Seeding, nucleation

Wet preparation of red iron oxides can involve either a hydrothermal process (see Hydrothermal processing) or a direct precipitation and growth of iron oxide particles on specially prepared nucleating seeds of Fe202- In the hydrothermal process, iron(II) salt is chemically oxidized to iron(III) salt, which is further treated by alkahes to precipitate a hydrated iron(III) oxide gel. The gel can be dehydrated to anhydrous hematite under pressure at a temperature around 150°C. 

Nucleation is initiated by secondary mechanisms involving the seed crystals or low super-saturation and high surface area of seed crystals eliminate or minimize nucleation seed crystals grow... 

The growth pathway of various fullerene- and graphene-type nano-objects may be related. They are synthesized in the vapor phase and often appear simultaneously on the same sample. A common growth mechanism with similar nucleation seeds may, therefore, lead to these different structures. 

Similarly, when rhombic red a-sulfur is heated above 100°C, it usually fails to exhibit the expected thermodynamic conversion to yellow /3-sulfur at 96°C. Instead, it persists as a superheated metastable phase up to 114°C (dashed line), where it exhibits an apparently normal melting point to the liquid form (unless extreme patience or a nucleating seed crystal of /3-sulfur is employed). The dashed lines in Fig. 7.5 therefore mark out metastable phase transition boundaries between forms of sulfur that are not true Gibbs free energy minima at the cited temperature and pressure (e.g., superheated a-sulfur and supercooled liquid sulfur at 114°C, 1 atm). The metastable phase domains can overlap stable phase domains in a quite complex and confusing manner. A kinetically facile metastable phase boundary will often appear more real and relevant to actual chemical phenomena than will the idealized boundary between (kinetically inaccessible) phases of lowest Gibbs free energy. 

Periodic nanoporous silicates have been prepared in a wide variety of conditions. Different sources of molecular, and non molecular silica have been used. This includes TEOS, TMOS, fumed, colloidal and precipitated silicas. Depending on the synthesis conditions, particularly on the nature of the silica source, crystallization may take place in seconds at subambient temperatures [82], or at room temperature [60,61,69,72,83]. However, in most cases the crystallization temperature was set in the 80 - 120 °C range. Liu et al. [84,85] found that the use of small amounts of colloidal particles (silica or titania) promotes the formation of ordered structures by providing nucleation seeds. The pH conditions varied from extremely acidic [60,61], to neutral [69,72] to very basic [48,49]. Ryoo and Kim [86]... 

Indeed, the smallest tube that we measured had a diameter of 10 A, which is of the size of C o- predicted to be the limiting case of vapor-grown graphitic tubes with monolayer thickness (14). Our observation of hemispherically capped 10 A tubes suggests that an incomplete cluster is the nucleation seed for these tubes. The 60 derived tube could be the core of possible multilayer concentric graphitic tubes. After the fullerene-based tube has been formed, further concentric shells can be added by graphitic cylindrical layer growth. 

The hydrolysis product is washed, treated with a Ti(III) solution to remove adsorbed heavy metal ions (Fe, Cr, Mn, V) or bleached with aluminum and acids and calcined at temperatures of ca. 1000°C. With doping or an appropriate choice of additive, latterly rutilization nucleating seeds, before calcination, anatase or rutile pigments can be produced in the calcination process. 

Figure 3 A typical microtubule assembly reaction is initiated by warming a solution of ice-cold tubulin dimers to 37°C in the presence of GTP. Tubulin dimers (adjacent white and gray circles) slowly form nucleating seeds (heptameric tubulin aggregate), which catalyze a rapid phase of microtubule elongation (growing microtubule) enroute to a steady state condition of microtubule formation and destruction. The assembly reaction is monitored by measuring the change in absorbance at 350 nm. In vitro incubation of microtubules with 2,5-HD or in vivo exposure of animals to 2,5-HD followed by tubulin purification yields pyrrolylated tubulin with altered assembly behavior. 2,5-HD-modified tubulin quickly forms numerous seeds, resulting in more rapid assembly into greater numbers of shorter microtubules compared to the control.

