Chemistry Reference
In-Depth Information
In many circumstances these conditions are not met and the equations are not
directly applicable to electrode reaction kinetics.
86,270,274
For example, when charge
transfer via surface states plays a significant role in the electrode processes, the first
assumption is violated. In this case as shown in Fig. 1.16a, the charge transfer process
is not directly between the levels in the bands and those in the solution. In the case of
a strong band bending the thickness of the space charge layer may become very small
so that electron tunneling through the space charge layer occurs as shown in Fig. 1.16b.
Thus, the second condition is violated because the electron tunnels from energy levels
that are distant from the band edges at the surface. Violation of the third assumption
may, for example, occur when the semiconductor is highly doped so that the capaci-
tance of the space charge layer near the flatband potential is comparable to that of the
Helmholtz layer and a significant fraction of the potential drops in the Helmholtz layer.
This may also occur when the density of surface states is high and the associated charge
is comparable to that of the Helmholtz layer.
Also, although Eqs. (1.69) and (1.70) resemble those for a Schottky barrier, there
are several important differences in the physical and chemical details: (1) charge trans-
fer between a semiconductor and a solution is a slow process, whereas that between a
metal and a semiconductor is fast; (2) the diffusion of redox species in the solution
toward the electrode surface is slow whereas that of charge carriers in metal is fast; (3)
the reduced and oxidized species of the redox couple as donors and acceptors can
change independently whereas the occupied and unoccupied states of the metal cannot
be changed artificially; (4) a Helmholtz layer is present between the semiconductor
electrode and the solution whereas no such layer exists at the metal/semiconductor
interface.
