Hollow structures possess a confinement function, which enables the handling and use of functional liquids. In this study, we focus on the confinement of water in the inner spaces of hollow SiO2 nanoparticles (HSNPs). Water is introduced into the hollow spaces passing through the mesopores of the shells. The introduced water is then confined by thickening the shell through a post-treatment using silicon alkoxides. HSNPs with a variety of shell thicknesses are prepared by tuning the synthetic conditions of the reaction media and the alkoxides. It is found that the amount of water retained in a hollow space is increased with shell thickness. The resultant water–HSNP nanocomposites exhibit effective far-infrared absorption. The facile confinement approach reported in this study will lead to highly functional complex nanocomposites, such as isolated metal nanoparticles–water–silica.

The present work is partially supported by JSPS KAKENHI Grant Number JP20K15368, MEXT Leading Initiative for Excellent Young Researchers, the Foundation for the Promotion of Ion Engineering, and the Izumi Science and Technology Foundation (2019-J-112).