Sol–Gel Mediated Synthesis of Fe₂O₃ Nanorods

Abstract

Nanorods as one-dimensional nanostructured materials have attracted much attention due to their peculiar properties, which originate from their high surface area and low dimensionality. [1,2] Recently, much effort has been devoted to the synthesis of various nanorods, including metals, [3,4] oxides, [5±8] and chalcogenides. [9] Among them, metal-oxide nanorods have been shown to possess many interesting properties that make them useful for a wide range of applications such as catalysts, [8,10] electrochromic devices, [11] magnetic storages, [12] lasers, and sensors. [13] Many solution-based syntheses of nanorods have utilized a thermal decomposition reaction of delicate and costly organometallic precursors in mixed surfactants. [3,9] A template-directed synthesis has been studied as an alternative route. [8,14] However, it may contaminate the nanorods by the successive removal process of the template materials. Here, we report a novel and easy route to Fe 2 O 3 nanorods by a sol-gel-mediated reaction of ubiquitous Fe 3+ ions in reverse micelles. A plausible mechanism is proposed for the formation of these nanorods, and it is expected that this synthetic technique can be extended to obtain other metal oxides.The reaction was carried out utilizing a sol±gel reaction inside reverse micelles and followed by crystallization by reflux. This sol±gel reaction, which we utilized here, was first reported by A. E. Gash et al.[ gel in a container. Instead of producing a container-sized monolithic gel, we prepared a countless number of monolithic gel particles inside reverse micelles using the reported sol±gel reaction and water-in-oil type microemulsion stabilized by an oleic acid surfactant. Scheme 1 represents the transformations of materials in a reverse micelle. First, we prepared Fe 2 O 3 monolithic gel particles with a narrow size distribution. This was possible by slow gelation initiated by a proton scavenger inside the reverse micelles. Each reverse micelle acts as a microreactor, which produced a monolithic gel particle in itself. Next, the gel particles were washed with a polar solvent to remove chloride impurities and excess surfactants and then air-dried. Now, each gel particle is composed of a porous network of amorphous Fe 2 O 3 (confirmed by X-ray diffraction (XRD) pattern) and its surface is roughly coated with surfactants. At this stage, scanning electron microscopy (SEM) images of gel particles showed shapeless aggregates and could not give any meaningful information. Then, the gel powder was treated in a highboiling-point solvent with reducing properties while controlling the temperature and the atmosphere, inducing crystallization [16] with a controlled phase depending on the conditions. During crystallization, the partially crystallized monolithic gel particles seem to fuse together in an end-to-end manner and grow into a nanorod. At the same time, a decrease of the particle size by crystallization seems to occur. Figure 1 shows the representative FTIR spectra of Fe 2 O 3 gels and a-Fe 2 O 3 (hematite...

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