Egon Matijević

Preparation of Highly Concentrated Stable Dispersions of Uniform Silver Nanoparticles

Stable concentrated aqueous dispersions of silver nanoparticles of narrow size distribution were prepared by reducing silver nitrate solutions with ascorbic acid in the presence of Daxad 19 (sodium salt of a high-molecular-weight naphthalene sulfonate formaldehyde condensate) as stabilizing agent. The latter has excellent ability to prevent the aggregation of nanosize silver at high ionic strength and high concentration of metal (up to 0.3 mol dm(-3)). The presence of the dispersing agent on the surface of silver particles was confirmed by ATR-FTIR spectroscopy and electrokinetic measurements, explaining both the negative charge over the entire pH range and the electrosteric effect responsible for their long-term stability. The other experimental conditions, i.e., the pH of the reacting solutions, the concentration of the stabilizing agent, and the metal/dispersant ratio, also have a significant impact on the size and stability of these dispersions. The final nanosize silver can be obtained as dried powder, and can be fully redispersed in deionized water by sonication.

Formation of monodispersed spindle-type hematite particles

Abstract Procedures have been developed which yield spindle-type colloidal hematite particles of narrow size distribution by forced hydrolysis of ferric chloride solutions at elevated temperatures. It was shown that small additions of phosphate or hypophosphite ions to such systems before aging had a significant effect on the resulting particle shape. Specifically, the axial ratio of the precipitated solids depended on the concentration of these anions in the original solutions.

Preparation and properties of monodispersed colloidal particles of lanthanide compounds: I. Gadolinium, europium, terbium, samarium, and cerium(III)

Colloidal lanthanide (Gd, Eu, Tb, Sm, Ce(III)) compounds consisting of particles of narrow size distribution were obtained by aging at elevated temperatures solutions of corresponding salts in the presence of urea. Cerium(III) yielded crystalline oxydicarbonate solids of ellipsoidal shape, while the other rare-earth elements produced sols of amorphous spherical particles of basic carbonates. The effects of various parameters (pH, temperature, nature of anions, aging time) on the properties of the resulting precipitates were studied systematically. The metal hydroxycarbonates were readily converted to the corresponding oxides, without a change in morphology, by calcination at 600°C. The size distribution, specific surface area, and weight loss on calcination, as well as optical, fluorescent, and electrokinetic properties of the precipitated solids were evaluated. An apparatus designed for continuous production of such sols is described.

Preparation of monodispersed metal particles

This review article deals with the preparation, characterization and mechanisms of formation of uniform simple and composite metal particles of different modal diameters and shapes. In principle, such dispersions can be obtained by reduction of metal ions, in uncomplexed or complexed state, with suitable reducing agents and appropriate additives. The effect of the redox potential in a given oxidation/reduction system on the nature of the final particles is discussed in some detail. In another approach monodispersed particles of metal compounds are prepared first and then reduced to pure metals either in a liquid or a gaseous medium. In doing so, one can produce powders of a given morphology by selecting the precursor particles of the desired shape. A special case is represented by coated particles, consisting of cores and shells of different chemical composition. Depending on the materials, it is possible to reduce either one or both components by appropriate chemical reactions.

Preparation and properties of uniform coated colloidal particles. VII. Silica on hematite

Abstract Spindle-type hematite particles of narrow size distribution were coated with uniform layers of silica by hydrolysis of tetraethylorthosilicate (TEOS) in 2-propanol. The effects of the reaction time and initial concentrations of ammonia and water on the thickness of the silica shell were investigated and the rate of the coating process, in terms of the TEOS concentration, was evaluated.

Tailoring the particle size of monodispersed colloidal gold

Abstract Monodispersed spherical gold particles ranging in modal diameter from 80 nm to 5 μm, were prepared by reducing tetrachloroauric(III) acid with iso-ascorbic acid in aqueous solutions at 20°C. The particle size was altered by changing the pH, which affected the composition of gold(III) solute complexes. The latter controlled the redox potential of the system, essential to the formation of the initial nanosize gold dispersions. Depending on the experimental conditions, the resulting primary particles remained either stable or they aggregated to form much larger uniform spheres. The mechanisms of the precipitation of the precursors (primary) particles and of their mutual interactions to yield the final dispersions are discussed.

Phase transformations of iron oxides, oxohydroxides, and hydrous oxides in aqueous media

Abstract The available information regarding the pathways of processes leading to precipitation of iron (hydrous) oxides in aqueous salt solutions is reviewed. The importance of the early hydrolysis stages in determining the nature and the morphology of the solid phases is discussed. In the second part, the phase transformation between various oxides and oxohydroxides in aqueous suspensions is described with emphasis on mechanistic considerations. Such phase transformations may proceed under milder conditions (e.g., at lower temperatures) by a dissolution-recrystallization mechanism than in dry powders, which can also influence the morphology and the size of the resulting particles. A proper control of experimental parameters has resulted in a number of well defined colloidal iron (hydrous) oxides with respect to their composition, structure, morphology, and size.

Preparation and properties of uniform coated colloidal particles: V. Yttrium basic carbonate on polystyrene latex

Abstract Cationic polystyrene latex was coated with uniform layers of yttrium basic carbonate by aging at elevated temperatures dispersions of the polymer colloid in the presence of aqueous solutions of yttrium nitrate, urea, and polyvinylpyrrolidone. The thickness and morphology of the deposited layers could be altered by suitably adjusting the reactant concentrations, aging time, and temperature. A mechanism for the formation of the coating shell is suggested.

Conducting polymer composites. I, Surface-induced polymerization of pyrrole on iron(III) and cerium(IV) oxide particles

Abstract Electrically conducting polypyrrole coatings on inorganic cores were obtained by deposition of pyrrole on catalytically active particles. The finely dispersed core materials were hematite (polyhedral and spindle-type), silica coated with hematite, and cerium(IV) oxide. The degree of polymer coverage could be controlled by varying the aging time and the properties of the carrier particles. The so prepared coated powders were examined by transmission electron microscopy and further characterized by elemental and thermogravimetric analyses, as well as by electrophoresis, X-ray diffraction, and conductivity measurements. The conductivities were comparable to those reported for pure polypyrrole and they increased with pressure.

Preparation of hollow spherical particles of yttrium compounds

Hollow spherical particles of yttrium compounds were obtained by coating a polystyrene latex with yttrium basic carbonate layers and subsequently calcining at elevated temperatures in air. In order to establish the optimum conditions for the preparation of such hollow spheres, temperature and heating rate were systematically varied. The size of voids and of shells could be altered by using as the starting powder particles covered with inorganic layers of different thickness.