Co-template method provides hierarchical mesoporous silicas with exceptionally ultra-low refractive indices

Abstract

In this study we prepared hierarchical mesoporous silicas through evaporation-induced self-assembly using the triblock copolymer poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) (F127) and the diblock copolymer poly(ethylene oxide-b-ε-caprolactone) (PEO-b-PCL) as co-templates. Small-angle X-ray scattering, transmission electron microscopy, and scanning electron microscopy revealed that these hierarchical mesoporous structures were greatly affected by the volume fraction of each component segment in the block copolymers of this co-template system. Furthermore, N₂ isotherm sorption analyses of our unique hierarchical mesoporous structures indicated that their BET surface areas reached as high as 781 m² g⁻¹, with pore volumes as high as 0.99 cm³ g⁻¹; they possessed a structure of regular spherical strip arrangements alternating with tetragonal cylinders. Using an n&k analyzer and a spin-coating method, we found that the samples displayed ultra-low refractive indices (ca. 1.08), suggesting that such hierarchical mesoporous silicas have potential for application in various fields (e.g., as anti-reflectance films for solar cells).