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J. Phys. Chem. B 1997, 101, 11106-11111
Surface Polymerization of Organic Monomers on Cu(II)-Exchanged Hectorite T. L. Porter* Department of Physics and Astronomy, Northern Arizona UniVersity, Flagstaff, Arizona 86011
M. P. Eastman and D. Y. Zhang Department of Chemistry, Northern Arizona UniVersity, Flagstaff, Arizona 86011
M. E. Hagerman Department of Chemistry, Union College, Schenectady, New York 12308 ReceiVed: January 29, 1997; In Final Form: October 1, 1997X
Exposing layered silicate materials such as hectorite and montmorillonite to aniline vapor results in spontaneous surface polymerization as well as intergallery polymerization of the organic monomer. The inorganicorganic assemblies produced in these reactions possess many unique chemical and electronic properties, including the ability to function as chemical sensors. We have studied the polymerization of aniline on the surface of Cu(II)-exchanged hectorite thin films. The in-situ nucleation and growth of these polymer films is studied for the first time using the technique of scanning force microscopy (SFM) phase contrast imaging. A novel mechanism for the nucleation and growth of the surface polymer film is proposed. The availability of Cu(II) cations via defects or faults in the layered silicate structure is crucial to the formation of the subsequent conductive polymer layer.
Introduction Layered silicates such as hectorite and montmorillonite possess unique structural and chemical properties that facilitate novel reactions with organic guests such as benzene, aniline, pyrrole, thiophene, and even certain amino acids and nucleotides.1-7 In many of these organic-inorganic hybrids, the organic compound provides the desired functional character while the inorganic component provides the stable host framework to adapt the material to different forms and applications. In the clay mineral hectorite, individual layers are formed by sandwiching a sheet of octahedrally coordinated Mg2+ ions between two silica sheets. Substitution of Li+ for Mg2+ in the octahedral sheet gives each three-sheet layer an overall negative charge (in the range 0.2-0.6 per formula unit). The final structure is formed by alternating these three-sheet layers with “gallery” layers containing water and exchangeable metal counterions. The gallery regions contain positive charge and thus serve to hold the individual negatively charged clay layers together through electrostatic forces.8,9 Faults, edges, and pores in the macroscopic clay fabric make these gallery regions accessible to external chemical agents. Even under standard atmospheric conditions, organic monomers such as benzene, aniline, thiophene, and pyrrole may intercalate into these regions and react with the gallery cations. Strong local electric fields in the interlayer regions enhance the oxidation potential of the transition metal ions and drive the reactions, such as the oxidation of benzene by Cu(II), which does not normally occur under ambient conditions.10,11 For example, we have shown that aniline intercalated into the intergallery regions of Cu(II)exchanged hectorite readily polymerizes to form nearly twodimensional sheets of polyaniline.12,13 In addition, a layer of surface polyaniline forms with the more common nanometerscale bundle structure observed in chemically prepared or electrochemically prepared polyaniline films.12,13 Polyaniline nanocomposite materials have also been successfully synthesized X
Abstract published in AdVance ACS Abstracts, December 1, 1997.
S1089-5647(97)00352-0 CCC: $14.00
Figure 1. Intergallery region of intercalated hectorite/aniline composite exposed using razor cleaving. A partial two-dimensional layer of polyaniline that has formed in the gallery region is visible. The dimensions of this image are 0.5 µm × 0.5 µm.
using the inorganic hosts MoO3 and FeOCl.14-16 Figure 1 shows a noncontact mode (NCM) scanning force microscope (SFM) image of a cleaved hectorite/polyaniline composite film, exposing a partial polyaniline layer that has formed in the hectorite intergallery region. The dimensions of this image are 0.5 µm × 0.5 µm. The surface chemical properties of layered silicates such as hectorite play a vital role in the understanding of the overall inorganic-organic host/guest system. These hybrid materials show promise in applications such as electronic, optical, photonic, and chemical sensing.9,13,17 Studies of the initial surface silicate-polymer formation process, as well as the microscopic physical and chemical properties of the finished composite, are needed in order to model and characterize sensing devices based on these materials. In this report, we use the technique of SFM phase-contrast imaging to study the in-situ © 1997 American Chemical Society
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Figure 2. Cu(II)-exchanged hectorite surface exposed to aniline vapor in microcell for 20 min (2a, top), 1 h (2b, first from the top), 2 h (2c, first from the bottom), and 24 h (2d, bottom). The leftmost images are topographical scans, with corresponding phase-contrast images on the right. In the phase-contrast images, darker regions correspond to polyaniline on the surface, while lighter regions indicate the underlying hectorite substrate.
