Synthesis of cerium/cobalt phosphate nanostructures in catanionic reverse micelles Yan Xing, Mei Li, Sean A. Davis, Avinash J. Patil and Stephen Mann* Received 6th April 2006, Accepted 28th April 2006 First published as an Advance Article on the web 15th May 2006 DOI: 10.1039/b604831e Nanofilaments/nanorods of cerium phosphate or cobalt phosphate are produced by reaction of aqueous hexadecyltrimethylammonium phosphate [(CTA) 3 PO 4 ] micelles with either [Ce(AOT) 3 ] or [Co(AOT) 2 ] reverse micelles prepared in isooctane. The nanostructures are hydrophobic, highly uniform in width (4–5 nm) and variable in length (up to 1200 nm). Whereas the particle width is constrained by the size of the catanionic reverse micelles, changes in the reaction conditions such as the concentration of the surfactant salts and [Ce(AOT) 3 ]/[(CTA) 3 PO 4 ] molar ratio, markedly influence the length of the inorganic nanostructures. The CePO 4 nanofilaments are crystalline and morphologically invariant with regard to changes in the reaction conditions, whereas the Co 3 (PO 4 ) 2 nanorods are amorphous and more sensitive to modifications in the surfactant salt molar ratio. The results are consistent with a mechanism based on the slow uni-directional aggregation/crystallization of nanoscale surfactant-coated inorganic building blocks. Introduction The synthesis of high aspect ratio inorganic nanostructures is an important and emerging field in various aspects of materials science. Anisometric small-scale structures exhibit novel properties that often differ from the bulk state, and therefore should be of technological value in numerous applications. 1–3 Although there are well established high temperature routes to many types of high aspect ratio nanostructures, more economic and environmentally benign approaches are of immediate consideration. In this regard, the use of complex fluids as organized reaction media for controlled inorganic nucleation and growth appears to be a promising approach. A number of different anisometric nanostructures have been prepared in reverse micelles and microemulsions, 4,5 and in most cases this involves a single amphiphilic component usually in the form of either a cationic or anionic long chain surfactant. In contrast, very limited work has been done using complex fluids prepared in the presence of both cationic and anionic surfactants. Lisiecki et al. synthesized copper nano- particles in water-in-oil microemulsions prepared using sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT), and added cetytri- methylammonium chloride (CTAC) to control the size of the crystals. 6 In similar studies, hexadecyltrimethylammonium bromide (CTAB) was added to AOT microemulsions to increase the stability of silver halide nanoparticles. 7 Recently, a few studies on the synthesis of high aspect ratio nano- structures using water-in-oil microemulsions prepared from combinations of two surfactants have been reported. For example, Simmons et al. described the synthesis of highly acicular cadmium sulfide particles obtained from a water-in-oil microemulsion containing an equimolar mixture of anionic AOT and the zwitterionic lipid, lecithin. 8 In other work, Qi et al. prepared BaWO 4 nanowires 9 and hierarchical superstructures consisting of BaCrO 4 nanobelts 10 using catanionic reverse micelles prepared from water, decane, and an equimolar mixture of anionic (undecylic acid) and cationic (decylamine) surfactants. In this paper, we report a new method for the synthesis of nanofilaments/nanorods of cerium phosphate or cobalt phos- phate by reaction of aqueous hexadecyltrimethylammonium phosphate [(CTA) 3 PO 4 ] micelles with either [Ce(AOT) 3 ] or [Co(AOT) 2 ] reverse micelles prepared in isooctane. On mixing the reaction solutions, hydration of the reverse micelles occurs by influx of water molecules and phosphate ions, and in order to maintain electroneutrality the cationic and anionic surfac- tants spontaneously associate, with the consequence that inorganic precipitation is mediated within the oil phase by a surrounding sheath of mixed amphiphilic molecules. Our results indicate that this process produces hydrophobic nanofilaments/nanorods of highly uniform width and variable size up to 1200 nm in length. The highly constrained width of the nanostructures is commensurate with the size of rod-like reverse micelles, whereas the length is markedly influenced by changes in the reaction conditions, such as the concentration of the surfactant salts and [Ce(AOT) 3 ]/[(CTA) 3 PO 4 ] molar ratio. We show that whereas the CePO 4 nanofilaments are crystalline and morphologically invariant with regard to changes in the reaction conditions, the Co 3 (PO 4 ) 2 nanorods are amorphous and more sensitive to modifications in the surfactant salt molar ratio. Our results are consistent with a mechanism based on the slow uni-directional crystallization or aggregation of nanoscale surfactant-coated inorganic clusters of cerium or cobalt phosphate, respectively, and suggest that coupling the in situ formation of catanionic micelles with inorganic deposition is a promising strategy for the room temperature preparation of a wide range of high aspect ratio nanostructures. Center for Organized Matter Chemistry, School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom. E-mail: s.mann@bris.ac.uk PAPER www.rsc.org/softmatter | Soft Matter This journal is ß The Royal Society of Chemistry 2006 Soft Matter, 2006, 2, 603–607 | 603