Silicon-29 cross-polarization/magic-angle-spinning NMR study of inorganic-organic hybrids: homogeneity of sol-gel derived hybrid gels

The structures of sol-gel derived hybrid gels prepared by co-hydrolysis of tetraethoxysilane (TEOS)-organotrialkoxysilane (RTES, RSi(OC₂H₅)₃, R = CH₃, C₅H₁₁, C₈H₁₇ and C₆H₅) mixtures (TEOS : RTES : CH₃CH₂OH : H₂O : HNO₃ = 0.5 : 0.5 : 10 : x : 0.3; x = 2, 5, 10 and 20) were characterized based on signal intensities of ²⁹Si CP/MAS NMR as a function of the contact time. The contact time dependencies of the signals for Q⁴ (Si(OSi)₄) units were successfully simulated by assuming distributed cross relaxation times (T_SiH) and an intrinsic ¹H spin-lattice relaxation time in the rotating frame (); the latter was different from the apparent value estimated by assuming single T_SiH. The distribution ranges of T_SiH for the TEOS-RTES gels broadened as the water content increased, suggesting that the Q⁴ units tended to be separated from the T units and that the local concentration of ¹H spins around the Q⁴ unit tended to decrease.     

Local structure of LuBO3 prepared via sol-gel process and its transformation with temperature: B-NMR and L-edge XAS studies

The present work deals with the temperature effect on the stability range of LuBO₃ prepared by sol-gel synthesis. Structural modifications from vaterite to calcite form versus the thermal treatments have been measured by X-ray diffraction and infrared spectroscopy. Several samples derived from the xerogel annealed at 400, 800 and 1200 °C were studied by ¹¹B solid state NMR and at the lutetium L_I-L_III edges by X-ray absorption spectroscopy to determine the local atomic arrangement around the rare earth and the boron ions. The predominance of BO₄ groups and the eightfold coordination of lutetium atoms in the LuBO₃ xerogel in samples treated up to 800 °C indicate an evolution of the xerogel to vaterite form after thermal treatment instead of the expected calcite form as established for samples prepared by solid state reaction.     

FTIR and absorption edge studies on tungsten oxide based precursor materials synthesized by sol-gel technique

The sol-gel technique has been employed for synthesizing three precursor materials for the deposition of tungsten oxide based electrochromic films. The ion-exchange route yields tungstic acid precursor (ITA) which has a strong tendency to polymerize rapidly. Addition of hydrogen peroxide and/or acetic acid suppresses the gelation process and an ethanol soluble precursor material (AIPTA) possessing a low degree of polymerization and high chemical stability is obtained. Another precursor material APTA endowed with similar desirable characteristics is produced from direct reaction between tungsten metal powder and a mixture of hydrogen peroxide and acetic acid. These materials were characterized by FTIR and absorption edge studies and compared in terms of their degree of polymerization and modes of association of various functionalities with the tungsten metal ion.     

Comparative study of the sol-gel processes starting with different substituted Si-alkoxides

In the present work a comparative study of the hydrolysis-polycondensation processes of different Si-substituted alkoxides, leading to hybrid materials with covalent -Si-C- bonds, was carried out. The following alkoxides were used: tetraethoxysilane (TEOS), methyltriethoxysilane (MTEOS) and vinyltriethoxysilane (VTEOS). Using gas chromatography coupled with mass spectrometry (CG-MS), nuclear magnetic resonance (²⁹Si-NMR) and infrared spectrometry (IR), information about the sol-gel process in the mentioned systems were obtained. The differences in the reactivity of the studied alkoxides are connected with the steric effect of the organic substituents. The reactivity of the alkoxides in the early stages of the hydrolysis-polycondensation process increased in the order TEOS < VTEOS < MTEOS.     

