Synthesis of manganese oxide sols by KMnO<Subscript>4</Subscript> reduction with polyvinyl alcohol in an aqueous medium

Manganese oxide sols were synthesized using the chemical condensation method by KMnO₄ reduction with polyvinyl alcohol in an aqueous medium. The resulting gels were characterized by the manganese oxide content of 0.07–0.19 wt %, pH 9.5–10.2, and ζ-potential of–(20.3–42.9) mV. Spectrophotometric, turbidimetric, and dynamic light scattering methods were employed to determine how the particle size, ζ-potential, pH, and manganese oxide content in the resulting sols were affected by the KMnO₄ concentration and the KMnO₄ : polyvinyl alcohol ratio. The most stable sols preserved stability for over180 days and remained stable up to 0.5 M KCl, CaCl₂, and AlCl₃ concentrations.     

Studies of Lead Zirconate Titanate Sol Ageing Part I: Factors Affecting Particle Growth

The formation and growth of polymeric particles during the hydrolysis and condensation of PbZr_0.3Ti_0.7O₃ (PZT 30/70) precursor solutions have been studied as functions of time by using photon correlation spectroscopy (PCS). Particle shape was deduced by measuring the rheological properties of the sols and was found to remain chain-like during sol ageing. Various factors that affect the ageing of PZT sols have been investigated. Ethylene glycol (EG) was found to greatly accelerate the hydrolysis and condensation of the acid-modified Pb-free Zr/Ti solutions but had little effect on the particle growth for the Pb-containing PZT sols. Temperature and H₂O concentration were found to have major effects on the ageing behaviors. The higher the temperature and H₂O concentration, the higher the particle growth rates and the faster the sol gels.     

Preparation of unsupported alumina membrane by sol-gel techniques

Unsupported alumina membranes were prepared by sol-gel technique using aluminum isoproxide. The influence of the hydrolysis conditions, the type and concentration of peptizant acid on the boehmite sols has been studied. The suitable hydrolysis temperature for the aluminum isoproxide was above 50°C. Crack-free unsupported alumina membranes were obtained by rapid gelation processing of sols. The boehmite gel membrane and γ-Al₂O₃ membrane formed exhibited (020) and (440) preferred orientation.     

Kinetic study on photochromism of WO3 aqueous sol and its enhancement accompanying spectral changes by the addition of TiO2 aqueous sol

Photochromism of a WO₃ aqueous sol has been investigated in a nitrogen atmosphere under controlled temperature. Effects of ageing of the WO₃ sols, concentrations of WO₃ sols or Cl⁻ ion and temperature on the coloring rate were examined. The coloring rate was the first-order with respect of the WO₃ concentration. The coloring process was accelerated by an addition of TiO₂ aqueous sols. Spectral changes were measured using the mixing sol with various molar ratios (γ) of WO₃ and TiO₂. The absorption spectra changed from those having the single peak at 775 nm to those with two peaks at 640 and 980 nm. Such spectral transformation was ascribed to the structural change of the WO₃ nanoclusters, depending on the γ value and the concentration of Cl⁻ ion.     

Synthesis of high surface area nanocrystalline anatase-TiO2 powders derived from particulate sol-gel route by tailoring processing parameters

