Electrochemical polymerization of pyrrole on Au(111) in sulphuric acid and sodium hexafluoroaluminate solutions monitored by electrochemical scanning tunneling microscopy

- The electropolymerization of pyrrole on Au(111) electrode was traced by EC-STM in two different electrolytes.
- The evolution from 2D isolated nano-islands to finally overlapping 3D globules was observed.
- The Au(111) substrate plays only a role of a support without any influence on the polymer adsorbate order.
- The role of the OH- chemisorption in the formation of the PPy overlayer is discussed.

The early stages of electrodeposition of polypyrrole (PPy) on an Au(111) surface are studied using in-situ scanning tunneling microscopy under the electrochemical control of the process. Shape, size, nucleation density and dimensionality of the early growth of the polymer phase was of interest. Electropolymerization took place in aqueous electrolytes containing multicharged anions. The diameter of the smallest detected surface particulates was of the order of a few Å in Na3AlF6 solution and a few tens of Å in pyrrole - H2SO4 solution.In H2SO4 solution, the very early stages of the electropolymerization lead to flat islands with a height of about 2.0 Å and a diameter of several tens of Ångstrom on Au(111) terraces, which indicates the dominance of two-dimensional polypyrrole structures. The surface population of these islands observed in sulphuric acid increases with the number, range, and decreasing potential scan rate of voltage cycles. At later stages and higher coverages the STM images show a further growth of overlapping PPy globules, which change from two- to three-dimensional structures. In aqueous Na3AlF6 solution, the STM images showed lifting of the Au(111) surface reconstruction, which may be due to the chemisorption of OH−. The early polypyrrole deposition in aqueous sodium hexafluoroaluminate, visible with nano-scale horizontal resolution, results predominantly in a 2D growth of the polymer globules having about 2.0 Å in height on the onset of the polymer deposition.

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