Phase segregation of non-stoichiometric aluminosilicate gels characterized by Al and Si MAS-NMR

Polymeric aluminosilicate gels with Al₂O₃/SiO₂ molar ratios of 1.3/2, 2.5/2, 3/2 and 4.7/2 were prepared by gelling a mixture of tetraethoxysilane and ethyl acetoacetate aluminium diisopropoxide. Mullite, without any other crystalline phase, directly crystallizes from the gel matrix at about 1000 °C for all investigated samples. The Al₂O₃ or SiO₂ crystalline phase can only be detected at relatively high temperatures accompanying the modification of mullite in the lattice structure. ²⁷Al and ²⁹Si MAS-NMR studies indicate that segregation of Al and Si atoms occurs in all samples below 900 °C in the amorphous state, regardless of the gel composition. However, the species of segregated units are very different and strongly dependent on the composition of the starting gels. This segregation may not cause the exothermic effect in differential thermal analysis examination, but it appears to be responsible for the composition change behavior of crystalline mullite with different heat treatment.     

Luminescence of CaS and MnS nanocrystallites co-activated sol-gel derived silica xerogel

CaS and MnS nanocrystallites co-activated sol-gel derived silica xerogel has been prepared by sol-gel processing. Their photoluminescence characteristics have been evaluated and compared with those of the undoped silica xerogel. Two emission bands have been observed from the doped sample, one at 440 nm while the other at 580 nm. CaS and MnS nanocrystallites embedded in sol-gel derived silica xerogel show sharp emission band. The novel luminescence phenomenon is attributed to the luminescent centers of CaS and MnS in the silica xerogel.     

Multinuclear NMR study of phosphosilicate gels derived from POCl3 and Si(OC2H5)4

Solid state ¹H, ²⁹Si and ³¹P MAS NMR have been used to investigate the microstructure of phosphosilicate gels prepared by a modified sol-gel method involving hydrolysis of silicon precursors in a solely aqueous environment at 50 °C. Gels with molar compositions 5, 10, 20 and 30 mol% P₂O₅ in P₂O₅-SiO₂ were studied. After drying to 400 °C the gels have very similar structures formed by a siloxane framework containing silanol groups and trapped molecules of orthophosphoric acid together with a very small amount, of pyrophosphoric acid. Unlike the gel samples previously synthesized by the hydrolysis of the silicon precursor in alcoholic solution at room temperature, the co-polymerization of phosphorus and silicon is much reduced. Although co-polymerization increases with phosphorus content, it still represents less than 50% of the phosphorus in the 30 mol% P₂O₅ gel. Furthermore there is no evidence for six-coordinated silicon in the glassy matrix.     

Effect of Y2O3 addition on the phase transition and growth of YSZ nanocrystallites prepared by a sol-gel process

The effect of Y₂O₃ addition on the phase transition and growth of yttria-stabilized zirconia (YSZ) nanocrystallites prepared by a sol-gel process with various mixtures of ZrOCl₂ · 8H₂O and Y(NO₃)₃ · 6H₂O ethanol-water solutions at low temperatures has been studied. DTA/TGA, XRD, SEM, TEM and ED have been utilized to characterize the YSZ nanocrystallites. The crystallization temperature of 3YSZ, in which Y₂O₃/(Y₂O₃ + ZrO₂) = 0.03, gel powders estimated by DTA/TG is about 427 °C. When 3YSZ and 5YSZ gels are calcined at 500-700 °C, their crystal structures as composed of coexisting tetragonal and monoclinic ZrO₂, and tetragonal phase decreases with calcination temperature increasing from 500 to 700 °C. Pure cubic ZrO₂ is obtained when added Y₂O₃ is greater than 8 mol%. A nanocrystallite size distribution between 10 and 20 nm is obtained in TEM observations.     

Magnetic resonances of Fe and Ni nanoparticles in films of silicon suboxide produced by ion irradiation of triethoxysilane gels containing Fe or Ni solute atoms

The structure and magnetic properties of composite films obtained by ion irradiation or thermal treatment of triethoxysilane gels containing iron or nickel in solution are investigated by means of X-ray diffraction, electron microscopy and electron spin resonance. Ion irradiated gels are converted into silica glasses containing metallic nanoparticles with a narrow range of sizes. These particles exhibit a magnetic resonance with a preferential alignment of magnetic moments perpendicular to the surface. This unusual out of plane anisotropy seem to be ascribed to an interaction of the moments with radiation defects. The magnetization in irradiated film is comparable to that in films of same materials submitted to a thermal conversion at 1000 °C in vacuum. But the latter films are porous and contain particles of metal and silicide or silicate with a wider range of sizes.     

