Ferricenium surface

Light is switched off at +0.5 V vs. SCE for the cathodic sweep. In (a) there is no added reductant (b), (c), and (d) contain 0.5mM ferrocene, 1, l -dimethylferrocene, and acetyl-ferrocene, respectively. Acetylferrocene does not attenuate the surface ferricenium surface ferrocene wave since it is not a sufficiently powerful reductant. Ferrocene and 1, l -dimelhylferrocene both attenuate the surface ferricenium - surface ferrocene wave. But l,l -dimethylferrocene is more effective under identical conditions despite the fact that the same, mass transport-limited, steady-state photocurrent is found for these two reductants. These data suggest that after the light is switched off the reduction of surface ferricenium is controlled partially by mass transport and partly by the electron transfer rate (see text). 

Rate of Reduction of Photogenerated, Surface-Confined Ferricenium by Solution Reductants... 

Scheme I Interface energetics for an n-type Si photoanode at the flat-band condition showing the formal potential for a surface-confined ferricenium/ferrocene reagent relative to the position of the top of the valence band, E ,and the bottom of the conduction band,E , at the interface between the Si substrate and the redox/electrolyte system. Interface energetics apply to an EtOH/0.1 M [n-Bu N]C10 electrolyte system.

Table I. Reduction of Surface-Confined Ferricenium by Iodide in Various Solvents at 298 K.

Table II, Rate Constants for Reduction of Surface-Ferricenium by Various Reductants at 298 K.

Photogenerated, surface-confined ferricenium can be reduced by a number of reductants including I-, Co(bipy)2 +s ferrocene, l,l -dimethylferrocene, phenylferrocene. Fe(rr indenyl)2> [Fe(CO)(n5-C5H5)]4, Ru(NH3)62+, Fe(CN)64 and dimethyldithiocarbamate. With the exception of I-, all generally reduce the surface-confined ferricenium with an observed heterogeneous rate constant of >0.06 cm/s which corresponds to a bimolecular rate constant > 6 x 10 M-1s-1, assuming that a monolayer ( 10-10 mol/cm2) of the surface-ferricenium participates in the... 

N. S. Lew is, A. B. Bocarsly, and M. S. Wrighton, Heterogeneous electron transfer at designed semi-conductor/liquid interfaces. Rate of reduction of surface-confined ferricenium centers by solution... 

Figure 4.22 Measured rate coefficients at 25 C for a ferrocene/ferricenium derivative tethered at a fixed distance ( 20 A) from a gold electrode surface within an alkane thiol monolayer for four different samples (symbols), together with rate coefficients calculated from eqs. 4.63 and4.64 with 2, = 0.85 eV and the prefactor (2n = 6.73 x tO s eV. From Chidsey (1991).

Thus the SECM is useful in probing heterogeneous kinetics at electrode surfaces, as well as other surfaces, like enzyme-containing membranes. The largest values of that can be measured are of the order of Did. Obviously this limit depends experimentally upon d, which is the characteristic length in SECM. Analogous measurements at a UME in bulk solution are similarly limited and provide a maximum on the order of DIa. For example, the for the ferrocene/ferricenium couple in MeCN, often used as a reference redox couple, as measured by SECM was 3.7 cm/s (22). 

The electrode is made of graphite, coated with 1,1 -dimethylferrocene. Glucose oxidase is immobilized on its surface by entrapment inside a polycarbonate membrane. When the electrode is poised at a potential sufficiently positive to produce the ferricenium ion, a response is obtained. The total current which flows is proportional to the amount of glucose which is present in the sample. 

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