Functional metal oxides with nanopores are promising for applications such as catalysts, sensors, and capacitors [1]. Although nanoporous metal oxides with various compositions and pore structures have been reported, the control of the crystallinity remains an unexplored area. Typically, the pore walls of nanoporous metal oxides are composed of aggregates of nanocrystals. If we can synthesize "single crystalline nanoporous metal oxides", the effect of grain boundaries can be reduced and properties such as electrical [2] and thermal conductivities can be greatly improved. Because the synthesis of such materials is quite difficult, novel synthesis methods are strongly demanded.
The preparation methods of “single crystalline nanoporous metal oxides” will be reported here. We have successfully prepared nanoporous indium tin oxide (ITO) and layered LiCoO₂, which are expected to exhibit promising oxide-based thermoelectric materials by using silica colloidal crystals as a template.
Single crystalline nanoporous ITO was prepared by crystal growth of ITO in the template by thermal decomposition of an appropriately selected precursor. The single crystalline nanoporous ITO showed higher electrical conductivity, if compared with those of porous ITO with small crystallites, while there was only a slight difference in the thermal conductivities.
The synthesis of nanostructured layered LiCoO₂ is known to be difficult, and there is only one example of the synthesis of macroporous layered LiCoO₂ composed of nanocrystals to the best of our knowledge [3]. The single crystalline nanoporous layered LiCoO₂ was synthesized by dividing the synthesis procedure into several steps to prevent the nanostructural collapse.
We thank JSPS for Grant-in-Aid for Scientific Research (A) (No. 19H00833) and JSPS Fellow (T.M. No. 19J13784).
[1] Y. Ren et al., Chem. Soc. Rev. 2012, 41, 4909.
[2] E. J. W. Crossland et al., Nature 2013, 495, 215.
[3] J. Peng et al., J. Solid State Electrochem. 2012, 16, 3079.

Takamichi Matsuno

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