Local pH-controlled reactivity investigations by thin-layer scanning tunnelling microscopy

A newly designed thin-layer STM (TLSTM) technique is presented based on a m icrofabricated probe combining an electrolytic STM probe with an annular ir idium oxide microelectrode operating as a potential-controlled generator or scavenger of H+. This TLSTM configuration can be used to combine in situ S TM studies of electrode processes with the simultaneous investigation of H flux integrals associated with the electrode reaction. In this paper, the technique is applied to investigate the H+ exchange behaviour of electropol ymerised polyaniline films and associated STM-relevant properties in 0.5 M NaClO4 + 10(-2) M HClO4. For a polyaniline film kept in the intermediate (e meraldine) oxidation state at 0 mV versus MSE it is shown that its thicknes s increases due to H+ insertion if the pH is lowered from pH 2 to ca. 1.6, and that its conductivity decreases significantly if the pH is increased fr om 2 to ca. 12. In a second experiment, where the exchange of H+ at pH 2 is monitored during film oxidation and reduction within the potential range o f both redox transfer steps, it is shown that significant exchange of H+ oc curs only in the interval of the second (emeraldine/pernigraniline) redox t ransfer. (C) 2001 Elsevier Science Ltd. All rights reserved.