Solid State Reactions

Solid-state reactions offer the possibility of generating nano-particles by controlled phase transformations or reactions of solid materials. The advantage of this technology is its simple production route. The solid-state reaction technique is a convenient, inexpensive, and effective preparation method of monodisperse oxide nanoparticles/nanocapsules in high yield. Controlled growth of nanoparticles and their self-assembly into one-, two-, three-dimensional arrays by chemical methods are currently attracting considerable attention for potential applications in nanodevices.

An overview of the low-temperature heterogeneous and solid reactions was presented as a result of synthesis dispersed and nanophased powders of carbides based on molybdenum, tungsten, or titanium [190]. In known methods of synthesis of nanophased powders by physical and chemical vapor deposition, solution chemistry materials are assembled from atoms (or molecules) to nanoparticles in a bottom-up approach. Such methods have a long history of use in catalysis and colloid chemistry. Among known methods of low-temperature synthesis of nanophased powders, the catalytic reduction and following carbidization, activation of gas phase, and mechanical attrition were discussed. Primary attention was paid to the questions of dispersion and coalescence under topochemical reactions [190].

The different methods for encapsulating crystalline materials inside fullerene related structures were reviewed

[191]. The relationships between the mode of encapsulation and the crystallization behavior obtained in each case were described. The mechanisms of morphological and orientational control of crystallite growth inside carbon nanotubes and the comparative encapsulation behavior of materials encapsulated by physical and catalytic methods were discussed. The encapsulation of defect tungsten oxide structures within inorganic fullerene-like structures was described.

Preparation methods of zeolite molecular sieve membranes and films with and without support were reviewed

[192]. Unsupported films were prepared by in-situ synthesis, casting of zeolite nanoparticles, and solid-state transformation, and supported films were obtained by in-situ synthesis, vapor-phase synthesis, secondary growth, casting of nano-particles, and their combinations or modifications.

Core-shell nanoparticles of metal oxides ([Fe2O3]MgO, [Fe2O3]CaO, [V2O3]MgO, and the other first-row transition metal shell materials coated on nanoparticles of MgO or CaO) were studied as destructive adsorbents for CCl4, CHCl=CCl2, C5H4Cl2, CN3P(O)(OCH3)2, and SO2 [193]. A catalytic effect due to the transition metal shell material was observed, where solid-state ion-ion exchange took place, thus allowing penetration into the MgO or CaO particles and thereby regenerating the transition metal oxide for additional catalytic action. Due to this catalytic effect, the destructive adsorption reaction became nearly stoichio-metric, and therefore much higher capacities for destruction/immobilization of the adsorbate under study were realized. The catalytic effects were due to the intermediacy of transition metal chlorides, phosphates, or sulfites, which were mobile and sought out reactive sites on the MgO or CaO nanoparticles.

High yields of nanocrystals at ambient temperature were attained for several inorganic salts such as ZnS and CuO [194]. Edone-step, solid-state reactions between easily obtained starting materials such as CuCl2 • 2H2O and NaOH were reported to give nanoparticles with narrow size distribution. The process was carried out in air and required no complex apparatus, reagents, or techniques and thus it showed potential for mass production of nanocrystals. All of the reactions involved hydrated salts, and the role of the water in the reaction mechanism was discussed.

Carbon-coated a-iron particles were prepared through a simple heat treatment of mixtures of Fe3O4 and polyvinylchloride at 1000 °C in an Ar flow [195]. Hollow carbon particles were obtained after HCl treatment of the particles to remove iron and iron compounds. According to thermo-gravimetric analysis of the Fe3O4-polyvinylchloride mixture, the formation of carbon-coated a-iron particles proceeds in four steps with weight loss, the first two steps being due to the pyrolysis and carbonization of polyvinylchloride, and the last two to the reduction of Fe3O4 to a-Fe. Using the other transition metal oxides, NiO, CoO, and Cu2O, carbon-coated respective metal particles were also prepared using the same procedure.

Nano-oxides (SiO2, CeO2, SnO2) were successfully synthesized by solid-state reactions at ambient temperature [196-198]. For instance, tin oxide (SnO2) nanocrystals were synthesized by two-step solid-state reactions [198]. In the first step, the SnO fine particles were synthesized from the reaction of SnCl2 • 2H2O precursor with KOH in the size range of 500 nm. Then the fine particles were oxidized into nanosized SnO2 crystals with O2. It is an environment friendly, convenient, inexpensive, and efficient preparation of SnO nanocrystals with grain size of 20 nm. Effects of calcination on the nanoparticles were studied. The mechanisms of the formation of nanomaterials by solid-state reactions at ambient temperature were primarily investigated. The shape and size control of nanoparticles with organic stabilizers were studied, which had one (or more) coordination group and long alkyl chain [199]. The organic stabilizers were used as the templates of nanoparticle self-assembly.

