Engineering of Silica Thin-Film Nanoporosity via Alkali-Ion-Assisted Reconstruction

We report a method to prepare patterned mesoporous silica-based films with fine control of the pore sizes and the porous fraction by combining the sol–gel approach with an alkali ion post-treatment. The strategy involves the porous mesostructure reconstruction of surfactant (cetyltrimethylammonium bromide or F127)-templated silica films by alkali metal ion diffusion into the inorganic matrix at 450 °C. Coatings with pores ranged from 2 to 100 nm; gradients of pore sizes and porous volumes as well as two-dimensional patterned structures were obtained by optimizing the alkali-ion-assisted reconstruction process (alkali ion amount, liquid deposition techniques, etc.). Na⁺, Li⁺, and K⁺ are demonstrated as efficient silica network modifiers to significantly and rapidly tailor the mesostructure of the films. Using spectroscopic ellipsometry, scanning electron microscopy, environmental ellipsometry porosimetry, atomic absorption spectroscopy, and phase diagram simulations, we identified two different alkali-ion-induced mechanisms: the densification of the silica framework and the separation of the silica matrix into two phases. Both mechanisms are easily controlled separately by adjusting the process conditions and the kind of ions used. This study, with the demonstration of an adaptive multiresponsive antireflective coating, shows that alkali-ion-assisted pore restructuration is complementary to the existing sol–gel techniques and is promising for the design of new functional smart materials.