Influence of phonon confinement, surface stress, and zirconium doping on the Raman vibrational properties of anatase TiO2 nanoparticles

Authors

  • Christian Lejon,

    1. FOI—Swedish Defence Research Agency, SE-901 82 Umeå, Sweden
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  • Lars Österlund

    Corresponding author
    1. FOI—Swedish Defence Research Agency, SE-901 82 Umeå, Sweden
    2. Department of Engineering Sciences, The Ångström Laboratory, Uppsala University, S-751 21 Uppsala, Sweden
    • Department of Engineering Sciences, The Ångström Laboratory, Uppsala University, P. O. Box 534, S-751 21 Uppsala, Sweden.
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Abstract

We present a comprehensive analysis of the Raman spectra of pure and zirconium-doped anatase TiO2 nanoparticles. To account for the wavenumber shifts of the Eg6) mode as a function of particle size (L) and dopant concentration (x), a modification of the standard phonon confinement model (PCM) is introduced, which takes into account the contribution of surface stress by means of the Laplace–Young equation. Together with X-ray diffraction (XRD) and transmission electron microscopy data, our analysis shows that the surface stress contribution to the observed blue shift of the Raman wavenumber is of the same magnitude as the spatial phonon confinement effect. Annealing experiments show that Zr-doped nanoparticles exhibit retarded grain growth and delayed anatase-to-rutile phase transition by up to 200 K compared to pure anatase TiO2. XRD shows that Zr doping leads to a unit cell expansion of the anatase structure. Applying the modified PCM to the x-dependent variations of the Eg6) Raman mode, the mode-Grüneisen parameter is found to increase abruptly at x > 0.07 with a concomitant mode softening. This coincides with the x range over which the Zr cations are reported to be displaced from their position in the tetrahedral lattice, and where Zr precipitation occurs upon annealing. The results have implications for the interpretation of Raman spectra of ionic metal oxide nanoparticles and how these are modified upon cation doping. Copyright © 2011 John Wiley & Sons, Ltd.

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