Electrochemical Control of Polyaniline Morphology as Studied by Scanning Tunneling Microscopy

The relation between structure and preparation conditions of polyaniline (PANI) was determined using scanning tunneling microscopy in air. The morphology of a PANI film grown with fast cyclic potential scans (20 V/s) was uniform and dense, whereas a film grown with slow scans (50 mV/s) over the same potential range with the same number of coulombs passed was amorphous and rough. The possibility of control of PANI morphology based on the electropolymerization kinetics is discussed. Electrochemical control of the morphology during the early stages of polyaniline (PANI) growth is demonstrated by variation of the deposition rate. The structures of thin (<500 A) electrochemically-formed PANI films were determined with a scanning tunneling microscope (STM) (1-5), The STM images were compared to those obtained with a scanning electron microscope (SEM). A uniform PANI structure was found when the film was electrodeposited with rapid cycling (20 V/s), but with slow scans (50 mV/s) a rougher, amorphous film was formed. Conducting polymers (6-10), such as polypyrrole, PANI, and polythiophene, have been investigated in an attempt to understand the effect of preparation conditions on polymer structure. The electrochemical deposition conditions, e.g., supporting electrolyte, solvent, and substrate, are important factors in determining the physical and chemical properties of the conducting films. Scanning electron microscopy (SEM) has been used to investigate the morphology of conducting polymers (6-10), but has not been of use in obtaining structural information about films less than a few hundred angstroms thick. Recently, the morphology of conducting polymers was investigated using an STM operated in air at a resolution of nm to p.m (11-15). In this letter, STM images of PANI deposited by cyclic potential sweeps provide insight into the structure-preparation relationship of films. An understanding of the electrochemistry and corresponding mechanism of film formation of PANI is difficult because of the complicated reaction kinetics involved in the electro-oxidation of aniline, particularly during the early stages of electrochemical deposition (16). To determine the effect of potential scan rate on film morphology, Pt film working electrodes were prepared by sputtering Pt on a glass slide. An STM image (500 nm x 500 nm) of this Pt film, see Fig. 1, showed a polycrystalline structure, with crystals in the size range 10-50 nm. A cyclic voltammogram recorded at 50 mV/s at a Pt film electrode in a solution containing 20 mM aniline in 1M H2SO4 is shown in Fig. 2a. Following initial oxidation of aniline at about +0.5 V vs. SMSE, at least five different reduction and oxidation steps are observed, as is typical of aniline electropolymerization under these conditions (16, 17). The major growth mechanism of PANI has been described as occurring via the formation of dimeric species from the nucleophilic attack of the aniline on the nitrene cation radical, resulting in three different dimerization products (i.e., head-to-head, head-to-tail, and tailto-tail), as well as competitive degradation reactions (16). PANI was deposited by passing a total anodic charge of approximately 35 mC/cm 2 (18). This yielded a film 450 _+ 50 A thick, as determined by electrochemical and spectroscopic ellipsometry measurements (19, 20). The typical surface morphology of a 5 x 5 i~m area of the resulting PANI is shown in the STM image, Fig. 2b. Unlike the fiber structures determined by SEM with micrometer thick films of PANI (9), most of the surface structures appeared rather amorphous and did not show good resolution. The SEM images obtained for the PANI, * Electrochemical Society Active Member. prepared using the same conditions as in Fig. 2b, were typically similar to the STM image in Fig. 2b; the thicker aggregates seen in Fig. 2d were rarely observed. In situ electrochemical STM of PANI also produced images similar to Fig. 2b (21). When the STM scanning area was decreased to 100 by 100 nm, small lumps could occasionally be seen, Fig. 2c. The diameters of lumps varied over a range of 20-80 nm. Figure 2b shows a large amount of either a void fraction or surface roughness. When the roughness was estimated by height measurement (e.g., peak to valley), the range of roughness was 200-400 ~, for a number of different spots and samples. Sometimes under higher resolution the bare Pt surface, as in Fig. 1, could be resolved. The difference between the thicknesses estimated from the electrochemical methods and STM measurements is probably caused by an underestimation of the electrode area and uncertainties in thickness measurements with the STM. When the electropolymerization .was performed with a scan rate of 20 V/s, the resulting cyclic voltammogram, Fig. 3a, and film morphology, Fig. 3b, were very different. Only one set of large peaks (B, B') is observed during the early stages of film growth. Smaller peaks (e.g., A, A') grow in during later scans. The oxidation peak (B) at 0.10 V vs. SMSE and the reduction peak (B') at -0.10 V are at the same potentials as that shown by the head-to-tail dimer, 4-amino-diphenylamine (22). In the first scan, the AEp for the B, B' couple is approximately 200 mV; this increases with continuous scanning, probably because of increased uncompensated iR drop as the PANI grows Fig. 1. STM image of the sputtered Pt film surface. Vb=200mV, i t=2.0nA. The scanned area is 500nm • 500 nm. Downloaded 28 Jan 2009 to 146.6.143.190. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp L72 J. Electrochem. Soc., Vol. 138, No. 12, December 1991 9 The Electrochemical Society, Inc.