Scanning probe microscopy studies of isocyanide functionalized polyaniline thin films

NanoStructured Materials. Vol. 4, No. 4, pp. 457-464, 1994 Copyright © 1994 Elsevier Science Ltd Printed in the USA. All rights reserved 0965-9773/94 $6.00 + .00

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SCANNING PROBE MICROSCOPY STUDIES OF ISOCYANIDE FUNCTIONALIZED POLYANIL1NE THIN FILMS

T.L. Porter Dept. of Physics, Northern Arizona University, Flagstaff, AZ 86011 A.G. Sykes and Y. Shi Dept. of Chemistry, Northern Arizona University, Flagstaff, Az 86011 (Accepted April 1994) Abstract-- The technique of scanning probe microscopy was used to study the nanometerscale morphological changes in isocyanide functionalized polyaniline films due to protonation in aqueous HCI, as well as exposure to lr ÷ cations in CH2C12 solution. Electropolymerized isocyanide functionalized aniline films exhibited a highly oriented fibrous structure, with individual strands averaging 25,4 in diameter. Upon protonation, the fibrous structure was lost, with the material reorganizing into oriented, elongated bundles of average diameter 200,4. Ir ÷ exposed unprotonated films exhibited an oriented, interlocking bundle structure, resulting from Ir + incorporation into the film matrix. The diameter of these bundles averaged 400 A. INTRODUCTION The functionalization of conducting polymer films by incorporation of various chemical species during polymerization results in the ability to synthesize materials with a wide range of desired chemical, electrical, or physical behaviors (1). One highly complex but very interesting system is the polyaniline family of conductive polymers. Polyaniline may be prepared and transformed between a number of different physical, chemical and conductive states. The conductivity of polyaniline may be highly controlled through chemical doping or post polymerization processing (2). The material may be processed in bulk form, or as electrochemically plated films, chemically prepared free-standing films cast from solution, or as thin films prepared by vacuum evaporation (3). Also, substituents may be incorporated into the material that alter any number of the chemical and electronic properties (4). In order to more fully understand the macroscopic properties of these materials, we must first attempt to understand the complicated interplay between the various microscopic features. Initial polymerization of aniline in the emeraldine oxidation state generally results in two different material types, depending on the precise method of formation (5,6). Films or bulk powders prepared in the conducting, HCI doped form are semi-crystalline in structure, with crystalline domain sizes in the range of 30 - 200 A (57). Polyaniline prepared in this manner is referred to as ES-I. Polyaniline prepared in base form (EB-II) may be either semicrystalline in powder form or amorphous when prepared as films or 457

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PORTER,AG SYKESANDY SHI

fibers. After protonation of emeraldine base, this material becomes more crystalline in structure (8,9). This conductive form is then referred to as ES-II. The macroscopic conductive properties of polyaniline depend upon charge transport phenomena on the microscopic scale. Factors such as the microscopic domain structure, crystalline structure, material defects and interchain coherence all affect charge transport on the microscopic scale. In the present study, we report on the nanometer-scale structural features of isocyanide functionalized polyaniline films prepared electrochemically. It has been shown that isocyanide complexes of Rh(I) and Ir(I) catalyze photochemical hydrogen atom transfer reactions (10-13), and also abstract halide atoms from environmentally toxic halocarbons (14). Isocyanide d 8 metal complexes also form adducts with both Lewis acids and bases (15-18), and catalyze photoevolution of dihydrogen in aqueous acid solutions (19). Synthesizing conductive polymer films incorporating isocyanide functionalities could act to bind metal ions, and manipulation of the local metal environment through the conductive matrix could be important in the control of the catalytic reactions mentioned above. Also, new electrode materials with applications as ion sensors and metal recovery or release systems may be possible. EXPERIMENTAL

3-isocyanoaniline powder was synthesized by first dissolving 30 grams of 1,3phenylenediamine in 400 ml chloroform (Figure 1). To this, 200 grams of potassium hydroxide flake dissolved in 800 ml H20/120 ml ethanol mixture was added. After 2 hours under reflux, the reaction mixture was cooled to room temperature and the water layer was separated from the chloroform layer and extracted using additional chloroform. The combined chloroform layers were washed with distilled water and then dried over anhydrous sodium sulfate powder. Chloroform was evaporated under reduced pressure and a dark, tarry residue was obtained. This residue was then extracted with ligroine, and the combined crystalline material was again recrystallized with hexane. This resulted in a pale, yellow solid material. Infrared, 1H NMR and mass spectroscopy successfully characterized the product (20).

