*2.5.1.6. Solvent type and concentration*

In general, after nucleation, the particles grow by molecular addition or aggregation and this particle growth is affected by the kinds of solvents used because particle interaction potential is different in each solvent. The increase in the amount and molecular weight of the alcohol was found to increase the size of the particles and the smallest size was obtained when no alcohol was used [71]. This is due to the fact that an increase in both concentration and molecular weight leads to a decrease in the dielectric constant of the solvent resulting in decreased stability and enhanced rate of re-aggregation and larger particle size. The formation of TiO2 from thermal hydrolysis of titanium tetrachloride in water/n-propanol mixtures was investigated by Park et al. [73]. The study showed that when powders were redispersed in various solvents the degree of aggregation increased in the order: methanol > ethanol > propanol suggesting that colloidal destabilization was the primary mechanism by which these alcohols influenced particle size.

In another study by Xu et al. [74], the photocatalytic activity of unsupported TiO2 gradually increased with increase in the chain of the solvent used in preparation due to the increase in the content of anatase and a decrease in particle size. They found out that when the solvent was changed from methanol to 2-pentanol, the content of anatase increased from 68 to 91% with a decrease in particle size from 11.6 to 10.5 nm. The increase in alcohol concentration in the sol-gel reaction mixture can slow down the hydrolysis rate and the resulting sol would possess a high content of amorphous TiO2 .
