A highly active alternative to Pt electrocatalysts for the oxygen reduction reaction (ORR), which... more A highly active alternative to Pt electrocatalysts for the oxygen reduction reaction (ORR), which is the cathode-electrode reaction of fuel cells, is sought for higher fuel-cell performance. Our theoretical modelling reveals that B-doped Pd (Pd-B) weakens the absorption of ORR intermediates with nearly optimal binding energy by lowering the barrier associated with O2 dissociation, suggesting Pd-B should be highly active for ORR. In fact, Pd-B, facile synthesized by an electroless deposition process, exhibits 2.2 times and 8.8 times higher specific activity and 14 times and 35 times less costly than commercial pure Pd and Pt catalysts, respectively. Another computational result is that the surface core level of Pd is negatively shifted by B doping, as confirmed by XPS, and implies that filling the density of states related to the anti-bonding of oxygen to Pd surfaces with excess electrons from B doping, weakens the O bonding to Pd and boosts the catalytic activity.
Chemical communications (Cambridge, England), Jan 4, 2016
A highly active Pd-P nanoparticle electrocatalyst for formic acid oxidation was synthesized using... more A highly active Pd-P nanoparticle electrocatalyst for formic acid oxidation was synthesized using NaH2PO2 as the reducing agent. The Pd-P nanoparticles were amorphous and exhibited higher specific and mass activity values compared to commercial Pd/C electrocatalyts and reported literature values. Furthermore, the Pd-P nanoparticles were found to be more durable than Pd/C electrocatalyts.
Topological insulator is composed of an insulating bulk state and time reversal symmetry protecte... more Topological insulator is composed of an insulating bulk state and time reversal symmetry protected two-dimensional surface states. One of the characteristics of the surface states is the locking between electron momentum and spin orientation. Here, we report a novel in-plane anisotropic magnetoresistance in topological insulator Bi 1.5 Sb 0.5 Te 1.8 Se 1.2 /CoFe heterostructures. To explain the novel effect, we propose that the Bi 1.5 Sb 0.5 Te 1.8 Se 1.2 /CoFe heterostructure forms a spin-valve or Giant magnetoresistance device due to spin-momentum locking. The novel in-plane anisotropic magnetoresistance can be explained as a Giant magnetoresistance effect of the Bi 1.5 Sb 0.5 Te 1.8 Se 1.2 /CoFe heterostructures.
Soft X-ray absorption spectroscopy (XAS), using fluorescence yield, was used to study the boron K... more Soft X-ray absorption spectroscopy (XAS), using fluorescence yield, was used to study the boron K near edge in MgB2 superconductor. The sample consists of MgB2 crystallites randomly oriented in a magnesium matrix. Abrasion of the sample surface in vacuum provides a surface relatively free of boroncontaining impurities. The intrinsic boron K near edge spectrum of the sample at a temperature of 295 K is identified. This spectrum is then compared in detail with a spectrum calculated using the full potential linearized augmented plane wave method. Features predicted by the theory appear near the expected energies, with qualitative agreement regarding shape and intensity.
Systematic studies of thermophysical properties of alloyed-123-structure high-Tc superconductors ... more Systematic studies of thermophysical properties of alloyed-123-structure high-Tc superconductors are required due to an increasing number of applications of these compounds rather than pure YBa 2Cu3O7-delta to improve bulk and grain boundary transport and other properties. Calculations made using consistent interatomic pair-potential parameters for 123 compounds give overall good agreement for lattice statics and dynamics. The lattice strain was found to affect substantially elastic constants, thermal expansion, congruent melting temperature, charge transfer, chain-oxygen order-disorder transition. Atomistic simulations show that the Schottky defect formation energy depends strongly on oxygen content, which directly determines the nature of space charges and probably affects transport properties of grain boundaries. Our calculations show that the most energetically favorable way is to substitute calcium ions at yttrium sites, accompanied by hole creation in the CuO2 plane around tensile strain regions. The increase of hole content and segregation of calcium help to improve the critical current density across grain boundaries. In addition, the thermodynamics calculations show the importance of Short-Range-Order in forming the cubic phase of Y-Ba oxy-fluoride, a precursor for the synthesis of YBCO compounds. The same origin leads to a transformation into ordered Y-Ba oxy-fluoride phase. Studies were also make of defect structure and electronic structure of the new superconductor MgB2. First-principles calculations were made of electronic structure, structure factors for X-ray diffraction, and soft X-ray absorption spectra. Besides, we have carried out first principles calculations on the electronic structure, structure factors, and X-ray absorption spectrum. These studies suggested that the electronic structure of MgB2 is highly anisotopic, which is the origin of the multi-superconducting-gaps in this compound. In addition, the thermodynamics calculations show the importance of Short-Range-Ordering in forming the cubic phase of Y-Ba oxy-fluoride as one precursor for synthesis of YBCO compounds. Exactly the same origin leads to the order-disorder transformation into ordered Y-Ba oxy-fluoride phase.
