Oxidation Process

The oxidation process oxides and deoxidizes directly the raw materials in the liquid phase or quasi-liquid phase state. The oxidation process can be used to prepare the nanoparticles

Figure 6. Scanning electron microscopy (SEM) photographs of preformed polystyrene latex particles coated with the conducting polymer (poly(3,4-ethylenedioxythiophene)) as an ultrathin overlayer. After [373], M. J. Burchell et al., J. Phys. D 32, 1719 (1999). © 1999, IOP Publishing.
Figure 7. TEM micrograph of polyaddition latex comprised of Epikote E828 and 4,4'-Diaminobibenzyl. After [374], K. Landfester et al., Macromol. Chem. Phys. 201, 1 (2000). © 2000, Wiley-VCH.

of metals, alloys, oxides, etc., in either water solutions or organic solutions. For water solutions, solutions of metal salts with different pH values can be deoxidized by the reducers in gases or liquids. Almost all the metal ions in the solutions can be deoxidized by H reduction to form nanopartices. Usually, surfactants are needed in order to avoid aggregation of the nanoparticles. Nanoparticles can also be synthesized by thermal decomposition or reduction of the organic or metal organic solutions. On the other hand, the oxidation process can be employed for coating the cover layer of the nanoparticles to form the nanocapsules and for studying directly the oxidation behavior of the nano-materials. The oxidation process is one of the most simple, cheap, and easily controlled processes for preparation of nanoparticles/nanocapsules.

A method was proposed for Si3N4 ceramics by forming a nanocomposite grain-boundary phase of SiC nanoparticles in the weak grain-boundary phase [375]. The SiC nano-particles were formed by in-situ reaction between an oxide phase on the surface of Si3N4 powder and carbon, which was coated on Si3N4 powder by thermal decomposition of methane (CH4) gas. The surface oxygen content increased by an oxidation process or decreased by a washing process. The precipitated SiC content depended on the coated carbon and surface oxygen content on the Si3N4 powder surface. A surface oxidized Si3N4 powder was regarded as an inferior powder and the oxidized powder was used to produce high quality Si3N4 ceramics by applying the carbon coating method.

The oxidation behavior of silver (Ag) nanometer particles (about 3 nm in diameter) within pores of a silica host in ambient air (298 K) was investigated [376]. Exposure to air resulted in a blueshift of the absorption edge with time, which was attributed to the surface oxidation of Ag nano-particles. In air at room temperature, oxidation occurred in surface layers of Ag particles in the form of a dense Ag2O film. Once the dense film was formed, further oxidation was restrained.

TeO2/ZnO and ZnS/ZnO coated nanoparticles of a few nm in diameter were created using an oxidation process of ZnTe and ZnS particles produced by a gas evaporation technique [377]. In the case of TeO2/ZnO production, the first phase formed on the ZnTe particle was a ZnO layer on the particle surface, and a part of the ZnTe particles was changed to Te. On further oxidation for a few hours in air at 300 °C, the Te region gradually changed to TeO2 with increasing ZnO layer. In the case of ZnS particles, the surface of the particles was gradually changed to ZnO by heat treatment above 700 °C.

Biferrocene-modified Au clusters, comprising a 2.2 ± 0.3 nm diameter Au core covered with 20 biferrocene-terminated thiolates and 75 octyl thiolates on average, were synthesized by a substitution reaction of octanethiol-modified clusters with biferrocene-terminated alkanethiol, (^(5)-C5H5)Fe(C10H8)Fe(^(5)-C5H4CO(CH2)7SH) [378]. The biferrocene-modified cluster underwent two-step oxidation reactions in NBu4ClO4+CH2Cl2 and the second oxidation process led to the formation of a uniform redox-active Au cluster film on electrode.

The electrocatalytic activities of two kinds of iridium oxide electrodes were studied for their ability to evolve hydrogen and oxygen in 1 M H2SO4 at room temperature [379]. The first kind of electrode was made of anodic iridium oxide nanoparticles prepared by cycling well-defined iridium metal nanoparticles supported on carbon between O2 and H2 evolution potentials. The oxidation process was followed by cyclic voltammetry and CO oxidation experiments. The mechanism for the two evolution reactions and for the two different electrodes was discussed.

Anodes of SnO were charged reversibly with Li to capacities greater than 600 mAh/g. The anode materials were characterized by Sn119 Mossbauer spectrometry at 11 and 300 K [380]. Trends in the valence of Sn were as expected when the Sn oxides were reduced in the presence of Li. The oxidation process of Sn was very much the reverse of the Sn reduction during the Li insertion.

Weblike aggregates of coalesced Si nanocrystals were produced by a laser vaporization-controlled condensation technique, showing luminescence properties that were similar to those of porous Si [381]. The oxidized Si nanoparticles did not exhibit the red photoluminescence that was characteristic of the surface-oxidized Si nanocrystals. A significant blueshift in the red photoluminescence peak was observed as a result of the slow oxidation process. As the nanoparticles oxidize, the radius of the crystalline core decreased in size, which gave rise to a larger bandgap and consequently to the observed blueshift in the photoluminescence band.

The function of oxide film during the thermal oxidation process of Zn nanoparticles was reported [382]. A study was reported concerning oxidation of SiC and amorphous silicon (a-Si) nanoparticles obtained by plasma-enhanced CVD [383, 384]. The oxidation behavior of the as-grown powder was understood in view of its polymeric structure containing a large amount of hydrogen, allowing rapid diffusion of oxygen.

