Nanostructured Polyamic Acid Membranes as Novel Electrode

Langmuir , 2005, 21 (15), pp 6891–6899 .... 1H NMR spectra were recorded on a Bruker AM 360 spectroscopic system .... Electrodeposition conditions: ...
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Langmuir 2005, 21, 6891-6899

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Nanostructured Polyamic Acid Membranes as Novel Electrode Materials Daniel Andreescu,† Adam K. Wanekaya,‡ Omowunmi A. Sadik,* and Joseph Wang§ Department of Chemistry, State University of New York at Binghamton, Post Office Box 6000, Binghamton, New York 13902-6000 Received January 18, 2005. In Final Form: April 27, 2005 This paper describes a new approach for the preparation of polyamic acid (PAA) composites containing Ag and Au nanoparticles. The composite film of PAA and metal particles were obtained upon electrodeposition of a PAA solution containing gold or silver salts with subsequent thermal treatment, while imidization to polyimide is prevented. The structural characterization of the films is provided by 1H NMR and Fourier transform infrared spectroscopy (FTIR), while the presence of metallic nanoparticles within the polymeric matrix was confirmed by scanning electron microscopy (SEM), cyclic voltammetry (CV), energy-dispersive X-ray analysis (EDX), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). This approach utilizes the unique reactivity of PAA by preventing the cyclization of the reactive soluble intermediate into polyimides at low temperature to design polymer-assisted nanostructured materials. The ability to prevent the cyclization process should enable the design of a new class of electrode materials by use of thermal reduction and/or electrodeposition.

Introduction The current widespread interest in metal nanomaterials is driven largely by potential applications in optical switches, catalysis and microelectronics, labels for biomolecules, biosensors, and surface-enhanced spectroscopies.1-3 The ability to functionalize the metal particles and control the size, shape, and stability at the nanoscale level is important for fundamental and industrial applications. This can be achieved by using polymeric membranes that provide an excellent dispersion and sequestering environment for metal particles, thus generating polymer metal nanocomposites with designed morphological, electrochemical, and structural properties. The potential advantage of such metal/polymer systems is that the size and distribution of dispersed metal nanoparticles can be readily controlled, based on the thermoplastic properties of the host polymers.4,5 Current methods for preparing nanoparticles dispersed in polymeric matrices include vapor deposition, electrochemistry, laser ablation, citric acid reduction, wet chemical synthesis, gold particle formation, self-assembly of nano* Corresponding author: e-mail [email protected]. † Present address: Center for Advanced Materials Processing, Clarkson University, P.O. Box 5814, Potsdam, NY 13699-5814. ‡ Present address: Department of Chemical and Environmental Engineering, University of California, Riverside, CA 92521. § Departments of Chemical & Materials Engineering and Chemistry, Arizona State University, Tempe, AZ 85287-5801. (1) Breimer, M. A.; Yevgeny, G.; Sy, S.; Sadik, O. A. Nano Lett. 2001, 1, 305. (2) Ueda, M.; Dietz, H.; Anders, A.; Kneppe, H.; Meixner, A.; Plieth, W. Electrochim. Acta 2002, 48, 377. (3) (a) Hernandez, M. M.; Pardave, M. P.; Batina, N.; Gonzalez, I. J. Electroanal. Chem. 1998, 443, 81. (b) Kakimoto, M.; Suzuki, T.; Konishi, T.; Imai, Y.; Iwamoto, Y.; Hino, T. Chem Lett. 1986, 823. (c) Sysel, P.; Konecna, V.; Volka, K. Eur. Polym. J. 1996, 32, 317. (d) Suzuki, T.; Kakimoto, M.; Konishi, T.; Imai, Y.; Iwamoto, Y.; Hino, T. Chem Lett. 1986, 395. (d) Sroog, C. In Macromolecular Synthesis; Moore, J. A., Ed.; John Wiley: New York, 1977; Vol. 1, p 295. (4) Shull, K. R.; Kellock, A. J. J. Polym Sci. B: Polym. Phys. 1995, 33, 1417. (5) Cole, D. H.; Shull, K. R.; Baldo, P.; Rehn, L. Macromolecules 1999, 32, 771.

