A Nanosheet‐Structured Three‐Dimensional Macroporous Material with High Ionic Conductivity Synthesized Using Glucose as a Transforming Template

The advancements of both top-down and bottom-up synthesis strategies have yielded very interesting findings towards the synthesis of nanosheets. The preparation of nanosheets with top-down methods typically involves the exfoliation of layerstructured materials. Examples include the nanosheet synthesis of transition-metal dichalcogenides (TMDs), transition-metal oxides (TMOs), and compounds such as BN, Bi2Te3, and Bi2Se3. [1a,2] As the parent layered compound is the essential requirement for exfoliations, nanosheet synthesis for the materials without layered structures have been excluded from the top-down method. On the other hand, the bottomup method also presents difficulties in the synthesis of nanosheet materials with structures other than layered compounds, such as cubic structures, because of the lack of intrinsic driving force for two-dimensional (2D) growth. Furthermore, the existing methods are not readily scaled up for large quantity preparations. Clearly, a method that can produce large scale nanosheets for a wide variety of materials is of particular significance. Ceria has found application in catalysis, solid oxide fuel cells, and oxygen sensors. In particular, when ceria is doped with other rare-earth ions, it can significantly increase the oxygen vacancies and ionic conductivities. Zeroand onedimensional ceria-based nanocrystals have been investigated using different approaches. However, it is difficult to achieve 2D structured ceria-based materials owing to their face-centered cubic structure (Figure 1a). Glucose has been used as a source for carbon materials, as it is readily available, environmentally benign, and easy to control. Upon heating in air, glucose undergoes successive reactions, that is, polymerization, carbonization, and combustion. Inspired by this feature, a novel simple one-pot glucose combustion route was devised to synthesize inorganic compounds with Sm0.2Ce0.8O1.9 (SDC) as an example. The glucose acts as both the fuel and a transforming template, leading to the formation of three-dimensional (3D) macroporous SDC with a 2D nanosheet microstructure. To our best knowledge, this microstructure was achieved for the first time. The SDC sample prepared through the combustion process is shown in Figure 1b; free-flowing yellow powders were obtained. The filled density of the SDC powders was as low as 0.020 gcm , which is only 0.28% of the theoretical density (7.15 gcm ) of the bulk SDC. Equivalently, the apparent volume of this SDC powder is about 357 times that of the same amount of bulk material. In general, materials produced through wet chemistry methods are dispersed in solvents; therefore, complicated separation processes are needed to obtain the solid materials, and agglomerations may occur. The combustion method shown herein offers a simple way to produce porous solid powders directly in large scales. The powder X-ray diffraction (XRD) pattern (Figure 1c) confirmed that the synthesized SDC sample exhibits the single-phase cubic fluorite structure (Fm3m ; JCPDS card no. 34-394). The strong peak intensity indicates that the sample Figure 1. a) The unit cell of ceria with a fluorite structure; b) porous SDC powders obtained by the glucose combustion method; c) the XRD pattern of the sample; d), e) SEM images of the sample; f) a nitrogen adsorption/desorption isotherm of the sample.