Results

The classical procedure for the activation of an electrode surface is to obtain a surface roughness by cycles of oxydation and reduction. This type of activation is easy to apply but does not give an opportunity to produce well-defined surfaces, whereas deposition of an SERS-active metal on an inert electrode offers the possibility of achieving the enhancement effect as well as controlling the surface structure. Some potential programs useful for the activation of the electrode surface are depicted in Fig. 1.

For the standard silver cyanide bath we obtained the activation by switching the potential from 0 V to a potential, in general -800 mV, where reduction of silver occured. After a couple of seconds an SER signal was observed, increasing in intensity within the following minutes. In general a braod maximum was obtained at a wavenumber of 2110 cm"1 (see Fig. 2). It was shown that the development of the Raman signal is connected with the development of the morphology of the surface, mainly depending on the initial state represented by the number of active sites and nuclei formed [1].

Fig. 1. Potential pulse programs used for SERS activation of the electrode by SER-active metal deposition on platinum.
Fig. 2. Development of the SER signal of the CN stretch vibration during silver deposition on platinum, electrolyte HI.
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Fig. 3. Development of the double-layer capacity and the SER intensity of adsorbed cyanide ions during silver deposition on platinum, electrolyte HI, E = 800 mVScE-

Fig. 4. Development of the double-layer capacity and the SER intensity of adsorbed cyanide ions during silver deposition on platinum, electrolyte n, E = 800 mVScE-

A comparison of the development of the SER intensity with the changes of the true surface area, represented by the double-layer capacity, is shown in Figs. 3 and 4. The two examples measured under similar conditions demonstrate the scattering of the general shape of Raman-time and capacitance-time plots. The good correlation in the shape of the curves found in the experiments leads to the assumption of a proportionality relation between the SER intensity and the „true area" of the surface.

Using the potential program described in Fig. 1(b) results in a splitting of the CN -stretch vibration of the broad cyanide peak into a large number of single peaks, shown in Fig. 5.

Fig. 5. SER spectrum of the CN" stretch vibration after silver deposition on platinum obtained by applying the potential pulse programming of Fig. 1(b) electrolyte I.

This splitting of the SER signal depends mainly on the pretreatment of the inert platinum electrode and on the type of activation. The optimal conditions to observe the splitting are not yet completely known but, for all cases, a single nucleation pulse in the higher cathodic regions for a short time followed by a potential where only deposition takes place is a useful procedure. The splitting was detected not only at the beginning of the film formation but could be observed during the whole time of growth; see Fig. 6. in order to elucidate the conditions for the observed splitting we varied both the nucleation time fa and concentration of the silver ions CAg+; two examples are shown in Figs. 7 and 8.

Fig. 6. SER spectra of the CN" stretch vibration during silver deposition on platinum obtained by applying the potential pulse programming of Fig. 1(b), electrolyte I.
Fig. 7. SER spectra of the CN" stretch vibration for different durations ?N of the nucleation pulse (electrolyte II): (a) iN = 3 s; (b) fa = 7 s.
Fig. 8. SER spectra of the CN" stretch vibration for different silver concentrations (iN (a) electrolyte I; (b) electrolyte HI.
Fig. 9. SER spectra measured at (a) -700 mV and (b) 0 mV: shift from 2110 cm"1; iN electrolyte HI.

Another example of this unusual behavior is shown in Fig. 9. The splitting could be conserved during the shift of the broad CN"1 signal from 2110 cm"1 to 2145 cm"1 evoked by a potential shift from -700 mV to 0 mV.

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