nucleation and growth process of aniline/polyaniline on the surface of Cu(II)-exchanged hectorite thin films. Experimental Section Cu(II)-exchanged hectorite was prepared by stirring sodium hectorite in a solution of 1.0 M CuSO4 for 24 h. The resulting material was washed with distilled water and then centrifuged
until a negative Ba2+ test for SO42- was obtained. Thin hectorite films were then prepared by suspending the material in deionized water and casting onto polished aluminum disks. All hectorite films were dried for a minimum of 24 h prior to organic vapor exposure. The dried Cu(II)-exchanged hectorite films were exposed to aniline vapor using a “microcell” apparatus supplied by Park
11108 J. Phys. Chem. B, Vol. 101, No. 51, 1997
Figure 3. 3200 Å × 3200 Å NCM topographical scan of the hectorite surface near a polyaniline island. The average lateral distance between neighboring nucleation sites is approximately 100 Å, while the lateral size of the small bundles ranges from less than 20 Å to about 100 Å.
Scientific Instruments, Inc. The microcell provides a completely sealed environment for the vapor (or liquid) exposure and subsequent in-situ imaging of the sample film. Both standard contact and noncontact SFM imaging modes are possible within the cell. For the present study, noncontact mode SFM with phase-contrast imaging enabled was used to obtain the images shown. The SFM instrument is an Auto-Probe CP from Park Scientific. The addition of phase contrast imaging allows for the discrimination of surface areas with differing material properties as well as collection of the usual surface topography information. The flow of aniline vapor over the hectorite surface took place at atmospheric pressure, with no prior vacuum preparation of the hectorite host. The aniline vapor was drawn from a beaker containing aniline in solution, and injected into the microcell using a standard syringe. Approximately 1 mL of aniline containing vapor was injected into the cell at 15 min intervals for a period of 24 h, with SFM imaging over the same surface location at 15 min to 2 h intervals. At the end of the 24 h period, the hectorite sample surface color had changed from light blue to gray-blue, indicating the possible formation of a surface polyaniline layer. Powder X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) spectroscopy were also used to characterize hectorite samples exposed to aniline vapor. In this previous study,12 XRD spectra indicated a net layer swelling of 2.5 Å owing to aniline incorporation and water loss from the hectorite intergallery region. This 2.5 Å net increase corresponds to the formation of approximately one to two gallery layers of polyaniline, depending on the precise intergallery polyaniline morphology. Coupled with the observed color change from blue to gray to black, EPR spectra (Bruker ESP-300E) during aniline exposure show that all Cu(II) cations are reduced, presumably to Cu(I), during the 24 h vapor exposure period, an indication that oxidative polymerization of the aniline is indeed occurring. Results and Discussion While previous studies on layered silicate/conductive polymer systems have clarified the formation of intergallery polymer formation,12,13,18 comparatively less attention has been paid to the formation of surface polymer layers in these same systems. The surface structure and composition of these inorganic/organic hybrids will undoubtedly play an important role in determining
Porter et al. the overall electrical and optical properties of sensing devices based on these materials. In the case of intergallery polymerization of the organic guest, oxidative polymerization of the intercalated monomer is assisted by the presence of metal cations (Cu2+ in the present case). The local electric field structure surrounding the organic guest (within the confines of the gallery region) is also important in the polymerization process, however. The precise nature of the electric field contribution has not yet been identified. In this study, we have monitored the in-situ reactions that occur on the surface of Cu(II)-exchanged hectorite thin films exposed to aniline vapor. In Figure 2, the surface of Cu(II)-exchanged hectorite is shown at various times as it is exposed to aniline vapor in the microcell. The dimensions of these images are all 1 µm × 1 µm. The time of aniline exposure for these scans was 20 min (Figure 2a), 1 h (Figure 2b), 2 h (Figure 2c), and 24 h (Figure 2d). The images were all acquired in noncontact mode over the same surface region. The leftmost images are topographical images, while those on the right are phase-contrast images. The vertical scales shown apply only to the topographical images. In standard noncontact mode (NCM), amplitude changes in the vibrating cantilever while immersed in the tip-surface force field provide the necessary error signal for the tip to follow the surface