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Articles Formation of Ultrathin Polypyrrole (PPY) Films on Alumina Particles Using Adsorbed Hexanoic Acid as a Template Gyoujin Cho,*,† Daniel T. Glatzhofer,*,‡ Bing M. Fung,‡ Wei-Li Yuan,§ and Edgar A. O’Rear§ Department of Chemical Engineering, Sunchon National University, 315 Maegok Sunchon, Chonnam 540-742, Korea, Department of Chemistry and Biochemistry, the Institute for Applied Surfactant Research, and the Department of Chemical Engineering and Materials Science, The University of Oklahoma, Norman, Oklahoma 73019 Received March 3, 1999. In Final Form: February 10, 2000 Thin films of polypyrrole (PPY) were deposited on alumina particles using hexanoic acid adsorbed on the particles as a template. The formation of the template on alumina was confirmed through studies of the adsorption of hexanoic acid at various pH values. The adsorption results could be fitted to the Langmuir adsorption isotherm at pH levels of 5 and 6, indicating monolayer adsorption. Solubilization studies of pyrrole on alumina particles with adsorbed hexanoic acid in aqueous solution at pH levels of 3, 4, 5, and 6 showed that a large amount of pyrrole was condensed at the interface at a pH level of 4, and subsequent polymerization can be carried out at the surface of alumina. In the atomic force microscopy studies, the morphology of the resulting PPY on alumina particles showed multigranular images similar to those of pure polypyrrole films and all alumina particles were completely coated by polypyrrole films. The contact conductivity of these PPY-alumina composites (4.7 × 10-2 s‚cm-1) is more than 3 times that of a pure polypyrrole powder, even with a very low amount (12 wt %) of polypyrrole incorporated onto alumina.
Introduction The discovery of organic conducting polymers (OCP) has led to the increasing displacement of both conventional metals and inorganic materials in a variety of applications, for example, optical displays, sensors, rechargeable batteries, membranes, electromagnetic interference shielding, nonlinear optics, and microelectronics.1 Unfortunately, the use of conducting polymers has often been impeded by their intractibility.2 Consequently, much effort has been directed toward improving the tractability of OCP using new methods. In these new methods, control of both microscopic and macroscopic structural homogeneity of the polymer is essential for the development of such materials with enhanced electronic, optical, and magnetic properties. Two methods currently exist for improving the tractability of OCP. One method involves mixing the conducting polymer of interest with a second inert material to form a conductive composite. In this way, the matrix material not only strengthens the composite (thereby providing a structural framework) but also serves to protect the polymer from both ambient moisture as well as air. This type of approach toward improved tractability * To whom correspondence should be addressed. † Sunchon National University. ‡ Department of Chemistry and Biochemistry and the Institute for Applied Surfactant Research, The University of Oklahoma. § School of Chemical Engineering and Materials, The University of Oklahoma. (1) Meyer, W. H. Adv. Mater. 1993, 5, 254. (2) Reynolds, J. R.; Pomerantz, M. In Electroresponsive Molecular and Polymeric Systems; Skotheim, T., Ed.; Marcel Dekker: New York, 1991; Vol. 2, p 187.
