Topic 4: SECM and Homogeneous kinetics

Question 1

• What is scanning electrochemical microscopy (SECM)?

Answer 1

• A scan probe technique giving high spatial electrochemical information about a surface

Question 2

• Draw a simple diagram of a SECM experiment

Answer 2

Question 3

• How big are the tips used in SECM?

Answer 3

• ~2 µm tip diameter wire, embedded in ~20 µm diameter insulator (RG = 10) – microelectrode probe

Question 4

• How does the limiting current change as the tip approaches an insulating surface and why?

Answer 4

• At E where i=i_lim tip approaches surface in z-direction
• As separation reduced, diffusion to electrode hindered causing a reduction of the steady state current.

Question 5

• What is negative feedback in SECM? Draw the I/d curve associated with it

Answer 5

• Tip blocks more diffusion of O to the tip as separation decreases, reducing current compared to i_bulk­
• i_bulk­ is the current when the electrode is far enough from the surface that hemispherical filed is not being perturbed (>10a from surface)

Question 6

• SECM surface is … meaning no … transfer can take place

Answer 6

• SECM surface is insulating meaning no electron transfer can take place

Question 7

• How is the negative feedback curve affected by RG

Answer 7

• RG >10: diffusion hindered more over large distances (as entire probe wider)
• RG < 10: diffusion largely unhindered, only at very close separation is there an effect

Question 8

• How is negative feedback mode in SECM used as a topographical imaging tool

Answer 8

• Analyte present in solution producing a current upon turning over
• Current response as a function of scanning distance, which is dependent on tip-substrate separation
• Larger gap –> diffusion easier as hemispherical profile less perturbed –> larger current; convert feedback response into topographical map

Question 9

• What governs x,y resolution of a topographical image in SECM

Answer 9

• For negative feedback, largely electrode but also contribution from insulating sheet surrounding electrode and its size

Question 10

• What is positive feedback in SECM? Draw the I/d curve associated with it, and compare it to that of negative feedback

Answer 10

• Tip is brought to within a few tip radii of a conductive substrate
• Red species formed at tip diffuses to substrate where it can be oxidised back to Ox
• Positive feedback produced by additional flux of Ox to the tip, causing current to increase
• Shorter tip-substrate separation distance, the larger the tip current

Question 11

• Describe the mechanism of positive feedback and compare it to that of negative

Answer 11

• In negative feedback A moves in and B moves out of the gap between the tip and insulating surface, and is hindered in doing to by a small gap
• In positive feedback B generate, via the tip, hits conduction surface and gives up electron and is converted back to A –> species generated locally

Question 12

• What is k_t dependent on in positive feedback?

Answer 12

• k_t = D/d
• D – diffusion coefficient
• d – tip-substrate separation

Question 13

• positive feedback is … limited as … transfer is very ….

Answer 13

• positive feedback is diffusion limited as electron transfer is very fast.

Question 14

• How is positive feedback affected by RG?

Answer 14

• Fairly independent, as zone where recycling is limited to region around electrode
• No hindering of diffusion where species must move in/out of gap

Question 15

• How do the diffusion profiles compare with positive and negative feedback?

Answer 15

• Centre of negative feedback shows very compressed 10a hemispherical field up close
• IN positive feedback, as A is being regenerated, lack of local depletion at substratemeansconcentration zone localised to electrode as more A is being put back in to solution.

Question 16

• What is the effect of varying the rate of electron transfer at the surface on the SECM approach curve?

Answer 16

• Rate of electron transfer at surface is how quickly B can give/take up an electron to surface/from electrode.
• Can measure k^o values in between two feedback limits as a finite rate of electron transfer
• If ET getting faster –> +ve feedback (v.v)

Question 17

• Describe an electrochemical system that undergoes a reaction with a homogeneous step. Use a sketch to aid your answer.

Answer 17

• R – e^- –> O⁺ –> P
• O⁺ in unstable so will decompose to product, P, with rate constant k
• Homogeneous as reaction of O⁺ –> P occurs in solution phase
• Heterogeneous R –> O⁺ at solid-solution phase

Question 18

• Describe and explain the relationship between time taken to cross the tip-substrate gap and lifetime of O⁺ in homogeneous kinetics.

Answer 18

• d²/D << 1/k (when time taken to cross gap << lifetime)
• All O reaches the substrate ad is reduced back to R
• Positive feedback
• d²/D >> 1/k (when time taken to cross gap >> lifetime)
• All O is converted to P
• Nothing to feedback on
• Between these limits kinetic measurements are possible

Question 19

• How can a tip crash on a rough surface in fixed height imagining mode be avoided?

Answer 19

• Independently measure surface topography whilst performing electrochemistry
• See papers

Question 20

• (?) SECM examples

Answer 20

• See rev lecture for what should be read and how much of it