Synthesis of liposome-templated titania nanodisks: Optical properties and photocatalytic activities

New spherical nanostructures of titania (TiO2) have been synthesized through formation of liposome-TiO2 nanocomposites by using egg lecithin lipid as a template, and their optical properties have been investigated with regard to the dynamics of surface charge carriers and photocatalytic activities by using UV-vis and photoluminescence (PL) spectroscopic techniques. On the basis of the measurements of X-ray diffraction, transmission electron microscopy, and atomic force microscopy, the spherical titania nanostructures are identified to be anatase crystalline nanodisks with an average diameter of 9 nm and height of 0.5 nm. The nanodisks have a large Brunauer-Emmett-Teller specific surface area of 227 m(2)/g. The FT-IR and X-ray photoemission spectra of the nanodisks confirm that the skeleton structure of the titania nanodisk is formed through H-bonding of the -Ti-O-Ti- network through tetrahedrally coordinated vacancies designated Ti-4(4+)-OH. Analysis of the UV-vis and PL spectra reveals that the band-gap energy is red-shifted to 3.02 eV from that of TiO2 natioparticle dots and its transition nature is exclusively indirect. The PL emission spectrum of the titania nanodisks exhibits a strong structural emission band around 420 unit with shoulders around 470 and 550 nm which is attributed to the transition from three different exciton-trapped surface states. In addition, another surface emission originating from the coordinatively unsaturated ions (Ti3+) is observed at 618 nm. These results suggest that coupling of the surface charge carriers with the lattice phonon of the nanostructures is so strong that the dominant route to charge recombination in titania nanodisks is nonradiative. Supporting the steady-state spectral observations, the decay profiles of the surface emission measured by using a fermosecond laser time-resolved PL system fit into a triexponential function with relatively longer lifetimes (20-30 ps, 1.1-1.5 ns, and 4.5-6.0 ns) as compared to those of simple nanoparticle dots, indicating that recombination of the charge carriers on the nanodisk surface is very prolonged. Being consistent with this, the photocatalytic efficiency for the reduction of methyl orange is much higher in the presence of the titania nanodisks than that observed in the presence of Degussa P-25.