Highly transparent silica aerogel thick films with hierarchical porosity from water glass via ambient pressure drying

Abstract

Highly transparent silica aerogel films of 2–3.6 μm thick and with the hierarchical porosity were deposited on fluorine doped tin oxide (FTO) conductive glass substrate via the sol–gel and ambient drying processes, by using water glass as the silicic source. An innovative aging process was developed by immersing the hydrogel film in large volume of tetraethoxysilane (TEOS)–ethanol solution. Obtained aerogel films possessed both the mesopores and the submicron pores, with the average pore size of 19.1 nm, the specific surface area of 530 m2 g−1, and the pore volume of 2.96 cm3 g−1. The leaching of silicic acid molecules from the hydrogel film into the bulk aging solution was responsible for the formation of submicron pores. The aging of the hydrogel film in TEOS–ethanol solution could greatly enhance the silica skeleton and resist the film shrinkage during the ambient drying process, and an aging time longer than 8 h was appropriate to obtain thick aerogel films without obvious structure collapse. The SiO2 content in the precursor should be kept at medium level, to avoid nonuniform film thickness or lower surface area. In addition, the aerogel film synthesized at optimal condition possessed good hydrophobicity (contact angle: 132°), high transparency (70–82% in 400–1200 nm band), and low dielectric constant (k = 1.42). The work demonstrates a facile and low-cost route toward transparent silica aerogel films with hierarchical pore structures, which can be used as the intermetal dielectrics or the template to synthesize novel functional materials requiring high transmittance, high surface area and effective mass transport.