LATIN AMERICAN SEMINAR OF ANALYSIS BY X-RAY TECHNIQUES (SARX 2012)

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

Congreso; XIII Latin American Seminary of Analysis by X-Ray Techniques; 2012

Ordered mesoporous silicates have attracted much interest since the discovery of M41S family of materials in 1992 because they opened new possibilities to prepare catalysts for reactions of relatively large molecules. In the search for larger pores, six years later new mesoporous silica, based on uniform hexagonal pores with a narrow pore size distribution and tunable pore diameters, was synthesized and named Santa Barbara Amorphous (SBA-15). The researches who synthesized these types of materials planned to use them as molecular sieves. Today, mesoporous silica pore sizes ranging from 2 to 30 nm can be obtained and the embedded nanoparticles have found applications in medicine, biosensors, and imaging. In this study we investigate by Small Angle X-ray Scattering (SAXS) and X-ray Powder Diffraction (XRPD) the structural characteristics of a SBA-15 matrix –modified to obtain larger pore size without altering its typical hexagonal mesoporous structure– loaded with pre-synthesized maghemite (γ Fe2O3) nanoparticles (NPs) of 2.5 ± 0.6 nm average diameters. The NPs size was determined by transmission electron microscopy and dynamic light scattering. 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 in 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 γ-Fe2O3 NPs loading and an enlargement of pores, as expected. In addition, the XRPD data exhibit broad lines consistent with the γ-Fe2O3 NPs presence. These findings are in agreement with Mössbauer results, which show that signals coming from the γ-Fe2O3 NPs and those from loaded SBA-15 reveal parameters belonging only to Fe3+ ions. In both samples, at low temperatures, the spectra display 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.