Single Crystal ZrO2 Nanosheets Formed by Thermal Transformation for Solid Oxide Fuel Cells and Oxygen Sensors

Monoclinic ZrO2 with a nanosheet structure has been successfully synthesized via the thermal trans- formation of NH4Zr2F9 using a high-temperature ionothermal synthesis. The samples were characterized by using X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and conductivity measurements with a multiprobe. The result indicates that the as-prepared samples have a single crystal sheet structure with a thickness of 4 nm. To the best of our knowledge, a thin film of single crystal monoclinic ZrO2 has not been reported, even though ZrO2 is an important material. In the absence of the ionic liquid (butyl-2,3-dimethylimidazolium tetrafluoroborate), the thermal transformation of NH4Zr2F9 produced porosity within the two-dimensional structure, whereas in the presence of the ionic liquid, the homogeneous sheet structure was maintained without porosity. The decomposition temperature of the ionic liquid and transformation temperature of NH4Zr2F9 were analyzed to investigate the thermal transformation process; the thermal transformation was completed before the combustion of the ionic liquid. The current vs voltage curve of a ZrO2 nanosheet indicates high resistivity and breakdown voltage (ca. 20 V), which are attributable to the single crystal structure without grain boundaries. The ZrO2 is utilized for solid oxide fuel cell and oxygen sensor; therefore, the single crystal nanosheet with high resistivity is expected to be applied to these fields. CeO2 single crystallized nanosheets were also obtained by the proposed thermal transformation method in an ionic liquid. The synthetic routes to obtain nanosheets are limited; however, the proposed method has the potential to expand the number of routes and enable the synthesis of nanosheet materials.