An X-ray photon spectroscopy method was used to investigate nanosize particles which worked as catalytical active surface probes for oxidation of benzene, toluene, and CO [385]. Active components in the process of benzene, toluene, and CO oxidation were Co and B compounds with oxygen featuring dangling bonds at the surface of nanoparticles. The nanosize dimensions of the synthesized boride particles played a substantial role in the oxidation process.

SO easily reduced Au3+ ions (1 : 1 equiv.) to monovalent Au+ species [386]. A salt composed of oxidized dye/Au+ was isolated. This salt was further reduced by another SO (2 : 1 equiv.) leading to gold atoms that coalesce into gold powder.

Pure y-Fe2O3 particles were continuously prepared by a continuous wave CO2 laser-induced pyrolysis of iron pen-tacarbonyl vapor in an oxidizing atmosphere [387]. Two different oxidation procedures were investigated: (1) airargon mixtures were employed to fill the apparatus; (2) air-ethylene mixtures were employed as carrier gas.

Electrooxidation of methanol in sulfuric acid solution was studied using Pt, Pt/Ni (1:1 and 3:1), Pt/Ru/Ni (5: 4:1 and 6:3.5:0.5), and Pt/Ru (1:1) alloy nanoparticle catalysts, in relation to methanol oxidation processes in the direct oxidation methanol fuel cell [388]. Pt/Ni and Pt/Ru/Ni alloys showed excellent catalytic activities compared to those of pure Pt and Pt/Ru. The role of Ni as a catalytically enhancing agent in the oxidation process was investigated. The presence of metallic Ni, NiO, Ni(OH)2, NiOOH, metallic Ru, RuO2, and RuO3 was confirmed.

Supramolecular polymer structures were assembled using the layer-by-layer, polyionic deposition technique [389]. Depositing alternating layers of a polycation, poly(diallyldimethylammonium chloride), and a polyanion, poly(styrenesulfonate), formed the thin films. The nucle-ation of nickel hydroxide nanoparticles was achieved using a cyclical absorption-oxidation process in which nickel ions were absorbed into the polymer matrix (which bind to the sulfonate groups) and then oxidized in an alkali solution.

3.3.9. Polymerization

Polymerization is a very common method for preparation of nanomaterials. During polymerization, usually, the formation of microemulsion is a very important factor which has been the focus of extensive research worldwide due to its importance in a variety of technological applications. These applications include enhanced oil recovery, combustion, cosmetics, pharmaceuticals, agriculture, metal cutting, lubrication, food, enzymatic catalysis, organic and bio-organic reactions, chemical synthesis of nanoparticles and nanocapsules, etc. In particular, nanocapsules consisting of an inorganic core and a polymer shell offer interesting prospects in various applications. Miniemulsions are specially formulated heterophase systems where stable nan-odroplets of one phase are dispersed in a second, continuous phase. It is delineated that miniemulsions rely on the appropriate combination of saturated high shear treatment, surfactants, and the presence of an osmotic pressure agent insoluble in the continuous phase. The droplet diameter is adjusted by the type and amount of surfactant, the volume fraction of disperse phase, and the general dispersion problem and lies between 30 and 500 nm. Since each of those droplets can be regarded as an individual batch reactor, a whole variety of polymerization reactions can be performed starting from miniemulsions, clearly extending the profile of classical emulsion polymerization. Actually, polymerization could be the only process or one of the multisteps of a whole preparation procedure. The polymerization process varies, depending on the kinds of the polymers used and the reactions. There are different polymerization processes, such as polymerization, co-polymerization, tri-polymerization, etc., depending on how many kinds of polymers are used. The polymerization process could be enhanced or adjusted by ultrasonic, plasma, microwave, etc. methods. The polymerization could be used with combination of other synthesization processes. As shown previously, during the chemical vapor deposition, solid-state reaction, sol-gel, precipitation, hydrothermal, solvent, latex, or oxidation process, the polmers could be used as the starting materials, such as precursors, surfactants, dispersions, etc. The polymerization could occur in all these kinds of processes for synthesis of nanoparticles and nanocapsules. Furthermore, it is easy to control the preparation parameters during the polymerization so that lot of different nanomaterials, like nanoparticles, nanocapsules, nanoporous, hollow, nano-wires, nanospheres, etc., can be synthesised. Figure 8 represents the design of water-soluble nanocapsules.

A review provided the features of microemulsions and their applications [390], which started with a brief introduction and focused on definition, structure, type, formation characteristics, stability, phase behavior, and the effect of additives, pressure, and temperature on the phase behavior of microemulsion. The physicochemical properties, state of water in the micropool, transport (electrical and hydrodynamic) behaviors, thermodynamics of formation, solubilization parameters, and uses and applications of microemulsions were presented.

Figure 9a represents the scheme of the formation of nanocapsules by miniemulsion polymerization and Figure 9b shows a TEM micrograph of nanocapsules of poly(styrene-co-acrylic acid) obtained by miniemulsion polymerization [391]. One major part of the work was concerned with the development of a convenient one-step syntheis of hollow polymer nanocapsules by the miniemulsion polymerization of different monomers in the presence of larger amounts of a hydrophobe. The idea was that hydrophobe and monomer formed a common miniemulsion, whereas the polymer was immiscible with the hydrophobe and demixed throughout polymerization to form a hollow structure surrounding the hydrophobe. It was found that the structure can be adjusted to cover the whole range from independent particles over partially engulfed structures to structurally integer nano-capsules of high uniformity [391].

An approach was presented for the identification of factors to permit the development of nanoparticle growth hormone

Figure 8. The design of water-soluble nanocapsules. Appropriately sized capsules are synthesised using dynamic combinatorial chemistry in an aim to develop efficient drug targeting systems. Image courtesy of S. Otto, Department of Chemistry, University of Cambridge, UK.
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