Some aspects of physico-chemical properties of TiO2 nanocolloids with respect to their age, size, and structure.
Aqueous colloidal solutions of quantum sized particles of titanium dioxide (Q-TiO2) undergo an aging process since the moment of their preparation. In course of time, the mean size of nanoparticles is gradually increasing and some of their physicochemical properties are changing as well. In the present study, the decrease of the blue spectral shift of the semiconductor absorption threshold was measured to determine the corresponding changes of band gap energy of the Q-TiO2 particles. In parallel, the decrease of the specific surface area of these particles was followed through their complexation with 2-coumaric acid. The formation kinetics and thermodynamic equilibrium of these surface charge-transfer complexes were investigated in detail by means of UV/vis absorption spectroscopy. Besides, the size and shape of the Q-TiO2 particles aged 2, 4, and 10 years were compared employing direct observation by means of high resolution transmission electron microscopy. On the basis of real particle images, a model of nanocrystalline anatase was developed. The specific surface areas estimated through complexation with 2-coumaric acid were confronted with the numbers of appropriate titanium atoms located on the particle surface that were calculated from a model of anatase nanocrystals of different sizes. The photocatalytic activity, which represents the most important quality of anatase from practical point of view, was repeatedly determined through photocatalytic degradation of 4-chlorophenol in a colloidal solution of Q-TiO2 particles during its aging at 4 degrees C for three years. The corresponding reaction rate was increasing rapidly in the first weeks; it almost tripled in 68 days, and afterward it approached a limiting value. On the whole, the initial value increased four times in three years. Voltammetry at hanging mercury drop as the last method for aging description did not show any significant change of voltammetric behavior in a short two months period. However, while compared to preceding results of a similar TiO2 system, new redox processes of Q-TiO2 colloids were observed in the negative potential range. Besides the reduction of surface protons reported previously, two new pairs of peaks appeared.
Kolár
M
M
J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3, 182 23 Prague 8, Czech Republic.
Mest'ánková
H
H
Jirkovský
J
J
Heyrovský
M
M
Subrt
J
J