Showing papers by "Jingbo Li published in 2008"
TL;DR: In this paper, single-crystal tetragonal α-MnO2 nanotubes have been successfully synthesized by a facile hydrothermal treatment of KMnO4 in the hydrochloric acid solution.
Abstract: Single-crystal tetragonal α-MnO2 nanotubes have been successfully synthesized by a facile hydrothermal treatment of KMnO4 in the hydrochloric acid solution. The sample has been characterized by X-ray powder diffraction, field emission scanning electron microscopy, transmission electron microscopy, and orientation dependent Raman spectroscopy, which indicates the nanotubes have high-quality crystalline and shape-dependent optical properties. The morphology evolution of the sample reveals that the nanotubes are formed via the solid nanorod by a chemical etching process.
232 citations
TL;DR: In this paper, Birnessite-type MnO2 nanowalls have been fabricated on Si(111) substrate by a facile solution method and a magnetic transition temperature of 9.2 K was determined.
Abstract: Birnessite-type MnO2 nanowalls have been fabricated on Si(111) substrate by a facile solution method. The XRD pattern indicates that the sample has the typical feature of turbostratic disorder and prefers to grow in the ab plane. The nanowalls are composed of thin flakes distributing uniformly over the surface of the Si(111) substrate. A magnetic transition temperature of 9.2 K is determined. Prominent magnetic anisotropy with the easy magnetization direction in the ab plane is observed at 5 K.
154 citations
TL;DR: The overall electro-catalytic activities of 20 wt % Pt/WO3 composites are significantly higher than that of commercial 20 wts% Pt/C catalyst and are comparable to the 20 wT % PtRu/C Catalyst in the potential region of 0.5-0.7 V.
Abstract: A new type of carbon-free electrode catalyst, Pt/mesoporous WO3 composite, has been prepared and its electrochemical activity for methanol oxidation has been investigated The mesoporous tungsten trioxide support was synthesized by a replicating route and the mesoporous composties with Pt loaded were characterized by using X-ray diffraction (XRD), nitrogen sorption, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) techniques Cyclic voltammetry (CV), line scan voltammetry (LSV) and chronoamperometry (CA) were adopted to characterize the electrochemical activities of the composites The mesoporous WO3 showed high surface area, ordered pore structure, and nanosized wall thickness of about 6−7 nm When a certain amount of Pt nanoparticles were dispersed in the pore structure of mesoporous WO3, the resultant mesostructured Pt/WO3 composites exhibit high electro-catalytic activity toward methanol oxidation The overall e
116 citations
TL;DR: In this paper, the electronic structures and magnetic properties of the anatase TiO2 doped with 3D transition metals (V, Cr, Mn, Fe, Co, Ni) were studied.
Abstract: We study the electronic structures and magnetic properties of the anatase TiO2 doped with 3d transition metals (V, Cr, Mn, Fe, Co, Ni), using first-principles total energy calculations based on density functional theory (DFT). Using a molecular-orbital bonding model, the electronic structures of the doped anatase TiO2 are well understood. A band coupling model based on d-d level repulsions between the dopant ions is proposed to understand the chemical trend of the magnetic ordering. Ferromagnetism is found to be stabilized in the V-, Cr-, and Co-doped samples if there are no other carrier native defects or dopants. The ferromagnetism in the Cr- and Co-doped samples may be weakened by the donor defects. In the Mn-, and Fe-doped samples, the ferromagnetism can be enhanced by the acceptor and donor defects, respectively.
77 citations
TL;DR: In this article, a simple and effective strategy, called the vacuum nano-casting route (VNR), was proposed for fabricating hollow mesoporous spheres with magnetic cores, the loading amount of magnetic cores and saturation magnetization value can be easily tuned by changing the concentration of iron nitrate solution used in the synthesis procedure.
Abstract: We present here a simple and effective strategy, called the vacuum nano-casting route (VNR), for fabricating hollow mesoporous spheres with magnetic cores. The loading amount of magnetic cores and saturation magnetization value can be easily tuned by changing the concentration of iron nitrate solution used in the synthesis procedure. After modification with 1,4-bis(triethoxysilyl)propane tetrasulfide (BTESPTS), the composite could be used as a highly selective absorbent of Hg2+, and was conveniently separated by an external magnetic field. The composites are characterized by X-ray diffraction, N2 sorption isotherms, transmission electron microscopy, energy-dispersive spectroscopy, vibrating-sample magnetometry, Fourier-transform infrared spectroscopy and inductively coupled plasma atomic emission spectroscopy.
