NANOMETER-SCALE ELECTROPOLYMERIZATION OF ANILINE USING THE SCANNING TUNNELING MICROSCOPE

The platinum tip of a scanning tunneling microscope is employed to dir ect the electropolymerization of aniline on nanometer-scale regions of a graphite surface which is immersed in an aqueous, aniline-containin g electrolyte. The electropolymerization of aniline is initiated by tw o bias voltage pulses which are applied in rapid succession: The first -(tip-negative) 6.0 V x 5 mu s-causes the formation of a pit in the gr aphite surface, and the second-(tip-negative) 3.0 V x 50 mu s-induces the oxidation and the subsequent polymerization of aniline at this pit to form a particle of polyaniline (PANI). These PANI particles varied from 100 to 600 Angstrom in diameter and 10 to 200 Angstrom in height . Nanoscopic PANI particles prepared by this method are shown to be el ectrochemically responsive: A comparison of in-situ STM images of indi vidual PANI particles acquired at sample potentials which are positive and negative of the PANI oxidation potential reveals that the volume of a particle is larger by approximate to 30% in the oxidized emeraldi ne form than in the reduced leucoemeraldine form as a consequence of t he higher solvent and anion content of the oxidized PANI particle. The results of in-situ STM experiments, ex-situ electrochemical measureme nts, and Monte Carlo simulations of transport collectively provide inf ormation on the mechanism of PANI particle growth. On the basis of the se data, it is postulated that the aniline involved in PANI particle s ynthesis is derived from an adsorbed monolayer at the graphite surface .