A simple one-step solid-state reaction in the presence of a suitable surfactant was developed for synthesizing PbS nanoparticles with diameters of 10-15 nm [200]. The surfactant C18H37O(CH2CH2O)10H in the formation of PbS nano-particles was discussed, which played an important role in the preparation of PbS nanoparticles.

Barium hexaferrite particles between 1 /m and 100 nm in diameter were prepared by solid-state reaction from different iron oxide precursors [201]. The characteristics of the iron oxide precursor in terms of particle size and its distribution were essential to prepare the hexaferrite at a significantly reduced temperature as well as to obtain particles with reduced size. When a goethite sample consisting of uniform particles of 200 nm length was used as iron oxide precursor, pure and uniform hexaferrite particles of around 100 nm diameter were obtained by heating up to 675 °C for 1 h the mixture of iron oxide and barium carbonate.

A form of fullerene-type carbon, named carbon nanoflask, was synthesized, using CO(CO)3NO, as a special precursor [202]. Upon its decomposition, the CO(CO)3NO was not only a source of carbon but also gave rise to fcc cobalt particles. After a careful purification process, the percentage of cobalt-filled carbon flasks was as high as 30%. The graphitic layers of the flask walls were over 100 nm thick, and much thicker than the flask cap. After an acid treatment of the sample, opened and empty carbon flasks were easily obtained.

Gold nanoparticles and nanowires encapsulated in carbon nanocapsules and nanotubes were spontaneously formed from one-dimensional self-organized gold nanoparticles on carbon thin films by annealing at 200-400 °C [203]. The one-dimensional arrangement of gold nanoparticles was strongly dependent on the adhesive force at the atomic step edges of amorphous carbon thin films. The gold crystals inside the nanotubes were distorted by the crystal growth of the nanowires. The result was expected as a fabrication technique for self-assembled ultra-large-scale-integration nano-wires and cluster-protected quantum dots protected by nanocapsules and nanotubes at scales beyond the limits of current photolithography.

Stepwise adsorption of polyelectrolytes was used for the fabrication of micro- and nanocapsules with determined size, capsule wall composition, and thickness [204]. The capsule walls made of polyelectrolyte multilayers exclude high molecular weight compounds. Assembly of lipid layers onto these polyelectrolyte capsules prevented the permeation of small dyes. Encapsulation of magnetite nanoparticles and the features of these capsules were discussed.

Co-filled carbon nanocapsules, formed by a heat treatment of the mixture of Co and diamond nanoparticles, were studied [205]. The heat treatment reduced the surface native oxide (Co3O4) of Co nanoparticles. The reduction was accompanied by graphitization of diamond nanoparticles, indicating that diamond nanoparticles in contact with the metallic Co were transformed into graphitic coating. In-situ TEM studies showed that the graphitic coating was formed in the heating process, not in the cooling process. Once the coating was completed, the number of the graphitic layers was almost constant on further heating and cooling. It was concluded that metallic Co particles simply acted as templates for graphitic coating.

Controlling the surface properties of nanoparticulate materials is necessary if they are exploited in applications such as colloidal crystals or biolabeling [206]. By tailoring the polymer flexibility and the electrostatic forces involved in polyelectrolyte adsorption onto highly curved gold surfaces, through variation of the total salt concentrations suspending the chains and spheres, the irreversible polyelectrolyte wrapping of gold nanoparticles can be affected. By consecutively exposing the nanoparticles to polyelectrolyte solutions of opposite charge, polyelectrolytes can be deposited in a layer-by-layer sequence, yielding gold nanoparticles coated with uniform polyelectrolyte multilayers. Self-supporting poly-electrolyte multilayered nanocapsules were formed after dissolution of the metallic core. The gold nanoparticle surface charge, created by the adsorption of anions, was insufficient to overcome the ionic and hydrophobic polyan-ion/polycation interactions, resulting in polymer desorption from the highly curved nanoparticle surface. One successful method to immobilize the charge on the gold nanoparticle surface was to covalently attach an anionic thiol before poly-electrolyte modification.

Formation of carbon nanocapsules with various clusters (SiC, Au, Fe, Co, Ge, and GeO2) by polymer pyrolysis was investigated [207], and nanocapsules with SiC and Au nanoparticles were produced by thermal decomposition of polyvinyl alcohol at around 500 °C in Ar gas atmosphere [164, 165, 207].

