Ji-Guang Li

Oxide ionic conductivity and microstructures of Sm- or La-doped CeO2-based systems

The concept of crystallographic index termed the effective index is suggested and applied to the design of ceria (CeO2)-based electrolytes to maximize oxide ionic conductivity. The suggested index considers the fluorite structure, and combines the expected oxygen vacancy level with the ionic radius mismatch between host and dopant cations. Using this approach, oxide ionic conductivity of Sm- or La-doped CeO2-based system has been optimized and tested under operating conditions of a solid oxide fuel cell. In the observation of microstructure in atomic scale, both Sm-doped CeO2 and La-doped CeO2 electrolytes had large micro-domains over 10 nm in the lattice. On the other hand, Sm or La and alkaline earth co-doped CeO2-based electrolytes with high effective index had small micro-domains around 1-3 nm in the microstructure. The large micro-domain would prevent oxide ion from passing through the lattice. Therefore, it is concluded that the improvement of ionic conductivity is reflected in changes of microstructure in atomic scale

Defect-Mediated Photoluminescence Dynamics of Eu3+-Doped TiO2 Nanocrystals Revealed at the Single-Particle or Single-Aggregate Level†

Lanthanide-doped materials are finding use in a wide variety of applications in optics as gain media for amplifiers and lasers and as biolabels, white-light emitters, and full-color phosphors for displays. Since direct excitation of the parity-forbidden intra-f-shell lanthanide ion crystal-field transitions is inefficient, it is anticipated that the luminescence of lanthanide ions incorporated in a wide-band-gap semiconductor lattice (e.g., ZnO and TiO2) could be sensitized efficiently by exciton recombination in the host (Figure 1). Recently, we synthesized Eu-doped TiO2 (TiO2:Eu ) nanocrystals by Ar/O2 radio-frequency thermal plasma oxidation and observed bright red emission either by exciting the TiO2 host with UV light of shorter wavelength than 405 nm or by directly exciting Eu at a wavelength beyond the absorption edge (405 nm, 3.06 eV) of TiO2. Various types of defect states have been considered to play an important role in energy transfer between TiO2 and the activating Eu ions. For example, with increasing annealing temperature, the photoluminescence (PL) intensity of visible emissions due to Eu ions increases at first but then decreases and reaches a maximum at an annealing temperature of 700 8C. In this respect, the luminescence of Eu depends critically on the locations of dopants in the host. However, the mechanism of the energy-transfer process from the defect energy levels of the host to dopants has not yet been clarified owing to several difficulties, such as the inhomogeneous distribution of ions in the material. Single-molecule (single-particle) fluorescence spectroscopy has already yielded new insight into the photophysics and photochemistry of inorganic and organic nanocrystals. There are, however, only a few reports on the PL behavior of lanthanide-doped materials. We have now investigated the PL dynamics of undoped TiO2 and TiO2:Eu 3+

Controlling the synthesis of TaC nanopowders by injecting liquid precursor into RF induction plasma

Abstract Thermal plasma processing has been used to synthesize nano-size powders through the condensation of reactant species from a vapor phase. Further development of this synthesis method will require the careful selection of an appropriate precursor and precise control of products species and their particle sizes. Direct introduction of liquid mist into thermal plasma gives us a wider choice of precursors than does vapor-phase precursor injection and lets us inject the precursors in larger amounts. In the present work, nano-size tantalum carbide powder was prepared from a liquid precursor, tantalum ethoxide Ta(OC2H5)5, by using r.f. thermal plasma. The liquid precursor was atomized to generate micron-sized mist droplets, and the mist was introduced into plasma. This atomized precursor evaporated quickly in the high-temperature plasma flame, and nanoparticles were formed as temperature decreased. The process was controlled by changing the hydrogen addition, process pressure, carrier gas flow rate for mist injection, and quenching condition. Adding hydrogen improved the powder quality by removing solid carbon, but excess hydrogen suppressed the formation of tantalum carbide. The quenching conditions gave significant effects on the reduction of particles size by two thirds and yielded average particle sizes as small as 8 nm.

Influence of nano-structural feature on electrolytic properties in Y2O3 doped CeO2 system

Abstract Doped ceria (CeO2) compounds are fluorite type oxides which show oxide ionic conductivity higher than yttria stabilized zirconia, in oxidizing atmosphere. As a consequence of this, considerable interest has been shown in application of these materials for ‘low temperature operation (500–650 8C)’ of solid oxide fuel cells (SOFCs). In this study, YxCe12xO22d (x ¼ 0:05; 0:1; 0:15; 0:2 and 0.25) fine powders were prepared using a carbonate co-precipitation method. The relationship between electrolytic properties and nano-structural features in the sintered bodies was examined. The micro-structures of Y0.05Ce0.95O1.975, Y0.15Ce0.85O1.925 and Y0.25Ce0.75O1.875 as representative three specimens have been investigated in more detail with transmission electron microscopy (TEM). The big diffuse scattering was observed in the background of electron diffraction pattern recorded from Y0.15Ce0.85O1.925 and Y0.25Ce0.75O1.875 sintered bodies. This means that the coherent micro-domain with ordered structure is in the micro-structure. While Y0.25Ce0.75O1.875 sintered body with low conductivity and high activation energy has big micro-domains, the micro-domain size in Y0.15Ce0.85O1.925 with high conductivity and low activation energy was much smaller than that of Y0.25Ce0.75O1.875. TEM observation gives us message that the size of coherent micro-domain with ordered structure would closely relate to the electrolytic properties such as conductivity and activation energy in the specimens. It was concluded that a control of micro-domain size in nano-scale in Y2O3 doped CeO2 system was a key for development of high quality solid electrolyte in fuel cell application.

