Colloidal Ceria Nanocrystals: A Tailor‐Made Crystal Morphology in Supercritical Water

Abstract

The synthesis of colloidal metal oxide nanocrystals with controlled shape is of fundamental and technological interest because in this way it is possible to tune their shape-dependent physical properties and thus consolidate their promising applications in optics, catalysis, biosensing, and data storage. Recently, the organic-solution phase [1][2][3] and liquid-solid-solution phase synthetic transfer routes [4] have been demonstrated to be versatile pathways toward such shape-controlled metal oxide nanocrystals. In all of these methods, organic surfactants play a key role in determining the growth and stability of nanocrystals. Combining this concept and the properties of supercritical water (SCW) will lead to a novel approach for the synthesis of metal oxide nanocrystals. For example, SCW is chemically stable and processing with it is environmentally benign; [5] it acts as a unique medium to aid in the spontaneous nucleation and crystallization of metal oxide nanoparticles; [6] and by using organic ligand molecules that are miscible with SCW, crystal growth can be limited and agglomeration can be inhibited in favor of small, well-dispersed particles. [7,8] As a well-known metal oxide, and because of its novel properties, ceria (CeO 2 ) has been extensively applied in catalysis, electrochemistry, and optics. For example, ceria nanocrystals have high oxygen-storage capability and act as an important component in three-way catalytic converters to clean up automotive exhausts.[9] To elevate catalytic activity, it is desirable to prepare ceria samples with a high surface area. So far, ceria nanocrystals with spherical, wire, rod, and tadpole shapes have been synthesized. [10] Along with this research, there is another recent trend aimed at tuning the ceria-crystal shape in order to expose reactive crystal planes for high reactivity.[11]Sayle et al. predicted in their theoretical study that the (100) surface is more reactive than (110) or (111) for the CeO 2 / YSZ(110) system (where YSZ is yttria-stabilized zirconia).[11a]Yan and co-workers synthesized ceria nanoparticles with various shapes using a hydrothermal method, and experimentally observed that cubic particles (ca. 36 nm) with exposed {100} crystal planes showed the highest oxygen-storage capacity.[11d]Despite these recent advances, it is still a great challenge to synthesize ceria nanocrystals of high quality in terms of uniform size, well-defined crystal shape, and ease of fabrication.Here, we report a simple and rapid approach for producing colloidal ceria nanocrystals on scales of less than 10 nm, with tailor-made ceria crystal planes, using organic-ligand-assisted supercritical water as the medium. The synthetic strategy is depicted in Figure 1. This strategy depends on: 1) sub-decananometer single-crystal formation in a supercritical hydrothermal process; [6] 2) the miscibility of the organic ligand molecules with high-temperature water, which is due to the lower dielectric constant of the water; [12] and 3) controlled nanocrystal growth from the ...

Synthetic versatility of nanoparticles: A new, rapid, one-pot, single-step synthetic approach to spherical mesoporous (metal) oxide nanoparticles using supercritical alcohols

“…However, these techniques generally require long reaction times (several hours) and complex multistep operations [1-43, 47, 48]. Recently, supercritical fluids have been applied in the synthesis of size-and morphology-controlled inorganic-organic hybrid metal and MO NPs [49][50][51][52][53][54][55][56][57][58][59][60][61]. All of these reported NP syntheses occur rapidly in supercritical fluids (several seconds to several minutes) [56][57][58][59][60][61].…”

Versatility of one-pot, single-step synthetic approach for spherical porous (metal) oxide nanoparticles using supercritical alcohols

“…Recently, sizeand morphology-controlled inorganic-organic hybrid metal and metal oxide nanoparticles have been synthesized in supercritical fluids. Supercritical ethanol (scEtOH) yields monodisperse metal nanoparticles [29], supercritical methanol (scMeOH) [30,31] or supercritical water (SCW) [32] yields zinc oxide (ZnO) nanoparticles, while SCW alone yields CeO 2 [33], TiO 2 [34], and magnetite (Fe 3 O 4 ) [35] nanoparticles. All of these reactions proceed in a short reaction time (several seconds to several minutes) [36][37][38][39][40][41].…”

Highly Concentrated Colloidal Dispersion of Decanoic Acid Self-assembled Monolayer-protected CeO2 Nanoparticles Dispersed to a Concentration of up to 77 wt % in an Organic Solvent

“…8 The decanoic acid-modified CeO 2 NPs were synthesized by a supercritical hydrothermal method described in the literature. 7,10,11 The procedure was exactly the same as that shown in ref. 7.…”