Preparation of polycyclodextrin hollow spheres by tem-plating gold nanoparticles was reported [208]. Oxidation of gold nanoparticles protected by thiolated S-cyclodextrin molecules led to formation of water-soluble polycyclodextrin nanocapsules held together by S-S bonds.

S-SiC nanorods were synthesized by the reaction of SiO and carbon nanocapsules [209]. For the synthesis of SiC nanorods, the reaction temperature and the ratio of SiO to carbon nanocapsules were important and the most appropriate temperature and ratio were around 1380 °C and 5:2, respectively. Most of the SiC nanorods were straight and had diameters of 30-150 nm while the SiC tips of the SiC nanorods were 100-400 nm in size. Each SiC nanorod had one kind of preferential axis direction, which was either parallel or normal to the [111] direction. A possible growth mechanism of the SiC nanorods was proposed.

Dispersed Eu3+ cations in an aqueous EuCl3 solution easily incorporated in pores in a hydrogenated porous AlO(OH) • aH2O boehmite powder [210]. H2 gas in pores and OH- anions from energized boehmite with pores coverted EuCl3 into Eu2O3 in pores as per the reaction 2

EuCl3 + 3/2H2 + 3 OH ^ Eu2O3 + 6 HCl, in a closed reactor at room temperature. The obtained Eu3+ :Al2O3 product consisted of dispersed Eu2O3 nanoparticles (crystallite size was about 30 nm) in an amorphous Al2O3 in a mesoporous structure. On heating, Eu2O3 nanoparticles dissolved in high surface energy pores and resulted in a complete amorphous structure of Eu3+:Al2O3 at about 700 K. A controlled reconstructive nucleation and growth into y-Al2O3 nanoparticles (crystallite size was about 6.5 nm) occurred from the amorphous state at about 1000 K.

The sol-gel method is a process, consisting of solution, sol, gel, solidification, and heat treatment of metal organic or inorganic compounds, which is applicable to preparation of metal oxides and other compounds [211, 212]. The processing parameters can be controlled well at the beginning step so that the uniformity can be achieved at the scale of micrometers, nanometers, and even at the level of molecules, and the micro/nanostructures of the materials can be controlled/designed. The advantages of the sol-gel method are high purity due to no need for mechanical mixing; good chemical homogeneity and structural/composition uniformity; small particle size and narrow size distribution; low process temperature; simple technology and equipment; etc. The mechanisms for the formation of the sol-gel are traditional colloid type, inorganic polymer type, and complex type. The sol-gel method can be used for producing nano-particles, nanocapsules, thin films, fibres, nanocomposites, and bulk materials. The sol-gel procedure allows coating of templates with complex shapes on the micrometer to nanometer scale, which some commonly used coating procedures cannot achieve. In addition, sol-gel coating techniques can be applied to delicate systems without disruption of their structure or functionality, for example, the coating of biocomplexes or organic aggregates, such as organogelators. Three-dimensional structures with elaborate pore architectures, such as polymer membranes and gels, can also be infiltrated with sol-gel solutions to achieve nanocoatings.

Carbon nanocapsules and nanotubes were formed by one-dimensional self-organization of gold nanoparticles, caused by the adhesive force at the step edge of amorphous carbon thin films [213]. The technique used was expected to be promising for self-assembly of nanowires and cluster-protected quantum dots at scales beyond the limits of current photolithography.

Investigation of encapsulation and solvatochromism of fullerenes in binary solvent mixtures was reported [214]. Fullerenes, when dissolved in certain binary solvent mixtures, were found to exhibit strong solvatochromism and an unusual chemical inertness. From ultraviolet-visible (UV-vis) optical absorption measurements, dynamic light scattering size measurements, and chemical tests on C60/pyridine/water mixtures, the origin of these unusual effects was the formation of monodispersive, spherical, and chemically inert C60 nanocapsules. Pyridine acted as a surfactant around fullerene molecules protecting them from chemical reagents. The hydrophobic surfactant interactions were thought to act in stabilizing the structures into chemically inert, uniformly sized particles.

Using vesicular polymerization, water-soluble polyelec-trolyte nanocapsules were prepared which were able to undergo a reversible swelling transition upon changing the pH and/or salt concentration [215]. The solvation of Nafion oligomer in equimolar water-methanol solution was studied by means of molecular dynamics simulations [216]. Star-shaped patterns, which were composed of rodlike nano-particles with diameters of 4-7 nm and lengths ranging from 150 to 200 /m, were prepared by a convenient microemulsion technique at room temperature [217]. The hydropho-bic carbon chain of surfactant played an important role in controlling the morphology of the product. This microemulsion method was relatively mild and of low toxicity, which provided a route to the production of other metal sulfide patterns at room temperature.