particle arrays, electron beam lithography, scanning tunneling microscopy- (STM-) assisted nanostructure formation, and nanosphere lithography.1,6-11 Examples of practical applications of metal nanostructured materials have been previously reported. For instance, Wrighton and co-workers12 deposited Pt and Pd into a viologen-based polymer used in semiconductor electrodes to improve hydrogen evolution. Pt microparticles electrodeposited into poly(4-vinylpyridine) or polyaniline films have been also applied in electrocatalysis (e.g., reduction of hydrogen, oxidation of methanol).13 In addition, electrochemical codeposition of Pt and Sn into polyaniline film generated Pt/Sn catalyst particles with higher activity for methanol oxidation as compared to Pt particles alone.14 In another application, Au nanoparticles cross-linked by a bipyridinium cyclophane and deposited onto a conductive indium-tin oxide (ITO) electrode have been used to enhance the detection of hydroquinone and various π-donor substrates and neurotransmitters.15 In this case, the organic cross-linker molecule acted as a receptor for π-donor substrates. Moreover, by controlling the nature and the orientation of functional groups present on the polymer, this could be used in biosensing and (6) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (7) Dabbousi, B. O.; Bawendi, M. G.; Onitsuka, O.; Rubner, M. F. Appl. Phys. Lett. 1995, 66, 1316. (8) Lamber, R.; Wetjen, S.; Schulz-Ekloff, G.; Baalmann, A. J. Phys. Chem. 1995, 99, 13834. (9) Beecroft, L. L.; Ober, C. K. Chem. Mater. 1997, 9, 1302. (10) Akamatsu, K.; Tsuboi, N.; Hatakenaka, Y.; Deki, S. J. Phys. Chem. B 2000, 104, 10168. (11) Wang, T. C.; Rubner, M. F.; Cohen, R. E. Langmuir 2002, 18, 3370. (12) (a) Bruce, J. A.; Murahashi, T.; Wrighton, M. S. J. Phys. Chem. 1982, 86, 1552. (b) Dominey, R. N.; Lewis, N. S.; Bruce, J. A.; Bookbinder, D. C.; Wrighton, M. S. J. Am. Chem. Soc. 1982, 104, 476. (13) (a) Bartak, D. E.; Kazee, B.; Shimazu, K.; Kuwana, T. Anal. Chem. 1986, 58, 2756. (b) Kost, K. M.; Bartak, D. E.; Kazee, B.; Kuwana, T. Anal. Chem. 1988, 60, 2379. (14) Hable, C. T.; Wrighton, M. S. Langmuir 1991, 7, 1305. (15) (a) Shipway, A. N.; Lahav, M.; Blonder, R.; Willner, I. Chem. Mater. 1999, 11, 13. (b) Lahav, M.; Shipway, A. N.; Willner, I. J. Chem. Soc., Perkin Trans. 2, 1999, 9, 1925.

10.1021/la050141k CCC: $30.25 © 2005 American Chemical Society Published on Web 06/09/2005

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molecular bioelectronics. For instance, Tang et al.16 developed a potentiometric immunosensor for diphtheria detection using a platinum electrode containing antidiphtheria immobilized via the metal nanoparticles (Au) incorporated within a polymeric matrix (polyvinyl butyral). An example of polymeric matrices used for incorporating metal nanoparticles is the polyamic acids (PAA) and their final forms, polyimides (PI). These matrices have been shown to act like redox polymers whose electrochemical and morphological characteristics are strongly dependent upon the type of solvent and supporting electrolyte.17,18 Compared to the imide form, the PAAs have significantly higher cation-complexing properties.19,20 This means that salts of noble metals having labile anions can react in situ to generate a metal-polymeric blend. Thus, a single crosslink between two molecules of PAA and noble metal composites can result in a stable, high molecular weight network and can be mass-produced. Although the metallization of polyimides for microelectronics application21-23 has been widely reported, neither PI nor its PAA precursors have been utilized for the preparation of inorganic nanoparticle hybrid materials. We recently described a rapid, one-step, single-phase synthesis of polymer-based nanocomposites. This approach is based on the reduction of Au(III) chloride by PAA and the subsequent capping and stabilization of the resulting gold nanoparticles with PAA.24 The PAA-capped gold nanoparticles showed no change in dispersion, nanoparticle density, size distribution, or absorption spectra. In this work, we report the synthesis, optimization, and characterization of the first PAA-metal nanoparticle composites as electrode materials. The uniqueness of this approach lies in the preparation of metal nanoparticles within electrodeposited PAA film at low temperature and the ability to prevent the cyclization process. The low temperature ensures that the thermal reduction occurs and imidization process is avoided. The nanocomposite membranes were deposited onto reticulous vitreous carbon (RVC) and solid electrodes while the polymer retains the carboxyl groups and exhibits high stability. This study includes the mechanism of polymer composite formation and the optimal conditions necessary to obtain good surface coverage during the deposition process. Finally, we provide the structural, electrochemical, and morphological characteristics of the resulting polymeric membranes by cyclic (16) Tang, D.; Yuan, R.; Chai, Y.; Zhang, L.; Zhong, X.; Liu, Y.; Dai, J. Sens. Actuators, B 2005, 104, 199. (17) (a) Tieke, B.; Gabriel, W. Polymer 1990, 31, 20. (b) Jin, B.; He, P.; Zhou, Y.; Chen, Y. Polym. Int. 2002, 51, 534. (c) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 333, 4213. (d) Dubas, S. T.; Schelenoff, J. B. Macromolecules 1999, 332, 8153. (e) Decher, G. Science 1997, 277, 1232. (f) Sullivan, D. M.; Bruening, M. L. Chem Mater. 2003, 15, 281. (18) (a) Kordas, K.; Bekesi, J.; Vajtai, R.; Nanai, L.; Leppavuori, S.; Uusimaki, A.; Bali, K.; Thomas, F.; George, F.; Galbacs, G.; Ignacz, F.; Moilanen, P. Appl. Surf. Sci. 2001, 172, 178. (b) Wang, X. C.; Zeng, H. Y.; Lim, G. C. Appl. Surf. Sci. 2002, 200, 165. (c) Yu, Z. J.; Kang, E. T.; Neoh, K. G. Polymer 2002, 43, 4137. (d) Yang, G. H.; Kang, E. T.; Neoh, K. G.; Zhang, Y.; Tan, K. L. Langmuir 2001, 17, 211. (e) Ang, A. K. S.; Kang, E. T.; Neoh, K. G.; Tan, K. L.; Cui, C. Q.; Lim, T. B. Polymer 2000, 41, 489. (19) Uebner, M.; Ng, K. M. J. Appl. Polym. Sci. 1988, 36, 1525. (20) (a) Yang, C. P.; Chen, Y. H. Angew. Makromolek. Chem. 1988, 160, 91. (b) Lupo, D.; Prass, W.; Scheunemann, U. Thin Solid Films 1989, 178, 403. (21) Kiang, M. H.; Lieberman, M. A.; Cheung, N. W.; Qian, X. Y. Appl. Phys. Lett. 1992, 60, 2767. (22) Murphy, M. M.; Van Herle, J.; McEvoy, A. J.; Thampi, K. R. J. Electrochem. Soc. 1994, 141, L94. (23) Kordas, K.; Nanai, L.; Galbacs, G.; Uusimaki, A.; Leppavuori, S.; Bali, K. Appl. Surf. Sci. 2000, 158, 127. (24) Sadik, O. A.; Andreescu, D.; Wanekaya, A.; Wang, J.; Mulchandani, A. In Book Abstract of 227th ACS National Meeting, Anaheim, CA, 28 March-1 April 2004; American Chemical Society: Washington, DC, 2004.