75 citations
TL;DR: In this article, the phase boundaries of the antiferromagnetic-paramagnetic transition were determined by temperature-dependent X-ray diffraction and the temperature dependences of expansion coefficients of the samples were obtained.
65 citations
TL;DR: In this paper, a solution-based method was used to synthesize anti ferromagnetic Co3O4 nanoparticles with diameter around 30nm and the phase identification by wide-angle X-ray powder diffraction indicates that the CO3O 4 nanoparticle has a cubic spinel structure with a lattice constant of 0.80843(2) nm.
Abstract: Anti ferromagnetic Co3O4 nanoparticles with diameter around 30nm have been synthesized by a solution-based method. The phase identification by the wide-angle X-ray powder diffraction indicates that the CO3O4 nanoparticle has a cubic spinel structure with a lattice constant of 0.80843(2) nm. The image of field emission scanning electron microscope shows that the nanoparticles are assembled together to form nanorods. The magnetic properties of Co3O4 fine particles have been measured by a superconducting quantum interference device magnetometer. A deviation of the Neel temperature from the bulk is observed, which can be well described by the theory of finite-size scaling. An enhanced coercivity as well as a loop shift are observed in the field-cooled hysteresis loop. The exchange bias field decreases with increasing temperature and diminishes at the Neel temperature. The training effect and the opening of the loop reveal the existence of the spin-glass-like surface spins. (C) 2008 Elsevier B.V. All rights reserved.
59 citations
TL;DR: In this paper, the size-and shape-dependent band gap of the TiO2 nanowires in rutile structure is calculated and the quantum confinement effect becomes significant for d < 25 angstrom, and a notable anisotropy exists that arises from the anisotropic of the effective masses.
Abstract: The quantum confinement effect, electronic properties, and optical properties of TiO2 nanowires in rutile structure are investigated via first-principles calculations. We calculate the size- and shape-dependent band gap of the nanowires and fit the results with the function E-g = E-g(bulk) + beta/d(alpha). We find that the quantum confinement effect becomes significant for d < 25 angstrom, and a notable anisotropy exists that arises from the anisotropy of the effective masses. We also evaluate the imaginary part of the frequency-dependent dielectric function [epsilon(2)(omega)] within the electric-dipole approximation, for both the polarization parallel [epsilon(parallel to)(2)(omega)] and the perpendicular [epsilon 1/2(omega)] to the axial (c) direction. The band structure of the nanowires is calculated, with which the fine structure of epsilon(parallel to)(2)(omega) has been analyzed.
50 citations
TL;DR: In this paper, pseudohydrogen atoms are used to passivate the surface dangling bonds, which remove the localized in-cap surface states in the rutile TiO2 QDs.
Abstract: The electronic structure of rutile TiO2 quantum dots (QDs) are investigated via the first-principles band structure method. We first propose a model to passivate the rutile TiO2 surfaces for the local density approximation calculations. In this model pseudohydrogen atoms are used to passivate the surface dangling bonds, which remove the localized in-cap surface states in the TiO2 QDs. As the size of the QD decreases, the band gap evolves as E-g(dot) = E-g(bulk) + 73.70/d(1.93), where E-g(dot) and d are the band gap and diameter of the QD, and E-g(bulk) is the band gap of the bulk rutile TiO2. The valence band maximum and the conduction band minimum states of the QDs are distributed mostly in the interior of the QDs, and they well inherit the atomic characteristics of those states of the bulk rutile TiO2.
46 citations
TL;DR: In this article, the bulk modulus of hexagonal AlxGa1-xN ternary alloys is calculated using first-principles methods, and the fully relaxed structures and the structures subjected to homogeneous biaaxial and uniaxial tension are investigated.
Abstract: Elastic constants, the bulk modulus, Young's modulus, band-gap bowing coefficients, spontaneous and piezoelectric polarizations, and piezoelectric coefficients of hexagonal AlxGa1-xN ternary alloys are calculated using first-principles methods. The fully relaxed structures and the structures subjected to homogeneous biaxial and uniaxial tension are investigated. We show that the biaxial tension in the plane perpendicular to the c axis and the uniaxial tension along the c axis all reduce the bulk modulus, whereas they reduce and enhance Young's modulus, respectively. We find that the biaxial and uniaxial tension can enhance the bowing coefficients. We also find that the biaxial tension can enhance the total polarization, while the uniaxial tension will suppress the total polarization. (C) 2008 American Institute of Physics.
