Precipitation

The precipitation method is a convenient and powerful technique for synthesizing nanoparticles and nanocapsules. Dissolvable salt solution consisting of one type or multitypes of ions could be hydrolyzed at a certain temperature when precipitator, like OH-, C2O^-, CO^-, etc., is added. The hydrates, hydrate oxides, or salt would form and separate out from solution. The oxide powders can be obtained by washing the anions in the solution and the solvent, and subsequently by pyrogenation or dehydration. When the pre-cipitator is added in the solution consisting of multitypes of cations, the complete precipitation of all the ions is the so-called co-precipitation method. The co-precipitation method consists of single-phase co-precipitation and mixture co-precipitation. Only a single compound or solid solution is the sediment in the former process, whereas the mixture deposits in the latter one. If one of the sediments were grown on the surface of others and the powders were in nanoscale, nanocapsules would form by employing the co-precipitation method. The advantages of the precipitation method are: high purity, high uniformity, small particle size, narrow size distribution, and structural/composition uniformity. Precipitation is a promising method for the economical production of commercial amounts of nano-particles/nanocapsules because it is fast and operable at ambient temperature.

ZnO nanoparticles in the size range from 2 to 7 nm were prepared by addition of LiOH to an ethanolic zinc acetate solution [237]. Nanoparticles of a model drug, viz. cholesteryl acetate, were prepared [238]. The cholesteryl acetate was dissolved in cyclohexane containing lecithin. The organic solution was emulsified in an aqueous solution containing a cosurfactant and a stable oil/water (o/w) emulsion resulted. The solvent was evaporated from the emulsion and cholesteryl acetate precipitated in the emulsion droplets. The size of the particles was almost unaffected by the concentration of cholesteryl acetate in cyclohexane.

A methodological description of heterogeneous polymerization processes, including suspension, emulsion, dispersion, and precipitation polymerization, was presented [239]. The discussion focused on the initial state of the polymerization mixture, the mechanism of particle formation, and the shape and size of polymer particles produced in different heterogeneous polymerization systems. The dependence of particle size and morphology on manufacturing parameters such as emulsifier, stabilizer, reactor design, and stirring speed was discussed. Special topics, including emulsifier-free (soapless) emulsion polymerization, seeded polymerization, and the formation of core-shell particles were covered.

Nanoparticles of barium ferrite (BaFe12O19) were synthesized using a method called microemulsion processing [240]. The aqueous cores (typically 5-25 nm in size) of water-cetyltrimethylammonium bromide-n-butanol-oct-ane microemulsions were used as constrained microreactors for the co-precipitation of precursor carbonates (typically 5-15 nm in size). The carbonates formed were separated, dried, and calcined to form nanoparticles (less than 100 nm) of barium ferrite.

Cubic structured CdS, CdSe, and CdTe, II-VI semiconductor nanoparticles were synthesized using aqueous solution precipitation at room temperature [241]. Thermal annealing caused an increase in particle size, a structural transition from the cubic to the bulk hexagonal structure for CdS and CdSe, and no structural transition for CdTe.

Well defined spherical particles of y-Fe2O3 were synthesized in the pores of a polymer matrix in the form of beads by an ion exchange and precipitation reaction [242]. The particle size distribution was Gaussian with an average diameter of 8 nm. The particles were superparamagnetic with a blocking temperature TB of about 55 K. The optical absorption edge of the mesoscopic system was redshifted with respect to single crystal films of y-Fe2O3 with an absorption tail extended deeply in the gap.

A kind of fullerene, a carbon nanoparticle encapsulating S-SiC grain, was precipitated during cooling Al2O3-Y2O3-CaO oxide melt containing SiC and C from 2023 K [243]. The SiC grains with a diameter of 5-20 nm were covered with 2-4 graphitic carbon layers with spacings of 0.34 nm.

Polyacrylamide-silver nanocomposites with silver nano-particles well dispersed in polyacrylamide matrix were synthesized by y-irradiation at ambient conditions in alcohol solvent [244]. The nanocomposites consisted of Mo phases, metallic silver, and polyacrylamide. The amount of silver present as metallic species in polyacrylamide matrix was measured by precipitation titration. The infrared (IR) spectrum confirmed the polymerization of acrylamide monomer and the formation of polyacrylamide in ethanol upon y-irradiation.

