Slow Aggregation of Titania Nanocrystals in Acidic Hydrosols
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Slow Aggregation of Titania Nanocrystals in Acidic Hydrosols Olga Pavlova-Verevkina1, Ludmila Ozerina1, Natalia Golubko1 and Abdelkrim Chemseddine2 1
Karpov Institute of Physical Chemistry, Obuha side street 3-1/12, building 6, 105064 Moscow, Russia Helmholtz-Zentrum Berlin fur Materialien und Energie GmbH, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany
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ABSTRACT The kinetics of slow aggregation of monodisperse TiO2 nanocrystals in the acidic hydrosols at room temperature was studied for months by turbidimetry. The dependence of the initial rate of aggregation on the pH was calculated. The comparison of results obtained by turbidimetry and small angle X-ray scattering permits to suppose that very loose aggregates form at the low pH in HCl solution. The dependencies obtained in this work for room temperature can be taken into consideration at the tuning of TiO2 nanoparticles morphology through thermal treatment of hydrosols. INTRODUCTION Aqueous dispersions of TiO2 nanoparticles (hydrosols) stabilized under acid conditions are generally used as precursors for the processing of nanostructured materials [1-4]. Protons of acid adsorb on TiO2 nanoparticles and the originating double electrical layer stabilizes particles and inhibits aggregation. On the other hand, an increase of concentration of acid or other electrolytes leads to compression of the double layer that destabilizes sols [4-5]. At critical concentrations of electrolytes Cc the sols fast coagulate, while at relatively low concentrations of electrolytes aggregation is very slow. The morphology of aggregates forming in the course of slow aggregation depends on structure of primary TiO2 nanocrystals, electrolytes concentration, pH, time of aggregation, and other parameters. Investigation of the kinetics of slow aggregation could help in the tuning of TiO2 nanoparticles morphology. Our research efforts are focused on the kinetics of slow aggregation of monodisperse TiO2 hydrosols stabilized by strong acids [6-8]. Evolution of turbidity spectra of the diluted sols destabilized by additions of HCl and KCl was investigated for months. (KCl was used because this electrolyte does not adsorb on TiO2 surface.) This investigation revealed that the slow structural changes in the sols were mainly due to aggregation of primary nanocrystals but not due to Ostwald ripening. The initial nonaggregated sols had the Rayleigh light scattering. In the course of aggregation the optical density gradually increased and the character of turbidity spectra quantitatively changed. In this paper, new results on influence of medium composition on kinetics of the slow aggregation are presented.
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EXPERIMENTAL DETAILS Nearly monodisperse TiO2 nanocrystals were obtained by stepped coagulation of polydisperse TiO2 hydrosols through additions of hydrochloric acid [6]. The isolated narrow fractions of nanocrystals were mixed with certain quantities of water that resulted in the formation of stable sols with pH=1. The stable sols were destabilized either by additions of HCl and KCl with co
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