KOI_YCHCI3 V

~NH 2

1,3-Phenylenediamine

"

3-Isocyanoaniline

Figure 1. Synthesis of 3-Isocyanoaniline.

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Anodic polymerization of this material was then performed in 0.1 M NaCIO4/MeCN solution. A standard three electrode, one compartment cell was used. All electrochemical potentials were recorded v s . the Ag/AgCI reference electrode (BAS). Electrolytes were dried with activated alumina, and solutions used were degassed with solvent-saturated nitrogen. Films were plated on Pt foil surfaces, with film thicknesses greater than 1 micron in order to obscure any substrate structural features. These f'flms were yellow-gold in color, free of defects such as pinholes, and nonelectrochromic with variation in potential (as polyaniline itself). All scanning probe images of the film surfaces were obtained with an Auto-Probe CP from Park Scientific Inslruments. The cantilevers used were of force constant 0.03 N/m, resonant frequency 19 kHz, and 180 microns in length. The actual scanning force used was less than 10 nN in all cases in order to minimize the distortion of the polymer surface features. Scanning speeds ranged from 1 micron/sec for larger areas to less than 200 A/sec for high resolution scans. All scans were repeatable, indicating little or no damage to the polymer surfaces during scanning. RESULTS AND DISCUSSION In a previous study (20), it was clearly demonstrated that electropolymerization of aniline containing the strong electron-withdrawing isocyanide group 3-Isocyanoaniline in non-aqueous media was possible. It was also demonstrated that these films readily coordinate Ir÷ cations from solution.

2 1.5 /Jrn 1

0.5 C 0

1

2

Figure 2. SPM image of the unprocessed surface of isocyanide-functionalized aniline film. Individual rod or strand-like features are visible, with a high degree of orientation. The average length of these strands is 1.25 + 0.25 p.m.

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Figure 2 shows a scanning probe microscopy (SPM) image of a 2.5 pm 2 region on the surface of an unprocessed isocyanide-functionalized aniline film. At this resolution, it is somewhat difficult to ascertain the film microstructure, however it is clear that there is a large degree of structural orientation. The film is not segregated into individual grains, with individual rod or strand-like structures dominating the film structure. From this and other large scale images, we have estimated the average length of these strands to be 1.25 + 0.25 ~un. In Figure 3 a high resolution scan of dimensions 2500 ,~2 is shown. The strand-like microstructure of the film is now clearly evident. The measured average diameter of the strands is 25 + 5/~. This strand diameter is in good agreement with other measured functionalized aniline strand dimensions (21). In this previous study, individual strands of electropolymerized poly-hydroxyaniline were imaged using the technique of scanning tunneling microscopy (STM). The diameter of these strands or coils was measured to be 20 + 5 ,/~. It was proposed that these strands are simple helical coils, with periodicity along the coil of about 30 it,. In this study, the diameter of individual coils is readily measurable, but any periodicity along the length of the strands is not seen. This may indicate that these are simply thin aggregates of linear polymer chains rather that helical coils of single polymer chains. Also of interest is the apparent degree of ordering in these films. In a study using the techniques of near x-ray absorption fine structure (NEXAFS) and infrared reflection-absorption spectroscopy (IRRAS), it was shown that initial layers of 3-methylthiophene would grow with preferential orientation parallel to the substrate (Pt) (22). This was due to the p-interaction with the metallic substrate. Ordering would then be imposed on subsequent film layers, with the degree

0

500

luuu

. . . .

Figure 3. High resolution SPM image of same film surface as in Figure 1. Individual polymer fibers or strands arranged in a close-packed structure are clearly visible. The average diameter of these strands is 25 + 5 .~.

