Raman spectroscopy of nanophase TiO 2
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J. Mater. Res., Vol. 4, No. 5, Sep/Oct 1989
Downloaded: 07 Jul 2014
IP address: 155.97.178.73
C. A. Melendres ef al.; Raman spectroscopy of nanophase TiO2
The assignment of the strong 144 cm ' feature seen in Figs. 1,2, and 3 for as-compacted samples to the anatase form of TiO2 is further substantiated by spectrum (d) of Fig. 3. In this spectrum, where the rutile bands have narrowed considerably (but not completely) following the anneal at 973 K and the 144 cm"1 band is still present, it is possible to identify a set of weak features (indicated by asterisks in Fig. 3) that are also attributable13 to anatase-type TiO 2 . (Specifically, these features are at the following approximate wavenumber values: 636, 514, 395, 325, and 196 cm"1.) It is noteworthy that these anatase bands are no longer discernible in spectra (e) and (f) of Fig. 3, providing additional evidence for the anatase-to-rutile transformation. It is deduced from the present Raman spectroscopy results that unannealed, as-compacted nanophase TiO2 is composed of both grains and grain boundaries, as seen by transmission electron microscopy,9 which are comprised of the local atomic structural units of rutile (short-range or long-range ordered), mixed with sporadically distributed, very small quantities of the anatase form of TiO2. Thus, since no Raman spectral components are observed other than those from rutile (with the exception of the sporadic anatase), it can be concluded that no 'gas-like' structural components, such as those suggested by Gleiter and coworkers,2'5'6 exist in the nanophase TiO2 samples studied here. Since grain size seems to be ruled out as a factor contributing to the sharpness (or lack thereof) of the rutile phonons at 600 and 418 cm"1, one is left with the plausible alternative that intragrain defects dominate the shaping of the spectral features. The view proposed here is that, regardless of grain size of the as-compacted nanophase material, the height-to-half-width of the 600 and 418 cm"1 bands primarily reflects the extent of the defects within the grains. Comparison of spectrum (a) in Fig. 1 with spectrum (a) in Fig. 2 and spectra (a) and (b) in Fig. 3 shows that the extent of these defects can vary from preparation to preparation of as-compacted nanophase TiO2, even for samples having the same average grain size. This variability should not be surprising, however, given (1) the manner in which the titanium powders were oxidized subsequent to their formation10 and (2) the normal occurrence of oxygen-deficient planar defects in rutile.17 The intensity response of the 240 cm" 1 band following the annealing process that sharpens the 600 and 418 cm"1 bands suggests that the 240 cm"1 Raman scattering may be due to a phenomenon that is much longer range in nature than the one causing broadening of the 600 and 418 cm"1 zone-center phonons in as-compacted nanophase TiO2. The 240 cnT 1 band is observed for both single and polycrystalline samples,1114"16 shows no polarization sensitivity in studies of oriented single crystals,15 and d
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