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voltammetry (CV), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM), while the metallic particles were evaluated by energydispersive X-ray analysis (EDX), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Experimental Section Reagents and Stock Solutions. All reagents are analytical grade unless otherwise stated. The following reagents were obtained from Aldrich Chemical Co. (Milwaukee, WI): 4,4′oxydianiline (ODA), triethylamine (TEA), pyromellitic dianhydride (PMDA), lithium perchloride (LiClO4), silver nitrite (AgNO3), sodium chloride (NaCl), gold chloride (AuCl3), sodium dodecyl sulfate (SDS), N,N′-dimethylacetamide (DMAc), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO-d6). Potassium bromide, infrared grade (KBr), and tetrahydrofuran (THF) were purchased from Acros Organics (New Jersey). Reticulous vitreous carbon (RVC) was from Destech Corp. (Tucson, AZ). Potassium phosphate dibasic (K2HPO4), potassium phosphate monobasic (KH2PO4), acetonitrile (ACN), and methanol (MeOH) were purchased from Fisher Scientific Co. In all cases, all organic solvents were deoxygenated with nitrogen for 10 min prior to use. All aqueous solutions were prepared with Nanopure water with a resistivity of 18 MΩ‚cm or better. Instrumentation. All electrochemical measurements were performed on an EG&G Princeton Applied Research 263A potentiostat/galvanostat equipped with M398 software. A conventional three-electrode system was employed throughout this work, which consists of a reticulous vitreous carbon electrode (RVC) (with a geometrical area of 1 cm2) or glassy carbon electrode (GCE) (with a geometrical area of S ) 0.1 cm2) as working electrodes, a Ag/AgCl reference electrode (BAS), and a platinum wire as auxiliary electrode. Infrared spectra were recorded with a Bruker Equinox 55 spectrophotometer equipped with OpusNT software version 2.06. 1H NMR spectra were recorded on a Bruker AM 360 spectroscopic system equipped with 8.45 T magnet and multinuclear and inverse detection capabilities at 360 MHz and 20 °C. SEM coupled with EDX measurements were carried out on a Princeton Gamma EcoScan. The crystallinity of silver and gold nanoparticles was assessed from powder XRD patterns obtained on a Siemens D5000 X-ray diffractometer with a Cu KR1 monochromatized radiation source (λ ) 1.541 78 Å) operated at 40 kV and 30 mA. XPS spectra were collected on a PHI 5500 modified multiprobe spectrometer with monochromatic Al KR radiation and operated at 1 × 10-10 Torr. Gravimetric measurements were performed with a Mettler Toledo electronic balance. Synthesis of Poly(amic acid). Synthesis of PAA was carried out in organic medium from 4,4′-oxydianiline (ODA) and 1,2,4,5benzenetetracarboxylic acid (PMDA) precursors. Since the type of solvent used for the preparation and deposition of the PAA film plays a critical role in obtaining homogeneous surface coverage and controlling polymer thickness and polymerization degree, we studied several solvents for the synthesis and characterization of the PAA. Depending on the solvent used, PAA can be prepared as a viscous liquid or a powder, and the resulting polymers were soluble either in aqueous or in organic solvent medium. For instance, when THF was used as solvent, the resulting PAA was a viscous liquid. In this case, the preparation procedure involves the following steps: 2.0024 g (0.01 mol) of ODA plus 42 mL of THF were stirred in a 0.5 L round-bottom flask. PMDA powder (2.1812 g, 0.01 mol) was added to the solution over 1 h. MeOH (35 mL) containing 0.02 mol (2.787 mL) of TEA was added. The solution was stirred for 24 h, resulting in a yellow viscous solution of PAA. The solid content of the resulting PAA was 9.8%. This viscous PAA was soluble in phosphate-buffered saline (PBS) and will be denoted as PAA(v) throughout this work. When ACN was used as solvent, another PAA derivative was obtained in a powdered form from the same ODA and PMDA. To prepare this PAA, 2.0024 g (0.01 mol) of ODA plus 157 mL of ACN were stirred till solvation. Then, 50 mL of ACN containing 2.1812 g of PMDA (0.01 mol) was added dropwise over 1 h and the solution was stirred for over 24 h. The resulting precipitates were filtered through a membrane under suction and finally

