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Solid State Electrochemistry

Margret Giesen

Since the invention of scanning probe microscopes (SPM), the direct observation of migration processes at surfaces has become possible. Frequently, however, atomic and molecular migration takes place rapidly compared to typical scan speeds of SPMs. At low temperatures, migration is slow; however, processes with high activation energies, important for the ion transport in electrodes and in electrode degradation, are frozen. To obtain a deeper insight into the relevant migration and degradation processes, experiments have to be performed at high temperatures where the SPM is much too slow to observe the diffusion of individual species.

In order to obtain quantitative information on dynamical processes at electrode surfaces with SPM, one has to make use of indirect methods: The migration of atomic and molecular species leads to shape variations of mesoscopic electrode structures in time which are analyzed by means of methods from statistical physics. From this analysis one can determine the atomic diffusion mechanism dominating the motion of the mesoscopic surface structures.

 

Video 1: Island coarsening in vacuum

The video shows a 150 x 150 nm² area of a Cu(111) surface at 41°C after deposition of about 20 monolayers of copper. The surface shows a certain roughness formed by vacancy holes, several layer deep grooves and multi-layer adatom islands. Due to atomic migration on the surface, the initially observed roughness degrades, holes and grooves are filled while upper layers in multi-layer islands vanish. From the statistical analysis of the time dependence of such migration and degradation phenomena one deduces information about the surface migration processes. The video shows the surface evolution during coarsening over a time span of approximately 13 hours.

Video 2: Au(100) reconstruction in electrolyte

The video shows a sequence of electrochemical scanning tunneling microscopy (EC-STM) images of a Au(100) electrode in 50 mM H2SO4 with monatomic high Au islands at room temperature. The surface area is 110x110 nm². At low electrode potentials, the Au(100) exhibits a quasi-hexagonal reconstruction with a higher atomic density in the surface layer than the normal (1x1) structure of the (100) surface. Increasing the electrode potential to more positive values, the reconstruction is lifted and monatomic high islands are formed. These islands are not static but reveal a high mobility caused by migration processes on the atomic level. For this video, the electrode potential has first been increased to +350 mV vs. the SCE (Saturated Calomel Electrode) reference to create islands on the Au(100) surface. Then, the potential has been stepped back again to -200 mV where the surface reconstruction is re-established. The islands decay due to the formation of the denser reconstructed layer. Reconstruction lines (which appear bright in the video) are formed and the islands are "eaten up".

Video 3: Au(100) island coarsening in electrolyte

The video shows a sequence of electrochemical scanning tunneling microscopy (EC-STM) images of a Au(100) surface in 50 mM H2SO4 with monatomic high Au islands at room temperature. The surface area is 85x85 nm². At low electrode potentials, the Au(100) exhibits a quasi-hexagonal reconstruction with a higher atomic density in the surface layer than the normal (1x1) structure of the (100) surface. Increasing the electrode potential to more positive values, the reconstruction is lifted and monatomic high islands are formed. These islands are not static but reveal a high level of mobility caused by migration processes on the atomic level. In this video, the electrode potential is +350mV vs. SCE (Saturated Calomel Electrode) reference. The islands' area does not remain constant: small islands decay and larger islands grow in time. Furthermore, coalescence events are observed.


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