Bottom-up synthesis of amorphous silica nanosheets

Amorphous silica nanosheets have recently emerged as a new class of two-dimensional (2D) materials owing to their unique structures and properties that differ significantly from those of bulk amorphous silica. While extensive studies have been conducted on substrate-supported amorphous silica nanosheets prepared by vapor-phase methods, the bottom-up synthesis of free-standing amorphous silica nanosheets in solution remains relatively unexplored. In this review, recent progress in the bottom-up synthesis of amorphous silica nanosheets is summarized, including hard-template approaches, interfacial templating methods, and surfactant-templating strategies. Particular attention is devoted to surfactant-directed synthesis, which has long been investigated as a route to two-dimensional silica structures. The formation mechanisms, structural characteristics, and limitations of conventional liquid-phase surfactant-templated systems are discussed. Furthermore, a recently developed solid-state surfactant templating method is introduced as a new strategy for producing free-standing amorphous silica nanosheets with molecularly controlled thickness. This method enables monolayer exfoliation, sub-nanometer-scale thickness control, and extension to organo-siloxane nanosheets with diverse compositions. The assembly of these nanosheets into densely packed ultrathin films and their potential applications are also described. Finally, current challenges and future perspectives for the synthesis, structural analysis, and functional development of amorphous silica nanosheets are discussed.