James L. Woolfrey

Formation of TiO2 Sols, Gels and Nanopowders from Hydrolysis of Ti(OiPr)4 in AOT Reverse Micelles

Titania sols, gels and nanopowders have been produced by the controlled hydrolysis of tetraisopropyltitanate (TPT) in sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles. Particle formation and aggregation have been investigated by photon correlation spectroscopy, the crystal phases by FT-Raman spectroscopy, and the crystallite dimensions of the precipitates by transmission electron microscopy. Nanoparticles could be produced at relatively high Ti(IV) concentrations (up to 0.05 mol dm−3). These nanoparticles aggregated into sols, with colloid sizes of 20–300 nm, eventually forming gelatinous precipitates. The kinetics of particle formation and aggregation were controlled by varying the primary process parameters [TPT], [H2O]/[AOT] (w0), and [H2O]/[Ti(IV)] (R), yielding a range of products including stable, transparent sols, precipitates and monolithic gels. The aggregation kinetics and physical properties of the sols depended strongly on w0. Different titania phases were produced, depending on w0; w0 ≤ 6 yielded amorphous particles, while w0 ≥ 10 produced anatase. The dimensions of the crystallites were comparable to those of the parent reverse micelles. A model was developed to interpret the effect of the primary process parameters on colloidal stability: (1) nucleation to form primary crystallites occurs by rapid hydrolysis and condensation reactions within the reverse micelle and (2) subsequent colloidal growth by aggregation occurs by reverse micellar exchange, where the rate of growth is governed by electrostatic and steric stability factors which increase as [AOT]/[TPT] (S) and residual [H2O]/[AOT] (wr) increase.

The Effect of Precursor Chemistry on the Crystallisation and Densification of Sol-Gel Derived Mullite Gels and Powders

Stoichiometric and silica-rich mullite gels and powders were prepared using four different sol-gel methods. Thermal analysis, X-ray powder diffraction and dilatometry techniques were used to investigate the thermal decomposition, crystallisation and sintering of these mullite precursor gels. The method of preparation, by controlled hydrolysis of various mixtures of tetraethylorthosilicate, aluminium sec-butoxide and aluminium nitrate, affected the texture of the gels, producing single-phase or diphasic samples.The crystallisation sequence of the gels depended on the composition and method of preparation. Single phase mullite crystallised from homogeneous gels at 980°C, while diphasic gels initially formed of a mixture of γ-Al2O3 spinel and mullite, or simple γ-Al2O3 spinel, which subsequently transformed to mullite at 1260°C.Dilatometry and density measurement were used to investigate the sintering of compacts formed by pressing powders prepared from gels precalcined at 500°C. Varying the heating rates from 2 to 10°C min-1 had little effect on the densification to 1500°C. However, the densification rate was sensitive to the degree of crystallinity and the amount and type of phases present at the sintering temperature. The presence of γ-Al2O3 spinel in the structure initially promoted densification, but the sintering rate was reduced considerably after mullite crystallised. Diphasic materials, especially those with an excess amount of silica in the original gel, sintered to higher densities due to the presence of excess silica promoting densification by viscous phase sintering.

Vibrational spectra of metal salts of bis(2-ethylhexyl)sulfosuccinate (AOT)

