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Nanometer-Scale Patterning of Surfaces Using Self-Assembly Chemistry. 3. Template-Directed Growth of Polymer Nanostructures on Organothiol Self-Assembled Mixed Monolayers William A. Hayes and Curtis Shannon* Department of Chemistry Auburn University, Auburn, Alabama 36849-5312 Received June 17, 1997. In Final Form: December 9, 1997 We report the template-directed growth of polyaniline nanostructures at Au electrodes modified with two-component self-assembled monolayers consisting of 4-aminothiophenol (4-ATP) and n-octadecanethiol (ODT). 4-ATP, which is more easily oxidized to the cation radical than aniline, serves as a nucleation site for the electrochemical deposition of polyaniline. Therefore, when 4-ATP/ODT mixed monolayers are oxidized in the presence of aniline, polyaniline features are formed selectively from islands of 4-ATP embedded in the ODT monolayer. The size and distribution of these features were characterized by AFM. In this work, we show that the feature density is a function of monolayer composition and that the feature height is dependent on the concentration of aniline in solution.
Introduction Self-assembly is a convenient synthetic route to densely packed, well-ordered organic monolayers and provides a rational basis for the modification of surface chemical and physical properties.1 For example, the surface free energy of metal substrates can be varied over a wide range by the adsorption of ω-terminated alkanethiol monolayers.2 For this class of adsorbate on Au, a relatively complete picture of the self-assembly process as well as the structure of the self-assembled monolayers (SAMs) has emerged over the past decade.3 SAMs also have been used to modify the chemical response of surfaces in contact with solutions,4 to study electron-transfer processes in electrochemical systems,5 and to impart molecular recognition capability to surfaces.6 SAMs often display unique patterns of chemical reactivity and are interesting as model organic surfaces.7 In * Address correspondence to this author: Telephone, 334-8446964; fax, 334-844-6959; e-mail address,
[email protected]. (1) (a) Whitesides, G. M.; Gorman, C. B. In Handbook of Surface Imaging and Visualization; Hubbard, A. T., Ed.; CRC Press: Boca Raton, FL, 1995; pp 713-32. (b) Allara, D. L. Biosens. Bioelectron. 1995, 10, 771. (c) Ulman, A. MRS Bull. 1995, 20, 46. (d) Buess-Herman, C. Prog. Surf. Sci. 1994, 46, 335. (e) Whitesides, G. M.; Ferguson, G. S.; Allara, D.; Scherson, D.; Speaker, L.; Ulman, A. Crit. Rev. Surf. Chem. 1993, 3, 49. (2) (a) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (b) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (c) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3359. (d) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (3) (a) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (b) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: Boston, 1991. (c) Poirer, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2859. (d) Perry, S. S.; Somorjai, G. A. Anal. Chem. 1995, 66, 403A. (4) Sun, L.; Johnson, B.; Wade, T.; Crooks, R. M. J. Phys. Chem. 1990, 94, 8869. (5) Bard, A. J.; Abruna, H. D.; Chidsey, C. E.; Faulkner, L. R.; Feldberg, S. W.; Itaya, K.; Majda, M.; Melroy, O.; Murray, R. W.; Porter, M. D.; Soriaga, M. P.; White, H. S. J. Phys. Chem. 1993, 97, 7147. (6) (a) Kepley, L. J.; Crooks, R. M.; Ricco, A. J. Anal. Chem. 1992, 64, 3191. (b) Jones, T. A.; Perez, G. P.; Johnson, B. A.; Crooks, R. M. Langmuir 1995, 11, 1318. (c) Nahir, T. M.; Clark, R. A.; Bowden. E. F. Anal. Chem. 1994, 66, 2595. (d) Haeussling, L.; Ringsdorf, H.; Schmitt, F.-J.; Knoll, W. Langmuir 1991, 7, 1837. (e) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426. (f) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312.
