Microstructure tuning of mesoporous silica prepared by evaporation-induced self-assembly processes: interactions among solvent evaporation, micelle formation/packing and sol condensation

Abstract

By varying the vapor pressure of solvents through changing the alkyl length of aliphatic alcohols or the aging temperature, the microstructures, including mesopore ordering length, porosity, and specific surface area, of silica templates are controllable in an evaporation-induced self-assembly (EISA) process. The microstructures of various silica powders are systematically characterized and compared by the small-angle X-ray scattering (SAXS), nitrogen adsorption/desorption isotherms, as well as scanning electron microscopic (SEM) and transmission electron microscopic (TEM) images. The microstructure ordering length of silica powders is found to decrease with reduction of the solvent evaporation rate tuned by the length of alkyl groups of alcohols, which is confirmed by the almost identical microstructure of silica powders prepared by using methanol and tetrahydrofuran (THF) which exhibit similar vapor pressures. The rate of micelle formation relative to the rate of silica precursor condensation, strongly depending on the solvent evaporation rate, determines the resultant organization of micelles and may cause morphological distortion and microdomain disorientation of silica templates. This concept is confirmed by the presence of completely and partially ordered microstructures of silica powders synthesized from the ethanol- and butanol-based precursor solutions, respectively, which are dried completely at a higher temperature in the aging process. The microstructure of mesoporous silica can thus be simply tuned by varying solvents and aging temperatures in the EISA process, which are promising for many applications.