The impact of surface coverage on the kinetics of electron transfer through redox monolayers on a silicon electrode surface

Abstract

The impact of the coverage of ferrocene moieties, attached to a silicon electrode modified via hydrosilylation of a dialkyne, on the kinetics of electron transfer between the redox species and the electrode is explored. The coverage of ferrocene is controlled by varying the coupling time between azidomethyl ferrocene and the distal alkyne of the monolayer via the copper assisted azide-alkyne cyclo addition reaction. All other variables in the surface preparation are maintained identical. What is observed is that the higher the surface coverage of the ferrocene moieties the faster the apparent rates of electron transfer. This surface coveragedependent kinetic effect is attributed to electrons hopping between ferrocene moieties across the redox film toward hotspots for the electron transfer event. The origin of these hotspots is tentatively suggested to result from minor amounts of oxide on the underlying silicon surface that reduce the barrier for the electron transfer.

Click Coupling Reactions on Flat and Nanostructured Hydrogen‐Passivated Silicon Surfaces

“…The low immobilization efficiency of dialkynes on porous substrate as suggested in this study, may hump its use in sensing application, make the surface more prone to oxidize and impede efficient electron transfer between the substrate and immobilized redox active compounds . Issues concerning click reaction on porous silicon substrates have previously been attributed to inactive copper catalyst due to complexation of the Cu(I) catalyst .…”

“…H‐terminated silicon surfaces have been turned into alkyne‐terminated “clickable” surfaces via hydrosilylation of terminal dialkynes, where one head group is grafted to the surface leaving a free terminal alkyne that may subsequently be coupled to an azide. The grafting of a n‐diyne followed by a copper catalyzed cycloaddition was first reported in 2008 by Ciampi et al Since then it is a frequently applied strategy to functionalize oxide free silicon electrodes and has been employed to immobilize redox active groups, to functionalize nano particles, solar energy devices, for monolayer induced doping, and patterning self‐assembled monolayer . The modified surfaces have shown remarkable stability and resistance to oxidation .…”

Forming Ferrocenyl Self‐Assembled Monolayers on Si(100) Electrodes with Different Alkyl Chain Lengths for Electron Transfer Studies

“…Here, the CPEs behaved very similar to a capacitor, as the power‐law modifiers have values 0.95≤ ϕ ≤0.99, where ϕ equal to unity indicates an ideal capacitor. By approximating the value of C ads by the corresponding Q value, the apparent electron‐transfer rate constant, k et , can be expressed as k et =(2R ct C ads ) −1 …”

“…It is hypothesized that the fast kinetic of electron transfer observed here is due to the presence of SiO x . hotspots . As shown by Seitz et al., existence of SiO x underneath the monolayer diminishes the Schottky barrier inside the semiconductor .…”

“…Ferrocene was used as it provides a well‐defined, and reversible, outer sphere one electron redox couple . In our first study on this topic, redox SAMs based on 1,8‐nonadiyne (C9‐SAM 2 a ) were formed according to the method outlined in Scheme . It was shown that the apparent rate of electron transfer ( k et ) increased with the coverage of ferrocene.…”