has been moderately successful, particularly in the preparation of sterically stabilized polypyrrole (PPY) colloids via dispersion polymerization.3 Since 1986, various research groups have reported the preparation of sterically stabilized colloidal dispersions of PPY incorporating either commercial polymers or metal oxide particles.3 Thin conducting films can then be fabricated from them. Although this approach significantly improves the processibility of these otherwise intractable materials, it still has shortcomings in obtaining the desired microscopic and macroscopic structural homogeneity. The second method to improve the tractability of OCP involves using a wide variety of host materials as templates including glasses,4 ceramics,5 and polymers6 for polypyrrole synthesis. The second method also provides a rare opportunity to control both the morphology and structure (3) (a) Barkland, R. B.; Liedberg, B. J. Chem. Soc., Chem. Commun. 1986, 1293. (b) Armes, S. P.; Miller, J. F.; Vincent, B. J. Chem. Soc., Chem. Commun. 1987, 288. (c) Armes, S. P.; Miller, J. F.; Vincent, B. J. Colloid Interface Sci. 1987, 118, 410. (d) Cawdery, N.; Obey, T. M.; Vincent, B. J. Chem. Soc., Chem. Commmun. 1988, 1189, and references therein. (e) Odegard, R.; Skotheim, T. A.; Lee, H. S. J. Electrochem. Soc. 1991, 138, 2930. (f) Digar, M. L.; Bhattacharyya, S. N.; Mandal, B. M. J. Chem. Soc., Chem. Commun. 1992, 18. (g) Armes, S. P.; Aldissi, M. Synth. Met. 1990. 37, 137. (4) Mehrotra, V.; Keddie, J. L.; Miller, J. M.; Giannelis, E. P. J. NonCryst. Solids 1991, 136, 97. (5) Mehrotra, V.; Giannelis, E. P. Solid State Commun. 1991, 77, 155. (6) (a) Armes, S. P.; Aldissi, M. J. Chem. Soc., Chem. Commun. 1989, 88. (b) Armes, J. P.; Aldissi, M.; Agnew, S. F.; Gottesfeld, S. Langmuir 1990, 6, 1745. (c) Ruckenstein, E.; Chen, J.-H. J. Appl. Polym. Sci. 1991, 43, 1209. (d) DeArmitt, C.; Armes, S. P. J. Colloid Interface Sci. 1992, 150, 134. (e) Maeda, S.; Carirns, D. B.; Armes, S. P. Eur. Polym. J. 1997, 33, 245. (f) Simmons, M. R.; Chaloner, P. A.; Armes, S. P. Langmuir 1998, 14, 611.
10.1021/la990247d CCC: $19.00 © 2000 American Chemical Society Published on Web 04/14/2000
Formation of Ultrathin PPY Films on Alumina Particles
of the composites. Recently, to enhance the microscopic structural homogeneity in the composite, both the LB wellordered semicrystalline method and chemisorbed selfassembled arrays (SAAs) have been used as templates for the direct deposition of thin PPY films on the surface of host materials.7 Although these processes present potential advantages for the preparation of the microscopically homogeneous composites (as well as controlling morphologies in other conducting polymers), more versatile, convenient, and inexpensive procedures to produce results comparable to those of both the LB and chemisorbed SAA methods are currently in demand. It has recently been reported that physisorbed SAAs possess well-ordered, quasi-two-dimensional structures comparable to the LB and chemisorbed SAA.8 Those physisorbed SAAs can then simply alter the intrinsic surface properties of substrates so that the morphology and structure of polymers formed on them are significantly influenced by both the aggregate structure and molecular orientation of the physisorbed SAA. In addition, the physisorbed SAA also plays an important role in both the nucleation and growth of polymer films.9 The use of physisorbed SAA as templates can therefore produce unique, thin polymer films coated substrates. To grow PPY films on the surface of substrates using physisorbed SAA as templates, pyrrole monomers should be diffused into the SAA or condensed onto the SAA before polymerization. However, our previous results showed that the formation of PPY films on substrates using SAA of surfactants is problematic because pyrrole does not partition sufficiently into the adsorbed surfactant bilayer structures.10 It was postulated that this difficulty may stem from both the high water solubility of pyrrole as well as the hydrophilic surface of adsorbed surfactant bilayers. Studying the solubilization of pyrrole in SDS admicelles showed that external influences (e.g., electrolyte concentration, pH, temperature, etc.) did not dramatically change the amount of solubilized pyrrole in adsorbed SDS bilayers.10 One possible solution to enhance the solubilization of pyrrole in adsorbed surfactant layers (and in turn decrease pyrrole’s water solubility) would be the use of N-alkylated pyrroles. The use of N-alkylated pyrroles, however, would increase the cost while simultaneously sacrificing the electrical conductivity of the deposited PPY on the substrate. To enhance the pyrrole concentration at solid-water interfaces, the surface of the solid should be modified using simple and inexpensive procedures suited for industrial applications. At this point, we chose alumina as the substrate and used organic acids to modify the alumina surface instead of surfactants because carboxylic groups can be “semichemically” adsorbed onto alumina surface.11 Hexanoic acid was chosen for adsorption and pyrrole solubilization because it has reasonable water solubility, is convenient, and has low cost for preparative-scale applications. The schematic process of PPY deposition on alumina particles using adsorbed hexanoic acid as a template is shown in Figure 1. Both the surface of acidic alumina and the dissociation of hexanoic acid under aqueous conditions are pHsensitive. Therefore, the adsorption of hexanoic acid onto (7) (a) Willicut, R. J.; McCarley, R. L. J. Am. Chem. Soc. 1994, 116, 10823. (b) Rozsnyai, L. F.; Wrington, M. S. Langmuir 1995, 11, 3913. (8) Soderlind, E. Langmuir 1994, 10, 1122. (9) Dunaway, D. J.; McCarley, R. L. Langmuir 1994, 10, 3598, and references therein. (10) Funkhouser, G. P.; Arevalo, M. P.; Glatzhofer, D. T.; O’Rear, E. D. Langmuir 1995, 11, 1443. (11) Kummert, R.; Stumm, W. J. Colloid Interface Sci. 1980, 75, 373.
