Single-Crystalline Ordered Mesoporous Indium Tin Oxides with Controlled Sn/In Ratio via Vapor-Phase Oxidation of Metal Chlorides within Silica Colloidal Crystals

Abstract

The properties of nanoporous metal oxides depend on their pore structure and the internal structure of the pore walls, and precise control of such factors is crucial. In particular, single-crystalline nanoporous metal oxides, in which the pore walls consist of a single crystal, exhibit characteristics of both single crystals and nanoporous materials that are distinct from conventional polycrystalline materials. Herein, we report the synthesis of single-crystalline mesoporous indium tin oxide (ITO) with a pore diameter of ~30 nm and a controlled Sn/In ratio. The synthesis was achieved using regularly arranged silica nanospheres (silica colloidal crystals) as a template, followed by crystal growth of ITO within the template by oxidation of metal chloride precursors. By increasing the oxygen-to-metal chloride ratio, crystal growth outside the template was suppressed and the volatilization of Sn species was reduced, thereby enabling an increase in the Sn/In molar ratio to 0.10, which is critical for electrical conductivity.