Quantcast

Synthesis and Characterisation of Cerium-Samarium Mixed Oxide Nanorods

November 29, 2006

By Bugayeva, N; Robinson, J

Mixed oxide nanoparticles of 80 mol.-% cerium and 20 mol.-% samarium with rod like morphology were synthesised via a chemical coprecipitation technique. X-ray diffraction (XRD) and high resolution TEM characterisation showed homogeneous distribution of both rare earth metal oxides throughout individual particles.

Keywords: Nanocrystalline materials, Cerium-samarium mixed oxide, Chemical synthesis, Rod-like morphology, TEM

Introduction

Cerium-samarium mixed oxide (CSO) has received much attention in recent years owing to its suitability as an ionically conductive material in solid oxide fuel cells (SOFC). In practice, yttria stabilised zirconia has been the most widely used material thus far. However, this material has certain limitations in the magnitude of electrical conductivity and requires high operating temperatures such as 1000C, which decrease the efficiency and stability of the cell. As an alternative solid electrolyte, cerium oxide based materials have shown great potential, allowing operating temperatures below 800C with a simultaneous increase in ionic conductivity.

Samarium has demonstrated the best characteristics among various types of dopants in the ceria lattice. This has been revealed by numerous studies on the electrical conductivity of ceria doped with different rare earth and alkaline earth elements.1,2 It has been also shown that the highest magnitude of electrical conductivity has been achieved at a Sm concentration of 20 mol.-%.

Grain size in sintered bodies and homogeneity of the dopant within grains are other important factors for high performance of the material. It has been found that a certain concentration of dopants improves homogeneity within the grain and as a consequence, improves ionic conductivity.3 It is also expected that particle morphology may influence the ionic conducting properties of sintered bodies. However, only a few studies have been reported thus far. The influence of spherical and elongated (elliptical) morphologies of samarium doped ceria particles on the sintering characteristics and electrical performance has been investigated.4 The conductivity for the materials prepared from spherical particles was higher than for those produced using powders of elongated particles, possibly owing to structural differences of the sintered bodies.

Synthetic methods producing various particle morphologies and promoting good Sm homogeneity within the ceria lattice have been reported, based on liquid phase techniques. The powders obtained have demonstrated better reactivity and homogeneity, although still required high sintering temperatures often owing to undesirable morphologies or agglomerations. Spherical and irregular flower like nanoparticles (10 mol.-%CSO) with good homogeneity have been synthesised via carbonate precipitation.5,6 A modified sol gel method has been employed for the preparation of 20 mol.-%CSO powder.7 The method involved treating the gel with octanol, a long chain high boiling point alcohol, in order to eliminate hard agglomerates. A hydrothermal synthetic route has also been shown to have potential for the preparation of 20 mol.-%CSO and a number of other rare earth and alkaline earth doped ceria powders.8

In this work, CSO nanoparticles with rod-like morphology are prepared via a chemical coprecipitation technique. This method yields well dispersed nano-particles with good homogeneity of samarium in the cerium oxide lattice throughout every particle.

Experimental

Nanoparticles of hydrated CSO with rod-like morphology were obtained via a chemical coprecipitation technique.9 The starting materials were CeCl^sub 3^.7H2O, SmCl^sub 3^ and concentrated aqueous NH^sub 3^ solution, which was diluted by the addition of deionised (DI) water to an equivalent concentration of 1-1 mol L^sup -1^ NH^sub 4^OH. The ammonia solution was taken in excess to achieve pH 9-5. First, 4-8 g CeCl^sub 3^.7H2O was dissolved in 290 g DI water; 0-8 g SmCl^sub 3^ was then slowly added while stirring; finally, 85 g of the aqueous NH^sub 3^ solution was added dropwise with vigorous stirring. A pink brown gel immediately formed. The gel was stirred for 20 min and left to age at room temperature in a sealed container. After aging for 20 h, the gel transformed into a purple/ white sol.

The obtained product was washed four times with dilute NH^sub 4^OH maintaining pH 9-5-10 throughout and then once with DI water. Washing was carried out by centrifugation at 3000 rev min^sup -1^. The obtained slurry was dried at 60C in air. Heat treatment was carried out in an unsealed porcelain crucible in an electric furnace at 700C for 2 h.

