The photocatalytic efficiency of the fabricated thin films
was evaluated over the degradation of methylene blue. Fig. 5
shows the comparative percent degradation of the dye by thin
films 1a–1f and 2a–2f. It can be seen that 1a showed best activity
(95%) under UV light exposure while 1f and 2f thin films
respectively degraded only 75% and 73% of the methylene
blue. The reason for the difference in % degradation is the difference
in band gap values, particle sizes and phases of the
TiO2 nanoparticles. The mechanism of photodegradation of
methylene blue under UV irradiation using TiO2 films has been explained by Khan et al. [3,6]. Accordingly, it was found that
the photogenerated electron reacts with dissolved oxygen in
water to produce superoxide radical anions, while the holes
created by excitation of electrons in valance bands have high
oxidation potential, which allows the formation of reactive
hydroxyl radicals. These reactive radicals and photogenerated
holes are strong and non-selective oxidants that lead to the
degradation of MB at the surface of TiO2 nanoparticles on
films.