The
apparent reaction rate constants increased with
increasing dip-coating cycles, which is attributed to the increase in the amount of TiO2 available to
participate in the photocatalytic reaction. However,
the apparent reaction rate constants decreased
when the number of dip-coating cycles
exceeded 15 which mainly results from the following
two factors: (1) increase in opacity and light
scattering of thick TiO2 films, leading to a decrease
in the passage of irradiation through the film, and
(2) TiO2 films partly breaking off with an increase
in dip-coating cycles, causing a decrease in the
amount of TiO2 available to participate in the photocatalytic reaction. Fig. 6 shows the variations
of the UV–Vis absorption spectra of X-3B
with the same irradiation time intervals (15 min)
for the film with 15 dip-coating cycles. A marked
absorption band at about 535nm is observed in
the spectrum before UV irradiation, whereas the
absorbance of the band decreased nearly to zero
after UV irradiation of 120 min. This result
typically indicates that X-3B was almost photocatalytically
decomposed by the TiO2 film. The
above data mean that the film with 15 dip-coating
cycles exhibited the highest photocatalytic activity
in comparison with other dip-coating cycles on the
same reaction conditions.