A Study of the Structure of CeO 2 Nanorods
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A Study of the Structure of CeO2 Nanorods Natalia Bugayeva School of Mechanical Engineering, The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia ABSTRACT Hydrated CeO2 nanoparticles of rod-like morphology have been obtained via a two-stage chemical precipitation method. The structure of the particles has been investigated using high resolution imaging techniques with a transmission electron microscope. The particles are found to exhibit five-fold symmetry and multiple twinning around {111} crystallographic planes. A structural model for the rod-like morphology is proposed. The mechanism of formation of such a structure is likely to depend on the unique chemistry of cerium and the preparation route.
INTRODUCTION CeO2-based materials have attracted considerable attention in recent years, particularly for applications such as environmental catalysts, where ceria has shown great potential due to its unique redox properties and high oxygen storage capacity. Doped ceria is also a promising material for use in a solid oxide fuel cell due to its high ionic conductivity. The oxygen storage capacity, redox activity and ionic conductivity of ceria are likely to be dependent on the morphology, surface area, lattice structure and defects. Considerable effort has therefore been made to synthesize high surface area ceria particles with unique morphologies and uniform sizes. For example, a needle-like morphology has been produced via a sol-gel method, though no study of a structure of the particles has been conducted [1]. The synthesis of CeO2 nanowires with diameter 70 nm has been reported [2]. The preparation method utilized a sol-gel process within porous anodic alumina templates at elevated temperature. The results of many studies indicate that the structure of ceria plays an important role in determining its oxygen storage/transport capacity. For example, results obtained for ceria films CeO2 (100) and CeO2 (111) indicate that oxygen is not easily removed from these surfaces [3]. However, it is believed that oxygen may be removed much more easily from high-index surfaces or from highly defective surfaces. It is possible that oxygen diffusion could be anisotropic. Thus the crystallographic orientation and internal structure may affect the rate of oxygen transport to the surface. Internal structure of nanoparticles as complex as multiply twinned particles (MTPs) has been found in a number of materials produced by different methods [4-8]. Multiple twinning commonly occurs in an fcc crystal lattice on {111} crystal planes. Basically, MTPs can be described as a collection of single crystal tetrahedra arranged in a certain order. Rod-like morphologies of particles are in some cases explained as large truncated decahedra [8], pentagonal prisms [7] or as two icosahedra in a twin orientation [9]. Mechanisms of the formation of MTPs are often described in terms of nucleation and layer-by-layer growth [10] or successive growth twinning [11], though this is still a question of debate.
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