assuming that one photon induces reaction or change in one mol-
ecule since a multiple-photon process and subsequent multiple-
electron transfer can be neglected in ordinary photoirradiation
conditions with relatively low photon flux. Confusion might arise
when this concept is applied to photocatalysis, in which a multi-
ple-photon process and at the same time radical chain reaction
may be included, especially in reaction in the presence of oxygen.
In a heterogeneous photocatalytic reaction, in which multiple
photons are absorbed due to the relatively large size of particles
compared with the size of molecules, multiple-electron (positive
hole) transfer may occur. For example, photocatalytic silver
metal deposition accompanied by molecular oxygen (O 2 ) libera-
tion proceeds with the following stoichiometry( 28 ):
4Agþþ2H 2 O!4AgþO 2 þ4Hþ ð 5 ÞFor the determination of quantum efficiency, it is necessary to
make an assumption on how many photons are required for the
reaction. One of the possible and the most frequently employed
assumptions is that four photons are required for liberation of
one oxygen molecule, four silver metal atoms, or four protons,
and when the O 2 yield is used, the quantum efficiency is calcu-
lated to be
4 nðÞO 2
nðÞphoton; ð 6 Þwherenis the number of molecules or photons. Thus, for the cal-
culation of quantum efficiency of heterogeneous photocatalytic
reactions, it is reasonable to consider the efficiency of utilization
of electrons–positive holes assuming that an electron and posi-
tive-hole pair is produced by absorption of a photon. However,
since neither a photoexcited electron nor a positive hole appears
in stoichiometry, the above-mentioned consideration may not
always be straightforward. For example, acetic acid dissolved in
air-saturated water is decomposed into carbon dioxide by an
appropriate suspended photocatalyst with the following proposed
stoichiometry:
CH 3 COOHþ2O 2 !2CO 2 þ2H 2 O: ð 7 ÞHow many photons are required for this reaction? Assuming that
only O 2 is reduced by photoexcited electrons in this reaction and
that reduction of an O 2 molecule requires four electrons, this
reaction is an eight-electron process. However, since the
photocatalytic reaction of acetic acid may include a radical chain
mechanism or at least addition of O 2 to intermediate radicals, an
412 B. OHTANI