Formation Mechanism of Amorphous TiO2 Spheres in Organic Solvents. 1. Roles of Ammonia

The roles of ammonia in the formation of monodisperse amorphous TiO(2) spheres through hydrolysis of titanium butoxide in its homogeneous solution of a mixed solvent of butanol/acetonitrile with ammonia have been investigated. The hydrolysis of titanium butoxide was originally so fast as to finish almost instantly within a few seconds even in the absence of ammonia that the effect of ammonia as an accelerator of the hydrolysis of titanium butoxide was not observed. Instead, ammonia was found to play key roles as an accelerator of the precipitation of the hydrolysis product, as an inhibitor of the coagulation of the growing hydroxide particles, and as a promoter for production of highly spherical particles. All of these effects are deemed to be mainly due to the reduced affinity between butanol and the hydroxide monomer joined with ammonia through hydrogen bonding. In addition, ammonia was revealed to work as a powerful accelerator of the condensation of the hydroxide monomer, which proceeds not in the solution phase but only in the individual particles during aging after precipitation. The accelerated condensation by ammonia was explained in terms of the promoted release of hydroxide ion of the monomer in each particle by the nucleophilic coordination of ammonia to its titanium ion. These mechanisms widely applicable to other sol-gel systems of transition metal oxides as well have been discussed in comparison with silica particle systems.