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September 13, 2006

Phases in ceria-zirconia binary oxide (1-x)CeO_2-xZrO₂ nanoparticles: the effects of particle size, the redox environment and the Ce³⁺ concentration

F. Zhang¹, C.-H. Chen¹, J.M. Raitano¹, R.D. Robinson¹, I.P. Herman¹, J.C. Hanson², W.A. Caliebe², S. Khalid², and S.-W. Chan^1
1Department of Applied Physics and Applied Mathematics, Materials Research Science and Engineering Center, Columbia University, New York, NY; ²Brookhaven National Laboratory, Upton, NY

Cerium oxide has been widely investigated as a key component in catalysts and as an electrolyte for solid oxide fuel cells because of its ability to release or store oxygen when in its cubic fluorite structure. This property, which is the alleged source of the oxygen storage capacity (OSC) of ceria, is much enhanced by a large surface area and a small particle size. Particle-size stability at high temperatures is a major issue for pure ceria, however. At high temperatures, ceria particles are easily coarsened, resulting in a smaller total surface area and a lower catalytic efficiency. Alloying with other metal oxides can halt this coarsening process; in particular, zirconia is most effective in this regard without significantly decreasing the oxygen activity. However, the zirconia content for most effective catalytic use will cause the binary oxide in micron-sized particles to contain a substantial amount of tetragonal phase that does not have the OSC properties, i.e. it does not supply and support the oxygen transport in and out of the solid oxide. In our study, nanoparticles of ceria-zirconia were found to have a stable cubic fluorite phase despite what was predicted by the normal “bulk” phase diagram. Furthermore, we proved that a reducing environment stabilizes the cubic phase to 90% zirconia.

(From left) Syed Khalid, Siu-Wai Chan, Jonathan C. Hanson, and Joan M. Raitano at the X7 beamline.

Cerium oxide in the cubic fluorite structure has been widely investigated because of its multiple applications, such as catalyst and electrolyte material of solid oxide fuel cells. In particular, the structural properties of binary oxide system of ceria and zirconia (CeO₂–ZrO2) are extensively studied because it retains the superb redox and oxygen storage capacity (OSC) properties, and prevents thermal instability against coarsening of CeO₂. However, its various phases have not been discussed in detail, particularly with crystallite size. In this study, we aimed to investigate the structural properties of its nanoparticles for phase information. Specifically, we aimed to address two areas: First, we looked for methods that can stabilize the c' phase for a higher zirconia concentration to lessen particle coarsening. Second, we ascertained the extent to which particle size affects phase stability. This has a significant impact on catalysis.

Phase information of ceria–zirconia nanoparticles observed in air is studied by x-ray diffraction, transmission electron microscopy, and Raman spectroscopy. Particle size and composition are varied. Both the metastable tetragonal t' phase and the monoclinic m phase are not observed. The nanoscale of the particles likely stabilizes the tetragonal t phase against the formation of the monoclinic phase even at 100% zirconia. As the particle size decreases, both the c-t" and the c'-t phase boundaries shift to higher zirconia concentrations. The zirconia solubility limit increases with decreasing particle size such that the c and t" phases can be sustained at higher concentrations of zirconia before the corresponding formation of the t" and t phases. Raman scattering and XRD results are consistent in determining the emerging compositions of the t phase in the 1200° and the 800°C samples. The nanoparticles show different phases from those of the bulk in the CeO₂–ZrO₂ binary system. Nanoparticles of 20 nm and smaller having 35%–40% zirconia in ceria are 100% c' and stable against coarsening. The t" phase also likely contributes to OSC. This shows that the range of c' phase can be extended to high ZrO₂ concentrations by decreasing the c' crystallite size alone.

Figure 1. Phase stability diagram of (1-x)CeO2-xZrO2 samples with various crystal-size of the c' phase. There are overlapping symbols to denote XRD and Raman techniques in phase identification. Open symbols are from the XRD results and the inside symbols are from Raman scattering. Open squares: 100% c' phase present from XRD, circles: t phase present from XRD, diamonds: only t phase present from XRD. Inner symbols, solid squares: only Raman peak 4 is present; vertical line: only Raman peaks 3 and 4 are present; crosses: only Raman peaks 1, 3 and 4 are present; dots: all Raman peaks 1-6 are present. At the top of the diagram are the phase fields identified at 800oC from reduced-quench-re-oxidized bulk/micron-size samples.

Figure 2. TEM lattice image of Ce0.8Zr0.2O2-y nanoparticles annealed previously at 900oC.

We observed that the range of the c phase can extend to high ZrO₂ concentrations by decreasing the c' crystallite size alone. Earlier, we used x-ray absorption near edge spectroscopy (XANES) to demonstrate that the Ce³⁺ concentration in ceria nanoparticles increases with a decreasing crystallite size. Here, we investigated the valence state of Ce with varying ZrO₂ concentration and annealing atmosphere to better understand the phase stability in this nanocrystalline binary oxide system.

Figure 2. Lattice parameter study from in-situ XRD of as-prepared (1-x)CeO2-xZrO2 in a reducing atmosphere as temperature increases -- each composition shown exhibit the cubic phase, c’.

We used x-ray absorption near edge spectroscopy (XANES), time-resolved high temperature x-ray diffraction (XRD), and room temperature XRD to study the stability of the cubic phase (c') of Ce_1–xZr_xO_2–y nanoparticles. Results from XANES at the Ce LIII edge and the Zr LIII edge indicate the same phase transition point of c -t for samples prepared in air. This is consistent with earlier results of XRD and Raman spectroscopy. The results show that the stability of the c' phase is directly related to the Ce³⁺ concentration. The percentage of the 3+ oxidation state of cerium was measured from the relative Ce³⁺ peak intensity at the Ce L_III edge in XANES. An 11% concentration of the larger Ce³⁺ ions, coupled with the smaller particle size, helps in releasing the local stress induced by the smaller Zr⁴⁺ ions and stabilizes the c' phase even under high zirconia concentrations of 40%–60%. XANES results at the Zr L_III edge supported the cubic phase stabilization. Under a reducing environment instead of in air, when the homogenization anneal was performed, the solubility limit of the cubic phase Ce_1–xZr_xO_2–y was extended to above 90% zirconia. The Ce³⁺ concentration increased, reaching 94% in Ce_0.1Zr_0.9O_2–y. Thus, the stability of c' phase is extended to higher ZrO₂ concentrations not by finer crystallite size alone but, more significantly, by a reducing environment.

BEAMLINES
X7B, X18B, X19A

FUNDING
U.S. Department of Energy
National Science Foundation

PUBLICATION
F. Zhang, J.M. Raitano, C. Chen, J.C. Hanson, W. Caliebe, S. Khalid, and S. Chan, “Phase Stability in Ceria-Zirconia Binary Oxide (1-x)CeO_2-xZrO₂ Nanoparticles: The Effect of Ce3+ Concentration and the Redox Envirnment,” J. of Appl. Physics, 99, 0843131-0843138 (2006).

FOR MORE INFORMATION
Siu-Wai Chan
Department of Applied Physics and Applied Mathematics
Columbia University
Email: sc174@columbia.edu