The vibrational spectra of sodium bis(2-ethylhexyl)sulfosuccinate (NaAOT), and NaAOT exchanged with alkali-metal (Li+, K+, Rb+, Cs), Ba2+ and Ce3+ and tetraphenylarsonium (TPA+) cations have been investigated. The effect of the charge-to-radius ratio of the counterion on the symmetric and antisymmetric sulfonate stretching modes has been studied to obtain a more comprehensive assignment of these modes for NaAOT. The symmetric sulfonate stretching mode shifted to lower wavenumber with increasing counterion radius, due to a decrease in the interaction between the cation and the SO3– group. A difference was observed between the IR and Raman spectra for the wavenumber of the symmetric sulfonate stretching mode, which was attributed to correlation coupling. This difference also decreased with increasing counterion radius. In the limiting case involving the large (TPA+) counterion, the symmetric sulfonate stretching mode appeared at the same wavenumber in the Raman and IR spectra. The spectrum of a model compound, bis(2-ethylhexyl)succinate (i.e. NaAOT without the sulfonate moiety), was used to assign bands in the 1300–1100 cm–1 region of the IR spectrum of NaAOT. This region consists of C–C and C–O stretching modes associated with the succinate backbone, and non-degenerate, antisymmetric stretching modes of the SO3– group. The correlation splitting yields four antisymmetric sulfonate stretching modes. The splitting decreases with increasing counterion radius, due to a reduction in the interaction between the cation and the SO3– group. The doubly degenerate, antisymmetric sulfonate stretching mode appeared as a single, broad band in the spectrum of (TPA)AOT. The carbonyl stretching mode of NaAOT appears as an asymmetric band between 1720 and 1740 cm–1. This asymmetry was attributed to different conformations about the acyl C–C bond. Weak interactions between the Na+ counterion and the carbonyl group may also contribute to the intensity of the lower-wavenumber band.

The influence of β-diketones on the induction times for hydrolysis of zirconium( IV ) alkoxides

The influence of low ligand/Zr mole ratios (ρ=0-0.1) of acetylacetone on the hydrolysis of Zr(OPr n ) 4 has been examined by investigating its effect on the induction time (t i ) before precipitation commences in the system. A remarkable feature of the data is the large increase in the induction time (reduction in hydrolysis/condensation rate) caused by addition of acetylacetone. This occurs even though only a maximum of one in eighty Zr-OR bonds have reacted with ligand and are no longer available for hydrolysis or further condensation. The effects on t i of five other β-diketones having varying steric properties (trifluoroacetylacetone, hexafluoroacetylacetone, benzoylacetone, dibenzoylmethane and dipivaloylmethane) are compared with that of acetylacetone, under similar concentration conditions, with ρ=0.1. Each shows a greater effect than acetylacetone, the largest change being caused by dibenzoylmethane which, under the conditions used, increases the induction time by a factor of 26 relative to acetylacetone. A mechanism rationalising the origin of the effect is discussed.

Formation and Gelation of Titania Nanoparticles from AOT Reverse Micelles

Titania nanoparticles have been produced by the controlled hydrolysis of tetraisopropyltitanate (TPT) in sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles. Particle formation and aggregation were investigated by static and dynamic light scattering and the chemical species by vibrational spectroscopy.The kinetics of particle formation and aggregation were controlled by varying [H2O]/[AOT] (w0), [H2O]/[Ti(IV)] and [AOT]/[Ti(IV)]. Nanoparticles, with diameters <10 nm, could be produced at relatively high Ti(IV) concentrations (up to 0.05 M). These nanoparticles aggregated into sols, with colloid sizes of 20 to 200 nm, eventually forming gelatinous precipitates.Different titania phases were produced, depending on the size of the micellar water pool; small pools (w0<6) yielded amorphous particles, while larger pools (w0>10) produced anatase.

Effect of processing variables on the structural evolution of silica gels

Constant shear rate and dynamic rheological measurements have been used to investigate the effect of shear on the processes controlling the structural evolution (size and volume fraction of clusters and the extent of cluster–cluster cross-linking) during the gelation of colloidal silica sols. In the absence of shear, the storage and loss moduli (G′ and G″, respectively) initially increase slowly prior to gelation, indicating that cluster growth and network formation are initially proceeding slowly, but then the system evolves rapidly, with cluster growth occurring at a slightly faster rate than network formation. In contrast, sols presheared for 4 h prior to gelation exhibit rapid increases in both G′ and G″ immediately after cessation of the applied shear, reflecting significant differences in the evolution of the gel structure. On aging, the viscoelastic properties of the unsheared and presheared samples are similar, indicating that their structures are comparable on the length scales (several microns) being probed by the frequency range used here. However, their chemical and microstructural properties differ significantly, due to differences in the intercluster bonds. The effect of shear rate, initial pH, colloid particle size, and volume fraction on the structure of the resulting sols and gels is discussed.