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Figure 1. Schematic illustration of the PPY deposition process on alumina particles using adsorbed hexanoic acid as a template at pH ) 4.
aluminum oxide particles from the aqueous phase has been studied at various pH levels. On the basis of the hexanoic acid adsorption results, the solubilization of pyrrole by the adsorbed hexanoic acid on alumina surfaces has been investigated. Finally, the resulting PPY-coated alumina was characterized using thermogravimetry, atomic force microscopy (AFM), and contact conductivity measurements. Experimental Section Materials. Hexanoic acid (99%, Aldrich) was dried over MgSO4 and fractionally distilled from CaSO4. Pyrrole (98%, Aldrich) was purified by passing it through a short column of basic alumina, activity grade I (Sigma). High surface area (155 m2/g) acidic alumina powder, activity grade I, was obtained from Aldrich Chemical Co. Reagent grade ammonium persulfate (Mallinckrodt) and hydrochloric acid (36-38%, EM science) were used without further purification. Hexanoic acid concentrations from the adsorption experiments and pyrrole concentrations from adsolubilization studies were determined using a Scientific Systems Model 520c Isocratic HPLC system equipped with a Showdex RI-71 refractive index detector and Wescan high-speed cation column with deionized water as an eluent. Adsorption Isotherms and Pyrrole Adsolubilization. For adsorption studies, 1 g of Al2O3 and 5 mL of solution containing various concentrations of hexanoic acid were put into separate vials and the pH level was adjusted as desired (pH 3-6) using reagent grade HCl and concentrated K2CO3 solutions. The vials were then sealed with Teflon septa. The samples were allowed to equilibrate at 30 °C in a thermostated water bath for 2 days. Adsorption isotherms were constructed by measuring the concentration of hexanoic acid in each solution before and after adsorption using HPLC. The amount of hexanoic acid adsorbed from solution onto the surface of the alumina powder was calculated using the relationship
G ) (Ci - Cf)V/m
(1)
where G is the number of moles of hexanoic acid adsorbed per gram of solid substrate at adsorption equilibrium, Ci is the initial molar concentration of hexanoic acid in solution before adsorption, Cf is the molar concentration of hexanoic acid in solution after
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Figure 2. Schematic diagram of the instrument for measuring the contact conductivity (σ). The values of σ were calculated using the equation σ ) l/(RA), where l is the length of the pressed alumina composite, R is the resistance, and A is the crosssectional area of the glass tube bone. adsorption, V is the volume of the solution, and m is the weight of the solid substrate. The solubilization of pyrrole on adsorbed hexanoic acid was studied at or near complete coverage of the alumina surface. Adsorption isotherms at pH levels of 3, 4, 5, and 6 were constructed using the hexanoic acid adsorption data, and the initial concentration of hexanoic acid to achieve complete coverage on alumina was calculated from the isotherms. To study pyrrole solubilization under different pH conditions, hexanoic acidadsorbed samples were first prepared by following the previously discussed procedures. After 2 days to establish equilibrium, 0.73 mmol of pyrrole was injected into each sample, which were then allowed to equilibrate further under the same conditions for 1 day. After an optimum pH level was found, the detailed study for the solubilization of pyrrole on adsorbed hexanoic acid was carried out under a selected pH level. For comparison, pyrrole adsorption on bare alumina was studied at the selected pH level as well. The final equilibrium concentrations were analyzed by HPLC (mobile phase: water); the adsolubilized amounts of pyrrole were determined using eq 1. Polymerization of Pyrrole at the Surface of Alumina. Polymerizations of pyrrole on adsorbed hexanoic acid and bare alumina were initiated by adding an equimolar amount of ammonium persulfate (based on pyrrole) into the pyrrole solubilized sample. In this work, we used 1 g of acidic Al2O3, 45 mM of hexanoic acid with 5 mL water, and various pyrrole concentrations at the optimum pH level (4.0) to study the relationship between the amounts of solubilized pyrrole and contact conductivity for polypyrrole-coated alumina powders. The contact conductivities were then measured using a two-probe technique (Figure 2). Atomic Force Microscopy (AFM). An AFM (Digital Instruments, Inc. Nano Scope II) was used to study the surface characteristics of PPY films on alumina powders. Bare alumina and PPY/alumina particles were mounted on scanning plates with glue and scanned as normal with forces usually on the order of 10-10 to 10-11 N.