1 XRD curve for hydrated 80Ce-20Sm (mol.-%) mixed oxide powders

TEM samples were prepared on holey carbon films by suspending the particles in methanol, placing a droplet of the suspension onto the carbon film and allowing the methanol to evaporate. TEM and high resolution TEM (HRTEM) studies were performed respectively using a JEOL 2000FX operating at 80 kV, a JEOL 3000F field emission gun TEM equipped with energy dispersive spectroscopy (EDS) detector and a Gatan image filter system operating at 300 kV. All data from the FEGTEM were acquired via a 1024 1024 pixel digital camera. Image processing and analysis were carried out with digital micrograph (supplied by Gatan Inc.). Fourier processing was employed to assist with image analysis.

Results and discussion

An X-ray diffraction (XRD) curve corresponding to the coprecipitated hydrated samarium (20 mol.-%) and cerium (80 mol.-%) mixed oxide powder aged at room temperature is shown in Fig. 1. Some of the diffraction peaks were identified as being from Sm(OH)^sub 3^, although these were slightly shifted indicating larger interplanar distances. Other diffraction peaks were identified as corresponding to either hydrated CeO^sub 2^ or mixed Ce-Sm hydrated oxide, which have similar patterns. A TEM image of the rod-like particles is shown in Fig. 2. The particles had a fibrous structure consisting of a number of whiskers joined in ‘bundles’ along their longitudinal axes. The size of the particles typically ranged from 40-100 nm in diameter and from 800-1900 nm in length. The surface area of the powder was 39 m^sup 2^ g^sup -1^.

From the XRD and TEM data it was impossible to determine if the identified phases existed together within each particle or within separate particles. To investigate the composition and distribution of the elements in single particles, electron energy loss spectroscopy (EELS) with elemental mapping was performed. The images of the elemental maps are shown in Fig. 3, where a is an image of the typical rod-like particles under investigation and the elemental maps for b cerium, c samarium and d oxygen, respectively. The elemental maps show that each particle consists of mixed hydrated cerium and samarium oxide. While at this resolution, cerium and oxygen appeared to be evenly distributed within the particles, the samarium signal showed variation across some of the particles which can be attributed to some degree of samarium inhomogeneity. The elemental distribution data together with the identified separate phases of the hydrated samarium and cerium oxides in the XRD data suggest the existence of nanodomains within the particles containing higher concentrations of either cerium or samarium that were partly resolved at the selected magnification.

2 Rod-like morphology of hydrated Ce-Sm oxide particles

A HRTEM micrograph of a hydrated Ce-Sm mixed oxide rod-like particle is shown in Fig. 4. The image resolves the crystal lattice revealing the polycrystalline fibrous nature of the particle. Fourier transform processing of the image shows a complex diffractogram (inset), which consists of a number of textured (arc like) spots. The diffuse appearance of these spots indicates certain variations in the lattice planar distances and angles. Additionally, the arrangement of the spots indicates a certain degree of structural alignment of the subunits. Three sets of textured spots forming the inner ring close to the centre give a measured planar distance of 0.323 nm. These spots can be identified as the reflections from {111} face centred cubic (fcc) hydrated cerium oxide lattice doped with samarium or (11-20) hexagonal close packed (hcp) and the equivalent crystallographic planes of the hydrated samarium oxide lattice doped with cerium. The calculated planar distances for pure hydrated cerium oxide and for pure hydrated samarium oxide are 0.312 and 0.318 nm, respectively. The second ring is formed by three sets of weak textured reflections with a measured planar distance of 0-273 nm. These spots originate from {200} fcc crystallographic planes of hydrated cerium oxide doped with samarium. The calculated interplanar distance of pure hydrated cerium is 0.270 nm. The outer ring of very weak textured spots is formed by the reflections from planes with spacing of 0.201 nm. They originate from either {220} fcc for hydrated cerium oxide doped with samarium or {30-30} hcp and the equivalent crystallographic planes for hydrated samarium oxide doped with cerium. The calculated planar distances for pure hydrated cerium oxide and pure hydrated samarium oxide a\re 0.191 and 0.184nm, respectively.

