Effects of alkaline media on the controlled large mesopore size distribution of bimodal porous silicas via sol-gel methods

A series of bimodal mesoporous silicas (BMMs) have been synthesized under different aqueous alkaline conditions, including sodium hydroxide (NaOH), ethylenediamine (EDA) as well as dipotassium phosphate (DKP). Their morphologies and textural properties were fully characterized by X-ray diffraction (XRD), scanning and transmission electron microscope (SEM/TEM), N2 adsorption/desorption isotherms, and Fourier transform infrared (FT-IR) spectroscopy. The results showed that a weak alkaline system was beneficial to obtain a low order degree of the mesopore structure with uniform spherical morphology, whereas a strong alkaline medium was favorable to achieve a high order degree of the mesopore structure with relatively larger but irregular nanoparticles. During the sol–gel process with combination of the template synthesis technique, the use of a strong base may accelerate the silicate condensation with irregular morphologies as a result. Also, it may accelerate the growth of the micellar (cetyltrimethyl ammonium bromide) phase with ordered arrangement. Therefore, the effects of different alkaline media on the mesopores and morphologies of these BMMs were remarkable. To account for the observation, a possible mechanism was proposed.