3. Results and discussion
3.1. Materials and characterization
Titanium dioxide colloid synthesized according to the procedure
published by Bahnemann et al. [15] is a stable, transparent
and colorless, acidic solution. Trials of its neutralization lead to
precipitation of white TiO2 aggregates already at pH above 4.
The materials are nanocrystalline, XRD and Raman spectra
show relatively broad bands (Figs. 2 and 3). The most characteristic
mode appearing in the Raman spectrum is the prominent peak at
155 cm1 (Eg) assigned to the anatase phase. It is shifted to higher
wavenumbers as compared to bulk TiO2 anatase, due to the size effect
[16]. The modes 516 and 640 cm1 can be assigned also to anatase
phase. The rutile phase is poorly represented by a weak peak
at 447 cm1
. Brookite may be identified by the mode at 245 cm1
(A1g). The XRD analysis does not enable determination of the exact
phase composition. The maximum around 2H = 25 can originate
either from anatase or brookite. The presence of brookite may be
confirmed by the peak at 2H = 30, while the specific signal of anatase
appears as a shoulder at ca. 55 [17]. The diffraction peaks
characteristic for both anatase and brookite at 2H = 48 and 54
are also present.
All modified nanomaterials form yellow, clear colloidal solutions
with a negligible level of light scattering. TEM pictures of colloids
presented in Fig. 4 show a uniform dispersion of
nanoparticles. The application of various modifiers nearly does
not influence the morphology of materials. Synthesized modified
colloids showed various stabilities in the dark at pH 7. The materials
based on catechol derivatives (Trn@TiO2 and Rtn@TiO2) were
stable for more than one year when kept in the dark. Materials
based on ascorbic acid were less stable under similar conditions,
due to desorption and/or oxidation of ascorbic acid proceeding
within days.
UV–vis absorption spectra of the unmodified TiO2 (pH 2) and
modified TiO2 colloids (pH 7), together with photographs of the
solutions, are shown in Fig. 5. For comparison, spectra of modifiers
are also shown. Spectra of the modified colloids differ significantly
from a mathematical sum of the spectra of components used during
the synthesis of colloids. The modified colloids exhibit a clear
absorption of visible light, extending from UV to ca. 500–600 nm.
This is a result of formation of surface charge-transfer complexes
[11].