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Structural Phase Diagrams of Oxide Nanocrystals: How Particle Size Changes Properties Complex oxide materials often have complex phase diagrams. This means that the structural phases depend on the details of the composition. Phase transition behavior should be different in nanocrystals and nanowires than in the bulk due to their much larger surface area and the variation of surface energy with phase and passivation. Professor Chan, in collaboration with Professor Herman and others, has observed this nano-effect in a study of the cubic phase of Ce1-xZrxO2-y, -- an important catalyst for energy applications -- using a variety of methods that utilize visible light, x-rays and electrons to probe structure and properties at room temperature and at the high temperatures at which these catalysts are used. With decreasing particle size, the phase boundary between the cubic phase (which is best for catalysis) and the tetragonal phase was observed to shift to higher zirconia concentrations (40-60%, dependent on particle size) in air than for the bulk material.
Another thing also happens when these particles become small. A smaller particle size increases the solubility of zirconia (ZrO2) in cubic phase ceria (CeO2) and also converts a small quantity of Ce4+ ions to Ce3+ ions. A higher concentration of larger Ce3+ ions helps to relieve the local stress induced by smaller Zr4+ ions and this stabilizes the cubic phase even under higher zirconia concentrations. This stabilization is very important in using these particles as catalysts at high temperature. Posted on: May 12, 2006 (For more, see J. Am. Ceram. Soc. 89, 1028-1036 (2006).) For more details please contact: Siu-Wai Chan |
 Phase diagram for Ce1-xZrxO2-y nanoparticles |
