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
m2/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 4Ti4+−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 nanoparticle dots and its transition nature is
exclusively indirect. The PL emission spectrum of the titania nanodisks exhibits a strong structural emission
band around 420 nm 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 femtosecond 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.
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