Counterion Effects in Liquid Crystal Templating of Nanostructured CdS

There have been several investigations on the synthesis of inorganics in molecular assemblies such as reverse micelles,1-4 LB films,5 vesicles,5 and block copolymers.6 Synthetic methodologies are also known for the formation of mesoporous materials7,8 based on the coassembly of surfactants and ionic species. After calcination these lead to the formation of mesoporous oxides9-11 or nonoxides such as SnS2. Typically, such mesoporous solids have periodic nanometer scale pores, and the mechanism of their formation involves a dynamic coassembly process of electrostatic charge matching in solution between ionic surfactants and precursors to inorganic solids. In contrast, we reported recently on the synthesis of nanostructured CdS templated directly with ion-doped liquid crystals.13,14 In both cases the mesoporous solid copied the symmetry and dimensions of its precursor mesophase. In the first case the hexagonal phase of oligoethylene oxide (10) oleyl ether doped with cadmium acetate or cadmium chloride was utilized. In the other system, nanostructured particles consisting of alternating sheets of CdS and oligomeric vinyl alcohol-based amphiphile were generated. In our present study we investigated the effect of different salt counterions on the synthesis of nanostructured CdS in a hexagonal liquid crystal. The salts studied were cadmium sulfate, cadmium perchlorate, and cadmium nitrate. In all precipitations the salt concentration was 0.1 M. In the methodology used here and in our previous work, H2S gas diffuses into the gellike mesophase leading to the formation of nanostructured CdS (Scheme 1). After precipitation was complete, the resulting CdS-mesophase composite was washed repeatedly with diethyl ether/ethanol (50/50) to remove byproducts and unbound amphiphile. Most importantly, as evidenced by transmission electron microscopy, the periodic nanostructure is not disrupted by this procedure. This is in contrast to the synthesis of nanosized semiconductors in phaseseparated copolymers,6 which would not exist in ordered arrays after the block copolymer template is removed. The characterization techniques included optical microscopy, X-ray diffraction, transmission and scanning electron microscopies, and elemental analysis. All ion-doped mesophases showed similar birefringent textures under the optical microscope. As shown in * To whom correspondence should be addressed. (1) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1990, 94, 1598. (2) Lianos, P.; Thomas, J. K. J. Colloid Interface Sci. 1987, 117, 505. (3) Petit, C.; Pileni, M. P. J. Phys. Chem. 1988, 92, 2282. (4) Pileni, M. P. J. Phys. Chem. 1993, 97, 6961. (5) Fendler, J. H. Membrane-Mimietic Approach to Advanced Materials; Springer-Verlag: New York, 1994. (6) Cummins, C. C.; Schrock, R. R.; Cohen, R. E.Chem. Mater. 1992, 4, 27. (7) Martin, C. R. Science 1994, 266, 1961. (8) Attard, G. S.; Glyde, J. C.; Göltner, C. G.Nature 1995, 378, 366. (9) Tanev, P. T.; Pinnavaia, T. J. Science 1996, 271, 1267. (10) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (11) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (12) Ozin, G. A. Supramol. Chem. 1995, 6, 125. (13) Braun, P. V.; Osenar, P.; Stupp, S. I. Nature 1996, 380, 325. (14) Osenar, P.; Braun, P. V.; Stupp, S. I. Adv. Mater. 1996, 8, 1022. VOLUME 9, NUMBER 7 JULY 1997