Sol Gel Synthesis

One of the remarkable feature of the starch is its gelling abilities in aqueous solution in the temperature range of 50-80°C, due to its content of linear amylose component. The mechanism of starch gelatinization is still an issue question, but nowadays it is accepted that gelation causes starch anionic, either by producing fatty acids or causing -OH group to dissociate (Yamamoto et al. 2006; Whistler et al. 1984; Chen and Jane 1994; Bertuzzi et al. 2007). This special characteristic of starch could be efficiently exploited in the less-expensive and highly-available variant of a sol-gel synthesis, in which the starch-gel acts also as a template for nanoparticles aggregations. It has in fact multiple functions, such as hosting, protecting, separating, binding and tailoring agent for the precursors and their corresponding nanooxides. The chelating action of the carbohydrate is essential for the starch-based sol-gel synthesis. The resulted colloidal complex precursors are usually obtained from the reaction of the inorganic salts of divalent ions or metal-organic compounds and starch that pre-organizes the morphology of the nanoparticles. Starch is removed by decomposition at significantly lower temperatures than the classic sol-gel chelators (ethylenediaminetetraacetic acid-EDTA, citric acid). The thermal decomposition of starch generates sometimes pores, being a well-known consolidating agent for porous ceramics (Kim et al. 2002; Garridoa et al. 2009; Alves et al. 1998; Gregorova and Pabst 2007; Lyckfeldt and Ferreira 1998; Kim 2005).

A typical sol-gel synthesis uses the complexing abilities of the starch in order to produce non-spherical particles, the particle shape being suitable to preserve the magnetic phase. g-Fe2O3 oxide (maghemite) crystallizes in the cubic inverse spinel structure, with vacancies distributed on the octahedral sites. For example, the ferrimagnetic behaviour of maghemite demands elongated ellipsoidal particles, for which nucleation could be controlled by complexing agents with a tailoring effect. A starch/ethyleneglycol mixture creates a suitable complexing medium to synthesize ellipsoidal mesoscopic g-Fe2O3 particles with an average size of 19 nm, while 10 nm in diameter spherical-shaped particles are obtained in a water/starch solution. The size/shape have a decisive influence on the optic and magnetic properties of the oxides, a red shift of the band-gap and an enhanced coercivity, induced by anisot-ropy, being observed for the non-spherical nanoparticles (Narayanan et al. 2008).

The reactions of KMnO4 and starch, with the formation of an intermediate gel, followed by a heating treatment at 400°C for 2 h, have afforded a layered-type structure of birnessite manganese oxide, MnO6 (Ramalingam et al. 2006). The resulted oxide is made-up from sheets of edge-sharing MnO6 octahedra, the interlayered distances, filled with water molecules and MnO(OH)2 species (identified in thermal analysis by decomposition processes in the 172-396°C temperature range) being approximated at 6.5 A.

An adapted sol-gel method, which consists in a reaction of titanium(IV) alkoxide with starch, under the mild conditions and non-polar cyclohexane, has led to mes-oporous nanocrystalline TiO2 materials (Kochkar et al. 2007). The starch that has only a stabilizing effect, was removed following two procedures: heat treatment and solvent extraction, the calcinated samples exhibiting superior textural properties. Nanosized TiO2 particles of 23 nm mean crystallite sizes, with a specific surface area of 94 m2 g-1 and pores dimensions varying between 9 and 15 nm, influenced by the Ti(IV)-alkoxide have been obtained.

