Communications Formation of Hollow Helicoids in Mesoporous Silica: Supramolecular Origami** By San Ming Yang, Igor Sokolov , Neil Coombs , Charles T. Kresge, and Geoffrey A. Ozin* In the past, helical shapes in nature have inspired inven- tions such as the water screw for agriculture, the retaining screw for wine presses, and architectural designs for spiral staircases. [1] Similarly, these days helix-shaped DNA, pro- teins and carbon nanotubes evoke great interest in biotech- nology and nanotechnology. [2±4] Also biomimetic synthesis of helical morphologies of calcium carbonate, barium sul- fate, and silica provides insight into morphogenesis of mineralized spiral forms in biology and ideas for new op- portunities in materials science. [5±9] Herein we describe the synthesis of hollow helicoids made of hexagonal mesopor- ous 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 micrometer- scale 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 left- and 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 su- pramolecular Origami theoretical model explains the crea- tion 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 ulti- mate 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 sil- ica curved shapes. [10] Control experiments demonstrate that a high concentration of ammonium and formate ions is es- sential 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 poly- merization-induced differential contraction of silicate mi- celle rods in a patch of silicate liquid-crystal film formed at the air±water interface becomes influential in hollow heli- coid formation. [11] Powder X-ray diffraction (PXRD) pat- terns in Figure 1 clearly define as-synthesized and calcined materials as the MCM-41 hexagonal mesoporous silica. [12] Scanning electron microscopy (SEM) images reveal morphologies consisting of extraordinary hollow helicoid shapes that resemble a screw thread, Figure 2a±c. Trans- mission electron microscopy (TEM) images, Figure 3a, on whole-mounted helicoids reveal that they are hollow with an approximately 1 mm thick ªshellº. They are composed of hexagonally close-packed ca. 5 nm diameter channels which have an ca. 1 nm silica wall and appear to spiral around the major axis of the helicoid. The dimensions of a collection of hollow helicoids have been analyzed by measurements of SEM images. It is strik- ing that diagnostic dimensions of the screw, namely pitch width and angle, flute width, and inside and outside diam- eter, are found to span a rather narrow range. Notably, the pitch angles center around 70 and 110, which correspond Adv. Mater. 1999, 11, No. 17 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim,1999 0935-9648/99/1712-1427 $ 17.50+.50/0 1427 Communications ± [*] Prof. G. A. Ozin, Dr. S. M. Yang,Dr. I. Sokolov,Dr. N. Coombs Materials Chemistry Research Group Lash Miller Chemical Laboratories University of Toronto 80 St. George Street, Ontario M5S 3H6 (Canada) Dr. C. T. Kresge Mobil Technology Company Strategic Research Center Paulsboro, NJ 08066-0480 (USA) [**] GAO is deeply indebted to the Canada Council for the award of an Isaac Walton Killam Foundation Research Fellowship (1995±97). SMY is grateful to the Croucher Foundation, Hong Kong, for the award of a PDF in support of his research while at the University of Toronto. Financial support of this work from Mobil Technology Com- pany is deeply appreciated. Fig. 1. Representative PXRD pattern for the a) as-synthesized and b) cal- cined hollow mesoporous silica morphologies.