Synthesis of Anisotropic Metal Oxide Nanoparticles via Non‑Aqueous and Non-Hydrolytic Routes

Due to their low cost, high stability and low toxicity, metal oxide nanomaterials are widely used for applications in various fields such as electronics, cosmetics and photocatalysis. There is an increasing demand thereby for nanoparticles with highly defined properties, in particular a narrow particle size distribution and a well-defined morphology. Such products can be obtained under high control via bottom-up synthesis approaches. Although aqueous processes are largely found in literature, they often lead to particles with low crystallinity and broad size distribution. Thus, there has been a growing trend towards the use of non-aqueous and non-hydrolytic synthesis routes. Through variation of the reaction medium and the use of adequate additives, such non-aqueous systems can be tuned to adapt the product properties, and especially to yield anisotropic nanoparticles with peculiar shapes and even complex architectures. Anisotropic particle growth enables the exposure of specific facets of the oxide nanocrystal, leading to extraordinary properties such as enhanced catalytic activity. Thus, there is an increasing demand for anisotropic nanoparticles with tailored morphologies. In this review, the non-aqueous and non-hydrolytic synthesis of anisotropic metal oxide nanoparticles is presented, with a particular focus on the different parameters resulting in anisotropic growth to enable the rational design of specific morphologies. Furthermore, secondary phenomena occurring during anisotropic particle growth, such as oriented attachment mechanisms, will be discussed.