Nanostructured Polyamic Acid Membranes dried at room temperature. The amount of PAA obtained was 3.351 g. This homogeneous powder is denoted as PAA(p) and was found to be soluble in DMAc, DMF, and DMSO. PAA will refer to both PAA(v) and PAA(p). The Mw of PAA was estimated by gel-permeation chromatography (10 000). Electrodeposition of PAA, PAA-Ag-, and PAA-AuModified Electrodes. The electrodeposition of PAA onto the electrode surface was performed following a galvanostatic procedure with a conventional three-electrode system (Ag/AgCl reference electrode, Pt auxiliary electrode, and RVC or GCE working electrode). The PAA film formation was carried out in either aqueous or organic medium depending on the PAA and solvent used. The electrodeposition of PAA and PAA-metal nanocomposite was carried out in a conventional 10 mL cell containing PAA (50 mg/mL) dissolved either in DMAc or in an aqueous medium of 0.1 M phosphate-buffered saline (PBS), pH 6.0, and supporting electrolyte (0.1 M LiClO4) with or without metal salt (0.1 M AgNO3 or AuCl3). After the electrodeposition step, the PAA-modified electrodes were removed from the polymerization solution and rinsed with the same organic solvent or PBS in which the film was developed. Then the electrodes were dried at 105 °C overnight in order to eliminate residual solvent molecules and allow thermal reduction process to proceed.25 The low-temperature ensures that the thermal reduction occurs and imidization process is avoided. Structural and morphological analysis was then performed to confirm the synthesis of the PAA salt, electrodeposited PAA, and PAA containing silver or gold (denoted as PAA-Ag and PAA-Au, respectively). The amount of electrodeposited PAA on RVC electrodes was determined by differential measurements of the weight gain by the electrode before and after the electrodeposition process. The thickness of the electrodeposited film can be approximated from the density (1.49 g/cm3) of PAA derived from ODA and PMDA (commercially Durimide 100).26 For an electrodeposition time of 5 min at 1 mA/cm3, the weighted polymer amount was 0.5 mg, which corresponds to a film thickness of ∼3.4 µm. The thickness of the electrodeposited film measured by SEM analysis in an electrode cross section was ∼3 µm, which is in good agreement with the calculated theoretical value (3.4 µm). Electrochemical Characterization of PAA-, PAA-Ag-, and PAA-Au-Modified Electrodes. Cyclic voltammetry (CV) was used to characterize the electrochemistry of the PAA film with or without metal particles. For these experiments we used GCE as working electrode. CVs were carried out before and after galvanostatic electrodeposition of the PAA membrane onto the surface of the GCE electrode. For these tests, the film was prepared from PAA(p) diluted in DMAc containing 0.1 M LiClO4 and Au or Ag salt (0.1 M AgNO3 or 0.05 M AuCl3). To check the electroactivity of the resulting polymeric membranes, the CV experiments were performed in 0.1 M PBS, pH 6, by scanning the potential at a scan rate of 50 mV/s, in the potential range where the two metal ions are electrochemically active. Film Characterization. Chemical structure and morphology of PAA-, PAA-Au-, and PAA-Ag-modified films were characterized by FTIR, NMR, SEM, EDX, XPS, and XRD techniques. Infrared spectra of the PAAs were performed for the polymer layer formed onto the electrode after electrodeposition and drying. This layer was removed from the electrode surface by peeling with a razor blade and then mixed with KBr. For PAA(v), KBr pellets were used and the data were analyzed by subtracting the THF spectrum. All FTIR spectra were recorded with a Bruker Equinox 55 spectrophotometer. 1H NMR measurements were achieved on sample films, powder PAA, or viscous PAA solution dissolved in DMSO-d6. SEM and EDX analysis were conducted in order to determine the distinct morphological structures and elemental composition of the PAA, PAA-Ag, and PAA-Au composite polymers on the RVC and GCE electrodes. The acceleration voltage was 20 kV with maximum magnification of 30000. XRD and XPS analysis of the composite materials was performed to confirm the chemical (25) Warner, J. D.; Pevzner, M.; Dean, C. J.; Kranbuehl, D. E.; Scott, J. L.; Broadwater, S. T.; Thompson, D. W.; Southward, R. E. J. Mater. Chem. 2003, 13, 1847. (26) www.archimico.com/products/4prob100.pdf.

Langmuir, Vol. 21, No. 15, 2005 6893 state of Ag and Au nanoparticles incorporated within the PAA membrane. Gravimetric measurements of PAA and noble metalmodified PAA films were performed to determine the mass changes following polymerization.

