Synthesis and Oxygen Storage Capacity of Two‐Dimensional Ceria Nanocrystals

Shape-controlled synthesis of inorganic nanomaterials has received great attention due to their unique shapedependent properties and their various applications in catalysis, electronics, magnetics, optics, and biomedicine. Among these nanomaterials, ultrathin twodimensional (2D) anisotropic nanomaterials are especially attractive due to their high surface-to-volume ratio and potential quantum size effects. A variety of approaches have been developed to prepare such nanomaterials. Typical methods include vapor deposition, templated synthesis, electrochemical deposition, sol–gel processing, and solvothermal/hydrothermal treatments. Solution-phase chemical synthesis has proven particularly effective in controlling the size and morphology of the nanomaterials. Ceria has been widely used in catalysis, optics, sensors, and solid oxide fuel cells. Due to its high oxygen storage capacity (OSC), which originates from easy conversion between CeO2 and CeO2 x, ceria has found its primary utilization in catalysis as an oxygen carrier. Ceria nanomaterials with various morphologies, mainly polyhedra, have been reported. Recently, 1D ceria nanostructures, such as nanowires, have also been reported. However, with the exception of one report on the preparation of nanosheets, well-controlled 2D ceria nanomaterials have not been explored and the comparison of the OSC properties between 3D and 2D structures has not been possible. On the other hand, the different properties of the (100), (110), and (111) ceria facets has been debated. There is no consensus on whether crystallographic orientation or particle size affects reactivities. Therefore, high-quality ceria nanocrystals selectively exposing different low Miller-index surfaces, are crucial to enabling experiments that resolve the controversy. Here we report a simple, robust solution-phase synthesis of ultrathin ceria nanoplates in the presence of mineralizers. The morphology of nanoplates can be easily controlled by changing reaction parameters, such as precursor ratio, reaction time, etc. In addition, we also prepare ceria nanomaterials in various 3D morphologies by hydrothermal and combustion methods. The OSC of our 2D ceria materials have been tested and compared to the OSC of their 3D counterparts. In brief, the synthesis of ceria nanoplates involves the thermal decomposition of cerium acetate at 320–330 8C in the presence of oleic acid and oleylamine as stabilizers and employs sodium diphosphate or sodium oleate as mineralizers. Transmission electron microscopy (TEM) images of ceria nanoplates are shown in Figure 1. Square ceria nanoplates (S-nanoplates, Figure 1a) with an edge length of 11.9 nm (s= 7%), are synthesized with sodium diphosphate as the mineralizer while elongated ceria nanoplates (Lnanoplates, Figure 1e) with a length of 151.6 nm (s= 9%) and a width of 14.3 nm (s= 12%), are produced with sodium oleate as the mineralizer. The nanoplates in both samples have a thickness of about 2 nm. As shown in Figure 1c and g, the stacks of nanoplates confirm that the sample consists of 2D plates rather than 3D cubes or rods. S-nanoplates readily form the demonstrated stacking arrays as seen in drop-cast TEM samples. L-nanoplates only form stacks by a selfassembly at a liquid–liquid (e.g. hexane–ethylene glycol) interface. The S-nanoplates also self-assemble to a ceria nanosheet at a hexane–acetonitrile interface, as shown in Figure 3a. High-resolution TEM (HRTEM) images of both nanoplates (Figures 1d,h and S1c in the Supporting Information) reveal an interplanar distance of 0.27 nm, consistent with the (200) lattice spacing of the ceria crystal. The fast Fourier transform (FFT) patterns confirm the {100} textures of ceria nanoplates. Plates (e.g. square plates) could be enclosed by either six (100) facets or a combination of two (100) facets and four (110) facets. As illustrated in Figure S1, our HRTEM images and simulations of HRTEM images suggest that our ceria nanoplates are enclosed by six (100) [*] D. Y. Wang, Y. J. Kang, Prof. C. B. Murray Department of Chemistry University of Pennsylvania, Philadelphia, PA 19104 (USA) E-mail: cbmurray@sas.upenn.edu