Marcos Augusto Bizeto

Layered niobate nanosheets: building blocks for advanced materials assembly

Layered materials with intracrystalline reactivity undergo intercalation and pillaring reactions to produce materials with useful properties for catalysis, electrodes for Li batteries and adsorbents. New possibilities for the use of layered inorganic solids came out from the layered structures capable of delamination. The exfoliated particles are considered a new class of nanomaterial based on single crystal nanosheets. Due to their unique morphological features and properties, these nanosheets can be used as building blocks for nanomaterials with innovative properties. In this feature article we describe the aspects related to layered niobate exfoliation and the new possibilities that arises from the use of niobate nanosheets in the manufacturing of thin films, layer-by-layer (LbL) assemblies, hybrid structures, sensors and other materials.

One-pot synthesis of a polyaniline–silver nanocomposite prepared in ionic liquid

A mixture of aniline and a silver salt in ionic liquid has afforded a composite of PANI, in its more conducting form, and silver nanoparticles through a single-step direct reaction, without addition of any acid species or template. An ionic liquid acted as the solvent and template for the nanostructured material. The silver salt (AgTf2N) used in this work is soluble in the chosen ionic liquid, which is not an usual characteristic. TEM analysis of the obtained composite showed the presence of rounded silver nanoparticles with average diameter size around 10–20 nm uniformly dispersed in the polymeric matrix. XRD corroborates the size of the metallic particles at the nanoscale. FTIR and UV-vis confirmed that PANI was obtained in emeraldine salt form. The composite is slightly soluble in DMSO, an unusual feature for bulk conducting polyaniline. The composite shows electrical conductivity 100 times higher than bulk PANI prepared by usual procedures and its electroactivity was studied by cyclic voltammetry. Therefore, in this study it was shown that Ag+, in ionic liquid, is able to quickly oxidize the aniline to PANI, which is not observed in another media.

Biopolymer-clay nanocomposites: cassava starch and synthetic clay cast films

Polymer-clay nanocomposites (PCN) based on cassava starch, synthetic hectorite clay and inverted sugar cane syrup (plasticizer) were prepared by solvent-assisted (casting) process producing transparent and homogeneous films. Small amounts of clay (5-15 wt.%) resulted mainly in exfoliated nanocomposites while large amounts (30 wt.%) promote the intercalated nanocomposites formation. FT-Raman bands sensitive to hydrogen bonding in starch granules are progressively shifted to lower wavenumbers as the clay content is raised. Nanocomposites show a similar thermal behavior up to 320 oC while the biomolecule decomposition at about 500 oC is dependent on the clay content. CO2 release at about 300 oC (non-oxidative decomposition of polymeric chains) decreases if compared to the gas delivery at ca. 500 oC, as the clay content is increased. Films with clay content higher than 10 wt.% show no substantial benefit for either elongation or resistance properties.

Chemical modification of niobium layered oxide by tetraalkylammonium intercalation

A modificacao quimica do material lamelar K 4 Nb 6 O 17 foi investigada sistematicamente atraves da reacao de sua forma protonica (H 2 K 2 Nb 6 O 17 ) em solucoes alcalinas contendo os cations tetrametilamonio (tma + ), tetraetilamonio (tea + ) ou tetrapropilamonio (tpa + ). A quantidade intercalada corresponde a 50% (para tma + ), 25% (para tea + ) e 15% (para tpa + ) da carga negativa do H 2 K 2 Nb 6 O 17 (considerando a troca ionica na regiao interlamelar I). As amostras de hexaniobato apresentam reflexoes basais (020) de 23,0, 26,3 e 26,5 A quando intercaladas, respectivamente, com tma + , tea + e tpa + . Aquecendo-se as amostras acima de 200-250 o C, observa-se a liberacao de CO 2 ; a reacao de eliminacao de Hofmann tambem e observada para as amostras de hexaniobato-tpa + . As imagens de microscopia eletronica de varredura mostram a presenca predominante de particulas em forma de placas; particulas em forma de bastoes tambem sao observadas nas amostras contendo ions volumosos. A reacao de intercalacao e promovida na ordem tma + > tea + > tpa + , enquanto a formacao de uma dispersao de particulas coloidais e facilitada na ordem inversa.

Structure of tetraalkylammonium ionic liquids in the interlayer of modified montmorillonite.

We perform molecular dynamics simulations of tetraalkylammonium ionic liquids confined in the interlayer of montmorillonite (MMT). We study the structure and energetics of the systems, which consist of cations with two different alkyl chain lengths and several ionic liquid concentrations. The results we obtained for the structure, namely the presence of a strong layering in all systems and the formation of nonpolar domains with interdigitated alkyl chains in some cases, are largely consistent with previous surface force balance experiments performed on similar systems. Finally, we show that swelling of the organo-modified MMT by a large amount of ionic liquid seems energetically favorable in all cases.

A one-pot synthesis of a ternary nanocomposite based on mesoporous silica, polyaniline and silver

The research on hybrid materials composed of inorganic and organic species in the nanometer range is motivated by the synergic combination of their native properties in a unique material. In this paper, we report a new one-pot synthesis of a ternary nanocomposite based on a conducting polymer (polyaniline – PANI) and silver, using mesoporous silica (MCM-41) as a hard template to promote the controlled growth of both polymer chains and silver nanoparticles. The presence of these reaction products was proved by the appearance of the characteristic diffraction peaks of face-centered cubic metallic silver phase in the X-ray diffractogram and by the presence of typical bands of polyaniline as emeraldine salt in the FTIR and UV-Vis spectra. The preservation of the ordered MCM-41 mesostructure after the polymerization reaction was also attested by XRD and TEM analysis and it was accompanied by a decreasing of the particle size as observed in the SEM images. N2 adsorption–desorption isotherms support the statement that PANI chains and AgNP have been incorporated mainly into the channel of MCM-41, a fact also attested by TEM images. The electrical conductivity of this nanocomposite is on the order of 10−3 S cm−1, which is one million times higher than the value obtained for a MCM-41/polyaniline composite. Although some reports about the isolated incorporation of polyaniline or silver nanoparticles into the MCM-41 pores are found in the literature, to our knowledge this is the first time that such nanocomposite preparation is reported.

A hybrid material assembled by anthocyanins from açaí fruit intercalated between niobium lamellar oxide

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.

Production of self-supported conductive films based on cellulose, polyaniline and silver nanoparticles

Synthetic efforts are being made to produce electrical conductive films based on a combination of cellulose matrix with conducting polymers. Improved mechanical properties and processability of the conducting polymers can be attained by promoting this combination, allowing the production of electrical conductive membranes or sheet of paper-like forms that have many technological uses. Cellulose films containing different amounts of polyaniline combined to silver nanoparticles were prepared in homogenous conditions by the dissolution of microcrystalline cellulose in 1-butyl-3-methylimidazolium chloride ionic liquid, followed by the mixing with dispersion of silver nanoparticles and polyaniline also prepared in an ionic liquid. Films with high electrical conductivities (23-34 S·cm-1) and homogeneous distribution of the constituents were produced. The high conductivity was promoted by the presence of polyaniline in its most conductivity state with the silver nanoparticles affording the electrical contact among these chains creating a conductive network spread throughout the insulating cellulose matrix.