J Lee

Synthesis and characterization of LiFePO4 and LiTi0.01Fe0.99PO4 cathode materials

Nanocrystalline LiFePO 4 and doped LiTi 0 . 0 1 Fe 0 . 9 9 PO 4 powders were synthesized via a sol-gel preparation route. High-resolution tunneling electron microscopy observation and energy dispersive spectroscopy, mappingshow the homogeneous distribution of dopant Ti cations in the crystals. Fe and O K-edge X-ray absorption near-edge structure (XANES) measurements show that Ti 4 + doping induces an increased unoccupied d-state in LiFePO 4 , resulting in an enhanced p-type semiconductivity. In situ Fe K-edge XANES measurements of Ti-doped and undoped LiFePO 4 electrodes have been performed to determine the change of Fe valence during the lithium intercalation and de-intercalation processes. Both LiFePO 4 and doped LiTi 0 . 0 1 Fe 0 . 9 9 PO 4 cathodes demonstrate good electrochemical performance.

Synthetic Utility of Ammonium Salts in a Cu-Catalyzed Three-Component Reaction as a Facile Coupling Partner

Ammonium salts were found to be a convenient and inexpensive reagent in the Cu-catalyzed three-component reaction with terminal alkynes and sulfonyl or phosphoryl azides leading to N-unprotected amidines. Thus obtained amidines bearing 2-bromobenzenesulfonyl moiety were efficiently cyclized by the Cu-catalyzed intramolecular N-arylation to give an important pharmacophore skeleton of 2H-1,2,4-benzothiadiazine 1,1-dioxides. Conveniently, two tandem catalytic procedures could be readily operated in one pot.

Band-gap narrowing of TiO2 doped with Ce probed with x-ray absorption spectroscopy

The electronic structure of TiO2 doped with Ce was investigated with x-ray absorption and photoluminescence (PL) spectra. Ce doping narrows the band gap of TiO2, as evidenced by a redshift in the absorption edge in the O K-edge spectrum and PL. Narrowing is ascribed to the formation of an impurity band, composed of Ce 4f (Ce3+/Ce4+) and oxygen defect states, that induces tailing of the conduction band. Band gap narrowing extends the photoactivity of TiO2 to visible light. The impurity band located below the minimum of the conduction band traps excited electrons and suppresses recombination, enhancing the photocatalytic activity.

Structural transformation of LiVOPO4 to Li3V2(PO4)3 with enhanced capacity.

In the present investigation, we report the transformation of alpha-LiVOPO 4 to alpha-Li 3V 2(PO 4) 3, leading to an enhancement of capacity. The alpha-LiVOPO 4 sample was synthesized by a sol-gel method, followed by sintering at 550-650 degrees C in a flow of 5% H 2/Ar. The structural transformation of a triclinic alpha-LiVOPO 4 structure to a monoclinic alpha-Li 3V 2(PO 4) 3 structure was observed at higher sintering temperatures (700-800 degrees C in a flow of 5% H 2/Ar). The alpha-Li 3V 2(PO 4) 3 phase was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermal gravimetric analysis, and X-ray absorption near edge spectrum (XANES) techniques. The valence shift of vanadium ions from +4 to +3 states was observed using in situ XANES experiments at V K-edge. The structural transformation is ascertained by the shape changes in pre-edge and near edge area of X-ray absorption spectrum. It was observed that the capacity was enhanced from 140 mAh/g to 164 mAh/g via structural transformation process of LiVOPO 4 to Li 3V 2(PO 4) 3.

Investigation on Mechanism of Catalysis by Pt-LiCoO2 for Hydrolysis of Sodium Borohydride Using X-ray Absorption

The synthesis of platinum nanoparticle loaded LiCoO2 (Pt-LiCoO2) was carried out successfully by an impregnation method followed by sintering at different temperatures. The catalytic role of Pt-LiCoO2 composite in hydrogen generation during hydrolysis of sodium borohydride (NaBH4) was studied for fuel cell applications. X-ray diffraction (XRD), transmission electron microscopy (TEM), and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) have been used to elucidate the structural and catalytic properties of Pt-LiCoO2. It was found that the 15 wt % of Pt nanoparticles on LiCoO2 sintered at 450 degrees C support showed the maximum efficiency for the catalysis reaction of hydrogen production. X-ray absorption near edge structure (XANES) analysis and extended X-ray absorption fine structure (EXAFS) analysis using a synchrotron radiation source were performed to carry out ex situ measurements in order to understand the mechanism of the catalytic process for the production of hydrogen during the hydrolysis of NaBH4. Co K-edge XANES showed a small percentage of cobalt in the metallic form after hydrogen generation which suggests the reduction of the cobalt during the hydrolysis of NaBH4.

