necessary for excitation. The positions of the upper valence band
and lower conduction band edge are given as electrochemical
potentials and apply for the anatase modification in contact
with water of pH 7. Surface defects and an inhomogeneous
electric field of a thin surface layer induce trapping of the
light-generated charges at the surface forming redox centers of
different chemical reactivity.
Centers which undergo primarily recombination are termed in
the scheme astrappedelectron–hole pairs (etr,htrþ) whereas
those involved in the IFET are named reactive electron–hole
pairs (er,hrþ). Only in a very few cases the existence of both
types of redox centers is experimentally proven( 3 ). In general,
only one type of electron–hole pairs is considered. In competition
with the primary processes depicted in Scheme 1, the photo-
generated charges may exchange electrons also with the semi-
conductor itself resulting in a deactivation of the photocatalyst.
Depending on the detailed reaction conditions like presence or
absence of oxygen, this photocorrosion termed process affords,
for example, in the case of the n-semiconductor zinc sulfide
elemental zinc and sulfur and zinc sulfate.
SCHEME 1. Schematic description of primary processes occurring
during a semiconductor photocatalyzed redox reaction. The thick verti-
cal bar symbolizes the solid/liquid interface. For the sake of simplicity
emissive and photocorrosive processes are omitted. (1) Light absorption
and primary charge recombination; (2) charge trapping at unreactive or
(3) reactive surface sites; (4, 5) secondary charge recombination; (6, 7)
IFET processes; (8) back electron transfer; (9, 10) secondary reactions.
374 HORST KISCH