contours without ever making contact with the surface. When the phase contrast mode is enabled, the phase of the vibrating cantilever relative to the cantilever driving signal is also monitored (and plotted in the case of Figure 2). Many physical properties of the underlying sample can affect the cantilever phase, including material stiffness, material adhesion properties, and surface water adsorption. In a simple two-component system, only small differences in one or more of these properties is sufficient to allow for material differentiation during phasecontrast scanning. Previous studies by us on the hectoriteaniline (polyaniline) system12 indicate that the cantilever phase generally lags the driving signal while the tip is over the “softer” polymer, as opposed to the more rigid silicate surface. This phase lag results in polymer regions appearing darker in the images than the surrounding hectorite substrate. Even at the very low aniline exposures used, the surface reaction proceeds rapidly (for example, a sample of Cu2+exchanged hectorite placed near an open beaker containing aniline solution and covered will react completely within 48 h, the hectorite being covered with a black layer of polyaniline). On the basis of this experience, we conclude that the microcell aniline exposure over a 24 h period results in at least several monolayers of physisorbed aniline. We also note that if the Cu2+-exchanged hectorite substrate is replaced by the naturally occurring Na+ hectorite, the reaction does not proceed. It is evident from the images in Figure 2 that a modified island growth, or modified Volmer-Weber growth, mode is the preferred mechanism for surface layer formation in this inorganic-organic system. Physisorption of the aniline monomer likely occurs over the entire hectorite surface, owing to the polar nature of the aniline molecule and the surface layer charge present in the hectorite layers. (Substitution of Li+ for Mg2+ in the octahedral sheet gives each three-sheet layer an overall negative charge in the range 0.2-0.6 per formula unit. This charge is compensated by the presence of intergallery cations in the bulk material, but not on the topmost surface layer.) These aniline molecules, weakly bound to the surface, have sufficient energy at room temperature to diffuse to more active surface sites where chemisorption and subsequent polymerization may then occur. For aniline to spontaneously polymerize on the Cu(II)-exchanged hectorite surface, we believe that two conditions must both be met. First, aniline monomers or oligomers
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SCHEME 1: Proposed Mechanism for the Polymerization of Aniline on the Surface of Cu2+ Exchanged Hectorite
on the surface must be in the presence of the Cu(II) cations in order to facilitate oxidation prior to polymerization. The possible presence of Cu(II) cations on the surface would not be sufficient alone, however, to account for the observed polymerization of the aniline adsorbate. Active surface regions, e.g., cracks, faults, or pores in the layered surface, and/or previously formed polyaniline on the surface must provide a “gettering” effect for additional aniline in order to further facilitate charge transfer and subsequent polymer growth. From Figure 2, we see that the lateral growth of the large islands apparently proceeds through the growth and subsequent coalescence of much smaller polymer bundles in the regions surrounding the polyaniline islands. This indicates that polymerized aniline has a “gettering” effect on the surface monomer, the vapor-phase monomer, or both.19-21 Even at the low aniline vapor pressure used in this experiment, the initial surface polymer has nearly formed a complete layer in 24 h. In Figure 3, a 3200 Å × 3200 Å NCM topographical scan of a surface region near the edge of a large polyaniline island is presented. Since this is a topographical scan, light or higher regions now correspond to the surface polymer as opposed to the underlying hectorite substrate. Dozens of small nucleation sites are visible, with the average distance between neighboring sites being approximately 100 Å. The lateral size of these tiny polymer bundles ranges from less than 20 Å to about 100 Å. Because of tip convolution effects present when scanning small