The results using tubulin purified from treated animals were confirmed and extended with microtubules treated in vitro with 2,5-HD (40). In vitro incubation with high concentrations of 2,5-HD generated a markedly altered tubulin that could assemble in the absence of added GTP, could readily nucleate the assembly of control tubulin, and was resistant to cold-induced disassembly. The induction of these 2,5-HD-induced assembly alterations required that the incubation take place with assembled microtubules. Negative-stain electron microscopy showed that 2,5-HD incubation followed by assembly led to shorter microtubules than control assemblies, a result explained by the treatment-related induction of numerous nucleating seeds. 

Crystallization processes using either added or nucleated seed can be designed to manufacture product with a desired product size distribution. The following steps must be taken to achieve this goal ... 

In the modified procedure, the organic solution was distilled under vacuum until the batch concentration exceeded the solubihty limit. In order to avoid undesited spontaneous nucleation, seed was charged at this point and the batch was aged to develop a seed bed. Following the seed age, toluene was fed and THF was distilled continuously under vacuum while maintaining the same batch volume. On completion of the distillation, additional hexane was added to minimize product loss in the mother hquor. The key parameters for this process are construction of the solubility map, determination of the seeding point, and control of the solvent evaporation rate to release the supersatrrration at the proper rate. 

The hydrothermal stability and acidity of aluminosilicate mesostructures can be improved substantially through the surfactant - directed assembly of protozeolitic aluminosilicate nanoclusters that normally nucleate (seed) the formation of microporous zeolites. Our results indicate that zeolite type Y, Beta, and ZSM-5 seeds are particularly effective at forming steam stable aluminosilicate mesostructures, which we generally denoted as MSU-S. 

Zhu et al. [109] proposed an environmentally innocuous method of preparation by using a single-step solvothermal route in ethanol solutions. The procedure leads to simultaneous rGO reduction and iron or cobalt oxide precipitation due to the fact that the GO/rGO layers act as heterogeneous nucleation seeds during the precipitation of the metal oxide nanocrystals. In a related approach, Han et al. [110] were able to obtain Li4Ti50i2 particles anchored to rGO by solvothermal treatment of H2O/ EtOH-based suspensions of graphite oxide and the oxide powder. The process involves reduction of GO and attachment of the mixed oxide nanoparticles within a single step. 

Acids can act as dopants for polyaniline therefore, if the nanofibers are first doped with these acids, and subsequently exposed to the metal ions, precipitates should be formed on the surface of the nanofibers, leading to inorganic-polyaniline nanofiber composites. This idea has been applied in Section 7.3.5 to improve the detection of H2S. Another possibility is to use nanofibers as nucleation seeds to collect inorganic nanoparticles from supersaturated solutions [148]. 

Bakr et al. reported the flow reactor synthesis of PbS quantum dots for applications in solar cells. They showed that the flow reactor products had comparable performance to the batch synthesized PbS nanoparticles. A dual-temperature-stage flow reactor synthesis was carried out to achieve optimum results. The flow reactor system is shown schematically in Fig. 11. In this method precursor A consists of lead oxide, oleic acid (OA), and octadecene (ODE) whereas precursor B contains bis(trimethylsilyl) sulfide (TMS) and ODE. The two precursors are injected under nitrogen. The mixed reactants proceed together to the nu-cleation stage that is temperature-controlled by thermocouple 1. The precursors react at the elevated temperatures to form nucleation seeds. The quantum dots are then isolated using acetone and re-dispersed in toluene. 

The weak Raman scattering from water allows the direct observation of the oxide species in the aqueous phase, the nanocrystalline nucleation seeds, and the solid phases present during zeolite synthesis, which are not as readily detectable with x-ray diffraction (XRD). The transformation of aluminosilicate gel to zeolite A was investigated with Raman spectroscopy by several researchers [150,179,180], but one outstanding investigation also... 

It is safe to assume, that cluster growth passes through coalescence of small nucleating seeds, melting with a rise in substrate temperature. In the framework of the same BOLS model, it is possible to determine the cluster melting temperature, knowing its size and m parameter by the following equation ... 

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