A novel sol-gel method for the synthesis of γ-aluminium oxide: development of the sol-gel transformation and characterization of the xerogel

A novel method for the synthesis of aluminium oxide gel has been developed, whereby the sol-gel transformation was investigated. Aluminium tri-sec-butoxide was used as precursor while acetone was chosen as solvent. The synthesis was carried out in a special reactor, which allowed the dosing of steam. ²⁷Al NMR spectroscopy showed that during the sol-gel process the signal at δ∼3 ppm increases strongly corresponding to the formation of hexacoordinated aluminium species. Beside hydrolysis and condensation reactions, the coordination of acetone to a strong Lewis acid aluminium site occurs, which was shown by FTIR and ²⁷Al NMR spectroscopy. Viscosimetric analysis showed that at the beginning of the sol-gel process short polymers are observed while before the gelation a three-dimensional polymer network is formed. After pyrolyzing the gel a high surface area γ-aluminium oxide xerogel was formed. The effect of heating on the morphology and structure was examined by nitrogen physisorption (BET and pore size distribution), XRD and ²⁷Al MAS NMR spectroscopy.     

Influence of additives and post-synthesis treatment on the structural properties of sol-gel prepared alumina-doped zirconia studied by EXAFS-spectroscopy and X-ray diffraction

Alumina- (5 and 10 wt%) doped zirconia samples were prepared by sol-gel processing of aluminum sec-butoxide and zirconium butoxide in the presence of acids (acetic, sulfuric or nitric acid) or organic additives (Pluronic P123). X-ray absorption spectroscopy, X-ray diffraction and nitrogen adsorption experiments showed that the structure of the synthesized mixed oxides is strongly influenced by the chemical synthesis parameters.     

Study of the phase transition and the thermal nitridation of nanocrystalline sol-gel titania films

Nanocrystalline titania films were prepared by a complexing agent-assisted sol-gel method and converted to titanium nitride by a thermal nitridation process. The effect of acetylacetone (AcAc), diethanolamine (DEA) and acid catalysts (HCl and HNO₃) on the structure and morphology of the heat-treated titania films and on their nitridation products was examined by FTIR spectroscopy, X-ray diffraction (XRD) and atomic force microscopy (AFM). The carbothermal reduction of titania during the nitridation process with the formation of carboxynitrides has been considered. The results showed that the oxide to nitride transition strongly depends on the complexing agent used to prepare the titania films. The XRD results indicated the dependence of the lattice parameter of the nitridation product on the complexing agent or acid catalyst: AcAc and DEA lead to TiN_x with a lattice parameter α close to the theoretical value, while with HCl the lattice parameter was found sensibly lower showing the presence of an oxynitride. These results are accounted for by the effect of complexing agents and acid catalysts on the size of both TiO₂ and TiN grains and the different reactivity of the anatase and rutile phases. The possibility of tailoring the composition and morphology of TiN films by using complexing agents is envisaged.     

F-19 NMR study of the ordering of high field strength cations at fluoride sites in silicate and aluminosilicate glasses

Several glasses were synthesized to explore cation ordering at fluorine sites in silicate and aluminosilicate glasses. Utilizing ¹⁹F NMR, we found a significant (at least ∼30% of the total intensity) amount of the fluorine signal to be due to F-Mg(n) (fluorine with an unknown number of exclusively Mg²⁺ nearest neighbors) bonding in the Mg-aluminosilicates. By combining this with previous data on Ca- and Ba-aluminosilicate glasses, we demonstrated a clear trend of an increasing amount of F-M(n) type bonding with increasing field strength of the network modifying cation. This indicates that the higher field strength cations can more effectively compete with Al³⁺ for fluorine bonds. The mixed-modifier (Na,La)-silicate glasses have primarily F-M(n) type bonding, with a pronounced preference for bonding to the higher field strength La³⁺ over Na⁺. In addition, Si-F bonding was found in the (Na,La)-silicate glasses at a level (∼2% of the total intensity) comparable to that found in other silicate glasses, suggesting that Si-F bonding is consistently present in silicate glasses. The (Na,La)-silicate glasses also had unusually short spin-spin relaxation times, suggesting short (similar to crystalline fluorides) fluorine-fluorine distances in environments associated with La³⁺.     

