Keywords: nonaqueous assembly, nonaqueous synthesis, solvothermal synthesis, metal oxides, nanoparticles, formation mechanisms, nanocrystals, surfactant-free processes, nanowires, mesoporous materials, nanoparticle formation, gas sensors, security, nanotechnology
Nonaqueous synthesis, assembly and formation mechanisms of metal oxide nanocrystals
Nonaqueous solution routes to metal oxide nanoparticles are a valuable alternative to the well-known aqueous sol-gel processes, offering advantages such as high crystallinity at low temperatures, robust synthesis parameters and avoidance of surfactants in order to control the crystal growth. In the first part of this paper we give an overview of the various solution routes to metal oxides in organic solvents, with a strong focus on surfactant-free processes developed in our group. In most of these synthesis approaches, the organic solvent plays the role of the reactant that provides the oxygen for the metal oxide, controls the crystal growth, influences particle shape and, in some cases, also determines the assembly behaviour. In general, these routes involve the reaction of metal oxide precursors such as metal halides, alkoxides, or acetylacetonates with benzyl alcohol, benzylamine or carbonyl compounds like ketones and aldehydes. Whereas the reaction between metal halides and benzyl alcohol enables the direct synthesis of crystalline nanoparticles via simple beaker chemistry, the other reaction systems require a solvothermal treatment at temperatures between 200C and 250C. The metal halidebenzyl alcohol system additionally allows for an insitu functionalisation process, where the surface of the nanoparticles can be modified during nanoparticle synthesis in order to tailor the solubility as well as the assembly behaviour. This is an important step towards the use of metal oxides as nanobuilding blocks for the fabrication of structures like nanowires or mesoporous materials. In the second part, various reaction pathways to nanoparticle formation are discussed. Solvothermal processes are not easy to monitor insitu. In order to elucidate possible formation mechanisms, we analyse the reaction solution obtained after synthesis of the nanoparticles as well as after reference experiments with altered reaction conditions. The organic species found in the mixtures allow us to propose possible formation mechanisms. As an important example, we will discuss the formation mechanism of ceria nanoparticles synthesised from cerium(III) isopropoxide and benzyl alcohol, involving a C-C bond formation between the isopropoxy ligand and benzyl alcohol. This reaction pathway was also found to lead to the formation of BaTiO3 nanoparticles. The last part of this paper deals with possible applications of metal oxide nanoparticles, especially with regard to gas sensing devices.