The kinetics of the
removal of stearic acid (SA) islands by photocatalytic
coatings is controversial, with some reporting that the islands fade
as their thickness,h, decreases with the irradiation
time,t, but maintain a constant area,a, −da/dt= 0, and others
reporting that −dh/dt= 0
and −da/dt= −constant,
i.e., the islands shrink, rather than fade. This study attempts to
understand the possible cause for these two very different observations
through a study of the destruction of a cylindrical SA island and
an array of such islands, on two different photocatalytic films, namely,
Activ self-cleaning glass, and a P25 TiO2coating on glass,
which have established uniform and heterogeneous surface activities,
respectively. In both cases, using optical microscopy and profilometry,
it is shown that, irrespective of whether there is as a single cylindrical
island or an array of islands,hdecreases uniformly
witht, −dh/dt= constant, and −da/dt=
0, so that the SA islands just fade. However, in a study of the photocatalyzed
removal of SA islands with a volcano-shaped profile, rather than that
of a cylinder, it is found that the islands shrink and fade. A simple
2D kinetic model is used to rationalize the results reported in this
work. Possible reasons for the two very different kinetic behaviors
are discussed. The relevance of this work to self-cleaning photocatalytic
films is discussed briefly.