Development of an all-optical method to measure surface diffusion at electrochemical interfaces

In this work, a new in situ method for measuring the surface diffusion rates of adsorbates on electrodes in electrochemical environment was developed.

Surface diffusion processes have been extensively studied on solids under vacuum conditions with a variety of different methods. In sharp contrast, only few studies exist to measure surface diffusion in electrochemical environment due to the lack of suitable methodology. We aim on partially filling this gap by introducing the all-optical in situ LOD methodology, which allows to measure surface diffusion rates over a wide range and thus overcomes limitations of established methods that are mostly restricted to slow diffusion rates. The method is based on the generation of a periodic spatial modulation of adsorbates using two interfering laser pulses. The subsequent equilibration of this pattern and thus the diffusion rate of adsorbates is probed by the linear optical diffraction (LOD) signal from a second laser.

First proof-of-principle measurements were performed on sulfur diffusion on the Pt(111) electrode. For this purpose, the optical and electrochemical properties of this systems were investigated first, including studies of the optical reflectance change induced by chemisorbed sulfur (Sad) and investigations of the Sad oxidation mechanism. The subsequent studies of Sad surface diffusion revealed potential and coverage dependent diffusion rates which were at least an order of magnitude faster than under vacuum conditions. Supported by numerical simulations, the implications of strong adsorbate-adsorbate interaction combined with different grating modulation depths to the temporal evolution of the LOD signal were demonstrated and data accuracy and pitfalls of the method were discussed. Based on these very first findings, further improvements for the developed in situ LOD methodology were proposed.


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