Nernstian behavior, materials

In the absence of suitable scavengers, recombination occurs within a few nanoseconds (19). Valence band holes (h+(vb)) have been shown to be powerful oxidants (20-231 whereas conduction band electrons (e (cb)) can act as reductants (24,251. The redox potentials of both, e and h+, are determined by the relative position of the conduction and valence band, respectively. Bandgap positions are material constants which have been determined for a wide variety of semiconductors (26). Most materials show "Nernstian" behavior which results in a shift of the surface potential by 59 mV in the negative direction with a pH increase of ApH = 1. Consequently electrons are better reductants in alkaline solutions while holes have a higher oxidation potential in the acid pH-range (26). Thus, with the right choice of semiconductor and pH, the redox potential of the e (cb) can be varied from +0.5 to -1.5 V (vs. NHE) and that of the h+(vb) from +1.0 to more than +3.5 V. This sufficiently covers the full range of redox chemistry of the H20/02 system (271. 

Extend the range of ferric species activities over which Pt electrode response is nernstian, and, provide evidence for the comparable behavior of another, physically and chemically distinct electrode material, wax-impregnated graphite. Experiments in laboratory and field situations indicate nernstian response of both Pt and WIG electrodes at pH s as high as 6.4, and thus total Fe(III) activities as low as 10 M. 

The functionalization of the monomer by a specific group has also been the route preferred by numerous groups to electrogen-erate pH-sensitive electroactive materials. ECP films derivatized in the 3-position with hydroquinone [335,336] and alkylcarboxyiic acid [337-342] functionalities exhibited an electrochemical behavior depending on the pH of the electrolytic medium. For example, poly(25) showed a sub-Nernstian potentiometric response of 46 mV pH at 25°C with a detection limit of 10 M [336]. 

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