A multinuclear solid state NMR study of the sol-gel formation of amorphous Nb2O5-SiO2 materials

Evolution of microstructure in mixed niobia-hybrid silica thin films from sol-gel precursors

The evolution of structure in sol-gel derived mixed bridged silsesquioxane-niobium alkoxide sols and drying thin films was monitored in situ by small-angle X-ray scattering. Since sol-gel condensation of metal alkoxides proceeds much faster than that of silicon alkoxides, the incorporation of d-block metal dopants into silica typically leads to formation of densely packed nano-sized metal oxide clusters that we refer as metal oxide building blocks in a silica-based matrix. SAXS was used to study the process of niobia building block formation while drying the sol as a thin film at 40-80°C. The SAXS curves of mixed niobia-hybrid silica sols were dominated by the electron density contrast between sol particles and surrounding solvent. As the solvent evaporated and the sol particles approached each other, a correlation peak emerged. Since TEM microscopy revealed the absence of mesopores, the correlation peak was caused by a heterogeneous system of electron-rich regions and electron poor regions. The regions were assigned to small clusters that are rich in niobium and which are dispersed in a matrix that mainly consisted of hybrid silica. The correlation peak was associated with the typical distances between the electron dense clusters and corresponded with distances in real space of 1-3 nm. A relationship between the prehydrolysis time of the silica precursor and the size of the niobia building blocks was observed. When 1,2-bis(triethoxysilyl)ethane was first hydrolyzed for 30 min before adding niobium penta-ethoxide, the niobia building blocks reached a radius of 0.4 nm. Simultaneous hydrolysis of the two precursors resulted in somewhat larger average building block radii of 0.5-0.6 nm. This study shows that acid-catalyzed sol-gel polymerization of mixed hybrid silica niobium alkoxides can be rationalized and optimized by monitoring the structural evolution using time-resolved SAXS.

Preparation and complex characterization of silica holmium sol-gel monoliths

Amorphous, sol-gel derived SiO(2) are known to biocompatible and bioresorbable materials. Biodegradable and inert materials containing radioactive isotopes have potential application as delivery vehicles of the beta radiation to the cancer tumors inside the body. Incorporation of holmium in the sol-gel derived SiO(2) could lead to the formation of a biodegradable material which could be used as carrier biomaterial for the radiation of radioactive holmium to the various cancer sites. The homogeneity of the prepared sol-gel silica holmium monoliths was investigated by Back Scattered Electron Imaging of Scanning Electron Microscope equipped with Energy Dispersive X-ray Analysis, X-ray Induced Photoelectron Spectroscopy and Nuclear Magnetic Resonance Spectroscopy. The biodegradation of the monoliths was investigated in Simulated Body Fluid and TRIS (Trizma pre-set Crystals) solution. The results show that by suitable tailoring of the sol-gel processing parameters holmium can be homogeneously incorporated in the silica matrix with a controlled biodegradation rate.

The (29)Si T(1) and T(2) NMR relaxation in porous paramagnetic material SiO(2)-MnO-Al(2)O(3)

The (29)Si spin-lattice relaxation in porous silica-based material 1, doped by ions Mn(2+) at a Si/Mn ratio of 3.5, is non-exponential, independent of magic-angle spinning (MAS) rates and governed by direct dipolar coupling between electron and nucleus where an electron relaxation time is estimated to be about 10(-8)s. In the absence of mutual energy-conserving spin flips (spin diffusion) in 1, the (29)Si T(2) time increases linearly with spinning rates. None was observed in diamagnetic porous system 2. The unexpected (29)Si T(2) dependence has been interpreted in terms of the large bulk magnetic susceptibility (BMS) effects. It has been shown that editing the (29)Si Hahn-echo MAS NMR spectra eliminates wide lines, belonging to (29)Si nuclei in the proximity of paramagnetic centers, and reduces the BMS broadenings in sideband patterns for nuclei remote from these centers.

