Identification of TiO2 clusters present during synthesis of sol-gel derived TiO2 nano-particles

Synthesis of titanium dioxide nanoparticles with controlled size distribution and morphology are of great interest for many applications i.e. photocatalysis and dye sensitized solar cells (DSSC). The sol-gel method has some advantages over other preparation techniques in the many parameters, which can be varied in order to obtain TiO2 particles with specific properties (Tailored materials). The TiO2 particles are formed by hydrolysis and condensation of titanium alkoxides. The properties of the final TiO2 particles are strongly affected by the kinetics of these reactions.1 Experimental parameters such as the temperature, pH, hydrolysis ratio h (H2O/alcoxide) and the nature of the alkoxy groups (R) can be used to control the kinetics of the hydrolysis and condensation.1-4 For h > 1.5 precipitation of titanium oxide particles will take place after an induction period in which slow particle growths is followed by a rapid precipitation. Understanding the phenomena occurring during the polymerisation step is of great importance, because they determine the properties of the final products.1 A scientific model for the transformation of initial formed titanium clusters after mixing to crystalline nano-particles is lacking but is highly important for full control of the synthesis and production of nano-particles. In this study titanium dioxide clusters present during nucleation and growth of sol-gel derived TiO2 nanoparticles was investigated by help of electro spray mass spectrometry (ESI-MS) and dynamic light scattering (DLS). Depending on the involved precursor TiO2 clusters of different sizes were identified (TTIP ~ 11-12 Ti atoms, TTB ~ 10-11 Ti atoms, and TTE ~ 5-7 Ti atoms).4 The Ti-O-Ti backbone/core of the titanium clusters were found to be quite stable after formation and do not easily break up into smaller clusters. The results of this investigation suggest that these clusters are used as building blocks in the growth during the induction period before precipitation of nanoparticles. The h-ratio (nH2O/nTi) and the pH were not found to influence the identity of the Ti clusters present during nucleation and growth of the sol-gel derived TiO2 nano-particles, although the time of the induction period was greatly influenced. The reactivity of the alkoxy group were found to be OEt > OPri > OBu. The difference in the reactivity of the alkoxides leading to difference in size of the primary clusters is generally believed to be due to the increasing size of the alkoxy group which causes sterical hindrance during the nucleophilic substitution reaction. The influence of pH and the alkoxy group on the resulting particle properties was investigated using differential scanning calometry (DSC), X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FT-IR), BET-adsorption isotherms and high-resolution transmission electron microscopy (HR-TEM).