Xinglong Dong

Catalytically active single-atom niobium in graphitic layers

Carbides of groups IV through VI (Ti, V and Cr groups) have long been proposed as substitutes for noble metal-based electrocatalysts in polymer electrolyte fuel cells. However, their catalytic activity has been extremely limited because of the low density and stability of catalytically active sites. Here we report the excellent performance of a niobium-carbon structure for catalysing the cathodic oxygen reduction reaction. A large number of single niobium atoms and ultra small clusters trapped in graphitic layers are directly identified using state-of-the-art aberration-corrected scanning transmission electron microscopy. This structure not only enhances the overall conductivity for accelerating the exchange of ions and electrons, but it suppresses the chemical/thermal coarsening of the active particles. Experimental results coupled with theory calculations reveal that the single niobium atoms incorporated within the graphitic layers produce a redistribution of d-band electrons and become surprisingly active for O2 adsorption and dissociation, and also exhibit high stability.

ZnO-based homojunction light-emitting diodes fabricated by metal-organic chemical vapor deposition

ZnO p–n homojunction light-emitting diodes were fabricated on p-GaAs substrates by metal-organic chemical vapor deposition. The current–voltage characteristics showed a diode characteristic between the n-type and p-type ZnO layers with a threshold voltage of 3.8 V. We present a room-temperature study of photoluminescence (PL) spectra and electroluminescence (EL) spectra. The PL spectra of both n-type and p-type ZnO exhibited strong near-band-edge emission and weak broad deep-level emission peaks. The EL spectra showed a strong broad deep-level emission when the injection current attained 40 mA, and a weak ultraviolet emission emerged when the current attained 150 mA.

Study on the properties of MgxZn1?xO-based homojunction light-emitting diodes fabricated by MOCVD

A MgxZn1−xO p–n homojunction light-emitting diode (LED) was fabricated on the GaAs substrate by metal–organic chemical vapour deposition (MOCVD). The I–V curve of the device exhibited typical rectifying behaviour of the p–n diode. The forward turn-on voltage was about 3.5 V. The photoluminescence spectra of the n-type and p-type MgxZn1−xO layers exhibited obvious near-band-edge emission peaks and weak deep-level emission peaks. In the electroluminescence (EL) spectra of the diode, a broad emission band from 2.3 to 2.8 eV can be observed. The emission band near 3.31 eV did not appear until the injection current reached 180 mA. The differences in the EL spectra between ZnO and MgxZn1−xO LEDs are also discussed.

Fabrication of nanostructured V2O5 via urea combustion for high-performance Li-ion battery cathode

Nanostructured vanadium pentoxide (V2O5) crusts were facilely synthesized via the combustion of a precursor by mixing commercial V2O5 with molten urea. The nanocrusts were transferred to nanorods during further annealing at 630 °C. Both the V2O5 nanocrusts and V2O5 nanorods were used preliminarily as a cathode material for Li-ion batteries. Their electrode performance was highly improved compared to commercial V2O5.

Manipulated electromagnetic losses by integrating chemically heterogeneous components in Fe-based core/shell architecture

Existing techniques for stabilizing and functionalizing metal nanostructures required precise control of complex procedures and probably introduce undesirable impurities. We herein report an arc-discharge chemical vapor deposition strategy for the synthesis of chemically heterogeneous core/shell metal/oxide nanocapsules Fe/TiFe2O4, Fe/MnFe2O4, and Fe/Al2O3. A universal formation mechanism based on the co-effect of oxygen potential and surface energy is further proposed, derived from fundamental thermodynamics. Such core/shell nanocapsules, integrated with tunable components, present an effective manipulability of microwave absorption at expected frequency, originating from the various dielectric behaviors of the heterogeneous oxide shells.

Formation mechanism and optical characterization of polymorphic silicon nanostructures by DC arc-discharge

Silicon nanoparticles (Si NPs), silicon nanosheets (Si NSs), and silicon nanoribbons (Si NRs) were fabricated by means of DC arc-discharge under diverse atmospheres (hydrogen, mixtures of hydrogen and inert gas). It is shown that these as-prepared Si NPs are approximately 5–50 nm in diameter, Si NSs are about 10–30 nm in width and about 2.8 nm in thickness, and Si NRs consist of fine sheets with a length as long as 200 nm, width of 13 nm, and thickness of 3.1 nm. BET measurements reveal that the specific surfaces are 110.9, 108.8, and 164.2 m2 g−1 for Si NPs, Si NSs, Si NRs, respectively. Formation mechanisms for polymorphic Si nanostructures are elucidated, revealing that the anisotropic or isotropic growth of Si nanostructures is greatly induced by high energetic inert gas and hydrogen atoms, and finally results in the formation of polymorphic Si nanostructures. A visible down-shift of Raman frequency for these Si nanostructures is mainly attributed to the size effect. The band gaps are experimentally measured as 2.89 eV (Si NPs), 2.92 eV (Si NSs), and 3.02 eV (Si NRs) for direct transition, and 1.99 eV (Si NPs), 1.26 eV (Si NSs), and 1.36 eV (Si NRs) for indirect transition. These are noticeably enlarged with respect to bulk Si (1.1 eV).

