Phase transitions in metal oxide nanoparticles as studied by Raman scattering.

摘要

Optical spectroscopy is used to study finite-size effects in three oxide nanoparticle systems: ceria/ceria-zirconia, hafnia-zirconia, and barium titanate.; In ceria, sub-band gap excitation causes emission in nanoparticles and micron-sized particles, when the laser intensity is increased beyond a threshold. A non-linear process, related to oxygen defects, is the source of the heating. The Stokes to anti-Stokes ratio is used to estimate the local temperature in the particles.; Raman scattering is used to investigate the size-dependent properties of ceria-zirconica (cerium-zirconium oxide, CexZr 1-xO2) nanocrystals for a range of temperatures, compositions, and gas environments. Cubic (c) and tetragonal (t and t″) phases of CexZr 1-xO2 nanocrystals are identified by the Raman spectrum and supporting x-ray diffraction data. As the particle size decreases, at room temperature the phase boundary between the cubic and tetragonal t″ phases extends the cubic and t″ phases to more Zr-rich compositions. Heating the nanocrystals in a reducing atmosphere showed that three very weak Raman peaks in ceria are due to tetragonal distortions (associated with tetragonal phase peaks 2 and 5/6) and not due to defects or second-order scattering.{09}At high laser powers, the appearance of strong visible emission and the loss of vibrational Raman intensity are evidence for a phase transition in sub-band gap laser-heated CexZr1-xO 2 nanocrystals, as has been seen for CeO2.; Raman scattering demonstrates that HfxZr 1-xO2 particles are solid solutions of hafnia-zirconia with no discernible segregation. A simple lattice dynamical model with composition-averaged cation mass and scaled force constants is used to understand how the Raman mode frequencies vary for these alloys. Background luminescence from these particles is minimized after oxygen treatment, suggesting possible oxygen defects in the as-prepared particles.; Phase transitions in barium titanate show size-dependent effects. As the particle size decreases the high temperature cubic phase becomes stable at lower temperatures. The tetragonal-cubic phase transition temperature in 50 nm particles is observed at 108°C, lower than the ∼120--130°C tetragonal (ferroelectric)-cubic (paraelectric) transition temperature for the bulk.