Synthesis and colloidochemical properties of cerium dioxide hydrosols stabilized by zirconium polycationic compounds

It is shown that cerium dioxide hydrosols can be stabilized by polycationic compounds of zirconium, and the optimum conditions of the stabilization are determined. Stable CeO₂ hydrosols with concentrations of higher than 20 wt % are prepared; the sizes and electrophoretic mobility of their particles, as well as sol stability in the presence of some electrolytes, are investigated. The nature of the aggregation stability of the stabilized cerium dioxide sols is discussed.     

Synthesis and aggregation stability of europium oxohydroxide hydrosols

Europium oxohydroxide hydrosols that are resistant to aggregation have been obtained by the condensation method. The density of dispersed phase particles, phase composition and dispersity of the sols, electrokinetic potential of the particles, and its dependence on medium pH have been determined. The effect of dispersion medium pH on the mechanism and reversibility of particle aggregation has been studied. The nature of the aggregation stability of the synthesized europium oxohydroxide hydrosols has been discussed.     

Synthesis and Some Properties of Cerium Dioxide Hydrosols

Cerium dioxide hydrosols are synthesized by peptizing with nitric acid a precipitate obtained by hydrolyzing cerium(III) nitrate. The synthesized sols are stable with respect to aggregation in both acidic (pH 1.5–3.0) and alkaline media (pH 9.0–11.0). The mean hydrodynamic radius of particles is about 25 nm. The isoelectric point corresponds to pH 6.2. The phase composition of sol particles is determined by X-ray diffraction at pH of the dispersion medium ranging from 1.5 to 3.0. The sol coagulation thresholds are determined in the presence of sodium nitrate and sulfate, as well as of mixed magnesium salt at various pH values of the dispersion medium. Assumptions are made concerning the nature of the aggregative stability of sols.     

Synthesis of hydrated tungsten(VI) oxide sols by peptization

A method has been developed for the synthesis of hydrated tungsten oxide hydrosols, with this method being based on potassium tungstate hydrolysis followed by peptization of the formed precipitate. The influence of the conditions of precipitation, aging, and washing of the precipitate on the particle phase composition and shape and the degree of precipitate peptization has been studied. Hydrosol-particle sizes have been determined by different methods. It has been found that the dispersed phase of the hydrosols consists mainly of platelike particles of hydrated tungsten oxide WO₃ · 2H₂O with a number-average size of 52 nm. The sols are stable to aggregation in a pH range of 3.0–4.5. The zeta potential of the particles ranges from–33 to–38 mV.     

Stable hydrosols for TiO<Subscript>2</Subscript> coatings

The optimum processing parameters required to synthesize, by hydrolysis of titanium isopropoxide (TIP), highly stable hydrosols composed of nanoparticles of the smallest possible size, are deduced both from data available in literature and from our own experiments. The colloids prepared in these conditions are composed of aggregates of anatase (~90%) and brookite crystallites (5–6 nm). They are suitable for coatings and have long-term stability (more than one year) in terms of polymorphic composition, crystallite and agglomerate size. Stable sols composed solely of anatase crystallites (4 nm) can be prepared by partially complexing the TIP by acetylacetone before hydrolysis. It is not possible to produce porous films with these colloids because they are stabilized by electrostatic repulsion which causes the particles to organize themselves, during the drying step, to form materials with a close packed structure. However, coatings with controlled porosity can be prepared from these stable sols through the post addition of polymers, like PEG or block copolymers.     

Synthesis of hydrosols of oxygen-containing cobalt compounds

A procedure is developed for the synthesis of hydrosols of oxygen-containing cobalt compounds. The hydrosols are stable to aggregation for several weeks in the absence of additional stabilizers. It is established that the synthesized hydrosols contain crystalline particles formed from a mixture of CoO(OH) and Co₃O₄. According to transmission electron microscopy data, the most probable diameter of the particles is about 30 nm. The hydrosols are stable to aggregation at pH 6.2?C8.0. In this pH range, the particles are positively charged and the values of their electrokinetic potential are no higher than 10 mV.     

Anomalous Concentration Dependences of Optical Density of Stable Nanodisperse Titanium Dioxide Hydrosols

Optical properties of two aggregative-stable nanodisperse TiO₂ hydrosols differed in sizes of crystalline particles were studied. These sols were found to have the anomalous concentration dependences of the optical density. It was suggested that these peculiarities are caused by an increase in structural order of the sols during their concentration.     

