Structure and optical properties of ordered mesoporous titania-silica composite thin films

Highly ordered mesoporous titania-silica (TiO2-SiO2) composite thin films have been prepared by spin-coating technique using poly(alkaline oxide) triblock copolymers EO20PO70EO20 (P123) as structure-directing agent. Low-angle X-ray diffraction analysis shows that the mesoporous composite thin films remain a long range periodic ordered structure even if the TiO2/SiO2 ratio in the thin films is as high as 80%. Wide-angle X-ray diffraction analysis reveals that the average grain size of TiO2 increases from 2.2 to 5.1 nm as the TiO2/SiO2 ratio increases from 20% to 80%. The crystal structure of TiO2 is identified to be the anatase phase. Transmission electron microscopy observations confirm that the mesoporous titania-silica composite thin films have a hexagonally ordered pore array nanostructure. Ultraviolet-visible absorption spectra give the evidence that the TiO2 nanocrystals as well as the four-coordinate Ti co-exist in the silica matrix. The semiconductor TiO2 nanocrystals in the silica matrix have an obvious blue shift phenomenon of the absorption edge. As the average TiO2 grain size increases from 2.2 to 5.1 nm, the band gap of the TiO2 nanocrystals in the mesoporous titania-silica composite thin films decreases from 3.9 to 3.45 eV.