Results and Discussion Figure 1 shows the chemistry of the deposition process of the PAA composite film, including the incorporation of silver. The presence of TEA in the growth solution is needed for neutralization of the carboxyl groups of the PAA to form the PAA salt and also to act as a surfactant.27 When a constant current is applied during the galvanostatic procedure, the PAA is oxidized via the amine groups and, under the influence of electric field, migrates to the anode, where it is deposited as a continuous PAA polymer film. In the presence of a metallic salt, the negatively charged polycarboxylate will couple to metal cation forming a metal-carboxylate salt (polyamate). Linde and Gleason28 demonstrated the formation of silver(I) polyamate in solution on the basis of the high affinity of silver ions. On the basis of this observation, we believe that the electrodeposited PAA-Ag film is generated as silver polyamate during the electrodeposition process. In the final step, an electrochemical or thermal reduction occurs at 105 °C overnight, when a metal-poly(amic acid) composite film is formed. This metallization process is generally accompanied by imidization of the PAA, which usually occurs at increasing temperature (160-400 °C).28 In this work, the imidization process is prevented by performing the thermal reduction at 105 °C. This process is avoided even after 5 days of thermal treatment. The chemical structure of the electrodeposited PAA and PAAmetal composite, the presence of metal particles within the polymer film, and the chemical state of the metal are studied and confirmed in the following sections. Fabrication of Nanostructured PAA Film. The main problem associated with the preparation of PAAmetal composites is that, during film formation, the metallization process occurs simultaneously with the imidization of PAA to produce polyimides, both of which are generated at high temperature. Thus, chemical imidization has to be avoided in order to retain the carboxyl groups for further derivatization of the particles. Several parameters are involved in this process, which could affect the PAA film formation and hence the nature of functional groups retained. The first and most important is the solvent used during synthesis, which acts as a plasticizer affecting the degree of polymerization and/or viscosity. The choice of the solvent is thus important for controlling film thickness and homogeneity. In addition, the type of solvent used to dissolve nonaqueous PAA emulsion is believed to influence the nature of the functional groups retained in the resulting polymer. For example, when the emulsions have been dissolved in solvents such as N,N′dimethylacetamide (DMAc) or N,N′-dimethylformamide (DMF), the carboxyl groups are retained. However, when solvents such as ethanol or acetone have been employed, precipitation of PAA occurs. Other important parameters include solvent composition,29 pH,30 and concentration of the supporting electrolytes.28,31 The PAA films obtained in this work have been prepared by an electrochemical procedure followed by thermal curing. (27) (a) Chen, Y.; Iroh, J. O. Polym. Eng. Sci. 1999, 39, 699. (b) Iroh, J. O.; Yuan, W. J. Appl. Polym. Sci. 1996, 59, 737. (28) Linde, H. G.; Gleason, R. T. J. Polym. Sci. Part B: Polym. Phys. 1989, 26, 1485. (29) (a) Iroh, J. O.; Levine, K. Eur. Polym. J. 2002, 38, 1547. (b) Jinwei, W.; Srinivasan, M. P. Synth. Met. 1999, 105, 1. (c) Levine, K.; Iroh, J. O. J. Mater. Chem. 2001, 11, 2248.

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Figure 1. Schematic representation of silver-composite PAA electrodeposited film.

Electrochemical Synthesis. Electrochemical synthesis of PAA films was carried out under galvanostatic conditions. It is well-known that the amount of polymer film electrodeposited onto an electrode surface is dependent on the composition of the growth medium (solvent, supporting electrolyte, surfactant) and other deposition conditions such as the deposition time and current (30) (a) Chan, S. H.; Wong, W. T.; Chan, W. T. Chem. Mater. 2001, 13, 4635. (b) He, L.; Wagner, S. R.; Illingsworth, M. L.; Jensen, A. J.; Yap, G. P.; Rheingold, A. L. Chem. Mater. 1997, 9, 3005. (c) Gibson, J. K.; Haire, R. C. Organometallic 1999, 18, 4471. (b) Southward, R.; Thompson, D. S.; Thompson, D. W.; St. Clair, A. K. J. Adv. Mater. 1996, 27, 2. (31) Echigo, Y.; Miki, N.; Tomioka, I. J. Polym. Sci. Part A: Polym. Chem. 1997, 35, 2493.

density.27 Thus, by varying these parameters it is possible to control the deposition process. In consequence, we studied the effect of these parameters on PAA film formation by performing electrochemical synthesis in both aqueous and organic solvent at different current densities and deposition times. Figure 2 shows the typical E-t curve obtained during the electrodeposition of PAA film from organic medium onto a RVC electrode at a current density of 1.0 mA/cm2. The polymer was deposited from a growth solution containing 50 mg/mL PAA(p) powder and 0.1 M LiClO4 in DMAc. The chronopotentiometric response indicates the occurrence of a steady-state electrodeposition process, which was controlled by the initial diffusion of PAA

Nanostructured Polyamic Acid Membranes

Figure 2. Chronopotentiometric curve for PAA electrodeposition onto RVC electrode. Electrodeposition conditions: [PAA(p)] ) 50 mg/mL; [LiClO4] ) 0.1 M in DMAc; Dc ) 1.0 mA/cm2.

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Figure 4. CV characterization of PAA(p) and PAA(p)-Au electrodeposited films on GCE. Experimental conditions: 0.1 M PBS at pH 6.0, 50 mV/s. Table 1. Vibrational Frequencies of PAA, PAA-Ag Composite Filma imide groupb

amide group compd

νCO-NH

νCOOH

νC-O-C

ν-C)O,C-N-C axial

PAA(p) PAA(v) Ag-PAA(v)

1653, 1540, 1410 1652, 1540, 1408 1653, 1510, 1410

1717, 1304 1717 1717

1225 1219 1224

NA NA NA

a Vibrational frequencies are given in reciprocal centimeters. Imide group missing for both at 1778 cm-1 and 1374 cm-1 (CN-C axial) confirmed the imidization process has been avoided at 105 °C Imides have 2 acyl gps bonded to a single nitrogen.

b

Figure 3. Potential-time (E-t) plot for Ag(I)-PAA(v) electrodeposition on RVC electrode as a function of the applied current density. Electrodeposition conditions: [PAA(v)] ) 50 mg/mL; [LiClO4] ) 0.1 M; [AgNO3] ) 0.1 M in PBS (pH ) 6.5); Dc ) 0.25, 0.5, 1.0, 2, and 2.5 mA/cm2 from lower to upper.

