Adsorption processes and spatiotemporal pattern formation during electrochemical reactions on Au(111) film electrodes – A surface plasmon resonance study

In this thesis the interaction of organic monolayers, which form condensed two-dimensional films on Au(111) electrodes, and different electrochemical reactions was studied, addressing questions which arise from two fields, namely electrochemistry and nonlinear dynamics. With surface plasmon resonance angle (SPRA) measurement and cyclic voltammetry, the adsorption of thymine, coumarin and camphor on Au(111) film electrodes was investigated with special focus on changes of the optical properties of the interface which are caused by a first-order phase transition that these adsorbates exhibit. Furthermore, the inhibiting properties of the condensed camphor layer on different electrochemical reactions was investigated. Among those, the systems camphor-periodate/Au(111) and hexacyanoferrate-(camphor)/Au(111) exhibited, for different reasons, particularly interesting properties and were investigated in some detail. The camphor-periodate/Au(111) system was shown to possess an S-shaped current-potential characteristics. Motivated by recent theoretical investigations, which predict that systems with an S-shaped current-potential characteristic exhibit spatial instabilities that give rise to stationary patterns, the lateral distribution of the camphor film was examined under different reaction conditions with surface plasmon imaging (SPI). Indeed, in wide parameter ranges, stationary adsorbate patterns could be observed that possessed all properties of so-called Turing patterns. These examinations allowed the unambiguous experimental confirmation of the existence of electrochemical Turing structures. In the absence of camphor, periodate reduction on Au(111) was found to possess an N-shaped current-potential characteristic. This system belongs thus to the large class of electrochemical N-NDR oscillators and was chosen as model system for studies on the impact of a negative global coupling on pattern formation in these systems. A negative global coupling was known to cause spatial instabilities in N-NDR systems, but studies with two-dimensional electrodes were missing. Such studies were carried out employing SPRI to obtain the spatial resolution and a negative resistance device (NRD) to introduce the negative global coupling into the system. The latter acts in the same way as the IR compensation of a conventional potentiostat does. Above a certain coupling strength, a spatial symmetry breaking was observed and in wide regions of the remaining parameters, gave rise to standing waves with a wave number 1 in one spatial dimension. In smaller parameter regions less symmetric spatio-temporal patterns were found. The experiments compare favourably with simulations. The hexacyanoferrate redox reaction exhibited a complex interaction with the adsorbed camphor monolayer, which, because of the widespread use of this reaction for the characterization of organic films, was investigated in more detail. If the electrode potential was kept negative to a threshold potential, the camphor adlayer slowed down the charge transfer rate, whereas for larger potentials the camphor film was replaced by a polymeric hexacyanoferrate adsorbate that transformed for longer reaction times to a Prussian-blue film. SPRA measurements revealed that the destruction of the camphor film occurred within the first voltage cycle. Furthermore, SPRA measurements in solutions without camphor provided evidence that in a neutral NaClO4 supporting electrolyte a bare Au surface does not exist in the presence of small amounts of ferri- or ferrocyanide in most of the potential ranges usually employed