I am coating some gold electrodes with different amounts of carbon nanotubes (CNT) to create a more electroactive surface with increased surface area, and I am evaluating the electrode behavior with ferrocene. When I test scan rate dependence at the bare Au electrode and thin CNT coating, I see a very clear linear correlation between peak current (Ipa) and square root of scan rate, indicating a diffusion-controlled process. However, I think thicker layers of the CNT film require some amount of diffusion of ferrocene into the film for full saturated signal - therefore I used an accumulation time of 1 minute before each CV cycle when measuring scan rate at this thicker film. Now, I actually see a better linear fit between Ipa and scan rate (R2 = 0.999, vs. R2 = 0.981 for square root of scan rate), indicating that this is now an adsorption-controlled process. Any thoughts as to why this thicker film would make the reaction adsorption-controlled?
Hi Ashley,
In my opinion this is perfectly possible. If you see in Sens. Actuators 202 (2014) 129, we see a difference between methylene blue on gold electrodes (thick-film instead of thin-film) with and without nanostructuration with amino functionalized MW CNTs. On gold electrodes the behavior is diffusion-controlled and with CNTs it become adsorption-controlled. Probably with a thin layer gold is still dominating and with a thick layer CNTs becomes dominant and adsorption phenomena control the mass transport.
Good luck and best regards,
Teresa
Hi Ashley Chapin,
To answer your question, I would like to ask you a few questions. 1) When you change a thin layer of CNT with a thick layer, did you notice after each scan (cyclic voltammetry) a shift of potential to positive values? If so, its electrode surface changes with each scan mean there is an adsorption phenomenon occurring. This phenomenon may be due to the fact that the surface of the electrode becomes very porous when you increase the amount of CNT, in this case, I think the reason you did not find the same for a thin layer of CNT, it can be provided by a small amount of CNT If not ( please see the response below) 2) for the accumulation time, did you also study the effect of the accumulation time for a CNT thin film? --If the results are similar, I think this phenomenon is normal because, when you apply a potential with a given accumulation time, you sometimes favor physical interactions (Physisorption) and / or chemical (chemisorption) on the sites of CNT layer, may have decreased the concentration of the analyte after each scan.
The values of R² that you showed, it's well remarked there is no a big difference I could say that the process of charge transfer becomes controlled by both ways (synergistic effect between Adsorption and diffusion)
Abdelghani
Good luck!
I have observed the same behevior for the electro oxidation of Dopamine on GCE and GCE/MWCNTs. I found the accumulation time is an effective factor. The thick film of MWCNTs on GCE changed the redox behevior of Dopamine from diff. cont. to ads. cont.
I think it is in good agreement with mass transfer phenomena followed by reaction in porous media.
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