molecules. The process was slightly affected when the PAA film was prepared from an aqueous solution [50 mg/mL PAA(v) diluted in PBS containing 0.1 M AgNO3]. For the same current density, for example, 1.0 mA/cm2, the electrodeposition equilibrium potential was 0.92 V in PBS compared to 1.4 V when DMAc was used as solvent. This result is a consequence of organic solvents stabilizing the cations. We examined the effect of applied current density and deposition time on the electrodeposition process at current densities ranging from 0.25 to 2.5 mA/cm2 while the deposition time was set at 5, 10, 15, and 20 min. Figure 3 shows the effect of current density on the chronopotentiometric responses of PAA(v)-Ag(I) onto the RVC electrode for a electrodeposition time of 5 min. As can be seen, the electrode potential increases with increasing applied current. This behavior suggests that the process obeys Faraday’s law of electrolysis. Thermal Curing. Because the supporting solvent of PAA represents ca. 80% of the resulting silver(I)-doped PAA films, the solvents must be removed from these composite materials by thermal curing.25 The thermal curing also ensures the formation of a compact membrane characterized by good mechanical stability. In addition, it is desirable to remove most of these solvents in order to eliminate film bubbling due to rapid solvent volatilization. However, because PAA strongly associates with most solvents, some cannot be easily removed and hence, the quality of the PAA film may be affected. Several literature sources showed that, depending on the solvent used, solvent removal from the PAA film could occur differ-

ently.31 In this work, we studied solvent removal on RVC electrodes coated with PAA(v)-Ag(I) in THF and PAA(p)Ag(I) in DMAc. The PAA-Ag(I)-modified electrodes were dried to constant weight in the oven in static air at 105 °C and the weight gain of the electrodes was determined gravimetrically by differential measurements before and after the electrodeposition process. The results showed that the solvent was removed gradually within the first 60 min, when more than 90% of the solvent was evaporated. The results also reveal a small influence of solvent volatility. For instance, the PAA(v)-Ag (I) film lost 95% of the solvent (THF) after 1 h at 105 °C, while the PAA(p)Ag(I) film lost about 85% (DMAc) during the same time. Literature data suggest that this process may occur simultaneously with the reduction of silver(I) to metallic silver particle sequestered within the polymeric films (Figure 1).25 However, the mechanism is currently not fully understood. Electrochemical Characterization. Electrochemical characterization of PAA-metal composites was carried out by cyclic voltammetry (CV) with a GCE as working electrode. Figure 4 shows the CV of the PAA- and PAA(p)Au-modified GCE electrode. The films were obtained via electrodeposition from a solution containing 25 mg/mL PAA(p) in DMAc in the absence or presence of 0.05 M AuCl3. CVs were performed in 0.1 M PBS at pH 6.0. As can be seen, electrodeposited PAA alone exhibits several peaks (0.4, 0.2, and -0.1 V), suggesting that this film is electroactive. For comparison, clean bare GCE is featureless as expected. The CV of the PAA(p)-Au exhibits several additional peaks at approximately 1.0 and 0.5 V that are characteristic of Au, whereas the peaks at 0.4, 0.2, and - 0.1 V are attributed to the poly(amic acid). There are slight shifts of these peak potentials in the positive direction after the incorporation of the Au. These shifts are consistent and reproducible. When Ag was incorporated within the PAA membrane, the CV showed a very

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Figure 5. XRD patterns of (a) PAA-Ag and (b) PAA-Au.

sharp anodic peak at around 0.2 V that is characteristic of the Ag0/Ag+ couple, confirming the presence of Ag in the electrodeposited PAA membrane. In a recent work, we studied the electrochemistry of silver ions and attributed these peaks to the redox couple Ag0/Ag+.32 Structural Characterization: (A) FTIR Spectroscopy. The FTIR spectra for PAA and PAA-Ag films were recorded in the range between 500 and 1800 cm-1 following electrodeposition and drying. Table 1 summarizes the IR spectra of PAA(p), PAA(v), and PAA(v)-Ag, recorded in a 1.5% mixture with KBr. The absorption bands that occur at 1717 cm-1 and a shoulder at 1304 cm-1 were assigned to the vibrational modes of carboxylic acid, while the bands occurring at 1653, 1540, and 1410 cm-1 were assigned to the vibrational mode of the amide group.27 The strong peak around 1225 cm-1 that appeared in both spectra is associated with a stretching vibration of the ether group.33 All these bands were still present in the PAA-Ag spectrum after thermal treatment. The absence of typical imide functional bands in the cured PAA and PAA-Ag films at 1778 cm-1 (for symmetric CdO) and 1374 cm-1 (C-N-C axial)34 confirmed that the imidization process has been avoided at 105 °C. Moreover, the similarity in both spectra suggests that the characteristic functional groups present (32) K’owino, I.; Agarwal, R.; Sadik, O. A. Langmuir 2003, 19, 4344. (33) Prasad, M. B.; Singh, D.; Misra, R. A. J. Polym. Mater. 1996, 13, 157. (34) Yu, K. K.; Yoo, Y. H.; Rhee, J. M.; Lee, M. H.; Yu, S. C. Mater. Res. Innovat. 2003, 7, 51.

in PAA basic molecule retained their integrity after electrodeposition and drying. (B) 1H NMR Measurements. 1H NMR measurements were performed to further clarify the chemical structure of PAA-metal composite films. The NMR spectrum of PAA-Ag showed the presence of the aromatic protons of PMDA at 7.96, 8.32, and 7.86 ppm and of the amine protons at 10.42 ppm. For a similar PAA synthesized in THF, Echigo et al.31 recorded major peaks at 7.97, 8.33, and 7.70 ppm, and for PAA obtained in THF in the presence of TEA, at 7.98, 8.20 and 7.80 ppm. The changes in chemical shifts due to the PMDA protons suggest the formation of a salt between PAA and TEA. These peaks were also observed for plain PAA(v) and PAA(p) film prepared in this work. For PAA(v)-Ag prepared in the presence of TEA and silver salt, the protons shifted to the same value as for PAA(v)-Ag, suggesting that the triethylammonium cations were replaced with Ag(I) when silver salt is present, thus confirming the structure shown in Figure 1. We believe that both cis and trans structures are adopted along the polymer chains. (C) X-ray Diffraction. XRD analysis of the composite materials was performed to determine the chemical state of silver and gold incorporated within the PAA membrane. The XRD patterns of PAA-Ag and PAA-Au powder are shown in Figure 5. The diffractograms exhibit the peaks characteristics of crystalline state for metals. The full width at half-maximum (fwhm) of the strongest characteristic reflection (111) was used to estimate the average