Ultrabroad near-infrared photoluminescence from Bi5(AlCl4)3 crystal

The Bi5(AlCl4)3 crystal, synthesized by a environmentally friendly room-temperature method using ionic liquids as reaction solvents, exhibits extremely broad near-infrared photoluminescence (PL) with a full width at the half maximum (FWHM) of >510 nm and an effective PL lifetime of 4.1 µs at 1160 nm. We envision that this study not only extends the understanding of photophysical properties of materials containing subvalent bismuth, but also may have promise for the design of novel photonic materials containing a wide array of p-block elements.

Control of particle size and phase formation of TiO2 nanoparticles synthesized in RF induction plasma

TiO2 nanoparticles have been synthesized in this work via Ar/O2 RF thermal plasma oxidation of atomized liquid precursors containing titanium tetrabutoxide and diethanolamine. Quench gases (Ar or He), either injected from the shoulder of the reactor (transverse injection) or injected counter to the plasma plume from the bottom of the reactor (counter-flow injection), are used to affect the quench rate and therefore the particle size and phase constituent of the resultant powders. The experimental results show that counter-flow injection is more effective in reducing the particle size, while He is more effective than Ar. As a result, well-dispersed TiO2 nanopowders with controllable phase structure (up to ~90% of anatase) and average particle size (down to 20 nm) are obtained. The experimental results are well supported by numerical analysis on the effects of the quench gas on flow pattern and temperature field of the thermal plasma as well as trajectory and temperature history of the particles.

Highly Fluorescent Silica‐Coated Bismuth‐Doped Aluminosilicate Nanoparticles for Near‐Infrared Bioimaging

For in vivo and deep-tissue imaging, near-infrared (NIR)-emitting nanoparticles (NPs) offer many advantages over visible-light-emitting NPs because the optical absorption and light scattering of biological media and tissue autofl uores-cence are minimal in the NIR region of the electromagnetic spectrum.

A Two-Step Hydrothermal Synthesis Approach to Monodispersed Colloidal Carbon Spheres

This work reports a newly developed two-step hydrothermal method for the synthesis of monodispersed colloidal carbon spheres (CCS) under mild conditions. Using this approach, monodispersed CCS with diameters ranging from 160 to 400 nm were synthesized with a standard deviation around 8%. The monomer concentration ranging from 0.1 to 0.4 M is in favor of generation of narrower size distribution of CCS. The particle characteristics (e.g., shape, size, and distribution) and chemical stability were then characterized by using various techniques, including scanning electron microscopy (SEM), FT-IR spectrum analysis, and thermalgravity analysis (TGA). The possible nucleation and growth mechanism of colloidal carbon spheres were also discussed. The findings would be useful for the synthesis of more monodispersed nanoparticles and for the functional assembly.

Recent progress in advanced optical materials based on gadolinium aluminate garnet (Gd3Al5O12)

Abstract This review article summarizes the recent achievements in stabilization of the metastable lattice of gadolinium aluminate garnet (Gd3Al5O12, GAG) and the related developments of advanced optical materials, including down-conversion phosphors, up-conversion phosphors, transparent ceramics, and single crystals. Whenever possible, the materials are compared with their better known YAG and LuAG counterparts to demonstrate the merits of the GAG host. It is shown that novel emission features and significantly improved luminescence can be attained for a number of phosphor systems with the more covalent GAG lattice and the efficient energy transfer from Gd3+ to the activator. Ce3+ doped GAG-based single crystals and transparent ceramics are also shown to simultaneously possess the advantages of high theoretical density, fast scintillation decay, and high light yields, and hold great potential as scintillators for a wide range of applications. The unresolved issues are also pointed out.

Low-temperature fabrication and electrical property of 10 mol% Sm2O3-doped CeO2 ceramics

Abstract Ten mol% Sm2O3-doped CeO2 solid-solution (20SDC) powders have been synthesized via carbonate coprecipitation using ammonium hydrogen carbonate (AHC) and urea as the precipitants, respectively. Characterizations were achieved by elemental analysis, X-ray diffractometry, differential thermal analysis/thermogravimetry, and FESEM. An amorphous hydroxyl carbonate precursor (Ce,Sm)(OH)CO3·2H2O having nanosized (~10 nm) spherical particles was formed with AHC, while a mixture of crystalline (Ce,Sm)2(CO3)2(OH)2·H2O and (Ce,Sm)2O(CO3)2·H2O phases exhibiting irregular particle morphologies was obtained with urea. Both the precursors convert to oxide solid solutions without any phase detected corresponding to Sm2O3 during calcination. The oxide powder processed via the AHC method can be sintered to >99% of the theoretical at a low temperature of 1200 ˚C, due to the good dispersion and ultrafine size (~15 nm) of the particles, while that from the urea method can only reach ,67.2% dense at the same temperature. Electrical conductivity of the densified ceramic was measured in air in the range 400—700 ˚C by the DC three-point method, and an activation energy of ~60.5 kJ/mol was derived from the experimental data.