We investigate the spin transport properties of iron-phthalocyanine (FePc) molecule sandwiched be... more We investigate the spin transport properties of iron-phthalocyanine (FePc) molecule sandwiched between two N-doped graphene nanoribbons (GNRs) based on the density functional theory and nonequilibrium Green's function methods. Our calculated results clearly reveal that the FePc molecular junction has high spin-filter efficiency as well as negative differential resistance (NDR). The zero-bias conductance through FePc molecule is dominated by the spin-down electrons, and the observed NDR originates from the bias-dependent effective coupling between the FePc molecular orbitals and the narrow density of states of electrodes. The remarkable high spin-filter efficiency and NDR are robust regardless of the edge shape and the width of GNRs, and the N-doping site in GNRs. These predictions indicate that FePc junction holds great promise in molecular electronics and spintronics applications.
Ferroelectric nanostructures can be formed by local switching of domains using techniques such as... more Ferroelectric nanostructures can be formed by local switching of domains using techniques such as piezo-force microscopy (PFM). Understanding lateral size effects is important to determine the minimum feature size for writing ferroelectric nanostructures. To understand these lateral size effects, we use the time-dependent-Ginzburg-Landau equations to simulate localized switching of domains for a PFM type and parallel-plate capacitor configurations. Our investigations indicate that fringing electric fields lead to switching via 90 deg domain wedge nucleation for thicker films while at smaller thicknesses, the polarization switches directly by 180 deg rotations. The voltage required to switch the domain increases by decreasing the lateral size and at very small lateral sizes the coercive voltage becomes so large that it becomes virtually impossible to switch the domain. In all cases, the width of the switched region extends beyond the electrodes, due to fringing.
Boron Carbide is one the hardest and lightest material that is also relatively easier to synthesi... more Boron Carbide is one the hardest and lightest material that is also relatively easier to synthesis as compared to other superhard ceramics like cubic boron nitride and diamond. However, the brittle nature of monolithic advanced ceramics material hinders its use in various engineering applications. Thus, strategies that can toughen the material are of fundamental and technological importance. One approach is to use nanostructure materials as building blocks, and organize them into a complex hierarchical structure, which could potentially enhance its mechanical properties to exceed that of the monolithic form. In this paper, we demonstrated a simple approach to synthesize one- and two-dimension nanostructure boron carbide by simply changing the mixing ratio of the initial compound to influence the saturation condition of the process at a relatively low temperature of 1500 degrees C with no catalyst involved in the growing process. Characterization of the resulting nano-structures shows B13C2, which is a superhard phase of boron carbide as its hardness is almost twice as hard as the commonly known B4C. Using ab-initio density functional theory study on the elastic properties of both B12C3 and B13C2, the high hardness of B13C2 is consistent to our calculation results, where bulk modulus of B13C2 is higher than that of B4C. High resolution transmission electron microscopy of the nanoflakes also reveals high density of twinning defects which could potentially inhibit the crack propagation, leading to toughening of the materials.