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of orientation decreasing as film thickness increases. In the present study, the higher than expected film orientation may be due to the isocyanide-Pt interactions in solution. Isocyanides display strong cr donation and extensive back acceptance of n-electrons from metals in low oxidation states (23). This interaction between the isocyanide group and the Pt substrate may not only enable efficient electro-oxidation of the monomer, but may also result in orientation of the first few polymer layers. Subsequent layers may grow in a template fashion, presumably with decreasing orientation and order as film thickness increases. Isocyanide-functionalized aniline films were also protonated by exposure to 1 M HC1 in solution. Total exposure time was 24 hr. The films were then dried for 24 hrs. prior to SPM imaging. Figure 4 shows a 2500 &2 SPM image of the film surface after protonation. The region shown here is characteristic of the entire film surface structure, which is not segregated into individual grains. The fibrous nanostructure of the unprotonated film has been completely replaced by an island or bundle structure. This type of reorganization upon protonation is typical ofpolyaniline and polyaniline based compounds (8, 24), and is a result of incorporation of protons into the polymer chains, as well as the added presence ofCl counterions in the polymer matrix. For pure polyaniline films or fibers, this EB-I1 to ES-II transition is also accompanied by a reordering of the polymer microstructure into a crystalline or semicrystalline state (6-8). In this model, each polymer bundle possesses sufficient crystallinity to allow for three-dimensional electron wave function delocalization, resulting in a polaronic lattice electronic structure. The overall macroscopic conductivity of the material is, however, still limited by poor charge transport between

2000-1500

A

1000 500 0 0

500

1000 A

1500

2000

Figure 4. Surface of isocyanide-functionalized aniline after protonation in aqueous HC1. The fibrous structure of the film has given way to more of an island or bundle structure. These bundles are still partially oriented in the original direction of film orientation. The average diameter of these bundles is 200/~.

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individual polymer bundles. The dimension of the individual bundles observed on the isocyanidefunctionalized films is on the order of 200 ~ laterally and 2500 ./k along the length of individual bundles. While the lateral bundle size is in good agreement with earlier studies on aniline and hydroxyaniline films (21, 24), the bundles in the present study are considerably more elongated. We feel that this result is due primarily to the increased structural order present in the isocyanoaniline films prior to protonation. In contrast, pure polyaniline films exhibit little or no structural coherence on the nanometer scale prior to protonation, but order themselves into cigar-shaped bundles of average dimension 250 ,h, after protonation (24). Unprotonated isocyanide-functionalized aniline films were also exposed to Ir+ in solution. A 1-2 mM solution of [Ir(COD)CI]2 (COD = 1,4-cyclooctadiene) in CH2C12 was degassed with dinitrogen and an electropolymerized isocyanoaniline film was then immersed. This exposure of the functionalized polymer to the Ir complex in solution binds metal ions to the modified surface (20), as with polyaniline. Metal ions typically form complexes with isocyanides through coordination of the isocyanide carbon which acts as a Lewis base. The films were then dried for 24 hours prior to SPM imaging. Figure 5 shows an image of a 10 ktm2 region on the film surface, while Figure 6 shows a high resolution scan of a region 2000 ~ in dimension. It is evident from these images that incorporation of the Ir÷ cations into the polymer matrix has resulted in structural reconstruction of the film surface morphology. Replacing the original fibrous structure is a structure consisting of interlocking, needle-like polymer bundles. The average length of the individual bundles is 1.25 + 0.25 Ixm, in accordance with the original length of the polymer fibers.

2

0 0

1

/~m

2

Figure 5. Overall surface structure of isocyanoaniline films after Ir exposure in solution. The material has readily incorporated the Ir cations, resulting in large-scale changes in the fibrous fdm structure.

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1

A

0

400

800

A

1200

1600

Figure 6. High resolution scan of the Ir exposed film. The fibrous structure has given way to a film structure consisting of interlocking polymer bundles. These bundles are highly elongated and partially oriented in the original film orientation direction. The average width of these bundles is 400 + 50 A. The average width of these bundles is now 400 + 50 A, however. Groups of individual fibers have coalesced laterally into these larger structures. The internal structure of these new needle-like bundles is sufficiently close-packed and uniform that SPM imaging on a smaller scale could not be achieved. These isocyanide functionalized films exhibit important properties in regard to applications as ion sensors, metal recovery systems, and control of certain catalytic reactions involving environmentally toxic halocarbons. A study of the microscopic structure of these materials and the relationship between this structure and the electronic and chemical properties of the films is needed in order to understand how these materials may be used to greater advantage in these applications. Further studies of these structural effects on aniline, as well as functionalized aniline films, are currently underway. CONCLUSIONS Films of isocyanide-functionalized polyaniline were polymerized using standard electrochemical methods. SPM scans on unprotonated, as-polymerized isocyanoaniline films reveal a nanometer-scale fibrous structure, The fiber diameters average 25 + 5 A, and their length 1.25 + 0.25 microns. The fibers show a large degree of orientation, due to large isocyanide-Pt interactions in the electrochemical solution. After protonation, the film material has coalesced into a nanometer-scale bundle structure similar to that seen in aniline and other functionalized aniline films after protonation. Exposure to Ir÷ cations in solution also results in a structural reconstruction

