Jun Zhang

Huazhong University of Science and Technology, Wu-han-shih, Hubei, China

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Publications (34)209.07 Total impact

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    ABSTRACT: Mesoscopic WO3 microspheres composed of self-assembly nanofibers were prepared by hydrothermal reaction of tungsten acid potassium and H2O2. The mesoscopic WO3 microspheres offer desired porous properties and large effective active areas provided by intertwining nanofibers, thereby resulting in excellent supercapacitive properties due to facile electrolyte flow and fast reaction kinetics. In three electrode configuration, mesoscopic WO3 microspheres exhibit specific capacitance value of 797.05 F g−1 at the current density of 0.5 A g−1 and excellent cycling stability without decay after 2000 cycles in 2 M H2SO4 aqueous solution. These values are superior to other reported WO3 composites. An asymmetric supercapacitor is constructed using the as-prepared WO3 mesoscopic microspheres as the positive electrode and the activated carbon as the negative electrode, which displays excellent electrochemical performance with a maximum energy density of 97.61 Wh kg−1 and power density of 28.01 kW kg−1. These impressive performances suggest that the mesoscopic WO3 microspheres are promising electrode materials for supercapacitor.
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    ABSTRACT: The synthesis of noble metal nanocrystals terminated with high-index facets has received increasing attention due to the remarkable improvement on their catalytic performance. Introducing a transition metal to the noble metal could result in a reduced cost and potentially improved properties. By keeping in mind both of the advantages, we have developed a new synthetic approach to fabricate size-controlled Pt3Fe concave nanocubes using a high-temperature organic solution system containing oleylamine and oleic acid. It was further demonstrated that the particle size and concavity could be controlled by a number of parameters such as ratio of oleylamine and oleic acid, physic-chemical properties of metal carbonyl, metal valence in the precursor and ratio of metal precursors. Catalytic tests show that the high-index-surface-terminated Pt3Fe concave nanocubes exhibit superior performance in both hydrogenation of styrene and reduction of 4-nitrophenol in comparison with their counterparts.
    05/2015; DOI:10.1002/cnma.201500048
  • Chenyu Wang · Carrie Siu · Jun Zhang · Jiye Fang
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    ABSTRACT: The assembly of nanocrystals into ordered structures called supercrystals or superstructures has become a pivotal frontier owing to numerous useful applications such as correlating the arrangements of atoms in macroscopic crystals and tuning the collective properties to meet the demands of various applications. In this article, recent progress in the preparation of three-dimensional superlattices of nanocrystals is outlined, with a particular emphasis on the driving forces and evolutionary routes beyond orderly assembly. First, the leading or repulsive forces that internally and externally govern the formation of three-dimensional supercrystals are systematically identified and discussed with respect to their origins and functions in three-dimensional self-organization. Then a synoptic introduction of commonly applied means of nanocrystal self-assembly based on growth scenarios such as droplet evaporation and a liquid/liquid interface is presented with specific cases and detailed analyses. Finally, the existing challenges and prospects for this field are briefly highlighted.
    Nano Research 01/2015; DOI:10.1007/s12274-015-0767-1 · 6.96 Impact Factor
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    ABSTRACT: Nonpolar (11 2 ¯ 0) a-plane MgZnO films were grown on different a-GaN/r-sapphire templates by pulsed laser deposition (PLD), where the growth temperature of GaN buffer layers varied from 700 °C to 1000 °C. High-quality a-plane MgZnO epitaxial film was deposited on the optimized 1000 °C a-GaN/r-sapphire template. Temperature-dependent PL measurements of a-plane MgZnO films reveal an S-type temperature dependence of the excitonic recombination energy. It is resulted that the excitons are localized in alloy-induced potential fluctuations at low temperature and the room-temperature quantum efficiency is calculated to be 9.2%. An involvement of band-tail states in the radiative recombination was considered, and a quantitative description of the blue temperature-induced shift was obtained assuming a Gaussian shape of the band tail.
    Optical Materials Express 11/2014; 4(11). DOI:10.1364/OME.4.002346 · 2.92 Impact Factor
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    ABSTRACT: The effects of substrate nitridation on the growth of nonpolar a-plane GaN directly deposited on r-plane sapphire by metalorganic chemical vapor deposition (MOCVD) were investigated. Using nitridation, highquality a-plane GaN with flat surface was acquired. On the contrary, if the nitridation layer was removed, the epitaxial a-plane GaN exhibited deep triangular pits and poor crystalline properties. This could be attributed to the fact that uniform-distributed AIN grains were introduced by nitridation, which might act as the nucleation layer for the following a-plane GaN growth. The effects of substrate nitridation on the evolutions of anisotropic morphologies and crystalline properties were also studied by artificially interrupting the growth at different stages. The consequences suggested the nitridation layer could contribute to surface coalescence of a-plane GaN. The reasons responsible for this phenomenon were probed by Raman spectrum, and a model was proposed to explicate the effects of nitridation on the growth of a-plane GaN.
