Control of Inorganic Layer Thickness in Self-Assembled Iron Oxide/Surfactant Composites Sarah H. Tolbert, ²,‡ Peter Sieger, ² Galen D. Stucky,* ,² Sheila M. J. Aubin, § Chi-Cheng Wu, § and David N. Hendrickson* ,§ Contribution from the Departments of Chemistry, UniVersity of California, Santa Barbara, California 93106-9510, and UniVersity of California, San Diego, La Jolla, California 92093 ReceiVed March 4, 1997 X Abstract: The synthesis and characterization of ordered, lamellar, iron oxide/surfactant composites in which the iron oxide layer thickness is selectively varied are presented. These new materials are prepared by the controlled precipitation and hydrolysis of aqueous iron cations into self-assembled iron/surfactant arrays. The use of redox chemistry to alter the solubility of iron oxide and thus control hydrolysis, solubility, and inorganic layer thickness is a key feature of the synthesis procedure. The composites show a layered structure as determined by X-ray diffraction and can be produced with approximately 3 to nearly 20 Å of iron oxide in alternation with surfactant bilayers. For samples with 10 Å or thicker iron oxide layers, magnetic susceptibility measurements and Mo ¨ssbauer spectroscopy indicate the presence of superantiferromagnetic domain structures with smaller domains observed in samples with thinner layers. The results are a first step toward the simple design of hierarchical nanostructured magnetic materials using cooperative, three-dimensional inorganic/organic self-organization. I. Introduction Recent developments in the synthesis of self-assembled inorganic/surfactant composites have opened up a new field in the study of composite materials. These compounds, exempli- fied by Mobil Corp.’s M41S silica-based composites, 1 tend to show periodicity on the 2-10 nm scale. 1,2 This size range is unique in that many compounds exhibit properties that are intermediate between those observed in isolated molecules and those seen in bulk solids. In addition, the formation of supramolecular assemblies with meso-scale periodicity is an important step in the quest to control periodicity on length scales between atomic and macroscopic dimensions. 3 Recently, the original surfactant/aluminosilicate composites 1,4 have been extended to a variety of transition metal oxides, 5-7 with potential catalytic applications, as well as to multiple combinations of metal oxides and silica. 8-11 The production of surfactant/ inorganic composite materials with size-controlled optical properties has also been achieved. 12 Here we present a new area: inorganic/surfactant composites with size-dependent magnetic properties. In this paper, the synthesis and characterization of layered iron oxide/surfactant composites are described. Iron oxide was chosen as the inorganic component for a number of reasons. In the first place, iron oxide is the only pure metal oxide to show strong ferrimagnetism at room temperature. While the majority of iron oxide and oxy-hydroxide phases are not ferro- or ferrimagnetic at room temperature, there is some possibility of accessing these potentially important phases. 13 Second, iron shows a range of chemical behavior in aqueous solution. 14,15 This diversity allows us to chose conditions which will favor specific products. Finally, aqueous iron has an easily accessible redox equilibrium: Fe(II) S Fe(III). 14 This feature is key to the synthesis scheme presented below as it allows the solution phase behavior of iron ions to be altered in a dramatic and well- defined way. The above ideas were combined to produce lamellar iron oxide/surfactant composites consisting of surfactant layers, alternating with approximately one, two, three, or six layers of iron oxide. The layer thickness is controlled by the differential solubility of Fe(II) and Fe(III) in aqueous solutions coupled with our ability to chemically convert between these species. The composites were characterized structurally by powder X-ray diffraction, compositionally by elemental analysis, and chemi- cally (oxidation state) by Mo ¨ssbauer spectroscopy. Both ² University of California, Santa Barbara. ‡ Permanent address: Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095-1569. § University of California, San Diego. X Abstract published in AdVance ACS Abstracts, August 1, 1997. (1) Kresge, C. T.; Leonowitz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. 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Soc. 1997, 119, 8652-8661 S0002-7863(97)00695-1 CCC: $14.00 © 1997 American Chemical Society