45 citations
TL;DR: In this paper, a thermodynamic assessment of the Cu-Ga system was carried out by the CALPHAD approach (CALculation of PHase Diagram), where the liquid phase, the solid solution phases of fcc, bcc, and hcp were taken into consideration in the optimization.
Abstract: A thermodynamic assessment of the Cu-Ga system was carried out by the CALPHAD approach (CALculation of PHase Diagram). The liquid phase, the solid solution phases of fcc (alpha-Cu) bcc (beta) and, hcp (zeta), the Solution compounds of Cu(9)Ga(4)_0 (gamma(o)), Cu(9)Ga(4)_1 (gamma(1)), Cu(9)Ga(4)_2 (gamma(2)) and Cu(9)Ga(4)_3 (gamma(3)), the stoichiometric compounds Of Cu(0.778)Ga(0.222) (zeta') and CuGa(2), and the terminal phase of orthorhombic-Ga were taken into consideration in the optimization. The substitutional solution model was used for the liquid phase and the solid solution phases of fcc, bcc and hcp. The solution compounds related to Cu(9)Ga(4) were described with the sublattice model. The order-disorder transformation between CugGa(4)_0 (gamma(0)) and Cu(9)Ga(4)_1 (gamma(1)) was considered. In the model description of Cu(9)Ga(4)_2 (gamma(2)) and Cu(9)Ga(4)_3 (gamma(3)), vacancies were considered. Differential scanning calorimetric analysis was conducted to obtain phase transformation temperatures and the enthalpies of fusion of Cu-Ga alloys. A set of self-consistent thermodynamic parameters were obtained and the calculated phase diagram and thermodynamic properties were presented and compared with experimental data. (C) 2008 Elsevier Ltd. All rights reserved.
TL;DR: In this paper, the phase diagram of the pseudo-binary system Bi2O3-Cr2O 3 has been reconstructed based upon X-ray diffraction measurement and differential thermal analysis.
TL;DR: In this paper, the Mg-Ga acceptor energy levels in GaN and random Al8In4Ga20N32 quaternary alloys were calculated using the first-principles band-structure method.
Abstract: The Mg-Ga acceptor energy levels in GaN and random Al8In4Ga20N32 quaternary alloys are calculated using the first-principles band-structure method. We show that due to wave function localization, the MgGa acceptor energy level in the alloy is significantly lower than that of GaN, although the two materials have nearly identical band gaps. Our study demonstrates that forming AlxInyGa1-x-yN quaternary alloys can be a useful approach to lower acceptor ionization energy in the nitrides and thus provides an approach to overcome the p-type doping difficulty in the nitride system.
TL;DR: The enhanced ionic conductivity, as well as the abnormal phase transitions, can be explained in terms of the existence of the highly conducting 7H polytype AgI and the formation of well-defined conduction paths in the composites.
Abstract: AgI nanowires/anodic aluminum oxide ( AgI NWs/AAO) composites have been fabricated by a simple approach, which involves the thermal melting of AgI powders on the surface of the AAO membrane, followed by the infiltration of the molten AgI inside the nanochannels. As-prepared AgI nanowires have corrugated outer surfaces and are polycrystalline according to scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. X-ray diffraction (XRD) shows that a considerable amount of 7H polytype AgI exists in the composites, which is supposed to arise from the interfacial interactions between the embedded AgI and the alumina. AC conductivity measurements for the AgI nanowires/AAO composites exhibit a notable conductivity enhancement by three orders of magnitude at room temperature compared with that of pristine bulk AgI. Furthermore, a large conductivity hysteresis and abnormal conductivity transitions were observed in the temperature-dependent conductivity measurements, from which an ionic conductivity as high as 8.0 x 10(2) Omega(-1) cm(-1) was obtained at around 70 degrees C upon cooling. The differential scanning calorimetry (DSC) result demonstrates a similar phase transition behavior as that found in the AC conductivity measurements. The enhanced ionic conductivity, as well as the abnormal phase transitions, can be explained in terms of the existence of the highly conducting 7H polytype AgI and the formation of well-defined conduction paths in the composites.
TL;DR: Wan et al. as mentioned in this paper investigated the ferroelectricity of rhombohedral PbTiO3 under uniaxial compression and showed that the Pb, Ti, and O atoms have different contributions to the total polarization with increasing the magnitude of uniao-xial compressive stress.
