Outer-Sphere Redox Species Characterization

For both redox species, as the boron concentration increases A p decreases until a value close to that synonymous with metal-like behavior is reached at boron concentrations 10 B atoms cm . Note the change in A p is much more dramatic with Ru(NH3)g and is evident for all boron dopant densities less 

FcTMA couples with respect to valence (fyg) and conduction ( (-g) bands for both O- and H-terminated semiconducting BDD. H-termination (O-termination) is known to induce a negative (positive) electron affinity, with a value of —1.3eV (-1-1.7 eV) measured in vacuum [84]. The presence of water molecules screening the C-H (C-O) surface dipole is expected to reduce the value of the electron affinity (z) toward less-negative (positive) values. We have chosen a value of approximately, j = -1.0eV and / =-1-1.3 eV for H- and 0-terminated surfaces, respectively. In the electrolyte region, the 

9 X 1()2 (pink line), and 3.2 x lO (green line) at a scan rate of 0.1 Vs for (i) the oxidation of 1 mM FcTMA and (ii) the reduction of 1 mM Ru(NHj)g5 in 0.1 M KNOj. (Taken from Ref [17] with permission.) The resulting peak-to-peak (Afp) separations are given for the differently doped electrodes in the two different redox mediator solutions. 

---

The third class of redox species are couples located near the conduction band of WSe2- The only outer-sphere example found, which is suitable for use in aqueous electrolytes, is Ru(NH3)e3+. Its reduction is characterized by an immediate onset upon accumulation in the semiconductor and a tafel slope of 130 mV/decade. The reduction mechanism appears to be direct reduction of the Ru(NH3)e3+ by electrons from the accumulation layer. The only member of the forth class of redox species is triiodide ion. It is characterized by adsorption onto the semiconductor surface as was demonstrated by the first application of chronocoulometry to a semiconductor electrode (another demonstration of the reproducibility and low background currents on... 

An [H + ] term in the rate law for reactions involving an aqua redox partner strongly suggests the participation of an hydroxo species and the operation of an inner-sphere redox reaction (Sec. 5.5(a)). Methods (a) and (b) are direct ones for characterizing inner-sphere processes, analyzing for products or intermediates which are kinetically-controlled. Method (c) is indirect. Other methods of distinguishing between the two basic mechanisms are also necessarily indirect. They are based on patterns of reactivity, often constructed from data for authentic inner-sphere and outer-sphere processes. They will be discussed in a later section. 

As discussed in Vol. 2, Chap. 4, experimental studies, mainly pioneered by Taube [11], revealed two different reaction pathways for redox reactions in solution (i) outer sphere mechanism characterized by weak interaction of the reactive species, with the inner coordination sphere remaining intact during the electron transfer, and reactions occurring through a common ligand shared by the metallic centers thus proceeding by an inner sphere mechanism. 

---