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Figure 6. XPS analysis of PAA-Ag assembly: (a) survey scan; (b) Ag 3d region.

crystallite size by applying the Scherrer equation.35 The crystallite size of Ag particles in PAA-Ag membrane was 22.5 nm, while the crystallite size for Au particles in PAAAu film was 28 nm. The values of the lattice constant were calculated from their corresponding XRD patterns. The lattice constants of Ag and Au particles were 4.086 and 4.087 Å, respectively. These are consistent with the standard values for Ag (a ) 4.086 Å), given by the PDF file 040783, and Au (a ) 4.078 Å), in PDF file 040784. (D) X-ray Photoelectron Spectroscopy. The XPS spectrum of PAA-Ag was collected in order to determine the composition of the mixture and the possible oxidation state(s) of the silver atoms. The material was pressed against soft indium foil and then analyzed at a base pressure of 1 × 10-10 Torr. Survey and high-resolution scans were collected at specific pass energies (185 and 11.9 eV, respectively). The high-resolution spectra for the C1s, Ag3d, O1s, and N1s signals were collected in this mode in order to measure accurate binding energies. The survey scan is presented in Figure 6a. Quantification of material was done by integrating the signals in the survey spectrum and using sensitivity factors for the levels selected for analysis. The binding energies were referenced to the C1s signal of adventitious carbon at 284.6 eV. The C1s signal shows a carboxylate signal at 288.7 eV in addition to a main signal centered at 284.6 eV. The N1s region of the spectrum contains a single N1s signal at 400.3 eV, consistent with nitrogen atoms in polymer structure. The high-resolution spectrum for the Ag3d signals gave a binding energy of 368.7 eV, associated with the position of Ag (Figure 6b). It is also noteworthy to mention that a slight shoulder is present on the lower (35) (a) Cullity, B. D.; Stock, S. R. Elements of X-ray diffraction, 3rd ed.; Prentice Hall Inc.: Upper Saddle River, NJ, 2001; Chapter 5, p 170. (b) Silvert, P. Y.; Herrera-Urbina, R.; Duvauchelle, N.; Vijayakrishnan, V.; Elbsissen, K. T. J. Mater. Chem. 1996, 6 (4), 573.

Figure 7. SEM micrographs recorded for electrodeposited films on the RVC electrode: (a) PAA(v) film, 3500×; (b) Ag-PAA(p) film, 25000×; (c) Ag-PAA(v) film, 4000×. Electrodeposition conditions: 5 min at Dc ) 1.0 mA/cm2 in 0.1 M PBS, pH 6.0.

binding energy sides of the Ag signals. This suggests that some Ag+ atoms in an oxide environment may also be present as well. Finally, the O1s region contains a major signal at 532.1 eV and a minor signal at 530.3 eV. The former signal is associated with oxygen in contact with carbon atoms and the latter is associated with an inorganic oxide (most likely Ag2O). The composition of the PAA-Ag measured yielded 72.8% C, 4.5% N, 15.0% O, and 2.2% Ag. Surface Morphology. SEM and EDX analysis were performed for PAA and PAA-metal composite films to determine the morphology of films and to identify the nature and distribution of metallic particles in PAA films (Figures 7 and 8). SEM analysis was carried out on PAA films electrodeposited onto the RVC electrodes with and

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Figure 8. SEM images of electrodeposited PAA(p)-Au film on RVC electrode at a magnification of (a) 3500× or (b) 30000×. Electrodeposition conditions: 5 min at Dc ) 2.0 mA/cm2.

without metallic particles, after drying at 105 °C. The magnification used was between 3500 and 30000. The SEM images showed that all the coatings were smooth and uniform, regardless of deposition conditions. Also, the carbon vitreous structures of electrodes were completely covered with a uniform PAA layer, containing few amorphous PAA clusters (Figure 7a). When the PAA films contain noble metallic particles, the SEM analysis illustrates a uniform coverage of the film and a homogeneous distribution of metal particles along the carbon network. Figure 6 shows the morphology of the PAA(p)-Ag deposited in DMAc (Figure 7b) and of PAA(v)-Au deposited in THF (Figure 7c). Figure 8 shows the SEM of PAA(p)-Au developed in DMAc. The SEM images of these PAA-metal composites reveal the presence of metallic particles on the surface of the electrode, confirming the incorporation of silver and gold within the PAA film. EDX analysis was then used to confirm that the bright particles on the images are indeed silver (and gold). For simple electrodeposited PAA, EDX measurements identified carbon and oxygen as the principal elements of this

