Formation of Hollow Helicoids in Mesoporous Silica: Supramolecular Origami

Abstract

In the past, helical shapes in nature have inspired inventions such as the water screw for agriculture, the retaining screw for wine presses, and architectural designs for spiral staircases. Similarly, these days helix-shaped DNA, proteins and carbon nanotubes evoke great interest in biotechnology and nanotechnology. Also biomimetic synthesis of helical morphologies of calcium carbonate, barium sulfate, and silica provides insight into morphogenesis of mineralized spiral forms in biology and ideas for new opportunities in materials science. Herein we describe the synthesis of hollow helicoids made of hexagonal mesoporous silica, a remarkable topology in the materials world. They have a hierarchical architecture comprised of 5 nm diameter channels that coil in the form of a micrometerscale tubular spiral. A population analysis of helicoid shapes defines a surprisingly narrow distribution of pitch and flute widths, pitch angles, inside and outside diameters, and significantly an equal number of leftand right-handed forms. Evidence is presented that morphogenesis involves polymerization-induced differential contraction of a patch of hexagonal silicate liquid-crystal film formed at the air± water interface, which can fold into a hollow helicoid. A supramolecular Origami theoretical model explains the creation and observed narrow distribution of mesoporous silica, hollow helicoid shapes. Mesoporous silica hollow helicoids were prepared by using cetyltrimethylammonium chloride (CTACl) as the surfactant micellar template and tetraethylorthosilicate (TEOS) as the silica precursor. An aqueous solution of CTACl, hydrochloric acid and formamide was aged for 48 h before adding TEOS, and the material was formed after 3 days in a quiescent state. The use of formamide in the synthesis is intentional because upon acid hydrolysis it yields ammonium chloride and formic acid to give an ultimate solution ca. pH 1.9 and an ionic strength that favors hollow helicoid formation. This solution pH is notably higher than the one used in the synthesis of mesoporous silica curved shapes. Control experiments demonstrate that a high concentration of ammonium and formate ions is essential for the formation of mesoporous silica at a pH close to two, which borders on the isoelectric point of aqueous silica. We believe that a low acidity and high ionic strength medium favor a slow rate of silicification, and hence polymerization-induced differential contraction of silicate micelle rods in a patch of silicate liquid-crystal film formed at the air±water interface becomes influential in hollow helicoid formation. Powder X-ray diffraction (PXRD) patterns in Figure 1 clearly define as-synthesized and calcined