The Influence of β-Diketones on the Hydrolysis and Growth of Particles from Zirconium(IV) n-Propoxide

The influence of low ligand/Zr mole ratios (ρ = 0–0.1) on the hydrolysis and growth of oligomers from Zr(OnPr)4 modified with a series of β-diketones (trifluoroacetylacetone, hexafluoroacetylacetone, benzoylacetone, dibenzoylmethane and dipivaloylmethane), and their subsequent aggregation to form uniform, dense spheres, has been investigated by light scattering and turbidometry. The addition of β-diketones results in a large increase in the induction time, ti (reduction in hydrolysis/condensation rates). A remarkable feature of the data is the dramatic reductions in rate observed even when a maximum of one in forty Zr–OR bonds have been replaced by the β-diketone and are no longer available for hydrolysis or further condensation. The largest effect is observed with dibenzoylmethane, which increases ti by a factor of 26 relative to acetylacetone.A mechanism rationalising the origin of the effect is discussed, which involves segregation of the β-diketone ligands on the surface of the growing particle, with subsequent particle growth restricted to those sites not occupied by the chelating ligands.

FT-Raman spectroscopy of titania hydrolysates

Abstract FT-Raman spectroscopy has been used to study the peptisation of titania hydrolysates produced from tetraisopropyltitanate (TPT), chemically modified by reaction with a number of carboxylic acids. During peptisation with nitric acid at 333 K, a colloidal sol of well crystallised anatase was formed. Similar sols were formed by heating at 333 K in the absence of HNO 3 , but at a slower rate. The rate of structural change depended upon the chain length of the modifying acid. Decreases in the anatase band widths occurred during peptisation indicating increased crystallinity upon formation of the colloid. FT-Raman spectroscopy has proven to be an ideal tool for the study of titania colloids.

Shear-Induced Restructuring of Colloidal Silica Gels

Continuous oscillatory experiments were used to investigate the viscoelastic properties and structural evolution of silica gels obtained from unsheared and presheared sols, at pH 8. The processes controlling structural evolution before, at and after the gel point are discussed. Although the viscoelastic properties of the aged gels are comparable (implying similar structures on length scales of several microns), their chemical properties and microstructures (<1 μm) are significantly different. This is directly related to the different aggregate structures formed in the unsheared and presheared systems (50 and 700 nm, respectively) and the associated concentration of surface hydroxyls available for cross-linking and formation of stable siloxane bonds.

Kinetics of colloid formation during the preparation of sol-gel zirconia - Code: C4

The formation of zirconia colloids by hydrolysing zirconium n-propoxide in n-propanol has been investigated by simultaneous, multi-angle static and dynamic light scattering, and vibrational spectroscopy, as a function of reactant concentration, water-to-alkoxide mole ratio and temperature.The overall hydrolysis/condensation reaction followed pseudo 2nd-order kinetics at 303 K, with an induction period of <1 to 24 hours. The induction period could be substantially reduced by increasing the temperature to 348 K. For hydrolysis with 3.6 moles of water per mole of alkoxide, the apparent activation energy was 24 kJ mol−1. Such a low activation energy implies that hydrolysis occurs readily over the temperature range investigated (303–348 K). During the induction period, processing with stoichiometric, or excess, water produced “oxy-hydroxides”, while “hydrated oxides” formed under water-deficient conditions.The hydrolysis reactions yielded zirconia colloids with equivalent spherical, z-averaged diameters of <200 nm. The colloids exhibited fractal dimensions of 3.0, with a low size-polydispersity, inferring the formation of dense, monodispersed spherical particles. SEM observations confirmed these results.