Synthesis and characterization of CeO₂ nanoparticles by low temperature hydrothemal and solvent thermal process

Abstract

Ceria has been aggressively explored for applications as a fuel cell electrolyte or in catalytic converter due to its high oxygen ion conductivity, or as a UV absorption material. It is proven that the properties and applications of ceria nanoparticles are related to their morphologies and sizes. This ability to control the shape and morphology of CeO₂ nanoparticles allows the corresponding tuning of their chemical and physical properties. Most of the applications require the use of non-agglomerated nanoparticles, as aggregated nano-particles lead to inhomogeneous mixing, poor sinterability and compromised properties. However, nano-crystals with a primary particle size < 5 nm have a strong tendency to agglomerate. In this work, nano-crystalline particles of CeO₂ have been synthesized by a low temperature hydrothermal and solvent thermal synthesis process. Using the precursors of Ce(NO₃)₃.6H₂O:NaOH in different mixing ratio, using polyvinylpyrrolidone (PVP) as the surfactant, the CeO₂ particles were synthesized via 24 h hydrothermal and solvent thermal process treatment at reaction temperature of 100 ^°C and 180 ^°C using Teflon-lined hydrothermal autoclave. We have optimized the conditions for the two synthesized methods, hydrothermal and solvent thermal, to yield highly crystallized particle with controllable shape, sizes and morphology. X-ray diffraction (XRD) and high-resolution transmission electron microscope (HR-TEM) analysis were used to characterize the crystalline and morphology of the synthesized CeO₂ nanoparticles. The optimal reaction condition to prepare the CeO₂ of the desired octahedron shaped fluorite structure was established. Based on the results, the hydrothermal synthesis method yields nanocrystalline CeO₂ sizes of ∼6 nm, while the solvent synthesis method yields nanocrystalline CeO₂ sizes of 2-3 nm at the optimal conditions. The hydrothermal synthesis method produced better particles in terms of crystallinity and morphology under HR-TEM. Temperature also plays a part in crystallinity and sizes of the CeO₂ nanoparticles. The crystallinity and size of the CeO₂ nanoparticles increases when using higher treatment temperature for both hydrothermal and solvent thermal methods. The growth mechanism of the shape and morphology of the CeO₂ will also be discussed.