Results and Discussion Adsorption Isotherm. The chemisorption of molecules from gases and solutions can be described by the Langmuir adsorption isotherm, which is applicable for the formation of monolayers on adsorbents12 and is expressed by
N ) NmKC/(1 + KC)
(2)
where N is the amount of adsorption per gram of solid adsorbent, Nm is the limiting adsorption of monolayer coverage, K is an adsorption equilibrium constant, and C is the equilibrium concentration of adsorbates. The parameters Nm and K can be calculated from adsorption isotherm data. For the adsorption of organic acids on alumina,13 it was determined that the carboxylic acid headgroups are (12) (a) Giles, C, H.; Smith, D. J. Colloid Interface Sci. 1974, 47, 755. (b) Giles, C. H.; D’Silva, A. P.; Easton, I. A. J. Colloid Interface Sci. 1974, 47, 766. (13) Mielczarski, J. A.; Cases, J. M.; Bouquet, E.; Barres, O.; Delson, J. F. Langmuir 1993, 9, 2370.
Figure 3. Adsorption isotherm of hexanoic acid on alumina at different pH levels.
adsorbed on the alumina surface by semi-chemical-bond formation at the water-alumina interface,14 not by ionic interactions as in the adsorption of surfactants. In fact, infrared studies measuring the adsorption of organic acids on aluminum oxide surfaces showed that while the carboxylic acid CdO adsorption (at 1703 cm-1) is absent in the adsorbate, a new, broad peak at a lower frequency (1608 cm-1) appeared. This result suggests the formation of strongly bound carboxylate surface species,15 with adsorption energies much larger than those of other organic compounds.16 For carboxylic acids with very small hydrophobic parts, such as acrylic acid and maleic acid, the adsorption on alumina obeys the Langmuir isotherm.17 For the adsorption of hexanoic acid from aqueous solutions onto the surface of alumina, the results of measurements conducted at pH levels of 3, 4, 5, and 6 are shown in Figure 3. As expected, the adsorption of hexanoic acid on alumina at pH levels of 5 and 6 obeys the Langmuir isotherm. The maximum amount of hexanoic acid adsorbed was found to be 1.3 mmol/g. If a complete monolayer coverage is assumed, the area occupied per molecule is 0.2 nm2, a reasonable value for hexanoic acid with an all-trans conformation. Visually, at the concentration range for the complete monolayer coverage from the isotherm at pH levels of 5 and 6, the alumina particles floated on top of the aqueous solution. This is indirect evidence for the formation of a hexanoic acid monolayer on alumina at pH levels of 5 and 6. However, the shapes of the isotherms of pH levels of 3 and 4 are different from simple Langmuir adsorption, and the data indicate that the adsorption is a multilayer phenomenon. Although the chain length of five carbons is not long enough to result in very large van der Waals interactions between the alkyl chains of hexanoic acid,18 the hydrophobic interaction between the alkyl chains at low pH (