addition, organic monolayers can be used as platforms to carry out chemical and electrochemical reactions which lead to novel two- and three-dimensional molecular architectures or enhanced performance. For example, the physical characteristics of electropolymerized materials can be altered by preadsorbing an organic monolayer at the working electrode. Rubinstein et al. have used 4-aminothiophenol (4-ATP) monolayers to increase the density of polyaniline electrodeposits on Au.8 Electropolymerization of an intact SAM containing a pyrrole group at the ω-terminus has been used by both McCarley9 and Collard10 as a method of preparing two-dimensional polypyrrole films covalently attached to a surface. SAMs containing more than one chemical species (i.e., mixed monolayers) hold great promise as a means of creating patterns of surface active groups.11 In a previous study, we reported the surface electrochemistry of 4-aminothiophenol/thiophenol mixed monolayers on Au and showed that the 4-ATP cation radical could be electrogenerated at the surface in aqueous media.12 In the two previous papers in this series, we investigated the phase (7) Porter, M. D.; Walczak, M. M. In Handbook of Surface Imaging and Visualization; Hubbard, A. T., Ed.; CRC Press: Boca Raton, FL, 1995; p 733. (8) (a) Sabatani, E.; Gafni, Y.; Rubinstein, I. J. Phys. Chem. 1995, 99, 12305. (b) Rubinstein, I.; Rishpon, J.; Sabatani, E.; Redondo, A.; Gottesfeld, S. J. Am. Chem. Soc. 1990, 112, 6135. (9) Willicut, R. J.; McCarley, R. L. Adv. Mater. 1995, 7, 759. Willicut, R. J.; McCarley, R. L. Langmuir 1995, 11, 296. (10) Sayre, C. N.; Collard, D. M. Polym. Mater. Sci. Eng. 1995, 72, 329. Sayre, C. N.; Collard, D. M. Langmuir 1995, 11, 302. (11) (a) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 6560. (b) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 3665. (c) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370. (d) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M.; Deutch, J. J. Phys. Chem. 1994, 98, 563. (e) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (f) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097. (g) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330. (h) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 1990. (i) Laibinis, P. E.; Fox, M. A.; Folkers, J. P.; Whitesides, G. M. Langmuir 1991, 7, 3167. (j) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (k) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1766. (l) Stranick, S. J.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636. (m) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705. (12) Hayes, W. A.; Shannon, C. Langmuir 1996, 12, 3688.
S0743-7463(97)00647-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/03/1998
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behavior of 4-aminothiophenol (4-ATP)/n-octadecanethiol (ODT) two-component SAMs as well as the electrochemistry of the 4-ATP phase domains.13 Specifically, we showed that 4-ATP islands indeed were electroactive and could be used to nucleate the electrodeposition of polyaniline from solution. Here, we discuss the formation of polyaniline nanostructures in more detail. First, we show that the density of polymer nanostructures is dependent on the composition of the template SAM. In particular, both the density and the average polyaniline feature diameter correlate with the size and number of 4-ATP phase domains we observed previously for these surfaces. Finally, we demonstrate that the feature height depends on the concentration of aniline in solution. Experimental Section Chemicals. 4-Aminothiophenol (Aldrich) was recrystallized 3 times from methanol (Fisher) and stored in the dark at 0 °C. Aniline (Fisher) was distilled over zinc metal and stored in the dark at 0 °C. Ethanol (200 proof, Florida Distillers), 70% HClO4 (Fisher), and n-octadecanethiol (97%, Aldrich) were used as received. Preparation of Au(111) Microbead Electrodes. A polycrystalline gold wire is melted and annealed to produce a 1-2 mm diameter bead containing large atomically flat Au(111) single crystal facets suitable for carrying out electrochemistry and atomic force microscopy experiments. The details of this procedure have been previously described.14 Monolayer Assembly. The faceted gold microbead was rinsed with ethanol and immersed for 3 h in an ethanol solution containing a total thiol concentration of 1 mM. Upon emersion, the specimen was rinsed with copious amounts of ethanol followed by a brief rinse with Millipore water. The Au bead was then immediately placed in the electrochemical cell. Electrochemistry. All electrochemistry experiments were carried out using a Pine AFRDE-5 bipotentiostat and a HewlettPackard 7015B X-Y recorder. A conventional three electrode configuration is employed, and all cell components are constructed from either Teflon or Kel-F. In all cases, the gold microbead was the working electrode, a platinum wire was the counter electrode, and Ag/AgCl was the reference electrode. When making electrochemical measurements, we made no attempt to mask the polycrystalline portions of the microbead electrodes. Therefore all electrochemistry is characteristic of SAMs adsorbed on polycrystalline Au surfaces. Deposition of aniline was carried out from a 0.01 M aniline solution in 0.5 M HClO4. The potential was cycled between -0.200 and +0.760 V for 15 min at a scan rate of 0.100 V s-1. After cycling, the working electrode was held at 0.0 V while it was emersed from the electrochemical cell. The electrode was then rinsed with Millipore water and subsequently allowed to air-dry under Ar before AFM experiments were performed. AFM. All AFM experiments were performed in air on a Park Scientific Instruments Autoprobe CP scanning probe microscope in noncontact mode (Park Scientific, Sunnyvale, CA). The tips used were commercially available model APUL-20AU-25 2 µm Ultralevers (Park Scientific). The resonant frequency of the tips was consistently between 80 and 90 kHz, and the drive amplitude was varied as necessary to obtain high signal-to-noise images. The scan rates used were 1-4 Hz on a 5 µm piezoelectric scanner, and the scan size was 1 µm × 1 µm or 1.6 µm × 1.6 µm. In all cases, error bars correspond to experiments performed on different gold balls as well as on different regions of the same facet. Typically, each experiment was repeated on 5-7 specimens.