Abstract: The ferroelectricity of rhombohedral PbTiO3 under uniaxial compression is investigated from first-principles study Upon compression, the ferroelectricity decreases until a critical stress of −29 GPa and then increases with a further increase of the magnitude of the uniaxial compressive stress We also find that uniaxial compression could enhance piezoelectricity and that the maximum piezoelectric coefficient d33 occurs at σ33=−49 GPa, which supports the experimentally observed piezoelectric behavior in rhombohedral Pb(Mg1/3Nb2/3O3)−032PbTiO3 [Q Wan, C Chen, and Y P Shen, J Appl Phys 98, 024103 (2005)] Our calculated results show that the Pb, Ti, and O atoms have different contributions to the total polarization with increasing the magnitude of uniaxial compressive stress, and that when −σ33>55 GPa, the Ti atoms no longer have contributions to the polarization, which leads to the changes of ferroelectricity and piezoelectricity
TL;DR: The subsolidus phase relations of the ternary system ZnO-B2O3-P2O5 were investigated by means of X-ray diffraction (XRD) and differential thermal analysis (DTA) methods as mentioned in this paper.
TL;DR: In this paper, the authors systematically study the mechanism of defect formation and electronic structures for 3D transition-metal impurities (V, Cr, Mn, Fe, and Co) doped in silicon nanowires.
Abstract: Using first-principles methods, we systematically study the mechanism of defect formation and electronic structures for 3d transition-metal impurities (V, Cr, Mn, Fe, and Co) doped in silicon nanowires. We find that the formation energies of 3d transition-metal impurities with electrons or holes at the defect levels always increase as the diameters of silicon nanowires decrease, which suggests that self-purification, i.e., the difficulty of doping in silicon nanowires, should be an intrinsic effect. The calculated results show that the defect formation energies of Mn and Fe impurities are lower than those of V, Cr, and Co impurities in silicon nanowires. It indicates that Mn and Fe can easily occupy substitutional site in the interior of silicon nanowires. Moreover, they have larger localized moments, which means that they are good candidates for Si-based dilute magnetic semiconductor nanowires. The doping of Mn and Fe atom in silicon nanowires introduces a pair of energy levels with t(2) symmetry. One of which is dominated by 3d electrons of Mn or Fe, and the other by neighboring dangling bonds of Si vacancies. In addition, a set of nonbonding states localized on the transition-metal atom with e symmetry is also introduced. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3000445]
TL;DR: In this paper, the electronic structure and optical gain of wurtzite ZnO nanowires were investigated in the framework of effective mass envelope function theory, and it was shown that as the elliptical aspect ratio e increases to a larger than a critical value, the hole ground states may change from optically dark to optically bright.
Abstract: The electronic structure and optical gain of wurtzite ZnO nanowires are investigated in the framework of effective-mass envelope-function theory. We found that as the elliptical aspect ratio e increases to be larger than a critical value, the hole ground states may change from optically dark to optically bright. The optical gain of ZnO nanowires increases as the hole density increases. For elliptical wire with large e, the y-polarized mode gain can be several thousand cm(-1), while the x-poiarized mode gain may be 26 times smaller than the former, so they can be used as ultraviolet linearly polarized lasers. (C) 2008 American Institute of Physics.
TL;DR: In this paper, the eutectic reaction between the NiZr and NZr2 compounds in the Ni-Zr binary phase diagram was evaluated according to previous experimental measurements and thermodynamic calculations.
Abstract: The eutectic reaction between the NiZr and NiZr2 compounds in the Ni – Zr binary phase diagram was evaluated according to the previous experimental measurements and thermodynamic calculations. A discrepancy between the calculations and the measurements exists. In order to determine definitely this eutectic temperature, Ni – Zr alloys of the compositions 65, 68, 72 and 74 wt.% Zr were prepared by melting in an arc furnace under a purified argon atmosphere. The phase transformation temperature was measured by Differential Scanning Calorimetry using both alumina and zirconia crucibles. At heating rates of 5, 10 and 20 K min− 1, the eutectic temperature between the NiZr and NiZr2 compounds was determined consistently to be 1022 ± 5 °C. Zirconia crucibles can be used in Differential Scanning Calorimetry measurements of zirconium-containing alloys but only below 1170 °C.
TL;DR: In this article, a facile one-pot synthetic approach, using oleic acid and oleylamine as composite stabilizers combined with high-temperature treatment in 1-octadecene, has been developed for the preparation of monodisperse and uniform lanthanum phosphate and europium-doped nanocrystals.
