acetic acid molecule could be decomposed by less than eight
electrons. It is impossible to calculate intrinsic quantum effi-
ciency, that is, efficiency of utilization of electron–positive-hole
pairs, only from the product yield. Therefore, quantum efficiency
is reported with the description that a given reaction is assumed
to proceed through a proposed multiple-electron process (e.g.,
eight for the acetic acid decomposition inEq. (7)).
Another problem for the determination of quantum efficiency
is the difficulty in determining the number of absorbed photons.
Unlike measurement for homogeneous solutions, solid materials
scatter incident photons to reduce the light intensity arriving at
a detector in a spectrophotometer. In the wavelength region in
which only some of the photons are absorbed, that is, around
the band-edge wavelength, it is difficult to measure the photo-
absorption efficiency. Therefore, apparent quantum efficiency
(photonic efficiency) has often been used instead of quantum effi-
ciency, and apparent quantum efficiency is calculated by the
number of incident photons rather than the number of photons
used for quantum efficiency calculation. Since quantum effi-
ciency is defined as efficiency of electron–positive-hole utiliza-
tion, apparent quantum efficiency is a product of efficiencies of
photoabsorption and electron–positive-hole utilization. Of
course, both quantum efficiency and apparent quantum effi-
ciency depend on the irradiation wavelength and sometimes on
the irradiation intensity, and thereby the data should be shown
with wavelength and preferably with intensity. In this sense,
such measurement must be performed by monochromatic irradi-
ation; irradiation with sharp-cut optical filters is inappropriate.
E. RATE-DETERMININGSTEP
Assuming that a certain reaction proceeds through a series of
steps without any branching reactions, the rate must be the
same as the rate of the slowest step, the rate-determining step,
that is, the overall activation energy is that of the rate-determin-
ing step (Fig. 9). This original definition cannot be directly
applied to photocatalysis. A possible reason is that reactions by
photoexcited electrons and positive holes occur in parallel, not
in series (Fig. 9). Considering the requirement of photocatalysis
for the same numbers of electrons and positive holes to be used
(consumed), it seems possible to compare the rates of electron
and positive-hole reactions. However, it seems that the overall
reaction rate is also influenced by recombination of e––hþ. In
ordinary photochemistry in homogeneous phase, steady
PHOTOCATALYSIS BY INORGANIC SOLID MATERIALS 413