COMMUNICATIONS Angew. Chem. Int. Ed. 2002, 41, No. 12 ¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 1433-7851/02/4112-2151 $ 20.00+.50/0 2151 Nucleation of MCM-41 Nanoparticles by Internal Reorganization of Disordered and Nematic-Like Silica±Surfactant Clusters** Sajanikumari Sadasivan, Christabel E. Fowler, Deepa Khushalani, and Stephen Mann* The synthesis of periodic mesostructures by self-assembly and cooperative organization of inorganic and organic species in aqueous media represents a significant benchmark in modern materials chemistry. [1±5] These template-directed processes generally involve charge matching interactions between inorganic oligomers and surfactant micelles, [6±8] and have been developed to produce a large number of hybrid mesophases and mesoporous replicas with surface areas of the order of 1000 m 2 g 1 . In spite of the advances in synthesis and applications of mesoporous materials, the mechanisms that give rise to periodic ordering in self-assembled hybrid systems are not well understood. Previous studies have focused on micellar structure just prior to precipitation, [9±12] or the growth of relatively large mesostructured crystals. [13±16] Details on the nucleation of the periodic mesophase have been difficult to obtain because of the relatively short timescales involved and the transient nature of the intermediates. Herein, we employ a controlled quenching process to sufficiently increase the nucleation period, such that high-resolution transmission electron microscopy (HR-TEM) can be used to reveal, for the first time to our knowledge, the structural stages associated with the nucleation of a hexagonally ordered silica ± surfactant mesophase (MCM-41). Recently, we reported the template-directed synthesis of single-domain MCM-41 nanoparticles with dimensions usu- ally less than 150 nm by the controlled quenching of alkaline reaction mixtures containing tetraethoxysilane (TEOS) and cetyltrimethylammonium bromide (CTAB). [17] By employing a constant (40 s) dilution step, followed by variable time delays prior to arresting the reaction by instantaneous neutralization to pH 7 (see Experimental Section), we now show that it is possible to determine the structural and shape transformations of the nanoparticles during the onset and development of mesophase ordering. The earliest stage in this process was investigated by preparing samples using neutral- ization periods of less than 3 min. Small-angle X-ray diffrac- tion (SAXRD) analysis showed an absence of long-range order with a broad low-angle reflection centered at 5.32 nm, consistent with swollen micelles and a low degree of silica condensation, and thermogravimetric analysis (TGA) meas- urements indicated that the hybrid mesophase contained 25 wt% of surfactant. Corresponding TEM images showed the presence of a homogeneous population of spherical nanoparticles with a mean size of 18 nm (s  4 nm). In contrast, extending the neutralization delay to 4 min pro- duced a mixture of two types of nanoparticles, 75% of which were spherical with increased mean diameter (35 nm, s  9 nm), whereas the remaining 25% were ellipsoidal and 67 (s  16) and 50 (s  6) nm in mean length and width, respectively. Individual spherical nanoparticles up to about 50 nm in size were shown by high-resolution TEM imaging to consist of a loosely packed aggregate of disordered silica±surfactant micelles (Figure 1). The micelles were in the form of short Figure 1. Initial stage in MCM-41 nucleation showing spherical clusters of disordered silica±surfactant micelles. Arrow highlights individual micelles. Scale bar  20 nm. rods, about 9.0 and 5.0 nm in length and width, respectively, that were ™stained∫ by a 1 to 2 nm-thick surface layer of silica/ silicate (Figure 1, arrow). Large numbers of nonaggregated spherical or slightly oblate silica-coated micelles were also imaged on the support film of the TEM grid (data not shown). Although drying artefacts cannot be ruled out, this suggests that for neutralization periods of less than 4 min the reaction mixture contained a high concentration of these discrete hybrid structures. In contrast with the disordered spherical aggregates, the ellipsoidal nanoparticles showed various degrees of internal mesostructural order. Ellipsoids that were relatively small (ca. 40 nm in length) had partially ordered interiors, which consisted of short rod-shaped silica-coated micelles preferentially aligned side-on within the nanoparticle (Figure 2a). Although some end-on fusion of the rods parallel to the minor axis of the ellipsoids was apparent, the surfactant cylinders were in general too short to traverse the entire [*] Prof. S. Mann, S. Sadasivan, Dr C. E. Fowler [] School of Chemistry University of Bristol Bristol BS8 1TS (UK) Fax: ( 44)117-925-1295 E-mail: s.mann@bris.ac.uk Dr D. Khushalani School of Physical Sciences University of Kent Canterbury (UK) [  ] Current address: Forsyth Institute, 140 The Fenway, Boston MA 02115 (USA) [**] We thank the University of Bristol for the support of an ORS award to S.S., and Dr. S. A. Davis for help with electron microscopy.