We present a first-principles study of the spin-polarized transport properties of three planar fo... more We present a first-principles study of the spin-polarized transport properties of three planar fourcoordinate Fe complexes with different noninnocent ligands sandwiched between two armchair (5,5) single-walled carbon nanotube (SWCNT) electrodes. Theoretical results clearly reveal that the current of the spin-up electrons through three examined molecular junctions is significant larger than that of the spin-down electrons. The low bias conductance is determined by the tail of transmission peak coming from the perturbed lowest unoccupied molecular orbital of the spin-up electrons. The high spin-filter efficiencies (up to 99.0%) are predicted for three planar four-coordinate Fe complexes coupled to SWCNTs.
NaOH-reacted graphite oxide film was prepared by decomposing epoxy groups in graphite oxide into ... more NaOH-reacted graphite oxide film was prepared by decomposing epoxy groups in graphite oxide into hydroxyl and -ONa groups with NaOH solution. Ultrafast carrier dynamics of the sample were studied by time-resolved transient differential reflection (△R/R). The data show two exponential relaxation processes. The slow relaxation process (∼2ps) is ascribed to low energy acoustic phonon mediated scattering. The electron-phonon coupling and first-principles calculation results demonstrate that -OH and -ONa groups in the sample are strongly coupled. Thus, we attribute the fast relaxation process (∼0.17ps) to the coupling of hydroxyl and -ONa groups in the sample.
An increasing number of applications of 123-structure high-Tc superconductors use compounds other... more An increasing number of applications of 123-structure high-Tc superconductors use compounds other than pure Y-123. In many bulk applications Y is replaced by Nd, Sm, or other rare-earth elements, and doping with Ca, Sr, and other alkaline earth elements has been utilized to improve grain boundary transport and other properties. In this paper we describe the development of shell models of ionic bonding in such materials and utilize them to discuss the energetics and thermodynamics of point defects, oxygen order, and the effects of stress, in comparison with the case of Y-123.
The time-dependent Ginzburg-Landau (TDGL) equations and phase field modeling have been used to de... more The time-dependent Ginzburg-Landau (TDGL) equations and phase field modeling have been used to describe various phenomena in ferroelectric materials, such as domain nucleation and evolution, and hysteresis. This work applies the TDGL model to explain the behavior of perovskite ferroelectric thin film with space charge. Results show that the presence of space charge at the surface significantly influences the switching process and domain structures in ferroelectric thin films. The role of space charge on size effects is also studied.
Uniform hierarchical microspheres scaffolded from ultrathin ZnGa2O4 nanosheets with over 99% expo... more Uniform hierarchical microspheres scaffolded from ultrathin ZnGa2O4 nanosheets with over 99% exposed facets were synthesized using an easy solvothermal route with ethylenediamine (en)/H2O binary solvents. Substitution of different chain length amines for en results in no formation of the nanosheet structures, indicating that the molecular structure of En is indispensable for the generation of two-dimensional structures. Inheriting both a high surface area of nanosheets and a high crystallinity of bulky materials allows the unique 3D hierarchical nanostructures to possess great CO2 photocatalytic performance. The normalized time-resolved traces of photo-induced absorption recorded from the nanosheet and meso-ZnGa2O4 indicate that the photo-excited carriers can survive longer on the nanosheet, which also contributes to the high photocatalytic activity of the ZnGa2O4 nanosheets.
Langmuir : the ACS journal of surfaces and colloids, Jan 27, 2016
Dendrite formation on the electrodes of a rechargeable battery during the charge-discharge cycle ... more Dendrite formation on the electrodes of a rechargeable battery during the charge-discharge cycle limits its capacity and application due to short-circuits and potential ignition. However, our understanding of the underlying dendrite growth and dissolution mechanisms is unclear. Here, the electrochemical growth and dissolution of silver dendrites on platinum electrodes immersed in an aqueous silver nitrate (AgNO3) electrolyte solution was investigated using in-situ liquid-cell transmission electron microscopy. The dissolution of Ag dendrites in an AgNO3 solution with added cetyltrimethylammonium bromide (CTAB) surfactant was compared to the dissolution of Ag dendrites in a pure aqueous AgNO3 solution. Significantly, when CTAB was added, dendrite dissolution proceeded in a step-by-step manner, resulting in nanoparticle formation and transient micro-growth stages due to Ostwald ripening. This resulted in complete dissolution of dendrites and "cleaning" of the cell of any silv...