structures such as these, the reported sizes may be overestimated (assuming a tip radius of 20-100 Å for the conical tips used). The small polymer bundles will begin to coalesce into a continuous film (and thus contribute to the overall growth of the large islands) when the bundle diameters all approach 100 Å. Near the top of this image, the underlying hectorite surface can also be seen. The hectorite surface is “pocked” with many small dark regions, which correspond to individual micropores or faults in the layered silicate surface structure. The lateral size and spacing of these micropores corresponds roughly to the size and spacing of the observed nucleating polymer bundles in the lower portion of this image. The depth of these micropores ranges from 10 to 20 Å, an indication that the faults are generally in the topmost silicate layer only. While much larger pores and faults (on the micron scale) have been previously reported,12,13 this is the first observation of defects in the hectorite surface on the scale of 100 Å or less. These micropores or faults in the hectorite surface are the most likely surface locations for the initial nucleation of the polyaniline bundles as seen in Figure 3. Physisorbed aniline diffusing to these sites would have a source of Cu(II) cations as well as a “suitable” structural site for the first oxidation and polymerization reactions to occur. The precise surface structural contribution to the polymerization process remains difficult to pinpoint, except that confinement within a surface fault or pore is clearly necessary for the initial polymerization reaction to
11110 J. Phys. Chem. B, Vol. 101, No. 51, 1997 occur (as mentioned earlier, aniline will not polymerize when simply in the presence of Cu(II) cations). Some type of surface or near surface effects must also come into play. We have previously postulated that strong local electric fields owing to the silicate microstructure assist in the necessary charge transfer for initial oxidation of aniline to occur.12,13 In one possible mechanism for the observed polymer formation, the reaction is initiated by the formation of an aniline radical cation, which is an oxidation product of aniline produced by oxidants such as Cu2+ (present in the clay) and H2O2 formed as a byproduct during the reaction. Specifically, one aniline molecule is initially oxidized by a Cu2+ atom to form a radical cation, which then reacts with a neutral aniline molecule to form a resonance-stabilized radical cation dimer (Scheme 1). The dimer then loses one proton to the environment. The resulting radical dimer then loses the para hydrogen atom via a hydrogen abstraction reaction by O2 in air (a) to form p-(phenylamino)aniline. The radical HOO• formed in reaction a then abstracts another hydrogen atom on the amino group to form hydrogen peroxide (b). As a result, a neutral dimer radical is formed which reacts with another aniline molecule (c). The same steps a-c repeat to produce polyaniline in its leucoemeraldine form. As the reaction proceeds, H2O2, which is formed in step a, may also serve as an oxidant in the generation of aniline radical cations, and Cu2+ may be regenerated via oxidation of Cu+ by O2 in air. As the individual polymer bundles continue to grow and coalesce, a continuous surface layer of polyaniline is formed. Additional exposure of this film to aniline vapor results in continued polymer growth. The thickness of the polymer film increases, and larger, discrete polymer bundles begin to aggregate. Finished films prepared in this maner (Figure 4a) exhibit the same nanometer-scale bundle or grain structure as polyaniline films prepared using more standard chemical or electrochemical methods.23 The role of the exchanged Cu(II) cations in the initial growth process of the surface polymer cannot be understated. Figure 4b shows the surface of an Na+ hectorite (in its natural state, hectorite contains intergallerey Na+ cations) film after 24 h exposure to aniline vapor. Without an initial Cu(II)-assisted polymerization mechanism on the surface, no polyaniline is formed. Finally, we make note of the fact that Cu(II)-exchanged hectorite/aniline composite films such as these exhibit promising electrical responses to organic vapors such as ethanol and hexane. When prepared as thin films on interdigitated arrays, the ac impedance of these devices clearly changes in response to the presence of these vapors,13 whereas Na-exchanged type films exhibit no electrical response.5 Conclusions The electrical properties of sensing devices based on inorganic-organic hybrid materials will depend not only on the bulk properties of the intercalated materials but also on the surface layers formed during device fabrication. When thin films of Cu(II)-exchanged hectorite are exposed to the organic monomer aniline, surface nucleation and polymerization proceed spontaneously, as does intergallery intercalation and polymerization. This polymerization does not occur in the case of naturally occurring Na+ hectorite substrates. Nucleation sites on the hectorite surface consist of small micropores or faults in the topmost silicate layer. Initial oxidation of aniline at these sites is facilitated by the presence of Cu(II) cations within the pores. In addition, local structural effects within the pores are probably needed for the initial reactions to occur. After the initial Cu(II)-assisted polymerization reactions have occurred, polymer chains continue to grow via aniline diffusion and
Porter et al.
