L. M. Gan

LUMINESCENCE CHARACTERISTICS OF IMPURITIES-ACTIVATED ZNS NANOCRYSTALS PREPARED IN MICROEMULSION WITH HYDROTHERMAL TREATMENT

Cu-, Eu-, or Mn-doped ZnS nanocrystalline phosphors were prepared at room temperature using a chemical synthesis method. Transmission electron microscopy observation shows that the size of the ZnS clusters is in the 3–18 nm range. New luminescence characteristics such as strong and stable visible-light emissions with different colors were observed from the doped ZnS nanocrystals at room temperature. These results strongly suggest that impurities, especially transition metals and rare-earth metals-activated ZnS nanoclusters form a new class of luminescent materials.

Synthesis and characterization of PtRu/C catalysts from microemulsions and emulsions

Nanosized PtRu catalysts supported on carbon have been synthesized from inverse microemulsions and emulsions using H2PtCl6 (0.025 M)/RuCl3 (0.025 M)/NaOH (0.025 M) as the aqueous phase, cyclohexane as the oil phase, and NP5 (poly(oxyethylene)5 nonyl phenol ether) or NP9 (poly(oxyethylene)9 nonyl phenol) as the surfactant, in the presence of Carbon Black suspended in a mixture of cyclohexane and NP5 + NP9. The titration of 10% HCHO aqueous solution into the inverse microemulsions and emulsions resulted in PtRu/C catalysts, in which the PtRu particles were nanometers in size. The catalysts were characterized by TEM, XRD and XPS and the metal particles were found to inherit the Pt fcc structure with Pt and Ru mostly in the zero valence oxidation states, amidst some Pt(II), Pt(IV) and Ru(IV). The cyclic voltammograms for methanol oxidation on these PtRu/C catalysts showed higher electrocatalytic activities for the two microemulsion derived catalysts than the emulsion-derived electrocatalyst.

Processing of hydroxyapatite via microemulsion and emulsion routes

Hydroxyapatite powders have been prepared by reacting CaCl2 and (NH4)2HPO4 in bicontinuous microemulsion, inverse microemulsion and emulsion, which have the same components as cyclohexane, non-ionic surfactant (NP5 + NP9) and aqueous solution. The characteristics of the resulting hydroxyapatite powders, such as the particle size, particle size distribution, chemical homogeneity and the degree of particle agglomeration, are strongly affected by the structure of the reaction medium. Both bicontinuous and inverse microemulsions led to the formation of much finer hydroxyapatite powders than that prepared from the emulsion composition. The two fine hydroxyapatite powders are sintered to a relative density of >95% theoretical density at 1000 degrees C, compared with a relative density of <73% theoretical density for the emulsion-derived one. The two microemulsion-derived hydroxyapatites also exhibit a higher sintered density and are more refined in grain size than that of the emulsion-derived one when sintered at 1200 degrees C for 2h.

Synthesis of vertically aligned carbon nanotubes on metal deposited quartz plates

Without plasma aid, we have successfully synthesized vertically aligned carbon nanotubes (CNTs) on iron-, cobalt- or nickel-deposited quartz plates by chemical vapor deposition with ethylenediamine as a precursor. The amine serves as both etching reagent for the formation of metal nanoparticles and carbon source for the growth of aligned carbon nanotubes. The carbon nanotubes were vertically aligned in high density on a large area of the plain silica substrates. The density and diameter of CNTs is determined by the thickness of the deposited metal film and the length of the tubes can be controlled by varying the reaction time. High-resolution transmission electron microscopy analysis reveals that the synthesized CNTs are multiwalled with a bamboo-like structure. Energy dispersive X-ray spectra demonstrate that the CNTs are formed as tip growths. Raman spectrum provides definite evidence that the prepared CNTs are multiwalled graphitic structure.

Processing of fine hydroxyapatite powders via an inverse microemulsion route

Abstract A submicrometer-sized hydroxyapatite powder has been synthesized via an inverse microemulsion processing route, which uses cyclohexane as the oil phase, mixed poly(oxyethylene) 5 nonyl phenol ether (NP-5) and poly(oxyethylene) 9 nonyl phenol ether (NP-9) as the surfactant phase, and an aqueous CaCl 2 solution as the water phase. In comparison with the hydroxyapatite powder prepared by directly reacting (NH 4 ) 2 HPO 4 with CaCl 2 in an aqueous solution, the microemulsion processing route led to a significant refinement in the particle size, particle size distribution, and the degree of particle agglomeration. XRD phase analysis indicates that a high-purity crystalline hydroxyapatite powder was obtained when the microemulsion-derived precursor was calcined at 800 °C. In contrast, there existed impurity phases such as β-Ca 3 (PO 4 ) 2 and CaO in the conventionally processed hydroxyapatite powder after calcination at various temperatures up to 1200 °C. This shows that the microemulsion processing route results in the formation of a more homogeneous ultrafine powder precursor than that obtained from the conventional processing route.

