VI. Design of Active PhotocatalystsA. PHYSICALPROPERTY–ACTIVITYCORRELATION
A.1. Nanostructured photocatalysts
There have been many reports on preparation of
photocatalysts with nanometer-sized structures, for example,
nanoparticles, nanoplates, nanocubes, nanorods, nanotubes or
nanowires, as well as their photocatalytic activities, and the
number of such reports is still increasing( 48 ). A possible reason
for such an explosive increase in studies on nanostructured
photocatalysts is popularization of low-priced pressure-tolerant
Teflon bottles for hydrothermal reactions. By using this type of
apparatus, a variety of inorganic compounds can be prepared
and morphology can be changed depending on the reaction
conditions. Scanning or transmission electron microscopic
images of those nanostructured photocatalysts (even non-
photocatalysts) are attractive and interesting. However, consid-
ering that we do not know what structural parameters of
photocatalysts govern the photocatalytic activity or how they
govern the photocatalytic activity, there seems to be no assured
reason why nanostructured materials, rather than ordinary
nonstructured ones, should be employed.
A.2. Dependence of photocatalytic activities on physical and
structural properties
It can be said that not only the above mentioned“nano-struc-
ture”but also other ordinary physical or structural properties
measured for photocatalysts have not been proved to be decisive
factors for the photocatalytic activities. It is true that
photocatalytic activities of photocatalysts of certain components
prepared or treated in different ways or under different
conditions may be different and this is because physical and
structural properties of those photocatalysts differ depending
on the preparation/treatment conditions, that is, physical and
structural properties must control the photocatalytic activity
( 49 ). A problem is we, at least the author, do not know how pro-
perties affect photocatalytic activity. A possible reason is that
those properties, though we do not know how many properties
are required for analysis, are changed at the same time. For
example, when titania photocatalysts are prepared by hydrolysis
of a titanium compound such as titanium(IV) sulfate or tetra
(2-propoxide) followed by calcination in air, higher-temperature
PHOTOCATALYSIS BY INORGANIC SOLID MATERIALS 421