Three-Dimensional Oriented Aggregation to a Single-Crystalline Nanoparticle

XRD pattern and TEM image of an array of CeO2 nanocubes

As a kind of important rare earth materials, CeO2 (ceria) has a variety of significant applications, such as catalysts for generation of hydrogen, catalysts for elimination of engine exhaust, abrasive materials for chemical-mechanical planarization, fuel cells, solar cells, etc. It is known that the CeO2 (100) surface is a kind of polar surface, which is not stable. It is difficult to directly acquire this kind of polar surface in experiment and only surface restructure can result in presence of CeO2 (100) surface. These unique properties provide CeO2 (100) surface with much higher catalytic activities than (110) and (111) surfaces. A research group led by Prof. Lian Gao of Shanghai Institute of Ceramics, Chinese Academy of Sciences has obtained a kind of ceria nanocubes with six {100} surfaces. These nanocubes have sizes from 4 to 20 nm. It seems that CeO2 (100) surface can stably exist when its dimensions are decreased to the nanometer level. Furthermore, the present CeO2 (100) surface of the nanocubes seems to be still polar. The dipole-dipole interaction from this kind of polar surfaces can drive these nanocubes to self-assemble into a regular array because long-range van der Waals forces become difficult to control for NPs whose dimensions are beyond molecular length scales (d>10 nm). Therefore, {200}-perfect-oriented monolayers or thickness-controlled films composed of ceria nanocubes can also result from this method.
 
Meanwhile, they have demonstrated that the three-dimensional oriented aggregation of nanoparticles can result in the formation of a new defect-free single-crystalline nanoparticle, exampled by the synthesis of ceria nanocubes. The idea utilizes the selective adsorption/desorption equilibrium of surfactant molecules on the surfaces of the nanocrystals. First, the adsorbed surfactant molecules on the surface of primary nanoparticles drive the self-assembly of particles. Second, the capillarity between nanoparticles, which is caused by the aggregation of nanoparticles, promotes the desorption of surfactant molecules from the surface of nanoparticles. Third, the oriented aggregation growth occurs and the primary nanoparticles “fuse” into one new single-crystalline nanoparticle. Because the three-dimensional aggregation of nanoparticles can enclose the hetero element or nanoparticles into the subsequent crystal growth process, the present process can be a new method for the doping of nanocrystal and the preparation of heteronanostructures. This idea is important because the smaller the size of a nanocrystal, the more difficult doping process becomes.
 
The research work has been published in the Journal of the American Chemical Society (Vol.128,p.9330, DOI: 10.1021/ja063359h). The significance of the research work was highly valued by the journal’s referees. “This oriented nanostructure is very important both in the understanding of nanomaterial crystallography, and in the exploration of nanocrystal applications in catalysis, magnetism and nanoelctronics,” they said.