Interface evolution in the platelet-like SiC@C and SiC@SiO2 monocrystal nanocapsules

Carbon-coated SiC@C nanocapsules (NCs) with a hexagonal platelet-like morphology were fabricated by a simple direct current (DC) arc-discharge plasma method. The SiC@C NCs were monocrystalline, 120–150 nm in size, and approximately 50 nm thick. The formation of the as-prepared SiC@C NCs included nucleation of truncated octahedral SiC seeds and subsequent anisotropic growth of the seeds into hexagonal nanoplatelets in a carbon-rich atmosphere. The disordered carbon layers on the SiC@C NCs were converted into SiO2 shells of SiC@SiO2 NCs by heat treatment at 650 °C in air, during which the shape and inherent characteristics of the crystalline SiC core were obtained. The interface evolution from carbon to SiO2 shells endowed the SiC@SiO2 NCs with enhanced photocatalytic activity due to the hydrophilic and transparent nature of the SiO2 shell, as well as to the photosensitive SiC nanocrystals. The band gap of the nanostructured SiC core was determined to be 2.70 eV. The SiC@SiO2 NCs degraded approximately 95% of methylene blue in 160 min under visible light irradiation.

Novel nanocapsules with Co–TiC twin cores and regulable graphitic shells for superior electromagnetic wave absorption

The synthesis of nanometer materials with unique structures and compositions has proven successful towards the attenuation of electromagnetic (EM) waves. However, it is still a challenge to form special nanostructures by integrating magnetic/dielectric loss materials into one particle due to the difficulties in coupling the heterogeneous components. Herein, we present the synthesis of novel nanocapsules (NCs) with Co–TiC twin cores encapsulated inside graphitic shells using an arc-discharge plasma method. The thickness of the graphitic shell could be controlled by quantitatively tuning the carbon source concentration. The optimal reflection loss (RL) values of the prepared NCs was −66.59 dB at 8.76 GHz with a low thickness of 2.56 mm. The bandwidth of RL ≤ −10 dB was up to 14.4 GHz, which almost covered the entire frequency band, namely, the S to Ku band (3.6 GHz to 18 GHz). This superior EM wave absorption was ascribed to the specific double-core shell nanostructures and effective impedance matching between the magnetic loss and dielectric loss originating from the combination of the magnetic Co and dielectric TiC/C.

Optical emission spectroscopy diagnosis of energetic Ar ions in synthesis of SiC polytypes by DC arc discharge plasma

Silicon carbides are basilic ceramics with proper bandgaps (2.4–3.3 eV) and unique optical properties. SiC@C monocrystal nanocapsules with different morphologies, sizes, and crystal types were synthesized via the fast and facile direct current (DC) arc discharge plasma method. The influence of Ar atmosphere on the formation of nanocrystal SiC polytypes was investigated by optical emission spectroscopy (OES) diagnoses on the arc discharge plasma. Boltzmann’s plot was used to estimate the temperatures of plasma containing different Ar concentrations as 10,582 K (in 2 × 104 Pa of Ar partial pressure) and 14,523 K (in 4 × 104 Pa of Ar partial pressure). It was found that higher energy state of plasma favors the ionization of carbon atoms and promotes the formation of α-SiC, while β-SiC is generally coexistent. Heat-treatment in air was applied to remove the carbon species in as-prepared SiC nanopowders. Thus, the intrinsic characters of SiC polytypes reappeared in the ultraviolet–visible (UV–vis) light absorbance. It was experimentally revealed that the direct bandgap of SiC is 5.72 eV, the indirect bandgap of β-SiC (3C) is 3.13 eV, and the indirect bandgap of α-SiC (6H) is 3.32 eV; visible quantum confinement effect is predicted for these polytypic SiC nanocrystals.

Adsorption Performance of Methylene Blue onto Nanoparticles of Carbon-Encapsulated Magnetic Nickel

Nanoparticles of carbon encapsulated nickel (Ni@C NPs) were in-situ synthesized by direct current arc-discharge plasma method through evaporating pure Ni in methane atmosphere. Transmission electron microscopy observation revealed that the nanoparticles (Ni@C NPs) exhibited an encapsulation structure with Ni metal as core and carbon 3-5 nm in thickness as shell. The BET surface area of the prepared Ni@C NPs is 38.82 m·g according to N2 adsorption-desorption isotherm. Surface modification with hydrogen peroxide was carried to graft oxygen-containing groups on carbon, which can improve the wettability and hydrophilicity of the Ni@C NPs. Then the effect of contact time, adsorption time and *国家重点基础研究发展计划2011CB936002和国家自然科学基金51271044、51331006、51171033资助项目。 2014年10月8日收到初稿; 2014年11月13日收到修改稿。 本文联系人:董星龙,教授