Ordering processes in electrochemical surface systems

In electrochemical systems one can induce phase transitions of the surface system by shifting the electrochemical potential. If the potential pulse is directly applied to the tip of an electrochemical STM, such phase transitions can be driven very fast. Since the distance between the tip and the surface is very small, the electrodes' double layers charge within a few nanoseconds.

We were able to dissolve up to about one half of a monolayer of Au from the topmost layer of a Au(111) surface, immersed into a KCl electrolyte within less than a microsecond. The ordering of the surface could be in situ followed by STM after the phase change. Since the change of the surface state occurred very fast, nucleation and growth of islands could be avoided and labyrinthine surface patterns, characteristic for spinodal decomposition of an unstable two-phase system are observed. Progressing ordering and self similar growth towards a stable island morphology occurred on a time scale of several minutes, following the initial spinodal decay [1].

Currently we are using electrochemical microcalorimetry (See more; verlinkt mit Thermodynamics and kinetics…) in combination with time resolved surface plasmon resonance spectroscopy in order to follow the nucleation and growth of Cu upon electrochemical deposition on Au. First results indicate that the heat released upon nucleation of Cu islands can be directly detected by calorimetry.

The excitation of surface plasmons, i.e., plasma vibrations at a surface, is particularly sensitive towards the properties of the interface between metal electrode and electrolyte [2]. Minute changes of the dielectric properties, e.g., by formation of a submonolayer of Cu can be detected. We record the plasma signal with a time resolution below 1 ms in order to obtain information on the progress of the deposition reaction.

Fig. 1: Schematic diagram of the so called ‘Kretschmann configuration’

Fig. 2/3: Assembling for surface plasmon resonance spectroscopy

Plasma vibrations are periodic oscillations of free electrons in conductors. As electromagnetic waves they are quantized and the corresponding quasiparticles are the so called plasmons.
We distinguish volume plasmons and surface plasmons. The former ones, i.e. plasmons in the volume of non-volatile matter, are longitudinal charge density waves, which can be excited exclusively by collisions (e.g. with electrons) and not by photons, i.e. transversal electromagnetic (plane) waves. In contrast surface plasmons propagate on an interface,
e.g. between a metal (solid) and a dielectric (liquid or gaseous). They contain a transversal oscillation component, hence they can be excited by light under well defined optical conditions. The characteristics of these surface plasmons are highly sensitive to modifications of these interfaces, e.g. by adsorption of atoms or molecules.

According Kretschmann´s configuration surface plasmons can be excited at the surface of thin gold films of about 50 nm thickness evaporated on top of a hemispherical cylinder (► fig. 1) ^[3]. This coupling device allows to fulfill both the conservation of energy and the momentum as can be seen in the dispersion relations of photons and surface plasmons (► fig. 4).

In a typical surface plasmon resonance experiment the reflected laser intensity is measured as a function of the angle of incidence. Under ideal conditions (optimal film thickness, ideal polarization of the laser beam, no interferences etc.) the reflected laser intensity drops to zero as the surface plasmons are generated (► fig. 5). The shape of the reflection curve (inclusive the angle of incidence, where resonance appears) is highly sensitive to the nature of the gold interface on which surface plasmons are excited.

We developed an experimental set up to measure the entire reflection curve with millisecond time resolution (► fig. 2 and 3) and use this technique for in situ electrochemical processes employing the gold film as a working electrode. This is presented in fig. 6, which shows the simultaneous recording of the cyclic voltammogram and the plasmon signal. The surface plasmon signal was taken at a constant angle of incidence at the maximum slope of the plasmon curve (► fig. 6).

[1] Schuster, R., D. Thron, M. Binetti, X.-H. Xia and G. Ertl, Two-dimensional nanoscale self-assembly in a gold surface by spinodal decomposition, Phys. Rev. Lett. 91, 066101 (2003)

[2] D. M. Kolb, 'The Study of Solid-Liquid Interfaces by Surface Plasmon Polariton Excitation' in 'Surface polaritons' ,eds. V. M. Agranovich and D. L. Mills, North-Holland (1982).

[3] D. M. Kolb, 'The Study of Solid-Liquid Interfaces by Surface Plasmon Polariton Excitation' in 'Surface polaritons' ,eds. V. M. Agranovich and D. L. Mills, North-Holland (1982).

Fig 4: Schematic diagram showing both the dispersion relation of surface plasmons (blue), which propagate on the interface metal / ambiance in χ -direction, and the (linear) dispersion relation of photons for the following cases: Photons going through the media vacuum (resp. air) and may reach the surface with any angle of incidence α < 90° (yellow) or propagate parallel to it (α < 90°, orange). In both cases no surface plasmons will be excited by reason that a common intersection between both curves is missing, but by changing through a media with ε_amb> 1 (red) the case of resonance could be fulfilled.

Fig 5:Graphical representation of the laser beams´ intensity (here: current I in μA) after reflection on a 50 nm thick gold film in dependence on the angle of incidence α_Pl. Two graphs are shown: On the one hand the run of the curve for the system „BK7-glass / golf film / air“ (blue), on the other hand for the system „BK7-glass / gold film / water“ (red).

Fig 6: Typical cyclic voltammogram (red) of a sulfuric copper sulfate system on a thin polycrystalline gold film. Beyond that the simultaneous measured corresponding laser intensity (blue) after reflection under a well-defined angle of incidence within the range of plasmon resonance on that film.