Stabilised titania sols were prepared using an additive free particulate sol-gel route, via electrostatic stabilisation mechanism, with various processing parameters. Peptisation temperature, 50°C and 70°C, and TiO₂ concentration, 0.1, 0.2 and 0.4 molar, were chosen as processing parameters during sol preparation. Results from TiO₂ particle size and zeta potential of sols revealed that the smallest titania hydrodynamic diameter (13 nm) and the highest zeta potential (47.7 mV) were obtained for the sol produced at the lower peptisation temperature of 50°C and lower TiO₂ concentration of 0.1 M. On the other hand, between the sols prepared at 70°C, smaller titania particles (20 nm) and higher zeta potential (46.3 mV) were achieved with increasing TiO₂ concentration up to 0.4 M. X-ray diffraction (XRD) and Brunauer-Emmett-Teller (BET) results of produced powders annealed at different temperatures showed that the 300°C annealed powder made from 0.1 M sol prepared at 50°C was a mixture of anatase and brookite, corresponding to a major phase of anatase (∼95% estimated), with the smallest average crystallite size of 1.3 nm and the highest specific surface area (SSA) of 193 m²/g. Furthermore, increasing TiO₂ concentration up to 0.4 molar for the sols prepared at 70°C resulted in decreasing the average crystallite size (1.9 nm at 300°C) and increasing SSA (116 m²/g at 300°C) of the powders annealed at different temperatures. Anatase-to-rutile phase transformation temperature was increased with decreasing peptisation temperature down to 50°C, whereas TiO₂ concentration had no effect on this transition. Anatase percentage increased with decreasing both peptisation temperature and TiO₂ concentration. Such prepared powders can be used in many applications in areas from photo catalysts to gas sensors.     

27Al NMR Study of Al-Speciation in Aqueous Alumina-Sols

A new method of preparing concentrated alumo-sols, the hydrolysis of Al-tri-sec-butylate in acidic aqueous media at 85°C, was studied in detail by varying the H₂O/Al- and NO₃ ⁻/Al-ratios in wide ranges. The components of the sols were characterized by ²⁷Al NMR spectroscopy. The pH-value depends on both the chosen H₂O/Al- and NO₃ ⁻/Al-ratio and on the aging time of the sols and reflects the composition of the sols. Al₁₃ polycations were detected in sols with a pH-value between 3.0 and 3.7. As a new result its presence was shown by NMR below 3.4. The Al₁₃ content of the sols increased with pH and the maximum fraction of Al₁₃ polycations was detected in the sol with the highest pH (3.7). Nearly 65% of the entire aluminium content of this solution is bound in the Al₁₃ polycations. Hence, a new synthetic method for the preparation of Al₁₃ ions containing sols was developed. Aging studies of the sols showed, that the Al₁₃ polycations were more stable in solutions with higher pH-value. Al₁₃ polycations were detected after an aging time of four months only in sols with a pH-value of 3.7. Tempering the aged sols at 40° to 80°C caused formation of Al₁₃ and also of Al₃₀ polycations.     

The effect of pH on aggregation kinetics of monodisperse silica sol in naCl solutions

The aggregation kinetics of monodisperse silica sols with a particle size of 220 nm in aqueous NaCl solutions is studied by flow ultramicroscopy in a pH range of 2.0–10.2. Slow coagulation of the sols is found to occur via the barrierless mechanism in the secondary potential minimum resulting from the summation of dispersion attractive and structural repulsive forces. The influence of pH and NaCl concentration on the parameters of the structural component of the energy of the interaction between SiO₂ particles is demonstrated.     

Studying the interaction between silica nanoparticles and metals by spectrophotometry

The optical absorption spectra of water silica sols containing nanoparticles (NPs) of metals (Ag, Pd, Fe, and Pt) are investigated. Silica sols are obtained from natural hydrothermal solutions via membrane concentration (ultrafiltration). Water sols of silica with specific sizes, pH values, ζ potentials of SiO₂ NP surfaces, and low concentrations of SiO₂ NPs are used. Plasmon resonance in optical absorption spectra is used to study the interaction between silica and metal NPs. Parameters of plasmon resonance (position, height, and half-width of optical absorption bands), from which the degree of interaction is assessed, are determined. Relationships between the optical properties of the surfaces of nanoparticle-size silica particles, the method of their production, and the effect of adsorbed metal particles on these properties are established.     