93Nb and 17O NMR chemical shifts of niobiophosphate compounds

Niobiophosphate compounds with a large range of niobium and oxygen environments were studied with (93)Nb and (17)O solid-state NMR. (93)Nb isotropic chemical shift of pure niobate Nb(ONb)(6), pure phosphate Nb(OP)(6) and mixed phosphate-niobate Nb(OP)(x)(ONb)((6-x)) (1<or= x<or=5) sites were measured at a high magnetic field (18.8T). (93)Nb chemical shifts were found to be sensitive to the variation of local charge on Nb, but not to the local bond geometry (i.e. crystallographic site and edge or corner connectivity). A systematic shift to high field of the (93)Nb chemical shift is measured when x increases. Then, (17)O NMR spectra of a series of enriched samples provided the chemical shift and quadrupolar parameters for 4 types of oxygen environment (P-O-P, P-O-Na, P-O-Nb and Nb-O-Nb). Finally, Nb-O-Nb sites were characterized by a large (17)O chemical shift anisotropy.

Solid-state NMR studies of 17O-enriched pyrophosphates

For the first time, 17O NMR studies were performed on 17O-enriched crystalline pyrophosphates (magnesium-, sodium- and barium-pyrophosphate) by means of triple-quantum magic-angle spinning (3QMAS) and double-rotation (DOR) in the high external field of 17.6 T. Oxygen atoms in bridging positions (P-OB-P) exhibit a significant higher quadrupole coupling constant compared to oxygen atoms in terminal positions (P-OT). With increasing cationic radius a higher value of the chemical shift of the terminal oxygen atoms is observed.

A new 17O-isotopic enrichment method for the NMR characterisation of phosphate compounds

Heating phosphate compounds under (17)O-enriched water vapour is an easy and rapid method to prepare homogeneously enriched and pure samples for the acquisition of (17)O NMR spectra with a good sensitivity.

23Na, 29Si, and 13C MAS NMR Investigation of Glass-Forming Reactions between Na2CO3 and SiO2

The glass-forming reactions between sodium carbonate (Na2CO3) and silica (SiO2) have been investigated by 23Na, 29Si, and 13C magic-angle spinning (MAS) NMR spectroscopy. The multinuclear MAS NMR approach identifies and quantifies reaction products and intermediates, both glassy and crystalline. A series of powdered batches of initial composition Na2CO3.xSiO2 (x = 1, 2) corresponding to a sodium metasilicate (Na2SiO3) and sodium disilicate (Na2Si2O5) stoichiometry were investigated after periods of isothermal and nonisothermal heat treatments at different temperatures. Analysis of the 23Na quadrupolar coupling parameters has identified the early reaction product in all cases as crystalline Na2SiO3. In the nonisothermal experiment, this reaction is preceded by an early silica-rich melt phase formed around 850 degrees C. The early reactions are controlled by solid-state Na+ diffusion across the reaction zone in the grain interface layer. Crystalline Na2SiO3 precipitates in the interface layer, increasing its thickness between the Na2CO3 and the SiO2 grains and slowing down the rate of Na+ migration. This creates a secondary phase, which is temperature dependent. At low temperatures, where Na+ migration is impaired, the production of Na2SiO3 ceases and silica-richer phases are precipitated. In the case of the sodium disilicate batch, where excess SiO2 is present, a secondary reaction of Na2SiO3 with SiO2 forming a glassy phase is observed. A transient carbon-bearing phase has been identified by 13C NMR as a NaCO3- complex loosely bound to bridging oxygens in the silicate network at the SiO2 grain surface.

93Nb NMR chemical shift scale for niobia systems

93Nb solid-state NMR spectra of a series of inorganic niobates with Nb in different oxygen coordination environments were measured. For all studied compounds the chemical shielding and quadrupole tensor parameters were determined using conventional and ultrahigh field NMR facilities, ultrahigh speed MAS, DQ STMAS, solid-echo and computer modeling. It has been demonstrated that the 93Nb isotropic chemical shift is sensitive to the coordination number of Nb sites. For the first time the 93Nb NMR chemical shift scale for NbOx polyhedra in solid materials has been proposed: for four-coordinated Nb sites, the isotropic shifts occur from -650 to -950 ppm; five-coordinated Nb sites have the isotropic shifts in the range of -900 to -980 ppm; for six-coordinated Nb sites the isotropic shifts vary from -900 to -1360 ppm; the shifts from -1200 to -1600 ppm are typical for seven-coordinated Nb sites; for eight-coordinated Nb sites the shifts are higher than -1400 ppm. The possible correlation between the value of the isotropic chemical shift and the ionic character of the NbOx-MOy polyhedra association has been suggested. The magnitude of the 93Nb quadrupole coupling constant depends on the local symmetry of Nb sites and may vary from hundreds of kHz to hundreds of MHz.