Pseudosolid, shear-thinning gel formation in binary dispersions of metal oxide nanoparticles at low volume fractions (original) (raw)
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Mixtures of fumed fractal metal oxide nanoparticles (np's) dispersed in water, at a solution pH where one species is positively charged and the other is negatively charged, form yield stress gels at volume fractions as low as 1.5%, due to electrostatic heteroaggregation into networks as confirmed by small-angle neutron scattering. These gels exhibit a measurable yield stress and an apparent viscosity that follows a power law relationship with shear rate. Rotational and oscillatory shear rheology is presented for binary mixtures of fumed silica, fumed alumina, and fumed titania in aqueous dispersions. Gels were characterized at various particle concentrations, solution pHs, mixture ratios, and salt concentrations. The strength of the gel network, as evaluated by the storage modulus and yield stress, is maximized when the mixture contains a mixture of particles with an approximately equal, but opposite, number of charged groups.
Journal of Solid State Chemistry, 1990
DEDICATED TO J. M. HONIG ON THE OCCASION OF HIS 65TH BIRTHDAY The steps in the evolution of colloidal gels during drying, aging, and crystallization into ceramic oxides are followed by high-resolution electron microscopy. Two systems are discussed in this paper. Colloidal ceric hydroxide gels in two morphologies were studied. In one, spherical gel particles with only shortrange order were observed to dehydrate to polycrystalline CeO, with large grain size. In the other a polycrystalline precipitate of CeO, was observed to increase its order without altering its grain size. Colloidal terbium hydroxycarbonate is also observed to have short-range order even in the earliest stages. In contrast to the cerium oxide these disordered materials are observed to crystallize and decompose in dynamic stages (even revealing reversibilities) through the intermediate dioxymonocarbonate to the oxide as carbon dioxide and water are removed. Observations are made by real-time video recording and time-lapse photography. o 1990 Academic press, IIIC.
Nonaqueous Sol–Gel Routes to Metal Oxide Nanoparticles
Accounts of Chemical Research, 2007
Sol-gel routes to metal oxide nanoparticles in organic solvents under exclusion of water have become a versatile alternative to aqueous methods. In comparison to the complex aqueous chemistry, nonaqueous processes offer the possibility of better understanding and controlling the reaction pathways on a molecular level, enabling the synthesis of nanomaterials with high crystallinity and well-defined and uniform particle morphologies. The organic components strongly influence the composition, size, shape, and surface properties of the inorganic product, underlining the demand to understand the role of the organic species at all stages of these processes for the development of a rational synthesis strategy for inorganic nanomaterials.
The Journal of Physical Chemistry B, 1999
The synthesis of semiconducting nanocrystals of manganese oxide of controlled sizes and their manipulation to form ordered arrays is described. Nanocrystalline mixed-valent manganese oxides have been prepared as colloidal solutions via reduction of tetraalkylammonium (methyl, ethyl, propyl, and butyl) permanganate salts in aqueous solutions with 2-butanol and ethanol. Reduction with the poorly water miscible 2-butanol produces aqueous colloids for the methyl, ethyl, and propyl systems, whereas 2-butanol colloids are produced for the butyl system. The colloids are reddish-brown, have an average manganese oxidation state of 3.70-3.79, and have been prepared in manganese concentrations up to 0.57 M. The sols will gel upon aging, and the gel time depends on the cation, the amount of alcohol, the temperature, and the concentration of manganese. Small angle neutron scattering (SANS) data indicate that the particles are disklike in shape with radii in the range 20-80 Å and are largely unassociated in solution. Thin films produced from evaporation of the colloid or spreading of the gel onto glass slides demonstrate long-range order, yielding an X-ray diffraction (XRD) pattern consistent with a structure of CdI 2-type layers of manganese oxide with tetraalkylammonium cations and water molecules interspersed between the layers. The SANS and XRD evidence are consistent with a mechanism of self-assembly of unassociated layers upon concentration and evaporation. Consistent with a mechanism of quantum confinement, UV/visible spectroscopy of the colloids reveals two absorbances, one near 220 nm and the other in the range 290-310 nm, blue-shifted from the maxima observed for bulk manganese oxide (400 nm). When the samples are aged, these bands shift to the red. The growth of particles in solution with aging, indicated by the shifts in absorbance, has been confirmed by SANS experiments.
