The self-assembly of twinned boehmite nanosheets into porous 3D structures in ethanol–water mixtures

Abstract

Surfactant molecules have high surface activity, and can therefore influence the self-assembly of nanomaterials. The self-assembly of twinned boehmite nanosheets into porous 3D superstructures will be greatly affected by the presence of surfactant monomer and micelles in various ethanol–water mixtures. It should be possible to influence the shape, thickness, and twinning of boehmite nanosheets by varying the concentration of the surfactant and ethanol in the synthesis mixture to obtain high porosity 3D superstructures. The critical micelle concentration (CMC) of cationic surfactant cetyl trimethylammonium bromide (CTAB) was determined in 0, 12.5, 25, 37.5, and 50 vol% ethanol–water mixtures. The effect of CTAB on boehmite particle morphology and superstructure formation during self-assembly was explored at the CMC, and at 20%, 15%, 10%, 5% below and above the CMC of CTAB in various ethanol–water mixtures. Boehmite nanosheets with controllable shape and thickness were successfully formed in various ethanol–water mixtures. Prior to micelle formation, the average thickness of nanosheets formed in 0 vol%, 25 vol%, and 50 vol% ethanol–water were 200 nm, 110 nm and 100 nm, respectively. Micelle formation reduced the availability of surfactant molecules for particle templating, broadening the nanosheet thickness distribution. Micelle formation was inhibited in 50 vol% ethanol–water due to the increase in Gibbs free energy needed to form micelles relative to the Gibbs free energy of micelles in 0 vol% or 25 vol% ethanol–water. The enthalpy-driven process gives greater control over the nanosheet thickness, producing particles with narrow thickness distributions. In general, the CMC represented the point at which control is lost over the thickness of the nanoplatelets; increasing the surfactant concentration above the CMC increased the thickness of the nanoplatelets. Particle twinning during crystal growth produced an interconnected 3D network of boehmite particles with high porosity (79–88%) and hydraulic permeability (62.4–809 mD). The addition of ethanol during synthesis increased the porosity and reduced the bulk density of the 3D superstructures by 8–11% and 26–28%, respectively, and yielded a ten-fold increase in the hydraulic permeability. The integrity of the porous 3D network was maintained upon calcination, suspension in water and deposition by filtration onto cellulose filter paper.