Peptization of Nanoparticulate Titania Sols Prepared Under Different Water–Alkoxide Molar Ratios

Abstract

Particulate titania sols with water–alkoxide molar ratios (r) of r=30:1 ([Ti4+]=1.146M), r=50:1 ([Ti4+]=0.818M), r=75:1 ([Ti4+]=0.579M), and r=100:1 ([Ti4+]=0.459M), have been synthesized. Both hydrolysis and peptization temperatures were maintained at 35°C in all cases. HNO3 was used as a catalyst and its amount was calculated to maintain the H+/Ti molar ratio at 0.2. The combination of the data measured with optical techniques such as laser diffraction, dynamic light scattering (DLS), and multiple light scattering with a near‐infrared light allows us to follow up the evolution of the peptization process and to unequivocally establish a procedure to quantify its time. This time increases as the water–alkoxide molar ratio increases. Stable monomodal nanoparticulate TiO2·xH2O sols with an average particle size of ∼20 nm, as measured by DLS were obtained after peptization, except for r=30:1. However, TEM observations of the xerogels treated at 300°C for 1 h show primary particles of ∼10 nm. This suggests that the synthesized sols are formed by agglomerated particles and DLS measurements provide particle sizes larger than the actual primary size. The viscosity of the sols increases with the Ti concentration, but it is very low (<5 mPa·s), except that of the 30:1 sol, where a transient structured fluid is formed that breaks with time and by shearing. The sols have considerably long‐term stability when preserved cool (<10°C). The anatase phase is obtained as a major phase in the xerogels, although a small peak of brookite is also present. The xerogels calcined at 300°C for 1 h show traces of rutile at r values of 50:1 and 30:1. This fact confirms that the anatase–rutile transformation is produced at lower temperatures when the molar ratio is lower, i.e., when the Ti concentration increases.