Figure 4. (a, top) NCM topographical scan of a Cu(II)-exchanged hectorite/polyaniline composite film surface. The film was exposed to saturated aniline vapor for 48 h. The nanometer-scale polymer grain structure is similar to that observed for chemically and electrochemically prepared polyaniline films. (b, bottom) Na+ hectorite film surface after similar aniline exposure. No surface polymerization has occurred. The dimensions of these images are 4 µm × 4 µm.
subsequent polymerization on the existing chain. Once a completed polyaniline layer has formed, thick surface films may be produced through additional aniline vapor exposure. When prepared on interdigitated arrays, these inorganic-organic composite films exhibit promising electrical, optical, and chemical sensing properties. Acknowledgment. The authors gratefully acknowledge funding from the National Science Foundation (DMR-9217526 and DMR-9703840), NSF RIMI Program (HRD-9105529), and Research Corp. Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society. M.E.H. is a Camille and Henry Dreyfus
Cu(II)-Exchanged Hectorite Foundation Fellow funded on a Dreyfus Scholar/Fellow grant to M.P.E. We would also like to thank R. A. Parnell for assistance with XRD data and R. W. Zoellner for other helpful discussions. References and Notes (1) Giannelis, E. P. Materials Chemistry; Interrante, L. V., Casper, L. A., Ellis, A. B., Eds.; American Chemical Society: Washington, DC, 1995. (2) Pinnavaia, T. J.; Mortland, M. M. J. Phys. Chem. 1971, 75, 3957. (3) Pinnavaia, T. J.; Hall, P. L.; Cady, S. S.; Mortland, M. M. J. Phys. Chem. 1974, 78, 994. (4) Mortland, M. M.; Boyd, S. A. EnViron. Sci. Technol. 1989, 23, 223. (5) Eastman, M. P.; Patterson, D. E.; Pannell, K. H. Clays Clay Min. 1984, 32, 327. (6) Rishpon, J.; O’Hara, P. J.; Lahav, N.; Lawless, J. G. J. Mol. EVol. 1982, 18, 179. (7) Odom, D. G.; Rao, M.; Lawless, J. G.; Oro, J. J. Mol. EVol. 1979, 12, 365. (8) Leidl, G. L. Sci. Am. 1986, 255, 127. (9) Ozin, G. A. AdV. Mat. 1992, 4, 612. (10) Vaia, R. A.; Ishii, H.; Giannelis, E. P. Chem. Mater. 1993, 5, 1694.
J. Phys. Chem. B, Vol. 101, No. 51, 1997 11111 (11) Cheetam, A. K. Science 1994, 264, 794. (12) Eastman, M. P.; Attuso, J. A.; Porter, T. L. Clays Clay Miner. In press. (13) Porter, T. L.; Eastman, M. P.; Hagerman, M. E.; Attuso, J. A.; Bain, E. D. J. Vac. Sci. Technol. Submitted. (14) Degroot, D. C.; Schindler, J. C.; Kannewurf, C. R.; Kanatzisis, M. G. J. Chem. Soc. Chem. Commun. 1993, 687. (15) Kerr, T.; Nazar, L. F. Chem. Mater. 1996, 8, 2005. (16) Kanatzidis, M. G.; Wu, C. G.; Marcy, H. O.; Kannewurf, C. R.; Kostikas, K.; Papaefthymiou, V. AdV. Mater. 1990, 2, 364. (17) Schollhorn, R. Chem. Mater. 1996, 8, 1747. (18) Krishnamoorti, R.; Vaia, R. A.; Giannelis, E. P. Chem. Mater. 1996, 5, 1728. (19) Wudl, F.; Angus, R. O.; Lu, F. L.; Allemand, P. M.; Vachon, D. J.; Nowak, M.; Liu, Z. X.; Heeger, A. J. J. Am. Chem. Soc. 1987, 109, 3677. (20) Jeon, D.; Kim, J.; Gallagher, M. C.; Willis, R. F.; Kim, Y.-T. J. Vac. Sci. Technol. 1991, B9 (2), 1154. (21) Genie`s, E. M.; Boyle, A.; Lapkowski, M.; Tsintavis, C. Synth. Met. 1990, 36, 139. (22) Shacklette, L. W.; Wolf, J. F.; Gould, S.; Baughman, R. H. J. Phys. Chem. 1988, 88 (6), 3955. (23) Porter, T. L.; Minore, D.; Stein, R.; Myrann, M. J. Polym. Sci. 1995, 33, 2167.
