inorganic chemistry

is not directly involved in the photocatalytic reaction, the use of
the term “active site” is inappropriate, and a relationship of
photocatalytic activities with active sites therefore cannot be
expected.

III. Principle of Photocatalysis

A. GENERALLYACCEPTEDEXPLANATION

The principle of photocatalysis is often explained with an illus-
tration likeFig. 2, a schematic representation of the electronic
structures of semiconducting materials, a band model. An elec-
tron in an electron-filled valence band (VB) is excited by photo-
irradiation to a vacant conduction band (CB), which is
separated by a forbidden band, a band gap, from the VB, leaving
a positive hole in the VB (Section III.B). These electrons and pos-
itive holes drive reduction and oxidation, respectively, of com-
pounds adsorbed on the surface of a photocatalyst. Such an
interpretation accounts for the photocatalytic reactions of semi-
conducting and insulating materials absorbing photons by the
bulk of materials. In the definition of “photocatalysis” given
above, however, no such limitation based on the electronic struc-
ture of a photocatalyst is included. For example, isolated

ΔG > 0

Uphill electron

transfer

ΔG < 0

e– e–

Downhill

electron transfer

Photocatalyst

CB

VB

Absorption ΔG < 0

e–

h+

ΔGe< 0

ΔGh< 0

Absorption ΔG > 0

CB

VB

h+

e– ΔGe< 0

ΔGh< 0

Energy

FIG. 2. Gibbs-energy change in photocatalytic reactions.

PHOTOCATALYSIS BY INORGANIC SOLID MATERIALS 399