EXAFS and XANES Investigations of CuFe 2 O 4 Nanoparticles and CuFe 2 O 4 -MO 2 (M ) Sn, Ce) Nanocomposites Venkata Krishnan,* ,†,‡ Ramakrishnan Kalai Selvan, §,| Chanassary Ouso Augustin, § Aharon Gedanken, | and Helmut Bertagnolli † Institute of Physical Chemistry, UniVersity of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany, Electropyrometallurgy DiVision, Central Electrochemical Research Institute, Karaikudi 630 006, India, and Department of Chemistry and Kanbar Laboratory for Nanomaterials, Bar-Ilan UniVersity Center for AdVanced Materials & Nanotechnology, Bar-Ilan UniVersity, Ramat-Gan 52900, Israel ReceiVed: May 15, 2007; In Final Form: August 1, 2007 Structural investigations were performed on combustion synthesized CuFe 2 O 4 nanoparticles and CuFe 2 O 4 - MO 2 (M ) Sn, Ce) nanocomposites in different compositions, by means of X-ray absorption fine structure spectroscopy. The studies on CuFe 2 O 4 nanoparticles reveal that the samples have a structure analogous to that of the bulk material, wherein all the copper ions occupy the octahedral sites and the iron ions are distributed between the tetrahedral and octahedral sites. The XAFS investigations on CuFe 2 O 4 -SnO 2 and CuFe 2 O 4 - CeO 2 nanocomposites show that the incorporation of the tetravalent metal ions in the spinel lattice does not alter the local structure around copper and iron in CuFe 2 O 4 nanoparticles. The X-ray diffraction pattern indicates CuFe 2 O 4 as a single phase in the nanoparticles and shows the incorporation of metal ions in the spinel structure in addition to the existence of MO 2 and CuFe 2 O 4 phases in CuFe 2 O 4 -MO 2 nanocomposites. The nanometric size of the as-synthesized materials has been confirmed by transmission electron microscopy studies. The high-resolution transmission electron microscopy investigations also confirm that CuFe 2 O 4 exists as a single phase and reveal the composite nature of CuFe 2 O 4 -MO 2 materials. 1. Introduction Magnetic nanomaterials have long been of scientific and technological interest. Among the magnetic materials, the spinel structured ferrites having general formula MFe 2 O 4 have been used in many industrial applications. By adjusting the chemical identity of the M 2+ cation, the magnetic configurations of MFe 2 O 4 can be molecularly engineered to provide a wide range of magnetic properties. 1 Due to this versatility, MFe 2 O 4 nano- materials have been increasingly investigated for nanomagnetism and have shown great potential for many important technological applications, ranging from information storage and electronic devices to medical diagnostics and drug delivery. 2 Several studies on pure nanoferrites such as Fe 3 O 4 , 3 NiFe 2 O 4 , 4 CoFe 2 O 4 , 5 ZnFe 2 O 4 , 6 and MnFe 2 O 4 7 have shown the supremacy of nanosize in view of their properties and applications. The term nanocomposite encompasses a variety of distinctly different mixed materials at the nanometer scale. Recently, several nanocomposites encompassing ferrites have been pre- pared and investigated for their interesting properties. These include the magnetic-metallic nanocomposites of Fe 3 O 4 -Au, 8 magnetic-oxide nanocomposites of CuFe 2 O 4 -SnO 2 , 9 magnetic- polymer nanocomposites of Fe 3 O 4 -polypyrrole, 10 and magnetic- sulfide nanocomposites of Fe 3 O 4 -PbS. 11 In the recent past, there has been considerable research on the structure and cation distribution in ferrite nanomaterials. Nilsen et al. 12 have studied the local structure of nanoparticulate ferrites including Fe 3 O 4 , NiFe 2 O 4 , CoFe 2 O 4 , and ZnFe 2 O 4 by means of X-ray absorption fine structure (XAFS) spectroscopy and illustrated that the bulk structure extends to the nanoregime. Carta et al. 13 have investigated the formation of CoFe 2 O 4 nanoparticles in an aerosol SiO 2 matrix using XAFS technique. The studies on cation distribution in synthetic binary, ternary, and quaternary ferrites of stoichiometry M 2+ M 2 3+ O 4 , where M 2+ ) Mg, Co, Ni, Zn and M 3+ ) Fe, Al, have been elaborately performed by Henderson et al. 14 using K-edge X-ray absorption spectroscopy. Among the ferrites, CuFe 2 O 4 has gained significant attention in recent years. 15,16 Qi et al. reported the preparation of CuFe 2 O 4 nanowalls using electrochemical methods, 17 and Du et al. reported the preparation of CuFe 2 O 4 nanorods and nanodisks, using reverse micelle and hydrothermal methods. 18 In addition, CuFe 2 O 4 nanoparticles have been prepared by co-precipitation method, 19 mechanical milling, 20 sol-gel method, 21 precipitation in polymer matrix, 22 etc., and these materials have a wide range of applications, including gas sensors, 23 CO 2 decomposition, 24 negative electrodes for Li-ion batteries, 25 hydrogen production, 26 magnetic adsorbents, 27 etc. The structural and magnetic evolu- tion in CuFe 2 O 4 caused by high-energy ball milling was investigated by Goya et al., 28 wherein the researchers evidenced that the milling process reduces the average grain size of CuFe 2 O 4 and induces cation redistribution between tetrahedral and octahedral sites. In CuFe 2 O 4 bulk materials, generally the copper ions occupy the octahedral sites and the iron ions occupy both the tetrahedral and octahedral sites. Among the octahedral sites, the copper ions are randomly distributed. The copper ion undergoes dsp 2 hybridization to form four square, covalent bonds in an octahedral interstice of the spinel lattice. Thus, it is bound to * Corresponding author. Telephone: +1 215 573 7995. Fax: +1 215 573 2112. E-mail: venkrish@sas.upenn.edu. † University of Stuttgart. ‡ Current address: Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, PA 19104. § Central Electrochemical Research Institute. | Bar-Ilan University. 16724 J. Phys. Chem. C 2007, 111, 16724-16733 10.1021/jp073746t CCC: $37.00 © 2007 American Chemical Society Published on Web 10/11/2007