Electronic structure of titania-based nanotubes investigated by EELS spectroscopy

Abstract

International audienceWe present an electron-energy-loss spectroscopy study of the O-K and Ti L-2,L-3 edges for anatase-, rutile-, and titania-based nanotubes. The titania-based tubes are composed of tetravalent titanium ions in an octahedral symmetry with the oxygen ligands, however the electronic structure does not correspond to that of either of the titania precursors. Crystal-field splitting is comparable with anatase but the 3d occupation number is closer to that of rutile. In addition, Ti L-2,L-3 spectroscopic signatures indicate that the octahedron connectivity of the tubes is different to that of the reference titania. We also report the first measurement at the scale of an individual nanometrical object of the charge-transfer satellites for the transition metal 2p edges. This may open new opportunities for the estimation of d-orbital occupation numbers for transition-metal or rare-earth elements occurring in local grains, phase-segregated regions, nanomaterials, or in the vicinity of interfaces

New fine structures resolved at the ELNES Ti-L2,3 edge spectra of anatase and rutile: Comparison between experiment and calculation

“…On the contrary, the Ti-L 2,3 edge fine structures, recorded from rutile nanoparticles, look different when exposure time increases from 4 to 20 s. After 20 s, the fine structures' shape stabilizes and does not evolve with time any more. Therefore, it is obvious that even after only 4 s, the ELNES spectra do not match the published ELNES/XANES spectra corresponding to bulk rutile [22][23][24][25][26][27][28][29][30][31][32][33]. By comparison to literature, we find that the spectrum recorded after 20 s presents more similarities to the TiO 2 -II phase spectrum recorded by Ruus et al [27] or to the brookite EELS spectrum recorded by Mitterbauer et al [22].…”

“…2b) and bulk rutile (Fig. 2c), recorded with different exposure times: 4, 12 and 20 s. At a first glance, one can observe that the shape of the spectra recorded from anatase nanoparticles is similar to the shape of bulk anatase spectra published in the literature by different authors using EELS [22][23][24][25] or XAS [26][27][28][29][30][31][32][33]. One can also observe that no significant changes occur when the exposure time is increased.…”

“…The first is related to the broadening of the Ti-L 3 t 2g band. While the anatase Ti-L 3 t 2g band full width at half maximum (FWHM) recorded using the ZEISS-CRISP is about 0.4170.1 eV for anatase and 0.46 70.1 eV for rutile, in spectra recorded using the CRISP monochromator ''off'', a cold field emission gun TEM or a conventional TEM [23][24][25], they are about a factor of 2-3 larger, respectively. On the contrary the Ti-L 3 t 2g band FWHM measured by XAS in SRS is quite similar (0.39 [26] to 0.48 eV [27]).…”

“…Up to now, anatase and rutile L 3 e g bands are reported to be composed of two peaks i.e. one sharp and one broadened and the L 2 e g band composed of a single broadened peak [22][23][24][25][26][27][28][29][30][31][32][33]. The energy positions (relative to the L 3 t 2g peak) of all the fine structures that were resolved in the present anatase Ti-L 2,3 ELNES spectra of anatase and rutile are listed in Tables 1 and 2 [23] are also indicated.…”

Influence of surface disorder, oxygen defects and bandgap in TiO2 nanostructures on the photovoltaic properties of dye sensitized solar cells

“…These peaks are labeled a, b, c and d and the corresponding spectral energy positions are listed in Table 2 for both as-prepared and hydrogenated samples. The L 3,2 edges of Ti has spin-orbit split L 3 and L 2 edges with a separation of $ 5 eV [47]. The L 3 edge originates due to the electron transition from 2p 3/2 orbital to empty 3d orbital of Ti.…”