Influence of quantum confinement, hydrostatic pressure and critical Zr dopant concentration on the Raman vibrational properties of Zr doped anatase TiO2 nanoparticles

• We present a comprehensive analysis of the Ramanspectra of pure and zirconium-doped anatase TiO2 nanoparticles. To accountfor the wavenumber shifts of the Eg(ν6)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 electronmicroscopy data, our analysis shows that the surface stress contribution to the observed blue shift of the Ramanwavenumber 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 themodified PCM to the x-dependent variations of the Eg(ν6) Raman mode, the mode-Gr ¨ uneisen 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

Nyckelord

TiO2 nanoparticles

Raman spectroscopy

phonon confinement

surface stress; zirconium doping

Engineering Science with specialization in Solid State Physics

Teknisk fysik med inriktning mot fasta tillståndets fysik

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