, formation of brownish red soluble oligomers in the solution is observed. Then, a dark polymer film is formed in the upper part of the substrate coated with ITO (the polymer layer is also formed at the air/water interface, but this phenomenon is not discussed in our paper). 2: A photograph presenting the selectively deposited POEA on patterned ITO/quartz substrate (upper part, POEA grown on ITO; lower part, bare quartz). This material is available free of charge via the Internet at http://pubs.acs.org. LA036475W