Zeta potential and the thickness of electrical double layer are
two important parameters describing the electrical surrounding
of nanoparticles in ionic media. An increase of any of them increases
the electrostatic repulsive interaction between nanoparticles
and therefore the stability of the colloid. Zeta potential may
be described as the potential of surface in an ionic environment
at the boundary between the Stern layer and the diffuse layer. It
is based on the charge displacement within the electric double
layer caused by an external force shifting the liquid phase tangentially
to the solid [6].
There are several approaches to surface modification of TiO2
nanoparticles and therefore to the stabilization of the colloid in
various solvents. For instance, dodecylamine, oleic acid, stearic
acid, dodecylbenzensulfonate are used to increase dispersibility
and stability of nanocrystals in organic solvents [7–9]. In aqueous
media polymeric dispersants containing different functional
groups are used to prevent nanoparticles from aggregation [10].
Molecules bound through hydroxyl or carboxylic groups to titanium
dioxide surface form sustainable, colorful surface complexes
[11–13]. Two neighboring binding groups improve efficiency of a
molecule complexation to TiO2 surface due to formation of a stable
ring, entropic effects associated with formation of chelate
complexes, and a further structure stabilization through additional
hydrogen bonds. Catechol, salicylic acid, phthalic acid and
their derivatives are ligands suitable for formation of such surface
moieties [14]. Surface complexation of appropriately selected
molecules can be used either for TiO2 sensitization to visible light
through formation of charge transfer complexes, or for protecting
nanoparticles from aggregation. Visible light activity, high transparency
and weak light scattering make these materials interesting
for various applications in the form of both colloidal solutions
and thin films. A possible drawback of such materials may be related
to their photostability, since organic modifiers undergo photocatalytic
oxidation in the presence of oxygen. Nevertheless, a
time-limited photostability of such systems may implicate some
short-term applications of colloids and coatings, for instance in
medicine.
The goal of the present work was to develop visible light active
nanocrystalline TiO2-based materials in the form of transparent
colloidal solutions and films using organic modifiers able to form
surface charge transfer complexes of titanium(IV). Highly dispersed
titania nanoparticles in solution have been used for preparation
of thin photo active coatings on glass substrate