Synthesis of CeO<Subscript>2</Subscript>-ZrO<Subscript>2</Subscript> hydrosols via peptization at elevated temperature

A method for synthesizing aggregation-stable CeO₂-ZrO₂ hydrosols with different particle compositions is developed based on the peptization of hydrated oxide precipitates at elevated temperature. It is shown that, by varying heat treatment time, sols can be obtained with particles that have different degrees of crystallinity and sizes of no larger than 6 nm.     

Synthesis and Some Properties of Sols Prepared by Hydrolysis of Lanthanum Nitrate

The procedure for synthesis of hydrosols by hydrolysis of lanthanum nitrate was developed. The range of pH values was established corresponding to the maximum aggregation stability of sols. The photon-correlation spectroscopy was applied to determine the average hydrodynamic radius of sol particles, equal to 50 nm. The thresholds of fast coagulation of sol in the presence of sodium nitrate and sodium sulfate were determined.     

On the Mechanism of Dissolution of Boehmite Nanocrystallites in the Acidic Medium

The process of dissolution of boehmite nanocrystallites in acidic hydrosols is investigated at a temperature of 90°C. The average size of crystallites is determined from the width of peaks on X-ray diffraction patterns. It was established that, during the dissolution of more than 50% of the dispersed phase of sols, the average size of crystallites decreases slightly. This means that a part of the crystallites is dissolved completely, whereas the size of crystallites of the other part virtually does not decrease. The mechanism of such an unusual behavior of sols is discussed. Reversibility of the compression of AlOOH bilayers in boehmite crystallites in the presence of electrolytes was established.     

A study of electrolytic coagulation of gold sols by the method of localized surface plasmon resonance spectroscopy

Surface plasmon resonance spectroscopy has been employed to study the aggregation of gold sols with average nanoparticle sizes of 15–35 nm under the action of an indifferent electrolyte (NaCl). The structure of resulting aggregates has been established as depending on the coagulation regime. In the regime of fast coagulation, anisotropic aggregates with branched structure are initially formed; then, they are transformed into denser aggregates with a lower degree of anisotropy. The change of the aggregate structure accelerates with an increase in hydrosol concentration. At the same time, slow coagulation obviously yields denser aggregates, the structure of which is independent of the sol concentration and particle size. A procedure has been proposed for estimating the critical coagulation concentration of gold hydrosols based on the analysis of variations in their extinction. It has been found that the critical coagulation concentration increases with a reduction in the sol concentration and gold nanoparticle size.     

Electrolyte-induced destabilization of hydrosols containing uniform TiO<Subscript>2</Subscript> nanoparticles

The electrolyte-induced destabilization of TiO₂ hydrosols consisting of anatase nanocrystals with a narrow particle size distribution is studied. It is established that, on the addition of HCl and KCl, the sols can undergo both fast and slow structural changes. It is shown that the thresholds of fast coagulation increase considerably as the nanoparticle size diminishes. The reversibility and probable mechanisms of the slow structural changes, whose rate drastically increase with the electrolyte concentration, are discussed. The obtained results make it possible to improve the method for fractionation of TiO₂ sols based on coagulation with electrolytes.     

Synthesis and Some Colloidal Properties of Hydrosols Prepared by the Hydrolysis of Copper(II) Nitrate

The synthesis of hydrosols of copper(II) basic salts by the hydrolysis of copper(II) nitrate in the presence of ammonia was developed. A pH range and an optimal molar ratio [Cu²⁺] : [OH^–] = 1.1 : 1.0, at which hydrosols stable with respect to sedimentation were formed, were determined. The average hydrodynamic radius of dispersed phase particles ranging from 220 to 280 nm was measured by the photon correlation spectroscopy. It was shown that the hydrosols can be stabilized by poly(vinyl alcohol), and its optimal concentration providing their aggregation stability was determined. The thresholds of fast coagulation of the initial and stabilized hydrosols in the presence of potassium chloride and ammonium sulfate were found.     

