INSTITUTO DE FISICA LA PLATA
Unidad Ejecutora - UE
congresos y reuniones científicas
Study of maghemite nanoparticles and SBA-15 embeddes ones
A. E. BIANCHI; I. O. PÉREZ DE BERTI; F. J. BENGOA; S G. MARCHETTI; G. PUNTE; R. C. MERCADER
Congreso; Internatonal Conference on the Applications of the Mossbauer Effect (ICAME 2013); 2013
Self-supported nanoparticles (NPs) of γ‑Fe2O3 (maghemite) of different sizes were characterized by Mössbauer spectroscopy (MS), X-ray powder diffraction (XRPD), transmission electron microscopy (TEM) and dynamic light scattering (DLS). Results allowed determining that the synthesis method  renders NPs with controlled size and narrow width of the size distribution. The smallest NPs produced (≈ 3 nm) were supported in a SBA-15 matrix modified to obtain larger pore size than the standard one . The solid was characterized using N2 adsorption, TEM, small-angle X-ray scattering (SAXS), DLS, thermo-gravimetric analysis and MS. The transmission Mössbauer spectra were taken at different temperatures from 17 to 298 K inside He closed-cycle cryogenic systems. The spectra were fitted taking into account hyperfine field distributions and relaxing magnetic signals. SAXS data for the pure SBA-15 matrix and from a loaded one were obtained at room temperature at the D11A- SAXS1 line workstation of the Brazilian Synchrotron Light Source, Campinas, Brazil, using a wavelength of 0.1488 nm. The range of q detected was 0.3 £ q £ 1.95 nm-1. Pure SBA-15 SAXS results allowed identifying four diffraction lines that could be indexed as (100), (110), (200), and (210) reflections with a cell parameter a = (12.65 ± 0.05) nm. This result indicates that the sample has a well-ordered two-dimensional (2D) mesostructure (p6mm). The loaded SBA-15 SAXS curve shows a higher slope and a smoother background that might be related to a decrease of the surface roughness. Analysis of the Bragg lines allows indexing the reflections (100), (110) and (200) yielding a cell parameter a = (12.84 ± 0.05) nm. These results support that the SBA-15 mesoporous structure was conserved after the loading of the γ-Fe2O3 NPs. An enlargement of the pores was observed, as expected. In addition, the XRPD data exhibit broad lines consistent with the γ-Fe2O3 NPs presence. The Mössbauer spectra of the self-supported γ-Fe2O3 NPs exhibited parameters belonging only to Fe3+ ions. At low temperatures, the spectra were fitted with magnetic relaxing signals that proved to originate in the γ-Fe2O3 and the Fe3+ ions coordinated to the oleic acid that coated the γ-Fe2O3 NPs. The γ-Fe2O3 NPs in the matrix were identified by their Mössbauer signals that also showed parameters that could only be assigned to Fe3+ ions coordinated to the oleic acid and to γ-Fe2O3. The combination of the above mentioned results with those yielded by the other techniques show that the γ‑Fe2O3/SBA-15 system has a narrow distribution of NPs sizes and keeps the structural properties of its components after impregnation. Acknowledgements: This work was partially carried out with funding from CONICET (PIP 0985 and PIP 0230), ANPCyT (PICTs Nº 22-38337 and 00549) and UNLP (projects X-571 and X-577).  I. O. Pérez De Berti et. al. ?PREPARACIÓN DE UN CATALIZADOR?SEMI-MODELO? DE NANOPARTICULAS DE γ-Fe2O3/SBA 15 PARA SER USADO EN LA SÍNTESIS DE FISCHER-TROPSCH?. VI Congreso de catalisis del Mercosur, 2012.  D. Zhao et al. Science, 279 (1998) 548 and J. Am. Chem. Soc. 120 (1998) 6024.