Mesostructured Iron Oxyhydroxides. 1. Synthesis, Local Structure, and Magnetism

Abstract

The synthesis, local structure, and magnetism of lamellar iron(III) oxyhydroxide−surfactant composites prepared by two different methods have been investigated in detail. In the first method, Fe(II) solutions are oxidized by H2O2 in the presence of C n H2 n +1OSO3 −Na+ surfactants (n = 10, 12, 14, 16, 18), leading to lamellar composites with an inorganic wall thickness of around 28 Å. When a second method is used, namely, aging an Fe(III) solution for selected times after slightly increasing the pH with NH3 and subsequent addition of the surfactant, the inorganic wall thickness can be tuned between 19 and 26 Å, employing the same surfactants. EXAFS analysis of the Fe K edge X-ray absorption spectra reveals that the local structure of the inorganic part is a reminder of those found for the bulk iron oxyhydroxides goethite and akaganéite; that is, [Fe(O,OH)6] octahedra are predominantly connected by common edges and corners, the ratio of edge to corner sharing being similar to the mentioned bulk oxyhydroxides. Whereas coordination numbers for the first oxygen coordination shell are around 6, confirming an octahedral (or distorted octahedral) coordination around the Fe ions, coordination numbers found for the second and third Fe···Fe neighbors are low (around 2), indicating the presence of a considerable amount of vacancies around the central absorber ion or, as an alternative description, a low degree of condensation of the oxyhydroxide. Complementary to the local structural picture given by EXAFS, Mössbauer spectra elucidate the inorganic iron oxyhydroxide walls to be built up by domains of different crystallinity. The crystallinity is sensitive to the synthesis conditions used in the preparation. For example, under aging in the presence of NH3, longer aging times and higher temperatures result in a larger overall crystallinity of the inorganic part. By carefully controlling the reaction parameters, the thickness of the inorganic layers can be varied from around 19 Å to around 30 Å; also, the blocking temperatures of these superparamagnetic compounds observed by zero-field-cooled magnetization measurements can be controlled in the range between 4 and 30 K.

Synthesis of acicular goethite with surfactants

“…Surfactants have received particular attention in this field due to their self-assembling capabilities [2]. There has been considerable success in the use of surfactants as templates for the synthesis of mesoporous materials [3] Laminar mesoporous structures of silica, [4] alumina [5] and more recently, iron oxide, [6] have been reported.…”

“…The method could potentially produce highly fibrous (acicular) goethite and lepidocrocite nanosized particles which has applications as precursors for heterogeneous catalysts, anticorrosion coatings and magnetic recording media [10,11].…”

Surfactant mediated nanoparticle assembly of catalytic mesoporous crystalline iron oxide materials

“…Some efforts have been devoted to the synthesis of ordered mesoporous transition metal oxides for their enormous potential in the fields of electromagnetics, photoelectronics, catalysis and separation [11][12][13]. Mesostructured lamellar iron (oxyhydr)oxide/surfactant composites have been synthesized by either oxidizing of Fe 2+ -containing solutions with H 2 O 2 or hydrolysis of Fe 3+ in the presence of C n H 2n+1 OSO 3 − surfactants [14][15][16]. They were unfortunately unstable and collapse upon surfactant removal, in spite of their super-antiferromagnetism.…”

Mesoporous Aluminosilicate Materials with Superparamagnetic γ‐Fe2O3Particles Embedded in the Walls

“…Although research has been performed on surfactant-based templating and sonochemical approaches to layered iron oxide/oxyhydroxide mesophases, [11,12] polymerderived magnetic bulk ceramics, [13] and bulk iron oxide silicates prepared through sol-gel techniques, [14][15][16][17] research on mesostructured iron-containing silicates is scarce and limited to surfactant-based systems where the iron compound was loaded after synthesis. [18,19] Backfilling the pores is a common technique to functionalize mesoporous materials, [20] but it requires more synthesis and characterization steps and, more importantly, risks clogging the pore structure.…”