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material (Figure 9a). The bright spot particles observed in Figure 6b for the electrodeposited PAA(p)-Ag composite film were identified by EDX to be silver (Figure 9b). These particles have consistent size, with diameters on the order of 100 nm or less. The silver particles in PAA(v)-Ag obtained from THF (Figure 7c) have an average size of 1 µm. The EDX spectra also showed the presence of Ag, C, and O as well as Na, P, and Cl resulting from PBS (Figure 9c), thus indicating the retention of phosphate in polymer film. Figure 9d shows the EDX spectrum of the PAA(p)Au film. The intense Au signal is an indication of the presence of Au particles with the film. The majority of these particles are consistent in size and shape. EDX analysis suggests that the supporting electrolyte plays an important role in size and distribution of metallic particles in PAA-metallic composite films. Summary of Results. We have prepared polyamic metal nanocomposite films using electrochemical deposition and thermal reduction. Electrodeposition was carried out from both aqueous (PBS) and nonaqueous (DMAc, DMF, DMSO) solutions utilizing PAAs derived from PMDA and ODA. Two forms of PAA were obtained in different conditions: the first one was a viscous solution that was prepared in THF containing a surfactant (TEA) and a precipitant (MeOH) and is soluble in PBS. The second was prepared in ACN as a yellow powder and is soluble in organic solvents such DMAc, DMF, and DMSO. A summary of the characteristics of the two PAAs studied in this work is presented in Table 2. The two PAAs were used to incorporate Ag and Au metallic particles in the polymeric matrix. The results presented in this work demonstrated that the solvent used in the electrodeposition process plays an important role in controlling the size and distribution of these particles within the PAA film. For instance, when PAA(v)-Ag was electrodeposited in PBS, a homogeneous distribution of silver metals with a size in the nanometer range was obtained, whereas when an organic solvent (DMAc) was used, the silver particles in PAA(p)-Ag were in the micrometer range. Using a similar procedure in DMAc allowed incorporation of Au particles within PAA(p) with a size in the micrometer range. Mechanistically, we believe that when the first solvent system was used, the TEA neutralizes the carboxylic groups of PAA.27 Thus, the electrons involved in the reduction of silver ions to metallic silver could be derived from the polymer with no changes in the basic mechanical and physical properties of the resulting PAA film. In the second system, the formation of silver-polyamate salt in the electrodeposition process is most probable.36 A possible explanation of this behavior was suggested by Warner et al.25 when studying the synthesis of hexafluoroisopropylidene-containing polyimide-silver nanoparticles. They believe that the process involves the formation of -COOAg groups in PAA. The coordination sites at Ag(I) involve the donor groups from the macromolecules, to generate uniform particle distribution. In the case of DMAc, the Ag(I) from polyamate is preponderant and permits the agglomeration process after reduction. The structural characterization (FTIR, NMR) of the films established the chemical structure and the presence of major functional groups of the PAA. These also confirmed that the cyclization process was prevented. In addition, morphological characterization of the film confirmed the presence and the distribution of metallic particles within the polymeric matrix, while X-ray diffraction provided evidence of the reduction of metal ions (36) Linde, H. G. J. Appl. Polym. Sci. 1990, 40, 613.

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Figure 9. EDX spectra of various electrodeposited films on RVC electrode: (a) PAA(v); (b) PAA(p)-Ag; (c) PAA(v)-Ag; (d) PAA(p)Au. Table 2. Summary of Results Obtained for the PAA, PAA Films, and PAA-Metal Composites solvent used for preparation physical state solubility PAA-metal composites particle size (Ag and Au)

PAA(v)

PAA(p)

THF (+TEA and MeOH) viscous liquid PBS homogeneous distribution of metallic particles nanometer range

ACN yellow powder DMAc, DMF, DMSO homogeneous distribution of metallic particles micrometer range

to metallic state for both Ag and Au. The XPS shows the presence of both metallic and oxidized states for silver in a PAA-Ag film. In summary, we demonstrated that by combining the electrodeposition process with thermal curing, PAA metallized films could be prepared onto carbon vitreous electrodes. Conclusion In conclusion, this work describes a new method for the formation of a PAA-Ag (and -Au) nanocomposite film as electrode materials by use of electrodeposition and thermal treatment. This approach utilizes the unique reactivity of PAA by preventing the cyclization of the reactive soluble intermediate into polyimides at low temperature to design polymer-assisted nanostructured materials. We demonstrated that by an appropriate selection of the experimental variables (solvent and deposition conditions) it is possible to create PAA-metal composite films with controlled morphology, distribution, and particle size. The results showed that the solubility of the synthesized PAA film depends greatly on the synthetic conditions and solvent used, and this greatly affects the characteristics of the composite film. These materials could find a wide range of application in many fields. For instance, the presence of free carboxylic groups in the PAA and PAA-metal electrode may allow

the utilization of this material as immobilization matrices in immunoassay, chromatographic stationary phases, biosensor devices, catalysts, and conductivity-enhanced electrode materials. The free carboxyl moieties exposed to the surface of these polymers could be chemically modified to achieve a high-density anchoring of biomolecules,37 while the presence of Au nanoparticles could be used to further attach a self-assembled monolayer. Acknowledgment. We acknowledge the Environmental Protection Agency through the STAR program (RD-83090601) for funding. We gratefully acknowledge Dr. R. Tatini for the determination of Mw by gel-permeation chromatography and Deborah Dietrich from the Geology Department at SUNYsBinghamton for ED-SEM analysis. We also thank Dr. Dan V. Goia from Center for Advanced Materials Processing, Clarkson University, Potsdam, NY, for SEM and XRD analysis and Jonathan Macko for working as summer intern on this project. O.A.S. acknowledges Harvard University for a Distinguished Radcliffe Fellowship LA050141K (37) (a) Andreescu, S.; Sadik, O. A. Anal. Chem 2004, 76, 552. (b) Yan, F.; Sadik, O. A. J. Am. Chem. Soc. 2001, 123, 11335. (c) Yan, F.; Sadik, O. A. Anal. Chem. 2001, 73, 5272.