TiO_2 nano-films have been prepared by dip coating in a reverse micelle solution at different periods of gelation. The morphologic and structural characteristics were examined by using XRD, SEM and AFM methods. Photocatalytic reduction of the dye methyl orange adsorbed on the TiO_2 nano-films was also studied. Molecular AM1 modeling calculation was carried out for understanding the behavior of the dye adsorption and decomposition on the films. The film made at an early state of gelation is thinnest and uniform in the distribution of nano- particles on the film. The methyl orange molecule may be adsorbed on the film only by the oxygen atom of the sulfonate group through electronic force and coordination from the oxygen atom to the surface of the film. Such a terminal group adsorption has little effect on the bond structure of the whole molecule and may not be benefit to the attacking of surface hydroxyl onto the reactant, resulting in low photocatalytic activity. The film made by repeating coating ten times in the early state of gelation shows high roughness due to the large aggregates formed by the interconnected TiO_2 nano-particles, and crackings can also be observed. But the film possesses a high capacity of adsorption and high photocatalytic activity for the dye. The methyl orange molecule may lie on the surface of the film in feedback coordination from surface hydroxyl to the conjugated π bond, besides the terminal group coordination, since the film possesses large defect and thus high density of active sites. Such an adsorption not only weakens the large conjugated π bond structure throughout the whole molecule but also favors the attacking of surface hydroxyl onto the adsorbed molecules. The film made at 6 hours of gelation by dip coating once shows higher roughness and the surface particles are of bigger agglomerates. The capacity and photoreactivity of the film are moderate since its defect and active density are also moderate.

Key words: TITANIUM DIOXIDE, NANOPHASE MATERIALS, METHYL ORANGE, PHOTOCATALYSIS, DEGRADATION, CATALYTIC ACTIVITY