The response of the diffusion layer to potentiostatic and galvanostatic stimulus of the substrate is described. The interaction of the SECM tip (an ultramicroelectrode (UME)) with the substrate relies on the diffusion of electroactive species between the tip and the substrate. This technique used the overlap of the diffusion layer of the tip with the substrate to ' produce a current response that varies with tip/substrate separation.
When the diffusion layer of the UME overlaps with the conductive substrate, an enhancement of current is seen due to diffusive feedback. In both cases of negative and positive feedback, the effect is a function of the point sample separation. The SECM uses diffuse electroactive species to investigate the electrochemical nature of the substrate (19a) as well as the separation between the tip and substrate. The fate of unstable species that diffuse into the tip/substrate gap can also be investigated (19a, 20).
The use of a disk UME for the probe tip in an SECM limits the approach of the tip to the substrate. Chapter three discusses the dynamics of the diffusion layer in the vicinity of the surface of an electrode in response to both a potentiostatic and a galvanostatic step.
Instrumentation
A series of relays was used to switch tip and substrate control between the STM electronics and the bipotentiostat. Another problem arises when monitoring the tip current as you approach the tip (or pull the tip away from) the substrate. However, an understanding of the diffusion layer dynamics for these cases can be developed through the use of simulations.
Definitions of the symbols used in the following equations can be found at the end of the chapter. At the beginning of the potential stage, the surface concentration of the product is fixed at C~ (DR I Do )112. Of primary importance is the dynamics of the diffusion layer in the STM region (about 1 Jlm).
Therefore, the reactant concentration is given by the same equation derived for the general case (Eq. 3.10). The introduction of a coupled chemical reaction prevents the derivation of an exact solution of the diffusion equation for the product (Eq. 3.15). As in the potentiostatic case, the concentration profiles within 1 Jlm of the electrode surface cluster closely.
A steady state must be reached before kt = 5.4 because the establishment of the steady-state surface concentration and the.
Time, s
Distance, 1-1m
Concentration profiles showing the approach to steady state of an intermediate species R generated on a planar electrode undergoing a galvanostatic step with 't = 1 s. Steady-state concentration profiles for intermediate R generated on a planar electrode undergoing a galvanostatic step on t=t=l sand with Do=5xiQ-6cm2/s, and k as indicated. Steady-state concentration profiles for intermediate R generated on a planar electrode undergoing a galvanostatic step on t=t=l sand with Do=5x1Q-6cm2/s, and k as indicated.
Distance, l!ffi
One set of experiments used the same galvanostatic step while placing the tip at different distances from the substrate electrode. In the second set of experiments, the point was held in a fixed position while the length of the. Upon completion of the galvanostatic step, the substrate is returned to potentiostatic control, the feedback electronics are turned on, and the tip is returned to tunneling distance.
A typical result is shown in figure 4.1, where the peak current as well as the substrate current and the potential are plotted. Since DMFc+ is no longer generated at the substrate electrode, the peak current decreases with time as the DMFc+ diffusion layer disappears into the bulk of the solution. When the tip is moved closer to the substrate, the current measured during the galvanostatic step increases (Fig. 4.3).
In this second set of experiments, the tip is held at a constant distance from the substrate electrode while the current step is varied. The first set of potential step experiments for the hexaamine complex investigates the effect on the current response of the tip due to changing the distance between the tip and the substrate. Ru(NH3)62+ is now consumed at the substrate electrode and the tip current returns to zero.
The growth of the diffusion layer of the reactant Ru(NH3)63+ can be monitored by adjusting the tip potential to -400 mV (Figure 4.12). In this case, there is a non-zero tip current before the potential step due to the reduction of Ru(NH3)63+ at the tip. As the tip moves closer to the ground, the tip current measured during the potential step increases (Figure 4.13).
As the tip gets closer to the substrate, the "rise time" of the tip current should be shorter. The faster tip current response as the tip moves to 0.16 f.lm of the substrate is clearly shown. Of the tip potentials shown in Figure 4.19, the best current response is observed at a tip potential set at -175 mV.
Using the peak current at -175 mV obtained from cyclic peak voltammograms for. Therefore, the measured peak current is actually a sum of the simulation curves, which would be further weighted by . By integrating a bipotentiostat, the tip and substrate potential can be controlled independently.
The behavior of the peak current for both the potentiostatic and galvanostatic stages of the substrate electrode agrees well with the theory of diffusion layer growth.