One of the most interesting applications of starch-assisted gel template synthesis is the cobalt-containing blue inorganic pigments. Thus, the gradual controlled insertion of Co2+ cations, the source of blue color, within the inorganic gahnite (zinc aluminate, ZnAl2O4) host lattice, in the presence of starch, have afforded nanosized CoxZn1-xAl2O4 (x=0, 0.1, 0.2, 0.4, 0.6, 0.8 and 1) (Visinescu et al. 2010) . The formationof the; starch-gel is crucial for keeping a higher degree of homogeneity, purity and also a narrow distribution of the oxide nanoparticles, while the combined complexing, structure-director and gelation effects of the carbohydrate is a powerful tool for controlling the aluminates particle shapes and sizes. The formed (Zn, Al, Co)-starch complex, which initially was encapsulated within the helical polysaccharide (in excess) matrix were converted, after a heat treatment of 800°C for 1 h, into spinelic mixed cobalt aluminates. The mean crystallite sizes vary with the cobalt amount in oxides, the higher is the Co2 + cations content the lower is the particle dimension [from 250 A for ZnAl2O4 (x=0) to 146 A for CoAl2O4 (x = 1)] (Fig. 5.2a). TEM investigations indicate the formation of small particles, homogeneous as shape and size and with a pronounced tendency to form agglomerates and even aggregates.

Several useful information have been extracted from the analysis of NIR-UV-Vis spectra, depicted in Fig. 5.2b: the tetrahedral sites are mainly occupied by the Co2+ cations and give the blue colored material (a three-structured band, with maxima centered at ca. 544, 595 and 622 nm in visible region assigned to the 4A2(F) ^ 4Tj(P) spin allowed transition for the tetrahedral Co(II) chromophores), the purity of the

Fig. 5.2 (a) Diagram of the mean crystallites size of CoxZn1-xAl2O4 oxides (calcinated for 1hat 800°C) vs. the cobalt (x) content; (b) Visible and near-infrared spectra for CoxZn1-xAl2O4 (x = 0.1, 0.2, 0.4, 0.6, 0.8, 1) oxides calcinated for 1 h at 800°C (Reproduced by permission from Visinescu et al. © 2010 Elsevier)

Fig. 5.2 (a) Diagram of the mean crystallites size of CoxZn1-xAl2O4 oxides (calcinated for 1hat 800°C) vs. the cobalt (x) content; (b) Visible and near-infrared spectra for CoxZn1-xAl2O4 (x = 0.1, 0.2, 0.4, 0.6, 0.8, 1) oxides calcinated for 1 h at 800°C (Reproduced by permission from Visinescu et al. © 2010 Elsevier)

spinelic phase (the absence of the Co3O4 is confirmed also by the XRD spectra) and the spinel-disorder dependence of the cobalt content. Thus, the rich-Co(II) oxide samples are characterized by the highest degree of spinel disorder, in which a percent of the Co(II) cations adopt an octahedral stereochemistry.

Doped nanocrystals of YVO4:Ln (Ln = Sm, Dy, with a concentration of Dy3+ and Sm3+ ions kept constant at 1%) have been obtained using starch as suitable and, at the same time, labile chelator in an adapted sol-gel routine (Zhang et al. 2008). The oxidation of some polyol (-OH) groups of the carbohydrate to aldehyde (-CHO), with an excess of HNO3, enhances the coordination abilities of the polysaccharide. The formation of a carbonylic gel-intermediate has been confirmed by the vibration spectra, in which the broad and strong band centered at 1630 cm-1 is attributed to the asymmetrical -COO- stretching frequencies, while its complete thermal conversion into the oxide is proved by the strong absorbtion at ca. 844 cm-1 characteristic to the orthovanadate anion (VO4 3-) . The exothermic reaction between the NO3 -anions (oxidant) and starch (reducing agent) at temperatures of 600-800°C, allow the rapid crystallization of the tetragonal pure phase of YVO4, with crystallite sizes ranging from 50 to 100 nm and with a nearly cobblestone-like morphology, at temperatures lower than in the analogue solid-state reaction.

Starch gel templates coupled with colloidal nanoparticles titanium(IV) oxide have been used to prepare TiO2 materials with a hierarchical meso/macropore organization (Iwasaki et al. 2004). The elasticity of starch, illustrated by its capacity to swell in water and to gradually shrunk by freezing without loosing its structural integrity, build a three-dimensional sponge-like structures with tunable pore sizes. The immersing of starch blocks into the TiO2 colloidal dispersions has led to macroporous TiO2-starch composites with extremely high photocatalytic activities for air purification.

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