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for the films. A large scale system of needle-like polymer bundles is observed. These bundles are highly interlocked, and have lengths similar to the fibers in the original films. The width of these new structures is measured to be 400 + 50 A.

ACKNOWLEDGEMENTS This work was supported by the NSF (DMR-9217525), Research Corporation, the Vice President for Research at NAU, and the Camille and Henry Dreyfus Foundation. We also gratefully acknowledge Johnson Matthey, Aesar/Alfa for a generous loan of iridium trichloride.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

E.M. Geni~s, A. Boyle, M. Lapkowski, C. Tsintavis, Synth. Met. 36, 139 (1990). A.P. Monkman, P. Adams, Synth. Met. 41,891 (1991). M. Angelopoulos, G.E. Asturias, S.P. Ermer, A. Ray, E.M. Scherr, A.G. MacDiarmid, M. Akhtar, Z. Kiss, A.J. Epstein, Mol. Cryst. Liq. Cryst. 160, 151 (1988). T.L. Porter, P.I. Oden and G. Caple, Surf. Sci. 259, 221 (1991). J.P. Pouget, M. Laridjani, M. E. J6zefowicz, A.J. Epstein, E.M. Scherr, A.G.MacDiarmid, Synth. Met. 51, 95 (1992). M.E. J6zefowicz, A.J. Epstein, X. Tang, Synth. Met. 46, 337 (1992). T.L. Porter, P.I. Oden, C.Y. Lee, G. Caple, J. Vac. Sci. Technol. A 10, 606 (1992). M. Laridjani, J.P. Pouget, A.G. MacDiarmid, A.J. Epstein, J. Phys. I France 2, 1003 (1992). T.L. Porter, K. Caple, G. Caple, Synth. Met. 60, 211 (1993). D.C. Smith, R.E. Marsh, W.P. Schaefer, T.M. Loehr, H.B. Gray, Inorg. Chem. 29, 534 (1990). D.C. Smith, H.B. Gray, Catalysis Letters 6, 195 (1990). D.C. Smith, H.B. Gray, Coord. Chem. Reviews 100, 169 (1990). M.G. Hill, K.R. Mann, Catalysis Letters 11,341 (1991). M.G. Hill, A.G. Sykes, K.R. Mann, Inorg. Chem. 32, 783 (1993). W.L. Gladfelter, H.B. Gray, J. Amer. Chem. Soc. 102, 5910 (1980). A.G. Sykes, K.R. Mann, J. Amer. Chem. Soc. 110, 8252 (1988). A.G. Sykes, K.R. Mann, J. Amer. Chem. Soc. 112, 7247 (1990). A.G. Sykes, K.R. Mann, Inorg. Chem. 29, 4449 (1990). K.R. Mann, N.S. Lewis, V.M.Miskowski, D.K. Erwin, G.S. Hammond, H.B. Gray, J. Amer. Chem. Soc. 99, 5525 (1977). A.G. Sykes, Y. Shi, T.R. Dillingham, T.L. Porter, G. Caple, J. Electroanal. Chem., submitted. T.L. Porter, C.Y. Lee, B.L. Wheeler, G. Caple, J. Vac. Sci. Technol. A9(3L 1452 (1991). X.Q. Yang, J. Chen, P.D. Hale, T. Inagaki, T.A. Skotheim, M.L. denBoer, Synth. Met. 28, C329 (1989). C. Elschenbroich, A. Salzer, Organometallics: a Concise Introduction (VCH Verlagsgasellschaft, Weinheim, 1989). T.L. Porter, M. Myrann, M. Bradley, Surf. Sci., submitted.