Development of Preparation Process of Fine Particles Using Reverse Micelles

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The mechanism of formation of TiO_2 ultrafine particles in aerosol OT(AOT)/isooctane reverse micellar system by hydrolysis of titanium tetrabutoxide (TTB) was investigated. The mesurement of change in UV-vis. absorption spectra showed that the increase of TTB concentration and water content ( = [H_2O]/[AOT]), and the decrease of surfactant concentration were found to facilitate particle formation. The particle formation was controlled by TTB concentration and the number of micelles, since a certain number of hydrolyzed molecules are required to form a stable nucleus in a micell. A reaction scheme of particle formation based on the number of micelles was proposed, which successfully explained the absorption spectra and particle diameter experimentally obtained.CdS and ZnS ultrafine particles were prepared by mixing two reverse micellar solutions containing Cd(NO_3)_2 (or Zn(NO_3)_2)and Na_2S, respectively. The particles less than 5nm in diameter were successfully prepared when the water content was less than 10. In the early stage (0-20ms), the particle formation and coagulation was controlled by the collision and redispersion process of the micelles. In the later stage (20ms-), the coagulation was dominated by distribution of the particles amongst the reverse micelles.The preparation of CdS-ZnS composite particle was also investigated. Zn_XCd_<1-X>S composite ultrafine particles were prepared by simultaneous precipitation in the reverse micellar system. ZnS-coated CdS ultrafine particles were prepared by sequential precipitation of ZnS on the CdS particles in the reverse micelles. These particles showed high photocatalytic activity for hydrogen generation by water cleavage in the reversed micellar system.