inorganic chemistry

particles as has been suggested by similarity of the action spec-
trum similar to the photoabsorption spectrum of the dye(37–39)
(Fig. 11). Another reason is that the absolute molar amount of
dye contained in the reaction system can be much smaller than
that of a solid photocatalyst. The concentration of dye in the solu-
tion should be relatively low since the absorption coefficient is
large. These two facts are closely related to the problem of how
we can prove a given reaction to be“photocatalytic,”as discussed
in Section II.A. The third reason is that the mechanism of dye deg-
radation is so complicated that efficiency of the photocatalytic
reaction, for example, quantum efficiency, cannot be measured.
Measuring the consumption (decrease) of a dye during photo-
irradiation requires only a spectrophotometer, but the use of dyes
as model compounds is inappropriate, and if dyes are used, care
must be taken in the analysis of experimental results.
An action spectrum is a plot of apparent quantum efficiency,
not quantum efficiency, against wavelength of light used for
apparent quantum efficiency measurement. Therefore, it is clear
that monochromatic light irradiation is required to record an
action spectrum. Usually, a grating-type monochromator is used
with a light source such as a xenon arc lamp. Interference-type
optical filters, transmitting at only a certain wavelength region,
are also used for monochromatic irradiation, though wavelengths
of possible irradiation are limited. Because of possible dependence
of apparent quantum efficiency on light intensity( 40 ), it is prefera-
ble to adjust the light intensity at each wavelength ( 41 ). Wave-
length-selective irradiation can be carried out using optical filters

(2) dye

(1) hν

Energy

(3) semi-

conductor

e–

e–

(4) O 2

CB

VB

FIG. 11. Possible mechanism of“dye-sensitized”photoinduced oxida-
tive decomposition of dye in the presence of oxygen. Components (1)–(4)
are necessary conditions for“dye-sensitized reaction”and they are also
necessary conditions for ordinary photocatalytic reaction.

418 B. OHTANI