Project ended 2002, report dated April 2003
Executive Summary
Uniform anatase-type TiO2 nanoparticles of different shapes have been formed by phase transformation of Ti(OH)4 gel matrix in the presence of shape controllers. For example, triethanolamine (TEOA) was found to change the morphology of TiO2 particles from cuboidal to ellipsoidal at pH above 11. The shape control can be explained in terms of the specific adsorption of TEOA to the crystal planes parallel to the c-axis of the tetragonal system in the alkaline range, as supported by the observation of preferential adsorption of TEOA to the crystal planes parallel to the c-axis at pH 11.5 and by the pH dependence of the adsorption to ellipsoidal particles. Diethylenetriamine (DETA) also modified the particle shape to ellipsoidal above pH 9.5 and the aspect ratio was much higher than with TEOA. The mechanism of the shape control could be explained in the same way as with TEOA, since analogous specific adsorption was observed with DETA as well. Similar shape control to yield ellipsoidal particles of a high aspect ratio was also achieved with other primary amines, such as ethylenediamine (ED), trimethylenediamine (TMD), and triethylenetetramine (TETA). However, secondary amines, such as diethylamine, and tertiary amines, such as trimethylamine and triethylamine, acted as a complexing agent of Ti(IV) ion to promote the growth of ellipsoidal particles of a low aspect ratio, rather than a shape controller to produce ellipsoids of a high aspect ratio. Sodium oleate and sodium stearate were found to modify the particle shape from round-cornered cubes to sharp-edged cubes. The mechanism was explained in terms of the reduction of the specific surface energies of the {001} and {100} planes of the tetragonal crystal system by the preferential adsorption of oleate or stearate ion to these planes, based on the adsorption experiment using ellipsoidal and cubic particles.