Some Aspects of Physico-Chemical Properties of TiO 2 Nanocolloids with Respect to Their Age, Size, and Structure M. Kola ´r ˇ, † H. Me ˇs ˇt’a ´nkova ´, † J. Jirkovsky ´, † M. Heyrovsky ´,* ,† and J. S ˇ ubrt ‡ J. HeyroVsky ´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejs ˇkoVa 3, 182 23 Prague 8, Czech Republic, and Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, 250 68 R ˇ ez ˇ, Czech Republic ReceiVed June 27, 2005. In Final Form: October 25, 2005 Aqueous colloidal solutions of quantum sized particles of titanium dioxide (Q-TiO 2 ) 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-TiO 2 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-TiO 2 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-TiO 2 particles during its aging at 4 °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 TiO 2 system, new redox processes of Q-TiO 2 colloids were observed in the negative potential range. Besides the reduction of surface protons reported previously, two new pairs of peaks appeared. Introduction Photocatalytic degradation of organic pollutants on the illuminated surface of titanium dioxide belongs to advanced oxidation processes that have been proposed as alternatives to classical procedures of water and air purification. 1-4 Knowledge of physicochemical properties of colloidal TiO 2 hence represents a precondition for successful application of the new procedures; the highly dispersed colloidal state has the important advantage of optical transparency, which enables investigation by means of absorption spectroscopy. In our previous work, 5,6 quantum- sized nanoparticles of titanium dioxide (Q-TiO 2 ) were prepared in the form of solid gels, which could be dissolved in water to form colloidal solutions. Such solutions contained nanoparticles of various sizes. The solutions of polydisperse composition showed interesting polarographic and voltammetric behavior that was already described and interpreted. 5,6 In the present study, the phenomenon of aging of Q-TiO 2 solutions has been investigated employing spectroscopic and electrochemical meth- ods, as well as performing tests of photocatalytic activity. It has been observed that physicochemical properties of Q-TiO 2 particles change in the course of aging of their colloidal solutions. This process most probably consists of slow recrystallization that should lead to somewhat bigger particles. As a consequence, the previously observed polarographic and voltammetric charac- teristics became more pronounced, and also additional new features appeared. Experimental Section The chemicals TiCl 4 (g99%, Fluka) and 2-coumaric acid (3- (2-hydroxyphenyl)-trans-propenic acid) (>97%, Fluka) were used without further purification, all others were analytically pure (p.a.): FeCl 3 ‚6H 2 O, 70% HClO 4 (Merck), NaClO 4 (Lachema Brno), and 4-chlorophenol (Fluka). The water used was distilled or bidistilled in quartz apparatus. The purity of water and of methanol as solvents for chromatography was HPLC grade (Merck). The preparation procedure of Q-TiO 2 nanoparticles was analogous to the previously used one: 5 Pure TiCl 4 (3.5 mL) was added dropwise, under vigorous magnetic stirring, to 900 mL of distilled water cooled to 1 °C. After 30 min of further slow stirring, the formed colloidal solution was dialyzed through a Spectrapor membrane against distilled water until the pH of the colloidal solution reached the value of 2.5. Then a necessary amount of distilled water was added to complete the total volume of the solution to 960 mL that corresponded to the final TiO 2 concentration of 33.3 mM. Contrary to the previous studies, 5,6 when the Q-TiO 2 particles were isolated in the form of solid gels by vacuum evaporation, this time the prepared colloidal solutions were kept as such in a refrigerator at about 4 °C. * To whom correspondence should be addressed. † J. Heyrovsky ´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic. ‡ Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic. (1) Kry ´sa, J.; Jirkovsky ´, J. J. Appl. Electrochem. 2002, 32, 591-596. (2) Bahnemann, D. W. In EnViromental Photochemistry; Boule, P., Ed.; Springer-Verlag: Berlin, 1999; pp 285-351. (3) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol. C: Photochem. ReV. 2000, 1,1-21. (4) Carp, O.; Huisman, C. L.; Reller, A. Prog. Solid State Chem. 2004, 32 (1-2), 33-177. (5) Heyrovsky ´, M.; Jirkovsky ´, J.; S ˇ truplova ´-Barta ´c ˇkova ´, M. Langmuir 1995, 11, 4300-4308. (6) Heyrovsky ´, M.; Jirkovsky ´, J.; S ˇ truplova ´-Barta ´c ˇkova ´, M. Langmuir 1995, 11, 4309-4312. 598 Langmuir 2006, 22, 598-604 10.1021/la058016w CCC: $33.50 © 2006 American Chemical Society Published on Web 12/13/2005