Synthesis of Liposome-Templated Titania Nanodisks:  Optical Properties and Photocatalytic Activities

Abstract

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.

Enhanced photocatalytic performance of g-C3N4 nanosheets–BiOBr hybrids

“…CNs shows a fundamental absorption edge rising at $424 nm, while BiOBr holds an onset of absorption at $444 nm. The band gap energy of both pure semiconductors can be estimated through the following formula [45]:…”

Single Nanoparticle Photoluminescence Studies of Visible Light-Sensitive TiO2and ZnO Nanostructures

“…Figure 4 shows TEM and AFM images of two-dimensional TiO 2 NDs synthesized by sol-gel method through formation of liposome-TiO 2 nanocomposites using egg-lecithin lipid as a template as previously established. 15 The TiO 2 NDs are anatase nanocrystals as seen from the well-defined SAED and 0.35nm lattice spacing (Figure 4(A) inset) with the average diameter of 9 nm and central height of ~2nm as determined from the cross sectional analysis of the AFM images. Figure 4 (C) shows the ensemble-averaged diffuse reflectance fluorescence emission (b) and excitation spectra of TiO2 NDs on slide glass, being attributed to recombination of electron-hole on multiple surface states for visible light absorption beyond 400 nm.…”

“…The present methodology and results would be useful to obtain surface exciton dynamics of other photoelectronic semiconductor nanostructures. The single nanoparticle spectroscopy was applied to a n ew class o f n an o stru ctu ral co m p o sites, Sn-porphyrin-intercalated TiO2 nanofibers (SnTTP-TiNF) fabricated by one-step hydrothermal reaction of a mixture solution of TiO2 anatase powder and a Sn-porphyrin, trans-dihydroxo [5,10,15,20] Supporting this speculation, the ensemble-averaged PL emission spectrum of the aqueous dispersion of SnTTP−TiNFs was observed to be quenched as compared to that of free TiNFs. However, free TiNFs exhibit broad surface emission bands which are overlapped with those of SnTTP and TiNFs, and it is difficult to clarify the pathway of the photoinduced electron transfer in SnTTP-TiNFs.…”

“…Therefore, to solve these problems, there have been many attempts to modify the surface state characteristics such as surface state density and electron/hole trapping kinetics by decreasing the degree of overlap of the electron and hole wave functions by synthesizing 1-or 2-dimensional single-crystalline nanostructures such as nanotubes (NTs), [12][13][14] nanowires (NWs) and nanodiscs (NDs). 15 In the nanostructures, the photoinduced electron transfer is speculated to be facilitated through the surface layer because of high aspect ratio and large surface area reducing inter-crystalline contacts of TiO 2 or ZnO.…”

Surface Modification Reaction of Photocatalytic Titanium Dioxide with Triethoxysilane for Improving Dispersibility

“…The onset absorption of the photocatalytic TiO2 is extrapolated to 3.02 eV corresponding to the band-gap energy. 23 It also exhibits a weak but significantly broadended band at longer wavelengths than 400 nm up to 532 nm, which might be caused to absorb visible light due to the mixture phase ratio of 75:25 in anatase and rutile.…”