In Situ Study of (1) Aqueous Room-Temperature Synthesis of Crystalline Nano-Ceria and (2) Nano-Zirconia—Transformation from Amorphous to Crystalline Phase(s)

In-situ studies are presented: (i) Ultra-Small-Angle X-ray Scattering (USAXS) of the formation and growth of nano-ceria (n-CeO₂) (ii) Wide Angle X-Ray Diffraction (WAXRD) of the amorphous-to-cubic crystallization of nano-ZrO₂ in a reducing environment. In the first study, in situ monitoring of characteristics from 1nm as a function of the temperature allowed for the identification of the nucleation and growth of ceria nano-crystals. Monitoring flow rate, temperature and pH of the suspension have permitted real-time studies of the nano-ceria crystalline synthesis from homogeneous solution over many hours. Incubation times for each temperature and minimum critical nucleus-size of 6nm have been identified which yield the interfacial energy of n-CeO₂ in the aqueous solution. Activation energy for growth has been measured to be ~0.5eV. In the second study, pair distribution function and reverse Monte Carlo simulation were applied to provide details of the local structure during the crystallization. The local structure of the amorphous phase bears resemblance to the short-range arrangement of cubic ZrO₂. The amorphous-to-cubic (a−c) crystallization of nanoZrO₂ in a reducing environment was studied. A reverse Monte Carlo (RMC) simulation provided details of the local structure during the crystallization process as well as partial Pair Distribution Function’s (PDF’s) of Zr−Zr and Zr−O during the crystallization. The number of Zr's next-nearest neighbors of Zr remains 12, whereas the number of O's as nearest neighbors of Zr increases from 6.7 to 7.3 as the amorphous evolves into a cubic structure, suggesting the persistence of a high concentration of oxygen vacancies. The simulated atomic local structure of the amorphous phase bears resemblance to the short-range arrangement of cubic ZrO₂, consistent with the results of X-ray Absorption Near Edge Spectroscopy (XANES) at Zr L_II and L_III. The amorphous-to-crystalline phase transformation is affected by the environment. Under an oxidizing condition, the amorphous phase crystallizes directly to tetragonal and subsequently to monoclinic zirconia.<br/><br/>1 J.Appl.Cryst., (2008). 41, pp 918–929 <b>DOI</b>:10.1107/S0021889808023078<br/><i>2 Chem. Mater.</i>, (2007), 19 (13), pp 3118–3126 <b>DOI: </b>10.1021/cm061739w