    Applied Surface Science 07/2014; 307:525-532. DOI:10.1016/j.apsusc.2014.04.069 · 2.54 Impact Factor
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    ABSTRACT: The advantages of blue InGaN light-emitting diodes (LEDs) with thickness-chirped barriers in the active region have been investigated by using the Crosslight APSYS programs. The results show that the output power of the proposed LED is increased 80% and the efficiency droop is decreased from 59% in conventional LED to 28% at the current of 250 mA. Based on the analysis of electrical and optical characteristics, these improvements are mainly attributed to the change of electrostatic field in the active region by using thickness-chirped barriers. In the even-numbered barriers, the fields are increased, which gets rid of more seriously bended valence band and results in decreased barrier heights for hole transport in the active region. Furthermore, the direction of electrostatic field in the last barrier is reversed to along the drift direction of holes, which not only can lead to upbended conduction band to rise the barrier height for electron escape but also can accelerate holes to increase the hole injection current. As a result, electrons blocking and holes injection are enhanced, and in turn, the performance of the proposed LED is improved.
    IEEE Photonics Journal 12/2013; 5(6):8200609-8200609. DOI:10.1109/JPHOT.2013.2285714 · 2.33 Impact Factor
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    ABSTRACT: The advantages of ultraviolet light-emitting diodes (LEDs) inserted with a p-AlGaN layer, whose Al mole composition is less than that of the last barrier, between the last barrier and the electron blocking layer have been investigated by using the Crosslight APSYS programs. The results show that the output power and the internal quantum efficiency of the proposed LEDs are improved. Furthermore, the efficiency droop is also mitigated effectively. Based on the analysis of electrical and optical characteristics, these improvements are mainly attributed to the relatively higher effective barrier height against the escape of electrons and an increased hole concentration in the quantum wells by inserting a hole reservoir near the active region. In addition, the optimized Al mole composition of this inserted layer has been also studied in detail, and the optimized Al mole composition has been achieved.
    IEEE Photonics Journal 08/2013; 5(4):1600310-1600310. DOI:10.1109/JPHOT.2013.2278520 · 2.33 Impact Factor
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    ABSTRACT: We explore the possibility of preparation of Pt3M (M = Fe, Ni and Co) nanocubes using carbon monoxide as an alternative protocol under different experimental conditions. The results suggest that the sole carbon monoxide flow at ambient pressure can enable the formation of binary Pt-M nanocrystals, but it is inadequate to facilitate the formation of Pt3M nanocubes in the given system under specified conditions.
    Chemical Communications 04/2013; 49(38). DOI:10.1039/c3cc41424h · 6.83 Impact Factor
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    ABSTRACT: Pt–Cu nanoctahedra were successfully synthesized through a simultaneous reduction of platinum(II) acetylacetonate and copper(I)/(II) chloride in the presence of tungsten hexacarbonyl, oleylamine and oleic acid at high temperature. Comparative studies of electrocatalysis on Pt nanocubes, the {111}-terminated Pt–Cu nanoctahedra and {100}-bounded Pt–Cu nanocubes with similar composition and size indicate that the activity of Pt–Cu nanocrystals towards formic acid oxidation is shape-dependent, and is higher than that of Pt nanocubes.
    Chemical Science 11/2012; 3(3):3302. DOI:10.1039/c2sc20514a · 9.21 Impact Factor
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    ABSTRACT: We report a Kirkwood-Alder transition in a system of nonspherical Pt(3)Cu(2) nanoctahedra coated with oleic acid and oleylamine ligands. Using both transmission electron microscopy tomography with 3D reconstruction analysis and synchrotron-based in-situ grazing-incidence small-angle X-ray scattering (GISAXS) techniques, we specifically determined that these nanoctahedra can assemble into an open structure in which the nanoctahedra are arranged tip-to-tip to form a bcc superlattice with a low packing efficiency. Using in-situ and real-time GISAXS, we further observed a "nanoctahedron crystallization" as a soft Kirkwood-Alder transition, that is, the soft nanoactahedra crystallize at a critical concentration and possess continuous crystalline states during a period of solvent evaporation. Finally, we found a reversible change of the superlattice constant during the solvent annealing and evaporation/drying processes.
    Journal of the American Chemical Society 07/2012; 134(34):14043-9. DOI:10.1021/ja304108n · 11.44 Impact Factor
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    ABSTRACT: Flexible electronics is an emerging and promising technology for next generation of optoelectronic devices. Herein, hierarchical three-dimensional ZnCo(2)O(4) nanowire arrays/carbon cloth composites were synthesized as high performance binder-free anodes for Li-ion battery with the features of high reversible capacity of 1300-1400 mAh g(-1) and excellent cycling ability even after 160 cycles with a capacity of 1200 mAh g(-1). Highly flexible full batteries were also fabricated, exhibiting high flexibility, excellent electrical stability, and superior electrochemical performances.
