Key Synthesis Parameters Research Articles

Self-assembly hydrothermal assisted synthesis of mesoporous anatase in the presence of ethylene glycol

Abstract Mesoporous nanocrystalline anatase was prepared hydrothermally employing P123 as structure-directing agent. Ethylene glycol was used as a key synthesis parameter to fine tune the morphology, crystal size and pore size of the resultant mesophases. The incorporation of EG in the synthesis gel resulted in the formation of 1–2 μm sphere-like shapes and led to an increase in the specific surface area from ∼95 to ∼170 m 2 /g, decrease in the average pore size from ∼11 to ∼4.8 nm, and decrease in the average crystallite size from ∼17 to ∼12 nm. These mesophases were used as photocatalysts for the UV degradation of methylene blue and methyl orange. The mesoporous anatase phases photodegraded MB ∼1.5–3× faster than commercially available P25 and showed limited photocatalytic behavior for methyl orange.

Carbon xerogels synthesized via phenol–formaldehyde gels

A new synthesis route for organic and carbon xerogels based on the low cost precursors phenol and formaldehyde was developed. The combination of n-propanol as solvent and acidic catalysis leads to homogeneous, highly porous monolithic samples. The conversion of the gel into an organic xerogel is performed via drying at ambient conditions. The organic samples as well as the carbon samples show densities between 0.29 and 1.17 g/cm 3 (porosities up to 88%). The impact of different synthesis parameters – such as the type and concentration of the catalyst, the mass content of phenol and formaldehyde in the initial solution, the molar ratio of phenol to catalyst or the molar ratio of formaldehyde to phenol – on the microstructure and morphology were evaluated and key synthesis parameters identified. Furthermore a monofunctional phenol was applied as an additional parameter to control the sol–gel process. Small-angle X-ray scattering (SAXS) measurements performed during the sol–gel transition provide information on the evolution of the gel structure. The carbons derived via pyrolysis of the organic xerogels were characterized with SAXS, nitrogen sorption and electron microscopy. Electron microscopy images show the typical network of cross linked primary particles that is also characteristic for resorcinol-based aerogels and xerogels. The as received carbon samples show specific surface areas up to 572 m 2/g, micropore volumes up to 0.24 cm 3/g, external surface areas up to 150 m 2/g and mesopore volumes up to 0.69 cm 3/g. Energy dispersive X-ray (EDX) reveals an almost pure carbon phase with only oxygen impurities.

Dynamic Scaling Properties of TeO2-Based Gels

Homogeneous, transparent, and mechanically rigid gels have been successfully synthesized in the tellurium isopropoxide-isopropanol-citric acid and water system. The sol to gel transition and the gels microstructure have been studied by using small angle X-ray scattering (SAXS) experiments. For any value of the two key synthesis parameters, which are the citric acid ratio and the alkoxide concentration, very small Te-rich elementary particles, about 1-1.5 nm in radius, form immediately when the water is added, leading to colloidal sols. During gelation, these elementary particles stick progressively together to build up fractal aggregates by a pure hierarchical aggregation process which has been identified as a reaction-limited cluster aggregation (RLCA) mechanism. The SAXS curve analysis, based on scaling concepts, shows that the gelling network exhibits a time and length scale invariant structure factor characterized by self-similarity. This self-similarity is also displayed for a wide range of chemical compositions and the gel microstructures only differ in their fractal aggregate size according to the tellurium isopropoxide concentration as well as the citric acid ratio.

Microwave-Assisted Synthesis of Water-Dispersed CdTe Nanocrystals with High Luminescent Efficiency and Narrow Size Distribution

Water-dispersed nanocrystals with high photoluminescence quantum yield (PLQY) and narrow size distribution are desirable for a variety of bioapplications. In this report, we present a facile method assisted by microwave irradiation for rapidly synthesizing high-quality CdTe nanocrystals in aqueous phase. The PLQY of CdTe nanocrystals prepared in very short time (15 min) reaches as high as 82%, and the fwhm value is merely 27 nm meanwhile. Moreover, the PLQY is increased to a remarkable 98% after further amelioration through the illumination method. The key synthesis parameters (molar ratio of reagents, pH values, reaction temperature, and time) are fully discussed, and the optimum conditions are illuminated in this paper. In addition, the mechanism about microwave irradiation improving the quality of nanocrystals is discussed as well.

Synthesis and Mechanism Study of Mesoporous SnO<SUB>2</SUB>/SiO<SUB>2</SUB> Composites

Mesoporous SnO2/SiO2 composite particles (Si/Sn < or = 0.25) sustainable to calcination up to 600 degrees C have been fabricated using a stepwise sol-gel technique on nonionic surfactant template (tetradecylamine, TDA). The newly designed preparation method involved the pre-formation of SnO2 sol solution from SnCl4. Subsequently, SnO2 nanocrystals were covered by the silicate species (from the hydrolysis of tetraethylorthosilicate, TEOS) in a pH controlled colloidal solution. Upon mixing with the surfactant solution, mesophase composite was obtained. After the removal of templates at various temperatures (400 to 600 degrees C), worm-like mesoporous SnO2/SiO2 with large specific surface area and pore volume as high as 362 m2/g and 0.33 cc/g were obtained, respectively. High thermal stability is mainly due to the effective inhibition of SnO2 crystal growth (mean crystallite size <30 A) by the amorphous SiO2 species at the grain boundaries. Formation of mesoporous silicate skeleton such as M41S family material was prevented. The obtained materials maintain the relatively narrow pore size distribution typically in the range of 30 to 70 A. Relations between material properties and key synthesis parameters (i.e. TDA/Si/Sn molar ratio and calcination temperature) were investigated by TGA, wide/small-angle X-ray scattering, (HR)TEM, BET, and FTIR techniques. Mechanisms on the mesostructure formation and crystal growth inhibition were also proposed with detailed discussion.

Sorption characteristics of radionuclides on synthetic birnessite-type layered manganese oxides

The removal of trace caesium (137Cs), trace strontium (89Sr, 90Sr/90Y) and trace cobalt (57Co) from aqueous solution by synthetic sodium and potassium birnessites has been studied by a batch technique. Distribution coefficients were determined as a function of pH in four solutions: water, 0.005 M sodium tetraborate, 0.005 M sodium citrate and 0.0001 M EDTA. Other effects studied were increasing sodium ion, potassium ion and complexing agent concentration. Key synthesis parameters (ageing time, MnO4−/Mn2+ ratio) were tuned in order to maximise trace cobalt sorption efficiencies. The optimised material was tested for the removal of caesium, strontium and cobalt from a nuclear power plant floor drain simulant. A sodium birnessite sample was also screened for the removal of 63Ni, 54Mn, 59Fe, 65Zn and 236Pu radionuclides.

“Figure/Ground” Study of Colloidal Silica Nanoparticles and Corresponding Microporous Xerogels

A critical hurdle in the development of sol−gel techniques for the preparation of microporous inorganic materials is elucidating the effect of key synthesis and processing parameters on the structure of the resulting sols. Characterization of colloidal particles with diameters of less than approximately 10 nm is problematic using conventional techniques such as dynamic light scattering. Similarly, quantitative determination of pore size distribution in the resulting materials using gas adsorption isotherms is difficult in the microporous regime. The present work extends these traditional methods with the objective of obtaining information both on the particle size in an inorganic sol and on the pore size distribution in the resulting microporous xerogel. Silica sols synthesized at pH 3 and an H2O/TEOS ratio of r = 83 were shown by atomic force microscopy to have a distribution of particle diameters centered around approximately 4−6 nm. Nitrogen and molecular probe adsorption studies on xerogels derived fr...