Effect of annealing on the structural and optical properties of indium-diffused Cd0.7Zn0.3Se thin films

Cadmium zinc-selenide (Cd0.7Zn0.3Se) thin films were deposited on the ITO substrate using chemical bath deposition techniques by optimizing the deposition parameters. The as-deposited films were annealed in air at 200, 300 and 400 °C for 1 h. The composition, surface morphology and structural properties of the as-deposited and annealed Cd0.7Zn0.3Se thin films were studied using x-ray photoelectron spectroscopy, scanning electron microscopy and x-ray diffraction techniques. The as-deposited films exhibited the hexagonal phase of CdSe and ZnSe. The films annealed at 200 °C showed the dominant cubic phase of CdSe and the hexagonal phase of ZnSe. However, the cubic structure of Cd0.7Zn0.3Se was transformed into a hexagonal structure after annealing at 300 and 400 °C. The lattice parameter a for the cubic structure was 6.0865 A, whereas for the hexagonal structure a varied from 4.3035 to 4.2938 A and c varied from 7.0916 to 6.9868 A. The optical absorption spectra were recorded within the range 350–800 nm. The optical band gaps were 2.08 eV, 2.03 eV, 1.91 eV and 1.72 eV for the as-deposited films and those annealed at 200 °C, 300 °C and 400 °C, respectively. The drastic decrease in the optical band gap at 300 and 400 °C was due to the indium diffusion into the Cd0.7Zn0.3Se matrix.

Effect of LiI Amount to Enhance the Electrochemical Performance of Carbon-Coated LiFePO4

We used the wet chemistry method with different amounts of LiI to prepare LiFePO 4 and carbon-coated LiFePO 4 (LiFePO 4 /C) samples. The lithiation process of LiI for preparing the LiFePO 4 is proposed. We found that the amount of LiI greatly affected the purities of products. The LiFePO 4 /C sample showed constant values of current density during potential cycling up to 35 cycles as compared to LiFePO 4 , suggesting that the LiFePO 4 /C has better electrochemical performance. The obtained maximum capacity for LiFePO 4 /C can be approached to the theoretical capacity of 170 mAh/g. Moreover, Brunauer-Emmett-Teller measurements of LiFePO 4 and LiFePO 4 /C showed a surface area of 6.7 and 50 m 2 /g, respectively. It is reasonable to believe that the excellent performance of the compound developed in our work can be attributed to the smaller particle size coated with conductive carbon achieved by the controlling LiI in the sol-gel method.

EXAFS study of double perovskite (Sr2-xCax)FeMoO6 (0 ≤ x ≤ 2.0) and Sr2CrMO6 (M = Mo, W) systems

The local structure of the double perovskite (Sr2-xCax)FeMoO6 (0 ≤ x ≤ 2.0) and Sr2CrMO6 (M = Mo, W) systems have been probed by extended X-ray absorption fine structure (EXAFS) spectroscopy at the Fe and Cr K-edges. It was found that the Fe-O distance decreases from 1.999(7) A (x = 0) to 1.991(1) A (x = 1.0) in (Sr2-xCax)FeMoO6 with a tetragonal phase (I4/m). When the x value is further increased from 1.5 to 2.0, the Fe-O bond distance decreases from 2.034(5) to 2.012(4) A with a monoclinic phase (P21/n). On the other hand, Cr-O, Cr-Sr and Cr-W bond distances in Sr2CrWO6 are all slightly longer than the corresponding bond distances in Sr2CrMoO6, which is attributable to the larger ionic radius of W5+ (0.62 A) than Mo5+ (0.61 A).

Crystal structure and electronic and thermal properties of TbFeAsO0.85

The crystal structure and the electronic and thermal properties of a high-quality polycrystalline TbFeAsO0.85 sample made by a high-pressure technique are investigated. The crystal structure, as determined by synchrotron X-ray powder diffraction, possesses a tetragonal unit cell (space group: P4/nmm) with lattice parameters of a=b=3.8851 A and c=8.3630 A. In order to elucidate the electronic structure and oxidation states of corresponding elements, X-ray absorption near-edge structure (XANES) spectra are presented. The XANES spectra confirm that the oxidation states of Fe, As, and Tb in the TbFeAsO0.85 sample are ∼Fe2+, ∼As3−, and ∼Tb3+, respectively, which are consistent with the previously reported band structure calculations. The n-type character of the charge carriers as revealed from XANES spectra is corroborated by the negative sign of the Seebeck coefficient (S) in the present study. The heat capacity (CP) measurement shows an anomaly in the vicinity of the superconducting transition temperature (T...

Formation of Hydrides in (Ti1−xZrx)Co2.00 (0 < x < 1) Pseudobinary Alloys

The exposure of (Ti(1-x)Zr(x))Co(2.00) intermetallic alloys to hydrogen at high pressure caused (Ti(1-x)Zr(x))Co(2.00) (x = 0.50-0.90) hydrides in the alloy. The crystalline structural, electronic, and magnetic properties of parent alloys and of their hydrides were determined by using XRD (X-ray powder diffraction) and XAS (X-ray absorption spectrometry) and by the use of SQUID (a superconducting quantum interference device). Hydrogenation did not alter the crystal structure of the parent alloy, but it did increase the volume of the unit cell. An in situ Co K-edge XAS study of the hydride revealed that the valence state of Co increased during discharge (which is the release of hydrogen from the hydride). Hydrogenation of the parent alloy also reduced the magnetic moment. A possible mechanism of discharge for the hydride is also proposed.