INSITU ELLIPSOMETRIC STUDY OF THE GROWTH AND ELECTROCHEMICAL CYCLING OF POLYPYRROLE FILMS ON PLATINUM

The growth and potential cycling of polypyrrole films on platinum has been examined using time-resolved in situ ellipsometry. The optical properties of the films produced depend critically on the experimental conditions used to grow the films. Films grown from "wet" acetonitrile, or acidic (< pH 2) aqueous solutions show very low values of n (1.25-1.35) at 633 nm, indicating the presence of a very strong absorption band in the near-infra-red (Lorentz oscillator model). Films grown in less optimal conditions (neutral aqueous solution, or with Fe(CN)6(3-)-containing electrolyte) have higher n values (1.45-1.5). Comparison of calculated (from charge per unit area data) and measured (ellipsometric) film thickness shows that films grown in aqueous media are somewhat denser than those grown in acetonitrile. On reduction, the film thickness increases for films grown under all conditions, but the change is greater for films grown from aqueous solution. Changes in n and absorbance, k, are, as expected, wavelength-dependent, but in general n greatly increases (to 1.65-1.80); this is also rationalized using the Lorentz oscillator model, since the wavelengths employed are to low energy of a strong absorption in neutral polypyrrole at ca 390 nm. On potential cycling from the reduced to the oxidized form, the film thickness decreases between ca -0.6 and -0.2 V, but thereafter, although charge is still being passed, changes much more slowly.