Dependence of nanocrystalline SnO2 particle size on synthesis route

Very fine SnO₂ powders were produced by (a) slow and (b) forced hydrolysis of aqueous SnCl₄ solutions and (c) hydrolysis of tin(IV)-isopropoxide dissolved in isopropanol (sol-gel route) and then characterized by X-ray powder diffraction, Fourier transform infrared and laser Raman spectroscopies, TEM and BET. The XRD patterns showed the presence of the cassiterite structure. As found from XRD line broadening the crystallite sizes of all powders were in the nanometric range. TEM results also showed that the sizes of SnO₂ particles in all powders are in nanometric range. Very fine SnO₂ powders showed different features in the FT-IR spectra, depending on the route of their synthesis. The reference Raman spectrum of SnO₂ showed four bands at 773, 630, 472 and 86 (shoulder) cm⁻¹, as predicted by group theory. Very fine SnO₂ powders showed additional Raman bands, in dependence on their synthesis. The broad Raman band at 571 cm⁻¹ was ascribed to amorphous tin(IV)-hydrous oxide. The additional Raman bands at 500, 435 and 327 cm⁻¹ were recorded for nanosized SnO₂ particles produced by forced hydrolysis of SnCl₄ solutions. However, these additional Raman bands were not observed for nanosized SnO₂ particles produced by slow hydrolysis of SnCl₄ solution or the sol-gel route. The aggregation effects of nanosized particles were considered in the interpretation of the Raman band at 327 cm⁻¹. The method of low frequency Raman scattering was applied for SnO₂ particle size determination. On the basis of these measurements it was concluded that the size of SnO₂ particles was also in the nanometric range and that, the sol-gel particles heated to 400 °C consisted of several SnO₂ crystallites.     

Solid-state NMR study of metastable immiscibility in alkali borosilicate glasses

In several series of lithium, sodium, and potassium borosilicate glasses whose compositions traverse known regions of liquid-liquid phase separation, we have applied triple-quantum magic-angle spinning (3QMAS) ¹¹B and ¹⁷O NMR to obtain high-resolution information about short-range structure and connections among various network structural units, and their variation with composition and thermal history. Oxygen-17 3QMAS spectra reveal changes in connectivities between silicate and BO₃ (^[3]B) and BO₄ (^[4]B) units, by quantifying populations of bridging oxygens such as B-O-B, Si-O-B and Si-O-Si, and of non-bridging oxygens. ^[3]B-O-Si and ^[4]B-O-Si as well as ^[3]B-O-^[3]B and ^[4]B-O-^[3]B linkages can be distinguished. ¹¹B MAS and 3QMAS at a magnetic field of 14.1 T allow proportions of several borate units to be determined, including ^[3]B in boroxol ring and non-ring sites and ^[4]B with 3 versus 4 Si neighbors. By combining the ¹¹B and ¹⁷O NMR results, detailed information on Si/B mixing in sodium borosilicates can be derived, showing, for example, that ^[4]B and non-ring ^[3]B tend to mix with silicate units, while ring ^[3]B is mainly connected to borate groups. In a preliminary study of the effects of varying alkali cation, potassium-containing glasses are similar to those in the sodium borosilicate system, but a lithium borosilicate seems to exhibit considerably greater chemical heterogeneity. In annealing experiments that converted an optically clear to obviously phase-separated glasses, the ratio of ^[3]B to ^[4]B does not change significantly, but part of the non-ring ^[3]B converts to ring ^[3]B as the degree of unmixing increases.     