A type of supramolecular compound, molybdenum-oxide-based composite was formed, consisting of magnetic nano-capsules with encapsulated keggin-ion electron reservoirs cross-linked to a two-dimensional network [218].

Ag colloid-containing coatings on soda lime glass and fused silica were prepared via the sol-gel process [219]. To incorporate Ag+ ions in the coatings homogeneously, they are stabilized by a functionalised silane (aminosi-lane) and then mixed with the basic sol prepared from 3-glycidoxypropyl trimethoxysilane and tetraethoxysilane. Crack-free and transparent coatings with thickness of 0.5 to 1.2 /m were obtained by heat treatment between 120 and 600 °C. The Ag-colloid formation was monitored by UV-vis spectroscopy as a function of temperature. The substrate had a deciding influence on the Ag-colloid formation caused by alkali diffusion from the substrate into the coating. Polycrystalline AgxOy nanoparticles formed during thermal densification in the coatings, which was accompanied by a vanishing of the yellow color of the coatings.

One-dimensional composite nanostructures (i.e., a S-SiC nanorod within a SiO2 nanorod) were synthesized by the combination of a carbothermal reduction at 1650 °C and a heating at 1800 °C of sol-gel derived silica xerogels containing carbon nanoparticles [220, 221]. The composite nanos-tructures were more than 20 / m in length. The diameters of the center thinner S-SiC nanorods were typically in the range of 10-30 nm, while the outside diameters of the corresponding thicker amorphous SiO2 nanorods were between 20 and 70 nm. Large quantities of SiC rod nuclei and the nanometer-sized nucleus sites on carbon nanoparticles were both favorable to the formation of much thinner S-SiC nanorods. The formation of the outer coaxial thicker amorphous SiO2 nanorod was from the combination reaction of decomposed SiO vapor and O2 during cooling.

Two sol-gel fabrication processes were investigated to make silica spheres containing Ag nanoparticles: (1) a modified Stober method for silica spheres below 1 / m size, and (2) a SiO2 film formation method on spheres of 3-7 /m size [222]. The spheres were designed to incorporate silver nanoparticles in a spherical optical cavity structure for the resonance effect. For the incorporation, interaction between [Ag(NH3)2]+ ion and SiOH was important. In the Stober method, the size of the silica spheres was determined by a charge balance of plus and minus ions on the silica surface. In the film formation method, the capture of Ag complex ion on the silica surface depended on whether the surface was covered with OH groups. After doping [Ag(NH3)2]+ into silica particles or SiO2 films on the spheres, these ions were reduced by NaBH4 to form silver nanoparticles. From plasma absorption at around 420 nm wavelength and TEM photographs of nanometer-sized silver particles, their formation inside the spherical cavity structures was confirmed.

Poly(N-vinyl 2-pyrrolidone)-protected gold clusters, incorporated into silica (SiO2) glass by a sol-gel process, were reported [223]. A sol-gel process was developed for fabricating inherently homogeneous nanocomposites involving precipitation of nanoparticles, coating the nanoparticles with a film of matrix phase, and powder formation and sintering [224].

Injectable and sprayable nanometer size hydrated silica particles encapsulating high molecular weight compounds such as [I-125]tyraminylinulin (mol wt 5 kD), fluorescein isothiocyanate-dextran (mol wt 19.6 kD), and horseradish peroxidase (mol wt 40 kD) were prepared [225]. The size of these particles was below 100 nm in diameter and the entrapment efficiency was found to be as high as 80%. Enzymes entrapped in these particles show MichaelisMenten kinetics and the catalytic reaction took place only after the diffusion of substrate molecules into the particles through the pores of the silica matrix. Peroxidase entrapped into silica nanoparticles showed higher stability toward temperature and pH changes compared to free enzyme molecules.

A chemically bound TiO2 layer was created on silica using a grafting method of chemical surface coating [226]. TiCl4 and H2O were used as reagents in successive cycles. The TiOx layer is constituted of nanoparticles of anatase homogeneously spread over the surface. Calcination of the TiO2-SiO2 materials led to a particle size increase. An extensive pore size analysis was undertaken to investigate the morphological changes as a function of the number of reaction cycles.

Nanosized Ce-doped silica particles (without and with Al addition) dispersed within pores of mesoporous silica host were synthesized by soaking and sol-gel techniques [227]. The dispersed phosphor particles were mainly located within the pores that were less than 4 nm in diameter. There existed two luminescence peaks at about 350 and 700 nm, respectively, for this phosphor in the dispersed or aggregated state. For the dispersed system, the luminescence intensities of both peaks are more than 14 times higher than those of the aggregated one, and the shoulders on the lower sides of the luminescence peaks disappear.