A highly active alternative to Pt electrocatalysts for the oxygen reduction reaction (ORR), which... more A highly active alternative to Pt electrocatalysts for the oxygen reduction reaction (ORR), which is the cathode-electrode reaction of fuel cells, is sought for higher fuel-cell performance. Our theoretical modelling reveals that B-doped Pd (Pd-B) weakens the absorption of ORR intermediates with nearly optimal binding energy by lowering the barrier associated with O2 dissociation, suggesting Pd-B should be highly active for ORR. In fact, Pd-B, facile synthesized by an electroless deposition process, exhibits 2.2 times and 8.8 times higher specific activity and 14 times and 35 times less costly than commercial pure Pd and Pt catalysts, respectively. Another computational result is that the surface core level of Pd is negatively shifted by B doping, as confirmed by XPS, and implies that filling the density of states related to the anti-bonding of oxygen to Pd surfaces with excess electrons from B doping, weakens the O bonding to Pd and boosts the catalytic activity.
Chemical communications (Cambridge, England), Jan 4, 2016
A highly active Pd-P nanoparticle electrocatalyst for formic acid oxidation was synthesized using... more A highly active Pd-P nanoparticle electrocatalyst for formic acid oxidation was synthesized using NaH2PO2 as the reducing agent. The Pd-P nanoparticles were amorphous and exhibited higher specific and mass activity values compared to commercial Pd/C electrocatalyts and reported literature values. Furthermore, the Pd-P nanoparticles were found to be more durable than Pd/C electrocatalyts.
Topological insulator is composed of an insulating bulk state and time reversal symmetry protecte... more Topological insulator is composed of an insulating bulk state and time reversal symmetry protected two-dimensional surface states. One of the characteristics of the surface states is the locking between electron momentum and spin orientation. Here, we report a novel in-plane anisotropic magnetoresistance in topological insulator Bi 1.5 Sb 0.5 Te 1.8 Se 1.2 /CoFe heterostructures. To explain the novel effect, we propose that the Bi 1.5 Sb 0.5 Te 1.8 Se 1.2 /CoFe heterostructure forms a spin-valve or Giant magnetoresistance device due to spin-momentum locking. The novel in-plane anisotropic magnetoresistance can be explained as a Giant magnetoresistance effect of the Bi 1.5 Sb 0.5 Te 1.8 Se 1.2 /CoFe heterostructures.
Soft X-ray absorption spectroscopy (XAS), using fluorescence yield, was used to study the boron K... more Soft X-ray absorption spectroscopy (XAS), using fluorescence yield, was used to study the boron K near edge in MgB2 superconductor. The sample consists of MgB2 crystallites randomly oriented in a magnesium matrix. Abrasion of the sample surface in vacuum provides a surface relatively free of boroncontaining impurities. The intrinsic boron K near edge spectrum of the sample at a temperature of 295 K is identified. This spectrum is then compared in detail with a spectrum calculated using the full potential linearized augmented plane wave method. Features predicted by the theory appear near the expected energies, with qualitative agreement regarding shape and intensity.
Systematic studies of thermophysical properties of alloyed-123-structure high-Tc superconductors ... more Systematic studies of thermophysical properties of alloyed-123-structure high-Tc superconductors are required due to an increasing number of applications of these compounds rather than pure YBa 2Cu3O7-delta to improve bulk and grain boundary transport and other properties. Calculations made using consistent interatomic pair-potential parameters for 123 compounds give overall good agreement for lattice statics and dynamics. The lattice strain was found to affect substantially elastic constants, thermal expansion, congruent melting temperature, charge transfer, chain-oxygen order-disorder transition. Atomistic simulations show that the Schottky defect formation energy depends strongly on oxygen content, which directly determines the nature of space charges and probably affects transport properties of grain boundaries. Our calculations show that the most energetically favorable way is to substitute calcium ions at yttrium sites, accompanied by hole creation in the CuO2 plane around tensile strain regions. The increase of hole content and segregation of calcium help to improve the critical current density across grain boundaries. In addition, the thermodynamics calculations show the importance of Short-Range-Order in forming the cubic phase of Y-Ba oxy-fluoride, a precursor for the synthesis of YBCO compounds. The same origin leads to a transformation into ordered Y-Ba oxy-fluoride phase. Studies were also make of defect structure and electronic structure of the new superconductor MgB2. First-principles calculations were made of electronic structure, structure factors for X-ray diffraction, and soft X-ray absorption spectra. Besides, we have carried out first principles calculations on the electronic structure, structure factors, and X-ray absorption spectrum. These studies suggested that the electronic structure of MgB2 is highly anisotopic, which is the origin of the multi-superconducting-gaps in this compound. In addition, the thermodynamics calculations show the importance of Short-Range-Ordering in forming the cubic phase of Y-Ba oxy-fluoride as one precursor for synthesis of YBCO compounds. Exactly the same origin leads to the order-disorder transformation into ordered Y-Ba oxy-fluoride phase.
