Design strategies for ceria nanomaterials: untangling key mechanistic concepts

The morphologies of ceria nanocrystals play an essential role in determining their redox and catalytic performances in many applications, yet the effects of synthesis variables on the formation of ceria nanoparticles of different morphologies and their related growth mechanisms have not been systematised. The design of these morphologies is underpinned by a range of fundamental parameters, including crystallography, optical mineralogy, the stabilities of exposed crystallographic planes, CeO_2-x stoichiometry, phase equilibria, thermodynamics, defect equilibria, and the crystal growth mechanisms. These features are formalised and the key analytical methods used for analysing defects, particularly the critical oxygen vacancies, are surveyed, with the aim of providing a source of design parameters for the synthesis of nanocrystals, specifically CeO_2-x. However, the most important aspect in the design of CeO_2-x nanocrystals is an understanding of the roles of the main variables used for synthesis. While there is a substantial body of data on CeO_2-x morphologies fabricated using low cerium concentrations ([Ce]) under different experimental conditions, the present work fully maps the effects of the relevant variables on the resultant CeO_2-x morphologies in terms of the commonly used raw materials [Ce] (and [NO₃^-] in Ce(NO₃)₃·6H₂O) as feedstock, [NaOH] as precipitating agent, temperature, and time (as well as the complementary vapour pressure). Through the combination of consideration of the published literature and the generation of key experimental data to fill in the gaps, a complete mechanistic description of the development of the main CeO_2-x morphologies is illustrated. Further, the mechanisms of the conversion of nanochains into the two variants of nanorods, square and hexagonal, have been elucidated through crystallographic reasoning. Other key conclusions for the crystal growth process are the critical roles of (1) the formation of Ce(OH)₄ crystallite nanochains as the precursors of nanorods and (2) the disassembly of the nanorods into Ce(OH)₄ crystallites and NO₃^--assisted reassembly into nanocubes (and nanospheres) as an unrecognised intermediate stage of crystal growth.