A detailed discussion of the voltammetric behavior of a 4-ATP monolayer in aqueous electrolyte containing aniline was given in an earlier report.12 Oxidation of adsorbed 4-ATP was shown to occur at 0.730 V, while the oxidation of solution phase aniline occurs at approximately 1.05 V. We reasoned that since adsorbed 4-ATP is about 260 mV easier to oxidize than aniline, it should be possible to initiate the growth of polyaniline preferentially at an adsorbed 4-ATP molecule if the electrode is held at the 4-ATP oxidation potential. Furthermore, at this potential there will be no precipitation of polyaniline from solution or growth of polymer from adventitious defects within the 4-ATP SAM because the potential is not sufficiently oxidizing to produce the aniline cation radical. Figure 1a shows that polyaniline can be grown at a 4-ATP modified Au electrode by cycling the potential between -0.200 and 0.760 V for 15 min in electrolyte containing 0.01 M aniline. Note that the positive scan limit is slightly positive of the 4-ATP oxidation potential but is significantly less positive than the oxidation potential of aniline. The two oxidative waves near 0.100 and 0.700 V are the characteristic voltammetric signatures of polyaniline and increase in intensity upon repeated cycling.15 Integrating the charge under the wave near 0.100 V in Figure 1a (outer trace) yields a value of 206 µC cm-2. Assuming an ideal Au(111) surface and a coulometric efficiency of one, this charge corresponds to 0.93 monolayers of polyaniline. The upper
(13) (a) Hayes, W. A.; Shannon, C. Template-Directed Growth of Polyaniline Nanostructures on Organothiol SAMS on Gold. In Scanning Tunneling Microscopy/Atomic Force Microscopy III; Cohen, S. H., Ed.; Plenum Press: New York, in press. (b) Hayes, W. A.; Kim, H.; Yue, X.; Perry, S. S.; Shannon, C. Langmuir 1997, 13, 2511. (14) Hsu, T. Ultramicroscopy 1988, 11, 167. Demir, U.; Shannon, C. Langmuir 1994, 10, 2794.
(15) (a) Wolf, J. F.; Forbes, C. E.; Gould, S.; Shacklette, L. W.; J. Electrochem. Soc. 1989, 136, 2887. (b) Seeger, D.; Kowalchyk, W.; Korzeniewski, C. Langmuir 1990, 6, 1527. (c) Syed, A. A.; Dinesan, M. K. Talanta 1991, 38, 815. (d) Ginder, J. M.; Epstein, A. S.; MacDiarmid, A. G. Synth. Met. 1989, 29, E395. (e) Stilwell, D. E.; Park, S.-M. J. Electrochem. Soc. 1988, 135, 2491. (f) Bacon, J.; Adams, R. N. J. Am. Chem. Soc. 1968, 90, 6596. See also ref 8.
Figure 1. (a, left) Electrochemical deposition of polyaniline at a 4-aminothiophenol (4-ATP) monolayer adsorbed on Au. Voltammetry characteristic of polyaniline is well-developed after several minutes of cycling between 0.00 and +0.760 V in 0.01 M aniline and continues to grow upon repeated cycling. The outer trace was obtained after 15 min of cycling. (b, right) The oxidative limit is not sufficiently positive to initiate polymerization of aniline from solution or at adventitious defects. Upper trace: Voltammetric response of a naked Au electrode subjected to the same conditions as in a. Lower trace: Voltammetric response of an ODT/Au electrode subjected to the same conditions as in a. In all cases, the supporting electrolyte was 0.5 M HClO4, the scan rate was 100 mV s-1, and the electrode area was 0.024 cm2.
