Synthesis of ceria-based nanomaterials

2 Synthesis and Assembly of Ceiia-Related Nanomaterials 6.2.1 Synthesis of ceria-based nanomaterials 

Ceria has a cubic phase with a fluorite structure. Therefore, common as-prepared ceria nanocrystals are isotropic in shape. On the other hand, the fluorite structure has a different atomic arrangement in different facets, which leads to distinct catalytic activity. Controlling the shape of nanocrystals can be used to tune the catalydc properties of such materials. The size of nanocrystals is another important factor in determining catalytic properties because of the surface effect. Various methods have been developed to control the size and shape of ceria nanomaterials. In this section, we will describe recent advances in the synthesis of this kind of nanomaterial and present some typical examples to introduce the characteristics of the various methods. 

As described in the introduction, OD ceria nanoparticles are easy to obtain because the cubic phase structure has no special favorite growth direction. 

The common synthesis techniques of nanocrystals can be easily applied to the synthesis of ceria nanocrystals. Because of the ultra-low solubility of ceria, basic co-precipitation can be applied directly and commercial ceria nanocrystals are obtained by this simple method. An alkali solution is added to a Ce(III) or Ce(IV) solution to initiate the hydrolysis of Ce(III) and Ce(IV) ions. Ce(OH)3 can be easily oxidized to Ce(OH)4. The as-produced Ce(OH)4 quickly loses HgO and forms ceria. Because of its ultra-low solubility, ceria nanocrystals are usually small and hard to grow, which is different from the usual recrystallization in the synthesis of other kinds of nanocrystals. 

Modified co-precipitation methods are also applied in the synthesis of ceria nanocrystals. NaOH, ammonia, urea, and Na2C03 can all be used as precipitation agents. Cetyl trimethylam-monium bromide (CTAB), carboxymethylcellulose sodium, and polyvinylpyrrolidone (PVP) are used as surfactants to control the size of the nanocrystals. 

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In the past decade, controlled synthesis of ceria based nanomaterials, such as obtaining the pure phase, doping for desirable composition, controlling uniform size, shape, and nanostructure, tuning surfaces, fabricating composites, assemblies and mesostructures, have been the targets of active research since ceria-based materials exhibits unique properties when their sizes are reduced to nanometer scale. 

There has been significant progress in the nanoscale controlled synthesis of ceria-based materials in recent decades. Different shapes and sizes of ceria have been prodnced with various synthesis methods developed by many research groups. However, it is not an exaggeration to say that the chemical synthesis of ceria-based nanomaterials remains a technique rather than a science because the atomistic details for the evolution from a precursor to the final well-defined nanostructures are far from being understood. The... 

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