JOURNAL OF MATERIALS SCIENCE 38 (2 0 0 3 ) 3315 – 3318 Aggregation kinetics of sol-gel process based on titanium tetraisopropoxide A. SOLOVIEV, H. JENSEN, E. G. SØGAARD Institute of Chemistry and Applied Engineering Science, Aalborg University Esbjerg, Niels Bohrs Vej 8, 6700 Esbjerg, Denmark E-mail: soloviev@aue.auc.dk A. V. KANAEV Laboratoire d’Ing ´ enierie des Mat ´ eriaux et des Hautes Pressions, C.N.R.S., Universit ´ e Paris-Nord, Av. J.-B. Cl´ ement, 93430 Villetaneuse, France The kinetics of hydrolysis and condensation of titanium tetraisopropoxide (TTIP) under neutral conditions has been investigated by a light scattering method for different TTIP and water concentrations. The evaluation of kinetics data confirmed the complex nature of the process, which includes hydrolysis, condensation and aggregation of primary particles. Instead of commonly used inverse value of induction time, the rate of an individual particle mass growth for the adequate description of kinetics during induction period was used. Taking into account the initial water consumption allowed a unified description of kinetic data in different ranges of reagent concentrations to be obtained. C  2003 Kluwer Academic Publishers 1. Introduction Metal oxide submicron particles with controlled size distribution and high specific area are of considerable interest for production of composite ceramics [1], pig- ments and components of coating films [2], as well as for catalytic and photocatalytic applications [3]. Whereas the sol-gel process to produce colloidal sil- ica is well understood [4], a clear and complete picture for the formation of transition metal oxide particles has not still been obtained. Understanding the phenomena occurring during the initial stages of particle growth seems to be especially important, because they can de- termine the properties of final products. Kinetics of the sol-gel process based on titanium metal alkoxides has been studied in a number of works [5–9]. Different empirical formulae for the dependence of the inverse value of the induction time as a function of reagent concentrations have been obtained. They are often used to determine the order of the hydrolysis reac- tion, which is found to be different for different ranges of reagent concentrations. This peculiarity has not yet been clearly explained. For description of induction pe- riod LaMer’s model [10] is often applied, where induc- tion time is explained by accumulation and supersatu- ration of alkoxyhydroxytitanium molecules (partially or completely hydrolysed molecules of titanium al- koxide). However, it has been shown by Karl Fisher titration method [8] and by Raman spectroscopy [11], that the initial hydrolysis is very fast compared with the induction period. Furthermore, non-monomeric re- action products have been observed during this period in X-ray absorption near edge structures (XANES) [12] and nuclear magnetic resonance (NMR) experiments [13]. From these studies it became clear that the induc- tion time in the case of TTIP based sol-gel process can not be directly related to the hydrolysis reaction, but that it reflects the kinetics of complex processes, in- cluding different types of inorganic condensation and aggregation. The presence of nanoparticles of radius 2–4 nanometers throughout the whole induction period of sol-gel process with TTIP was observed by using dynamic light scattering (DLS) [14–16, 17]. The early-stage processes in hydrolysis and con- densation of zirconium alkoxides has been studied by Fourier transform infrared spectroscopy (FTIR) and small angle X-ray scattering (SAXS) [18]. A rapid flow- through mixing cell was used for monitoring the reac- tion within a millisecond time scale. This investiga- tion showed the fast hydrolysis/condensation reactions (<1 sec) and formation of primary particles with a ra- dius about 3–4 nm. Strictly speaking the term “induction period” must denote a period between the beginning of the reaction and the nucleation of particles. However, in this ar- ticle the traditional definition of the induction period accepted in sol-gel chemistry as the period until the ap- pearance of a significant turbidity in the system will be used. The analysis of TTIP based sol-gel reaction kinet- ics is presented. The early stage rapid hydrolysis and condensation reactions, which precede the formation of primary particles are not studied. Instead, interest is the kinetics for the rest of the induction period, which consists, as hypothesized later, in aggregation of addi- tionally hydrolyzed primary particles. First a conclusive parameter for the description of the kinetics during this 0022–2461 C  2003 Kluwer Academic Publishers 3315