Results and Discussion
Nanometer-Scale Patterning of Surfaces
trace in Figure 1b shows that the growth of polyaniline does not occur when a naked Au electrode is subjected to the same treatment: The characteristic polyaniline voltammetry is not observed; in addition, no significant changes in this featureless response were observed for up to 30 min of potential cycling. The lower trace in Figure 1b shows a cyclic voltammogram of an ODT modified Au electrode in 0.01 M aniline solution. As with naked Au, the ODT modified electrode could be cycled for long periods without the appearance of the characteristic polyaniline voltammetry. This result clearly demonstrates that there is no deposition of polyaniline at adventitious defects within the ODT SAM at these potentials. The large decrease in the double layer capacitance (note the scale change) is characteristic of ODT monolayers. There is no increase in the double layer capacitance with cycling under these conditions, indicating no significant oxidative desorption of the monolayer. We were interested in investigating the electrochemical behavior of 4-ATP in a two-component SAM. Extrapolating from the behavior of a pure monolayer, we reasoned that it should be possible to effect selective deposition of polyaniline at 4-ATP molecules or islands imbedded within an electrochemically blocking SAM, resulting in the production of small polymer features covalently anchored to the surface. We have previously shown that the phase behavior in 4-ATP/ODT mixed monolayers is complex and that some phase separation occurs under most conditions.13b Specifically, if the concentration fraction of 4-ATP in the assembly solution is less than or equal to about 0.8, the monolayer consists of islands of 4-ATP approximately 50-70 nm in diameter embedded in a matrix of ODT. In addition, we find that the islands are stable as a function of time after removal from the assembly solution and that their size and distribution are not strongly dependent on the assembly time.16 In all of the following experiments, two-component SAMs consisting of varying amounts of 4-ATP and n-octadecanethiol (ODT) were subjected to the identical electrochemical treatment: 15 min of cycling between -0.200 and 0.760 V at 0.100 V s-1. The specimens were emersed under potential control at 0.00 V, and noncontact AFM was used to detect the presence of polyaniline features on the electrolyzed samples. Images were acquired from one of the Au(111) facets on the annealed Au microbead electrode. In Figure 2a we show a noncontact mode AFM image of a 65% 4-ATP/ODT mixed SAM after electrochemical cycling for 65 min in 0.06 M aniline. This image is characterized by a low density of randomly dispersed features averaging 40 ( 65 nm in diameter and 6.2 ( 6.8 nm in height. The average feature density is 20 ( 4 µm-2 and is extremely reproducible. If polymer growth is indeed templated by adsorbed 4-ATP molecules, then the density of surface features should scale with the coverage of 4-ATP in the monolayer. Figure 2b shows the topography of a 80% 4-ATP/ODT mixed monolayer after electrolysis. In this image, the average diameter and height of the features are 70 ( 67 and 6.4 ( 6.5 nm, respectively. The feature density has increased substantially to 75 ( 5 µm-2, consistent with templated growth of the polymer. Particularly striking is the correlation of the size, shape, and density of the electrodeposited polymer features with the 4-ATP islands we observed previously using frictional force microscopy.13b To demonstrate that these features are due to the formation of polyaniline, we repeated this experiment in the absence of solution phase aniline. In Figure 2c we (16) Kim, H.; Yue, X.; Hayes, W. A.; Perry, S. S.; Shannon, C. Manuscript in preparation.
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Figure 2. Noncontact AFM images of electrolyzed mixed monolayers showing the formation of polyaniline nanostructures. Electrolysis conditions were the same as in Figure 1b: (a, top) 65%/35% 4-ATP/ODT electrolyzed in 0.01 M aniline; (b, middle) 80%/20% 4-ATP/ODT electrolyzed in pure electrolyte (no aniline); (c, bottom) 65%/35% 4-ATP/ODT electrolyzed in pure electrolyte (no aniline). Note: percentages refer to the composition of the assembly solution, not the SAMs.
show a micrometer scale AFM image of a 65% 4-ATP/ ODT SAM cycled 60 times in pure electrolyte. No features are formed under these conditions: the surface appears
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consistent with our model of surface directed growth. If, instead, polyaniline were precipitating from solution upon reaching some critical oligomer size, the height of the features would be expected to be independent of the aniline concentration in solution.
Figure 3. Dependence of apparent average feature height on aniline concentration. In principle, the apparent feature height corresponds to the difference in height between the top of the ODT chains and the top of the polyaniline nanostructures.
atomically flat, reflecting the topography of the underlying Au(111) substrate. Finally, we investigated the influence of aniline concentration on the average feature size. We observed a slight increase (roughly 10%) in the average diameter of the features as the aniline concentration was increased over an order of magnitude. This is as expected if islands of 4-ATP are acting to template polymer growth. On the other hand, we find that the average feature height is a linear function of the aniline concentration. In Figure 3 the average feature height is plotted as a function of the concentration of aniline in solution. This result is
Conclusion In summary, we have shown that mixed monolayer techniques can be used in conjunction with adsorbatedirected electrochemical deposition to produce arrays of nanometer sized features of polymeric material on surfaces. The size and distribution of the polyaniline features can be controlled by changing the electrolysis conditions and the monolayer composition. These systems have many potential applications. For example, arrays of nanometer sized polymer features could serve as the basis for novel amperometric chemical sensors. In addition, our results suggest the interesting possibility of using template directed growth as a means of indirect visualization of the structure of mixed SAMs.17 These areas are currently under investigation in our laboratory. Acknowledgment. The financial support of this research by the National Science Foundation (Grant OSR9553348), the Society of Analytical Chemists of Pittsburgh, and Auburn University is gratefully acknowledged. We wish to thank Prof. Rik Blumenthal (Auburn University) for valuable discussions during the preparation of this manuscript. LA9706476 (17) (a) Sun, L.; Crooks, R. M. Langmuir 1993, 9, 1951. (b) Sun, L.; Crooks, R. M. J. Electrochem. Soc. 1991, 138, L23.