3 Elemental mapping studies on hydrated 80Ce-20Sm (mol.-%) oxide rod-like particles: a image; b cerium map; c samarium map; d oxygen map

4 HRTEM image of single rod like particle of hydrated Ce-Sm oxide and its diffractogram

Because the diffractogram was derived from the image of a discrete particle, the observed increase in interplanar spacing suggests that either the hydrated cerium oxide lattice was uniformly doped with samarium throughout the particle or nanoscale regions rich with hydrated cerium oxide doped with samarium and hydrated samarium oxide doped with cerium coexist within individual particles. This is consistent with the results obtained from XRD data and elemental distribution studies.

5 XRD curve for 80Ce-20Sm (mol.-%) mixed oxide powder after heat treatment at 700C for 2 h

The development of rod-like morphology can be attributed to anisotropy of the cerium and samarium trihydroxides unit cell and experimental parameters including pH, temperature and ionic environment.

Cerium trihydroxide is unstable and oxidises upon prolonged atmospheric exposure to cerium tetrahydroxide Ce(OH)^sub 4^ with cubic structure. Oxygen diffusion within the particles takes place resulting in the formation of the Ce(OH)^sub 4^ and Sm(OH)^sub 3^ mixed compound. It is possible that this oxidation process contributes to some phase segregation and therefore, lower homogeneity in as precipitated particles was found in XRD, EELS elemental distribution and HRTEM studies.

The powder was heat treated at 700C for 2 h. After heat treatment for only one phase, 80Ce-20Sm (mol.-%) mixed oxide was observed, the corresponding XRD curve is shown in Fig. 5, which reveals the typical pattern for this composition. Evidently, heat treatment allowed sufficient diffusion to occur, resulting in the formation of a solid solution with homogeneous distribution of samarium throughout the cerium oxide lattice in each particle. The particles also retained their rod-like morphology after heat treatment as shown by the TEM micrograph in Fig. 6.

6 Rod-like morphology of Ce-Sm oxide particles after heat treatment at 700C for 2 h

Conclusion

Mixed oxide nanoparticles of 80 mol.-% cerium and 20 mol.-% samarium with rod-like morphology were prepared via a chemical coprecipitation technique. This method yielded well dispersed nanorods with high homogeneity of samarium in the cerium oxide lattice throughout every particle. Homogeneity was improved further by moderate heat treatment, which resulted in formation of a complete solid solution with preservation of rod like morphology of the particles. This morphology may facilitate fabrication of thinner layers of conducting electrolytes and consequently, further improve the performance of intermediate temperature SOFC.

References

1. H. Inaba and H. Tagawa: Solid Stale Ionics, 1996, 83, 1.

2. X. Qi, Y. S. Lin, S. T. Holt and S. L. Swartz: J. Mater. Sci., 2003, 38, 1073.

3. M. Mogensen, N. M. Sammes and G. A. Tompsett: Solid State Ionics. 2000, 129, 63.

4. T, Mori, Y. Wang, J. Drennan, G. Auchterlonie, J. G. Li and T. Ikegami: Solid Slate Ionics. 2004, 175, (1-4), 641.

5. Y. Wang, T. Mori, J. G. Li and Y. Yajima: Sci. Tech. Adv. Mater., 2003, 4, 229.

6. J. G. Li, T. Ikegami and T. Mori: Ada Mater., 2004, 52, 2221.

7. G. B. Jung, T. J. Huang, M. H. Huang and C. L. Chang: J. Mater. Sci. 2001, 36, 5839.

8. K. Yamashita, K. V. Ramanujachary and M. Greenblatt: Solid State Ionics, 1995, 81, 53.

9. Provisional Patent Application 2005901334.

N. Bugayeva*1 and J. Robinson2

1 School of Mechanical Engineering, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia

2 Advanced Nanotechnology Ltd, 108 Radium Street, Welshpool, WA 6106, Australia

* Corresponding author, email natalia @ mech.uwa.edu.au

Copyright Institute of Materials Oct 2006

(c) 2006 Materials Science and Technology; MST. Provided by ProQuest Information and Learning. All rights Reserved.




comments powered by Disqus