We investigate the spin transport properties of iron-phthalocyanine (FePc) molecule sandwiched be... more We investigate the spin transport properties of iron-phthalocyanine (FePc) molecule sandwiched between two N-doped graphene nanoribbons (GNRs) based on the density functional theory and nonequilibrium Green's function methods. Our calculated results clearly reveal that the FePc molecular junction has high spin-filter efficiency as well as negative differential resistance (NDR). The zero-bias conductance through FePc molecule is dominated by the spin-down electrons, and the observed NDR originates from the bias-dependent effective coupling between the FePc molecular orbitals and the narrow density of states of electrodes. The remarkable high spin-filter efficiency and NDR are robust regardless of the edge shape and the width of GNRs, and the N-doping site in GNRs. These predictions indicate that FePc junction holds great promise in molecular electronics and spintronics applications.
Ferroelectric nanostructures can be formed by local switching of domains using techniques such as... more Ferroelectric nanostructures can be formed by local switching of domains using techniques such as piezo-force microscopy (PFM). Understanding lateral size effects is important to determine the minimum feature size for writing ferroelectric nanostructures. To understand these lateral size effects, we use the time-dependent-Ginzburg-Landau equations to simulate localized switching of domains for a PFM type and parallel-plate capacitor configurations. Our investigations indicate that fringing electric fields lead to switching via 90 deg domain wedge nucleation for thicker films while at smaller thicknesses, the polarization switches directly by 180 deg rotations. The voltage required to switch the domain increases by decreasing the lateral size and at very small lateral sizes the coercive voltage becomes so large that it becomes virtually impossible to switch the domain. In all cases, the width of the switched region extends beyond the electrodes, due to fringing.
Boron Carbide is one the hardest and lightest material that is also relatively easier to synthesi... more Boron Carbide is one the hardest and lightest material that is also relatively easier to synthesis as compared to other superhard ceramics like cubic boron nitride and diamond. However, the brittle nature of monolithic advanced ceramics material hinders its use in various engineering applications. Thus, strategies that can toughen the material are of fundamental and technological importance. One approach is to use nanostructure materials as building blocks, and organize them into a complex hierarchical structure, which could potentially enhance its mechanical properties to exceed that of the monolithic form. In this paper, we demonstrated a simple approach to synthesize one- and two-dimension nanostructure boron carbide by simply changing the mixing ratio of the initial compound to influence the saturation condition of the process at a relatively low temperature of 1500 degrees C with no catalyst involved in the growing process. Characterization of the resulting nano-structures shows B13C2, which is a superhard phase of boron carbide as its hardness is almost twice as hard as the commonly known B4C. Using ab-initio density functional theory study on the elastic properties of both B12C3 and B13C2, the high hardness of B13C2 is consistent to our calculation results, where bulk modulus of B13C2 is higher than that of B4C. High resolution transmission electron microscopy of the nanoflakes also reveals high density of twinning defects which could potentially inhibit the crack propagation, leading to toughening of the materials.