Functional Metal Oxide Nanosystems by a Hybrid CVD/Sol–Gel Approach
Chemical Vapor Deposition, 2007
The present contribution is devoted to outlining some recent results from an innovative approach to metal oxide nanosystems. The proposed synthesis pathway is based on three successive steps: i) preparation of a porous xerogel layer (host) via sol-gel (SG); ii) CVD of a guest phase on the above matrix, making it possible to achieve an intimate host/guest intermixing; iii) ex situ thermal treatment under suitable conditions, aimed at tailoring the system properties as a function of the desired application. In particular, the above route has been adopted in two specific cases, LaCoO 3 and CeO 2 -ZrO 2 nanostructured thin films, which are acquiring increasing importance in the development of fuel cell technology. The most significant results, obtained by glancing incidence X-ray diffraction (GIXRD), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and transmission electron microscopy (TEM), will be critically discussed, emphasizing the achievement of tailored material properties by design.
Ultrastructural evolution during gelation of TiO2-SiO2 sols
Journal of Non-Crystalline Solids, 1992
Small angle X-ray scattering was used to examine in situ formation of mixed TiO2-SiO 2 gels. In order to elaborate the homogeneous solution, either ultrasonic radiation or alcoholic dilution of the precursors was carried out. The evolution of the typical sizes calculated at low and high q-regions were correlated. This lead to an approximate model for the aggregation process. The fit of the experimental data to a simple growth law was attempted allowing a kinetic rate constant to be estimated. This permits the evaluation of the differences induced in titanium doped silica sono-and classic gels.
Genesis of a Metal Oxide Gel by the Sol-Gel Process: Evidence from Scanning Electron Microscopy
Chemistry of Materials, 1994
Scanning electron microscopy (SEM) was used to characterize the products of sol-gel synthesis with Al(O-s-Bu)S and Mg(0Et)z in the presence of acetic acid and sec-butyl alcohol. Water was not included among the reactants, but it formed in the synthesis mixture. The gel formed slowly and was crystalline. The material was observed a t various stages of the sol-gel process, as illustrated by a single SEM image of a region of the sample. The image shows a honeycomb structure (which is not well characterized) that is the intermediate. The image also shows small particles that are formed from the honeycomb structure and indicates their coalesce into larger particles, which constitute the gel product.
Sol phase and sol–gel transition in SnO 2 colloidal suspensions
Journal of Applied Crystallography, 2000
The effect of concentration on the structure of SnO 2 colloids in aqueous suspension, on their spatial correlation and on the gelation process was studied by small angle x-ray scattering (SAXS). The shape of the experimental SAXS curves varies with suspension concentration. For diluted suspensions ([SnO 2 ] ≤ 0.13 mol L-1), SAXS results indicate the presence of colloidal fractal aggregates with an internal correlation length ξ ≅ 20 Å, without any noticeable spatial correlation between them. This suggests that the aggregates are spatially arranged without any significant interaction like in ideal gas structures. For higher concentrations ([SnO 2 ] = 0.16, 0.32, and 0.64 mol L-1), the colloidal aggregates are larger (ξ = 24 Å) and exhibit a certain degree of spatial correlation between them. The pair correlation function corresponding to the sol with the highest concentration (0.92 mol L-1) reveals a rather strong short range order between aggregates, characteristic of a fluid-like structure, with an average nearestneighbor distance between aggregates d 1 = 125 Å and an average second-neighbor distance d 2 = 283 Å. The pair distribution function remains essentially invariant during the sol-gel transition, suggesting that gelation involves the formation of a few points of connection between the aggregates resulting in a gel network constituted by essentially linear chains of clusters..