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 transformation 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.