3.3. Coatings on glass
Nanocrystalline TiO2 was successfully used for preparation of
thin films on glass substrates. Coatings obtained by spin-coating
technique were characterized by a good homogeneity and transparency. Coating at spinning rate 8000 rpm yielded films of
ca. 25 ± 5 nm thickness, as estimated by spectroscopic ellipsometry
measurements. After annealing at 200 C the films appeared very
resistant against scratching. The investigated films are smooth
and compact, composed of approximately a monolayer of nanoparticles
(Fig. 8).
Several attempts have been made to synthesize similar coatings
using modified TiO2 colloids. Unfortunately, all prepared films
were characterized by a lack of homogeneity and a weak adhesion
to the substrate. Moreover, unmodified films were more resistant
to water and scratching than those composed of modified materials.
Much better results have been achieved applying the two-step
procedure, involving neat TiO2 deposition followed by impregnation
of the formed film with the solution of a modifier. UV–vis
spectra of impregnated films, exhibiting a visible light absorption,
are shown in Fig. 5d.
The results of contact angle measurements for thin TiO2 films
are shown in Table 1. The contact angle measured for non-irradiated
TiO2 film was ca. 17 and was lower than the angle measured
for the uncoated glass (ca. 32). As expected, upon irradiation with
visible light no significant change in the contact angle for unmodified
TiO2 thin film was recorded, since unmodified TiO2 does not
absorb in this spectral range. All modified titania films show a
higher hydrophilicity than unmodified coatings. Values of contact
angle correspond to the chemical structure of modifiers and decrease
in the order: Trn@TiO2, Rtn@TiO2, Asc@TiO2. After 90 s of
irradiation with visible light the surface of all modified TiO2 films
became more hydrophobic due to degradation of organic modifiers
exposed to a high intensity light beam. Upon illumination organic
ligands are oxidized as a result of electron injection into the conduction
band of titania (compare results of photocurrent generation)
and reactive oxygen formation (compare the influence of
oxygen on photocurrent generation). Measured contact angles increase
and become similar to those observed for neat TiO2 coating.
3.4. Photocatalytic activity of the coatings
The efficiency of photocatalytic reactions using unmodified and
modified titanium dioxide coatings was evaluated by monitoring
the intensity of fluorescence peak at 425 nm emitted by excited
2-hydroxyterephthalic acid (TAOH). All coatings appeared active
w when excited with visible light (Fig. 9). Tiron-modified TiO2 is
clearly the most active material upon irradiation with visible light,
as after 60 min the amount of formed TAOH is sixfold higher than
for unmodified TiO2. Ascorbic acid and rutin modified TiO2 exhibited
ca. 1.5–2 times faster TAOH generation than neat TiO2. It is
noteworthy that the highest difference in activity for tiron-modi-
fied and unmodified coatings is observed at the beginning of irradiation,
when photobleaching of the sensitizer is negligible. This
result correlates with the integrated photocurrent efficiency which
is the highest for Trn@TiO2 under these irradiation conditions
(k > 400 nm, compare Fig. 7b).