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

摘要

A newly designed thin-layer STM (TLSTM) technique is presented based on a microfabricated probe combining an electrolytic STM probe with an annular iridium oxide microelectrode operating as a potential-controlled generator or scavenger of H~+. This TLSTM configuration can be used to combine in situ STM 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 electropolymerised polyaniline films and associated STM-relevant properties in 0.5 M NaClO_4+ 10~(-2) M HC1O_4. For a polyaniline film kept in the intermediate (emeraldine) oxidation state at 0 mV versus MSE it is shown that its thickness 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 from 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 of both redox transfer steps, it is shown that significant exchange of H~+ occurs only in the interval of the second (emeraldine/pernigraniline) redox transfer.