Basic structural aspects about the layered hexaniobate of K 4 Nb 6 O 17 composition and its proton-exchanged form were investigated mainly by spectroscopic techniques. Raman spectra of hydrous K 4 Nb 6 O 17 and H 2 K 2 Nb 6 O 17 .H 2 O show significant modifications in the 950-800 cm -1 region (Nb-O stretching mode of highly distorted NbO 6 octahedra). The band at 900 cm -1 shifts to 940 cm -1 after the replacement of K + ion by proton. Raman spectra of the original materials and the related deuterated samples are similar suggesting that no isotopic effect occurs. Major modifications were observed when H 2 K 2 Nb 6 O 17 was dehydrated: the relative intensity of the band at 940 cm -1 decreases and new bands seems to be present at about 860-890 cm -1 . The H + ions should be shielded by the hydration sphere what preclude the interaction with the layers. Removing the water molecules, H + ions can establish a strong interaction with oxygen atoms, decreasing the bond order of Nb-O linkage. X-ray absorption near edge structure studies performed at Nb K-edge indicate that the niobium coordination number and oxidation state remain identical after the replacement of potassium by proton. From the refinement of the fine structure, it appears that the Nb-Nb coordination shell is divided into two main contributions of about 0.33 and 0.39 nm, and interestingly the population, i.e., the number of backscattering atoms is inversed between the two hexaniobate materials.
. Aquecendo-se as amostras acima de 200-250 o C, observa-se a liberação de CO 2 ; a reação de eliminação de Hofmann também é observada para as amostras de hexaniobato-tpa + . As imagens de microscopia eletrônica de varredura mostram a presença predominante de partículas em forma de placas; partículas em forma de bastões também são observadas nas amostras contendo íons volumosos. A reação de intercalação é promovida na ordem tma + > tea + > tpa + , enquanto a formação de uma dispersão de partículas coloidais é facilitada na ordem inversa.Chemical modification of the layered K 4 Nb 6 O 17 material was systematically investigated through the reaction of its proton-exchanged form (H 2 K 2 Nb 6 O 17 ) in alkaline solutions containing tetramethylammonium (tma + ), tetraethylammonium (tea + ) or tetrapropylammonium (tpa + ) cations. The intercalated amount reaches 50% (for tma C, CO 2 evolution is observed; Hofmann elimination reaction is also detected for hexaniobate-tpa + samples. Scanning electron microscopy images show the predominance of plate-like particles; stick-like particles are also observed for samples containing bulky ions. The intercalation reaction is promoted in the order tma + > tea + > tpa + , while the formation of a dispersion of colloidal particles is facilitated in the inverse order. Keywords: layered niobate, hexaniobate, tetraalkylammonium, intercalation IntroductionNew materials have been prepared combining chemical species that show unlike properties such as organic, inorganic and biochemicals in order to develop systems with improved or unique performance. However, the design of these hybrid materials requires chemical strategies to compatibilize so dissimilar species at the nanoscopic domain. One plausible approach demands the use of chemical reactions or processes that enhance the physicochemical interactions among the counterparts.Considering inorganic phases, suitable reactions to increase the interaction with organic species consist in (i) functionalization of the inorganic surfaces through covalent bonds with appropriated pendent groups or (ii) ion exchange of charged species that neutralize inorganic surfaces by proper ions.1 Chemical modification of inorganic nanostructures or nanocrystals has deserved special attention in the recent literature.1,2 Organicinorganic and biochemical-inorganic hybrid materials can be explored in diversified studies concerning the Shiguihara et al. 1367 Vol. 21, No. 7, 2010 preparation of nanostructured thin-films, macro-or mesoporous solids, biomaterials, biosensors, polymer nanocomposites, catalysts and photocatalysts, devices for generation of photocurrent or photoluminescence, and hierarchical structures. 1,2Among the inorganic materials, the layered frameworks can produce nanostructured hybrid materials by intercalation of guest species between the layers or reassemblage of the anisotropic nanosheets produced in an exfoliation process. Layered niobates are an important class of inorganic materials that have some interesting characteristics such as...
Organic-inorganic hybrid materials can be prepared dispersing organic species into well-defined inorganic nanoblocks. This paper describes the immobilization of natural dyes from the extract of the Brazilian açaí-fruit into two types of layered hexaniobate precursors derived from H(2)K(2)Nb(6)O(17): (i) colloidal dispersion of niobate exfoliated nanoparticles and (ii) niobate pre-intercalated with tetraethylammonium cations (TEA(+)). The restacking of exfoliated particles in the presence of açaí anthocyanins promotes their intercalation and produces stacked layers showing large basal spacing (ca. 50 A). The TEA(+) pre-intercalated niobate provides particles with lower content of dye species than the exfoliated precursor but with higher degree of organization and regularity according to X-ray diffraction data and images obtained by electron microscopies. Vibrational (FTIR and Raman) and (13)C NMR spectroscopies indicate the presence of flavylium cations in the hybrid materials and spectral profiles characteristic of glycosylated anthocyanidins. According to thermal analysis results, the purplish hybrids materials are more stable than the free açaí-dyes. One hybrid sample was heated under air up to 170 degrees C and maintained at this temperature for 240 min. No weight loss events were observed and the sample retained its original color, indicating that the intercalation of anthocyanin into hexaniobate increases its thermal stability. Considering the structural, chemical, optical and thermal properties of the synthesized hybrid materials, they might be good candidates to be investigated for future specialized applications.
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