X-ray scattering studies of polymeric zirconium species in aqueous xerogels

X-ray scattering in the small angle range was used to investigate the structure and size of the polymeric species present in zirconium acetate (ZrAc)-based gel powders (GP) and thin films (TF). The formation of amorphous zirconia aggregates/linear polymers was studied by measuring their gyration radii and their correlation and hydrodynamic lengths via Guinier, `longrods', Zimm and Porod plots. Scattering data obtained in this investigation suggest that both amorphous ZrAc-based GP and TF contain `tetrameric' units, arranged in a cylindrical fashion, that polymerize upon thermal treatment. Housdorff fractal dimensionality, D, of amorphous zirconia GP (D∼2.25) and TF (D∼2.5) indicates a diffusion-limited aggregation mechanism. Tricyclic cluster aggregates (Porod D∼1.85) with a M_w∼12 000 were found within amorphous ZrAc-based GP. This structure is preserved over the entire curing range. In contrast, a linear polymeric chain (Porod D∼1.52), with a M_w∼20 000 below 165 °C and ∼13 000 at higher temperatures, was suggested for amorphous ZrAc-based TF.     

Ordered structure and preferred orientation of boehmite films prepared by the sol-gel method

Several boehmite films were produced using the sol-gel route. The drying process, the film structure evolution and the final texture of the samples were investigated by means of combined X-ray diffraction and weight variation. A remarkable (0 1 0) preferred orientation (about 97% of the crystallites) was found for xerogel films with thickness ranging from 1.5 to 24 μm. The ordered structure is already present in the sol state and is appeared and enhanced during drying, before gelling. This phenomenon may be attributed to the particular boehmite structure that induces special crystallite organization in the aquagel.     

Structural investigation of SnO-B2O3 glasses by solid-state NMR and X-ray photoelectron spectroscopy

Local structure of the SnO-B₂O₃ glasses was investigated using several spectroscopic techniques. ¹¹B MAS-NMR spectra suggested that BO₄ tetrahedral units maximized at around the composition with 50 mol% SnO. The BO₄ units were still present at compositions with high SnO content (67 mol% SnO), suggesting that SnO acted not only as a network modifier but also as a network former. O1s photoelectron spectra revealed that the addition of small amounts of SnO formed non-bridging oxygens (NBO) (B-O?Sn) and the amounts of NBO increased with an increase in SnO content. ¹¹⁹Sn Mössbauer spectra indicated that Sn was present only as Sn(II) in the glasses. The structure of the SnO-B₂O₃ glasses was compared with that of conventional alkali borate glasses and lead borate glasses. The thermal and viscous properties of these glasses were discussed on the basis of the glass structure revealed in the present study.     

Vibrational spectroscopic studies of sol-gel derived physical and chemical bonded ORMOSILs

Fourier transform infrared (FT-IR) and FT-Raman Spectroscopy have been utilized to characterize the structure of physical and chemical bonded ORMOSILs (organically modified silicates) and to compare with the physical properties, reported earlier. The classic and sono ORMOSILs were synthesized by sol-gel method via traditional- and sono-catalysis routes, respectively. The physical and chemical bonding is established by Poly(ethylene glycol) (PEG) and Poly(dimethoxysilane) (PDMS), respectively. Existence of band at ∼560 cm⁻¹ (FT-IR) and ∼490 cm⁻¹ (FT-Raman) for the sono and classic gels with 5-10 wt% of PEG indicates the presence of structural defect due to the four-membered siloxane rings. Application of long molecular PEG chain and PDMS increases this defect. Modification in the intensity and peak position of siloxane (Si-O-Si) bands is found correlative with the physical properties. The xerogels with 5-10 wt% of PEG possess a large number of residual surface silanol groups than the xerogels with long molecular PEG chains and PDMS and is found related with the ν(OH) band at ∼3470 cm⁻¹. These results are attributed to the way of hydrolysis-condensation reactions and discussed with the help of Percolation and DLVO models.