The biocatalytic systems from nanocapsules containing a-chymotrypsin in the inner aqueous cavities were prepared [245], which acted in both the organic solvent and the aqueous medium. For such encapsulation, the reversed hydrated micelles from N,N-diallyl-N,N-didodecyl ammonium bromide (DDAB) in cyclohexane (w0 = 22), including a-chymotrypsin, were polymerized by UV initiation. After precipitation by acetone, these nanocapsules were moved into the aqueous medium with the aid of ionic or non-ionic surfactants. In this case, unilamellar liposomes were formed. They had the inner monolayer from the poly-DDAB network and the outer one predominantly from surfactant molecules. According to the light-scattering data, the average outer diameter of nanocapsules equals 20 nm. The vesicular "coated" a-chymotrypsin was used for study of enzymatic activity.

The lattice dynamics of nanoparticles of Co embedded in Ag, created by ion implantation and subsequent annealing, were investigated [246]. A type of material that was able to produce giant strain in a nonhomogeneous magnetic field was developed [247]. In these magnetic-field-sensitive gels (ferrogels) fine colloidal particles having superparamag-netic behavior were incorporated into a highly swollen elastic polymer network. Magnetic properties were interpreted on the basis of a core-shell model.

Mossbauer spectroscopy in combination with atomic scale modeling was used in order to gather a comprehensive understanding of the growth and the dynamics of cobalt nanoprecipitates in silver [248]. The modeling made use of classical molecular dynamics in the canonical ensemble by means of the Rahman-Parinello technique.

Organic nanoparticles of cholesterol, Rhovanil, and Rhodiarome were synthesized in different microemulsions [AOT/heptane/water; tritonidecanol/water; cetyltrimethy-lammonium bromide (CTABr)/hexanol/water] by direct precipitation of the active compound in aqueous core under continuous ultrasound treatment [249]. The size of the nano-particles depended on different parameters such as the concentration of the organic molecules and the diameter of the water cores, which was related to the ratio R = [H2O][surfactant].

Polyvinyl alcohol coated magnetic particles (PVA fer-rofluids) were synthesized by chemical co-precipitation of Fe2+/Fe3+ salts in 1.5 mol/L NH4OH solution at 70 °C in the presence of PVA [250]. The resultant colloidal particles had core-shell structures, in which the iron oxide crystallites formed the cores and PVA chains formed the shells. The hydrodynamic diameter of the colloidal particles was in the range of 108 to 155 nm, which increased with increasing PVA concentration from 5 to 20 wt%. The size of the magnetic cores was relatively independent of PVA concentration.

Nanoparticles of various compositions were fabricated by chemical processes [251], like sol-gel processes and precipitation processes, and surface modifiers acted as vehicles to tailor surface chemistry, to tailor the £ potential, and to avoid agglomeration.

A line was drawn from the controlled preparation and understanding of Langmuir monolayers to the fabrication of hollow capsules [252]. Polymeric ultrathin films were permeable to small molecules, which could be used to prepare capsules after coating depolymerizable colloids. The inside of the capsule was designed such that it was distinguished by inner walls and pH in the interior in order to perform a specific chemistry. As an application example, selective precipitation of a dye molecule as a model for drug loading was presented.

Precipitation in the nanostructured Cr3C2/NiCr coatings was investigated [253]. Cr2O3 particles with an average size of 8.3 nm were observed in the nanostructured Cr3C2/NiCr coatings exposed to elevated temperatures. In addition to the precipitation of oxide particles, the phase transformations in the original NiCr amorphous phase were always observed in the as-sprayed nanostructured Cr3C2/NiCr coatings. The increases in microhardness and scratch resistance and decrease in coefficient of friction of the nanostructured coatings were attributed to a high density of oxide nano-particles precipitating within the coating as the exposure temperature increased.

A composite film of titanium dioxide (TiO2) nanoparticles and hydrolyzed styrene-maleic anhydride alternating copoly-mer (HSMA) was obtained on a substrate when a TiCl4 solution was heated at 80 °C with a spin-cast thin HSMA film present in the solution [254]. The TiO2 nanoparticles discretely dispersed on the polymer layer, which were dom-inantly rutile phase, of a spherical shape and 18-20 nm in diameter. In contrast, mainly amorphous TiO2 powders were obtained from the identical TiCl4 solution by drying the solution with the absence of the HSMA film. The TiO2 nano-particles deposited on the polymer layer were regarded to contain polymer chains, and a multilayered core-shell model was suggested for the formation of these composite nano-particles.

Nanostructured coatings on metals, plastics, and textiles have numerous applications, for example, as antifogging and self-cleaning coatings as well as protective coatings against corrosion, heat, or wear. The preparation at low temperature of dense nanostructured tetragonal ZrO2 coatings via a modified emulsion precipitation method was presented [255]. The method, which involved the controlled preparation, crystallization, and densification of nonagglomerated ZrO2 nanoparticles, opened up the possibility of applying nanocoatings of high-melting oxides to steel or plastic, where low temperatures were required.

