Study of self supported and of SBA-15 embedded maghemite nanoparticles

A. E. BIANCHI; I. O. PÉREZ DE BERTI; J. F. BENGOA; S. G. MARCHETTI; R. C. MERCADER; G. PUNTE

Congreso; International Conference on the Applications of the Mössbauer Effect; 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 [1] 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 [2].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).[1] 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.[2] D. Zhao et al. Science, 279 (1998) 548 and J. Am. Chem. Soc. 120 (1998) 6024.