The Fe(CN)₆^3-/Fe(CN)₆^4- charge transfer reaction on Au(1 1 1) revisited in the presence and absence of a two-dimensional, condensed organic film

We present studies on the electron transfer rate (etr) of the Fe(CN)₆^3-/4- redox reaction on Au(1 1 1) electrodes in the presence and absence of a condensed two-dimensional camphor layer in NaClO₄ electrolytes of different ionic strength. The experiments were carried out employing cyclic voltammetry, capacitance measurements and surface plasmon resonance (SPR) measurements. The interaction between the two-dimensional organic film and the Fe(CN)₆^3-/4- redox couple depended strongly on the most positive potential of the experiment. If U was kept negative of a threshold potential U_th = -0.2 V vs. Hg | Hg₂SO₄, the camphor adlayer slowed down the charge transfer rate, and the effect was more pronounced the smaller was the conductivity of the electrolyte. For potentials larger than U_th the camphor film was initially replaced by a polymeric hexacyanoferrate adsorbate that transformed after longer reaction times to a Prussian-white/Prussian-blue film, respectively. The initial destruction of the camphor film could be followed sensitively from changes in the cyclic voltammogram and in the capacitance and occurred within the first voltage cycle. SPR measurements allowed the transformation of the 'precursor' hexacyanoferrate film to a Prussian-white/Prussian-blue film to be monitored. Moreover, SPR measurements in solutions without camphor provided evidence that in a neutral NaClO₄ supporting electroyte a bare Au surface does not exist in the presence of small amounts of Fe(CN)₆^3- or Fe(CN)₆^4- ions in most of the potential ranges usually employed.