Electrochromism of NiO-TiO2 sol gel layers

Films of NiO-TiO₂ with Ni concentration of 100, 90, 87, 83, 75, 66, 50 and 33 mol% have been obtained via the sol-gel route by dip coating technique and sintered in air between 250 and 500°C using ethanolic sols of nickel acetate tetrahydrate (Ni(CH₃COO)₂·4H₂O) and titanium n-propoxide (Ti(O-CH(CH₃)₂)₄) precursors. Xerogels obtai- ned by drying the sols have been studied up to 900°C by thermal analysis (DTA/TG) coupled to mass and IR spectroscopy. The crystalline structure and morphology of the layers in the as deposited, bleached and colored states were determined by X-ray diffractometry, scanning electron microscopy and transmission electron microscopy Their electrochromic properties have been studied in 1 M KOH aqueous electrolyte as a function of the layer composition, thickness and sintering temperature. Deep brown colour with reversible transmittance changes have been obtained using cycling voltammetry and chronoamperometry processes. The best composition to get stable sols, a high reversible transmittance change and fast switching times (<10 s) was obtained with double NiO-TiO₂ layers 160 nm thick having 75% Ni molar concentration, and sintered between 300 and 350°C. The mechanism of coloration and morphology transformation of the layer during cycling are discussed in terms of an activation and degradation period. The results are in agreement with the accepted Bode model.     

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.     

On the Influence of Metal Alkoxides on the Epoxide Ring-Opening and Condensation Reactions of 3-Glycidoxypropyltrimethoxysilane

The extent of the epoxide ring-opening, the formation of ethyl ether groups as one reaction product of the ring-opening and the condensation degree of RSi(O_0.5)₃ units in sols and gels of the system 3-glycidoxypropyltrimethoxysilane (GPTS)-1.5H₂O-0.01/0.1/1.0 metal alkoxide [Si(OEt)₄, Sn(OBu^t)₄, Al(OBu^s)₃, Al(OEtOBu)₃, Ta(OEt)₅, Ti(OEt)₄, Zr(OBuⁿ)₄] in ethanol has been examined by means of liquid- and solid-state ₁₃C and ₂₉Si NMR spectroscopies. The results reveal a strong epoxide ring-opening effect of Al-alkoxides in hybrid sols after 24 h reaction time and of Zr-, Ta-, Al- and Sn-alkoxides in corresponding hybrid gels already at low concentration (1 mole%). The ring-opening rate increases in sols with higher metal alkoxide concentration (10 mole%) but decreases at 50 mole% concentration of Al-, Ti- and Zr-alkoxides. The ring-opening activity of metal alkoxides in 10 mole% hybrid sols increases after 24 h reaction time in the order Si(OEt)₄ < Ti(OEt)₄ < Zr(OBuⁿ)₄ < Ta(OEt)₅, Sn(OBu^t)₄, Al(OBu^s)₃, Al(OEtOBu)₃. The 24 h hybrid sols and gels contain considerable amounts (up to 90%) of ethyl ether groups as reaction product of the ring-opening reaction which lowers the formation of polyether bonds. The condensation degree (c.d.) of RSi(O_0.5)₃ units of GPTS-1.5H₂O sols with 10 mole% of metal alkoxides increases up to 80% after 7 h reaction time in the order: Si(OEt)₄ Sn(OBu^t)₄ < Zr(OBuⁿ)₄ < Al(OBu^s)₃ < Al(OEtOBu)₃ < Ta(OEt)₅ < Ti(OEt)₄. An additional increase in c.d. up to 90% follows after the thermal sol-gel transformation. Generally, maximum activity of metal alkoxides in ring-opening and condensation reactions was found in sols and gels with 10 mole% additives. The effect of water on the epoxide ring-opening and on c.d. is discussed. Furthermore, the activity of metal alkoxides is compared with corresponding nanoscaled metal oxides.     

