Elsevier

Journal of Electroanalytical Chemistry

Volume 541, 16 January 2003, Pages 163-174
Journal of Electroanalytical Chemistry

The Fe(CN)63−/Fe(CN)64− charge transfer reaction on Au(111) revisited in the presence and absence of a two-dimensional, condensed organic film

https://doi.org/10.1016/S0022-0728(02)01428-6Get rights and content

Abstract

We present studies on the electron transfer rate (etr) of the Fe(CN)63−/4− redox reaction on Au(111) 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)63−/4− redox couple depended strongly on the most positive potential of the experiment. If U was kept negative of a threshold potential Uth=−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 Uth 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)63− or Fe(CN)64− ions in most of the potential ranges usually employed.

Introduction

The impact of organic films on the rate of electron transfer reactions (etr) has been of considerable interest during past decades not only because of fundamental questions but also owing to the practical aspects. Among the basic questions are the mechanism of electron tunneling across the film, which may be elastic or resonant, the effect of the film on the potential distribution close to the electrode, or how an inhibiting effect is distributed between changes in the probability of the electron transition and the activation energy of an etr. More applied points include the protective properties of organic films against corrosion or the utilization of condensed films in studies of biologically relevant electron exchange processes between redox centers which occur across large segments of organic molecules, such as cytochromes.

The majority of the early studies was carried out with mercury electrodes. Most of the organic films (e.g. thymine [1], coumarin [2], [3], quinolines [4], and camphor [5], [6]) were found to inhibit etr considerably. These studies also revealed that the details of the inhibition depended crucially on the structure of the film, the type of electrode process and even more subtle properties of the experiments, such as the nature and, in some cases, even the conductivity of the base electrolyte.

More recent studies focused on the inhibiting features of solid electrodes covered by organic films, in particular of self-assembled monolayers (SAMs) on Au electrodes [7], [8], [9], [10], [11], [12], [13], [14], [15]. If the monomers which form the film are sufficiently large, such as octadecanethiol, the films provide effective blocking of the electrochemical reactivity [15], [16]. Thus, the degree of the inhibition of an etr is frequently used as a measure for structural defects of SAMs [16], [17].

In many of these studies, the Fe(CN)63−/4− redox couple was used to probe the extent of the inhibition of the electron transfer by an organic monomolecular film. Yet, a few indications can be found in the literature, that the Fe(CN)63−/4− reaction causes some destruction of the organic film [18]. As far as the reaction of the hexacyanoferrate(III)/hexacyanoferrate(II) redox couple on bare Au electrodes is concerned, it is known to depend sensitively on the composition and concentration of the electrolyte [19], [20], [21], [22], [23]. Furthermore, despite extensive studies, there is still some dispute over whether there is some adsorption of a hexacyanoferrate species [24] as well as over the nature of the adsorbate, which was considered to be either one or both of the redox species Fe(CN)63−/4− [25], [26] itself or a more complex ‘reaction product’ formed from these species [21], [27], [28], [29], [30], [31].

In this paper we study the hexacyanoferrate redox reaction on a Au(111) electrode covered by a condensed physisorbed camphor film. The camphor film was shown effectively to inhibit the reduction of IO4 [32], i.e. a reaction in which the electron transfer is accompanied by the transfer of a proton.

The ability of camphor to form a physisorbed two-dimensional film on Au(111) over a large potential interval was first reported by Kolb and coworkers [33]. Negative to this potential interval, the Au surface is nearly adsorbate free, the transition between both regions being accompanied by a first-order phase transition (PT), which manifests itself in a needle-like peak pair in the cyclic voltammogram. We demonstrate that this signature of the PT represents a very sensitive probe for the intactness of the camphor film and allows the detection of traces of a polymeric hexacyanoferrate complex that replaces camphor. This hexacyanoferrate adsorbate is further investigated by employing SPR measurements in the presence and absence of camphor. The studies show clear limitations on the use of the Fe(CN)63−/4− redox reaction in the characterization of organic surface films with respect to their capability to inhibit etrs. Furthermore, they allow conclusions to be drawn on the type of hexacyanoferrate adsorbate that forms on the electrode in neutral perchlorate electrolytes.

Section snippets

Experimental

The cyclic voltammetric (CV) experiments were performed with a Au(111) single crystal (MaTeck) in the hanging-meniscus geometry. The mounting of the single-crystal allowed the electrode to be rotated in the hanging-mensicus geometry with a variable rotation rate and the height of the meniscus to be adjusted reproducibly (Fig. 1a). The construction of the mounting was similar to that described by Cahan and Villullas [34]. The single crystal was flame-annealed (butane flame) prior to each

Camphor adsorption

Fig. 2a represents a cyclic voltammogram (CV) of Au(111) in 50 mM NaClO4 and 5 mM camphor solution in a potential region between −1.0 and 0.2 V. The two characteristic sharp peak pairs encompass the potential region (region II) in which camphor forms a condensed, physisorbed film on the electrode surface. The needle-like peaks are a manifestation of a first-order PT of the camphor film. In region I, the camphor coverage is negligible, while in region III a chemisorbed camphor phase exists. Upon

Conclusions

The PT peaks in the CV of camphor on Au(111) proved to be a sensitive probe for the interaction between the two-dimensional physisorbed film and a redox couple. CVs and capacitance measurements of the Au(111)  camphor, Fe(CN)63−(4−), ClO4-system revealed that camphor inhibits the reaction as long as the electrode potential is more negative than 0.2 V versus Hg  Hg2SO4, the decrease in the reaction rate being more pronounced in electrolytes of low ionic strength (0.1 and 0.02 M) than in those of

Acknowledgements

We thank K. Doblhofer for many fruitful discussions.

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