Chemistry Reference
In-Depth Information
where is the number of molecules that reach the electrode surface, the transi-
tion coefficient which strongly depends on the distance of the reacting molecules from
the surface, and the densities of empty states and occupied states in the
electrode, respectively, and and the densities of occupied energy states
and empty states in the electrolyte defined by Eqs. (1.11) and (1.12). The anodic current,
consists of an electron transfer current into the conduction band, and corre-
sponding to a transfer into the valence band. Similarly, the cathodic current, consists
of the contribution from the conduction band, and from the valence band, The
contributions of conduction and valence bands to the anodic and cathodic currents are
illustrated in Fig. 1.14.
The magnitude of the currents depends on the overlap between the levels in the
semiconductor bands and those in the solution. For a material having a large band gap
the overlap generally involves only the conduction band or the valence
band, whereas for a small band gap there may be overlap of a redox couple with both
the conduction band and the valence band so that the contributions from both bands,
i.e., and are significant. In general, the electron transfer probability is mainly deter-
mined by the energy correlations between the band edges of the semiconductor, and
and principal energy levels of the redox couple, and If is closer to
than to electron transfer in the conduction band is the more likely process. In
the opposite case, electron transfer in the valence band is more likely.
In the case of the conduction band process, the density of empty states,
in the conduction band can be taken to be constant because only a few of them are occu-
pied so that using Eq. (1.11) the conduction band anodic current,
can be expressed as