    Nano Letters 05/2012; 12(6):3005-11. DOI:10.1021/nl300794f · 13.59 Impact Factor
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    ABSTRACT: We report a robust method for synthesis of monodisperse PbSeTe single ternary alloy and core/shell heterostructured nanocubes, respectively. The key synthetic strategy to produce such different classes of nanocubes is to precisely control the time of reaction and successive growth. The crystallinity, shape/size distributions, structural characteristics, and compositions of as-prepared nanocubes, both ternary alloy and core/shell, were carefully studied. A plausible growth mechanism for developing each type of lead chalcogenide nanocubes is proposed. These delicately designed PbSeTe nanoscale architectures offer tunable compositions in PbSeTe ternary alloy and nano-interfaces in core/shell nanocubes, which are the critical factors in controlling thermal conductivity for applications in thermoelectrics.
    Journal of the American Chemical Society 11/2011; 133(44):17590-3. DOI:10.1021/ja207763p · 11.44 Impact Factor
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    ABSTRACT: Flexible and transferable TiO(2) nanorods cloths (TNRCs) were synthesized from a fast and catalyst-free microwave heating route by using carbon cloth as an efficiently sacrificial template. The as-synthesized TNRCs were assembled by numerous aligned TiO(2) nanorods with diameters of about 100 nm. The good transferability and flexibility make it possible to be transferred to any substrate for further device applications. As an example, we transferred the TNRCs to a FTO substrate to make dye-sensitized solar cells, which exhibited an improved efficiency of around 2.21% assisted by TiCl(4) treatment. The transferable TNRCs were also configured as high-performance photodetectors. Illuminated by UV light with a wavelength of 365 nm, the current was found significantly enhanced, and an I(UV)/I(dark) of about 60, a rise time of nearly 1.4 s, and a decay time of 6.1 s were obtained. Moreover, they were also configured as flexible and recyclable photocatalysts with good photocatalytic performance for the degradation of methylene blue solution under UV light irradiation.
    ACS Nano 09/2011; 5(10):8412-9. DOI:10.1021/nn203315k · 12.88 Impact Factor
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    ABSTRACT: We present a structural study of Pt(3)Ni nanoctahedron superlattice, prepared through both drop-casting and controlled solvent evaporation approaches. In this superlattice system containing ∼10.6 nm side-length Pt(3)Ni nanoctahedra, we observed a body-centered cubic (bcc) packing structure in both local superlattices and statistically averaged superlattice ensembles using transmission electron microscopic tomography and grazing-incidence small-angle X-ray scattering techniques, respectively. Within the superstructure, it was directly observed that nanoctahedra are orientated along the superstructure axes through sharing their vertices. We found that this arrangement of a bcc superstructure with nanoctahedra connecting through their vertices is dependent on neither the processing pathway nor the substrate under our experimental conditions. With such a very low packing density and ultrahigh surface area, this type of self-organized superstructure possesses unique features for future applications.
    Nano Letters 06/2011; 11(7):2912-8. DOI:10.1021/nl201386e · 13.59 Impact Factor
  • Jun Zhang · Zhaoyong Sun · Jiye Fang
    MRS Online Proceeding Library 01/2011; 942. DOI:10.1557/PROC-0942-W08-25
  • Zhaoyong Sun · Jun Zhang · Ming Zhang · Jiye Fang
    MRS Online Proceeding Library 01/2011; 942. DOI:10.1557/PROC-0942-W08-24
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    ABSTRACT: Regular square FeSe x nanoflakes with tetragonal PbO-type phase and selenium deficiency have been grown from an inorganic iron source, i.e. ferrous chloride and selenium trioctylphosphine through a simple solution-based route in the co-solvent of oleylamine and oleic acid. The phase and structure are identified by powder X-ray diffraction and electron microscopy. The square nanoflakes are revealed to be bound by two larger {001} planes and four equivalent smaller {100} side surfaces. The appearance of the larger {001} planes and flaky morphology inherently results from the greatest atom density on {001} lattice planes and the largest d 001 spacing. The addition of the organic acid, i.e. oleic acid, at a proper stage during heating of the starting materials, is of great importance to the yield of the tetragonal PbO-type phase. Interestingly, the chemical composition and the edge length of the single crystalline FeSe x nanoflakes can be tailored readily by using an organic diol which is suggested to serve as a reducing agent and chemical ligand. Thus, the deficiency of selenium can be highly reduced and the top-bottom surface area be enlarged.
    CrystEngComm 11/2010; 12(12). DOI:10.1039/C005097K · 3.86 Impact Factor
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    ABSTRACT: A facile, reliable, and robust synthetic method to prepare high-quality Pt3Fe nanocubes is reported. Results of investigation on these Pt3Fe nanocubes in shape-/size-distribution, crystallinity, structural characteristics, compositions, electrochemical activity towards small organic molecule oxidation as well as capability of assembling into simple cubic supercrystals are presented, and a possible formation mechanism of the cubic nanocrystals at high temperature in oleylamine/oleic acid solution is also proposed. This study indicates that the catalyst performance may be improved by controlling the shape of particles.
    Advanced Functional Materials 11/2010; 20(21):3727 - 3733. DOI:10.1002/adfm.201000679 · 11.81 Impact Factor