Abstract: A facile one-pot synthetic approach, using oleic acid and oleylamine as composite stabilizers combined with high-temperature treatment in 1-octadecene, has been developed for the preparation of monodisperse and uniform lanthanum phosphate and europium-doped lanthanum phosphate nanocrystals. In particular, with the present synthetic approach, the size of the resulting nanocrystals could be tuned precisely and continuously from 3.5 to 6.5 nm by seed-mediated epitaxial growth. The as-obtained uniform nanocrystals with hydrophobic surfaces, which show efficient photoluminescence, could be easily dispersed in nonpolar solvents. More importantly, these nanocrystals can also be easily modified to water-dispersed ones with hydrophilic surfaces for potential use in in vitro imaging in bioanalysis. In addition, a synthetic mechanism for these monodisperse nanocrystals is presented and discussed.
TL;DR: In this paper, a mesoporous tungsten trioxide support was synthesized by a replicating route and the mesostructured Pt/WO3 composites with Pt loaded were characterized by using X-ray diffraction (XRD), nitrogen sorption, field emission scanning electron microscopy (FE-SEM), TEM and energy-dispersive Xray spectroscopy (EDS) techniques.
Abstract: A new type of carbon-free electrode catalyst, Pt/mesoporous WO3 composite, has been prepared and its electrochemical activity for methanol oxidation has been investigated. The mesoporous tungsten trioxide support was synthesized by a replicating route and the mesoporous composties with Pt loaded were characterized by using X-ray diffraction (XRD), nitrogen sorption, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) techniques. Cyclic voltammetry (CV), line scan voltammetry (LSV) and chronoamperometry (CA) were adopted to characterize the electrochemical activities of the composites. The mesoporous WO3 showed high surface area, ordered pore structure, and nanosized wall thickness of about 6−7 nm. When a certain amount of Pt nanoparticles were dispersed in the pore structure of mesoporous WO3, the resultant mesostructured Pt/WO3 composites exhibit high electro-catalytic activity toward methanol oxidation. The overall e...
TL;DR: K(2)Zn(3) was synthesized by solid state reaction and its crystal structure was determined by ab initio method from powder X-ray diffraction (XRD) data.
Abstract: K(2)Zn(3)(P(2)O(7))(2) was synthesized by solid state reaction and its crystal structure was determined by ab initio method from powder X-ray diffraction (XRD) data. The title compound was determined to be orthorhombic with space group P2(1)2(1)2(1), Z=4, and lattice parameters a=12.901(8) angstrom, b=10.102(6) angstrom, and c=9.958(1) angstrom. Values of lattice parameters from 303 to 573 K were measured by temperature-dependent XRD. Thermal expansion coefficients alpha(0), lattice parameters, and cell volume at 0 K were determined to be alpha(0)(a)=1.62327X 10(-4)/K, a(0)=12.855(4) angstrom, alpha(0)(b) = 1.17921 X 10(-4)/K, b(0)=10.070(8) angstrom, alpha(0)(c)=2.62364X 10(-4)/K, c(0)=9.880(4) angstrom, and alpha(0)(V) = 6.599 X 10(-2) /K, V(0) = 1278.967(0) angstrom(3). The specific heat equation as a function of temperature was determined to be C(p)=0.77115 +0.00231 T-1241.60027T(-2)- 1.4133 X 10(-6)T(2) (J/K g), for temperatures from 198 to 710 K. The melting point estimated from the mu-DTA heating curve is 795 degrees C. (C) 2008 International Centre for Diffraction Data. [DOI: 10.1154/1.2992517]
TL;DR: In this paper, a new compound, Na2Zn5(PO4)(4), was identified in the system ZnO-Na2O-P2O5 and high-quality crystal was obtained by the melt method.
Abstract: A new compound, Na2Zn5(PO4)(4), was identified in the system ZnO-Na2O-P2O5 and high-quality crystal was obtained by the melt method. The crystal structure of this compound was solved by direct method from single crystal X-ray diffraction data. The structure was then refined anisotropically using a full-matrix least square refinement on F-2 and the refinement converged to R-1 = 0.0233 and wR(2) = 0.0544. This compound crystallizes in the orthorhombic system with space group Pbcn, lattice parameters a=10.381(2) angstrom, b=8.507(1) angstrom, c = 16.568(3) angstrom and Z = 4. The structure is made up of 3D [Zn5P4O16](n)(2n-) covalent framework consisting of [Zn4P4O16](n)(4n-) layers. The powder diffraction pattern of Na9Zn21(PO4)(17) is explained by simulating a theoretical pattern with NaZnPO4 and Na2Zn5(PO4)(4) in the molar ratio of 1:4 and then by Rietveld refinement of experimental pattern. Na2Zn5(PO4)(4) melts congruently at 855 degrees C and its conductivity is 5.63 x 10(-9) S/cm. (C) 2007 Elsevier Inc. All rights reserved.