Two-dimensional (2D) anisotropic nanostructures of metal oxides and semiconductors, especially ultrathin nanosheets (thicknesses< 5 nm), have attracted tremendous attention because of their unusual properties derived from exceptionally small thickness and possible quantum size effects.[1] Typical methods for preparing nanosheets mainly involve the delamination of layer-structured materials.[2] The difficulty in solution-phase synthesis of free-standing nanosheets most likely arises from the fact that metal oxides and semiconductors with a cubic crystal structure, including ceria (CeO2), have no intrinsic driving force for 2D anisotropic growth. Although it has been shown that nanocrystals made of semiconductors, such as CdTe and CdSe, can selforganize into free-standing nanosheets in a solution phase, they remained assemblies of nanocrystals with an overall polycrystalline structure even after ageing for a long period of time (ca. one month).[3] The solution-phase synthesis of single-crystal nanosheets has met with limited success and still remains a grand challenge.

Ceria has been widely used in catalysis, solid oxide fuel cells, oxygen sensors, and chemical mechanical planarization processes.[4] Nanocrystals of ceria are of particular interest owing to an increased oxygen vacancy that promotes catalytic activity.[5] Ceria nanocrystals have been synthesized by various methods, such as solution precipitation,[6] solvothermal synthesis,[7] microemulsion,[8] microwave-assisted heating,[9] and sonochemical treatment.[10] However, the products of these syntheses have been restricted to nanocrystals with polyhedral structures. Recently, one-dimensional ceria nanostructures, such as nanowire and nanorod, have been synthesized by a nonhydrolytic sol–gel process or alcohothermal route.[11, 12] However, 2D nanosheets of ceria have not been reported and, as a result, the influence of quantum size effects on the properties of 2D ceria nanostructures remains largely unexplored.