Actually, it is not easy to find a photocatalyst showing activity higher than that of P25, and
P25 has therefore been used as a de-facto standard titania photocatalyst; photocatalytic
reactions of P25 have been reported in more than thousand papers since 1990 [1
In a scientific sense, isolation of anatase and rutile crystallites from P25 is necessary
to determine the crystalline composition and to check the synergetic effect of anatase and
rutile. Isolation of rutile by selective dissolution of anatase from P25 has been reported by
Ohno et al. [5]. Recently, we have developed a method to isolate anatase by preferable
dissolution of rutile from P25 using a mixed solution of hydrogen peroxide and ammonia [6].
Our attempts to determine the crystalline composition and to check the synergetic effect of an
anatase-rutile mixture using isolated anatase and rutile powder as standards are reported in
this paper.
]. It is well
known that P25 is composed of anatase and rutile crystallites, the reported ratio being
typically 70:30 or 80:20, but it seems that nobody knows the exact crystalline composition [2],
presumably due to a lack of methodology for determination of crystalline contents in
nanometer-sized particulate samples [ 3 ]. Another myth, a hypothesis not proved
scientifically, regarding P25 is that the co-presence of anatase and rutile crystallites induces
the high level of photocatalytic activity; transfer of photoexcited electrons and positive holes
between interconnecting anatase and rutile particles may enhance charge separation and hence
improve the efficiency of utilization of electron-hole pairs [4]. However, as far as the
authors know, there have been no reports showing enhancement of activity compared with
pure anatase and rutile parts of P25 or with the anatase and rutile particles without any
electrical contact: no data have been shown for expected lower activity of pure anatase and
rutile particles alone. Thus, despite the long history of photocatalysis studies, it seems that
we have not yet known what P25 is from the standpoints of structural and photocatalytic
characteristics.