We present a first-principles study of the spin-polarized transport properties of three planar fo... more We present a first-principles study of the spin-polarized transport properties of three planar fourcoordinate Fe complexes with different noninnocent ligands sandwiched between two armchair (5,5) single-walled carbon nanotube (SWCNT) electrodes. Theoretical results clearly reveal that the current of the spin-up electrons through three examined molecular junctions is significant larger than that of the spin-down electrons. The low bias conductance is determined by the tail of transmission peak coming from the perturbed lowest unoccupied molecular orbital of the spin-up electrons. The high spin-filter efficiencies (up to 99.0%) are predicted for three planar four-coordinate Fe complexes coupled to SWCNTs.
NaOH-reacted graphite oxide film was prepared by decomposing epoxy groups in graphite oxide into ... more NaOH-reacted graphite oxide film was prepared by decomposing epoxy groups in graphite oxide into hydroxyl and -ONa groups with NaOH solution. Ultrafast carrier dynamics of the sample were studied by time-resolved transient differential reflection (△R/R). The data show two exponential relaxation processes. The slow relaxation process (∼2ps) is ascribed to low energy acoustic phonon mediated scattering. The electron-phonon coupling and first-principles calculation results demonstrate that -OH and -ONa groups in the sample are strongly coupled. Thus, we attribute the fast relaxation process (∼0.17ps) to the coupling of hydroxyl and -ONa groups in the sample.
An increasing number of applications of 123-structure high-Tc superconductors use compounds other... more An increasing number of applications of 123-structure high-Tc superconductors use compounds other than pure Y-123. In many bulk applications Y is replaced by Nd, Sm, or other rare-earth elements, and doping with Ca, Sr, and other alkaline earth elements has been utilized to improve grain boundary transport and other properties. In this paper we describe the development of shell models of ionic bonding in such materials and utilize them to discuss the energetics and thermodynamics of point defects, oxygen order, and the effects of stress, in comparison with the case of Y-123.
The time-dependent Ginzburg-Landau (TDGL) equations and phase field modeling have been used to de... more The time-dependent Ginzburg-Landau (TDGL) equations and phase field modeling have been used to describe various phenomena in ferroelectric materials, such as domain nucleation and evolution, and hysteresis. This work applies the TDGL model to explain the behavior of perovskite ferroelectric thin film with space charge. Results show that the presence of space charge at the surface significantly influences the switching process and domain structures in ferroelectric thin films. The role of space charge on size effects is also studied.
Uniform hierarchical microspheres scaffolded from ultrathin ZnGa2O4 nanosheets with over 99% expo... more Uniform hierarchical microspheres scaffolded from ultrathin ZnGa2O4 nanosheets with over 99% exposed facets were synthesized using an easy solvothermal route with ethylenediamine (en)/H2O binary solvents. Substitution of different chain length amines for en results in no formation of the nanosheet structures, indicating that the molecular structure of En is indispensable for the generation of two-dimensional structures. Inheriting both a high surface area of nanosheets and a high crystallinity of bulky materials allows the unique 3D hierarchical nanostructures to possess great CO2 photocatalytic performance. The normalized time-resolved traces of photo-induced absorption recorded from the nanosheet and meso-ZnGa2O4 indicate that the photo-excited carriers can survive longer on the nanosheet, which also contributes to the high photocatalytic activity of the ZnGa2O4 nanosheets.
Langmuir : the ACS journal of surfaces and colloids, Jan 27, 2016
Dendrite formation on the electrodes of a rechargeable battery during the charge-discharge cycle ... more Dendrite formation on the electrodes of a rechargeable battery during the charge-discharge cycle limits its capacity and application due to short-circuits and potential ignition. However, our understanding of the underlying dendrite growth and dissolution mechanisms is unclear. Here, the electrochemical growth and dissolution of silver dendrites on platinum electrodes immersed in an aqueous silver nitrate (AgNO3) electrolyte solution was investigated using in-situ liquid-cell transmission electron microscopy. The dissolution of Ag dendrites in an AgNO3 solution with added cetyltrimethylammonium bromide (CTAB) surfactant was compared to the dissolution of Ag dendrites in a pure aqueous AgNO3 solution. Significantly, when CTAB was added, dendrite dissolution proceeded in a step-by-step manner, resulting in nanoparticle formation and transient micro-growth stages due to Ostwald ripening. This resulted in complete dissolution of dendrites and "cleaning" of the cell of any silv...
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