Colloid stability of electrostatically stabilized sols

 The dependence of the stability of SiO₂ and Al₂O₃ sols on the pH of their medium has been studied. Vapor adsorption isotherms on powders were prepared from acidic and basic sols, the immersion enthalpy (heat) of samples containing preadsorbed water have been determined and, also, the reversibility of the sol ⇌ xerogel transformation, i.e. the peptizability of the powders, has been investigated. Based on the flocculation values determined with KCl, the sols have been classified into three groups. The stability of highly hydrophilic sols (acidic SiO₂- and Al₂O₃-sols) is ensured by a thick continuous diffuse lyosphere formed around the particles, as the continuity principle by Ostwald–Buzágh suggests. In this case, no electric charge is needed for ensuring stability. These sols are thermodynamically stable (group 1). Sols with medium stability are stabilized by the electrical double layer around the particles and by 1–2 layers of adsorbed water. The flocculation value of these sols is determined by the electrostatic interaction, whereas the peptizability of the flocs is related to formation of water layers. Such sols are the basic SiO₂- and Al₂O₃-sols (group 2). Sols of low stability are of hydrophobic nature. Their flocculation value with 1:1 electrolytes is smaller than 0.1 molkg^-1. The transformation process sol ⇒ floccule ⇒ xerogel is mostly irreversible. There are a lot of such sols (group 3). The existence of a hydrosphere is proved by the almost identical value of the hydration energy for both the acidic and the basic SiO₂ sols, in spite of an order of magnitude difference in the flocculation value. The remnants of adsorbed water after drying hinders sintering of the particles and ensures the peptizability of powders. The highly hydrophilic sols (sequence of hydrophilicity: SiO₂> Al₂O₃>FeO OH ⋅ 0.5 H₂O) are all oxides which are formed in acidic media. Received: 25 May 1997 Accepted: 13 October 1997     

Effects of diethanolamine on the evolution of silver/titanium dioxide sol–gel process

In order to clarify the effects of diethanolamine (DEA) in the silver (Ag)/titanium dioxide (TiO₂) sol–gel process, sols with and without DEA, and films derived from these sols were prepared. The samples were investigated by X-ray diffraction, transmission electron microscopy, electron diffraction and optical absorption spectra. The results showed that metallic Ag clusters were formed in the sol with DEA and was absent in the sol without DEA. This indicated that DEA worked not only as the stabilizer but also as the reduce agent in Ag/TiO₂ sol–gel process. After annealed, Ag metallic nanoparticles were generated in the films derived from both the sols with and without DEA. The particles in the films derived from the sol with DEA were smaller than those from the sol without DEA. This can be ascribed to the limitation of the growth of Ag cluster formed in the sol with DEA during heat treatment. Mechanisms for the formation of metallic Ag in the Ag/TiO₂ sols and films were discussed. The effects of DEA in the sols and films were studied in detail.     

Studies in the rheology of clay-water systems

The specific viscosity of sols of series of English, Mediterranean and American montmorillonites of varying exchangeable sodium content have been followed as a function of clay concentration; and the effects of variation in particle size and shape, p_H, etc. have been investigated. The part played by heterocoagulation has been studied, by comparing the properties of sodium montmorillonite sols dispersed in distilled water with those dispersed in polyphosphate solutions. The specifie viscosity —corrected for changes in relative volume caused by immobilisation of solvent by clay-water interaction —follows the Schulz-Blaschke relation for concentrated systems of uncharged anisometric particles; and the intrinsic viscosity rises with increasing plate diameter-thickness ratio.     

Influence of polyvinylpyrrolidone (PVP) on alumina sols prepared by a modified Yoldas procedure

Al₁₃ polycations containing alumina sols, prepared by a modified Yoldas procedure were mixed with polyvinylpyrrolidone (PVP). Although Al speciation in freshly prepared sols was not affected by PVP addition the decay rate of Al₁₃ polycations was slightly decreased in PVP-containing sols. PVP does not show any influence on particle size and particle growth. The influence of PVP addition on viscosity and flow behavior of modified Yoldas sols depends on their solids content and molar ratio.     