Ellipsoidal mesoscale "eggshells" were prepared by a template-directed synthesis using a sol-gel precursor [228]. TiO2 shells of micrometer sizes would have great potential as containers in microencapsulation and their ellipsoidal shape would provide them with a range of interesting optical, mechanical, and hydrodynamic properties.

Nanohybrids containing nonstoichiometric zinc ferrite of spinel structure in an amorphous silica matrix exhibited a fundamentally different structure to that formed by stoichio-metric zinc ferrite [229]. A unique cluster glass structure, where nanocrystallites of zinc ferrite existed in amorphous Fe-rich pockets, occurred at the significantly high Fe/Zn molar ratio of 10. The occurrence of zinc ferrite crystallites due to the Zn2+ deficiency, together with the confinement of the silica matrix, suppressed the nucleation and crystallization of a-Fe2O3 to a temperature above 900 °C, preserving the cluster glass structure.

A route was described for the synthesis of nanomet-ric Ni particles embedded in a mesoporous silica material with excellent potential for catalytic applications [230]. Mesoporous silica with a surface area in the range of 202280 m2/g with narrow pore size distribution and Ni nano-particles (particles in the range of 3-41 nm) were obtained in a direct process. A different approach was adopted to process such a nanocomposite, which was based on the formation of a polymer with the silicon oxianion and nickel cation chelated to the macromolecule structure and on the control of the pyrolysis step. The CO/CO2 atmosphere resulting from the pyrolysis of the organic material promoted the reduction of the Ni citrate.

The hybrid materials were investigated, consisting of nonagglomerated iron oxide particles hosted in silica aerogels [231]. The composite material can be produced as a monolith, in any shape, and with different dilutions of the iron oxide phase. Two sol-gel chemistry routes followed: a solution of Fe(NO3)3 • 9H2O was added either to the silica gel or to the initial sol, and the iron salt provided the water required for the gel polymerisation. To obtain monolithic aerogels, the gels were dried by hypercritical solvent evacuation. On the other hand, some gels were dried by slow and controlled evaporation of the solvent, resulting in xero-gels. Several heat treatments were performed and the iron oxide particle phase, growth mechanism, and crystallinity were analyzed.

Mesoporous nanocrystalline titanium dioxide with narrow pore size distribution was prepared by a sol-gel technique, when butanediol mixed with tetrapropylothotitanate was used as precursor [232]. The aging time for the synthesis had an evident influence on the phase transition of TiO2 and the nucleation process. A very fine network texture made from uniform nanoparticles was revealed. The mesoporous structure of as-prepared titania was maintained after a heat treatment at 350 and 400 °C for 1 h, exhibiting a significant thermal stability. Four titania crystal phases (amorphous, anatase, anatase-rutile, rutile) were observed at different calcination temperatures.

y-Fe2O3/SiO2 nanocomposites were prepared using a sol-gel procedure, starting from iron nitrate and triethyl orthosilicate [233]. The addition of acids to the sols resulted in a way to increase particle size, keep iron concentrations low, and narrow the particle size distribution of y-Fe2O3 in the glass composite. The addition of 0.56 mmol HCl (approximate to 0.1 M) to a solution having an Fe/Si molar ratio of 18% led to an increase of y-Fe2O3 particle size from 6 to 13 nm together with a remarkable decrease in the polydispersity degree of the particle size from 66% to 15%. The iron oxide crystalline phase, the particle size and shape, and the homogeneity of the resulting nanocomposites were studied. The influence of acid addition on the size of the magnetic particles was found to depend on the matrix microstructure, the charge environment, and the presence of counteranions of the acids.

A study was reported of the formation of particles of Ni-Zn ferrites embedded in a xerogel SiO2 matrix [234]. Initial solutions were prepared mixing tetraethyl orthosilicate, distilled water, ethanol, and three different nitrates: iron, nickel, and zinc. Formation of Ni0.5Zn05Fe2O4 as well as structural modifications of the SiO2 matrix induced by these particles were discussed. The composites of y-Fe2O3/SiO2 and a-Fe2O3/SiO2 were reported, starting from three different iron precursors: iron nitrate, iron chloride, and nanometric Fe particles prepared by aqueous chemical reduction [235].

Nanocoating, the covering of materials with a layer on the nanometer scale or covering of a nanoscale entity, to form nanocomposites and structured materials using the sol-gel process was reviewed [236]. Templates from spherical nano-particles to complex bicontinuous networks were discussed, where either the coated material or the structured inorganic hollow frame resulting after removal of the template were of interest in fields of application ranging from information storage to catalysis.

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