Chemistry Reference
In-Depth Information
materials. The magnitude of the Helmholtz potential is determined by the adsorption/
desorption processes, which in many situations is a function of pH. By adjusting the
can be made zero and the pH at
pH of the solution,
is called the point of
zero charge (pzc).
The measured potential
and thus
and
can be varied through external
polarization.
is the open-circuit potential without external polarization. When the semiconductor has
no excess charge, there is no space charge region and the bands are not bent. The elec-
trode potential under this condition is called the flatband potential
is the applied potential when the electrode is externally polarized and
The flatband
potential is an important quantity for a semiconductor electrode because it connects the
energy levels of the carriers in the semiconductor to those of the redox couple in the
electrolyte and it connects the parameters that can be experimentally determined to
those derived from solid-state physics and electrochemistry. It can generally be
expressed as
The value of the flatband potential of a semiconductor is quantitatively related to its
electron affinity
for silicon):
880,953
n
where is defined as the energy of the conduction band at the point of zero charge
(pzc), that is, when the potential drop in the Helmholtz layer is zero. This means that
the electron affinity in the electrolyte equals the band edge plus the energy level of
the reference level on the absolute scale at the point of zero charge. Figure 1.6