Preparation of highly stable TiO<Subscript>2</Subscript> sols and nanocrystalline TiO<Subscript>2</Subscript> films via a low temperature sol–gel route

Highly stable TiO₂ sols were prepared by adjusting the water-to-titanium molar ratio to ~4 in the process of hydrolysis and condensation of titanium isopropoxide in ethanol with HNO₃. Particularly, long-term stable TiO₂ sols were prepared without adding any chemical additives. Anatase TiO₂ nanocrystallites with sizes of 3–5 nm in diameter were uniformly dispersed in the stable sol. Crystallized TiO₂ films were successfully deposited on Si (100) using the stable sol via a dip-coating process with low temperature curing at as low as 100 °C. The synthesized TiO₂ sols and films are promising for use in flexible or dye-sensitized solar cells.     

Orientation of tungsten trioxide thin films fabricated by sol–gel method using aqueous sols of colloidal layered tungstates

Oriented monoclinic WO₃ thin films were fabricated by sol–gel method using aqueous sols of colloidal layered tungstates. The colloidal tungstate sols were prepared by reacting different alkylamines with layered tungstic acid H₂WO₄ in water. With decreasing the alkyl chain length of the alkylamines, the colloidal layered tungstate became smaller. Alkylamines with a short alkyl chain provided transparent aqueous sols. Furthermore, the WO₃ thin films fabricated from the obtained aqueous sols had a high (100)-orientation. However, upon annealing H₂WO₄ crystals applied on a glass substrate with the tungstate layers parallel to the substrate, highly (001)-oriented WO₃ layers were obtained. Since both of the A- and C-planes of WO₃ have a similar structure to the layers of H₂WO₄, the orientation of the WO₃ thin films and layers probably resulted from the topotactic structural conversion of the tungstates. Interestingly, the preferential orientation of the thin films was dependent on the presence or absence of interlayer alkylamines in the tungstates.     

Colorimetry of sulphur anions

Summary The colorimetric method for the determination and evaluation of polythionates above trithionate has been adapted for use in conjunction with sulphur sols. This has been achieved simply by increasing the period of cyanolysis. The method gives good results for the concentration of sulphur. The n values in the formula S _n O ₆ ^2– , which also represents sulphur sols, are not very accurate, but the accuracy might be increased by high precision colorimetry. However, they do serve to distinguish between polythionates and colloidal sulphur.Part III.: Urban, P. J.: Z. analyt. Chem. 180, 110 (1961).     

Polymer approach to supported silica membranes

Polymeric silica sols were used to prepare membranes on commercial -Al₂O₃ supports with nominal support pore diameters of 4.0 nm. Aging of the silica sols was shown to be effective to form discrete membrane layers. He/N₂ selectivity factors exceeding ideal Knudsen values were observed when the sols were prepared under conditions in which the condensation rate was minimized. We propose that the average pore size of the membrane depends on the balance of capillary pressure and modulus during membrane deposition. The condensation rate might influence the breadth of the pore size distribution.     

Preparation and characterization of TiO2 sols and their UV-cured hybrid thin films on plastic substrates

In this research, TiO₂ sols were synthesized via a sol?Cgel reaction at room temperature followed by heating under reflux. Hybrid thin films were prepared using the TiO₂ sols and dipentaerythritol hexacrylate on poly(methyl methacrylate) substrates via spin coating followed by UV-curing. The images of transmission electron microscopy (TEM) and results of dynamic light scattering (DSL) showed that some originally synthesized TiO₂ nanoparticles aggregated while many small-sized (~5?nm) TiO₂ nanoparticles still existed after reflux heating. The synthesized TiO₂ sols showed poor photocatalytic ability, which might avoid degradation of organic moieties in the hybrids. The refractive indices of the hybrid thin films were increased from 1.66 to 1.82 while the water contact angles on the thin films were increased from 70.2° to 87.7° with the increment of TiO₂ content. Increasing the heating time of the TiO₂ sol resulted in an increase in the refractive index and contact angle.