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

详细信息    查看全文

• 作者：G. Wirnsberger ; K. Gatterer ; H. P. Fritzer ; W. Grogger ; B. Pillep ; P. Behrens ; M. F. Hansen ; and C. Bender Koch
• 刊名：Chemistry of Materials
• 出版年：2001
• 出版时间：May 2001
• 年：2001
• 卷：13
• 期：5
• 页码：1453 - 1466
• 全文大小：315K
• 年卷期：v.13,no.5(May 2001)
• ISSN：1520-5002

文摘

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_nH_2n+1OSO₃^-Na⁺ surfactants (n = 10, 12, 14, 16, 18), leading to lamellar composites with an inorganicwall 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 additionof the surfactant, the inorganic wall thickness can be tuned between 19 and 26 Å, employingthe same surfactants. EXAFS analysis of the Fe K edge X-ray absorption spectra revealsthat the local structure of the inorganic part is a reminder of those found for the bulk ironoxyhydroxides goethite and akaganéite; that is, [Fe(O,OH)₆] octahedra are predominantlyconnected by common edges and corners, the ratio of edge to corner sharing being similar tothe mentioned bulk oxyhydroxides. Whereas coordination numbers for the first oxygencoordination shell are around 6, confirming an octahedral (or distorted octahedral) coordination around the Fe ions, coordination numbers found for the second and third Fe···Feneighbors are low (around 2), indicating the presence of a considerable amount of vacanciesaround the central absorber ion or, as an alternative description, a low degree of condensationof the oxyhydroxide. Complementary to the local structural picture given by EXAFS,Mössbauer spectra elucidate the inorganic iron oxyhydroxide walls to be built up by domainsof different crystallinity. The crystallinity is sensitive to the synthesis conditions used inthe preparation. For example, under aging in the presence of NH₃, longer aging times andhigher temperatures result in a larger overall crystallinity of the inorganic part. By carefullycontrolling the reaction parameters, the thickness of the inorganic layers can be varied fromaround 19 Å to around 30 Å; also, the blocking temperatures of these superparamagneticcompounds observed by zero-field-cooled magnetization measurements can be controlled inthe range between 4 and 30 K.