Spontaneous core-shell and shell-shell reactivities of thiolate-capped nanoparticles were exploited for assembling nanoparticle network thin films via an exchange-cross-linking-precipitation route [256]. Gold nanoparticles of two different core sizes (2 and 5 nm) capped with decanethiolates and alkylthiols of two different functionalities, 1,9-nonanedithiol (NDT) and 11-mercaptoundecanoic acids (MUA), were studied as the assembly components. The film formation and growth involved intercore covalent Au-thiolate bonding at both ends of NDT, or intershell non-covalent hydrogen bonding at carboxylic acid terminals of MUA shells.

Synthesis and coating of superparamagnetic monodis-persed iron oxide nanoparticles were carried out by a chemical solution method [257]. A controlled coprecipi-tation technique was used to prevent undesirable critical oxidation of Fe2+. The obtained Fe3O4 nanoparticles were coated with sodium oleate.

Doped ZnS nanoparticles were synthesized using a chemical coprecipitation of Zn2+, Mn2+, Cu2+, and Cd2+ with sulfur ions in aqueous solution [258]. The diameter of the particles was close to 2-3 nm. The unique luminescence properties, such as the strength (its intensity is about 12 times that of ZnS nanoparticles) and stability of the visible-light emission, were observed from ZnS nanoparticles codoped with Cu2+ and Mn2+.

A method of preparing CdS nanoparticles inside MCM-41 was described [259]. This method made use of the unique interfacial properties of selectively functionalized ordered mesoporous materials to deliver cadmium ions inside the mesopores of MCM-41 via an ion-exchange reaction, thereby minimizing uncontrolled precipitation reactions of cadmium ions outside the mesopores. The general methodology can also be used to prepare other sulfide, telluride, and oxide nanoparticles.

Co3(BO3)2/surfactant composites were prepared by the controlled precipitation of aqueous cobalt cations together with surfactant [260]. The composite showed a layered structure, in which the Co3(BO3)2 layers of about 6 A in thickness were in alternation with surfactant bilayers. There was a freezing temperature at low temperature (<6 K) and the temperature was weakly frequency dependent. All the experimental observations suggested that the multilayers were spin glass in nature.

Thermoresponsive, core-shell poly-N-isopropylacrylam-ide (p-NIPAm) nanoparticles (microgels) were synthesized by seed and feed precipitation polymerization, and the influence of chemical differentiation between the core and shell polymers on the phase transition kinetics and thermodynamics was examined [261]. The core-shell architecture was a powerful one for the design of colloidal "smart gels" with tunable properties. The addition of small concentrations of a hydrophobic monomer (butyl methacrylate, BMA) into the particle shell produced large decreases in the rate of ther-moinduced particle collapse.

BN tassellike and treelike nanostructures were synthesized through a CVD method [262]. The tassellike morphology was made up of a BN bamboo-shaped nanotube and numerous polyhedral particles attached onto it. The tree consisted of a BN main stem and many BN nanotube branches growing outward from it. The attachments, either particles or nanotube branches, remained adhered to the primary stems even after 15 min sonication treatment, indicating the high stability of these nanostructures. The formation of these unusual structures was proposed to arise from a two-stage deposition process: First, primary BN stems were formed, followed by subsequent precipitation of amorphous clusters onto the rough outer surfaces. Second, polyhedral particles or BN nanotubes nucleated and grew on the outer surface as a result of further deposition from the vapor phase.

An approach to incorporate different polymers into micro- and nanocapsules fabricated by means of layer-by-layer adsorption of oppositely charged polyelectrolytes on colloidal particles was proposed [263]. This method comprised two stages. At first, the polymers, which were supposed to be incorporated, precipitated on the surface of colloidal particles. This can be done either by complexa-tion of polyelectrolytes with multivalent ions or by adding miscible nonsolvents. Then stable layer-by-layer assembled polyelectrolyte shells were formed. After core decomposition, the inner polymer molecules were released from the wall but were captured by the outer shell and floated in the capsule interior. The possibilities to encapsulate a wide class of charged and noncharged polymers were demonstrated on such examples as sodium poly(styrene sulfonate) as a polyanion, poly(allylamine hydrochloride) as a polycation, and Dextrane as noncharged water soluble polymer.