Synthesis and characterization of Eu:Y2O3 nanoparticles

The red-emitting nanophosphor Eu:Y2O3 was synthesized using the the microemulsions method. The microemulsion system was composed of petroleum ether (60-80??C), nonionic surfactants NP5/NP9, aqueous yttrium nitrate/europium nitrate and ammonium hydroxide solution. The nanoparticles were studied by thermal analysis, x-ray diffraction, transmission electronic microscopy, scanning electron microscopy and photoluminescence. The size of the nanoparticles was in the range 10-100?nm, and showed a narrow size distribution, high crystallinity and special luminescent properties. Compared with the phosphors prepared by the conventional method, the quenching concentration of Eu was raised remarkably. For this type of nanophosphor, quenching starts at a Eu concentration of 10% (mol%), while a value of 6-8% was obtained for the conventional one (Tao?Y 1996 Mater. Lett. 28 137-40). Based on this study, we have successfully prepared some promising nanophosphors.

Photoluminescence and structural characteristics of CdS nanoclusters synthesized by hydrothermal microemulsion

Spherical and uniform CdS nanoclusters were synthesized by hydrothermal microemulsion. The reaction of Cd2+ ions with S2− ions generated from the decomposition of thioacetamide proceeded in water microdroplets. The mean diameter of the CdS nanoclusters can be varied from 20 to 80 nm by increasing the reaction temperature from 30 to 120 °C. XRD results indicate that the resulting CdS nanoclusters have a reduced and distorted hexagonal lattice compared to bulk materials. Two intense luminescence bands, i.e., green and red, were observed to coexist in the CdS nanoclusters. Their peak positions and relative intensities were found to be sensitive to the size and structure of nanoclusters. These emissions are attributed to surface defects (green emission) and the Cd–Cl composite vacancies (red emission).

Photoluminescence and electroluminescence from copper doped zinc sulphide nanocrystals/polymer composite

Cu-doped ZnS nanocrystals were prepared in an inverse microemulsion at room temperature as well as under a hydrothermal condition. X-ray diffraction analysis showed that the diameter of the Cu-doped ZnS nanocrystals particles was about 9 nm. These particles showed a strong photoluminescence intensity and a broad emission band from 490 to 530 nm. The half-width of emission was about 60 nm. Cu-doped ZnS nanocrystals/polymethylmethacrylate composite as a light-emitting layer was used to fabricate a single layer structure electroluminescent device which had low turn on voltage (less than 5 V). The green light of electroluminescence was observed at room temperature. The electroluminescence and photoluminescence spectra were nearly identical at room temperature.

Nanosized hydroxyapatite powders from microemulsions and emulsions stabilized by a biodegradable surfactant

Ultrafine hydroxyapatite powders have been successfully synthesized in inverse microemulsions and emulsions consisting of petroleum ether as the oil phase, 1.0 M CaCl 2 solution as the aqueous phase and biodegradable KB6ZA as the surfactant. The titration of 0.6 M (NH 4 ) 2 HPO 4 aqueous solution into the inverse microemulsion and emulsions containing 25.0 and 35.0 wt% aqueous phase, resulted in hydroxyapatite precursors that were nanometer sized and more or less spherical in morphology. However, they underwent a considerable degree of particle coarsening when calcined at 650 °C for 6 h. A nanocrystalline hydroxyapatite powder, which exhibited a dendritic agglomerate morphology, was synthesized in an oil-in-water emulsion containing 90.0 wt% aqueous phase. It shows an average particle size of 25 nm upon calcination at 650 °C for 6 h, as little particle coarsening and growth in crystallite size were observed at the calcination temperature. The inverse microemulsion- and emulsion-derived hydroxyapatites exhibit a degree of type B carbonate substitution, which cannot be eliminated by calcination in air at 650 °C.

A novel method for the synthesis of perovskite-type mixed metal oxides by the inverse microemulsion technique

Nanoparticles of lanthanum-nickel, lanthanum-copper and barium-lead oxalates with the metal molar ratios of 1∶1, 2∶1 and 1∶1, respectively, have been successfully synthesized in inverse microemulsions. These metal oxalate particles of about 20 nm diameter were readily calcined into single-phase perovskite-type LaNiO3, La2CuO4 and BaPbO3. The calcination temperatures for these metal oxalates were generally 100–250°C lower than those for the metal oxalates prepared by the conventional aqueous solution precipitation method. The substantial reduction in the calcination temperatures is attributed to the formation of uniform, near-spherical nanoparticles of the metal oxalate precursors obtained by the unique inverse microemulsion technique.