I think the following paper may help a bit to understand about the mechanism of feedback mode: J. Phys. Chem., 1992, 96 (4), pp 1861–1868, http://pubs.acs.org/doi/abs/10.1021/j100183a064?journalCode=jpchax

Hi Luca,

Thanks for your comment.

It's true that the current is too high for sub-micro electrode. This is another question which we are going to deal with because the SECM tip was commercially bought. However, I think the increase of current during scanning the metal surface is still valid even in high current magnitude, right?

Thanks a lot.

 Hi Indira,

Thanks for your information.

Actually I have read some paper mentioned about the feedback mode. However, seldom of them are mentioned whether the conductive substrate need to be applied a voltage with positive feedback or not. Or even without the applied voltage, the species generating from the tip would automatically transfer the hole/ electron to the conductive substrate. Could you help for this question?

Thanks a lot. 

Have you actually tried to run  approach curves? In your description you write that the tip has contact with the substrate - this should not happened. Instead, the gap between tip and substrate should become as small as possible. 

Positive feedback is based on a recycling mechanism of a redox pair. For example, if the potential on the tip is so that a species is oxidised, the potential of the substrate should be set to a level where the oxidised species is recycled, i.e. reduced. So the applied potential at the substrate plays a crucial role.

When the tip is approaching to a conductive substrate by monitoring the current of a redox reaction, you can either observe a positive feedback (facile electron transfer of the substrate) or a kinetic controlled kinetic (less facile electron transfer of the substrate) when the substrate is left at open circuit potential. You do not have to apply a potential to get these trends.

From your CV, the charging current (curve separation) is too big, try to polish your electrode with appropriate alumina powders.

Dear Tseng,

First, your comparison is not correct, what you are presenting in the first image is an approaching curve (current vs. distance); meanwhile, the second image presents a cyclic voltammetry (current vs. potential). Second, as everybody mentioned, your current range is too high for a UME, you should obtain a current response in the order of 10-9 A. Third, as Dr. Yin Jing pointed, the charge current between the forward and backward scan is too separated, you should obtain almost the same current curves for both scans. I can suggest you clean in a proper way your electrode by polishing it softly but also to read a lot about the technique in order that you understand the purpose of the approach curve, but also about the reductive or oxidative potential that you are applying to your UME which will depend on the redox pair that you are employing.

Best!