A method for the preparation of composite nanoparticles SiO2/ZnO by the controlled double-jet precipitation technique was described [264]. On the silica (SiO2) surface, the ZnO nanoparticles were coated as thin layers or nanoclus-ters, depending on the reaction conditions.

Various Au/Fe2O3 catalysts were prepared by the copre-cipitation method, and CO oxidation was studied at ambient temperature and in the presence of water vapor in the feed

[265]. The precipitation method and the calcination temperatures had a significant effect on the catalytic performance of CO oxidation. The stability was related to the particle size of metallic gold and a-Fe2O3 and the oxidation state of gold and the iron crystalline phase. The sintering of the gold particles, the reduction of oxide gold to metallic gold, the accumulation of carbonate, and a decrease in the specific surface area were observed during the reaction.

The process control of precipitation, due to the rapidity of the involved processes of mixing, nucleation, growth and agglomeration, and the stabilization against agglomeration represented challenges to this method. Schwarzer et al. showed how these challenges can be successfully handled

[266]. The focus was set on how to tailor the particle-size distribution in continuous precipitation [266]. Precipitation experiments with barium sulfate in a T-mixer were presented. It was found that the size of the precipitated primary particles was strongly dependent on the mixing intensity. On increasing the mixing intensity, it was possible to generate particles of approximately 50 nm in diameter. The second challenge, to stabilize the particles against agglomeration, was successfully met by adsorbing potential-determining ions on the particle surfaces (i.e., by increasing repulsive particle interactions). Thus, stable suspensions of barium sulfate nanoparticles were obtained.

Strontium ferrite nanoparticles were prepared by copre-cipitation in a polyacrylic acid (PAA) aqueous solution [267]. The average diameter of the mixed hydroxide precipitates was 3.1 nm. The average diameters of the strontium ferrite nanoparticles calcined at 700 and 800 °C were 34 and 41 nm, respectively.

The effects of the interactions of metal ions with lipoic acid-capped Ag and Au nanoparticles were studied [268], which were dependent on the metal ion concentration. First, in the dilute regime, there was reversible chelation of the metal ions, causing a marked dampening of the plasmon resonance band of the nanoparticles, but there was no aggregation. The magnitude of plasmon dampening depended on the nature as well as the concentration of the metal ions. In the intermediate concentration regime, aggregation occurs, but in the high concentration regime, there was precipitation.

Nanoparticles of a series of magnetic ferrites (MFe2O4, M = Zn, Ni, Co, Cu, Mn) were fabricated by hydrothermal precipitation [269]. The results on the mechanochemical synthesis of CaCO3, Cr2O3, and Nb2O5 nanopowders were reported [270]. The volume fraction of the matrix phase was crucial to the formation of separate, unagglomerated particles. With Cr2O3 and Nb2O5, amorphous particles were formed by mechanochemical reaction and a low temperature heat treatment was required for crystallization.

A heterogeneous precipitation method was described for obtaining nanocomposite powders consisting of Ni nano-particles homogeneously dispersed within y-Al2O3 [271]. The amorphous Al(OH)3 was nucleated on the surfaces of NiO nanoparticles and crystallized to y-Al2O3 at 900 °C. The porosity of y-Al2O3 allowed the growth of NiO

nanoparticles during calcinations. After the calcined nano-composite powders were selectively reduced at 700 °C in a hydrogen atmosphere, NiO nanoparticles were converted to Ni with the size of 25-35 nm, which were uniformly dispersed in the y-Al2O3 matrix.

Nanosized nickel aluminate spinel particles were synthesized with the aid of ultrasound radiation by a precursor approach [272]. Sonicating an aqueous solution of nickel nitrate, aluminum nitrate, and urea yielded a precursor that on heating at 950 °C for 14 h yielded nanosized NiAl2O4 particles with a size of 13 nm and a surface area of about 108 m2 g-1.

The precipitation and condensation of submicrometer organic particles were reviewed [273]. The importance of physical state effects was discussed, and the role of com-partmentalization in controlling particle size was introduced. The thermodynamic driving forces for precipitation and phase transformation were briefly reviewed. The use of emulsification as a primary step in producing small particle dispersions was described with photographic and pharmaceutical applications. Precipitation driven by solvent shifting was illustrated and applications in preparing organic-inorganic composites and protein coacervation were described. Miscible solvent-nonsolvent induced precipitation was outlined and followed by related applications using supercritical fluid technology. The applications of organic particle precipitation in reverse microemulsion systems were described. The applicability of gas condensation methods and of precipitation in submicrometer hollow spheres and dye entrapment in submicrometer polymer gel networks were discussed.

0 0

Post a comment