Results and Discussion

In the in-situ electrochemical STM and electrochemical AFM investigations the first steps of the cyclovoltammetric formation of PPy could be seen. The polymer formed nuclei at a definite distance from steps on the HOPG working electrode surface (Fig. 1). This can be interpreted as follows. Pyrrole monomers are oxidized at the most reactive centers on the WE surface such as steps and the defect sites of the single crystal surface. These oxidized pyrrole molecules form oligomers which are desorbed from the electrode and react with unoxidized pyrrole molecules in the electrolyte solution. The oligomers so formed react with further monomers until they reach a critical size where they become insoluble in the electrolyte. They are precipitated on the WE surface as the nuclei for further polymerization processes. Due to a pH gradient from the highly reactive sites towards the planar electrode surface, the path length for becoming insoluble is nearly constant for each molecule. From this behavior the parallel orientation of the nuclei from steps of the nuclei on the HOPG surface can be understood. After the formation of the nucleation centers polypyrrole starts to grow at these centers and forms islands. In Fig. 2 the polymeric growth of such an island close to an HOPG step can be seen as a sequence of AFM images. The single crystal step is clearly seen in the upper left corner of the sequence. On the right side of the images the growth of the polymer island can be followed. The small knots on the right-hand side of the islands in Fig. 2 become larger with increasing time. Also, the height of the whole island increases during the polymerization process. With further cyclovoltammetric pyrrole oxidation the island increases in diameter and height. The step on the working electrode appears to be unchanged. The potentiostatic growth of PPy at different pH values shows a dependence on the velocity of growth. Without pH buffer, the PPy formation is dominated by rapid three-dimensional growth. The protons formed cause an additional chemical polymerization. The polymer islands formed expand very fast. This leads to rough surfaces because the growth of several separated nuclei causes extreme differences in height. These nuclei overlap very fast. By usage of phosphate buffers the growing velocity can be decreased. At a pH of 5, single polymer islands on the graphite surface can be detected by scanning probe microscopy. In comparison with potentiostatic polymerization the films are smoother those without a pH-buffer system. The growing velocity is decreased even further at a pH of 7. Neither electrochemical STM nor electrochemical AFM shows the formation of polypyrrole nuclei on the electrode surface with tosylate electrolyte in high- resolution or in large-scale investigations. The PPy films produced by potentiostatic and potentiodynamic methods were washed with distilled water and dried overnight. Afterwards they were replaced

Fig. 1. PPy nuclei at a definite distance from HOPG steps at cyclovoltammetric conditions after 14 cycles (0-500 mV vs. Ag/AgCl at 500 mV/s) investigated by in-situ electrochemical STM.

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Fig. 1. PPy nuclei at a definite distance from HOPG steps at cyclovoltammetric conditions after 14 cycles (0-500 mV vs. Ag/AgCl at 500 mV/s) investigated by in-situ electrochemical STM.

again in the electrochemical SPM cells and tosylate electrolyte was added to the system without pyrrole monomers. The polymer film covered the working electrode. The surface morphology was investigated by in-situ electrochemical SPM at different potentials. It could be observed that not only does a potentiostatically produced fresh PPy film appear rougher than the potentiodynamically produced film, but this roughness also shows a dependence on the applied potential (Fig. 3). At more oxidizing potentials the polymer surface gets rougher until a saturation value is reached at the beginning of the overoxidation of the PPy film. This increasing roughness at increasing potentials is reversible to some extent. The behavior of the film inchanging its surface structure is interpreted as a gradient of the counter ions in the molecular tissue of the polymer. By oxidizing the PPy film, more counter ions diffuse into the film surface. These counter ions (tosylate) are diffusing into the polymer tissue to compensate the positive charge

Fig. 2. Island growth of PPy at an HOPG step (scan size 606.3x606.3 nm2, height 100 nm) investigated by in situ electrochemical AFM (-100 -350 mV vs. Ag/AgCl at 20 mV/s).

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Fig. 2. Island growth of PPy at an HOPG step (scan size 606.3x606.3 nm2, height 100 nm) investigated by in situ electrochemical AFM (-100 -350 mV vs. Ag/AgCl at 20 mV/s).

on the polymer chains. When the charge on the film surface is balanced, it becomes more and more difficult for the counter ions to diffuse into the film. For these reasons, the surface tensions differ from the tensions in the bulk polymer. This difference in the tensions leads to a further contraction which also causes an increase in the surface roughness.

A change in the surface roughness of potentiodynamically produced films is not observed (Fig. 4). From the equilibrium potential towards oxidizing potentials and also towards reducing potentials, no change of the films roughness can be observed. Moreover, films which were produced potentiostatically and which are more than one day old do not show any increase in the roughness from the applied potential. This, together with the higher roughness values for potentiostatically produced PPy films, leads in general to the conclusion that in fresh films the polymer chains are formed with a larger disorder than in potentiodynamically produced films. This disorder causes more unsaturated chain endings. After polymerization has finished it takes time to saturate these reactive centers in the polymer bulk. By applying an oxidizing potential

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