Slow aggregation and disaggregation of TiO2 nanocrystals in aqueous HCl solutions

The kinetics of slow aggregation and disaggregation of the plate-shaped TiO₂ nanocrystals in aqueous HCl solutions at room temperature was studied using turbidimetry and small-angle X-ray scattering. The rate of slow aggregation of the diluted TiO₂ hydrosols with 0.3 ≤ pH ≤ 3 was compared. The sols with pH about 1 were the most stable and the aggregation rate increased at the decrease of pH from 1 to 0.3 and at the increase of pH from 1 to 3. The reversible loose aggregates slowly formed at the low pH, while the more stable aggregates grew at higher pH.     

Synthesis of copper(II) oxide hydrosols

A procedure is developed for the synthesis of copper(II) oxide hydrosols. It is shown that the main parameters that govern the reproducibility of the synthesis results and the aggregation stability of synthesized hydrosols are the temperature of copper(II) nitrate hydrolysis and the concentration of a peptizing agent. Dispersed phase particles are shown to have a cylindrical shape with a prevailing length-to-diameter ratio of approximately 2.7.     

Effects of ageing conditions and block copolymer concentration on the stability and micellization of P123-Ti<Superscript>4+</Superscript> sols prepared by the templating method

In the work reported here we studied, for the first time, the effects of ageing conditions (temperature and time) on the stability and micellization of two sets of sols with different P123 block copolymer (BC) concentrations by use of dynamic light scattering (DLS). Further, for a comparative study the aggregation/clustering behavior of pure P123 in IPA solution was also examined by DLS, and ¹H NMR, and Raman spectroscopy at room temperature. Different ageing regimes applied to the samples prepared included: (i) in-situ ageing of the sols in the sealed capillary cell of the DLS system at room temperature for 0, 1, 2, 3, 4, 5, 6, and 19 h, (ii) ageing of the sols in a Teflon-lined sealed vessel at 50, 55, 60, and 65 °C for 30 min, and (iii) isothermal ageing of the synthesized sols at 50 °C for 1, 4, 8, and 14 days. On the basis of the results obtained it is shown that both ageing time and temperature have remarkable effects on the clustering and aggregation of unimer/micelles formed in the sol system studied. Further, quantitative analysis of interparticle potential energy carried out for the prepared sol at low concentration confirmed that steric interactions play the major role among the other contributing sources of energy. This is mainly related to the presence of BC and the complex polymer structure formed by acetyl acetone (ACAC). Moreover, on the basis of the calculated stability ratio for this sol, we believe that reaction-limited cluster aggregation (RLCA) is possibly the major governing mechanism. Finally, kinetic and dynamic aspects of the micellar aggregates formed are also discussed in this paper.     

Visible light-induced reduction of Ag+ cations in silver hydrosols

It is shown that, in silver hydrosols that are produced by the reduction of metal cations under the action of borohydride and do not contain electron-donor additives, the photoinduced reduction of Ag⁺ ions and photoinduced oxidation of nanoparticles occur at pH 9.0 and 4.0, respectively. The effect of ionic composition of sols on the direction of the photoinduced process makes it possible to assume that OH^? anions reduce silver cations on the surface of nanosized nuclei.     

Synthesis and Use of Highly Dispersed Zinc Oxide

Methods for synthesis of zinc oxide hydrosols by peptization and condensation were developed. The basic colloid-chemical properties of the sols were determined: electrokinetic properties, size and phase composition of particles, and stability of hydrosols against introduction of electrolytes. The possibility of obtaining antibacterial and UV-protecting cosmetic preparations from the hydrosols obtained was demonstrated.     

Preparation of Thermostable Highly Dispersed Titanium Dioxide from Stable Hydrosols

A large set of stable nanodispersed TiO₂ hydrosols differing in particle structure and dispersion medium composition was synthesized. For highly dispersed TiO₂ samples obtained by calcination of dried sols at 500°C phase compositions, sizes of primary crystallites, and specific surface areas were found. The factors affecting thermal stability of TiO₂ nanoparticles were analyzed. The sol containing the most thermostable nanoparticles was used to produce a highly efficient catalyst for cyclohexanone ammoximation.     

Colloidal properties of CeO<Subscript>2</Subscript>-ZrO<Subscript>2</Subscript> hydrosols

CeO₂-ZrO₂ hydrosols are synthesized and the size, shape, phase composition, density, and electrophoretic mobility of particles are studied. The pH ranges of the stability of hydrosols and the thresholds of their fast coagulation in the presence of some electrolytes are determined. The nature of the aggregation stability of CeO₂-ZrO₂ hydrosols is discussed.