inorganic chemistry

chemical species, not having the above mentioned band struc-
ture, on or in a solid can be a photocatalyst, and even when a
bulk material is used, the photoabsorption and resultant
photocatalytic reaction may proceed at a localized site when,
for example, photocatalysts are photoirradiated at a wavelength
near the band gap. An example is a gold-modified titania
photocatalyst which induces “photocatalytic” decomposition of
organic compounds under aerated conditions by photoabsorption
of surface-plasmon resonance of gold particles ( 8 ). Therefore, the
interpretation using a band model is not always adequate for
understanding photocatalysis. In this sense, the term“heteroge-
neous photocatalytic reaction (photocatalysis)”seems better than
“semiconductor photocatalytic reaction”based on the electronic
band structure.

B. BANDSTRUCTURE ANDEXCITATION

An important point in general understanding of the mecha-
nism of photocatalysis is that photoabsorption and (e––hþ) gener-
ation (Fig. 3) are inextricably linked; a VB electron is not excited
after photoabsorption. This interband (band-to-band) excitation
is often illustrated by three bands, CB, forbidden band (band
gap) and VB, in which an electron moves vertically from the
VB to CB, that is, no spatial change in the position of electron,
though the author sometimes encounters misunderstanding that

LUMO

HOMO

Absorption

Semiconductor / Atom/molecule

insulator

h

Conduction band (CB)

Valence band (VB)

e

Absorption

Surface?

Energy

FIG. 3. Photoabsorption by transition of electrons in the VB or
HOMO to the CB or LUMO in a semiconductor/insulator or atom/mole-
cule, respectively.

400 B. OHTANI